Shufang Ma

Synthesis of large-scale GaN nanobelts by chemical vapor deposition

GaN nanobelts were synthesized in mass by direct reaction of metallic gallium with flowing ammonia using nickel as a catalyst. X-ray diffraction, field emission scanning electron microscope equipped with an energy dispersive spectroscopy, and high resolution transmission electron microscopy were used to characterize the products. The results showed that the products are of hexagonal wurtzite structure. The widths of nanobelts were in the range of 50–200nm with the thickness ranging from 3to10nm and their lengths up to several tens of microns. The catalytic growth mechanism of the GaN nanobelts was also discussed.

Synthesis, characterization and growth mechanism of flower-like vanadium carbide hierarchical nanocrystals

Novel flower-like vanadium carbide (V8C7) hierarchical nanocrystals have been successfully synthesized in a large-scale hydrothermal process using a mixture of diethanolamine (HN(C2H4OH)2, DEA) and V2O5 powder. The products were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), selected-area electron diffraction (SAED) and energy-dispersive spectra (EDS). The results showed that flower-like products consist of very thin sheets with an average thickness of 10 nm and the nanosheets were a uniform single-crystalline V8C7 phase. It was found that reaction temperature and type of capping molecules have a crucial influence on the morphology of the products. A possible growth mechanism of the flower-like V8C7 hierarchical nanocrystals is proposed and discussed.

Preparation and characterization of SiC@CNT coaxial nanocables using CNTs as a template

Novel one-dimensional SiC@carbon nanotube (CNT) coaxial nanocables have been successfully fabricated on a large scale by using a carbothermal chemical vapor deposition method. Sol–gel-derived silica xerogels containing commercial multi-wall carbon nanotubes (MWCNT) were used as silicon and carbon sources. The obtained product was characterized by SEM, HRTEM, Raman spectroscopy and XRD. The coaxial nanocables have been found to be composed of a 40–100 nm diameter carbon nanotube as the core, surrounded by a thick SiC outlayer. The inner nanotube corresponded to the multi-walls of the carbon nanotube with a lattice spacing of 0.34 nm. The PL spectrum revealed that the SiC@CNT nanocables have two broad emission bands centered at 461 nm and 573 nm which can be attributed to the quantum confinement effect and the morphology effects. The morphology of the product was tuned by simply altering the reaction temperature. In the formation of SiC@carbon nanotube coaxial nanocables, it was proposed that CNTs acted as a template to confine the reaction, which resulted in the continuous SiC outlayer growth on the CNT surface to form SiC@CNT coaxial nanocables.

The evolution of a GaN/sapphire interface with different nucleation layer thickness during two-step growth and its influence on the bulk GaN crystal quality

The role of the nucleation layer thickness on the GaN crystal quality grown by metal organic chemical vapor deposition is explored. The surface morphologies of a low-temperature GaN nucleation layer (NL) investigated by Atomic Force Microscopy shows the nuclei grain size increases with increasing thickness. After annealing, island-like morphologies of the low-temperature GaN NL are obtained. Increasing the NL thickness is beneficial for obtaining larger island size, however, the uniformity of the island size is deteriorated. The high-resolution X-ray diffraction analysis reveals that bulk GaN crystal properties are closely connected with NL thickness, which can be well explained by the dislocation generation and propagation process in the GaN films. All the obtained results indicate that the NL thickness effectively controls the size and density of the islands and thus determines the crystal properties of GaN films.

Superior Mechanical Properties of AlCoCrFeNiTi x High-Entropy Alloys upon Dynamic Loading

High-entropy alloys with composition of AlCoCrFeNiTix (x: molar ratio; x = 0, 0.2, 0.4) under quasi-static and dynamic compression exhibit excellent mechanical properties. A positive strain-rate sensitivity of yield strength and the strong work-hardening behavior during plastic flows dominate upon dynamic loading in the present alloy system. The constitutive relationships are extracted to model flow behaviors by employing the Johnson-Cook constitutive model. Upon dynamic loading, the ultimate strength and fracture strain of AlCoCrFeNiTix alloys are superior to most of bulk metallic glasses and in situ metallic glass matrix composites.

Investigation of the growth temperature on indium diffusion in InGaAs/GaAsP multiple quantum wells and photoelectric properties

InGaAs/GaAsP multiple quantum wells (MQWs) structures were grown by metal–organic chemical vapor deposition and the effect of the growth temperature on the interfacial crystal quality was characterized by high-resolution X-ray diffraction and photoluminescence. The surface roughness of MQWs was measured by atomic force microscopy for evaluating the surface quality. The existence of an indium diffusion zone (InGaAsP) between InGaAs and GaAsP was demonstrated by secondary ion mass spectrometry profiles in the growth direction. The results suggest the different diffusion widths originate from the growth temperature variation. A smoother surface and higher quality interface of MQWs was obtained at 650 °C growth temperature. Furthermore, the phenomena of current self-oscillations were confirmed through current–voltage measurements at room temperature, which can be attributed to the negative differential resistance effect. The influence of the growth temperature on the crystal quality of InGaAs/GaAsP MQWs was used for the optimization of technological parameters.