Nitrogen doped epitaxial graphene on 4H-SiC(0001) – Experimental and theoretical study

The experimental and theoretical investigations of morphological and electronic properties of nitrogen-doped epitaxial graphene grown by chemical vapor deposition on 4H-SiC(0001) are discussed. It is shown that presence of nitrogen significantly affects the graphene growth process and leads to an increase in the concentration of defects (in the form of holes). Macro- and nanoscale investigations confirm the formation of pyridinic-N, pyrrolic-N and graphitic-N configurations within graphene layers. The relative concentrations of these configurations change as a function of global nitrogen concentration. Additionally, it is reported that the incorporated nitrogen results in inhomogeneous doping and a few nanometers wide spatial modification of the local density of states. Finally, the SiC substrate is also modified during the nitrogen doping process. To quantify the impact of the substrate modification on electronic structure of graphene the non-intercalated and hydrogen-intercalated doped graphene layers are compared. The presented complementary study sheds light on properties of N-doped graphene and its dependence on nitrogen concentration.     

Correlation between atomistic morphology and electron transport properties in defect-free and defected graphene nanoribbons: An interpretation through Clar sextet theory

Structural, electronic and transport properties of defect-free, defected and functionalized armchair and zig-zag graphene nanoribbons (GNRs) are investigated with density functional theory and non-equilibrium Green’s function calculations and rationalized in terms of Clar’s theory of the aromatic sextet. Calculations suggest a tight relationship between the transport properties of nanoribbons and the underlying bond patterning as described by valence bond and Clar sextet theory. Namely, armchair GNRs exhibit a strong dependence of the transport properties on the ribbon width, as a consequence of different valence bond representations. The occurrence of localized defects involving electron pairs does not significantly alter this behavior. Conversely, transport properties of zigzag GNRs are less affected by morphological details, such as width and occurrence of defects, as expected from the application of Clar’s theory. However, controlled edge functionalization and morphology modifications in zigzag GNRs can potentially lead to localization of aromatic sextets and, consequently, to strong changes in the transport properties. Our work indicates Clar sextet theory as a powerful and accurate tool to rationalize and predict the electronic and transport properties of complex carbon nanostructures based on GNRs. These principles can be extended to the design of novel systems with potential applications in nanoelectronics.     

Tailoring the performance of graphene-based supercapacitors using topological defects: A theoretical assessment

Graphene-based materials have been proposed as promising electrodes for electric double layer capacitors. Recently, it has been found that one of the limitations of graphene electrodes is the finite quantum capacitance at low applied voltage. In this work, we investigate the impact of having point-like topological defects in graphene on the electronic structure and quantum capacitance. Our results clearly show that the presence of defects, such as Stone Wales, di-vacancies, and di-interstitials, can substantially enhance the quantum capacitance when compared to pristine graphene, which is found to be due to defect-induced quasi-localized states near the Fermi level. In addition, the charging behavior tends to be asymmetric around the neutrality point. We also discuss the possibility of tuning the electronic structure and capacitance through mixtures of these defects. Our findings suggest that graphene-based electrodes with topological defects may demonstrate noteworthy capacitance but should be carefully selected for use as either the positive or negative electrode.     

Lateral homogeneity of the electronic properties in pristine and ion-irradiated graphene probed by scanning capacitance spectroscopy

In this article, a scanning probe method based on nanoscale capacitance measurements was used to investigate the lateral homogeneity of the electron mean free path both in pristine and ion-irradiated graphene. The local variations in the electronic transport properties were explained taking into account the scattering of electrons by charged impurities and point defects (vacancies). Electron mean free path is mainly limited by charged impurities in unirradiated graphene, whereas an important role is played by lattice vacancies after irradiation. The local density of the charged impurities and vacancies were determined for different irradiated ion fluences.     

Large area buffer-free graphene on non-polar (0 0 1) cubic silicon carbide

Graphene is, due to its extraordinary properties, a promising material for future electronic applications. A common process for the production of large area epitaxial graphene is a high temperature annealing process of atomically flat surfaces from hexagonal silicon carbide. This procedure is very promising but has the drawback of the formation of a buffer layer consisting of a graphene-like sheet, which is covalently bound to the substrate. This buffer layer degenerates the properties of the graphene above and needs to be avoided.We are presenting the combination of a high temperature process for the graphene production with a newly developed substrate of (0 0 1)-oriented cubic silicon carbide. This combination is a promising candidate to be able to supply large area homogenous epitaxial graphene on silicon carbide without a buffer layer.We are presenting the new substrate and first samples of epitaxial graphene on them. Results are shown using low energy electron microscopy and diffraction, photoelectron angular distribution and X-ray photoemission spectroscopy. All these measurements indicate the successful growth of a buffer free few layer graphene on a cubic silicon carbide surface. On our large area samples also the epitaxial relationship between the cubic substrate and the hexagonal graphene could be clarified.     

Superfast Self‐Healing and Photothermal Active Hydrogel with Nondefective Graphene as Effective Additive

2D graphene with high quality holds great promise in improving the performance of the hydrogels owing to its exceptional electronic, thermal, and mechanical properties. However, the structure defects existed in graphene restrict its further applications. Herein, a simple and green method of fabricating defect‐free graphene nanosheets with the assistance of supercritical carbon dioxide (SC CO₂) is designed. The graphene nanosheets directly assemble with acrylic acid monomer and clay, and a flexible semitransparent hydrogel is fabricated. Benefiting from the excellent properties of the defect‐free graphene, the hydrogel exhibits the high mechanical performance, superfast self‐healing capability, excellent conductivity, and super photothermal conversion efficiency. According to the advantages above, the graphene/poly(acrylic acid)/clay hydrogels can be used for intelligent sensors for disease diagnosis, artificial electronic skin, and military stealth materials in the near future.     

Tunable bands in biased multilayer epitaxial graphene

We have studied the electronic characteristics of multilayer epitaxial graphene under a perpendicularly applied electric bias. Ultraviolet photoemission spectroscopy measurements reveal that there is notable variation of the electronic density-of-states in valence bands near the Fermi level. Evolution of the electronic structure of graphite and rotational-stacked multilayer epitaxial graphene as a function of the applied electric bias is investigated using first-principles density-functional theory including interlayer van der Waals interactions. The experimental and theoretical results demonstrate that the tailoring of electronic band structure correlates with the interlayer coupling tuned by the applied bias. The implications of controllable electronic structure of rotationally fault-stacked epitaxial graphene grown on the C-face of SiC for future device applications are discussed.     

Redox-switchable devices based on functionalized graphene nanoribbons

The possibility of tuning the electronic properties of graphene by tailoring the morphology at the nanoscale or by chemical functionalization opens interesting perspectives towards the realization of devices for nanoelectronics. Indeed, the integration of the intrinsic high carrier mobilities of graphene with functionalities that are able to react to external stimuli allows in principle the realization of highly efficient nanostructured switches. In this paper, we report a novel approach to the design of reversible switches based on functionalized graphene nanoribbons, operating upon application of an external redox potential, which exhibit unprecedented ON/OFF ratios. The properties of the proposed systems are investigated by electronic structure and transport calculations based on density functional theory and rationalized in terms of valence-bond theory and Clar's sextet theory.     

Formation of high-quality quasi-free-standing bilayer graphene on SiC(0 0 0 1) by oxygen intercalation upon annealing in air

We report on the conversion of epitaxial monolayer graphene on SiC(0 0 0 1) into decoupled bilayer graphene by performing an annealing step in air. We prove by Raman scattering and photoemission experiments that it has structural and electronic properties that characterize its quasi-free-standing nature. The (6√3 × 6√3)R30° buffer layer underneath the monolayer graphene loses its covalent bonding to the substrate and is converted into a graphene layer due to the oxidation of the SiC surface. The oxygen reacts with the SiC surface without inducing defects in the topmost carbon layers. The high-quality bilayer graphene obtained after air annealing is p-doped and homogeneous over a large area.     

Strong interaction between graphene edge and metal revealed by scanning tunneling microscopy

The interaction between a graphene edge and the underlying metal is investigated through the use of scanning tunneling microscopy (STM) and density functional theory (DFT) calculations and found to influence the geometrical structure of the graphene edge and its electronic properties. STM study reveals that graphene nanoislands grow on a Pt(1 1 1) surface with the considerable bending of the graphene at the edge arising from the strong graphene-edge–Pt-substrate interactions. Periodic ripples along the graphene edge due to both the strong interaction and the lattice mismatch with the underlying metal were seen. DFT calculations confirm such significant bending and also reproduce the periodic ripples along the graphene edge. The highly distorted edge geometry causes strain-induced pseudo-magnetic fields, which are manifested as Landau levels in the scanning tunneling spectroscopy. The electronic properties of the graphene edge are thus concluded to be strongly influenced by the curvature rather than the localized states along the zigzag edge as was previously predicted.     

Supercapacitors based on high-quality graphene scrolls

High-quality graphene scrolls (GSS) with a unique scrolled topography are designed using a microexplosion method. Their capacitance properties are investigated by cyclic voltammetry, galvanostatic charge-discharge and electrical impedance spectroscopy. Compared with the specific capacity of 110 F g(-1) for graphene sheets, a remarkable capacity of 162.2 F g(-1) is obtained at the current density of 1.0 A g(-1) in 6 M KOH aqueous solution owing to the unique scrolled structure of GSS. The capacity value is increased by about 50% only because of the topological change of graphene sheets. Meanwhile, GSS exhibit excellent long-term cycling stability along with 96.8% retained after 1000 cycles at 1.0 A g(-1). These encouraging results indicate that GSS based on the topological structure of graphene sheets are a kind of promising material for supercapacitors.     

Electrostatic transparency of graphene oxide sheets

The interaction of graphene oxide of varying reduction degrees with dielectric and metallic surfaces is probed in this study, in order to assess the influence that the supporting substrate has on the electronic properties of as-produced graphene oxide and its reduced form. Lateral inhomogeneities in the distribution of substrate trapped charged impurities are found to affect the electronic properties of reduced graphene oxide, giving rise to significant in-plane variations of the local electrostatic potential on reduced one-layer sheets supported on dielectric substrates. On the contrary, no such surface potential fluctuations are identified on as-produced graphene oxide sheets, or on graphene oxide layers deposited on a metallic substrate. Thicker, two-layer reduced graphene oxide sheets show effective screening of the electrostatic effects caused by charge impurities trapped in the substrate. The current study provides a useful account of the limitations that device performance could face when attempting to tune the electronic structure of graphene oxide via functionalization, highlighting the role of substrate-related disorder affecting the behaviour of nanodevices. The role of the substrate is particularly important for applications where electronic properties of graphene oxide are especially targeted, such as transparent conducting films, sensors and electrochemical devices.     

Anisotropic behavior of hydrogen in the formation of pentagonal graphene domains

Variously shaped graphene domains are of significant interest since the electronic properties of pristine graphene are strongly dependent on its size, shape, and edge structures. With the consideration that the reactivity of graphene is governable by the p-electron structure at its edge, a number of attempts have been made to grow variously shaped graphene domains and to define their edge structures. In this work, we explored the anisotropic behavior of hydrogen in the formation of graphene domains during atmospheric pressure chemical vapor deposition. As increasing the hydrogen or reducing the methane partial pressure, the formation of pentagonal graphene domains was accelerated through anisotropic growth and etching. Their edge structures were characterized using polarization-dependent D and G peak Raman spectroscopy. This work contributes significantly to improving graphene-based engineering by allowing graphene shapes and domain edges to be tuned, and also provides greater insight into the electronic properties of graphene devices.     

Buffer layer free graphene on SiC(0 0 0 1) via interface oxidation in water vapor

Intercalation of various elements has become a popular technique to decouple the buffer layer of epitaxial graphene on SiC(0 0 0 1) from the substrate. Among many other elements, oxygen can be used to passivate the SiC interface, causing the buffer layer to transform into graphene. Here, we study a gentle oxidation of the interface by annealing buffer layer and monolayer graphene samples in water vapor. X-ray photoelectron spectroscopy demonstrates the decoupling of the buffer layer from the SiC substrate. Raman spectroscopy is utilized to investigate a possible introduction of defects. Angle-resolved photoemission spectroscopy shows that the electronic structure of the water vapor treated samples. Low-energy electron microscopy (LEEM) measurements demonstrate that the decoupling takes place without changes in the surface morphology. The LEEM reflectivity spectra are discussed in terms of two different interpretations.     

Electromechanical properties of armchair graphene nanoribbons under local torsion

While graphene nanoribbons are prone to twist intrinsically, the effect of local twist on the electromechanical properties remains unexplored. By using the density functional theory in combination with the nonequilibrium Green’s function method, we investigate the responses of structural evolution and electrical transport of armchair graphene nanoribbons to local torsion. We show that local twist can alter their transport properties significantly. The current at a given bias can switch on/off or change many times with twist angle, which is related with twist-induced changes in electronic structures of graphene nanoribbons. Our results can provide a valuable guideline for design and implementation of graphene nanoribbons in nanoelectromechanical systems and devices.     

石墨烯的制备及应用现状

石墨烯具有独特的结构和性质,从石墨烯的成果获得诺贝尔奖以来,其相关研究便引起人们的极大兴趣.本文总结近年石墨烯的研究现状,比较全面地介绍了石墨烯的制备方法和其应用情况.详细介绍了微机械剥离法,液相或气相直接剥离法,氧化还原法,化学气相沉积法,取向附生法,晶体外延生长法,化学分散法,电化学方法,电弧法,爆炸法,自组装法制备石墨烯.以及石墨烯在催化过程,太阳能电池,电子信息,电化学,生物医药等方面的应用.结合本课题组的应用主要用于催化反方向,作了一定的描述.     

The electronic properties of superatom states of hollow molecules

Electronic and optical properties of molecules and molecular solids are traditionally considered from the perspective of the frontier orbitals and their intermolecular interactions. How molecules condense into crystalline solids, however, is mainly attributed to the long-range polarization interaction. In this Account, we show that long-range polarization also introduces a distinctive set of diffuse molecular electronic states, which in quantum structures or solids can combine into nearly-free-electron (NFE) bands. These NFE properties, which are usually associated with good metals, are vividly evident in sp(2) hybridized carbon materials, specifically graphene and its derivatives. The polarization interaction is primarily manifested in the screening of an external charge at a solid/vacuum interface. It is responsible for the universal image potential and the associated unoccupied image potential (IP) states, which are observed even at the He liquid/vacuum interface. The molecular electronic properties that we describe are derived from the IP states of graphene, which float above and below the molecular plane and undergo free motion parallel to it. Rolling or wrapping a graphene sheet into a nanotube or a fullerene transforms the IP states into diffuse atom-like orbitals that are bound primarily to hollow molecular cores, rather than the component atoms. Therefore, we named them the superatom molecular orbitals (SAMOs). Like the excitonic states of semiconductor nanostructures or the plasmonic resonances of metallic nanoparticles, SAMOs of fullerene molecules, separated by their van der Waals distance, can combine to form diatomic molecule-like orbitals of C(60) dimers. For larger aggregates, they form NFE bands of superatomic quantum structures and solids. The overlap of the diffuse SAMO wavefunctions in van der Waals solids provides a different paradigm for band formation than the valence or conduction bands formed by interaction of the more tightly bound, directional highest occupied molecular orbitals (HOMOs) or the lowest unoccupied molecular orbitals (LUMOs). Therefore, SAMO wavefunctions provide insights into the design of molecular materials with potentially superior properties for electronics. Physicists and chemists have thought of fullerenes as atom-like building blocks of electronic materials, and superatom properties have been attributed to other elemental gas-phase clusters based on their size-dependent electronic structure and reactivity. Only in the case of fullerenes, however, do the superatom properties survive as delocalized electronic bands even in the condensed phase. We emphasize, however, that the superatom states and their bands are usually unoccupied and therefore do not contribute to intermolecular bonding. Instead, their significance lies in the electronic properties they confer when electrons are introduced, such as when they are excited optically or probed by the atomically sharp tip of a scanning tunneling microscope. We describe the IP states of graphene as the primary manifestation of the universal polarization response of a molecular sheet and how these states in turn define the NFE properties of materials derived from graphene, such as graphite, fullerenes, and nanotubes. Through low-temperature scanning tunneling microscopy (LT-STM), time-resolved two-photon photoemission spectroscopy (TR-2PP), and density functional theory (DFT), we describe the real and reciprocal space electronic properties of SAMOs for single C(60) molecules and their self-assembled 1D and 2D quantum structures on single-crystal metal surfaces.     

Point defect-induced transport bandgap widening in the downscaled armchair graphene nanoribbon device

We report on ab initio study of the electronic states and transport characteristics for armchair graphene nanoribbon devices with point defects. Geometric optimization of the point defect channel revealed the self-organizing property of the graphene. Density of state (DOS) calculation shows the point defect-induced gap states. However, the quantum transport calculations revealed that these defect-induced gap states are not contributing to the carrier transport across the channel. This is clarified further from the wavefunction analysis, which shows the spatial localization of the wavefunction around the defect regions. Most importantly, the widening of the transport bandgap with the increase in number of point defects was found even though the DOS bandgap remains unchanged. The impact of these point defects on the device characteristics varies depending on their type and location in the channel. The orientation of the defects plays a vital role in the carrier scattering, which is a crucial factor to be considered in downscaled devices.     

Wafer-scale homogeneity of transport properties in epitaxial graphene on SiC

Magnetotransport measurements on Hall bar devices fabricated on purely monolayer epitaxial graphene on silicon carbide (SiC/G) show a very tight spread in carrier concentration and mobility across wafer-size dimensions. In contrast, SiC/G devices containing bilayer graphene domains display variations in their electronic properties linked to the amount of bilayer content. The spread in properties among devices patterned on the same SiC/G wafer can thus be understood by considering the inhomogeneous number of layers often grown on the surface of epitaxial graphene on SiC.     

Resonant Raman spectra of graphene with point defects

The Raman spectra of graphene with three different types of point defects, namely, a mono-vacancy, a di-vacancy, and a Stone-Wales defect, was calculated within a non-orthogonal tight-binding model using supercells of graphene with a single defect. The defects were found to modify the electronic structure and the phonons of graphene giving rise to new optical transitions and defect-related phonons. Based on the calculated Raman spectra, we determined the Raman lines that can serve as signatures of the specific defects. The comparison of the calculated Raman intensity of the graphitic (G-) band of perfect graphene and graphene with defects shows that the intensity can be enhanced up to one order of magnitude by the presence of defects.