Graphene Interfaces Research Articles

Computational Insights into Schottky Barrier Heights: Graphene and Borophene Interfaces with H- and H́-XSi2N4 (X = Mo, W) Monolayers.

The two-dimensional (2D) semiconducting family of XSi2N4 (X = Mo and W), an emergent class of air-stable monolayers, has recently gained attention due to its distinctive structural, mechanical, transport, and optical properties. However, the electrical contact between XSi2N4 and metals remains a mystery. In this study, we inspect the electronic and transport properties, specifically the Schottky barrier height (SBH) and tunneling probability, of XSi2N4-based van der Waals contacts by means of first-principles calculations. Our findings reveal that the electrical contacts of XSi2N4 with metals can serve as the foundation for nanoelectronic devices with ultralow SBHs. We further analyzed the tunneling probability of different metal contacts with XSi2N4. We found that the H-phase XSi2N4/metal contact shows superior tunneling probability compared to that of H́-based metal contacts. Our results suggest that heterostructures at interfaces can potentially enable efficient tunneling barrier modulation in metal contacts, particularly in the case of MoSi2N4/borophene compared to MoSi2N4/graphene and WSi2N4/graphene in transport-efficient electronic devices. Among the studied heterostructures, tunneling efficiency is highest at the H and H́-MoSi2N4/borophene interfaces, with barrier heights of 2.1 and 1.52 eV, respectively, and barrier widths of 1.04 and 0.8 Å. Furthermore, the tunneling probability for these interfaces was identified to be 21.3 and 36.4%, indicating a good efficiency of carrier injection. Thus, our study highlights the potential of MoSi2N4/borophene contact in designing power-efficient Ohmic devices.

Electrochemical Li+ Insertion/Extraction Reactions at LiPON/Epitaxial Graphene Interfaces.

Redox reactions of the Li+ insertion/extraction from one to two interlayers of graphene (Gr) on area-defined single-crystalline SiC substrates are investigated using lithium phosphorus oxynitride glass (LiPON) as the solid-state electrolyte. Unlike an organic liquid electrolyte, this glassy electrolyte does not induce a reduction current and excludes the desolvation reaction of Li+. Gr electrodes with less than two Gr layers show a single reduction peak and one or two oxidation peaks below +0.21 V (vs Li+/Li), differing distinctly from those of graphite and multilayer Gr, which display multiple peaks (multiple stage transitions). However, this finding aligns with the conventional understanding that graphite stage structure transitions proceed with stepwise increases or decreases in the number of Gr layers between adjacent Li-inserted interlayers. Cyclic voltammetry measurements indicate the presence of surface capacity due to Li+ adsorption/desorption at the LiPON/Gr interface. Moreover, Li+ insertion and extraction induce different charge transfer resistances at the level of a single interlayer. These sensitive measurements are achieved using high-quality epitaxial Gr and LiPON electrolyte, which prevent the formation of a solid electrolyte interphase and the desolvation reaction of Li+. Similar measurements using bilayer Gr produced by chemical vapor deposition coupled with a Gr transfer method and an ethylene carbonate/dimethyl carbonate liquid electrolyte are not reliable. Thus, the proposed method is effective for electrochemical measurement of Gr electrodes with a controlled number of layers.

General Capacitance Upper Limit and Its Manifestation for Aqueous Graphene Interfaces

Double-layer capacitance (Cdl) is essential for chemical and biological sensors and capacitor applications. The correct formula for Cdl is a controversial subject for practically useful graphene interfaces with water, aqueous solutions, and other liquids. We have developed a model of Cdl, considering the capacitance of a charge accumulation layer (Cca) and capacitance (Ce) of a capacitance-limiting edge region with negligible electric susceptibility and conductivity between this layer and the capacitor electrode. These capacitances are connected in series, and Cdl can be obtained from 1/Cdl = 1/Cca + 1/Ce. In the case of aqueous graphene interfaces, this model predicts that Cdl is significantly affected by Ce. We have studied the graphene/water interface capacitance by low-frequency impedance spectroscopy. Comparison of the model predictions with the experimental results implies that the distance from charge carriers in graphene to the nearest molecular charges at the interface can be ~(0.05–0.1)nm and is about a typical length of the carbon-hydrogen bond. Generalization of this model, assuming that such an edge region between a conducting electrode and a charge accumulating region is intrinsic for a broad range of non-faradaic capacitors and cannot be thinner than an atomic size of ~0.05 nm, predicts a general capacitance upper limit of ~18 μF/cm2.

Molecular dynamics simulation of reinforcement mechanism of graphene/aluminum composites and microstructure evolution

Graphene/Aluminum (Gr/Al) composites with different graphene chirality and distribution patterns were established. The contribution of graphene interfaces to the mechanical properties of composites was investigated by using molecular dynamics methods. The composites strengthen about 10% of Young's modulus and 5% of tensile yield strength compared with aluminum. Influenced by the chiral structures of graphene, the fracture strain of zigzag composite is larger, and the yield stress of armchair composite is higher. Folding deformation of graphene promotes the yielding of the composites under compressive loading. Even if the graphene breaks, the graphene interface can effectively limit the dislocation motion, and the twins can only expand around the graphene. The results of both tensile and compressive tests showed that the uniform distribution of graphene was beneficial to enhance the mechanical properties of the composites. Graphene hinders the motion of pre-dislocations more than pinning dislocations. Therefore, the Bauschinger effect of composite is lower than that of pure aluminum under tension-compression loading.

A Study on Dislocation Mechanisms of Toughening in Cu-Graphene Nanolayered Composite.

We investigated the role of graphene interfaces in strengthening and toughening of the Cu-graphene nanocomposite by a combination of in situ transmission electron microscopy (TEM) deformation and molecular dynamics (MD) simulations. In situ TEM directly showed that dislocation plasticity is strongly confined within single Cu grains by the graphene interfaces and grain boundaries. The weak Cu-graphene interfacial bonding induces stress decoupling, which results in independent plastic deformation of each Cu layer. As confirmed by the MD simulation, the localized deformation made by such constrained dislocation plasticity results in the nucleation and growth of voids at the graphene interface, which acts as a precursor for crack. The graphene interfaces also effectively block crack propagation promoted by easy delamination of Cu layers dissipating the elastic strain energy. The toughening mechanisms revealed by the present study will provide valuable insights into the optimization of the mechanical properties of metal-graphene nanolayered composites.

Osmotic Transport at the Aqueous Graphene and hBN Interfaces: Scaling Laws from a Unified, First-Principles Description.

Osmotic transport in nanoconfined aqueous electrolytes provides alternative venues for water desalination and "blue energy" harvesting. The osmotic response of nanofluidic systems is controlled by the interfacial structure of water and electrolyte solutions in the so-called electrical double layer (EDL), but a molecular-level picture of the EDL is to a large extent still lacking. Particularly, the role of the electronic structure has not been considered in the description of electrolyte/surface interactions. Here, we report enhanced sampling simulations based on ab initio molecular dynamics, aiming at unravelling the free energy of prototypical ions adsorbed at the aqueous graphene and hBN interfaces, and its consequences on nanofluidic osmotic transport. Specifically, we predicted the zeta potential, the diffusio-osmotic mobility, and the diffusio-osmotic conductivity for a wide range of salt concentrations from the ab initio water and ion spatial distributions through an analytical framework based on Stokes equation and a modified Poisson-Boltzmann equation. We observed concentration-dependent scaling laws, together with dramatic differences in osmotic transport between the two interfaces, including diffusio-osmotic flow and current reversal on hBN but not on graphene. We could rationalize the results for the three osmotic responses with a simple model based on characteristic length scales for ion and water adsorption at the surface, which are quite different on graphene and on hBN. Our work provides fundamental insights into the structure and osmotic transport of aqueous electrolytes on 2D materials and explores alternative pathways for efficient water desalination and osmotic energy conversion.

Molecular Dynamics-Based Cohesive Law for Epoxy–Graphene Interfaces

Molecular dynamics simulations are used to obtain mode I and mode II fracture energies and cohesive laws for bulk epoxy and interfaces formed between epoxy and single-layer graphene (SLG), multilayer graphene (MLG), and multilayer graphene oxide (MLGO). The elastic moduli and ultimate tensile and shear strengths of epoxy–graphene interfaces are calculated from uniaxial tension and simple shear loadings. The results show that Young’s modulus and the ultimate tensile strength increase relative to bulk epoxy, whereas the shear modulus and ultimate shear strength are reduced. Failure of epoxy–graphene interfaces in tension occurs due to the formation of voids in the epoxy. Failure in shear is due to tangential slipping at the interface. Under mixed-mode conditions, the shear modulus and shear strength decrease with increasing tensile load. The critical energy release rate $$G_{\text{c}}$$ for the studied epoxy–SLG/MLG/MLGO systems is obtained using a continuum fracture mechanics approach and is found to be significantly lower than for bulk epoxy. All of the results are combined to define mode I and II cohesive laws for bulk epoxy and epoxy–SLG/MLG/MLGO interfaces that can be used in theoretical models and numerical methods, such as finite elements, that employ cohesive zones.

Transformative Electronics: Design Strategy for Transformative Electronic System toward Rapid, Bidirectional Stiffness Tuning using Graphene and Flexible Thermoelectric Device Interfaces (Adv. Mater. 10/2021)

In article number 2007239, Byung Jin Cho, Jae-Woong Jeong, and co-workers present a design strategy for transformative electronics capable of rapid bidirectional stiffness tuning. Transformative electronics are built on a gallium platform interfaced with graphene and a flexible thermoelectric device (f-TED). The graphene works as a catalyst to mitigate the supercooling phenomenon in the liquid-to-solid transition of gallium, and the f-TED offers active temperature control to help accelerate the rigid–soft transformation of the overall system.

Understanding the Strength of the Selenium–Graphene Interfaces for Energy Storage Systems

We present comprehensive first-principles density functional theory (DFT) analyses of the interfacial strength and bonding mechanisms between crystalline and amorphous selenium (Se) with graphene (Gr), a promising duo for energy storage applications. Comparative interface analyses are presented on amorphous silicon (Si) with graphene and crystalline Se with a conventional aluminum (Al) current collector. The interface strengths of monoclinic Se (0.43 J m-2) and amorphous Si with graphene (0.41 J m-2) are similar in magnitude. While both materials (c-Se, a-Si) are bonded loosely by van der Waals (vdW) forces over graphene, interfacial electron exchange is higher for a-Si/graphene. This is further elaborated by comparing the potential energy step and charge transfer (Δq) across the graphene interfaces. The interface strength of c-Se on a 3D Al current collector is higher (0.99 J m-2), suggesting a stronger adhesion. Amorphous Se with graphene has comparable interface strength (0.34 J m-2), but electron exchange in this system is slightly distinct from monoclinic Se. The electronic characteristics and bonding mechanisms are different for monoclinic and amorphous Se with graphene as they activate graphene via surface charge doping divergently. The implications of these interfacial physicochemical attributes on electrode performance have been discussed. Our findings highlight the complex electrochemical phenomena in Se interfaced with graphene, which may profoundly differ from their "free" counterparts.

Design Strategy for Transformative Electronic System toward Rapid, Bidirectional Stiffness Tuning using Graphene and Flexible Thermoelectric Device Interfaces.

Electronics with tunable shape and stiffness can be applied in broad range of applications because their tunability allows their use in either rigid handheld form or soft wearable form, depending on needs. Previous research has enabled such reconfigurable electronics by integrating a thermally tunable gallium-based platform with flexible/stretchable electronics. However, supercooling phenomenon caused in the freezing process of gallium impedes reliable and rapid bidirectional rigid-soft conversion, limiting the full potential of this type of "transformative" electronics. Here, materials and electronics design strategies are reported to develop a transformative system with a gallium platform capable of fast reversible mechanical switching. In this electronic system, graphene is used as a catalyst to accelerate the heterogeneous nucleation of gallium to mitigate the degree of supercooling. Additionally, a flexible thermoelectric device is integrated as a means to provide active temperature control to further reduce the time for the solid-liquid transition of gallium. Analytical and experimental results establish the fundamentals for the design and optimized operation of transformative electronics for accelerated bidirectional transformation. Proof-of-concept demonstration of a reconfigurable system, which can convert between rigid handheld electronics and a flexible wearable biosensor, demonstrates the potential of this design approach for highly versatile electronics that can support multiple applications.

Strongly Absorbing Nanoscale Infrared Domains within Strained Bubbles at hBN-Graphene Interfaces.

Graphene has great potential for use in infrared (IR) nanodevices. At these length scales, nanoscale features, and their interaction with light, can be expected to play a significant role in device performance. Bubbles in van der Waals heterostructures are one such feature, which have recently attracted considerable attention, thanks to their ability to modify the optoelectronic properties of two-dimensional (2D) materials through strain. Here, we use scattering-type scanning near-field optical microscopy (sSNOM) to measure the nanoscale IR response from a network of variously shaped bubbles in hexagonal boron nitride (hBN)-encapsulated graphene. We show that within individual bubbles there are distinct domains with strongly enhanced IR absorption. The IR domain boundaries coincide with ridges in the bubbles, which leads us to attribute them to nanoscale strain domains. We further validate the strain distribution in the graphene by means of confocal Raman microscopy and vector decomposition analysis. This shows intricate and varied strain configurations, in which bubbles of different shape induce more bi- or uniaxial strain configurations. This reveals pathways toward future strain-based graphene IR devices.

Modulation of Peptide–Graphene Interfaces via Fatty Acid Conjugation

AbstractMaterials‐binding peptides have the capability to specifically recognize 2D nanomaterials and their modification via covalent attachment of nonnatural moieties offers exciting under‐explored possibilities to tune and exploit this recognition property. Here, an integrated suite of experimental approaches is used to reveal how conjugation of a fatty‐acid chain modifies surface adsorption of a graphene‐binding peptide, P1, achieved using quartz crystal microbalance measurements to access biomolecule adsorption free energies at the aqueous graphene interface, atomic force microscopy to investigate the overlayer morphologies, and circular dichroism spectroscopy to probe peptide secondary structures in the unadsorbed state. To complement these data, replica‐exchange with solute tempering molecular dynamics simulations predict the conformations of these biomolecules in both the surface‐bound and unbound states. Conjugation of the fatty acid dramatically modifies the surface‐adsorbed conformations. However, these conformational changes do not lead to substantial differences in the graphene binding strengths for each of the biomolecules studied, due to enthalpic/entropic compensation arising from changes to the contributions of peptide/graphene, peptide/fatty acid, and fatty acid/graphene interactions. These findings provide a fundamental basis for guiding future modification of materials‐binding peptides to adapt their binding propensities for applications using 2D nanomaterials.

Tuning Neuronal Circuit Formation in 3D Polymeric Scaffolds by Introducing Graphene at the Bio/Material Interface.

2D cultures are useful platforms allowing studies of the fundamental mechanisms governing neuron and synapse functions. Yet, such models are limited when exploring changes in network dynamics due to 3D-space topologies. 3D platforms fill this gap and favor investigating topologies closer to the real brain organization. Graphene, an atom-thick layer of carbon, possesses remarkable properties and since its discovery is considered a highly promising material in neuroscience developments. Here, elastomeric 3D platforms endowed with graphene cues are exploited to modulate neuronal circuits when interfaced to graphene in 3D topology. Ex vivo neuronal networks are successfully reconstructed within 3D scaffolds, with and without graphene, characterized by comparable size and morphology. By confocal microscopy and live imaging, the 3D architecture of synaptic networks is documented to sustain a high rate of bursting in 3D scaffolds, an activity further increased by graphene interfacing. Changes are reported in the excitation/inhibition ratio, potentially following 3D-graphene interfacing. A hypothesis is thus proposed, where the combination of synapse formation under 3D architecture and graphene interfaces affects the maturation of GABAergic inhibition. This will tune the balance between hyperpolarizing and depolarizing responses, potentially contributing to network synchronization in the absence of changes in GABAergic phenotype expression.

Water Structure, Dynamics, and Sum-Frequency Generation Spectra at Electrified Graphene Interfaces

The properties of water at an electrified graphene electrode are studied via classical molecular dynamics simulations with a constant potential approach. We show that the value of the applied electrode potential has dramatic effects on the structure and dynamics of interfacial water molecules. While a positive potential slows down the reorientational and translational dynamics of water, an increasing negative potential first accelerates the interfacial water dynamics before a deceleration at very large magnitude potential values. Further, our spectroscopic calculations indicate that the water rearrangements induced by electrified interfaces can be probed experimentally. In particular, the calculated water vibrational sum-frequency generation (SFG) spectra show that SFG specifically reports on the first two water layers at 0 V but that at larger magnitude applied potentials the resulting static field induces long-range contributions to the spectrum. Electrified graphene interfaces provide promising paradigm systems for comprehending both short- and long-range neighboring aqueous system impacts.

Bioelectrocatalysis on Anodized Epitaxial Graphene and Conventional Graphitic Interfaces

AbstractGraphitic materials exhibit significant anisotropy due to the difference in conductivity in a single layer and between adjacent layers. This anisotropy is manifested on epitaxial graphene (EG), which can be manipulated on the nanoscale in order to provide tailor‐made properties. Insertion of defects into the EG lattice was utilized here for controllable surface modification with a model biocatalyst and the properties were quantified by both electrochemical and optical methods. A comparative evaluation of the electrode reaction kinetics on the enzyme‐modified 2D material vs conventional carbon electrode materials revealed a significant enhancement of mediated bioelectrocatalysis at the nanoscale.

Substitutional Nitrogen Defects Enhance Thermal Transport across Graphene Interfaces

We present on the thermal and electrical transport properties in nitrogen-doped single layer graphene. After synthesis via atmospheric pressure chemical vapor deposition under various parameter conditions, we characterize the degree to which nitrogen defects are present in these systems. Transmission electron microscopy exemplifies the fact that that the structural integrity and homogeneity of the doped graphene lattice is maintained. Using Raman spectroscopy, we find that the intensity of the D peak is directly correlated to the nitrogen substitutional concentration in these systems, consistent with other works in the literature. Further, using Raman spectroscopic mapping, we find that the doping effect is homogenous, important for microelectronic applications. Finally, we characterize the thermal and electrical transport properties in these systems using time-domain thermoreflectance and transfer line method, respectively. In all, the culmination of these characterization techniques provides a basis for nitrogen-doped graphene use in nanoelectronic applications contingent on optimizing boundary conductances from a thermal and electrical perspective.

Atomistic molecular dynamics simulation of grapheneisoprene nanocomposites

A series of atomistic molecular dynamics simulations were performed for 4-mer isoprene molecules confined between graphene sheets with varied graphene separations in order to observe the finite size effects arose from the van der Waals interactions between the isoprene oligomer, forming high density shells at the graphene interfaces. For the small confinement regions, 1.24 nm and 2.21 nm, local density of isoprene at the graphene interface becomes higher than 10 times the bulk density. These results provided the further insights towards the rational design of nanostructures based on graphene sheets and natural rubber polymer.

Structural, Electronic, and Transport Properties of Hybrid SrTiO3-Graphene and Carbon Nanoribbon Interfaces

Hybrid materials composed of different functional structural units offer the possibility of tuning both the thermal and electronic properties of a material independently. Using quantum mechanical calculations, we investigate the change in the electronic and thermoelectric transport properties of graphene and hydrogen-terminated carbon nanoribbons (CNRs) when these are placed on the SrTiO3 (001) surface (STO). We predict that both p-type and n-type composite materials can be achieved by coupling graphene/CNR to different surface terminations of STO. We show that the electronic properties of graphene and CNR are significantly altered on SrO-terminated STO but are preserved upon interaction with TiO2-terminated STO and that CNRs possess distinct electronic states around the Fermi level because of their quasi-one-dimensional nature, leading to a calculated Seebeck coefficient much higher than that of a pristine graphene sheet. Moreover, our calculations reveal that in the TiO2-SrTiO3/CNR system there is a favorable electronic level alignment between the CNR and STO, where the highest occupied molecular orbital of the CNR is positioned in the middle of the STO band gap, resembling n-type doping of the substrate. Our results offer design principles for guiding the engineering of future hybrid thermoelectric materials and, more generally, nanoelectronic materials comprising oxide and graphitic components.

Effect of the Formation of Ambient Ice-like Water Ad- Layersat the Interface of Different Layer Graphene on SiO2/ Si and Si Substrates

Objectives: In this work, we probe the effect of ambient conditions on few layer and multi-layer graphene interfaces in Si and SiO2/Si substrates. Methods/Statistical Analysis: Scanning probe microscopy techniques have been employed to study the topography of the few layer graphene and multi-layer graphene interfaces by Intermittent-Contact Atomic Force Microscopy (IC-AFM) and the changes in the surface gradient of electrical forces have been probed by electrical force microscopy. A study of different thickness of mechanically exfoliated graphene layers, with and without folds, deposited on SiO2/Si and Si substrates is presented here. Findings: The number of ambient ice-like water ad-layers are determined by measurement of the heights of the graphene layers using IC-AFM, while as Electrical Force Microscopy (EFM) is used to determine the nature of the electrical interaction forces at the graphene interfaces on the different substrates. It is found that with increasing thickness of graphene on Si substrates, the electrostatic interaction increases on account of larger number of ambient ice-like water ad-layers formed. Application/Improvements: The electrical nature of Graphene interfaces on hydrophilic and hydrophobic substrates have an important role for the graphene-based devices operating under ambient.

Molecular dynamics simulations of pyrrolidinium and imidazolium ionic liquids at graphene interfaces.

Understanding the electrode-electrolyte interface is essential in the battery research as the ion transport and ion structures at the interface most likely affect the performance of a battery. Here we investigate interfacial structures of three ionic liquids: 1-ethyl-3-methylimidazolium dicyanamide ([C2mim][dca]), 1-butyl-3-methylimidazolium dicyanamide ([C4mim][dca]) and N-butyl-N-methylpyrrolidinium dicyanamide ([C4myr][dca]) at a charged and uncharged graphene interface using molecular dynamics simulations. We find that these ionic liquids (ILs) behave differently both in the bulk phase and near a graphene interface and we find that this difference is apparent in all types of analyses performed here. First, a partial density analysis in the direction perpendicular to the surface of the electrodes, which, in the cases near a negatively charged graphene, reveals that the pyrrolidinium system is generally more layered than the imidazolium systems. Second, a 2D topographic structure analysis of the IL species in the inner layer near a negatively charged graphene surface, which reveals that the pyrrolidinium system exhibits a quasi-hexagonal surface configuration of the cations, while the imidazolium systems show linearly arranged groups of cations. Third, a 3D orientation-preference analysis of cation rings near the negative graphene electrode, which shows that the pyrrolidinium rings prefer to lie parallel to the electrode surface while the imidazolium rings prefer to stand on the electrode surface at high tilt angles. Extending the imidazolium alkyl chain was found to reduce the number of imidazoliums that can link up into linearly arranged groups in the inner layer 2D structures. Our results support earlier experimental findings and indicate that the interfacial nanostructures may have a significant influence on the electrochemical performance of IL-based batteries.