Zhiqiang Qi

Low defect concentration few-layer graphene using a two-step electrochemical exfoliation

Low defect concentration few-layer graphene (FLG) sheets were fabricated by a two-step electrochemical intercalation exfoliation, including a graphite foil pretreatment in sodium hydroxide solution and a subsequent further exfoliation in sulfuric acid solution. During the process, the pretreatment results in the expansion of the graphite foil and in turn facilitates the final exfoliation in sulfuric acid solution. The results show that the I(D)/I(G) of the obtained FLG sheets is as low as 0.29 while maintaining relatively high yield, more than 56%. In addition, the oxygen content in the FLG sheets is 8.32% with the C/O ratio of 11.02.

Large-scale growth of density-tunable aligned ZnO nanorods arrays on GaN QDs

An effective approach for growing large-scale, uniformly aligned ZnO nanorods arrays is demonstrated. The synthesis uses a GaN quantum dot (QD) template produced by a self-assembled Stranski-Krastanow mode in metal organic chemical vapor deposition, which serves as a nucleation site for ZnO owing to the QDs high surface free energy. The resultant ZnO nanorods with uniform shape and length align vertically on the template, while their density is easily tunable by adjusting the density of GaN QDs, which can be adjusted by simply varying growth interruption. By controlling the density of ZnO nanorod arrays, their optical performance can also be improved. This approach opens the possibility of combining one-dimensional (1D) with 0D nanostructures for applications in sensor arrays, piezoelectric antenna arrays, optoelectronic devices, and interconnects.

AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors with reduced leakage current and enhanced breakdown voltage using aluminum ion implantation

This letter has studied the performance of AlGaN/GaN metal-insulator-semiconductor high electron mobility transistors on silicon substrate with GaN buffer treated by aluminum ion implantation for insulating followed by a channel regrown by metal–organic chemical vapor deposition. For samples with Al ion implantation of multiple energies of 140 keV (dose: 1.4 × 1014 cm−2) and 90 keV (dose: 1 × 1014 cm−2), the OFF-state leakage current is decreased by more than 3 orders and the breakdown voltage is enhanced by nearly 6 times compared to the samples without Al ion implantation. Besides, little degradation of electrical properties of the 2D electron gas channel is observed where the maximum drain current IDSmax at a gate voltage of 3 V was 701 mA/mm and the maximum transconductance gmmax was 83 mS/mm.

Synthesis of ZnO nanowire array film on Mg doped gallium nitride substrate

ZnO nanowire array films, composed of well aligned ZnO nanowires ∼200 nm in diameter and 1 μm in length, were successfully synthesised on Mg doped gallium nitride by hydrothermal method. In addition, the films possess quite flatten surface. In the synthesised process, there was no catalyst that had been used. Growth conditions were comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled with vertically aligned nanowire may have potential applications as UV light emitting diodes.

Influence of high-temperature postgrowth annealing under different ambience on GaN quantum dots grown via Ga droplet epitaxy

The influence of high-temperature postgrowth annealing (PGA) under NH3/H2, NH3/N2 ambience on the morphologies and nitridation degree of GaN/AlN QDs grown via droplet epitaxy is investigated. The results show that the size and density of GaN QDs changes with different ambiences and the NH3/N2 ambience is demonstrated as a necessary condition for maintaining optimal QD morphology by suppressing the migration and evaporation of Ga atoms and preventing the GaN decomposition. Moreover, the PGA process can effectively enhance the nitridation and crystallization of GaN QDs and the photoluminescence performance has been effectively improved after annealed under NH3/N2 ambience.

Fabrication and Characterization of Vertically Aligned ZnO Nanorod Arrays via Inverted Monolayer Colloidal Crystals Mask

The periodically ordered ZnO nanorod (NR) arrays have been successfully synthesized via a hydrothermal approach on the silicon substrates by templating of the TiO2 ring deriving from the polystyrene (PS) nanosphere monolayer colloidal crystals (MCC). With the inverted MCC mask, sol–gel-derived ZnO seeds could serve as the periodic nucleation positions for the site-specific growth of ZnO NRs. The large-scale patterned arrays of single ZnO NR with good side-orientation can be readily produced. According to the experimental results, the as-integrated ZnO NR arrays showed an excellent crystal quality and optical property, very suitable for optoelectronic applications such as stimulated emitters and ZnO photonic crystal devices.Graphical Abstract

Improved Optical Performance of InAs Quantum Dot Structure via Suitable Manipulation of GaAs Cap Layer Growth

The influences of the different growth methods of GaAs cap layer on the self-assembled InAs quantum dots were investigated via photoluminescence and transmission electron microscopy. A two-step growth technique, consisting of a low temperature pulsed atomic layer epitaxy and a high temperature conventional continuous growth method, could improve the surface morphology and photoluminescence intensity due to the enhancement of Ga atomic mobility and the reduction in dislocation density. Furthermore, the photoluminescence peak shows a red shift instead of a blue shift due to the strain relaxation.

Synthesis of ZnO Nanowire Array Film on Mg-Doped Gallium Nitride Substrate by Simple Hydrothermal Method

ZnO nanowire array films, composed of well-aligned ZnO nanowires with about 200 nm in diameter and 1 μm in length, were successfully synthesized on Mg-doped gallium nitride by hydrothermal method. The films possess quite flattened surface. In the synthesized process, there was no catalyst used. Growth conditions are comprehensively discussed in the process of aqueous solution method. It was found that the length of ZnO nanowires and the thickness of the film could be tunable by altering solution concentration and growth time. Such ZnO film assembled of vertically aligned nanowire may have potential applications as UV light-emitting diodes.

Plasmonic enhanced AlGaN solar-blind ultraviolet photodetectors with size-tunable Al nanoparticles

High-performance AlGaN solar-blind UV photodetectors were realized by incorporating size-tunable Al nanoparticles. The highest peak responsivity can reach 2.02A/W at 269 nm. The effects and enhancement mechanism of Al nanoparticles are investigated.