bluuebunny/crossref_metadata_2025 huggingface dataset:
ZnO Thin Film Transistors for RF Applications
Authors: Bayraktaroglu Burhan , Leedy Kevin , Neidhard Robert
Published: January 2010

Nanocrystalline ZnO thin films grown by the pulsed laser deposition technique were used to fabricate high performance thin film transistors suitable for RF applications. It was shown that drain current on/off ratios of higher than 1×1012, sub-threshold voltage swing values lower than 100 mV/decade and hysteresis-free operation could be maintained with films grown across a wide temperature range (25°C to 400°C). Films grown at 200°C have the lowest surface roughness and result in devices with the highest current density operation. Devices with 1.2 μm gate lengths and Au-based gate metals had record current gain and power gain cut off frequencies of fT = 2.9 GHz and fmax = 10 GHz, respectively.
Manipulation of Surface Charge on GaN
Authors: Ferguson Josephus D. , Foussekis Michael A. , Ruchala Monika D. , Moore J. C. , Reshchikov Michael A. , Baski Alison A.
Published: January 2010

We have characterized the surface charge on a variety of GaN samples using two surface potential techniques, conventional Kelvin probe and scanning Kelvin probe microscope (SKPM). Kelvin probe was primarily used to measure the change in surface potential under UV illumination, otherwise known as the surface photovoltage (SPV). Due to band bending near the semiconductor surface of about 1 eV in dark conditions, the SPV signal for n-type GaN typically reaches 0.5 to 0.6 eV upon switching on UV light. This value can slowly decrease by up to 0.3 eV during UV illumination in air ambient for 2-3 hours. We report that samples with many hours of ambient UV exposure do not show this slow decrease during SPV measurements, consistent with the UV-induced growth of a thicker surface oxide that limits charge transfer. In addition to prolonged UV exposure, the surface contact potential was also manipulated by local charge injection. In this procedure, the surface is charged using a metallized atomic force microscope tip which is scanned in contact with the sample. Subsequent SKPM measurements indicate an increase or decrease in the surface contact potential for the charged region, depending on the applied voltage polarity. Measurements of the discharge behavior in dark for these regions show a logarithmic time behavior, similar to the decay behavior during our observations of SPV transients after switching off the light. As expected, illumination of the surface increases the discharge rate and restores the charged area to its original state.
Properties of Digital Aluminum Gallium Nitride Alloys Grown via Metal Organic Vapor Phase Epitaxy
Authors: Rodak L. E. , Korakakis D
Published: January 2010

Deep Ultra Violet (UV) emitters are of particular interest for applications including, but not limited to, biological detection and sterilization. Within the III-Nitride material system, Aluminum Gallium Nitride (AlxGa1-xN) alloys are the most promising for UV device fabrication due to the wide, direct band gap. The growth of high quality AlxGa1-xN alloys via Metal Organic Vapor Phase Epitaxy (MOVPE) is challenging due to large sticking coefficient of the Al species compared to that of Ga and also the high reactivity of Al precursors. As a result, films are often characterized by large dislocation densities, cracks, and poor conductivity. Digital alloy growth, or Short Period Superlattices (SPS), consisting of layers of binary or ternary alloys with a period thickness of a few monolayers has been shown to be a viable means of growing high quality ternary alloys via Metal Organic Vapor Phase Epitaxy (MOVPE). In certain materials, such as AlGaInP, the electronic properties of digitally grown alloys differ considerably from the equivalent random alloy. Specifically, the bandgap has been shown to differ significantly from the equivalent random alloy. As a result, digital alloy growth presents the potential to further engineer material properties. However, the influence of digital growth on the electronic properties of III-Nitride alloys has not been extensively characterized. This study focuses on Aluminum Gallium Nitride (AlxGa1-xN) alloys grown using a digital technique via MOVPE. The influence of the growth technique over a wide range of compositions is reported along with the electronic properties.
Thermodynamic Analysis and Purification for Source Materials in Sublimation Crystal Growth of Aluminum Nitride
Authors: Du Li , Edgar James
Published: January 2010

Source material purification according to a thermodynamic analysis is reported for the sublimation crystal growth of aluminum nitride in an inert reactor. OAlOH is strongly favored over all other possible oxygen containing compounds in both the Al-O-H-N and Al-O-H-C-N systems, while Al2O, proved to be the most favorable oxygen containing gas species for Al-O-N system in previous study, become secondary favorable gas species. A low temperature (<1200 °C) treatment is effective in eliminating oxygen and hydrogen from the source powder. Carbon monoxide is another important oxygen containing gas species in the Al-O-H-C-N system, and is favored over Al2O at certain temperature and pressure. Carbothermal reduction with intentionally added carbon (graphite) can further reduce the oxygen concentration. Experiments show that high-temperature sintering minimizes the oxygen concentration and reduces the specific surface area of the source. With only 5.5% of mass loss, the purification produced a source with low O, H, and C concentrations of 0.018 wt%, 6ppm, and 0.006wt%.
X-ray Diffraction Study of InGaN/GaN Superlattice Implanted with Eu3+ Ions
Authors: Ebdah Mohammad Ahmad , Kordesch Martin E. , Anders Andre , Jadwisienczak Wojciech M.
Published: January 2010

In this work, europium implanted InGaN/GaN SL with a fixed well/barrier thickness ratio grown by metal-organic chemical-vapor deposition (MOCVD) on GaN/(0001) sapphire substrate were investigated. The as-grown and Eu ion implanted InGaN/GaN SLs were annealed at different temperatures ranging from 600°C to 950°C in nitrogen ambient. The quality of the SL interfaces in undoped and implanted structures has been investigated by X-ray diffraction (XRD) at room temperature. The characteristic satellite peaks of SLs were measured for the (0002) reflection up to the second order in the symmetric Bragg geometry. The XRD simulation spectrum of the as-grown SL agrees well with the experimental results. The simulation results show x=0.06 atomic percent the InGaN well sub-layers, with thicknesses of 2.4 and 3.3 nm for single InGaN well and GaN barrier, respectively. It was observed that annealing of the undoped SL does not significantly affect the interfacial quality of the superstructure, whereas, the Eu ion implanted InGaN/GaN SL undergo partial induced degradation. Annealing the implanted SLs shows a gradual improvement of the multilayer periodicity and a reduction of the induced degradation with increasing the annealing temperature as indicated by the XRD spectra.
Fluorescent Nanodiamonds: Effect of Surface Termination
Authors: Kratochvílová Irena , Taylor Andrew , Kovalenko Alexander , Fendrych Frantisek , Řezáčová Vladimira , Petrák Vaclav , Zalis Stanislav , Šebera Jakub , Nesládek Milos
Published: January 2010

It has been reported that physico-chemical properties of diamond surfaces are closely related to the surface chemisorbed species on the surface. Hydrogen chemisorption on a chemical vapor deposition grown diamond surface is well-known to be important for stabilizing diamond surface structures with sp3 hybridization. It has been suggested that an H-chemisorbed structure is necessary to provide a negative electron affinity condition on the diamond surfaces. Negative electron affinity condition could change to a positive electron affinity by oxidation of the H-chemisorbed diamond surfaces. Oxidized diamond surfaces usually show characteristics completely different from those of the H-chemisorbed diamond surfaces. The unique electron affinity condition, or the surface potential, is strongly related to the chemisorbed species on diamond surfaces. The relationship between the surface chemisorption structure and the surface electrical properties, such as the surface potential of the diamond, has been modelled using DFT based calculations.
Nanodiamond Particles in Electronic and Optical Applications
Authors: Shenderova Olga , Hens Suzanne Ciftan , Vlasov Igor , Borjanovic Vesna , McGuire Gary
Published: January 2010

Current use of nanodiamond (ND) particles in electronic-related applications is mostly restricted to their role in seeding of substrates for growth of diamond films by chemical vapor deposition (CVD). While it is a niche application, nanometer-sized diamond particles are indispensable in this role. Seeding of substrates using a novel slurry of detonation nanodiamond particles in dimethylsulfoxide (DMSO) and methanol is one of the topics of this article. At the same time, optical applications of NDs, particularly development of photoluminescent NDs for biomedical applications is one of the most popular current research topics. In this paper perspectives for the use of detonation NDs and specifically the role of surface functionalization in imparting photoluminescent properties to detonation NDs as well as enhanced photoluminescence of proton irradiated ND-polydimethylsiloxane composites are discussed.
Simulations of CVD Diamond Film Growth Using a Simplified Monte Carlo Model
Authors: May Paul William , Harvey Jeremy N. , Allan Neil L. , Richley James C. , Mankelevich Yuri A.
Published: January 2010

A simple 1-dimensional Monte Carlo (KMC) model has been developed to simulate the chemical vapour deposition (CVD) of a diamond (100) surface. The model considers adsorption, etching/desorption, lattice incorporation, and surface migration along and across the dimer rows. The reaction probabilities for these processes are re-evaluated in detail and their effects upon the predicted growth rates and morphology are described. We find that for standard CVD diamond conditions, etching of carbon species from the growing surface is negligible. Surface migration occurs rapidly, but is mostly limited to CH2 species oscillating rapidly back and forth between two adjacent radical sites. Despite the average number of migration hops being in the thousands, the average diffusion length for a surface species is <2 sites.
Optical Monitoring of Nanocrystalline Diamond with Reduced Non-diamond Contamination
Authors: Remes Zdenek , Kromka Alexander , Izak Tibor , Purkrt Adam , Vanecek Milan
Published: January 2010

Previously, the nanocrystalline grain boundaries were often contaminated by the “non-diamond phase” with the photo-ionization threshold at 0.8 eV. Here, we present the optical spectra of the NCD films grown on transparent substrates by the microwave plasma enhanced chemical vapor deposition (CVD) at a relatively low temperature below 600°C. The transmittance and reflectance spectra are useful to evaluate the film thickness, the surface roughness and the index of refraction. The direct measurement of the optical absorptance by the laser calorimetry and photothermal deflection spectroscopy (PDS) provides high sensitive methods to measure the weak optical absorption of thin films with rough surface. The optical measurements indicate the high optical transparency of our standard, nominally undoped 0.2-0.3 μm thick NCD film with low non-diamond content. However, the optical scattering is rather high in UV and needs to be reduced.
Capacitive Field-effect (bio-)chemical Sensors Based on Nanocrystalline Diamond Films
Authors: Bäcker Matthias , Poghossian Arshak , Abouzar Maryam H. , Wenmackers Sylvia , Janssens Stoffel D. , Haenen Ken , Wagner Patrick , Schöening Michael Josef
Published: January 2010

Capacitive field-effect electrolyte-diamond-insulator-semiconductor (EDIS) structures with O-terminated nanocrystalline diamond (NCD) as sensitive gate material have been realized and investigated for the detection of pH, penicillin concentration, and layer-by-layer adsorption of polyelectrolytes. The surface oxidizing procedure of NCD thin films as well as the seeding and NCD growth process on a Si-SiO2 substrate have been improved to provide high pH-sensitive, non-porous thin films without damage of the underlying SiO2 layer and with a high coverage of O-terminated sites. The NCD surface topography, roughness, and coverage of the surface groups have been characterized by SEM, AFM and XPS methods. The EDIS sensors with O-terminated NCD film treated in oxidizing boiling mixture for 45 min show a pH sensitivity of about 50 mV/pH. The pH-sensitive properties of the NCD have been used to develop an EDIS-based penicillin biosensor with high sensitivity (65-70 mV/decade in the concentration range of 0.25-2.5 mM penicillin G) and low detection limit (5 μM). The results of label-free electrical detection of layer-by-layer adsorption of charged polyelectrolytes are presented, too.
Interaction of Hydrogen and Oxygen with Nanocrystalline Diamond Surfaces
Authors: Haensel Thomas , Ahmed Syed Imad-Uddin , Uhlig Jens , Koch Roland , Garrido José A. , Stutzmann Martin , Schaefer Juergen A.
Published: January 2010

Nanocrystalline diamond films (NCD) are strong candidates for applications in a wide variety of fields. An important concern in all these applications is to understand the properties of variously prepared NCD surfaces. This contribution is focussed on the surface science study of hydrogen and oxygen containing NCD films using X-ray photoelectron spectroscopy (XPS) as well as high resolution electron energy loss spectroscopy (HREELS). Previous studies have demonstrated that hydrogen, oxygen, and gases from the ambient environment as well as water can result in drastic surface changes affecting conductivity, wettability, tribological properties, etc. In this contribution we analyzed differently prepared NCD surfaces as a function of parameters such as the annealing temperature under ultrahigh vacuum conditions (UHV). We are able to identify the thermal stability of a number of species at the interface, which are related to different characteristics of C-H, C-OH, C=O, and C=C bonds. Furthermore, a formation of graphitic-like species appears at higher annealing temperatures. An atomic hydrogen treatment was also applied to the NCD surface to obtain further information about the surface composition.
Study of Growth Kinetics of Ultra-Long Carbon Nanotube Arrays through Wall Number Statistical Investigation
Authors: Cui Xinwei , Wei Weifeng , Chen Weixing
Published: January 2010

The unique properties of carbon nanotubes (CNTs) strongly depend on their structures. In this study, the growth kinetics of ultra-long multi-walled CNT (MWCNT) arrays by water-assisted chemical vapor deposition (WACVD) has been investigated based on the statistical studies of CNT wall number. It was found that the kinetics of MWCNT arrays in WACVD demonstrated a lengthening and thickening growth. In the linear lengthening stage, CNT wall number remains constant and catalysts preserve the activity; while in the thickening stage, CNTs thicken substantially through the gas phase-induced thickening process and catalysts start to deactivate. The effects of ethylene and hydrogen flow rates on the MWCNT array growth have also been studied. It was found that by changing ethylene flow rate, different linear lengthening stages corresponding to different CNT wall numbers could be obtained. These findings provide experimental solutions to fabrication MWCNT arrays with both selective heights and controllable wall numbers by WACVD.
Electron Beam Induced Two-State Noise in Carbon Nanotubes
Authors: Chan Jack , Burke Brian G. , Hu Chong , Campbell Joe , Harriott Lloyd , Williams Keith A.
Published: January 2010

Discrete current switching is induced in carbon nanotubes by electron beam irradiation. Switching amplitudes of 3% to 6% are observed at room temperature. Switching is created by electron beam exposure with dosage as low as 1000 pC/cm. Relative switching amplitude remains constant as the bias voltage varies, suggesting that current fluctuation is dominated by mobility fluctuation. Changes in the noise power spectral density following electron beam exposure will be discussed.
Comparing Field Emission Stability of Lithography-free, Modified Multi-walled Carbon Nanotubes
Authors: Pandey Archana , Prasad Abhishek , Moscatello Jason , Yap Yoke Khin
Published: January 2010

Field emission from carbon nanotubes (CNTs) has been known for more than a decade but there is no commercialized product available in the market. Apparently, we need to improve our basics understanding on stable field emission from CNTs. Here we compared the field emission properties of as grown vertically-aligned multi-walled carbon nanotubes (MWCNTs) to two types of modified MWCNTs: 1) Conical bundles of opened-tip MWCNTs, and 2) Opened-tip MWCNTs embedded in poly-methyl methacrylate (PMMA). We found that both types of modified MWCNTs have lower emission thresholds and better emission stability than the as grown samples. Among these modified samples, MCNTs embedded in PMMA has lower emission thresholds and better emission stability. We attributed these improvements to the filling of spacing between MWCNTs with PMMA that has higher dielectric constant than vacuum.
Photocatalytic Segmented Nanowires and Single-step Iron Oxide Nanotube Synthesis: Templated Electrodeposition as all-round Tool
Authors: Maas Michiel G. , Rodijk Eddy J.B. , Maijenburg Wouter , ten Elshof Johan E , Blank Dave H.A.
Published: January 2010

Templated electrodeposition was used to synthesize silver-zinc oxide nanowires and iron oxide (Fe2O3) nanotubes in polycarbonate track etched (PCTE) membranes. Metal/oxide segmented nanowires were made to produce hydrogen gas from a water/methanol mixture under ultraviolet irradiation. It was observed that gas production increased during irradiation.
Iron oxide nanotubes were synthesized via a gel synthesis route, avoiding clogging of the membrane pores during growth. The nanotubes formed without thermal after-treatment. Transmission electron microscopy (TEM) analysis and selected area electron diffraction (SAED) revealed a completely amorphous iron oxide structure. By demonstrating the synthesis of photocatalytically active segmented nanowire and nanotubes without post-treatment steps, templated electrodeposition can be a versatile and low cost tool for nanowires with designed functionality or fast nanotube synthesis.
Effect of hydrogen plasma treatment on the luminescence and photoconductive properties of ZnO nanowires
Authors: Li Yanbo , Uchino Ryohei , Tokizono Takero , Paulsen Alexander , Zhong Miao , Shuzo Masaki , Yamada Ichiro , Delaunay Jean-Jacques
Published: January 2010

ZnO nanowires with strong green emission synthesized by chemical vapor deposition were treated using hydrogen plasma. The effect of hydrogen plasma treatment was studied by means of photoluminescence and photoconductivity. A strong passivation of the green emission and a significant enhancement of the near band edge emission were found after the hydrogen plasma treatment. The conductivity of the nanowires in dark was increased by more than 3 orders of magnitude. The photoconductivity also increased after the hydrogen plasma treatment. The observed changes in the luminescence and photoconductive properties of the ZnO nanowires were likely caused by hydrogen atoms occupying both oxygen vacancies and interstitial sites.
FIB-deposited carbon-based superconducting nanowires with Tc ˜ 7 K
Authors: Dhakal Pashupati , McMahon Gregory , Norris Liam , Oh Jeong Il , Naughton Michael J.
Published: January 2010

We have fabricated carbon-containing nanowires by a gallium focused ion beam-induced deposition process from the precursor phenanthrene, C14H10. The electrical conductivity of the nanowires is only weakly temperature dependent below 300K, and reveals a superconducting state below Tc ˜ 7 K. We have measured the temperature dependence of the resistive upper critical field Hc2(T), and from those data, estimate the zero temperature critical field and coherence length to be 8.8 T and 6.1 nm, respectively. The Tc of this material is approximately 40% higher than that in any other FIB/direct write nanowire, such as those based on W(Ga), and thus offers the possibility of fabricating superconducting direct-write nanostructures that function at liquid helium temperature.
Spectrophotometric Investigation on the Growth Mechanism of a Single Tapered CuO Nanowire
Authors: Chatterjee Anindita , Huang C-Y , Liu S-B , Wu S-Y , Cheng Chia-Liang
Published: January 2010

In this study, well separated tapered CuO nanowires have been synthesized on copper grid using an easy and cost effective thermal oxidation method. A set of spectroscopic investigations have been performed on one single tapered CuO nanowire using Energy Dispersive X-ray, confocal Raman and confocal Photoluminescence spectroscopy to get insight into the mechanism of growth of the nanowires. Energy Dispersive X-ray studies reveal crystallization process of CuO nanowires occur from Cu/Cu2O mixed phase state to pure CuO structure. Raman measurements indicate a little sharpening of the Raman peaks with increasing growth temperature of the nanowires starting from 400°C to 600°C. Photoluminescence studies were carried out by mapping along the length of the nanowire to investigate the growth as well as optical properties of a single tapered CuO nanowire. As the diameter of the single tapered CuO nanowire decreases, the green emission of the nanowire gradually shifts towards the higher energy side. A steady blue shift of 20 nm of the photoluminescence peak has been attributed to the nanosize effect of the tapered nanowire along the length and enhanced surface defects.
Synthesize of Metal-semiconductor Segmented Nanowire by Electrodeposition
Authors: Eom HyeonJin , Yoo Bongyoung
Published: January 2010

Recently, thermoelectric (TE) is ignited by enhancement of nano-science and engineering that uncovers the possibility of increasing of figure of merit (ZT). As a candidate for thermoelectric materials, metal could not be considered because of their high thermal conductivity. However, according to other research, it is feasible to decrease thermal conductivity of also metal without much degradation of the electrical conductivity, which strongly implies that nanoscale metal can be utilized to improve thermoelectric properties of devices, especially power factor. Semiconductor-semiconductor superlattice nanowire structure has been studied as one of the best candidates for thermoelectric devices, because of their photon scattering characteristics at the interfaces. In this research, instead of semiconductor-semiconductor superlattice structure, metal-semiconductor superlattice structure was synthesized, and its microstructure and electrical properties are investigated. With this structure, improvement of electrical conductivity as well as degradation of thermal conductivity with phonon scattering would be anticipated. Metal-semiconductor superlattice nanowires were acquired with electrochemical displacement method, which can exchange the material which has low reduction potential with more noble materials in aqueous electrolyte. To achieve this, Ni-Ag superlattice structure was firstly prepared as metal-metal superlattice structure. The electrolyte for the electrodeposition of Ni-Ag segmented nanowries was consisted of 0.3 mol/L C6H5Na3O7, 0.7mol/L NiSO4 and AgNO3. Electrodeposition was performed galvanostatically in two electrode cell configuration with anodized aluminum oxide(AAO) template. Cu layer was deposited at the one side of AAO template as a working electrode, platinum coated titanium stripe as counter electrode. The composition of electrodeposits could be controlled by applied potential which is directly related the applied current density. Ni-rich phase could be obtained when the applied current density was over 10mA/cm2 and Ag-rich phase at 0.5mA/cm2. After acquiring Ni-Ag segmented nanowire structure, we synthesized the metal-semiconductor segemented nanowries (Ag-BiTe segmented nanowire) by using galvanic-displacement method. Because standard reduction potential energy of nickel is more negative (-0.25V) than that of BiTe in Bi ion and Te ion contained bath, it is possible to make displacement of Ni with BiTe using galvanic displacement. The morphology was investigated by SEM and composition profile was observed by EDS. In conclusion, nanoscale metal-semiconductor segmented nanowires were synthesized by electrodeposition and electrochemical displacement method. Their structural characteristics and electrical properties were investigated to understand the possibility of this segmented nanowires as a thermelectrical materials.
Synthesis and Structural Characterization of Ultra-thin Flexible Au Nanowires
Authors: Kisner Alexandre , Heggen Marc , Tillmann Karsten , Mourzina Yulia , Offenhäeusser Andreas
Published: January 2010

Au nanowires (AuNWs) were produced by electroless reduction of HAuCl4 in a micellar structure formed by oleylamine and investigated by means of high-resolution transmission electron microscopy (HRTEM). Micrometer long ultra-thin flexible AuNWs with 1—2 nm diameter and AuNWs with about 12 nm diameter and a few hundred nm length were produced. Their extremities show a characteristic bulging. In contradiction with previous work, the bodies of the 12 nm nanowires are defect-free along the axial direction, their extremities, however, show the presence of twin boundaries. Ultra-thin AuNWs were often found as bundles presenting lengths of few micrometers. Although they are stable in solution for months, they were found to be quite sensitive to electron beam irradiation during HRTEM experiments, with a tendency to break up into face centered cubic (fcc) Au droplets. It is proposed that the micellar configuration of oleylamine plays a fundamental role in the atomic arrangement of nanowires. Finally, we anticipate our results to be a starting point for a more realistic experimental investigation of surface effects on the mechanical properties of ultra-thin nanowires with high aspect ratio, which have been only widely exploited theoretically.
Electrodeposited Metallic Nanowires as a Scanning Probe Tip
Authors: Motoyama Munekazu , Prinz Fritz B.
Published: January 2010

This paper describes lateral deformations of electrodeposited Ni nanowires as an atomic force microscope (AFM) probe tip. The maximum end deflections for nanowires to break the elasticity, which beam mechanics indicates, were applicable to our observed results.
Optically Controlled Nanomagnets
Authors: Mackowski Sebastian , Gurung Tak , Karczewski Grzegorz , Jackson Howard E , Smith Leigh
Published: January 2010

We report on single dot photoluminescence imaging and spectroscopy at B=0T on magnetically doped CdMnTe self-assembled quantum dots with average Mn concentration of several percent. Quasi-resonant excitation with circularly polarized laser leads to formation of magnetic polarons with magnetization induced by the laser light. In this case all quantum dots are polarized in the same direction. In contrast, when the dots are populated using above the barrier excitation, with randomly polarized excitons, the resultant magnetization is random and varies from dot to dot. These experiments demonstrate a way to control the magnetization of magnetically doped quantum dots by means of light excitation. In addition, they point towards extremely long spin memory times in these structures, reaching hundreds of microseconds, making CdMnTe quantum dots promising candidates for local magnetic field sources on the nanoscale.
Photodetectors Fabricated from Strain-free GaAs Coupled Quantum Dots
Authors: Wu Jiang , Shao Dali , Manasreh Omar , Li Alvason Zhenhua , Wang Zhiming M , Salamo Gregory J.
Published: January 2010

In this article, an alternative strain-free growth mode is presented where GaAs coupled-quantum dots are grown on lattice matched AlGaAs. The coupled quantum dots were grown at 550 °C in a molecular beam epitaxy system. The GaAs quantum dots were characterized by using a photoluminescence technique and an atomic force microscope. The photodetector was fabricated into normal incident configuration and photoconductivity spectra were measured covering the mid-infrared spectrum of 2.0 – 8.0 micron (intersubband transitions) and the visible-near-infrared spectrum of 0.5 – 0.9 micron (interband or exciton transitions). The photoresponse spectra in mid-infrared spectral range were found to exist at temperatures lower than 80 K, while the photoresponse spectra in the visible-near-infrared range were observed at temperatures as high as 300 K.
Efficient Energy Transfer Between CdS Quantum Dots in Layer-by-layer Self-assembled Films
Authors: Shimura Kunio , Kim DaeGwi , Nakayama Masaaki
Published: January 2010

We have investigated efficient energy transfer (ET) between CdS quantum dots (QDs) measuring photoluminescence dynamics in layer-by-layer (LBL) self-assembled films. The assembly of negatively charged colloidal QDs and positively charged polyelectrolytes results in QD/polymer multilayers. Furthermore, to reveal how the ET rate depends on the distance between CdS QDs, we fabricated bilayer structures consisting of differently sized CdS QDs. It is experimentally verified that ET between the donor and acceptor QDs is conclusively dominated by the dipole-dipole interaction.
Dynamic Control of Plasmon-Exciton Coupling in Au Nanodisk–J-Aggregate Hybrid Nanostructure Arrays
Authors: Zheng Yue Bing , Juluri Bala Krishna , Jensen Linlin , Jensen Lasse , Huang Tony Jun
Published: January 2010

We report the dynamic control of plasmon-exciton coupling in Au nanodisk arrays adsorbed with J-aggregate molecules by incident angle of light. The angle-resolved spectra of an array of bare Au nanodisks exhibit continuous shifting of localized surface plasmon resonances. This characteristic enables the production of real-time, controllable spectral overlaps between molecular and plasmonic resonances, and the efficient measurement of plasmon-exciton coupling as a function of wavelength with one or fewer nanodisk arrays. Experimental observations of varying plasmon-exciton coupling match with coupled dipole approximation calculations.
Supporting metal catalysts on modified carbon nanocones to optimize dispersion and particle size
Authors: Matelloni Paolo , Grant David M. , Walker Gavin S.
Published: January 2010

Carbon nanocones are the fifth allotropic form of carbon, first synthesized in 1997. They have been selected for investigating hydrogen storage capacity, because initial temperature programmed desorption experiments found a significant amount of hydrogen was evolved at ambient temperatures. The aim of this work was to study the effect of impregnation conditions on metal catalyst dispersion and to investigate whether the metal loaded cones had improved hydrogen storage characteristics. Pre-treatment of carbon nanocones with hydrogen peroxide was carried out, followed by metal decoration in aqueous solution by an incipient wetness technique. Two methods of reducing the metal catalyst have been applied: in hydrogen at room temperature (RT) and in an aqueous solution of NaBH4. XRD confirmed the complete metal reduction and TEM showed that the reduction technique affected the catalyst dispersion. Very fine dispersions of ca. 1 nm diameter metal clusters at catalyst loadings of 5 wt.% were achieved and high dispersions were retained for loadings as high as 15 wt.%. Hydrogen uptakes at RT were measured and an increase with metal loading was observed.
The Ammonia-Driven Phase Transition in Bulk and Nanostructured Potassium Graphite KC24
Authors: Lovell Arthur , Kurban Zeynep , Bennington Stephen M. , Srinivas Gadipelli , Skipper Neal T. , Smith Ron I. , Jenkins Derek , Howard Christopher A. , Chapon Laurent
Published: January 2010

We report the synthesis of nanostructured stage-2 potassium graphite, KC24, by intercalation of turbostratic graphite nanofibers produced from an electrospun polymer, and compare its properties with exfoliated graphite-based KC24. The nanostructured KC24sample has low crystalline order and slightly increased interlayer spacing of 8.76 Å, compared with 8.65 Å in the bulk sample, indicating minimal registration of the graphite planes. Time-resolved time-of-flight neutron diffraction on both nanostructured and bulk KC24under ammoniation is suggestive of a more homogeneous and faster pressure-modulated phase transition to the ternary ammoniated potassium-graphite in the nanostructured material. Following ammoniation, negligible hydrogen uptake is observed at 50 K.
First-principles Calculations of Phonon and Thermodynamic Properties of Hydrogen Storage α-LaNi5H
Authors: Saito Shigeki , Katagiri Masahiko , Tserolas Vasileios , Nakamura Jun , Onodera Hidehiro , Ogawa Hiroshi
Published: January 2010

The phonon distribution of hydrogen storage α-LaNi5H with 4h, 6m, 12n, and 12o interstitial hydrogen was calculated by using first-principles potential surfaces with a 2×2×2 supercell model in order to investigate structural and thermodynamic properties. Frequency shifts due to the phonon contribution from the internal energies of 12n < 6m < 12o < 4h appeared in specific modes originating from interstitial hydrogen and in the upper-edge modes with nickel-lattice motion. The thermodynamic stability of 12n interstitial hydrogen in α-LaNi5H due to the wide XZ storage space can be explained by its phonon amplitudes and the charge density around nickel-bonded hydrogen.
Atomistic Simulation on Hydrogen Storage in Metallic Nanoparticles
Authors: Ogawa Hiroshi , Kayanuma Megumi , Katagiri Masahiko
Published: January 2010

Hydrogen storage in metallic nanoparticles was investigated by classical molecular dynamics and parameter physics. We observed phenomenological variation due to the differences in potential parameters of metal-hydrogen pair and crystal lattices. Three patterns of hydrogen distribution in both b.c.c. and f.c.c. nanoparticles were observed: non-absorbing, homogeneously-absorbing and heterogeneously-absorbing. In the last case, hydrogen atoms distribute just beneath the particle surface to form a hydrogen-rich layer. This layer prevents the diffusive motions of hydrogen atoms into the nanoparticle. We also carried out long simulation runs up to 1 nm to observed the structural variation of nanoparticles due to hydrogenation. Generation of grain boundaries was observed in b.c.c nanoparticles with the condition of strong metal–hydrogen interaction. Most of the grain boundaries were symmetric-tilt type and migrated inside the particle to reduce the interface energies. Formation of grain boundary was not observed in f.c.c. nanoparticles.
Lattice strain effects in graphane and partially-hydrogenated graphene sheets
Authors: Morris James Robert , Averill Frank W. , He HaiYan , Pan Bicai , Cooper Valentino R. , Peng Lujian
Published: January 2010

This paper presents a brief review of recent developments in the studies of fully hydrogenated graphene sheets, also known as “graphane,” and related initial results on partially hydrogenated structures. For the fully hydrogenated case, some important discrepancies exist between published first-principles calculations, and between calculations and experiment, with qualitative differences on whether or not the graphene sheet expands or contracts upon hydrogenation. The lattice change has important effects on partially hydrogenated structures. First-principles calculations of ribbon structures, with interfaces between graphane and graphene regions, show that the interfaces have substantial misfit strains. Calculating the interfacial energy must carefully account for the strain energy in the neighboring regions, and for sufficiently large regions between interfaces, defects at the interface that relieve the strain may be energetically preferable. Tight-binding simulations show that at ambient temperatures, segments of graphene sheets may spontaneously combine with atomic hydrogen to form regions of graphane. Small amounts of chemisorbed hydrogen distort the graphene layer, due to the lattice misfit.
Large H Absorption in Nd2Ni2In; Magnetism in a New Structure Type
Authors: Havela Ladislav , Mašková Silvie , Daniš Stanislav , Stelmakhovych Olha , Miliyanchuk Khrystyna
Published: January 2010

Several hydrides of Nd2Ni2In with various H concentrations up to 6 H atoms/f.u. have been synthesized. Nd2Ni2In crystallizes in the tetragonal Mo2FeB2 structure type which can be changed upon hydrogenation to an orthorhombic structure, space group Pbma. The unit-cell volume increases by 15.5 % for Nd2Ni2InH4.5 and by 21.6 % for Nd2Ni2InH6. The triangular rare-earth lattice can give rise to magnetic frustration in case of antiferromagnetic interactions in Nd2Ni2In. The frustration is removed upon hydrogenation due to the orthorhombic distortion. The magnetic ordering temperature decreases with H concentration from 8 K in Nd2Ni2In down to 3.5 K in Nd2Ni2InH6.
Hydrogen Sorption in Magnesium Nanoparticles: Size- and Surface-related Phenomena
Authors: Pasquini Luca , Callini Elsa , Piscopiello Emanuela , Montone Amelia , Jensen Torben René , Vittori Antisari Marco , Bonetti Ennio
Published: January 2010

The aim of this work is the investigation of the metal-hydride transformation in magnesium (Mg) nanoparticles both as a function of particle size and in response to surface functionalization by clusters of transition metals (TM): Pd, Ni, Ti.

Mg nanoparticles were synthesized by the inert-gas condensation technique, which yields single crystals with six-fold symmetry whose average size can be controlled by tuning the inert gas pressure. After the synthesis the nanoparticles were passivated by slow exposure to oxygen, obtaining a core-shell morphology where a metallic core is coated by a MgO shell of about 5 nm thickness.

The material structure was investigated by Transmission Electron Microscopy (TEM), also in High Resolution (HRTEM) mode, and by X-Ray Diffraction (XRD). The sorption kinetics were analysed by a volumetric Sievert apparatus, which also allowed for a determination of the activation energies.

Small nanoparticles (≈35 nm) display interesting kinetics with gravimetric capacity of 4.5 wt.% at saturation, limited by the oxide fraction. Hydride formation proceeds by one-dimensional growth controlled by diffusion through the hydride, while the reverse transformation to metal involves interface-controlled three-dimensional growth of nuclei formed at constant rate.

On the contrary, large nanoparticles (≈450 nm) exhibit very low reactivity due to reduced probability of hydrogen dissociation/recombination and nucleation at the particle surface. For this reason, large nanoparticles were surface-decorated by TM through in situ evaporation in the inert-gas condensation chamber. This procedure results in clusters of 3-4 nm located over a portion of the MgO shell, as shown by XRD and HRTEM on Pd-decorated sample. This treatment results in dramatically improved hydrogen sorption behavior. In fact, previously inert nanoparticles now exhibit of up to 5.6 wt.%.

Real-time diffraction studies using Synchrotron Radiation were carried out during hydrogen desorption on the Pd-decorated nanoparticles. We clearly show that a Mg-Pd intermetallic phase is formed after the first heating treatment and takes active part in the transformation.
Structure and Stability of Multivalent Metal Tetraborohydrides
Authors: Łodziana Zbigniew , Borgschulte Andreas , Germaud Robin , Remhof Arndt , Züttel Andreas
Published: January 2010

Metal tetrahydroborates remain interesting materials as potential hydrogen storage media. In the present paper, we analyze thermodynamic stability of borohydrides of Al and Zr by means of extensive density functional calculations. We show that solid phases of these compounds are formed by dispersive Van der Waals forces. These compounds are thermodynamically unstable at room temperature with respect to decomposition to boron and hydrogen. Their stability is explained by formation of diborane as a necessary step in the decomposition path, pointing out to the kinetic factors that are important for the stability analysis of metal borohydrides.
Theoretical Study on N Doping in Carbon Materials for Hydrogen Storage
Authors: Kayanuma Megumi , Ikeshoji Tamio , Ogawa Hiroshi
Published: January 2010

Interaction between nitrogen-substituted graphene-like compounds and hydrogen was investigated using ab initio molecular orbital method in the aspect of hydrogen storage. We adopted coronene as a model compound for fragmented graphene-like carbon materials and compared the interaction between hydrogen and pure or N-substituted coronenes by changing nitrogen positions. Among the assumed 19 N-substituted models, polarozabilities and HOMO–LUMO gaps were compared to evaluate physisorption and chemisorption energies. As for chemisorption, two N-substituted models were selected and closely examined to reveal the dependence on both nitrogen-substitution and hydrogen-adsorption positions. Potential energy surfaces as a function of H–H bond length and H2–coronen distance showed that the barrier height for hydrogen chemisorption strongly depends on N-substitution positions. The chemisorbed products of N-substituted coronenes are stabilized or destabilized compared with the pure carbon case depending on the sites of N-substitution and H-adsorption. These results suggest that N-substitution at certain positions possibly improve hydrogen storage properties of graphene-like materials.
Electrochemical Charge for the Formation of Metal Hydrides from LiH+M (M=Mg, Al)
Authors: Hanada Nobuko , kamura Akito , Suzuki Hiroshi , Ichikawa Takayuki , Kojima Yoshitsugu , Takai Kenichi
Published: January 2010

For a formation of metal hydride of MgH2 or AlH3 under room temperature and ambient pressure, the cathode electrodes of metal and lithium hydride are electrochemically charged with Li anode electrodes in the system of Li-ion extraction. For MgH2 formation, the VC (Voltage-Composition) curve of Mg + 2LiH during charge shows a plateau voltage at 0.6 V until the final composition of 1.05 Li extraction. After charge MgH2 phase is observed by the XRD measurement. Therefore MgH2 is produced by the electrochemical charge from Mg and LiH. For AlH3 formation, Al + 3LiH is charged until the final composition of 0.6 Li at a plateau voltage of 0.8 V which corresponds to the reaction between Al and LiH for the formation of AlH3. In the XRD profile after charge AlH3 phase is not detected although the intensities of Al and LiH decrease compared with these before charge, which suggests the reaction leading to the formation of AlH3.
Time and Frequency Resolved Hydrogen Dynamics in deuterated LiBH4
Authors: Gremaud Robin , Andresen Esben R. , Ramirez-Cuesta Anibal J. , Refson Keith , Łodziana Zbigniew , Züttel Andreas , Hamm Peter , Borgschulte Andreas
Published: January 2010

We study deuterated tetrahydroborate LiBH4in the low-temperature (LT) phase, focusing on the BH (BD) stretching vibrations, by infrared (IR) and IR-pump-probe spectroscopy. Calculated infrared spectra by density functional theory allow us to disentangle the contributions coming from all the coexisting BH4-nDn(n = 0…4) molecular units. We report on anharmonicity and vibrational lifetime (1.6 ps) of the BD vibrations, as well as on the BH2D2orientational relaxation by polarization anisotropy IR-pump-probe measurements. We find a constant anisotropy for at least 5 ps confirming the slow reorientational motion and low thermal disorder of the BH4 unit in the LT phase of this potential hydrogen storage material.
First-Principles Study on Hydrogen Atom Hopping in NaAlH4
Authors: Wang Hao , Tezuka Akinori , Ogawa Hiroshi , Ikeshoji Tamio
Published: January 2010

Hydrogen vacancy effect on the activation energy for self-diffusion is investigated by NEB method. The path was calculated by moving a hydrogen atom from a nearby complex into the vacancy in another complex. Compared with the substitution enthalpy of hydrogen vacancy, the activation energy for self-diffusion is easier to achieve during the dehydrogenation process.
Improvement in Power Density of Rechargeable Air Battery using Hydrogen Storage Alloy
Authors: Morimitsu Masatsugu , Kondo Takahito , Osada Naoki , Takano Koji
Published: January 2010

A novel class of secondary battery comprising MH and air electrodes was developed for potential uses in high power density and high energy density applications such as electric or hybrid vehicles and power storage units supporting fuel cell and solar power systems. The air electrode consisted of nickel-based gas diffusion electrode using Ir2Bi2O7-zas oxygen evolution and reduction catalyst. Coin-type of cells using alkaline solutions as electrolyte were designed and fabricated, and the charge-discharge behaviors were evaluated with constant current operation. The discharge voltage and power density were improved by using a thin film membrane, in which the electrolyte was impregnated, between the air and MH electrodes, and the maximum power density was comparable to that of commercially available Ni-MH secondary battery. The MH utilization and the current efficiency of a charge-discharge cycle were found to be more than 90%.
New B,N-hydrides: Characterization and Chemistry
Authors: Anderson Paul , Chater Philip , David Bill , Evans Ian , Kersting Alexandra
Published: January 2010

Three different classes of boron and nitrogen containing light metal complex hydrides have been investigated, resulting from the reactions of LiNH2 with LiBH4, NaNH2 with NaBH4 and MHx (where M = Li, Na and Ca) with NH3BH3. A rich variety of new phases has been identified, which exhibit modified decomposition pathways and onset temperatures of hydrogen desorption as low as 40°C. In each case the composition of phases formed has been examined in detail and the products of thermal decomposition—solid and gaseous—have been determined.
IR and XRD Amorphous Synthesis Residue Showing Zeolitic Micropores
Authors: Chiang Anthony S.T. , Fun Sewn-Yi , Wu Jan-Sen , Tsai Tseng-Chang
Published: January 2010

In the preparation of zeolite nanocrystals from the hydrothermal reaction of clear solution, the zeolite products are typically collected by high speed centrifugation. For Beta zeolite, the crystalline yield is often low, thus a good fraction of silicates reminds in the supernatant. These XRD amorphous materials turn out to be uniform nanoparticles that, after calcination, showed similar micropore structure as that of the collected beta zeolite nanocrystals. TPD measurements of hexane isomers further indicated that both the crystalline and the amorphous products were more selective toward the smaller hexane molecule.