Rui Huang

Probing the electron states and metal-insulator transition mechanisms in molybdenum disulphide vertical heterostructures

The metal-insulator transition is one of the remarkable electrical properties of atomically thin molybdenum disulphide. Although the theory of electron-electron interactions has been used in modelling the metal-insulator transition in molybdenum disulphide, the underlying mechanism and detailed transition process still remain largely unexplored. Here we demonstrate that the vertical metal-insulator-semiconductor heterostructures built from atomically thin molybdenum disulphide are ideal capacitor structures for probing the electron states. The vertical configuration offers the added advantage of eliminating the influence of large impedance at the band tails and allows the observation of fully excited electron states near the surface of molybdenum disulphide over a wide excitation frequency and temperature range. By combining capacitance and transport measurements, we have observed a percolation-type metal-insulator transition, driven by density inhomogeneities of electron states, in monolayer and multilayer molybdenum disulphide. In addition, the valence band of thin molybdenum disulphide layers and their intrinsic properties are accessed.

Detection of interlayer interaction in few-layer graphene

Research Grants Council of Hong Kong [604112, N_HKUST613/12, 16302215, HKUST9/CRF/08, CRF_HKU9/CRF/13G]; Raith-HKUST Nanotechnology Laboratory electron-beam lithography facility [SEG HKUST08]

Nitrogen deep accepters in ZnO nanowires induced by ammonia plasma

Nitrogen doping in ZnO nanowires was achieved through ammonia plasma treatment followed by thermal annealing. The strong dependence of the red light emission from the nanowires excited by 2.4u2009eV on the nitrogen concentration, suggests that the red light emission originates from nitrogen related defects. The mechanism responsible for the red light emission is in good agreement with the deep-acceptor model of nitrogen defects, clarifying that nitrogen atoms caused deep accepters in ZnO nanowires. Based on this model, the enhanced green emission from defects in nitrogen-doped samples (excited by 325u2009nm line) can be well explained by the increase of the concentration of activated oxygen vacancies resulting from the compensation of nitrogen deep acceptors.

Multiple magnification effects of Ce 3+ ions on near-infrared persistent luminescence of Cr-doped LaAlO 3

Ce3+/Cr3+ co-doped LaAlO3 for near-infrared (NIR) long lasting phosphors were synthesized through solid-state reaction. Incorporation of Ce3+ ions into Cr3+-doped LaAlO3 significantly enhanced the NIR persistent luminescence by more than one order of magnitude compared with LaAlO3 doped with Cr3+. Detailed analysis of the photoluminescence, photoluminescence excitation, and Thermo-luminescence spectra, as well as the persistent decay behavior of Ce3+/Cr3+ co-doped LaAlO3, indicated that the improvement of NIR persistent luminescence at around 735 nm (Cr3+: 2E→4A2 transition) is not only originated from a persistent energy transfer process from Ce3+ ions to Cr3+ ions, but also attributed to the extra efficient traps created by incorporation of Ce3+ ions. The current work develops an alternative approach toward the efficient NIR Cr3+-doped non-gallate long-persistence phosphors.

Luminescence enhancement of ZnO-core/a-SiN x :H-shell nanorod arrays

We report a remarkable improvement of photoluminescence from ZnO-core/a-SiN(x):H-shell nanorod arrays by modulating the bandgap of a-SiN(x):H shell. The a-SiN(x):H shell with a large bandgap can significantly enhance UV emission by more than 8 times compared with the uncoated ZnO nanorods. Moreover, it is found that the deep-level defect emission can be almost completely suppressed for all the core-shell nanostructures, which is independent of the bandgaps of a-SiN(x):H shells. Combining with the analysis of infrared absorption spectrum and luminescence characteristics of NH(x)-plasma treated ZnO nanorods, the improved photoluminescence is attributed to the decrease of nonradiative recombination probability and the reduction of surface band bending of ZnO cores due to the H and N passivation and the screening effect from the a-SiN(x):H shells. Our findings open up new possibilities for fabricating stable and efficient UV-only emitting devices.

Effective control of photoluminescence from ZnO nanowires by a-SiN x :H decoration

The a-SiNx:H with a large bandgap of 3.8 eV was utilized to decorate ZnO nanowires. The UV emission from the a-SiNx:H-decorated ZnO nanowires are greatly enhanced compared with the undecorated ZnO nanowire. The deep-level defect emission has been completely suppressed even though the sample was annealed at temperatures up to 400 °C. The incorporation of H and N is suggested to passivate the defect states at the nanowire surface and thus result in the flat-band effect near ZnO surface as well as reduction of the nonradiative recombination probability.

Defect Emission and Optical Gain in SiCxOy:H Films

Luminescent SiCxOy:H films, which are fabricated at different CH4 flow rates using the plasma-enhanced chemical vapor deposition (PECVD) technique, exhibit strong photoluminescence (PL) with tuning from the near-infrared to orange regions. The PL features an excitation-wavelength-independent recombination dynamics. The silicon dangling bond (DB) defects identified by electron paramagnetic resonance spectra are found to play a key role in the PL behavior. The first-principles calculation shows that the Si DB defects introduce a midgap state in the band gap, which is in good agreement with the PL energy. Moreover, the band gap of a-SiCxOy:H is found to be mainly determined by Si and C atoms. Thus, the strong light emission is believed to result from the recombination of excited electrons and holes in Si DB defects, while the tunable light emission of the films is attributed to the substitution of stronger Si-C bonds for weak Si-Si bonds. It is also found that the light emission intensity shows a superlinear dependence on the pump intensity. Interestingly, the film exhibits a net optical gain under ultraviolet excitation. The gain coefficient is 53.5 cm-1 under a pumping power density of 553 mW cm-2. The present results demonstrate that the SiCxOy system can be a very competitive candidate in the applications of photonics and optoelectronics.