Monolayer Graphene Research Articles

Exact Green’s function for 2D dirac oscillator in constant magnetic field within curved snyder space, and its thermal properties

Following the path integral approach, and in the context of curved Snyder space, we formulate the Green function for a (1+1)-dimensional Dirac oscillator system subject to a homogeneous magnetic field. Using the radial coordinates transformation the Green function and the electron propagator are calculated. Consequently, the exact bound states and their corresponding spectral energies are extracted. Our analysis has revealed that, under specific conditions when mω¯→mωc/2 and c → V F , the behavior of the Dirac oscillator system in the presence of a uniform magnetic field within the SdS algebra closely resembles the dynamics of the monolayer graphene problem in the same algebraic framework. At high temperatures, the thermodynamic properties of the electron gas in the four cases of deformation parameters were extracted. The effect of the deformation parameters on these properties are tested, and also the limit cases for small parameters were inferred.

The conductivity effect of the top coating on optical properties of thin Cu(Ag)-layered structures

This study examines the optical properties of thin Cu (Ag)-layered structures covered with protective layers based on graphene, titanium (TiO2), or aluminium (Al2O3) oxides. The objective is to investigate the impact of these coatings on the optical behaviors of underlying metallic layers, specifically in the spectral range of excitation of surface plasmon resonances. Combining the methods of spectroreflectometry and spectro-ellipsometry was used to analyze the optical characteristics of the hybrid metal-oxide-graphene films. The study shows that graphene, due to its exceptional electrical conductivity and unique optoelectronic properties, significantly modifies the optical behavior of investigated structures. It includes notable changes in refractive and absorption indices, and optical conductivity indicating potential for enhancing light-matter interactions in plasmonic-graphene layered structures with the aim to apply as biosensor. It is important that addition of TiO2 and Al2O3 layers has also strong effects on the optical properties, which are relevant to their respective applications in the fields of optoelectronics and microelectronics. Employing the effective medium approximation and the Tauc–Lorentz model promotes deeper understanding the interplay between interband and intraband electronic transitions at the nanoscale level. It was revealed that the layer thickness of constituted materials and their individual dielectric functions together with addition of a graphene monolayer commit the significance for altering the optical properties of hybrid layered structures. The obtained results are important for the fields of plasmonics and nanotechnology, providing insights for designing sensors and devices with improved optical characteristics.

Float-stacked graphene–PMMA laminate

Semi-infinite single-atom-thick graphene is an ideal reinforcing material that can simultaneously improve the mechanical, electrical, and thermal properties of matrix. Here, we present a float-stacking strategy to accurately align the monolayer graphene reinforcement in polymer matrix. We float graphene-poly(methylmethacrylate) (PMMA) membrane (GPM) at the water–air interface, and wind-up layer-by-layer by roller. During the stacking process, the inherent water meniscus continuously induces web tension of the GPM, suppressing wrinkle and folding generation. Moreover, rolling-up and hot-rolling mill process above the glass transition temperature of PMMA induces conformal contact between each layer. This allows for pre-tension of the composite, maximizing its reinforcing efficiency. The number and spacing of the embedded graphene fillers are precisely controlled. Notably, we accurately align 100 layers of monolayer graphene in a PMMA matrix with the same intervals to achieve a specific strength of about 118.5 MPa g−1 cm3, which is higher than that of lightweight Al alloy, and a thermal conductivity of about 4.00 W m−1 K−1, which is increased by about 2,000 %, compared to the PMMA film.

Hexagonal Boron Nitride Spacers for Fluorescence Imaging of Biomolecules

AbstractFluorescence imaging is an invaluable tool to investigate biomolecular dynamics, mechanics, and interactions in aqueous environments. Two‐dimensional materials offer large‐area, atomically smooth surfaces for wide‐field biomolecule imaging. Despite the success of graphene for on‐chip biosensing and biomolecule manipulation, its strong fluorescence‐quenching properties pose a challenge for biomolecular investigations that are based on direct optical readouts. Here, we employ few‐layer hexagonal boron nitride (hBN) as a precisely tailorable fluorescence spacer between labelled lipid membranes and graphene substrates. By stacking high‐quality hBN crystals in the 10–20 nm thickness range on monolayer graphene, we observe distance‐dependent fluorescence intensity variations. Remarkably, with hBN spacers as thin as 20 nm, the fluorescence intensity is comparable to bare SiO2/Si substrates, while the intensity was reduced to 60 % and 80 % with ~10 nm and ~16 nm hBN thicknesses respectively. We confirm that pre‐determined hBN thicknesses can be employed to control the non‐radiative energy transfer properties of graphene, with fluorescence quenching following a d−4 distance‐dependent behaviour. This seamless integration of electronically active and dielectric van der Waals materials into vertical heterostructures enables multifunctional platforms addressing the manipulation, localization, and visualization of biomolecules for fundamental biophysics and biosensing applications.

Passive waveguide lasers Q-switched by evanescent-field interaction with high-energy ion-irradiated monolayer graphene

The transfer of two-dimensional (2D) nanomaterials as saturable absorbers to the surface of optical waveguides is extremely attractive for increasing the laser damage threshold of 2D nanomaterials and the laser power of pulsed waveguide lasers. We report on the design and fabrication of passively Q-switched waveguide lasers based on monolayer graphene covered on the surface of neodymium-doped yttrium aluminum garnet (Nd:YAG) cladding waveguides under high-energy carbon ion irradiation. The monolayer graphene is forced into close contact with the waveguide surface by energetic carbon ions, which leads to an excellent coupling of the evanescent wave with graphene. As a result, highly efficient Q-switched pulsed waveguide lasers with high output power, short pulse duration, and high repetition rate are realized under both TE and TM polarized optical pumping. Among all lasers in this experiment, the shortest pulse duration of 38.7 ns is provided by the ion-irradiated larger waveguide system under TE optical pumping.

Micropatterned Strain Domain in Graphene on Cu Substrates: Correlation with Facet Structures

Metallic catalysts have emerged as prominent materials for graphene growth because the choice of the substrate significantly influences the properties of graphene. Among metallic catalysts, Cu is a highly promising catalytic substrate because it facilitates the growth of monolayer graphene. However, the growth process often induces strain and structural deformations that affect the electronic properties and device performance of graphene. In this study, we present the first observation of micropatterned strain domains in graphene grown on Cu substrates using chemical vapor deposition. Distinct strain‐induced shifts and broadening in the Raman peaks by employing Raman spectroscopy and atomic force microscopy are revealed, demonstrating the unique structural characteristics within the patterned domains. Furthermore, a strong correlation between the strain patterns in graphene and the facet structures of the underlying Cu surface is established. Our findings reveal intriguing variations in strain within the patterned domains and along the line boundaries, offering valuable insights into the intricate interactions between graphene and the Cu surface. These observed strain patterns and their spatial correlations with the Cu facet structures provide crucial guidance for designing graphene‐based devices with tailored strain engineering.

Mechanical properties of bilayer WS2 and Graphene-WS2 Hybrid composites by molecular dynamics simulations

In the present work, by using molecular dynamics (MD) simulations, we investigate the mechanical properties of different nanostructures that may be core elements in next generation flexible/wearable photovoltaic devices, namely double layer WS2 nanosheets (DLNS), graphene/WS2 (layer) composites and graphene/WS2 nanotube (NT) composites. Our results reveal that the mechanical properties of DLNS deteriorate when compared to those of monolayer WS2. Owing to graphene’s reinforcement action, the mechanical properties of graphene/WS2 (layer) composite with both layers deformed are superior than those of WS2, even though inferior than those of bare graphene. If stress is applied only to the graphene layer, the graphene/WS2 composite retains the most of the strength and toughness of monolayer graphene, decreasing the fracture strength and Young’s modulus by only 9.7% and 16.3%, respectively. Similarly, in the case of the graphene/WS2 NT composite the mechanical strength and toughness experience a reduction compared to monolayer graphene, specifically by 15% and 53% for fracture strength and Young’s modulus, respectively. Considering the market’s keen interest in nanomaterials, particularly van der Waals (vdW) ones, for flexible and wearable photovoltaic devices, the findings presented here will significantly enhance the effective utilization of vdW composites.

Graphene Failure under MPa: Nanowear of Step Edges Initiated by Interfacial Mechanochemical Reactions.

The low wear resistance of macroscale graphene coatings does not match the ultrahigh mechanical strength and chemical inertness of the graphene layer itself; however, the wear mechanism responsible for this issue at low mechanical stress is still unclear. Here, we demonstrate that the susceptibility of the graphene monolayer to wear at its atomic step edges is governed by the mechanochemistry of frictional interfaces. The mechanochemical reactions activated by chemically active SiO2 microspheres result in atomic attrition rather than mechanical damage such as surface fracture and folding by chemically inert diamond tools. Correspondingly, the threshold contact stress for graphene edge wear decreases more than 30 times to the MPa level, and mechanochemical wear can be described well with the mechanically assisted Arrhenius-type kinetic model, i.e., exponential dependence of the removal rate on the contact stress. These findings provide a strategy for improving the antiwear of graphene-based materials by reducing the mechanochemical interactions at tribological interfaces.

Extreme Thermal Insulation and Tradeoff of Thermal Transport Mechanisms between Graphene and WS2 Monolayers.

Controlling and understanding the heat flow at a nanometer scale are challenging, but important for fundamental science and applications. Two-dimensional (2D) layered materials provide perhaps the ultimate solution for meeting these challenges. While there have been reports of low thermal conductivities (several mW m-1 K-1) across the 2D heterostructures, phonon-dominant thermal transport remains strong due to the nearly-ideal contact between the layers. Here, this work experimentally explores the heat transport mechanisms by increasing the interlayer distance from perfect contact to a few nanometers and demonstrates that the phonon-dominated thermal conductivity across the WS2/graphene interface decreases further with the increasing interlayer distance until the air-dominated thermal conductivity increases again. This work finds that the resulting tradeoff of the two heat conduction mechanisms leads to the existence of a minimum thermal conductivity at 2.11nm of 1.41×10-5W m-1 K-1, which is two thousandths of the smallest value reported previously. This work provides an effective methodology for engineering thermal insulation structures and understanding heat transport at the ultimate small scales.

An investigation into the thermal rectification in one-dimensional asymmetric systems

The thermal rectification phenomenon in a one-dimensional asymmetric system of two thin films with different temperature-dependent thermal conductivities is studied. Based on the traditional thermal diffusion equation, the optimization condition for maximizing the thermal rectification ratio is re-examined; a new sufficient condition is found that the interface temperatures in the forward and reverse heat transfer directions should be equal, regardless of how the thermal conductivities depend on temperature. In other words, the thermal rectification ratio is maximized when the temperature dependence of the thermal conductivity over the entire operating temperature range is fully exploited. This sufficient condition can be used to find out the optimal geometric parameters of nanostructures which determine the effective thermal conductivity. An illustration of an asymmetric system composed of monolayer graphene sheet (GS) and graphene nanomesh (GNM) is presented. This study is beneficial to the development of thermal devices such as thermal resistors, thermal diodes, and thermal logic circuits.

On the local aspect of valley magnetic moments

Valley magnetic moments play a crucial role in valleytronics in 2D hexagonal materials. Traditionally, insights drawn from the study of quantum states in homogeneous bulks have led to a widespread belief that only materials with broken structural inversion symmetry can exhibit nonvanishing valley magnetic moments. This belief, however, limits the scope of relevant applications, especially for materials with inversion symmetry, such as gapless monolayer graphene, despite its advantage in routine growth and production. This work revisits valley-derived magnetic moments in a broad context covering inhomogeneous structures as well. It generalizes the notion of a valley magnetic moment for a state from an integrated quantity to the local field called the “local valley magnetic moment” with space-varying distribution. It explores the local magnetic moment analytically both within the Dirac model and through a symmetry argument. Numerical investigations are conducted within the tight-binding model. Overall, we demonstrate that the breaking of inversion symmetry in the electron probability distribution leads to nonvanishing local magnetic moments. This probability-based breaking can occur in both structural inversion symmetric and symmetry-broken structures. In suitable inversion-symmetric structures with inhomogeneity, e.g., zigzag nanoribbons of gapless monolayer graphene, it is shown that the local moment of a state can be nonvanishing while the corresponding integrated moment is subject to the broken symmetry constraint. Moreover, it is demonstrated that the local moment can interact with space-dependent magnetic fields, resulting in field effects such as valley Zeeman splitting. Such effects can be exploited for local valley control as a conduit for the implementation of valleytronics.

Raman characteristics of graphene/quartz and graphene/Ag nanoparticles/quartz substrate: Laser power dependence

The sensitivity of graphene surface to adjacent conditions plays an important role that modifies the performance and characteristics of graphene devices working under ambient conditions. Quartz is a dielectric transparent material with excellent optical transmission and therefore graphene/quartz is widely used in graphene device technology. Here, graphene/quartz and graphene/plasmonic nanoparticle/quartz structures were investigated using Raman spectroscopy for applications in nanotechnology like graphene quartz fiber (GQF) multifunctional electrode. Optically induced shift of the Fermi level of graphene monolayer on quartz substrates and on Ag nanoparticles (NPs) distributed on quartz substrates was tracked by increasing the applied laser power. Strained graphene attached to quartz substrate, thermal effects, and charge transfer were discussed using the evolution of the G-peak characteristics. Well distributed Ag NPs on quartz substrate are shown by scanning electron microscopy; an interesting property of plasmonic nanoparticles on quartz substrates. The graphene on single Ag NP verified using Raman mapping images by the surface enhanced Raman G-peak on the Ag NP. The Ag NP was driven to the plasmonic resonance by selecting the laser probe wavelength and the localized surface plasmons excited in Ag NPs were inferred by the transmission and photoluminescence spectra. Investigations of graphene on quartz and on plasmonic nanoparticles/quartz substrates can help for GQF hybrid structures which combines the enhanced thermal and electrical conductivity of graphene/plasmonic nanoparticles in addition to the extraordinary properties of quartz fibers suitable for sensing applications.

Thermal transports of 2D phosphorous carbides by machine learning molecular dynamics simulations

Carbon phosphide is a newly discovered two-dimensional semiconductor material which wrinkles and has a significant carrier mobility. Due to lack an accurate force field, the use of molecular dynamics to study its phonon-dominated thermal conductivity which lead to inaccurate results. At present, the use of machine learning to construct a high-precision force field has become the mainstream research method to solve this problem. The main work of this study is to construct a comprehensive training sets for Phosphorus-Doped Graphene (PCn) (n = 3, 5, 6) and to use the fitted potential to calculate the related thermal properties. The research found that (PC5) exhibited anisotropic behavior, with a thermal conductivity of 106.6 Wm−1K−1 in the y-direction and 63.6 Wm−1K−1 in the x-direction. In comparison, (PC6) and (PC3) showed isotropic behavior, with thermal conductivity of approximately 104 Wm−1K−1 and 76.83 Wm−1K−1, respectively. Compared to monolayer graphene, the lower thermal conductivity of PCn is mainly attributed to phonon-phonon scattering effects, which are limited by the regular wrinkled structure. Additionally, low-frequency phonon have been found to have a significant impact on the thermal performance of PCn. Furthermore, we investigated the influence of uniaxial strain on the PC6 and observed an increase in the thermal conductivity with increasing strain. This study used key computational and analytical techniques, including phonon dispersion relations, homogeneous nonequilibrium molecular dynamics method, spectral thermal conductivity analysis. These findings provide a theoretical basis for understanding the thermal transport properties of PCn and will guide its potential applications value.

Switchable visible and near-infrared light bifocal lens based on hybrid graphene-metasurface structures

We propose a wideband visible (VIS) and near infrared (NIR) light reflective flat lens with switching capability base on hybrid graphene-metasurface structure. This tunable flat lens structure consists of a slotted gold reflector substrate, a graphene monolayer and TiO2 nanofins that provide excellent focusing performance and tunability in the VIS and NIR range. In order to achieve the switching feature, a bi-functional device capable of switching reflection/absorption modes is required. To this end, the gold substrate with two cross slots has been used to create absorption properties and a graphene layer has been used to change the intensity of absorption and reflection. The switching property for incident light with circular polarization arises from uniformly changing the chemical potential of the graphene layer. By setting the chemical potential of graphene, μc, equal to 1.1 eV, we obtained the reflection property. The reflection mode exhibits a broadband (715–800 nm) optical response with high polarization conversion efficiency (more than 70 %). On the other hand, by choosing μc = 0.75 eV, the absorption feature is created at the wavelength of 700–780 nm. The reflected light can be focused on an arbitrary position above the structure by rotating the TiO2 nanofins with the help of Pancharatnam-Berry (PB) phase theory. In addition to obtaining the single and dual focus tunable flat lenses, we have also presented a multifunction configuration that can switch between different modes, such as non-focus, single and dual focus modes. Such switchable flat lenses can have various applications in imaging and sensing systems.

Electrically tunable dual-channel absorber based on a graphene integrated slanted grating cavity

In this paper, we propose a tunable dual-channel absorber based on a monolayer of graphene combined with a slanted grating. A single-channel perfect absorption of a monolayer graphene structure at 850 nm was initially achieved in a Fabry-Perot cavity resonance. The absorption rate of graphene was enhanced by nearly 24.5 times. Furthermore, the symmetry of the grating is broken to excite the quasi-bound states in the continuum (quasi-BICs) resonance mode, resulting in another absorption channel at 878.8 nm, thus achieving a perfect dual-channel absorber. The absorption half-width and absorption wavelength can be adjusted by tuning the grating thickness and the electro-optical effect of the DAST material. Furthermore, adjusting the polarization angle of the incident light enables the transition between single- and dual-channel absorbers. The proposed device has potential applications in the fields of high-performance graphene-based optoelectronic devices and optical switching systems.

Enhanced photocatalytic properties of s-triazine-based-g-C3N4/BlueP and g-C3N4/G/BlueP vdW heterostructures: A DFT study

Constructing a Z-scheme heterostructure from selected semiconductors has been proved to be an effective route for attaining the strong redox capability and broad light absorption range simultaneously. In this work, we investigated the electronic, optical and photocatalytic properties of g-C3N4/BlueP and g-C3N4/G/BlueP van der waals heterostructures (vdWHs) by density functional theory (DFT) calculations. The staggered band alignment and the interfacial built-in electric field originated from the charge transfer from g-C3N4 to BlueP depict the Z-scheme photocatalytic mechanism of the g-C3N4/BlueP vdWH. Meanwhile, the vdWH shows an enhanced absorbance at the ultraviolet and visible light range compared to the individual g-C3N4 and BlueP monolayers. Furthermore, the intercalation of Graphene monolayer between g-C3N4 and BlueP is proved to modulate the interfacial built-in electric field, while retaining the staggered band alignment simultaneously. The significant enhancement of the visible light region absorption efficiency promoted by the gap-state of intercalated Graphene manifests the co-catalytic effect of Graphene and the applicable prospect of the constructed ternary heterostructure in photocatalysis area.

Crater-free monolayer graphene above the 2D Si film on SiC(0001) formed via SiSn cointercalation and Sn deintercalation

When Si atoms are intercalated under the graphene-like zero layer (ZL) generated on the SiC(0001) substrate, the resulting quasi-free-standing monolayer graphene (QFMLG) has lots of crater-like defects due to a strong Si-C reaction. Here, a method for creating crater-free QFMLG above a two-dimensional Si film is investigated using scanning tunneling microscopy and photoemission spectroscopy. The method involves forming a SiSn interfacial film as a precursor via cointercalation performed by sequential Sn and Si deposition on the ZL at room temperature and annealing at 650–700 °C. The Sn atoms prevent reactive Si atoms from directly contacting the ZL, which would otherwise cause SiC defects. The sample is then annealed at 750 °C to selectively remove the Sn atoms from the SiSn interfacial film, leaving a well-ordered Si film. The SiSn film composed of a bottom Si layer and top Sn atoms has a short-range-ordered “2 × 2” or a long-range-ordered 2×7 superstructure depending on the annealing temperature and the Sn coverage. Both the superstructures are semiconducting and induce less n-doped QFMLG than the pure Si film.

Manipulation of magnetic anisotropy of 2D magnetized graphene by ferroelectric In2Se3

The capacity to externally manipulate magnetic properties is highly desired from both fundamental and technological perspectives, particularly in the development of magnetoelectronics and spintronics devices. Here, using first-principles calculations, we have demonstrated the ability of controlling the magnetism of magnetized graphene monolayers by interfacing them with a two-dimensional ferroelectric material. When the 3d transition metal (TM) is adsorbed on the graphene monolayer, its magnetization easy axis can be flipped from in-plane to out-of-plane by the ferroelectric polarization reversal of In2Se3, and the magnetocrystalline anisotropy energy (MAE) can be high to −0.692 meV/atom when adopting the Fe atom at bridge site with downward polarization. This may be a universal method since the 3d TM-adsorbed graphene has a very small MAE, which can be easily manipulated by the ferroelectric polarization. As a result, the inherent mechanism is analyzed by the second variation method.

Pressure-dependent thermal conductivity transient measurement of graphene

The thermal properties of graphene determine the thermal damage evaluation of graphene diaphragm and the lifetime of graphene-based devices. The conventional transient measurement method tends to result in the low-precision thermal conductivity (κ) for suspended monolayer graphene and complex interferometry setup. Hence, in order to improve κ measurement of graphene, a simple, miniaturized and portable Fabry-Perot (F–P) resonator with 10-layer graphene is proposed to perform the actuation and vibration detection of the suspended graphene. Then, the κ value can be calculated by means of laser-heated graphene's thermomechanical response, along with the established model for thermal time constant (τ) of a uniformly heated circular film. A quadratic fit is applied to the measured κ at different air pressures and an average fitting curve is obtained, thus determining that the measured κ relatively decreases by 13.32% with the increasing air pressure outside the F–P cavity from ∼22 kPa to ∼101 kPa. Subsequently, the COMSOL heat transfer simulation confirms that convective heat transfer is the primary reason for the external air pressure affecting transient measurements of κ of graphene. Hence, after taking the intercept of the average fitting curve, the predicted κ at 0 Pa of 10-layer suspended graphene is calculated to be 194.48 W/(m·K), which is close to the fitting curve calculated from previously reported κ of graphene with different layers. This study contributes to accurate transient measurement of κ in graphene, and is also instructive for investigating transient thermal process in other two-dimensional materials.

Significance of Porous Graphene for Gas Sensing and Charge Storage Applications

In recent, the air pollution problem has become more serious issue due to pernicious effects on human health. One of the most harmful pollutants, carbon monoxide (CO) has been adsorbed on the graphene for gas-sensing applications. In order to overcome these challenges, the proposed work demonstrates the impact of different numbers of CO molecules adsorption for different widths of graphene. The analysis suggests that the tetralayer demonstrates ∼69.2% better strength in adsorption than monolayer graphene for multiple pollutants, and is also found to be suitable for band-gap-related applications. Moreover, graphene has been studied as an electrode material due to its high surface area and good conductivity. The higher conductivity of porous graphene results in higher charge storage capability due to more conducting channels that play a major role in calculating the parasitic values. For more conducting channels, it is observed that the percentage change in quantum resistance and inductance are reduced by 21.62% and 26.67%, respectively for 10 atoms w.r.t. 8 atoms wide porous graphene. The quantum capacitance is increased by 11.5% for 10 atoms w.r.t. 8 atoms wide porous graphene that results in improved charge storage capability of graphene.