The optical responsivity in IR-photodetector based on armchair graphene nanoribbons with p–i–n structure

Armchair graphene nanoribbons (A-GNRs) are an alternative material to use in novel infrared photodetectors, because of their tunable energy gap in the infrared spectrum, and their high quantum efficiency. In this paper, an A-GNR p–i–n structure with all three structural families, different width, and different number of layers to use in IR detectors have been investigated. With calculating the band structure and energy gap using the tight-binding model and by including the edge deformation, the optical absorption in the single electron approximation has been obtained by calculating the optical conductance. Finally, we have calculated the quantum efficiency and the optical responsivity of A-GNR based IR photodetector as a function of incident photon energy, temperature, nanoribbon width and the number of layers. Results show that the responsivity of the A-GNR based IR photodetector increase by increasing the width and number of layers and decrease by increasing the temperature.     

Theoretical calculation of excitonic binding energies and optical absorption spectra for Armchair graphene nanoribbons

In this paper the excitons of armchair graphene nanoribbons with layers of different width and thickness have been investigated. In this investigation, the band structure and energy gap of armchair graphene nanoribbons have been calculated using a tight-binding model including edge deformation effects (all edge atoms have been passivated with hydrogen atoms). Also, by calculating the conductance in armchair graphene nanoribbons (A-GNRs) optical absorption of armchair graphene nanoribbon in the single-electron approximation has been obtained. Finally, the binding energy of excitons in armchair graphene nanoribbons has been calculated using the Wannier model, Hartree-Fock approximation and the Bethe-Salpeter equation.     

Effect of electrical contact on performance of WSe2 field effect transistors

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) such as tungsten diselenide (WSe₂) have spead many interesting physical properties, which may become ideal candidates to develop new generation electronic and optoelectronic devices. In order to reveal essential features of 2D TMDCs, it is necessary to fabricate high-quality devices with reliable electrical contact. We systematically analyze the effect of graphene and metal contacts on performance of multi-layered WSe₂ field effect transistors (FETs). The temperature-dependent transport characteristics of both devices are tested. Only graphene-contacted WSe₂ FETs are observed with the metal-insulator transition phenomenon which mainly attributes to the ultra-clean contact interface and lowered contact barrier. Further characterization on contact barrier demonstrates that graphene contact enables lower contact barrier with WSe₂ than metal contact, since the Fermi level of graphene can be modulated by the gate bias to match the Fermi level of the channel material. We also analyze the carrier mobility of both devices under different temperatures, revealing that graphene contact can reduce the charge scattering of the device caused by ionized impurities and phonon vibrations in low and room temperature regions, respectively. This work is expected to provide reference for fabricating 2D material devices with decent performances.     

Tunable localized surface plasmon resonances in one-dimensional h-BN/graphene/h-BN quantum-well structure

The graphene/hexagonal boron-nitride(h-BN) hybrid structure has emerged to extend the performance of graphenebased devices. Here, we investigate the tunable plasmon in one-dimensional h-BN/graphene/h-BN quantum-well structures.The analysis of optical response and field enhancement demonstrates that these systems exhibit a distinct quantum confinement effect for the collective oscillations. The intensity and frequency of the plasmon can be controlled by the barrier width and electrical doping. Moreover, the electron doping and the hole doping lead to very different results due to the asymmetric energy band. This graphene/h-BN hybrid structure may pave the way for future optoelectronic devices.     

基于石墨烯纳米带的齿形表面等离激元滤波器的研究

提出了一种基于石墨烯纳米带的齿形表面等离激元波导滤波器, 并且用时域有限差分法研究了这种结构. 单个齿形的滤波器可以实现带阻滤波, 其滤波特性可以用基于散射矩阵的解析模型解释. 滤波器的透射谱特性可以通过调节齿的长度、宽度以及石墨烯的化学势来改变. 由于石墨烯的化学势可以用门电路来调节, 这种结构的滤波器可以在器件加工完成后灵活地调节其工作波长. 同时研究了多齿滤波器, 这种结构可以实现宽带滤波, 文中对具有不同齿数、周期的滤波器的透射谱进行了细致的研究. 研究结果对实现基于石墨烯的大规模集成光电子器件提供了重要的理论参考.     

Energy band-gap engineering of graphene nanoribbons

We investigate electronic transport in lithographically patterned graphene ribbon structures where the lateral confinement of charge carriers creates an energy gap near the charge neutrality point. Individual graphene layers are contacted with metal electrodes and patterned into ribbons of varying widths and different crystallographic orientations. The temperature dependent conductance measurements show larger energy gaps opening for narrower ribbons. The sizes of these energy gaps are investigated by measuring the conductance in the nonlinear response regime at low temperatures. We find that the energy gap scales inversely with the ribbon width, thus demonstrating the ability to engineer the band gap of graphene nanostructures by lithographic processes.     

Terahertz absorption window and high transmission in graphene bilayer

A detailed theoretical study of terahertz (THz) optical absorption and transmission in graphene bilayer is presented. Considering an air/graphene/dielectric-wafer system, it is found that there is an absorption window in the range 3-30 THz and the optical transmission is up to 96%. Such an absorption window is induced by different transition energies required for inter- and intra-band optical absorption in the presence of the Pauli blockade effect. As a result, the position and width of this THz absorption window depend sensitively on temperature and carrier density of the system. These results are pertinent to the applications of recently developed graphene systems as novel optoelectronic devices such as THz photodetectors.     

Perfectly conducting channel and universality crossover in disordered graphene nanoribbons

The band structure of graphene ribbons with zigzag edges have two valleys well separated in momentum space, related to the two Dirac points of the graphene spectrum. The propagating modes in each valley contain a single chiral mode originating from a partially flat band at the band center. This feature gives rise to a perfectly conducting channel in the disordered system, if the impurity scattering does not connect the two valleys, i.e., for long-range impurity potentials. Ribbons with short-range impurity potentials, however, through intervalley scattering display ordinary localization behavior. The two regimes belong to different universality classes: unitary for long-range impurities and orthogonal for short-range impurities.     

A review on silicene — New candidate for electronics

Silicene-the silicon-based counterpart of graphene-has a two dimensional structure that is responsible for the variety of potentially useful chemical and physical properties. The existence of silicene has been achieved recently owing to experiments involving epitaxial growth of silicon as stripes on Ag(001), ribbons on Ag(110), and sheets on Ag(111). The nano-ribbons observed on Ag(110) were found-by both high definition experimental scanning tunneling microscopy images and density functional theory calculations-to consist of an arched honeycomb structure. Angle resolved photo-emission experiments on these silicene nano-ribbons on Ag(110), along the direction of the ribbons, showed a band structure which is analogous to the Dirac cones of graphene. Unlike silicon surfaces, which are highly reactive to oxygen, the silicene nano-ribbons were found to be resistant to oxygen reactivity.On the theoretical side, recent extensive efforts have been deployed to understand the properties of standalone silicene sheets and nano-ribbons using both tight-binding and density functional theory calculations. Unlike graphene it is demonstrated that silicene sheets are stable only if a small buckling (0.44 Å) is present. The electronic properties of silicene nano-ribbons and silicene sheets were found to resemble those of graphene.Although this is a fairly new avenue, the already obtained outcome from these important first steps in understanding silicene showed promising features that could give a new future to silicon in the electronics industry, thus opening a promising route toward wide-range applications. In this review, we plan to introduce silicene by presenting the available experimental and theoretical studies performed to date, and suggest future directions to be explored to make the synthesis of silicene a viable one.     

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional(2D) layered materials have potential applications in nanoelectronics, nanophotonics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes(LEDs), and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.     

Electronic and transport properties of boron-doped graphene nanoribbons

We report a spin polarized density functional theory study of the electronic and transport properties of graphene nanoribbons doped with boron atoms. We considered hydrogen terminated graphene (nano)ribbons with width up to 3.2 nm. The substitutional boron atoms at the nanoribbon edges (sites of lower energy) suppress the metallic bands near the Fermi level, giving rise to a semiconducting system. These substitutional boron atoms act as scattering centers for the electronic transport along the nanoribbons. We find that the electronic scattering process is spin-anisotropic; namely, the spin-down (up) transmittance channels are weakly (strongly) reduced by the presence of boron atoms. Such anisotropic character can be controlled by the width of the nanoribbon; thus, the spin-up and spin-down transmittance can be tuned along the boron-doped nanoribbons.     

非近邻跳跃对扶手椅型石墨烯纳米带电子结构的影响

根据π电子的紧束缚模型,将电子的次近邻和第三近邻跳跃能考虑在内,得到扶手椅型石墨烯纳米带（AGRNs）能带结构的解析解.讨论了由次近邻和第三近邻电子跳跃引起的能带和能隙变化,发现次近邻和第三近邻跳跃分别对带隙产生增大和减小的影响. 比较了边界弛豫与非近邻跳跃之间的互相竞争关系. 当纳米带的宽度n为奇数时,二维石墨面的紧束缚模型中所固有的van Hove奇异性表现为AGRNs中的无色散带. 当AGRNs宽度增加时,能谱趋向于二维石墨烯时的能谱结构. 关键词： 扶手椅型石墨烯纳米带 非近邻跳跃 边界弛豫 电子结构     

光器件应用改性Ge的能带结构模型

Ge为间接带隙半导体,通过改性技术可以转换为准直接或者直接带隙半导体.准/直接带隙改性Ge半导体载流子辐射复合效率高,应用于光器件发光效率高;同时,准/直接带隙改性Ge半导体载流子迁移率显著高于Si半导体载流子迁移率,应用于电子器件工作速度快、频率特性好.综合以上原因,准/直接带隙改性Ge具备了单片同层光电集成的应用潜力.能带结构是准/直接带隙改性Ge材料实现单片同层光电集成的理论基础之一,目前该方面的工作仍存在不足.针对该问题,本文主要开展了以下三方面工作:1)揭示了不同改性条件下Ge材料带隙类型转化规律,完善了间接转直接带隙Ge实现方法的相关理论; 2)研究建立了准/直接带隙改性Ge的能带E-k模型,据此所获相关结论可为发光二极管、激光器件仿真模型提供关键参数; 3)提出了准/直接带隙改性Ge的带隙调制方案,为准/直接带隙改性Ge单片同层光电集成的实现提供了理论参考.本文的研究结果量化,可为准/直接带隙改性Ge材料物理的理解,以及Ge基光互连中发光器件有源层研究设计提供重要理论依据.     

Large-area graphene synthesis and its application to interface-engineered field effect transistors

This article reviews recent advances in the large-area synthesis of graphene sheets and the applications of such sheets to graphene-based transistors. Graphene is potentially useful in a wide range of practical applications that could benefit from its exceptional electrical, optical, and mechanical properties. Tremendous effort has been devoted to overcoming several fundamental limitations of graphene, such as a zero band gap and a low direct current conductivity-to-optical conductivity ratio. The intrinsic properties of graphene depend on the synthetic and transfer route, and this dependence has been intensively investigated. Several representative reports describing the application of graphene as a channel and electrode material for use in flexible transparent transistor devices are discussed. A fresh perspective on the optimization of graphene as a 2D framework for crystalline organic semiconductor growth is introduced, and its effects on transistor performance are discussed. This critical review provides insights and a new perspective on the development of high-quality large-area graphene and the optimization of graphene-based transistors.     

Effects of the impurity–impurity and impurity–host interactions on the charge density and the related processes

Transitions via impurities, in addition to transitions from the valence band to the conduction band in semiconductors increase the generation of electron–hole pairs. These transitions lead to alterations in the electronic density around the impurities, and as a consequence, to modifications in the forces on the impurities. In many cases it leads to an increase in the non-radiative recombination. The understanding of these processes at an atomic scale is very important for the physics of doped semiconductors, and technologically, for optoelectronic and spintronic devices. Using simple approximate models based on first-principles, the charge density and the related processes are analyzed. The results show that an increase in the impurity–host interaction or an increase in the concentration of the impurity leads to small variations in the density around the impurities for different charge states and a decrease in the effective Coulomb repulsion. These facts reduce the mechanism of non-radiative recombination via multi-phonon emission.     

Tunable graphene-based mid-infrared band-pass planar filter and its application

We have designed and proposed the edge modes supported by graphene ribbons and the planar band-pass filter consisting of graphene ribbons coupled to a graphene ring resonator by using the finite-difference time-domain numerical method.Simulation results show that the edge modes improve the electromagnetic coupling between devices. This structure works as a novel, tunable mid-infrared band-pass filter. Our studies will benefit the fabrication of planar, ultra-compact nano-scale devices in the mid-infrared region. A power splitter consisting of two output ribbons that is useful in photonic integrated devices and circuits is also designed and simulated. These devices are useful for designing ultra-compact planar devices in photonic integrated circuits.     

Improvement of sensitivity of graphene photodetector by creating bandgap structure

Graphene has aroused large interest in optoelectronic applications because of its broad band absorption and ultrahigh electron mobility. However, the low absorption of 2.3% seriously limits its photoresponsivity and restricts the relevant applications. In this paper, a method to enhance the sensitivity of graphene photodetector is demonstrated by introducing electron trapping centers and creating a bandgap structure in graphene. The carrier lifetime obviously increases, and more carriers are collected by the electrodes. Compared with intrinsic graphene detector, the defective graphene photodetector possesses high photocurrent and low-driving-voltage, which gives rise to great potential applications in photodetector area.     

The influence of impurity formation on electron inelastic scattering of suspended graphene

This study reports on controlling the formation of nanoimpurities on suspended graphene to investigate the inelastic scattering of electrons using a two‐phonon Raman process. Results were analyzed by transmission electron microscopy (TEM) and scanning Raman spectroscopy in the same region of suspended graphene. The findings revealed that the area with a higher concentration of impurities shown in the TEM image corresponds directly to the area with a lower integrated intensity and a wider full width at half maximum in the Raman mapping of the 2D band and vice versa. The same trend is also apparent in the 2D′ and D + D″ bands. In conclusion, the results are explained by an increase in the electronic scattering rate due to impurities, which affects two‐phonon Raman scattering. Combining the TEM image and Raman mapping image effectively demonstrates how electron behavior is affected by the distribution of impurities in graphene systems. Copyright © 2012 John Wiley & Sons, Ltd.     

Strain engineering of graphene nanoribbons: pseudomagnetic versus external magnetic fields

Bandgap opening due to strain engineering is a key architect for making graphene’s optoelectronic, straintronic, and spintronic devices. We study the bandgap opening due to strain induced ripple waves and investigate the interplay between pseudomagnetic fields and externally applied magnetic fields on the band structures and spin relaxation in graphene nanoribbons (GNRs). We show that electron-hole bands of GNRs are highly influenced (i.e. level crossing of the bands are possible) by coupling two combined effects: pseudomagnetic fields (PMF) originating from strain tensor and external magnetic fields. In particular, we show that the tuning of the spin-splitting band extends to large externally applied magnetic fields with increasing values of pseudomagnetic fields. Level crossings of the bands in strained GNRs can also be observed due to the interplay between pseudomagnetic fields and externally applied magnetic fields. We also investigate the influence of this interplay on the electromagnetic field mediated spin relaxation mechanism in GNRs. In particular, we show that the spin hot spot can be observed at approximately B = 65 T (the externally applied magnetic field) and B₀ = 53 T (the magnitude of induced pseudomagnetic field due to ripple waves) which may not be considered as an ideal location for the design of straintronic devices. Our analysis might be used for tuning the bandgaps in strained GNRs and utilized to design the optoelectronic devices for straintronic applications.     

Real-space pseudopotential calculations for graphene dots embedded in hexagonal boron nitride

A major challenge for graphene-based applications is the creation of a tunable electronic band gap as would be present for traditional semiconductor alloys. Since hexagonal boron nitride has a very similar lattice structure to graphene, it is a natural candidate for modifying the electronic structure of graphene by forming a hybrid phase sheet containing domains of graphene and hexagonal boron nitride, as has been done experimentally. Here we investigate the properties of such hybrid sheets using pseudopotential-density functional theory implemented in real space. We find for a graphene dot comparable in size to those observed in experiment, the band gap of the sheet is not significantly modified.