Electron Transport In Graphene Research Articles

Klein tunneling degradation and enhanced Fabry-Pérot interference in graphene/h-BN moiré-superlattice devices

Hexagonal boron-nitride (h-BN) provides an ideal substrate for supporting graphene devices to achieve fascinating transport properties, such as Klein tunneling, electron optics and other novel quantum transport phenomena. However, depositing graphene on h-BN creates moiré superlattices, whose electronic properties can be significantly manipulated by controlling the lattice alignment between layers. In this work, the effects of these moiré structures on the transport properties of graphene are investigated using atomistic simulations. At large misalignment angles (leading to small moiré cells), the transport properties (most remarkably, Klein tunneling) of pristine graphene devices are conserved. On the other hand, in the nearly aligned cases, the moiré interaction induces stronger effects, significantly affecting electron transport in graphene. In particular, Klein tunneling is significantly degraded. In contrast, strong Fabry-Pérot interference (accordingly, strong quantum confinement) effects and non-linear I-V characteristics are observed. P-N interface smoothness engineering is also considered, suggesting as a potential way to improve these transport features in graphene/h-BN devices.

Effect of Charge Density Wave on the Electronic Transport in Graphene

Effect of Charge Density Wave on the Electronic Transport in Graphene

Electronic transport in graphene with out-of-plane disorder

Real-world samples of graphene often exhibit various types of out-of-plane disorder–ripples, wrinkles and folds–introduced at the stage of growth and transfer processes. These complex out-of-plane defects resulting from the interplay between self-adhesion of graphene and its bending rigidity inevitably lead to the scattering of charge carriers thus affecting the electronic transport properties of graphene. We address the ballistic charge-carrier transmission across the models of out-of-plane defects using tight-binding and density functional calculations while fully taking into account lattice relaxation effects. The observed transmission oscillations in commensurate graphene wrinkles are attributed to the interference between intra- and interlayer transport channels, while the incommensurate wrinkles show vanishing backscattering and retain the transport properties of flat graphene. The suppression of backscattering reveals the crucial role of lattice commensuration in the electronic transmission. Our results provide guidelines to controlling the transport properties of graphene in presence of this ubiquitous type of disorder.

Spatiotemporal Observation of Quasi-Ballistic Transport of Electrons in Graphene.

We report spatiotemporal observations of room-temperature quasi-ballistic electron transport in graphene, which is achieved by utilizing a four-layer van der Waals heterostructure to generate free charge carriers. The heterostructure is formed by sandwiching a MoS2 and MoSe2 heterobilayer between two graphene monolayers. Transient absorption measurements reveal that the electrons and holes separated by the type-II interface between MoS2 and MoSe2 can transfer to the two graphene layers, respectively. Transient absorption microscopy measurements, with high spatial and temporal resolution, reveal that while the holes in one graphene layer undergo a classical diffusion process with a large diffusion coefficient of 65 cm2 s-1 and a charge mobility of 5000 cm2 V-1 s-1, the electrons in the other graphene layer exhibit a quasi-ballistic transport feature, with a ballistic transport time of 20 ps and a speed of 22 km s-1, respectively. The different in-plane transport properties confirm that electrons and holes move independently of each other as charge carriers. The optical generation of ballistic charge carriers suggests potential applications for such van der Waals heterostructures as optoelectronic materials.

TETHYS: A simulation tool for graphene hydrodynamic models

We present a simulation tool, TETHYS (i.e., Two-dimensional Emitter of THz, Hydrodynamic Simulation), developed with the aim of studying hydrodynamic models for electronic transport in graphene, being particularly tailored for the simulation of the configuration of top-gated graphene field-effect transistors (GFET) near room temperature. We present the model governing the system and briefly discuss its validity and physical parameters; then we proceed to discuss the algorithms and numerical methods used in its implementation, namely, two-step Richtmyer and weighted forward-time centred-space schemes, which address the hyperbolic and parabolic parts of the model, respectively. An abridged usage guide and description of the output are also provided, as well as two illustrative simulation cases: the Dyakonov–Shur instability and the boundary layer of pulsatile electric flow. The simulation program here brought forward is uncomplicated and lightweight, yet robust and accurate, having a wide variety of applicable scenarios and configurations to explore. Program summaryProgram Title: TETHYSCPC Library link to program files:https://doi.org/10.17632/s28zwgpnpw.1Developer's repository link:github.com/pcosme/TETHYS-Graphene-Hydrodynamic-SimulationLicensing provisions: MITProgramming language: C++Nature of problem: The main objective of this code is to simulate and analyse the electronic conduction on gated (mono-layer) graphene resorting to a hydrodynamic model [1]. Note that, given the Dirac-like dispersion relation of such material, the mass of the carriers is ill-defined; and, in fact, the mass of the fluid element varies with the number density, which sets this system apart from other 2D electron fluids [2]. The model assumes enough doping, so that only one kind of carriers, electrons or holes, participates in the dynamics of the system. Moreover, the simulation is limited to the fully degenerate limit.Thus, these assumptions correspond to the situation of a regular GFET and this application can be used to explore atypical phenomena arising from the effects of uniform and static magnetic field and Hall viscosity, and even spatially non-uniform gate capacitance.Furthermore, this software was designed with the intent of examining unstable nonlinear modes that may lead to the emission of radiation in the THz range [3].Solution method: The fluid equations (continuity, momentum and energy transport) are solved with a split-operator technique, with a second-order finite-volume Lax–Wendroff method [4] for the hyperbolic part and a forward-time centred-space of second-order stencil to tackle the dissipation and heat diffusion. That is, the fields of number density, velocity/momentum density and temperature are calculated over a rectangular grid and stored at a HDF5 file for further processing.Then, the obtained field profiles are used to compute the relevant macroscopic electronic quantities, such as drain-to-source current or dissipated power, and, particularly, the electric dipole moment of the charge distribution, which can subsequently be used to characterize the emitted far-field radiation. This is mostly done by numerically integrating the data in space, with the necessary time derivatives obtained by Gaussian convolution.

Quantum energy-transport and drift-diffusion models for electron transport in graphene: an approach by the wigner function

The present work aims at formulating quantum energy-transport and drift-diffusion equations for charge transport in graphene from a quantum hydrodynamic model proposed in Luca and Romano (Ann. Phys. 406:30–53, 2019), obtained from the Wigner-Boltzmann equation via the moment method. Here, we only sketch the main ideas and show the most relevant formulas. Further developments are under current investigation by the authors. The interested reader can find the omitted details in the references. In analogy with the semiclassical case, we are confident that the energy-transport and drift-diffusion models have mathematical properties which allow an easier numerical treatment.

Machine learning approach to the recognition of nanobubbles in graphene

Since the local and elastic strain induced by nanobubbles largely affects the transport properties of graphene, detecting and probing nanobubbles are important processes for research on electronic transport in graphene. In this study, we propose a means to recognize the presence of nanobubbles in graphene by analyzing electronic properties based on a machine learning approach. Our machine learning algorithm efficiently classifies the density of states spectra by the height and width of the nanobubbles, even in cases with a substantial magnitude of noise. The machine-learning-based analysis of electronic properties proposed in this study may introduce a changeover in the probing of nanobubbles from image-based detection to electrical-measurement-based recognition.

GRAPHENE - A NEW AGE OF MATERIAL: A REVIEW

Abstract - We all be familiar with two well-known forms of Carbon which are diamond and graphite. Diamond is a one of the toughest and hard material in the earth. Graphite is well-known as most brittle material. But one thing had changed forever, when two physicists A.K.Geim and K.S. Novoselov had got noble prize for “Graphene”. On the basis of his research work published title” The rise of Graphene”. His work shows that Graphene is 200 times stronger than cast iron. It have so many desirable qualities to use in our daily life. Graphene, becoming eminent as a true 2-D material, had got ever-increasing attention due to its unmatched physicochemical properties (excellent conductivity, high surface area, high mechanical strength ,ease of functionalization and mass production). Graphene is a extraordinarily amazing material with countless highest extents to its name. It is the thinnest material in the universe and the strongest yet calculated. Its charge carriers exhibit massive inherent mobility, have the negligible effective mass (it is zero) and can travel micrometer-long distances without scattering at room temperature. Graphene be able to uphold current densities 6 orders higher than copper, shows record thermal conductivity and stiffness, is impermeable to gases and reconciles such contradictory qualities as brittleness and ductility. Electron transport in graphene is described by a Dirac-like equation.

Transient performance analysis of graphene FET gated via ionic solid by numerical simulations based on tight-binding method and Nernst–Planck–Poisson equations

We investigate the electrical transport characteristics of graphene channel field-effect transistors (FETs) gated via ionic solid (IS), where the conventional gate insulator, such as SiO2, has been replaced by solid electrolytes, such as LiP3O4. In this study, we focus on (i) the gate controllability of the current in comparison to conventional graphene FETs with SiO2 as an insulating material and (ii) the transient characteristics of the drain current and time required to switch on the current. We employ the tight-binding formalism and Boltzmann equation to calculate the electronic band structure and the electronic transport in graphene, while the Nernst–Planck–Poisson equations have been employed to calculate the time-dependent charge distribution in solid electrolytes and the resulting electric double layer formation at the graphene/IS and IS/gate interfaces. Our simulations have shown that graphene FET gated via IS shows superior gate controllability more than SiO2-gated graphene FET with the insulator thickness of 1 nm, and the saturated drain current is insensitive to the IS thickness. Moreover, the time required to switch on the drain current is proportional to the thickness of IS, and the limited number of Li+ ion vacancies in IS is preferable in obtaining faster switching than the case of unlimited vacancy cases while keeping the superior gate controllability.

Hydrodynamic Approach to Electronic Transport in Graphene: Energy Relaxation

In nearly compensated graphene, disorder-assisted electron-phonon scattering or “supercollisions” are responsible for both quasiparticle recombination and energy relaxation. Within the hydrodynamic approach, these processes contribute weak decay terms to the continuity equations at local equilibrium, i.e., at the level of “ideal” hydrodynamics. Here we report the derivation of the decay term due to weak violation of energy conservation. Such terms have to be considered on equal footing with the well-known recombination terms due to nonconservation of the number of particles in each band. At high enough temperatures in the “hydrodynamic regime” supercollisions dominate both types of the decay terms (as compared to the leading-order electron-phonon interaction). We also discuss the contribution of supercollisions to the heat transfer equation (generalizing the continuity equation for the energy density in viscous hydrodynamics).

Quantum transport, electronic properties and molecular adsorptions in graphene

Molecular sensor applications are used in different fields including environmental monitoring and medical diagnosis. Graphene, a single atomic layer consisting of the hexagonally arranged carbon material, is one of the most promising materials for ideal channels in field-effect transistors to be used as electronic sensing applications owing to its lightweight, mechanical robustness, high electronic conductivity and large surface-to-volume ratio. This paper provides a review of molecular adsorptions, electronic properties and quantum transport of graphene based on the first-principles density-functional study. The adsorption properties of environmentally polluting or toxic molecules and electronic transport of graphene are revealed. The possibility of detecting these molecules selectively is also discussed for designing the graphene-based sensor applications.

Semi-analytical approach to transport gaps in polycrystalline graphene.

Electron transport in graphene is dominated by its Dirac-like charge carriers. Grain boundaries add a geometric aspect to the transport behavior by coupling differently oriented grains. In the phase coherent limit this aspect allows to relate the transport properties to two factors: the electronic structure of individual grains around the Dirac points and the orientation relation of the Dirac cones within the grain boundary Brillouin zone. Based on this picture it is possible to quantify the size and strain modulation of transport gaps without the need for explicit transport calculations within the non-equilibrium Green functions formalism. In this work we present a semi-analytical method that exploits this picture. Our method can explore arbitrary grain misorientations in the presence of an external strain providing valuable information about the electronic properties of individual grain boundaries.

Graphene as a Schottky Barrier Contact to AlGaN/GaN Heterostructures.

Electrical and noise properties of graphene contacts to AlGaN/GaN heterostructures were studied experimentally. It was found that graphene on AlGaN forms a high-quality Schottky barrier with the barrier height dependent on the bias. The apparent barrier heights for this kind of Schottky diode were found to be relatively high, varying within the range of φb = (1.0–1.26) eV. AlGaN/GaN fin-shaped field-effect transistors (finFETs) with a graphene gate were fabricated and studied. These devices demonstrated ~8 order of magnitude on/off ratio, subthreshold slope of ~1.3, and low subthreshold current in the sub-picoamperes range. The effective trap density responsible for the 1/f low-frequency noise was found within the range of (1–5) · 1019 eV−1 cm−3. These values are of the same order of magnitude as reported earlier and in AlGaN/GaN transistors with Ni/Au Schottky gate studied as a reference in the current study. A good quality of graphene/AlGaN Schottky barrier diodes and AlGaN/GaN transistors opens the way for transparent GaN-based electronics and GaN-based devices exploring vertical electron transport in graphene.

Performance improvement of perovskite heterojunction solar cell using graphene

Abstract The performance of a perovskite solar cell (PSC) is improved with a graphene electron transport layer (ETL) using Silvaco simulation software. Factors affecting solar cell performance such as absorber layer thickness, carrier mobility, doping concentration and carrier transport layer thickness have been studied. Higher carrier mobility, and fabrication cost are the most important factors toward the increment of the PSC utilization. Graphene is used here as an ETL due to its high carrier mobility, high optical transparency and good optical, mechanical and electrical properties at a low cost. The simulation is based on glass/ITO/Graphene/CH3NH3PbI3/Spiro-OMeTAD/Au structure which yields to photovoltaic (PV) parameters of Voc = 1.094 V, Jsc = 21.24 mA/cm2, FF = 90.185%, and power conversion efficiency of 20.95%. Moreover, the optimum thicknesses of 600 nm and 21 nm are determined for the perovskite absorber and ETL, respectively.

Oxygen intercalated graphene on SiC(0001): Multiphase SiOx layer formation and its influence on graphene electronic properties

Low-dimensionality materials are highly susceptible to interfaces. Indeed, intercalation of different chemical species in between epitaxial graphene and silicon carbide (SiC), for instance, may decouple the graphene with respect to the substrate due to the conversion of the buffer layer into a graphene layer. O-intercalation is known to release the strain of such 2D material and to lead to the formation of high structural quality AB-stacked bilayer graphene. Nonetheless, this interface transformation concomitantly degrades graphene electronic transport properties. In this work we employed different techniques in order to better understand the structure of the graphene/SiC interface generated by O-intercalation and to elucidate the origin of the poor electronic properties of graphene. Experimental results revealed the formation of a SiO2 rich layer with a defective transition layer in between it and the SiC, which is characterized by the existence of silicon oxycarbide structures. Scanning tunneling spectroscopy measurements revealed an extensive presence of electronic states just around the Fermi level all over the sample surface, which may suppress the charge carriers mobility around this region. According to theoretical calculations, such states are mainly due to the formation of silicon oxicarbides within the interfacial layer.

Electronegative ligands enhance charge transfer to Mn12 single-molecule magnets deposited on graphene

Combining single-molecule magnets (SMMs) and emergent two-dimensional substrates such as graphene may lead to device configurations that are promising for spintronics and quantum computing. However, to fully exploit the unique features of SMMs anchored to two-dimensional substrates, the choice of ligand attachments, which could affect the magnetic and electronic properties, is critical. In this work, we focus on hybrid junctions comprising CVD-grown graphene and [Mn12O12(O2CR)16(H2O)4](R=CH3,CHCl2) SMMs with different ligands. We find that [Mn12O12(O2CCH3)16(H2O)4] SMMs barely change the graphene’s conductivity, while [Mn12O12(O2CCHCl2)16(H2O)4] SMMs with more electronegative ligands, by means of charge transfer, remarkably modify the electronic transport in graphene as revealed by gate-voltage dependent magnetotransport measurements.

Perovskite Solar Cells: SrTiO3/Al2O3‐Graphene Electron Transport Layer for Highly Stable and Efficient Composites‐Based Perovskite Solar Cells with 20.6% Efficiency (Adv. Energy Mater. 2/2020)

Perovskite Solar Cells: SrTiO₃/Al₂O₃‐Graphene Electron Transport Layer for Highly Stable and Efficient Composites‐Based Perovskite Solar Cells with 20.6% Efficiency (Adv. Energy Mater. 2/2020)

Enhanced Optical Absorption and Interfacial Carrier Separation of CsPbBr3/Graphene Heterostructure: Experimental and Theoretical Insights.

Controlling effective separation of carriers at the interface is a key element to realize highly efficient halogenated perovskite-based optoelectronic devices. Here, a comprehensive study of interfacial properties for CsPbBr3 nanocrystals (NCs)/graphene heterostructure is performed by the combination of theoretical and experimental methods. Enhanced visible light absorption is observed experimentally in the CsPbBr3 NCs/graphene heterostructure. The strong photoluminescence quenching phenomenon and improved photoresponse prove the efficient interfacial charge transfer from the perovskite CsPbBr3 NC layer to the graphene side. Significantly, theoretical calculations suggest that an intrinsic built-in electric field, pointing from graphene toward CsPbBr3, promotes the separation of photoinduced carriers at the CsPbBr3 NCs/graphene interface and simultaneously inhibits the recombination of electron-hole pairs. Thus, the high optoelectronic performance can be obtained in the CsPbBr3 NCs/graphene heterostructure, as shown in our experiment. Moreover, the CsPbBr3 NCs/graphene heterostructure exhibits smaller effective mass than that of CsPbBr3 NCs, indicating that the heterostructure does possess a high carrier mobility, which can further accelerate the separation of photogenerated carriers. Furthermore, the calculated results reveal that, accounting for the presence of the stronger built-in electric field, larger band bending value, and smaller effective mass, the PbBr2/graphene interface can realize the separation of the photoinduced carriers more effectively than the CsBr/graphene interface and thus more efficiently facilitate electron transfer from the perovskite optical absorber side to the graphene electronic transport side. Our findings provide valuable insight into perovskite/graphene-based photodetector devices via the interface engineering project.

Terahertz Nonlinear Optics of Graphene: From Saturable Absorption to High‐Harmonics Generation

AbstractGraphene has long been predicted to show exceptional nonlinear optical properties, especially in the technologically important terahertz (THz) frequency range. Recent experiments have shown that this atomically thin material indeed exhibits possibly the largest nonlinear coefficients of any material known to date, paving the way for practical graphene‐based applications in ultrafast (opto‐)electronics operating at THz rates. Here the advances in the booming field of nonlinear THz optics of graphene are reported, and the state‐of‐the‐art understanding of the nature of the nonlinear interaction of graphene with the THz fields based on the thermodynamic model of electron transport in graphene is described. A comparison between different mechanisms of nonlinear interaction of graphene with light fields in THz, infrared, and visible frequency ranges is also provided. Finally, the perspectives for the expected technological applications of graphene based on its extraordinary THz nonlinear properties are summarized. This report covers the evolution of the field of THz nonlinear optics of graphene from the very pioneering to the state‐of‐the‐art works. It also serves as a concise overview of the current understanding of THz nonlinear optics of graphene and as a compact reference for researchers entering the field, as well as for the technology developers.

Effect of strained barrier on valley polarization in magnetic-modulated graphene

In this paper, we investigate the valley-dependent electron transport in graphene modulated by a strained barrier which can induce a large valley polarization in this structure. Our results show that the valley polarization can be easily controlled by changing the strength, the position and/or the width of the strained barrier. This remarkable feature could be used in the fabrication of valleytronic devices based on graphene controlled by the strained barrier.