Session H11: Focus Session: Graphene Structure, Stacking, Interactions: Interfaces and Adsorbates

Confinement of organic solvents by wet transfer of graphene

Transfer of graphene grown by chemical vapor deposition (CVD) on Cu requires a polymer support for the graphene and wet-etching of the Cu growth substrate. After etching, the polymer/graphene film must go through several solvent baths to remove contaminants. Water is commonly used for this cleaning due to its capability as a solvent and its ability to support the polymer/graphene film through high surface tension. By contrast, common organic solvents have lower surface tension, causing the film to tear and fold within the liquid. To bypass this challenge, we have implemented a polymer-bound truss to reinforce the polymer/graphene film, which avoids the need of proper solvent surface tension. We have transferred graphene grown on Cu using common organic solvents like methanol, isopropanol, ethylene glycol, and dimethyl sulfoxide for the final transfer liquids. This process traps the solvents between the graphene and the final substrate. Confinement effects are determined via optical microscopy, atomic force microscopy, and Raman spectroscopy for both the trapped solvent molecules and the graphene. Our procedure opens up the possibility of confining biological materials suspended in organic solvents under graphene.   

Photo-assisted polymerization of one-dimensional molecular arrays on epitaxial graphene

Self-assembled monolayers provide opportunities to tailor the materials properties of graphene and template subsequent chemical functionalization at molecular length scales. In particular, 10,12 pentacosadiynoic acid (PCA) is found to assemble into one-dimensional arrays on epitaxial graphene on SiC(0001) as revealed by ultra-high vacuum scanning tunneling microscopy (UHV-STM). Furthermore, UV-triggered polymerization of PCA in UHV is achieved, leading to distinct conformational changes at the molecular scale. The sub-5 nm widths of these one-dimensional polymers make them promising candidates for templating the formation of graphene nanoribbons with significant band gaps. Molecular resolution UHV STM characterization of the structure and electronic properties of this nanostructured chemical functionalization strategy will be presented.   

Chemical reactivity imprint lithography on graphene: Controlling the substrate influence on electron transfer reactions

The chemical functionalization of graphene enables control over electronic properties and interactions with other materials. Graphene's chemical reactivity is strongly influenced by the underlying substrate. In this paper, we show a stark difference in the rate of electron transfer chemistry with aryl diazonium salts on monolayer graphene supported on a broad range of substrates. Reactions proceed rapidly when graphene is on SiO$_{2}$ and Al$_{2}$O$_{3}$ (sapphire), but negligibly on alkyl-terminated and hexagonal boron nitride (hBN) surfaces. The effect cannot be explained by the overall graphene doping levels alone, and can instead be described using a reactivity model accounting for substrate-induced electron-hole puddles in graphene. Raman spectroscopic mapping is used to characterize the effect of the substrates on graphene. Reactivity imprint lithography (RIL) is demonstrated as a technique for spatially patterning chemical groups on graphene by patterning the underlying substrate, and is applied to the covalent tethering of proteins on graphene.   

Graphene on Metals: Interface Structure and Defects

The epitaxial growth of graphene on metal substrates is one of the major methods of graphene production for electronic applications. Therefore, the metal/graphene interface interactions as well as the graphene defects appeared during the growth affect in a substantial way the electronic properties of both graphene and graphene/metal contacts, which are both important for device applications. Structural and electronic properties of simple and complex graphene/metal as well as graphene/metal-alloy interfaces were investigated using first principles density functional theory. The point defect structures in graphene on metal substrate were studied and compared with those in free standing graphene.   

Topologically Frustrated Bonding in Dual-sided Adsorption to an Atomically Thin Membrane

A seamless sp2 atomically thin layer cuts space in half, and prevents penetration of atoms through the sheet, while still allowing cross-sheet charge transfer. This geometrical frustration separates charge donating (e.g. alkali) and charge accepting (e.g. halogen) ions in opposite subspaces and generates a collective planar dipole. In this unusual geometry we observe new physics and chemistry. For graphene layer the uncompensated Coulomb interactions generate a system with multiple nearly degenerate structures which are either metallic or small-gap semiconductors, in contrast to the insulating behavior of unfrustrated salt crystals. When the layer is changed to h-BN, the collective dipole imposes a large Stark effect on the halogen and alkali-derived valence and conduction bands, resulting in a large band gap tuning with areal adsorbate density.   

Correlation between adatom adsorption properties and growth morphology of metal on graphene

We present a systematic study of various metal adatom adsorption on graphene by \textit{ab initio} calculations. The correlation between the adatom adsorption properties and the growth morphology of the metals on graphene is investigated. We show that the growth morphology is related to the ratio of the adsorption energy to the bulk cohesive energy (E$_{a}$/E$_{c})$ of the metals and the diffusion barrier ($\Delta $E) of the metal adatom on graphene. The growth morphology is also affected by the strain induced by metal adsorption on graphene. We also show that most of the metal nanostructures on graphene are thermally stable again coarsening. The first-principles calculations are consistent well with the observations from recent experiments.   

Lithium Intercalation Induced Decoupling of Epitaxial Graphene on SiC(0001): Electronic Property and Dynamic Process

This work presents first-principles investigations of the dynamic process of lithium (Li) penetration through the buffer layer on 6H-SiC(0001) surface, as well as the Li-insertion induced change of electronic structure. It is found that the penetration is kinetically forbidden for perfect buffer layer, because of the size confinement of its honeycomb structure. From the analysis of rate coefficient under the experimental conditions, topological defects no smaller than 8-membered ring are predicted to be essential for Li intercalation. Along with the Li insertion, the electronic property of the buffer layer is changed from n-type doping (Li-adsorption) to that of quasi-free-standing graphene (Li-intercalation). It is the electron injection by Li that results in the dissociation of the Si-C bonds and the decoupling of Li-intercalated buffer layer from the substrate. Moreover, we demonstrate the influence of such topological defects on the electronic property of epitaxial graphene, which provides some useful hints for understanding the observed gap and midgap state behavior.   

Single atom doping of graphene -- A theoretical study

We present first principles results and analysis for the electronic structure of chemically modified graphene. We show the relationship between different physical parameters and the electronic band structure of the modified material and its doping level. Finally, we discuss the possible effects of a substrate and of charge transfer patterns for device applications.   

Hydrogen adsorption induced structural and electronic changes in graphene grown on metal substrate

Graphene hydrogenation proposed to open a band gap has also been shown to be the case for graphene on metal substrates. Our carbon specific soft x-ray (photoelectron, absorption, emission) spectroscopy studies on single and few layer graphene on Pt(111) do not indicate band opening due to hydrogenation. The graphene layer is weakly interacting with the Pt(111) substrate but hydrogenation induces structural changes which lead to observation of density of states at the Fermi level (contrary to band opening hypothesis) due to strong hybridization with substrate. Hydrogenation observed to occur only on the surface atoms of few layers of graphene induces interlayer carbon-carbon bonding due to structural distortions initiated at the surface, i.e. propagation of sp$^{3}$ hybridization to underneath carbon layers. This structure is stabilized due to hybridization of the carbon atoms in the bottom layer with the Pt(111) substrate.   

Vibrational Spectrum of ``Crystalline Graphane''

Since the discovery of graphene, a flat monolayer of carbon atoms, a great interest was attracted to synthesis of chemically modified carbon sheets. In particular, it was proposed that graphane, representing a graphene sheet saturated by hydrogen adsorbed from both sides, would be stable [1], and this prediction was confirmed by TEM [2]. Recently, hydrogenated graphite with a composition close to CH has been synthesized by exposing the graphite to gaseous hydrogen at P=2 to 7 GPa and T=350 to 450$^{o}$C [3]. The formation of hydrographite is accompanied by a 40{\%} increase in the $c$-parameter of the unit cell. The IR spectrum shows a strong band near 2850 cm$^{-1}$ due to stretching vibrations of the C-H covalent bonds. In the present work, we studied the vibrational spectrum of hydrographite by inelastic neutron scattering. The obtained spectrum is very similar to that calculated for a single graphane plane [4]. This suggests a weak interaction between the graphane layers in hydrographite, so it could be considered as a ``crystalline graphane'' material. 1. J.O. Sofo et al., PRB \textbf{75}, 153401 (2007). 2. D.C. Elias et al., Sci. \textbf{323}, 610 (2009). 3. I.O. Bashkin et al., Int. Symp. Metal-Hydr. Syst., Reykjavik, Iceland, 2008. 4. G. Savini et al., PRL \textbf{105}, 037002 (2010).   

Direct determination of the dominant scatterer in graphene on silicon oxide

Previously the density of native scatterers in graphene on silicon oxide was shown to be proportional to the number of adsorption sites for atomic hydrogen [1]. However, this study provided limited information about the sites in graphene with affinity to atomic hydrogen. We employed a detailed temperature programmed desorption study on hydrogen-dosed graphene sheets. The determined desorption energy is used to reveal the nature of the dominant scatterer in graphene on silicon oxide. \\[4pt] [1] J. Katoch, J.H. Chen, R.Tsuchikawa, C.W. Smith, E.R. Mucciolo, and M. Ishigami, \textit{Uncovering the dominant scatterer in graphene sheets on SiO$_{2}$}, Physical Review B Rapid Communications, 82, 081417 (2010).   

Electron Heat Capacity of Nitrogen Doped Graphene in Low Temperature

We calculate the electron heat capacity of nitrogen doped graphene with a simple method. There are four kinds of bonds to be considered, the pi and anti-pi bonds of C-C and C-N. And the extra electrons are treated as free constituents. We found a small amount of difference in the electron heat capacity between a pure graphene and a thin film CNx. Nevertheless, with a precise measurement on the electron heat capacity of the thin film or layer structure of CNx, we can determine the doping concentration of nitrogen.   

Chemical reactions and thermal stability of oxygen impurities on graphene

Oxygen as an impurity is known to degrade conductivity in graphene, but annealing at moderate temperature reverses the effect. Here we report first-principles calculations of oxygen binding and reactions on graphene that elucidate the underlying physics. We find that two O atoms can form an O dimer that can desorb from graphene with an overall activation barrier of 1.3 eV. Oxygen can also be removed in a more complicated reaction in which C atoms in graphene are consumed. We find that structural defects such as Stone-Wales defect and grain boundaries show enhanced binding to O atoms due to the local strain, facilitating the O reaction. If H atoms coexist, an O atom can bind to an H atom forming an OH group, which can also be removed by thermal annealing due to the weak binding, resulting in defect-free graphene.   

Efficient adsorbate transport on graphene by electromigration

Chemical functionalization of the surface of graphene holds promise for various applications ranging from nanoelectronics to surface catalysis and nano-assembling. In many practical situations it would be beneficial to be able to propel adsorbates along the graphene sheet in a controlled manner. We propose to use electromigration as an efficient means to transport adsorbates along the graphene surface. Within the tight-binding approximation for graphene, parametrized by density functional theory calculations, we estimate the contributions of the direct force and the electron wind force to the drift velocity of electromigration and demonstrate that the electromigration can be rather efficient. In particular, we show that the drift velocity of atomic oxygen covalently bound to graphene can reach up to 4 cm/s for realistic graphene samples. Further, we discuss ways to dynamically, i.e., during experiment, control the efficiency of electromigration by charging and/or local heating of graphene.