Graphene Interface Research Articles

Atomic-scale manipulation of buried graphene–silicon carbide interface by local electric field

Precision of scanning tunneling microscopy (STM) enables control of matter at scales of single atoms. However, transition from atomic-scale manipulation strategies to practical devices encounters fundamental problems in protection of the designer structures formed atop the surface. In this context, STM manipulation of subsurface structures on technologically relevant materials is encouraging. Here, we propose a material platform and protocols for precise manipulation of a buried graphene interface. We show that an electric field from the STM tip reversibly controls breaking and restoring of covalent bonds between the graphene buffer layer and the SiC substrate. The process involves charge redistribution at the atomically sharp interface plane under the epitaxial graphene layer(s). This buried manipulation platform is laterally defined by unit cells from the corresponding (6×6)SiC moiré lattice of the epitaxial graphene. Local and reversible electric-field-induced patterning of graphene heterostructures from the bottom interface creates an alternative architecture concept for their applications.

Facile synthesis of nanostructured Ni/NiO/N-doped graphene electrocatalysts for enhanced oxygen evolution reaction.

Electrocatalysts containing a Ni/NiO/N-doped graphene interface have been synthesised using the ligand-assisted chemical vapor deposition technique. NiO nanoparticles were used as the substrate to grow N-doped graphene by decomposing vapours of benzene and N-containing ligands. The method was demonstrated with two nitrogen-containing ligands, namely dipyrazino[2,3-f:2',3'-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile (L) and melamine (M). The structure and composition of the as-synthesized composites were characterized by XRD, Raman spectroscopy, SEM, TEM and XPS. The composite prepared using the ligand L had NiO sandwiched between Ni and N-doped graphene and showed an overpotential of 292 mV at 10 mA cm-2 and a Tafel slope of 45.41 mV dec-1 for the OER, which is comparable to the existing noble metal catalysts. The composite prepared using the ligand M had Ni encapsulated by N-doped graphene without NiO. It showed an overpotential of 390 mV at 10 mA cm-2 and a Tafel slope of 78.9 mV dec-1. The ligand-assisted CVD route demonstrates a facile route to control the microstructure of the electrocatalysts.

Versatile Pickering emulsion gel lubricants stabilized by cooperative interfacial graphene oxide-polymer assemblies.

Although a large number of water- and oil-based gel lubricants have found extensive potential applications in industrial and biomedical fields, developing new-type emulsion-based gel lubricants that may effectively integrate their characteristics and preponderances remains a significant challenge. Here a water-in-oil Pickering emulsion gel lubricant that is able to combine the high colloidal stability of traditional Pickering emulsions, the good swelling and corrosion resistance of oil-based gel lubricants, and the high cooling capacity of water-based gel lubricants prepared from a binary mixture of aqueous graphene oxide (GO) dispersion and diamino-functionalized polydimethylsiloxane oil solution in a broad concentration, pH, and water volume fraction range is reported. It can provide favourable lubrication for the Si3N4/steel and Si3N4/silicone tribopairs either in air or under water owing to the formation of a sturdy adsorbed oil film and ball-bearing actions of the GO particles. It can also be printed into various colourful 2D and 3D geometries upon direct extrusion into water, thanks to its water-in-oil nature and inherent shear-thinning and thixotropic properties, which further shows good prospects in underwater operations and artificial biomimetic organs. Our study may provide new insights into the design and preparation of novel semi-solid materials for diverse industrial, engineering, and biomedical applications.

Direction-sensitive graphene flow sensor

Graphene flow sensors not only hold great prospects for applications but also encounter many difficulties, such as unwanted electrochemical phenomena, low measurable signal, and limited dependence on the flow direction. This study proposes an approach allowing for the detection of a flow direction-dependent electric signal in aqueous solutions of salts, acids, and bases. The key element in the proposed solution is the use of a reference electrode which allows external gating of the graphene structure. Using external gating enables to enhance substantially the amplitude of the flow-generated signal. Simultaneous measurement of the reference electrode current allows us to recover a flow-direction-sensitive component of the flow-induced voltage in graphene. The obtained results are discussed in terms of the Coulomb interaction and other phenomena, which can be present at the interface of graphene with the aqueous solution.

Additive-free electroless deposition on graphene/copper foil: Photo-induced and defect-assisted approach for environmentally friendly plating

The environmental toxicity of chemicals used in electroless plating process is often overlooked. In fact, the wastewater from the electroless plating process contains metal complexes, reducing agents and stabilizers that can create hazard to our environment. To address this concern, an additive-free electroless deposition process on graphene/Cu foil is proposed. The defect structure of graphene plays a pivotal role by acting as a reservoir for concentrating photoelectrons generated from ultra-thin cuprous oxide (Cu2O) located at the interface of graphene and Cu foil upon exposure to light irradiation. In the Cu ion solution, Cu ions were anchored on the defect sites and formed a Cu-C4 structure with graphene. The Cu-C4 sites accumulate abundant photoelectrons and trigger the reduction of Cu ions to form island-like growth of Cu, which immediately was oxidized to Cu2O, leaving only a small portion of metallic Cu. Concurrently, the presence of Cu ions induces the generation of additional defect sites on graphene, providing further growth sites for Cu deposition. This process continues until the entire graphene surface is covered by the deposited Cu2O. Ultimately, the deposition of metallic oxides with lower redox potential can be achieved. The proposed deposition mechanism assisted by irradiation of photocatalyst and defect structure of graphene material provides a truly environmentally friendly electroless plating process. By eliminating the need for additives and significantly reducing the environmental hazards associated with electroless plating, this method holds great promise for sustainable materials manufacturing.

Characterizing and controlling infrared phonon anomaly of bilayer graphene in optical-electrical force nanoscopy

We, for the first time, report the nanoscopic imaging study of anomalous infrared (IR) phonon enhancement of bilayer graphene, originated from the charge imbalance between the top and bottom layers, resulting in the enhancement of E1u mode of bilayer graphene near 0.2 eV. We modified the multifrequency atomic force microscope platform to combine photo-induced force microscope with electrostatic/Kelvin probe force microscope constituting a novel hybrid nanoscale optical-electrical force imaging system. This enables to observe a correlation between the IR response, doping level, and topographic information of the graphene layers. Through the nanoscale spectroscopic image measurements, we demonstrate that the charge imbalance at the graphene interface can be controlled by chemical (doping effect via Redox mechanism) and mechanical (triboelectric effect by the doped cantilever) approaches. Moreover, we can also diagnosis the subsurface cracks on the stacked few-layer graphene at nanoscale, by monitoring the strain-induced IR phonon shift. Our approach provides new insights into the development of graphene-based electronic and photonic devices and their potential applications.

Enhanced interfacial adhesion of patterned Cu-graphene nanolayered composite

In this study, graphene was physically patterned to allow for Cu-Cu contacts to serve as anchoring points, thereby preventing complete delamination at the interface, and the effect of the inclusion of Cu-Cu contacts on bending fatigue response was evaluated. Direct patterning and roll-based dry transfer process of CVD-grown graphene was used to fabricate patterned Cu-Gr nanolayered composites. Bending fatigue tests were performed to determine the optimal configuration of patterning that can lead to enhanced reliability. The graphene was shown to hinder crack formation and propagation, where the cracks were absorbed at the graphene interface through localized delamination while the patterned area with Cu-Cu contact served as anchoring points to prevent macroscopic delamination. Therefore, the optimized graphene patterning design proposed in this study was shown to provide a good balance in crack absorption at the interface and prevention of global delamination that can significantly enhance the reliability under bending fatigue.

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.

Manipulation of surface plasmon polariton fields excitation at quantum-size slit in a dielectric and graphene interface

The dispersion relation of surface plasmon polaritons is controlled and modified at the interface of atomic and Graphene medium. The beats patterns and moire fringes spacing are modified from vectorial character of the waves and parameters of the driving fields coupled with system energy levels. The beats patterns are investigated with the influence of control fields and its parameters. It is reported that the phase shifts Φ1,2 are the strong functions of the probe detuning and distance x along the interface. The light and surface plasmon polariton waves polarization is a strong oscillation function of both the position and time variation as well as driving fields parameters. The maximum value of phase sensitivity SΦ1 is 600 and SΦ2 is 400 radian per unit change of refractive index of the atomic medium at x=20λ. This work is particularly useful for surface plasmon resonance sensors technology.

Water flow in a polymeric nanoslit channel with graphene and hexagonal boron nitride wall coatings: An atomistic study

Low flow resistance is essential for the design of nanofluidic platforms. Here, we present a comparative atomic-level study analyzing the fundamentals of water flow enhancement in a polymeric nanochannel due to the implementation of monatomic thick coatings—specifically, graphene and hexagonal boron nitride (hBN). Our results obtained employing large scale non-equilibrium molecular dynamics simulations and continuum models, revealing significant values of slip lengths of ∼29 and ∼6 nm for graphene- and hBN-coated nanochannels, respectively. Equilibrium molecular dynamics simulations, using the Green–Kubo relation, show the significant effect that the partial charges of hBN coating layer have on the water–wall friction. In addition, consistent values of the slip length are obtained from independent sets of equilibrium and non-equilibrium molecular dynamics simulations, confirming that the computed interfacial friction coefficients hold across flow regimes where water molecules no longer occupy the most energetically stable zones at the interface. Hence, the lower interfacial friction observed in the graphene-coated channel leads to a higher water flow enhancement than the one computed in the hBN-coated channel. We also show that the natural undulations of two-dimensional honeycomb-like materials, implemented as wall coatings, remain largely unhindered due to strong interfacial coupling facilitated by π−π stacking between the underlying aromatic polymer substrate and coating monolayers. This is particularly relevant for graphene coatings, which display significant out-of-plane thermal rippling that further enhances water flow. This observation is supported by a stronger atomic-scale vibrational coupling at the water–graphene interface compared to that computed at the water–hBN interface.

Dynamically tuning friction at the graphene interface using the field effect

Dynamically controlling friction in micro- and nanoscale devices is possible using applied electrical bias between contacting surfaces, but this can also induce unwanted reactions which can affect device performance. External electric fields provide a way around this limitation by removing the need to apply bias directly between the contacting surfaces. 2D materials are promising candidates for this approach as their properties can be easily tuned by electric fields and they can be straightforwardly used as surface coatings. This work investigates the friction between single layer graphene and an atomic force microscope tip under the influence of external electric fields. While the primary effect in most systems is electrostatically controllable adhesion, graphene in contact with semiconducting tips exhibits a regime of unexpectedly enhanced and highly tunable friction. The origins of this phenomenon are discussed in the context of fundamental frictional dissipation mechanisms considering stick slip behavior, electron-phonon coupling and viscous electronic flow.

Effects of Graphene Interface on Potassiation in a Graphene–Selenium Heterostructure Cathode for Potassium-Ion Batteries

Effects of Graphene Interface on Potassiation in a Graphene–Selenium Heterostructure Cathode for Potassium-Ion Batteries

Single-Layer Graphene/Germanium Interface Representing a Schottky Junction Studied by Photoelectron Spectroscopy.

We investigated the interfacial electronic structure of the bidimensional interface of single-layer graphene on a germanium substrate. The procedure followed a well-established approach using ultraviolet (UPS) and X-ray (XPS) photoelectron spectroscopy. The direct synthesis of the single-layer graphene on the surface of (110) undoped Ge substrates was conducted via chemical vapor deposition (CVD). The main graphitic properties of the systems were identified, and it was shown that the Ge substrate affected the electronic structure of the single-layer graphene, indicating the electronic coupling between the graphene and the Ge substrate. Furthermore, the relevant features associated with the Schottky contact's nature, the energy level's alignments, and the energy barrier's heights for electron and hole injection were obtained in this work. The results are useful, given the possible integration of single-layer graphene on a Ge substrate with the complementary metal-oxide-semiconductor (CMOS) technology.

In-situ synthesized graphene reinforced copper matrix composites with enhanced thermal properties and corrosion resistance

Graphene is expected to comprehensively improve the thermal conductivity and corrosion resistance of metals to satisfy the need of adverse electrochemical environments. However, there are some major concerns in the mixing methods such as, low quality of graphene, non-uniform dispersion, and poor interface bonding with the metal matrix. In this study, we propose an effective approach to fabricate Cu-graphene (Cu/G) nanocomposite by sintering in-situ growth of graphene on Cu nanoparticles. Results confirmed the firmly adhesive graphene on Cu surface, which closely linked Cu particles to building a bulk, particularly binding between Cu particles and graphene flakes is largely improved by the sintered progress. The as-prepared Cu/G heat spreader demonstrated higher thermal conductivity (376 W/(m·K) at room temperature and lower thermal expansion. Additionally, the short-time corrosion resistance showed that Cu/G can build a graphene network to avoid the penetration of active species and enhance the passivation layer. This work endows Cu/G integral materials possessing good thermal conductivity and corrosion inhibition, which can effectively serve as potential offshore materials.

Three-terminal vertical ferroelectric synaptic barristor enabled by HZO/graphene heterostructure with rebound depolarization

Ferroelectric switching devices employing doped hafnium oxide (HfO2)-based thin films have expanded their range of electronic applications from densely-packed memory to brain-inspired artificial synapses. Here, we introduce a novel and distinctive three-terminal vertical device using a heterogeneous stack of hafnium-zirconium-oxide (HZO) thin film and graphene, called ferroelectric synaptic barristor (FSB), which can function as a scalable artificial synapse for energy-efficient neuromorphic computing. Electrostatic gating in the FSB can simultaneously reorient the polarization degree and the direction of HZO as well as the Schottky barrier and the trapping degree at the graphene interface. With these peculiar controlling factors, this process mimics crucial synaptic characteristics with rebound depolarization (RD), which converts an incoming inhibitory pre-spike into cell excitation. Using the RD function mediated by both ferroelectric switching and trap-mediated charge transport, the FSB exhibited a stable long-term potentiation/depression for 5000 identical pulse schemes with excellent linearity, high yield (47/48 cells), and a good state-retaining ability for 5000 s. The learning ability and energy consumption based on a single neural network were evaluated using MNIST handwritten digits and fashion patterns. The FSB with RD exhibited better accuracy (90.03% and 74.65% for handwritten digits and fashion patterns, respectively), fast and low-power learning ability than that without RD. Our FSB device can bring the advantages of conventional ferroelectric synaptic devices while mitigating their ingrained technical and operational issues.

Ionic Liquid–Graphene Interface: Effect of Anions on the Fermi Level

Since energy conversion and storage processes take place at the electrolyte–electrode interface, it is important to develop experimental and theoretical procedures to understand the interfacial nanostructure in graphene-based electrochemical storage devices where ionic liquids (ILs) are used as electrolytes. In this contribution, the impact of the anions of imidazolium-based ILs on the IL–graphene interface as well as on the electronic structure of graphene is investigated. Raman spectroscopy unveils that 1-butyl-3-methylimidazolium ILs having smaller anions induce n-type doping, while ILs with larger anions have a negligible effect on the doping. Molecular modeling simulations reveal that changes in the electrostatic potential at the IL–graphene interface are responsible for the n-type doping.

Reduced Absorption Due to Defect-Localized Interlayer Excitons in Transition-Metal Dichalcogenide-Graphene Heterostructures.

Associating atomic vacancies to excited-state transport phenomena in two-dimensional semiconductors demands a detailed understanding of the exciton transitions involved. We study the effect of such defects on the electronic and optical properties of WS2-graphene and MoS2-graphene van der Waals heterobilayers, employing many-body perturbation theory. We find that chalcogen defects and the graphene interface radically alter the optical properties of the transition-metal dichalcogenide in the heterobilayer, due to a combination of dielectric screening and the many-body nature of defect-induced intralayer and interlayer optical transitions. By analyzing the intrinsic radiative rates of the subgap excitonic features, we show that while defects introduce low-lying optical transitions, resulting in excitons with non-negligible oscillator strength, they decrease the optical response of the pristine-like transition-metal dichalcogenide intralayer excitons. Our findings relate excitonic features with interface design for defect engineering in photovoltaic and transport applications.

Twist pz Orbital and Spin Moment of the Wavy-Graphene/L10-FePd Moiré Interface

A crystallographically heterogeneous moiré interface of hexagonal graphene (Gr) and a tetragonal L10-FePd alloy is bonded via van der Waals (vdW) forces. Robust interfacial perpendicular magnetic anisotropy was discovered at the L10-FePd side of the Gr/L10-FePd heterogeneous interface (H. Naganuma et al. ACS Nano, 2022, 16, 4139). This study focuses on the Gr side of the Gr/L10-FePd interface. X-ray absorption spectroscopy measurements of Gr from two different angles demonstrated that in addition to the π* orbital peak being observed at a glance incident angle (θΑ = 30°), it was also observed at a nominal incident (NI) angle (θΑ = 90°). The appearance of the π* peak at NI is attributed to the wavy Gr and strong bonding of the chemisorption-type vdW force. The densities of states of px, py, and pz from first-principles calculations indicate another reason for the π* peak at NI. The pz orbital of C twists into the x–y plane owing to its chemical bond with Fe or perturbation of the Fe ion to the C orbital. This twisted pz orbital appears near the Fermi level. Thus, the π* peak at NI can be interpreted to appear owing to three reasons: (i) the wavy Gr, (ii) the twisted pz orbital near the Fermi level, and (iii) the chemisorption-type vdW force. The X-ray magnetic circular dichroism of the C K-edge and first-principles calculations revealed that wavy Gr has a spin magnetic moment of 0.018 μB/C atoms but no orbital magnetic moment.

Atomistic simulation and experimental verification of tribological behavior of high entropy alloy/graphene composite coatings

Dissatisfactory yield strength and wear have been critical issues in FCC single phase high entropy alloy (HEA) limiting wide applications as advanced engineering materials, in spite of their attractive mechanical and physical properties. Here we show by introducing graphene layer, the tribological properties of Fe20Ni20Cr20Co20Cu20/graphene composite coatings can be noticeably enhanced. In this work, molecular dynamics simulations were performed to probe the tribological mechanisms of HEA/graphene composite coatings. It was revealed that the reasonable choice of single-layer graphene geometry and embedding position play an important part in friction reduction and anti-wear of the coatings. The graphene interface not only cause robust dislocation blockage from HEA, but also reduces the accumulation of atoms at surface during sliding. Besides, the influence of temperature on the tribological behavior of the coatings is explored. The tribological performances of the coatings were also investigated by the experimental nanoscratch test, which are consistent with the simulation results. The present work provides a theoretical and technical support for the design of wear-resistant self-lubricating coatings, promoting the engineering applications of HEA coating materials.

Self-assembly of exfoliated graphene flakes as anticorrosive coatings for additive manufactured steels

This study demonstrates the feasibility of using liquid exfoliation of expandable graphite into multilayer exfoliated graphene flakes (EGFs) to form a self-assembled thin film on an air–water interface. The film can coat the surface of additive manufactured (AM) steel substrates to enhance surface properties, specifically AM 316 stainless-steel (AM316), AM 8620 steel (AM8620), and samples of the same alloys made by conventional manufacturing (CM) processes. Liquid exfoliation offers a high yield route for an EGF coating that can cover up to 95% of the sample surface with a single application. The thin, flexible EGFs can coat a rough AM metal surface, while the highly intact crystal lattice protects covered areas against diffusing ions and prevents localized corrosion compared to similar coatings made with graphene oxide (GO) or reduced GO (rGO). The EGF coating exploits a unique self-assembly process without surfactants or stabilizers, wherein the hydrophobicity and hydrophilicity of EGFs arrange the flakes, then van der Waals (vdW) forces bond them together and to the substrate for a coherent anti-corrosive coating. Electrochemical measurements indicate lower corrosion potential for coated samples, nanoindentation measurements show surface hardness protection against corrosive attack, and weight loss measurements demonstrate long-term protective capabilities. Density functional theory calculations, using the optB88-vdW exchange functional of the graphene and iron (111) interface demonstrated stronger binding and shorter interface distance compared to the well-studied graphene/nickel (111) interface. These calculations and experimental results can further elucidate the superior performance of EGF thin film coatings on high iron content steels, especially AM steels.