Electronic Properties Of Graphene Research Articles

(Invited) Plasma Designing of Hybrid 2D Graphene: A Green Approach for Future Energy Materials

The global research interest in two-dimensional (2D) materials and their derivatives has increased significantly, driven by the requirements of application-oriented materials for different sectors, including energy storage. Among the various classes of material groups, 2D layered materials have emerged as a critical research area due to their high aspect ratio, open edge boundaries, high surface-to-volume ratios, and mechanical and electrical properties. Graphene is a widely researched 2D class material with an atomically thin structure, excellent electronic properties, very high intrinsic mobility and exceptional thermal and electrical conductivity1. Even though the research on the electronic properties of graphene is explored very well and in a matured state, the studies on non-electronic properties are still in the budding phase. One of the significant obstacles to extending the studies of graphene to different fields is the cost-effective and environmentally friendly large-scale synthesis and processing of high-quality graphene. Thus, a cutting-edge approach for probing graphene structures at the atomic scale is required to produce high-quality graphene for next-generation applications. As an alternative progressive technique, plasma-enabled methods have emerged as safe and green approaches to tailor different graphene structures at the nanoscale. During graphene production, plasma assembles the nanostructures from gaseous into a solid form or converts solid precursor to graphitic form 2,3. Besides, plasma techniques open up the possibility of tailoring the properties by crafting heteroatoms to the graphene lattice, which is beneficial for energy storage applications4. Therefore, this paper introduces the advantages of plasma-enabled techniques in designing graphene-based materials with different morphologies and orientations and tailoring them at the nanoscale using heteroatoms. Such plasma-assisted techniques ensure the high structural quality of graphene and controllability in the hybrid morphologies, making them frontrunner energy storage applications. Plasma also offers the production of graphene directly on any substrate surface, gives a new direction to designing binder-free electrodes for energy-related applications, and could be used as a competitive alternative approach to the widely known wet chemical procedures to design advanced graphene electrode materials for next-generation energy storage devices.Reference Geim, A. K. Graphene: Status and prospects. Science (1979) 324, 1530–1534 (2009).Dias, A. et al. N-Graphene-Metal-Oxide(Sulfide) hybrid Nanostructures: Single-step plasma-enabled approach for energy storage applications. Chemical Engineering Journal 430, 133153 (2022).Jagodar, A. et al. Growth of graphene nanowalls in low-temperature plasma: Experimental insight in initial growth and importance of wall conditioning. Appl Surf Sci 643, 158716 (2024).Santhosh, N.M. et al. N-Graphene Nanowalls via Plasma Nitrogen Incorporation and Substitution: The Experimental Evidence. Nanomicro Lett 12, 53 (2020).

Effects of the in-plane uniaxial and biaxial strains on the electronic and optical properties of the graphene/β-Si3N4 heterostructure

van der Waals heterostructures can enable adjustment of graphene's electronic properties, which is important for the application of graphene-based electronic devices. The structural, electronic and optical properties of heterostructures composed of graphene and β-Si3N4 under different strain conditions are studied in this paper using first-principle calculation. The result shows that the heterojunction constituted with β-Si3N4 opens a small bandgap at the Brillouin zone Γ, which is about 0.099 eV, and the bandgap is highly sensitive to the direction and magnitude of strain. Under uniaxial compressive (tensile) strain, the heterojunction bandgap increases linearly, reaching 1.901 eV and 1.614 eV at −11 % and 11 %, respectively; under biaxial strain, the heterojunction bandgap decreases linearly with the compressive strain, decreasing to 0.087 eV at −11 %, and increases linearly with the tensile strain, reaching 0.113 eV at 11 %. And the rate of the bandgap under uniaxial strain is larger than that under biaxial strain, which suggests that the uniaxial strain regulation is more effective. Optical property study shows that uniaxial (biaxial) compressive strain improves the light absorption of the heterojunction in the blue-ultraviolet region band, especially when the uniaxial compressive strain reaches 9 %, the light absorption of the heterojunction increases significantly in the whole spectral interval. These research results will provide a theoretical basis for the practical applications of graphene and β-Si3N4 heterostructures in the fields of pressure sensors and optical modulators.

Atomic-scale characterization of defects in oxygen plasma-treated graphene by scanning tunneling microscopy

Defects in graphene are important nanoscale pathways for metal atoms to enter the interface between epitaxial graphene and SiC in order to form stable ultrathin metal layers with new exotic physical properties. However, the atomic-scale details of defects that mainly govern the intercalation process remain modest. In this work, we present the first atomic investigation of point defects generated by oxygen plasma treatment on epitaxial graphene grown on SiC using low-temperature scanning tunneling microscopy, corroborated by density functional theory calculations. We found a broad spectrum of point defects that varies in size, shape, and symmetry and is dominated by triangular species. Tunneling spectroscopy identified defect-induced states in the vicinity of the Fermi level that significantly perturb the graphene electronic properties at the defect site. Based on the well-defined defect symmetry, we simulated the local density of states of the triangular defects and their corresponding scanning tunneling microscopy images which further helped us to identify the exact atomic configurations of monovacancy defects. The combination of atomic-scale scanning tunneling microscopy experiments and reliable density functional theory simulations provides ultimate microscopic details and opens a new way to identify the atomic configurations of defects in oxygen plasma-treated graphene. Our work might shed light on precise control of defect engineering in graphene for metal intercalation by controlling the defect types based on a deep understanding of each configuration.

Chlorinated graphene and graphene nanoribbons: A density functional theory study

Functional groups based on halides, such as fluorine (F), chlorine (Cl), bromine (Br), and iodine (I), are crucial for understanding the chemical reactivity of graphitic nanomaterials. Except for I, halogens exhibit electronegativity greater than carbon (C); therefore, charge transfer from carbon to halogen is expected. First-principles density functional theory calculations were performed to determine the role of different Cl-functional groups (methyl-trichloride, ethyl-trichloride, chloride, acyl-chloride, vinyl-chloride, acetyl hypochlorite, chloramines, sulfonyl chloride, and more) on the electronic properties of graphene and graphene nanoribbons (GNRs). GNRs with zigzag edges (ZGNRs) and armchair edges (AGNRs) were studied. We analyzed the optimized structures, band structure, density of states, cohesive energy, and band gap. Our results revealed that the based-Cl functional groups can provide an alternative route to activate the borders and surfaces of sp2 carbon materials. Methyl-trichloride and acyl-chloride can induce magnetism and metallicity. Chloride and acyl-chloride are the most energetically stable functional groups attached to the edges. Surprisingly, methyl-trichloride or acyl-chloride functionalizing the surface of the AGNRs showed a direct (indirect) band gap for states with spin-up (spin-down). The results of aromatic (chlorobenzene- and dichlorobenzene-like structures) functionalization considering F, Cl, Br, and I are also shown. Finally, –F2 and –ClF functionalization cases are discussed.

Measuring the Capacitance of Carbon in Ionic Liquids: From Graphite to Graphene.

The physical electrochemistry of the carbon/ionic liquids interface underpins the processes occurring in a vast range of applications spanning electrochemical energy storage, iontronic devices, and lubrication. Elucidating the charge storage mechanisms at the carbon/electrolyte interface will lead to a better understanding of the operational principles of such systems. Herein, we probe the charge stored at the electrochemical double layer formed between model carbon systems, ranging from single-layer graphene to graphite and the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI). The effect of the number of graphene layers on the overall capacitance of the interface is investigated. We demonstrate that in pure EMIM-TFSI and at moderate potential biases, the electronic properties of graphene and graphite govern the overall capacitance of the interface, while the electrolyte contribution to the latter is less significant. In mixtures of EMIM-TFSI with solvents of varying relative permittivity, the complex interplay between electrolyte ions and solvent molecules is shown to influence the charge stored at the interface, which under certain conditions overcomes the effects of relative permittivity. This work provides additional experimental insights into the continuously advancing topic of electrochemical double-layer structure at the interface between room temperature ionic liquids and carbon materials.

Graphene Fermi Level-Guided Attachment of Single Exoelectrogens and Induced Interfacial Doping.

Graphene's exceptional electronic and mechanical properties make it a promising material for bioelectronic applications; however, understanding its interaction with electrogenic bacteria is crucial to harness its full potential. This study investigates the interface between electrogenic bacteria and graphene with Raman spectroscopy by analyzing the distinctive spectral fingerprints to understand electron energy and distribution via this non-destructive and label-free method. We find that the presence of bacteria induces a distinct red-shift in the G peak positions of graphene, indicating electron doping. Correspondingly, the bacteria demonstrate a predilection for attachment on hole-rich sites on the graphene sheet, evidenced by the comparative analysis of pre- and post-spatial Raman mapping, revealing their consistent presence within the hole-doped 2D peak position range of 2673.89-2675.43 cm-1. This affinity of bacteria is due to the overall higher Fermi level (∼4.9 ± 0.2 eV) of these regions, which favors electron transfer. These findings demonstrate the potential of leveraging the graphene's electronic properties in engineering graphene-based biosensors. Tuning graphene's charge carrier concentration would enable the promotion or prevention of bacterial attachment, facilitating capture of specific bacteria or development of antimicrobial surfaces. This approach enables clean, efficient, and accurate study of graphene-based bacterial systems, driving significant advancements and enhancing their performance.

Visible-frequency plasmonic enhancement at the edge of graphene/h-BN heterostructures on silicon substrate

Heterostructures of graphene and hexagonal boron nitride (G/h-BN) have been widely studied for controlling and utilizing graphene electronic properties. Here we characterize specific optical and electronic properties of G/h-BN heterostructures made of a high-quality single layer chemical vapor deposition (CVD) graphene laid over h-BN flakes, with focus on plasmonic effects. We compare the G/h-BN properties on Si and SiO2 substrates by micro-Raman spectroscopy mapping, Kelvin probe force microscopy, optical and atomic force microscopy. We observe highly enhanced Raman intensity (up to 280 %) from Si as well as graphene along the G/h-BN edge. It is attributed to localized concentration of electrons in graphene and suitable perpendicular orientation of plasmonic vibrations at the edge. The plasmonic Raman enhancement occurs under a visible light excitation (532 nm) and the effect can be tuned by the h-BN flake thickness (10–150 nm). The enhancement is specific to G/h-BN/Si structures, on G/h-BN/SiO2 structures the Raman signal is suppressed while I2D/IG ratio is increased. Vice versa, change of surface potential under visible light illumination (photovoltage) is on G/h-BN/Si negligible (within 10 mV) compared to the G/h-BN/SiO2 structures. These results open new prospects for broad utilization of localized visible plasmonic effects in graphene.

Atomistic insights into the nucleation and growth of hexagonal boron nitride and graphene heterostructures.

Graphene and hexagonal boron nitride (hBN) are two-dimensional (2D) materials with a similar atomic structure but drastically different although complementary electronic properties. The large-scale synthesis of h-BN/graphene heterostructures with high crystallographic quality is required to fully benefit of the graphene electronic properties. In this study, we examine numerically the interaction of graphene precursors on hBN and of hBN precursors on graphene to gain deep insight of the CVD and MBE growth mechanism of graphene/hBN heterostructures. Density functional theory (DFT) calculations reveal the adsorption and diffusion behaviors for B, N, and C atoms on these surfaces. In particular, the adsorption energy is found to be similar to the diffusion barriers, except for the nearly free diffusion of B atoms on both graphene and hBN. We have also investigated the transition from individual atoms to graphene or h-BN seeds by considering the stability of linear chains as well as branched and ring seeds. Furthermore, for larger clusters, the triangular h-BN domains are found to be equally thermodynamically stable on graphene regardless of their orientation. These findings provide preliminary hints for the ability of graphene to grow on hBN layers and hBN layer on graphene.

Catalytic Activity of Horseradish Peroxidase Immobilized on Pristine and Two‐Photon Oxidized Graphene

AbstractBiosensors based on graphene and bio‐graphene interfaces have gained momentum in recent years due to graphene's outstanding electronic and mechanical properties. By introducing the patterning of a single‐layer graphene surface by two‐photon oxidation (2PO), the surface hydrophobicity/hydrophilicity and doping can be varied at the nanoscale while preserving the carbon network, thus opening possibilities to design new devices. In this study, the effect of 2PO on the catalytic activity of the noncovalently immobilized enzyme horseradish peroxidase (HRP) on single‐layer graphene‐coated Si/SiO2 chips is presented. To monitor the activity continuously, a simple well‐plate setup is introduced. Upon controllable 1–2‐layer immobilization, the catalytic activity decreases to a maximum value of 7.5% of the free enzyme. Interestingly, the activity decreases with increasing 2PO area on the samples. Hence, the HRP catalytic activity on the graphene surface is locally controlled. This approach can enable the development of graphene‐bio interfaces with locally varying enzyme activity.

Dispersion consisting entirely of trilayer graphene provides film of high specific capacitance

The excellent electrical conductivity and tunable bandgap make trilayer graphene (TG) an attractive material for energy storage and electronic applications. The variation in electronic properties of graphene with number of layers present, severely limits the utility of scalable and cost-effective electrochemical exfoliation of graphite for TG synthesis, as the produced graphene is layer-polydispersed. The limitation is further accentuated by the oxygen functionalization, high defect density, and smaller sizes of the produced graphene. Here we produce a dispersion consisting entirely of TG through tailored electrochemical exfoliation of graphite to single-stage III graphite bisulfate (GB) particles, enabling subsequent selective cleaving to TG in the dispersion-favorable solvent DMF. A detailed morphological, structural, and chemical analysis revealed large size of flakes (average size: ∼ 96 μm2) with low defect density (ID∕IG ratio < 0.1) and high C/O ratio (26.78), along with excellent optoelectronic properties (transmittance: ∼ 92.2 % and conductivity: ∼ 2.17 × 106 S m−1) comparable to CVD grown TG. The specific capacitance of TG film is ∼ 259 F g−1 indicating its suitability for energy storage applications.

Modification on Flower Defects and Electronic Properties of Epitaxial Graphene by Erbium.

Manipulating the topological defects and electronic properties of graphene has been a subject of great interest. In this work, we have investigated the influence of Er predeposition on flower defects and electronic band structures of epitaxial graphene on SiC. It is shown that Er atoms grown on the SiC substrate actually work as an activator to induce flower defect formation with a density of 1.52 × 1012 cm-2 during the graphitization process when the Er coverage is 1.6 ML, about 5 times as much as that of pristine graphene. First-principles calculations demonstrate that Er greatly decreases the formation energy of the flower defect. We have discussed Er promoting effects on flower defect formation as well as its formation mechanism. Scanning tunneling microscopy (STM) and Raman and X-ray photoelectron spectroscopy (XPS) have been utilized to reveal the Er doping effect and its modification to electronic structures of graphene. N-doping enhancement and band gap opening can be observed by using angle-resolved photoemission spectroscopy (ARPES). With Er coverage increasing from 0 to 1.6 ML, the Dirac point energy decreases from -0.34 to -0.37 eV and the band gap gradually increases from 320 to 360 meV. The opening of the band gap is attributed to the synergistic effect of substitution doping of Er atoms and high-density flower defects.

The synergistic regulation effect on the structure and electronic properties of graphene by methane plasma, Stone-Wales defect and equibiaxial strain

The synergistic regulation effects on the structure and electronic properties of graphene by methane plasma, Stone-Wales defect and equibiaxial strain has been investigated. It is found that the graphene containing Stone-Wales defect (SWG77) has metallicity, when surface adsorbing a CH3+ion, CH3+@SWG77 is a semiconductor with 0.339 eV indirect band gap. The stress-strain curve results indicate that adsorbing different plasma fragments in methane may influence to varying degrees on the ultimate strain of the SWG77 system. The hole mobility has an enhanced tendency as the tensile strain increases for the CH3+@SWG77 system. In addition, the hole mobilities seem to reach saturation when applied equibiaxial strain increase to 2 % or larger. Applying equibiaxial strain on the system substrate, the band gaps of CH3+@ SWG77 and 2CH3+@ SWG77 systems are almost unaffected. In addition, the concentration of CH3+ ion has a certain effect on the light absorption coefficient, reflectivity, and energy loss of the SWG77 system in the visible light region (1.62 eV–3.11 eV) and ultraviolet regions (3.11 eV–5.50 eV). Therefore, defect graphene with appropriate carrier mobility and band gap may be designed by adjusting the equiaxial strain and methyl concentration, which provides useful theoretical support for the development of optical electrical multifunctional electronic devices based on graphene, and is conducive to promoting the application of graphene materials in the field of band gap engineering.

Graphene on SiO2/Si and Al2O3 under thermal annealing and electric current: Competition of dopant desorption and conformation to substrate

In this work, we study phonon and electronic properties of graphene on SiO2/Si and Al2O3 by simultaneous Raman and electrical measurements in the temperature range from room temperature to 550 °C, or at voltages from 20 to −20 V. The dependencies of G and 2D peak parameters and electrical resistance on temperature and voltage made it possible to observe in situ a competition between the p-type adsorbate removal from graphene surface and substrate-induced doping due to graphene-substrate conformality increase, both stimulated by either ambient or Joule heating. The analyzed parameters were dominated by the conformality increase, with the hole density increasing significantly and unidirectionally, while resistance and I-V curves fluctuated due to the competition. Having calculated Raman peak shift temperature coefficients, resistance temperature coefficients, total variations of carrier density, resistance and strain, we show that Al2O3 substrate can be used to reduce the desorption barrier, the overall doping, the impact on graphene resistance and on phonon anharmonicity – however, it should be used with regard to the possibility of introducing strain or stronger doping after longer treatments. The conformality effects should be taken into account when performing annealing, as well as graphene applications for sensors or strong electric currents.

A first-principles study of structural, electronic and transport properties of aluminium and phosphorus-doped graphene

First-principles simulations are utilised for computing the structural, electronic, and transport characteristics of pure graphene and graphene doped with aluminium and phosphorus. The results demonstrate that by substituting Al and P atoms for carbon atoms in graphene, the material's band gap may be effectively opened. When Al or P atom replaces a carbon atom, a notable band gap value of 1.49 eV or 0.40 eV has been detected. Additionally, the thermal conductivity, Seebeck coefficient, carrier concentration, power factor, electrical conductivity, and dimensionless figure of merit of pure graphene, aluminium and phosphorus-doped graphene are examined. Our study presents the theoretical basis for using a doping mechanism with heteroatoms to improve graphene's electronic and transport properties. Al/P doped graphene has the potential to significantly improve thermoelectric efficiency due to its electronic properties and low thermal conductivity.

Functionalization of graphene for precise control of the structure and electronic properties of carbon materials

The structure and electronic properties of graphene are fundamental to those of carbon materials and are significantly influenced by the presence of nonideal structures such as defects, impurities, and edges, which can be controlled through the functionalization of graphene by chemical species. According to the energy scale of the interactions between graphene and these species, three routes for the functionalization of graphene were examined: the proximity effects of adsorbed molecules, ion irradiation, and covalent bonding. The sign and concentration of the charge carrier, and its scattering in graphene were precisely controlled by the adsorption of molecules on it or the chemical modification of the surface of the supporting substrate. Irradiation with an ion beam modified the electronic structure of graphene and its electrical conductivity in a well-defined manner. Direct chemical modification induced the formation of covalent bonds with guest species and significantly changed the topology of the honeycomb lattice of graphene, resulting in localized spin magnetism and catalytic activity originating from spin-polarized localized states emerging at the Fermi energy.

Compressive strain engineering of strong and sensitive pseudomagnetic fields in buckled graphene nanobubbles

The formation of pseudomagnetic fields is a well-known consequence due to lattice strain, which has considerable effects on the electronic properties of graphene. To this end, strain engineering remains an effective route to enable unconventional electronic properties. Particularly, pseudomagnetic fields due to compressive strain have demonstrated unique advantages such as remarkably high intensities at relatively low magnitude of strains (e.g., 5%) in several experiments. For example, pseudomagnetic fields as high as 300 T have been observed in buckled graphene nanobubbles. Recently, strong pseudomagnetic fields as high as 108 T have been measured in periodically buckled monolayer graphene, responsible for its band flattening and correlated states. Nevertheless, the general features and sensitivities of compressive strain-induced pseudomagnetic fields have been rarely explored, posing challenges for realizing their full potential. In this paper, we carry out large-scale molecular dynamics simulations to explore the properties of pseudomagnetic fields in buckled graphene nanobubbles, which are the basic representative graphene structures under compressive strains. We reveal that compressive strain can induce strong and sensitive pseudomagnetic fields for a variety of nanobubble shapes and boundary relaxation conditions. We also discuss the effect of the microscopy probe tip, which is frequently used to tune the morphologies and strain patterns. These results may offer guidance for designing electronic two-dimensional structures enabled by compressive strain engineering.

Probing the graphene/substrate interaction by electron tunneling decay

The electronic properties of graphene can be modified by the local interaction with a selected metal substrate. To probe this effect, Scanning Tunneling Microscopy is widely employed, particularly by means of local measurement via lock-in amplifier of the differential conductance and of the field emission resonance. In this article we propose an alternative, reliable method of probing the graphene/substrate interaction that is readily available to any STM apparatus. By testing the tunneling current as function of the tip/sample distance on nanostructured graphene on Ni(100) and Ir (100), we demonstrate that I(z) spectroscopy can be quantitatively compared with Density Functional Theory calculations and can be used to assess the nature of the interaction between graphene and substrate. This method can expand the capabilities of standard STM systems to study graphene/substrate complexes, complementing standard topographic probing with spectroscopic information.

Graphene-based materials and their applications in electrolyte-gated transistors for sensing

Graphene is a two-dimensional material with remarkable physicochemical characteristics. In particular, graphene is highly sensitive to its electronic and electrostatic environment. For these reasons, among various applications of graphene, its use as a sensitive material in field-effect transistors (FETs) is widely studied, especially for biodetection purposes. So, this review aims to introduce the reader to the state of the art of graphene-based materials for their applications in electrolyte-gated graphene field-effect transistors (EGGFETs) for biosensing applications. Among numerous ways to produce graphene-based materials for such devices, some of the most important, widely used methods are presented herein. After that, the working principle of graphene-based FETs is discussed and the various ways to tune graphene's electronic properties are presented. The fabrication methods are discussed, with an emphasis on inkjet printing which allows printing of EGGFETs from aqueous graphene oxide inks, providing that it is subsequently reduced to adjust the mobility of charge carriers and the doping state. Finally, some of the recent works concerning the use of FETs in biosensing applications are reviewed.

Observation of Robust and Long-Ranged Superperiodicity of Electronic Density Induced by Intervalley Scattering in Graphene/Transition Metal Dichalcogenide Heterostructures.

Two-dimensional (2D) h-BN and transition metal dichalcogenides (TMDs) are widely used as substrates of graphene because they are insulating, atomically flat, and without dangling bonds. Usually, it is believed that such insulating substrates will not affect the electronic properties of graphene, especially when the moiré pattern generated between them is quite small. Here, we present a systematic study of the electronic properties of graphene/TMD heterostructures with the period of the moiré pattern <1 nm, and our results reveal an unexpected sensitivity of electronic properties in graphene to the 2D insulating substrates. We demonstrate that there is a robust and long-ranged superperiodicity of electronic density in graphene, which arises from the scattering of electrons between the two valleys of graphene in the graphene/TMD heterostructures. By using scanning tunneling microscope and spectroscopy, three distinct atomic-scale patterns of the electronic density are directly imaged in every graphene/TMD heterostructure.

Ab Initio Dynamics of Graphene and Graphyne Electrodes in Vacuum and in the Presence of Electrolytes

Graphene and graphyne electrodes are both relevant for electrochemical energy storage applications due to their unique physical, chemical, and electronic properties. Graphyne has been considered even more attractive than graphene due to its larger specific surface area, greater electronic mobility, and intrinsic band gap. The analyses presented here reveal relevant features of graphyne in an interaction with an electrolyte that can be useful for practical applications. Although there is already a considerable volume of work on graphyne, the analysis of its properties taking into account the dynamics of its network and the interaction with the condensed medium has not yet been presented. In this work, ab initio molecular dynamics simulations were performed to study the energetic, structural, spectroscopic, and electronic properties of graphene and graphyne electrodes in vacuum and in the presence of an electrolyte (EMIM-BF4). A detailed analysis of the electrode–electrolyte interaction is reported for both systems. Structural features are presented in terms of both radial and spatial distribution functions and angular orientation distributions. Power spectral analyses and molecular orbital maps provided important insights into the intermolecular interactions in graphene and graphyne systems, describing the main differences as well as revealing similarities between them.