Nanomechanics of Self-Assembled Monolayers on Nanoscale Gold Films
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1086-U08-27
Nanomechanics of Self-Assembled Monolayers on Nanoscale Gold Films Milca I Aponte-Roman, and Adrian B Mann Materials Science & Engineering, Rutgers, The State University of New Jersey, 607 Taylor Road, Piscataway, NJ, 08854 ABSTRACT Self-assembled monolayers (SAMs) are thin organic films formed by a single layer of molecules adsorbed on a substrate. Since their discovery the preparation of these molecular assemblies has attracted the attention of interfacial researchers interested in controlled wetting of surfaces, adhesion, friction, chemical sensing, and high resolution lithography. However, little effort has gone into understanding how this nano-layer affects the mechanics of the underlying surface. In this study the nanomechanics of alkanethiolate SAMs on Au (111) films has been investigated using nanoindentation techniques. The research is aimed at establishing the effect of a SAM on the measured mechanical properties of the Au film. The SAMs considered for this study were made from 1-decanethiol, 96% (CH3(CH2)9-SH). Nanoindentation experiments were performed using a Triboindenter (Hysitron Inc., MN) using displacement control mode. Comparisons were made between the mechanical behavior of the Au films, the Au films plus SAMs and the glass substrate. A range of maximum indentation displacements were used. During the nanoindentation tests the load-displacement curves and the apparent mechanical properties were found to depend on the presence of the SAM film. Surprisingly, the effects of the SAM layer are seen even when the nanoindentation displacement is orders of magnitude greater that the SAM thickness. Many of the effects of the SAM can be explained by changes in the contact geometry and the ability of the SAM to sustain compressive loads when it is in a confined volume. The results and conclusions are potentially relevant to all thin adsorbed organic films, including protein layers on biomaterial surfaces and lubricants on engineering components. INTRODUCTION Very thin, organic surface layers play a vital role in many tribological processes, but how they modify the mechanical behavior of a surface is difficult to quantify. Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the spontaneous adsorption of amphiphilic molecules onto hydrophilic surfaces by the immersion of a substrate into a solution of an active surfactant. The production of SAMs can be traced back to 1946 when Zisman et al. [1] found that certain types of polar organic molecules are adsorbed from solutions in non-polar solvents to form well-oriented monolayers on solid surfaces. Zisman’s films were nearly closepacked and formed approximately vertically oriented molecules. In 1983, Nuzzo and Allara published an extension to Zisman et al’s approach, showing that dilkyldisulfides form oriented monolayers on gold surfaces, and exposed both high- and low-energy surfaces, depending on the tail group present [2]. Later the same was found for sulfides [3] and thiols [4]. SAMs can be prepared using different types o
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