Denver, CO, May 7-12, 2006

Quantum Confinement Phenomena in Metallic Conductance of Monoatomic Nanobridges

Maria Hepel, Department of Chemistry, State University of New York at Potsdam, 44 Pierrepont Ave., Potsdam, NY 13676, Fax: 315-267-3170, hepelmr@potsdam.edu

Abstract.  A variety of new nanoscale devices and phenomena have been recently studied for novel sensory applications and microelectronic designs [1,2].
In this work, the quantum conductance phenomena in narrow metal nanobridges were investigated. The nanobridge devices with small atomic-wide gap were formed by a novel controlled electroplating method on a nanolithographed quartz wafer. The metallic contact nanowires were grown across the gap by field evaporation. The field-induced formation and field-stabilization of monatomic Ni nanobridges are described. Thus formed Ni nanowires appeared to be unstable in the absence of electric field and disassembled quickly under no field conditions. The minimum quantum conductance associated with monatomic constriction in the nanobridge was observed. Exceptionally stable monatomic quantum nanobridges (QNB) were obtained by chemical treatment. They were investigated in the dry state and in the electrolyte solutions. The quantum conductance with zero temperature coefficient was observed at low electric field strength E (bias voltage |V_b| < 0.3 V) but thermionic conductance dominated at higher E (|V_b| > 0.8 V) leading to the conductance increase with temperature, opposite to the behavior of a bulk Ni metal. The mechanism of the observed phenomenon is presented.
    To gain further insights into the thermionic barrier formation in QNB, we have performed quantum mechanical calculations, using semi-empirical method, for model Ni nanobridge atom clusters (base : nanowire : base = 6:n:6 atoms, with n = 1...5). They have shown that in longer joining nanowires (n > 1) the lowest unoccupied molecular orbitals (LUMO) are predominantly concentrated over base electrodes (reservoirs) rather than over the nanowire constriction, while the short (monatomic) nanowires are densely populated with low lying LUMO's. The results of calculations suggest also that the location of the quantum confinement (the constriction) may not necessarily be at the nanowire center (as generally assumed) but rather at the joints of nanowire with base electrodes where the wire width is the smallest. This is indicated by the analysis of Fermi surface calculated for E = 5.35 eV for nanobridge metal clusters.
    The mechanism of thermionic conductance in nanobridge devices, which was observed at higher bias is presented and discussed in detail.

Figure 1. The i-E characteristic of a dry Ni nanobridge junction showing a no-metallic-conductance region (a), jump into conductance (b), and ohmic quantum metallic contact (d) at negative bias.