Graphene, the new nanocarbon

Graphene is an ideal two-dimensional crystal with the advantages of high conductivity, unique physical and chemical properties, and high specific surface area. Especially, because of its super excellent electronic properties, graphene may substitute the traditional semiconductor silicon material and carbon nanotube, thus creating a new nanoscale electronic device. In addition, multilayer graphene with ultra-wide spectral absorption characteristics and unique photoelectric properties is an ideal material for photovoltaic devices. However, the zero band gap and semi-metality of graphene both limit its application in space detectors such as the microelectronic industries and satellites. Opening and regulating the graphene band gap by physical methods has become one of the key means to further expand its applications. Research work has shown that the doping of elements can significantly change the electronic structure of graphene, thereby regulating the optical properties of graphene. In order to provide an insight into electronic properties of graphene and tune its electronic band structure and optical properties effectively, electronic and optical properties of Ni-doped multi-layer graphene are studied and a number of interesting results are obtained. The calculation are carried out by the CASTEP tool in Materials Studio software based on the first-principles of ultrasoft pseudopotential of density functional theory. The models of three typical doping positions relative to carbon atoms are constructed. After structural optimization, it is obtained that " above the center of two carbon atoms” is the most stable doping structure. By using the method of local density approximation, the band structure, density of states, dielectric constant, reflectivity and refractive index of the models are calculated. The results show that an enhanced energy band gap can be achieved after nickel-doping, and reach up to 0.604 eV. Besides, peaked phenomenon of density of states at Femi level can be observed, which is accomplished by enhancing the plasma energy. Furthermore, the calculations show that the imaginary part of permittivity and refractive index increase after nickel-doping, suggesting that the optical absorbing performance is improved. All these results provide theoretical guidance for further exploring the optical properties of graphene.

Graphene covered metal nanoparticles constitute a novel type of hybrid material, which provides a unique platform to study plasmonic effects, surface-enhanced Raman scattering (SERS), and metal-graphene interactions at the nanoscale. Such a hybrid material is fabricated by transferring graphene grown by chemical vapor deposition onto closely spaced gold nanoparticles produced on a silica wafer. The morphology and physical properties of nanoparticle-supported graphene are investigated by atomic force microscopy, optical reflectance spectroscopy, scanning tunneling microscopy and spectroscopy (STM/STS), and confocal Raman spectroscopy. This study shows that the graphene Raman peaks are enhanced by a factor which depends on the excitation wavelength, in accordance with the surface plasmon resonance of the gold nanoparticles, and also on the graphene-nanoparticle distance which is tuned by annealing at moderate temperatures. The observed SERS activity is correlated with the nanoscale corrugation of graphene. STM and STS measurements show that the local density of electronic states in graphene is modulated by the underlying gold nanoparticles.

Graphene is a two-dimensional material that has special characteristics. The electronic properties of graphene show zero band gap conditions. The magnetic properties of graphene can be created by modifying the electronic properties through atomic substitution. In this research, we study the magnetic and electronic properties of single-layer graphene substituted with boron (B) atoms, because it has almost the same atomic radius as carbon (C) atoms, resulting in only small lattice deformation. The spin-polarized density functional theory (DFT) method implemented in the Quantum Espresso package was selected to perform the calculations. The simulated models are a 4×4×1 supercell of pristine graphene structure consisting of 32 C atoms and boron-substituted graphene with a variety number of atoms (B = 1 and 2 atoms). The results of band gap energy obtained after the structure was optimized are 0.19 and 0.21 eV (spin-down and spin-up) for G-B and 0.36 and 0.37 eV (spin-down and spin-up) for G-2B. Boron substitution in graphene opens the bandgap and shifts the Fermi energy level. It also influences the magnetic moment of the graphene layer, estimated at 0.22 and 0.06 μB/cell for G-B and G-2B, respectively. This research shows that modifying graphene by substituting boron makes the graphene material semiconductive and weakly magnetic.

A DFT study of electronic and thermoelectric properties of arsenic doped graphene

Doping with nitrogen atom is an effective way to modify the electronic and magnetic properties of graphene. In this paper, we studied the effect of the number of dopant atoms on the electronic and magnetic properties of the two most common nitrogen bond configurations in N-doped graphene, that is, graphitic and pyridinic, using density functional theory (DFT). We found that the formation of graphitic and pyridinic configurations can initiate the transition of the electronic properties of graphene from semimetal to metal with n-type conductivity for the graphitic configuration and p-type conductivity for the pyridinic configuration. The formation of a bandgap-like structure was observed in both configurations. The bandgap increased with the increase in the number of dopant atoms. We also observed that the formation of graphitic configuration did not cause a transition to the magnetic properties of graphene even though the number of dopant atoms was increased. In the pyridinic configuration, the increase in the number of dopant atoms caused graphene to be paramagnetic, with the remarkable total magnetic moment of 0.400 μB per cell in the pyridinic-N3 model. This study provides a deeper understanding of the modification of electronic and magnetic properties of N-doped graphene by controlling the bond configuration and the number of nitrogen dopants.

The structural and mechanical properties of an epitaxial graphene (EG) monolayer thermally grown on top of a 6H–SiC(0001) surface were studied by combined dynamic scanning tunneling microscopy (STM) and frequency modulation atomic force microscopy (FM-AFM). Experimental STM, dynamic STM and AFM images of EG on 6H–SiC(0001) show a lattice with a 1.9 nm period corresponding to the (6 × 6) quasi-cell of the SiC surface. The corrugation amplitude of this (6 × 6) quasi-cell, measured from AFM topographies, increases with the setpoint value of the frequency shift Δf (15–20 Hz, repulsive interaction). Excitation variations map obtained simultaneously with the AFM topography shows that larger dissipation values are measured in between the topographical bumps of the (6 × 6) quasi-cell. These results demonstrate that the AFM tip deforms the graphene monolayer. During recording in dynamic STM mode, a frequency shift (Δf) map is obtained in which Δf values range from 41 to 47 Hz (repulsive interaction). As a result, we deduced that the STM tip, also, provokes local mechanical distortions of the graphene monolayer. The origin of these tip-induced distortions is discussed in terms of electronic and mechanical properties of EG on 6H–SiC(0001).

Geometric imperfections may be created during the production process or setting borders of single-layer graphene sheets (SLGSs). Vacancy defects are an instance of geometric imperfection, so investigating the effect of these vacancies on the mechanical properties of single-layer graphene is extremely important. Since very few studies have been conducted on the structure of imperfect graphene (with the vacancy defect) as an anisotropic structure, further study of this defective structure seems imperative. Due to the vacancy defects and for the proper assessment of mechanical properties, the graphene structure should be considered anisotropic in certain states. The present study investigates the effects of site and size of vacancy defects on the mechanical properties of graphene as an anisotropic structure using the lekhnitskii interaction coefficients and Molecular Dynamic approach. The effect of temperature on the severity of the SLGS becoming anisotropic is also investigated in this study. The results reveal that the amount of temperature has a big effect on the severity of the structure getting anisotropic even for a graphene without any defects. The effect of aspect ratio, temperature and also size and site of vacancy defects on the material properties of the graphene are studied in this research work. According to the present study, using material properties of flawless graphene for imperfect structure can lead to inaccurate results.

The unique properties of vertically-oriented graphene (VG) are discussed in this chapter. VG is intrinsically graphene, but it also possesses unique structural features, i.e., being arranged perpendicularly to the substrate surface. Therefore, VG possesses not only the properties of graphene but also some unique characteristics induced by its oriented arrangement. We start this chapter with a brief introduction of some general properties of graphene, which is deemed reasonable and necessary before we elaborate on the uniqueness of VG. To illustrate the attractive characteristics of VG, we compare VG with planar (or horizontal) graphene structures and emphasize the benefits that can be brought about due to VG’s vertical orientation. The unique properties of VG are summarized at the end of this chapter. Understanding of the VG’s uniqueness is critical to appreciating why VG has attracted so much interest and is also an essential step toward tailoring VG properties for various applications.

This chapter discusses the structure and properties of graphene and its characterization. The chapter begins by exploring the atomic and electronic structure of graphene, followed by its optical, mechanical and electronic properties, such as the quantum hall effect and the Klein paradox. In the last part of the chapter, characterization of graphene through various techniques, such as atomic force microscope, scanning electron microscope, and transmission electron microscope, are discussed. The chapter aims to provide the fundamental understanding of the properties of graphene and its characterization.

Transient absorption spectroscopy as a promising optical tool for the quality evaluation of graphene layers deposited by microwave plasma

Graphene is the thinnest 2-D material which can be regarded as a single layer of graphite. The unique electrical, mechanical and optical properties of graphene can be used in many technological applications. 2-D nanomaterials with semiconducting properties are of great interest since they can be applied in electronics industry. Pure graphene is a zerogap semiconductor or semimetal, since the electron states just cross the Fermi energy. However, the electronic properties of graphene can be tuned by doping boron or nitrogen atoms. Understanding the electronic properties in terms of density of states and band structure of doped graphene is of great relevance today. In our work, we have analyzed the electronic properties of boron and nitrogen doped graphene using Density Functional Theory (DFT). The stability and charge analysis of doped structures have been studied. The Local Density Approximation (LDA) calculations have been used to find the total energies of the structures. In addition to the electronics industry, doped graphene also has great potential to adsorb gas molecules. Therefore, we have analyzed the H2 molecule adsorption in pure, B-doped and N-doped graphene.

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.

Graphene and its derivatives belong to one of the most intensively studied materials. The radical reaction using halogen derivatives of arene-diazonium salts can be used for effective control of graphene's electronic properties. In our work we investigated the influence of halogen atoms (fluorine, chlorine, bromine and iodine) as well as their position on the benzene ring towards the electronic and electrochemical properties of modified graphenes. The electronegativity as well as the position of the halogen atoms on the benzene ring has crucial influence on graphene's properties due to the inductive and mesomeric effects. The results of resistivity measurement are in good agreement with the theoretical calculations of electron density within chemically modified graphene sheets. Such simple chemical modifications of graphene can be used for controllable and scalable synthesis of graphene with tunable transport properties.

The properties of graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of platinum (Pt) metal on exfoliated single, bi- and trilayer graphene and on chemical vapor deposition-grown single-layer graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that Pt affects the structure as well as the electronic properties of graphene. The shift of peak frequencies, intensities and widths of the Raman bands were analyzed after the deposition of Pt with different thicknesses (1, 3, 5 nm) on the graphene. The shifts in the G and 2D peak positions of the Raman spectra indicate the n-type doping effect by the Pt metal. The doping effect was also confirmed by gate-voltage dependent resistivity measurements. The doping effect by the Pt metal is stable under ambient conditions, and the doping intensity increases with the increasing Pt deposition without inducing a severe degradation of the charge carrier mobility.

Substrate effect on the electronic properties of graphene on vicinal Pt(1 1 1)