Unique Surface Characteristics of Cyclohexanethiol Self-Assembled Monolayers on Au(111)

Abstract

Surface structure and conformational change of cyclohexanrthiol (CHT) SAMs on a gold surface was investigated by scanning tunneling microscopy (STM) and surface-enhanced Raman scattering (SERS). It was revealed that the spontaneous adsorption of CHT on Au(111) leads to long-range two-dimensional ordered SAMs. High-resolution STM imaging clearly reveals that CHT SAMs have an oblique unit cell containing two different molecular features, indicating the presence of two energetically stable conformers, i.e., the equatorial and axial chair conformers. Two conformers in CHT SAMs were confirmed by SERS study. We also found that the surface structures CHT SAMs on gold are quite similar regardless of the solution temperature of CHT solution used for SAM preparation, contrary to the case of alkanethiol SAMs.

I. Introduction Self-assembled monolayer (SAMs) of alkanethiols on gold surfaces have attracted many researches because they fundamental interest to surface science and the high possibility of their practical application. SAMs are known to be formed as a result of chemical bond between gold substrate atom and sulfur atom of the molecule. In recent years, many studies have focused on various aromatic thiol SAMs to understand the packing structure, adsorption kinetics and orientations of the molecules adsorbed on the Au (111) surface. As a result, it was found that the packing arrangement and the ordering of aliphatic or aromatic thiol SAMs are strongly influenced by interplay of various interactions, the interchain van der waals interaction and interchain end group interactions [1]. On the other hand, it was reported that the surface morphology of alkanethiol SAMs was significantly influenced by the temperature of thiol solution used for SAM preparation [2]. In this work, we examined the temperature effect of solution for CHT SAM formation using scanning tunneling microscopy (STM) and surface-enhanced Raman spectroscopy (SERS). II. Experiment Cyclohexanethiol (C6H11SH) was purchased from Tokyo Chemical Industry (Tokyo, Japan) and used without further purification. The Au(111) substrates on mica were prepared by vacuum deposition, as described previously [3]. CHT SAMs was formed by dipping Au(111) substrates into freshly prepared 1 mM ethanol in CHT solutions of the corresponding compounds for 1 hour or 1 day. The temperatures of solutions were -20, 4, 20, and 50 ℃. STM measurements were carried out with a NanoScope E. All STM images were obtained in air in the constant current mode. Raman spectra were obtained using a Renishaw Raman confocal system model 1000 spectrometer equipped with an integral microscope (Leica DM LM).

III. Results & Discussion It was revealed that the temperature of solution for alkanethiol significantly affects the size and number of domain and vacancy islands (VIs) during the self-assembly processes. Especially, alkanethiol SAMs have many small sizes of domains and VIs at low temperature of - 20 ℃. However, surprisingly, CHT SAMs form well-ordered structure with a large size of few domains and VIs even at low temperature of - 20 ℃, as shown in Fig. 1. The molecularly resolved STM image in Fig. 1b shows two different molecular spots in the unit cell, indicating two conformers, i.e., the equatorial and axial chair conformers. We consider that the remarkable differences in surface structures between alknethiol and CHT SAMs results from a large difference in van der Waals interactions between alkyl groups because CHT molecules have very low lateral interactions due to the high flexibility of cyclic ring compared with alkanethiol SAMs.

References (1) J. Noh and M. Hara, Langmuir, 18, 9111 (2002). (2) R.Yamada, H. Wano, and K.Uosaki, Langmuir 16, 5523 (2000). (3) J. Noh, T. Murase, K. Nakajima, H. Lee, and M. Hara, J. Phys. Chem. B 104, 7411 (2000).