Elucidating the influence of the monolayer interface versus bulk on the macroscopic properties (e.g., surface hydrophobicity, charge transport, and electron transfer) of organic self-assembled monolayers (SAMs) chemically anchored to metal surfaces is a challenge. This article reports the characterization of prototypical SAMs of n-alkanethiolates on gold (CH3(CH2)nSAu, n = 6-19) at the macroscopic scale by electrochemical impedance spectroscopy and contact angle goniometry, and at the molecular level, by infrared reflection absorption spectroscopy. The SAM capacitance, dielectric constant, and surface hydrophobicity exhibit dependencies on both the length (n) and parity (nodd or neven) of the polymethylene chain. The peak positions of the CH2 stretching modes indicate a progressive increase in the chain conformational order with increasing n between n = 6 and 16. SAMs of nodd have a greater degree of structural gauche defects than SAMs of neven. The peak intensities and positions of the CH3 stretching modes are chain length independent but show an odd-even alternation of the spatial orientation of the terminal CH3. The correlations between the different data trends establish that the chain length dependencies of the dielectric constant and surface hydrophobicity originate from changes in the polymethylene chain conformation (bulk), while the odd-even variation arises primarily from a difference in the chemical composition of the interface related to the terminal group orientation. These findings provide new physical insights into the structure-property relation of SAMs for the design of ultrathin film dielectrics as well as the understanding of stereostructural effects on the electrical characteristics of tunnel junctions.

The ability of organic self-assembled monolayers (SAMs) to act as insulating barriers to electron transfer, ion transport, or molecular diffusion is critical to their application in a variety of technologies. The use of appropriate analytical tools to characterize the dielectric properties of these molecular thin films is important for the control of structural defects and establishing structure–property relations. In this context, we analyze the ionic permeability and dielectric response of SAMs formed from a homologous series of n-alkanethiolates (CH3(CH2)nS, where n = 9, 11, 13, 15, 17, and 19) on gold using the immittance quantities of the complex impedance, capacitance, and permittivity available from the same electrochemical impedance spectroscopy (EIS) measurement. The most sensitive parameters and frequency range for characterizing the capacitive behavior and assessing the ion-blocking quality of the SAMs under non-Faradaic conditions are identified. We also investigate the effect of chain length on the interfacial capacitance and dielectric constant of ionic insulating SAMs. The advantages of the capacitance quantity and related permittivity data over traditional impedance representations and equivalent electric circuit modeling are discussed.

Large-area metal/molecule/metal junctions based on self-assembled monolayers (SAMs) of organothiolates are of interest for investigations of charge transport across ultrathin organic films and the fabrication of molecular electronic devices. Several recent reports demonstrate an odd-even dependency in the rate of charge transport through SAMs of the insulating n-alkanethiolates (CH3C n S)1,2 or the redox-active ferrocenylalkanethiolates (FcC n S)3. This parity effect has been attributed to differences in the orientation of the terminal functional group and alkyl chain packing in SAMs consisting of an odd versus even number of methylene repeat units. We have sought experimental evidence for odd-even distinctions in the molecular organization of these SAMs using infrared reflection-adsorption spectroscopy (IRRAS), electrochemistry, and contact angle goniometry. The findings of the investigations will be presented in this talk and the molecular origin of the parity effect of SAM macroscopic properties will be discussed. References: 1. 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. 2. Chen, J.; Giroux, T. J.; Nguyen, Y.; Kadoma, A. A.; Chang, B. S.; VanVeller, B.; Thuo, M. M., Understanding Interface (Odd–Even) Effects in Charge Tunneling using a Polished EGaIn Electrode. Phys. Chem. Chem. Phys. 2018, 20, 4864-4878. 3. Thompson, D.; Nijhuis, C. A., Even the Odd Numbers Help: Failure Modes of SAM-Based Tunnel Junctions Probed via Odd-Even Effects Revealed in Synchrotrons and Supercomputers. Acc. Chem. Res. 2016, 49, 2061-2069.

Large-area metal/molecule/metal junctions based on self-assembled monolayers (SAMs) of organothiolates are of interest for the investigation of charge transport across ultrathin organic films and the fabrication of molecular electronic devices such as diodes and switches. Several reports demonstrate an odd-even dependency in the rate of charge transport across SAMs of the redox-active ferrocenylalkanethiolates Fc(CH2) n S1, 2 and insulating n-alkanethiolates CH3(CH2) n S3-6. This parity effect is attributed to small changes in the orientation of the terminal functional group (ferrocene or methyl) and in the alkyl chain packing in SAMs consisting of an odd versus even number of methylene repeat units (n odd or n even). These odd-even differences ultimately impact device performance. For instance, junctions comprising gold-supported Fc(CH2) n S SAMs (Figure 1) of n even present lower leakage currents and rectify current, while those of n odd do not.1, 2 We have sought experimental evidence for odd-even distinctions in the molecular organization of these SAMs using electrochemistry,7 infrared reflection-absorption spectroscopy (IRRAS), contact angle goniometry, and surface stress measurements8. The findings of these investigations will be presented in this talk. References: 1. Nerngchamnong, N.; Yuan, L.; Qi, D.-C.; Li, J.; Thompson, D.; Nijhuis, C. A., The Role of van der Waals Forces in the Performance of Molecular Diodes. Nat. Nanotechnol. 2013, 8, 113-118. 2. Yuan, L.; Thompson, D.; Cao, L.; Nerngchangnong, N.; Nijhuis, C. A., One Carbon Matters: The Origin and Reversal of Odd–Even Effects in Molecular Diodes with Self-Assembled Monolayers of Ferrocenyl-Alkanethiolates. J. Phys. Chem. C 2015, 119, 17910-17919. 3. 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. 4. Thuo, M. M.; Reus, W. F.; Nijhuis, C. A.; Barber, J. R.; Kim, C.; Schulz, M. D.; Whitesides, G. M., Odd−Even Effects in Charge Transport across Self-Assembled Monolayers. J. Am. Chem. Soc. 2011, 133, 2962-2975. 5. Chen, J.; Giroux, T. J.; Nguyen, Y.; Kadoma, A. A.; Chang, B. S.; VanVeller, B.; Thuo, M. M., Understanding Interface (Odd–Even) Effects in Charge Tunneling using a Polished EGaIn Electrode. Phys. Chem. Chem. Phys. 2018, 20, 4864-4878. 6. Jiang, L.; Sangeeth, C. S. S.; Nijhuis, C. A., The Origin of the Odd–Even Effect in the Tunneling Rates across EGaIn Junctions with Self-Assembled Monolayers (SAMs) of n-Alkanethiolates. J. Am. Chem. Soc. 2015, 137, 10659-10667. 7. Feng, Y.; Dionne, E. R.; Toader, V.; Beaudoin, G.; Badia, A., Odd–Even Effects in Electroactive Self-Assembled Monolayers Investigated by Electrochemical Surface Plasmon Resonance and Impedance Spectroscopy. J. Phys. Chem. C 2017, 121, 24626-24640. 8. Dionne, E. R.; Dip, C.; Toader, V.; Badia, A., Micromechanical Redox Actuation by Self-Assembled Monolayers of Ferrocenylalkanethiolates: Evens Push More Than Odds. J. Am. Chem. Soc. 2018, 140, 10063-10066. Figure 1