Unconventional fractional quantum Hall effect in monolayer and bilayer graphene

The commensurability condition is applied to determine the hierarchy of fractional fillings of Landau levels in monolayer and in bilayer graphene. The filling rates for fractional quantum Hall effect (FQHE) in graphene are found in the first three Landau levels in one-to-one agreement with the experimental data. The presence of even denominator filling fractions in the hierarchy for FQHE in bilayer graphene is explained. Experimentally observed hierarchy of FQHE in the first and second Landau levels in monolayer graphene and in the zeroth Landau level in bilayer graphene is beyond the conventional composite fermion interpretation but fits to the presented nonlocal topology commensurability condition.     

Patterning nanoroads and quantum dots on fluorinated graphene

Using ab initio methods we have investigated the fluorination of graphene and find that different stoichiometric phases can be formed without a nucleation barrier, with the complete “2D-Teflon” CF phase being thermodynamically most stable. The fluorinated graphene is an insulator and turns out to be a perfect matrix-host for patterning nanoroads and quantum dots of pristine graphene. The electronic and magnetic properties of the nanoroads can be tuned by varying the edge orientation and width. The energy gaps between the highest occupied and lowest unoccupied molecular orbitals (HOMO-LUMO) of quantum dots are size-dependent and show a confinement typical of Dirac fermions. Furthermore, we study the effect of different basic coverage of F on graphene (with stoichiometries CF and C₄F) on the band gaps, and show the suitability of these materials to host quantum dots of graphene with unique electronic properties.     

Fabrication of quantum-dot devices in graphene

Abstract

We describe our recent experimental results on the fabrication of quantum-dot devices in a graphene-based two-dimensional system. Graphene samples were prepared by micromechanical cleavage of graphite crystals on a SiO₂/Si substrate. We performed micro-Raman spectroscopy measurements to determine the number of layers of graphene flakes during the device fabrication process. By applying a nanofabrication process to the identified graphene flakes, we prepared a double-quantum-dot device structure comprising two lateral quantum dots coupled in series. Measurements of low-temperature electrical transport show the device to be a series-coupled double-dot system with varied interdot tunnel coupling, the strength of which changes continuously and non-monotonically as a function of gate voltage.     

Sn量子点/石墨烯复合材料的合成及储锂性能

Sn基材料是目前高容量锂离子电池电极材料研究的热点,但循环性能较差阻碍了其大规模应用.以氧化石墨烯为载体,通过化学还原法在载体表面成功均匀负载＜10 nm的Sn量子点,合成Sn量子点/石墨烯(SnQds/rGO)复合电极材料.结果 表明,Sn质量分数为90wt％的SnQds/rGO复合材料具有良好的综合电化学性能,首次...     

Nonlinear optical interactions of laser modes in quantum cascade lasers

We overview the results of recent experimental and theoretical studies of nonlinear dynamics of mid-infrared quantum cascade lasers (QCLs) associated with nonlinear interactions of laser modes. Particular attention is paid to phase-sensitive nonlinear mode mixing which turns out to be quite prominent in QCLs of different kinds and which gives rise to frequency and phase locking of laser modes. Nonlinear phase coupling of laser modes in QCLs leads to a variety of ultrafast and coherent phenomena: synchronization of transverse modes, beam steering, the RNGH multimode instability, and generation of mode-locked ultrashort pulses.     

A quantum description of linear,and non-linear optical interactions in arrays of plasmonic nanoparticles

A quantum theory for describing the interaction of photons and plasmons, in one- and two-dimensional arrays is presented. Ohmic losses and inter-band transitions are not considered. We use macroscopic approach, and quantum field theory methods including S-matrix expansion, and Feynman diagrams for this purpose. Non-linear interactions are also studied, and increasing the probability of such interactions, and its application are also discussed.     

Probing Defect‐Induced Midgap States in MoS2 Through Graphene–MoS2 Heterostructures

Crystalline defects in MoS₂ may induce midgap states, resulting in low carrier mobility. These midgap states are usually difficult to probe by conventional transport measurement. The quantum capacitance of single‐layer graphene is sensitive to defect‐induced states near the Dirac point, at which the density of states is extremely low. It is reported that the hexagonal‐boron nitride/graphene/MoS₂ sandwich structure facilitates the exploration of the properties of those midgap states in MoS₂. Comparative results of the quantum capacitance of pristine graphene indicate the presence of several midgap states with distinct features. Some of these states donate electrons while some states lead to localization of electrons. It is believed that these midgap states originate from intrinsic point defects such as sulfur vacancies, which have a significant impact on the property of the MoS₂/graphene interface. They are responsible for the contact problems of metal/MoS­₂ interfaces.     

磷氮掺杂石墨烯层封装FeP纳米颗粒的增强ORR催化性能

开发高效的非贵金属氧还原反应(ORR)催化剂来替代铂基催化剂受到了广泛关注.设计合成在碱性电解质和酸性电解质中均表现出高催化活性的非贵金属催化剂仍然是一个挑战.在本文中,我们通过前驱体热解法制备了一种纳米复合催化剂(FeP@PGL),该催化剂由氮掺杂的碳纳米片以及镶嵌在片层上的磷掺杂石墨烯层封装磷化铁(FeP)纳米颗粒组成.FeP@PGL催化剂表现出优异的ORR催化性能,在碱性介质中的起始电位和半波电势分别高达1.01 V和0.90 V vs.RHE;在酸性介质中的起始电位和半波电势分别高达0.95 V和0.81 V vs.RHE.通过详细的电子显微和谱学表征,我们发现碳纳米片基质与包裹纳米颗粒的碳包裹层存在组成的差别,磷掺杂主要发生在包裹FeP纳米颗粒的石墨烯层上.封装的FeP纳米颗粒与外层磷掺杂石墨烯层之间存在界面电荷转移,并且通过界面相互作用降低了催化剂表面的功函数.FeP和磷掺杂石墨烯层之间的界面协同作用对于增强催化剂ORR活性至关重要.本文不仅证明了封装型FeP基纳米复合催化剂在氧还原反应上的应用价值,而且为界面电荷转移效应及其在ORR过程中的作用提供了实验证据.     

An approach to prepare uniform graphene oxide/aluminum composite powders by simple electrostatic interaction in water/alcohol solution

The homogenous dispersion of graphene in Al powders is a key challenge that limits the development of graphene-reinforced metal matrix composites with high performance. Here, uniform distribution of graphene oxide (GO) coated on flake Al powders were obtained by a simply stirring and ultrasonic treatment in the water/alcohol solution. The effect of water volume content on the formation of GO/Al composite powders was investigated. The results showed that GO adsorbed with synchronous reduction on the surface of Al powders, but when the water content was higher than 80% in the solution, Al powders were totally changed into Al(OH)₃. With optimizing the water content of 60% in the solution, reduced GO was homogenously coated onto the surface of flake Al powders. The formation mechanism can be ascribed to the balance control between the liquid/solid interaction and the hydrolysis reaction.     

Driving Spin and Charge in Quantum Wells by Surface Acoustic Waves

Recent experiments have shown the potential of surface acoustic waves as a mean for transporting charge and spin in quantum wells. In particular, they have proven highly effective for the coherent transport of spin‐polarized wave packets, suggesting their potential in spintronics applications. Motivated by these experimental observations, the spin and charge dynamics in a quantum well under surface acoustic waves is theoretically studied. It is shown that the dynamics acquires a simple and transparent form in a reference frame co‐moving with the surface acoustic wave. The results, e.g., the calculated spin relaxation and precession lengths, are in excellent agreement with recent experimental observations.     

Facile synthesis of graphene quantum dots/ZSM-5 type metalosilicate composites and evaluating their performance in photocatalytic degradation of methylene blue and electrochemical water splitting

Graphene quantum dots (GQDs)/ZSM-5 type cobaltosilicate and GQDs/ZSM-5 type chromosilicate composites were prepared by two methods; direct and solid state dispersion (SSD) method. The composites were characterized using XRD, FT-IR, UV–vis absorption, and SEM techniques. The photocatalytic activities of composites were evaluated by degradation of methylene blue (MB) under both UV and vis light irradiation. The composites prepared by direct method showed better photocatalytic activity and the GQDs/chromosilicate composite had a superior performance. This composite could remove about 96 % and 87 % of MB at first 30 min under UV and vis light, respectively. The results of photocatalytic degradation kinetic studies revealed that the composites follow the pseudo-second-order kinetic model (R² = 0.99) with rate constants ranging from 0.295 to 0.581 g mg^?1 min^?1 for different composites prepared by direct method. The GQDs/metalosilicate composites were also examined for the electrochemical water splitting. The GQDs/cobaltosilicate composites required lower overpotentials for HER and OER in comparison to the pure GDQs and cobaltosilicate. Moreover, the charge transfer resistance and Warburg impedance was lower than the corresponding values obtained for the pure cobaltosilicate and GQDs. These new GQDs-based composites are expected to open new windows in materials science and electrocatalytic-related processes.