Molecular Self-Assembly at Bare Semiconductor Surfaces: Characterization of a Homologous Series of n-Alkanethiolate Monolayers on GaAs(001)
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- 作者：Christine L. McGuiness ; Daniel Blasini ; John P. Masejewski ; Sundararajan Uppili ; Orlando M. Cabarcos ; Detlef Smilgies ; David L. Allara
- 刊名：ACS Nano
- 出版年：2007
- 出版时间：August 2007
- 年：2007
- 卷：1
- 期：1
- 页码：30-49
- 全文大小：665K
- 年卷期：0
- ISSN：1936-086X
文摘
Structural trends for a homologous series of n-alkanethiolate self-assembled monolayers (SAMs), CnH2n+1S– with 12 ≤ n ≤ 19, on GaAs(001), studied by a combination of grazing incidence X-ray diffraction and infrared spectroscopy, along with ancillary probes, show an overall decay in organization with decreasing n, with the largest changes occurring below n = 15−16. The long-chain monolayers form a mosaic structure with ≤10 nm domains of molecules organized in an incommensurate pseudo-hcp arrangement with nearest neighbor distances of 4.70 and 5.02 Å, a 21.2 Å2 area per chain, two chains per subcell in a herringbone packing with a chain tilt angle of 14°, and preferential domain alignment along the substrate [11̄0]([1̄10]) step edge direction. In contrast, for n < 14 no evidence of translational ordering is seen and the alkyl chains exhibit a loss of conformational ordering and coverage relative to the n > 16 cases. A 4′-methyl-biphenyl-4-thiolate companion SAM shows evidence for ordered structures but with lattice parameters close to those expected for a structure commensurate with the intrinsic GaAs(001) square lattice. These trends are explained on the basis of competitions between lattice, interfacial, and intermolecular forces controlling the nanoscale structures of the SAMs. Overall these results provide an important aspect of understanding the effects of SAM formation on surface properties such as electronic and chemical passivation.