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We give the results of density functional calculations for graphene with a widely varying fluorine adsorptions. We give a systematic analysis of the adsorption energies, lattice constants, bulk modulus, bandgap openings, and magnetic properties. We find that a number of different adsorption geometries and a range of physical properties can occur for each adsorbate coverage. The systems are found to range from metallic to semiconducting with widely vary band gaps, and a number of interesting magnetic phases are found. We expect that many of these structures may occur in real materials systems. Further that a listing of the properties found here may help in determining what fluorinated graphenes are produced experimentally.
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In this paper, we investigate the adsorption of water monomer on fluorinated graphene using state-of-the-art first principles methods within the framework of density functional theory (DFT). Four different methods are employed to describe the interactions between water and the carbon surface: The traditional DFT calculations within the generalized gradient approximation (GGA), and three types of calculations using respectively the semi-empirical DFT-D2method, the original van der Waals density functional (vdW-DF) method, and one of its variants. Compared with the adsorption on pristine graphene, the adsorption energies of water on fluorinated graphene are significantly increased, and the orientations of water diploe moment are notably changed. The most stable configuration is found to stay right above the top site of the C atom which is bonded with F, and the dipole moment of water molecule aligns spontaneously along the surface normal.
While numerous methods have been proposed to produce semiconducting graphene, a significant bandgap has never been demonstrated. The reason is that, regardless of the theoretical gap formation mechanism, disorder at the sub-nanometer scale prevents the required chiral symmetry breaking necessary to open a bandgap in graphene. In this work, we show for the first time that a 2D semiconducting graphene film can be made by epitaxial growth. Using improved growth methods, we show by direct band measurements that a bandgap greater than 0.5 eV can be produced in the first graphene layer grown on the SiC(0001) surface. This work demonstrates that order, a property that remains lacking in other graphene systems, is key to producing electronically viable semiconducting graphene.
By using four layered graphene/gallium nitride (GaN) Schottky diodes with an undoped GaN spacer, we demonstrate highly effective gating of graphene at low bias rendering this type of structure very promising for potential applications. An observed Raman G band position shift larger than 8.5 cm-1 corresponds to an increase in carrier concentration of about 1.2x10^13 cm-2. The presence of a distinct G band splitting together with a narrow symmetric 2D band indicates turbostratic layer stacking and suggests the presence of a high potential gradient near the Schottky junction even at zero bias. The subbands characterized by the highest Raman energies correspond to the largest concentration of electrons. An analysis based on electroreflectance measurements and a modified Richardson equation confirmed that graphene on n-GaN separated by an undoped GaN spacer behaves like a capacitor at reverse bias. At least 60% of G subband position shifts occur at forward bias, which is related to a rapid reduction of electric field near the Schottky junction. Raman micromapping shows a high uniformity of gating efficiency on the surface. Therefore, our studies demonstrate the usefulness of few layer turbostratic graphene deposited on GaN for tracing electron-phonon coupling in graphene. Multilayer graphene also provides uniform and stable electric contacts. Moreover, the observed bias sensitive G band splitting can be used as an indicator of charge transfer in sensor applications in the low bias regime.
Recent synthesis of fluorinated graphene introduced interesting stable derivatives of graphene. In particular, fluorographene (CF), namely fully fluorinated chair conformation, is found to display crucial features, such as high mechanical strength, charged surfaces, local magnetic moments due to vacancy defects and a wide band gap rapidly reducing with uniform strain. These properties, as well as structural parameters and electronic densities of states are found to scale with fluorine coverage. However, most of the experimental data reported to date neither for CF, nor for other CnF structures complies with the results obtained from first-principles calculations. In this study, we attempt to clarify the sources of disagreements.
We present a method for decoupling epitaxial graphene grown on SiC(0001) by intercalation of a layer of fluorine at the interface. The fluorine atoms do not enter into a covalent bond with graphene, but rather saturate the substrate Si bonds. This configuration of the fluorine atoms induces a remarkably large hole density of p approx 4.5 times 1013 cm-2, equivalent to the location of the Fermi level at 0.79 eV above the Dirac point ED .
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