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Electron-electron interactions (EEI) play a pivotal role in the transport behavior of carriers confined in a two-dimensional system. The strength of the interaction can be tuned by changing carrier concentration in a conventional two-dimensional system, while it expectedly cannot be controlled in monolayer graphene with a linear dispersion relation since the interaction parameter is not related to carrier concentration. Here, an anomalous carrier-concentration dependence of the EEI is observed in epitaxial graphene on SiC. From quantum transport measurement, a logarithmic temperature dependence of the Hall coefficient is obtained, which is a manifestation of the EEI. From the logarithmic temperature dependence, the EEI-related factor K e e is extracted. Unexpectedly, the extracted K e e shows an increase on increasing carrier concentration, which is further confirmed by analyzing logarithmic correction to the Drude conductivity. This can be attributed to the carrier-concentration dependence of Fermi-liquid constant F 0 σ due to gas absorption, which offers a route to control the EEI in graphene.

Bilayer graphene has attracted interest for its unique properties, including interesting electrical behavior when one layer is slightly rotated relative to the other. However, the quality of large-area bilayer graphene is often limited by the layer-plus-island growth mode in which islands of thicker graphene present as unavoidable impurities. Here, we report the observation of the layer-by-layer, Frank-van der Merwe (FM) growth mode in bilayer graphene where multilayer impurities are suppressed. Instead of the conventional surface adhesive energy, it is found that interface adhesive energy is possible to be tuned with an oxidative pretreatment. The FM-grown bilayer graphene is of AB-stacking or with small-twisting-angle (θ = 0-5°), which is more mechanically robust compared to monolayer graphene, facilitating a free-standing wet transfer technology.

We report on magnetotransport in a high-quality graphene device, which is based on monolayer graphene (Gr) encapsulated by hexagonal boron nitride (hBN) layers, i.e., hBN/Gr/hBN stacks. In the vicinity of the Dirac point, a negative magnetoconductance is observed for high temperatures > ~ 40 K, whereas it becomes positive for low temperatures ≤ ~ 40 K, which implies an interplay of quantum interferences in Dirac materials. The elastic scattering mechanism in hBN/Gr/hBN stacks contrasts with that of conventional graphene on SiO2, and our ultra-clean graphene device shows nonzero magnetoconductance for high temperatures of up to 300 K.

The complex nature of filling factor ν = 0 of monolayer graphene is studied in magnetotransport experiments. As a function of perpendicular magnetic field a metal-insulator transition is observed, which is attributed to disorder-induced Landau level broadening in the canted antiferromagnetic phase. In the metallic regime a separation of the zeroth Landau level appears and signs of the quantum spin Hall effect are seen near ν = 0. In addition to local transport, nonlocal transport experiments show results being consistent with helical edge transport.

We investigated the thermoelectric (TE) properties of the MoS2 monolayer-graphene heterostructure which consists of the MoS2 monolayer and graphene. The electronic structures of the MoS2 monolayer-graphene heterostructure are mainly contributed from graphene and the MoS2 monolayer for the valence band maximum and conduction band minimum, respectively. The change in the electronic structures near the Fermi level is responsible for the fact that the calculated Seebeck coefficients S and electrical conductivity σ/τ of MoS2 monolayer-graphene are largely affected from those of graphene and the MoS2 monolayer. Its power factor S 2σ/τ is increased compared to those of graphene and the MoS2 monolayer at an electron concentration of 1011 to 1012 cm-2, which corresponds to a three-dimensional concentration of 3 × 1018 to 3 × 1019 cm-3. We also demonstrated that the MoS2 monolayer shows the p-type TE behavior, while the MoS2 monolayer-graphene heterostructure is given to the n-type TE material. The current study provides a strategy to improve TE properties of the MoS2 monolayer through the formation of the MoS2 monolayer-graphene heterostructure.

In this article, silicene nanoribbon (SiNR)-based tunneling field effect transistor (TFET) is investigated for low power digital applications. A comprehensive study on the role of gate dielectric and device parameters, such as drain doping, channel length of SiNR TFETs, are carried out to see the performance of digital applications. To achieve higher ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio and lower subthreshold swing (SS), a detailed investigation is carried out on the equivalent oxide thickness (EOT), channel length, and doping of drain. A digital performance metric comparison of SiNR TFETs with germanium–silicon (Ge-Si), monolayer graphene nanoribbon (MLGNR), and bilayer graphene nanoribbon (BLGNR) TFETs has been performed. It is concluded that SiNR TFETs made of Al2O3 gate dielectric with 0.86 nm effective oxide thickness, 0.5 mM ( $8\,\times \,10^{12}\,\,cm^{-2}$ ) drain doping concentration, and 5 mM ( $8\,\times \,10^{13}\,\,cm^{-2}$ ) source doping concentration exhibits lower intrinsic delay, higher ${I}_{ \mathrm{\scriptscriptstyle ON}}$ , higher ${I}_{ \mathrm{\scriptscriptstyle ON}}/{I}_{ \mathrm{\scriptscriptstyle OFF}}$ ratio, and lower SS at 25 nm channel length.

The present work intends to discover the influences of 60Co gamma (γ) ray-irradiation on frequency-dependent dielectric features of Graphene/Silicon Schottky diode with an insulator layer. Graphene (Gr) nanosheets have been synthesized by chemical vapor deposition (CVD) to build a Gr-based p-type Si Schottky diode. The diode was irradiated at 30 kGy and 60 kGy doses. The study has been performed at 300 K in the voltage range −6 V to +6 V at dark conditions both at 400 kHz low-frequency and 900 kHz high-frequency. The experimental results showed that dielectric features of the structure are dependent on the radiation dose and applied voltage and to be a strong function of frequency.

The probability of terahertz transitions between Landau levels for various magnetic fields at 4.2 K is calculated at different positions of Fermi levels corresponding to different concentrations of...

We calculate the plasmon frequency and damping rate in a double-layer system made of monolayer graphene and GaAs quantum well at finite temperature using the random-phase-approximation dielectric function and taking into account the inhomogeneity of the dielectric background of the system. We show that the temperature, interlayer correlation parameters and dielectric background inhomogeneity affect significantly the plasmon frequencies and damping rates of the system. At low temperatures, acoustic (optical) plasmon frequency increases (decreases) with the increase of temperature. We also find that damping rates of both plasmon modes increase remarkably compared to the zero-temperature case.

Phase contrast transmission electron microscopy (TEM) is a powerful tool for imaging the local atomic structure of materials. TEM has been used heavily in studies of defect structures of 2D materials such as monolayer graphene due to its high dose efficiency. However, phase contrast imaging can produce complex nonlinear contrast, even for weakly-scattering samples. It is therefore difficult to develop fully-automated analysis routines for phase contrast TEM studies using conventional image processing tools. For automated analysis of large sample regions of graphene, one of the key problems is segmentation between the structure of interest and unwanted structures such as surface contaminant layers. In this study, we compare the performance of a conventional Bragg filtering method to a deep learning routine based on the U-Net architecture. We show that the deep learning method is more general, simpler to apply in practice, and produces more accurate and robust results than the conventional algorithm. We provide easily-adaptable source code for all results in this paper, and discuss potential applications for deep learning in fully-automated TEM image analysis., 12 pages, 6 figures

The authors demonstrate higher-order topological corner states and related fractional charge in monolayer graphene.

Monolayer graphene aligned with hexagonal boron nitride ($h\text{\ensuremath{-}}\mathrm{BN}$) develops a gap at the charge neutrality point (CNP). This gap has previously been extensively studied by electrical transport through thermal activation measurements. Here, we report the determination of the gap size at the CNP of $\mathrm{graphene}/h\text{\ensuremath{-}}\mathrm{BN}$ superlattice through photocurrent spectroscopy study. We demonstrate two distinct measurement approaches to extract the gap size. A maximum of $\ensuremath{\sim}14\text{ }\text{ }\mathrm{meV}$ gap is observed for devices with a twist angle of less than 1\ifmmode^\circ\else\textdegree\fi{}. This value is significantly smaller than that obtained from thermal activation measurements, yet larger than the theoretically predicted single-particle gap. Our results suggest that lattice relaxation and moderate electron-electron interaction effects may enhance the CNP gap in $\mathrm{graphene}/h\text{\ensuremath{-}}\mathrm{BN}$ superlattice.

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.

The conventional thermal chemical vapor deposition (T-CVD) equipment takes a long time to synthesize a thin film, which is the stumbling block in the commercialization of monolayer graphene. Rapid thermal chemical vapor deposition (RT-CVD) equipment is suitable for rapid graphene synthesis on most surfaces of large area transition metals. The initial graphene domains synthesized by RT-CVD were visualized and compared with graphene synthesized by T-CVD. The number of graphene domains grown by RT-CVD was approximately 12.31% larger than those synthesized by T-CVD. The surface of the thermally treated metal substrate determines the density and shape of the graphene domains due to the reduced surface irregularities and hence flattens. As a result of the thermal treatment of the copper foil, the ID/IG value of RT-CVD graphene decreased by 12.5%. Thus, it was confirmed that the electrical characteristics were improved as the average value of the sheet resistance reduced by 15.8%.

We developed the forced convection (FC)-PECVD method for the synthesis of graphene, in which a specially designed blowing plasma source is used at moderate gas pressure (1–10 Torr) and the distribution of reactive radicals reaching the substrate surface can be controlled by forced convection. Self-limiting growth of graphene occurs on copper foil, and monolayer graphene growth with a few defects is achieved even at low temperatures (

Molecular dynamic simulations are performed to study the wrinkling behaviors of a rectangular graphene sheet with various chirality angles subject to in-plane shear displacements. The results indicate that the 1/4-power laws, which are for the average out-of-plane amplitude and half-wavelength of the wrinkles as functions of shear strain, are found to be valid for the graphene with all different chirality angles. Moreover, the dependence of wrinkling amplitude and half-wavelength of graphene on the applied shear strain is insensitive to the chirality. It is expected that the research could promote applications of graphene-based materials or nano-mechanical systems in engineering.

Monolayer graphene exhibits extraordinary properties owing to the unique, regular arrangement of atoms in it. However, graphene is usually modified for specific applications, which introduces disorder. This article presents details of graphene structure, including sp2 hybridization, critical parameters of the unit cell, formation of σ and π bonds, electronic band structure, edge orientations, and the number and stacking order of graphene layers. We also discuss topics related to the creation and configuration of disorders in graphene, such as corrugations, topological defects, vacancies, adatoms and sp3-defects. The effects of these disorders on the electrical, thermal, chemical and mechanical properties of graphene are analyzed subsequently. Finally, we review previous work on the modulation of structural defects in graphene for specific applications., Graphical Abstract

The main purpose of the present article is an investigation of electronic properties of Ag-decorated graphene (Ag-G), in the presence of ambient O 2 molecules, when methane gas is adsorbed. To pursue this aim, we employ the dispersion-corrected density functional theory (DFT-D2) in which the Van der Waals interactions, as well as spin-polarization-mode, are explicitly activated. Our calculations, thereby, demonstrate the stable geometrical configuration, the corresponding parameters and the adsorption energy of CH 4 / ( O 2 -Ag- G ) . For a comparison, moreover, we also report the geometrical properties and adsorption energies for CH 4 / ( O 2 - G ) , CH 4 / ( Ag - G ) and O 2 / ( Ag - G ) . We then proceed by calculating the charge transfer and charge redistribution, as a result of adsorption, for the aforementioned combinations. The manner of charge transfer and redistribution serves to justify the electronic spin-polarized band structures and densities of states, which we also report. Our results then demonstrate that the adsorption energy of CH 4 onto Ag-decorated graphene, even in the presence of ambient O 2 , is by far larger than the undecorated one. In addition, considerations of the band structures, reveal an increase in the response of O 2 -Ag-G to the presence of CH 4 in comparison with that of O 2 -G. The last point, in combination with the enhancement of adsorption energy, provide strong motivation for the use of Ag-decorated graphene as a low-cost and highly sensitive methane gas detector.

In this work, the optical features of the monolayer graphene surrounded by two high and low Tc superconductors or in other words a superconductor-graphene-superconductor (SGS) junction is analyzed in the visible region as a function of photon energy, chemical potential, temperature, and incident angle. Regarding that, a theoretical formalism was presented to consider the effect of optical conductivity of graphene layer on the interface of two superconductor media. The results show different behaviors for the introduced SGS junction in the visible region for both s and p polarizations. The calculated results may help the development of graphene- and superconductor-based optical devices.

We use the none quilibrium molecular dynamics method to investigate the thermal conductivity of both suspended and supported graphene at room temperature. Significant size (both length and wide) effect is observed in thermal conductivity of suspended monolayer graphene. And it is found that the thermal conductivity value would converge to a constant value as further increasing size. In the case of supported graphene, we find the thermal conductivity is much lower than that of its suspended counterpart, and the suppressed size effect on thermal transport is obtained. The underlying physics of the size effect on both suspended and supported graphene is investigated and presented in terms of phonon transport analysis. These results show the nature of phonon transport in graphene and provide a useful guide for tailoring and engineering of the thermophysical properties of graphene in thermo-electronic and photonic devices.

Although there are several kinds of thermal interfacial materials used in the electronic semiconductor industry, such as thermal grease, thermal glue, thermal gap filler, thermal pad and thermal adhesive, the problem of heat dissipation still remains a challenge. In this context, chemical vapor deposition of graphene on copper foils in vacuum has recently become considered as a wonderful hybrid material (graphene/copper/graphene) for more demanding thermal management applications, thanks to the unique properties of graphene in comparison with other materials. We found that the thermal properties of copper films change as graphene is deposited on top of the copper surface. Especially, a single atomic plane of graphene can significantly increase the film’s thermal conductivity. Our graphene on copper foil was analyzed and measured by optical microscopy, Raman spectroscopy, scanning electron microscopy and heat transfer technique. This stack of graphene/copper/graphene materials may play a very important role as a potential material with superior thermal conductivity to replace traditional copper shim thermal pads in current electronic devices.

Monolayer graphene, successfully isolated in 2004, is the first member of the class of materials called two-dimensional (2D) materials. Since then, 2D materials such as hexagonal boron nitride (h-BN), transition metal dichalcogenides (TMDs), silicene and phosphorene have been extensively investigated owing to their extraordinary mechanical, chemical, and physical properties. Furthermore, some of these 2D materials are expected to result in novel high-frequency and optical phenomena. In this review, we highlight the recent progress of the state-of-the-art research on graphene applications on the scale of terahertz (THz) emitting, detecting and modulating, and summarize similar THz applications of h-BN, TMDs, silicene and phosphorene.

Graphene, a single layer of carbon atoms arranged in the form of hexagonal lattice, has many intriguing optical and electrical properties. However, due to the atomic layer thickness, light-matter interactions in the monolayer graphene are naturally weak when the light is normally incident to the material. To overcome this challenge, waveguide-integrated graphene optoelectronic devices have been proposed and demonstrated. In such coplanar configurations, the propagating light in the waveguide can significantly interact with the graphene layer integrated on the surface of the waveguide. The combination of photonic integrated circuits and graphene also enables the development of graphene devices by using silicon photonic technology, which greatly extends the scope of graphene’s application. Moreover, the waveguide-integrated graphene devices are fully CMOS-compatible, which makes it possible to achieve low-cost and high-density integration in the future. As a result, the area has been attracting more and more attention in recent years. In this paper, we introduce basic principles and research advances of waveguide-integrated graphene optoelectronics.

Hydrogen storage in solids is becoming an ever more important technology. We theoretically design a completely new material for hydrogen storage, which is quite different from the present materials. This kind of material, base on monolayer graphene, is thought to be extremely flexible. First-principles calculations and kinetic theory of gases are employed to evaluate this material. The results demonstrate that it is promising for hydrogen storage. More than anything, the hydrogen storage capacity of this material is about 4 mmol g−1 at 300 K and 1 atm.

A recently derived formula for complex conductivity of the monolayer graphene is analyzed. We show that the real and imaginary parts in this formula obey the Kramers and Kronig dispersion relations which are a good test for validity of the formula for complex conductivity of monolayer graphene. We consider also an additional test for this formula, sensitive to the integral characteristic of the conductance such as the famous f sum rule. We write it in the two dimensional form and show that it fulfils identically if we admit the cyclotron mass as an effective one and take the principal value of the integral. We find a deep relation between the graphene complex optical conductivity singularities and electrons Zitterbewegung in graphene. Namely, the value of Zitterbewegung frequency is related with the recently found magnitudes of the inductance L and capacitance C by the Thomson formula., 8 pages

The atomic structure and electronic properties of monolayer graphene on a curved multi-nano-vicinal Pt (1 1 1) substrate are investigated with a low-energy electron diffraction, scanning tunneling microscopy and angle-resolved photoemission spectroscopy. Despite graphene grows continuously over the terraces and step bunching areas, the spatial periodicity also varies by varying the vicinal angle of the substrate. This superperiodicity has been evidenced by splitting of first order Pt spots in LEED and from STM. The photoemission spectroscopy unravels that the linearly dispersing π band of graphene is affected by the step periodicity as evidenced by the opening of bandgaps. The gaps opening occurs at the intersection of the main graphene band with Umklapp bands due to the superperiodicity of the one-dimensional nanostructured substrate. The energy and momentum locations of the minigaps change with the superperiodicity, which is related to the spatial periodicity and vicinal angle of the substrate. Our results show a simple way to tune the electronic properties of epitaxial graphene by tuning the substrate vicinality.

Homogeneity is crucial to material applications. Although monolayer graphene was successfully prepared more than a decade ago, to date, the growth of adlayer-free monolayer with a large area remains challenging. Here, we report the synthesis of metre-size adlayer-free monolayer graphene film on Cu80Ni20 foil via atmospheric pressure chemical vapour deposition. Experimental results show that the growth parameter window of adlayer-free monolayer graphene on Cu80Ni20 is very wide, which makes the growth of adlayer-free monolayer graphene easy and controllable. A growth model of adlayer-free monolayer graphene on Cu80Ni20 substrate is also proposed.

The coupling strength between two-dimensional (2D) materials and substrate plays a vital role on thermal transport properties of 2D materials. Here we systematically investigate the influence of vacuum thermal annealing on the temperature-dependence of in-plane Raman phonon modes in monolayer graphene supported on silicon dioxide substrate via Raman spectroscopy. Intriguingly, raising the thermal annealing temperature can significantly enlarge the temperature coefficient of supported monolayer graphene. The derived temperature coefficient of G band remains mostly unchanged with thermal annealing temperature below 473 K, while it increases from −0.030 cm−1/K to −0.0602 cm−1/K with thermal annealing temperature ranging from 473 K to 773 K, suggesting the great impact of thermal annealing on thermal transport in supported monolayer graphene. Such an impact might reveal the vital role of coupling strength on phonon scattering and on the thermal transport property of supported monolayer graphene. To further interpret the thermal annealing mechanism, the compressive stress in supported monolayer graphene, which is closely related to coupling strength and is studied through the temperature-dependent Raman spectra. It is found that the variation tendency for compressive stress induced by thermal annealing is the same as that for temperature coefficient, implying the intense connection between compressive stress and thermal transport. Actually, 773 K thermal annealing can result in 2.02 GPa compressive stress on supported monolayer graphene due to the lattice mismatch of graphene and substrate. This study proposes thermal annealing as a feasible path to modulate the thermal transport in supported graphene and to design future graphene-based devices.

We propose a valley-dependent electromotive force in a graphene device based on a deformed graphene in the presence of magnetic field. The coexistence of the strain and magnetic field in the presented mechanism leads to a valley dependent electromotive force in zero Fermi energy. In the non-zero Fermi energies strain alone could induce a valley dependent electromotive force while the magnetic field alone could not induce any electromotive force in zero and any level of Fermi energy. The calculations are based on the Berry curvature approaches.

Flexible substrate consisted of PMMA-supported monolayer graphene with sandwiched Ag-nanoflowers (G/AgNFs/PMMA) for ultrasensitive, reproducible, and stable surface-enhanced Raman scattering (SERS) detection is reported. Graphene templated micro-current-assisted chemical reduction method was employed to support AgNFs growth, and uniform-distribution AgNFs with all directions nanotips can generate tremendous enhancement factor and intensive hotspots. The minimum detectable concentration (i.e., detection limit) for rhodamine 6G (R6G) in-situ detection by covering this as-synthesized G/AgNFs/PMMA flexible substrate can be as low as 10−14M. Moreover, graphene can effectively stabilize the SERS signals and protect AgNFs from oxidation, endowing this flexible substrate a long-term stability with maximum intensity deviation lower than 10%, for the quantitative measurements from spot-to-spot or substrate-to-substrate. In order to trial its practical applications with various real-world surfaces, the in-situ SERS detection of phenylalanine@apple, adenosine aqueous solution and methylene-blue@fish was performed by covering this G/AgNFs/PMMA flexible substrate. Clear Raman peaks can be obtained for all the selected samples with concentration of 10−10M and, importantly, good linear relationship between Raman intensity and molecular concentration indicates the potential application of the G/AgNFs/PMMA flexible substrate in quantitative determination. Thus, this high-efficiency and low-cost flexible SERS substrate may provide a new way for the molecular trace-detection in food security and environmental protection.

We report variable range hopping (VRH) transport in monolayer epitaxial graphene grown on SiC. A crossover from Efros–Shklovskii (E-S) VRH to Mott VRH can be observed by increasing the magnetic field applied perpendicular to the plane of graphene or by a low-temperature vacuum annealing process. Our new experimental results suggest that monolayer graphene on SiC is an interesting platform for probing VRH and crossover between different VRH regimes in a strongly disordered system.

A new graphene/silver nanowire (AgNW) hybrid transparent conductive films (TCFs) was prepared by wet-transferring a chemical vapor deposition (CVD)-grown monolayer graphene onto pristine spin coati...

In this study, a room-temperature acetone gas sensor based on monolayer graphene is proposed with the improvement by ultraviolet (UV) illumination. The monolayer graphene film was grown by low pressure chemical vapor deposition and then transferred to the interdigital gold electrode by the standard transferred method. To test the acetone sensing properties, the concentration of 300, 600 and 900 ppb acetone provided by a gas standards generator is applied in the testing chamber. The resistance changes following a doping type modification of graphene between illumination and acetone gas. A 1.9% change in short time illumination on graphene could be obtained at 600 ppb and room temperature, which could make this a candidate for biomedical application in the future. In the meantime, a mechanism is proposed to explain whole behavior based on photo-induced molecular desorption.

Highly ordered vertically aligned ZnO nanorod arrays are fabricated via a recyclable SiOx-mask-patterned GaN template by hydrothermal method. The morphology of ZnO nanorods can be modulated, and the mechanism is studied for the first time. The modulation depends on the nucleation state, which can be tuned by varying PH value through adding ammonia. Tight contact is observed between a monolayer graphene and the smooth surface of the arrays due to the uniform height and horizontal c-plane of nanorods. The ZnO nanorod arrays are characterized by XRD and Room-temperature PL.

Thermodynamic properties of graphene bilayers are studied by path-integral molecular dynamics (PIMD) simulations, considering quantization of vibrational modes and anharmonic effects. Bilayer graphene has been studied at temperatures between 12 and 1500~K for zero external stress, using the LCBOPII effective potential. We concentrate on the thermal expansion, in-plane and out-of-plane compressibility, and specific heat. Additional insight into the meaning of our results for bilayer graphene is obtained from a comparison with data obtained from PIMD simulations for monolayer graphene and graphite. They are also analyzed in view of experimental data for graphite. Zero-point and thermal effects on the in-plane and "real" area of bilayer graphene are studied. The thermal expansion coefficient $\alpha_{xy}$ of the in-plane area is negative at low temperatures and positive for $T \gtrsim$ 800~K. The minimum $\alpha_{xy}$ is $-6.6 \times 10^{-6}$ K$^{-1}$ at $T \approx 220$~K. Both in-plane ($\chi_{xy}$) and out-of-plane ($\chi_z$) compressibilities of graphene bilayers are found to increase for rising temperature, and turn out to be lower than that corresponding to monolayer graphene and higher than those found for graphite. At 300 K, we find for the bilayer $\chi_{xy} = 9.5 \times 10^{-2}$ \AA$^2$/eV and $\chi_z = 2.97 \times 10^{-2}$ GPa$^{-1}$. Results for the specific heat obtained from the simulations are compared with those given by a harmonic approximation for the vibrational modes. This approach is noticeably accurate at temperatures lower than 200~K., Comment: 14 pages, 9 figures

Materials can suffer mechanical fatigue when subjected to cyclic loading at stress levels much lower than the ultimate tensile strength, and understanding this behaviour is critical to evaluating long-term dynamic reliability. The fatigue life and damage mechanisms of two-dimensional (2D) materials, of interest for mechanical and electronic applications, are currently unknown. Here, we present a fatigue study of freestanding 2D materials, specifically graphene and graphene oxide (GO). Using atomic force microscopy, monolayer and few-layer graphene were found to exhibit a fatigue life of more than 109 cycles at a mean stress of 71 GPa and a stress range of 5.6 GPa, higher than any material reported so far. Fatigue failure in monolayer graphene is global and catastrophic without progressive damage, while molecular dynamics simulations reveal this is preceded by stress-mediated bond reconfigurations near defective sites. Conversely, functional groups in GO impart a local and progressive fatigue damage mechanism. This study not only provides fundamental insights into the fatigue enhancement behaviour of graphene-embedded nanocomposites, but also serves as a starting point for the dynamic reliability evaluation of other 2D materials. Mechanical fatigue occurs under cyclic stress much lower than the tensile strength, but this has not been investigated for 2D materials. Here, graphene is found to have a fatigue life of 109 cycles.

We study Coulomb drag in a system consisting of a carbon nanotube (CNT) and monolayer graphene. Within the Fermi liquid theory we calculate the drag resistivity and find that the dimensional mismatch of the system components leads to a dependence of the drag rate on the carrier density, temperature, and spacing, which is substantially different from what is known for graphene double layers. Due to the competing effects of forward and backward scattering, we identify new features of the drag dependence on the electron density, which allows us to control their relative contribution to the drag resistivity., Comment: 5 pages, 5 figures

In this study of electronic transport in devices where flakes of the layered antiferromagnetic insulator $\ensuremath{\alpha}$-RuCl${}_{3}$ are placed on top of monolayer graphene, a significant charge transfer is found to occur, with graphene becoming strongly hole doped as electrons move into the $\ensuremath{\alpha}$-RuCl${}_{3}$. Due to imperfect contact between the two materials, transport through intrinsic graphene is observed in parallel, which unexpectedly reveals signatures of magnetic transitions in the neighboring $\ensuremath{\alpha}$-RuCl${}_{3}$.

This work addresses a fundamental question: To what extent is graphene graphite? In particular does 2D graphene have many of the same 3D mechanical properties as graphite, such as the bulk modulus and elastic constant c_{33}? We have obtained, for the first time, unambiguous Raman spectra from unsupported monolayer graphene under pressure. We have used these data to quantify the out-of-plane stiffness of monolayer graphene, which is hard to define due to its 2D nature. Our data indicate a first physically meaningful out-of-plane stiffness of monolayer graphene, and find it to be consistent with that of graphite. We also report a shift rate of the in-plane phonon frequency of unsupported monolayer graphene to be 5.4 cm^{-1} GPa^{-1}, very close to that of graphite (4.7 cm^{-1} GPa^{-1}), contrary to the previous value for supported graphene. Our results imply that monolayer graphene has similar in-plane and out-of-plane stiffnesses, and anharmonicities to graphite.

Atomic-scale stick–slip friction of monolayer graphene on a copper substrate is computationally studied at a wide range of temperatures, which reveals strong temperature dependence of friction force and friction mode. The increase in temperature distorts regular stick–slip behavior and causes a nonlinear decrease of friction force, demonstrating a friction behavior transition from athermal friction to thermally activated. By analyzing atomic morphologies, the true contact area, defining the number of atoms interacting across the interface, shows a weak temperature dependence, yet the quality of interfacial contact substantially varies with temperature. Spatial distributions of atomic friction force uncover the significant effect of Moire superlattices, that act as strong pinning sites at low temperatures, partly change to pushing due to nonconcurrent atomic jumps at high temperatures, which leads to a chaotic friction mode with reduced lateral force. Additionally, we demonstrate the role of superlattices in strengthening friction and dictating the periodic stick–slip motion of interfacial sliding.

The relative orientation between atomic lattices in twisted bilayer graphene opens up a whole new field of rich physics. So, the study of self-assembled twisted bilayer graphene gives deep insight into its underlying growth process. Cuts in monolayer graphene via the atomic force microscopy technique are used to start self-assembly and to generate a folding process. The final configurations for this self-assembly process are investigated. Here, the focus is on structures that arise from one cut. During the self-assembly, these structures not only move forward but also rotate. As it turns out, the final positions for all studied structures can be assigned to commensurate interlayer configurations.

In this paper, the spin-dependent transport and the spin polarization properties for graphene superlattice with Rashba spin–orbit interaction (RSOI) in the presence of zigzag and armchair direction strain are studied. It is found that for the zigzag direction strain the angular range of the spin-inversion can be efficiently controlled by the strain strength. In addition, the efficiency of spin-inversion and spin-dependent conductivity decreases by increasing the strain strength. When the armchair direction strain is applied to a monolayer graphene superlattice the spin polarization can be observed and increases by increasing the strain strength, whereas for the zigzag direction strain it is zero.