Photovoltaic properties of GaAsP core-shell nanowires on Si(001) substrate

We report on the growth and electro-optical studies of photovoltaic properties of GaAsP nanowires. Low density GaAsP nanowires were grown by Au assisted MOVPE on Si(001) substrates using a two step procedure to form a radial p-n junction. The STEM analyses show that the nanowires have cubic structure with the alloy composition GaAs?.??P?.?? in the nanowire core and GaAs?.??P?.?? in the shell. The nanowire ensembles were processed in the form of sub-millimeter size mesas. The photovoltaic properties were characterized by optical beam induced current (OBIC) and electronic beam induced current (EBIC) maps. Both OBIC and EBIC maps show that the photovoltage is generated by the nanowires; however, a strong signal variation from wire to wire is observed. Only one out of six connected nanowires produce a measurable signal. These strong fluctuations can be tentatively explained by the variation of the resistance of the nanowire-to-substrate connection, which is highly sensitive to the quality of the Si-GaAsP interface. This study demonstrates the importance of the spatially resolved charge collection microscopy techniques for the diagnosis of failures in nanowire photovoltaic devices.     

Site-controlled fabrication of dimension-tunable Si nanowire arrays on patterned (001)Si substrates

In this study, we fabricated well-ordered arrays of site-controlled, vertically-aligned Si nanowires on the desired areas of pre-patterned (001)Si substrates by employing the nanosphere lithographic technique in combination with the Au-assisted selective etching process. The results of transmission electron microscopy and selected-area electron diffraction analysis show that the Si nanowires that fabricated on the patterned (001)Si substrates have a single-crystalline nature and form along the [001] direction. The length of the Si nanowires was found to increase linearly with the Au-assisted etching time. Scanning electron microscopy images clearly revealed that by adjusting the sizes of the nanosphere template and the etching temperature and time, the diameter and length of the patterned Si nanowires could be effectively tuned and accurately controlled. Furthermore, the diameters of the Si nanowires produced at various temperatures and time were found to be relatively uniform over the entire length. The combined approach presented here provides the capability to fabricate a variety of size-, length-tunable 1D Si-based nanostructures on various patterned Si-based substrates.     

Aligned arrays of biodegradable poly(epsilon-caprolactone) nanowires and nanofibers by template synthesis

We demonstrate that the technically simple, low-cost, and rapid method of template synthesis can be used to create arrays of nanowires and nanofibers from the biocompatible, biodegradable polymer poly(epsilon-caprolactone) (PCL). PCL substrates introduced into a standard laboratory oven at temperatures as low as 65 degrees C are able to form nanostructures. Nanostructure morphology can be controlled, and complex patterning can be achieved. Solvent-free and low-temperature fabrications also allow for the encapsulation of therapeutics for sustained release.     

Thermal and tensile properties of Si/Ge core-shell and superlattice nanowires

The Stillinger-Weber potential-based MD (Molecular dynamics) method is used to simulate the heating-up and axial tension of Si/Ge core-shell and superlattice nanowires; according to the simulative results, the differences in their thermal and mechanical properties are discussed. The results show the following: (1) The Si/Ge superlattice nanowire is more thermally stable than the core-shell one, and their melting points are 1160 and 1320 K, respectively. (2) The Si/Ge core-shell nanowire has higher elastic module than the super-lattice one. (3) Under tension, the super-lattice nanowire has better antideformation capability than the core-shell one but has comparative antiloading capability.     

Thermal and tensile properties of Si/Ge core-shell and superlattice nanowires

The Stillinger-Weber potential-based MD (Molecular dynamics) method is used to simulate the heating-up and axial tension of Si/Ge core-shell and superlattice nanowires; according to the simulative results, the differences in their thermal and mechanical properties are discussed. The results show the following: (1) The Si/Ge superlattice nanowire is more thermally stable than the core-shell one, and their melting points are 1160 and 1320 K, respectively. (2) The Si/Ge core-shell nanowire has higher elastic module than the super-lattice one. (3) Under tension, the super-lattice nanowire has better antideformation capability than the core-shell one but has comparative antiloading capability.     

Self-doped conducting core-shell poly(styrene/pyrrole) nanoparticles via two-stage shot-growth

Self-doped conducting core-shell poly(styrene/pyrrole) (poly(St/Py)) nanoparticles were successfully prepared by a one-pot synthetic route in both Fe(3+)-catalyzed oxidative polymerization and emulsifier-free emulsion polymerization. Modified two-stage shot-growth method was introduced to obtain higher doping level of the self-doped conducting core-shell poly(St/Py) nanoparticles. The particle size and core-shell morphology of the resulting particles before and after two-stage shot-growth were investigated by SEM and TEM analyses. Surface charge density of the particles highly increased after two-stage shot-growth and was measured by zeta-potential analysis. The self-doped core-shell nanoparticles showed a high conductivity after two-stage shot-growth.     

Crystalline structures and misfit strain inside Er silicide nanocrystals self-assembled on Si(001) substrates

The morphology and crystalline structure of Er silicide nanocrystals self-assembled on the Si(001) substrate were investigated using scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). It was found that the nanowires and nanorods formed at 630?°C has dominant hexagonal AlB(2)-type structure, while inside the nanoislands self-organized at 800?°C the tetragonal ThSi(2)-type structure is prevalent. The lattice analysis via cross-sectional high-resolution TEM demonstrated that internal misfit strain plays an important role in controlling the growth of nanocrystals. With the relaxation of strain, the nanoislands could evolve from a pyramid-like shape into a truncated-hut-like shape.     

Effect of the bimodal size distribution on the optical properties of self-assembled Ge/Si(001) quantum dots

In situ reflection high-energy electron diffraction, atomic force microscopy and photoluminescence spectroscopy have been combined to investigate the effects of a bimodal size distribution and of the pyramid/dome transition on the optical properties of related Ge/Si layers. It is shown that the wetting layers are inhomogeneous in thickness due to the lateral diffusion of Ge from 2D layers towards islands, while no change is observed in the island-related photoluminescence. These results obtained indicate that 3D islands are, in their early nucleation stages, formed by consuming Ge from 2D layers, and that island luminescence energy is not sensitive to the vertical confinement inside islands.     

Synthesis and characterization of self-assembled ZnO nano-dots grown on SiNx/Si(001) substrates by radio frequency magnetron sputtering

This paper reports the synthesis and characterization of self-assembled ZnO nano-dots deposited on SiN_x/Si(001) substrates by radio frequency magnetron sputtering. The effect of the working pressure on the microstructure of the as-grown ZnO thin films was examined. At a working pressure of 6 × 10⁻³ Torr, a flat layered structure was dominant with a preferred orientation of the ZnO(0002) plane, while ZnO nano-structures were observed on the samples grown at 2 × 10⁻² Torr. This was attributed to the columnar growth that facilitated the nucleation of ZnO nano-structures on the growing surfaces. Hexagonal nano-pyramids were formed, which then transformed into nano-dots as the film became thicker. The ZnO nano-dots were uniform and well dispersed, exhibiting distinct photoluminescence spectra due to the quantum confinement effect.     

Co-delivery of drugs and DNA from cationic core-shell nanoparticles self-assembled from a biodegradable copolymer

Non-viral gene-delivery systems are safer to use and easier to produce than viral vectors, but their comparatively low transfection efficiency has limited their applications. Co-delivery of drugs and DNA has been proposed to enhance gene expression or to achieve the synergistic/combined effect of drug and gene therapies. Attempts have been made to deliver drugs and DNA simultaneously using liposomes. Here we report cationic core-shell nanoparticles that were self-assembled from a biodegradable amphiphilic copolymer. These nanoparticles offer advantages over liposomes, as they are easier to fabricate, and are more readily subject to modulation of their size and degree of positive charge. More importantly, they achieve high gene-transfection efficiency and the possibility of co-delivering drugs and genes to the same cells. Enhanced gene transfection with the co-delivery of paclitaxel has been demonstrated by in vitro and in vivo studies. In particular, the co-delivery of paclitaxel with an interleukin-12-encoded plasmid using these nanoparticles suppressed cancer growth more efficiently than the delivery of either paclitaxel or the plasmid in a 4T1 mouse breast cancer model. Moreover, the co-delivery of paclitaxel with Bcl-2-targeted small interfering RNA (siRNA) increased cytotoxicity in MDA-MB-231 human breast cancer cells.     

Fabrication of Fe(3)O(4) nanoparticle arrays via patterned template assisted self-assembly

Magnetic nanoparticle arrays have been fabricated by combining chemically synthesized Fe(3)O(4) nanoparticles with a diblock copolymer template substrate consisting of self-assembled polystyrene (PS) dots in a polymethylmethacrylate (PMMA) matrix. The influence of the volume fraction of the Fe(3)O(4) suspending solution and the withdrawal speed of the template on the formation of array structures was investigated. A small volume fraction of the nanoparticles and low withdrawal speed play an important role in the fabrication of the patterned arrays of nanoparticles via template assisted self-assembly. Below a withdrawal speed of 0.5?mm?s(-1) and a nanoparticle volume fraction below 0.05?vol% (in particular, at extremely high dilutions of less than 0.01?vol%), the selective deposition of one to several nanoparticles on every single PS dot becomes possible.     

Optical properties of GaN epilayers grown on Si (111) and Si (001) substrates

We report the observation of bright photoluminescence (PL) emission from two types of GaN epilayers grown by molecular beam epitaxy (MBE). Wurtzite phase GaN/Si (111) epilayers are grown by gas source MBE process, whereas cubic phase GaN epilayers are grown on (001) Si covered by thin SiC film in the process of Si annealing in propane prior to the GaN growth. PL emissions are identified based on the results of detailed PL and time-resolved PL investigations. For the wurtzite phase GaN we observe an efficient up in the energy transfer from bound to free excitons. This process is explained by a large difference in the PL decay times for two types (free and bound (donor, acceptor)) of excitonic PL emissions. For cubic phase GaN we confirm recent suggestion that acceptors have smaller thermal ionization energies than those in the wurtzite phase GaN.     

Composition and atomic ordering of Ge/Si(001) wetting layers

A combination of X-ray diffraction with anomalous X-ray scattering at the Ge K edge and specular reflectivity measurements is used to reveal both composition and atomic ordering in Ge:Si wetting layers. By comparing the intensity distribution close to the (400) and (200) surface reflections we show that the Ge wetting layer is composed of a SiGe alloy which exhibits atomic ordering. Due to the Si interdiffusion the wetting layer thickness is larger than the nominal 3 ML Ge deposition. The chemical depth distribution is obtained from X-ray reflectivity measurements and confirms the enhanced Ge interdiffusion. These phenomena evidence the crucial interplay between surface kinetics and intermixing in SiGe thin films and nanostructures on Si(001) substrates.     

The facile synthesis of CdSe hollow nanoparticles and necklace-like nanowires from a CdO sacrificial template via chemical reaction in aqueous solution

This paper describes a powerful and facile synthesis of CdSe hollow nanostructures via a sacrificial template method employing the Kirkendall diffusion effect for void formation and an anion exchange reaction for selenization. The CdSe hollow nanowires and nanoparticles were easily transformed from a sacrificial template of CdO nanowires that were formed by thermal Ostwald ripening. The distinctive structures of CdO sacrificial templates were developed through thermodynamically spontaneous process. The morphology of each CdSe nanostructure could be controlled by one of each CdO sacrificial template which was calcined at several different temperatures. Their microstructure was analyzed by field-emission scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, and their optical properties were compared using light absorption. The suggested process for synthesis of hollow nanostructures could also be applied to the formation of other hollow chalcogenide compounds.     

Oriented immobilization of IgG on hydroxylated Si(001) surfaces via protein-A by a multiple-step process based on a self-assembly approach

The aim of this study was the oriented immobilization of IgG molecules on the silicon surfaces. A multiple-step procedure was applied for oriented immobilization of IgG in this study. After hydroxylation of the Si(001) surfaces, 3-glycidoxypropyltrimethoxysilane (GPTS) molecules were self-assembled onto these substrates. Dipping time and GPTS concentration were found to be effected by on both layer thicknesses and water-contact angles. 2,2′-(ethylenedioxy)diethylamine (EDA) molecules were then covalently attached to the silicon surface with GPTS molecules. There was no effect of concentration on the formation of EDA molecules on the surfaces, while EDA deposition increased with the dipping time significantly. Imaging ellipsometry and atomic force microscopy (AFM) images exhibited aggregate formation at this step. Protein-A molecules were bound to the free amino groups of EDA molecules on the substrate surface, especially onto the aggregates by using a carbodiimide (i.e., EDAC) as the activating agent. We were able to immobilize IgG molecules in an oriented form onto the protein-A attached surfaces, especially in the regions, where EDA aggregates are located.     

Ni(3)Si(Al)/a-SiO(x) core-shell nanoparticles: characterization, shell formation, and stability

We have used an electrochemical selective phase dissolution method to extract nanoprecipitates of the Ni(3)Si-type intermetallic phase from two-phase Ni-Si and Ni-Si-Al alloys by dissolving the matrix phase. The extracted nanoparticles are characterized by transmission electron microscopy, energy-dispersive x-ray spectrometry, x-ray powder diffraction, and electron powder diffraction. It is found that the Ni(3)Si-type nanoparticles have a core-shell structure. The core maintains the size, the shape, and the crystal structure of the precipitates that existed in the bulk alloys, while the shell is an amorphous phase, containing only Si and O (SiO(x)). The shell forms around the precipitates during the extraction process. After annealing the nanoparticles in nitrogen at 700?°C, the tridymite phase recrystallizes within the shell, which remains partially amorphous. In contrast, on annealing in air at 1000?°C, no changes in the composition or the structure of the nanoparticles occur. It is suggested that the shell forms after dealloying of the matrix phase, where Si atoms, the main constituents of the shell, migrate to the surface of the precipitates.     

Macro- and micro-strain in GaN nanowires on Si(111)

We analyze the strain state of GaN nanowire ensembles by x-ray diffraction. The nanowires are grown by molecular beam epitaxy on a Si(111) substrate in a self-organized manner. On a macroscopic scale, the nanowires are found to be free of strain. However, coalescence of the nanowires results in micro-strain with a magnitude from ± (0.015)% to ± (0.03)%. This micro-strain contributes to the linewidth observed in low-temperature photoluminescence spectra.     

Selection of microemulsion composition via study of phase behavior for synthesis of stable monodisperse platinum nanoparticles and optimization of experimental parameters

ABSTRACT

Monodisperse platinum nanoparticles were synthesized by microemulsion technique. Feasibility of formation of microemulsion with chosen proportions of components is a common problem with this method. Here, prior to preparation of microemulsion for synthesis purpose, systematic study was carried to check the suitability of selected microemulsion system by establishing phase diagram for ternary system at constant temperature on triangular coordinates. Temperature dependency of microemulsion was studied by preparing phase diagram of temperature against mass fraction of aqueous phase in ternary mixture. Both these triangular coordinate diagram at constant temperature and temperature dependency plots helped to choose proper concentrations of components in the system. Effects of molar concentration of water-to-surfactant ratio, platinum salt concentration, continuous oil phase concentration, and presence of cosurfactant with surfactant on particle size were investigated for selected system of components. Synthesized platinum nanoparticles were characterized by dynamic light scattering and transmission electron microscope. Light backscattering profiles obtained by Turbiscan were used to judge the stability of microemulsions. Further, partial weightage of affecting parameters on size of synthesized particle were studied by Taguchi orthogonal array method.     

The study of the interactions between graphene and Ge(001)/Si(001)

The interaction between graphene and germanium surfaces was investigated using a combination of microscopic and macroscopic experimental techniques and complementary theoretical calculations.Density functional theory (DFT) calculations for different reconstructions of the Ge(001) surface showed that the interactions between graphene and the Ge(001) surface introduce additional peaks in the density of states,superimposed on the graphene valence and conduction energy bands.The growth of graphene induces nanofaceting of the Ge(001) surface,which exhibits well-organized hill and valley structures.The graphene regions covered by hills are of high quality and exhibit an almost linear dispersion relation,which indicates weak graphene-germanium interactions.On the other hand,the graphene component occupying valley regions is significantly perturbed by the interaction with germanium.It was also found that the stronger graphene-germanium interaction observed in the valley regions is connected with a lower local electrical conductivity.Annealing of graphene/Ge(001)/Si(001) was performed to obtain a more uniform surface.This process results in a surface characterized by negligible hill and valley structures;however,the graphene properties unexpectedly deteriorated with increasing uniformity of the Ge(001) surface.To sum up,it was shown that the mechanism responsible for the formation of local conductivity inhomogeneities in graphene covering the Ge(001) surface is related to the different strength of graphene-germanium interactions.The present results indicate that,in order to obtain high-quality graphene,the experimental efforts should focus on limiting the interactions between germanium and graphene,which can be achieved by adjusting the growth conditions.     

Growth and characterization of epitaxial anatase TiO2(001) on SrTiO3-buffered Si(001) using atomic layer deposition

Epitaxial anatase titanium dioxide (TiO₂) films have been grown by atomic layer deposition (ALD) on Si(001) substrates using a strontium titanate (STO) buffer layer grown by molecular beam epitaxy (MBE) to serve as a surface template. The growth of TiO₂ was achieved using titanium isopropoxide and water as the co-reactants at a substrate temperature of 225-250 °C. To preserve the quality of the MBE-grown STO, the samples were transferred in-situ from the MBE chamber to the ALD chamber. After ALD growth, the samples were annealed in-situ at 600 °C in vacuum (10^− 7 Pa) for 1-2 h. Reflection high-energy electron diffraction was performed during the MBE growth of STO on Si(001), as well as after deposition of TiO₂ by ALD. The ALD films were shown to be highly ordered with the substrate. At least four unit cells of STO must be present to create a stable template on the Si(001) substrate for epitaxial anatase TiO₂ growth. X-ray diffraction revealed that the TiO₂ films were anatase with only the (004) reflection present at 2θ = 38.2°, indicating that the c-axis is slightly reduced from that of anatase powder (2θ = 37.9°). Anatase TiO₂ films up to 100 nm thick have been grown that remain highly ordered in the (001) direction on STO-buffered Si(001) substrates.