Abstract

Functionalization of nanocarbon materials with heteroatoms is of paramount interest as doping of carbon with electron withdrawing groups results in change of electrochemical properties of the potential catalyst. Adding fluorine, as the most electronegative element into the doping process next to boron is expected to have significant effect on the design of novel nanocarbon‐based electrocatalysts. In this paper boron and fluorine co‐doped reduced graphene oxide/few‐walled carbon nanotube (BF‐rGO/FWCNT) catalysts are synthesized via simple and low‐cost direct pyrolysis method using boron trifluoride diethyl etherate (BTDE). Composition analysis confirmed that boron and fluorine have been grafted onto the carbon support. Rotating disk electrode (RDE) measurements revealed that BF‐rGO/FWCNT has remarkable electrocatalytic activity toward the oxygen reduction reaction (ORR) both in alkaline and acid media. The onset potential of the best BF‐rGO/FWCNT catalyst was 50 mV more positive in alkaline and 600 mV more positive in acidic media compared with un‐doped rGO/FWCNT. The half‐wave potential was 100 mV more positive in alkaline media and 700 mV more positive in acidic media in comparison with un‐doped rGO/FWCNT.

Demonstrating extremely large reduction of cross-plane thermal conductivity of graphite by co-intercalating MnCl₂ and FeCl₃ and identifying the mechanism through its thickness and temperature dependences.

Pre-oxidation of Ni substrates before sulfidation resulted in a highly active multifunctional anode catalyst capable of ensuring energy-saving hydrogen production by water-small molecule coelectrolysis.

An extended hydrothermal treatment of Ni with restricted Te content resulted in a highly active tri-functional electrocatalyst performing the HER, OER, and MOR in alkaline medium.

A critical experimental analysis finding the answer to the question, ‘Can we use any reference electrode irrespective of its pH-compatibility?’.

Liquid ammonia is able to release hydrogen via electrolysis with a theoretical voltage of 0.077 V. However, the actual operation voltage is much higher (around 1–2 V) due to the large overpotential of the anodic reaction. We systematically investigated Pt, Ir, Ru, Ni, Co, Fe, Ta, and Ti as the anodic catalyst, and observed a volcano correlation when plotting the current density against the formation enthalpy of metal nitrides per nitrogen ΔfHMN (kJ/mol-N). ΔfHMN reflects the strength of metal-nitrogen bond, and Ru having a moderate ΔfHMN showed the highest catalytic activity. Then, the activity enhancement via microstructure control was examined. The sputtered Pt and Ni films having a columnar structure with a rough surface showed 7- and 13-times higher current density than the Pt and Ni plates, respectively. An anodic current density of 0.45 mA/cm2 was achieved for the sputtered Ni film at an anodic potential as low as 0.3 V vs H2/NH3.

The MgH2-Nb2O5-carbon nanotube (CNT) composite was fabricated to accommodate the volume change of particles during the hydrogen storage cycles by holding the MgH2-Nb2O5 particles within the sponge-like matrix of the CNT. This allowed for preservation of the composite structure and led to more stable hydrogen sorption properties during 20 cycles, as compared to without CNT. To investigate this effect of CNT on the cyclic stability of MgH2-Nb2O5, CNT and expanded graphite (EG) were added to MgH2-Nb2O5 via ball milling. The MgH2-Nb2O5-CNT powder showed stable cyclic performance, similar to the MgH2-Nb2O5-CNT composite, whereas the MgH2-Nb2O5-EG powder exhibited cyclic degradation similar to MgH2-Nb2O5. From SEM-EDS, it was found that the C/Mg ratio of the surface of the MgH2-Nb2O5-CNT powder was higher than that of the MgH2-Nb2O5-EG powder. Thus, the fibrous CNT on the surface of the MgH2 particles could be responsible for the greater cyclic stability of the MgH2-Nb2O5-CNT composite.

A Ag aerogel-like film (referred to as aerogel hereafter), used as a thermal interface material with enhanced performance and thermal stability, is reported. An aerogel/foil/aerogel "monocomposite"is created by rapidly forming and depositing Ag nanoparticles on both faces of a Ag foil by the gas-evaporation and particle-deposition method in a few minutes. The monocomposite, having an aerogel of a low packing ratio (<10%) with a clean surface, exhibits low thermal resistance (18 mm2 K W-1) when sandwiched with Cu rods. The monocomposite exhibits excellent thermal stability and significantly reduced thermal resistance (3.0 mm2 K W-1) at 246 °C, keeping its low thermal resistance (2.8 mm2 K W-1) after the temperature is decreased to 51 °C; further, the resistance reduces to 2.1 mm2 K W-1 after 10 cycles of heating and cooling (100-200 °C). The Ag monocomposite, having high performance comparable to that of the benchmark In sheet and excellent thermal stability at a temperature higher than the melting point of In, will enable high-power and high-temperature operation of computing and power devices.

A perspective with an explicit account of the appropriate screening of water splitting electrocatalysts advocating dos and don’ts!

Cyclic and linear sweep voltammetry techniques substantially misjudge the performance of water splitting electrocatalysts due to their transient nature that forbids the interface from reaching a steady-state. This misjudgment leads to the potentially detrimental yet unwittingly falsified data accumulation in the literature that requires immediate attention. Alternatively, sampled-current voltammetry (SCV) constructed from steady-state responses is advised to be widely adopted for screening electrocatalysts that are actually destined for steady-state operations. To show that this exaggeration is universal, a well-characterized activated SS, coprecipitated Co(OH)₂, and Pt foil electrodes are studied for OER and HER in 1.0 M KOH. The results urge that it is time to adopt a relatively more precise alternative technique such as SCV.

This perspective details every single aspects of iRu drop correction in controlled-potential electrocatalysis from fundamentals to the best practices.

A critical perspective that questions the use of PtX₂ for the HER when we have a better performing Pt/C while analysing the potential ways in which PtX₂ can actually be better than Pt/C.

Controlling catalyst-particle formation is essential for the growth of single-wall carbon nanotube (SWCNT) arrays with improved alignment, areal mass, and height. We have previously reported the positive effect of CO2 on SWCNT growth via chemical vapor deposition, and in this study, we found its negative effect on catalyst-particle formation during annealing. A Fe (1 nm)/AlOx (15 nm) catalyst that was sputter-deposited on SiO2/Si substrates demonstrated a prolonged lifetime and enabled the growth of SWCNT arrays with better alignment, twice the height, and three times higher areal mass when the catalyst was annealed under 10 vol% H2/Ar without CO2 than with 1 vol% CO2. Detailed analysis indicated that the Fe particles could remain partially oxidized during annealing in H2 with mildly oxidative CO2, resulting in the bulk diffusion of Fe into the AlOx layer. In contrast, Fe is reduced sufficiently in H2 in the absence of CO2, thereby remaining on the AlOx surface and active for SWCNT growth. The findings of this study emphasize the importance of maintaining a highly reductive atmosphere during annealing to achieve active catalyst particles with a higher number density and longer lifetime.

Needs of a material for thermal management in reduced-sized electronic devices drive single-walled carbon nanotubes (SWCNTs) to be one of the most promising candidates due to their excellent thermal properties. Many numerical and experimental studies have reported on understanding thermal properties of the SWCNT for thermal device applications. In the present study, thermal diffusivity and conductivity of SWCNT forests in the axial direction with various volume fractions of SWCNTs were measured by laser flash analysis technique and the same properties of an individual SWCNT were derived. The volume fraction was controlled up to 25 vol% by biaxial mechanical densification which maintains SWCNT alignment. While the thermal diffusivity of SWCNT forests was almost constant, it increased when the volume fraction was higher than 17 vol%, suggesting that some SWCNTs, which did not serve as a thermal path in the case of lower volume fraction, were connected with the other SWCNTs. Through the increase of inferred thermal conductivity equivalent to an individual SWCNT after 17 vol%, we also realized that the enhancement of volume fraction of SWCNT forest diminished thermal boundary resistance in good agreement with the tendency inferred from the reported numerical data. (C) 2021 Elsevier Ltd. All rights reserved.

We propose using hydrogen as a heat transfer medium to supply waste heat from hydrogen-driven devices to hydrogen storage tanks. In our model, MgH is used in the form of porous sheets, set in parallel in the tank, and heat is supplied via hot hydrogen flowed through the interspaces between the porous sheets. Feasibility of the hydrogen desorption reaction in this process was verified numerically. Hydrogen efficiently carried heat to the stack of porous MgH sheets via convective heat transfer and then carried heat into the porous MgH sheets via conductive heat transfer through the pores owing to its high thermal conductivity. We found that the hydrogen desorption is also fast enough to allow the supplied heat to be used efficiently to drive the endothermic hydrogen desorption reaction. It was understood that the thickness of the MgH sheet and hot hydrogen flow speed affected hydrogen desorption. These factors can be evaluated by using the dimensionless number of τ /τ which is the ratio of the space time to the time constant for heat transfer in the MgH sheet. Under τ /τ > 0.01 range, both the reaction and heat transfer are fast enough, the hydrogen desorption is limited by heat supply, and hydrogen desorption amount is proportional to the heat supplied to the reactor. The tank structure and operating conditions can be designed by using the dimensionless number of τ /τ . 2 2 2 2 s h 2 s h s h

© 2020 Elsevier Ltd A carbon nanotube forest with a length of 14 cm grew with an average growth rate of 1.5 μm s−1 and a growth lifetime of 26 h. Several key factors to realize this unprecedented long growth such as catalyst conditions, growth conditions in chemical vapor deposition, and reactor system were clarified. It was found that the combination of the catalyst system of iron/gadolinium/aluminum oxide (Fe/Gd/Al2Ox) and the in situ supplements of Fe and Al vapor sources at very low concentration was crucially important. A cold-gas system, where only the substrate is heated while keeping the gas at room temperature, was employed to suppress unnecessary reactions and depositions. The long carbon nanotube forest enabled macroscopic measurements of the tensile and electrical properties of the carbon nanotube wires, and it gave several important insights for industrial applications of the carbon nanotubes in the future.

Electrocatalytic oxygen evolution reaction (OER) catalyzed by non-precious metals and their compounds in alkaline medium is an attractive area of energy research for the generation of hydrogen from water. The 3d transition metals, particularly, Ni and Co show better OER activity than others in alkaline medium. Ni and Co based oxygen-evolving catalysts (OECs) experience an enormous enhancement in the OER activity either by incidental or intentional Fe doping/incorporation. To account for this, different roles of Fe that it exerts when incorporated into these OECs are reported to be responsible. Unfortunately, the conclusions drawn in many related studies are often contradictory to one another. Important contradictory conclusions are: 1) a few studies claim Fe is the active site and Ni/Co are inactive while other studies claim Ni/Co and Fe act together in OER, 2) a few studies claim Fe stays unoxidized while a few shows evidence for the existence of Fe , and 3) a few studies suggest Fe is the faster site in Ni/Co OEC matrices for OER but fail to explain similar effects observed with other OER matrices. Many critical experimental and theoretical investigations have been made recently to reveal this magical Fe effect and the results of those studies are coherently presented here with critical discussion. This review is presented as it is inevitable to know the critical roles of Fe effect in Ni/Co based OECs to succeed in energy efficient hydrogen generation in alkaline medium. 3+ 4+ 3+

Transition metal hydroxides (M-OH) and their heterostructures (X|M-OH, where X can be a metal, metal oxide, metal chalcogenide, metal phosphide, etc.) have recently emerged as highly active electrocatalysts for hydrogen evolution reaction (HER) of alkaline water electrolysis. Lattice hydroxide anions in metal hydroxides are primarily responsible for observing such an enhanced HER activity in alkali that facilitate water dissociation and assist the first step, the hydrogen adsorption. Unfortunately, their poor electronic conductivity had been an issue of concern that significantly lowered its activity. Interesting advancements were made when heterostructured hydroxide materials with a metallic and or a semiconducting phase were found to overcome this pitfall. However, in the midst of recently evolving metal chalcogenide and phosphide based HER catalysts, significant developments made in the field of metal hydroxides and their heterostructures catalysed alkaline HER and their superiority have unfortunately been given negligible attention. This review, unlike others, begins with the question of why alkaline HER is difficult and will take the reader through evaluation perspectives, trends in metals hydroxides and their heterostructures catalysed HER, an understanding of how alkaline HER works on different interfaces, what must be the research directions of this field in near future, and eventually summarizes why metal hydroxides and their heterostructures are inevitable for energy-efficient alkaline HER.

We recently reported the fastest anodization method (just 80 s) of all for accessing a denser array of Cu(OH)2-CuO nanoneedles on a Cu foil substrate by applying a constant potential of 0.864 V vs a reversible hydrogen electrode in 1.0 M KOH that delivered a better activity for the methanol oxidation reaction (MOR). In this study, we show that the strength of the KOH solution used for anodization alters the size, morphology, surface chemistry, electrochemical accessibility of Cu sites, and the subsequent MOR activity trend. Intriguingly, an increase in KOH solution strength shortens the time of anodization from 80 s (1.0 M KOH) to 20 s with 3.0 M KOH, which in turn drastically reduces to just 6 s with 6.0 M KOH. As of now, this is the shortest time ever achieved for the anodic growth of Cu-OH/O nanoneedles on a Cu substrate. A set of detailed and comparative physical and electrochemical characterizations reveal positive relationships between anodization pH and anodization current, the size of Cu-OH/O nanoneedles grown, rate of growth, electrochemical accessibility of Cu sites, and electrocatalytic MOR activity. Thus, this study provides a universal approach to control the size of Cu-OH/O nanoneedles, electrochemical accessibility of Cu sites, and their subsequent MOR activity.

Carbon nanotube (CNT)/n-Si heterojunction solar cells were fabricated based on solution processes. CNT film with high transparency of 90% and low sheet resistance of ∼115 Ω/sq was fabricated from commercially available CNT powder via dispersion-filtration process using poly(p-styrene-sulfonic acid) (PSS) as both dispersant and dopant. Heterojunction was then fabricated by attaching the CNT-PSS film onto an n-Si wafer. The CNT/n-Si solar cell showed an enhanced efficiency of 11.7% with the PSS-doping compared with the device without doping (7.7%). The device showed further improvements in efficiency to 14.1% with an antireflective coating layer of sulfonated polytetrafluoroethylene (Nafion) and stability to 1000 h in ambient air without additional protection. The solution-based strategies using polymeric acids for efficiency and stability enhancement will open new avenues for the low-cost CNT/Si heterojunction solar cells.

© 2020 Elsevier B.V. Carbon nanotube/silicon solar cells have increasingly attracted attention owing to their high efficiency and cost-effectiveness. Herein, we demonstrated a significant improvement in the performance of the carbon nanotube/silicon solar cell by applying solution-processable MoOx. The MoOx acts as both an efficient chemical dopant and an anti-reflection coating, resulting in a reduction in the series resistance and an enhancement of the short-circuit current density of the cell. As a result, the power conversion efficiencies increased to 10.0%, which is a 39% increase from the pristine value (7.2%). The device showed considerable stability, maintaining the power conversion efficiency at 80% of its original value for two months in the air without any protective layer. The simple solution process at room-temperature in this study can lower the preparation cost and pave a way for the practical application of carbon nanotube/silicon solar cells.

Electrochemical hydrogen peroxide synthesis using two-electron oxygen electrochemistry is an intriguing alternative to currently dominating environmentally unfriendly and potentially hazardous anthraquinone process and noble metals catalysed direct synthesis. Electrocatalytic two-electron oxygen reduction reaction (ORR) and water oxidation reaction (WOR) are the source of electrochemical hydrogen peroxide generation. Various electrocatalysts have been used for the same and were characterized using several electroanalytical, chemical, spectroscopic and chromatographic tools. Though there have been a few reviews summarizing the recent developments in this field, none of them have unified the approaches in catalysts' design, criticized the ambiguities and flaws in the methods of evaluation, and emphasized the role of electrolyte engineering. Hence, we dedicated this review to discuss the recent trends in the catalysts' design, performance optimization, evaluation perspectives and their appropriateness and opportunities with electrolyte engineering. In addition, particularized discussions on fundamental oxygen electrochemistry, additional methods for precise screening, and the role of solution chemistry of synthesized hydrogen peroxide are also presented. Thus, this review discloses the state-of-the-art in an unpresented view highlighting the challenges, opportunities, and alternative perspectives.

In this study, we show a simple two-step surface engineering method that uses chemical oxidation (using KOH and NaClO in 1:2 M ratio)-assisted leaching of metals (Cr, Mn, and Ni) from the surface and an electrochemical potentiostatic activation enabled resurfacing of only catalytically active Ni and Mn of the alloy. Such surface-engineered stainless steel 304 (SS-304-Ox-ECA) foils rich in Ni(OH)2 and multivalent Mn oxides were found to have a coarse texture with uniform nanostructures. As a result of leached Cr, resurfaced catalytically active sites improved roughness with nanotexturing and enhanced the charge-transfer ability. The SS-304-Ox-ECA foil has become a high-performance HER electrocatalyst that delivered 400 mA cm-2 higher current density at -0.8 V versus RHE and demanded 210 mV lower overpotential for a current density of 100 mA cm-2 than pristine SS-304 foils in 1.0 M KOH. A smaller Tafel slope (90 mV dec-1) and a higher double-layer capacitance (2Cdl = 0.784 μF cm-2) further justified that the activity enhancement is also due to the improved HER kinetics and increased electrochemical surface area. This catalytic electrode of high abundance and low cost is a promising candidate for cost-efficient hydrogen production from water.

Non-oxide/hydroxide hydrogen evolution reaction (HER) catalysts undergo hydroxylation to a significant extent even under reductive condition when exposed to alkali. Actual role of such hydroxylation in alkaline HER electrocatalysis is not previously given any significance. In this study, we report an intriguing finding that nickel sulfide a well-known HER electrocatalyst when subjected to anodic potential cycling covering Ni and Ni redox couple led to accelerated hydroxylation accompanying surface amorphization. As a result, improved electrochemical surface, better HER kinetics, and better charge transfer were achieved that lowering the HER overpotential by 110 mV at 100 mA cm . This surface amorphized and hydroxylated nickel sulfide exhibited excellent stability upon both galvanostatic and potentiostatic electrolysis for over 50 h. Besides, it also showed a lower Tafel slope (120 mV dec ), higher relative ECSA in terms of 2C (3.85 µF cm ), and higher electrochemical accessibility for Ni sites (3.7 × 10 cm ) which further advocate the superiority of our way of improving HER activity of a non-oxide/hydroxide catalyst. Thus, this study open up new avenues for re-examining other non-oxide/hydroxide catalysts in alkaline HER for benefiting the energy and cost-efficient hydrogen generation. 2+ 3+ 2 -1 −2 17 −2 dl

Oxygen evolution reaction (OER) is the bottleneck for realizing energy-efficient hydrogen production through water electrolysis in both acid and alkali. Alkaline OER electrocatalyzed by Ni and Co hydroxides are well known which showed unexpected enhancement with the addition of Fe. We found that the commercially procured Cu foam containing trace amount of Ni (~1.5 wt.%) upon anodization formed Cu(OH) –CuO nanowires with conceivable formation of Ni(OH) and experienced a notable enhancement in its OER activity. When sufficient amount of Fe was intentionally supplemented during anodization, OER activity of the same was further improved. Specifically, as a combined result of anodization in KOH and in Fe supplemented KOH, overpotential at 50 mA cm was lowered by 153 mV. Such an activation also improved the kinetics of OER by lowering the Tafel slope by 100 mV dec . With these, it has been shown here that a moderately active OER catalyst i.e., Cu(OH) –CuO/Cu formed upon the anodization of Cu foam can be turned into a highly active catalyst just by utilizing the trace Ni that it already contains and intentionally supplementing sufficient amount of Fe. 2 2 2 −2 −1

A swift potentiostatic anodization method for growing a 5-7 μm tall nanoneedle array of Cu(OH)2-CuO on Cu foil within 100 s has been developed. This catalytic electrode when screened for methanol oxidation electrocatalysis in 1 M KOH with 0.5 M methanol, delivered a current density as high as 70 ± 10 mA cm-2 at 0.65 V versus Hg/HgO which is superior to the performance of many related catalysts reported earlier. The observed activity enhancement is attributed to the formation of both Cu(OH)2-CuO nanoneedle arrays of high active surface area over the metallic Cu foil. In addition, the Cu(OH)2-CuO/Cu electrode had also exhibited excellent stability upon prolonged potentiostatic electrocatalytic oxidation of methanol while retaining the charge-transfer characteristics. Growth of such highly ordered assembly of Cu(OH)2-CuO nanoneedles within a minute has never been achieved before. When compared to its oxygen evolution reaction activity, the addition of 0.5 M methanol has lowered the overpotential at 10 mA cm-2 by 334 mV, which is significant. This encourages the use of methanol as a sacrificial anolyte for energy-saving production of H2 from water electrolysis.

Electrochemical water splitting powered by electrical energy derived from renewable sources is a green and faster way of producing bulk hydrogen with the highest purity. Unfortunately, the cost-inefficiency associated with energy loss (as overpotential) and costs of electrode materials have been forbidding this technology to surpass the currently dominant industrial process (steam reforming of hydrocarbons). With the recent evolution of transition metal chalcogenides, efficient commercial electrochemical water splitting is not too far. Transition metal chalcogenides are better in the hydrogen evolution reaction (HER) than pristine metals as they have negatively polarized chalcogenide anions with relatively lower free energy for proton adsorption. Moreover, chalcogenides are relatively easy to prepare and handle. Several metal chalcogenides have been reported with good HER activity among which Ni chalcogenides are reported to be exceptional ones. In recent years, growth of the nickel chalcogenide catalysed HER is massive. This review is devoted to bringing out a comprehensive understanding of what had happened in the recent past of this field with highlights on future prospects. In addition, we have also briefed the key physico-chemical properties of these materials and highlighted what one should anticipate while screening an electrocatalyst for electrochemical water splitting.

In the near future, sustainable energy conversion and storage will largely depend on the electrochemical splitting of water into hydrogen and oxygen. Perceiving this, countless research works focussing on the fundamentals of electrocatalysis of water splitting and on performance improvements are being reported everyday around the globe. Electrocatalysts of high activity, selectivity, and stability are anticipated as they directly determine energy- and cost efficiency of water electrolyzers. Amorphous electrocatalysts with several advantages over crystalline counterparts are found to perform better in electrocatalytic water splitting. There are plenty of studies witnessing performance enhancements in electrocatalysis of water splitting while employing amorphous materials as catalysts. The harmony between the flexibility of amorphous electrocatalysts and electrocatalysis of water splitting (both the oxygen evolution reaction [OER] and the hydrogen evolution reaction [HER]) is one of the untold and unsummarized stories in the field of electrocatalytic water splitting. This Review is devoted to comprehensively discussing the upsurge of amorphous electrocatalysts in electrochemical water splitting. In addition to that, the basics of electrocatalysis of water splitting are also elaborately introduced and the characteristics of a good electrocatalyst for OER and HER are discussed.

© 2019 American Chemical Society. Multi-millimeter-tall vertically aligned single-wall carbon nanotube (VA-SWCNT) forests were grown using Fe/Gd/Al2Ox catalyst with high initial growth rate of ∼2 μm s-1 and long catalyst lifetime of ∼70 min at 800 °C. The addition of Gd with a nominal thickness of 0.3 nm drastically prolonged the catalyst lifetime. The analysis of the VA-SWCNT forests by a transmission electron microscope showed that the average diameter of the SWCNTs grown with Gd is constant from the top to the bottom of the forests, while it monotonically increased without Gd. This indicated that Gd suppresses the structure change of the Fe nanoparticles in the lateral direction during the CNT growth. By X-ray photoelectron spectroscopy, it was found that the longer catalyst lifetime with Gd stems from the suppression of the interaction between Fe and C resulting in the smaller structure change of the Fe nanoparticles.

We investigated the heat transfer behavior of thermally conductive networks with one-dimensional carbon materials to design effective heat transfer pathways for hybrid filler systems of polymer matrix composites. Nano-sized few-walled carbon nanotubes (FWCNTs) and micro-sized mesophase pitch-based carbon fibers (MPCFs) were used as the thermally conductive materials. The bulk density and thermal conductivity of the FWCNT films increased proportionally with the ultrasonication time due to the enhanced dispersibility of the FWCNTs in an ethanol solvent. The ultrasonication-induced densification of the FWCNT films led to the effective formation of filler-to-filler connections, resulting in improved thermal conductivity. The thermal conductivity of the FWCNT-MPCF hybrid films was proportional to the MPCF content (maximum thermal conductivity at an MPCF content of 60 wt %), indicating the synergistic effect on the thermal conductivity enhancement. Moreover, the MPCF-to-MPCF heat transfer pathways in the FWCNT-MPCF hybrid films were the most effective in achieving high thermal conductivity due to the smaller interfacial area and shorter heat transfer pathway of the MPCFs. The FWCNTs could act as thermal bridges between neighboring MPCFs for effective heat transfer. Furthermore, the incorporation of Ag nanoparticles of approximately 300 nm into the FWCNT-MPCF hybrid film dramatically enhanced the thermal conductivity, which was closely related to a decreased thermal interfacial resistance at the intersection points between the materials. Epoxy-based composites loaded with the FWCNTs, MPCFs, FWCNT-MPCF hybrids, and FWCNT-MPCF-Ag hybrid fillers were also fabricated. A similar trend in thermal conductivity was observed in the polymer matrix composite with carbon-based hybrid films.

© 2019, The Author(s). Solution-based heterojunction technology is emerging for facile fabrication of silicon (Si)-based solar cells. Surface passivation of Si substrate has been well established to improve the photovoltaic (PV) performance for the conventional bulk Si cells. However, the impact is still not seen for the heterojunction cells. Here, we developed a facile and repeatable method to passivate the Si surface by a simple 1-min annealing process in vacuum, and integrated it into the heterojunction cells with poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) or carbon nanotube (CNT). A thin and dense oxide layer was introduced on the Si surface to provide a high-quality hole transport layer and passivation layer. The layer enhanced the power conversion efficiency from 9.34% to 12.87% (1.38-times enhancement) for the PEDOT:PSS/n-Si cells and from 6.61% to 8.52% (1.29-times enhancement) for the CNT/n-Si cells. The simple passivation is a promising way to enhance the PV performance of the Si cells with various solution-based heterojunctions.

Abstract

The development of semitransparent photoanodes is required for the construction of tandem photoelectrochemical (PEC) water splitting cells incorporating photocathodes. However, the poor stability of transparent conductive oxides at high temperatures hampers the growth of non‐oxide photoanodes with intense visible light absorption. In this work, semitransparent Ta₃N₅ thin film photoanodes were prepared on quartz glass substrates coated with carbon nanotubes (CNTs) by sputtering and thermal nitridation. This process makes use of the high thermal and chemical stability as well as the tunable conductivity and transmittance of CNT substrates. The photoanodic current produced by these Ta₃N₅ photoanodes at negative potentials is also enhanced by surface modification with Mg species. Conductive semitransparent CNT substrates such as these will assist in the development of new tandem PEC cells for water splitting.

© 2019 Elsevier Ltd We report a high-productivity chemical vapor deposition (CVD) process of graphene by extending the reaction field to three dimensions (3D) and shortening the CVD time to a few minutes. A large Cu foil (5 × 30 cm2) is rolled up and set in a small reactor (3.4 cm in diameter), and a continuous graphene film is obtained uniformly in a short time (1.5 min) by using C2H4 as a more reactive carbon source than the popular CH4. The graphene transferred onto a quartz glass showed optical transmittances of 94.8–96.7% (550 nm) with sheet resistances of 0.78–1.68 kΩ sq−1 (without doping) and 0.3 kΩ sq−1 (with doping by HNO3 vapor). Compared with the previous reports for fast and/or large-scale CVD, our method realized similarly high productivity of 100 cm2 min−1 based on the CVD time despite of the small reactor, and higher productivity of 0.03 cm2-graphene per cm3-reactor per minute based on the reactor volume and total time for high temperature processing (15 min for heating, 1 min for annealing, and 1.5 min for CVD). The knowledge obtained here on the CVD conditions and packing ratio of Cu foils (0.55 cm2 per cm3-reactor) is reusable for designing large-scale graphene production processes.

© 2019 Elsevier B.V. Direct formation of graphene films on dielectric substrates is investigated by the “etching-precipitation” method which converts metal-carbon mixed films to graphene films by etching metal away by Cl 2 at 600–650 °C. Here we report a new approach for improved control of the layer number and continuity of the graphene films. Reactive sputtering of Fe in C 2 H 4 /Ar enabled fine control of the carbon concentrations and thicknesses of the initial Fe-C films, which yielded continuous multilayer graphene films of controllable average layer numbers of ~10–40, low resistivity down to ~240 μΩ cm, and high Raman G-band to D-band intensity ratio up to 16 directly on SiO 2 substrates. We also show that the carbon concentration of the initial Fe-C films determines the film continuity and crystallinity of the graphene.

The CO2-assisted chemical vapor deposition (CVD) is reported as a versatile method for millimeter-tall vertically-aligned single-wall carbon nanotube (VA-SWCNT) arrays when compared with the famous H2O-assisted one. The mild oxidant CO2 enabled the VA-SWCNT growth with mostly equivalent structures and yield when it was added at a much higher concentration (0.3–1 vol%) than H2O (50 ppmv). Furthermore, CO2 showed a clear advantage for the uniform growth when 18 substrates (10 × 10 mm2) were loaded in one batch. The areal yield of VA-SWCNTs decreased drastically from 1.6 to 0.4 mg cm−2 for the first 4 substrates with 50 ppmv H2O because of its depletion whereas it decreased more mildly from 1.6 to 0.8 mg cm−2 for the whole 18 substrates with 1.0 vol% CO2. The gradual decrease in the SWCNT yield with 1.0 vol% CO2 was caused by the change in the carbon source depending on its position. The mixed feed of 0.30 vol% C2H2 (being converted to SWCNTs gradually) and 3.0 vol% C2H4 (yielding C2H2 gradually) realizes the uniform growth of VA-SWCNTs for the whole 18 substrates. The CO2-assisted CVD with optimized carbon feed is promising for the uniform growth of millimeter-tall SWCNTs in large areas.

© 2018 The Royal Society of Chemistry. A highly sensitive interdigitated electrode (IDE) with vertically aligned dense carbon nanotube forests directly grown on conductive supports was demonstrated by combining UV lithography and a low temperature chemical vapor deposition process (470 °C). The cyclic voltammetry (CV) measurements of K4[Fe(CN)6] showed that the redox current of the IDE with CNT forests (CNTF-IDE) reached the steady state much more quickly compared to that of conventional gold IDE (Au-IDE). The performance of the CNTF-IDE largely depended on the geometry of the electrodes (e.g. width and gap). With the optimum three-dimensional electrode structure, the anodic current was amplified by a factor of ∼18 and ∼67 in the CV and the chronoamperometry measurements, respectively. The collection efficiency, defined as the ratio of the cathodic current to the anodic current at steady state, was improved up to 97.3%. The selective detection of dopamine (DA) under the coexistence of l-ascorbic acid with high concentration (100 μM) was achieved with a linear range of 100 nM-100 μM, a sensitivity of 14.3 mA mol-1 L, and a limit of detection (LOD, S/N = 3) of 42 nM. Compared to the conventional carbon electrodes, the CNTF-IDE showed superior anti-fouling property, which is of significant importance for practical applications, with a negligible shift of the half-wave potential (ΔE1/2 < 1.4 mV) for repeated CV measurements of DA at high concentration (100 μM).

© 2018 The Authors Flame synthesis enables the mass-production of carbon black and fullerene but not of carbon nanotubes (CNTs) due to the narrow window for producing CNTs while preventing tar generation. We report a flame-assisted chemical vapor deposition method, in which a premixed flame is used for the instantaneous generation of floating catalysts, the heating of the gas, and the growth of single-wall CNTs (SWCNTs) using a furnace at the downstream of the flame. This method yields high quality SWCNTs with a small average diameter of 0.96 nm, a small diameter deviation of 0.21 nm, and a high carbon purity of ∼90 wt%. Multiple parameters affect the SWCNT production significantly, which are investigated systematically and optimized carefully. The effects and possible mechanisms of the key parameters are discussed.

A carbon nanotube (CNT) web electrode comprising magnetite spheres and few-walled carbon nanotubes (FWNTs) linked by the carboxylated conjugated polymer, poly[3-(potassium-4-butanoate) thiophene] (PPBT), was designed to demonstrate benefits derived from the rational consideration of electron/ion transport coupled with the surface chemistry of the electrode materials components. To maximize transport properties, the approach introduces monodispersed spherical Fe3O4 (sFe3O4) for uniform Li+ diffusion and a FWNT web electrode frame that affords characteristics of long-ranged electronic pathways and porous networks. The sFe3O4 particles were used as a model high-capacity energy active material, owing to their well-defined chemistry with surface hydroxyl (-OH) functionalities that provide for facile detection of molecular interactions. PPBT, having a π-conjugated backbone and alkyl side chains substituted with carboxylate moieties, interacted with the FWNT π-electron-rich and hydroxylated sFe3O4 surfaces, which enabled the formation of effective electrical bridges between the respective components, contributing to efficient electron transport and electrode stability. To further induce interactions between PPBT and the metal hydroxide surface, polyethylene glycol was coated onto the sFe3O4 particles, allowing for facile materials dispersion and connectivity. Additionally, the introduction of carbon particles into the web electrode minimized sFe3O4 aggregation and afforded more porous FWNT networks. As a consequence, the design of composite electrodes with rigorous consideration of specific molecular interactions induced by the surface chemistries favorably influenced electrochemical kinetics and electrode resistance, which afforded high-performance electrodes for battery applications.

The authors regret that the incorrect version of the Graphical Abstract was included with this article. The figures within the article are correct. The correct Graphical Abstract appears below: [Figure presented] The authors would like to apologise for any inconvenience caused.

Millimeter-tall vertically aligned carbon nanotubes (VA-CNTs) were grown directly on Al substrates. Atmospheric pressure chemical vapor deposition is performed at 600 °C, which is well below the melting point of Al (660 °C), using Fe catalyst and C2H2 as a highly reactive carbon feedstock. The CNT height was sensitive to the C2H2 concentration and 0.06 vol% was optimum for balanced growth rate and catalyst lifetime, yielding 0.06 mm-tall VA-CNTs in 2 h. The CO2 addition at 1.8 vol% to the C2H2/Ar gas significantly enhanced the CNT growth, yielding 1.1 mm-tall VA-CNTs in 12 h. CO2 shows this remarkable effect when added in large excess to C2H2, differently from the well-known method of “small addition of water.” Moreover, the resulting VA-CNTs showed electrical contact with the Al sheets with resistance of ≤0.7 Ω cm−2. The effect of CO2 is systematically studied and discussed.

Hybrid supercapacitors, which combine the advantages of supercapacitors and rechargeable batteries, have the potential to meet the demands of both high-energy and -power in electrochemical energy storage systems. However, the energy density of the hybrid supercapacitors has been limited because of the low capacity of the activated carbon cathode. Here we introduce a high-capacity carbon cathode containing plenty of oxygen functional groups that are redox-active towards both Li- and Na-ions. This functional carbon has an ultra-thin two-dimensional structure that has significant advantages in utilizing the redox reactions. The functional carbon cathode can exhibit very high capacities of ∼250 mA h g-1 in Li-cells and ∼210 mA h g-1 in Na-cells. A hybrid supercapacitor consisting of the two-dimensional functional carbon cathode with a commercial level loading density of ∼9.3 mg cm-2 and a Si-based anode delivers a high-energy density of ∼182 W h kg-1 at a high-power density of 1 kW kg-1.

Although lithium-sulfur (Li-S) batteries are a promising secondary power source, it still faces many technical challenges, such as rapid capacity decay and low sulfur utilization. The loading of sulfur and the weight percentage of sulfur in the cathode usually have a significant influence on the energy density. Herein, we report an easy synthesis of a self-supporting sulfur@graphene oxide-few-wall carbon nanotube (S@GO-FWCNT) composite cathode film, wherein an aluminum foil current collector is replaced by FWCNTs and sulfur particles are uniformly wrapped by graphene oxide along with FWCNTs. The 10 wt% FWCNT matrix through ultrasonication not only provided self-supporting properties without the aid of metallic foil, but also increased the electrical conductivity. The resulting S@GO-FWCNT composite electrode showed high rate performance and cycle stability up to ∼385.7 mA h gelectrode -1 after 500 cycles and close to ∼0.04% specific capacity degradation per cycle, which was better than a S@GO composite electrode (353.1 mA h gelectrode -1). This S@GO-FWCNT composite self-supporting film is a promising cathode material for high energy density rechargeable Li-S batteries.

Monocrystalline, low-defect density Si thin films were successfully fabricated via epitaxy with 1 minute rapid vapor deposition on a porous seed layer. Zone heating recrystallization reduced the surface roughness of the seed layer to sub-nanometer size. The critical effect of roughness on the defect density of epitaxial films was confirmed.

Rapid gas-evaporation method is proposed and developed, which yields Si nanoparticles (SiNPs) in a few seconds at high yields of 20%-60% from inexpensive and safe bulk Si. Such rapid process is realized by heating the Si source to a temperature &gt;= 2000 degrees C, much higher than the melting point of Si (1414 degrees C). The size of SiNPs is controlled at tens to hundreds nanometers simply by the Ar gas pressure during the evaporation process. Self-supporting films are fabricated simply by co-dispersion and filtration of the SiNPs and carbon nanotubes (CNTs) without using binders nor metal foils. The half-cell tests showed the improved performances of the SiNP-CNT composite films as anode when coated with graphitic carbon layer. Their performances are evaluated with various SiNP sizes and Si/CNT ratios systematically. The SiNP-CNT film with a Si/CNT mass ratio of 4 realizes the balanced film-based capacities of 618 mAh/g(film), 230 mAh/cm(3), and 0.644 mAh/cm(2) with a moderate Si-based performance of 863 mAh/g(Si) at the 100th cycle. (C) 2017 Elsevier B.V. All rights reserved.

Epitaxial copper (Cu) films yield graphene with superior quality but at high cost. We report 1-3 μm thick epitaxial Cu films prepared on c plane sapphire substrates in 10-30 s, which is much faster than that of the typical sputtering method. Such rapid deposition is realized by vapor deposition using a Cu source heated to 1700-1800 °C, which is much higher than its melting point of 1085 °C. Continuous graphene films, either bilayer or single-layer, are obtained on the epitaxial Cu by chemical vapor deposition and transferred to carrier substrates. The sapphire substrates can be reused five to six times maintaining the quality of the epitaxial Cu films and graphene. The mechanisms and requirements are discussed for such quick epitaxy of Cu on reused sapphire, which will enable high-quality graphene production at lower cost.

Flame-synthesized (CoO)(x)(Al2O3)(1-x) spinel nanopowders with primary particles of similar to 20 nm were used to grow small diameter carbon nanotubes (CNTs). The nanopowders with x &lt;= 035 grew few CNTs whereas that with x = 0.65 grew CNTs efficiently. Low crystalline and large-diameter multi-wall CNTs grew by annealing and chemical vapor deposition (CVD) at 800 degrees C for similar to 10 min, whereas single-wall CNTs with high crystallinity (G-band to D-band intensity ratio of 20-100 by Raman spectroscopy) grew by annealing and CVD at &gt;= 1000 degrees C for similar to 10 s. The excess Co in the spinel reduced and segregated to form multiple Co nanoparticles on the surface of the single primary alumina nanoparticles in similar to 10 s, yielding SWCNTs in similar to 10 s. Such flame synthesized nanopowders, reduced and activated by H-2, provide CNTs from C2H2, all in ten-second time scale, and as such are promising for practical, high-through-put production of small-diameter CNTs. (C) 2016 Elsevier Ltd. All rights reserved.

Hole doping into carbon nanotubes can be achieved. However, the doped nanotubes usually suffer from the lack of air and moisture stability, thus, they eventually lose their improved electrical properties. Here, we report that a salt of the two-coordinate boron cation Mes(2)B(+) (Mes: 2,4,6-trimethylphenyl group) can serve as an efficient hole-doping reagent to produce nanotubes with markedly high stability in the presence of air and moisture. Upon doping, the resistances of the nanotubes decreased, and these states were maintained for one month in air. The hole-doped nanotube films showed a minimal increase in resistance even upon humidification with a relative humidity of 90%. (C) 2017 The Japan Society of Applied Physics

Self-polymerized dopamine is a versatile coating material that has various oxygen and nitrogen functional groups. Here, we demonstrate the redox-active properties of self-polymerized dopamine on the surface of few-walled carbon nanotubes (FWNTs), which can be used as organic cathode materials for both Li-and Na-ion batteries. We reveal the multiple redox reactions between self-polymerized dopamine and electrolyte ions in the high voltage region from 2.5 to 4.1 V vs. Li using both density functional theory (DFT) calculations and electrochemical measurements. Free-standing and flexible hybrid electrodes are assembled using a vacuum filtration method, which have a 3D porous network structure consisting of polydopamine coated FWNTs. The hybrid electrodes exhibit gravimetric capacities of similar to 133 mA h g(-1) in Li-cells and similar to 109 mA h g(-1) in Na-cells utilizing double layer capacitance from FWNTs and multiple redox-reactions from polydopamine. The polydopamine itself within the hybrid film can store high gravimetric capacities of similar to 235 mA h g(-1) in Li-cells and similar to 213 mA h g(-1) in Na-cells. In addition, the hybrid electrodes show a high rate-performance and excellent cycling stability, suggesting that self-polymerized dopamine is a promising cathode material for organic rechargeable batteries.

Herein, we propose lithium ion batteries (LIBs) without binder or metal foils, based on a three-dimensional carbon nanotube (CNT) current collector. Because metal foils occupy 20-30 wt% of conventional LIBs and the polymer binder has no electrical conductivity, replacing such non-capacitive materials is a valid approach for improving the energy and power density of LIBs. Adding only 1 wt% of few-wall CNTs to the active material enables flexible freestanding sheets to be fabricated by simple dispersion and filtration processes. Coin cell tests are conducted on full cells fabricated from a 99 wt% LiCoO2-1 wt% CNT cathode and 99 wt% graphite-1 wt% CNT anode. Discharge capacities of 353 and 306 mAh g(graphite)(-1) are obtained at charge-discharge rates of 37.2 and 372 mA g(graphite)(-1) respectively, with a capacity retention of 65% at the 500th cycle. The suitability of the 1 wt% CNT-based composite electrodes for practical scale devices is demonstrated with laminate cells containing 50 x 50 mm(2) electrodes. Use of metal combs instead of metal foils enables charge-discharge operation of the laminate cell without considerable IR drop. Such electrodes will minimize the amount of metal and maximize the amount of active materials contained in LIBs. (C) 2016 Elsevier B.V. All rights reserved.

Biomass derived carbon materials have been widely used as electrode materials; however, in most cases, only electrical double layer capacitance (EDLC) is utilized and therefore, only low energy density can be achieved. Herein, we report on redox-active carbon spheres that can be simply synthesized from earth-abundant glucose via a hydrothermal process. These carbon spheres exhibit a specific capacity of similar to 210 mA h g(CS)(-1), with high redox potentials in the voltage range of 2.2-3.7 V vs. Li, when used as positive electrode in lithium cells. Free-standing, flexible composite films consisting of the carbon spheres and few-walled carbon nanotubes deliver high specific capacities up to similar to 155 mA h g(electrode)(-1) with no obvious capacity fading up to 10 000 cycles, proposing to be promising positive electrodes for lithium-ion batteries or capacitors. Furthermore, considering that the carbon spheres were obtained in an aqueous glucose solution and no toxic or hazardous reagents were used, this process opens up a green and sustainable method for designing high performance, environmentally-friendly energy storage devices.

A flexible organic-light-emitting diode (OLED) with capability to show 16 million colors is fabricated on plastic barrier-film substrate, which can produce arbitrary shape with arbitrary colors, suitable for artistic expressions. Independently controlled red, green, and blue light-emitting layers are stacked vertically, so that no visible structure can be observed even with magnifiers from right-in-front measurement. In the past, large voltage drop of intermediate electrode was preventing this approach to be applied to actual electronic devices. However, according to the surface mobility control using Fick's law analysis, low sheet resistance 7.34 Omega/square on plastic film is developed, so that 7.17-cm(2) area emission is successfully achieved. With optical length optimization for each color stack, more than 100% color reproduction in National Television Committee Standard is achieved by stack design. The device can be used for colored illumination, as well as for organic-light-emitting display pixels for three times emission than the conventional pixel design. The device is fabricated on plastic substrate, so that the polychromatic OLED device is manufacturable with roll-to-roll production line.

Carbon nanotube (CNT)-silicon (Si) heterojunction solar cells are fabricated with surface-textured Si substrates. Using a dilute alkaline solution, common etchant in the Si solar cell industry, we formed a pyramidal texture on the Si substrate surface. The texture effectively enhances the absorption of the incident light, improving the short-circuit current density by similar to 1.3-fold, up to 33.1 mA cm(-2). We fabricated CNT-Si solar cells with a power conversion efficiency (PCE) of 10.4% without any anti-reflective coatings or doping of the CNTs. Moreover, the CNT films were prepared from commercialized CNT agglomerates by a mild solution-based process, which is well suited for the fabrication of CNT-Si solar cells with large area. We also achieved a PCE of 9.57% for a flat cell with careful removal of surfactant from and doping by nitric acid of the CNT films. These findings suggest that with the combination of surface-textured Si and solution-processed CNT films, efficient and low-cost CNT-Si solar cells may be realized.

To overcome the tradeoff between the gravimetric capacitance and loading density of pseudocapacitive MnO2, we electrodeposited MnO2 nanoparticles on the carbon nanotube (CNT) surfaces in 18-37 mu m thick self-supporting CNT papers. We examined the electrodeposition conditions including constant potential, constant current, and potential pulses, and obtained MnO2-CNT hybrid electrodes containing MnO2 nanoparticles uniformly deposited at 60-90 wt% with an expanded CNT matrix. The MnO2-CNT hybrid electrode with a thickness of 62 mu m, density of 1.09 g cm(-3), areal mass of 6.75 mg cm(-2), and 82 wt% MnO2 load showed a total gravimetric capacitance of 120 and 51 F-total g(electrode)(-1), volumetric capacitance of 131 and 56 F-total cm(-3) and areal capacitance of 0.81 and 0.34 Ftotal cm(-2) at scan rates of 2 and 200 mV s(-1), respectively. The large thickness, moderately high mass density, and fairly conductive CNT matrix realized such high values of gravimetric, areal and volumetric capacitances that are important for practical devices.

We developed a film deposition method which yielded continuous polycrystalline Si films with large lateral grain sizes of over 100 mu m and thicknesses of similar to 10 mu m in 1 min on growth substrates other than silicon wafers in a single-step process. The silicon source is heated to similar to 2000 degrees C, much higher than the melting point of Si, which enables a high deposition rate. Controlling the temperature of the growth substrate, initially above and later below the melting point of Si, allows the seamless lateral to vertical growth of crystalline silicon grains. Thermally and chemically stable substrates of quartz glass and alumina with a 0.1 mu m-thick amorphous carbon layer were effective; liquid silicon wetted well by forming a thin SiC interlayer while substrates stayed stable. Such large-grain polycrystalline silicon films synthesized rapidly in 1 min may be used for low-cost, stable and flexible thin film photovoltaic cells.

Crumpled graphene is known to have a strong aggregation-resistive property due to its unique 3D morphology, providing a promising solution to prevent the restacking issue of graphene based electrode materials. Here, we demonstrate the utilization of redox-active oxygen functional groups on the partially reduced crumpled graphene oxide (r-CGO) for electrochemical energy storage applications. To effectively utilize the surface redox reactions of the functional groups, hierarchical networks of electrodes including r-CGO and functionalized few-walled carbon nanotubes (f-FWNTs) are assembled via a vacuum-filtration process, resulting in a 3D porous structure. These composite electrodes are employed as positive electrodes in Li-cells, delivering high gravimetric capacities of up to similar to 170 mA h g(-1) with significantly enhanced rate-capability compared to the electrodes consisting of conventional 2D reduced graphene oxide and f-FWNTs. These results highlight the importance of microstructure design coupled with oxygen chemistry control, to maximize the surface redox reactions on functionalized graphene based electrodes.

A fabrication process of monocrystalline thin films for Si solar cells was developed that uses epitaxial growth and layer transfer process for low fabrication cost and high photoelectric conversion efficiency. Fabrication of high crystalline epitaxial thin films requires controlling the surface roughness and pore size of the seed layer in nano-level, double layer porous Si (DLPS). By using this zone heating recrystallization (ZHR) method, in which the top surface of DLPS is selectively annealed by scanning upper lamp heater, we successfully reduced the surface roughness and pore size. By using the rapid vapor deposition (RVD) method, we fabricated an epitaxial monocrystalline Si thin film in 1 mm on the ZHR-treated DLPS and then easily detached this thin film from the substrate. The crystal distortion of epitaxial Si thin films can be controlled by using ZHR to smooth and change only the top surface of DLPS.

To achieve denser and taller carbon nanotube (CNT) arrays on Cu foils, catalyst and chemical vapor deposition (CVD) conditions were carefully engineered. CNTs were grown to similar to 50 mu m using Fe/TiN/Ta catalysts in which Ta and TiN acted as diffusion barriers for Cu and Ta, respectively. A tradeoff was found between the mass density and height of the CNT arrays, and CNT arrays with a mass density of 0.30 g cm(-3) and height of 45 mu m were achieved under optimized conditions. Thermal interface materials (TIMs) with CNT array/Cu foil/CNT array structures showed decreasing thermal resistance from 86 to 24 mm(2) K W-1 with increasing CNT array mass densities from 0.07-0.08 to 0.19-0.26 g cm(-3) for Cu and Al blocks with surfaces as rough as 20-30 mu m. The best CNT/Cu/CNT TIMs showed thermal resistance values comparable to that of a typical indium sheet TIM. (C) 2015 The Japan Society of Applied Physics

Dispersion-printing processes are essential for the fabrication of various devices using carbon nanotubes (CNTs). Insufficient dispersion results in CNT aggregates, while excessive dispersion results in the shortening of individual CNTs. To overcome this tradeoff, we propose here a repetitive dispersion-extraction process for CNTs. Long-duration ultrasonication (for 100 min) produced an aqueous dispersion of CNTs with sodium dodecylbenzene sulfonate with a high yield of 64%, but with short CNT lengths (a few mu m), and poor conductivity in the printed films (similar to 450 S cm(-1)). Short-duration ultrasonication (for 3 min) yielded a CNT dispersion with a very small yield of 2.4%, but with long CNTs (up to 20-40 mu m), and improved conductivity in the printed films (2200 S cm(-1)). The remaining sediment was used for the next cycle after the addition of the surfactant solution. 90% of the CNT aggregates were converted into conductive CNT films within 13 cycles (i.e., within 39 min), demonstrating the improved conductivity and reduced energy/time requirements for ultrasonication. CNT lines with conductivities of 1400-2300 S cm(-1) without doping and sub-100 mu m width, and uniform CNT films with 80% optical transmittance and 50 Omega/sq sheet resistance with nitric acid doping were obtained on polyethylene terephthalate films. (C) 2015 Elsevier Ltd. All rights reserved.

We report micrometre-thick porous Si-Cu anodes that are rapidly co-deposited on Cu current collectors in 1 mm. This rapid deposition is realized by heating Si and Cu powders to similar to 2000 degrees C and elevating their vapour pressures, while the porous and amorphous anode structure is realized by keeping the substrates at 100 degrees C. The films spontaneously form a 2-4.5-mu m-thick composition gradient that changes from a Cu-rich region at the bottom to a Si-rich region at the top of the film, because of the higher vapour pressure for Cu than Si. A small addition of 5 wt% Cu to the Si source enhances the cycle performance of the film remarkably in a half-cell test, yielding a gravimetric capacity of 1250 mAh g(film)(-1), a volumetric capacity of 1956 mAh cm(film)(-3), and an areal capacity of 0.96 mAh cm(anode)(-2) at the 100th cycle. However, excess addition of Cu causes partial Si crystallization in the films, which results in poorer cycle performance. While further improvement is needed, this rapid vapour deposition method yields Si-Cu films with compositional gradients on Cu current collectors in 1 min using inexpensive and safe Si and Cu powder sources, and is attractive for practical Si-based anode fabrication. (C) 2015 Elsevier B.V. All rights reserved.

Electrochemical energy-storage devices have the potential to be clean and efficient, but their current cost and performance limit their use in numerous transportation and stationary applications. Many organic molecules are abundant, economical and electrochemically active; if selected correctly and rationally designed, these organic molecules offer a promising route to expand the applications of these energy-storage devices. In this study, polycyclic aromatic hydrocarbons are introduced within a functionalized few-walled carbon nanotube matrix to develop high-energy, high-power positive electrodes for pseudocapacitor applications. The reduction potential and capacity of various polycyclic aromatic hydrocarbons are correlated with their interaction with the functionalized few-walled carbon nanotube matrix, chemical configuration and electronic structure. These findings provide rational design criteria for nanostructured organic electrodes. When combined with lithium negative electrodes, these nanostructured organic electrodes exhibit energy densities of similar to 350 Wh kg(electrode)(-1) at power densities of similar to 10 kW kg(electrode)(-1) for over 10,000 cycles.

A novel "etching-precipitation" method is proposed and developed for the direct synthesis of graphene on dielectric substrates. In this method, graphene precipitates from the Fe-C solid solution film during selective etching of Fe using Cl-2 gas. Few-and multi-layer graphene is fabricated directly on quartz glass and SiO2/Si substrates without Fe residue at a growth temperature of 500-650 degrees C, which is a significantly lower temperature than used in the conventional chemical vapor deposition method. The 6- to 7-layer graphene synthesized at 650 degrees C shows a volume resistivity of 80-140 mu Omega cm. The average number of layers can be easily controlled in a linear fashion with the initial carbon feed, which is proportional to the thickness of the starting Fe-C films. Line-patterned multi-layer graphene is also fabricated by simply pre-patterning the starting Fe-C film although its structure is somewhat different from typical graphene ribbons. "Etching-precipitation" will be a practical route to synthesize graphene with micro-patterns directly onto device substrates of arbitrary sizes. (C) 2014 Elsevier Ltd. All rights reserved.

A simple process is presented that realizes carbon nanotube (CNT) arrays that meet the process and structure requirements for use in large-scale integrated circuits. Ni particles are formed densely on a conductive TiN layer on SiO2/Si substrates through nucleation and growth by sputtering, which was stopped prior to percolation of the Ni particles. Ni particles as dense as 2.8 x 10(12) cm(-2) were formed after annealing at 400 degrees C and chemical vapor deposition (CVD) was carried out at 400 degrees C by feeding C2H2 at partial pressures as low as 0.13-1.3 Pa so as not to kill the catalyst. Scanning electron microscopy with energy dispersive X-ray spectroscopy revealed the mass density of the arrays to be as high as 1.1 g cm(-3). High resolution transmission electron microscopy showed the densely packed CNTs with an average wall number of eight. Atomic force microscopy of the root of the CNT arrays transferred to a SiO2/Si substrate enabled direct counting of individual CNTs, revealing areal densities of CNTs and CNT walls as high as 1.5 x 10(12) and 1.2 x 10(13) cm(-2), respectively. The simple process, using conventional sputtering and CVD apparatus, with carefully engineered conditions offers a route for practical application of CNTs. (C) 2014 Elsevier Ltd. All rights reserved.

We report the rapid vapour deposition of 3-14 mu m-thick porous Si anodes on Cu current collectors in 10-60 s. Such rapid deposition was achieved by heating the Si source to over 2000 degrees C, well above the melting point of Si, while the porous structure was realized in the deposited Si films by keeping the Cu collector at a much lower temperature of 100-500 degrees C. The adhesion between the Cu collectors and Si films was enhanced by forming a CuSix intermixed layer by post-deposition annealing as well as surface treatment of the Cu collectors. Half-cell measurements showed that the porous Si anodes without post-annealing degraded in a few cycles. A markedly improved cycle performance (1000 mA h g(Si)(-1) and 0.66 mA h cm(anode)(-2) at the anode for the 50th cycle) was achieved for post-annealed 3.5 mu m-thick porous Si films. Rapid vapor deposition of micrometre-thick porous Si films using inexpensive, safe Si powder is a practical route to fabricate high-capacity anodes for lithium ion batteries.

Various capacitive particles are currently available for use in electrochemical capacitors, but their electrical conductivities are typically low and require enhancement. Carbon nanotube (CNT) matrices can be used to fabricate self-supporting electrodes without binding or conducting additives. Herein, liquid dispersion and subsequent vacuum filtration were used to prepare thick (similar to 100 mu m) hybrid electrodes of activated carbon (AC) and CNTs. Factors including CNT type, AC particle size, solvent, and surfactant strongly affected the capacitance and rate performance of the hybrid electrodes. Different solvents and types of CNTs were best suited for pure CNT electrodes and AC-CNT hybrid electrodes; single-wall CNTs (SWCNTs) with a strong dispersant produced CNT electrodes with the best performance among pure CNT electrodes. Meanwhile, few-wall CNTs (FWCNTs) with a weak dispersant produced AC-CNT hybrid electrodes with the best performance among hybrid electrodes. Addition of 10 wt% FWCNTs to AC yielded a self-supporting hybrid electrode with improved performance (132 F g(-1), 58 F cm(-3), 0.70 F cm(-2) at 100 mV s(-1)) compared with that of a conventional AC electrode with conducting and binding additives (74 F g(-1), 26 F cm(-3), 0.60 F cm(-2) at 100 mV s(-1)), and a pure electrode of expensive SWCNTs (52 F g(-1), 26 F cm(-3), 0.26 F cm(-2) at 100 mV s(-1)).

To establish a method for sub-second conversion of acetylene to sub-millimeter-long carbon nanotubes (CNTs), we have proposed and developed an internal heat-exchange reactor for fluidized-bed chemical vapor deposition (FBCVD). This reactor enabled sufficient heating of the reaction gas and uniform heating of the bed of alumina beads at a space velocity as high as 3600 h(-1). The direct feeding of the catalyst vapors (aluminum isopropoxide for the alumina support layer and ferrocene for the iron particles) to the bed separately from the other gases, which were fed through the heat-exchange and preheating zone and the distributer, enabled the careful control of the catalyst particles deposited on the beads. By decreasing the acetylene feed concentration and preventing the deactivation of small Fe particles, we realized semi-continuous production of 99.6-99.8 wt%-pure, sub-millimeter-long, few-wall CNTs with an average diameter of 6.5 nm at a carbon yield of 42%. The FBCVD reactor with an internal heat-exchanger can be scaled-up for practical mass production with uniform and energy-saving heating. (C) 2014 Elsevier Ltd. All rights reserved.

We propose a technique for one-step micropatterning of as-grown carbon-nanotube films on a plastic substrate with sub-10 mu m resolution on the basis of the dry transfer process. By utilizing this technique, we demonstrated the novel high-performance flexible carbon-nanotube transparent conductive film with a microgrid structure, which enabled improvement of the performance over the trade-off between the sheet resistance and transmittance of a conventional uniform carbon-nanotube film. The sheet resistance was reduced by 46% at its maximum by adding the microgrid, leading to a value of 53 Omega/sq at a transmittance of 80%. We also demonstrated easy fabrication of multitouch projected capacitive sensors with 12 x 12 electrodes. The technique is quite promising for energy-saving production of transparent conductor devices with 100% material utilization.

Self-supporting hybrid electrodes were fabricated through the systematic combination of activated carbon (AC), a low cost capacitive material, with sub-millimetre long few-wall carbon nanotubes (FWCNTs). After an easy three-step (mixing, dispersion and filtration) process, robust self-supporting films were obtained, comprising 90% AC particles wrapped in a 3-dimensional FWCNT collector. The 10% FWCNTs provide electrical conductivity and mechanical strength, and replace heavier metal collectors. The FWCNT matrix effectively improved the capacitance of the inexpensive, high surface area AC to 169 F g(-1) at a slow scan rate of 5 mV s(-1), and to 131 F g(-1) at a fast scan rate of 100 mV s(-1), in fairly thick (similar to 200 mu m) electrodes. Connection to a metallic collector at the film edge only, which significantly reduced the use of metal, retained much larger capacitance for the AC-FWCNT hybrid film (107 F g(-1)) than for the conventional AC electrode with binder and conductive filler (3.9 F g(-1)) at a practical voltage scan rate, 100 mV s(-1). Transport measurements in three-and two-electrode cells show that the FWCNT matrix can greatly enhance the conductivity of the AC-based films.

We examined the use of low purity H-2 (96 vol % H-2 with 4 vol % CH4) in chemical vapor deposition (CVD) using a C2H2 feedstock, and obtained vertically aligned single-wall carbon nanotubes (VA-SWCNTs) with unexpectedly smaller diameters, larger height, and higher quality compared with those grown using pure H-2. During the catalyst annealing, carbon deposited at a small amount from CH4 on the Fe particles, which kept them small and dense. During CVD, CH4 prevented the Fe particles from coarsening, resulting in an enhanced growth lifetime and suppressed diameter increase of growing SWCNTs. These effects were observed only for CH4, and not for C2H4 or C2H2. (H-4-assisted CVD is an efficient and practical method that uses H-2 containing CH4 that is available as a byproduct in chemical factories.

Vertically-aligned carbon nanotubes (VA-CNTs) were rapidly grown from ethanol and their chemistry has been studied using a "cold-gas" chemical vapor deposition (CVD) method. Ethanol vapor was preheated in a furnace, cooled down and then flowed over cobalt catalysts upon ribbon-shaped substrates at 800 degrees C, while keeping the gas unheated. CNTs were obtained from ethanol on a sub-micrometer scale without preheating, but on a millimeter scale with preheating at 1000 degrees C. Acetylene was predicted to be the direct precursor by gas chromatography and gas-phase kinetic simulation, and actually led to millimeter-tall VA-CNTs without preheating when fed with hydrogen and water. There was, however a difference in CNT structure, i.e. mainly few-wall tubes from pyrolyzed ethanol and mainly single-wall tubes for unheated acetylene, and the by-products from ethanol pyrolysis possibly caused this difference. The "cold-gas" CVD, in which the gas-phase and catalytic reactions are separately controlled, allowed us to further understand CNT growth. (C) 2012 Elsevier Ltd. All rights reserved.

We report the very rapid growth of carbon nanotubes (CNTs) at high temperatures that can be tolerated by glass substrates. Glass substrates with metal microelectrodes and sputtered catalysts are heated by a pulsed current in a chemical vapour deposition gas environment for 0.5-1 s to synthesize CNTs of several micrometres in height without damaging the glass substrate. CNTs with structures from single-walled to multi-walled and morphologies from entangled networks to vertically aligned forests are grown simply by changing the nominal thickness of the catalyst, and such CNTs grown selectively on the microelectrodes worked as field emitters for cathodoluminescence. Rapid, easy growth of patterned CNT arrays on glass substrates without using furnaces/heaters or vacuum pumps will be useful for various applications of CNTs. (C) 2012 Elsevier Ltd. All rights reserved.

The rapid growth method for vertically aligned, single walled carbon nanotube (SWCNT) arrays on flat substrates was applied to a fluidized-bed, using ceramic beads as catalyst supports as a means to mass produce sub-millimeter-long SWCNT arrays. Fe/Al2Ox catalysts were deposited on the surface of Al2O3 beads by sputtering and SWCNTs were grown on the beads by chemical vapor deposition (CVD) using C2H2 as a feedstock. Scanning electron microscopy and transmission electron microscopy showed that SWCNTs of 2-4 nm in diameter grew and formed vertically aligned arrays of 0.5 mm in height. Thermogravimetric analysis showed that the SWCNTs had a catalyst impurity level below 1 wt.%. Furthermore, they were synthesized at a carbon yield as high as 65 at.% with a gas residence time as short as &lt;0.2 s. Our fluidized-bed CVD, which efficiently utilizes the three-dimensional space of the reactor volume while retaining the characteristics of SWCNTs on substrates, is a promising option for mass-production of high-purity, sub-millimeter-long SWCNT arrays. (C) 2011 Elsevier Ltd. All rights reserved.

Binder-free and self-standing carbon nanotube (CNT) electrodes of tens of microns in thickness have been assembled via a vacuum-filtration process of oxidized few-walled CNTs (FWNTs), with different amounts of oxygen functional groups on FWNTs. Sub-millimetre long FWNTs can provide high electrical conductivity and mechanical strength in self-standing porous networks by reducing the number of junctions among FWNTs. We show that the gravimetric capacity of FWNT electrodes in lithium cells can be enhanced by increasing oxygen functional groups on FWNTs, which results from Faradaic reactions between lithium ions and surface oxygen functional groups. These self-standing FWNT electrodes (free of binder/additive and current collector) can provide a high gravimetric energy of similar to 200 W h kg(-1) at a high power of similar to 10 kW kg(-1), showing promise as the positive electrode for lightweight, high-power lithium batteries.

Preparation of single-walled carbon nanotubes (SWNTs) has been advanced by controlling several parameters including the catalyst and the catalyst support material. Although zeolite has been frequently used as a catalyst support material for the synthesis of SWNTs, detailed surface properties of previously employed zeolites and thus their role as a catalyst support material have not been sufficiently clarified yet. In this study, a clean b-plane surface of silicalite-1, which is a siliceous MEI-type zeolite, was used as a model substrate for the synthesis of SWNTs. The amount of active cobalt used for SWNT generation was smaller than the initially sputtered amount, and XPS measurements revealed diffusion of cobalt into the zeolite framework. The diffused cobalt was found to interact strongly with the silica framework of zeolite. The diffusion coefficient of cobalt in silicalite-1 zeolite was larger than that in thermally oxidized SiO2 formed on a Si substrate. This difference was ascribed to the microporous structure and lower density of zeolite. In this study, the state of the cobalt catalyst and the interaction between cobalt and the crystalline zeolite substrate is presented and discussed.

Millimeter-tall single-walled carbon nanotube (SWCNT) forests were grown by chemical vapor deposition (CVD) from C2H2/H2O/Ar using Fe/Al-Si-O catalysts. Using combinatorial catalyst libraries coupled with real-time monitoring of SWCNT growth, the catalyst and CVD conditions were systematically studied. The keys for this growth are to maintain the C2H2 pressure below its upper limit to prevent the killing of the catalysts and to grow the SWCNTs before the catalyst particles lose their activity because of coarsening through Ostwald ripening. Lower temperatures lead to lower limits for the C2H2 pressure which result in lower growth rates but also lead to even lower coarsening rates which result in even longer growth lifetimes. Using these principles, we grew 4.5-mm-tall SWCNT forests in 2.5 h at 750 degrees C. (C) 2011 Elsevier Ltd. All rights reserved.

In addition to the structural control of individual carbon nanotubes (CNTs), the morphological control of their assemblies is crucial to realize miniaturized CNT devices. Microgradients in the thickness of catalyst are used to enrich the variety of available self-organized morphologies of CNTs. Microtrenches were fabricated in gate/spacer/cathode trilayers using a conventional self-aligned top-down process and catalyst exhibiting a microgradient in its thickness was formed on the cathode by sputter deposition through gate slits. CNTs, including single-walled CNTs, of up to 1 mu m in length were grown within 5-15 s by chemical vapor deposition. The tendency of thin CNTs to aggregate caused interactions between CNTs with different growth rates, yielding various morphologies dependent on the thickness of the catalyst. The field emission properties of several types of CNT assemblies were evaluated. The ability to produce CNTs with tailored morphologies by engineering the spatial distribution of catalysts will enhance their performance in devices. (C) 2011 The Japan Society of Applied Physics

A simple and fast dispersion method that incorporates heating is used to disperse long (more than 10 AM) single-walled carbon nanotubes (SWCNTs) with minimal defects. The method enables a dispersed solution of SWCNTs to be produced in less than 10 min in only three steps: (1) addition of the dispersant, (2) heating, and (3) grinding. The dispersion method does not require sonication, which shortens the SWCNTs and can generate surface defects. SWCNT films were prepared from the dispersed solution, and the films exhibited a resistance of 380 Omega/sq at a transparency of 64.8%. This dispersion method can be easily scaled up, making it useful for the preparation of dispersed SWCNTs for commercial and industrial applications. (C) 2011 Elsevier Ltd. All rights reserved.

A semi-continuous fluidized-bed process is reported which rapidly converts acetylene into carbon nanotubes (CNTs). Catalysts are first immobilized on ceramic beads and CNTs are then grown on the beads and then separated from them in a repetitive process accomplished within a single reactor simply by switching gases at a fixed temperature. CNTs of 6-10 nm diameter, three walls on average, 0.4 mm length and 99 wt.% purity were synthesized at an yield of over 70% in a reactor residence time shorter than 0.3 s. The easy and efficient production of such CNTs with in situ separation from the catalysts may accelerate the development of CNT-based nanotechnology industries. (C) 2011 Elsevier Ltd. All rights reserved.

We performed a systematic study of the nanostructure and magnetic properties of FePt on templates of either (200)-oriented polycrystalline TiN underlayers with in-plane grain sizes from 5.8 to 10 nm (poly-TiN) or highly (200)-textured TiN underlayers epitaxially grown on single-crystalline MgO (100) substrates (epi-TiN). For small nominal FePt thicknesses (0.7-8.0 nm), FePt forms particulate films with the magnetic easy axis perpendicular to the film plane on every template TiN underlayer. In addition, the coercivity of nominally 1.4-nm-thick FePt at 300 K in the out-of-plane direction increases from 5.3 kOe for 5.8-nm-sized poly-TiN to 12.9 kOe for 10-nm-sized poly-TiN and reaches 16.3 kOe for epi-TiN, which shows that the coercivity strongly depends on the degree of the c-axis orientation. For larger FePt nominal thicknesses (16-64 nm), FePt particles percolate and form continuous films, and the direction of the easy magnetic easy axis becomes random. The coercivity of nominally 64-nm-thick FePt at 300 K in the out-of-plane direction is still as large as 8.8 kOe for 10-nm-sized poly-TiN, but it drastically decreases to 0.5 kOe for epi-TiN. The absence of in-plane texture in the FePt layer on the poly-TiN suppresses the decrease in coercivity, which prevents domain-wall displacement. (C) 2011 American Vacuum Society. [DOI: 10.1116/1.3575155]

Millimeter-tall vertically aligned single-walled carbon nanotubes (SWCNTs) were grown in 10-15 min by chemical vapor deposition from C(2)H(2)/Ar with or without water addition using Fe catalyst supported on an Al-Si-O underlayer. Using combinatorial catalyst libraries coupled with the real-time monitoring of SWCNT growth, the catalyst and chemical vapor deposition conditions were systematically examined, and millimeter-tall SWCNTs were obtained even without water addition. The key for millimeter-scale growth of SWCNTs is to limit the C(2)H(2) supply to below a certain partial pressure to retain an active catalyst. Water prolongs the catalyst lifetime under excess C(2)H(2) supply, whereas it deactivates small catalyst particles and degrades the quality of SWCNTs at the same time. We also observed a gradual increase in the diameter of SWCNTs with growth because of the coarsening of catalyst particles and found that water had no effect on this phenomenon. We demonstrate millimeter-tall SWCNTs grown by simply using C(2)H(2)/Ar gas without water addition, which revealed the mysterious role of water, and we show a practical route for the large-scale production of SWCNTs.

Two processes for the fabrication of polycrystalline CoSi2 thin films based on the codeposition of Co and Si by sputtering were studied and compared. The first process involved "annealing after deposition", where Co and Si are codeposited at ambient temperature and then crystallized by annealing. This process yielded randomly oriented plate-like CoSi2 grains with a grain size that is governed by the nanostructure of the as-deposited film. Polycrystalline CoSi2 thin films were obtained at a process temperature of 170 degrees C, which was much lower than the annealing temperature of 500 degrees C needed for Co/Si bilayers. The second process involved "heating during deposition", where Co and Si are codeposited on heated substrates. This process yielded CoSi2 grains with a columnar structure, and the grain size and degree of (1 1 1) orientation are temperature dependent. The sheet resistance of the resulting films was determined by the preparation temperature regardless of the deposition process used, i.e. "annealing after deposition" or "heating during deposition". Temperatures of 500 degrees C and higher were needed to achieve CoSi2 resistivity of 40 mu Omega cm or lower for both processes. (C) 2010 Elsevier B.V. All rights reserved.

Carbon nanotube (CNT) emitters are of interest for inclusion in cold cathodes and field emission displays. CNT field electron emitters self-organized on substrates with an Fe/Al2O3 catalytic/supporting layer, which accelerates CNT growth, are characterized using combinatorial libraries. A variety of morphologies are formed on single substrates by C2H2 thermal chemical vapor deposition for 10 s at ambient pressure. Degradation of field emission decreases upon prolonged operation. Raman signals from thinner single-walled CNTs predominantly degrade during operation. Controlling the number of protruding thin CNTs is crucial to extracting current and ensuring sustainability. Thin CNTs protruding from CNT ensembles formed on a substrate with a multimodal distribution of catalyst particles show good field emission (FE) properties with practical sustainability. A potential design for self-organized thin CNTs fabricated by the current process is discussed on the basis of the combinatorial evaluation for field emission and 3D electric field simulations.

The rapid growth dynamics of millimeter-tall, vertically aligned single-walled carbon nanotubes (VA-SWCNTs) was studied using a simple real-time monitoring method By using combinatorial catalyst libraries, VA-SWCNT growth curves under various catalyst conditions were obtained in a single chemical vapor deposition (CVD) run VA-SWCNTs grew at constant or gradually decreasing rates for several minutes and then abruptly ceased growth This unusual behavior of the growth occurred under wide ranges of catalyst and CVD conditions (C) 2010 The Japan Society of Applied Physics

Millimeter-tall vertically-aligned carbon nanotubes (VA-CNTs) were grown from ethanol under ambient pressure by Co-catalyzed chemical vapor deposition (CVD), with systematic optimization of the CVD temperature and catalytic conditions using combinatorial catalyst libraries. We investigated the use of both aluminum oxide and silicon oxide as underlayers for the Co catalyst and found that VA-CNTs grew to millimeter heights in 15-30 min when the pyrolysis of ethanol was carried out at high temperatures (2-850 degrees C) and long residence times (&gt;= 10 s). Thick Co catalytic layers (&gt;= 1.3 nm) produced (sub)millimeter-tall multi-walled VA-CNTs on both the aluminum oxide and silicon oxide underlayers. However, thin Co catalytic layers (0.62-1.0 nm) produced (sub)millimeter-tall VA-CNTs, which consisted mainly of single-walled CNTs, only on the aluminum oxide underlayers. Stripe patterns were found in the VA-CNTs near the substrate on both aluminum oxide and silicon oxide, indicating some instability prior to growth termination. The possible roles of aluminum oxide in growing millimeter-tall single-walled VA-CNTs were discussed. (C) 2010 Elsevier Ltd. All rights reserved.

Establishing fabrication methods of carbon nanotubes (CNTs) is essential to realize many applications expected for CNTs. Catalytic growth of CNTs on substrates by chemical vapor deposition (CVD) is promising for direct fabrication of CNT devices, and catalyst nanoparticles play a crucial role in such growth. We have developed a simple method called "combinatorial masked deposition (CMD)'', in which catalyst particles of a given series of sizes and compositions are formed on a single substrate by annealing gradient catalyst layers formed by sputtering through a mask. CMD enables preparation of hundreds of catalysts on a wafer, growth of single-walled CNTs (SWCNTs), and evaluation of SWCNT diameter distributions by automated Raman mapping in a single day. CMD helps determinations of the CVD and catalyst windows realizing millimeter-tall SWCNT forest growth in 10 min, and of growth curves for a series of catalysts in a single measurement when combined with real-time monitoring. A catalyst library prepared using CMD yields various CNTs, ranging from individuals, networks, spikes, and to forests of both SWCNTs and multi-walled CNTs, and thus can be used to efficiently evaluate self-organized CNT field emitters, for example. The CMD method is simple yet effective for research of CNT growth methods. (C) 2010 The Japan Society of Applied Physics

Diameter increase was found during the rapid, vertically aligned growth of millimeter-long single-walled carbon nanotubes (SWCNTs). The diameters continuously increased on average from 1.7nm at the top to 3.7nm at the bottom of vertically aligned SWCNTs. The SWCNT structures ranged from straight and bundled at the top to bent and isolated and even collapsed at the bottom. Our findings show the importance of suppressing catalyst coarsening to obtain thin SWCNTs, as well as a potential production route of thick SWCNTs and edgeless bilayer graphene ribbons. (C) 2010 The Japan Society of Applied Physics

Carbon nanotube (CNT) emitters were formed on line-patterned cathodes in microtrenches through a thermal CVD process. Single-walled carbon nanotubes (SWCNTs) self-organized along the trench lines with a submicron inter-CNT spacing. Excellent field emission (FE) properties were obtained: current densities at the anode (J(a)) of 1 mu A cm(-2), 10mA cm(-2) and 100 mA cm(-2) were recorded at gate voltages (V(g)) of 16, 25 and 36 V, respectively. The required voltage difference to gain a 1: 10 000 contrast of the anode current was as low as 9 V, indicating that a very low operating voltage is possible for these devices. Not only a large number of emission sites but also the optimal combination of trench structure and emitter morphology are crucial to achieve the full FE potential of thin CNTs with a practical lifetime. The FE properties of 1D arrays of CNT emitters and their optimal design are discussed. Self-organization of thin CNTs is an attractive prospect to tailor preferable emitter designs in FE devices.

We present a two-dimensional combinatorial investigation of resonant Raman and fluorescence enhancement in silver and gold sandwich structures. Gold and silver, separated by a thin alumina spacer layer, were deposited in two orthogonal gradients, with a thickness from a few hundred to a few nanometers, covering the range from island films through percolation to continuous films. Resonant Raman spectra of Rhodamine 6G adsorbed on the substrate surface were recorded in a 13 x 13 matrix using an automated scanning stage. The most Raman-active substrates were composed of silver (8 nm)-on-alumina (7 nm)-on-gold (5 nm). They consist of both gold and silver discontinuous nanoparticle films separated by alumina, forming a labyrinthine network structure. Gold-on-alumina-on-silver substrates also displayed increased activity compared with that of goldon-alumina substrates. Maximum fluorescence intensity was observed on silver films nominally 35 nm thick covered by 8 nm of alumina. The efficacy of the combinatorial method to correlate multiple aspects of the measurements and reduce uncertainties is demonstrated.

Utilizing a combinatorial method, we used spectroscopic ellipsometry to determine the dielectric functions of silver island films over a large range of sizes and morphologies from the percolation threshold down to average particle size smaller than 5 nm. We measured films on silicon substrates with 2 and 20 nm oxide layers and compared the surface-enhanced Raman scattering properties of the films. As expected, the films on 20-nm-thick oxide substrates showed increased Raman counts due to reduced damping of the plasmon resonance; however, the optical absorption was greater in the films on 2 nm oxide. The maximum Raman scattering was observed for average particle diameters of 13.6 and 25 nm and interparticle spacings of 3.3 and 4.1 nm for the 2 and 20 nm oxide substrates, respectively. The use of a combinatorial method resulted in significantly reduced uncertainties by avoiding multiple sample preparations and allowed unambiguous identification of optimal film parameters for the different substrates.

We show evidence of a dependence of the enhancement of the Raman scattering cross section on the length of the gradient in graded silver island films. A factor-of-three increase in the Raman signal is observed for gradients with length of the order of 0.5 mm when compared to gradients of the order of 9 mm. Scanning electron microscopy reveals the nanostructure of the two films to be statistically similar. We attribute the observation to differences in plasmon hybridization in the gradients arising from long range structural differences.

We realized simple fabrication of carbon nanotube field emitters for backlight units. Carbon nanotubes were directly grown on cathode lines patterned on low-strain glasses by atmospheric pressure CVD with pulse electrical heating of cathodes for I second. Field emission current density was as high as 5.6 mA/cm(2) at 3.3 V/mu m.

Sustainability of human beings in the 21st century requires development of renewable energy systems based on technology innovation. Chemical engineering plays a key role in promoting technology innovation relating to environmental and energy systems. The technological domains to which chemical engineering has contributed have shift from petrochemicals to functional materials and devices. An example of the key devices expected in the future is a combination of solar cells and Li-ion batteries, in which the indispensable materials are silicon and carbon. The shape and nanostructure of materials must be controlled to fabricate highly efficient devices at a low cost. Single-walled carbon nanotubes (SWNT) and spherical silicon solar cells (SSSC) with a semi-concentration reflector system are discussed as examples of future materials and devices. Chemical engineering is responsible for technology innovation through mass production, product quality control, materials recycling, high-quality device fabrication, and structuring knowledge.

Carbon nanotubes (CNTs) were grown directly on substrates by alcohol catalytic chemical vapor deposition using a Co-Mo binary catalyst. Optimum catalytic and reaction conditions were investigated using a combinatorial catalyst library. High catalytic activity areas on the substrate were identified by mapping the CNT yield against the orthogonal gradient thickness profiles of Co and Mo. The location of these areas shifted with changes in reaction temperature, ethanol pressure and ethanol flow rate. Vertically aligned single-walled CNT (SWCNT) forests grew in several areas to a maximum height of ca. 30 mu m in 10 min. A pure Co catalyst yielded a vertically aligned SWCNT forest with a bimodal diameter distribution. The effects of Mo on the formation of catalyst nanoparticles and on the diameter distribution of SWCNTs are discussed and Mo as thin as a monolayer or thinner was found to suppress the broadening of SWCNT diameter distributions. (C) 2008 Elsevier Ltd. All rights reserved.

Our group recently reproduced the water-assisted growth method, so-called "SuperGrowth," of millimeter-thick single-walled carbon nanotube (SWNT) forests by using C2H4/H-2/H2O/Ar reactant gas and Fe/Al2O3 catalyst. In this current work, a parametric study was carried out on both reaction and catalyst conditions. Results revealed that a thin Fe catalyst layer (about 0.5 nm) yielded rapid growth of SWNTs only when supported on Al2O3, and that Al2O3 support enhanced the activity of Fe, Co, and Ni catalysts. The growth window for the rapid SWNT growth was narrow, however. Optimum amount of added H2O increased the SWNT growth rate but further addition of H2O degraded both the SWNT growth rate and quality Addition of H-2 was also essential for rapid SWNT growth, but again, further addition decreased both the SWNT growth rate and quality. Because Al2O3 catalyzes hydrocarbon reforming, Al2O3 support possibly enhances the SWNT growth rate by supplying the carbon source to the catalyst nanoparticles. The origin of the narrow window for rapid SWNT growth is also discussed.

Field emission properties of carbon nanotubes (CNTs) were comparatively evaluated by using combinatorial CNT libraries. The libraries were prepared by combinatorial masked deposition of a Co catalytic layer on Al2O3/Si substrates and subsequent CNT growth by chemical vapor deposition from ethanol. Each library reproduced various types of single- and multiwalled carbon nanotubes with different morphologies and a variety of field emission properties. Combinatorial evaluations immediately identified the CNTs preferable as field emitters. The results obtained from individual field emission evaluations for samples with a constant nominal Co thickness agreed well with the results obtained from comparative evaluations for combinatorial CNT libraries. The results revealed that protrusive single-walled carbon nanotubes with a moderate interspacing showed the best field emission properties.

We demonstrate a production method for self-organized arrays of metal nanoparticles and aligned nanowires. Ion beam-sputtered Si/SiO2 substrates are used as templates for metallic vapor deposition, forming aligned arrays of 5-20 nm silver and cobalt nanoparticles with a period of 35 nm. The 20 nm diameter cobalt nanowires with lengths in excess of a micrometer are produced under appropriate conditions. All processing steps can be integrated into a single vacuum chamber and performed in a matter of minutes at mild temperatures. This inherently scalable technique can be extended to a range of substrate materials, array patterns, and nanoparticle materials. (C) 2008 American Institute of Physics.

The relationships among the nominal thickness of Co catalyst, the structure of the catalyst particles, and the structure of carbon nanotubes (CNTs) growing from the catalyst during chemical vapor deposition were investigated. Various morphologies of CNTs such as individuals, random networks parallel to the surface of the substrate (&apos;grasses&apos;), and vertically aligned forests of single- and multi-walled carbon nanotubes were grown by only varying the nominal thickness of catalyst under the same reaction condition. These different morphologies at the same growth time were due to the different areal density rather than to the length of CNTs. With increasing nominal thickness of catalyst, the catalyst particles changed in diameter while their areal density remained relatively almost constant. The change in diameter possibly affected the number ratio of active catalyst particles to the whole particles, which in turn affected the areal density of CNTs and yielded the various morphologies. Longer growth time increased the CNT length, which caused further change in CNT morphologies from individuals to grasses and grasses to forests. (C) 2008 Elsevier B. V. All rights reserved.

Field emission properties of single-walled carbon nanotubes (SWCNTs), which were prepared through alcohol catalytic chemical vapor deposition for 10-60s, were characterized in a diode configuration. Protrusive bundles at the top surface of samples act selectively as emission sites. The number of emission sites was controlled by emitter morphologies combined with texturing of Si substrates. SWCNTs grown on a textured Si substrate exhibited a turn-on field as low as 2.4 V/mu m at a field emission current density of 1 mu A/cm(2). Uniform spatial luminescence (0.5 cm(2)) from the rear surface of the anode was revealed for SWCNTs prepared on the textured Si substrate. Deterioration of field emission properties through repetitive measurements was reduced for the textured samples in comparison with vertically aligned SWCNTs and a random network of SWCNTs prepared on flat Si substrates. Emitter morphology resulting in improved field emission properties is a crucial factor for the fabrication of SWCNT-electron sources. Morphologically controlled SWCNTs with promising emitter performance are expected to be practical electron sources.

The growth mechanism of epitaxial CoSi2 was studied using Co/Ti/Si multilayer solid phase reaction. Results showed that phase formation was controlled by diffusion of Co through the growing CoSix, although at the early stage of CoSi2 growth the diffusion of Co could be controlled by a Ti layer. A reactive deposition technique was also evaluated by using a conventional magnetron sputtering system. Results showed that an epitaxial CoSi2 layer was formed by controlling the Co sputtering rate not to exceed the Co diffusion rate through CoSix. However, the surface of CoSi2 became rough when the deposition rate was much slower than the Co diffusion rate through CoSix. The roughness was caused by the formation Of CoSi2 (111) facets at the interface between CoSi2 and the Si substrate. Si/CoSi2/Si double heteroepitaxial structures were fabricated when Si and Co were sequentially sputter-deposited on a Si (100) substrate. (c) 2007 Elsevier B.V. All rights reserved.

The relationships among the nominal thickness of Co catalysts, the structure of the catalyst particles, and the structure of carbon nanotubes (CNTs) growing from the catalysts were investigated. A gradient thickness profile of Co was prepared using a combinatorial method and then subjected to alcohol catalytic chemical vapor deposition at 700 degrees C. In the deposited sample, two active regions appeared on either side of an inactive region. In the active regions, mainly single-walled carbon nanotubes (SWNTs) or multi-walled carbon nanotubes (MWNTs) grew, depending on the nominal Co thickness (SWNTs grown at a Co thickness of about 0.1 nm, and MWNTs grown at a Co thickness of about 1.5 nm). However, neither SWNTs nor MWNTs grew efficiently at a moderate Co thickness (similar to 0.4 nm). This dependence of CNT growth on the initial Co thickness is explained by the different mechanisms of catalyst particle formation from sub-nanometer-thick and nanometer-thick Co films.

L1(0)-FePt is a promising material for high-density perpendicular magnetic recording media. The authors previously reported that c-axis oriented L1(0)-FePt nanoparticle monolayers can be formed on (200)-oriented polycrystalline template TiN underlayers on SiO2 by using a conventional sputtering method. In this study, TiN nanostructures, such as the degree of (200) orientation, were improved by first depositing a buffer layer, such as amorphous Si onto SiO2, and the grain size could be controlled by adjusting either the deposition temperature or TiN thickness. When FePt nanoparticles were formed on a template TiN underlayer with a buffer layer of amorphous Si, both their degree of c-axis orientation and their magnetic properties were improved; FePt nanoparticles with nominal thickness of 1.4 nm had coercivity of 12.9 kOe in the out-of-plane direction at 300 K. (c) 2007 American Vacuum Society.

A parametric study of so-called "super growth" of single-walled carbon nanotubes (SWNTs) was done by using combinatorial libraries of iron/aluminum oxide catalysts. Milli meter-thick forests of nanotubes grew within 10 min, and those grown by using catalysts with a thin Fe layer (about 0.5 nm) were SWNTs. Although nanotube forests grew under a wide range of reaction conditions such as gas composition and temperature, the window for SWNT was narrow. Fe catalysts rapidly grew nanotubes only when supported on aluminum oxide. Aluminum oxide, which is a well-known catalyst in hydrocarbon reforming, plays an essential role in enhancing the nanotube growth rates.

A simple, practical method was developed to automatically align single crystalline Si nanocones (SiNCs) vertical to a Si substrate. Double heteroepitaxial structure of Si/CoSi2 on a Si (100) substrate was prepared by sputtering, and Si was then deposited on the surface via chemical vapor deposition with SiH2Cl2/H-2 reaction gas. When Si was deposited at 900 degrees C, SiNCs were fabricated vertical to the substrate, had a tip curvature of about 100 nm, and had a number density of (0.9-35) X 10(8)/m(2). A CoSi2 nanocrystal was clearly visible on the tip of each SiNC. These CoSi2 nanocrystals were formed by agglomeration of the Si/CoSi2 layer, and catalyzed the Si growth during chemical vapor deposition. In conclusion, the alignment of the fabricated SiNCs could be controlled by utilizing agglomeration in the Si/CoSi2/Si double heteroepitaxial structure. (c) 2007 American Vacuum Society.

Fe, Co, and Ni are catalytically effective for growing single-walled carbon nanotubes (SWNTs). On substrates, however, Ni tends to yield only multi-walled carbon nanotubes. Because enhanced surface diffusion at the elevated growth temperature required for deposition might cause coarsening of Ni catalyst nanoparticles, adjusting the nominal Ni thickness should be crucial for controlling the particle size. Using our previously developed combinatorial method, we prepared a thickness profile of Ni on a quartz glass (SiO(2)) substrate and found that Ni nanoparticles catalyzed the growth of SWNTs by chemical vapor deposition only when nominal Ni thickness was in the monolayer range. (c) 2006 Elsevier B.V. All rights reserved.

A simple yet versatile combinatorial method to discover binary metal nanoparticle catalysts was developed. In this method, the nominal thickness of component metals can be independently screened for a wide range by simply setting a mask with a slit above a substrate during sputter-deposition. Using this method, we prepared a catalyst library with Mo (0.2-4 nm) and Co (0.2-8 nm) thickness profiles on a SiO(2)/Si wafer and discovered active catalysts that grow vertically aligned single-walled carbon nanotubes by alcohol catalytic chemical vapor deposition. (C) 2005 Elsevier Ltd. All rights reserved.

The pulsed laser induced phase transition of gold nanoparticles in aqueous solution was observed via a transient absorption on nanosecond time scales and longer. Gold nanoparticles were excited with an intense picosecond laser pulse (355 nm, 30 ps), and the subsequent changes were monitored using two continuous wave laser wavelengths (488 and 635 nm). On the nanosecond time scale, below 6.3 mJ cm(-2), no change was observed; however, in the low fluence region between 6.3 and 17 mJ cm(-2), gold nanoparticles produced a bleach signal (488 nm) attributed to the melting of the gold nanoparticles, which decreased linearly with increasing laser fluence. Laser fluences above 17 mJ cm(-2) resulted in a strong absorption at both wavelengths, which is ascribed to vaporization of gold nanoparticles rather than solvated electrons (ejected from gold nanoparticles) or light scattering. The decay of both signals was faster than the 5 ns time resolution used in our experimental system. On the microsecond time scale, increase in absorbance at 635 nm was observed with a time constant of 1.0 mu s, while no change was observed at 488 nm. It is considered that this increase is attributed to the formation of smaller gold nanoparticles resulting from pulsed laser induced size reduction of initial gold nanoparticles.

Scientific publications written in natural language still play a central role as our knowledge source. However, due to the flood of publications, obtaining a comprehensive view even on a topic of limited scope, from a stack of publications is becoming an arduous task. Examples are presented from our recent experiences in the materials science field, where information is not shared among researchers studying different materials and different methods. To overcome the limitation, we propose a structured keywords method to reinforce the functionality of a future e-library.

c-Axis oriented face-centered-tetragonal (fct)-FePt magnetic nanoparticles are a promising candidate for. high density perpendicular magnetic recording media. In this study, TiN was investigated as a seed layer to achieve c-axis orientation of fct-FePt nanoparticles. First, a (200)-oriented, polycrystalline TiN layer with grain size around 10 nm was prepared by reactive sputter-deposition at 873 K on SiO(2), and then FePt was sputter-deposited at 973 K on it. Both in-plane and out-of-plane X-ray diffraction revealed that FePt had fct structure with c-axis orientation. Plan-view field emission scanning electron microscopy showed that FePt formed well-isolated nanoparticles. The particle diameter increased with increasing nominal thickness of FePt, and it was similar to the size of the TiN grains when nominal thickness was 1.4 nm. Cross-sectional transmission electron microscope images indicated that single FePt nanoparticles grew on single TiN grains, namely, one nanoparticle per grain, with an epitaxial relationship. Superconducting quantum inference device measurement at 300 K revealed that the FePt nanoparticles had coercivity of 6.2 and 0.8 kOe for the out-of-plane and in-plane directions, respectively. The FePt nanoparticle monolayer sputter-deposited on polycrystalline TiN seed layer is a promising candidate for perpendicular magnetic recording media.

The initial stage during vapor deposition has been extensively studied in physical vapor deposition (PVD) processes, and nucleation theories have been successfully used to model island nucleation processes during PVD. Compared with the extensive research in PVD, there has been less work on understanding the initial stage in chemical vapor deposition (CVD) processes, despite the technological and commercial importance of CVD-based manufacturing systems. In this work we briefly review the nucleation theories developed for PVD processes and consider the validity of them for modeling the initial stage of CVD processes. One characteristic of CVD processes is the existence of an incubation time. Recent research indicates that the incubation time can be caused by the different reactivity of precursors nucleating on substrates and islands. We proposed process indices to evaluate the relative importance of sticking probabilities and desorption of adsorbates on the incubation time. The differing precursor reactivity between islands and substrates may also affect the island growth mode. This situation in CVD processes differs from that in PVD processes, for which current nucleation theories were developed, and therefore prevents the direct application of PVD nucleation theories to CVD processes. Therefore, to model CVD processes, a nucleation model is needed that is sensitive to the different reactivity of precursors to islands and substrates. &COPY; 2004 Elsevier B.V. All rights reserved.

Enhanced surface diffusion at the growth temperature of single-walled carbon nanotubes (SWNTs) can cause coarsening of metal catalysts. By balancing the nominal thickness and surface diffusion length of metals, metal nanoparticles of desirable size are expected to form spontaneously under the SWNTs growth conditions. Our combinatorial method, using a library of nominally 0.001 to 1 nm thick sputter-deposited cobalt patterns, identified in a single experimental run that cobalt nanoparticles from submonolayers can catalyze the growth of high-quality SWNTs. (c) 2005 American Institute of Physics.

Materials science is divided into disciplines based on the properties of bulk materials. However, common rules that govern phenomena at nanoscales are eliminating those boundaries between disciplines. in order to support more effective education, research, development, and manufacturing in materials science, a Japanese national project for structuring knowledge of materials nanotechnology is underway. Examples of the effect of structuring knowledge on the relationships between processes and material structures are shown for vapor-deposition processes and nanoparticle coating and drying processes. A knowledge platform, into which this generalized nanotechnology knowledge is to be integrated, is outlined in this paper. The key purpose of structuring knowledge is to stimulate idea generation based on a fundamental and general understanding of underlying mechanisms. (C) 2004 Published by Elsevier Ltd.

Chemical vapor deposition (CVD) of tungsten is an important process to make interconnections in advanced integrated-circuit devices. As device dimensions continue to decrease, incomplete nucleation inside the trenches and via holes is becoming a crucial issue. In this work, micro-Auger electron spectroscopy with in-plane spatial resolution was applied for the first time to study the nucleation and growth process of W islands. Results showed that W grew slowly and uniformly on TiN surfaces up to about one-monolayer coverage, and then W islands nucleated and started to grow rapidly. This transition from layer to island shows that W grew by Stranski-Krastanov mode during CVD on TiN from WF6 and SiR4. Drastic difference might exist in chemical reactivity between the-initial W layer on TiN surfaces and the W islands, causing the change in W growth rate.

Cauliflower-like protrusions formed in CVD processes under diffusion-limited conditions have been studied both experimentally and theoretically. Both approaches indicate that the difference in diffusion fluxes to the film and to the protrusions controls the growth of such protrusions. However, direct comparisons of these two approaches have never been done, probably due to the complexity of the theoretical models. To simplify model protrusion growth, we developed a one-dimensional (1D) analytical model by hypothesizing the diffusion of growth species in the boundary layer above a growing film. Based on this model, we propose a non-dimensional quantity, k(s)f/D, as an index of protrusion growth (D is the diffusion coefficient of the growth species, k(s) is the surface reaction-rate coefficient, and f is film thickness). This index represents more directly the protrusion growth than does the previously proposed index, the Damkohler number, Da = k(s)delta/D, where delta is boundary layer thickness. To obtain smooth, protrusion-free films, D/k(s) should be kept larger than the desired film thickness. By controlling the process conditions to satisfy this index, we successfully fabricated protrusion-free films with SiC deposition from dichlorodimethylsilane (DDS).

Aiming to realize a conductive passivation layer for copper interconnection, the solid-gas reactions of cobalt films with silane and with disilane to form cobalt silicides are experimentally investigated. X-ray photoelectron spectroscopy revealed that cobalt silicides layers of up to 6 nm thickness can be selectively formed in the reaction at 473-673 K within 5 min without detectable silicon deposition on silicon dioxide, a common inter-metal dielectric layer. Rapid thermal oxidation experiments revealed that the silicided cobalt layers had better anti-oxidation performance than untreated cobalt layers, and the effect of silicidation was to suppress copper out-diffusion through the cobalt layers. Because cobalt-based alloys can be selectively electroless-plated on copper, selective silicidation of cobalt layers will be easily incorporated into device processing.

Cu thin films were grown by sputter deposition on SiO2 substrates with a Cr underlayer that is known to improve the adhesion between Cu and SiO2. The initial stage of Cu growth was investigated using transmission electron microscopy. Results showed that non-wetting spherical Cu nanoislands were formed with a random crystalline orientation on Cr/SiO2, and evolved into a randomly oriented polycrystalline thin film. These results were then compared with our previous results on the initial growth of Cu on SiO2 with and without a Ti underlayer. A quantitative model was proposed to explain the difference in dependence of the wettability of microscopic nanoislands and that of the adhesion of macroscopic thin films on interfacial interactions and surface energies.

Vapor deposition is a complex process, including gas-phase, surface, and solid-phase phenomena. Because of the complexity of chemical and physical processes occurring in vapor deposition processes, it is difficult to form a comprehensive, fundamental understanding of vapor deposition and to control such systems for obtaining desirable structures and performance. To overcome this difficulty, we present a method for simplifying the complex description of such systems. One simplification method is to separate complex systems into multiple elements, and determine which of these are important elements. We call this method abridgement. The abridgement method retains only the dominant processes in a description of the system, and discards the others. Abridgement can be achieved by using process indices to evaluate the relative importance of the elementary processes. We describe the formulation and use of these process indices through examples of the growth of continuous films, initial deposition processes, and the formation of the preferred orientation of polycrystalline films. In this paper, we propose a method for representing complex vapor deposition processes as a set of simpler processes. (C) 2004 Elsevier B.V. All rights reserved.

Communication: Mechanisms controlling the deposition incubation time of Si on SiO2 are studied. Two major mechanisms are considered to affect the incubation time: desorption of adsorbate that is not captured by existing islands, and differing sticking probabilities between islands and substrate. Results indicate that desorption of adsorbate seems to have less contribution and the sticking probability mechanism controls island growth (Figure), and therefore is the main cause for the incubation time in silane CVD processes.

A gas mixture of tungsten hexafluoride (WF6) and silane (SiH4) is generally used to form the initial layer of tungsten (W) on titanium nitride (TiN). However, the nucleation mechanism is still not clear, thus making it difficult to optimize such processes for complete filling of via holes. Therefore, in this study, we examined the nucleation process by laser-reflection measurements, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). These measurements indicate that W nucleation has two stages: monolayer formation followed by nucleation of three-dimensional (3D) islands. The monolayer formation can be expressed as Langmuir-type adsorption, and proceeds with the reduction of WF6 by Ti on TiN substrates. After monolayer formation, nucleation of 3D islands occurs and islands rapidly grow. These processes were quantitatively modeled using a simple rate equation. The results of our model agree well with our measurements of the deposited amount and coverage of islands.

The structural evolution of tantalum nitride (TaN) films deposited by reactive rf magnetron sputtering were investigated in detail by using transmission electron microscopy (TEM) and x-ray diffractometry (XRD) for a wide range of thickness from 2 nm to 2 mum under various N-2 /Ar flow ratios from 0 to 20 vol % on both amorphous SiO2 (a-SiO2) and randomly oriented polycrystalline fee TaN (poly-fcc-TaN) substrates. Although the films had various crystalline structures [including tetragonal Ta, bee Ta(N), and fee TaN] of different preferred orientation (PO) and had amorphous phases depending on deposition conditions, the formation mechanism of these structures was systematically explained by mapping them on 2D graphs of film thickness vs N-2 /Ar flow ratio. The texture map of films deposited on a-SiO2 substrates reflected both nucleation and growth stages, whereas that of films deposited on poly-fcc-TaN substrates reflected mainly the growth stage. Comparison of these two maps allowed the nucleation and growth processes to be separately discussed. For films deposited. at 4 vol % N-2 /Ar ratio on a-SiO2 substrates, an amorphous phase initially appeared when the film thickness was 1.8-3.5 nm. When the film thickness was about 7 nm, nucleation occurred to form fee TaN without any PO. When the thickness was about 100 nm, (111) PO appeared. Finally, when the thickness exceeded 200 nm, (200) PO dominated the film. Cross-sectional TEM micrographs revealed that evolutionary selection growth occurred when the film was 200-nm-thick to cause the PO change. (111) PO was preferred at relatively low (2-3 vol %) and high (greater than or equal to10 vol %) N-2/Ar ratios, whereas (200) was preferred at medium N-2/Ar ratio (4-7 vol %). (C) 2004 American Vacuum Society.

Deposition flux is an important factor that determines the structures of, vapor-deposited materials. However, controlling this flux over a wide range is difficult using only a single apparatus. In-this work, we developed a simple method, called combinatorial masked deposition (CMD), that enables a series of deposition fluxes and their respective distribution to be realized on a single sample by just setting a mask with holes of different sizes above a substrate. The degree of reduction in deposition flux can be controlled by the hole size and distance between the given point and the hole. The characteristics and applicability of CMD were evaluated by two experiments. In the first experiment, Cu nanoparticles were formed by sputter-deposition on a-SiO2 at different Cu deposition fluxes. The nanoparticles had a higher number density and smaller size when deposited at 0.80 nm/s for 2.5 s than when deposited at 0.014 nm/s for 140 s. In the second experiment, metal-induced crystallization of amorphous Si (a-Si) was done with spatially distributed Ni additives. The CMD method can realize a series of Ni flux distributions and was successfully used to form 100 different profiles of Ni concentration on a single sample, thus enabling efficient screening of concentration profiles to enhance grain size. (C) 2003 Elsevier B.V. All rights reserved.

The ever growing ULSI device integration now requires ultra thin films as low as several nano-meter level. The so-called layer by layer growth is the ideal for making thin films for these requirements but island growth often occurred in metal CVD to deposit Al, W and Cu thin films on barrier layers, mostly composed of TiN or Ta/TaN films. We have made in-situ monitoring of initial nucleation behavior using laser light reflectivity measurement. This technique enables us to monitor the incubation period during which no nuclei formation is observed. The surface adsorbates during the incubation period was monitored by XPS and we found that the reactivity of initial surface and the film precursor is determining the incubation period.

We propose a chemical vapor deposition (CVD) process with closed gas recycling for making low-cost, crystalline silicon thin films for solar cells, which connects chlorosilane synthesis from Si and HCl with Si thin-film growth by CVD from chlorosilanes. In this work we studied the formation of chlorosilanes by the reaction of Si with HCl at temperatures ranging from 623 to 723 K. The reaction rate is time dependent, and many pores are formed on the surface of particles after reaction. These pores are active sites for chemical reactions, and the reaction rates increase with increasing pore area. The rate can be correlated with the conversion ratio of Si, and the temporal evolution of the reaction rate can be explained by a reaction model called the shrinking-core model with growing pores. By using this model, we estimated the reaction rates per unit area of activated surfaces and converted them into a rate equation that can be used for the reactor design. The incubation time of the reaction can be shortened by pretreating the Si particles in a fluidized bed, which probably creates defects in the native oxide layers on the particles, which in turn become reactive sites. (C) 2004 The Electrochemical Society.

Texture control of sputter-deposited nitride films has provoked a great deal of interest due to its technological importance. Despite extensive research, however, the reported results are scattered and discussions about the origin of preferred orientation (PO) are sometimes conflicting, and therefore controversial. The aim of this study is to acquire a clear perspective in order to discuss the origin of PO of sputter-deposited nitrides. Among nitrides, we focus on titanium nitride (TiN), aluminum nitride (AlN), and tantalum nitride (TaN), which are three commonly used nitrides. First, we collected reported experimental results about the relation between operating conditions and PO, because PO is considered to be determined by film formation processes, such as surface diffusion or grain growth, which is affected by operating conditions. We also collected reported results about such PO-determining processes. Then, we categorized the PO-determining processes into an initial stage and a growth stage of film deposition, and further categorized each stage into a vapor-solid interface and a solid-solid interface. Then, we related each stage and interface to film morphology and to PO-determining processes. Finally, based on existing results, previous models, and proposed schema, we discuss the origin of PO. Based on previous experimental results on film morphology, PO of nitride films occurred in the growth stage at the vapor-solid interface, where the sticking process of the precursor and the surface diffusion process determine PO, rather than in the initial stage and in the growth stage at the solid-solid interface. TiN (002) PO, however, seems to be caused in the initial stage at the solid-solid interface. (C) 2003 American Vacuum Society.

The effects of substrate heating and substrate biasing on. the initial stage of nonepitaxial heterogeneous growth of TiN on Si(111) was studied by using high-resolution transmission electron microscopy. Although TiN films deposited at room temperature (RT) undergo a transition from continuous amorphous films to polycrystalline films with three-dimensional grains when the film thickness is increased from similar to1 to 2 nm, crystallization occurred at a substrate temperature, T-s = 570 K, even for film thicknesses less than 1 nm. Compared with growth at T-s = RT, at T-s = 570 K, the initial lateral grain size was only slightly larger, and the grains tended to be spherical and discontinuous at higher film thickness. At a substrate bias voltage, V-b = -70 V-b the grains were laterally larger and planar. At a film thickness of 50 nm, the films deposited at V-b = -70 V showed the thermodynamically favored (200) preferred orientation, whereas the films deposited at T-s = 570 K showed (111) preferred orientation with a weak (200) peak. (C) 2003 American Vacuum Society.

The structure and morphology of nanosized Cu islands grown by sputter deposition on clean SiO2 substrates and Ti-underlayered SiO2 substrates are investigated using transmission electron microscopy. On SiO2, spherical Cu islands with a random crystalline orientation are formed, whereas on Ti/SiO2, semispherical islands with a preferred &lt;111&gt; crystalline orientation are formed. Moreover, the Cu islands on Ti/SiO2 have smaller sizes, shorter interisland distances, and a higher number density than those on SiO2. These structural and morphological changes at the nanoscale are discussed from the viewpoint of interfacial interactions. Our study suggests that by using an appropriate metal underlayer, it is possible to fabricate nanosized islands with the desired wettability, crystalline orientation, as well as morphology of island ensembles. (C) 2003 American Institute of Physics.

The initial growth stage of titanium nitride (TiN) deposited by reactive magnetron dc sputtering onto (111)-oriented Si substrates was investigated by using high-resolution transmission electron microscopy (HRTEM). During the initial growth stage, a continuous amorphous layer was observed when the deposited film was less than I nm thick. Crystal nucleation occurred from the amorphous layer when the film grew to about 2 nm thick. No pref erred orientation was found for the initial crystal nuclei. The growth of the crystal grains depended on the N-2 partial pressure, P-N2. Increasing P-N2 from 0.047 to 0.47 Pa enhanced lateral grain growth and coalescence between grains. For P-N2 = 0.47 Pa, planar grains with a large lateral dimension were found formed by grain growth and coalescence; inducing a (200) film orientation. For films formed at P-N2 = 0.47 Pa, an amorphous interlayer 1.5-1.8 nm thick formed between the TiN layer and Si substrate, and was indicated to be primarily SiNx by x-ray photoelectron spectroscopy and HRTEM. This interlayer was less than 0.5 nm thick in films formed at P-N2 = 0.047 Pa. (C) 2003 American Vacuum Society.

The growth of sputter-deposited Cr thin films on amorphous SiO2 during the early stages was studied using transmission electron microscopy. Amorphous three-dimensional islands were first formed, and then they grew with continuously increasing density and slowly increasing size as the deposition proceeded. When these islands began to coalesce at a nominal film thickness of 2.3-3.0 nm, they abruptly crystallized into randomly oriented crystalline nuclei. The depth profile analysis by x-ray photoelectron spectroscopy indicates the existence of interfacial Cr-O interactions. After excluding-the possibilities of kinetic limitation and interfacial mixing, a thermodynamic model was employed to explain the size-dependent amorphous-to-crystalline transition. Our results suggest that the interfacial-interaction-induced strain relaxation at island/substrate interfaces might result in the thermodynamic stabilization of substrate-supported amorphous islands below a critical size. (C) 2003 American Institute of Physics.

In the absence of epitaxy between a film and a substrate, the preferred orientation of polycrystalline films can often be explained by the "evolutionary selection rule", which states that grains with the fastest growing direction normal to the substrate envelope the other grains and determine the final orientation of the film. However, the mechanism determining the fastest growing plane and the factors affecting the growth rates of each plane are still not well understood. We examined existing experimental results in the previous literatures and found a correlation between process conditions and preferred orientation for poly-Si and poly-SiC thin films. We present a model based on Langmuir-type adsorption for predicting preferred orientation, which agrees well with experimental results in the previous literatures for Si and SiC.

The growth of metals on TiO2(1 10) at one monolayer coverage is classified into three-dimensional island, twodimensional layer, and transition growth zones via two thermodynamic parameters, the heat of formation of metal oxides, -Delta(f)H(oxide of M)(0), and the heat of sublimation of metals, -Delta(f)H(metal, per mol of metal)(0) (both expressed per mol of metal), which are easily obtainable. These two parameters represent the strength of metal/TiO2(110) interfacial interactions and the strength of metal/metal lateral interactions, respectively. Such classification is based on the thermodynamic criteria that the growth mode of metals on TiO2(110) is determined by metal/TiO2 interfacial free energy and metal surface free energy. Compared with the conventional approach that only uses the heat of formation of metal oxides, -Delta(f)H(oxide of O)(0) (expressed per mol of oxygen), our model provides a clearer and more comprehensive vision of the growth mode of metals on TiO2(110) and the factors affecting the growth mode. The approach described in this study can also be applied to other metal/reducible oxide systems. (C) 2002 Elsevier Science B.V. All rights reserved.

We investigated the mechanism that determines the preferred orientation of polycrystalline silicon carbide (SiC) films prepared by CVD from a mixture of dichlorodimethylsilane (DDS) and He, X-ray diffraction (XRD) measurements indicated that the major growth direction is either the (220) or the (111) plane. We developed a numerical model for predicting the preferred orientation, assuming Langmuir-type adsorption and reaction of the growth species. This model suggests that the (111) plane appears under reaction-limited deposition, while the (220) plane appears under adsorption-limited deposition. Our experimental and numerical results show good qualitative agreement with experimental results for films prepared from methyltrichlorosilane (MTS) and H2.

The cone structure formation in epitaxial Si films produced by chemical vapor deposition (CVD) was reported. Measures to prevent cone formation were proposed by studying the cause of the different growth rates between the cone and the film. The shape of the cone was analyzed to precisely determine the growth rate of each crystallographic plane.

The initial growth and texture formation mechanism of titanium nitride (TiN) films were investigated by depositing TiN films on (111) silicon substrates by using reactive magnetron sputtering of a Ti metallic target under a N-2/Ar atmosphere, and then analyzing the films in detail by using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Two power sources for the sputtering, dc and rf, were compared. At the initial growth stage, a continuous amorphous film containing randomly oriented nuclei was observed when the film thickness was about 3 nm. The nuclei grew and formed a polycrystalline layer when the film thickness was about 6 nm. As the film grew further, its orientation changed depending on the deposition conditions. For dc sputtering, the appearance of (111) or (200)-preferred orientations depended on the N-2 partial pressure, and the intensity of the preferred orientation increased with increasing film thickness. For rf sputtering, however, when the film thickness was small (for instance, about 20 nm), the film showed (200) orientation, independent of the N2 partial pressure, and further growth caused the film to orient to the (111) orientation when the N-2 partial pressure was low (about 0.015 Pa). The results indicated that preferred orientation of TiN films is controlled by a competition between kinetic and thermodynamic effects. (C) 2002 American Vacuum Society.

The effect of interfacial interactions on the initial growth of Cu on clean SiO2 and 3-mercaptopropyltrimethoxysilane (MPTMS)-modified SiO2 substrates by sputter deposition was studied using transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray photoelectron spectroscopy. Plasma damage during sputter deposition makes surfaces of MPTMS-modified SiO2 substrates consist of small MPTMS islands several tens of nanometers in diameter and bare SiO2 areas. These MPTMS islands are composed of disordered multilayer MPTMS aggregates. The initial growth behavior of Cu on MPTMS-modified SiO2 substrates differs from that on clean SiO2 substrates, although Cu grows in three-dimensional-island mode on both of them. After a 2.5-monolayer Cu deposition on clean SiO2 substrates, spherical Cu particles were formed at a low number density of 1.3 x 10(16) /m(2) and at a long interparticle distance of 5 nm. In contrast, after the same amount of deposition on MPTMS-modified SiO2 substrates, Cu particles preferentially grow on MPTMS islands at a high number density of 3.9 x 10(16) /m(2) and at a short interparticle distance of 3 run, but do not grow on bare SiO2 areas. The increased number density and the decreased interparticle distance indicate that Cu has a lower mobility on MPTMS islands on MPTMS-modified SiO2 substrates than on clean SiO2 substrates. This difference in Cu mobility is attributed to the enhanced interfacial interactions between Cu and S on MPTMS islands on MPTMS-modified SiO2 substrates via the formation of Cu-S bonds, compared with the relatively weak interfacial interactions between Cu and Si or O on clean SiO2 substrates. (C) 2002 American Vacuum Society.

Communication: Systematic creation of abnormal structures in silicon epitaxial growth is studied. The abnormalities are induced by randomly dispersing 2-4 mum sized diamond particles onto silicon substrates When the epitaxial plane is not the fastest growing one, elongated crystallites form a radial pattern from the particles enveloped in the epitaxial film, resulting in cone structures much larger than the seed particles

We investigated the mechanism that determines the preferred orientation of polycrystalline silicon carbide (SiC) films prepared by CVD from a mixture of dichlorodimethylsilane (DDS) and He. X-ray diffraction (XRD) measurements indicated that the major growth direction is either the (220) or the (111) plane. We developed a numerical model for predicting the preferred orientation, assuming Langmuir-type adsorption and reaction of the growth species. This model suggests that the (111) plane appears under reaction-limited deposition. while the (220) plane appears under adsorption-limited deposition. Our experimental and numerical results show good qualitative agreement with experimental results for films prepared from methyltrichlorosilane (MTS) and H-2.

Communication: The deposition of trumpet-like protrusions during CVD of SiC is reported. The protrusions are deposited at the flow stagnation point of an impinging flow reactor. SEM analysis of the structures (Figure) indicates that they consist of aggregates of 1 mum sized particles surrounded by a smooth and dense film, which suggests a possible growth mechanism that produces preferential thermophoretic deposition on the protrusions rather than on the surface of the films.

Pb-Ti-Nb-O ferroelectric thin films with various Nb additions are grown on a Pt/Ti/SiO2/Si substrate at 400 degreesC by metal-organic (MO) CVD. A high density of dome-like surface protrusions is observed by scanning electron microscopy (SEM) in all the as-prepared films. Both the shape and the size of the surface defects are found to be Nb-content-dependent. The internal microstructure of the protrusions is further characterized by cross-section transmission electron microscopy (XTEM). The origins of these surface defects are discussed, based on the substrate hillock as well as the crystallization behavior of the film forming precursors during MOCVD. The development of the observed surface defects is modeled using a two-dimensional vector analysis.

Self-assembled 3-mereaptopropyltrimethoxysilane (MPTMS, (CH3O)(3)SiCH2CH2CH2SH) layers on hydroxyl-terminated silicon oxide (SiO2) were prepared at MPTMS concentrations ranging from 5 x 10(-3) to 4 x 10(-2) M. The surface structure and morphology of MPTMS layers were characterized by X-ray photoelectron spectroscopy (XPS), contact angle measurements, scanning electron microscopy (SEM), and atomic force microscopy (AFM). We found that the MPTMS layers on SiO2 consisted of dispersed domains 20-200 run in diameter, instead of continuous, flat monolayers. With increasing MPTMS concentration, the domain shape changed from flat to steep. Flat domains were composed of well-ordered monolayers with thiol headgroups uniformly distributed on the uppermost surface, whereas steep domains were composed of disordered polymers with randomly distributed thiol headgroups on the uppermost surface. These results indicate that MPTMS molecules show good self-assembly at an MPTMS concentration of 5 x 10(-3) M, but not above this concentration. The effect of MPTMS concentration on the structure and morphology of MPTMS layers might be due to the competition between self-polymerization and surface dehydration reactions, which depends on the trace quantity of water in the solvent and on the SiO2 surface. Our research further indicates that MPTMS and water concentrations are the controlling parameters for preparing well-ordered, self-assembled MPTMS monolayers on SiO2. (C) 2001 Elsevier Science B.V All rights reserved.

Exhaust gas from a diesel engine is one of the main causes of air pollution. It is difficult to reduce NO under the excess O-2 concentration so that effective method of NO reduction has not been developed. In this study, nickel on the porous glass (VYCOR(R) glass) was examined as a catalyst for NO reduction. This catalyst was effective for NO reduction where 35% of NO was converted to N-2 under excess O-2 concentration (3%) in the tubular fixed-bed reactor. Adsorption rates of NO and O-2 gases were measured in order to examine the mechanism of selective reduction of NO by porous VYCOR catalyst. It was proved that NO adsorption rate was much higher than O-2 adsorption rate, so that difference of adsorption rate was one of the main causes of NO reduction. As a short lifetime of the-catalyst was the serious disadvantage of the tubular fixed-bed reactor, another type of catalytic reactor was proposed and tested. It is a membrane reactor consisting of a porous VYCOR catalyst tube and a quartz tube. NO reduction under excess O-2 concentration was also achieved in the membrane reactor. Maximum conversion from NO to N-2 was 45%. The possibility of longer lifetime catalyst could be found.

In the current study, we grew Pb-Ti-Nb-O (PTN) thin films on Pt/Ti/SiO2/Si substrate by metallorganic chemical vapor deposition (MOCVD) at temperatures ranging from 400 to 620 degreesC. The thin films obtained were examined by X-ray diffraction and cross-sectional transmission electron microscopy. It was revealed that both the PTN film and its interface with the underlying Pt layer were quite sensitive to variations in the deposition temperature. The considerable change in surface morphology of PTN thin films with an enhanced deposition temperature is discussed in relation to the surface hillocks formed on the Pt layer by aggregation of the out-diffused Pt atoms during MOCVD. (C) 2001 The Electrochemical Society.

The gas-phase hydroxyl(OH) radical emission was observed in the thermal decomposition of lithium hydroxide: (LIOH). This phenomenon was investigated in a vacuum flow tube reactor at around 2 Torr by the temperature programmed reaction (TPR) experiments at 500-1300 K. The production of OH and other gaseous products was quantitatively investigated by the laser induced fluorescence method and quadrupole mass spectrometry, respectively. The TPR spectra of OH had a peak at 1100-1200 K, which largely exceeded the gas-phase thermodynamic equilibrium. The origin of OH was supposed to be either the surface OH groups on Li2O or the residual LiOH in the LiOH/Li2O solid solution. OH production exceeding the thermodynamic equilibrium was explained by means of the partial equilibrium in the reaction: LiOH + 1/4O(2) &lt;-&gt; 1/2Li(2)O + OH. This phenomenon can be a new route for the OH production from H2O and O-2 in cyclic reactions of lithium compounds.

Hydroxyl radical desorption in the heterogeneous catalytic reactions of water or hydrogen with oxygen was examined by laser-induced fluorescence spectroscopy. The catalytic activities of Pt, Al2O3, and basic metal oxides (MgO, CaO, SrO, BaO) supported on Al2O3 were studied in the pressure range 0.1-10 Torr and the temperature range 1100-1300 K. In the case of OH generation from water, the catalytic activities of Pt and all oxides except Al2O3 were very high and the OH concentration reached the equilibrium value within a residence time of 4 ms. In the case of hydrogen oxidation, differences in the catalytic behavior were clearly observed. The surface reaction mechanisms and the effects of Al2O3 and MgO on gas-phase ignition are discussed.