Xia-Xia Liao

Tuning the indirect-direct band gap transition of SiC, GeC and SnC monolayer in a graphene-like honeycomb structure by strain engineering: A quasiparticle GW study

We have calculated the electronic properties of graphene and SiC, GeC and SnC monolayers in a two-dimensional graphene-like honeycomb structure under various strained conditions using first principles calculations based on density functional theory and the quasiparticle GW approximation. Our results show that the indirect–direct band gap transition of group-IV carbides can be tuned by strain, which indicates a possible new route for tailoring the electronic properties of ultrathin nanofilms through strain engineering.

Getting high-efficiency photoluminescence from Si nanocrystals in SiO2 matrix

Silicon nanocrystals in SiO2 matrix are fabricated by plasma enhanced chemical vapor deposition followed by thermal annealing. The structure and photoluminescence (PL) of the resulting films is investigated as a function of deposition temperature. Drastic improvement of PL efficiency up to 12% is achieved when the deposition temperature is reduced from 250u200a°C to room temperature. Low-temperature deposition is found to result in a high quality final structure of the films in which the silicon nanocrystals are nearly strain-free, and the Si/SiO2 interface sharp. The demonstration of the superior structural and optical properties of the films represents an important step towards the development of silicon-based light emitters.

Silicon nanowires grown on a pre-annealed Si substrate

Polycrystalline Si nanowires (poly SiNWS) were successfully synthesized by plasma-enhanced chemical vapor deposition (PECVD) at 440degreesC using silane as the Si source and Au as the catalyst. The diameters of Si nanowires range from 15 to 100nm. The growth process indicates that to fabricate SiNWS by PECVD, pre-annealing at high temperature is necessary. A few interesting nanowires with Au nanoclusters uniformly distributed in the body of the wire were also produced by this technique

Study on Er3+ emission from the erbium-doped hydrogenated amorphous silicon suboxide film

The erbium-doped hydrogenated amorphous silicon suboxide films containing amorphous silicon clusters were prepared. The samples exhibited photoluminescence peaks at around 750 nm and 1.54 mum, which could be assigned to the electron-hole recombination in amorphous silicon clusters and the intra-4f transition in Er3+, respectively. Correlations between the intensities of these two photoluminescence peaks and oxidation and dehydrogenation of the films during annealing were studied. It was found that the oxidation is triggered by dehydrogenation of the films even at low annealing temperatures, which decisively changes the intensities of the two photoluminescence peaks. On the other hand, the increase of Er content in the erbium-doped hydrogenated amorphous silicon suboxide film will enhance Er3+ emission at 1.54 mum, while quench amorphous silicon cluster emission at 750 nm, such a competitive relationship, was also observed in the erbium-doped silicon nanocrystals embedded in SiO2 matrix. Moreover, we found that Er3+ emission is not sensitive to whether silicon clusters are crystalline or amorphous. The amorphous silicon clusters can be as sensitizer on Er3+ emission as that of silicon nanocrystals

Tailoring of polar and nonpolar ZnO planes on MgO (001) substrates through molecular beam epitaxy

Polar and nonpolar ZnO thin films were deposited on MgO (001) substrates under different deposition parameters using oxygen plasma-assisted molecular beam epitaxy (MBE). The orientations of ZnO thin films were investigated by in situ reflection high-energy electron diffraction and ex situ X-ray diffraction (XRD). The film roughness measured by atomic force microscopy evolved as a function of substrate temperature and was correlated with the grain sizes determined by XRD. Synchrotron-based X-ray absorption spectroscopy (XAS) was performed to study the conduction band structures of the ZnO films. The fine structures of the XAS spectra, which were consistent with the results of density functional theory calculation, indicated that the polar and nonpolar ZnO films had different electronic structures. Our work suggests that it is possible to vary ZnO film structures from polar to nonpolar using the MBE growth technique and hence tailoring the electronic structures of the ZnO films.PACS: 81; 81.05.Dz; 81.15.Hi.

High-performance n-type organic thin-film phototransistors based on a core-expanded naphthalene diimide

High-performance n-type organic phototransistors (OPTs) based on a core-expanded naphthalene diimide are reported in this letter. The photo responsivity is as high as 27000 AW−1 and photocurrent/dark-current ratio reaches 1.1u2009×u2009107 under a white light illumination with a power density of 107u2009μWcm−2. In such OPTs, persistent photoconductivity (PPC) is observed, which can be instantly eliminated by a positive gate voltage pulse. This is explained in terms of trapped photo holes in the channel. In the light on-off switching test, such PPC also leads to well-reproducible memory effect in the OPTs.

Optoelectronic properties of a fullerene derivative containing adamantane group.

A fullerene derivative linked with an adamantane cage, [6,6]-phenyl-C61-butyric acid 1-adamantane methyl ester (PC61BAd), has been designed and synthesized. Systematic investigations on its organic field effect performance, photovoltaic properties, and corresponding thermal stability have been made. In OFET device, the electron mobility (μe) of PC61BAd was found to reach a value as high as 0.01 cm(2)/V·s with a high on-off (Ion/Ioff) ratio of 4.9 × 10(6) that is useful for logic device applications. In the organic photovoltaic devices of P3HT:PC61BAd, the power conversion efficiency (PCE) was found to reach 3.31 % in the optimized device. More importantly, the active layer of P3HT:PC61BAd was found to exhibit superior thermal stability over that of P3HT:PC61BM. After heating at 150 °C for 20 h, the P3HT:PC61BAd device still showed a PCE of 2.44 %, demonstrating the applicability of PC61BAd as an acceptor material for the preparation of thermally stable organic solar cells. X-ray diffraction and atomic force microscopy were employed to probe the structure and morphology of PC61BAd and to rationalize its performance as an organic electronic material.

Indan-C60: from a crystalline molecule to photovoltaic application

Crystalline Indan-C60 and its photovoltaic application were studied. Microsheets and aloe-like micro-nano superstructures can be assembled from Indan-C60. Indan-PC61BM derived from Indan-C60 was further investigated as an acceptor for OPV devices, which shows a higher Voc, FF, and PCE than those obtained using PC61BM.

Tuning the thermal conductivity of strontium titanate through annealing treatments

Strontium titanate (SrTiO is a promising n-type material for thermoelectric applications. However, its relatively high thermal conductivity limits its performance in efficiently converting heat into electrical power through thermoelectric effect. This work shows that the thermal conductivity of SrTiO can be effectively reduced by annealing treatments, through an integrated study of laser flash measurement, scanning electron microscopy, Fourier transform infrared spectroscopy, x-ray absorption fine structure, and first-principles calculations. A phonon scattering model is proposed to explain the reduction of thermal conductivity after annealing. This work suggests a promising means to characterize and optimize the material for thermoelectric applications.

Strong red light emission from silicon nanocrystals embedded in SiO/sub 2/ matrix

In this study, silicon nanocrystals embedded in SiO/sub 2/ matrix were formed by conventional plasma enhanced chemical vapor deposition (PECVD) followed by high temperature annealing. The formation of silicon nanocrystals (nc-Si), their optical and micro-structural properties were studied using various experimental techniques, including Fourier transform infrared spectroscopy, micro-Raman spectra, high resolution transmission electron microscopy and x-ray photoelectron spectroscopy. Very strong red light emission from silicon nanocrystals at room temperature (RT) was observed. It was found that there is a strong correlation between the PL intensity and the substrate temperature, the oxygen content and the annealing temperature. When the substrate temperature decreases from 250 /spl deg/C to RT, the PL intensity increases by two orders of magnitude.