Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session Y37: Focus Session: Probing Novel Nanostructures |
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Sponsoring Units: DMP Chair: Daniel Ralph, Cornell University Room: Baltimore Convention Center 340 |
Friday, March 17, 2006 8:00AM - 8:12AM |
Y37.00001: Highly Ordered Graphene for Two Dimensional Electronics Rui Feng, J.R. Hass, E.H. Conrad, X Li, C. Berger, W.A. De Heer, T. Li, P.N. First, C.A. Jeffrey Many of the same electrical properties of carbon nanotubes can be realized by confining grapheme sheets to nano-dimensions. Scalability issues of nanotube devices can therefore be overcome by lithographic patterning of graphene films if thin well ordered graphene films can be grown. We have been able to grow ultrathin epitaxial graphite films that show remarkable 2D electron gas (2DEG) behavior by thermal decomposition of both Si-face and C-face 4H-SiC. Early studies of the structure of both surfaces of 4H-SiC showed that graphite grown on the C- face was rotationally disordered compared to the Si-face. For this reason the C-face graphite was ignored as a potential substrate for graphitic devices. However, our surface X-ray scattering measurements show that while the orientational order is reduced, the coherent film size of the C-face graphite is more than three time greater than the Si-face. Part of the reason is that the Si-face step density increases dramatically with graphitization while the C-face shows little change in step structure. These observations are consistent with the improved 2D conductivity measurement on the C-face graphite films. [Preview Abstract] |
Friday, March 17, 2006 8:12AM - 8:24AM |
Y37.00002: 2d electronic gas properties of epitaxial graphene Claire Berger, Cecile Naud, Zhimin Song, Xuebin Li, Walt de Heer We present transport measurement on multi-layered epitaxial graphene grown on SiC. The films, a few to a few dozen layers thick, can be lithographically patterned and show remarkable 2d electron gas properties. In high mobility samples (up to 10$^{4}$ cm$^{2}$/Vs) perpendicular magnetoresistance measurements indicate micrometer long electronic phase coherence lengths at 4K, comparable to the sample size. Pronounced Shubnikov --de Haas oscillations are consistent with graphene-like electronic dispersion relation. A novel low temperature electronic phase transition was also observed. Most recent development of his ongoing research will be presented. [Preview Abstract] |
Friday, March 17, 2006 8:24AM - 8:36AM |
Y37.00003: STM studies of graphite microdevices Elena Polyakova, George Flynn, Yuanbo Zhang, Mina Fazlollahi, Philip Kim A novel scanning tunneling microscope (STM) has been designed and built to study transport phenomena in mesoscopic conductive films, self-assembled monolayers, and nanostructures on insulating substrates under ambient conditions. In this work we present experimental STM studies of monocrystalline ultrathin graphite films including single graphite sheets (graphene) at the atomic scale. Electronic transport is possible only in the lateral direction for graphene. Graphite microscopic devices ($\sim $ 10 $\mu $m) have been prepared by mechanical exfoliation followed by deposition of macroscopic gold electrodes over the graphite film and its surrounding insulator. Evolution of STM images during the transition from multilayer to single graphene sheets as well as the dependence of STM images on tunneling conditions near the Fermi energy will be discussed. [Preview Abstract] |
Friday, March 17, 2006 8:36AM - 9:12AM |
Y37.00004: Low temperature scanning tunneling microscopy of metallic and organic nanostructures Invited Speaker: Low temperature scanning tunneling microscopy (LT-STM) is capable of both characterizing and manipulating atomic-scale structures at surfaces. It thus provides a powerful experimental tool to gain fundamental insight into how electronic properties evolve when controlling size, geometry, and composition of nanometric model systems at the level of single atoms and molecules. The experiments discussed in this talk employ a Cu(111) surface onto which perfect nanostructures are assembled from native adatoms and organic molecules. Using single Cu adatoms as building blocks, we obtain zero-, one-, and two-dimensional quantum objects (corresponding to the discrete adatom, monatomic adatom chains, and compact adatom assemblies) with intriguing electronic properties. Depending on the structure shape and the number of incorporated atoms we observe the formation of characteristic quantum levels which merge into the sp-derived Shockley surface state in the limit of extended 2D islands; this state exists on many surfaces, such as Cu(111). Our results reveal the natural linkage between this traditional surface property, the quantum confinement in compact adatom structures, and the quasi-atomic state associated with the single adatom. In a second step, we study the interaction of pentacene (C$_{22}$H$_{14})$ with Cu adatom chains serving as model quantum wires. We find that STM-based manipulation is capable of connecting single molecules to the chain ends in a defined way, and that the molecule-chain interaction shifts the chain-localized quantum states to higher binding energies. The present system provides an instructive model case to study single organic molecules interacting with metallic nanostructures. The microscopic nature of such composite structures is of importance for any future molecular-based device realization since it determines the contact conductance between the molecular unit and its metal ''contact pad''. [Preview Abstract] |
Friday, March 17, 2006 9:12AM - 9:24AM |
Y37.00005: Imaging Electron Flow in Two-Dimensional Electron Gases M. P. Jura, M. A. Topinka, A. R. Sciambi, D. H. Lo, D. Goldhaber-Gordon We present images of electron flow from a quantum point contact (QPC) into a surrounding two-dimensional electron gas (2DEG). We resolve flow patterns associated with the one-dimensional modes responsible for quantized conductance through the QPC. The imaging technique relies on scanning a charged tip over the 2DEG and simultaneously measuring the conductance through the sample [1]. We show images of electron flow in different GaAs/AlGaAs heterostructures, and we discuss which features of 2DEGs can affect both application of this technique and the observed current flow paths. [1] M. A. Topinka \textit{et al.}, Nature 410, 183 (2001). [Preview Abstract] |
Friday, March 17, 2006 9:24AM - 9:36AM |
Y37.00006: Probing Single-Electron Charging and Dissipation of Au Nanocrystals with Electric Force Microscopy J. Zhu, M. Brink, P. L. McEuen We synthesize and link Au nanocrystals to carbon nanotube (CNT) field effect transistors through thiol and aromatic ring bi- functionalized molecules. AFM images taken at room temperature using tapping mode and at 77 K using force microscopy both show specific binding of Au nanocrystals onto the CNTs. At 77 K, the small size of the Au nanocrystals (d $\sim$ 12 nm) leads to quantum dot phenomena. A metalized AFM tip perturbs the electrostatic potential of the dot and brings electrons onto the dot one by one. The CNT serves as a charge transfer line and a reservoir. We study the force, frequency shift, and dissipation generated by this single-electron motion using AFM-based electric force microscopies. We observe Coulomb oscillations of the Au nanocrystal and derive its capacitances to the gates, i.e. the backgate, the AFM tip, and the anchoring CNT, through charge addition spectra. The total capacitance of the Au nanocrystals was found to be approximately 1.6 aF, corresponding to a charging energy of 100 meV. Most interestingly, we observe a typical power dissipation of $\sim$ 10 aW, extracted from the decrease of the quality factor Q of the AFM cantilever, for many charging events. We discuss the possible origins of the observed dissipation. [Preview Abstract] |
Friday, March 17, 2006 9:36AM - 9:48AM |
Y37.00007: Quantitative Investigation of Quantum Dots Using Frequency Shift Microscopy at Low Temperatures. Markus Brink, Jun Zhu, Paul L. McEuen We use frequency shift microscopy (FSM), an AFM-based scanned probe technique, to study quantum dots formed in carbon nanotubes at 4K. FSM requires the quantum dot to be coupled to only one charge reservoir, allowing us to probe quantum dots in single-terminal geometries; furthermore, FSM requires only weak coupling between the quantum dot and the charge reservoir, permitting measurements in poorly conducting regimes that are inaccessible to transport measurements. Charging events of individual quantum dots are detected with single-electron sensitivity down to the few electron regime. We describe a general method to extract the charging energy of a quantum dot from frequency shift measurements. Coupled dots show avoided crossings in FSM charge addition spectra, from which we extract their mutual capacitance. [Preview Abstract] |
Friday, March 17, 2006 9:48AM - 10:00AM |
Y37.00008: Measurements of electron-in-a-box level spectra in chemically-synthesized metal nanoparticles Ferdinand Kuemmeth, K.I. Bolotin, D.C. Ralph We incorporate chemically-synthesized metal nanoparticles into a single electron transistor geometry such that tunneling spectroscopy can be used to measure the electron energy levels within a single nanoparticle at dilution refrigerator temperatures. This technique gives better control over the size and shape of the nanoparticle than previous studies of level spectra in metals. We use a monolayer of dithiols or propylamines as a self-assembled tunnel barrier on top of a back-gated pair of gold electrodes, separated by a nm-sized gap. Into the gap we trap gold nanospheres 5 to 15 nm in diameter from a citrate based colloid solution, with control provided by adjusting the pH. Previous experiments on gold nanoparticles found a wide range of g-factors for Zeeman splitting in an applied magnetic field, with some values inconsistent with expected orbital contributions. Our measurements clarify the relationship between g-factors and level spacing through systematic variation of the particle's size and composition. [Preview Abstract] |
Friday, March 17, 2006 10:00AM - 10:12AM |
Y37.00009: Single-electron transport through a Mn$_{12}$ (2-thiophenecarboxylate) single-molecule magnet Christopher M. Ramsey, Enrique del Barco, Eduardo Mucciolo, Firoze Haque, Saiful Khondaker, Michael Leuenberger, Abhudaya Mishra, George Christou We report single-electron transport measurements on Mn$_{12}$ based single-molecule magnet, which has been functionalized with 2-thiophenecarboxylate ligands that bind to gold. The self-assembly of these molecules was confirmed by scanning probe microscopy and XPS measurements. Because it is well known that the molecule's environment within the crystal can have a profound influence on the quantum properties of the system, it is important to study the quantum spin dynamics in individual isolated molecules. Single electron transistor devices have been prepared for this purpose by electron beam lithography and electromigration. The transport properties of a single, isolated Mn$_{12}$(2-thiophenecarboxylate) molecule were measured down to mK temperatures in a 3-D superconducting vector magnet with arbitrary field direction. The data are characteristic of a molecular single-electron transistor device where the SMM bridges the gap between two gold nanoelectrodes. Magnetic field and temperature dependence as well as theoretical aspects will be discussed. [Preview Abstract] |
Friday, March 17, 2006 10:12AM - 10:24AM |
Y37.00010: Local electronic structure of a quantum point contact observed with STM Katsumi Nagaoka, Snin Yaginuma, Tadaaki Nagao, Tomonobu Nakayama A quantum point contact (QPC) is realized when a width of an electron conduction channel is comparable to the Fermi wavelength of electrons in a material. Since the first observation that QPCs show the conductance quantization in units of G$_{0}$=2e$^{2}$/h, QPCs have been mainly used in transport measurements. But the quantized states of confined electrons, which are the origin of the quantized conductance in the transport measurements, have not been observed directly so far. We report the direct observation of electronic structures of QPCs using an STM spectroscopy at 77K. The QPCs are formed across boundary gaps between a multi-domain Bi film on a Si(111)-$\beta \surd $3x$\surd $3-Bi substrate. Since the QPCs are formed in the direction parallel to the substrate, the direct observations of the atomic and the local electronic structures by using STM are possible. The electronic structure of the QPCs is neither sensitive to the external electric field induced by the STM tip nor to the length of the bridging part, and is well-explained by electronic confinement in a one-dimensional cylindrical potential well. [Preview Abstract] |
Friday, March 17, 2006 10:24AM - 10:36AM |
Y37.00011: Spectral diffusion in the tunneling spectra of ligand-stabilized undecagold clusters Thomas P. Pearl, Rachel K. Smith, Sanjini U. Nanayakkara, Paul S. Weiss, Gerd H. Woehrle, James E. Hutchison Electronic properties of isolated, solution-derived, and ligand-stabilized undecagold clusters (Au$_{11}$L$_{10})$ in both cryogenic (4 K, UHV) and ambient conditions were measured using scanning tunneling microscopy and spectroscopy. Clusters were immobilized on alkanethiolate self-assembled monolayer matrices with inserted dithiol tethers. At low temperature, Au$_{11}$L$_{10}$ clusters demonstrated Coulomb blockade, with zero-conductance gaps resulting from quantum size effects, and spectral hopping. While these clusters were immobilized for probing, we hypothesize that these assemblies may be dynamic and influence their charge transport significantly. Diffusion in the conductance resonances of the tunneling spectra of Au$_{11}$L$_{10}$ clusters is observed for a \textit{single }cluster, as well as across multiple clusters. The observed spectral diffusion is independent of tunneling conditions and varies significantly for spectra recorded with identical setpoint/tunneling junction conditions. [Preview Abstract] |
Friday, March 17, 2006 10:36AM - 10:48AM |
Y37.00012: Influence of Linker Molecules on Charge Transport through Self-Assembled Single-Nanoparticle Devices. Al-Amin Dhirani, Amir Zabet-Khosousi, Yoshinori Suganuma, Kenneth Lopata, Paul-Emile Trudeau We investigate electrical characteristics of single-electron electrode/nanoisland/electrode devices formed by alkanedithiol assisted self-assembly. Contrary to predictions of the orthodox model for double tunnel junction devices, we find a significant ($\sim $five fold) discrepancy in single-electron charging energies determined by Coulomb blockade (CB) voltage thresholds in current-voltage measurements versus those determined by an Arrhenius analysis of conductance in the CB region. The energies do, however, scale with particle sizes, consistent with single-electron charging phenomena. We propose that the discrepancy is caused by a multi-barrier junction potential that leads to a voltage divider effect. Temperature and voltage dependent conductance measurements performed outside the blockade region are consistent with this picture. We simulated our data using a suitably modified orthodox model. [Preview Abstract] |
Friday, March 17, 2006 10:48AM - 11:00AM |
Y37.00013: Absence of Even-Odd Effect in Kondo Resonance in Single-Molecule Transistors Dong-Hun Chae, Zhen Yao, Jeong T. Lee, Jonathan L. Sessler We have observed the Kondo effect in single-electron transistors incorporating individual cyclo[$n$]pyrrole molecules. In contrast to the usual spin-1/2 Kondo effect, the zero-bias Kondo resonance in these devices appears for both even and odd number of electrons. We suggest a model in which instead of alternate spin filling, two successive electrons are added to the molecule with parallel spins due to strong exchange interaction within the molecule. [Preview Abstract] |
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