Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Graphene Quantum Dots |
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Sponsoring Units: DMP Chair: Vincent Meunier, Rensselaer Polytechnic University Room: 605 |
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F30.00001: Graphene quantum dots: localized states, edges and bilayer systems Invited Speaker: Klaus Ensslin Graphene quantum dots show Coulomb blockade, excited states and their orbital and spin properties have been investigated in high magnetic fields. Most quantum dots fabricated to date are fabricated with electron beam lithography and dry etching which generally leads to uncontrolled and probably rough edges. We demonstrate that devices with reduced bulk disorder fabricated on BN substrates display similar localized states as those fabricated on the more standard SiO$_2$ substrates. For a highly symmetric quantum dot with short tunnel barriers the experimentally detected transport features can be explained by three localized states, 1 in the dot and 2 in the constrictions. A way to overcome edge roughness and the localized states related to this are bilayer devices where a band gap can be induced by suitable top and back gate voltages. By placing bilayer graphene between two BN layers high electronic quality can be achieved as documented by the observation of broken symmetry states in the quantum Hall regime. We discuss how this method can be exploited to achieve smoother and better tunable graphene quantum devices. This work was done in collaboration with D. Bischoff, P. Simonet, A. Varlet, Y. Tian, and T. Ihn. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F30.00002: Multiple quantum dot behavior in short and wide graphene devices, with disorder Joseph Lambert, Steven Carabello, Roberto Ramos Quantum dot (QD) behavior in graphene has been investigated previously in several different types of systems. These systems range from QDs etched in graphene, to impurity induced QDs in graphene nanoribbons. Here, we report on QD behavior in a new system where the graphene channel between two superconducting leads is short (a few hundred nanometers long) and wide (5-10 microns across). Measurements of conductance as a function of gate voltage and bias voltage at temperatures between 20 mK and 10 K revealed long-range tapestry patterns extending across a wide range of voltages, from -60 to $+$20 volts. Applying filtering techniques reveals Coulomb diamond features of varying sizes, suggestive of multiple QDs contributing to the conductance. The minimum conductance values for our devices range from G$_{min}\approx $ 40 e$^{2}$/h to 100 e$^{2}$/h, which are several orders magnitude larger than in typical QD systems. For several samples, measurements of conductance versus gate voltage show a broad and relatively flat minimum conductance region $\Delta $V$_{g}\approx $ 10V to 20V wide, with a center that is shifted in gate voltage to V$_{g}\approx $ -10V to -20V. This indicates impurity doping and the formation of electron/hole puddles on the graphene surface. The Coulomb diamonds uncovered by filtering is consistent with the presence of several low-barrier QDs in parallel. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F30.00003: Wigner localization in a graphene quantum dot with a mass gap Karina Andrea Guerrero Becerra, Massimo Rontani The role of electron-electron interactions in graphene is an open issue that impacts on the operation of quantum dots (QDs) and other graphene-based devices. Whereas electrons in bulk graphene allegedly behave as noninteracting particles except for subtle effects, there is strong evidence that electrons in carbon-based nanostructures--nanotubes--form Wigner molecules [Nat. Phys. 9, 576 (2013)]. Besides, a significant effort is presently devoted to minimize the role of disorder in next-generation graphene QDs. Here we show theoretically that Dirac electrons in a clean, circular graphene QD with a mass gap induced by the breaking of sublattice symmetry form a Wigner molecule for realistic values of device parameters. The evidence is the combined analysis of many-body energies, one-body densities, and pair correlation functions obtained through the exact diagonalization of the interacting Dirac-Weyl Hamiltonian. This method, which uses two different sublattice envelopes and includes both inequivalent Dirac cones, allows us to take all many-body correlations into account. The experimental signature of Wigner localization is the suppression of the fourfold periodicity of the filling sequence and the quenching of excitation energies, accessible through Coulomb blockade spectroscopy. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F30.00004: Narrow graphene nanoribbons with atomically precise armchair edges: Solution synthesis and characterization Alexander Sinitskii Although graphene is a semimetal, a substantial electronic band gap could be found in narrow graphene nanoribbons (GNRs) with atomically precise armchair edges and widths less than 2 nm. Different top-down approaches typically yield ribbons with widths \textgreater 10 nm and have a limited control over the edge structure in GNRs. Much narrower GNRs with atomically precise edges could be synthesized by a surface-assisted bottom-up approach. This method provides small amounts of GNRs of exceptional quality, but it cannot be used to produce large quantities of GNRs for bulk applications. Therefore, a complimentary chemical approach for bulk quantities of high-quality GNRs is in order. This talk will be focused on a recently developed bottom-up approach for gram quantities of narrow GNRs that are less than 2 nm wide and have atomically precise armchair edges. STM studies of these GNRs show that their structural quality is comparable to that of surface-synthesized GNRs. These nanoribbons have a bandgap of about 1.3 eV, which makes them promising for applications in field-effect transistors with high on-off ratios, as well as bulk applications, including coatings, composites and photovoltaic devices. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F30.00005: Single channel ballistic transport in epitaxial graphene nanoribbons Claire Berger, Ming Ruan, Jens Baringhaus, Frederik Edler, James Palmer, Zelei Guo, John Hankinson, Christoph Tegenkamp, Walt A. de Heer We present transport results on high quality epitaxial graphene nanoribbons about 40 nm in width, with annealed edges, grown on sidewall SiC. The nanoribbons are produced directly in their final shape with no post-graphene growth patterning. We show that the nanoribbons are neither semiconductors, nor have a transport gap, but are single channel room temperature ballistic conductors. The graphene ribbons behave as electronic waveguides or quantum dots. The low-temperature transport properties of top-gated ribbons indicates that transport is dominated by two components of the ground state transverse waveguide mode, one that is ballistic and temperature independent, and a second thermally activated component that appears to be ballistic at room temperature and insulating at cryogenic temperatures. These properties appear to be related to the lowest energy quantum states in the charge neutral ribbons.\\[4pt] [1] J. Baringhaus et al. ArXiv: 1301.5354. Nature (in print).\\[0pt] [2] M. Sprinkle et al. Nature Nanotech 5, 727 (2010).\\[0pt] [3] Y. Hu et al. J. Phys. D: Appl. Phys 45 154010 (2012).\\[0pt] [4] M. Ruan, MRS Bulletin 37, 1146 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F30.00006: Transport properties of nanoconstrictions in armchair graphene ribbons Igor Romanovsky, Constantine Yannouleas, Uzi Landman The transport properties of nanoconstrictions and quantum-point contacts formed in atomically precise segmented armchair graphene nanoribbons ( SaGRs) are investigated using a tight-binding non-equilibrium Green's function (TB-NEGF) approach and relativistic quantum-field theory modeling.\footnote{% I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B {\bf 87}, 165431 (2013)} The TB behavior is accounted for by a one-dimensional Dirac-transfer-matrix (DTM) model using variable-mass (scalar-field) barriers assigned to the junctions between the nanoribbon segments. It is shown that the topology of the junctions (sharp versus smooth) and the ratio of length over width of the constriction are the principal factors influencing the height of the mass barriers, and thus they control the extent of trapping and confinement by the constriction of graphene's relativistic carriers, even in the case of all-metallic SaGRs. A rich variety of transport patterns ensues, ranging from ballistic quantized conductance to resonant tunneling associated with Coulomb blockade. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F30.00007: Atomic-scale measurements of graphene nanoribbon edge properties Patrick Han, Katsuya Iwaya, Susumu Shiraki, Naoki Asao, Taro Hitosugi, Paul Weiss Graphene edges are predicted to be a type of defects that can be utilized to tailor both the electronic and the magnetic properties of graphene structures. However, to date, there is little experimental result on how graphene size and structure affect these edge properties. For this purpose, we fabricate defect-free graphene nanoribbons (GNRs) by self-assembly of organic precursor molecules on a Cu(111) single-crystal surface in ultrahigh vacuum. We use low-temperature scanning tunneling microscopy to image and measure the electronic properties of these ribbons, comparing GNR edges and centers. We discuss the results of our fabrication process and of our local spectroscopic measurements of individual GNRs. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F30.00008: Electronic transport in graphene ribbons with a Gausssian deformation Ramon Carrillo, Daiara Faria, Andrea Latg\'{e}, Francisco Mireles, Nancy Sandler The coupling of geometrical and electronic properties is a promising avenue to engineer conduction properties in graphene. Confinement added to strain allows for interplay of different transport mechanisms with potential device applications. In particular, strain-predicted to produce localized states similar to those in an external magnetic field--can be tailored for desired transport properties. To investigate specific strain signatures on transport in confined geometries, we focus on graphene nanoribbons with different edge terminations and circularly symmetric deformations. In particular, we study nanoribbons with an inhomogeneous, out of plane Gaussian bump deformation, connected to reservoirs, with and without external magnetic field. We use the tight-binding approximation with the deformation described by elasticity theory. Using the recursive Green function algorithm, we calculate the local density of states and obtain the Landauer conductance. An enhancement of the density of states in the deformed region, similar to the one appearing with constant fields in confined regions is observed. We show how these confined states give rise to peculiar features in the emerging Landau levels and discuss their effect on the overall conductance. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F30.00009: Bandgap Engineering of Bottom-up Synthesized graphene nanoribbon junctions Zahra Pedramrazi, Yen-Chia Chen, Chen Chen, Danny Haberer, Ting Cao, Dimas Oteyza, Felix Fischer, Steven Louie, Michael Crommie Bandgap engineering is a key concept in electronic device fabrication, through which various types of semiconductor heterostructures have been realized. However, as the size of electronic building blocks is approaching the physical limits of well-established top-down methods, the need for alternative strategies towards electronic devices becomes apparent. Considering the recent progress in bottom-up synthesis of graphene nanoribbons (GNRs), components with single-atom thickness and sub-2 nm width may be realized based on GNRs. The electronic properties of GNRs are crucially depending on their width and edge geometry, and it has been predicted that intra-ribbon bandgap engineering may be achieved by varying width or doping at desired positions. Here, we demonstrate the successful realization of bottom-up narrow-wide GNR junctions, consisting of covalent bonding of armchair segments having either 7 or 13 carbon dimer lines across the width (i.e. the n$=$7 and n$=$13 segments are ``welded together'' at the atomic scale). We study the resultant~7-13~junctions with scanning tunneling microscopy (STM) and spectroscopy (STS), and identify distinct electronic structures in different GNR segments. We have further performed first-principles calculations to support our experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F30.00010: Monomer Doping of Self-Assembled Graphene Nanoribbons for Band Gap Alignment Christopher Bronner, Stephan Stremlau, Marie Gille, Felix Brau{\ss}e, Anton Haase, Stefan Hecht, Petra Tegeder In order to exploit the technologically interesting electronic properties of graphene, several concepts have been discussed which would lead to the opening of a band gap. One approach is spatial confinement of the charge carriers in quasi-one-dimensional graphene nanoribbons (GNRs). The band gap of a GNR scales inversely with its width and particularly nanometer-scale widths are desirable for application e.g. in transistor devices. Since the electronic properties of GNRs depend critically on their structure, precise synthesis is necessary but challenging for conventional methods such as lithography. In contrast, self-assembly from molecular precursors is an intriguing approach which has been employed to fabricate defect-free GNRs with well-defined widths and edge structures. Only this high level of structural precision allows introduction of dopant atoms at specific doping sites and concentrations in the graphene lattice. Nitrogen doping has been known to shift the band structure of GNRs with respect to the Fermi level which is interesting for GNRs in contact with electrodes and other device materials. Using surface-sensitive electron spectroscopies we demonstrate a continuous down-shift of the band structure with increased nitrogen doping of the monomers. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F30.00011: Polarization Dependent Optical Responses of Graphene Nanoribbons Ting Cao, Sangkook Choi, Steven Louie The optical response of an anisotropic system depends on light's polarization direction. In this study, we perform first-principle calculations on polarization dependent optical absorption spectra of graphene nanoribbons at the RPA and GW-BSE level. We observe significant polarization dependent features. We demonstrate the many-body origins of these features. We also discuss the polarization dependent optical responses of other carbon nanostructures, and connect our work to experimental measurements. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F30.00012: A structural and electronic comparison of armchair and zigzag epitaxial graphene sidewall nanoribbons Meredith Nevius, F. Wang, I. Palacio, A. Celis, A. Tejeda, A. Taleb-Ibrahimi, W. de Heer, C. Berger, E. Conrad Graphene grown on sidewalls of trenches etched in SiC shows particular promise as a candidate for post-Si CMOS electronics because of its ballistic transport, exceptional mobilities, low intrinsic doping, and the opening of a large band gap. [1,2] However, before definitive progress can be made toward epitaxial graphene-based transistors, we must fully understand the nuances of graphene ribbon growth on different SiC facets. We have now confirmed that sidewall ribbons grown in graphene's two primary crystallographic directions (``armchair'' and ``zigzag'') differ greatly in both structure and electronic band-structure. We present data from both geometries obtained using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), photoemission electron microscopy (PEEM), micro-ARPES and dark-field micro-ARPES. We demonstrate that while graphene grows on stable facets of trenches oriented for armchair edge growth, trenches oriented for zigzag edge growth prefer narrow ribbons of graphene on the (0001) surface near the trench edge. The structure of these zigzag edge graphene ribbons is complex and paramount to understanding their transport. [1] J. Baringhaus et al. arXiv:1301.5354 Nature to be published [2] J. Hicks et al. Nature Physics (2012). This work was supported by the NSF under grants DMR-1005880 and DMR-0820382, the W. M. Keck Foundation and the Partner University Fund from the Embassy of France. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F30.00013: Interference between Fano resonances for impurities in graphene nanoribbon C.H. Chiu, C.S. Chu The presence of impurities in a gapless armchair graphene nanoribbon (AGNR) is expected to exhibit Fano resonances in the conductance $G$ of the AGNR. For instance, in the low energy range, when the only propagating subband is gapless, an impurity with an on-site potential $V$\textgreater 0 will give rise to the formation of the Fano resonance just above the first hole-like subband. This Fano characteristics, however, is masked in the total $G$ by the contribution from the first hole-like subband. In this work, we show that two neighboring impurities of the same type could recover the peak-dip Fano characteristics by shifting the peak-part of the Fano structure away from the first hole-like subband. Factors that affect, separately, the peak- and the dip-part of the Fano profile will be elucidated and discussed. Furthermore, the interference of the Fano resonances from two neighboring impurities, and the sensitivity to their respective locations in the sublattice, will be studied in detail. For comparison, we also consider the case of gapped AGNRs. [Preview Abstract] |
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