Person:

Barber, Jabulani Randall

This paper applies statistical methods to analyze the large, noisy data sets produced in measurements of tunneling current density (J) 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 J0). For data that contain significant numbers of outliers (i.e., most measurements of charge transport), commonly used statistical techniques—e.g., summarizing data with arithmetic mean and standard deviation and fitting data using a linear, least-squares algorithm—are prone to large errors. The paper recommends statistical methods that distinguish between real data and artifacts, subject to the assumption that real data (J) are independent and log-normally distributed. Selecting a precise and accurate (conditional on these assumptions) method yields updated values of β and J0 for charge transport across both odd and even n-alkanethiols (with 99% confidence intervals) and explains that the so-called odd–even effect (for n-alkanethiols on Ag) is largely due to a difference in J0 between odd and even n-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.

Tunneling junctions having the structure AgTS–S(CH2)n−1CH3//Ga2O3/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 AgTS–S(CH2)n−1CH3//Ga2O3/EGaIn junctions. (i) The oxide is ∼0.7 nm thick on average, is composed mostly of Ga2O3, 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.

This paper compares the J(V) 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 Ga2O3 surface film) produce indistinguishable data. The junctions incorporated SAMs of SCn–1CH3 (with n = 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 J(V) with statistically large numbers of data (N = 400–1000). The devices with the top electrode stabilized in microchannels also made it possible to conduct measurements of J(V) as a function of temperature, almost down to liquid nitrogen temperatures (T = 110–293 K). The J(V) characteristics were independent of T, 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 nC–1 (0.80 Å–1), and the pre-exponential factor, J0 (which is a constant that includes contact resistance), of 3.0 × 102 A/cm2. 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 J0, the value of J0 estimated using the Ga2O3/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 SAMs—including their molecular and supramolecular structure, and their interfaces with the electrodes—dominate charge transport in both types of EGaIn junctions. These results establish that studies of J(V) characteristics of AgTS-SAM//Ga2O3/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 Ga2O3 surface film, or to the work functions of the metals.

This paper compares charge transport across self-assembled monolayers (SAMs) of n-alkanethiols containing odd and even numbers of methylenes. Ultraflat template-stripped silver (AgTS) surfaces support the SAMs, while top electrodes of eutectic gallium−indium (EGaIn) contact the SAMs to form metal/SAM//oxide/EGaIn junctions. The EGaIn spontaneously reacts with ambient oxygen to form a thin (1 nm) oxide layer. This oxide layer enables EGaIn to maintain a stable, conical shape (convenient for forming microcontacts to SAMs) while retaining the ability to deform and flow upon contacting a hard surface. Conical electrodes of EGaIn conform (at least partially) to SAMs and generate high yields of working junctions. Ga2O3/EGaIn top electrodes enable the collection of statistically significant numbers of data in convenient periods of time. The observed difference in charge transport between n-alkanethiols with odd and even numbers of methylenes — the “odd−even effect” — is statistically discernible using these junctions and demonstrates that this technique is sensitive to small differences in the structure and properties of the SAM. Alkanethiols with an even number of methylenes exhibit the expected exponential decrease in current density, J, with increasing chain length, as do alkanethiols with an odd number of methylenes. This trend disappears, however, when the two data sets are analyzed together: alkanethiols with an even number of methylenes typically show higher J than homologous alkanethiols with an odd number of methylenes. The precision of the present measurements and the statistical power of the present analysis are only sufficient to identify, with statistical confidence, the difference between an odd and even number of methylenes with respect to J, but not with respect to the tunneling decay constant, β, or the pre-exponential factor, J0. This paper includes a discussion of the possible origins of the odd−even effect but does not endorse a single explanation.

Analysis of rates of tunneling across self-assembled monolayers (SAMs) of n-alkanethiolates SCn (with n = number of carbon atoms) incorporated in junctions having structure AgTS-SAM//Ga2O3/EGaIn leads to a value for the injection tunnel current density J0 (i.e., the current flowing through an ideal junction with n = 0) of 103.6±0.3 A·cm–2 (V = +0.5 V). This estimation of J0 does not involve an extrapolation in length, because it was possible to measure current densities across SAMs over the range of lengths n = 1–18. This value of J0 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 Ga2O3 layer, and that of the AgTS-SAM, determine values of the effective electrical contact area that are 10–4 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 J0(+0.5 V) = 107.6±0.8 A·cm–2, which is compatible with values reported for junctions using top-electrodes of evaporated Au, and graphene, and also comparable with values of J0 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–1; β = 0.92 ± 0.02 nC–1) falls within the consensus range across different types of junctions (β = 0.73–0.89 Å–1; β = 0.9–1.1 nC–1). A comparison of the characteristics of conical Ga2O3/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.

This communication describes the use of embossing, and “cut-and-stack” methods of assembly, to generate microfluidic devices from omniphobic paper, and demonstrates that fluid flowing through these devices behaves similarly to fluid in an open-channel microfluidic device. The porosity of the paper to gasses allows processes not possible in devices made using PDMS or other non-porous materials. Droplet generators and phase separators, for example, could be made by embossing “T”-shaped channels on paper. Vertical stacking of embossed or cut layers of omniphobic paper generated three-dimensional systems of microchannels. The gas permeability of the paper allowed fluid in the microchannel to contact and exchange with environmental or directed gases. An aqueous stream of water containing a pH-indicator, as one demonstration, changed color upon exposure to air containing HCl or NH3 gases.

This manuscript describes the use of explosions to power a soft robot—one composed solely of organic elastomers (e.g., silicones). The robot has three pneumatic actuators (pneu-nets) in a tripedal configuration. Explosion of a stoichiometric mixture of methane and oxygen within the microchannels making up the actuators produced hot gas that rapidly inflated the pneu-nets, and caused the robot to launch itself vertically from a flat surface (e.g., to jump). A soft flap embedded in the pneu-net acted as the valve of a passive exhaust system, and allowed multiple sequential actuations. The flame and temperature increase from the explosions are short-lived, and do not noticeably damage the robots over dozens of actuation cycles.

This paper investigates the influence of the atmosphere used in the fabrication of top electrodes from the liquid eutectic of gallium and indium (EGaIn) (the so-called “EGaIn” electrodes), and in measurements of current density, J(V) ((A/cm^2)), across self-assembled monolayers (SAMs) incorporated into (Ag/SR//Ga_2O_3)/EGaIn junctions, on values of J(V) obtained using these electrodes. A gas-tight measurement chamber was used to control the atmosphere in which the electrodes were formed, and also to control the environment in which the electrodes were used to measure current densities across SAM-based junctions. Seven different atmospheres—air, oxygen, nitrogen, argon, and ammonia, as well as air containing vapors of acetic acid or water—were surveyed using both “rough” conical-tip electrodes, and “smooth” hanging-drop electrodes. (The manipulation of the oxide film during the creation of the conical-tip electrodes leads to substantial, micrometer-scale roughness on the surface of the electrode, the extrusion of the drop creates a significantly smoother surface.) Comparing junctions using both geometries for the electrodes, across a SAM of n-dodecanethiol, in air, gave (log |J|mean = −2.4 \pm 0.4) for the conical tip, and (log |J|mean = −0.6 \pm 0.3) for the drop electrode (and, thus, (\Delta log |J| \approx 1.8)); this increase in current density is attributed to a change in the effective electrical contact area of the junction. To establish the influence of the resistivity of the (Ga_2O_3) film on values of J(V), junctions comprising a graphite electrode and a hanging-drop electrode were compared in an experiment where the electrodes did, and did not, have a surface oxide film; the presence of the oxide did not influence measurements of (log |J(V)|), and therefore did not contribute to the electrical resistance of the electrode. However, the presence of an oxide film did improve the stability of junctions and increase the yield of working electrodes from ∼70% to ∼100%. Increasing the relative humidity (RH) in which J(V) was measured did not influence these values (across methyl ((CH_3)^-) or carboxyl ((CO_2H)^-) terminated SAMs) over the range typically encountered in the laboratory (20%–60% (RH)).

This paper describes a method of fabrication that generates small arrays of tunneling junctions based on self-assembled monolayers (SAMs); these junctions have liquid-metal top-electrodes stabilized in microchannels and ultraflat (template-stripped) bottom-electrodes. The yield of junctions generated using this method is high (70−90%). The junctions examined incorporated SAMs of alkanethiolates having ferrocene termini (11-(ferrocenyl)-1-undecanethiol, SC({11})Fc); these junctions rectify currents with large rectification ratios (R), the majority of which fall within the range of 90−180. These values are larger than expected (theory predicts R ≤ 20) and are larger than previous experimental measurements. SAMs of n-alkanethiolates without the Fc groups (SC({n−1})CH(_3), with n = 12, 14, 16, or 18) do not rectify (R ranged from 1.0 to 5.0). These arrays enable the measurement of the electrical characteristics of the junctions as a function of chemical structure, voltage, and temperature over the range of 110−293 K, with statistically large numbers of data (N = 300−800). The mechanism of rectification with Fc-terminated SAMs seems to be charge transport processes that change with the polarity of bias: from tunneling (at one bias) to hopping combined with tunneling (at the opposite bias).