Monolayer Graphene Research Articles

Application of Monolayer Graphene to Cryo-Electron Microscopy Grids for High-resolution Structure Determination.

In cryogenic electron microscopy (cryoEM), purified macromolecules are applied to a grid bearing a holey carbon foil; the molecules are then blotted to remove excess liquid and rapidly frozen in a roughly 20-100 nm thick layer of vitreous ice, suspended across roughly 1 µm wide foil holes. The resulting sample is imaged using cryogenic transmission electron microscopy, and after image processing using suitable software, near-atomic resolution structures can be determined. Despite cryoEM's widespread adoption, sample preparation remains a severe bottleneck in cryoEM workflows, with users often encountering challenges related to samples behaving poorly in the suspended vitreous ice. Recently, methods have been developed to modify cryoEM grids with a single continuous layer of graphene, which acts as a support surface that often increases particle density in the imaged area and can reduce interactions between particles and the air-water interface. Here, we provide detailed protocols for the application of graphene to cryoEM grids and for rapidly assessing the relative hydrophilicity of the resulting grids. Additionally, we describe an EM-based method to confirm the presence of graphene by visualizing its characteristic diffraction pattern. Finally, we demonstrate the utility of these graphene supports by rapidly reconstructing a 2.7 Å resolution density map of a Cas9 complex using a pure sample at a relatively low concentration.

Reflectionless graphene perfect absorber based on parity symmetric unidirectional guided resonance.

We propose a type of reflectionless graphene perfect absorber (GPA) in which the reflection channel is forbidden, while the transmission channel is open. Peak absorption of 99.97% in the near-infrared is numerically demonstrated for monolayer graphene loaded on a one-dimensional silicon photonic crystal slab with rhomboid cross sections that supports parity symmetric unidirectional guided resonances (UGRs). Based on the proposed GPA, a transmissive optical modulator with a modulation depth of about 28 dB and an insertion loss of 0.31 dB by varying the Fermi energy level graphene from 0.3 eV to 0.7 eV is numerically presented. Remarkably, the design strategy can be straightforwardly applied to other two-dimensional (2D) materials. Our study may find promising applications in 2D material-based optical modulators and filters.

Gaussian and Gaussian-pulsed-like Fermi velocity graphene structures

Gaussian and Gaussian-related structures are quite attractive due to its versatility to modulate the electronic transport, including its possibility as electron filters. Here, we show that these non-conventional profiles are not the exception when dealing with Fermi velocity barriers in monolayer graphene. In particular, we show that Gaussian Fermi velocity graphene barriers (G-FVGBs) and Gaussian-pulsed-like Fermi velocity graphene superlattices (GPL-FVGSLs) can serve as electron band-pass filters and oscillating conductance structures. We reach this conclusion by theoretically studying the transmission and transport properties of the mentioned structures. The study is based on the continuum model, the transfer matrix method and the Landauer–Büttiker formalism. We find nearly flat transmission bands or pass bands for G-FVGBs modulable through the system parameters. The pass bands improve as the maximum ratio of Fermi velocities () increases, however its omnidirectional range is reduced. These characteristics result in a decaying conductance (integrated transmission) with . The integrated transmission remains practically unaltered with the size of the system due to the saturation of the electron pass band filtering. In the case of GPL-FVGSLs the GPL profile results in regions of high transmission probability that can merge as flat transmission minibands if the pulse fraction and the superlattice parameters are appropriately tuned. The GPL profile also results in conductance (integrated transmission) oscillations that can be multiplied or reduced in number by adjusting the pulse fraction as well as the superlattice parameters.

Graphene-based ultralow-profile microwave Fresnel lens

The development of ultrathin Fresnel lenses has revolutionized the lens technology and provided great prospects for the miniaturization of electromagnetic systems. Despite considerable advances in microtechnology, a scalable fabrication strategy to achieve ultralow profile microwave lenses is yet to be developed. Here, we report on a novel strategy to create ultrathin and essentially flat Fresnel lenses using graphene rings. We show that monolayer or multilayer graphene can be microstructured into a Fresnel lens to utilize the unique transmission properties of graphene. We design different Fresnel lenses and investigate their performance in the microwave regime both numerically and experimentally. It is shown that one can change the intensity of the focused wave without shifting the focusing point by altering the number of the graphene layers. The good agreement between our experimental results and the results of numerical simulations demonstrates the feasibility of atom-thick (about 10−7 of the operating wavelength) graphene-based lenses and shows their potential for ultra-compact integration.

Morphology of Bi(110) quantum islands on epitaxial graphene

Proximitized 2D materials present exciting prospects for exploring new quantum properties, enabled by precise control of structures and interfaces through epitaxial methods. In this study, we investigated the structure of ultrathin coverages formed by depositing high-Z element bismuth (Bi) on monolayer graphene (MLG)/SiC(0001). By utilizing electron diffraction and scanning tunneling microscopy, ultrathin Bi nanostructures epitaxially grown on MLG were studied. Deposition at 300 K resulted in formation of needle-like Bi(110)-terminated islands elongated in the zig-zag direction and aligned at an angle of approximately 1.75∘ with respect to the MLG armchair direction. By both strain and quantum size effects, the shape, the orientation and the thickness of the Bi(110) islands can be rationalized. Additionally, a minority phase of Bi(110) islands orthogonally aligned to the former ones were seen. The four sub-domains of this minority structure are attributed to the formation of mirror twin boundaries, resulting in two potential alignments of Bi(110) majority and minority domains with respect to each other, in addition to two possible alignments of the majority domain with respect to graphene. Notably, an annealing step at 410 K or lowering the deposition temperature, significantly increases the concentration of the Bi(110) minority domain. Our findings shed light on the structural control of proximitized 2D materials, showcasing the potential for manipulating 2D interfaces.

Negative refraction achieved by inducing chirality in monolayer graphene through Raman gain mechanism

Negative refraction achieved by inducing chirality in monolayer graphene through Raman gain mechanism

Metallurgical slag modified monolayer graphene hybrid SA-based composite phase change materials for high thermal conductivity

Phase change materials are thermal energy storage materials with high heat storage density, low temperature loss and long cycle life. However, they also show a weak light absorption ability and low thermal conductivity, limiting their applicability for solar thermal conversion and storage technologies. In this paper, in order to improve the relatively low thermal conductivity of stearic acid and enhance its thermal conductivity, SGC-T composites with enhanced thermal conductivity were prepared using metallurgical slag, via the melt blending method. The SEM and EDS, surface scanning tests, were used to determine the morphology characteristics of the composite phase change materials and verify the addition of metallurgical slag. The physical phase composition, structure, and absorption coefficients were analyzed by XRD, FT-IR, and UV-Vis-NIR analyses. The thermophysical properties were analyzed by DSC. The thermal stability was explored using TG testing. The thermal conductivity was explored by measuring the thermal conductivity coefficient value of the samples at the testing temperature of 25 ℃ using the hot disk method/transient flat plate heat source method. The heat storage and exothermic capabilities of the samples were evaluated by the photo-thermal warming system/infrared thermography test. The results showed that the thermal conductivity of SGC was 0.663 W·(m·K)−1, and that of SGC-2 T, SGC-6 T, SGC-10 T, SGC-14 T, were 0.695 W·(m·K)−1, 0.758 W·(m·K)−1, 0.824 W·(m·K)−1 and 0.872 W·(m·K)−1, respectively, which are comparable to SGC. Compared with SGC, the thermal conductivity values of the four composite phase change materials increased by 4.8%, 14.3%, 24.3%, and 31.5%, respectively. SGC-14 T has the best heat transfer performance, which significantly improves the heat transfer performance during latent heat storage and release. Therefore, the SGC-14 T material reported in this work is a promising candidate in the field of energy-saving conversion, and further expands the applicability of phase change materials.

Ionic liquid/metal salt mixtures at the graphene interface: A density functional theory approach

The interaction of graphene sheets with metal cations is very relevant because of the modifications that are induced in the electrochemical properties of the 2D material. Ionic liquids are a promising kind of materials with several applications in electrochemical devices, so understanding how they affect the graphene-metal interaction is key for their practical implementation. Therefore, we have studied the adsorption of a mixture of an ionic liquid and a metal salt on a graphene surface by means of first-principles calculations. Several metals were chosen to analyze the effect they have on the optoelectronic properties of the graphene monolayer and to understand the trends in their adsorption behavior. We have characterized the ground state configurations in terms of their binding energies and the distance between the metal atom and the graphene layer. From the analysis of the charge transfer behavior, calcium and magnesium have been identified as the species that transfer the highest and the lowest amount of charge, respectively, which is related to their ionization energies. Band structure diagrams and projected density of states calculations also show that the energy shift of the Dirac cone increases with the degree of charge transfer. In addition, a stronger interaction of magnesium with the ionic liquid compared to that of other metal atoms was observed. The analysis of several electromagnetic parameters indicated an anisotropic behavior for electric fields polarized both perpendicular and parallel to the graphene layer. Our density functional theory study offers fundamental insights into the adsorption behavior of ionic liquids mixed with metal ions on monolayer graphene.

Graphene binding on black phosphorus enables high on/off ratios and mobility.

Graphene is one of the most promising candidates for integrated circuits due to its robustness against short-channel effects, inherent high carrier mobility and desired gapless nature for Ohmic contact, but it is difficult to achieve satisfactory on/off ratios even at the expense of its carrier mobility, limiting its device applications. Here, we present a strategy to realize high back-gate switching ratios in a graphene monolayer with well-maintained high mobility by forming a vertical heterostructure with a black phosphorus multi-layer. By local current annealing, strain is introduced within an established area of the graphene, which forms a reflective interface with the rest of the strain-free area and thus generates a robust off-state via local current depletion. Applying a positive back-gate voltage to the heterostructure can keep the black phosphorus insulating, while a negative back-gate voltage changes the black phosphorus to be conductive because of hole accumulation. Then, a parallel channel is activated within the strain-free graphene area by edge-contacted electrodes, thereby largely inheriting the intrinsic carrier mobility of graphene in the on-state. As a result, the device can provide an on/off voltage ratio of >103 as well as a mobility of ∼8000cm2 V-1 s-1 at room temperature, meeting the low-power criterion suggested by the International Roadmap for Devices and Systems.

High‐Quality van der Waals Epitaxial CsPbBr3 Film Grown on Monolayer Graphene Covered TiO2 for High‐Performance Solar Cells

Two‐dimensional materials have been widely used to tune the growth and energy‐level alignment of perovskites. However, their incomplete passivation and chaotic usage amounts are not conducive to the preparation of high‐quality perovskite films. Herein, we succeeded in obtaining higher‐quality CsPbBr3 films by introducing large‐area monolayer graphene as a stable physical overlay on top of TiO2 substrates. Benefiting from the inert and atomic smooth graphene surface, the CsPbBr3 film grown on top by the van der Waal epitaxy has higher crystallinity, improved (100) orientation, and an average domain size of up to 1.22 μm. Meanwhile, a strong downward band bending is observed at the graphene/perovskite interface, improving the electron extraction to the electron transport layers (ETL). As a result, perovskite film grown on graphene has lower photoluminescence (PL) intensity, shorter carrier lifetime, and fewer defects. Finally, a photovoltaic device based on epitaxy CsPbBr3 film is fabricated, exhibiting power conversion efficiency (PCE) of up to 10.64% and stability over 2000 h in the air.

Resonant amplification of slow surface plasmon polaritons in a DC current pumped semiconductor/graphene waveguide with a groove defect

We propose the principle of a planar surface plasmon polariton amplifier composed of a complex waveguide structure based on a semiconductor thin film separated from a dielectric substrate by a graphene monolayer. The amplification of surface plasmon polaritons in that waveguide in the terahertz regime is driven by a direct current in the graphene layer under a synchronism condition that allows an efficient energy exchange from the collective flux of charge carriers in graphene and the surface electromagnetic wave in the semiconductor film. Positive feedback required for resonant amplification is achieved through surface plasmon polariton reflections at the edges of the active waveguide, one of which is formed by a local thickness defect (a groove) at the upper surface of the semiconductor film that also provides an exit channel for the energy of the amplified surface wave to be transferred to an adjacent passive thin film semiconductor waveguide. We determine the conditions required for the resonant amplification of surface plasmon polaritons in that complex structure, which are essentially governed by the geometry of the groove as well as by the characteristic parameters of the active and passive waveguides.

Elastic properties and mechanical stability of bilayer graphene: molecular dynamics simulations

Graphene has become in last decades a paradigmatic example of two-dimensional and so-called van-der-Waals layered materials, showing large anisotropy in their physical properties. Here, we study the elastic properties and mechanical stability of graphene bilayers in a wide temperature range by molecular dynamics simulations. We concentrate on in-plane elastic constants and compression modulus, as well as on the atomic motion in the out-of-plane direction. Special emphasis is placed upon the influence of anharmonicity of the vibrational modes on the physical properties of bilayer graphene. We consider the excess area appearing in the presence of ripples in graphene sheets at finite temperatures. The in-plane compression modulus of bilayer graphene is found to decrease for rising temperature, and results to be higher than for monolayer graphene. We analyze the mechanical instability of the bilayer caused by an in-plane compressive stress. This defines a spinodal pressure for the metastability limit of the material, which depends on the system size. Finite-size effects are described by power laws for the out-of-plane mean-square fluctuation, compression modulus, and spinodal pressure. Further insight into the significance of our results for bilayer graphene is gained from a comparison with data for monolayer graphene and graphite.Graphic abstract

Landau levels for a Weyl pair in a monolayer medium and thermal quantities

In this paper, we consider a Weyl pair under the effect of an external uniform magnetic field in a monolayer medium without considering any charge-charge interaction between the particles. Choosing the interaction of the particles with the magnetic field in the symmetric gauge we seek for an analytical solution of the corresponding form of a one-time fully-covariant two-body Dirac equation derived from quantum electrodynamics via the action principle. As it is usual with two-body problems, we separate the relative motion and center of mass motion coordinates. Assuming the center of mass is at rest, we derive a matrix equation in terms of the relative motion coordinates without considering any group theoretical method. This equation gives a wave equation in exactly soluble form and accordingly we obtain the spinor components and complete energy eigen-states (in closed form) for such a spinless composite structure. Our results not only give exact Landau levels for such a Weyl pair in a monolayer medium but also show the considered system behaves as a two-dimensional harmonic oscillator. Furthermore, our findings give exactly the excited states of a Weyl particle under the effect of uniform external magnetic field in a monolayer graphene sheet and there is no imprint to distinguish these modes from each other. This means that the performed experiments based on Landau levels for a monolayer graphene sheet may actually involve many-body effects. Our results provide a suitable basis to analyze the associated thermal quantities and accordingly we discuss the thermal properties by determining free energy, total energy, entropy and specific heat for the composite system in question.

Theoretical study on structural, electronic, transport and thermoelectric properties of Si/Ge doped graphene

First-principles simulations are used in this study to show the structural, electronic, transport, and thermoelectric features of silicon and germanium-doped monolayer graphene. We employed a supercell of 2 × 2 × 1 graphene that contains 8 C atoms. One C atom was substituted by Si/Ge at a doping concentration of 12.5 %. The results obtained show that the band gap of pure graphene may be effectively opened and converted into a direct bandgap semiconductor material by replacing the C atom with Si/Ge atoms. Phonon dispersion provides a representation of the dynamical stability of all three structures. Graphene's thermal conductivity is also decreased when heteroatoms are added. Additionally, the Seebeck coefficient, the power factor, the electrical conductivity, and the figure of merit of pure, silicon, and germanium-doped graphene were investigated. This study offers a theoretical basis for using a heteroatom-doping process to improve the performance of graphene's electronic, transport, and thermoelectric characteristics.

Thermally tunable electromagnetic surface waves supported by graphene loaded indium antimonide (InSb) interface

The thermal agitation plays a vital role in tunability of optoelectronic, structural and chemical characteristics of the temperature sensitive materials. Graphene enables the THz optics, due to its unprecedent controlling characteristics over the traditional materials. The influence of temperature on the monolayer graphene is very negligible due to its low free charge carrier density, to enhance the thermal sensitivity of graphene, the graphene loaded temperature sensitive material interface has been proposed. A theoretical analysis has been carried out on temperature dependent propagation characteristics of electromagnetic surface waves supported by the graphene loaded semi-infinite indium antimonide (InSb). The InSb has been taken as temperature sensitive material. The Drude model has been used for the modeling of InSb in the THz region while the modeling of the graphene has been done by random phase approximation-based Kubo’s formulism. To realize the graphene loaded indium antimonide interface, the impedance boundary conditions (IBCs) have been employed. The numerical analysis has been conducted to analyze the influence of temperature on the characteristics of electromagnetic surface waves i.e., dispersion curve, effective mode index (Neff), penetration depth (δ), propagation length (Lp), phase speed (Vp) and field profile, propagating along the graphene loaded InSb. In all the numerical results, the temperature variation has been considered from 200 to 350 K. It has been concluded that the graphene–InSb interface provides more temperature assisted tunability to the interfacial surface modes, commonly known as surface waves, as compared to monolayer graphene. Further, the graphene parameters can play a vital role in the dynamical tuning of electromagnetic surface waves in THz to IR frequency range. The numerically computed results have potential applications in designing of thermo-optical waveguides, temperature assisted communication devices, thermo-optical sensors and near field thermal imaging platforms.

Predicting the miniaturization limit of vertical organic field effect transistor (VOFET) with perforated graphene as a source electrode

Vertical organic field effect transistors (VOFETs) are of paramount importance due to their fast switching speed, low power consumption, and higher density on a chip compared to lateral OFETs. The low charge carrier mobility in organic semiconductors and longer channel lengths in lateral OFETs lead to higher operating voltages. The channel length in VOFETs can be less than 100 nm which reduces the size of the channel and hence the operating voltages. Another important factor in the operation of VOFETs is the thickness and width of the source electrode. The channel length, source electrode thickness and width sets the miniaturization limit of the VOFETs. The graphene monolayer can be exploited as a source electrode due to its thinness, high carrier mobility, and metallic behaviors. However, for better gate modulation, perforations in the source material are desired. Here, we simulate the VOFET having perforated graphene monolayer as a source electrode and n-type organic semiconductor N, N′-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) as an active channel material, while aluminum as a drain electrode to predict the best-miniaturized device. The miniaturization limit of such a VOFET has a limit to the gate opening/perforation in which the minimum source width is 10 nm, as in the sub 10 nm range graphene starts behaving like a semiconductor. The subthreshold swing, deduced from the drain current (J D) versus gate voltage (V G) graph, advocates the limit of the organic semiconductor height/channel length to 50 nm, while 50 nm for the gate.

How Do Substrates Affect the Friction on Graphene at the Nanoscale?

Substrates supporting two-dimensional materials are omnipresent in micro/nano electromechanical systems. Moreover, substrates are indispensable to all nanotribological experimental systems. However, substrates have rarely been taken into account in first-principles simulations of nanotribological systems. In this work, we investigate the effects of substrates on nanofriction by carrying out first-principles simulations of two systems: (a) one graphene monolayer sliding on another one supported by a metal substrate, denoted as the Gr-Gr/Metal system; and (b) a diatomic tip sliding on a graphene monolayer supported by a metal substrate, named the Tip-Gr/Metal system. Each substrate is made of triatomic layers constituting the minimum period and obtained by cutting a metal through its (111) surface. By varying metal substrates and analyzing the results of the first-principles simulations, it follows that (i) the fluctuation in the sliding energy barriers of the two systems can be modified by changing substrates; (ii) the adsorption type and the pressure affect friction; (iii) the presence of a substrate varies the interfacial binding strength; and (iv) the modulation of friction by substrates lies in altering the interface electron density. These results provide an answer to the important question of how substrates affect the friction on graphene at the nanoscale.

Thermodynamic Window for Size-Controlled Pore Formation in Graphene for Large-Scale Molecular Sieves.

Nanopores in graphene monolayers are a promising option for molecular separation applications, such as desalination and carbon capture. Graphene's atomic thickness allows for an optimal balance between molecular selectivity and permeability, while its chemical stability and robust mechanical properties make it appealing for a wide range of commercial applications. However, scaling to large areas with controlled pore size distribution is an open challenge in ultrathin membranes. Here, using first-principles calculations, we identify a suitable thermodynamic window in a chemical vapor deposition system for directly growing graphene monolayers with a controlled pore size distribution. As an example, our calculations show that a postgrowth annealing step with a supersaturation range of 19.7-25 kJ/mol at 1000 K results in the creation of a controllable pore density at graphene grain boundaries, with pore sizes falling within the range of 5-8 Å. Such pores isolate hydrated Cl ions from water molecules, effectively desalinating seawater. Thus, it allows the design of targeted synthesis of large-scale 2D layers for membrane applications.

Superconductivity in monolayer and few-layer graphene. III. Impurity-induced subgap states and quasiparticle interference patterns

Superconductivity in monolayer and few-layer graphene. III. Impurity-induced subgap states and quasiparticle interference patterns

Superconductivity in monolayer and few-layer graphene. I. Review of possible pairing symmetries and basic electronic properties

Superconductivity in monolayer and few-layer graphene. I. Review of possible pairing symmetries and basic electronic properties