### Session P21: Focus Session: Nano-Graphene

 Wednesday, March 17, 2010 8:00AM - 8:12AM P21.00001: Edge effects on impurity states in carbon nanoribbons Jie Jiang , Wenchang Lu , Piotr Boguslawski , Jerry Bernholc We investigated the electronic structure and spin polarization of nitrogen-doped carbon nanoribbons with DFT. We find enhanced segregation in zigzag nanoribbons, due to interplay between impurity states in the valence bands and the edge states. The magnetization is partially quenched by the doping. We also find that the three armchair nanoribbons (ARs) families 0, 1, 2 as mod (n+1, 3)=0, 1, 2 behave differently in doping. In family 1, the impurity level is in the band gap. Its ionization energy decreases with the increase of ribbon width and it oscillates with the dopant position. The joint effects of high mobility and width-dependent ionization energy make the family 1 AR attractive n- type semiconductors for electronic devices. The impurity level in families 0 and 2 is a resonance in the conduction bands, which strongly influences the transport properties by decreasing the conductivity near the impurity level. Wednesday, March 17, 2010 8:12AM - 8:24AM P21.00002: A twist on graphene nanoribbons John Mintmire , Junwen Li , Daniel Gunlycke , Carter White Although two-dimensional graphene might exhibit some microscopic structure, viewed macroscopically it is essentially flat. In contrast, many one-dimensional materials are helical in nature. Because of the strong $sp^2$ bonds in graphene nanoribbons, these ribbons are generally assumed to be flat like graphene. In this presentation, first-principles calculations on graphene nanoribbons are presented that suggest that narrow ribbons could in fact be helical depending on size and edge species. Twisting nanoribbons is also shown to affect the electronic properties near the Fermi level. If the twist is large enough, our calculations show that it might even be possible to reduce the semiconducting band gap to well below room temperature, effectively making the ribbons metallic. Wednesday, March 17, 2010 8:24AM - 8:36AM P21.00003: Electron transport in top-gated graphene nanoribbons Melinda Han , Inanc Meric , Kenneth Shepard , Philip Kim We report on studies of electron transport in top-gated graphene nanoribbons. Graphene nanoribbon devices are fabricated from mechanically exfoliated graphene. A metal top gate is then fabricated with a thin gate dielectric over both the graphene nanoribbon and the wide graphene leads, avoiding the formation of p-n junctions in the channel. The increased capacitive coupling suppresses the effects of Coulomb charging, and allows us to quantify the relative contribution of Coulomb interactions in the low-bias transport. Additionally, high bias transport in top-gated nanoribbons shows good field effect transistor characteristics, including strong saturation behavior for ribbons with a range of lengths. Wednesday, March 17, 2010 8:36AM - 8:48AM P21.00004: Localization length in edge-disordered armchair-edge graphene nanoribbons Daniel Gunlycke , Carter White To find a conductance in a graphene nanoribbon that is of the order $2e^2/h$, the length of the ribbon should be chosen to be shorter than its localization length. The localization length is an intrinsic property that depends on the disorder in the material. Arguably the most serious form of disorder in graphene ribbons is edge disorder. Edge disorder could be topological as well as environmental. In this presentation, analytical expressions for the localization length for ribbons with both types of disorder will be presented. The expressions show that the maximum localization length near the Fermi level scales with the square of the width of the ribbon. The origin of this dependence, which could partly explain why it is challenging to obtain large conductances in narrow ribbons, will be discussed. The analytical expressions are in excellent agreement with numerical transport calculations that are based on multi-band tight-binding Hamiltonians. Wednesday, March 17, 2010 8:48AM - 9:00AM P21.00005: Current {\&} Heat Transport in Graphene Nanoribbons: Role of Non-Equilibrium Phonons Gary Pennington , Daniel Finkenstadt The conducting channel of a graphitic nanoscale device is expected to experience a larger degree of thermal isolation when compared to traditional inversion channels of electronic devices. This leads to enhanced non-equilibrium phonon populations which are likely to adversely affect the mobility of graphene-based nanoribbons due to enhanced phonon scattering. Recent reports indicating the importance of carrier scattering with substrate surface polar optical phonons in carbon nanotubes$^{1}$ and graphene$^{2,3}$ show that this mechanism may allow enhanced heat removal from the nanoribbon channel. To investigate the effects of hot phonon populations on current and heat conduction, we solve the graphene nanoribbon multiband Boltzmann transport equation. Monte Carlo transport techniques are used since phonon populations may be tracked and updated temporally.$^{4}$ The electronic structure is solved using the NRL Tight-Binding method,$^{5}$ where carriers are scattered by confined acoustic, optical, edge and substrate polar optical phonons. [1] S. V. Rotkin et al., \textit{Nano Lett.} \textbf{9}, 1850 (2009). [2] J. H. Chen, C. Jang, S. Xiao, M. Ishigami and M. S. Fuhrer, \textit{Nature Nanotech.} \textbf{3}, 206 (2008). [3] V. Perebeinos and P. Avouris, arXiv:0910.4665v1 [cond-mat.mes-hall] (2009). [4] P. Lugli et al., \textit{Appl. Phys. Lett.} \textbf{50}, 1251 (1987). [5] D. Finkenstadt, G. Pennington {\&} M.J. Mehl, \textit{Phys. Rev. B} \textbf{76}, 121405(R) (2007). Wednesday, March 17, 2010 9:00AM - 9:12AM P21.00006: Production and Characterization of Graphene Nanoribbons Gabriel Setzler , L.K.K.D. Chamath , Seungsoo Kim , Hyeun Joong Yoon , Mark Ming-Cheng Cheng , Zhixian Zhou We have produced graphene nanoribbons by sonicating various types of graphite in solution. Atomic force microscopy (AFM) was used to characterize individual graphene Nanoribbons deposited onto Si/SiO2 substrates. Monolayer and few layer graphene nanoribbons were observed. Field effect transistor devices of individual nanoribbons were fabricated, and their electrical transport properties were measured. Possible mechanisms of graphene nanoribbon formation and electrical transport data on graphene nanoribbon devices will be discussed. Wednesday, March 17, 2010 9:12AM - 9:24AM P21.00007: Capacitance of graphene nanoribbons I.V. Zozoulenko , J.W. Klos , A.A. Shylau We present an analytical theory for the gate electrostatics and the classical and quantum capacitance of graphene nanoribbons (GNRs) and compare it with the exact self-consistent numerical calculations based on the tight-binding p-orbital Hamiltonian within the Hartree approximation [1]. We demonstrate that the analytical theory is in a good qualitative (and in some aspects quantitative) agreement with the exact calculations. There are however some important discrepancies that are traced to the quantum mechanical effects leading to the significant modification of the self-consistent charge distribution in comparison to the noninteracting electron description. The role of electron-electron interaction in the electronic structure and the capacitance of the GNRs is discussed. Finally, we discuss an experimental extraction of the quantum capacitance from experimental data.\\[4pt] [1] A. Shylau, J. W. Klos and I. V. Zozoulenko, Phys. Rev. B, in press (arXiv:0907.1040v1 [cond-mat.mes-hall]) Wednesday, March 17, 2010 9:24AM - 9:36AM P21.00008: Stability of graphene nanoribbon edges under high temperature joule heating Xiaoting Jia , Mario Hofmann , Vincent Meunier , Bobby Sumpter , Jessica Campos-Delgado , Jose Romo-Herrera , Jing Kong , Mauricio Terrones , Mildred Dresselhaus Graphene nanoribbon edges have generated a lot of research interests recently, due to the different electronic properties of the ribbons arising from zigzag and armchair edges. Recent progress has shown that atomically smooth graphene nanoribbon edges can be produced using joule heating. In order to fully understand the joule heating process and study the stability of graphene nanoribbon edges, we investigated the temperature that is reached during the joule heating process using metal particles. Metal particles on a suspended ribbon melt and evaporate with enough resistive joule heating, thereby providing a temperature calibrator of the ribbon surface. The successive melting process also provides a temperature gradient along the ribbon length. Thermodynamic calculations are carried out to estimate the melting point of the nanoparticles as a function of decreasing size. We also investigated the different edge junctions that were formed after the joule heating process. Our results showed that certain types of zigzag-armchair edge junctions are more dominant. Another type of zigzag-armchair edge junction, which is unstable, was found to reconstruct to form a stable edge junction during joule heating. Wednesday, March 17, 2010 9:36AM - 9:48AM P21.00009: Half-metallic Armchair Graphene Nanoribbon Fumiyuki Ishii , Keisuke Sawada , Mineo Saito Among a variety of applications of graphenes, spintronics applications are considered to be hopeful. For an example, spin transport has been experimentally observed by using graphene layers [1]. There are two types of shaped edge in graphene nanoribbon (GNR), one of them is armchair GNR (AGNR) and the other is zigzag GNR (ZGNR). Armchair edges are often observed compared with the zigzag edges [2]. However, the AGNR is believed to have the nonmagnetic structure whereas the magnetic properties in ZGNR attracted much attention. In this study, we perform first-principles density functional calculation on dehydrogenated AGNR. Surprisingly, we find that dehydrogenated AGNR has magnetic states in the case of carrier doping. Magnetic state of the carrier-induced AGNR has ferromagnetic chains at the two edges having the same directions of the magnetic moments. We conclude that the carrier-doped AGNR is half-metallic. [1] M. Ohishi et al., Jpn. J. Appl. Phys. 46, L605 (2007). [2] Y. Kobayashi et al., Phys. Rev. B 71, 193406 (2005). Wednesday, March 17, 2010 9:48AM - 10:00AM P21.00010: Thermal transport in graphitic nanostructures: Analytic force constants and first principles calculations Yifeng Chen , Thushari Jayasekera , B. D. Kong , K. W. Kim , M. B. Nardelli Graphitic nanostructures such as graphene, nanoribbons and carbon nanotubes have shown to be potential candidates for device applications that may revolutionize the future of nanoelectronics. However, very little is known about their thermal properties. In fact, understanding the heat transfer at the nanoscale is essential for optimal thermal management and heat removal in device applications. In this talk, we will discuss an efficient approach to compute the phonon contribution to thermal transport in a broad range of carbon nanostructures using models based on analytic force constants and validate them against state of the art ab initio calculations based on Density Functional Theory. This work was supported, in part, by the NERC/NIST SWAN-NRI and the DARPA/HRL CERA programs. Wednesday, March 17, 2010 10:00AM - 10:12AM P21.00011: A three dimensional approach to selective growth and photo-lithographic fabrication of graphene nanoribbon on SiC Ming Ruan , Mike Sprinkle , Xiaosong Wu , Yike Hu , Miguel Rubio-Roy , John Hankinson , Claire Berger , Walt de Heer We present a unique three-dimensional process for fabrication of epitaxial graphene devices on hexagonal silicon carbide. Pre-patterning of SiC substrate allows graphene growth on SiC crystal planes other than typical (000-1) and (0001) planes. Selective graphitization on these crystal planes are reported, which enables definition of graphene nanoribbon by standard photolithographic processing. Measurement of devices demonstrates the electronic viability of graphene grown on these SiC crystal planes and over SiC step edges, suggesting technologically practical methods of obtaining semiconducting graphene nanoribbons. Fabrication of $>$10,000 transistors on a 0.24cm$^{2}$ chip illustrates the scalability of this process. Wednesday, March 17, 2010 10:12AM - 10:24AM P21.00012: Low Temperature Transport in Networks Based on Multi-layer Graphene Nanoribbons Ashkan Behnam , Jason Johnson , Yanbin An , Amlan Biswas , Ant Ural We fabricate and characterize networks composed of narrow but long multi-layer graphene nanoribbons. Fabrication is based on chemical processing of expandable graphite and vacuum filtration of the produced ribbons. We analyze the structure of the networks by various electron and optical imaging techniques, then pattern the networks into four point probe structures using photolithography and plasma etching and measure their resistivity down to 4.2 K. Due to the disordered nature of the networks, resistivity depicts insulating behavior explained by Mott Variable Range Hopping (VRH) at low temperatures. We also investigate the dependence of the network resistivity on electric and magnetic fields. VRH theory can explain most of the magnetoresistance data, although carrier-carrier interaction also becomes important at high fields and the lowest temperatures. Resistivity also decreases sharply at electric fields higher than 10 V/cm. A high localization radius is extracted from this dependence, which is likely due to the high conductance of the nanoribbons and/or good electrical contact between them. The multi-layer graphene nanoribbon networks depict favorable electrical properties that advocate their use for applications such as bolometers, photodetectors, and gas sensors. Wednesday, March 17, 2010 10:24AM - 10:36AM P21.00013: Adatom Ordering and Tunable Band Gap in Graphene Andrey Shytov , D.A. Abanin , L.S. Levitov The unique electronic properties of graphene make it an attractive candidate for future nano-electronics applicaitons. However, the gapless semi-metallic character of graphene band structure is a major obstacle for graphene electronics. This talk will describe a proposal to use controlled chemical adsorption of adatoms or molecules similar to that employed in a recent work on hydrogenated graphene\footnote{D. C. Elias et al., Science {\bf 323}, 610 (2009)}, as a tool to open a band gap in this material. The gap is induced by Bragg scattering on a modulation resulting from adatom ordering. By comparing electron-mediated interactions\footnote{A. V. Shytov, D. A. Abanin, L. S. Levitov, Phys. Rev. Lett. {\bf 103}, 016806 (2009), and to be published} of different adatom configurations as a function of graphene doping, we find that a state can be realized in which adatoms reside on one of the two graphene sublattices. In such a state, owing to sublattice symmetry breaking, Dirac fermions acquire a band gap, which scales as a square root of adatom concentration. For realistic system parameters, we find that sublattice ordering can take place at temperatures as high as few hundred Kelvin. Our findings show that adatom adsorption provides a way to create a new semiconducting form of graphene with a tunable band gap.