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
Defining the Value of Injection Current and Effective Electrical Contact Area for EGaIn-Based Molecular Tunneling Junctions
Analysis of rates of tunneling across
self-assembled monolayers
(SAMs) of <i>n</i>-alkanethiolates SC<sub><i>n</i></sub> (with <i>n</i> = number of carbon atoms) incorporated
in junctions having structure Ag<sup>TS</sup>-SAM//​Ga<sub>2</sub>O<sub>3</sub>/​EGaIn leads to a value for the injection
tunnel current density <i>J</i><sub>0</sub> (i.e., the current
flowing through an ideal junction with <i>n</i> = 0) of
10<sup>3.6±0.3</sup> A·cm<sup>–2</sup> (<i>V</i> = +0.5 V). This estimation of <i>J</i><sub>0</sub> does
not involve an extrapolation in length, because it was possible to
measure current densities across SAMs over the range of lengths <i>n</i> = 1–18. This value of <i>J</i><sub>0</sub> is estimated under the assumption that values of the geometrical
contact area equal the values of the effective electrical contact
area. Detailed experimental analysis, however, indicates that the
roughness of the Ga<sub>2</sub>O<sub>3</sub> layer, and that of the
Ag<sup>TS</sup>-SAM, determine values of the effective electrical
contact area that are ∼10<sup>–4</sup> the corresponding
values of the geometrical contact area. Conversion of the values of
geometrical contact area into the corresponding values of effective
electrical contact area results in <i>J</i><sub>0</sub>(+0.5
V) = 10<sup>7.6±0.8</sup> A·cm<sup>–2</sup>, which
is compatible with values reported for junctions using top-electrodes
of evaporated Au, and graphene, and also comparable with values of <i>J</i><sub>0</sub> estimated from tunneling through single molecules.
For these EGaIn-based junctions, the value of the tunneling decay
factor β (β = 0.75 ± 0.02 Å<sup>–1</sup>; β = 0.92 ± 0.02 nC<sup>–1</sup>) falls within
the consensus range across different types of junctions (β =
0.73–0.89 Å<sup>–1</sup>; β = 0.9–1.1
nC<sup>–1</sup>). A comparison of the characteristics of conical
Ga<sub>2</sub>O<sub>3</sub>/​EGaIn tips with the characteristics
of other top-electrodes suggests that the EGaIn-based electrodes provide
a particularly attractive technology for physical-organic studies
of charge transport across SAMs
Statistical Tools for Analyzing Measurements of Charge Transport
This paper applies statistical methods to analyze the
large, noisy data sets produced in measurements of tunneling current
density (<i>J</i>) through self-assembled monolayers (SAMs)
in large-area junctions.
It describes and compares the accuracy and precision of procedures
for summarizing data for individual SAMs, for comparing two or more
SAMs, and for determining the parameters of the Simmons model (β
and <i>J</i><sub>0</sub>). For data that contain
significant numbers of outliers (i.e., most measurements of charge
transport), commonly used statistical techniquesî—¸e.g., summarizing
data with arithmetic mean and standard deviation and
fitting data using a linear, least-squares algorithmî—¸are prone
to large errors. The paper
recommends statistical methods that distinguish between real data
and artifacts, subject to the assumption that real data (<i>J</i>) are independent and log-normally distributed. Selecting a precise
and accurate (conditional on these assumptions) method yields updated
values of β and <i>J</i><sub>0</sub> for charge transport
across both odd and even <i>n</i>-alkanethiols (with 99%
confidence intervals) and explains that the so-called odd–even
effect (for <i>n</i>-alkanethiols on Ag) is largely due
to a difference in <i>J</i><sub>0</sub> between odd and
even <i>n</i>-alkanethiols. This conclusion is provisional,
in that it depends to some extent on the statistical model assumed,
and these assumptions must be tested by future experiments
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