Session V20: Graphene Synthesis, Characterization and Applications

Local Conductance Variation in Graphene Devices: a Scanning Gate Microscope Study

Despite much works have been done on the geometric structures of ripples, defects and edge atoms in a graphene device, there has been no report showing the direct correlation between the structures and the transport property. Unlike scanning tunneling microscopy or other electron microscopes, Scanning Gate Microscope (SGM) is a unique microscopic tool with which the local electronic structure and the transport property of a device can be measured simultaneously. We have performed a transport measurement in nanometer scale using a scanning gate microscope. We have found the nanoscopic pictures of electron and hole puddles and the role of graphene- device edges in the transport measurements. These experimental findings were successfully explained with a theoretical model.   

Features of electrostatic landscape on graphene surfaces

Electronic inhomogeneities in graphenes on the substrate are important for understanding the limiting factors for future device applications. One can study the length scale and distribution of potentials on single or few layers graphene sheets with local probe methods. We employ scanning probe imaging techniques such as AFM and EFM in ambient conditions and scanning electron microscopy for correlating studies of topological and electrostatic variations in a variety of mechanically cleaved graphenes and chemically modified graphene sheets. Enhanced contrast in phase signal, that is tunable via tip-substrate bias, indicates variations in the potential distribution which differ form the topographic landscapes. The nature of these structures has yet to be explored. Authors would like to thank for graphene oxide samples provided by L. Cote, J. Huang (MSE, Northwestern University).   

Molecular Beam Deposition and Raman Characterization of Large Area Graphitic layers

We report on the use of a carbon Molecular Beam Deposition (MBD) method in an ultra high vacuum system for the fabrication of large area graphitic layers on nickel films. Elongated substrates are used for the growth of samples with a gradient in carbon thickness. After carbon deposition, the samples are annealed at temperatures from 800C to 1000C for 30 min and cooled down to RT. The graphitic film has been successfully transferred onto a transparent glass by wet etching of the Ni. Raman measurements of the graphitic layers on Ni and on glass show very similar spectra with clear G and D* resonances associated with high quality thin graphitic layers.   

Centimeter-scale, Highly-Ordered, Continuous Graphene on Metal Surface and Its Moir\'e Template Effect

The limited size and quality of the single- and multi-layer graphene synthesized with the existing methods hinders the investigation of the intrinsic physical properties of the graphene and realization of its potential applications. We will present a new method for synthesizing large scale single layer graphene by thermal annealing of ruthenium single crystal containing carbon. Low energy electron diffraction (LEED) and scanning tunneling microscopy (STM) indicate the graphene grows to as large as millimeter dimensions with good long-range order, continuity, and perfect crystallinity. Such high quality sample has allowed a single layer graphene to greatly modulate the thermoelectric potential and polarity of the system. Furthermore, the adsorption behavior of clusters and molecules using the graphene-based Moir\'e pattern will also be presented.   

Direct Chemical Vapor Deposition of Single and Few--Graphene Layers on Dielectric Surfaces

Direct deposition of graphene on various dielectric substrates is demonstrated using a single-step chemical vapor deposition process. Single and few-layer graphene is formed through surface catalytic decomposition of hydrocarbon precursors on thin copper films pre-deposited on dielectric substrates. The copper films de-wet and evaporate during or immediately after graphene growth, resulting in graphene deposition directly on the bare dielectric substrates. Scanning Raman mapping and spectroscopy, scanning electron microscopy, and atomic force microscopy confirm the presence of continuous graphene layers on tens of micron square metal-free areas. The revealed growth mechanism opens new opportunities for deposition of higher quality graphene films on dielectric materials.   

Photo-exfoliation of graphene from graphite: An {\em ab initio} study

Mass production of high-quality graphene monolayers is an essential prerequisite for producing graphene devices [1]. Flaws of current synthesis techniques, such as chemical exfoliation combined with sonication[2] are remaining contaminants, and CVD synthesis [3] suffers from the influence of the substrate [3]. The best quality graphene monolayers are still obtained using the scotch-tape technique [1], which is ill-suited for mass production. By performing {\em ab initio} TDDFT-MD calculations, we have identified irradiation by ultra-short laser pulses as a suitable technique to produce graphene monolayers by photo-exfoliating graphite without the above drawbacks. Our simulations indicate that exposing graphite to 800~nm laser pulses with a suitable pulse shape and intensity may cause detachment of single graphene layers due to a non-equilibrium charge redistribution in a vibrationally cold substrate [4] \newline [1] K. S. Novoselov {\em et al.}, Science {\bf 306}, 666 (2004), and supporting mat. \newline [2] X. Li {\em et al.}, Science {\bf 319}, 1229 (2008). \newline [3] A. Reina {\em et al.}, Nano Lett. {\bf 9}, 30 (2008); X. Li {\em et al.}, Science {\bf 324}, 1312 (2009). \newline [4] Y. Miyamoto, H. Zhang, and D. Tom\'anek, submitted.   

Suspending graphene using a quadrupole ion trap

Creating a technology to suspend, manipulate, and probe an isolated graphene layer in vacuum may have applications ranging from new growth and fabrication strategies to fundamental measurements of these materials when they are completely uncoupled from a substrate. A sufficiently charged piece of graphene will remain flat due to electrostatic repulsion and can be confined using ac electric fields in a quadrupole trap. I will describe a trap built for this purpose which uses a design borrowed from atomic physics that is optimized for easy optical and physical access to the trapped particle [1]. Charged particles are injected into the trap using electrospray emission [2] of graphene suspended in organic solvents[3] and are probed with a 532 nm laser. I will provide details of the trap design and present preliminary data on characterization of the trapped graphene particles. \\[4pt] [1] Maiwald \textit{et al}., Nature Physics \textbf{5}, 551 (2009). \\[0pt] [2] Pearson \textit{et al}., Phys. Rev. A \textbf{73}, 032307 (2006). \\[0pt] [3] Hernandez \textit{et al}., Nature Nanotechnology \textbf{3}, 563 (2008).   

Catalytic selectivity of Transition Metal Functionalized 2-Dimensional Graphene

Energy production is expected to be one of the most significant challenges in the 21$^{st}$ century. Therefore it is important to develop and explore routes that reduce the dependence on fossil fuels and ensure energy security for future generations. We have performed DFT calculation of binding of small molecules on graphene sheets as mediated by chemisorbed transition metals (TM). The preliminary results show that molecules like N$_{2}$, CO and O$_{2}$ form stable bonds with the TM. In contrast, H$_{2}$O$_{2}$ decomposes and forms a stable TM-(OH)$_{2}$ complex. These two different behaviors may explain selectivity of some carbon based catalytic materials toward O$_{2}$. In either case the charge density is affected only locally and hence electronic transport properties are likely dominated by the underlying carbon structure. However, the results also support the notion that the use of the 6 -- 8 orders of magnitude more abundant 3$^{rd}$ row TM's in energy generation may form a viable alternative to expensive and comparatively rare transition elements such as platinum.   

Complex evolution of the electronic structure from polycrystalline to monocrystalline graphene: generation of a new Dirac point

First calculations, employed to address the properties of polycrystalline graphene, indicate that the electronic structure of tilt grain boundaries in this system [1-4] displays a rather complex evolution towards graphene bulk, as the tilt angle decreases, with the generation of a new Dirac point at the Fermi level, and an anisotropic Dirac cone of low energy excitations. Moreover, the usual Dirac point at the {\bf K} point falls below the Fermi level, and rises towards it as the tilt angle decreases. Further, our calculations indicate that the grain-boundary formation energy behaves non-monotonically with the tilt angle, due to a change in the the spatial distribution and relative contributions of the bond-stretching and bond-bending deformations associated with the formation of the defect.\\[4pt] [1] L.~B.~Biedermann {\textit et al.}, Phys.~Rev.~B {\bf 79}, 125411 (2009). \\[0pt] [2] S.~S.~Datta {\textit et al.}, Nanoletters {\bf 9}, 7 (2009). \\[0pt] [3] P.~Simonis {\textit et al.}, Surf.~Sci. {\bf 511}, 319 (2002). \\[0pt] [4] G.~Gu {\textit et al.}, Appl.~Phys.~Lett. {\bf 90}, 253507 (2007).   

Thermal Properties of Graphene Sheet Composites

Recent reports about extremely high values for the thermal conductivity of single-layer graphene sheets (GS) that outperform carbon nanotubes (CNT) in heat conduction give rise to the expectation that GS nano-composites could be able to make the unfulfilled promise of CNT composites a reality. The presence of resistance to the transfer of heat at the inclusion- matrix interface, known as the Kapitza resistance, is the reason for achieving worse results than anticipated with CNT composites. In this work, we investigate the effective conductivity of GS composites by means of off-lattice Monte- Carlo algorithms. This method is more efficient than conventional algorithms and faster than molecular dynamics. We will present the methodology, which is used to study the effects of GS orientation, dispersion and volume fraction on the effective thermal conductivity of the GS composites. The discussion will include a comparison between theoretical predictions of the value of the thermal resistance at the GS- polymer interface relative to the CNT-polymer interface based on the acoustic mismatch theory, and a comparison between CNT composites and GS composites with similar volume fraction and similar dispersion pattern of the nano-inclusions in the composite matrix.   

Non-adiabatic Interactions in Reduced Graphene Oxide

Thermal reduction of Graphene Oxide (GO) has been studied using infrared absorption spectroscopy to evaluate the reduction pathways, including oxygen and carbon removal. GO is an insulator due to the presence of various oxygen species. Thermal reduction of GO leads to oxygen incorporation in the basal plane at the edges. Upon annealing to 850\r{ }C in vacuum, a strong IR band is observed at $\sim $800 cm$^{-1}$, assigned to the remaining ether oxygen ($\sim $5-8 at{\%}) located at the edges. The strength of its C-O-C asymmetric stretch is due the modulation of extended electronic states associated with the presence of oxygen in the vicinity of the Fermi level. This strong non-adiabatic interaction leads to intensity enhancement of 10-100 times larger than purely vibrational modes.   

Graphitic Switches

Four-probe dc current-voltage (I-V) characteristics were measured for 80-250 nm thick graphite samples with the lateral size ranging between $\sim $ 30 microns and 0.5 mm. All studied samples possess break junctions made by means of mechanical deformation or using focused ion beam (FIB). The measurements were performed in the temperature interval 2 K $\le $ T $\le $ 300 K and applied magnetic field up to 9 T. Nonlinear (I $\sim $ V$^{n}$, n $>$ 1) and hysteretic I-V curves were recorded even at T = 300 K. The results revealed the switching effect in I-V curves at applied current of a few microamps that could be reduced further by magnetic field. The results indicate that graphite is a promising material for switches that can be used in memory devices.   

Gas Adsorption Properties of Graphene-Oxide-Frameworks and Nanoporous Benzene-Boronic Acid Polymers

There has been a recent resurgence in graphene oxide research as a potential route to large scale graphene synthesis. Recent research has also used dehydration reactions of boronic acids for the formation of covalent organic frameworks (COFs) and other new nanoporous materials. We are trying to synthesize graphene-oxide-frameworks (GOFs) by linking the OH groups on graphene oxide with benzene-boronic acids. Our initial x-ray studies indicate that the benzene-boronic acids are successfully incorporated into graphene-oxide (GO) layers expanding the interlayer spacing up to 12 Ang. We also found that the amorphous phases of bare dehydrated benzene-boronic acid polymers (amorphous borocarbons, ABCs) show quite interesting and unusual hydrogen adsorption behavior. The diffusion of hydrogen into the sample is thermally activated. While there is no adsorption at 30 K, the rate of excess adsorption increases with increasing temperature up to 70 K. We will present detailed high-pressure isotherms of H2/CO2/Methane at different temperatures of these interesting new GOF materials and dehydrated boronic acid polymers.   

The Electronic Structure on Two-Dimensional Graphane from Ab Initio Theory

Using density functional theory (DFT) we have investigated the electronic structure of the intrinsic two-dimensional graphane and fluorine-substituted graphane. We found the conduction band minimum of graphane has nearly free electron (NFE) properties similar, but in some respects different, than the recently calculated image potential states of graphene and superatom states of fullerenes.\footnote{V. M. Silkin, J. Zhao, F. Guinea, E. V. Chulkov, P. M. Echenique, and H. Petek, Phys. Rev. B \textbf{80}, 121408 (2009)}$^,$\footnote{M. Feng, J. Zhao, and H. Petek, Science \textbf{320}, 359 (2008)} The electronic structure of fluorine-substituted graphane is very sensitive to the doping configuration. The band gap of fluorine doped graphane can be tuned by different doping functionalization. Our study gives new insights into engineering of the electronic structure of two-dimensional semicoductors by surface chemical functionalizaton.   

Angular momentum and pseudospin in graphene

Any quantum mechanical problem with two levels can be treated by analogy with the spin 1/2 system. For instance, treating the proton and neutron as `up' and `down' states of the same particle leads to the fruitful concept of nuclear isospin. Because the honeycomb structure of graphene has two inequivalent atoms per unit cell, a pseudospin variable arises in the solution of the graphene Hamiltonian. This variable is commonly thought to be analogous to a spin 1/2 angular momentum. Unlike isospin, however, pseudospin is intimately connected to rotations in real space. Furthermore, graphene electrons couple to the electromagnetic field through their pseudospin: the pseudospin flip seen in electron-hole recombination creates a spin-1 photon. The natural conclusion is that the pseudospin is a real angular momentum. Implications of this identification for condensed matter and particle physics will be discussed.