Our motivation for this project is to develop a system with which to study charge transport (by through-bond tunneling and hopping) through thin organic films (self-assembled monolayers, SAMs) with nanometer-scale dimensions, to use this system to correlate charge transport characteristics with the structures (chemical, physical, and electronic) of the organic film, and to make these correlations the basis for a qualitative physical-organic (and eventually quantitative molecular-orbital based) theory of charge transport. This subject is fundamental to two areas of science and technology: i) Charge transport is mechanistically important in potential application, especially in organic and printed electronics. Although these systems will not necessarily use SAMs, they provide, in our opinion, the best model systems with which to understand fundamentals of tunneling in organic matter. ii) Charge transport in living cells (especially in the respiratory chain, and in membrane-embedded redox systems) is an important and incompletely understood area of mechanistic biochemistry. SAMs will also provide elementary models for the very complex systems of biology.
Our work focuses on SAMs supported on metal films and contacted with a “soft” top-electrode based on a eutectic of gallium and indium (EGaIn); this electrode enables non-damaging electrical contact with the SAMs, but its success is based, in part, on a thin film (~0.7 nm) of Ga2O3 that forms spontaneously on its surface (see Figure 1).
The focus of the work has been on generating statistically significant numbers of data, and analyzing them rigorously using sound statistics. We can summarize an exhaustive set of studies in the following conclusions; i) The use of template-stripped (TS) silver films gives dramatically better results than rougher, evaporated electrodes. ii) AgTS/SAM//SAM/Hg junctions give interpretable results, and are much better-defined than other systems with evaporated top electrodes, but give noisy data and give working junctions in low (25 %) yields. These junctions also require two layers of SAMs and a surrounding liquid bath to be stable, and their instability (to amalgamation, mechanical noise, and other parameters) makes them undesirably difficult to work with. iii) Use of AgTS/SAM//EGaIn junctions eliminates most of the instability found in other systems. EGaIn junctions have high yields (~80%). It is straightforward to incorporate complex organic functionality into the SAMs, although, of course, not correspondingly easy to identify the structure of these SAMs. Many aspects of this work have been described; here we touch only on those relevant to future work.
The following sections sketch the major accomplishments and conclusions from our work:
1. Current Rectification. Ferrocene-terminated alkanethiol (HS(CH2)11Fc) SAMs show unexpectedly large rectification (“rectification ratio” R = J(+V)/J(-V) ~ 100-500. By synthetically varying the position of the Fc moiety within the SAM, and thus the degree of asymmetry in the junction, we have established that rectification can result from a single, accessible molecular orbital placed asymmetrically between two electrodes. Rectification increased with increasing asymmetry as demonstrated with varying alkyl-spacers. This result is in agreement with theoretical predictions, and in contrast to some assertions that a donor-bridge-acceptor structure is necessary for rectification. These studies also establish rectification as a particularly attractive system to examine experimentally, since many of the experimental artifacts that still complicate these measurements of SAMs disappear in rectification measurements. We have also demonstrated half-wave rectifiers (Figure 2) among other interesting diode-like behavior.
2. Odd/Even Effects. A number of surface characteristics (e.g., wettabilty) of alkanethiolate SAMs on AgTS depend on whether the alkyl chain has an even or odd number of carbons. We compared J(V) measurements for SAMs derived from even and odd alkanethiols. We collected data from thiols, SCn (where n=9 to 19), and observed that for both—odd and evens, the values of J decreased exponentially with chain length; the trend lines for the two were, however, statistically different: the current density, J, for an extrapolated common thickness was greater for even than odd SAMs by a factor of approximately half an order of magnitude. Our statistical analyses indicated that odds and evens are different (Figure 3).
3. Characterization of the Ga2O3/EGaIn Electrode. We investigated the composition, structure and morphology of the EGaIn based electrodes using TOF-SIMS MS, XPS, SEM, and Auger spectroscopy. The oxide layer was predominantly a thin layer (~0.7 nm) of Ga2O3. Contact characteristics of the electrode were measured using two techniques: inverted optical microscopy and capacitance measurements. These measurements agreed that the area of contact between an EGaIn conical tip and the surface of the SAM is ~25% of contact area calculated on the basis of an assumed uniform circular contact. Based on temperature-dependent measurements, charge-transport through the layer of Ga2O3 is classically resistive. We calculated a resistivity of approximately 105 O-cm, a reasonable value, and one that implies a significant density of defects in the layer. The important conclusion from these studies is that, in a typical SAM-based junction, the layer of Ga2O3 is a factor of ~104 less resistive than the SAM. We therefore conclude that the SAM determines the J(V) characteristics of such a junction, and that Ga2O3/EGaIn is therefore a useful electrode for conducting physical-organic studies of charge-transport through SAMs or other structures.
(1) Yoon, H. J.; Bowers, C. M.; Baghbanzadeh, M.; Whitesides, G. M. "The Rate of Charge Tunneling is Insensitive to Polar Terminal Groups in Self-Assembled Monolayers in AgTSS(CH2)nM(CH2)mT//Ga2O3/EGaIn Junctions", J. Am. Chem. Soc. 2014, 136, 16-19.
(2) Liao, K.-C.; Yoon, H. J.; Bowers, C. M.; Simeone, F. C.; Whitesides, G. M. "Replacing AgTSSCH2-R with AgTSO2C-R in EGaIn-Based Tunneling Junctions Does Not Significantly Change Rates of Charge Transport", Angew. Chem., Int. Ed. 2014, 53, 3889-3893.
(3) Yoon, H. J.; Liao, K.-C.; Lockett, M. R.; Kwok, S. W.; Baghbanzadeh, M.; Whitesides, G. M. "Rectification in Tunneling Junctions: 2,2'-Bipyridyl-terminated n-Alkanethiolates", J. Am. Chem. Soc. 2014, 136, 17155-17162.
(4) Baghbanzadeh, M.; Simeone, F. C.; Bowers, C. M.; Liao, K.-C.; Thuo, M.; Baghbanzadeh, M.; Miller, M. S.; Carmichael, T. B.; Whitesides, G. M. "Odd-Even Effects in Charge Transport across n-Alkanethiolate-Based SAMs", J. Am. Chem. Soc. 2014, 136, 16919-16925.