Session A22: Graphene Applications

Transfer-free fabrication methods for graphene based devices

Integration of graphene into modern electromechanical systems necessitates processes that are both clean and fully automatable. Currently, methods for fabricating graphene based devices typically require a manual liquid transfer process that can easily damage the sheet. We discuss two transfer-free device fabrication techniques that are directly applicable to nanoscale electronics as well as NEMS resonators. Graphene is grown directly onto the final device substrate by using evaporated copper as a catalyst. The graphene is then patterned into the desired shape and the copper is removed by either a liquid or completely dry etch process. By using these techniques we are able to produce both ultra long graphene channels ($>$0.3 mm) as well as suspended graphene devices. Resulting arrays of graphene FETs are produced in high yield ($>$95{\%}) and exhibit uniform electrical properties including current saturation and moderate mobility.   

Heat Removal with Graphene Lateral Heat Spreaders

Device downscaling leads to higher chip power densities. A possible approach for heat removal from the localized hot spots is incorporation to chips of materials with high thermal conductivity. Recently, graphene and few-layer graphene (FLG) were proposed for heat removal owing to their superior thermal conductivity [1]. To evaluate the feasibility of this approach we simulated numerically heat propagation in SOI-based chip with and without graphene layers. It was found that incorporation of graphene or FLG can lead to substantial reduction of the hot spot's temperature [2]. The obtained results and are important for the design of graphene heat spreaders and interconnects [3]. \\[4pt] [1] A.A. Balandin, et al., Nano Lett., 8, (2008); S. Ghosh, et al., Appl. Phys. Lett., 92, (2008) \\[0pt] [2] S. Subrina, et al., Electron Dev. Lett., December (2009) \\[0pt] [3] A.A. Balandin, ``New materials can keep chips cool,'' IEEE Spectrum, October 2009   

Functionalizing graphene by embedded boron clusters

We present results from an ab initio study of B$_7$ clusters implanted into graphene [1,2]. Our model system consists of an alternating chain of quasiplanar B$_7$ clusters. We show that graphene easily accepts these alternating B$_7$-C$_6$ chains and that the implanted boron components may dramatically modify the electronic properties. This suggests that our model system might serve as a blueprint for the controlled layout of graphene based nanodevices, where the semiconducting properties are supplemented by parts of the graphene matrix itself, and the basic metallic wiring is provided by alternating chains of implanted boron clusters. [1] A. Quandt, C. \"Ozdo\u{g}an, J. Kunstmann, and H. Fehske, Nanotechnology \textbf{19}, 335707 (2008). [2] A. Quandt, C. \"Ozdo\u{g}an, J. Kunstmann, and H. Fehske, phys. stat. solidi (b) \textbf{245}, 2077 (2008).   

Facile and Scalable Route to Wafer-Size Patterned Graphene

Graphene-based nanoelectronic devices are promising as an alternative to silicon-based nanodevices in the future. Producing graphene sheets and patterned structures as device building blocks is an important step to graphene-based nanodevice fabrication. It is highly desirable to assemble graphene sheets at specific locations and into desired patterns on large scale. Several methods have been reported for patterning graphene nanostructure. However, all of these methods involve either sophisticated instruments or were rather high cost and low throughput, hindering their large-scale fabrication and practical applications. We developed a simple and efficient method to covalently immobilize graphene on silicon wafers. Patterned structures were fabricated where the feature sizes could be conveniently controlled from micron to millimeters. The formation of patterned graphene layers was confirmed by Raman spectroscopy, optical and atomic force microscopy. Evidence of covalent bond formation was provided by X-ray photoelectron spectroscopy. In addition, this method can be readily applied to other substrates. This approach represents a new route for solution-based graphene fabrication, allowing graphene sheets and patterned graphene structures to be fabricated on virtually any surface.   

Electrostatic Transfer of Monolayer Graphene Grown on Copper Foil

The growth of graphene on metal substrates, in particular, single-layer graphene on copper, suggests technological applications requiring graphene transfer from the growth substrate. However, it is difficult to transfer a large area of single-layer graphene with high quality, although using a multistep polymethyl-methacrylate (PMMA) based process looks promising. Transferring graphene by etching away the metal graphene growth substrate, as is the case with nickel and copper, can incorporate residues from the wet etching step, affecting graphene quality. Here, we demonstrate a simple method of large-area, single-layer, and high quality graphene transfer by electrostatic force. Using graphene grown by chemical vapor deposition directly on copper foils, we can transfer millimeter-sized, mostly single-layer graphene onto different substrates. We also note transfer of some bilayer graphene. By varying the electrostatic force with different electric fields, this technique furthers our understanding of the interaction between graphene and the copper film. Through Raman spectroscopy, atomic force microscopy, and scanning electron microscopy, we examine the number of defects and wrinkles in the transferred graphene layers, which gives us information about the graphene growth process on copper.   

Characterization of Stacked Transparent-Conductive Graphene Films

Graphene is a 2D material that exhibits very interesting optical and electronic properties. In this work, several graphene layers, prepared by the chemical decomposition of a carbon containing gas on a metal surface, were transferred and stacked on different kinds of substrates. The effect of the number of layers on both conductivity and transparency was studied. The stacked layers were doped with different materials which lead to an increase in the conductivity of the layers. AFM, SEM, XPS and sheet resistance measurements were used for characterization of the graphene layers. The improved conductivity of the stacked films can be of great value for some optical and electronic applications.   

pi-pi Functionalization of Graphene: Avenue for building Ultra-sensitive Graphene BioSensors

The tremendous attention received by graphene has been attributed to its sp$^{2}$ hybridized carbon atoms arranged in a 2-D honeycomb lattice structure with a high density of $\pi $ electrons confined within the quasi-planar, atomically-thick sheet. These structural and electronic attributes impart graphene with remarkable electrical, mechanical, and optical properties. Currently, covalent functionalization of graphene is carried out starting from graphene oxide (GO). This process deteriorates graphene's superior electrical properties by (i) opening up a band gap via removal of pi-electrons and (ii) increasing carrier scattering due to (a) the distorted structure produced by conversion of planar sp$^{2}$ to tetrahedral sp$^{3}$ carbons, (b) the charged impurities introduced and (c) the vacancy defects formed via removed carbon atoms. There is an immediate need for a functionalization mechanism, which retains the sp$^{2}$ carbons and the low scattering density on graphene's lattice structure. Here we present the electrical and interfacial characterization of graphene functionalized via pi-pi bonding mechanism that produces a minimal change in carrier density and scattering (10$^{4}$ fold reduced carrier scattering). We will present the functionalization and characterization of several biomolecular groups on graphene and show the bio-detection properties.   

DNA-decorated graphene chemical sensors

We measure the sensing response of DNA functionalized graphene to various analytes. Graphene is the current flagship nanomaterial and has been actively studied as a chemical sensor since shortly after it was isolated. Increasingly sophisticated device processing has revealed that some early measurements of graphene chemical sensing have been amplified by unintentional functionalization. We start with chemically clean graphene transistors and purposefully functionalize them to allow chemical sensing responses not found using pristine graphene. By using different DNA sequences during our functionalization, we are able to change the chemical sensitivity of the graphene. The resulting devices show fast response times, complete recovery at room temperature and discrimination between several similar analytes. This work has been supported by the IC Postdoc program, REU and the Nano/Bio Interface Center.   

Layer Transferred Graphene for Solar Cell Applications

Indium tin oxide (ITO) used commonly as a transparent electrode has proven to be unfavorable for the eventual commercialization of organic photovoltaic devices. We have investigated graphene grown on copper by CVD and transferred to arbitrary substrates as a possible replacement for ITO. Graphene-covered copper foils (Alfa Aesar) were first coated in photoresist (Shipley 1805) and the copper was removed with a ferric chloride based etchant. After cleaning in water the photoresist and graphene was transferred to the substrate of choice and the photoresist was removed with acetone. The transferred graphene were found to show a Hall mobility higher than 2000 cm$^{2}$/Vs at room temperatures and optical absorbance of 3.2{\%} at 550nm and 2.5{\%} at 900nm. We used scanning Raman spectroscopy to characterize the thickness of the graphene and found that 90+{\%} of layer transferred material is single layer graphene. The remainder was found to consist of clusters of bi- or multi-layer graphene of a typical size ranging from 0.5 to 2 $\mu $m. We fabricated organic hybrid solar cells utilizing this material as a transparent electrode. Results including a comparison between graphene and ITO devices fabricated using the same procedure and efforts to improve the efficiency of such graphene hybrid solar cells will be presented. Work supported in part by NSF.   

Bacterium Wrapping with Graphene for Non-destructive TEM Imaging and Raman Enhancement

\textit{Graphene} - a single atom thick layer of sp$^{2}$ carbon atoms arranged in a honeycomb lattice - exhibits atomic impermeability, high electric conductivity and mesoscale flexibility. We demonstrate that graphene's functionalization with cell-adhesion-peptides makes it highly specific to bacterial cells and stable in suspensions. Upon interaction with live gram-positive bacterial cells, these graphene-peptide microcarpets (GPMCs) instantaneously ($<$ 1 min) wrap the bacterial cells. Dye permeation studies confirm the impermeability of the GPMCs, consistent with the theory. Further, concurrent microscopic and spectroscopic analysis of the wrapping process would also be presented. Time-lapse TEM imaging studies on both wrapped and unwrapped bacteria, showed a $\sim $ 30 {\%} reduction in efflux of the cellular material from the wrapped bacteria. We will also present the Raman spectroscopy results, showing a $\sim $ 400 {\%} enhancement of the scattering signal after graphene wrapping.   

Real-time observations of Ag nanoparticle etching in ultraclean suspended graphene

We describe a range of experimental conditions under which we observe unprecedented long-term stability in suspended graphene membranes under intense electron beam irradiation. The stability and lack of beam-induced contamination permits the study of high-temperature catalytic etching of graphene sheets by Ag nanoparticles along graphene symmetry directions in a Titan ETEM where we observe rich and surprising behavior at video frame rates and near atomic resolution. We discuss the possibilities of controlling this type of catalytic patterning for the definition of e.g. graphene nanoribbons or narrow channels.   

Graphene-based materials and their physical properties

We are growing graphene on metal substrates and learning about growth mechanisms and also the physical properties of the graphene including when transferred from such substrates to arbitrary other substrates, including TEM support films, oxide layers, and so on. This talk will present a status report on progress since our recent series of publications in Science, Nano Letters, APL, and so on, in 2008 and 2009. Our publications can be, e.g., obtained at http://bucky-central.me.utexas.edu/publications.htm   

Metal-Semiconductor Interfaces and Patterns in Functionalized Graphene

Functionalization offers a novel way to modify the electronic and magnetic properties of graphene. Specific topology is essential to achieve devices with the desired features. Using density functional theory, we demonstrate stability of several such configurations, (in single and double sided functionalized graphene) and analyze their electronic and magnetic properties. We show that ``nanoroads'' [1] and ``nanodots'' [2] of pristine graphene can be carved in the electrically insulating matrix of fully hydrogenated carbon sheet (graphane) [1]. Such one-dimensional roads display individual characteristics and, depending upon zigzag or armchair orientation, can be metallic or semiconducting. Furthermore, the wide enough zigzag roads become magnetic with energetically similar ferro- and antiferromagnetic states. Engineering magnetic, metallic, and semiconducting elements within the same mechanically intact sheet of graphene presents a new opportunity for applications. \newline [1] A. K. Singh and B. I. Yakobson, Nano Lett., \textbf{9}, 1540 (2009). \newline [2] A. K. Singh, E. S. Penev, and B. I. Yakobson submitted.   

Liquid Separation by a Graphene Membrane

The behavior of liquids separated by a one atom thick membrane (graphene) is studied by using extensive molecular dynamics (MD) simulations at different conditions. With the help of appropriate empirical potentials, we investigate two liquid phases forming distinct systems: XGY, where X represents water or benzene, G represents a graphene sheet, and Y represents water, benzene or acetonitrile. Our MD simulations reveal important changes in the wettability patterns of these liquids near graphene. For instance, water-graphene-water exhibits strong density oscillations in a thin interfacial region of ~2.3 nm. In the cases of separate benzene and/or acetonitrile the oscillating-density interfacial region extends beyond ~3 nm, under similar thermodynamic conditions. Interestingly, our findings indicate that a liquid in one side of the membrane can affect the degree of wetting on the other side. Also, we show that high pressure effects, up to 10 kbar, can lead liquid water to be highly ordered along the normal direction of the graphene sheet.   

Investigation of High Frequency Performance of Graphene field effect transistor Using a Self Consistent Transport Model

Extremely high field effect mobility together with the high surface coverage makes graphene a promising material for high frequency electronics application. We investigate the intrinsic high frequency performance of graphene field effect transistors using a self consistent transport model. The self-consistent transport model is based on a nonuniversal diffusive transport that is governed by the charged impurity scattering owing to the presence of the charged impurities on the substrate. Experimentally feasible top gated transistor geometry is used for the calculations. The output and transfer characteristics of graphene field effect transistors are characterized as function of impurity concentration and dielectric constant of the gate insulator. Important high frequency device parameters such as transconductance, output resistance and power gain have been investigated. These results reveal the essential design considerations of the graphene transistors for radio frequency operations.