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Herein, we report the self-aligned growth of GaN nanorods on different orientations of sapphire like c-, a-, r- and m-plane substrates by hydride vapor phase epitaxy. Vertical c-axis orientation of GaN NRs is obtained on c-plane [0001] and a-plane $$ \left[ { 1 1\bar{2}0} \right] $$ sapphire and a skew or inclined NRs on r-plane, and inclined intertwined but self-aligned NR array was formed on m-plane sapphire. GaN (002) and (004) peaks were obtained on c- and a-plane sapphire, whereas (110), (103), and (103) only were observed on r- and m-planes, respectively. In the case of r- and m-plane-grown GaN, A1 transverse optical mode is dominant, and the A1 longitudinal optical mode is suppressed. Conversely, in the case of c- and a-plane, it is reversed. The probable reason is the optical mode vibrations difference along the differently inclined NRs surfaces. In addition, the specimen exhibits surface optical modes too. The optical behavior of GaN NR on m-sapphire shows an intensity variation when measured in different angular rotations of the specimen by photoluminescence which is because of the higher area of excitation in the case of axial surfaces and lower area of excitation in radial surface. Their epitaxial crystallographic relationship with the substrates and the reasons for the self-aligned orientations are discussed. The anomalies found in the optical behavior are attributed to Raman antenna effect and so on. The self-aligned intertwined GaN NRs find suitable applications in polarizer.

We have investigated the structural properties of AlGaSb films grown on Si (1 0 0) substrates with multi-buffer layers (GaSb/AlSb/GaSb) inserting a thin AlSb and InSb quantum dots (QDs) inter-buffer layer. Atomic force microscopy measurements of the buffer layers with the QDs inter-buffer layer showed the mirror-like surface with a low defect density. Also, the surface roughness of a resulting AlGaSb film on the buffer layers with the QDs inter-buffer layer is decreased by a factor of about 10 compared with the roughness of the film without the QDs interlayer. In addition, X-ray diffraction rocking curve result showed that the structural properties of AlGaSb layer with the QDs interlayer were improved significantly. We suggest that the significant reduction of the dislocation density in the AlGaSb film was due to the dislocations being prevented from propagating into the AlGaSb overlayer by the thin AlSb and InSb QDs interlayer.

layers showed that the surface had uniformly-sized quantum dots with a low defect density. The surface roughness of a GaSb film with a thin GaSb interlayer grown at a low temperature was decreased by a factor of about 5 compared with the roughness of the GaSb film without the thin GaSb interlayer. In addition, double-crystal X-ray diraction and photoluminescence results showed that the structural and the optical properties of the GaSb layer with the GaSb interlayer were improved significantly. We suggest that the significant reduction of the dislocation density in the GaSb film was due to the dislocations being prevented from propagating into the GaSb overlayer by the thin GaSb interlayer.

Dislocation defects in GaN epilayers grown on Si substrates were investigated using cathodoluminescence (CL) and atomic force microscopy (AFM). The dislocation densities of the films were deduced from two-dimensional CL image measurements and ranged from 1 × 10 cm−2 to 5 × 10 cm−2. These values are larger than those reported for GaN epilayers grown on sapphire substrates. The surface morphology and dislocations in the GaN epilayers were investigated by using AFM. From a comparative study of the CL and the AFM measurements, the GaN epilayers with lower dislocation densities are seen to have good surface morphologies with smaller densities of surface defects.

The variation of the strain and structural properties of InN layers grown by molecular beam epitaxy on AlN/Si(1 1 1) substrates were investigated using reflection high-energy electron diffraction (RHEED), atomic force microscopy, scanning electron microscopy, photoluminescence, and X-ray diffraction. The RHEED intensity, the thickness of the InN wetting layer, and the lattice constant of the InN during its initial growth stage were found to be most dependent on the indium flux. Although when using a high indium flux and high growth temperature, growth would be expected to follow the Volmer–Weber growth mode during the early growth stages, in the case of the nitrogen-rich conditions, the initial InN layer grows according to the Stranski–Krastanov mode. The emission peaks are present at 0.83 eV (1500 nm) and 0.79 eV (1569 nm) for the samples made using low and high indium fluxes, respectively. These results provide important information about single-crystal hexagonal InN.

Morphological evolution of Si nanowires (Si-NWs) grown on Si (001) substrates is explored. The Si-NWs are fabricated by nanoscale Au-Si island-catalyzed rapid thermal chemical vapor deposition. The Au-Si islands (10-50 nm in dia.) are formed by deposition of Au thin film (1.2-3.0 nm) at room temperature and followed by annealing at 700oC. The Si-NWs are grown by exposure them to a mixture of gasses of SiH4 and H2. We found a critical thickness of the Au film for Si-NW nucleation at a given growth condition. Also, we observed variation in the growth rate and the dimension of the NWs depending on the growth pressure and temperature. The resulting NWs are ~30-100nm in diameter and ~0.4-5.0μm in length. Most of the NWs were aligned along the direction. The morphological and dimensional evolution of the Si-NWs is discussed in terms of kinetics (atomic diffusion mechanism) and energetics (surface and interface energies).

Liquid-phase exfoliation is likely to be feasible for practical fabrication of few-layer MoS2 nanosheets in large quantities. However, this method generally involves the organic solvents with high boiling point; new strategy using low-boiling-point solvents to obtain high MoS2 concentration is still highly required. In this study, using the strategy of Hansen solubility parameters (HSP), a method based on exfoliation of MoS2 in chloroform/acetonitrile mixtures is demonstrated to fabricate high concentration MoS2 nanosheet solution. The highest concentration of few-layer MoS2 nanosheets and nanoparticles up to 0.4 mg/ml is achieved with the optimum composition of mixture. The MoS2 nanosheet thin film is also investigated in terms of their sensing properties towards humidity. The exfoliated MoS2 based thin film sensor exhibited excellent sensitivity, quick response and recovery, and good reproducibility comparing to their bulk counterpart. The excellent sensing performance of exfoliated MoS2 is generally attributed to the high surface-to-volume-ratio and increased ratio of edge sites and basal plane sites after exfoliation.

Adsorption equilibria of two amino acids (phenylalanine and tryptophan) and two antibiotics (penicillin G and cephalosporin C) from aqueous solutions onto non-ionic polymeric sorbents (XAD-4 and XAD-16) were investigated under various experimental conditions such as pH, temperature and organic solvents. The assumption that amino acids adsorbed on polymeric sorbents were desorbed by competitive adsorption with organic solvent as a desorbate was verified using binary adsorption data for amino acids (phenylalanine and tryptophan) and organic solvents (isopropyl alcohol and methanol) on XAD-4 and XAD-16. The experimental data were predicted by using multicomponent adsorption models of an Extended-Langmuir (EL) equation and an ideal adsorbed solution theory (IAST) based on the Langmuir equation as a single-component isotherm. Copyright © 2004 Society of Chemical Industry

A separation technique for amino acids, phenylalanine and tryptophan, from aqueous solution was studied in a column that was packed with a polymeric resin, XAD-16. This technique is based on a cyclic operation that has three typical steps such as adsorption, desorption, and washing. In particular, the desorption step for amino acids from the resin was carried out by using organic solvents, isopropyl alcohol and methanol. The desorption mechanism was assumed to be a competitive adsorption between amino acids and solvents, and the ideal adsorbed solution theory (IAST) based on the Langmuir equation as a single component isotherm was used in describing multicomponent equi- libria. Adsorption and desorption breakthrough curves of the two amino acids were measured under various experi- mental conditions such as concentration, flow rate, and column length, in order to check the feasibility of the resin as a medium for the separation of amino acids. It was found that this separation technique could be a promising one for this purpose. Also, a simple dynamic model was formulated to describe both adsorption and desorption breakthrough curves of amino acids.

Photoluminescence (PL) measurements performed on as-grown, hydrogenated, and annealed n-type ZnO bulk samples investigated the origins of their yellow (2.10 eV) and green (2.43 eV) emission bands. After hydrogenation, the defect-related peak at 2.10 eV was no longer present in the room temperature PL spectrum, the peak intensity at 2.43 eV was unchanged, and the intensity of the emission peak at 3.27 eV increased significantly. These results indicate that yellow band emission is due to oxygen vacancies, as the emission peak at 2.10 eV disappears when hydrogen atoms passivate these vacancies. The emission peak at 2.43 eV originates from complexes between oxygen vacancies and other crystal defects. We discuss the shallow donor impurities arising due to these hydrogen atoms in the ZnO bulk sample.

The surface potential variations on ZnO nanocrystal surfaces were investigated by using Kelvin probe force microscopy. It was found that the nanocrystal surface contains randomly distributed circular pits with a diameter of ~100 nm and a depth of ~30 nm and that the surface potential within the pits is lower than that in the vicinity of the pits. We discuss the correlation between the observed pits and speck-like defects on a ZnO nanocrystal surface associated with the emission at 3.31 eV. We suggest that the reduced surface potential is caused by the local acceptor-like states concentrated near the structural defects.

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