Session J11: Focus Session: Graphene Structure, Stacking, Interactions: Density Functional Theory

Electronic structure, electron-phonon coupling and superconductivity in metal-doped few-layer graphene

We systematically investigate from first-principles how the electronic properties and electron-phonon (el-ph) coupling change in metal (Ca, Li) doped graphene as we tune the number of graphene layers from single layer to three layers and to the case of bulk graphite. We find that the Fermi level and el-ph constant can be tuned by the number of layers. In particular, the number of graphene layers has a large effect on the inter-layer metal free-electron-like states, which give rise to large el-ph constant in these systems. Surprisingly, the el-ph coupling in Ca intercalated tri-layer graphene system is stronger than one in the superconducting Ca-doped graphite. Our results suggest the possibility of high T$_{c}$ superconductivity in metal doped few-layer graphene for nanodevice applications.   

First-principles investigation of lithium doped of bilayer graphene and Lithium Intercalated Carbon Nanotubes

We have performed first-principles calculations based on density-functional theory for understanding of the structural and electronic properties of Li doped bilayer graphene and Li intercalated CNTs especially addressing the controversial charge transfer state between Li and C. We have checked the possible adsorption, substitution and intercalation of Li by using a bilayer graphene system both with AB stacking (12 different initial configurations) and AA stacking (8 different initial configurations). All calculations are repeated both with LDA and GGA exchange-correlation potential, even though the values of binding energies are different, their order and corresponding physical picture are same from both of the functionals as well as the stackings. In conclusion, we can summarize that Li prefers the hollow site adsorption geometry and it prefers intercalation but not the substitution. In these adsorption modes, almost 0.9 electron of Li atom is transferred to neighboring carbon atoms network leaving positively charged core behind.   

Ab Initio Study of the Interactions between Dopant Atoms in Graphene

We present a first-principles computational study of the interactions between the boron (B) and nitrogen (N) dopant atoms in graphene. Our calculations are carried out using density functional theory combined with the generalized gradient approximation for the exchange-correlation functional. The total energies, equilibrium geometries, electronic charge distributions, and densities of states of doped graphene sheets are examined in cases of B-B, N-N, and B-N co-doped graphene. We find the B-B and N-N interactions to be repulsive and the B-N interaction to be attractive. In all cases studied, dopant-dopant interactions appear to have a relatively short range. The interaction energy between the two dopant atoms is found to be inversely proportional to the square of the separation distance. We interpret these results in terms of structural relaxation and electronic charge transfer.   

Ab Initio Study of the Interactions between Dopant Atoms and Point Defects in Graphene

We apply a first-principles computational method based on density functional theory to study the interaction of substitutional boron and nitrogen atoms with Stone-Wales defects and vacancies in graphene. Our calculations are carried out using a pseudopotential technique combined with the generalized gradient approximation for the exchange-correlation functional implemented in the SIESTA electronic structure package. Graphene sheets are modeled by periodic supercells containing up to 160 atoms. The equilibrium geometries, total energies, electronic structures, and densities of states of doped and defective graphene sheets are examined as a function of the separation distance between dopant atoms and point defects in graphene. The results of our study demonstrate the presence of attractive interaction between dopant atoms and point defects in graphene. The interaction energy decreases rapidly with increasing the dopant-defect separation distance.   

Many-body study of cobalt adatoms adsorbed on graphene

Interest in the adsorption of transition metal adatoms on graphene has grown rapidly. The interaction between magnetic adatoms and graphene may have applications in designing spintronics devices. Several theoretical and experimental studies have examined Co adatoms on graphene. Calculations of Co--graphene systems have largely been done at the density functional theory (DFT) level, with local or semi-local functionals and with an empirical Hubbard on-site repulsion $U$ (LDA+U). We use auxiliary-field quantum Monte Carlo (AFQMC), in combination with DFT and quantum chemistry methods, to examine the effects of electron correlation in Co--graphene systems, without adjustable parameters. Binding energy curves for Co--graphene and model structures will be presented, and their implications on the electronic and structural properties will be discussed.   

Adsorption of group VA atoms in single and double vacancy of graphene

Dopants can greatly affect the electronic structure of graphene. In this study we report on the effect of group VA elements associated with single vacancy (SV) and double vacancy (DV) on the electronic structure of graphene. The results of our density-functional-theory (DFT) computations predict strong and exothermic chemisorption of group VA atoms to SV and DV in graphene (M@SV/DV, where M=N, P, As, Sb, or Bi) which leads to large, systematic and diverse effects on its electronic structure. We find that N@SV has a small band gap below Fermi level (E$_{F})$ and P@SV has semiconducting and magnetic properties consistent with previous studies. Our preliminary band structure computations show that As@SV induces $\sim $100{\%} spin polarization close to E$_{F}$ while Sb@SV and Bi@SV are metallic. In contrast, N@DV has a large spin polarization near E$_{F}$ while all other DV defects are predicted to be metallic. These results suggest that especially As@SV and N@DV may have interesting applications in spintronics and nanoelectronics.   

Stability and electronic structure of the functionalized graphene layers

We present results of extensive ab initio calculations in the framework of the density functional theory of the graphene layers (GLs) doped with nitrogen and boron, and also functionalized with simple OH, COOH, NH$_{n}$, and CH$_{n}$ molecules. We calculate binding energies, heat of formation, resulting local deformations (characteristic sp$^{3}$ rehybridization of the bonds induced by fragments), electronic structure, elastic properties (Young's modulus and Poisson ratio), and conductance of doped and functionalized GLs as a function of the density of the functionalizing systems. Generally, the stability of the functionalized graphene layers decreases with the growing concentration of functionalizing molecules and we determine the critical density of the molecules that can be chemisorbed on the surface of GLs. We find out that the GLs functionalization leads in many cases to the opening of the graphene band gap and can be therefore utilized in graphene devices. In particular, the zero band gap in K-point of the pristine single GL increases to 0.11, 0.12, 0.25, and 0.24 eV for the GL functionalized with OH, NH, NH$_{2}$, and COOH, respectively.   

Band Structures of bilayer graphene with experimentally measured twisting angles

Using density functional theory (DFT) and tight binding calculations, we study the band structure of bilayer graphene as a function of twisting angles and compare the results to spectroscopic measurements.To test the accuracy of DFT, we first compare the band structure with DFT and the G$_0$W$_0$ approximation for several bilayer graphene systems with special twisting angles.Our DFT results agree with previous DFT calculations for the weak coupling between the two layers and the band structure.Calculations of the quasiparticle dispersion using the G$_0$W$_0$ approximation show that DFT underestimates the Fermi velocity of bilayer graphene and the bandgap away from the $K$ point.Scaling of the DFT band structure by an empirical parameter that depends on the twist angle accounts for most of the difference between the DFT and the G$_0$W$_0$ approximation. Based on the G$_0$W$_0$ calculations, we fit a set of tight-binding parameters for the interlayer coupling Hamiltonian. Using this tight-binding model we study the band structure of bilayer graphene for twist angles that closely match experimental systems and have larger unit cells. The results of these band structure calculations explain the experimentally observed G band resonance in Raman spectroscopy for specific twist angles.   

From graphene to graphite: A first-principles study

The multilayer graphene and graphite are systematic studied via a first-principles method based on density functional theory and general gradient approximation. The hexagonal, Bernal and rhombohedral stacking types are considered. The total energy, the electronic structure and the strength of the multilayer graphene and graphite are analyzed to study the process of graphene to graphite. We predict the number of graphene multilayers shall be greater than 15 layers to behave as a graphite bulk.   

Accurate computational studies of carbon doped two-dimensional boron-nitride

Advances in development of atomic-layer crystals with a plethora of new materials are greatly extending the range of possible applications of these two-dimensional (2D) materials. One of these materials is the hexagonal structure of boron nitride (h-BN). Hexagonal BN has a wide band gap and a lattice constant similar to that of graphene. We show that even small quantities of C atoms can offer new functionalities and transform h-BN to be an amazing playground for 2D physics. Large-scale accurate density-functional-theory calculations with the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional reveal the electronic and the magnetic properties of h-BN with substitutionally embedded carbon atoms. Results of local magnetic moments induced by substitution and their interactions are presented for low C concentrations. We also show the electronic structures of quantum dots made of carbon nano-domains for applications in optics and opto-electronics.   

Electronic and Structural Properties of Graphene Dots in h-BN

A major challenge for graphene-based applications is the creation of a tunable electronic band gap as would be present for traditional semiconductor alloys. Since hexagonal boron nitride has a very similar structure to graphene, it is a natural candidate to modify the electronic structure of graphene by forming a hybrid phase sheet containing domains of graphene and hexagonal boron nitride, as has been done experimentally. Here we investigate the properties of such hybrid sheets by using pseudopotential-density functional theory implemented in real space. We find for a graphene dot comparable in size to those observed in experiment, the band gap of the sheet is not significantly modified. Moreover, when the size of graphene dot decreases below $\sim$13\AA, strong structural instabilities of the graphene domain occur.   

Hybrid density functional study of 2D graphene-boron nitride (BCN) nanostructures

Graphene has attracted enormous research interest in the last few years because of its intriguing physics as well as application potential. Recent synthesis of BCN nanostructures by doping graphene with a wide bandgap insulator boron nitride (BN) has unveiled new possibilities for this material [1]. BCN nanostructures are semiconductors and possess interesting properties that are distinct from the parent compounds. Reliable theoretical estimates can predict the feasibility and usefulness of still largely unexplored BCN nanostructures, and provide a route to engineer their properties. We study electronic structures of a variety of 2D BCN nanostructures using hybrid functional HSE in density functional theory (DFT). We show that their properties can be gradually tuned and are sensitive to composition and the type of configurations. In agreement with experimental observation, a strong tendency to phase-segregate exists for low concentration of BN in graphene. We also investigate magnetic properties of graphene containing substitutional nitrogen atoms, and their suitability for magnetic devices.\\[4pt] [1]. L. Ci et al., Nature Materials 9, 430 (2010).   

First-principles and Tight-binding Investigations of Graphene-like Systems

We have performed first-principles calculations of graphene-like systems with large unit cells of the order of 100 atoms. These are mostly quasi-circular pieces of graphene with or without hydrogen atoms to passivate dangling bonds. We have also introduced nitrogen replacements of carbon atoms to explore possibilities of creating a quantum-mechanical analog of the split-ring resonator used in negative refractive index metamaterials. In addition, vibrational spectra were calculated to check the stability of the small flakes. Furthermore, we have used the NRLMOL DFT code with extensions to treat an AC magnetic field coupled to spin and orbital moments. We simulate the graphene-like system to a circuit model to show the cancellation of individual loop currents and the emergence of an edge ballistic current. A tight-binding Hamiltonian was fitted to our NRLMOL results using the NRLTB method. The TB approach starts with an exact fit to the benzene molecule and the resulting TB parameters are transferable to the larger molecules matching well the HOMO-LUMO gap found by the DFT. The NRLTB scheme allows to calculate the electronic spectra of much bigger systems, and examples with results for systems with more than a thousand atoms will be presented.   

Quasiparticle Band Gaps of Graphene and Graphone on Hexagonal Boron Nitride Substrate

Graphene holds great promise for post-silicon electronics; however, it faces two main challenges: opening up a band gap and finding a suitable substrate material. Graphene on hexagonal boron nitride (hBN) substrate provides a potential system to overcome these challenges. While theoretical studies suggested a possibility of a finite band gap of graphene on hBN, recent experimental studies find no band gap. We have studied graphene-hBN system using the first-principles density functional method and the many-body perturbation theory within GW approximation [1]. A Bernal stacked graphene on hBN has a band gap on the order of 0.1 eV, which disappears when graphene is misaligned with respect to hBN. The latter is the likely scenario in realistic devices. In contrast, if graphene supported on hBN is hydrogenated, the resulting system (graphone) exhibits band gaps larger than 2.5 eV. The graphone band gap is due to chemical functionalization and is robust in the presence of misalignment, however, it reduces by about 1 eV due to the polarization effects at the graphone/hBN interface.\\[4pt] [1] N. Kharche and S. K. Nayak, \textit{Nano Lett.}, DOI: 10.1021/nl202725w, (2011).