Molecular Self-Assembly at Bare Semiconductor Surfaces:  Preparation and Characterization of Highly Organized Octadecanethiolate Monolayers on GaAs(001)

• Published in: Journal of the American Chemical Society Vol. 128; no. 15; pp. 5231 - 5243
• Main Authors: McGuiness, Christine L, Shaporenko, Andrey, Mars, Carole K, Uppili, Sundararajan, Zharnikov, Michael, Allara, David L
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 19.04.2006
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja058657d

Through rigorous control of preparation conditions, organized monolayers with a highly reproducible structure can be formed by solution self-assembly of octadecanethiol on GaAs (001) at ambient temperature. A combination of characterization probes reveal a structure with conformationally ordered alkyl chains tilted on average at 14 ± 1° from the surface normal with a 43 ± 5° twist, a highly oleophobic and hydrophobic ambient surface, and direct S−GaAs attachment. Analysis of the tilt angle and film thickness data shows a significant mismatch of the average adsorbate molecule spacings with the spacings of an intrinsic GaAs(001) surface lattice. The monolayers are stable up to ∼100 °C and exhibit an overall thermal stability which is lower than that of the same monolayers on Au{111} surfaces. A two-step solution assembly process is observed:  rapid adsorption of molecules over the first several hours to form disordered structures with molecules lying close to the substrate surface, followed by a slow densification and asymptotic approach to final ordering. This process, while similar to the assembly of alkanethiols on Au{111}, is nearly 2 orders of magnitude slower. Finally, despite differences in assembly rates and the thermal stability, exchange experiments with isotopically tagged molecules show that the octadecanethiol on GaAs(001) monolayers undergo exchange with solute thiol molecules at roughly the same rate as the corresponding exchanges of the same monolayers on Au{111}.