Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session B11: Focus Session: Transport Properties of Nanostructures I: Surface Assemblies and Films |
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Sponsoring Units: DMP Chair: Douglas Natelson, Rice University Room: 305 |
Monday, March 16, 2009 11:15AM - 11:51AM |
B11.00001: Following elemental chemical steps by imaging molecular orbitals by STM Invited Speaker: Probing the electronic structure of molecules by STM is complicated by the strong interaction of the molecular orbitals with the metal substrate.[1,2] Adsorbing the molecule on an ultrathin insulating film alleviates this problem and allows direct imaging of molecular orbitals in real space.[1] This approach will be illustrated by two examples: characterizing the operation of a single-molecule switch and controlling and monitoring the formation of an organometallic complex. The inner hydrogens in the central cavity of a naphthalocyanine molecule can be switched between two equivalent positions.[3] This process (hydrogen tautomerization reaction) can be initiated in a controlled fashion by excitation induced by the inelastic tunnelling current. The tautomerization reaction can be followed by resonant tunnelling through the LUMO of the molecule and is expressed as considerable changes in the conductivity. In addition, we demonstrate a coupling of the switching process so that the charge injection in one molecule induces tautomerization in an adjacent molecule. The other example will consider constructing an organometallic complex from individual organic molecules and metal atoms by STM manipulation on the ultrathin insulating film. The manipulation process and the associated changes in the molecular orbitals (energy, spatial extension, symmetry) can be followed STM imaging and spectroscopy. \\[4pt] [1] J. Repp et al. Phys. Rev. Lett. 94, 026803 (2005).\\[0pt] [2] X.H. Qiu, G.V. Nazin, W. Ho Science 299, 542 (2003).\\[0pt] [3] P. Liljeroth, J. Repp, G. Meyer Science 317, 1203 (2007). [Preview Abstract] |
Monday, March 16, 2009 11:51AM - 12:03PM |
B11.00002: Apparent anomaly in electron scattering in Ag nanostructures* Ellen D. Williams, Chenggang Tao, William G. Cullen Electron scattering from diffusing atoms can be visualized via the effects of the corresponding force that biases atomic motion, the ``electromigration force''. Using a combination of scanning tunneling microscopy and scanning electron microscopy, we investigate electron scattering via the biased motion of monatomic islands and C$_{60}$-decorated steps on Ag(111) surfaces in the presence of large current density (j$_{bulk}$ = 6.7x10$^{9}$ A/m$^{2})$. For monatomic adatom islands, the biased motion is opposite to the current direction and thus parallel to the direction of momentum transfer (the ``wind force'' direction), while vacancy islands move oppositely. The measured drift velocity v as a function of the island radius R, vR = 1.9nm$^{2}$/s, yields an anomalously large$^{1}$ effective force per boundary atom $\sim $0.06 meV/nm . An effective scattering force of similar magnitude is also observed via current-induced curvature of C$_{60}$ decorated line-boundaries. Possible mechanisms for this effect, including current crowding, charge transfer and local heating, will be discussed. 1. A. Bondarchuk, et al. Phys. Rev. Lett. \textbf{99, 206801 (2007)} [Preview Abstract] |
Monday, March 16, 2009 12:03PM - 12:15PM |
B11.00003: Orbital-resolved polaron states in CdSe dots and rods probed by scanning tunnelling spectroscopy Peter Liljeroth, Zhixiang Sun, Ingmar Swart, Christophe Delerue, Dani\"el Vanmaekelbergh Despite the extensive knowledge of phonons in semiconductor crystals, the polaron states formed by the coupling between phonons and single electronic orbitals have not been measured directly due to the negligible spacing between the energy levels in conventional semiconductors. This can be overcome in semiconductor nanocrystals that have discrete energy levels due to quantum confinement. Here, we present scanning tunnelling spectroscopy results on CdSe rods and dots showing the single- electron polaron energy levels with their phonon replica. We measure the spacing and intensity of the replica, and derive the electron-phonon coupling strength for different orbital symmetries. The effect of multiple added electrons on the coupling strength can be assesses under shell-filling conditions. Our results show the formation of polaron eigenstates arising from Fröhlich coupling of an electron to longitudinal phonons with a coupling strength that depends considerably on the size and shape of the nanocrystals. The results are important for understanding electron transport in zero and one-dimensional semiconductors and the intra-band relaxation of hot carriers in quantum dots. [Preview Abstract] |
Monday, March 16, 2009 12:15PM - 12:27PM |
B11.00004: Morphology and transport properties of self-assembled, ligand-exchanged PbSe nanocrystal arrays Ching-Tzu Chen, Weon-kyu Koh, Christopher Murray, Chang C. Tsuei Self-assembled PbSe nanocrystal (NC) arrays have shown strong potential as a viable candidate for producing large-scale quantum dot superlattices. Such superlattices not only have significant technological implications, but they also serve as a model system for simulating strongly correlated transition-metal oxides. At present, highly-ordered PbSe NC arrays can be reproducibly prepared on structured Si-substrates by drop-casting PbSe solution in controlled environments. The as-grown superlattice films are nearly insulating, and post-processing ligand exchange is necessary to induce conduction. However, the lack of understanding of the ligand exchange processes has been a bottleneck to reliably producing highly-ordered conductive arrays. In this talk, we report on in-depth characterizations of the PbSe NC films treated with various ligand molecules in different solvents. The effect of a range of ligands and solvents on the film morphology will be discussed in details. Preliminary temperature-dependent transport and noise studies will be presented. [Preview Abstract] |
Monday, March 16, 2009 12:27PM - 12:39PM |
B11.00005: Tunneling Spectroscopy of Ultrathin Insulating Films: Cu$_2$N on Cu(100) Charles Ruggiero, Taeyoung Choi, Jay Gupta Insulating films of only a few atomic layers offer insight into the evolution of electronic structure at the nanoscale. We report scanning tunneling microscopy (STM) studies of one monolayer Cu$_2$N films grown on Cu(100). Our tunneling spectra indicate that Cu$_2$N acts as an insulator, with a band gap that exceeds 4~eV [1]. We study changes in this electronic structure with size, ranging from few-atom islands to complete films. We find that the conduction band edge first emerges in few-atom islands, and shifts toward lower energy with increasing island size. Images of the local density of states show standing wave patterns consistent with the confinement of electrons to these 2D islands. Measurements of the tunneling barrier height and image potential states indicate that the Cu$_2$N work function is $\sim $0.9~eV larger than bare Cu. This suggests a significant surface dipole, consistent with charge transfer predicted by theory. http://www.physics.ohio-state.edu/$\sim $jgupta \\[3pt] [1] C.D. Ruggiero, T. Choi, J.A. Gupta, Appl. Phys. Lett. \textbf{91}, 253106 (2007). [Preview Abstract] |
Monday, March 16, 2009 12:39PM - 12:51PM |
B11.00006: Ambipolar Ballistic Electron Emission Microscopy (BEEM) Studies of Gate-field Modified Schottky Barriers(SBs). Y.L. Che, J.P. Pelz Gate-field modified SBs are important for ``Schottky Barrier FETs'' [1], and could be used to control spin and charge injection into other semiconductor device structures. We have made the first \textit{ambipolar} BEEM measurements on Au/Si SBs that can be changed from effective $n$-type to $p$-type by applying a positive or negative back-gate bias, respectively. Samples were fabricated using SIMOX silicon-on-insulator wafers (35nm Si/150nm SiO$_{2}$/p-Si substrate), with Ti/Au and Pt pads as ohmic contacts for $n$-type and $p$-type operation, respectively. The local SB heights at 80K for electrons and holes were measured \textit{at the same location} to be $\sim $0.785eV and $\sim $0.323eV respectively, which correspond to intrinsic SBHs of $\sim $0.84eV and $\sim $0.36eV after accounting for image force lowering. These sum to 1.20eV, close to the $\sim $1.17eV Si bandgap at 80K. We will discuss on-going measurements of the dependence of the local SBH on temperature, back-gate bias, Si film thickness, and bias between the Schottky and ohmic contacts. Future work will investigate local variations of the conduction and valence bands due to local ``geometry-induced'' electric fields in nanostructured contacts. Work supported by National Science Foundation Grants No. DMR-0505165 and DMR-0805237.[1] S. Heinze, et al., Phys. Rev. Lett. 89, 106801 (2002). [Preview Abstract] |
Monday, March 16, 2009 12:51PM - 1:03PM |
B11.00007: Temperature dependence of lateral hot-electron spreading in Au films using Cross-Sectional Ballistic Electron Emission Microscopy C. Marginean, J.P. Pelz Cross-sectional ballistic electron emission microscopy (XBEEM) was used to investigate the \textit{temperature dependence} of hot-electron lateral spreading in metal films. A sequence of GaAs QWs of 1 to 15 nm width (separated by 200nm Al$_{0.3}$Ga$_{0.7}$As barrier layers) were cleaved \textit{ex situ,} and then 10 nm-thick of Au was thermally deposited on the cleaved edge to form Au Schottky barrier (SB) ``nanoaperture'' contacts [1]. Previous XBEEM results showed an unexpectedly large hot-electron lateral spreading at room temperature consistent with multiple electron scattering inside the metal film [2]. If phonon scattering of hot-electrons is significant in Au films (as previously suggested [3]), then the lateral spreading should increase at lower temperature. However, we found that the lateral spreading at 80K was almost the same as at room temperature, suggesting that electron-phonon scattering is not the dominant scattering mechanism. We will also discuss the temperature dependence of the BEEM current\textit{ amplitude}, as well as Monte-Carlo simulations of the lateral spreading process. Work supported by NSF Grant No. DMR-0505165. [1] C. Tivarus \textit{et al}., PRL \textbf{94}, 206803 (2005)~ [2] C. Tivarus \textit{et al}., APL \textbf{87}, 182105 (2005) [3] L. D. Bell, PRL \textbf{77}, 319007 (1996) [Preview Abstract] |
Monday, March 16, 2009 1:03PM - 1:15PM |
B11.00008: Classical size effect in nanometric Cu films: the dominant role of grain boundary scattering D. Choi, T. Sun, A. Warren, B. Yao, A. Darbal, K. Barmak, M. Toney, R. Peale, K. Coffey Surface and grain boundary electron scattering contribute significantly to resistivity as the dimensions of polycrystalline metallic conductors are reduced to, and below, the electron mean free path. In this work, a methodology is reported to independently evaluate surface and grain boundary scattering in encapsulated polycrystalline Cu thin films, with thicknesses of 28-158 nm, grain sizes of 35-466 nm, and interface roughnesses of 0.2-2 nm. The film resistivity, measured at both room temperature and at 4.2 K, is compared for samples having different grain sizes and film thicknesses. The resistivity contribution from grain boundary scattering is found to be dominant in SiO$_{2}$/Cu/SiO$_{2}$ and Ta/ SiO$_{2}$/Cu/Ta/SiO$_{2}$ films. Resistivity data for a third set of samples, namely SiO$_{2}$/TaSiN$_{x}$/Cu/TaSiN$_{x}$/SiO$_{2}$, will also be presented. [Preview Abstract] |
Monday, March 16, 2009 1:15PM - 1:27PM |
B11.00009: Crystal Orientation Imaging of Nanometric Metal Films in the Transmission Electron Microscope A. Darbal, K. Barmak, N. T. Nuhfer, D. J. Dingley, G. Meaden, J. Michael, T. Sun, B. Yao, K. R. Coffey A reliable method for orientation mapping of nanocrystals is crucial to the study of the impact of grain boundaries on resistivity increase of metal films as thickness is reduced (classical size effect). Here we report on the use of the Automated Crystallography (ACT) system for high-resolution grain and orientation mapping in the TEM. The samples for the study were a 50 nm-thick Pt film annealed at 800\r{ }C and a 40 nm-thick Cu film annealed at 450\r{ }C. In ACT, the diffraction pattern for a given point is constructed by analyzing its intensity variation in a series of dark field images obtained using hollow-cone illumination. The reconstructed diffraction pattern for every point is indexed to obtain the orientation map. The sensitivity of the orientation imaging results to details of sample preparation, data acquisition and choice of indexing parameters is discussed. [Preview Abstract] |
Monday, March 16, 2009 1:27PM - 1:39PM |
B11.00010: Controlling the assembly and electronic properties of solution processed CNT devices: From large area arrays to individual CNT Saiful I. Khondaker, Paul Stokes, Yodchay Jompol, Shashank Shekhar Single walled carbon nanotubes (SWNTs) are considered to be ideal components for nanoelectronic devices because of their exception electronic properties. Integration of these nanostructures into electronic circuits requires the precise positioning of them in different architectures. Here we will summarize our recent progress on the directed assembly of SWNTs using AC dielectrophoresis (DEP). SWNTs are assembled from a surfactant free commercially available aqueous solution using a non uniform electric field. By controlling the electric field strength, frequency, density of solution and novel fabrication techniques, we are able to control the assembly of SWNT from dense arrays mimicking electric flux to single SWNT devices with high yield. Electronic transport properties of field effect transistors and single electron transistors fabricated from such assembly will be discussed. [Preview Abstract] |
Monday, March 16, 2009 1:39PM - 1:51PM |
B11.00011: Probing the doping state of suspended carbon nanotube films by photo-induced voltage response Benoit St-Antoine, David Menard, Richard Martel A study of the photovoltage (PV) properties of suspended carbon nanotube films was undertaken. Although absorbance exhibits resonance features, changing the photoexcitation using lasers at two different wavelengths did not impact the magnitude of the observed behaviour. Moreover, increasing the thermalization by exposing the films to nitrogen resulted in a steep decrease of the PV with increasing pressures. Interestingly and contrary to previously reported studies, we also found that the PV could be maximized by illuminating the films at a short distance from the metallic contacts. All these findings reveal that the PV response of suspended films in vacuum is mainly driven by thermal mechanisms. Finally, we were able to reverse the sign of the PV by changing the doping state of the carbon nanotube films. Thus, monitoring the magnitude of the PV appears as a powerful tool for evaluating the doping state of suspended films. [Preview Abstract] |
Monday, March 16, 2009 1:51PM - 2:03PM |
B11.00012: Carrier Type and Transport Characteristics of a-B$_{4}$C Marcus Sky Driver, Saad Janjua, Sudarshan Karki, Dae Heum Yeoun, Anthony Caruso Boron carbide has many technological applications, including radiation hard semiconducting applications; the most popular of which is voltaic transduction. Structural defects are known to exist, whose implications in defining the majority carrier type is important, yet unclear. Gaining greater insight into the local physical structure and transport character is crucial toward optimizing the voltaic behavior. Subsequently, gaining information about the majority carrier and carrier concentration from Hall effect and band structure from photoemission spectroscopy gives insight into the electronic structure and transport of boron-rich carbides. Boron carbides are predominately p-type due to their electron deficiency, but it has been suggested that the electron as a majority carrier may also exist. Preliminary studies including Hall effect, photoemission and extended X-ray absorption fine structure will be discussed within the context of amorphous boron carbides with respect to the carrier properties and physical structure for various growth conditions, within the context of local structural defects. [Preview Abstract] |
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