Mercury−Mercury Tunneling Junctions. 1. Electron Tunneling Across Symmetric and Asymmetric Alkanethiolate Bilayers

Abstract

Electron tunneling experiments involving Hg−Hg junctions incorporating two alkanethiolate monolayers are described. Formation of a symmetric junction (Hg−SC n −C n S−Hg) is accomplished by bringing in contact two small (3 × 10-3 cm2) mercury drop electrodes in a 5−20% (v/v) hexadecane solution of an alkanethiol. Formation of asymmetric junctions (Hg−SC n −C m S−Hg) and junctions containing n-alkane-3-thiopropanamide bilayers are also described. Tunneling currents in the junctions were measured for voltage biases extending to ±1.5 V. The currents decrease exponentially with the junction thickness yielding the tunneling decay constant, β = 0.89 ± 0.1 per CH2. The decay constant exhibits only a weak dependence on the voltage bias suggesting that electron tunneling follows a through-bond mechanism. Tunneling currents in the n-alkane-3-thiopropanamide bilayer junctions were larger than those in alkanethiolate junctions with the same number of atoms suggesting that introduction of an amide group increases the strength of the electronic coupling through these types of σ-bonded systems.

Charge transport along phenylenevinylene molecular wires

“…in a break junction [10] or between mercury drops. [11] These methods provide insight in the transport of charges through the molecule, however, the results of such measurements are to a large extent determined by effects related to the contacts between electrodes and the molecule. In many cases the conductance can be considered to occur via a bridge-mediated superexchange mechanism in which the charge 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 F o r P e e r R e v i e w O n l y 4 tunnels from one electrode to the other, without becoming localized on the molecular bridge.…”

Adsorption of Alkylthiol Self-Assembled Monolayers on Gold and the Effect of Substrate Roughness: A Comparative Study Using Scanning Tunneling Microscopy, Cyclic Voltammetry, Second-Harmonic Generation, and Sum-Frequency Generation

“…Furthermore, many authors [44][45][46][47]58 also use alkylthiols with similar purities, and therefore our results are directly relevant to those studies, if not of general applicability. 3.3.…”

Hybrid Molecomputer Using Vapor Phase Assembly

“…10,17 FTIR investigation showed that the orientation of the alkanethiol SAMs on Au(111) surfaces are tilted ~ 30° from the surface normal. 18 Electronic transport through alkanethiol SAMs have also been characterized by STM, 19,20 conducting atomic force microscopy, [21][22][23][24] mercury-drop junctions, [25][26][27][28] cross-wire junctions, 29 and electrochemical methods. [30][31][32] These investigations are exclusively at ambient temperatureclearly useful -but insufficient for an unambiguous claim that the transport mechanism is tunneling (of course expected, assuming that the Fermi levels of the contacts lie within the large HOMO-LUMO gap).…”

“…[11][12][13] Previous work on Langmuir-Blodgett alkane monolayers 48 exhibited a significant impurity-dominated transport component, complicating the analysis. I(V) measurements on self-assembled alkanethiol monolayers have also been reported; [19][20][21][22][23][24][25][26][27][28][29]49 however all of these measurements were performed at fixed temperature (300 K) which is insufficient to prove tunneling as the dominant mechanism. To describe the transport through a molecular system having HOMO and LUMO energy levels, one of the applicable models is the Franz two-band model.…”