The resistivity of different films and structures containing fluorinated graphene (FG) flakes and chemical vapor deposition (CVD) grown graphene of various fluorination degrees under tensile and compressive strains due to bending deformations was studied. Graphene and multilayer graphene films grown by means of the CVD method were transferred onto the flexible substrate by laminating and were subjected to fluorination. They demonstrated a weak fluorination degree (F/C lower 20%). Compressive strains led to a strong (one-two orders of magnitude) decrease in the resistivity in both cases, which was most likely connected with the formation of additional conductive paths through fluorinated graphene. Tensile strain up to 3% caused by the bending of both types of CVD-grown FG led to a constant value of the resistivity or to an irreversible increase in the resistivity under repeated strain cycles. In the case of the structures with the FG thin film printed on polyvinyl alcohol, a stable bipolar resistive switching was observed up to 6.5% of the tensile strain (bending radius was 2 mm). The excellent performance of the crossbar memristor structures under tensile strain shows that the FG films and structures created from suspension are especially promising for flexible electronics.
Within the framework of stochastic reactive molecular dynamics simulations we develop a statistical method for generating fluorinated graphene structures with desirable fluorine distribution. Electronic transport properties of fluorinated graphene in a wide range of functionalization degree and system ordering are investigated. We have found a strong correlation between irregularities in fluorine distribution and electronic properties. In particular, proposed consideration allows us to reproduce both the experimentally observed electron-hole asymmetry in transport properties of fluorinated gra…
Charge doping induced phase transitions in hydrogenated and fluorinated graphene
我们发现，电荷掺杂可以诱导氢化和氟化石墨烯中三种不同的吸附物相之间的转变。通过结合从头算，近似密度泛函理论和紧束缚计算，我们确定了过渡从岛的C $_8$ H $_2$和C $_8$ F $_2$的随机吸附分布周围的掺杂水平$\pm 0.05$ e/C-原子。此外，在具有随机吸附物覆盖的情况下，电荷掺杂被示出为触发有序转变，其中当掺杂水平超过吸附物浓度时，亚晶格对称性自发地被破坏。再杂化能和晶格畸变能使得仅从一侧共价官能化的石墨烯最容易发生这两种相变。与聚集和有序转变相关的能量增益超过室温热能。
This work analyzes carrier transport, the relaxation of non-equilibrium charge, and the electronic structure of fluorinated graphene (FG) films with graphene quantum dots (GQDs). The FG films with GQDs were fabricated by means of chemical functionalization in an aqueous solution of hydrofluoric acid. High fluctuations of potential relief inside the FG barriers have been detected in the range of up to 200 mV. A phenomenological expression that describes the dependence of the time of non-equilibrium charge emission from GQDs on quantum confinement levels and film thickness (potential barrier par…
In this paper, we investigate the adsorption of water monomer on fluorinated graphene using state-of-the-art first principles methods within the framework of density functional theory (DFT). Four different methods are employed to describe the interactions between water and the carbon surface: The traditional DFT calculations within the generalized gradient approximation (GGA), and three types of calculations using respectively the semi-empirical DFT-D2method, the original van der Waals density functional (vdW-DF) method, and one of its variants. Compared with the adsorption on pristine graphen…
In this paper we present a successful approach for the generation of partially fluorinated graphene structures. A computationally simple model optimized on a large DFT dataset quickly and precisely predicts experimentally observed structures. From the analysis of the structural diversity of fluorinated graphene in a wide range of synthesis temperatures, the general structural patterns are identified and the conditions for their achievement are determined. In addition, to facilitate further studies of fluorinated graphene, we present a ready-to-use GenCF code that implements the described struc…
Graphene functionalization by hydrogen and fluorine has been proposed as a route to modulate its reactivity and electronic properties. However, until now, proposed systems present degradation and limited hydrogen adsorption capacity. In this study, combining first-principles calculations based on density functional theory (DFT) and reactive molecular dynamics, we analyze the tuning of hydrogen adsorption and electronic properties in fluorinated and hydrogenated monolayer graphenes. Our results indicate that fluorine adsorption promotes stronger carbon$-$hydrogen bonds. By changing the concentr…
A systematic study is conducted on the fluorination-enriched essential properties of 2D graphene and 1D graphene nanoribbons using the first-principles method. The combined effects, which arise from the significant chemical bonds in C-C, F-C and F-F bonds, the finite-size quantum confinement, and the edge structure, can greatly diversify geometric structures, electronic properties and magnetic configurations. By the detailed analyses, the critical orbital hybridizations in determining the essential properties are accurately identified from the atom-dominated energy bands, the spatial charge di…
The graphene-enhanced Raman scattering of Rhodamine 6G molecules on pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates was studied. The uniformity of the Raman signal enhancement was studied by making large Raman maps. The relative enhancement of the Raman signal is demonstrated to be dependent on the functional groups, which was rationalized by the different doping levels of pristine, fluorinated and 4-nitrophenyl functionalized graphene substrates. The impact of the Fermi energy of graphene and the phonon energy of the molecules was considered together for the first t…
Notwithstanding numerous density functional studies on the chemically induced transformation of multilayer graphene into a diamond-like film, a comprehensive convincing experimental proof of such a conversion is still lacking. We show that the fluorination of graphene sheets in Bernal (AB)-stacked bilayer graphene (AB-BLG) grown by chemical vapor deposition on a single crystal CuNi(111) surface triggers the formation of interlayer carbon-carbon bonds, resulting in a fluorinated diamond monolayer (F-diamane). Induced by fluorine chemisorption, the phase transition from AB-BLG to single layer di…
The control of strain in two-dimensional materials opens exciting perspectives for the engineering of their electronic properties. While this expectation has been validated by artificial-lattice studies, it remains elusive in the case of atomic lattices. Remarkable results were obtained on nanobubbles and nano-wrinkles, or using scanning probes; microscale strain devices were implemented exploiting deformable substrates or external loads. These devices lack, however, the flexibility required to fully control and investigate arbitrary strain profiles. Here, we demonstrate a novel approach makin…
Chemical vapor deposition (CVD) has been widely adopted as the most scalable method to obtain single layer graphene. Incorporating CVD graphene in planar devices can be performed via well established wet transfer methods or thermal adhesive release. Nevertheless, for applications involving 3D shapes, methods adopted for planar surface provide only a crude solution if a continuous, tear-free, wrinkle-free graphene layer is required. In this work, we present the fabrication and characterization of PDMS-supported 3D graphene probes. To accommodate 3D geometries, we perform CVD on catalysts posses…
The electronic and optical properties of graphene can be precisely tuned by generating deterministic arrangements of strain features. In this paper, we report the formation of widespread and controlled buckling delamination of monolayer graphene deposited on hexagonal boron-nitride promoted by a significant squeezing of the graphene flake and induced by polymeric micro-actuators. The flexibility of this method offers a promising technique to create arbitrary buckling geometries and arrays of wrinkles which could also be subjected to iterative folding-unfolding cycles. Further development of th…
Confinement of electrons in graphene to make devices has proven to be a challenging task. Electrostatic methods fail because of Klein tunneling, while etching into nanoribbons requires extreme control of edge terminations, and bottom-up approaches are limited in size to a few nanometers. Fortunately, its mechanical flexibility raises the possibility of using strain to alter graphene's properties and create novel straintronic devices. Here, we report transport studies of nanowires created by linearly-shaped strained regions resulting from individual folds formed by layer transfer onto hexagonal…
The phase diagram of isotropically expanded graphene cannot be correctly predicted by ignoring either electron correlations, or mobile carbons, or the effect of applied stress, as was done so far. We calculate the ground state enthalpy (not just energy) of strained graphene by an accurate off-lattice Quantum Monte Carlo (QMC) correlated ansatz of great variational flexibility. Following undistorted semimetallic graphene (SEM) at low strain, multi-determinant Heitler-London correlations stabilize between $\simeq$8.5% and $\simeq$15% strain an insulating Kekule-like dimerized (DIM) state. Closer…
The extremely high thermal conductivity of graphene has received great attention both in experiments and calculations. Obviously, new feature in thermal properties is of primary importance for application of graphene-based materials in thermal management in nanoscale. Here, we studied the thermal conductivity of graphene helicoid, a newly reported graphene-related nanostructure, using molecular dynamics simulation. Interestingly, in contrast to the converged cross-plane thermal conductivity in multi-layer graphene, axial thermal conductivity of graphene helicoid keeps increasing with thickness…
We present a fabrication method for reliably and reproducibly forming electrical contacts to 2D materials, based on the tri-layer resist system. We demonstrate the applicability of this method for epitaxial graphene on silicon carbide (epigraphene) and the transition metal dichalcogenides (TMDCs) molybdenum disulfide ($MoS_2$). For epigraphene, the specific contact resistances are of the order of $ρ_c$ ~ $50$ $Ωμm$, and follow the Landauer quantum limit, $ρ_c \propto n^{-1/2}$, with $n$ being the carrier density of graphene. For $MoS_2$ flakes, our edge contacts enable field effect transistors…
The fabrication of electrochemical double-layer capacitors (EDLCs) with high areal capacitance relies on the use of elevated mass loadings of highly porous active materials. Herein, we demonstrate a high-throughput manufacturing of graphene/nanotubes hybrid EDLCs. Wet-jet milling (WJM) method is exploited to exfoliate the graphite into single/few-layer graphene flakes (WJM-G) in industrial volume (production rate ~0.5 kg/day). Commercial single/double walled carbon nanotubes (SDWCNTs) are mixed with graphene flakes in order to act as spacers between the graphene flakes during their film format…
Flexoelectricity is a form of electromechanical coupling that has recently emerged because, unlike piezoelectricity, it is theoretically possible in any dielectric material. Two-dimensional (2D) materials have also garnered significant interest because of their unusual electromechanical properties and high flexibility, but the intrinsic flexoelectric properties of these materials remain unresolved. In this work, using atomistic modeling accounting for charge-dipole interactions, we report the intrinsic flexoelectric constants for a range of two-dimensional materials, including graphene allotro…
Van der Waals heterostructure based on layered two-dimensional (2D) materials offers unprecedented opportunities to create materials with atomic precision by design. By combining superior properties of each component, such heterostructure also provides possible solutions to address various challenges of the electronic devices, especially those with vertical multilayered structures. Here, we report the realization of robust memristors for the first time based on van der Waals heterostructure of fully layered 2D materials (graphene/MoS2-xOx/graphene) and demonstrate a good thermal stability lack…
