Electron transport through Al-ZnO-Al: an {\it ab initio} calculation

The electron transport properties of ZnO nano-wires coupled by two aluminium electrodes were studied by {\it ab initio} method based on non-equilibrium Green's function approach and density functional theory. A clearly rectifying current-voltage characteristics was observed. It was found that the contact interfaces between Al-O and Al-Zn play important roles in the charge transport at low bias voltage and give very asymmetric I-V characteristics. When the bias voltage increases, the negative differential resistance occurs at negative bias voltage. The charge accumulation was calculated and its behavior was found to be well correlated with the I-V characteristics. We have also calculated the electrochemical capacitance which exhibits three plateaus at different bias voltages which may have potential device application.Comment: 10 pages, 6 figure

Shot noise of spin current and spin transfer torque

We report the theoretical investigation of noise spectrum of spin current and spin transfer torque for non-colinear spin polarized transport in a spin-valve device which consists of normal scattering region connected by two ferromagnetic electrodes. Our theory was developed using non-equilibrium Green's function method and general non-linear $S^\sigma-V$ and $S^\tau-V$ relations were derived as a function of angle $\theta$ between magnetization of two leads. We have applied our theory to a quantum dot system with a resonant level coupled with two ferromagnetic electrodes. It was found that for the MNM system, the auto-correlation of spin current is enough to characterize the fluctuation of spin current. For a system with three ferromagnetic layers, however, both auto-correlation and cross-correlation of spin current are needed to characterize the noise spectrum of spin current. Furthermore, the spin transfer torque and the torque noise were studied for the MNM system. For a quantum dot with a resonant level, the derivative of spin torque with respect to bias voltage is proportional to $\sin\theta$ when the system is far away from the resonance. When the system is near the resonance, the spin transfer torque becomes non-sinusoidal function of $\theta$. The derivative of noise spectrum of spin transfer torque with respect to the bias voltage $N_\tau$ behaves differently when the system is near or far away from the resonance. Specifically, the differential shot noise of spin transfer torque $N_\tau$ is a concave function of $\theta$ near the resonance while it becomes convex function of $\theta$ far away from resonance. For certain bias voltages, the period $N_\tau(\theta)$ becomes $\pi$ instead of $2\pi$. For small $\theta$, it was found that the differential shot noise of spin transfer torque is very sensitive to the bias voltage and the other system parameters.Comment: 15pages, 6figure

Excitonic energy transfer in light-harvesting complexes in purple bacteria

Two distinct approaches, the Frenkel-Dirac time-dependent variation and the Haken-Strobl model, are adopted to study energy transfer dynamics in single-ring and double-ring light-harvesting systems in purple bacteria. It is found that inclusion of long-range dipolar interactions in the two methods results in significant increases in intra- or inter-ring exciton transfer efficiency. The dependence of exciton transfer efficiency on trapping positions on single rings of LH2 (B850) and LH1 is similar to that in toy models with nearest-neighbor coupling only. However, owing to the symmetry breaking caused by the dimerization of BChls and dipolar couplings, such dependence has been largely suppressed. In the studies of coupled-ring systems, both methods reveal interesting role of dipolar interaction in increasing energy transfer efficiency by introducing multiple intra/inter-ring transfer paths. Importantly, the time scale (~4ps) of inter-ring exciton transfer obtained from polaron dynamics is in good agreement with previous studies. In a double-ring LH2 system, dipole-induced symmetry breaking leads to global minima and local minima of the average trapping time when there is a finite value of non-zero dephasing rate, suggesting that environment plays a role in preserving quantum coherent energy transfer. In contrast, dephasing comes into play only when the perfect cylindrical symmetry in the hypothetic system is broken. This study has revealed that dipolar interaction between chromophores may play an important part in the high energy transfer efficiency in the LH2 system and many other natural photosynthetic systems.Comment: 14 pages 9 figure

Translanguaging Practices and Ideologies: Lao Students’ Identity Construction on WeChat and Facebook

In the shifting paradigm of the world economy, China has become one of the popular destinations for international students crossing the border and receiving high education. To reinforce the regional cooperation between China and its neighbouring countries, China’s border provinces have been discursively constructed as the platform for intiating international communication. Given the transformed positioning of China’s Southwest provinces, Yunnan has turned itself into an educational hub receiving international students from Southeast and South Asia. Based on a longitudinal ethnography with five Lao students receiving China’s higher education between September 2019 and July 2021, this study examines the ideological meanings of their translanguaing practices on WeChat and Facebook. The multiple types of data were collected through participant observation online and offline, WeChat and Facebook screenshots and semi-structured interviews. The study finds that translanguaging practices are often deployed by Lao students during their stay in China and in Laos. Their translanguaging practices contain different language forms and patterns, and display various types of social meanings including the intertextuality of the local voice, the identity construction of language learners and global citizens, and the sociocultural inbetweenness. This study indicates that Lao students tend to perform their transnational identities online and their translanguaging practices intersects with social, cultural, political and economic factors

Facile synthesis of Mn3O4-reduced graphene oxide hybrids for catalytic decomposition of aqueous organics

Mn3O4-reduced graphene oxide (rGO) hybrids were synthesized, and their catalytic performance in heterogeneous activation of peroxymonosulfate (PMS) to oxidize a target pollutant, Orange II, in aqueous solutions was investigated. The surface morphology and structure of the Mn3O4-rGO hybrids were characterized by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA). Through an in situ chemical deposition and reduction, Mn3O4-rGO hybrids with Mn3O4 nanoparticles at an average size of 29.2 nm were produced. The catalytic activity in Orange II oxidative decomposition was evaluated in view of the effects of various processes, pH, PMS concentration, Orange II concentration, and temperature. The combination of Mn3O4 nanoparticles with graphene sheets leads to a much higher catalytic activity than that of pure Mn3O4 or rGO. Graphene was found to play an important role in Mn3O4 dispersion and decomposition of Orange II. Typically, 30 mg/L of Orange II could be completely oxidized in 120 min at 25 °C and 0.05 g/L of Mn3O4-rGO hybrids, showing a promising application of the catalyst in the oxidative degradation of aqueous organic pollutants. The efficiency of Orange II decomposition increased with increasing temperature (25-55 °C), pH (4.0-11.0), and PMS dosage (0.25-1.5 g/L), but it decreased with increasing initial Orange II concentration (30-90 mg/L). Mn3O4-rGO hybrids exhibited stable performance without losing activity after four successive runs

Realization of valley-spin polarized current via parametric pump in monolayer

Monolayer ${\mathrm{MoS}}_2$ is a typical valleytronic material with valley-spin locked valence bands. We numerically investigate the valley-spin polarized current in monolayer ${\mathrm{MoS}}_2$ via adiabatic electron pumping. By introducing an exchange field to break the energy degeneracy of monolayer ${\mathrm{MoS}}_2$ , the top of its valence bands is valley-spin polarized and tunable by the exchange field. A device with spin-up polarized left lead, spin-down polarized right lead, and untuned central region is constructed through applying different exchange fields in the corresponding regions. Then, equal amount of pumped currents with opposite valley-spin polarization are simultaneously generated in the left and right leads when periodically varying two pumping potentials. Numerical results show that the phase difference between the pumping potentials can change the direction and hence polarization of the pumped currents. It is found that the pumped current exhibits resonant behavior in the valley-spin locked energy window, which depends strongly on the system size and is enhanced to resonant current peaks at certain system lengths. More importantly, the pumped current periodically oscillates as a function of the system length, which is closely related to the oscillation of transmission. The effects of other system parameters, such as the pumping amplitude and the static potential, are also thoroughly discussed

Fabrication of Fe3O4/SiO2 core/shell nanoparticles attached to graphene oxide and its use as an adsorbent

Amino-functionalized Fe3O4/SiO2 core/shell nanoparticles were synthesized by reacting Fe3O4 nanoparticles with tetraethyl orthosilicate and (3-aminopropyl) triethoxysilane to introduce amino groups on the surface. The amino groups on the Fe3O4/SiO2 were reacted with the carboxylic groups of graphene oxide (GO) with the aid of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide and N-hydroxysuccinnimide to form Fe3O4/SiO2––GO nanoparticles. The structural, surface, and magnetic characteristics of the material were investigated by scanning and transmission electron microscopy, energy-dispersive X-ray spectrometry, powder X-ray diffraction, Fourier transform infrared spectroscopy, and thermogravimetric analysis. Adsorption equilibrium and kinetics of methylene blue on the Fe3O4/SiO2––GO were studied in a batch system. The maximum adsorption capacities were found to be 97.0, 102.6, and 111.1 mg g−1 at 25, 45, and 60 °C, respectively. A second-order kinetic equation could best describe the sorption kinetics. Thermodynamic parameters indicated that the adsorption of methylene blue onto the material was thermodynamically feasible and could occur spontaneously

Novel Two-Dimensional Layered MoSi2Z4 (Z = P, As): New Promising Optoelectronic Materials

Very recently, two new two-dimensional (2D) layered semi-conducting materials MoSi2N4 and WSi2N4 were successfully synthesized in experiments, and a large family of these two 2D materials, namely MA2Z4, was also predicted theoretically (Science, 369, 670 (2020)). Motivated by this exciting family, in this work, we systematically investigate the mechanical, electronic and optical properties of monolayer and bilayer MoSi2P4 and MoSi2As4 by using the first-principles calculation method. Numerical results indicate that both monolayer and bilayer MoSi2Z4 (Z = P, As) present good structural stability, isotropic mechanical parameters, moderate bandgap, favorable carrier mobilities, remarkable optical absorption, superior photon responsivity and external quantum efficiency. Especially, due to the wave-functions of band edges dominated by d orbital of the middle-layer Mo atoms are screened effectively, the bandgap and optical absorption hardly depend on the number of layers, providing an added convenience in the experimental fabrication of few-layer MoSi2Z4-based electronic and optoelectronic devices. We also build a monolayer MoSi2Z4-based 2D optoelectronic device, and quantitatively evaluate the photocurrent as a function of energy and polarization angle of the incident light. Our investigation verifies the excellent performance of a few-layer MoSi2Z4 and expands their potential application in nanoscale electronic and optoelectronic devices