Session A12: Graphene: Electronic Structure and Interactions - Adsorbates, Doping and Defects

Transport study of electrochemically decorated and intercalated graphene

Due to the surface-only properties of graphene, the decoration and/or intercalation of single, bi- and multi-layer graphene with foreign atoms can severely modify its electronic interactions, similar to those observed in its 3D counterpart the graphite intercalation compounds. Supported by a highly increased density of state due to a strong charge transfer above 10$^{14}$ cm$^{-2 }$into the graphene $\pi $-bands, certain adatoms are expected to induce strong electronic interactions to the graphenes own Dirac fermions, where theoretical predictions reach from the Kondo-effect and magnetism to as far as superconductivity in graphene. In this study we will present evidence of specific adsorption and intercalation of diverse atomic species by electrochemical means. We will present a detailed transport study, including resistivity-, Hall- and magneto-resistivity measurements of single-, bi- and multi-layer graphene devices which were subjected to electrochemical doping by a variety of electrolytes and ionic species such as Li$^{+}$, ClO$_{4}^{-}$, Cs$^{+}$, Ca$^{2+}$, etc.   

Electronic detection of phase transition of adsorbed water on graphene

Graphene is sensitive to overlayers on its surface through charge transfer and also through changes in dielectric constant, which can alter the scattering by static disorder in graphene. We performed transport measurement on graphene devices \textit{in situ} in ultra-high vacuum at low temperature. Water vapor is introduced to the chamber and is adsorbed on graphene at low temperature. After deposition of a few monolayers of water, the resistivity of graphene at fixed gate voltages is measured as the temperature is raised from 40K to room temperature. Sharp features in the temperature-dependent resistivity mark reproducible, irreversible (upon re-cooling) changes in the sample which we interpret as phase transitions in the adsorbed water overlayer, likely corresponding to dewetting and desorption. This work has been supported by the University of Maryland NSF-MRSEC under Grant No. DMR 05-20471 with supplemental funding from NRI.   

\textit{GW} study of the effect of various defects on the band gap of fluorographene

Recently synthesized fluorographene, fully fluorinated graphene in a chair configuration, is a wide band-gap ($E_{g})$ semiconductor with an experimental optical band gap of $\sim$3 eV. However, first-principles calculations have shown that pristine fluorographene should have E$_{g}$ of 5.4 to 7.5 eV. To explain this discrepancy, we have studied the effect of F vacancies, a Stone-Wales (SW) defect, C single vacancies and C double vacancies on E$_{g}$ of fluorographene using density functional theory and the \textit{GW} approximation. F vacancies and a SW defect are not likely to affect $E_{g}$ of fluorographene, whereas a C single vacancy with a doubly fluorinated C atom, a C double vacancy, and a C double vacancy with two doubly fluorinated C atoms lead to a \textit{GW} band gap of $\sim$4 eV, which is consistent with the optically measured E$_{g}$, and they are energetically more favorable than other C vacancies at a wide range of chemical potential of F ($\mu_{F})$.   

Controlled electrochemical functionalization of epitaxial graphene

Chemical functionalization is a promising means of modifying graphene for applications ranging from nanoelectronics to transparent electrodes. Various schemes have been demonstrated, but control over functionalization density with well-specified molecules is still a challenge. We report on the controlled electrochemical functionalization of epitaxial graphene with trifluoromethylphenylene (CF$_3$Ph), where the functionalization density was controlled by the electron injection rate. CF$_3$Ph peaks were observed in x-ray photoemission spectroscopy, along with binding energy shifts consistent with bonding between CF$_3$Ph and graphene. A maximum functionalization density of one molecule per six graphene carbons was inferred from the peak intensities. Spectroscopic Raman mapping revealed increasing graphene D:G peak intensity ratios that scaled with increasing functionalization-induced localized defects. While a minimal shift in the $\pi$ orbital structure and the emergence of CF$_3$Ph related features were observed in ultraviolet photoemission spectroscopy, a work function increase by $0.5$ eV in CF$_3$Ph-graphene suggests a shift of the electron distribution towards the CF$_3$ moieties on the surface. This work has positive implications for transparent electrode applications.   

Adsorption of NH$_{2}$ on Graphene in the Presence of Primary Defects

The primary amine, NH$_{2}$, is of interest as a linker between graphene and organic molecules in novel biotechnologies using graphene platforms. We are using \textit{ab initio} electronic structure calculations to study NH$_{2}$ adsorption on graphene. We find that the adsorption energy on pristine graphene is on the order of 0.778 eV, a relatively weak bond. We are interested in situations in which the bonding of NH$_{2}$ is stronger and are studying systems in which NH$_{2}$ adsorbs near defects. We find the adsorption energy of a NH$_{2}$ molecule near a second NH$_{2}$ molecule is as high as 1.037 eV and that the adsorption near a substitutional N atom is 1.063 eV. We find that there is a RKKY-like interaction between the adsorbate molecules in the case of two NH$_{2}$. We will also give results for NH$_{2}$ adsorption near other defects.   

Influence of subsurface hydrogen on the properties of single layer graphene grown on Ru(0001)

Graphene has aroused tremendous interest due to its remarkable electronic and mechanical properties. The lack of a band-gap, however, causes a serious challenge for implementing graphene as a material for electrical switches and therefore creative ways of inducing this band-gap are needed. We will present a STM/LEED/DFT study of the single layer graphene on Ru(0001) system in the presence of hydrogen. Structural studies show arrays of Moire superlattices with sizes ranging from 0.9 to 3.0 nm in the presence of hydrogen on the compact surface of ruthenium. First principle calculations help explain the appearance of these arrays of graphene reconstructions driven by the H presence at the Ru(0001) interface, and furthermore, predict the appearance of a bandgap with values correlated with the Moire superstructure sizes in the presence of hydrogen.   

Graphene symmetry-breaking with molecular adsorbates: modeling and experiment

Graphene's structure and electronic properties provide a framework for understanding molecule-substrate interactions and developing techniques for band gap engineering. Controlled deposition of molecular adsorbates can create superlattices which break the degeneracy of graphene's two-atom unit cell, opening a band gap. We simulate scanning tunneling microscopy and spectroscopy measurements for a variety of organic molecule/graphene systems, including pyridine, trimesic acid, and isonicotinic acid, based on density functional theory calculations using VASP. We also compare our simulations to ultra-high vacuum STM and STS results.   

Overcoming Doping Difficulty in Graphene via Substrate

Controlling the type and density of charge carriers by doping is the key step for developing graphene electronics. However, direct doping of graphene is rather challenge. Using first-principles method we find that doping could be strongly enhanced in epitaxial graphene grown on silicon carbide substrate. Compared to free-standing graphene, the formation energies of the dopants can decrease by as much as 8 eV. The type and density of the charge carriers of epitaxial graphene layer can be effectively manipulated by suitable dopants and surface passivation. More importantly, contrasting to the direct doping of graphene, the charge carriers in epitaxial graphene layer are weakly scattered by dopants due to the spatial separation between dopants and conducting channel.   

Calculations of double-sided coverage of transition metals on graphene

We study the properties of transition metal atoms adsorbed in high coverage on graphene using first principles density functional theory. While there have been many studies on single-sided coverage of adatoms on graphene, we focus on coverage of both sides of graphene. We have observed systems with significantly stronger binding with double-sided coverage than systems with only single-sided coverage. We discuss the effect of double-sided coverage on the electronic structure of these systems.   

Excited Carriers Relaxation and Hydrogen Dissociation on Hydrogenated Graphene: A Theory

Using \textit{ab initio} molecular dynamics coupled with time-dependent density functional theory (TDDFT), we show that the energy transfer of photo-excited carriers into atomic kinetic energy on hydrogenated graphene depends sensitively on the surface H coverage. Here, the energy transfer rate plays a crucial role in the determination of the H dissociation dynamics from graphene. In the low density ``isolated'' hydrogen atom limit, the energy transfer is significantly suppressed 80 fs after the excitation. Thus, it is difficult to dissociate hydrogen due to the faster energy dissipation from H into carbon backbone, despite that initially the H kinetic energy had increased to around 1.5 eV and the C-H bondlength had starched to 2.4 {\AA}. In sharp contrast, at the high-density graphane limit, an efficient energy transfer channel is established when the C-H bondlength exceeds 1.4 {\AA}. A fraction of the H readily dissociates within 15 fs. This is because ionized H forms a charged layer that expels, and as such accelerates the H ions with higher initial thermal velocities flying away. Our study thus reveals the importance of performing TDDFT calculations for excited carrier dynamics as from the widely adopted ground-state or constrained DFT dynamics one would expect the C-H bonds in graphane to be significantly stronger, due to full surface passivation, than that of isolated H.   

Theory of hydrogen induced giant spin-orbit coupling in graphene

Adatoms seem for now the most perspective way of increasing and controlling spin-orbit coupling in graphene. Hydrogen in articular is a role representative, as it gives both lattice deformation and covalent bonding, both contributing towards sigma-pi hybridization needed for the increase of the spin-orbit coupling around K. To establish the relevant physics of the H induced spin-orbit coupling in graphene, we have performed systematic calculations of hydrogenated graphene. We found that the magnitude of the coupling is of the order of meVs, exactly what is needed to explain the experimental data on spin relaxation. Doing both first-principal calculations and tight-binding modeling we calculate the spin-orbit splittings and introduce an effective hopping model that can be used in realistic investigations.   

Understanding electron-phonon interactions in doped graphene: the case of Li-intercalated graphite

The recent explosion of research on doped graphene systems together with the discovery of superconductivity in CaC$_6$ has reignited the interest in graphite intercalation compounds (GICs). While it is generally agreed that the superconductivity observed in GICs is BCS-like, there is still much controversy over which electrons and which phonons are primarily involved in the electron-phonon (e-ph) coupling leading to superconductivity. Moreover, thanks to the close similarity between the electronic structure of GICs and doped graphene, the study of e-ph interactions in GICs provides a unique approach to help elucidate the complex e-ph interactions in doped graphitic systems. We present inelastic x-ray scattering measurements of the high energy ($\sim$ 200 meV) graphitic phonons in LiC$_6$ across the Brillouin zone. The LiC$_6$ phonons are much softer than in pure graphite, as the electron doping destabilizes the C-C bonds. We observe large phonon broadening for all phonons at the graphite Brillouin zone center, suggestive of unusual e-ph interaction phenomena. We discuss our results in the light of the e-ph coupling reported from angle-resolved photoemission spectroscopy and in relation to strong non-adiabatic effects observed using Raman scattering.   

p-n codoping induced improvement of adsorption, magnetism, and electronic structure in 3d transition metal adatoms on graphene

Generating ferromagnetism and preserving its unique properties in graphene are crucial to the development of graphene-based spintronics. Using first-principles calculations, we investigate the effects of p-n codoping method on absorption, magnetic properties, and of electronic structures 3d transition metal adatoms (TMs, i.e., Fe, Co, and Ni) on graphene. It is found that p-n codoping can strengthen the adsorption of TMs on graphene, and enhance the magnetic moments of Fe and Co adatoms on graphene. It can also cause Ni to transition from nonmagnetic to magnetic states. Furthermore, magnetic coupling between two p-n pairs is also explored. Electronic structure analysis indicates that p-type dopant turns graphene into an electron-deficient system, and compensates for the shift in Fermi level caused by adsorption of TMs. Therefore, p-n codoping can bring about increases in the magnetic moment and adsorption of TM-adsorbed graphene systems while preserving the unique properties of graphene.   

Directed Assembly of Molecules on Graphene/Ru(0001)

Recently, the graphene monolayers have been seen to adopt a superstructure - moir\'e pattern - on Ru(0001). By using low temperature scanning tunneling spectroscopy, we identified the laterally localized electronic states on this system. The individual states are separated by 3 nm and comprise regions of about 90 carbon atoms. This constitutes a highly regular quantum dot-array with molecular precision. It is evidenced by quantum well resonances with energies that relate to the corrugation of the graphene layer. By using scanning tunneling microscopy/spectroscopy, we demonstrate the selective adsorption and formation of ordered molecular arrays of FePc and pentacene molecules on the graphene/Ru(0001) templates. With in-depth investigations of the molecular adsorption and assembly processes we reveal the existence lateral electric dipoles in the epitaxial graphene monolayers and the capability of the dipoles in directing and driving the molecular adsorption and assembly. When increasing the molecular coverage, we observed the formation of regular Kagome lattices that duplicate the lattice of the moir\'e pattern of monolayer graphene.   

Study on the Adsorption of Small Gas Molecules on graphene by the Density Functional Theory Calculations

The absorption of different small gas molecules on graphene is investigated based on the pseudopotential method within the density functional theory formalism. The preferred adsorption site (among the top, bridge, and hollow positions) and orientations of these molecules on the graphene surface are analyzed and the related adsorption energies are calculated. The charge transfer between the absorpted molecules and the graphene is discussed as well.