J. Phys. Chem. B 2005, 109, 22971-22975
22971
Enhanced Lubricity in Mixed Alkanethiol Monolayers Lu Zuo,† Ye Xiong,‡ Xincheng Xie,‡ and Xudong Xiao*,† Department of Physics, The Hong Kong UniVersity of Science and Technology, Hong Kong, China, and Department of Physics, Oklahoma State UniVersity, Stillwater, Oklahoma 74078 ReceiVed: June 16, 2005; In Final Form: August 28, 2005
The frictional properties of two-component mixed alkanethiol monolayers were investigated with frictional force microscopy. With large chain length differences, the two types of molecules segregated into large islands and reveal similar frictional properties as those of single-component monolayers. With small chain length differences, the completely mixed monolayers display a decreased friction as compared with single-component monolayers. This observation indicates that development of gauche kinks in the top portion of the mixed monolayers can enhance lubrication rather than increase friction, perhaps due to suppression of other energy dissipation channels.
Introduction One of the goals of molecular-scale tribology is to correlate the tribological properties (adhesion, friction, and wear) with the atomic or electronic structures of the contact surfaces, to identify the energy dissipation mechanisms during friction.1-6 For self-assembled monolayers (SAMs), the effects of the hydrocarbon chain length,7-11 the ordering of the monolayers,7,11,12 and the termination groups13 on friction have been extensively studied. The role of gauche kinks in energy dissipation has been intensively examined.7-10,14-17 While theoretical simulations have recently shed light on the understanding of the effects of chain length and monolayer ordering on friction, disagreements between experiments and simulations still exist and remain to be resolved.11 In this paper, we report an investigation of the frictional properties of two-component mixed alkanethiol self-assembled monolayers (SAMs) with different chain lengths. By examining single-component alkylsilane monolayers with different chain lengths self-assembled on mica, it has been found7,8 that the friction becomes higher with shorter chain lengths. This relationship was interpreted as a result of the quality of monolayer assembly, with the shorter chain having more gauche defects. It was thus taken as evidence that gauche defects are an energy-dissipation channel.7 In the present experiment, we mix two types of alkanethiol molecules with different chain lengths and self-assemble them on Au(111). The friction of mixed monolayers, either Langmuir-Blodgett films18 or selfassembled films,13,19 has been studied before. However, the attention was basically focused on monolayer systems with similar molecular structures but different terminating groups.13,18,20,21 Mixed monolayers with the same termination groups facing the contact have seldom been investigated.19 Choosing mixed alkanethiol monolayers self-assembled on Au(111) with the same termination groups has a number of advantages: (i) the formed SAM is commensurate with the Au(111) substrate,22 which provides a good measure of the quality of the monolayer as compared to alkylsilane self* Author to whom correspondence should be addressed. E-mail:
[email protected]. † The Hong Kong University of Science and Technology. ‡ Oklahoma State University.
assembled monolayers which do not form two-dimensional ordered lattices; (ii) the compact packing in the lower part of the mixed monolayers strongly suppresses the development of gauche kinks in the respective part of the hydrocarbon chain by steric constraints and any gauche kinks can therefore be controlled to be in the top portion of the mixed monolayers; (iii) by varying the chain length difference and/or the composition, the density of the gauche kinks can be controlled; and most importantly (iv) by using the same terminal group for the alkanethiol molecules in the mixed monolayers, the chemical effect due to different termination groups is eliminated. Any observed effect can be easily related to the molecular chain length and the difference between the chain lengths of the two constituent molecules. Thus, the mixed alkanethiol monolayers provide us with an ideal system to investigate the role of gauche kinks in energy dissipation. Experimental Section Alkanethiols, CH3(CH2)n-1SH (abbreviated as Cn), were selfassembled on atomically flat Au(111) surfaces.22,23 These alkanethiol materials were purchased from Aldrich with a nominal purity of 97% and used without further purification. The atomically flat gold substrates were produced by thermal evaporation of ∼1600 Å gold (99.99%) thin films on muscovite mica at 300 °C under a pressure of
