Session U6: Focus Session: Graphene - Intercalation, Doping, Characterization

Analysis of the intercalation of oxygen at the Ru(0001)-Graphene interface

The process whereby oxygen intercalates at the Ru(0001)-Graphene interface, resulting in systematic electronic decoupling of the graphene layer from the metallic substrate, depends on the interplay between graphene adhesion on the surface and the oxygen adsorption energy. We use density functional theory based calculations, including the effect of van der Waals interaction, to compare the energetics of competing phases in this process. We report three key findings. First, the van der Waals interaction makes a significant contribution to the binding of graphene to Ru(0001). Second, we assess the thermodynamic driving force between uniform oxygen phases on the clean surface and those intercalated at the interface. Third, we consider a series of local 1x1 oxygen patches centered on the raised region of the Ru(0001)-Graphene moir\'e which illustrate a series of stages in the decoupling of graphene from the Ru(0001) surface.   

Optical conductivity in bromine-intercalated graphite

Graphite intercalation compounds have a long and interesting history, with surprising thermal, electrical, and magnetic properties. In this study highly oriented pyrolytic graphite (HOPG) samples were exposed to bromine vapor for times between 20 and 100 minutes. The reflectance was measured using FTIR spectrophotometer, in the~far and mid infrared~at temperatures between~10 K and 300 K. With increasing the bromination time the reflectance in infrared region increases significantly, that gives rise to the increase of optical conductivity of the material calculated by Kramers-Kronig technique. The variation of scattering rate and charge carrier density in different temperatures for different intercalation times can lead to better understanding of the drastic enhancement of electrical conductivity in the material.   

Rb-intercalated bilayer graphene studied by high-resolution ARPES

To elucidate the electronic structure at the thinnest limit of the graphite intercalation compound (GIC) C$_8$Rb, we have performed high-resolution angle-resolved photoemission spectroscopy (ARPES) and low-energy electron diffraction (LEED) on Rb-intercalated bilayer graphene fabricated by in-situ evaporation of Rb atoms onto graphene grown epitaxially on SiC. Using LEED, the creation of an intercalated layer with in-plane geometry identical to bulk GICs was confirmed by the observation of a 2x2 spot pattern consistent with Rb intercalation. From ARPES measurement, we found that the Dirac point is at a binding energy of approximately 1 eV, compared to 0.4 eV in pristine epitaxial graphene on SiC [1]. The Fermi surface of this material was also measured. The critical differences between C$_8$Rb, its sister compound C$_8$K, and pristine bilayer graphene will be examined herein.\\[4pt] [1] T. Ohta et al, Science 313, 951-4 (2006).   

First principles study of Stage-1 graphene intercalates, IBr and ICl

In this study we examine, from a first-principles approach, the properties of 2 graphene intercalant systems namely, iodine monochloride (ICl-GIC) and iodine monobromide (IBr-GIC). These materials are being explored as possible interlayer dielectric candidates for 2D-to 2D-tunnel FETs (TFETs) and Bilayer pseudospin FETs (BiSFETs). To do so we employ density functional theory (DFT). Both these intercalants are stage-1 and acceptor type. We first put forth a structural description of these compounds that intercalate 2 successive layers of graphene, stacked AA type as obtained upon relaxation. Subsequently we describe the electronic structure of ICl-GIC and IBr-GIC and use it to predict the device suitability of these intercalants. It is seen that adding a layer of these GIC's to a single layer of graphene does not disturb graphene electronic spectra except for opening a small gap and introducing doping. With the second graphene layer added, coupling between the graphene layers becomes evident through a small amount of band splitting.   

Charge Density Waves on the Graphene Sheets of the Heavily-Doped Superconductor Graphitic Intercalate CaC$_6$

The electronic properties of graphitic materials can be readily tuned by adding charge carriers, and high levels of doping can even lead to superconductivity. We used scanning tunnelling microscopy to investigate the graphene-terminated surface of the superconducting graphitic material CaC$_6$ at temperatures well above T$_c$=11.5K [1]. We find two distinct surface types that show atomic resolution: one exhibits the expected structure of a graphene lattice superimposed on a hexagonal Ca superlattice while the other has stripes with a period three times that of the underlying Ca superlattice. A periodic distortion was found in the Ca atoms matching the periodicity of the electronic contrast on the graphene sheet, though no displacements of the carbon lattice were detected. Spectroscopic measurements reveal an energy gap in the electronic structure that can be directly associated with the stripe periodicity. This provides strong evidence that the stripes correspond to a charge density wave (CDW) in a graphitic system that also superconducts at lower temperatures, offering an excellent test bed for studying the relationship between these two important phenomena. [1] K.C. Rahnejat et al., Nat. Commun. 2, 558 (2011).   

Phonon-mediated superconductivity in graphene by lithium deposition

Graphene is the physical realization of many fundamental concepts and phenomena in solid-state physics. However, in the list of graphene's many remarkable properties, superconductivity is notably absent. If it were possible to find a way to induce superconductivity, it could improve the performance and enable more efficient integration of a variety of promising device concepts. To this end, we explore, by first-principles DFT calculations, the possibility of inducing superconductivity in a graphene sheet by doping its surface with alkaline metal adatoms [1], in a manner analogous to which superconductivity is induced in graphite intercalated compounds (GICs). As for GICs, we find that the electrical characteristics of graphene are sensitive to the species of adatom used. However, unlike GICs, we find that lithium atoms should induce superconductivity in graphene at a higher temperature than calcium.\\[4pt] [1] G. Profeta, M. Calandra, F. Mauri, Nature Physics 8, 131-134 (2012)   

Si on epitaxial graphene on SiC: intercalation and graphene-SiC transformation

The interface between epitaxial graphene and bulk SiC plays a dominant role in both the growth and transport properties of graphene on SiC. The differences in diffusion of Si through graphene on the two polar SiC surfaces is related to the different nucleation of Si diffusion channels on the two graphene-SiC interfaces. In this work we use LEEM, XPEEM and XPS to study how the excess Si at the graphene-vacuum interface reorders itself at high temperatures. We show that silicon deposited at room temperature onto multilayer graphene films grown on the SiC(000$\bar{1}$) surface rapidly diffuses to the graphene-SiC interface when heated to temperatures above 1020 $^{\circ}$C. The Si that does intercalate into the interface can be removed back out to the graphene-vacuum boundary by heating the sample to 1200 $^{\circ}$C. Most of the Si evaporates at this temperature, however, a significant amount of Si reacts with the graphene at the vacuum interface and form a relative stable reconstructed ($2\times2$) SiC structure. At significantly higher Si concentrations, graphene at the vacuum interface transforms to SiC.   

Silicon Layer Intercalation and Interface Properties between Graphene and Metal hosts

Graphene is being considered as a contender as the reference material with extraordinary properties for a post-CMOS technology. The availability of high quality and large scale single crystal graphene is fundamental for it to fulfill its promise in electronic applications. Graphene is usually grown on a metallic substrate from which it has to be transferred before it can be used. However, uncontrolled shear and strain, associated with the transfer and the presence of extended domains, lead to unavoidable tearing, rendering it useless for scalable production. We propose a way to overcome this bottleneck and produce high quality, free standing graphene by intercalating Si in graphene epitaxially grown on metals, like Ru(0001) {\&} Ir(111). This G/Si/metal architecture, produced by the silicon-layer intercalation approach (SIA), was characterized by STM/STS, Raman, and angle resolved electron photoemission spectroscopy (ARPES) and proves the high structural and electronic qualities of the new composite. The SIA eliminates the need for the graphene transfer and also allows for an atomic control of the distance between the graphene and the metal. References: 1. Jinhai Mao, Yeliang Wang, H.-J. Gao, et al., Appl. Phys. Lett. 100, 093101 (2012) (Cover). 2. Lei Meng, Yeliang Wang, H.-J. Gao, et al., Appl. Phys. Lett. 100, 083101 (2012).   

Na induced changes in the electronic band structure of graphene grown on C-face SiC

Studies of the effects induced on the electron band structure after Na deposition, and subsequent heating, on a C-face 2 MLs graphene sample will be presented. Na deposition shifts the Dirac point downwards from the Fermi level by about 0.5 eV due to electron doping. After heating at temperatures from around 120 to 300\textordmasculine C, the $\pi $-band appears considerably broadened. Collected Si 2p and Na 2p spectra then indicate Na intercalation in between the graphene layers and at the graphene SiC interface. The broadening is therefore interpreted to arise from the presence of two slightly shifted, but not clearly resolved, $\pi $-bands. Constant energy photoelectron distribution patterns, E(kx,ky);s , extracted from the clean 2MLs graphene C-face sample look very similar to earlier calculated distribution patterns for monolayer, but not Bernal stacked bi-layer, graphene. After Na deposition the patterns extracted at energies below the Dirac point appear very similar so the doping had no pronounced effect on the shape or intensity distribution. At energies above the Dirac point the extracted angular distribution patterns show the flipped, ``mirrored,'' intensity distribution predicted for monolayer graphene at these energies. An additional weaker outer band is also discernable at energies above the Dirac point, which presumably is induced by the deposited Na.   

First-Principles Modeling of Low-Energy Electron Diffraction of Few Layer Graphene

We present calculations of the low-energy electron microscopy (LEEM) spectra of few layer graphene (FLG) systems using our newly developed theoretical approach based on density-functional theory (DFT). The traditional analysis using multiple scattering off muffin-tin potentials is replaced with a Bloch wave matching approach using self-consistent potentials via DFT to better describe the LEEM spectra, especially in the low energy range. Our calculated results for free-standing FLG exhibit oscillations in reflectivity for energies between 0 and 7 eV, in good agreement with the experimental LEEM spectra of FLG observed on various substrates. The number of oscillations is correlated to the number of graphene layers, a fact often used to determine the number of graphene layers in a sample region. We have calculated FLG on Ni(111)-(1x1) and find that the FLG features dominate those of the bare Ni(111) when two graphene layers are added, as seen in experiments. Our results show that the valleys in the LEEM spectra due to graphene appear only with more than one graphene layer, consistent with our results for free-standing FLG.   

Theory of low-energy electron reflectivity from graphene

We have developed a self-consistent description of low-energy electron reflectivity spectra, yielding results that compare well with experimental data for graphene on SiC and on Cu substrates (obtained by our group as well as by other groups [1]). Our approach utilizes wavefunctions for a thin multilayer graphene slab, computed with a first-principles method. By combining wavefunctions for positive and negative wavevectors, we forms states with only outgoing character on one side of the slab, and hence deduce the electron reflectivity. For free-standing n-layer graphene, we obtain the reflectivity curves that show n-1 reflectivity minima over the energy range 0 - 10 eV. The minima are shown to arise from states with wavefunctions localized between the graphene layers (not on the layers, as previously suggested [1]). For graphene on a substrate, we match the states on one side of the graphene slab to bulk states of the substrate. For graphene on Cu(111) substrates, we find the same set of reflectivity minima as for free-standing graphene, together with an additional minimum whose location varies with the graphene-Cu separation. Hence, this separation can be deduced by comparing experimental and theoretical spectra. [1] H. Hibino et al., Phys. Rev. B \underline {77}, 075413 (2008).   

Bandgap opening in bilayer graphene via molecular doping

We report the emergence of an electronic bandgap in bilayer graphene through the interaction with physisorbed molecules. The bandgap is found to scale linearly with induced carrier density though a slight asymmetry is found between n-type dopants where the bandgap varies as 47 meV/10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$ and p-type dopants where the bandgap varies as 38 meV/10$^{\mathrm{13}}$ cm$^{\mathrm{-2}}$. The n-type dopant molecules include tetrathiafulvalene (TTF), cobaltocene and decamethylcobaltocene (DMC) and p-type dopant molecules include NO$_{\mathrm{2}}$, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and 3,6-difluoro-2,5,7,7,8,8-hexacyano-quinodimethane (F2-HCNQ). Ammonia is found to be weak amphoteric dopant on bilayer graphene, as it is on single layer graphene, where the charge transfer depends on the orientation of the N atom relative to the upper graphene layer. The bandgap opening is explained in terms of the asymmetric charge distributions on the upper graphene layer which is in contact with the molecules. The high binding energy found upon adsorption of some of these molecules results in an attractive way to a permanent bandgap and when combined with a variable external electric field can either close the gap or widen it still further.   

Substrate Screening Effects in \textit{ab initio} Many-body Green's Function Calculations of Doped Graphene on SiC

Understanding many-electron interaction effects and the influence of the substrate in graphene-on-substrate systems is of great theoretical and practical interest. Thus far, both model Hamiltonian and ab initio GW calculations for the quasiparticle properties of such systems have employed crude models for the effect of the substrate, often approximating the complicated substrate dielectric matrix by a single constant. We develop a method in which the spatially-dependent dielectric matrix of the substrate (e.g., SiC) is incorporated into that of doped graphene to obtain an accurate total dielectric matrix. We present ab initio GW $+$ cumulant expansion calculations, showing that both the cumulant expansion (to include higher-order electron correlations) and a proper account of the substrate screening are needed to achieve agreement with features seen in ARPES. We discuss how this methodology could be used in other systems.   

Electronic Strengthening of Graphene by Charge Doping

Graphene is known as the strongest 2D material in nature, yet we show that moderate charge doping of either electrons or holes can further enhance its ideal strength by up to $\sim$17\%, based on first principles calculations. This unusual electronic enhancement, versus conventional structural enhancement, of material's strength is achieved by an intriguing physical mechanism of charge doping counteracting on strain induced enhancement of Kohn anomaly, which leads to an overall stiffening of zone boundary K$_{1}$ phonon mode whose softening under strain is responsible for graphene failure. Electrons and holes work in the same way due to the high electron-hole symmetry around the Dirac point of graphene, while over doping may weaken the graphene by softening other phonon modes. Our findings uncover another fascinating property of graphene with broad implications in graphene-based electromechanical devices.   

Incremental Tuning of Graphene's Fermi Level by Chemical Doping

We report a simple, scalable method for fine tuning the Fermi level of CVD-grown graphene, through controlled chemical doping by the addition of the polymer polyethyleneimine (PEI) to the graphene surface. Graphene samples initially showed $p$-type behavior before doping. By dropcasting a low concentration solution of PEI in methanol onto graphene, the hole concentration was lowered. Repeated applications to the same sample shift the Fermi level of the graphene through the Dirac point, yielding an increasingly $n$-type sample. The graphene mobility increases with each application of PEI solution due to charge screening effects. Additionally, the magnetoresistance becomes increasingly linear near the Dirac point, consistent with the existence of charge puddles in neutral graphene.