GraphITA 2011

Frontmatter

Study of Graphene Growth Mechanism on Nickel Thin Films

Abstract

Since chemical vapor deposition of carbon-containing precursors onto transition metals tends to develop as the preferred growth process for the mass production of graphene films, the deep understanding of its mechanism becomes mandatory. In the case of nickel, which represents an economically viable catalytic substrate, the solubility of carbon is significant enough so that the growth mechanism proceeds in at least two steps: the dissolution of carbon in the metal followed by the precipitation of graphene at the surface. In this work, we use ion implantation to dissolve calibrated amounts of carbon in nickel thin films and grow graphene films by annealing. Observations of those graphene films using transmission electron microscopy , directly on the growth substrate as well as transfered on TEM grids, allowed us to precisely study the mechanisms that lead to their formation.

L. Baraton, Z. He, C. S. Lee, J. L. Maurice, C. S. Cojocaru, Y. H. Lee, D. Pribat

Elastic Moduli in Graphene Versus Hydrogen Coverage

Abstract

Through continuum elasticity we define a simulation protocol addressed to measure by a computational experiment the linear elastic moduli of hydrogenated graphene and we actually compute them by first principles. We argue that hydrogenation generally leads to a much smaller longitudinal extension upon loading than the one calculated for ideal graphene. Nevertheless, the corresponding Young modulus shows minor variations as function of coverage. Furthermore, we provide evidence that hydrogenation only marginally affects the Poisson ratio.

E. Cadelano, L. Colombo

Electrical Response of GO Gas Sensors

Abstract

In this paper we report a study of the electrical response to \({\rm NO}_{2},\,{\rm CO,\,H}_{2}\hbox{O}\) and \({\rm H}_{2}\) of a graphene oxide (GO) based gas sensor. The device has been operated in the temperature range 25–\(200^{\circ}\hbox{C}\) at different gases concentrations (1–200 ppm). Micro structural physical features of the GO sensing films were characterized by Raman and X-Ray Photoelectron Spectroscopy, and by Scanning Electron Microscopy. The GO based sensor has shown high sensitivity to \(\hbox{NO}_{2}\) (down to 1 ppm) at \(150^{\circ}\hbox{C}\) operating temperature, analogous to a p-type response mechanism of inorganic gas sensors. The \(\hbox{NO}_{2}\) adsorption/desorption has been found to be reversible, but with increasing desorption time when decreasing the operational temperature. Negligible response to CO, \(\hbox{H}_{2}\) and \(\hbox{H}_{2}\hbox{O}\) has been observed. The observed gas sensing performance of the GO based sensor is similar to the best one reported in literature for carbon nanotubes.

C. Cantalini, L. Giancaterini, E. Treossi, V. Palermo, F. Perrozzi, S. Santucci, L. Ottaviano

Spectral Properties of Optical Phonons in Bilayer Graphene

Abstract

Recent optical measurements in bilayer graphene have reported a strong dependence of a phonon peak intensity, as well of the asymmetric Fano lineshape, on the charge doping and on the band gap, tuned by gate voltage. In this paper we show how these features can be analyzed and predicted on a microscopic quantitative level using the charge-phonon theory applied to the specific case of graphene systems. We present a phase diagram where the infrared activity of both the symmetric (\(E_g\)) and antisymmetric (\(E_u\)) phonon modes is evaluated as a function of doping and gap, and we also show a switching mechanism can occur between these two modes as dominant channels in the optical response. The exploiting of the gate dependence of the phonon peak intensity and lineshape asymmetry in the optical conductivity provides thus a new suitable tool to characterize multilayer graphenes and to investigate the role of the underlying electron-lattice interaction.

E. Cappelluti, L. Benfatto, A. B. Kuzmenko

A New Wide Band Gap Form of Hydrogenated Graphene

Abstract

We propose a new form of partially hydrogenated graphene in which hydrogen atoms lay in para position to each other, forming a honeycomb-shaped superlattice. This arrangement is shown to be favored by progressive preferential sticking events, while its particular lattice symmetry guarantees the presence of a wide band gap. With the help of first principles DFT and many-body calculations we find this structure to be an insulator, similarly to graphane.

S. Casolo, G. F. Tantardini, R. Martinazzo

Tailoring the Electronic Structure of Epitaxial Graphene on SiC(0001): Transfer Doping and Hydrogen Intercalation

Abstract

Graphene grown on the (0001) basal plane of silicon carbide, i.e. on the SiC(0001) surface, is an extremely promising candidate for future nano-electronic applications. However, hurdles such as strong electron doping and low carrier mobility might sensibly limit the prospects of graphene on SiC(0001). In this work we present and discuss two different approaches that allow for a precise tailoring of the band-structure of graphene on SiC(0001): non-covalent functionalization of the graphene surface with a strong acceptor molecule, i.e. tetrafluorotetracyanoquinodimethane (F4-TCNQ), and passivation of the SiC interface via hydrogen intercalation. Both approaches effectively eliminate the intrinsic n-type doping in graphene and might have a positive impact in the charge carrier mobility. The molecular functionalization approach also leads to an enlargement of the band-gap of bilayer graphene to more than double of the original value. Hydrogen intercalation yields graphene layers decoupled from the SiC substrate and hence quasi-free standing. Furthermore, this work investigates a combination of the two approaches and demonstrates that quasi-free standing bilayer graphene can be hole doped by depositing F4-TCNQ.

C. Coletti, S. Forti, K. V. Emtsev, U. Starke

Interface Electronic Differences Between Epitaxial Graphene Systems Grown on the Si and the C Face of SiC

Abstract

We use the local density approximation of the density functional theory to perform a comparative analysis between the bonding interactions of the epitaxial graphene/SiC interface in the case of Si and C face growth [i.e. growth on the SiC(0001) and the \(\hbox{SiC}(000\bar{1})\) surfaces respectively]. We argue that when the SiC substrate below the graphene films reconstructs with no additional adatoms, the observed electronic differences are the outcome of an interplay between \(sp^2\) and \(sp^3\) hybridization of the interface atoms. We find a strong preferential disposition towards an \(sp^2\) hybridization for the case of the C face, whereas towards the \(sp^3\) scheme for the Si face. Notwithstanding purely quantitative, this mismatch is important and reflects the strength of the \(\pi\) bond in Si and C.

I. Deretzis, A. La Magna

Towards a Graphene-Based Quantum Interference Device

Abstract

We propose a new quantum interference device based on a graphene nanoring attached to two leads. A lateral gate voltage applied across the nanoring creates an electric field perpendicular to the current flow and shifts energy levels of the two arms of the ring. A charge carrier being injected from the source at a given energy couples therefore to different states of the arms. Those states can be in or out of phase at the drain, resulting in interference effects, which allow for a fine control of the current between the two leads by the gate voltage. We find also that electron transport depends on the type of edges (zigzag or armchair) of the nanoring and discuss the effects of edge imperfections on the performance of the quantum interference device.

J. Munárriz, A. V. Malyshev, F. Domínguez-Adame

High Field Quantum Hall Effect in Disordered Graphene Near the Dirac Point

Abstract

We investigate on the conductance properties of low mobility graphene in the quantum Hall regime at filling factor less than \({\textit v}=2.\) For this purpose, we compare the high-field longitudinal and Hall resistances of two graphene samples with different mobility. We show that the presence of “charge density puddles”, most probably due to charged impurities, particularly affect the fundamental high field electronic properties of graphene. In particular, the Hall resistance plateau at \(R_{XY}=h/2e^2\) is unstable and shows a non-monotonic behaviour when the system is driven close to the Dirac point. This phenomenon is ascribed to as Fermi level pinning in the Landau Level sub-bands of graphene, in the presence of disorder.

W. Escoffier, J. M. Poumirol, M. Amado, F. Rossella, A. Kumar, E. Diez, M. Goiran, V. Bellani, B. Raquet

Graphene Edge Structures: Folding, Scrolling, Tubing, Rippling and Twisting

Abstract

Conventional three-dimensional crystal lattices are terminated by surfaces, which can demonstrate complex rebonding and rehybridisation, localised strain and dislocation formation. Two-dimensional crystal lattices, of which graphene is the archetype, are terminated by lines. The additional available dimension at such interfaces opens up a range of new topological interface possibilities. We show that graphene sheet edges can adopt a range of topological distortions depending on their nature. Rehybridisation, local bond reordering, chemical functionalisation with bulky, charged, or multi-functional groups can lead to edge buckling to relieve strain, folding, rolling and even tube formation. We discuss the topological possibilities at a two-dimensional graphene edge, and under what circumstances we expect different edge topologies to occur. Density functional calculations are used to explore in more depth different graphene edge types.

V. V. Ivanovskaya, P. Wagner, A. Zobelli, I. Suarez-Martinez, A. Yaya, C. P. Ewels

Axial Deformation of Monolayer Graphene under Tension and Compression

Abstract

The mechanical response of single layer graphene is monitored by simultaneous Raman measurements through the shift of either the G or 2D optical phonons, for low levels of tensile and compressive strain. In tension, important physical phenomena such as the G and 2D band splitting are discussed. The results can be used to quantify the amount of uniaxial strain, providing a fundamental tool for graphene-based nanoelectronics. In compression, graphenes of atomic thickness embedded in plastic beams are found to exhibit remarkable high compression failure strains. The critical buckling strain for graphene appears to be dependent on the flake size and geometry with respect to the strain axis. It is shown that the embedded flakes can be treated as ideal plates and their behavior can be described by Euler mechanics.

K. Papagelis, O. Frank, G. Tsoukleri, J. Parthenios, K. Novoselov, C. Galiotis

Morphological and Structural Characterization of Graphene Grown by Thermal Decomposition of 4H-SiC (0001) and by C Segregation on Ni

Abstract

In this paper, we present a nanoscale morphological and structural characterization of few layers of graphene grown by thermal decomposition of off-axis 4H-SiC (0001) and by C segregation on Ni thin films from a solid carbon source. Transmission electron microscopy in different configurations, i.e. cross-section and plan view, was used to get information on the number of graphene layers as well as on the rotational order between the layers and with respect to the substrate. Atomic force microscopy was used to study the changes in the surface morphology produced by thermal annealing. In particular, the density and the height of peculiar corrugations (wrinkles) in the few layers of graphene, formed during the cool down in the thermal process, were investigated.

F. Giannazzo, C. Bongiorno, S. di Franco, R. Lo Nigro, E. Rimini, V. Raineri

Synthesis of Graphene Films on Copper Substrates by CVD of Different Precursors

Abstract

In the present work, graphene films of the order of \(1\,\hbox{cm}^{2}\) were grown on copper foil substrates by CVD using hydrogen/methane or hydrogen/argon/ethanol mixtures as gas precursors. The growth processes were performed near \(1\hbox{,}000^{\circ}\hbox{C}\) both at atmospheric and low pressures. A system for the fast cooling of the sample, based on the fast extraction from the hot zone of the furnace, was implemented allowing for rapid decrease of the temperature below \(600^{\circ}\hbox{C}\) in few seconds. Samples grown under different conditions were analyzed by SEM, Raman spectroscopy and XPS with the aim to assess their characteristics and to refine the growth process.

R. Giorgi, Th. Dikonimos, M. Falconieri, S. Gagliardi, N. Lisi, P. Morales, L. Pilloni, E. Salernitano

Lattice Gauge Theory for Graphene

Abstract

We propose a lattice gauge model for graphene, described in terms of tight binding electrons hopping on the honeycomb lattice interacting with a three-dimensional quantum U(1) gauge field. The infrared fixed point of the theory is analyzed by exact Renormalization Group methods. We find that the interacting response functions have a large distance decay described by anomalous critical exponents, which vary continuously with the strength of the electron-photon interaction. The dominant excitations at low energies turn out to be the Kekulé distortion, the charge density wave and the staggered magnetization. External fields coupled to the corresponding local order parameters are dramatically enhanced by the interactions. We also derive a non-BCS gap equation, suggesting that spontaneous emergence of Kekulé, staggered density or magnetization can arise at intermediate values of the electromagnetic coupling.

A. Giuliani, V. Mastropietro, M. Porta

A Chemists Method for Making Pure Clean Graphene

Abstract

Even before Geim and Novoselov’s Nobel Prize in Physics 2010 "for groundbreaking experiments regarding the two-dimensional material graphene". the interest of physicists in graphene was enormous compared to that of chemists. This probably results from the absence of a well-established large scale method to produce graphene. Therefore, the most important role chemists can play is the establishment of an inexpensive and simple wet-chemical method for making graphene. Herein, we describe an intercalation method to make clean graphene that has good electrical properties. The new method is based on an earlier procedure to make expanded graphite. Our method leads to the production of graphene.

S. Malik, A. Vijayaraghavan, R. Erni, K. Ariga, I. Khalakhan, J. P. Hill

The Effect of Atomic-Scale Defects on Graphene Electronic Structure

Abstract

Graphene, being one-atom thick, is extremely sensitive to the presence of adsorbed atoms and molecules and, more generally, to defects such as vacancies, holes and/or substitutional dopants. This feature, apart from being directly usable in molecular sensor devices, can also be employed to tune graphene electronic properties. Here we focus on those basic features of atomic-scale defects that can be useful for material design. Starting with isolated \(p_z\) defects, we analyse the electronic structure of the defective substrate and how it determines the chemical reactivity towards adsorption (chemisorption) of atomic/molecular species. This is shown to produce non-random arrangement of adatoms on the surfaces. Then, we consider the reverse problem, that is how to use defects to engineer graphene electronic properties. In particular, we show that arranging defects to form honeycomb-shaped superlattices (what we may call “supergraphenes”) a sizeable gap opens in the band structure and new Dirac cones are created right close to the gap region. These possible structures might find important technological applications in the development of graphene-based logic transistors.

R. Martinazzo, S. Casolo, G. F. Tantardini

Ritus Method and SUSY-QM: Theoretical Frameworks to Study the Electromagnetic Interactions in Graphene

Abstract

We study the Dirac fermion propagator in (2 + 1) dimensions in the background of external magnetic fields in a quantum electrodynamics framework. In graphene, the massless limit of our findings is of direct relevance. We present the Ritus formalism to obtain the fermion propagator in the general case. We show how, for some class of external static magnetic fields, the graphene hamiltonian can be cast into a hamiltonian with quantum mechanical supersymmetric properties, leading us to an exactly solvable model. As an example of these two formalisms we work out the canonical case of a uniform constant magnetic field.

G. Murguía, A. Raya

Transmission Electron Microscopy Study of Graphene Solutions

Abstract

In this paper we present the transmission electron microscopy characterization of the graphene membranes produced by exfoliation of KC 8 in N-methyl-pyrrolidone. This approach allows processing of large quantities of material, with a yield of 35% of the starting graphite, producing large, micron size, membranes composed of only few graphenes. The characterization of the solutions reveals the crumpled folded structure of the graphene membranes and their stacking structure.

L. Ortolani, A. Catheline, V. Morandi, A. Pénicaud

Strain Effect on the Electronic and Plasmonic Spectra of Graphene

Abstract

Within the tight binding approximation, we study the dependence of the electronic band structure and of the plasmonic spectrum of a graphene single layer on the modulus and direction of applied uniaxial strain. While the Dirac cone approximation, albeit with a deformed cone, is robust for sufficiently small strain, band dispersion linearity breaks down along a given direction, corresponding to the development of anisotropic massive low-energy excitations. We evaluate of the dynamical polarization and recover the plasmonic spectrum, within the random phase approximation (RPA). We explicitly consider local field effects (LFE). Thus, we are able to account for plasmons of any wavelength as a function of strain.

F. M. D. Pellegrino, G. G. N. Angilella, R. Pucci

Chemically Derived Graphene for Sub-ppm Nitrogen Dioxide Detection

Abstract

One of the most extraordinary properties of the graphene, the high sensitivity to the adsorption/desorption of gas molecule, is still at the very beginning of its exploitation. The ability to detect the presence even of a single interacting molecule relies on the two-dimensional nature of graphene, that allows a total exposure of all its atoms to the adsorbing gas molecules, thus providing the greatest sensor area per unit volume. Nevertheless, due to the complexity of the entire process, starting from the graphene synthesis and/or isolation up to the introduction into the proper device architecture, the fabrication of the single graphene flake based chemical sensor is still challenging. Herein a simple approach to fabricate a sensitive material based on chemically exfoliated natural graphite is presented. The devices were tested upon sub-ppm concentrations of \(\hbox{NO}_2\) and show the ability to detect this toxic gas at room temperature in actual environmental conditions.

T. Polichetti, E. Massera, M. L. Miglietta, I. Nasti, F. Ricciardella, S. Romano, G. Di Francia

Study of Interaction Between Graphene Layers: Fast Diffusion of Graphene Flake and Commensurate-Incommensurate Phase Transition

Abstract

Temperature-activated diffusion of a graphene flake on a graphene layer and a commensurate-incommensurate phase transition in bilayer graphene are investigated using the classical potential for graphene developed recently on the basis of first-principles calculations with the van der Waals correction. It is shown that rotation of graphene flakes to incommensurate states can significantly contribute to diffusion of the flakes consisting of several tens of atoms even at room temperature. Formation of incommensurability defects in bilayer graphene upon stretching of one of the layers is observed by molecular dynamics simulations.

I. V. Lebedeva, A. A. Knizhnik, A. M. Popov, Yu. E. Lozovik, B. V. Potapkin

Organic Functionalization of Solution-Phase Exfoliated Graphene

Abstract

Graphene has received increasing attention due to its unique physicochemical properties. The ability to control the size and dispersion of graphene sheets in a variety of organic solvents and water is probably the most essential technological challenge in the future of this exciting material. In this work we describe the recent efforts on the chemical functionalization of graphene produced by solution-phase exfoliation of pristine graphite. This approach could lead to new materials with well-defined properties and to the production of graphene in large-scale quantities.

M. Quintana, C. Bittencourt, M. Prato

UV Lithography On Graphene Flakes Produced By Highly Oriented Pyrolitic Graphite Exfoliation Through Polydimethylsiloxane Rubbing

Abstract

Graphene is a promising candidate in sensing applications; indeed thanks to its two-dimensionality, it provides the highest surface volume ratio, allowing all its atoms to be totally exposed to the adsorbing gas molecules. Due to several technological limits in production and manipulation of graphene as well as the device fabrication, the synthesis of graphene is still far from a well-settled process. This work aims to illustrate an approach for the graphene preparation that is based on the mechanical exfoliation and circumvents some difficulties encountered in a graphene based nanodevice production. Relying on that fabrication technique a chemiresistive sensor device was prepared. Preliminary findings showed that the device responds to a toxic gas such as \(\hbox{NO}_2,\) up to a few ppm, and reducing gases, such as \(\hbox{NH}_3,\) to few hundred ppm, at room temperature in controlled environments.

F. Ricciardella, I. Nasti, T. Polichetti, M. L. Miglietta, E. Massera, S. Romano, G. Di Francia

Photonic Crystal Enhanced Absorbance of CVD Graphene

Abstract

In the first part of this work we describe a chemical vapour deposition (CVD) method developed for graphene synthesis. Graphene samples with a controlled amount of layers have been prepared and transferred onto different substrates. The samples obtained have been characterized by several optical techniques. Optical absorption spectroscopy was used for estimation of the number of deposited graphene layers and the Raman spectra confirmed the presence of a high quality graphene monolayer. In the second part of the work we present a general concept of graphene integration with photonic crystals (PC) for enhancement of the optical absorbance of graphene. We describe a design approach, computer simulations of optical properties and a fabrication process of PC slabs. The experimental details of graphene combination with PC structures and the optical characterization of devices are described.

M. Rybin, M. Garrigues, A. Pozharov, E. Obraztsova, C. Seassal, P. Viktorovitch

Ab Initio Studies on the Hydrogenation at the Edges and Bulk of Graphene

Abstract

The opening of a band gap in graphene through chemical functionalization and realization of nanostructures, is an important issue for technological applications. Using first principles density functional theory, we show that how one can modify the electronic structure of bulk and nanoribbons of graphene by hydrogenation. It is shown that the hydrogenation of bulk graphene occurs through the formation of compact hydrogenated C islands. This also paves a unique way to realize zigzag and armchair nanoribbons at the interfaces between hydrogenated and bare C atoms and opens up the possibility to tune the band gap by controlling the width of the graphene-graphane interface. Moreover, we have studied the stability of hydrogenated edges of nanoribbons at finite temperature and pressure of hydrogen gas. It is shown that a dihydrogenated edge, which opens up a gap, can be stabilized under certain thermodynamic conditions.

S. Haldar, S. Bhandary, P. Chandrachud, B. S. Pujari, M. I. Katsnelson, O. Eriksson, D. Kanhere, B. Sanyal

Engineering of Graphite Bilayer Edges by Catalyst-Assisted Growth of Curved Graphene Structures

Abstract

This work addresses the edge engineering in graphite sheets with simple experimental method. Using catalytic chemical vapor deposition of methane on graphite, we demonstrated that curved graphene structures (CGS) can be grown at the edges of topmost graphite bilayer sheet steps, connecting the edges of two stacked layers. Scanning tunneling microscopy (STM) data showed that the growth of CGS results in the conversion of the abruptly terminated bilayer step edges into atomically smooth crystalline structures. Structural similarities of CGS and graphene folding were discussed. The presented approach is promising for the controlled growth and modification of graphite edges, as well as for engineering the edge characteristics of graphene systems at the atomic scales.

I. N. Kholmanov, C. Soldano, G. Faglia, G. Sberveglieri

“Flatlands” in Spintronics: Controlling Magnetism by Magnetic Proximity Effect

Abstract

Carbon atoms in graphene gain magnetic moments when in contact with magnetic substrates, following the macroscopic substrate alignment even at ambient temperature (Weser, M. et al.: Appl. Phys. Lett. 96, 012504 (2010)). On the other hand, magnetically doped topological insulators are ferromagnetic only at low temperatures (\(T_c=13\,\hbox{K}\) in \(\hbox{\it Bi}_{2-x}\hbox{\it Mn}_{x}\hbox{\it Te}_{3}\)) (Hor, Y.S. et al.: Phys. Rev. B 81(81), 195203 (2010)). Here we report chemical selective polarization dependent X-ray experiments on \(\hbox{{\it Fe}/{\it Bi}}_{2-x}\hbox{\it Mn}_{x}\hbox{\it Te}_{3}\) interfaces, where we followed the temperature dependence of the magnetic properties of Mn doped topological insulator in proximity with magnetic iron film. We find the presence of robust long range ferromagnetism in Mn induced by the magnetic proximity effect and maintained up to the room temperature (Vobornik, I. et al.: Nano. Lett. 11(10), 4079 (2011)). These results trace path to interface-controlled ferromagnetism in novel (graphene and topological insulators) “flatlands”.

I. Vobornik, J. Fujii, G. Panaccione, M. Unnikrishnan, Y. S. Hor, R. J. Cava

Graphite Nanopatterning Through Interaction with Bio-organic Molecules

Abstract

We investigated the interaction of the HOPG surface with aqueous solutions of different bio-organic molecules. Upon interaction with the solution, we observe the formation of extended nanopatterned domains on the HOPG surface. The domains, consisting of parallel rows, are oriented according to a three-fold symmetry and are characterized by the same row periodicity, irrespectively of the specific molecules under investigation. The interpretation of the nanopattern in terms of a restructuring of the graphite topmost layer following the interaction with the molecular solution is discussed.

A. Penco, T. Svaldo-Lanero, M. Prato, C. Toccafondi, R. Rolandi, M. Canepa, O. Cavalleri

Backmatter