Molecular Origin of the Odd–Even Effect of Macroscopic Properties of n-Alkanethiolate Self-Assembled Monolayers: Bulk or Interface?

Abstract

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.

Mechanistic Framework for the Formation of Different Sulfur Species by Electron Irradiation of n-Dodecanethiol Self-Assembled Monolayers on Au(111) and Au(100)

“…The second and third reactions involve, respectively, the gain or loss of an electron by the thiyl radical. We calculated the energy required to form the anion RS − ( ) from RS • and an electron on Au (equations [1] and [2]): [4] = + where is the work function of the Au(hkl) surface, and is the electronic affinity of the radical in vacuum, calculated as (−2.71 eV). Lastly, the cation formation, draining --…”

“…Moreover, it has been observed a renewed interest in fundamental research about thiolate-SAMs. For instance, the molecular origin of the odd-even effects 4 and whether a dipolar group modifies the electron-tunneling across SAMs 5 have been recently studied. The interest has been mainly triggered due to the wide range of applications of thiolate-SAMs in molecular electronic devices.…”

Modulated Rectification of Carboxylate-Terminated Self-Assembled Monolayer Junction by Humidity and Alkali Metal Ions: The Coupling and Asymmetric Factor Matter

“…The spectral features at 1550 and 1483 cm –1 are the asymmetric (ν a (COO – )) and symmetric (ν s (COO – )) stretching vibrations of COO – , respectively. We observed that the carboxylate peaks near 1483 cm –1 appear in C12, 14, and 16 SAMs, but not in the C11 SAM, which may be related to the odd–even effect − and will be further studied in future work.…”

Bilayer versus Polymeric Artificial Water Channel Membranes: Structural Determinants for Enhanced Filtration Performances

“…The odd−even effect on macroscopic dimensions of formed nanoparticles arises primarily from a difference in the chemical composition at the interfaces and is related to the terminal group orientation as previously described for self-assembled monolayers (SAMs). 18 The surface hydrophobicity originates from changes in the polyethylene chain orientation as n odd presents a "methyl-like" surface, exhibiting higher contact angles (greater surface hydrophobicity) than n even of comparable chain length, exposing an "ethyl-like" surface.…”