α-Fe2O3 nanorod arrays were fabricated by a low-temperature aqueous chemical growth (ACG) technique and followed by an annealing process. For the surface doping of α-Fe2O3 nanorods, β-FeOOH nanorods obtained via ACG were coated with a thin layer of Cr3+ precursor solution by spin coating, and then underwent the annealing treatment in air. Conducting polymer polypyrrole (PPy) decorated α-Fe2O3 nanorods were prepared by electrodeposition method using malic acid contained pyrrole aqueous solution. Primary results showed that the photocurrents of α-Fe2O3 nanorod array photoanodes were greatly enhanced by surface doping of Cr3+, as well as PPy decoration. This might be due to the retarded charge recombination and promoted surface reaction rate of photogenerated holes with water. Further investigation on surface modification of α-Fe2O3 nanorod array photoanodes is currently conducted in our group.

We have prepared n-type hydrogenated microcrystalline silicon oxide films (n μc-SiO:H) and investigated their structural, electrical and optical properties. Raman spectra shows that, amorphous phase of the n μc-SiO:H films tends to increase when the CO2/SiH4 ratio increases from 0 to 0.28 resulting in a reduction of the crystalline volume fraction (Xc) from 70 to 12%. Optical bandgap (E04) becomes gradually wider while dark conductivity and refractive index (n) continuously drop with increasing CO2/SiH4 ratio. The n μc-SiO:H films have been practically applied as a n layer in top cell of a-SiO:H/μc-Si:H micromorph silicon solar cells. We found that, open circuit voltage (Voc) and fill factor (FF) of the cells gradually increased, while short circuit current density (Jsc) remained almost the same value with increasing CO2/SiH4 ratio for n top layer deposition up to 0.23. The highest initial cell efficiency of 10.7% is achieved at the CO2/SiH4 ratio of 0.23. The enhancement of the Voc is supposed to be due to a reduction of reverse bias at sub cell connection (n top/p bottom interface). An increase of shunt resistance (Rsh) which is caused by a better tunnel recombination junction contributes to the improvement in the FF. Quantum efficiency (QE) results indicate no difference between the cells using n top μc-SiO:H and the cells with n top μc-Si:H layers. These results reveal that, the n μc-SiO:H films in this study do not work as an intermediate reflector to enhance light scattering inside the solar cells, but mainly play a key role to allow ohmic and low resistive electrical connection between the two adjacent cells in the micromorph silicon solar cells.

Controlled ZnO nanostructures were grown on a flexible substrate for the future development of smart sensing tags. Thermolysis-assisted chemical solution deposition was used to grow ZnO nanorods at 85°C from 0.01mol of Zinc nitrate hexahydrate and HMT (Hexamethyltetramine) solution. To promote and modulate the ZnO nanorods, R.F. sputtered ZnO seed layers were deposited on polyimide substrates at various film thicknesses in the range of 8 to 160 nm. The optimum processing conditions to fabricate ZnO nanostructures have been investigated to examine the growth behaviors and to correlate the process parameters with the morphological characteristics. When the ethanol gas sensitivities were measured at different thickness of ZnO seed layers before growing ZnO nanorods, the highest sensitivity was obtained at 40 nm thick ZnO film at 300°C where the film thickness is similar to the Debye length. When ZnO nanorods were grown on such a ZnO seed layer, the sensitivities were more heavily influenced by the ZnO nanostructures rather than the thickness of the seed layer probably due to the dominant proportion of carrier density involved with the gas absorption.

The advanced electrodes for detecting organophosphate pesticides were prepared by modification of the gold (Au) electrode with the reduced graphene oxide/ionic liquid (RGO/IL) nanohybrids. Due to the cationic and anionic parts, the ILs on RGO sheets provide the amount of functional groups for dispersion of hybrids and immobilization of organophosphorus hydrolase (OPH) enzymes. After the immobilization of OPH on the RGO/IL-modified Au electrodes, the modified electrodes represent faster electron transfer than that of Au electrode, resulting in high performance of biosensor with response time (~ 10 s) and sensitivity (4.56 nA μM−1). In addition, the OPH/RGO/IL-modified Au electrode displayed good stability and reproducibility.

We use transient absorption methods to characterize the sequential two-photon absorption in a quantum-dot super-lattice based intermediate band solar cell (QD-IBSC). Using collinear, orthogonally polarized beams generated from an Optical Parametric Oscillator (OPO) at varying time delay, tuned stepwise from 1050nm to 1250nm, we use the solar cell photocurrent as a direct measure of the transient absorption by measuring the differential photo-current as a function of time delay between two energetically degenerate, 100fs pulses. For comparison, we measure the pulse autocorrelation in the same geometry using a GaAsP photodiode, where all observed photocurrent is derived from instantaneous two-photon absorption. Our measurements show that at high intensity, the measurement is dominated by instantaneous two photon absorption, with a simultaneous sequential two-photon photocurrent which persists beyond the pulse overlap. Our measurements demonstrate the method can reveal carrier dynamics in a working QD-IBSC, and their dependence on energy. The method could potentially give details of the band structure formed in the QD-IBSC. Such knowledge may benefit device development and future designs of IBSCs based on QD superlattices or alternative intermediate band materials or device structures.

The pseudoelastic behavior of Fe3Al and Fe3Ga alloys with the D03 structure is reviewed. In general, pseudoelasticity of shape memory alloys is based on a thermoelastic martensitic transformation. However, pseudoelasticity regardless of the martensitic transformation is found to take place in D03-ordered Fe3Al and Fe3Ga alloys. For instance, a 1/4<111> superpartial dislocation in Fe3Al alloys moves independently dragging an antiphase boundary (APB). During unloading, the APB pulls back the superpartial to decrease its energy resulting in pseudoelasticity, which is called “APB pseudoelasticity”. Moreover, D03-type Fe3Ga alloys were found to demonstrate three types of pseudoelasticity based on the dislocation motion, twinning and martensitic transformation depending on the chemical composition, degree of D03 order, loading axis, stress sense and deformation temperature. The mechanism of the pseudoelasticities in the D03-type intermetallics is discussed based on some in situ observations. The effects of chemical composition, deformation temperature and crystal orientation on the pseudoelastic behaviors are also discussed.

A high strength ferritic steel with finely dispersive precipitates was investigated to reveal the fundamental strengthening mechanisms in this alloy. Using energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscope (TEM), fine carbides with an average diameter of 10 nm were observed in the ferrite matrix of the 0.08%Ti steel, and some cubic M23C6 precipitates were also observed at the grain boundaries and the interior of grains. The dual precipitate structure of finely dispersive TiC precipitates in the matrix and coarse M23C6 at grain boundaries provides combined matrix and grain boundary strengthening. The resulting yield stress is two or three times higher than that of conventional Ti-bearing high strength hot-rolled sheet steels. The effect of the particle size, particle distribution and intrinsic particle strength have been investigated through dislocation dynamics (DD) simulations and the relationship for resolved shear stress for single crystal under this condition has been presented using simulation data. The results show that the finely dispersive precipitates can strengthen the material by pinning the dislocations up to a certain shear stress and retarding the recovery as well as annihilation of dislocations. The DD results also show that strengthening is not only a function of the density of the nano-scale precipitates but also of their size. This size effect is explained using a mechanistic model developed based on dislocation-particle interaction.

Thin films nano-crystalline zirconia of ~ 300 nm thick were deposited on Si substrate, and the samples were irradiated with 2 MeV Au+ ions at temperatures of 160 and 400 K, up to fluences of 35 displacements per atom. The films were then studied using glancing incidence x-ray diffraction, Rutherford backscattering, secondary ion mass spectroscopy and transmission electron microscopy. During the irradiation, cavities were observed to form at the zirconia/silicon interface. The morphology of the cavities was found to be related to the damage state of the underlying Si substrate. Elongated cavities were observed when the substrate is heavily damaged but not amorphized; however, when the substrate is rendered amorphous, the cavities become spherical. As the ion dose increases, the cavities then act as efficient gettering sites for the Au. The concentration of oxygen within the cavities determines the order in which the cavities getter. Following complete filling of the cavities, the interface acts as the secondary gettering site for the Au. The Au precipitates are determined to be elemental in nature due to the high binding free energy for the formation of Au-silicides.

This paper presents our material studies on hydrogenated microcrystalline silicon (μc-Si:H) and microcrystalline silicon-germanium alloy (μc-Si1-x Gex :H) thin films for the development of high efficiency p-i-n junction solar cells. In μc-Si:H solar cells, we have evaluated the structural properties of the intrinsic μc-Si:H layers grown by plasma-enhanced chemical vapor deposition at high deposition rates (>2 nm/s). Several design criteria for the device grade μc-Si:H are proposed in terms of crystallographic orientation, grain size and grain boundary passivation. Meanwhile, in μc-Si1-x Gex :H solar cells, we have succeeded in boosting the infrared response of solar cell upon Ge incorporation up to x∼0.2. Nevertheless, a degradation of solar cell parameters is observed for large Ge contents (x>0.2) and thick i-layers (> 1 μm), which is attributed to the influence of the Ge dangling bonds that act as acceptorlike states in undoped μc-Si1-x Gex :H. To improve the device performance, we introduce an oxygen doping technique to compensate the native defect acceptors in μc-Si1-x Gex :H p-i-n solar cells.

Polyaniline (PANI) is one of the most interesting conducting polymers with a wide and controllable conductivity range, synthesized easily via chemical or electrical route, stable chemically and environmentally, having high absorption in the visible range and high mobility of charge carriers. Under different conditions, PANI morphology can be controlled yielding to the creation of nano-tubes, belts, rods, fibers and particles.

In this study, the chemical oxidative polymerization which consists of mixing aniline hydrochloride (A-HCl) with ammonium peroxydisulfate (APS) was used to synthesize HCl doped PANI. Fixing the weight ratio A-HCl/APS defined by the IUPAC while varying their quantities leads to the formation of PANI nanoparticles with variable diameters. In addition, PANI nano-needles of 60 nm average diameter at the center are also obtained. Different methods are used to investigate of 1-D morphologies. The electrical conductivity of bulk PANI pellets is measured using the four-point probe technique. The absorption in the visible range of PANI particles and nano-needles is determined by UV-Vis spectroscopy. XRD analysis was performed to study the effect of PANI particle size and morphology on the crystallinity of the powder. Such structures could be used in hybrid solar cells for higher conversion efficiencies.

A CdSxTe1-x layer forms by interdiffusion of CdS and CdTe during the fabrication of thinfilm CdTe photovoltaic (PV) devices. The CdSxTe1-x layer is thought to be important because it relieves strain at the CdS/CdTe interface that would otherwise exist due to the 10% lattice mismatch between these two materials. Our previous work [1] has indicated that the electrical junction is located in this interdiffused CdSxTe1-x region. Further understanding, however, is essential to predict the role of this CdSxTe1-x layer in the operation of CdS/CdTe devices. In this study, CdSxTe1-x alloy films were deposited by radio-frequency magnetron sputtering and coevaporation from CdTe and CdS sources. Both radio-frequency-magnetron-sputtered and coevaporated CdSxTe1-x films of lower S content (x<0.3) have a cubic zincblende (ZB) structure akin to CdTe, whereas those of higher S content have a hexagonal wurtzite (WZ) structure like that of CdS. Films become less preferentially oriented as a result of a CdCl2 heat treatment at ∼400°C for 5 min. Films sputtered in a 1% O2/Ar ambient are amorphous as deposited, but show CdTe ZB, CdS WZ, and CdTe oxide phases after a CdCl2 heat treatment. Films sputtered in O2 partial pressure have a much wider bandgap than expected. This may be explained by nanocrystalline size effects seen previously [2] for sputtered oxygenated CdS (CdS:O) films.

We address the issue of decreasing band-gap with increasing atomic number, inherent in semiconducting materials, by introducing a concept we call dimensional reduction. The concept leads to semiconductor compounds containing high atomic number elements and simultaneously exhibiting a large band gap and high mass density suggesting that dimensional reduction can be successfully employed in developing new γ-ray detecting materials. As an example we discuss the compound Cs2Hg6S7 that exhibits a band-gap of 1.65eV and mobility-lifetime products comparable to those of optimized Cd0.9Zn0.1Te.

Nano-colloids and nano-crystals doped with ions of rare-earth elements have recently attracted a lot of attention of scientific community. This attention is due to unique physical, chemical and optical properties attributed to nanometer size of the particles. They have great potential of being used in applications spanning from new types of lasers, especially blue and UV lasers, phosphorous display monitors, optical communications, and fluorescence imaging. In this paper we investigate the infrared-to-visible upconversion luminescence in bulk crystals doped with ytterbium and holmium co-doped and ytterbium and thulium co-doped NaYF4 upconversion phosphors. The phosphors were prepared by using simple co-precipitation synthetic method. The initially prepared phosphor has very weak upconversion fluorescence. The fluorescence significantly increased after the phosphor was annealed at a temperature of 600 0C. Nanocolloids of this phosphor were obtained using methanol as solvents and they were utilized as laser filling medium in photonic crystal fibers. Under 980 nm laser excitation very strong upconversion signals were obtained for ytterbium and holmium co-doped phosphor at 541 nm, 646 nm and 751 nm, and 376 nm, 476 nm, 646 nm, 696 nm and 803 nm for ytterbium and thulium co-doped phosphor. The particle sizes of the nanocolloids were analyzed using Atomic Force Microscope. The reported nanocolloids are good candidates for fluorescent biosensing applications and also as a new laser filling medium in fiber lasers.

Mo-Si-B alloys respond to high temperature oxidation in two distinct stages. First, there is a transient stage with an initial high recession rate that corresponds to the evaporation of volatile MoO3 due to the oxidation of the molybdenum rich phases. The steady state stage of the oxidation begins when a borosilica layer that initiated in the transient period becomes continuous and protects the alloy from further rapid oxidation. Then, the oxidation rate is limited by oxygen diffusion through the borosilicate layer. In order to improve the oxidation performance of the Mo-Si-B alloys, it is necessary to minimize the transient stage. The three phases, Mo (solid solution), Mo3Si (A15) and Mo5SiB2 (T2), composing the Mo-Si-B alloys play different roles in the transient stage. The interaction of the three phases with a reduced microstructure scale can reduce considerably the transient oxidation stage. As a further approach to inhibit the transient stage, a kinetic biasing strategy has been developed to capitalize on the reactions between different phases to develop useful reaction products and alloy compositions that evolve toward a steady state of a compatible system. In order to achieve a compatible interface coating together with enhanced oxidation resistance, a pack cementation process has been adopted to apply diffusion coatings. Two areas are highlighted for successful coating applications on Mo-Si-B alloys and robust high temperature oxidation resistance: development of metal-rich silicide + borosilicide high-temperature coating and in-situ thermal-barrier + borosilica coatings.

We report here swift heavy ion (SHI) irradiation induced effects on structural and surface properties of III-nitrides. Tensile strained Al(1-x)InxN/GaN Hetero-Structures (HS) were realized using Metal Organic Chemical Vapour Despotion (MOCVD) technique with indium composition as 12%. Ion species and energies are chosen such that electronic energy deposition rates differ significantly in Al(1-x)InxN and are essential for understanding the ion beam interactions at the interfaces. Thus the samples were irradiated with 80 MeV Ni6+ and 100 MeV Ag7+ ions at varied fluence (1×1012 and 3 ×1012 ions/cm2) to alter the structural properties. Under this energy regime, the structural changes in Al(1-x)InxN would occur due to the intense ultrafast excitations of electrons along the ion path. We employed different characterization techniques like High Resolution X- ray Diffraction (HRXRD) and Rutherford back scattering spectrometry (RBS) for composition, thickness and strain. HRXRD and RBS experimental spectra have been fitted with Philip’s epitaxy SIMNRA code, which yields thickness and composition from compound semiconductors. The surface morphology of pristine and irradiated samples is studied and compared by Atomic Force Microscopy (AFM).

Composition, bonding state, and electrical properties of CNx films formed by electro-chemical deposition using liquid acrylonitrile were studied. X-ray photoelectron spectra reveal that C, N, and O are major components of the deposited films. From analysis of C 1s and N 1s spectra, the major bonding state in the CNx film is attributed to a mixture of C≡N and partially hydrogenated C=N bond. Metal-insulator-semiconductor capacitors incorporating the CNx insulating layers are fabricated to evaluate the electrical properties of the deposited films. The lowest dielectric constant k of the CNx film is determined to be 2.6 from the accumulation capacitance and the thickness of the film. It is demonstrated that the CNx film formed by electrochemical deposition is a promising low-k material for use in ultralarge-scale integration multilevel interconnections.

We present a first-principles lattice dynamics for the assembly of the transition-metal (M)-encapsulated Sin clusters in amorphous phase (a-MSin), which has been proposed as a potential candidate for the channel material of the next-generation thin-film transistors (TFTs) [N. Uchida et al., Appl. Phys. Express1, 121502 (2008)]. The shape of calculated vibrational density of states (VDOS) curve of a-MoSi10 is similar to the counterpart of the high pressure phase of a-Si (HPA-Si) although the present systems are obtained as a result of pressure relaxation. Its radial distribution function (RDF) among Si themselves is characterized by the absence of a gap between the first and second shells, which is also the case in . We further present the VDOS of a-WSi10, whose curve shape is again similar to that of HPA-Si. A difference between a-MoSi10 and a-WSi10 is that the W-atom displacement components extracted from the vibration eigenvectors are mainly distributed over a lower frequency range (< ~ 150 cm-1) than the Mo counterpart (~ 150 cm-1 to ~ 300 cm-1). This may be attributed to a larger atomic mass of W than Mo.

A femtosecond (fs) pulse duration is shorter than many physical/chemical characteristic times, such as the electron-photon relaxation time, which makes it possible to control electron dynamics. This paper reviews our recent progress which proposes to change electron dynamics (selective excitation/ionization) and electron densities/temperatures in materials to control the following properties and processes: 1) the transient (femtosecond-to-picosecond time scale), localized (nanometer-to-micrometer length scale) material properties, 2) the corresponding photon absorption process, and 3) phase change mechanisms, by manipulating fs pulse-train number/delay for high-precision micro/nanoscale manufacturing.

Membranes of epitaxial SiC have been used as a means of eliminating the leakage current into the Si substrate during circular transmission line model (CTLM) measurements. In the n+-3C-SiC/Si wafers, the Si substrate was etched in a patterned window with dimensions up to 10 mm × 15 mm2. An array of CTLM metal contacts was then deposited onto the upper surface of the n+-SiC membrane. The CTLM contacts on the membrane have shown an ohmic current/voltage response while electrodes located on the adjacent substrate were non-ohmic. Values of ρc were measured directly on the membranes. These results have shown a significant increase in the current flow below the metal contacts due to the presence of the Si substrate.