Session Q24: Focus Session: Transport in Nanostructures V: Optics, Mechanics, and Networks

Simultaneous measurements of single-molecule electrical conduction and Raman response

Electronic conduction through single molecules is affected by the molecular electronic structure as well as by other information that is extremely difficult to assess, such as bonding geometry and chemical environment. The lack of an independent diagnostic technique has long hampered single-molecule conductance studies. We report on simultaneous measurements of single molecule electronic conduction and Raman response in a nanogap geometry using paramercaptoaniline. The measurements show strong correlations in the time variation of the conductance and the Raman spectrum. The Raman changes include ``blinking'', spectral diffusion, and changes in Raman mode structure. Conduction in nanogaps is known to be dominated by a single or at most a few molecules, demonstrating the correlated Raman response is also single molecule in nature. These observations establish that multimodal sensing of individual molecules is possible in these mass-producible nanostructures.   

Local luminescence characterizations of ZnO nanostructures and their electrical transport characteristics

Local luminescence characterizations using cathodoluminescence (CL) emissions from individual ZnO nanostructures with diameters of 30-100 nm are investigated to correlate their optical and electrical properties. Two types of ZnO nanostructures with high and low charge carrier densities are identified from electronic transport measurements and concomitant CL characterizations. The results demonstrate that local luminescence characterizations can provide information about inhomogeneities in electrical and optical properties among ZnO nanostructures.   

Transport properties of a novel molecular rotor

Rotary motion around a molecular axis has been controlled by electron transfer process and by photoexcitation. The basis of the motion is intramolecular rotation of a ligand (3,8-di-ethynyltrityl-1, 10-phenanthroline) around a copper axle. The asymmetric copper system is synthesized by immobilizing a ``stator'' to a silicon support. The ``rotator,'' 3,8-di-ethynyltrityl-1, 10-phenanthroline is complexed to the metal center, Cu (I) or Cu (II) serving as an ``axle''. The Cu (I) system structure is tetrahedral, but that of Cu (II) is square planar. The interconversion of the two provides the basis for controlled, rotational motion. Hysteresis is observed in the different region of the applied voltage for different stators. The peak of the bisP-Si shifts to the left compared to that of the phen-Si stator because of the larger energy gap of phen-Si. The energy states of the Cu (I) and Cu (II) are extracted from the transport measurement results.   

Simultaneous Measurements of Force and Conductance through Single Molecular Junctions

We measure the conductance of single molecules attached to gold electrodes by repeatedly forming and breaking junctions between a gold substrate and a gold-coated cantilever in a modified atomic force microscope (AFM). While transport through single molecular junctions has been investigated, we gain additional information through the simultaneous recording of the forces required to break these junctions. Specifically, the force traces show elastic and plastic deformation processes hidden in conductance measurements. Our single molecular conductance measurements reproduce the results obtained previously and we find that the forces required to break gold point-contacts is consistent with published results. Furthermore, we present a comparison of measured forces required to break the Au-N bond formed in our single molecule junctions and density functional theory (DFT) results.   

Dielectrophoretic Trapping of Au Nanoparticles using High Quality Nanogap Electrodes

In the past decade, single molecule-based electronic devices have drawn enormous attention. One of the great challenges to be overcome is the fabrication of well-defined, uncontaminated nanogap electrodes, and the subsequent assembly of individual molecules or nanoparticles onto the contacts. A promising route to this goal is the feedback controlled electromigration (FCE) process, which can be used to create stable, metal-particle free nanogap contacts at temperatures ranging from 4--300~K.$^{1,2}$ Here we describe experiments where Au nanoparticles (NPs) are assembled inside FCE fabricated nanogaps by using positive AC dielectrophoresis (DEP). Specific challenges relating to circuit design for efficient DEP and solutions to these challenges are discussed. Additionally, substrate interactions are found to substantially influence dielectrophoretic assembly. Methods for controlling DEP assembly by modification of the electrostatic interaction between NPs and substrate surfaces are explored. Funding: NSF-NSEC/NBIC DMR-0425780. $^{1 }$D. R. Strachan et al., Appl. Phys. Lett. \textbf{86} 043109 (2005). $^{2 }$D. R. Strachan et al., Nano. Lett. \textbf{86} 043109 (2006).   

Nanomechanical Shuttling of Electrons

A nanoscale metal island that is oscillating between two opposing electrodes on a vibrating string can be used to mechanically actuate an electic current. We have realized such an electron shuttle on a doubly clamped high Q silicon nitride beam subject to high tensile strain. The shuttle is operated acoustically which guarantees complete decoupling of the measured signal from the drive at arbitrary source drain voltages. We have investigated the shuttling current across the island as a function of the applied voltage bias under resonant actuation. Since the tunnel resistance increases exponentially with distance, charge transfer between the island and an electrode only occurs at the turning points of the shuttle motion. Therefore transport across the island is strictly sequential, so that the shuttle can be theoretically described using a single electron box model during contact time. We have observed excellent agreement between our measured data and theoretical calculations which suggests that a crossover to the Coulomb blockade regime should be observed for smaller sample dimensions and lower temperatures.   

Measuring the momentum of a nanomechanical oscillator using tunnel junctions

We present a way to measure the momentum $p$ of a nanomechanical oscillator\footnote{C. B. Doiron, B. Trauzettel, C. Bruder. arXiv:0707.2709.}. The momentum detector is based on two tunnel junctions in an Aharonov-Bohm-type setup, where one of the tunneling amplitudes depends on the motion of the oscillator and the other one does not. The coupling between the first tunnel junction and the oscillator is assumed to be linear in the position $x$ of the oscillator $t(x) = t_0 + t_1 \hat{x}$. However, the presence of two junctions can, under certain conditions, lead to an effective imaginary coupling $t(x) = t_0 + i t_1 \hat{x}$. By calculating the equation-of-motion for the density matrix of the coupled (oscillator+tunnel junction) system\footnote{A.A Clerk, S. Girvin. Phys. Rev. B {\bf 70}, 121303 (2004).}, we show that in this case the finite-frequency current noise of the detector is proportional to the momentum spectrum of the oscillator.   

Vibronic enhancement of phonon heat conductance

We have studied the electron transport and phonon heat transport through single molecular bridge-junctions in terms of a newly proposed self-consistent theory. Due to the inelastic coupling between electrons and phonons, the two transport properties are closely correlated, which are treated on an equal footing way. By using the theory we have studied the two problems, i.e., (1) the non-equilibrium phonon effect on the electron transport and (2) the inelastic vibronic coupling effect on the phonon heat conductance. We have discussed dissipation processes of the inelastic energy accompanying the electronic conduction through the bridge-junction. Ref) Y. Asai, Phys. Rev. Lett. 93, 246102 (2004); 94, 099901(E) (2005). Y. Asai and H. Fukuyama, Phys. Rev. B 72, 085431 (2005). Y. Asai, submitted.   

Anisotropic Phonon Propagation in Nanoporous Alumina

Self-ordered nanoporous alumina membranes contain highly ordered hexagonal arrays of cylindrical holes. Phononic crystals based on nanoporous alumina with various porosities represent a composite medium for rich elastic wave propagation phenomena due to their periodicity and acoustic impedance contrast between alumina and infiltrated materials. It allows the manipulation of the high frequency acoustics as probed by Brillouin light scattering. In-plane and out-of-plane (perpendicular to the holes) propagation of the elastic waves are distinctly different. While the former reveals an effective medium and localization behavior, the latter selects the medium filling the holes. Band structure theoretical calculations provide a semiquantitative description of the new experimental findings.   

Phononic Properties of Periodic 1D Multilayer Polymer Films

The evolution of phonon dispersion relation with composition and periodicity in 1D periodic multilayer nanoscale polymer films is studied using high resolution Brillouin light scattering. An increase in complexity of the dispersion relation as the lattice constant $d$ becomes comparable to the phonon wavelength (q$^{-1})$ is observed. Films with large $d$ include phonons propagating within individual layers, as opposed to delocalized phonons moving throughout an effective homogeneous medium in films with small $d$. Temperature dependent measurements of the sound velocities reveal the presence of distinct glass transition temperatures in support of the distinct propagation of phonons through the periodic medium in agreement with theoretical predictions. The structure related elastic excitations are determined by the product of the layer thickness and the phonon wave vector qd and hence all layer guided modes are superimposed in a reduced plot of the phase velocity v vs qd.   

Dephasing of the weak localization correction in networks of quantum dots

We consider the corrections to the conductance of networks of quantum dots due to electron-electron interactions. Interaction corrections are calculated to first order in the propagator associated with the capacitive coupling among the dots. We focus in particular on calculating the dephasing correction to the weak localization correction and the Altshuler-Aronov correction to the conductance.   

Fractal conductance fluctuations of classical origin

The coherent conductance through mesoscopic structures is well known to show reproducible fluctuations with the variation of an external parameter (e.g. a magnetic field). These fluctuations are caused by interference effects and can be described semiclassically. In systems with mixed (regular and chaotic) classical dynamics {\em fractal} conductance curves are found\footnote{R. Ketzmerick, \textit{Phys. Rev. B} \textbf{54}, 10841 (1996)}. Experiments that study the transition from coherent to incoherent transport showing a change of the fractal dimension with the coherence-length\footnote{A.P. Micolich et al., \textit{Phys. Rev. Lett.} \textbf{87}, 036802 (2001)}, however, seemed to contradict the semiclassical theory of the fractal scaling. We show that there is no contradiction but that the classical dynamics itself already leads to fractal conductance curves\footnote{ H. Hennig, R. Fleischmann, L. Hufnagel and T. Geisel, \textit{Phys. Rev. E} \textbf{76}, 015202 (2007)} explaining the experimental observations. Moreover, we predict fractal classical conductance fluctuations not only in systems with mixed phase space but in purely chaotic systems.   

Electron Transport through Models for Small-World Nanomaterials

We investigate the quantum transport of (spinless) electrons through simplified models related to small-world nanomaterials. We employ a tight-binding Hamiltonian, and obtain the transmission coefficient from a matrix solution of the associated time-independent Schr\"{o}dinger Equation. The system studied corresponds to $d=1$ semi-infinite input and output leads, connected to a `blob' of $N$ atoms. We first present exact results for $N$ inter-connected atoms, a fully-connected graph. The exact solution, for any $N$, is given both for symmetric and non-symmetric connections between the `blob' and the input/output. We then present numerical results obtained by removing some of the connections within the $N$-site `blob', thereby approaching transport through a small-world nanomaterial [1-4]. \newline [1] S. \c{C}ali\c{s}kan, M.A. Novotny, and J.I. Cerd\'{a}, J. Appl. Phys., \textbf{102}, 013707 (2007). \newline [2] M.A. Novotny \textit{et al.}, J. Appl. Phys., \textbf{97}, 10B309 (2005). \newline [3] M.A. Novotny and S.M. Wheeler, Braz. J. Physics \textbf{34}, 395 (2004). \newline [4] J. Yancey, M.A. Novotny, and S.R. Gwaltney, 2008 March Meeting presentation.   

Atomic scale contact formation: A combined Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM) study

We are investigating contact formation at the atomic scale, in particular the interplay of forces and conductivity [1]. As it has been shown (e.g. in the case of C60 in between a STM tip and an Au(111) sample [2]), the conductivity in molecular junctions depends strongly on the contact geometry. In order to fully characterize the junction, we use a homebuilt ultra high vacuum (UHV) (p $<$ $10^{-10}$ mbar) microscope which runs in simultaneous scanning tunneling microscope (STM) and atomic force microscope (AFM) modes. Additionally we image the STM tip structure with field ion microscopy (FIM) prior to using it in our experiments [3]. In order to realize a controlled contact we use the STM tip as one electrode and the sample as counter electrode. We are investigating bare Au(111) samples and W STM tips as an example of a nano metal-metal contact and one C60 molecule sandwiched between the W-tip and the Au(111) sample as a model for a controlled metal-molecule-metal contact. We will present new measurements of I(z), F(z) and dI/dV (z) curves of the above mentioned systems, where z is the tip-sample separation as well as images of the sample and tip structure. [1] Sun et. al. PRB 71 193407, 2005 [2] De Menech et. al. PRB 73, 155407, 2006 [3] Lucier et. al. PRB 72, 235420, 2005   

Dissipation in metallic nano-mechanical resonators at millikelvin temperatures.

A magneto-motive detection scheme has often been employed to study dissipation in semiconducting nano-mechanical resonators at millikelvin temperatures. This scheme involves placing the beam in a large applied magnetic field and driving the mechanical resonance by sending rf currents through a thin metal electrode plated on the beam. The motion is then detected by measuring the induced e.m.f in the electrode. As a first step in understanding possible damping effects due to metallic films, we have fabricated free standing metallic resonators. We have fabricated doubly clamped beams out of $Au$ and $Au/Pd$ alloys (dimensions length $3-5 \mu m $ ,thickness $\approx 50nm$ and width $\approx 200 nm $). Beams of this dimension have a resonant frequency around $5-10$ MHz. We will report measurements, performed in a dilution refrigerator,of the quality factor $Q$ in these resonators as a function of temperature and magnetic field (up to 15 T).