Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session X21: Focus Session: Graphene: Devices |
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Sponsoring Units: DMP Chair: Vasili Perebeinos, IBM Room: Portland Ballroom 251 |
Thursday, March 18, 2010 2:30PM - 2:42PM |
X21.00001: Graphene as a platform to study 2D electronic transitions Vincent Bouchiat, Brian Kessler, Caglar Girit, Alex Zettl The easily accessible 2D electron gas in graphene provides an ideal platform on which to tune, via application of an electrostatic gate, the coupling between electronically ordered dopants deposited on its surface. To demonstrate this concept, we have measured arrays of superconducting clusters deposited on Graphene capable to induce via the proximity effect a gate-tunable superconducting transition. Using a simple fabrication procedure based on metal layer dewetting, doped graphene sheets can be decorated with a non percolating network on nanoscale tin clusters. This hybrid material displays a two-step superconducting transition. The higher transition step is gate independent and corresponds to the transition of the tin clusters to the superconducting state. The lower transition step towards a real zero resistance state exhibiting a well developped supercurrent, is strongly gate-tunable and is quantitatively described by Berezinskii-Kosterlitz-Thouless 2D vortex unbinding. Our simple self-assembly method and tunable coupling can readily be extended to other electronic order parameters such as ferro/antiferromagnetism, charge/spin density waves using similar decoration techniques. [1] B. M. Kessler, C.O. Girit, A. Zettl, and V. Bouchiat, Tunable Superconducting Phase Transition in Metal-Decorated Graphene Sheets submitted to PRL, arXiv:0907.3661 [Preview Abstract] |
Thursday, March 18, 2010 2:42PM - 2:54PM |
X21.00002: Graphene Nanoconstriction Field Effect Transistor Ye Lu, Brett Goldsmith, Douglas Strachan, A.T. Charlie Johnson We report an approach to fabricate monolayer graphene nanoconstriction field effect transistors (NCFETs) with critical dimensions below 10 nm, a regime that is not accessible by conventional nanolithography. We start by fabricating a gold nanowire on top of mechanically-exfoliated monolayer graphene. We use Feedback Controlled Electromigration to form a nanoconstriction in the gold wire, which is then used as an etch mask for the graphene during an oxygen plasma patterning step. We observe the opening of a confinement-induced energy gap as the NCFET width is reduced, as evidenced by a sharp increase in the NCEFT on/off ratio, which can be as large as 1100 at room temperature for the thinnest devices. Such devices deliver up to 100microampere current at 50mV bias with an on state resistance of 50kilo ohm, which is at least an order of magnitude lower than graphene nanoribbon FETs with similar on/off ratio. This lower resistance is due to large area contacts in our devices. [Preview Abstract] |
Thursday, March 18, 2010 2:54PM - 3:06PM |
X21.00003: Channel Length Scaling Effects in Graphene Field-Effect Transistor Inanc Meric, Jyotsna Chauhan, Melinda Han, Philip Kim, Jing Guo, Kenneth L. Shepard We present measurement and analysis of the current-voltage characteristics in the high-bias regime of graphene field-effect transistors of different channel lengths. The devices are fabricated with a gate dielectric process based on a polyvinyl alcohol adsorption layer, enabling reliable top-gates with the highest reported efficiency. Device characteristics are strongly determined by velocity saturation of the carriers, the zero-bandgap density-of states, contact doping, and tunneling. Contact doping strongly reduces the effective channel length in the absence of ``spacer'' between the gated channel region and the contacts. Surface polar optical phonon scattering determines saturation velocities down to short channel lengths. At the shortest channel lengths, band-to-band tunneling degrades device output conductance and transconductance. [Preview Abstract] |
Thursday, March 18, 2010 3:06PM - 3:42PM |
X21.00004: Graphene Electronics and Optoelectronics Invited Speaker: Because of its zero band-gap, the current in nonolayer graphene cannot be effectively switched off as in conventional semiconductors. However, the limited field tuning of the current afforded by its DOS and its exceptionally fast carrier transport can be used to build advanced high-frequency analog devices. I will discuss the device physics, fabrication and operation of RF graphene transistors with cut-off frequencies up to 100GHz. Despite its gapless nature, built-in fields at graphene-metal contacts can also be used to construct ultrafast photoconductors. Such devices will be demonstrated and utilized for error-free detection of GBit/s optical data streams. Finally, we will discuss the electrical bandgap opening in bilayer graphene and demonstrate bilayer transistors operating at room temperature. Co-workers: Y.-M. Lin, F. Xia, T. Mueller, K. Jenkins, D. Farmer, C. Dimitrakopoulos, H.-Y. Chiu, A. Valdes-Garcia, A. Grill. [Preview Abstract] |
Thursday, March 18, 2010 3:42PM - 3:54PM |
X21.00005: Mechanism for energy dissipation and current saturation in graphene transistors Ashley DaSilva, Ke Zou, Jainendra Jain, Jun Zhu We have studied the high-field transport in graphene FETs on an amorphous SiO$_{2}$ substrate by a combination of theory and experiment. A theoretical treatment using the Boltzmann equation and a drifted Fermi distribution function gives an excellent quantitative account of the experiments without any adjustable parameters. Our study establishes that the surface optical (SO) phonons of the substrate dominate the high-field transport properties in a broad range of parameters. The efficient heat dissipation directly from electrons to the substrate results in a cooling of the electrons, and hence an enhancement of the drift velocity despite the additional scattering mechanism, thus explaining a lack of saturation in current experiments. Our results suggest that one may exploit knowledge of the substrate properties to control the high field transport in graphene. [Preview Abstract] |
Thursday, March 18, 2010 3:54PM - 4:06PM |
X21.00006: Large-area Graphene-PDMS Hybrid Structures for Multifunctional Applications Swastik Kar, Xiaohong An, Edward Joseph, Trevor Simmons, Saikat Talapatra, Morris Washington, Saroj Nayak We report on the fabrication, characterization and applications of flexible, transparent, electrically conducting hybrid structures using thin films of functionalized graphene and PDMS. These structures can be directly used as conductometric sensors for organic vapors and thin-film atmospheric pressure sensors. The graphene present in the hybrids are pre-functionalized by 1-Pyrenecarboxylic acid, which is a known fluorophore that exhibits strong absorbance peaks even at trace amounts in the UV regions. As a result, these flexible membranes are nearly transparent in the visible region, but absorb a significant amount of UV light, rendering them useful as flexible UV filters. Their conductance was also found to be quite sensitive to the presence of light, and hence can be used as flexible photodetectors. [Preview Abstract] |
Thursday, March 18, 2010 4:06PM - 4:18PM |
X21.00007: Multi-scale modeling of nanotransistors Anders Blom, Kurt Stokbro This talk will discuss the current state-of-the-art in atomic-scale modeling of electronic devices. Over the last decade, combined DFT and NEGF methods have become an established approach for describing the non-equilibrium transport properties of e.g. nanotubes, graphene, and molecular electronics structures, involving system geometries with two leads, or electrodes. The ballistic tunneling current through the scattering or contact region can be computed at a finite bias applied between these electrodes. In order to simulate more device-like geometries it is however also necessary to consider the influence of various types of electrostatic gates and dielectric screening regions. To describe such systems realistically also requires a dramatically larger simulation cell, since the gates are typically positioned at distances that are an order of magnitude larger than the features of the active nanoscale device region itself. We will present how a multi-scale approach which includes an advanced description of the electrostatic environment, as implemented in the atomic-scale modeling platform Atomistix ToolKit from QuantumWise, can be used to model nanoscale field-effect type transistor structures and single-electron transistor devices. [Preview Abstract] |
Thursday, March 18, 2010 4:18PM - 4:30PM |
X21.00008: Ultrathin Graphene Membranes as Flexible Electrodes for Electrochemical Double Layer Capacitors Saikat Talapatra, Swastik Kar, Rakesh Shah, Sujoy Ghosh, Xiaohong An, Trevor Simmons, Morris Washington, Saroj Nayak We will present the results of our investigations of electrochemical double layer capacitors (EDLCs) or supercapacitors (SC) fabricated using graphene based ultra thin membranes. These EDLC's show far superior performance compared to other carbon nanomaterials based EDLC's devices. We found that the graphene based devices possess specific capacitance values as high as 120 F/g, with impressive power densities ($\sim $105 kW/kg) and energy densities ($\sim $9.2 Wh/kg). Further, these devices indicated rapid charge transfer response even without the use of any binders or specially prepared current collectors. Our ultracapacitors reflect a significant improvement over previously reported graphene-based ultracapacitors and are substantially better than those obtained with carbon nanotubes. [Preview Abstract] |
Thursday, March 18, 2010 4:30PM - 4:42PM |
X21.00009: Low Temperature Photocurrent Measurements in Graphene Devices George Nazin, Yan Zhang, Liyuan Zhang, Peter Sutter The Dirac-like chiral nature of charge carriers in graphene has been linked to a number of unusual charge transport phenomena, including suppression of localization and minimum conductivity. A crucial ingredient to understand such phenomena in graphene is the ability to correlate the charge transport characteristics with the corresponding internal potential landscape and band-bending. With a scanning optical microscope operated at LHe temperature we have measured lateral photo-current in graphene-based devices. Spatial maps of photocurrent obtained using this approach contain information about the distribution of lateral electrostatic fields in these devices. At room temperature, band-bending induced by metal contacts has been observed. At cryogenic temperature, formation of electron-hole puddles leads to spatially inhomogeneous maps of photocurrent, which become very sensitive to the applied gate voltage. [Preview Abstract] |
Thursday, March 18, 2010 4:42PM - 4:54PM |
X21.00010: Low-Frequency Noise in Top-Gate Graphene Transistors Guanxiong Liu, William Stillman, Sergey Rumyantsev, Michael Shur, Alexander Balandin Electronic and sensor applications of graphene require low levels of the low-frequency noise. The unavoidable noise up-conversion results in serious limitations for practical use. Here we report results of our experimental investigation of flicker noise in the top-gate graphene field-effect transistors. We prepared graphene flakes by mechanical exfoliation and verified their quality with Raman spectroscopy. The hafnium-oxide top-gate dielectric was grown by the atomic layer deposition. The measurements revealed a low level of the noise with the spectrum close to 1/f [1-2]. The analysis of its gate dependence allowed us to identify the noise sources. The work at UCR was supported by DARPA -- SRC FENA and IFC. \\[4pt] [1] Q. Shao, G. Liu, et al., Electron Dev. Lett., 30, 288 (2009). \\[0pt] [2] G. Liu, et al., Appl. Phys. Lett., 95, 033103 (2009). [Preview Abstract] |
Thursday, March 18, 2010 4:54PM - 5:06PM |
X21.00011: Terahertz Spectroscopy of Large-Area Graphene L. Ren, T. Arikawa, B.F. Cruz, M.Z. Jin, J. Kono, Z. Jin, J.M. Tour, A.K. Wojcik, A.A. Belyanin, Y. Takemoto, K. Takeya, I. Kawayama, M. Tonouchi Graphene is predicted to possess exotic nonlinear electromagnetic properties, which may lead to novel terahertz (THz) applications. THz dynamic conductivity measurements allow us to probe the dynamics of such 2D quantum relativistic Dirac fermions (DF). Here, we used THz time-domain spectroscopy (TDS) to measure large-area graphene grown by chemical vapor deposition. Raman spectroscopy, as well as optical absorption spectroscopy from the ultraviolet to the far-infrared, was used to determine the number of layers and assess the sample quality. THz complex dynamic conductivity was extracted and exhibited a non-Drude frequency dependence between 0.2 and 2.0 THz, which will be discussed by taking into account both interband and intraband dynamics. Gate voltage will be further added to the graphene to tune its Fermi level. Also a magneto-THz-TDS setup with a wider frequency range of up to 6 THz will be used to study time-domain cyclotron resonance of DF. [Preview Abstract] |
Thursday, March 18, 2010 5:06PM - 5:18PM |
X21.00012: Graphene based switch Hang Zhang, Wenzhong Bao, Zeng Zhao, Gang Liu, Jairo Volesco, Lei Jing, Brian Standley, Marc Bockrath, Chunning Lau We investigate both suspended and non-suspended SiO2 substrate-supported graphene based switch devices, which can be switched ``OFF'' by applying pulses of 8V and ``ON'' by pulses of 4V. In addition to the I-V characteristics, we also investigate the statistical behavior of the switching rate and ON state conductance. Conductance switching is observed in suspended as well as non-suspended devices. Our experimental results suggest that the observed conductance switching in SiO2-supported devices results from a combination of the switching behavior of the underlying SiO2 substrate as well as the switching behavior of the graphene break junction alone. [Preview Abstract] |
Thursday, March 18, 2010 5:18PM - 5:30PM |
X21.00013: Fabrication of High Quality Suspended Graphene devices Zeng Zhao, Wenzhong Bao, Hang Zhang, Lei Jing, Velasco Jairo, Gang Liu, Chun Ning Lau Suspended graphene devices have been demonstrated to have ultra-high mobility, enabling the recent obsevation of fractional quantum Hall effect. Here we report a lithography-free technique for fabrication of suspended graphene devices. Graphene sheets are exfoliated over pre- defined trenches on the substrate. Hard shadow masks are etched from Si wafers and aligned to the trenches. Metals are evaporated through the masks to form clean suspended devices, eliminating contaminants introduced by lithography and etching processes. The completed devices have high mobilities $\sim $100,000 cm$^2$/Vs. [Preview Abstract] |
Thursday, March 18, 2010 5:30PM - 5:42PM |
X21.00014: Resonant Energy Transfer from Individual Semiconductor Nanocrystals to Graphene Zheyuan Chen, Stephane Berciaud, Tony F. Heinz, Louis E. Brus Graphene is a promising material for the fabrication of transparent electrodes in photovoltaic devices. In order to evaluate its potential, however, we must understand the mechanism of charge and energy transfer from a light harvesting material to such graphene layers. Here we focus on the latter process and demonstrate strong resonant energy transfer from individual CdSe nanocrystals to graphene layers. We observe resonant energy through the presence of strong quenching (by a factor of nearly 100) of the luminescence of individual CdSe nanocrystals when placed upon a single-layer graphene sheet. The quenching efficiency increases with thickness for few-layer graphene sheets and is found to saturate at a value of $\sim $600 for bulk graphite. Our results are in good qualitative agreement with a simple theoretical model using the dipole approximation and a tight-binding description of the graphene electronic structure [1]. \\[4pt] [1] R.S. Swathi and K.L. Sebastian, \textit{J. Chem. Phys.} \textbf{130}, 086101 (2009) [Preview Abstract] |
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