Session W12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Exfoliation and Doping

Graphene Carrier Control and Band Gap Formation through Stacked Graphene Sheets

Graphene's use in RF transistors and frequency doublers is attractive since its high mobility and high saturation velocity translate into operation at high frequencies while utilizing little power. However, further graphene development for device applications is hindered by high metal contact resistance, poor control of channel conductivity, and the absence of a band gap. In this talk, I will present our efforts at NRL to address these challenges using two strategies: 1) substitutional insertion of group III-V atoms into graphene's lattice to control the carriers and 2) through a synthetic means to create bilayer graphene with a band gap. Substitutional incorporation of atoms into graphene can result in doping, if their concentration does not drastically affect the $\pi$-network. Using selective oxidation to remove C atoms from the graphene lattice, we are able to backfill the C vacancies using molecular beam deposition of dopants with controllable ultra-low fluxes. We will show that boron and phosphorus dopants can provide extra holes and electrons to the graphene $\pi$-network, respectively, modifying the carrier concentration in transport measurements. Bernal-stacked graphene bilayers have a relatively small band gap ($\sim$few meV). However, if the symmetry of the system is broken by the application of a large applied electric field, the band gap can be increased ($\sim$250 meV). Alternatively, we find it is possible to obtain such large built-in electric fields when graphene sheets of opposite doping are stacked. By bonding a p-type, CVD-grown graphene monolayer transferred from Cu to an n-type, epitaxially grown graphene monolayer on SiC, we formed a p-n graphene bilayer. Transport measurements and modeling of the resulting electric field generated by opposite doping of the graphene sheets indicate the creation of a 100-300 meV band gap in the synthetic bilayer.   

Atomic characterization of monolayer doped graphene sheets synthesized by chemical vapor deposition

Large-area, high-quality monolayer nitrogen (or boron)-doped graphene sheets were synthesized on copper foils by a modified chemical vapor deposition (CVD) apparatus. As-grown graphene sheets could be easily transferred from copper foils onto different substrates (e.g. silicon/silicon dioxide wafers). Compared with pristine graphene, nitrogen (or boron)-doped graphene shows strong D-band caused by doping and structural defects formed within the lattice. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the defects in the doped graphene samples arrange in different geometrical configurations. The localized states in the valence and conduction bands are in accordance with the type of dopant and bonding type. These experimental results are in agreement with first principles calculations of LDOS of doped graphene and STM image simulations.   

Graphene melting by molecular dynamics simulations

Mechanisms of melting of graphene were studied by molecular dynamics (MD) using two different interatomic potentials: the Reactive Empirical Bond Order (REBO) and recently developed Screened Environment Dependent (SED) -REBO potentials. Melting was investigated in two-dimensional (2-D) and three-dimensional (3-D) coordinate space. It was shown that the loss of long-range order and melting proceeds through generation and in-plane aggregation of Stone-Wales (S-W) defects in REBO-graphene, followed by the formation of a complex 3D network of carbon chains. Although S-W defects are also formed in the SED-REBO-graphene, they do not cluster. Instead, the melting proceeds through the formation of dangling bonds and vacancies. The melting temperature of graphene using REBO was found to be 5,200 K, whereas for SED-REBO it is lower by $\sim $800K. The melting in 2-D occurs at higher temperatures compared to 3-D because of in-plane geometrical constraints.   

Unexpected Structures for Intercalation of Sodium in Epitaxial SiC-Graphene Interfaces

We show using scanning tunneling microscopy and spectroscopy and calculations from first principles that several different intercalation structures exist for Na in epitaxial graphene on SiC(0001). Intercalation takes place rapidly at room temperature and tunneling spectroscopy shows that it significantly electron dopes the graphene. Upon annealing above room temperature a quite different intercalation structure is formed which removes the carbon-rich interface layer and transforms this into a second graphene layer. In addition, we find that direct deposition of Na onto the carbon rich buffer layer graphene precursor decouples it from the SiC substrate leading to formation of a new sheet of graphene. This interface-layer decoupling is unambiguously demonstrated by transforming bare buffer layer to a graphene layer. Our observations show that intercalation in graphene is fundamentally different than in graphite and provides a very versatile approach to metal-graphene functionality and electronic-property control.   

Electron phonon renormalization in N-doped graphene

Current research efforts are aimed at controlling the electronic properties graphene sheets using electron (or hole) doping for successful device fabrication. The presence of strong coupling between electronic and vibrational properties in graphene greatly assists Raman spectroscopy in probing the dopant-induced electronic energy changes. Previously, Raman spectroscopy was employed as a tool to probe the electron and phonon renormalization in doped single-walled carbon nanotubes (SWNT). It was found that the increase in electron velocity in?uences lattice vibrations locally near a negatively charged defect. These local renormalization effects were observed to result in a new effectively downshifted (up-shifted) Raman peak below the G' peak for n-doped (p-doped) SWNTs. In case of graphene, we find that the several Raman features for CVD grown N-doped graphene vary depending upon local dopant bonding environment. For instance, non-graphitic dopants (pyridinic, pyrrolic) were observed to result in highly intense D {\&} D'-band unlike the graphitic dopants. We explain these results in terms of the zig-zag (armchair) edges formed by graphitic (non-graphitic) bonding environment of the dopant.   

Transport properties of pristine and doped graphene

Graphene has attracted a lot of attention for various applications recently. Chemically exfoliated graphene is one of the best methods to prepare good quality and large amount of few-layer graphene sheets. We prepared pristine and doped graphene using chemical exfoliation through high energy tip sonication technique. The exfoliated graphene was later sintered for studying thermoelectric properties. The thermopower of these samples exhibits valleys, tentatively assigned to phonon drag, which shift towards higher temperature upon vacuum annealing and electron doping. Such a similar behavior was previously observed in doped carbon nanotubes. The effects of vaccum annealing and doping upon the fundamental behavior of thermoelectric power and thermal conduction of graphene will be presented.   

Experimental investigation of carrier quantum confinement in graphene quantum dots and nanoribbons

Recently, Zhu \textit{et al.} (Chem. Comm., 2011. \textbf{47}(24), 6858) and Pan \textit{et al}. (Adv. Mat., 2010. \textbf{22}(6), 734) have successfully synthesized graphene quantum dots (width $<$10 nm). They probed the carrier quantum confinement in such GQDs using photoluminescence spectroscopy (PL). However, a self-consistent explanation for the observed PL spectra is lacking. Interestingly, we find that the organic reducing agents used for synthesizing GQDs have PL signature similar to the GQDs themselves. Thus, deconvoluting solvent effects is extremely important to achieve further progress in synthesis and application of GQDs. We studied the PL behavior of hydrothermally synthesized graphene nanoribbons (GNRs) and GQDs to understand the solvent effects and the underlying mechanism for the observed PL. We will discuss the effects of size, shape and morphology on the PL behavior of GQDs and GNRs.   

Scanned Probe Measurements of Graphene on Ferroelectrics

We present results on scanned probe measurements of graphene and few-layer graphite (FLG) on ferroelectric thin films. The graphene was mechanically exfoliated onto the PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$ and other ferroelectric films, and its topography and polarization were characterized using atomic force microscopy, Kelvin Probe (surface potential) Force Microscopy, and Piezoresponse Force Microscopy. We discuss how graphene can be used as a top electrode for ferroelectric materials, as changing the potential of the graphene can cause the ferroelectric region beneath it to switch polarization. We demonstrate that the change in polarization is reversible. We also show how the surface potential of FLG on PZT depends on the number of layers of graphene. *The authors acknowledge Grant DMR-1124696, Sub-Award 235743-3668.   

Can Structured Mixed Solvents be Used for Graphene Exfoliation?

Using a combination of computational and experimental techniques we investigate graphene exfoliation and suspension in C6H6, C6F6 and their mixtures. Our MD simulations show that an equimolar mixture of C6H6/C6F6 has the highest affinity for graphene. This is manifested in the formation of translational and orientational order normal to the graphene surface, with no translational ordering parallel to the graphene surface. The solvent structure is driven by quadrupolar interactions and consists of stacks of alternating C6H6/C6F6 molecules rising from the surface of the graphene. These stacks give rise to density oscillations in registry with the graphene surface. The period of the density oscillations is on the order of 3.4 {\AA}, corresponding to the van der Waals diameter of carbon and this ordered structure extends 30 {\AA} from each side of the graphene sheet. To experimentally verify the results of the molecular dynamics simulations we use dynamic contact angle measurements. These measurements demonstrate an increase in solvent affinity for HOPG in the case of 1:1 mixtures in comparison with pure components. The quality of the exfoliated material and flakes after sonication is verified by AFM, SEM, and TEM techniques. The graphene sheets produced in the equimolar mixture can be freeze-dried at room temperature, (T=300K) producing sponge-like graphene structures held together entirely by graphene sheet interactions and reflecting the structure of the graphene sheets in solution.   

Fabrication and Characterization of folded Graphene on Graphite

Graphene is one of the most studied carbon compounds due to its electronic properties. Applying an electric pulse between a Graphite surface and STM tip the upper layer of a Graphite surface could be folded due to mechanical and electrostatic forces forming voids, bend, move, and decouple or transfer part of the tip to the surface. Ab-initio electrostatic calculations and MD thermal simulations support our experimental method. Due to the different new discovered and studies physical-effects in Graphene edges, which can significantly influence the overall electronic and magnetic properties of Graphene nanostructures, this results may be exploited as an method to obtain different structures. Here we study Graphene flakes obtained with this Method. Folded Graphene's structure and electronic properties are studied to determine its degree of coupling to the graphite substrate. Cross-sectional analysis of the fold shown reveals that it consists of a single sheet of graphite folded. Graphene edges and folded line reveals new electronic and magnetics properties. Among others the possibility of a multi folded Graphene sheet system gains importance due to the opportunity of getting a Semi-Carbon-Nanotube-Graphene device.   

Controlling desorption of H, O, atoms and OH group from graphene by pulse laser

Possibilities of reduction of graphene oxide and one-side dehydrogenation of graphane (H-terminated graphene) with using ultra-short pulse ($\sim$2 fs) laser are discussed. We have performed molecular dynamics (MD) simulation of O-, H-, and OH-adsorbed graphene sheets induced by electronic excitation upon irradiation with laser pulse. The time-dependent density functional theory treating real-time propagation of electron wave functions combined with the Ehrenfest approximation for the MD was employed and FPSEID [1] code was used to check the energy conservation rule under dynamical external field [2]. We found asymmetric pulse shape as a function of time causes an efficient desorption of O atoms and OH groups from graphene which can be applicable for reduction of graphene oxide alternative to chemical and thermal treatment. Meanwhile, such asymmetric pulse shape is beneficial for one-side H-desorption from graphane that will trigger further structural changes such as spontaneous shrink/rippling or heterogeneous termination on side-by-side.\\[4pt] [1]O. Sugino, Y. Miyamoto, PRB{\bf 59}, 2579, (1999);B{\bf 66}, 089901(E) (2002)\\[0pt] [2]Y. Miyamoto, H. Zhang, PRB{\bf 77}, 165123 (2008)   

Graphene on Au-coated SiO$_{x}$ substrate: Its visibility and intrinsic core-level photoemission

With the motivation of precisely and intrinsically characterizing a exfoliate graphene using photoelectron spectroscopy, a conducting substrate having high optical contrast is greatly desired. Here, we demonstrate that exfoliated graphene can be optically visible on a thin 9-nm Au-coated SiO$_{x}$ substrate, and can be easily conducted into scanning photoelectron microscopy/spectroscopy (SPEM/S) studies. Because of the elimination of charging effect, precisely core-level characterization of exfoliated graphene is presented with different numbers of layers. Consequently, the usage of Au-coated SiO$_{x}$ substrate serves a simple but effective method to study pristine graphene by photoelectron spectroscopy and other electron-detection techniques.   

Stage-1 intercalation compounds of few graphene layers by anhydrous ferric chloride

Anhydrous ferric chloride (FeCl3) was used to intercalate few graphene layers into stage-1 intercalation compounds. The intercalant, staging, stability, and doping of the resulting intercalation compounds are characterized by Raman scattering. The G peak of pure stage-1 compounds upshifts to $\sim $1626 cm-1, which is similar to that of heavily-doped monolayer graphenes by 18M sulfuric acid. A single Lorentzian line shape for the 2D band of stage-1 compounds were observed, which indicates that each layer behaves as a decoupled heavily doped monolayer. By performing Raman measurements at different excitation energies, we show that, for a given doping level, the variation of the 2D intensity relative to the G peak with excitation energy allows one to assess the Fermi energy. This allows us to estimate a Fermi level shift of up to $\sim $0.85 eV, which agrees well with that estimated from the 2D/G intensity ratio and is close to $\sim $0.9 eV measured in stage-1 GICs by electron energy loss spectroscopy. The stage-1 intercalation compound of few graphene layers is thus ideal test-beds for the physical and chemical properties of heavily doped graphenes.