Suikong Hark

Aligned ZnO/CdTe Core−Shell Nanocable Arrays on Indium Tin Oxide: Synthesis and Photoelectrochemical Properties

Vertically aligned ZnO/CdTe core-shell nanocable arrays-on-indium tin oxide (ITO) are fabricated by electrochemical deposition of CdTe on ZnO nanorod arrays in an electrolyte close to neutral pH. By adjusting the total charge quantity applied during deposition, the CdTe shell thickness can be tuned from several tens to hundreds of nanometers. The CdTe shell, which has a zinc-blende structure, is very dense and uniform both radially and along the axial direction of the nanocables, and forms an intact interface with the wurtzite ZnO nanorod core. The absorption of the CdTe shell above its band gap ( approximately 1.5 eV) and the type II band alignment between the CdTe shell and the ZnO core, respectively, demonstrated by absorption and photoluminescence measurements, make a nanocable array-on-ITO architecture a promising photoelectrode with excellent photovoltaic properties for solar energy applications. A photocurrent density of approximately 5.9 mA/cm(2) has been obtained under visible light illumination of 100 mW cm(-2) with zero bias potential (vs saturated calomel electrode). The neutral electrodeposition method can be generally used for plating CdTe on nanostructures made of different materials, which would be of interest in various applications.

Temperature dependent photoluminescence study on phosphorus doped ZnO nanowires

We report temperature dependent photoluminescence studies on phosphorus doped ZnO nanowires. The shape of the spectra is very similar to those of phosphorus doped ZnO films. The photoluminescence spectrum at 10K is dominated by neutral acceptor bound exciton (AX0) emissions. The acceptor binding energy determined also agrees with the corresponding value in phosphorus doped films. Studies on the AX0 intensity show two quenching channels, associated with the thermal dissociations of AX0 to a free exciton and of shallow residual donors. The residual donors revealed provide a clue for the difficulty in p doping of ZnO.

The epitaxial growth of ZnS nanowire arrays and their applications in UV-light detection

Compared to random nanowires, vertically aligned nanowire arrays (NWAs) are especially useful as functional parts in future nanowire based devices. Here, vertically aligned ZnS NWAs have been grown on the GaAs (111)B substrates by metal–organic chemical vapor deposition. Ga was chosen as the catalyst and dopant of the ZnS nanowires during growth. The density of nanowires was controlled and their diameter was slightly tuned. The NWAs have been assembled into a photodetector by a low cost and effective method. An individual ZnS nanowire photodetector has also been fabricated for comparison. The two photodetectors are sensitive to UV but not to visible light. Under the same conditions, the photocurrent of the NWA photodetector is more stable, and five orders larger than that of the nanowire photodetector. In addition, the ZnS NWA photodetector has a fast response and an excellent repeatability, showing its potential for fast, visible-blind, UVA detection.

Structure control of CdS nanobelts and their luminescence properties

Single crystalline CdS (hexagonal) nanobelts have been synthesized using thermal evaporation at temperatures ranging from 1000to1200°C. These nanobelts universally grow along the [120] crystalline direction. The nanobelts fabricated at 1000°C appear to be much more uniform than those at 1200°C. Moreover, the surface of the nanobelts grown at 1200°C is rough compared to that of the former, which is due to the secondary growth of CdS crystallites on the nanobelts’ surfaces at higher temperatures, as induced by the surface polarity. Cathodoluminescence and photoluminescence studies disclose the different electronic structure qualities of the two samples. The growth mechanisms of the nanobelts and the luminescence differences of the two samples are discussed.Single crystalline CdS (hexagonal) nanobelts have been synthesized using thermal evaporation at temperatures ranging from 1000to1200°C. These nanobelts universally grow along the [120] crystalline direction. The nanobelts fabricated at 1000°C appear to be much more uniform than those at 1200°C. Moreover, the surface of the nanobelts grown at 1200°C is rough compared to that of the former, which is due to the secondary growth of CdS crystallites on the nanobelts’ surfaces at higher temperatures, as induced by the surface polarity. Cathodoluminescence and photoluminescence studies disclose the different electronic structure qualities of the two samples. The growth mechanisms of the nanobelts and the luminescence differences of the two samples are discussed.

Self-catalytic growth of single-phase AlGaN alloy nanowires by chemical vapor deposition

Wurtzite structured AlGaN alloy nanowires over the entire compositional range have been grown on silicon (100) substrates by chemical vapor deposition using NH3 vapor and a mixture of Al powders and molten Ga droplets. The morphology, structure, and compositions were characterized by scanning and transmission electron microscopies, energy dispersive x-ray analysis, and micro-Raman scattering. The nanowires grow along the ⟨0001⟩ direction and remain as a single phase alloy, despite their small diameter. At comparable diameters, previous studies had shown that AlGaN nanowires would have spontaneously phase separated. The growth of the nanowires is suggested as via a self-catalytic vapor-liquid-solid mechanism.

High-yield synthesis of In2−xGaxO3(ZnO)3 nanobelts with a planar superlattice structure

Homologous compound In2−xGaxO3(ZnO)3 (x ≈ 0.18) nanobelts were successfully synthesized in high yield by a simple thermal vapour method for the first time. Two sets of peaks appear in X-ray diffraction pattern. One is attributed to wurtzite ZnO, while the other is indexed to rhombohedral In2−xGaxO3(ZnO)3 (x ≈ 0.18) with lattice constants a = 0.335 nm and c = 4.24 ± 0.01 nm within experimental errors. Cross-section high-resolution transmission electron microscopy observations and XRD data confirmed the formation of a planar superlattice structure in the nanobelts, which consists of alternate stacking of In–O layers and In1−xGax/Zn–O blocks along the [0001] direction, which is normal to the growth direction, leading to the (0001) plane as a surface.

Preparation of ZnO/In2O3(ZnO)n heterostructure nanobelts

Bent In2O3(ZnO)n/ZnO (n = 4 and 5) heterostructure nanobelts were successfully synthesized by a simple thermal vapor method for the first time, using Au as a catalyst. The heterostructures were characterized by transmission electron microscopy and nanoprobe X-ray energy dispersive. The relationship between the strain induced by lattice mismatch and the radius of curvature of the nanobelts with heterostructure is discussed in terms of geometric mathematics. A formation mechanism of the heterostructures is suggested. Such a structure could be an ideal model system for exploring novel phenomena with potential applications and studying size and dimension dependent properties at the nanoscale.

Further evidence for the quantum confined electrochemistry model of the formation mechanism of p−‐type porous silicon

Two types of p− porous silicon (PS) were formed in HF solutions of different concentrations. One type with nanoscale (NS) dimensions of about 3 nm and the other with dimensions of about 5 nm. PS samples formed in the lower concentration of HF were anodized again in the higher concentration of HF and vice versa. The photoluminescence peak position and, thus, the size of NS units of PS were found to be related to the concentration of HF in which the PS is formed, independent of the forming time. The larger NS units of PS can be further electrochemically etched by anodization, while the smaller ones cannot. These results give a confirming evidence for the quantum confined electrochemistry model of the formation mechanism of PS based on the quantum confinement effect and classical electrochemical theory [S. L. Zhang, K. S. Ho, Y. T. Hou, B. D. Qian, P. Diao, and S. M. Cai, Appl. Phys. Lett. 62, 642 (1993)].

Fabrication and optical properties of vertically aligned ZnSe nanowire arrays catalyzed by Ga and Ag

Vertically aligned ZnSe nanowire arrays have been grown on GaAs(111)B substrates by metal–organic chemical vapor deposition using Ga and Ag catalysts. Regardless of the catalysts, the nanowires are of zinc blende structure and grow along the 〈111〉B direction. However, the catalysts were found to affect the optical properties of the arrays. The dominant peak in the near-band-edge emission spectrum (10 K) of Ga-catalyzed nanowires is attributed to excitons bound to Ga donors and the corresponding peak in the spectrum of Ag-catalyzed nanowires to Ag acceptor complexes. Thus, Ga and Ag can serve both as an effective catalyst and a dopant for the growth of ZnSe nanowires.

Synthesis, micro-structural and magnetic properties of Mn-doped ZnO nanowires

Single-crystalline Mn-doped ZnO nanowires of different compositions were synthesized by chemical vapor deposition. Their micro-structures were characterized by high-resolution transmission electron microscopy and X-ray diffraction. The results reveal that Mn doping leads to production of planar and point defects, lattice expansion and disorder. These micro-structural features are also reflected by characteristic changes in the Raman spectra of the nanowires, such as the red-shift and asymmetric broadening of the E2H mode, the appearance of an additional mode, and the anomalous enhancement of the E1(LO) mode. All the doped nanowires show ferromagnetism with a Curie temperature above 350 K. The small effective magnetic moment of ∼0.02 μB/Mn suggests the presence of isolated Mn ions and/or antiferromagnetic superexchange interactions. The long-range ferromagnetic interaction is understood in terms of two possible mechanisms, including the percolation of bound magnetic polarons and surface-hole-mediated ferromagnetic coupling.