bluuebunny/crossref_metadata_2025 huggingface dataset:
Optimal Morphology for a Composite Solar Cell
Authors: Burlakov V. M. , Briggs G. A. D. , Sutton A. P.
Published: April 2011

Charge transport in a composite solar cell (CSC) is analysed by considering independent and random migration of the photo-generated electrons/holes over interpenetrating conducting networks. It is shown that besides an interface area and the materials parameters the efficiency (average number of the carriers reaching the electrode per time unit) of the CSC depends on the effective dimensionality of the conducting channels. Our analysis shows that the 1-d network is ∼20% more effective than the 3-d one, and therefore the morphology of the 1-d type for the organic phase within a 3-d inorganic one is preferable for the CSC. It is shown that the CSC with bulk generation of excitons is potentially more efficient than a dye-sensitised solar cell. The highest efficiency of collection of photo-generated carriers for the CSC at highest possible current for the given material properties cannot exceed ∼40% and 34% for 1-d and 3-d networks respectively.
Photoelectric Cells of Merocyanine Langmuir-Blodgett Films Utilizing Surface Plasmon Excitations
Authors: Kato Keizo , Takahashi Futoshi , Shinbo Kazunari , Kaneko Futao , Wakamatsu Takashi
Published: April 2011

Short-circuit photocurrents (ISC) due to surface plasmon (SP) excitations have been investigated for the photoelectric cells using Langmuir-Blodgett (LB) films of merocyanine (MC) dye. The MC dye exhibits p-type conduction, and the Schottky and Ohmic contacts are obtained at the interfaces between MC LB films and Al thin films and between MC LB films and Ag thin films, respectively. Since the Schottky diodes show the photoelectric effects, the Schottky photoelectric cells have been constructed. The cells with two kinds of structures, that is, prism/Al/MC/Ag (type I) and prism/MgF2/Al/MC/Ag (type II), have been prepared. In the attenuated total reflection (ATR) method, the types I and II have the Kretschmann and both the Kretschmann and Otto configurations, respectively. SP has been resonantly excited at the interface between Ag and air for the type I and at the interfaces between MgF2 and Al between Ag and air for the type II. The ATR and the ISC properties have been simultaneously measured as a function of the incident angles of the laser beams. The peaks of the ISC have corresponded to the resonant angles of the ATR curves. The electric fields and optical absorptions in the cells have been also calculated using the dielectric constants and the film thicknesses obtained from the ATR measurements. The calculated absorptions in the MC layers as a function of the incident angles have corresponded to the results of ISC. It has been estimated that the ISC for both types I and II could be enhanced by the excitations of SP in the ATR configurations.
A Novel Conformational Searching Technique applied to small polymer units of 3-phenyl-1-ureidonitrile
Authors: Silver Ari , Sohlberg Karl
Published: April 2011

A novel conformational searching scheme is employed to identify low energy structures for low-degree polymers of 3-phenyl-1-ureidonitrile, a proposed thin film data storage material. Vibrational analysis of the predicted polymer structures is consistent with experimental reports and supports the experimentally proposed polymerization mechanism.
Photosensitized Reaction of Poly(L-lactic Acid) via Two-Photon Ionization of Dopant
Authors: Sakai Wataru , Wakabayashi Kosuke , Nishinaka Hiroshi , Kawamoto Hiroki , Kinoshita Motoi , Tsutsumi Naoto
Published: April 2011

We here studied photosensitized radical reaction of poly(L-lactic acid) (PLA) using spectrophotometry, electron spin resonance (ESR), gel permeation chromatography (GPC) measurements, and semiempirical molecular orbital calculation. PLA doped with N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD) was irradiated at 77 K using UV light. After the irradiation, we observed a new broad absorption band that is assigned to TMPD radical cation produced by two-photon ionization. ESR spectrum of the irradiated sample also indicated presence of main chain scission radical of PLA as well as the TMPD radical cation. By increasing the irradiation time, the ESR spectrum changed by another radical species which is produced by dehydrogenation of PLA main chain. Spectral simulation for the obtained ESR spectra revealed changes of relative amounts. The GPC measurements clearly indicated decrease in molecular weight of PLA after the irradiation. The semiempirical molecular orbital calculation was used to confirm the reaction scheme of radical anion decomposition.
Iodine Release from Silver Iodide under Reducing Condition with Iron-Bearing Minerals
Authors: Xia X. , Inagaki Y. , Hattori A. , Idemitsu K. , Arima T.
Published: April 2011

Iodine release from silver iodide (AgI) to water was evaluated under reducing conditions in the presence of iron-bearing minerals, goethite (FeOOH), magnetite (Fe3O4) and Wüstite (FeO). The release tests were performed in a glove box purged with gas mixture (Ar+5%H2), and the concentrations of dissolved iodine, silver and iron were measured. The apparent equilibrium concentration of dissolved iodine was 4.0 ×10−8, 4.3×10−8 mol/l for the tests with FeOOH and Fe3O4, respectively. The values were almost the same as in the test without minerals. For the test with FeO, the concentration of dissolved iodine was 5.4×10−3 mol/l, which is five orders of magnitude higher than for the other tests. Solid phase analyses by using XRD and SEM/EDS indicated that metallic silver precipitated on the surface of the original AgI for the test with FeO and on the surface of Fe3O4 for the test with Fe3O4, but not for that with FeOOH. These results showed that only FeO could reduce AgI effectively to increase iodine release. The amount of dissolved Fe(II) was a key factor affecting AgI reduction instead of redox potential of solution. Kinetic processes may also play an important role in AgI reduction and associated iodine release under reducing condition.
Diogenic Transmutation Effects in a Crystalline Aluminosilicate Ceramic: a Tem Study
Authors: Fortner Jeffrey , Aase Scott , Reed Don
Published: April 2011

We demonstrate the use of transmission electron microscopy (TEM) to study the effects of beta-decay of radioactive 137Cs to 137Ba in crystalline pollucite (CsAlSi2O6). Most prior work on radiation effects in materials has focused on structural damage from alpha radiation. Beta radiation, on the other hand, causes little atomic displacement, but the decay transmutation, that is, the radioactive decay of a radioisotope to an isotope of another element, results in progeny with different the valence and ionic radius. Cesium-137, a fission product of uranium, is a major contaminant at U.S. Department of Energy production facilities. Pollucite is an aluminosilicate ceramic with potential use for long-term storage of 137Cs. We focused on one of several available 137Cs sources originally fabricated in the 1970s and 1980s. These sources were small, sealed, stainless steel capsules containing pollucite in which varying amounts of the natural Cs had been replaced by radioactive 137Cs (t1/2 = 30.13 years). The sample chosen for TEM examination, aged for nearly 20 years, contained the most radiogenic barium and was expected to show the largest radiation effects. Bright field transmission images revealed a homogeneous crystalline matrix, with no evidence of distinct Ba phases or ex-solution phenomena resulting from the 137Cs transmutation. Electron diffraction patterns obtained from several portions of the sample were consistent with literature values for pollucite. These data suggest that little substantial damage was done to the crystal structure of this sample, despite the transmutation of nearly 1.5% of the total cesium to barium over the elapsed 20 years. Although our observations are limited, to our knowledge these are the only available data in which transmutation effects have been isolated from other radiation damage phenomena.
Influence of Salinity on The Hydraulic Conductivity of Compacted Bentonite
Authors: Cho Won-Jin , Lee Jae-Owan , Kang Chul-Hyung
Published: April 2011

The effect of salinity on hydraulic conductivities of compacted bentonites with various dry densities was studied. The hydraulic conductivities increase with increasing salinity only when the dry density of bentonite is relatively low. The degree of increase becomes more remarkable at a lower dry density of bentonite. For bentonites with densities of 1.0 Mg/m3 and 1.2 Mg/m3, the hydraulic conductivities associated with the 0.4 M NaCl solution increase beyond those associated with the fresh water solution by factor of 7 and 3 times, respectively. However, for the bentonite with dry density higher than 1.4 Mg/m3, the salinity has an insignificant effect on the hydraulic conductivities, and the hydraulic conductivity is nearly constant within the salinity range of 0 to 0.4 M NaCl.
Element Partitioning in a Pyrochlore-Based Ceramic Nuclear Waste form
Authors: Gieré R. , Hatcher C. , Reusser E. , Buck E. C.
Published: April 2011

Pyrochlore-rich ceramics containing additional zirconolite, brannerite and rutile have been developed for immobilizing excess weapons plutonium. This study reports data for a ceramic containing small amounts of Al and Mo in addition to the major oxide components of Ti, U, Ca, Hf, Gd and Ce. Hafnium and Gd are added as neutron absorbers, Al and Mo represent impurities. Quantitative electron microprobe data demonstrate that UO2 is strongly partitioned into brannerite (45 wt%), which is present as euhedral crystals with inclusions of unreacted UO2. Pyrochlore forms the groundmass and has an average UO2 content of 28 wt%. Zirconolite contains only 15 wt% UO2, but is significantly more effective in accommodating Hf and Gd than both brannerite and pyrochlore. Zirconolite incorporates U together with Hf and Ce in one structural site, whereas brannerite and pyrochlore accommodate U with Gd in a site that is distinct from that occupied by Hf. Incorporation of Gd into zirconolite takes place via a coupled substitution involving Al, thus explaining the high Al2O3 contents (3 wt%). Molybdenum was not detected in the major oxides, and it might be present in the accessory rutile. Although the studied waste form was designed to incorporate Pu, the present dataset is also valuable because immobilization of highly fissile U (e.g., U-233) might need to be considered in the future.
An Overview of the H12 Performance Assessment in Perspective
Authors: Miyahara Kaname , Makino Hitoshi , Kato Tomoko , Wakasugi Keiichiro , Sawada Atsushi , Ijiri Yuji , Takasu Aki , Naito Morimasa , Umeki Hiroyuki
Published: April 2011

The H12 performance assessment (PA) provided a test for the robustness of a HLW repository system concept based on structured siting and design, taking account of a wide range of potentially suitable Japanese geological environments. The generic nature of the host rock in the H12 assessment means, however, that emphasis is placed verymuch on strong EBS performance. The assessment included a comprehensive evaluation of uncertainty and potentially detrimental factors, including perturbations due to external events and processes. Despite the considerable uncertainty at the current stage of the Japanese program, a safety case that is adequate for the aims of the assessment can be made by a strategy of employing conservatism where there is uncertainty and stressing the reliability and effectiveness of the performance of the near-field. The aim of this paper is to present the H12 PA in a way which makes the PA process clearer and the implications of the results more meaningful, both to workers within the PA field and to a wider technical audience.
Determination of liquid-solid partition coefficients (Kd) of radionuclide anionic species from a contaminated aquifer
Authors: Carona François , Haas Michael K. , Cooper Edward L. , Robertson David E.
Published: April 2011

Contaminated groundwater from a former liquid waste discharge area at the Chalk River Laboratories was treated with ion exchange resins to isolate aqueous extracts of selected radionuclides (60Co, 106Ru, 137Cs, 238Pu, 239/240Pu, 241Am and 244Cm). The extracts were then mixed with native uncontaminated soil material to determine their liquid-solid partitioning coefficients (Kd). Separate Kd values of the same elements were also obtained with the usual short-term “batch” technique for comparison, but using different radioisotopes as tracers (57Co and 134Cs), spiked to the extracts. The comparison included separate Kd determinations with 239/240Pu and 241Am as tracers, added to uncontaminated groundwater.

In all cases, the radionuclides originally present in the contaminated groundwater extracts exhibited low Kd values, compared to the values obtained in the “batch” method with tracers (57Co, 134Cs, 239/240Pu and 241Am). The difference was up to two orders of magnitude. This was attributed to differences in aqueous speciation of the nuclides in the contaminated groundwater, allowing limited interactions of complexed radionuclide species with soil material. Our results indicated that all the radionuclides were predominantly in anionic form in the groundwater, whereas in the tracer “batch” experiment, 57Co and 134Cs were predominantly cationic, Pu and Am were predominantly anionic. Hydrolysis partially explains the dominant anionic character of Am and Pu in the tracer experiments, however dissolved organics are suspected to dominate the speciation of the radionuclides in contaminated groundwater. Our experiment implies that in some cases, if Kd values obtained using the “batch” method with tracers are used in transport models, radionuclide transport could be underestimated.
Fabrication of Stimulus-Responsive Polymeric Nanostructures by Proximal Probes
Authors: Ahn Sang-Jung , Lee Woo-Kjung , Zauscher Stefan
Published: April 2011

The triggered control of interfacial properties on the nanometer scale holds significant promise for actuation in bio-nanotechnology applications where polymeric actuators may manipulate the transport, separation, and detection of biomolecules. To fabricate patterned, stimulus-responsive polymer brushes we have developed several methods that combine surface initiated polymerization (SIP) with dip-pen nanolithography (DPN). Surface-confined, stimulus-responsive polymer brush nanopatterns were fabricated by amplification of DPN patterned, self-assembled monolayers of 16-mercaptohexadecanoic acid on gold surfaces by SIP of N-isopropylacrylamide (NIPAAm). While free radical polymerization yielded only short polymer brushes (DP<50), atom transfer free radical polymerization (ATRP) produced thick, uniform polymer brushes. For free radical polymerization the thickness of the polymer brush layer is a function of the lateral feature size and the initiator density and is maximal at pattern boundaries.
Biotemplate Directed 2-Dimensional Nanostructure Assembly
Authors: Yu Seungju M. , Mo Xiao , Krebs Mark P.
Published: April 2011

There are manifest scientific and technological interests in organizing nanocrystals (NC) into controlled architectures and work is rapidly expanding in exploiting opportunities through bio-assembly processes. In this research, we have applied biologically assembled nano-scale template to direct formation of precisely defined 2D nanocrystal arrays. We explored purple membrane (PM), a naturally occurring membrane protein (bacteriorhodopsin, BR) crystal patch from halobacteria, as a precisely structured nanometer-scale template. We have developed genetically engineered PMs that display unique functional group (cystein) on the membrane surfaces with precisely defined nano-scale symmetry. These reactive functional groups were used as specific anchoring sites for NC immobilization. We show that the cystein mutants of BR mutated at the surface of the PM are capable of forming stable PM-like hexagonal crystal lattices in the host cell membrane. These crystal patches are easily purified by ultracentrifugation and can be used to react with functionallized gold nanoclusters. We also present the variation of crystal lattice by detergent treatment and recrystallization techniques. The PM has been known to posses good materials properties and is expected to allow formation of robust nanocrystal array that can withstand variety of optical and electrical characterization conditions.
Fabrication and Imaging of Protein Crossover Structures
Authors: LaGraff John R. , Zhao Yi-Ping , Graber David J. , Rainville Dan , Wang Gwo-Ching , Lu Toh-Ming , Chu-LaGraff Quynh , Szarowski Don , Shain William , Turner James N.
Published: April 2011

Proteins often deform, dehydrate or otherwise denature when adsorbed or patterned directly onto an inorganic substrate, thus losing specificity and biofunctionality. One method used to maintain function is to pattern the protein of interest directly onto another underlying protein or polypeptide that acts as a buffer layer between the substrate and the desired protein. We have used microcontact printing (μcp) to cross-stamp orthogonal linear arrays of two different proteins (e.g., IgG, poly-lysine, protein A) onto glass substrates. This created three separate types of protein-substrate microenvironments, including crossover structures of protein one on protein two. We report preliminary fluorescent microscopy and scanning force microscopy characterization of these structures, including commonly encountered structural defects.
Development of Polyimide-based Flexible Tactile Sensing Skin
Authors: Engel Jonathan , Chen Jack , Liu Chang , Flachsbart Bruce R. , Selby John C. , Shannon Mark A.
Published: April 2011

We present a novel microfabrication process for realizing a new type of flexible sensory “smart skin”. In this work, we focus on demonstration of a skin containing a two dimensional array of tactile sensors using polyimide and metal strain gauges. A novel polymer microfabrication approach coupled with surface release methods is demonstrated. The process yields flexible sensory skins in a low cost, efficient manner. Experimental characterization of the devices is also presented. The demonstrated sensors use metal-film strain gauges in a multiplexed two-dimensional array of tactile pixels (taxels) embedded in a polyimide thin film membrane to detect force distribution on the flexible skin. The arrays have been used to image force distributions and could be used with slip-detection friction measurement for robotic gripping application.
Large Area Printing of Organic Transistors via a High Throughput Dry Process
Authors: Blanchet Graciela B. , Loo Yueh-Lin , Rogers J. A. , Gao F. , Fincher C. R.
Published: April 2011

Organic electronic systems offer the advantage of low weight and flexibility at potentially lower cost. Although the fabrication of functioning plastic transistors using approaches such as ink jet, lithography and stamping has been described i1–3, chemically compatible materials that allow for the sequential application of liquid layers is a technical barrier. Material issues maybe the Achilles heel of ultimately printing organic electronic devices as newspapers today, at high speeds and in a reel to reel process. We introduce a novel process–thermal transfer–a non-lithographic technique that enables printing multiple, successive layers via a dry additive process. This method is capable of patterning a range of organic materials at high speed over large areas with micron size resolution and excellent electrical performance. Such a dry, potentially reel-to-reel printing method may provide a practical route to realizing the expected benefits of plastics for electronics. We illustrate the viability of thermal transfer and the ability to develop suitable printable organics conductors by fabricating a functioning 4000 cm2 transistor array.
High Performance Organic Field Effect Transistor Withanovel Top-And-Bottom Contact (TBC) Structure
Authors: Yoshida Manabu , Uemura Sei , Hoshino Satoshi , Kodzasa Takehito , Haraichi Satoshi , Kamata Toshihide
Published: April 2011

We studied the features of the newly developed Top and Bottom Contact Field Effect Transistor (TBCFET) with organic semiconductor layers. TBCFETs with ca.0.5μm channel length (L) were fabricated and their transistor properties were measured. The output drain-source currents (IDS) of TBCFET were 1 to 2 orders of magnitude higher than those of ordinary planar type FETs with 100μm channel length. On the other hand, because of the TBC structure, off current tends to become larger in the TBCFET. Therefore, in order to solve this off-current problem, we intentionally formed the Schottky junction at the top electrode/semiconductor interface. As a result, the off current became about 2 orders of magnitude smaller than before the formation of the Schottky junction.
Tunnel-Coupled Quantum Dots: Atomistic Theory of Quantum Dot Molecules and Arrays
Authors: Bryant Garnett W. , Aizpurua Javier , Jaskolski W. , Zielinski Michal
Published: April 2011

An understanding of how dots couple in quantum dot molecules and arrays is needed so that the possibilities for tailored nanooptics in these systems can be explored. The properties of tunnel-coupled dots will be determined by how the dots couple through atomic-scale junctions. We present an atomistic empirical tight-binding theory of coupled, CdS nanocrystal artificial-molecules, vertically and laterally coupled InAs/GaAs self-assembled dots, and arrays of InAs/GaAs self-assembled dots. Electron states follow the artificial molecule analogy. The coupling of hole states is much more complex. There are significant departures from the artificial molecule analogy because the interdot hole coupling is determined by the hole envelope functions, as for the electron states, and by the hole atomic state near interdot interfaces.
Charged Excitons in Self-assembled Quantum Dots
Authors: Warburton R. J. , Urbaszek B. , McGhee E. J. , Schulhauser C. , Högele A. , Karrai K. , Govorov A. O. , Garcia J. M. , Gerardot B. D. , Petroff P. M.
Published: April 2011

We have succeeded in generating highly charged excitons in InAs self-assembled quantum dots by embedding the dots in a field-effect heterostructure. We discover an excitonic Coulomb blockage: over large regions of gate voltage, the exciton charge remains constant. We present here a summary of the emission properties of the charged excitons.
Shape Engineered InAs Quantum Dots with Stabilized Electronic Properties
Authors: Tokranov V. , Yakimov M. , Katsnelson A. , Dovidenko K. , Lamberti M. , Oktyabrsky S.
Published: April 2011

We have studied the influence of overgrowth procedure and a few monolayer-thick AlAs overlayer on the properties of self-assembled InAs quantum dots (QDs) using scanning electron microscopy (SEM) and photoluminescence (PL). PL spectroscopy was used to optimize optical properties of the QDs by shape engineering (QD truncation) through adjustment of the thickness of overlayers and temperature of the subsequent heating. QDs with 6 nm - thick overlayer with subsequent heating up to 560°C was found to have the highest PL intensity at room temperature and the lowest FWHM, 29 meV. Ground state energy of the truncated QDs is very stable against variations of growth parameters. 1.23 μm edge-emitting laser of triple-layer QD structure demonstrated room temperature threshold current density, 74 A/cm2.
Silicon Epitaxial Regrowth Passivation of SiGe Nanostructures Pattered by AFM Oxidation
Authors: Bo Xiang-Zheng , Rokhinson Leonid P. , Sturm J. C.
Published: April 2011

SiGe quantum devices were demonstrated by AFM oxidation and selective wet etching with features size down to 50 nm. To passivate the devices and eliminate the interface states between Si/SiO2, low temperature regrowth of epitaxial silicon over strained SiGe has been tested. The silicon regrowth on Si0.8Ge0.2 was done by rapid thermal chemical vapor deposition (RTCVD) at 700 °C using a hydrogen pre-cleaning process at 800 °C and 10 torr. SIMS analysis and photoluminescence (PL) of strained SiGe capped with epitaxial regrown silicon show a clean interface. Nano-gaps between doped SiGe filled and overgrown with epitaxial silicon show an electrical insulating property at 4.2 K.
Wavelength Selective Charge Storage in self-assembled InGaAs-GaAs Quantum Dots
Authors: Kroutvar M. , Ducommun Y. , Finley J. J. , Bichler M. , Abstreiter G. , Zrenner A.
Published: April 2011

We present the first demonstration of spectrally selective charge storage in self-assembled quantum dots that utilizes optical charge generation and readout of the stored charge distribution. Resonant optical excitation charges only a sub-ensemble of the inhomogeneous broadened dots. The optical readout process directly recovers the spectral distribution of stored charges. Wavelength selectivity is demonstrated in a two-color experiment exhibiting charge storage in two independent QD sub-ensembles. Resonant charge storage is observed at least up to 85K depending on excitation energy. Lack of time evolution of the spectral emission indicates charge storage times Δt much longer than 25μs.
Synthesis of Crystalline Silicon Nanoparticles in Low-Pressure Inductive Plasmas
Authors: Bapat Ameya , Kortshagen Uwe , Campbell Stephen A. , Perrey Christopher R. , Barry Carter C.
Published: April 2011

Amorphous silicon has been used for a wide variety of electronic applications including thin film transistors and energy conversion devices. However, these devices suffer greatly from defect scattering and recombination. A method for depositing crystalline silicon would be highly desirable, especially if it can be remotely created and deposited on any kind of substrate. Our work aims at synthesis and deposition of mono-disperse, single crystal silicon nanoparticles, several tens of nm in diameter on varied substrates. Synthesis of nanocrystals of 2–10 nm diameter has been previously reported but larger particles were amorphous or polycrystalline. This work reports the use of an inductively coupled low-pressure plasma to produce nanocrystals with diameters between 20–80 nm. Electron microscopy studies confirm that the nanocrystals are highly oriented diamond-cubic silicon.
Raman Analysis of Carbon Nanotube Bundles for Bio-electronic Applications
Authors: Chou Shin. G. , Dresselhaus Mildred. S. , Humphreys Elen , Chung Sung Yoon , VanderSande John , Chiang Yet Ming , Swan A. K. , Ünlü M. S. , Goldberg B. B.
Published: April 2011

Three different types of carbon nanotubes being considered for bio-recognition experiments were studied using resonance Raman spectroscopy. Raman spectroscopy, taken using several laser excitation energies, has been shown to provide an effective characterization tool for these carbon nanotubes. The technique yields structural information that both complements and corroborates structural information obtained using electron microscopy techniques, such as TEM, SEM, and AFM.
Carbon Nanotubes Grown on Metallic Wires by Cold Plasma Technique
Authors: Sarangi D. , Karimi A.
Published: April 2011

Carbon nanotubes on metallic wires may be act as electrode for the field emission (FE) luminescent devices. Growing nanotubes on metallic wires with controlled density, length and alignment are challenging issues for this kind of devices. We, in the present investigation grow carbon nanotubes directly on the metal wires by a powerful but simple technique. A novel approach has been proposed to align nanotubes during growth. Methane, acetylene and dimethylamine have been used as source gases. With the same growth conditions (viz. pressure, growth temperature and plasma) methane does not produce any nanotube but nanotubes grown with dimethylamine show shorter length and radius than acetylene. The effect of temperature to control the radius, time to control the density, plasma conditions to align the nanotubes has been focused. Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and Rutherford Back Scattering (RBS) are used to characterize the nanotubes.
Electron Irradiation on Amorphous Silicon
Authors: Cheng Ju-Yin , Murray J.
Published: April 2011

For the first time we use fluctuation electron microscopy to study the effect of electron irradiation on amorphous silicon. Before showing the results, we compare two variable coherence methods, hollow cone and dark field. These techniques have been successfully implemented in the Philips CM12, JEOL 4000 and Hitachi 9000 microscopes. The image fluctuations for ion-implanted and vacuum-deposited amorphous silicon can be reliably measured under the conditions used in these microscopes.
Self-Organized ZnO Nanosize Islands with Low-Dimensional Characteristics on SiO2/Si Substrates by Metalorganic Chemical Vapor Deposition
Authors: Kim Sang-Woo , Fujita Shizuo , Fujita Shigeo
Published: April 2011

Self-organized ZnO nanosize islands on thermally grown SiO2 layers on Si (111) with low-dimensional quantum characteristics were realized by metalorganic chemical vapor deposition. Investigation by atomic force microscopy showed that the density and size of the ZnO nanosize islands were changed by the growth conditions. In macroscopic photoluminescence measurements at 10 K using a 325 nm He-Cd laser, we observed the broad spectra with band tails, which were located at the higher energy with respect to band edge emission of ZnO thin films with the free exciton emission located at about 3.38 eV. These results indicate that these ZnO nanosize islands have low-dimensional quantum effect characteristics.
Phase Transitions in Octanethiol-Capped Ag, Au and CdS Nanocluster Assemblies
Authors: Ellis A. V. , Vijayamohanan K. , Ryu C. , Ramanath G.
Published: April 2011

We describe phase transitions in assemblies of octanethiol (OT)-capped nanoclusters of Ag and Au and CdS of sizes ranging from 2 to 5 nm, created by a new variant of the Brust synthesis method, without the use of phase transfer agents. We probed the stability of these assemblies by a combination of UV-Visible spectrometry, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and in situ polarized microscopy (PLM). Our results show that Ag nanoclusters form a crystalline assembly at ambient temperatures. Upon heating, these assemblies undergo two reversible phase transitions corresponding to melting of (i) a phase comprised of excess thiols that are not linked to the nanoclusters, and (ii) the nanocluster assembly, at ∼60 and 124 °C, respectively. In contrast, Au nanocluster assemblies are softer, waxy solids, and show sub-zero melting transitions. Both these assemblies show no observable mass loss up to ∼180 °C. Cds nanocluster assemblies are also waxy solids, but show a nonreversible melting transition at 137 °C, with simultaneous mass loss due to OT desorption. From our results the thermal stability of the nanoclusters was OT-Ag > OT-Au > OT-CdS.
Vertical Alignment of Single-Walled Carbon Nanotubes on Chemically Functionalized Silicon Substrates
Authors: Lee Ha-Jin , Park Hyeyoung , Koo Sunyoung , Lee Haiwon
Published: April 2011

Carbon nanotubes have been widely investigated as an essential component for fabricating nanoelectronic devices and for their numerous applications. We investigated the vertical alignment of single-walled carbon nanotubes (SWNTs) on chemically functionalized Si surfaces using chemical reactions between chemical groups in SWNTs and surfaces. For controlling the high selectivity of a specific chemical reaction, a pre-patterned 3-D nanostructure was used by using AFM anodization lithography for achieving the vertical alignment of SWNTs. To consider the subsequent chemical reaction with chemically modified Si surface, the carboxylic acid groups were converted into acid chloride groups followed by the reaction with chemically functionalized surfaces. The protruded structures on pre-patterned areas strongly suggest the vertically oriented SWNTs, and the distribution of the vertically aligned SWNTs becomes denser and their heights are longer with prolonged reaction time. The aspect ratio of SWNTs aligned on -OH functionalized surface is independent from the reaction time (H/W = 0.2). After random alignment of relatively shorter nanotubes on the substrate, their bundle size increases with increasing the reaction time due to strong van der Waals interaction between the lateral sides of nanotubes described as ‘nucleation growth’. Longer tubes get also adsorbed on the surface by increase in the bundle size and nanotube length, simultaneously. Based on the chemical reactions of the modified SWNTs with functionalized surfaces, selective attachments of SWNTs were carried out onto pre-patterned surfaces. Detailed characterization of aligned SWNTs will be discussed.
Optically Driven Micromanipulation Tools Fabricated by Two-photon Microstereolithography
Authors: Maruo Shoji , Ikuta Koji , Korogi Hayato
Published: April 2011

Light-driven micromanipulators have been developed by two-photon microstereolithography. The manipulators are driven and controlled by optical trapping. The torque of micromanipulator was successfully controlled on the order of femto-newton by adjusting the focal position of the trapped laser beam. Nanotweezers and a nanoneedle with probe tip of diameter 250 nm were fabricated and driven in a liquid. Such remote-controlled manipulation tools provide a unique and effective handling method of biological samples such as living cell, protein and DNA.
Control of indium surface segregation in GaAs layer on InGaP grown by MOVPE
Authors: Fukushima Y. , Nakano Y. , Shimogaki Y.
Published: April 2011

In order to fabricate good hetero-interface of GaAs/InGaP system by metal-organic vapor phase epitaxy (MOVPE), we studied the indium atom distribution by X-ray photoelectron spectroscopy (XPS). The systematic XPS depth profile analyses of InGaP/GaAs revealed that InGaP surface contains an excess amount of indium atoms. The excess indium atoms diffuse into GaAs layer and cause surface segregation. We have developed a novel gas switching sequence to switch the growth from InGaP to GaAs. That is, after the growth of InGaP, trimethylgallium (TMGa) was pre-introduced to the reactor to terminate the excess amount of indium atoms on InGaP layer. Then tertiarybutylarsine (TBAs) was allowed to flow on the InGaP surface to initiate GaAs growth. This new sequence reduced the amount of indium atoms at the interface and contributed to suppress the inter-diffusion and surface segregation of indium atoms.
Lateral Epitaxial Overgrowth of InAs on (100) GaAs Substrates
Authors: Suryanarayanan Ganesan , Khandekar Anish A. , Hawkins Brian E. , Kuech Thomas F. , Babcock Susan E.
Published: April 2011

The microstructure of epitaxial InAs thin films grown by MOCVD on mask-patterned “LEO” (lateral epitaxial overgrowth) GaAs and on unpatterned GaAs substrates was studied using double-crystal x-ray diffraction, scanning electron microscopy and cross-sectional transmission electron microscopy. This paper describes the improvement in crystal quality (factor of 20 reduction in x-ray rocking curve width), the order of magnitude reduction in dislocation density, and the rearrangement of the remaining extended defects that were observed in the LEO material when compared to the film grown on the unpatterned wafer.
Preparation of Optoelectronic Devices Based on AlN/AlGaN Superlattices
Authors: Holtz M. , Kipshidze G. , Chandolu A. , Yun J. , Borisov B. , Kuryatkov V. , Zhu K. , Chu S. N. G. , Nikishin S. A. , Temkin H.
Published: April 2011

We present results on growth and fabrication experiments of AlN/AlGaN superlattices for ultraviolet (UV) optoelectronic devices. Superlattices with extremely short periods have been studied. The AlN “barrier” layers are 0.5 nm thick, and the AlxGa1-xN “wells” are 1.25 nm thick, with x ∼ 0.08. This combination gives an average AlN mole fraction of 0.63 across one full period. The superlattice periods, AlN mole fractions, and energy gaps are determined using TEM, X-ray diffraction, and optical reflectance. They are all consistent with each other. For device fabrication, p-i-n structures are grown doped with Si (n-type) and Mg (p-type). The acceptor activation energy of ∼ 0.2 eV is found. Mesa structures are plasma etched using chlorine chemistry. Etch rates of AlN are ∼ 1/3 those of GaN under identical circumstances. Etch rates of 250 nm/min are used for the device structures. A light emitting diode, with primary emission at 280 nm is reported, and a detector with sensitivity edge at 260 nm are reported.
Electrical isolation of p-type GaAsN epitaxial layers by ion irradiation
Authors: Gao Q. , Muller J. , Deenapanray P. N. K. , Tan H. H. , Jagadish C.
Published: April 2011

The evolution of sheet resistance (Rs) of p-type conductive GaAs(1-x)Nx epilayers (x = 0.6%, 1.4%, and 2.3%) exposed to MeV 1H+, 7Li+, 12C+, and 16O+ ions and the stability of the formed electrical isolation during post-irradiation annealing were investigated. Results show that the threshold dose (Dth) to convert a conductive layer to highly resistive one close-to-linearly depends on original free carrier concentration and inversely depends on the number of irradiation-generated atomic displacements, and is independent of the nitrogen content in GaAsN layers. Increasing beam flux of 12C+ results in a lower Dth, whereas 1H+ beam flux does not affect it, showing the influence of collision cascade density. Results also show that irrespectively of the ion mass, the stability of electrical isolation formed in GaAsN is dependent on the ratio of the concentration of irradiation-created carrier traps to Dth. The electrical isolation can be preserved up to 550°C when the accumulated dose (D) is greater than 3.3 Dth.
Extremely Large Er Excitation Cross Section in Er,O-Codoped GaAs Light Emitting Diodes Grown by Organometallic Vapor Phase Epitaxy
Authors: Fujiwara Yasufumi , Koizumi Atsushi , Inoue Kentaro , Urakami Akira , Yoshikane Taketoshi , Takeda Yoshikazu
Published: April 2011

Room-temperature Er-related electroluminescence (EL) properties have been investigated in Er,O-codoped GaAs (GaAs:Er,O) light emitting diodes (LEDs) grown by organometallic vapor phase epitaxy (OMVPE). Under forward bias, characteristic emission due to a luminescence center consisting of Er coordinated by O and As was clearly observed at room temperature, while the Er-related EL was undetectable under reverse bias. At lower current densities, the EL intensity increased linearly with the current density. Subsequently, the intensity exhibited a tendency to saturate at higher current densities. By analyzing the behavior with a fitting according to rate equations, the excitation cross section of Er ions due to current injection was determined to be approximately 10-15 cm2, which is by five orders in magnitude larger than that for optical excitation in Er-doped fiber amplifiers (10-20∼10-21 cm2).
Quantum Efficiency Modeling of Amorphous/Crystalline Silicon Heterojunction Photovoltaic Devices
Authors: Khalvati F. , Sivoththaman S.
Published: April 2011

Amorphous/crystalline silicon (a-Si/c-Si) heterojunctions are of particular importance in photovoltaic (PV) energy conversion in a cost-effective way. This is principally due to the low temperature (low-T) nature of the process. In this work, we have analyzed a (n)a-Si/(i)a-Si/(p)c-Si heterojunction solar cell structure using theoretical models for internal quantum yield (IQY) and I-V behavior. We considered low-quality (low bulk lifetime), cheaper substrates. Thin, low bulk lifetime substrates in combination with a low-T bulk passivation scheme and low rear surface recombination can lead to a cost effective device fabrication process with competitive conversion efficiency.
Photosensitive Amorphous Si Thin Films Prepared by Magnetron Technology.
Authors: Khlyap Galina , Brytan Victor
Published: April 2011

Electric field – induced effects are studied in thin films of amorphous Si grown by magnetron sputtering performed in continuous and pulse modes. Current-voltage characteristics are measured under the room temperature in different spectral ranges. It is shown that the investigated dependencies are of exponential character in all range of applied bias. Good photosensitivity was revealed by the samples prepared in continuous mode in the near-IR and visible interval. The samples grown by the pulse magnetron technology were shown room-temperature photosensitivity in near-IR range after 2000C hydrogenation.
Raman and magneto transport studies of MBE grown β-FeSi2, β-(Fe1-xCrx)Si2, and β-(Fe1-xCox)Si2
Authors: Srujana A. , Wadhawan A. , Srikala K. , Gorman B.P. , Cottier R.J. , Zhao Wei , Littler C.L. , Perez J.M. , Golding T.D. , Birdwell A.G. , Henrion W. , Rebien M. , Stauss P. , Glosser R.
Published: April 2011

Iron disilicide shows great promise as a silicon based light emitter operating in the 1.3 to 1.5 μm wavelength range. However, there exists a number of questions related to the band structure and the ability to alloy and controllably dope the material both n and p type. In this paper we present Raman and magneto transport studies on β-FeSi2, β-(Fe1-xCrx)Si2, and β-(Fe1-xCox)Si2 grown by MBE. By comparing the spectra obtained for undoped and doped samples we provide a general overview of the effects of doping on the crystallinity of the material. The temperature dependent (4K<T<300K) magneto transport illustrates that Cr is a p-type dopant and Co is an n-type dopant in β-FeSi2.The temperature dependence of the resistivity indicates that the transport properties at higher temperatures are determined by free carriers whereas at lower temperatures impurity band conduction prevails.
Design of a 364 nm Electrically Pumped Multi-Quantum Well Continuous Wave Nitride Vertical Cavity Surface Emitting Laser
Authors: Luke Shelia C. , Motayed Abhishek , Christou Aris
Published: April 2011

Short-wavelength vertical cavity surface emitting lasers (VCSELS) are typically important for high-density optical memory and optical imaging systems. An AlGaN/GaN multiquantum well (MQW) VCSEL was designed for operation at wavelength of 364 nm. The design process consisted of careful selection of materials and optimization of the parameters for distributed Bragg reflector (DBR) stacks, active region, and calculation of the threshold current density, external quantum efficiency, and threshold carrier concentration. InGaN/AlGaN material system was selected for constructing DBR stacks in order to minimize power dissipation and to achieve high reflectivity. A reflectivity of 99% was calculated for 33 pairs in the bottom DBR stack and 97% was calculated for 30 pairs in the top DBR stack. An external quantum efficiency of 85% has been achieved through parametric optimization. Results were comparable to that of the similar structures based on the review of recent literatures.
Optical Properties of Al0.70Ga0.30As:Er,Yb
Authors: Uekusa Shin-ichiro , Tanaka Isao , Arai Tomoyuki.
Published: April 2011

Erbium (Er) ions were co-implanted with ytterbium (Yb) into Al0.70Ga0.30As substrates and we realized an increase in the intensity of Er intra-4f-shell luminescence. The photoluminescence (PL) intensity of Er-related dominant peak (1538.2nm) was enhanced by co-implanted Yb. The thermal quenching was improved. PL intensity of Yb-related emission was decreased. We studied the transfer energy and the optical sensitization of Yb ions co-implanted with Er ions in Al0.70Ga0.30As. Energy transfers from 2F5/2 (the first excited state) → 2F7/2 (the ground state) of Yb3+ to 4I13/2 (the first excited state) → 4I15/2 (the ground state) of Er3+ were observed by PL excitation (PLE) and selectively excited PL (SPL).
Rotating Anisotropies Without Superparamagnetic Grains in Exchange Bias Systems
Authors: Schulthess T. C.
Published: April 2011

We introduce a random field model with anisotropic interfacial exchange for ferromagnet / antiferromagnet bilayers and calculate average coupling energies and coupling directions for different grain or domain sizes. The model is shown to reconcile Malozemoff's random field Ising model for exchange bias with the spinflop coupling mechanism. Furthermore, we find that for small and mid size domains, the average coupling direction strongly varies between different interfacial configurations. We show how this behavior can lead to rotating anisotropies in systems with very strong magnetocrystalline anisotropies, where the use of superparamagnetic grains or domains in the antiferrofmagnet is not justified.
Role of Spin Momentum Current in Magnetic Non-Local Damping of Ultrathin Film Structures
Authors: Woltersdorf G. , Urban R. , Heinrich B.
Published: April 2011

Non-local damping was investigated by Ferromagnetic Resonance (FMR) using ultrathin magnetic single and double layer structures prepared by Molecular Beam Epitaxy (MBE). The double layer structures show magnetic damping which is caused by spin transport across a normal metal spacer (N). In double layer structures a thin Fe layer, F1, was separated from a thick Fe layer, F2, by a Au(001) spacer. The interface magnetic anisotropies separated the FMR fields of F1 and F2 by a big margin allowing one to investigate FMR in F1 while F2 had a negligible angle of precession, and vice versa. The Fe films in magnetic double layers acquire non-local interface Gilbert damping. It will be shown that the precessing magnetic moments act as spin pumps and spin sinks. This concept was tested by investigating the FMR linewidth around an accidental crossover of the resonance fields for the layers F1 and F2. There is another possible mechanism for non-local damping which is based on a “breathing Fermi surface” of the spacer. The temperature dependence of the non-local damping indicates that this mechanism is weak in Au spacers. Surprisingly the Au spacer acts as an additional impedance for the spin pump mechanism. Finally, it will be shown that electron-electron correlations in a Pd spacer can lead to a significant enhancement of the non-local damping.
Wide-gap P-type Conductive Properties in Layered Oxychalcogenides
Authors: Ueda Kazushige , Takafuji Kouhei , Hiramatsu Hidenori , Ohta Hiromichi , Hirano Masahiro , Hosono Hideo , Kawazoe Hiroshi
Published: April 2011

Several Cu(I)-containing layered oxysulfides were selected as candidates for wide-gap p-type semiconductors by extending a concept of a materials design for transparent p-type conducting oxides. The electrical and optical properties of the selected oxysulfides were investigated, and their electronic structures were analyzed by energy band calculations. LaCuOS, Sr2Cu2ZnO2S2 and Sr2CuGaO3S were found to be wide-gap p-type semiconductors, and LaCuOS showed the largest energy gap (Eg=3.1eV) among these layered oxysulfides. It was also found that LaCuOS shows band edge emission under uv excitation at room temperature, which is consistent with the results of the energy band calculations that LaCuOS has a direct-allowed-type energy gap at Γ point. In further materials research, analogous layered oxychalcogenides such as LaCuOSe and LnCuOS (Ln=Pr, Nd) were found to show similar optical and electrical properties to those of LaCuOS. Therefore, it is considered that the layered crystal structure and the electronic structure are responsible for the wide-gap p-type conductive properties in these materials.