Controlled synthesis of AlN/GaN multiple quantum well nanowire structures and their optical properties.

We report the controlled synthesis of AlN/GaN multi-quantum well (MQW) radial nanowire heterostructures by metal-organic chemical vapor deposition. The structure consists of a single-crystal GaN nanowire core and an epitaxially grown (AlN/GaN)(m) (m = 3, 13) MQW shell. Optical excitation of individual MQW nanowires yielded strong, blue-shifted photoluminescence in the range 340-360 nm, with respect to the GaN near band-edge emission at 368.8 nm. Cathodoluminescence analysis on the cross-sectional MQW nanowire samples showed that the blue-shifted ultraviolet luminescence originated from the GaN quantum wells, while the defect-associated yellow luminescence was emitted from the GaN core. Computational simulation provided a quantitative analysis of the mini-band energies in the AlN/GaN superlattices and suggested the observed blue-shifted emission corresponds to the interband transitions between the second subbands of GaN, as a result of quantum confinement and strain effect in these AlN/GaN MQW nanowire structures.

Growth mechanism of GaN nanowires: preferred nucleation site and effect of hydrogen.

The growth mechanism of epitaxial GaN nanowires grown using particle-mediated chemical vapour deposition was investigated. By examining the diameter-dependent growth rate of GaN nanowires, we show that the kinetic reaction-limited growth of GaN nanowires originates from the combination of mono-nuclear and poly-nuclear growth rather than the Gibbs-Thompson effect. We present a generalized nucleation-mediated growth model to describe the diameter dependence of the nanowire growth rate and show that the nucleation of sources occurs at the vapour/liquid/solid three-phase boundary. From the same model, we demonstrate that increased hydrogen concentration in the carrier gas reduces the supersaturation, leading to a reduced GaN nanowire growth rate. Our approach can be applied to other nanowire materials systems, and it allows the determination of the preferred nucleation site during nanowire growth.

Position controlled nanowire growth through Au nanoparticles synthesized by galvanic reaction.

Semiconductor nanowires have emerged as promising materials for fundamental studies in quantum-confined systems and applications in nanophotonics and electronics, but major challenges remain in controlling nanowire properties, including their position and size. Here, we report a simple and efficient electrochemical process that combines galvanic reaction and electron-beam lithography to selectively synthesize gold nanoparticles that are consequently used for the growth of ordered GaAs nanowire arrays with pre-defined diameter and position. Size and density control of gold nanoparticles is achieved on non-patterned GaAs substrates by changing the reaction time and concentration of Au(3 + ) ions during the galvanic reaction. Spontaneous formation of localized etch pits is observed when the galvanic reaction is constrained to lithography-defined substrate regions, which confines small-diameter Au nanoparticles during the high temperature growth of GaAs nanowire arrays and enables epitaxial growth of well-ordered nanowire structures.

Direct correlation between structural and optical properties of III-V nitride nanowire heterostructures with nanoscale resolution.

Direct correlation of structural and optical properties on the nanoscale is essential for rational synthesis of nanomaterials with predefined structure and functionality. We study optical properties of single III-V nitride nanowire radial heterostructures with measured spatial resolution of <20 nm using cathodoluminescence (CL) technique coupled with scanning transmission electron microscopy (STEM). Enhanced carrier recombination in nanowire quantum wells and reduced light emission from regions containing structural defects were directly observed. Using newly developed parallel-detection-mode CL-STEM, we show that optical properties can vary within a single nanowire heterostructure as a function of nanowire morphology.

Controlled growth of ternary alloy nanowires using metalorganic chemical vapor deposition.

We report the growth and characterization of ternary AlxGa1- xAs nanowires by metalorganic chemical vapor deposition as a function of temperature and V/III ratio. Transmission electron microscopy and energy dispersive X-ray spectroscopy show that, at high temperatures and high V/III ratios, the nanowires form a core-shell structure with higher Al composition in the nanowire core than in the shell. We develop a growth model that takes into account diffusion of reactants and decomposition rates at the nanowire catalyst and stem to describe the compositional difference and the shell growth rate. Utilizing this model, we have successfully grown compositionally uniform Al0.16Ga0.84As nanowires. The ability to rationally tune the composition of ternary alloy nanowires broadens the application range of nanowires by enabling more complex nanowire heterostructures.

Mono-layer of Ni(100-x)Fe(x) nanoparticles fabricated on a polyimide film under different curing atmospheres.

A mono-layer of nano-sized metal particles was prepared on the surface of a polyimide film by simply depositing a thin film of Ni80Fe20 on top of the polyamic acid that was spin coated onto a Si wafer. During thermal imidization of the polyamic acid film, Fe was selectively etched by reacting with the carbonyl group of the polyamic acid to leave behind uniformly distributed Ni-rich metallic particles. The average diameter of the particles was 4 nm and the particles were confined into a single layer on top of the polymer film. Moreover, it was also shown that the morphology of the nanoparticles can be substantially altered by curing the precursor film in a hydrogen atmosphere, without significantly damaging the polymer film. Thus produced nanoparticles lay exposed on top of the electrically insulating and chemically stable polymer film so that it is possible that the nanoparticles can be directly used for fabricating a nonvolatile flash memory device or as a template for building functional nano-structures.

Nanoparticles fabricated by selective reaction of Fe100-xPtx alloy films during imidization of polyamic acid.

Nanoparticles with different morphology and composition were fabricated inside a polyimide (PI) matrix based on selectively oxidizing a layer of Fe(100-x)Pt(x) alloy metal film sandwiched between two PI precursor layers. Gamma-Fe2O3, Pt, and Fe3Pt nanoparticles were formed in a monolayer between two PI layers, depending on the alloy film composition and curing conditions. These particles were well-crystallized and sized between 4 and 10 nm. X-ray photoelectron spectroscopy confirmed that Fe in the film preferentially reacted with the organic matrix whereas Pt remained metallic throughout the curing process, which enabled fabrication of particles different morphology and composition. This process can be easily extended to other alloy films, which provides an opportunity to fabricate nanoparticles relatively easily with desired composition and morphology embedded in an inert organic matrix.

Synthesis of iron oxide nanoparticles in a polyimide matrix.

A monolayer of gamma-Fe(2)O(3) nanoparticles embedded in a polyimide (PI) matrix was fabricated by oxidizing an Fe metal film between two PI precursor layers. There was a critical Fe thickness ( approximately 7 nm) above which a continuous layer of gamma-Fe(2)O(3) film was formed in the PI film. Below the critical Fe thickness, the oxide film broke up into fine particles whose size was approximately 8 nm with narrow size distribution. It was further shown that these nanoparticles could have metallic cores, surrounded by an oxide layer. This method offers a unique way of covering a large surface area with fine magnetic oxide nanoparticles for potential application in high-density data-storage media.

Fabrication of Ni nanoparticles by selective oxidation of permalloy thin film during imidization of polyamic acid.

Ni nanoparticles embedded in a polyimide (PI) matrix were fabricated by selectively oxidizing a layer of Ni(80)Fe(20) metal film sandwiched between two PI precursor layers. Ni nanoparticles, formed in a monolayer between two PI layers, had an average particle size of approximately 5 nm. X-Ray photoelectron spectroscopy confirmed that Fe in the film was preferentially consumed, resulting in the formation of Ni nanoparticles.