# Molecular Electronics

How does charge tunnel through a molecule? Can we control charge tunneling with molecular design? These problems are important in two general areas (1) electronics: charge tunneling (and other quantum effects) begins to dominate as the feature sizes of electronic components approach the molecular scale, and (2) Biochemistry: charge transport is essential for the function of the respiratory chain, many membrane-embedded redox systems, and DNA repair enzymes.

We have developed a system to study charge transport through a single layer of molecules (self-assembled monolayers, SAMs). Using the physical organic approach, we correlate charge transport characteristics with the structure (chemical, physical, and electronic) of the molecules. Our approach uses metal films as a bottom electrode and a liquid metal alloy (based on a eutectic of gallium and indium (EGaIn)) as a top electrode. The SAM grown on the metal film acts as the insulator through which charge tunnels. The “soft” liquid-metal top electrode enables non-damaging electrical contact with the SAMs. The top electrode maintains a static contact area because a thin film (~0.7 nm) of Ga2O3 forms spontaneously on its surface, which holds the liquid metal in place (see Figure 1).

 Figure 1. a) Illustration of the apparatus used to make measurements of currents across SAMs. b) A picture of a working AgTS/SAM//Ga2O3/EGaIn junction. The junction is fabricated by gently lowering the EGaIn tip onto a substrate bearing the SAM, and contact is confirmed by convergence of the tip with its reflected image on the substrate to give a closed electrical circuit. c). An illustration of the anatomy of a perfect junction showing the van der Waals interface between the SAM and the EGaIn/oxide top-electrode and how the deformable liquid metal conforms to features on the surface.
Key Results:

The following sections sketch the major accomplishments and conclusions from our work:

1. Super-exchange. We observe anomalously high rates of charge tunneling across SAMs of oligoglycine and oligo-(ethylene glycol) (Figure 2). These rapid rates of tunneling can be rationalized by the superexchange model, and correlate with the presence of high-lying, delocalized, occupied orbitals formed from the π-orbitals of the peptide bonds in oligoglycine and the oxygen lone pairs in oligo-(ethylene glycol). SAMs of both oligo-(ethylene glycol) and oligoglycine should be considered as members of a new class of organic electronic materials: good conductors by tunneling, but poor conductors by electronic drift. The conductivity of these compounds is also relevant to questions concerning the ability of biopolymers (both proteins and nucleic acids) to support charge tunneling. Moreover, the observation that calculated interactions between neighboring amide bonds correlate with experimentally observed rates of charge transport suggests that the superexchange model will be useful as a predictive tool in designing molecules with tunneling barriers engineered for potential and topography.

 Figure 2. Left: Schematic illustration of the junctions in the form of AuTS/S(CH2CH2O)nCH3/Ga2O3/EGaIn (n = 1 – 7). Right: Plot of the Gaussian mean values of log|J| at +0.5 V versus the number of non-hydrogen atoms in the backbone of the molecule, starting with carbon next to the sulfur atom and counting to the final non-hydrogen atom (for example, number of atoms for HS CH2CH2O)2CH3 is 7). The dashed line represents transport data collected for SAMs of AuTS/S(CH2)nH. The solid line indicates the fit from Simmons equation to the data collected for SAMs of AuTS/S(CH2CH2O)nCH3. The error bars represent the standard deviation of the Gaussian mean values.

2. 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 3) among other interesting diode-like behavior.

3. 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 4).

4. 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.

References:

(1) Baghbanzadeh, M.; Pieters, P. F.; Yuan, L.; Collison, D.; Whitesides, G. M. The Rate of Charge Tunneling in EGaIn Junctions Is Not Sensitive to Halogen Substituents at the Self-Assembled Monolayer//Ga2O3 Interface. ACS Nano, 2018.

(2) Rothemund, P.; Bowers, C. M.; Suo, Z.; Whitesides, G. M.Influence of the Contact Area on the Current Density Across Molecular Tunneling Junctions Measured with EGaIn Top-Electrodes. Chemistry of Materials, 2018, 30, 129-137.

(3) Baghbanzadeh, M.; Bowers, C. M.; Rappoport, D.; Zaba, T.; Li. Y.; Kang, K.; Liao, K. C.; Gonidec, M.; Rothemund, P.; Cyganik, P.; Aspuru-Guzik, A.; Whitesides, G. M.; Anomalously Rapid Tunneling: Charge Transport Across SAMs of Oligoethylene Glycol. J. Am. Chem. Soc., 2017, 139, 7624-7631.

(4) Bowers, C. M.; Rappoport, D.; Baghbanzadeh. M.; Simeone, F. C.; Liao. K. C.; Semenov, S. N.; Zaba, T.; Cyganik, P.; Aspuru-Guzik, A.; Whitesides. G. M. Tunneling across SAMs Containing Oligophenyl Groups. J. Phys. Chem. C, 2016, 120, 11331-11337.

(5) Liao, K. C.; Hsu, L.; Bowers, C. M.; Rabitz, H.; Whitesides, G. M.; Molecular Series-Tunneling Junction. J. Amer. Chem. Soc., 2015, 137, 5948-5954.

(6) Liao, K.C.; Bowers, C. M.; Yoon, H. J.; Whitesides, G. M. Fluorination, and Tunneling across Molecular Junctions. J. Amer. Chem. Soc., 2015, 137, 3852-3858.

(7) Baghbanzadeh, M.; Bowers, C. M.; Rappoport, D.; Zaba, T.; Gonidec, M.; Al-Sayah, M.; Cyganik, P.; Aspuru-Guzik, A.; Whitesides, G. M. Charge Tunneling along Short Oligoglycine Chains. Angew. Chemie. 2015, 54, 14743-14747.

(8) Bowers, C. M., Liao, K. C.; Zaba, T.; Rappoport, D.; Baghbanzadeh, M.; Breiten, B.; Krzykawska, A.; Cyganik, P.; Whitesides, G. M. Characterizing the Metal-SAM Interface in Tunneling Junctions. ACS Nano, 2015, 9, 1471-1477.

(9) Mirjani, F., Thijssen, J. M.; Whitesides, G. M.; Ratner, M. A. Charge Transport across Insulating Self-Assembled Monolayers: Non-Equilibrium Approaches and Modeling to Relate Current and Molecular Structure. ACS Nano, 2014, 8, 12428-12436.

(10) 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.

(11) 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.

(12) 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.

(13) 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.