CdTe Nanowires by Au-Catalyzed Metalorganic Vapor Phase Epitaxy

Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE

The fabrication of high-efficiency GaAsP-based solar cells on GaAs wafers requires addressing structural issues arising from the materials lattice mismatch. We report on tensile strain relaxation and composition control of MOVPE-grown As-rich GaAs_1-xP_x/(100)GaAs heterostructures studied by double-crystal X-ray diffraction and field emission scanning electron microscopy. Thin (80-150 nm) GaAs_1-xP_x epilayers appear partially relaxed (within 1-12% of the initial misfit) through a network of misfit dislocations along the sample [011] and [011-] in plane directions. Values of the residual lattice strain as a function of epilayer thickness were compared with predictions from the equilibrium (Matthews-Blakeslee) and energy balance models. It is shown that the epilayers relax at a slower rate than expected based on the equilibrium model, an effect ascribed to the existence of an energy barrier to the nucleation of new dislocations. The study of GaAs_1-xP_x composition as a function of the V-group precursors ratio in the vapor during growth allowed for the determination of the As/P anion segregation coefficient. The latter agrees with values reported in the literature for P-rich alloys grown using the same precursor combination. P-incorporation into nearly pseudomorphic heterostructures turns out to be kinetically activated, with an activation energy E_A = 1.41 ± 0.04 eV over the entire alloy compositional range.

Single-Atom-Kernelled Nanocluster Catalyst

To propose the concept of single-atom-kernelled nanocluster, we synthesized a Pd-based trimetal nanocluster with a single-Ag atom-kernel for the first time by introducing some steric hindrance factors and employing a joint alloying strategy that combines the coreduction with an antigalvanic reduction (AGR). Although the AGR-derived Pd-based trimetal nanoclusters with single-silver atom kernels have low contents of gold, they show higher activity and selectivity than those of the bimetal precursor nanocluster in the electrocatalytical reduction of CO₂ to CO. Furthermore, it is revealed that the kernel single atoms from both Au₄Pd₆(TBBT)₁₂ and Au₃AgPd₆(TBBT)₁₂ are not the active sites for catalysis, but greatly influence the catalytical performance by effecting the electronic configuration. Thus, it is demonstrated that the single-atom-kernelled nanocluster can not only improve the precious metal utilization (even to 100%) but also better the properties and provide insight into the structure-property correlation for metal nanoclusters.

Bimetallic PdCu Nanoparticles for Electrocatalysis: Multiphase or Homogeneous Alloy?

Crystal phase structure of bimetallic alloy is an important factor determining the electrocatalytic activity toward oxidation of energy molecules. In this paper, PdCu bimetallic NPs with similar element composition and different crystal phase structural features have been synthesized hydrothermally by adjusting the content of ethylenediaminetetraacetic acid disodium salt (EDTA-2Na). Multiphase PdCu NPs composed of pure Pd and alloy phase are obtained with a low concentration (even as low as zero) of EDTA-2Na in synthetic systems while homogeneous PdCu alloy NPs are formed in the presence of EDTA-2Na with a high concentration. The catalytic activity of ethanol electrooxidation is increased from 3.1 mA·cm^-2 of pure Pd NPs, to 3.6 mA·cm^-2 of multiphase PdCu NPs, and to 5.0 mA·cm^-2 of homogeneous PdCu alloy NPs (about 2360 mA mg_Pd^-1). The surface composition and structural stability of homogeneous PdCu NPs were much less damaged during electrochemical measurements. Based on the experimental data, the formation mechanism of multiphase and homogeneous PdCu NPs and their structure-property relationship have been discussed.

Nanowire Electronics: From Nanoscale to Macroscale

Semiconductor nanowires have attracted extensive interest as one of the best-defined classes of nanoscale building blocks for the bottom-up assembly of functional electronic and optoelectronic devices over the past two decades. The article provides a comprehensive review of the continuing efforts in exploring semiconductor nanowires for the assembly of functional nanoscale electronics and macroelectronics. Specifically, we start with a brief overview of the synthetic control of various semiconductor nanowires and nanowire heterostructures with precisely controlled physical dimension, chemical composition, heterostructure interface, and electronic properties to define the material foundation for nanowire electronics. We then summarize a series of assembly strategies developed for creating well-ordered nanowire arrays with controlled spatial position, orientation, and density, which are essential for constructing increasingly complex electronic devices and circuits from synthetic semiconductor nanowires. Next, we review the fundamental electronic properties and various single nanowire transistor concepts. Combining the designable electronic properties and controllable assembly approaches, we then discuss a series of nanoscale devices and integrated circuits assembled from nanowire building blocks, as well as a unique design of solution-processable nanowire thin-film transistors for high-performance large-area flexible electronics. Last, we conclude with a brief perspective on the standing challenges and future opportunities.

Self-Catalyzed CdTe Wires

CdTe wires have been fabricated via a catalyst free method using the industrially scalable physical vapor deposition technique close space sublimation. Wire growth was shown to be highly dependent on surface roughness and deposition pressure, with only low roughness surfaces being capable of producing wires. Growth of wires is highly (111) oriented and is inferred to occur via a vapor-solid-solid growth mechanism, wherein a CdTe seed particle acts to template the growth. Such seed particles are visible as wire caps and have been characterized via energy dispersive X-ray analysis to establish they are single phase CdTe, hence validating the self-catalysation route. Cathodoluminescence analysis demonstrates that CdTe wires exhibited a much lower level of recombination when compared to a planar CdTe film, which is highly beneficial for semiconductor applications.

Growth of wurtzite CdTe nanowires on fluorine-doped tin oxide glass substrates and room-temperature bandgap parameter determination

The growth of CdTe nanowires, catalyzed by Sn, was achieved on fluorine-doped tin oxide glass by physical vapor transport. CdTe nanowires grew along the 〈0001〉 direction, with a very rare and phase-pure wurtzite structure, at 290 °C. CdTe nanowires grew under Te-limited conditions by forming SnTe nanostructures in the catalysts and the wurtzite structure was energetically favored. By polarization-dependent and power-dependent micro-photoluminescence measurements of individual nanowires, heavy and light hole-related transitions could be differentiated, and the fundamental bandgap of wurtzite CdTe at room temperature was determined to be 1.562 eV, which was 52 meV higher than that of zinc-blende CdTe. From the analysis of doublet photoluminescence spectra, the valence band splitting energy between heavy hole and light hole bands was estimated to be 43 meV.