10.1021/jacs.5b00448.s001
Kung-Ching Liao
Liang-Yan Hsu
Carleen M. Bowers
Herschel Rabitz
George M. Whitesides
Molecular
Series-Tunneling Junctions
2015
American Chemical Society
SAM
density
junction
groups R 1
R 1
R 2
AgTS
R 2 units
tunneling decay constants
HOMO
6H
2015-05-13 00:00:00
article
https://acs.figshare.com/articles/Molecular_Series_Tunneling_Junctions/2167333
Charge transport through junctions
consisting of insulating molecular
units is a quantum phenomenon that cannot be described adequately
by classical circuit laws. This paper explores tunneling current densities
in self-assembled monolayer (SAM)-based junctions with the structure
Ag<sup>TS</sup>/O<sub>2</sub>C–R<sub>1</sub>–R<sub>2</sub>–H//Ga<sub>2</sub>O<sub>3</sub>/EGaIn, where Ag<sup>TS</sup> is template-stripped silver and EGaIn is the eutectic alloy of gallium
and indium; R<sub>1</sub> and R<sub>2</sub> refer to two classes of
insulating molecular units(CH<sub>2</sub>)<sub><i>n</i></sub> and (C<sub>6</sub>H<sub>4</sub>)<sub><i>m</i></sub>that are connected in series and have different tunneling
decay constants in the Simmons equation. These junctions can be analyzed
as a form of series-tunneling junctions based on the observation that
permuting the order of R<sub>1</sub> and R<sub>2</sub> in the junction
does not alter the overall rate of charge transport. By using the
Ag/O<sub>2</sub>C interface, this system decouples the highest occupied
molecular orbital (HOMO, which is localized on the carboxylate group)
from strong interactions with the R<sub>1</sub> and R<sub>2</sub> units.
The differences in rates of tunneling are thus determined by the electronic
structure of the groups R<sub>1</sub> and R<sub>2</sub>; these differences
are <i>not</i> influenced by the order of R<sub>1</sub> and
R<sub>2</sub> in the SAM. In an electrical potential model that rationalizes
this observation, R<sub>1</sub> and R<sub>2</sub> contribute independently
to the height of the barrier. This model explicitly assumes that contributions
to rates of tunneling from the Ag<sup>TS</sup>/O<sub>2</sub>C and
H//Ga<sub>2</sub>O<sub>3</sub> interfaces are constant across the
series examined. The current density of these series-tunneling junctions
can be described by <i>J</i>(<i>V</i>) = <i>J</i><sub>0</sub>(<i>V</i>) exp(−β<sub>1</sub><i>d</i><sub>1</sub> – β<sub>2</sub><i>d</i><sub>2</sub>), where <i>J</i>(<i>V</i>) is the current density (A/cm<sup>2</sup>) at applied
voltage <i>V</i> and β<sub><i>i</i></sub> and <i>d</i><sub><i>i</i></sub> are the parameters
describing the attenuation of the tunneling current through a rectangular
tunneling barrier, with width <i>d</i> and a height related
to the attenuation factor β.