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Langmuir 2005, 21, 8270-8277
Chain-Length Dependence of Metastable Striped Structures of Alkanethiols on Au(111) Carmen Munuera,† Esther Barrena,†,‡ and Carmen Ocal*,† Departamento de Intercaras y Crecimiento, Instituto de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Cientı´ficas (CSIC), Cantoblanco 28049 Madrid, Spain Received November 30, 2004. In Final Form: May 20, 2005 The chain-length dependence of metastable striped phases of alkanethiols films partially covering the gold surface has been determined by means of atomic force microscopy. These structures are obtained from solution and consist of molecules adsorbed with their carbon chains flat on the surface. The stripes run parallel to the next-nearest-neighbor direction of Au(111) and have been found to always coexist with islands of upright molecules. The stripe spacing changes linearly with molecular length differently than twice the chain length. This dependence is discussed in terms of both interdigitation and herringbone-like lamella models. With time and under ambient conditions, these phases transform, without increasing coverage, by aggregation of the lying flat molecules to the preexisting islands with upright configuration.
Introduction The spontaneous adsorption of organosulfur compounds on metal surfaces has attracted wide attention in recent years, as they have shown to be promising materials for microelectronics and biotechnology devices.1 Self-assembled monolayers (SAMs) formed by alkanethiols on gold have become one of the most accepted candidates for technical applications due to their well-ordered, closepacked structure, high stability, and the easy control of their surface properties by introducing different functional groups at the exposed molecular end. In particular, biologically relevant functionalities are probably the most exciting properties of SAMs.2 By using the appropriate SAM as a “substrate”, biorelevant species can be adsorbed and immobilized. If lateral structuring is included, other fascinating applications are possible.3 However, the relative simplicity of the self-assembly method conceals the complexity of intermolecular and interfacial interactions involved in the formation of the monolayer.4 Moreover, the molecular packing quality significantly influences the final chemical and physical properties of these SAMs, such as the passivation efficiency, lubricity, and frictional coefficient.5,6 Thus, a better understanding of the structure and stability of the different phases observed at room temperature is essential for potential applications of organic thin films. Throughout their growth process, alkanethiols (CH3(CH2)n-1SH, referred to as Cn from now on) on Au(111) exhibit a rich variety of structures before converging to the densely packed high-coverage structure of * Author to whom correspondence may be addressed. E-mail:
[email protected]. † Instituto de Ciencia de Materiales de Madrid. ‡ Current address: Max-Planck-Institut fu ¨ r Metallforschung, Heissenbergstr. 3, 70569 Stuttgart, Germany. (1) Schreiber, F. Prog. Surf. Science 2000, 65, 151; Schreiber, F. J. Phys.: Condens. Matter. 2004, 16, R881 and references therein. (2) Plant, A. L.; Chen, C. S.; Groves, J. T.; Parikh, A. N. Langmuir 2003, 19, 1449, special issue on biomolecular interfaces. (3) Kra¨mer, S.; Fuierer, R. R.; Gorman, C. B. Chem. Rev. 2003, 103, 4367. (4) Ulman, A. An Introduction to Ultrathin Organic Films From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, 1991. (5) Carpick, R. W.; Salmeron, M. Chem. Rev. 1997, 97, 1163. (6) Lee, S.; Shon, Y.-S.; Colorado, R., Jr.; Guenard, R. L.; Lee, T. R.; Perry, S. S. Langmuir 2000, 16, 2220.
the complete monolayer. It is well established that the lowest-energy configuration consists of molecules ordered in a close-packed (x3 × x3)R30° lattice commensurate with the underlying surface. Optimal packing of the n-alkane component of the SAM is achieved by tilting the molecular axis with respect to the surface normal (∼30°). This saturation phase yields the highest molecular density with about 21.6 Å2/molecule7-13 and may appear modulated by a c(4 × 2) superstructure.9-11,14 Among the different configurations observed, particular attention has been devoted in the literature to the socalled striped phases, where the molecules arrange, forming rows on the gold surface. These low-density structures can be prepared by various methods such as gas-phase deposition,13,15-17 brief immersion in dilute solutions (
