Studies Of Structure And Thermal Desorption Of Thiol Self-Assembled Monolayers Using Infrared Reflection Spectroscopy

Self-assembled monolayers (SAMs) are formed by the spontaneous chemisorptions of functional organic molecules with active groups (e.g. SH, S–S, COOH) on many different solid substrates (e.g. Au, Ag, Cu, Si). The most thoroughly studied and characterized system is organic sulfide self-assembled system, especially alkanethiol CH₃(CH₂)n-1SH (abridged as Cn) on a Au surface for the reasons of simple preparation, stable structure and high degree of order, where the driving force is the strong S-Au bond. Thermal stability of the monolayer and defects in the monolayer are two main factors to influence its application. Many groups have studied these aspects using techniques such as X-ray, Scanning Tunneling Microscopy, Infrared Spectroscopy and Molecular Dynamics simulation procedure, but have no consensus.Structure characteristics of alkanethiol SAMs with different chain lengths (C₁₆, C₁₂ and C₁₀) at room temperature and after annealing in the air are investigated using grazing angle reflection-absorption infrared spectroscopy.At room temperature, alkyl chains in long thiol SAMs are dense packing and the SAMs are with high degree of order. However, in the thiol SAMs with short alkyl chains, the order of the SAMs decreases, and alkyl chains are loose packing. Besides, there are some defects existing in the SAMs. The temporal stability of the SAMs preserved in air condition at room temperature is well, although a few gauche defects will be introduced in the monolayers.SAMs with different chain lengths have different annealing properties, because of different initial states at room temperature, such as gauche concentrations and degrees of order. Untilting of alkyl chains, aggregation of gauche defects and continuous rotation phase separately play leading function during different thermal annealing range. When the thermal annealing temperature is elevated to a certain point, desorption of partial alkyl chains occurs in all the three kinds of monolayers. The desorption temperature is between 362³69K for short thiol SAMs (C₁₀ and C₁₂), while 371³83K for long thiol SAMs (C₁₆ ), suggesting better thermal stability of long thiol SAMs.