Tianxin Li

Single InAs Nanowire Room-Temperature Near-Infrared Photodetectors

Here we report InAs nanowire (NW) near-infrared photodetectors having a detection wavelength up to ∼1.5 μm. The single InAs NW photodetectors displayed minimum hysteresis with a high Ion/Ioff ratio of 10(5). At room temperature, the Schottky-Ohmic contacted photodetectors had an external photoresponsivity of ∼5.3 × 10(3) AW(-1), which is ∼300% larger than that of Ohmic-Ohmic contacted detectors (∼1.9 × 10(3) AW(-1)). A large enhancement in photoresponsivity (∼300%) had also been achieved in metal Au-cluster-decorated InAs NW photodetectors due to the formation of Schottky junctions at the InAs/Au cluster contacts. The photocurrent decreased when the photodetectors were exposed to ambient atmosphere because of the high surface electron concentration and rich surface defect states in InAs NWs. A theoretical model based on charge transfer and energy band change is proposed to explain this observed performance. To suppress the negative effects of surface defect states and atmospheric molecules, new InAs NW photodetectors with a half-wrapped top-gate had been fabricated by using 10 nm HfO2 as the top-gate dielectric.

Interlayer Transition and Infrared Photodetection in Atomically Thin Type-II MoTe2/MoS2 van der Waals Heterostructures

We demonstrate the type-II staggered band alignment in MoTe2/MoS2 van der Waals (vdW) heterostructures and an interlayer optical transition at ∼1.55 μm. The photoinduced charge separation between the MoTe2/MoS2 vdW heterostructure is verified by Kelvin probe force microscopy (KPFM) under illumination, density function theory (DFT) simulations and photoluminescence (PL) spectroscopy. Photoelectrical measurements of MoTe2/MoS2 vdW heterostructures show a distinct photocurrent response in the infrared regime (1550 nm). The creation of type-II vdW heterostructures with strong interlayer coupling could improve our fundamental understanding of the essential physics behind vdW heterostructures and help the design of next-generation infrared optoelectronics.

Distinct Photocurrent Response of Individual GaAs Nanowires Induced by n-Type Doping

The doping-dependent photoconductive properties of individual GaAs nanowires have been studied by conductive atomic force microscopy. Linear responsivity against the bias voltage is observed for moderate n-doped GaAs wires with a Schottky contact under illumination, while that of the undoped ones exhibits a saturated response. The carrier lifetime of a single nanowire can be obtained by simulating the characteristic photoelectric behavior. Consistent with the photoluminescence results, the significant drop of minority hole lifetime, from several hundred to subpicoseconds induced by n-type doping, leads to the distinct photoconductive features. Moreover, by comparing with the photoelectric behavior of AlGaAs shelled nanowires, the equivalent recombination rate of carriers at the surface is assessed to be >1 × 10(12) s(-1) for 2 × 10(17)cm(-3) n-doped bare nanowires, nearly 30 times higher than that of the doping-related bulk effects. This work suggests that intentional doping in nanowires could change the charge status of the surface states and impose significant impact on the electrical and photoelectrical performances of semiconductor nanostructures.

A genetic algorithm study on the most stable disordered and ordered configurations of Au38–55

Abstract We used the genetic algorithm, with a Gupta n -body potential, to study structures of the ground states and near ground ordered states of medium-sized Au n ( n from 38 to 55) clusters. It is found that the most stable configurations are mainly disordered with the ordered isomers very close in energy to the ground state. The lowest-lying ordered structure changes from close packed mode to icosahedron-like structure with increasing cluster size. A common neighbor analysis (CNA) was applied to demonstrate the structural evolvement.

Preparation and characterization of Al-doped quasi-aligned ZnO submicro-rods

Al-doped quasi-aligned ZnO submicro-rods were prepared on heavily doped n-type Si(111) substrates by the thermal evaporation method. Electrical transport measurements indicate the Al-doped ZnO submicro-rods have better conductivity than the undoped submicro-rods. The photoluminescence excitation spectrum of the annealed sample indicates an energy level ~100 meV below the conduction band minimum, which may be due to the localized state of the Al impurity. The annealed sample exhibits a strong green emission centred at 2.45 eV that can be seen by the naked eye and an infrared emission centred at 1.10 eV. The intensity ratios of the deep-level green emission to the ultraviolet emission are 0.86 and 30 for the as-grown and annealed samples, respectively. Moreover, two emissions at 2.2 and 1.96 eV were discriminated from the PL spectrum of the as-grown Al-doped ZnO submicro-rods, which were not observed in the pure ZnO submicro-rods and the annealed Al-doped sample.

Thermal behavior of Cu–Co bimetallic clusters

Abstract Based on a Gupta-like many-body potential, we have investigated the thermal behavior of Co–Cu bimetallic clusters with n =18, 19, 20 by using Monte Carlo techniques. A relation between cluster melting temperature and Cu concentration is obtained. We find the melting behavior of a bimetallic cluster strongly depends on the component materials, stoichiometries, size and structure of the cluster. The structural transition between isomers or degenerated states happens in the 18-atom clusters. Local melting occurs in the Co 13 Cu 5 cluster because of the segregation of Cu and a novel layered structure.

Self-Induced Uniaxial Strain in MoS2 Monolayers with Local van der Waals-Stacked Interlayer Interactions

Strain engineering is an effective method to tune the properties of electrons and phonons in semiconductor materials, including two-dimensional (2D) layered materials (e.g., MoS2 or graphene). External artificial stress (ExAS) or heterostructure stacking is generally required to induce strains for modulating semiconductor bandgaps and optoelectronic functions. For layered materials, the van der Waals-stacked interlayer interaction (vdW-SI) has been considered to dominate the interlayer stacking and intralayer bonding. Here, we demonstrate self-induced uniaxial strain in the MoS2 monolayer without the assistance of ExAS or heterostructure stacking processes. The uniaxial strain occurring in local monolayer regions is manifested by the Raman split of the in-plane vibration modes E2g(1) and is essentially caused by local vdW-SI within the single layer MoS2 due to a unique symmetric bilayer stacking. The local stacked configuration and the self-induced uniaxial strain may provide improved understanding of the fundamental interlayer interactions and alternative routes for strain engineering of layered structures.

Melting properties of noble metal clusters

Abstract The melting behaviors of noble metal clusters Ag 55 , Cu 55 and Au 55 have been investigated by molecular-dynamics simulations with an empirical many-body potential. Comparing with bulk materials, the enthalpy changes Δ H m during solid–liquid transitions of clusters drop remarkably as well as the melting temperatures T m , which are customarily due to the size confinement for clusters. Furthermore, the effect of limited size on melting-like transition is quite different among the three clusters. Gold cluster has been found to suffer the most from the size confinement effect, which can be illustrated by a broad transition manner and drastic decrease of T m and H m . The dependence of size confinement effect on substance can be attributed to the characteristic long-range correlation of interaction potentials.

Structural transitions of Au55 isomers

Abstract The thermodynamic behavior of near ground-state isomers of Au 55 clusters are investigated through molecular-dynamic simulations using an n -body effective potential. During relaxation at certain energies, non-reversible structural transitions (from cuboctahedron to icosahedron, and then to disordered configuration) occur well below the melting temperature. We show in this Letter on a picosecond scale the motion of the individual atoms in a cluster during these transformations, demonstrating that the processes involved are either collective distortions of the entire cluster or migration of individual or small groups of atoms. In both cases the cohesive energies of the system go smoothly without any significant barrier during transitions.

Scanning capacitance microscopy investigation on InGaAs/InP avalanche photodiode structures: Light-induced polarity reversal

Cross-sectional scanning capacitance microscopy is applied to study the carrier distribution as well as its variation under irradiation in an InGaAs/InP avalanche photodiode. The photocarriers excited by the stray light of atomic force microscope laser beam lead to a dramatic deviation of the dC/dV profile in the unintended-doped absorption layer, and even cause the reversal of signal polarity. The existence of surface potential and its impact on the spreading of photocarriers near the cleaved face are demonstrated as the main origins of the light-induced dC/dV reversal. The effect provides experimental information on the distribution property of photoelectric process in devices.Cross-sectional scanning capacitance microscopy is applied to study the carrier distribution as well as its variation under irradiation in an InGaAs/InP avalanche photodiode. The photocarriers excited by the stray light of atomic force microscope laser beam lead to a dramatic deviation of the dC/dV profile in the unintended-doped absorption layer, and even cause the reversal of signal polarity. The existence of surface potential and its impact on the spreading of photocarriers near the cleaved face are demonstrated as the main origins of the light-induced dC/dV reversal. The effect provides experimental information on the distribution property of photoelectric process in devices.