Session T37: Focus Session: Graphene Structure, Dopants, and Defects: Nanoribbons

Atomically Precise Bottom-up Fabrication of Graphene Nanoribbons

Graphene nanoribbons (GNRs) -- narrow stripes of graphene -- are predicted to exhibit remarkable properties making them suitable for future electronic applications. Contrary to their two-dimensional (2D) parent material graphene, which exhibits semimetallic behavior, GNRs with widths smaller than 10 nm are predicted to be semiconductors due to quantum confinement and edge effects. Despite significant advances in GNR fabrication using chemical, sonochemical and lithographic methods as well as recent reports on the successful unzipping of carbon nanotubes into GNRs, the production of sub-10 nm GNRs with chemical precision remains a major challenge. In this talk, we will present a simple GNR fabrication method that allows for the production of atomically precise GNRs of different topologies and widths [1]. Our bottom-up approach consists in the surface-assisted coupling of suitably designed molecular precursors into linear polyphenylenes and their subsequent cyclodehydrogenation, and results in GNRs whose topology, width and edge periphery are defined by the precursor monomers. By means of STM and Raman characterization, we demonstrate that this fabrication process allows for the atomically precise fabrication of complex GNR topologies. Furthermore, we have developed a reliable procedure to transfer GNRs fabricated on metal surfaces onto other substrates. It will for example be shown that millimeter sized sheets of crosslinked GNRs can be transferred onto silicon wafers, making them available for further processing, e.g. by lithography, prototype device fabrication and characterization. \\[4pt] Coauthors: Pascal Ruffieux, Rached Jaafar, Marco Bieri, Thomas Braun, and Stephan Blankenburg, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland; Matthias Muoth, ETH Zurich, Department of Mechanical and Process Engineering, 8092 Zurich, Switzerland; Ari P. Seitsonen, University of Zurich, Physical Chemistry Institute, 8057 Zurich, Switzerland; Moussa Saleh, Max Planck Institute for Polymer Research, 55124 Mainz, Germany; Ivan Shorubalko, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland; Shuping Pang, Xinliang Feng, and Klaus M\"ullen, Max Planck Institute for Polymer Research, 55124 Mainz, Germany; and Roman Fasel, Empa, Swiss Federal Laboratories for Materials Science and Technology, 3602 Thun and 8600 D\"ubendorf, Switzerland and University of Bern, Department of Chemistry and Biochemistry, 3012 Bern, Switzerland. \\[4pt] [1] J. Cai \textit{et.al}, Nature \textbf{466,} 470-473 (2010)   

Templated graphene nanoribbon growth on SiC

We demonstrate a photo-lithography fabrication method of graphene nanoribbon. Epitaxial graphene is grown selectively on SiC (1-10n) facets. For this, SiC is patterned to define 3-dimensional structures. Epitaxial graphene nanoribbons grow preferentially on the exposed sidewalls recrystallized facets that avoids post-processing lithography damage of graphene ribbons. Graphene ribbons narrower than 30nm were produced with this method and all-graphene interconnected structures are fabricated. Metal contacts are evaporated on large graphene areas seamlessly connected to nanoribbons. Transport measurement shows gap opening and high mobility. SiC crystal faceting was also explored. Low index crystal facets where found to be energetically favored.   

Influence of size effects and substrate morphology on the conductance of epitaxial graphene nanoribbons

To utilize graphene's superior electrical properties and achieve transistor operation comparable to that of silicon, the properties of graphene nanoribbons need to be better understood and optimized. Lithographically patterned nanoribbons suffer from line edge roughness which can result in a detrimental effect on the graphene conductivity. In addition to edge-induced scattering, the morphology of the silicon carbide substrate appears to have a strong effect on the line width scaling behavior. In this talk, we present experimental data that clearly shows the interplay between substrate morphology and line edge roughness in epitaxial graphene nanoribbons. Resistivity is shown to strongly increase as nanoribbon line width is reduced, although the line width at which this behavior sets in varies depending upon the substrate morphology. We also propose a model which can be used to predict the dependence of graphene nanoribbon resistivity on line width.   

Large-scale production of Graphene Nanoribbons with controlled width: Electrical Properties of Graphene Nanoribbon Films

In this talk, we will demonstrate a novel large scale production (10$^{7}$ ribbons/ sec) scheme for several microns long, smooth-edged graphene nanoribbons (GNRs) with controlled widths (from 5 -- 50 nm). We will then present detailed structural, optical and electrical properties of GNR-films $\sim $ 100 nm thick produced from 5, 15, and 45 nm wide GNRs; including their band-gap evolution and electrical transport mechanism. The high throughput method to synthesize GNR of high-quality will be a quantum leap in the graphene research. The work indents to bridge the gaps in the understanding of monodisperse-GNR film properties.   

Correlated crystallographic etching of graphene and nanoribbon formation

Catalytic etching is a promising method for constructing crystallographically defined graphene structures such as nanoribbons. Catalytic etching experiments are performed and shown to contain significant correlation yielding crystallographic graphene nanoribbons. This correlation is investigated as a function of etching conditions and compared to simulations with possible sources discussed. Supported in part by NSF Award No. DMR-0805136, the Kentucky NSF EPSCoR program, the University of Kentucky Center for Advanced Materials, and the University of Kentucky Center for Nanoscale Science and Engineering.   

Aligned, ultra-long graphene nanoribbon network fabrication by nanowire etch masks

Patterning semi-metallic graphene into quasi one-dimensional structures known as nanoribbons (GNRs) can open a $\sim $0.5 eV bandgap by quantum confinement [1]. To circumvent GNR lithographic difficulties, Si nanowires (NWs) were used previously as an etch mask for exfoliated graphene [2], but with no scalability or alignment control. Conversely, we transfer $\sim $1 in$^{2}$ graphene sheets off copper to silicon dioxide, giving us a template for array fabrication. We meniscus align both Au NWs ($<$w$>$=20 nm, $<$l$>$=400 nm) and Ag NWs ($<$w$>$=200 nm, $<$l$>$=10 $\mu $m), respectively, on the graphene surface. By reactive ion etch (RIE), we remove the unmasked graphene, and we etch the NWs. Based on the starting NWs, the resulting GNR arrays have lengths ranging from 200 nm to tens of microns, and widths from 10 nm to 250 nm. We create single GNRs that can span micron-separated contacts and GNR networks, similar to a graphene nanomesh. Using atomic force microscopy and Raman spectroscopy, we determine that we have monolayer GNRs with a high disorder intensity I$_{D}$/I$_{G}\sim $1, indicating rough edges and graphene grain boundaries, which are deleterious to transport. [1] K.A. Ritter and J.W. Lyding, Nat. Mat. 8, 235 (2009). [2] J. Bai et al., Nano Lett. 9, 2083 (2009).   

Polymer electrolyte enhanced performance in graphene nanoribbon field-effect transistors

Graphene nanoribbon Field-effect transistors were fabricated from unzipped multiwall carbon nanotubes on Si/SiO2 substrate by standard electron beam lithography and metal deposition. A small drop of polymer electrolyte consisting of poly(ethylene oxide) and lithium perchlorate was applied to the graphene nanoribbon devices. Electrical transport properties of the polymer electrolyte covered devices were measured using both the Si-back-gate and polymer-electrolyte-gate configurations. We observed dramatic increase of carrier mobility, significant reduction of the peak-width of the resistance as a function of the back-gate voltage, and the shift of the charge neutrality point toward zero gate-voltage in polymer electrolyte covered graphene nanoribbon devices. These experimental results will be presented and discussed in the context of ionic and dielectric screening of charged impurities on or near the graphene nanoribbons.   

Enhanced Conductance Fluctuation by Quantum Confinement Effect in Graphene Nanoribbons

Conductance fluctuations are usually unavoidable in graphene nanoribbons (GNR) due to the presence of disorder along its edges. By measuring the low-frequency noise in GNR devices, we find that the conductance fluctuations are strongly correlated with the density-of-states of GNR [1]. In single-layer GNR, the gate-dependence of noise shows peaks whose positions quantitatively match the subband positions in the band structures of GNR. This correlation provides a robust mechanism to electrically probe the band structure of GNR, especially when the subband structures are smeared out in conductance measurement.\\[4pt] [1]. G. Xu et al. Nano Lett. 2010, 10, 4590--4594.   

Doping Level Dependence of Transfer Characteristic of n-type Graphene Nanoribbon Field Effect Transistors

By performing first principles calculations and electron transport simulations, we demonstrate that the transfer curves of graphene nanoribbon field effect transistors can be controlled by changing the concentration of potassium atoms and cobaltocene molecules doping, or nanoribbon edge carbon atoms substitution by nitrogen. We reveal that Dirac point shift downward from 0 to -12 V when the impurity concentration increase from 0 to 1.37{\%}, while the transfer curves maintain bipolar characteristics with reasonably high on/off ratios. Moreover, we observed strong charge transfer from the adsorbed atoms and molecules that facilitates n-type characteristics in graphene nanoribbons. Thus, we suggest that an effective way to achieve tunable n-type graphene nanoribbons field effects transistors is to dope them with electron donors.   

The search for stable sp2 zigzag edge graphene nanoribbon termination

The zig-zag edge of a graphene ribbon has attracted much attention, since it is predicted to support a spin-polarized edge state. However, it is difficult to produce thermodynamic conditions that favor a pure sp$^2$ termination of a graphene sheet, since the edge carbons generally prefer to bond to two hydrogen atoms, in sp$^3$ hybridization, rather than one hydrogen, as sp$^2$. We examine several candidate alternative termination groups which can modify the thermodynamics of various edge configurations to favor the sp$^2$ edge termination. Ab-initio calculations demonstrate these alternative terminations can support robust edge states across a broad range of synthetic conditions.   

High resolution thermal properties study of Joule self-heated graphene nanoribbon

We present high resolution thermal properties of Joule self-heated graphene nanoribbons (GNRs) by scanning thermal microscope (SThM) which enables local temperature survey within 100 nm spatial resolution. In order to calibrate the SThM probes, we employ the micro-Raman spectroscopy to measure the temperature distribution across a standard graphene device as a function of applied electrical power. This calibrated SThM measurement allows us to scrutinize the temperature distributions in GNRs attributed to Joule heating variation in the channel due to the locally enhanced scattering which forms hot spot formation. We also estimate the junction thermal resistance between GNR and SiO$_{2}$ substrate from the temperature distribution of the GNR devices. In addition, we will discuss simultaneous SThM and scanning Kelvin probe microscopy study of high quality exfoliated graphene on hexagonal boron nitride substrate.   

Thermal transport in graphene nanoribbons: R-Matrix theory approach

We have developed a new theoretical tool based on R-Matrix theory to calculate phonon scattering on the atomic scale. As device sizes shrink, boundary and interface scattering have become bottlenecks to thermal transport. Therefore, calculating thermal transport considering the atomistic constitution of a device is very important. In this R-Matrix approach, we consider a finite region, which is the main scattering center of the system, connected to semi-infinite leads. We develop interior region solutions (normal modes of the finite system) and lead solutions(periodic waves with dispersion) independently that can be matched at predefined boundaries to extract the transmission probabilities of each phonon modes in the lead. In this work we demonstrate the implementation of the theory for graphene nanoribbons.   

Strong suppression of thermal conductivity in edgedisordered graphene nanoribbons: Order-N methodology and thermoelectric properties

We investigate electron and phonon transport through edge disordered graphene nanoribbons. Electronic transport is calculated using Green's functions[1] while for phonons we develop an efficient linear scaling method [2-3] which is based on the Chebyshev polynomial expansion of the time evolution operator and the Lanczos tridiagonalization scheme. We show that edge disorder dramatically reduces phonon thermal transport in both armchair and zigzag ribbons, while in zigzag graphene nanoribbons edge disorder is only weakly detrimental to electronic conduction. The behavior of the electronic and phononic elastic mean free paths points to the possibility of realizing an electron-crystal coexisting with a phonon-glass. The calculated thermoelectric figure of merit (ZT) values qualify zigzag graphene nanoribbons as a promising material for thermoelectric applications. \\[4pt] [1] H. Sevin\c{c}li and G. Cuniberti Phys. Rev. B 81, 113401 (2010). [2] W. Li, H. Sevin\c{c}li, G. Cuniberti and S. Roche, Phys. Rev. B 82, 041410 (2010). [3] W. Li, H. Sevin\c{c}li, S. Roche and G. Cuniberti, arXiv:1011.1116