Properties Of Monolayer Graphene Research Articles

Cumulated Effects of Magnetic Field, Electron–Phonon and Spin–Orbit Interaction on Thermodynamics Properties of Mono Layer Graphene

Cumulated Effects of Magnetic Field, Electron–Phonon and Spin–Orbit Interaction on Thermodynamics Properties of Mono Layer Graphene

Correlation of Valence electron structure and properties of monolayer graphene and MX2 (M=Mo, W; X=S, Se, Te): Empirical Electron Theory (EET) investigation

The atomically thin layers of transition-metal dichalcogenide (TMDC) materials have garnered considerable attention due to their exceptional electrical, optical, mechanical, and thermal properties. Hence, it is important to investigate the mechanism of their excellent properties. In this paper, the study is focused on the correlation between valence electron structures (VESs) and mechanical as well as thermal properties of graphene and MX2 (M = Mo, W; X = S, Se, Te) for revealing their essential mechanisms of properties with an empirical electron theory (EET). A model of Young's modulus is built for the monolayer graphene and MX2 (M = Mo, W; X = S, Se, Te) based on the VES, which has been verified by the observed ones of elements in the 4th to 6th periods in the periodic table of elements. The calculated bond lengths and mechanical and thermal properties of graphene and MX2 are in good agreement with experimental ones. The study reveals that the thermal and mechanical properties of MX2 strongly depend on their valence electron structures. It shows that the melting point, cohesive energy, thermal conductivity, and Young's modulus are modulated by covalence electron pair nA, the averaged covalence electron per atom nc/atom, covalence electron pair nA and linear density of covalent electron on the strongest bond ρl, respectively. The study helps explain the thermal and mechanical properties of two-dimensional (2D) materials and also supplies a reference for their design with high performance by modulating their valence electron structures.

Critical View on Buffer Layer Formation and Monolayer Graphene Properties in High-Temperature Sublimation

In this work we have critically reviewed the processes in high-temperature sublimation growth of graphene in Ar atmosphere using closed graphite crucible. Special focus is put on buffer layer formation and free charge carrier properties of monolayer graphene and quasi-freestanding monolayer graphene on 4H–SiC. We show that by introducing Ar at higher temperatures, TAr, one can shift the formation of the buffer layer to higher temperatures for both n-type and semi-insulating substrates. A scenario explaining the observed suppressed formation of buffer layer at higher TAr is proposed and discussed. Increased TAr is also shown to reduce the sp3 hybridization content and defect densities in the buffer layer on n-type conductive substrates. Growth on semi-insulating substrates results in ordered buffer layer with significantly improved structural properties, for which TAr plays only a minor role. The free charge density and mobility parameters of monolayer graphene and quasi-freestanding monolayer graphene with different TAr and different environmental treatment conditions are determined by contactless terahertz optical Hall effect. An efficient annealing of donors on and near the SiC surface is suggested to take place for intrinsic monolayer graphene grown at 2000 ∘C, and which is found to be independent of TAr. Higher TAr leads to higher free charge carrier mobility parameters in both intrinsically n-type and ambient p-type doped monolayer graphene. TAr is also found to have a profound effect on the free hole parameters of quasi-freestanding monolayer graphene. These findings are discussed in view of interface and buffer layer properties in order to construct a comprehensive picture of high-temperature sublimation growth and provide guidance for growth parameters optimization depending on the targeted graphene application.

Synthesis and Properties of Monolayer Graphene (MLG)-Covered Fe(111)

Fe undergoes a corrosion process under aerobic conditions. Oxidized forms of Fe lose malleability, durability, and other critical physical properties. For spintronic applications, alloy forms or ca...

Adsorption-induced spontaneous curvature and superflexibility in monolayer graphene

Graphene is considered an enabling material for next generation sensors, due to the remarkable properties derived from its atomic thickness. However, these promises come with acute sensitivity to parasite adsorbates, that may hinder reaching the full potential of commercial devices. Herein we report a detailed study of the dramatic effect of adsorbed carbon chains on the effective mechanical properties of graphene monolayers supported by different liquids. We first extract the equivalent bending stiffness of graphene/alcohol bilayers by observing the spontaneous wrinkling of graphene sheets floating on solutions of increasing alcohol contents. We find gains in flexibilities of more than three orders of magnitude compared to expected and previously reported values. We then implement the role of frustrated adsorption in classical membrane models and predict quantitatively the wrinkling transition with no fitting parameters. We also predict a simultaneous transition in alcohol adsorbed on the wrinkled graphene monolayer. We finally characterize this dual transition by detailed combined optical analysis and confocal Raman spectroscopy, and confirm the predictions of the presented model as well as the claims of superflexibility.

Effect of spatial distribution of nanopores on mechanical properties of mono layer graphene

Since the discovery of graphene, it has immense popularity among scientists and researchers due to its superior mechanical and electrical properties. In the present study, the effects of the spatial distribution of nanopore defects on the mechanical properties of the single-layer graphene sheet (SLGS) are investigated. Based on Tersoff potential functions, molecular dynamics (MD) simulations are conducted to perform the uniaxial deformation of defected graphene. The nanopore defects are induced intentionally at various spatial locations on a pristine graphene sheet for studying the variation in its mechanical properties such as fracture strength, Young’s modulus and failure strain. The results illustrate that the mechanical properties are predominantly dependent on the spatial locations of the defects. It is also observed that the mechanical properties are slightly higher in case of zigzag direction than armchair direction but it decreases with the presence of defects in both the cases. In the consequence, it is suggested to consider spatial locations of defects while fabricating nanodevices with graphene.

Temperature and Size Effect on the Electrical Properties of Monolayer Graphene based Interconnects for Next Generation MQCA based Nanoelectronics

Graphene interconnects have been projected to out-perform Copper interconnects in the next generation Magnetic Quantum-dot Cellular Automata (MQCA) based nano-electronic applications. In this paper a simple two-step lithography process for patterning CVD monolayer graphene on SiO2/Si substrate has been used that resulted in the current density of one order higher magnitude as compared to the state-of-the-art graphene-based interconnects. Electrical performances of the fabricated graphene interconnects were evaluated, and the impact of temperature and size on the current density and reliability was investigated. The maximum current density of 1.18 ×108 A/cm2 was observed for 0.3 μm graphene interconnect on SiO2/Si substrate, which is about two orders and one order higher than that of conventionally used copper interconnects and CVD grown graphene respectively, thus demonstrating huge potential in outperforming copper wires for on-chip clocking. The drop in current at 473 K as compared to room temperature was found to be nearly 30%, indicating a positive temperature coefficient of resistivity (TCR). TCR for all cases were studied and it was found that with decrease in width, the sensitivity of temperature also reduces. The effect of resistivity on the breakdown current density was analysed on the experimental data using Matlab and found to follow the power-law equations. The breakdown current density was found to have a reciprocal relationship to graphene interconnect resistivity suggesting Joule heating as the likely mechanism of breakdown.

Field Effect in Monolayer Graphene Associated with the Formation of Graphene–Water Interface

Establishing the features of interfacial effects on the electrical conductivity of graphene is crucial for successful design of novel graphene-based electronic devices, including chemical sensors and biosensors. We study electrical properties of monolayer graphene, prepared by thermal decomposition of silicon carbide in argon, in the field-effect transistor and the four-probe geometries. Alterations in the electrical properties of graphene in response to placing a quantity of water on its surface followed by removal of the water are investigated. In these geometries, the field effect is shown to play a key role in the way the electrical properties of graphene are affected by the formation of the graphene–water interface.

Layer-dependent band alignment of few layers of blue phosphorus and their van der Waals heterostructures with graphene

The structural and electronic properties of few layers of blue phosphorus and their van der Waals heterostructures with graphene were investigated by means of first-principles electronic structure calculations. We study the four energetically most stable stacking configurations for multilayers of blue phosphorus. For all of them, the indirect band-gap semiconductor character, are preserved. We show that the properties of monolayer graphene and single-layer (bilayer) blue phosphorus are preserved in the van der Waals heterostructures. Further, our results reveal that under a perpendicular applied electric field, the position of the band structure of blue phosphorus with respect to that of graphene is tunable, enabling the effective control of the Schottky barrier height. Indeed, for the bilayer blue phosphorene on top of graphene, it is possible to even move the system into an Ohmic contact and induce a doping level of the blue phosphorene. All of these features are fundamental for the design of new nanodevices based on van der Waals heterostructures.

Theoretical Model Study of Interplay of Coulomb Interaction and Electron-Phonon Interaction in the Thermal Properties of Monolayer Graphene

We propose here a tight-binding (TB) model Hamiltonian for monolayer graphene-on-substrate describing the nearest-neighbor-hopping, on-site Coulomb interaction on the sub-lattices and the electron-phonon interaction under the high-frequency limit of phonon vibration. Applying Lang-Firsov canonical transformation, the electron and phonon systems are decoupled in the atomic Hamiltonian, such that the effective Coulomb interaction and effective nearest-neighbor-hopping integral respectively appear as $\tilde {U}=U-2t_{1}\lambda $ and $\tilde {t}_{1}=t_{1}e^{\frac {-t_{1}\lambda } {\omega _{0}}}$ , where U, t1, λ and ω0 are respectively Coulomb energy, nearest-neighbor-hopping integral, electron-phonon (e-ph) coupling and phonon frequency. The effective Coulomb interaction in the Hamiltonian is considered within mean-field approximation. The Hamiltonian is solved by Zubarev’s Green’s function technique. The temperature-dependent electronic entropy and specific heat are calculated from the free energy of graphene system and are computed numerically. The temperature-dependent electronic specific heat exhibits a charge gap peak at room temperature arising due to the effect of Coulomb interaction and electron-phonon interaction. The evolution of these peaks in specific heat is investigated by varying the model parameters of the system.

Electro‐oxidized Monolayer CVD Graphene Film Transducer for Ultrasensitive Impedimetric DNA Biosensor

AbstractWe report the effect of electrochemical anodization on the properties of monolayer graphene as the main aim of this research and consequently using the resulting label‐free impedimetric biosensor for DNA sequences detection. Monolayer graphene was grown by chemical vapor deposition (CVD) with methane as precursor on copper foil, transferred onto a glassy carbon electrode and electrochemically anodized. Raman spectroscopy and X‐Ray photo electron spectroscopy revealed enhancement of defect density, roughness and formation of C−O−C, C−O−H and C=O functional groups after anodization. Amine‐terminated poly T probe was linked covalently to the carboxylic groups of anodized graphene by the zero‐length linker to fabricate the impedance‐based DNA biosensor. The anodized graphene electrode demonstrated a superior performance for electrochemical impedance detection of DNA. The DNA biosensor showed a large linear dynamic range from 2.0×10−18 to 1.0×10−12 M with a limit of detection of 1.0×10−18 M using electrochemical impedance spectroscopy (EIS) method. Equivalent circuit modeling shows that DNA hybridization is detected through a change in charge transfer resistance.

Application of RLGC modeling in probing the electrical properties of monolayer graphene at microwave frequencies

The S-parameters of the microstrip line loaded with graphene on glass and graphene on quartz as a superstrate are measured from 10 MHz to 26.5 GHz. These parameters have shown only a little difference compared to glass or quartz loaded microstrip line. Then, de-embedded S-parameters of superstrate loaded microstrip line are modeled as the transmission line (RLGC) parameters and these parameters are obtained with frequency. Their comparison with unloaded microstrip line shows large changes in R and G whereas, in L and C parameters, no significant changes have been noticed with superstrates. In these parameters, larger percentage changes for graphene on glass have showed a poor contact between two than graphene and quartz. Changes in R and G parameters confirmed that graphene has modified the surface resistance and effective relative permittivity on loading the microstrip line.

Effects of environmental conditions on the ultrafast carrier dynamics in graphene revealed by terahertz spectroscopy

A thorough understanding of the stability of graphene under ambient environmental conditions is essential for future graphene-based applications. In this paper, we study the effects of ambient temperature on the properties of monolayer graphene using terahertz time-domain spectroscopy as well as time-resolved terahertz spectroscopy enabled by an optical-pump/terahertz-probe technique. The observations show that graphene is extremely sensitive to the ambient environmental conditions and behaves differently depending on the sample preparation technique and the initial Fermi level. The analysis of the spectroscopic data is supported by van der Pauw and Hall effect measurements of the carrier mobility and carrier density at temperatures comparable to those tested in our THz spectroscopic experiments.

Ultrafast terahertz responses in monolayer graphene

ABSTRACTWe theoretically investigate the ultrafast terahertz (THz) properties of monolayer graphene. The analytical formulations of the photon carrier, electric polarization and optical current are obtained by solving the Bloch-equations in present of the ultrafast THz Gaussian pulse. Graphene shows a large nonlinear and ultrafast optical response at THz frequencies due to the gapless and relativistic Dirac particles with nearly linear energy dispersion. It is found that the photon carrier density, electric polarization and optical current density increase with increasing the frequency of the THz pulse. These theoretical results are in agreement with recent experimental findings. This study confirms further that graphene exhibits important features and is relevant to the applications in the ultrafast THz fields.

Enhancing the Electrochemical and Electronic Performance of CVD‐Grown Graphene by Minimizing Trace Metal Impurities

AbstractThe presence of unwanted impurities in graphene is known to have a significant impact on its physical and chemical properties. Similar to carbon nanotubes, any trace metals present in graphene will affect the electrocatalytic properties of the material. Here, we show by direct electroanalysis that traces of copper still remain in transferred CVD (chemical vapor deposition)‐grown graphene (even after the usual copper etching process) and strongly influence its electrochemical properties. Subsequently, we use a real‐time electrochemical etching procedure to remove more than 90 % of the trace metal impurities, with a clear improvement in both the electrochemical and electronic‐transport properties of monolayer graphene.

Electronic and transport properties of monolayer graphene defected by one and two carbon ad-dimers

Using density functional theory combined with non-equilibrium Green’s function method, we have investigated the electronic and transport properties of graphenes defected by one and two carbon ad-dimers (CADs), placed parallel to the graphene lattice. Addition of these CADs to graphenes creates 3D paired pentagon–heptagon defects (3D-PPHDs). The band structure, density of states (DOS), quantum conductance, projected DOS, as well as the current–voltage characteristic per graphene super-cells containing each type of 3D-PPHD are calculated. The local strain introduced to graphene by 3D-PPHDs forces the C-bonds in the dimers to hybridize in sp 3-like rather than sp 2-like orbitals, creating localized states at the center of the corresponding defect below the Fermi energy. Simulations show that the zero-bias conductances per super-cells containing defects created by one and two CADs exhibit dip about ~0.579 and ~0.253 eV below their corresponding Fermi levels, respectively. These can be attributed to the localized states around the same energy levels. Simulations also show that the enhanced carriers scatterings within the graphenes defected by the 3D-PPHDs have increased their overall resistances, as compared with the pristine graphene. Moreover, the current–voltage characteristic calculated per super-cell for each case shows that the current for those containing one and two CADs, at an applied voltage of 0.5 V, is ~5 and 13 % less than the current calculated for the pristine super-cell of the same size.

The selection rule of graphene in a composite magnetic field

The generalized tight-binding model with exact diagonalization method is developed to calculate the optical properties of monolayer graphene in the presence of composite magnetic fields. The ratio of the uniform magnetic field and the modulated one accounts for a strong influence on the structure, number, intensity and frequency of absorption peaks, and thus the extra selection rules that are subsequently induced can be explained. When the modulated field increases, each symmetric peak, under a uniform magnetic field, splits into a pair of asymmetric peaks with lower intensities. The threshold absorption frequency exhibits an obvious evolution in terms of a redshift. These absorption peaks obey the same selection rule that is followed by Landau level transitions. Moreover, at a sufficiently strong modulation strength, the extra peaks in the absorption spectrum might arise from different selection rules.

Effect of Oxygen Plasma on the Optical Properties of Monolayer Graphene

The significant alteration of absorption (A) of monolayer graphene under mild oxygen plasma exposure has been observed. The first important effect is the reduction of the excitonic resonance peak at 4.64 eV. Secondly, in the near infrared range, A is gradually suppressed below an exposure-dependent threshold in sense that A << A0. Quantity A0 (given by πα and α is the fine structure constant) denotes constant absorption and relates to universal optical conductivity σ0. The suppression of A0 can be thought as the weakening of electron-hole interaction as displayed by the reduction of the excitonic resonance peak at 4.64 eV. The weakening of this interaction is due to the disorder introduced by the oxygen plasma exposure.

Monte Carlo study of the electron transport properties of monolayer graphene within the tight-binding model

We study the effect of Coulomb interaction between charge carriers on the properties of graphene monolayer, assuming that the strength of the interaction is controlled by the dielectric permittivity of the substrate on which the graphene layer is placed. To this end we consider the tight-binding model on the hexagonal lattice coupled to the non-compact gauge field. The action of the latter is also discretized on the hexagonal lattice. Equilibrium ensembles of gauge field configurations are obtained using the Hybrid Monte-Carlo algorithm. Our numerical results indicate that at sufficiently strong coupling, that is, at sufficiently small substrate dielectric permittivities e<4, and at sufficiently small temperatures T<10^4 K the symmetry between simple sublattices of hexagonal lattice breaks down spontaneously and the low-frequency conductivity gradually decreases down to 20-30% of its weak-coupling value. On the other hand, in the weak-coupling regime (with e>4) the conductivity practically does not depend on e and is close to the universal value s=1/4.

Physics of Graphene

A brief review is given on exotic electronic and transport properties of monolayer graphene from a theoretical point of view.