Session T50: Focus Session: Mesoscopic Materials and Devices II

Guided growth of horizontal nanowires: A new path to self-integrated nanosystems

The large-scale assembly of nanowires with controlled orientation on surfaces remains one of the most critical challenges toward their integration into practical devices. We report the vapor-liquid-solid growth of perfectly aligned, millimeter-long, horizontal GaN [1] and ZnO [2] nanowires with controlled crystallographic orientations on different planes of sapphire and other substrates [3]. The growth directions, crystallographic orientation and faceting of the nanowires vary with each surface orientation, as determined by their epitaxial relationship with the substrate, as well as by a graphoepitaxial effect that guides their growth along surface steps and grooves. Despite their interaction with the surface, these horizontally grown nanowires display few structural defects, exhibiting optical and electronic properties comparable to those of vertically grown nanowires. Guided GaN nanowires and ZnO nanowires present general similarities and a few interesting differences, which shed light into the guided growth mechanism. The controlled horizontal growth of nanowires of different materials on different substrates proves the generality of the guided growth approach. Recently, we demonstrated the feasibility of massively parallel ``self-integration'' of NWs into functional systems based on guided growth, including hundreds of sing-NW based field-effect transistors made all at once, and complex logic circuits, such as a 3-bit address decoder [4]. These examples highlight the potential of guided growth for the large-scale integration of nanowires into practical devices. \\[4pt] [1] D. Tsivion, M. Schvartzman, R. Popovitz-Biro, P. von Huth, E. Joselevich, \textit{Science}, \textbf{333}, 1003 (2011).\\[0pt] [2] D. Tsivion, M. Schvartzman, R. Popovitz-Biro, E. Joselevich, \textit{ACS Nano}, \textbf{6}, 6433 (2012).\\[0pt] [3] D. Tsivion, E. Joselevich, \textit{Nano Lett.}, 13, 5491 (2013).\\[0pt] [4] M. Schvartzman, D. Tsivion, D. Mahalu, O. Raslin, E. Joselevich, \textit{Proc. Nat. Acad. Sci. USA}, \textbf{110}, 15195 (2013).   

Effect of Metallic Nanoparticle Decoration on Graphene Oxide Conductivity

Light and strong single-atom-thick carbon derivatives attract a wealth of attention from the research community due to their potential applications. Development of compatible satellite technologies for all-carbon nanoelectronic circuitry is vital for progress in practical applications. Graphene oxide (GO), the closest graphene relative, with its high surface area, unique atomic-layer properties, chemical inertness, and excellent bio-compatibility, has been tested for the applications in energy storage, flexible electronics, sensing technologies, and photovoltaics. GO conductivity enhancement by nanoparticle decoration can drastically improve the field effect transport of charge carriers in thin film transistors. In this study, GO, synthesized using modified Hummer's method, was functionalized with Ag nanoparticles using a two-step sonochemical procedure. Ag nanoparticles were shown to effectively migrate and redistribute when exposed to other carbon allotropies, such as carbon nanotubes and carbon dots. Studies of the effect of Ag precursor concentration and further nanoparticle migration on the conductivity of Ag/GO composites will be discussed within the context of charge carrier transport mechanisms.   

STM Studies of Graphene Grown on Non-Polar Surfaces of SiC

The unconventional electronic properties of graphene make it a highly promising candidate for the realization of nano-electronic circuits. Large-area epitaxial graphene (EG) grown by thermal decomposition of a SiC surface is a very promising candidate in this respect. So far the focus of the EG on SiC surfaces is mainly on the polar surfaces of SiC(0001) and SiC(000-1). In order to further understand the properties of EG on SiC and to correlate differences between surfaces of SiC, it is essential to study EG grown on non-polar surfaces SiC as well and to characterize them in detail. Here we present our studies of EG grown on the non-traditional, non-polar 6H-SiC(1-100) surface (m-plane) and (11-20) surface (a-plane) using scanning tunneling microscopy (STM). We show that there are regions of few layer and twisted multilayer graphene. Our STM images display the characteristic moire pattern corresponding to a twist angle of the top layer relative to the layer underneath. Combining the STM images and ball-and-stick model, we also determine the location of the graphene grain boundary and the manner in which the grains with different tilted angles patch together.   

Tuning the Band Gap of Graphene Nanoribbons Synthesized from Molecular Precursors

A prerequisite for future graphene nanoribbon (GNR) applications is the ability to fine-tune the electronic band gap of GNRs. Such control requires the development of fabrication tools capable of precisely controlling width and edge geometry of GNRs at the atomic scale. Here we report a technique for modifying GNR band gaps via covalent self-assembly of a new species of molecular precursors that yields n $=$ 13 armchair GNRs, a wider GNR than those previously synthesized using bottom-up molecular techniques. Scanning tunneling microscopy and spectroscopy reveal that these n $=$ 13 armchair GNRs have a band gap of 1.4 eV, 1.2 eV smaller than the gap determined previously for n $=$ 7 armchair GNRs. Furthermore, we observe a localized electronic state near the end of n $=$ 13 armchair GNRs that is associated with hydrogen-terminated sp2-hybridized carbon atoms at the zigzag termini.   

Evidence for pairing fluctuations in the Coulomb drag resistance of a GaAs/Graphene electron-hole bilayer

We report on experiments in a novel double-layer system composed by an ordinary two-dimensional electron gas (2DEG) in a GaAs heterostructure and a two-dimensional hole gas in a graphene monolayer placed on top of GaAs. Owing to the relatively short distance between the two layers we were able to measure the Coulomb drag in both the graphene and 2DEG layers. We discuss, in particular, the temperature evolution of the measured drag resistivity in the 2DEG. While the drag follows the expected quadratic temperature dependence at values above $T\approx 8K$, at lower temperature it displays a remarkable logarithmic increase. These data suggests the occurrence of electron-hole excitonic fluctuations as the double layer approaches the condensation temperature.   

Direct Determination of Mid-Gap States in Molecular and Nanocrystalline Films

We present a novel approach to directly measure the local electronic density of states (DOS) in the bandgap of monolayer films of organic molecules and semiconductor nanocrystals, by combining Kelvin probe force microscopy (KPFM) and field-effect transistor (FET). By tuning the molecule-dielectric surface chemistry or the nanocrystal surface ligand passivation, the mid-gap DOS can be dramatically changed. The correlation of the local DOS with field-effect transport measurements reveals both the spatial and energetic charge transport pathway.   

Mechanical control of frontier orbital alignment in single molecule junctions

The unique interplay between electronic and mechanical properties of molecular electronic systems offers opportunities to create novel devices and investigate new phenomena not seen in conventional electronics. Here we will discuss the electromechanical properties of a simple molecular system, a single 1,4-benzenedithiol (BDT) molecule bound to the Au electrodes of a STM, which display a counterintuitive increase in conductance when stretched [1]. This conductance behavior is attributed to the coupling of frontier molecular orbitals with the electrodes. By stretching and compressing BDT molecular junctions we are able to show that weakening of the coupling between molecule and electrode causes the energy of the frontier orbital to approach that of the Fermi level, moving the transport towards resonance. This effect is measured experimentally by recording conductance vs. stretching distance trace for single molecule junctions. Additionally, conductance-voltage and IETS spectra indicate that stretching and compressing a single BDT junction causes reversible change in the molecule-electrode coupling. Finally, transition voltage spectroscopy shows the change in energy of the frontier orbital as a single molecule junction is stretched. The measured behavior of BDT is a consequence of the molecular scale interactions that dominate mesoscopic transport and set molecular electronics apart from conventional electronics.\\[4pt] [1] C. Bruot, J. Hihath, and N. Tao, Nature Nanotechnology 7, 35 (2012).   

Mesoscopic electrons driven by quantum microwave states I: squeezed states

Motivated by recent experiments where superconducting microwave circuits have been coupled to electrons in semiconductor nanostructures [1-3], we consider theoretically the general problem of a mesoscopic conductor (such as a quantum point contact) driven by quantum states of a microwave field in a cavity. We show that even in the simplest case of a coherent state, there are significant corrections to the dc current over the completely classical treatment used in standard photon-assisted tunnelling theory. The case of a squeezed microwave field leads to even more striking deviations. Our calculations incorporate both the use of quantum-optics phase-space methods, and also a general Keldysh formalism that allows a more complete description. \\[4pt] [1] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck, and J. R. Petta, Nature \textbf{490}, 380 (2012). \\[0pt] [2] T. Frey, P. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, and A. Wallraff, Phys. Rev. Lett. \textbf{108}, (2012). \\[0pt] [3] M. Delbecq, V. Schmitt, F. Parmentier, N. Roch, J. Viennot, G. Feve, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. \textbf{107}, 256804 (2011).   

Mesoscopic electrons driven by quantum microwave states II: nonclassical light and noise

Motivated by recent experiments where superconducting microwave circuits have been coupled to electrons in semiconductor nanostructures [1-3], we consider theoretically the general problem of a mesoscopic conductor driven by a quantum microwave field. We focus here on perhaps the most dramatic case, where the microwave field is prepared in a highly non-classical cat state. We consider both signatures of this nonclassical light on the dc current through the conductor, as well as additional features which emerge in the low-frequency current noise. Our calculations incorporate both the use of quantum-optics phase-space methods, and also a general Keldysh formalism that allows a more complete description.\\[4pt] [1] K. D. Petersson, L. W. McFaul, M. D. Schroer, M. Jung, J. M. Taylor, A. A. Houck, and J. R. Petta, Nature \textbf{490}, 380 (2012).\\[0pt] [2] T. Frey, P. Leek, M. Beck, A. Blais, T. Ihn, K. Ensslin, and A. Wallraff, Phys Rev Lett \textbf{108}, (2012).\\[0pt] [3] M. Delbecq, V. Schmitt, F. Parmentier, N. Roch, J. Viennot, G. Feve, B. Huard, C. Mora, A. Cottet, and T. Kontos, Phys. Rev. Lett. \textbf{107}, 256804 (2011).   

Measurement of the typical persistent current in gold rings at high magnetic fields

Theory has long predicted the existence a dissipationless persistent current (PC) in rings made of a normal (i.e., non-superconducting) conductor. The PC is usually detected via its magnetic moment (i.e., without connection to leads or deliberate excitation) and therefore provides an important testbed for understanding the equilibrium properties of conductors. At low magnetic fields, the PC is predicted to be a sensitive probe of electron-electron interactions, non-equilibrium effects, and variety of other interesting phenomena. In contrast, at high magnetic fields the PC is expected to be accurately described by a simple single-electron theory. Previously, our group used a torque magnetometry technique to measure PC in aluminum rings in the presence of a strong magnetic field, and found good agreement with the single-electron theory of PC. In this talk we describe new measurements of very large arrays of gold rings. We will present measurements of these rings in high magnetic fields, where we find good agreement with the single-electron theory (including Zeeman and spin-orbit coupling effects). We will also describe the prospect for measuring these arrays at low magnetic field, where many-body and non-equilibrium effects may dramatically alter the PC.   

Measurements of the normal state persistent current in Au rings at high and low magnetic fields

Flux biased normal metal rings smaller than the phase coherence length can sustain persistent current (PC). We employ cantilever torque magnetometry to detect PC with high sensitivity, efficient background rejection, and in an electromagnetically clean environment. Previously, our group focused on the high magnetic field regime, where the PC is well described by single-particle theory. However at low magnetic field (few flux quanta) interaction effects are expected to be dominant. ~Previous low field studies by other groups employing SQUID and resonator-based techniques have found that Au, Ag, Cu, and GaAs rings show a large diamagnetic average PC, indicative of attractive e-e interactions. One possible explanation is that the superconductivity that would normally arise from this interaction is suppressed by a small number of magnetic impurities ($\sim$ 1 ppm), while the interaction-enhanced persistent current is not [1]. In this talk we will describe measurements of Au rings. We have fabricated arrays of 100,000 rings with 125 nm radius on ultrasensitive silicon cantilevers. At high magnetic fields, we find that the PC agrees with single-particle theory. We also describe the results at low field, expected to give further insight into the many body ground state of this system. \\[4pt] [1] H. Bary-Soroker, O. Entin-Wohlman and Y. Imry, Phys. Rev. Lett. 101, 057001 (2008).   

Heterogeneous atomic-scale break junctions for spin-based electronics

We discuss properties of atomic-scale contacts formed by breaking a thin film of Au, containing embedded ferromagnetic nanoparticles made from Ni, with diameters ranging from 2-5nm. The contacts are made by using a feedback-based electromigration technique. The breaking process leads to the observation of plateaus in conductance versus time plots, which we attribute to discrete atomic rearrangements. These discrete steps are studied using conductance histograms in order to compare the effects of interspersed nanoparticles in the junctions. Comparisons of Au films and Au-Ni composites lead to significant differences in conductance histograms, indicating that Ni plays a role in the process of breaking. Magneto-resistance measurements of Au-Ni composite contacts are investigated to determine the viability of this fabrication process for nm-scale spin-based electronics.