Metal-assisted chemical etching is a simple and low-cost silicon nanowire fabrication method which allows control of nanowire diameter, length, shape and orientation. In this work, we fabricated well-ordered silicon nanowire array by patterning gold thin film by nanosphere lithography and etching single crystalline silicon wafer by metal-assisted chemical etching technique. We investigated relation between etched solution concentration and nanowire morphology, wafer crystal orientation, etching rate. This well-ordered silicon nanowires arrays have the potential applications in many fields but especially next generation energy related applications from solar cells to lithium-ion batteries.

We demonstrate the newly developed technique Photomodulated Rayleigh Scattering spectroscopy in order to probe the electronic band structure of single semiconductor nanowires. We show that both the electronic transition energies and nanowire diameter can be measured simultaneously and with high accuracy in a single non-destructive measurement. We demonstrate our results for zincblende GaAs as well as wurtzite InP nanowires where we probed the band gaps and transition energies at both room and low temperatures. This technique should advance the study of optical properties of single nanowires as well as other types of nanostructures.

Composites of single-walled carbon nanotubes (SWNTs) and polyaniline (PAni) were synthesized using different approaches. SWNT/PAni nanocomposite with controlled core/shell morphology was achieved. Our chemical sensing tests showed that such core/shell morphology resulted in superior sensor performance, with an increased sensitivity to acetone vapors, and a reversible detection of hydrazine vapors. The reversible detection of parts-per-billion concentrations of hydrazine offers promise for a portable solid-state detector that has potential application in aerospace.

Water-splitting to form hydrogen was examined by using strontium titanate (SrTiO3) nanofibers as photocatalysts. SrTiO3 nanofibers were fabricated by hydrothermal treatment of amorphous titanium dioxide nanofibers, which were electrospun from the mixture of polyvinylpyrrolidone (PVP), titanium(IV) butoxide, and acetylacetone. The hydrothermal treatment involved the reaction of amorphous TiO2 nanofiber template with strontium hydroxide octahydrate (Sr(OH)2·8H2O) for 20 hours at 120 ºC. The product was calcined to form crystalline SrTiO3 nanofibers, which were characterized via Scanning Electron Microscopy (SEM)/Energy Dispersive Spectroscopy (EDS) and tested their photocatalytic activities for the water splitting. The hydrogen production with the fabricated SrTiO3 nanofibers was found to be 6.1 μmol·h-1·g-1 catalyst, which is twice that of commercially available SrTiO3 nanoparticles (3.0 μmol·h-1·g-1 catalyst).

Silicon and Germanium nanowires (NWs) have shown a strong ability to enhance both the absorption and scattering of light. Tailoring the optical properties of Si or Ge NWs can be obtained by adjusting the nanowire diameter. Another parameter that can be used is the chemical composition of silicon-germanium (Si1-xGex-NWs) alloys. In this work, we perform a numerical study on the optical properties of single Si1-xGex-NWs based on the Lorenz-Mie theory. The effects of Ge composition, light polarization and angle of incidence on the nanowire optical properties are investigated.

SiGe nanowires of different Ge atomic fractions up to 15% were grown and ex-situ n-type doped by diffusion from a solid source in contact with the sample. The phenomenon of dielectrophoresis was used to locate single nanowires between pairs of electrodes in order to carry out electrical measurements. The measured resistance of the as-grown nanowires is very high, but it decreases more than three orders of magnitude upon doping, indicating that the doping procedure used has been effective.

Finemet-type amorphous alloys are good soft magnetic materials due to their amorphous-nanocrystalline structure with close to zero magnetostriction. A very sensitive method for controlling relaxation and crystallization processes in such alloys is proposed. It consists in precise DSC measurements of heat capacity peak in the vicinity of the Curie temperature TC. Time-temperature dependencies of TC for microwires in comparison with ribbon shaped amorphous alloys were studied. Relaxation of atomic structure of amorphous phase during annealing was accompanied by an increase of TC. σ-shaped time dependencies are characterized by at least two relaxation processes, corresponding apparent values of activation energy were estimated. Decomposition of amorphous phase and redistribution of components between amorphous phase and growing nanocrystals affect the shape and position of the DSC peak at TC as well.

Study of microwires with glass coating revealed the influence of internal stress on the shape and position of the Curie peak on DSC curve: increase of internal strain tensions leads to suppression of the TC peak.

The oscillating piezoelectric field of a surface acoustic wave (SAW) is employed to transport photoexcited electrons and holes in GaAs nanowires (NWs) transferred to a SAW beam line on a LiNbO3 crystal. We show that carriers generated in the NW by a focused light spot can be acoustically transported to a second location, where they recombine emitting short light pulses. The results presented here demonstrate the high-frequency manipulation of carriers in NWs without the use of electrical contacts, which opens new perspectives for applications in opto-electronic devices operating at GHz frequencies.

An experimental investigation about the thermoelectric properties of heavily doped p ad n-type nanocrystalline silicon nanowires (NWs) is described. The NWs are produced with low cost CMOS compatible processes, highly customizable in terms of cross-section and placement, which enables the fabrication of both stacked NWs in nearly vertical arrays within nanostructured templates built with SiO2/Si3N4 thin films and individual, freestanding NWs suited for thermal conductivity measurements. The cross-section dimensions of the investigated NWs range between 30 and 120 nm in size and up to about 2 cm in length. The structure of the NWs, as shown by SEM/TEM observations, is nanocrystalline with average size of the nanocrystals in one dimension that is comparable with the nanowire diameter. On the NWs, Seebeck coefficient, electrical resistivity and thermal conductivity have been measured, yielding thermoelectric figure of merit (ZT) values of 0.2 at 300 K for the best case.

Si nanowires (NWs) have been fabricated by Ag-assisted electroless etching technique using an HF/AgNO3 aqueous solution. Scanning electron microscopy (SEM) measurements have revealed that a highly dense array of Si NWs with length of ∼1.4 μm is formed over the surface of both n-type and p-type Si (100) substrates. Following the fabrication of Si NWs, electron-beam evaporated p-type AgGa0.5In0.5Se2 thin film was deposited on the n-type Si NWs to form p-n heterojunction solar cells. The fabricated solar cells yield a 5.50% power conversion efficiency under AM (1.5) illumination.

The use of semiconductor nanowires as new material building blocks for developing original devices is conditioned by the controllability of their growth. An important challenge is to form nanowires which include heterostructures of predictable dimensions. This objective requires a precise knowledge of the growth kinetics which appears much more complex for nanowires than for standard two-dimensional layers. Here, we present a method which provides detailed information on nanowire formation. The method is implemented with InP1-xAsx nanowires grown by Au-catalyzed molecular beam epitaxy. Controlled and periodic modulations of the incident vapor phase are generated. Due to these modulations, the nanowires show small and short oscillations of composition along their growth axis. These oscillations furnish a time scale which is recorded in the nanowire solid phase. The instantaneous growth rate and the total length of individual nanowires at any time of the growth are accessible. Moreover, the distribution of the oscillation lengths contains the nucleation statistics. This statistics is shown to be strongly sub-Poissonian, which indicates that some regulation mechanism operates. The rapid depletion of group V atoms in the catalyst drop which follows the growth of each ML could explain the self-regulation of nucleation events.

Spinel cobalt ferrite nanotubes and nanowires of about five micrometers in length were fabricated using anodic aluminum oxide (AAO) templates of 20 – 200 nm pore diameters and sol-gel processing. A cobalt ferrite sol was prepared by mixing the acetic acid solution of cobalt (II) acetate and ethanol solution of iron (III) acetylacetonate. The templates filled with precursor were obtained after they were dipped into the sol and dried in air. The template/precursor composites were sintered in air at 500 ºC to form cobalt ferrite phase, which was verified by XRD. The morphology of the nanostructures determined by SEM revealed that the cobalt ferrite nanotubes were formed in the channels of 100 nm and 200 nm diameters in the templates, whereas the nanowires were formed in the 20 nm channels of the templates. The magnetic measurement of cobalt ferrite nanowires by a SQUID magnetometer showed that the nanowires are superparamagnetic at room temperature. The room temperature measurement of magnetization versus the applied field on the nanowire arrays in 20 nm channels of templates showed that the coercivity is 1.57 kOe and 1.47 kOe for the nanowire axis parallel and perpendicular to the applied field, respectively, indicating that the nanowire arrays are nearly magnetically isotropic. However, the coercivity of cobalt ferrite nanowires fabricated in this work is much larger than those in the similar systems reported in the literatures.

Silicon Nanowires (Si-NWs) are obtained by vapor-liquid-solid growth using an inductively coupled chemical vapor deposition system which works at temperatures lower than 400 °C. Gold nanodots are used as metal catalyst. The selective growth of Si-NWs on the gold nanodots is obtained by controlling the contribution coming from the uncatalyzed growth on the bare Si substrate. In this way the final NW length can be controlled, and it is not influenced by the thickness of the uncatalyzed layer. The important parameter ruling the NW growth is found to be the plasma power which governs the dissociation of the Si precursor gas. Final NW lengths of 1 μm are obtained at temperatures of 380 °C with a thickness of uncatalyzed layer equal to zero. Also the NW density is addressed in this work and it is optimised by increasing the gold equivalent thickness. The NW density is increased from 2.9×108 to 1.3×1010 cm-2, when the gold equivalent thickness passes from 1.8 nm to 2.2 nm.

Vector magneto-optical generalized ellipsometry on passivated ferromagnetic permalloy slanted columnar thin films is reported. The nanostructured thin film was grown by electron-beam glancing angle deposition and subsequently coated with a thin Al2O3 layer by atomic layer deposition. Magneto-optical generalized ellipsometry data have been acquired while an external magnetic field H with constant amplitude was rotated with respect to the sample (spatial hysteresis loops). The magneto-optical coupling parameters, which are proportional to the sample magnetization M, reveal intriguing anisotropic magnetic behavior of the slanted columnar thin film. Three-dimensional graphs of the magneto-optical coupling parameters with respect to spatial hysteresis loops shown here are representative for the sample magnetization.

Silicon nanowires (NWs) are promising thermoelectric materials as they offer large reductions in thermal conductivity over bulk Si without a significant decrease in the Seebeck coefficient or electrical conductivity. In this work, interference lithography was used to pattern a square lattice photoresist template over 2 cm x 2 cm Si substrates. The resulting vertical Si NW arrays were 1 μm tall with a packing density of ~15%, and the diameter of the Si NWs were 80 - 90 nm. The Si NW arrays were then embedded in spin-on glass (SOG) to form a dense composite material with a measured thermal conductivity of 1.45 W/m-K at 300 K. Devices were fabricated for cross-plane Seebeck coefficient measurements and the Si NW/SOG composite was found to have a Seebeck coefficient of roughly -284 μV/K, which is similar to bulk Si with the same doping. We also report a combined power generation of 29.3 μW from both the Si NW array and Si substrate with a temperature difference of 56 K and 50 μm x 50 μm device area.

The selective formation of porous silicon in nanowires is observed in Si/Ge epitaxial layers along Ge layers grown by molecular beam epitaxy on a Si(100) substrate after metal-assisted chemical etching in aqueous HF-H2O2 solution. We assume that Ge layers serve as channels for a hole current out of the semiconductor to sustain the dissolution reaction. The tunnelling of holes through the potential barrier at the semiconductor surface is assumed to be the dominating mechanism of the hole transfer to the electrolyte.

We present comparative studies of optical properties of GaN nanowires (NWs) obtained by two different self-formation techniques: Plasma-Assisted Molecular Beam Epitaxy (PAMBE) growth; and plasma etching of GaN layers deposited by Metal-Organic Vapor Phase Epitaxy (MOVPE). The effects of the coalescence process on grown NW and plasma-induced defects in etched NWs have been studied by photoluminescence (PL) and Raman scattering. In MBE grown NWs, the coalescence-associated defects are extended toward the NW top for intermediate Ga flux. Using High Resolution Electron Microscopy of reactive plasma etching (RIE) NWs, it was found that NWs obtained with an optimal combination of inductive (ICP) and capacitive (RF) plasma are free of extended structural defects. The PL efficiency is strongly increased in plasma etched NWs. However, plasma-induced point defects have to be taken into account for explaining the changes of the PL spectra. Less plasma-induced degradation is observed for high ICP/RF power ratios.

A hybrid assembly was built using ZnO nanowire (NW) arrays and colloidal CdSe quantum dots (QDs) stabilized by 3-mercaptopropionic acid (MPA). The QDs were chemically linked to the nanowires through the bonds formed between the outgoing carboxyl groups of the QD stabilizers and the zinc ions on the nanowire surface. An efficient clustering attachment of the QDs was achieved via partial removal of the stabilizers of the QDs. The photoconductivity of the NW/QD assembly was investigated by selective excitation of the CdSe QDs. Oxygen desorption from the nanowire surface enhances the photoconductivity and a model involving electron transfer between the QDs and the nanowires is proposed to explain the experimental results.

Al2O3 and AlN nanotubes were fabricated by depositing conformal thin films via atomic layer deposition (ALD) on electrospun nylon 66 (PA66) nanofiber templates. Depositions were carried out at 200°C, using trimethylaluminum (TMAl), water (H2O), and ammonia (NH3) as the aluminum, oxygen, and nitrogen precursors, respectively. Deposition rates of Al2O3 and AlN at this temperature were ∼1.05 and 0.86 Å/cycle. After the depositions, Al2O3- and AlN-coated nanofibers were calcinated at 500°C for 2 h in order to remove organic components. Nanotubes were characterized by using X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). AlN nanotubes were polycrystalline as determined by high resolution TEM (HR-TEM) and selected area electron diffraction (SAED). TEM images of all the samples reported in this study indicated uniform wall thicknesses.