2维光子晶体中的掺杂效应数值研究

 把平面波展开方法(PWM)用于2维光子晶体掺杂情况下透射特性的研究,计算得到了粗细不同的空气柱掺杂、相同半径不同介质柱掺杂、不同柱体形状掺杂情况下2维光子晶体的透射系数与入射光频率的关系曲线。结果表明2维光子晶体的禁带的宽度、位置、透过率与掺杂体半径、掺杂体介电常数、掺杂体柱体几何形状等因素有关,掺杂因素相差越大透射曲线变化越明显,特别是损耗介质的掺入更使得禁带可调要素增加。     

On transmittance and localization of the electromagnetic wave in two-dimensional graphene-based photonic crystals

Two-dimensional graphene-based photonic crystal (GPC) formed by a periodic array of the homogeneous dielectric cylinders etched in the alternating graphene and dielectric layers and its inverse counterpart are considered. The transmittance of the photonic crystal is obtained. The waveguide due to the localization of the electromagnetic wave on the lattice defect that breaks the translational symmetry of the GPC of two different topologies is studied. The different topologies of GPC are characterized by different photonic band structures with different widths of photonic band gaps (PBG) and provide different frequencies for the localized electromagnetic wave due to the defect. The frequencies of the localized mode for both type of the GPC, located inside the lowest PBG, are in the range of THz or tens of THz depending on the topology of the GPC. It is shown that the photonic band gap always can be tuned by changing the chemical potential of graphene to provide formation of the localized photonic mode due to the defect. The technological advantages of the GPC, as well as the opportunity to tune the PBG and the frequency of the localized electromagnetic wave in the terahertz region of spectrum for the GPC are discussed.     

Rippling and the external electric field on conductance in corrugated graphene nanoribbons molecular device

Molecular devices constructed using corrugated graphene nanoribbons (GNRs) are proposed in the paper. Recursive Green's function calculations show that the intrinsic ripples in graphene and the external electric field energy play important roles on the electron transport properties. Negative differential resistance is observed in zigzag corrugated GNRs. With the wavelength of the ripples decreasing, both the zigzag and armchair corrugated GNRs exhibit ON/OFF characteristics. On applying external electric field, current decreases dramatically in zigzag corrugated GNRs. These findings show that corrugated GNRs can be used to design functional nanoscale devices.     

Voltage-driven electronic transport and shot noise in armchair graphene nanoribbons

We study theoretically shot noise and minimal conductivity of electrons by evanescent states penetrating through clean graphene nanoribbons (GNRs). With increasing of the barrier voltage, we find that the minimum conductivity will increase to 4e²/πh and the maximum Fano factor will increase to 1/3. More interestingly, quantum oscillations can be tuned by the gate voltage and separated by tuning the barrier voltage     

Theory of the integer quantum Hall effect in graphene

A Hall resistivity formula for the 2DES in graphene is derived from the zero-mass Dirac field model adopting the electron reservoir hypothesis. The formula reproduces perfectly the experimental resistivity data [K.S. Novoselov, et al., Nature 438 (2005) 201]. This perfect agreement cannot be achieved by any other existing models. The electron reservoir is shown to be the 2DES itself.     

石墨烯纳米片电子结构和量子输运特性第一原理研究

用基于密度泛函理论的原子紧束缚方法计算研究单层石墨烯纳米圆片和纳米带的电子结构,并结合第一原理和非平衡函数法计算量子输运特性.通过电子能态和轨道密度分布研究纳米碳原子层的电子成键状态,结合电子透射谱、电导和电子势分布分析电子散射与输运机制.石墨烯纳米带和纳米圆片分别呈现金属和半导体的能带特征,片层边缘上电极化分别沿垂直和切向方向,电子电导出现较大的差异,来源于石墨烯纳米圆片边缘的突出碳原子环对电子的强散射.石墨烯纳米带的电子透射谱表现为近似台阶式变化并在费米能级处存在弹道电导峰,而石墨烯纳米圆片的电子能带和透射谱在费米能级处开口并且因量子限制作用呈现更加离散的多条高态密度窄能带和尖锐谱峰.     

Interacting electrons on a two-dimensional surface: planar vs. spherical geometries

The center-of-mass excitations are identified in the spectrum of electrons on a two-dimensional surface in the lowest Landau level. The correspondence between the quantum numbers labeling electron states on a sphere and on a plane is drawn. The excitation spectrum on a sphere with increasing radius is shown to converge to that on a plane. In particular, in the presence of a lateral confinement (i.e. for the case of quantum dots), identical series of magic states appear in both cases. Also, a class of interaction potentials which lead to the fractional quantum Hall effect in an extended system are identified.     

Nonlocal heat conduction in suspended graphene

In order to investigate the mechanism of the extremely high thermal conductivity of the suspended graphene, a nonlocal heat conduction model of heat conduction in two-dimensional materials under a temperature gradient along the length direction is proposed. This model shows that the heat transport of the suspended graphene along the length direction under the nonlocal effect in the width and thickness directions demonstrates a similarity with the viscous Poiseuille flow between two parallel plates. Based on this model, the dimensional crossover of heat conduction in few-layer graphene is demonstrated and the impact of temperature variation on the thermal conductivity of graphene is investigated. The obtained results show a good agreement with the experimental ones. The proposed model indicates that the combination of the Poiseuille phonon transport and boundary conditions on the upper and lower surfaces of the suspended graphene plays a decisive role on the ultra-high heat conductivity of graphene.     

Disordered two-dimensional electron systems with chiral symmetry

We review the results of our recent numerical investigations on the electronic properties of disordered two dimensional systems with chiral unitary, chiral orthogonal, and chiral symplectic symmetry. Of particular interest is the behavior of the density of states and the logarithmic scaling of the smallest Lyapunov exponents in the vicinity of the chiral quantum critical point in the band center at E=0. The observed peaks or depressions in the density of states, the distribution of the critical conductances, and the possible non-universality of the critical exponents for certain chiral unitary models are discussed.     

Fabrication and demonstration of square lattice two-dimensional rod-type photonic bandgap crystal optical intersections

This paper reports fabrication and demonstration of optical intersections in two-dimensional (2D) rod-type photonic crystal (PhC) structures. High resolution and aspect ratio 2D square lattice PhC waveguide intersections were designed and fabricated for application at the optical communication wavelengths centered at 1550 nm. In the silicon processing front, challenges resolved to overcome issues of drastically reduced process windows caused by the dense PhC rods arrays with critical dimensions (CDs) reduced to only a few hundred nanometers were addressed not only in terms of critical process flow design but also in the development of each processing module. In the lithographic process of deep ultraviolet laser system working at 248 nm, PhC rods of sub-lithographic wavelength CDs (115 nm in radii) were realized in high resolution, even near periphery regions where proximity errors were prone. In the deep etching module, stringent requirements on etch angle control and low sidewall scallops (undulations arising from time multiplexed etch and passivation actions) were satisfied, to prevent catastrophic etch failures, and enable optical quality facets. The successfully fabricated PhCs were also monolithically integrated with large scale optical testing fiber grooves that enabled macro optical fiber assisted coupling to the micro scale PhC devices. In the optical experiments, the transmission and crosstalk properties for the PhC intersection devices with different rod radii at the center of the PhC optical waveguides crossings were measured with repeatability. The properties of the PhC intersections were therefore optimized and verified to correspond well with first principle finite difference time domain simulations.     

Weak localization of two-dimensional Dirac fermions beyond the diffusion regime

We develop a microscopic theory of the weak localization of two-dimensional massless Dirac fermions, which is valid in the whole range of classically weak magnetic fields. The theory is applied to calculate the magnetoresistance caused by the weak localization in graphene and topological insulators.     

Gas-surface interactions on two-dimensional crystals

Two-dimensional (2D) crystals have developed into a popular mainstream research topic which is interesting for basic research and many applications. Gas-surface interactions, as reviewed here, are important for catalysis including noble metal-free catalysts, materials science, and surface science as well as environmental and energy technologies. Basic science concerns fundamental differences of 2D crystals and bulk materials as well as e.g. how the substrate of epitaxial 2D crystals affects their surface properties.Most of the attention so far obtained (gas-phase) water adsorption which always was an evergreen in surface science. However, studies about small inorganic/organic molecule adsorption (CO, CO₂, NO_x, O₂, H, rare gases, H₂S, SO₂, alkanes, benzene, alcohols, thiophene, etc.) and surface reactions on 2D crystals (CO oxidation, ethylene epoxidation, oxygen reduction reaction, SO₂ and H₂SO₃ oxidation) started to appear in the literature as well. This review describes all of these probe molecules, but focuses on experimental and theoretical surface science model studies usually conducted at ultra-high vacuum (UHV).The review focusses on graphene and functionalized graphene (graphene oxide, N-graphene, etc.) since the bulk of the literature deals with that system. However, included in fair detail are also many other 2D crystals such as silicatene, zeolite films (doped silicatene), metal dichalcogenides (such as MoS₂, WS₂), boron nitride, MXenes, germanene, silicene, TiO₂, graphane, graphone, and portlandene.As a prototypical example, in recent projects, the wetting properties of e.g. graphene for water were controversially discussed. Therefore, a long chapter is devoted to water on graphene. That dispute was originally based on contact angle measurements at ambient pressure. In the meanwhile detailed surface science works including theoretical modelling are available. Literature on other carboneous surfaces such as HOPG (see list of acronyms) will be considered as a reference. Related works are also visible for other inorganic 2D crystals such as silicatene, i.e., 2D-SiO₂, or 2D-MoS₂ as well as functionalized 2D crystals (i.e. graphene oxide, N-doped graphene, graphane, etc.). Hydrophobic systems also are interesting for applications.Although included in this review, but not described in very detail are electro chemistry studies, projects in the liquid phase, photo-chemistry, high pressure catalysis, and pure engineering studies (membranes, separation, fuel cells). However, in comparison with 2D crystals and to perhaps motivate related UHV surface chemistry projects in the future many of these projects were included to some extent.As a broader objective, this review summarizes the currently available knowledge needed to extend the use of 2D materials beyond the utilization of their remarkable electronic and mechanical properties.     

Investigation on fluorescence quenching of dyes by graphite oxide and graphene

Rhodamine B (Rh B), eosin (E) and methylene blue (MB) were used as a probe to investigate the molecular structure and charge of the dyes on the sensitized efficiency of graphite oxide (GO) and graphene (G). The structure of the prepared GO and G were characterized by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. To study the electron transfer between dyes and GO or G, UV-vis absorption spectra (UV-vis), steady state fluorescence spectra (FL) and time resolved fluorescence spectra have been determined. It has been found that the electron transfer from the excited dyes to G was more efficient than to GO, and the transfer from excited MB to G was easier than to Rh B and E, because of the different electrostatic attraction between the dye and G.     

Classical motion in two-dimensional crystals

The classical motion of an electron of high enough energy in a two-dimensional crystal is diffusive for many potentials with Coulomb singularities. A simple model of the dynamics is developed which predicts the dependence of the diffusion constantD on the particle energyE in the high-energy limit:D(E)const·E ^3/2. This diffusion law is checked for a concrete crystal by numerically integrating the Hamilton equations for an ensemble of initial conditions. Finally this method is compared with other models of the classical dynamics in a crystal, especially the Sinai billiard.     

Enhanced gas sensor based on nitrogen-vacancy graphene nanoribbons

We study the electron transport of nitrogen-vacancy zigzag graphene nanoribbons (ZGNRs) absorbing gas molecules. It is found that the nitrogen-vacancy ZGNRs are more sensitive to the gas molecules than the pristine ZGNRs. The gas molecules absorbed on the three-nitrogen vacancies lead to sharp resonant peaks on conductance, while those absorbed on the four-nitrogen vacancies lead to anti-resonant dips. Each kind of gas molecule can be detected by its own unique (different energy) resonant peaks (or dips). This indicates that the nitrogen vacancy can enhance the sensitivity to gas molecules, i.e., nitrogen-vacancy ZGNRs can serve as better gas sensors.     

Transport properties in a monolayer graphene modulated by the realistic magnetic field and the Schottky metal stripe

Based on the transfer-matrix method, a systematic investigation of electron transport properties is done in a monolayer graphene modulated by the realistic magnetic field and the Schottky metal stripe. The strong dependence of the electron transmission and the conductance on the incident angle of carriers is clearly seen. The height, position as well as width of the barrier also play an important role on the electron transport properties. These interesting results are very useful for understanding the tunneling mechanism in the monolayer graphene and helpful for designing the graphene-based electrical device modulated by the realistic magnetic field and the electrical barrier.     

Shot noise fluctuations in disordered graphene nanoribbons near the Dirac point

Random fluctuations of the shot-noise power in disordered graphene nanoribbons are studied. In particular, we calculate the distribution of the shot noise of nanoribbons with zigzag and armchair edge terminations. We show that the shot noise statistics is different for each type of these two graphene structures, which is a consequence of the presence of different electron localizations: while in zigzag nanoribbons electronic edge states are Anderson localized, in armchair nanoribbons edge states are absent, but electrons are anomalously localized. Our analytical results are verified by tight binding numerical simulations with random hopping elements, i.e., off diagonal disorder, which preserves the symmetry of the graphene sublattices.     

Reprint of : Shot noise fluctuations in disordered graphene nanoribbons near the Dirac point

Random fluctuations of the shot-noise power in disordered graphene nanoribbons are studied. In particular, we calculate the distribution of the shot noise of nanoribbons with zigzag and armchair edge terminations. We show that the shot noise statistics is different for each type of these two graphene structures, which is a consequence of the presence of different electron localizations: while in zigzag nanoribbons electronic edge states are Anderson localized, in armchair nanoribbons edge states are absent, but electrons are anomalously localized. Our analytical results are verified by tight binding numerical simulations with random hopping elements, i.e., off diagonal disorder, which preserves the symmetry of the graphene sublattices.     

Surface adhesion properties of graphene and graphene oxide studied by colloid-probe atomic force microscopy

Surface adhesion properties are important to various applications of graphene-based materials. Atomic force microscopy is powerful to study the adhesion properties of samples by measuring the forces on the colloidal sphere tip as it approaches and retracts from the surface. In this paper we have measured the adhesion force between the colloid probe and the surface of graphene (graphene oxide) nanosheet. The results revealed that the adhesion force on graphene and graphene oxide surface were 66.3 and 170.6 nN, respectively. It was found the adhesion force was mainly determined by the water meniscus, which was related to the surface contact angle of samples.     

Atomistic simulations of divacancy defects in armchair graphene nanoribbons: Stability,electronic structure,and electron transport properties

Using the first principles calculations associated with nonequilibrium Green?s function, we have studied the electronic structures and quantum transport properties of defective armchair graphene nanoribbon (AGNR) in the presence of divacancy defects. The triple pentagon–triple heptagon (555–777) defect in the defective AGNR is energetically more favorable than the pentagon–octagon–pentagon (5–8–5) defect. Our calculated results reveal that both 5–8–5-like defect and 555–777-like defect in AGNR could improve the electron transport. It is anticipated that defective AGNRs can exhibit large range variations in transport behaviors, which are strongly dependent on the distributions of the divacancy defect.