5 research outputs found
Comparison of SAM-Based Junctions with Ga<sub>2</sub>O<sub>3</sub>/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions
This paper compares the <i>J</i>(<i>V</i>)
characteristics obtained for self-assembled monolayer (SAM)-based
tunneling junctions with top electrodes of the liquid eutectic of
gallium and indium (EGaIn) fabricated using two different procedures:
(i) stabilizing the EGaIn electrode in PDMS microchannels and (ii)
suspending the EGaIn electrode from the tip of a syringe. These two
geometries of the EGaIn electrode (with, at least when in contact
with air, its solid Ga<sub>2</sub>O<sub>3</sub> surface film) produce
indistinguishable data. The junctions incorporated SAMs of SC<sub><i>n</i>–1</sub>CH<sub>3</sub> (with <i>n</i> = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver
electrodes. Both methods generated high yields of junctions (70–85%)
that were stable enough to conduct measurements of <i>J</i>(V) with statistically large numbers of data (<i>N</i> =
400–1000). The devices with the top electrode stabilized in
microchannels also made it possible to conduct measurements of <i>J</i>(V) as a function of temperature, almost down to liquid
nitrogen temperatures (<i>T</i> = 110–293 K). The <i>J</i>(<i>V</i>) characteristics were independent of <i>T</i>, and linear in the low-bias regime (−0.10 to 0.10V);
the current density decreased exponentially with increasing thickness
of the SAM. These observations indicate that tunneling is the main
mechanism of charge transport across these junctions. Both methods
gave values of the tunneling decay coefficient, β, of ∼1.0 <i>n</i><sub>C</sub><sup>–1</sup> (∼0.80 Å<sup>–1</sup>), and the pre-exponential factor, <i>J</i><sub>0</sub> (which is a constant that includes contact resistance),
of ∼3.0 × 10<sup>2</sup> A/cm<sup>2</sup>. Comparison
of the electrical characteristics of the junctions generated using
EGaIn by both methods against the results of other systems for measuring
charge transport indicated that the value of β generated using
EGaIn electrodes is compatible with the consensus of values reported
in the literature. Although there is no consensus for the value of <i>J</i><sub>0</sub>, the value of <i>J</i><sub>0</sub> estimated using the Ga<sub>2</sub>O<sub>3</sub>/EGaIn electrode
is compatible with other values reported in the literature. The agreement
of experimental values of β across a number of experimental
platforms provides strong evidence that the structures of the SAMsî—¸including
their molecular and supramolecular structure, and their interfaces
with the electrodesî—¸dominate charge transport in both types
of EGaIn junctions. These results establish that studies of <i>J</i>(<i>V</i>) characteristics of Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junctions are dominated by
the structure of the organic component of the SAM, and not by artifacts
due to the electrodes, the resistance of the Ga<sub>2</sub>O<sub>3</sub> surface film, or to the work functions of the metals
Comparison of SAM-Based Junctions with Ga<sub>2</sub>O<sub>3</sub>/EGaIn Top Electrodes to Other Large-Area Tunneling Junctions
This paper compares the <i>J</i>(<i>V</i>)
characteristics obtained for self-assembled monolayer (SAM)-based
tunneling junctions with top electrodes of the liquid eutectic of
gallium and indium (EGaIn) fabricated using two different procedures:
(i) stabilizing the EGaIn electrode in PDMS microchannels and (ii)
suspending the EGaIn electrode from the tip of a syringe. These two
geometries of the EGaIn electrode (with, at least when in contact
with air, its solid Ga<sub>2</sub>O<sub>3</sub> surface film) produce
indistinguishable data. The junctions incorporated SAMs of SC<sub><i>n</i>–1</sub>CH<sub>3</sub> (with <i>n</i> = 12, 14, 16, or 18) supported on ultraflat, template-stripped silver
electrodes. Both methods generated high yields of junctions (70–85%)
that were stable enough to conduct measurements of <i>J</i>(V) with statistically large numbers of data (<i>N</i> =
400–1000). The devices with the top electrode stabilized in
microchannels also made it possible to conduct measurements of <i>J</i>(V) as a function of temperature, almost down to liquid
nitrogen temperatures (<i>T</i> = 110–293 K). The <i>J</i>(<i>V</i>) characteristics were independent of <i>T</i>, and linear in the low-bias regime (−0.10 to 0.10V);
the current density decreased exponentially with increasing thickness
of the SAM. These observations indicate that tunneling is the main
mechanism of charge transport across these junctions. Both methods
gave values of the tunneling decay coefficient, β, of ∼1.0 <i>n</i><sub>C</sub><sup>–1</sup> (∼0.80 Å<sup>–1</sup>), and the pre-exponential factor, <i>J</i><sub>0</sub> (which is a constant that includes contact resistance),
of ∼3.0 × 10<sup>2</sup> A/cm<sup>2</sup>. Comparison
of the electrical characteristics of the junctions generated using
EGaIn by both methods against the results of other systems for measuring
charge transport indicated that the value of β generated using
EGaIn electrodes is compatible with the consensus of values reported
in the literature. Although there is no consensus for the value of <i>J</i><sub>0</sub>, the value of <i>J</i><sub>0</sub> estimated using the Ga<sub>2</sub>O<sub>3</sub>/EGaIn electrode
is compatible with other values reported in the literature. The agreement
of experimental values of β across a number of experimental
platforms provides strong evidence that the structures of the SAMsî—¸including
their molecular and supramolecular structure, and their interfaces
with the electrodesî—¸dominate charge transport in both types
of EGaIn junctions. These results establish that studies of <i>J</i>(<i>V</i>) characteristics of Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junctions are dominated by
the structure of the organic component of the SAM, and not by artifacts
due to the electrodes, the resistance of the Ga<sub>2</sub>O<sub>3</sub> surface film, or to the work functions of the metals
The SAM, Not the Electrodes, Dominates Charge Transport in Metal-Monolayer//Ga<sub>2</sub>O<sub>3</sub>/Gallium–Indium Eutectic Junctions
The liquid–metal eutectic of gallium and indium (EGaIn) is a useful electrode for making soft electrical contacts to self-assembled monolayers (SAMs). This electrode has, however, one feature whose effect on charge transport has been incompletely understood: a thin (approximately 0.7 nm) filmconsisting primarily of Ga<sub>2</sub>O<sub>3</sub>that covers its surface when in contact with air. SAMs that rectify current have been measured using this electrode in Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn (where Ag<sup>TS</sup> = template-stripped Ag surface) junctions. This paper organizes evidence, both published and unpublished, showing that the molecular structure of the SAM (specifically, the presence of an accessible molecular orbital asymmetrically located within the SAM), not the difference between the electrodes or the characteristics of the Ga<sub>2</sub>O<sub>3</sub> film, causes the observed rectification. By examining and ruling out potential mechanisms of rectification that rely either on the Ga<sub>2</sub>O<sub>3</sub> film or on the asymmetry of the electrodes, this paper demonstrates that the structure of the SAM dominates charge transport through Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junctions, and that the electrical characteristics of the Ga<sub>2</sub>O<sub>3</sub> film have a negligible effect on these measurements
Replacing −CH<sub>2</sub>CH<sub>2</sub>– with −CONH– Does Not Significantly Change Rates of Charge Transport through Ag<sup>TS</sup>-SAM//Ga<sub>2</sub>O<sub>3</sub>/EGaIn Junctions
This paper describes physical-organic studies of charge
transport
by tunneling through self-assembled monolayers (SAMs), based on systematic
variations of the structure of the molecules constituting the SAM.
Replacing a −CH<sub>2</sub>CH<sub>2</sub>– group with
a −CONH– group changes the dipole moment and polarizability
of a portion of the molecule and has, in principle, the potential
to change the rate of charge transport through the SAM. In practice,
this substitution produces no significant change in the rate of charge
transport across junctions of the structure Ag<sup>TS</sup>-SÂ(CH<sub>2</sub>)<sub><i>m</i></sub>XÂ(CH<sub>2</sub>)<sub><i>n</i></sub>H//Ga<sub>2</sub>O<sub>3</sub>/EGaIn (TS = template
stripped, X = −CH<sub>2</sub>CH<sub>2</sub>– or −CONH–,
and EGaIn = eutectic alloy of gallium and indium). Incorporation of
the amide group does, however, increase the yields of working (non-shorting)
junctions (when compared to <i>n</i>-alkanethiolates of
the same length). These results suggest that synthetic schemes that
combine a thiol group on one end of a molecule with a group, R, to
be tested, on the other (e.g., HS∼CONH∼R) using an amide-based
coupling provide practical routes to molecules useful in studies of
molecular electronics
Electrical Resistance of Ag<sup>TS</sup>–S(CH<sub>2</sub>)<sub><i>n</i>−1</sub>CH<sub>3</sub>//Ga<sub>2</sub>O<sub>3</sub>/EGaIn Tunneling Junctions
Tunneling junctions having the structure Ag<sup>TS</sup>–SÂ(CH<sub>2</sub>)<sub><i>n</i>−1</sub>CH<sub>3</sub>//Ga<sub>2</sub>O<sub>3</sub>/EGaIn allow physical–organic
studies
of charge transport across self-assembled monolayers (SAMs). In ambient
conditions, the surface of the liquid metal electrode (EGaIn, 75.5
wt % Ga, 24.5 wt % In, mp 15.7 °C) oxidizes and adsorbs―like
other high-energy surfaces―adventitious contaminants. The interface
between the EGaIn and the SAM thus includes a film of metal oxide,
and probably also organic material adsorbed on this film; this interface
will influence the properties and operation of the junctions. A combination
of structural, chemical, and electrical characterizations leads to
four conclusions about Ag<sup>TS</sup>–SÂ(CH<sub>2</sub>)<sub><i>n</i>−1</sub>CH<sub>3</sub>//Ga<sub>2</sub>O<sub>3</sub>/EGaIn junctions. (i) The oxide is ∼0.7 nm thick on
average, is composed mostly of Ga<sub>2</sub>O<sub>3</sub>, and appears
to be self-limiting in its growth. (ii) The structure and composition
(but not necessarily the contact area) of the junctions are conserved
from junction to junction. (iii) The transport of charge through the
junctions is dominated by the alkanethiolate SAM and not by the oxide
or by the contaminants. (iv) The interface between the oxide and the
eutectic alloy is rough at the micrometer scale