Surface Absorption of Monolayers
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individual molecule and the structure of the assembly it forms, but also the relationship between a given microscopic molecular property and its manifestation in the appropriate macroscopic property, arising from a spatial and thermal average of the microscopic property over the assembly structure. Such understanding is prerequisite to any progress in the development of new materials using the molecular design approach and selfassembly. Moreover, understanding the mutual relationships between chemisorption symmetry, intermolecular interactions, ^-functional groups, and monolayer structure in self-assembled monolayers is important not only for designing a new material, but also for the engineering of surfaces (Figure 1). SAMs SAMs are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface (Figure 2). This simple process makes SAMs inherently manufacturable and thus technologically attractive for building superlattices and for surface engineering. The order in these two-dimensional
Monolayer Structure
Chemisorption Symmetry
co-Functional Groups Intermolecular Interactions
Figure 1. The mutual interactions among chemisorption symmetry, intermolecular interactions, RS"Au + • Au° + —H 2 . (2) That the adsorbing species is the thiolate (RSI has been shown by XPS,73'89"91 Fourier transform infrared (FTIR) spectroscopy, 8889 Fourier transform mass spectrometry,92 electrochemistry, 93 and Raman spectroscopy.9495 Early electrondiffraction studies (both high-96'97 and low-energy98) of alkanethiolate monolayers on Au(lll) surfaces show that the symmetry of sulfur atoms is hexagonal with an S • • • S spacing of 4.97 A and a calculated area per molecule of 21.4 A2.
Helium diffraction 99 and atomic force microscopy (AFM)100 studies confirmed that the structure formed by docosanethiol on A u ( l l l ) is commensurate with the underlying gold lattice and is a simple (V3xV3)R30° overlayer (Figure 3). However, recent studies have revealed that the chemisorption picture is more complex than had been thought before. UHV scanning tunneling microscopy (STM) studies have revealed the existence of a 2D liquid phase at room temperature consisting of butane (CH3(CH2)3S ) and hexanethiolate (CH3(CH2)5S~) monolayers on Au(lll). 101 No 2D liquid was observed for octane (CH 3 (CH 2 ) 7 S~) and decanethiolate (CH3(CH2)9S ) monolayers. The short-chain homologues exhibited slow desorption of surface thiolate that led to the nucleation and growth of ordered domains having a unit cell of pX V 3 (8 < p < 10). On the other hand, both octane and decanethiolate form densely packed SAMs with a c(4 X 2) superlattice of a ( V 3 x V 3 ) R 3 0 ° lattice.101-104 The STM study also revealed facile transport of surface gold atoms in the presence of the liquid phase. This suggests that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/ desorption processes may have an important role in the formation of the final equilibrium structure of the monolayer. Support
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