Thermal Study of Accumulation of Conformational Disorders in the Self-Assembled Monolayers of C₈ and C₁₈ Alkanethiols on the Au(111) Surface

Abstract

The thermal stability of short alkanethiol CH(3)(CH(2))(7)SH (C(8)) and long C(18) self-assembled monolayers (SAMs) is investigated using grazing angle reflection-absorption infrared spectroscopy, cyclic voltammetry, and molecular dynamics simulation. We track the disordering of SAM by untilting and gauche defect accumulation with increasing temperature in the 300-440 K range, a range of interest to tribology. Molecular dynamics simulation with both fully covered and partially covered C(6), C(8), and C(18) monolayers brings out the morphological changes in the SAM, which may be associated with the observed thermal stability characteristics. The molecular dynamics simulations reveal that short-chain C(6) and C(8) alkanethiols are more defective at lower temperature than the long-chain C(18) alkanethiol. With increasing temperature disorder in the SAM, as reflected in both untilting and gauche defect accumulation, tends to saturate at temperatures below 360 K for short-chain SAMs such that any further increase in temperature, until desorption, does not lead to any significant change in conformational order. In contrast the disorder in the long-chain C(18) SAM increases monotonically with temperature beyond 360 K. Thus, in a practical range of temperature, the ability of a SAM to retain order with increasing thermal perturbations is governed by the state of disorder prior to heat treatment. This deduction derived from molecular dynamics simulation helps to rationalize the significant difference we have observed experimentally between the thermal response of short- and long-chain thiol molecules.

Use of Eyring Equation to Explore the Frictional Responses of a –CF3 and a –CH3 Terminated Monolayers Self-Assembled on Silicon Substrate

“…A steady increase in residual peak frequency, that we observe with peak heattreatment temperature, indicates a general increase in disorder. The increase that we observe in integral intensity and untilting with peak temperature indicates an increase in gauche defect population [21,46,47]. Comparing the spectra of the methylene (d -) and methyl (r -) we further deduce that while gauche defects accumulate at the terminal end till about 90°C (peak temperature), at higher peak temperatures they penetrate significantly into the backbone.…”

Probing How Defects in Self-assembled Monolayers Affect Peptide Adsorption with Molecular Simulation

“…Though molecular simulation can offer unique insights into the consequences of SAM structural imperfections, it has only rarely been done [5][6][7][8][9]; limitations of small simulation cell sizes and/or insufficient sampling times have prevented the explicit exploration of defects in typical SAM modeling studies [4]. We have employed the enhanced sampling method parallel tempering metadynamics using the well-tempered ensemble (PTMetaD-WTE), which we have used successfully in several prior studies to study peptide/protein adsorption at interfaces [10][11][12].…”

Stability of large-area molecular junctions

“…Phase transitions and desorption of alkanethiol monolayers on Au(1 1 1) have been observed at elevated temperatures with STM [20,21,[24][25][26][27]. The two dimensional melting temperature of alkanethiol SAMs increases with increasing chain length of the alkane backbone [22,23,27]. The transition temperature in the junctions being independent of chain length can therefore not yet be explained.…”

“…Thermal stability of SAMs on Au surfaces has been studied extensively [17][18][19][20][21][22][23]. Phase transitions and desorption of alkanethiol monolayers on Au(1 1 1) have been observed at elevated temperatures with STM [20,21,[24][25][26][27].…”