Session Z11: Focus Session: Graphene Devices - Hybrid Structures

Nano-patterning of fluorinated graphene by electron beam

The development of transparent electronics is reliant on achieving high conductivity materials with a gate tuneable carrier mobility and low contact resistance at the interface with metals. Graphene --a layer of carbon atoms in a honeycomb lattice- offers just such a possibility. Functionalizing graphene with fluorine induces the opening of a band gap in the otherwise semimetallic graphene. Here we demonstrate that fluorinated graphene --a wide gap semiconductor with sp3 electron orbital hybridization- can be selectively reduced to graphene by electron-beam irradiation. We employ this functionality to pattern conductive nanostructures in a sheet of fluorinated graphene, realizing transparent graphene-based electronic devices such as nanoribbons without the need for etching of graphene. Electrical transport experiments over a wide range of temperatures (from 300K to 4K) of the ribbons show a transport gap whose size is inversely proportional to the width of the patterned ribbons. In this gap, electrons are localized, and charge transport is dominated by variable range hopping. Charging effects constitute a significant portion of the activation energy, and we find that the activation energy scales well with the width of the ribbons [Nano Lett. 11, 3912 (2011)].   

Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes

We report on a new method based on feedback controlled electroburning to controllably form nanogaps in few-layer graphene [1]. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. Furthermore, molecules are deposited in the nanogaps. These molecular junctions display gateable IV-characteristics at room temperature. Gateable transport through molecules contacted between the electrodes demonstrates the potential of room-temperature operation of our molecular devices. Combined with the observed stability in time, our study shows that few-layer graphene nanogaps are an interesting alternative to metal electrodes. [1] Ferry Prins, Amelia Barreiro, Justus Ruitenberg, Johannes Seldenthuis, N\'{u}ria Aliaga-Alcalde, Lieven Vandersypen, Herre van der Zant, Nanoletters 11 (2011) 4607 - 4611   

Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics

We present atomistic calculations$^1$ of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules may be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitate control of the device conductance in a transistor effect with high on/off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.\\ $^1~$ A. Bergvall, K. Berland, P. Hyldgaard, S. Kubatkin, and T. Lofwander, Phys. Rev. B. {\bf 84}, 155451 (2011).   

The Role of Surfactant Adsorbates on Hysteresis and Carrier Mobility in Graphene Transistors

Understanding the role of polar and ionic adsorbates on the transport characteristics of graphene transistors is important for the development of graphene-based sensor devices and printable electronics using graphene solutions. We have investigated the effects of commonly used surfactants for graphene dispersion in aqueous solution on transport characteristics of graphene transistors. The adsorbates are found to transfer electrons to graphene, scatter carrier transport, and induce more electron-hole puddles when the graphene is on an SiO$_{2}$ substrate. We relate the change in transport characteristics to specific properties of a series of anionic, cationic, and neutral surfactants using a modification of a self-consistent transport theory. To understand the effects of surfactant adsorbates trapped on either side of the graphene, suspended devices were fabricated. Strong hysteresis is observed when both surfaces were exposed to the surfactants, attributable to their function as charge traps. This work is the first to demonstrate the control of hysteresis, allowing us to eliminate it for sensor and device applications or enhance it for non-volatile memory applications.   

Model results for graphene electrodes in molecular junctions

Graphene is a material with a high potential for nanoelectronic applications as transparent conductive electrode. In view of a graphene-based electronics, a possible way to bridge graphene electrodes is by molecular linkers either organic molecules or graphene-derived systems such as graphene nanoribbons (GNR). In the present work, we investigated the electronic and conductive properties of such junctions for two different devices. First we modeled a photochromic switch based on diarylethene molecules, which can be activated/deactivated by light. We found a large on/off current ratio, which can be improved by modifying the functional groups of the molecule. We then investigated the properties of graphene/GNR/graphene junctions, showing that their conductive properties can be tailored by changing the length and width of the GNR. The calculations have been carried out by using the non-equilibrium Green's function method combined with density functional theory.   

Graphene Chemical sensors on Flexible Substrates

Thanks to its all-surface 2D structure combined with a very high carrier mobility, Graphene is a very promising candidate for high sensitivity and low noise chemical sensing. Indeed, graphene devices can perform electrical detection for chemical sensing in a wide variety of applications, including pH monitoring in electrolytes and glucose measurements in blood. Furthermore, the fabrication of low cost and flexible sensors can be made possible by enabling the integration of graphene on plastic substrates. Our group has developed the first solution-gated graphene field effect transistor (SGFET) on a flexible substrate, PolyEthylene Naphthalate (PEN). For this purpose, graphene grown by chemical vapor deposition is transferred on the PEN substrate, where the metal contacts had already been evaporated. The characterization of our devices in a phosphate buffer solution displays good transconductance around 0.9 mS.mm$^{-1}$. pH measurements have been performed using these devices and a sensitivity as high as 22 mV/pH have been demonstrated. In addition, long term pH monitoring was demonstrated in these devices. Our on-going work focuses on studying the influence of the substrate as well as the presence of residues on the graphene surface in the pH sensing mechanism.   

Graphene field-effect biosensor arrays

We report on the study of chemical and biological sensing using graphene field-effect transistor (GFET) arrays. Large-scale single layer graphene sheets are synthesized by low-pressure chemical vapor deposition on copper. We fabricate GFET arrays capable of operating in solutions by passivating the graphene channel with a thin oxide layer. This oxide layer also serves as the electrolyte gate dielectrics and the sensing surface. The GFET arrays exhibit an average field-effect mobility of $\sim $ 5000 cm$^{2}$/Vs and small hysteresis in gate sweeps. We demonstrate the sensing operation of the GFET via measuring the pH value of phosphate buffer solutions. Gate sweeps indicate an approximately linear shift of the Dirac point with increasing pH, with an average slope of +46mV/pH. The viability of using GFETs to detect the specific binding of biomolecules will also be discussed.   

A model of large volumetric capacitance in graphene supercapacitors based on ion clustering

Electric double layer supercapacitors are promising devices for high-power energy storage based on the reversible absorption of ions into porous, conducting electrodes. Graphene is a particularly good candidate for the electrode material in supercapacitors due to its high conductivity and large surface area. In this paper we consider supercapacitor electrodes made from a stack of graphene sheets with randomly-inserted ``spacer" molecules. We show that the large volumetric capacitances $C > 100$ F/cm$^3$ observed experimentally can be understood as a result of collective intercalation of ions into the graphene stack and the accompanying nonlinear screening by graphene electrons that renormalizes the charge of the ion clusters.   

Morphology-controlled graphene aerogel for energy storage

The development of new anode/cathode materials with highly conductive, non-corrosive, high specific surface area and high porosity for energy storage devices is highly desirable. Graphene aerogels has been focused emergently recently due to novel properties of the graphene. However, the aerogel-based application performance strongly depends on the morphology and structure of the graphene aerogels. The graphene aerogels with low-density have thinner struts, a different distribution of particle sizes, and less internal connectivity. This, in turn, changes the way the material can transport electric charge. As a result, the highest surface area graphene aerogels end up having the worst electrical conductivity, and the most conductive graphene aerogels have lowest surface areas. So the best designs of the developed graphene aerogel nanostructures in terms of pore size, porosity, density and mechanical properties for energy storage devices are essential. In this work, we develop a new fabrication method of graphene aerogels with well-controlled morphology and high electrical conductivity from graphene oxide through the supercritical drying process. The morphology and electrical conductivity of the graphene aerogels are controlled by the precursor contents and the synthesis conditions. The experimental results are very useful for experimentalists deciding the best graphene aerogel nanostructures for their needs.   

Electrostatic control of many-body interactions in graphene: Observation of the effects of doping on the saddle-point exciton

Significant excitonic effects were recently identified in the optical response of graphene through the asymmetric resonance feature at 4.62 eV in the optical conductivity. The peak, which is red-shifted by nearly 600 meV from the predicted band-to-band transition energy,\footnote{Mak et al. Phys. Rev. Lett. 106, 046401, (2011).} can be considered as a saddle-point exciton. Here we report a systematic study of this excitonic feature as a function of the doping density, for densities extending up to the metallic regime\footnote{Efetov \& Kim, Phys. Rev. Lett. 105, 256805, (2010).} ($\sim$ $10^{14} cm^{-2}$). With increasing density of either electrons or holes, the excitonic resonance is found to shift to the red and to become more symmetric in form. These experimental features agree very well with a recent ab-initio GW-Bethe-Salpeter calculation\footnote{Felipe H. da Jornada, J. D., Steven G. Louie. Private communications. (2011).} and can be understood as a consequence of enhanced ``metallic'' screening of the graphene dielectric function.\footnote{Hwang, E. H. \& Das Sarma, S. Phys. Rev. B 75, 205418} In addition, analysis of the width of the excitonic peak provides information on the quasiparticle lifetime. Mechanisms for the inferred rapid quasiparticle decoherence will be discussed.   

Chemically Tunable Transport Phenomena of Functionalized Graphene

We present an ab initio multiscale study and quantum transport simulations using the Kubo formalism [1] of chemically modified graphene based materials, whose properties are tuned by changing the density and nature of grafted molecular units. Depending on the nature of the introduced molecular bonding different conduction mechanism are obtained, including transition from weak to strong Anderson localization [2,3], as well as spin-dependent phenomena [4] and magnetoresistive fingerprints [5]. \\[4pt] References: [1] H. Ishii, F. Triozon, N. Kobayashi, K. Hirose, and S. Roche, C. R. Physique 10, 283 (2009) [2] N. Leconte, J. Moser, P. Ordejon, H. Tao, A. Lherbier, A. Bachtold, F. Alsina, C.M. Sotomayor Torres, J.-C. Charlier, and S. Roche, ACS Nano 4, 7, 4033-4038 (2010) [3] N. Leconte, A. Lherbier, F. Varchon, P. Ordejon, S. Roche, and J.-C. Charlier (accepted in PRB) [4] N. Leconte, D. Soriano, S. Roche, P. Ordejon, J.-C. Charlier, and J.J. Palacios, ACS Nano 5, 5, 3987-3992 (2011) [5] D. Soriano, N. Leconte, P. Ordejon, J.-C. Charlier, J.J. Palacios, and S. Roche, Phys. Rev. Lett. 107, 016602 (2011)   

First-principles study of hybrid Graphene/MnO$_2$ bilayers

Oxide nanosheets are an important and promising component for creating new materials. Their capacitance properties are particularly appealing for electric batteries. Manganese oxide nanosheets are abundant, environmental friendly, have low cost, and high electrochemical activity. However MnO$_2$ poor electrical conductivity and chemical stability limits its applicability as electrode material. Hybrid graphene/oxide nanostructures have been proposed to overcome these difficulties. Here, density functional theory calculations are performed to better understand the electronic properties of heterobilayers made from graphene and MnO$_x$ monolayers. The charge transfer between graphene and MnO$_2$ monolayers are analyzed and related to the presence of oxygen vacancies in different concentrations, which are known to induce atomic reconstructions and phase transformations of the oxide. Magnetic properties of the heterobilayers will also be discussed.   

Boron Nitride Nanoribbons: Synthesis and Future Directions

Boron Nitride Nanoribbons (BNNR) have been theorized to have many interesting electrical and magnetic properties and edge states, but these characteristics have not been experimentally verified due to challenges in synthesis and purification. We have produced BNNRs by longitudinally splitting~boron nitride nanotubes (BNNT) using potassium vapor as an intercalant. Due to the strong interactions between boron nitride sheets, separation of nanoribbons from their parent tubes is challenging. We have used various solvent systems to assist with separation of the ribbons with the goal of probing their properties.   

Electron-Hole Polarization Dynamics in Graphene Oxide

Graphene oxide (GO) has been shown to emit broadband visible and near infrared photoluminescence (PL). Here we use polarization sensitive optical spectroscopy to study spectral diffusion and temporal dynamics of electron-hole polarization in this material. Steady state polarization memory measurements show strong polarization memory close to the excitation energy, which weakens gradually in moving toward lower emission energies. To understand the dynamics underlying this behavior, we also perform time-resolved PL studies using an optical Kerr gate with sub-picosecond temporal resolution. Polarization memories show ultrafast dynamics within the PL lifetime in solid GO preparations where the incident light lies fully in the plane of the GO flakes. Using additional knowledge gained from optical anisotropy measurements, we discuss the relevance of our polarization memory data to the origins of PL in these systems.