Deprotecting Thioacetyl-Terminated Terphenyldithiol for Assembly on Gallium Arsenide Dmitry A. Krapchetov, † Hong Ma, ‡ Alex K. Y. Jen, ‡ Daniel A. Fischer, § and Yueh-Lin Loo* ,†,| Chemical Engineering Department, UniVersity of Texas at Austin, Austin, Texas 78712, Materials Science and Engineering Department, UniVersity of Washington, Seattle, Washington 98195, Materials Science and Engineering Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, and Chemical Engineering Department, Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed August 7, 2007 We characterize the assembly of terphenyldithiol (TPDT) on gallium arsenide (GaAs) from ethanol (EtOH) and tetrahydrofuran (THF) as a function of ammonium hydroxide (NH 4 OH) concentration. NH 4 OH facilitates the conversion of thioacetyl end groups of the TPDT precursor to thiolates in the assembly solution. The final structure of TPDT assembled on GaAs is sensitive not only to the assembly solvent but also to NH 4 OH concentration. In the presence of low concentrations of NH 4 OH (1 mM), TPDT assemblies from EtOH are oriented upright. The same assemblies are less upright when adsorption is carried out at higher NH 4 OH concentrations. In THF, TPDT does not adsorb significantly on GaAs at low NH 4 OH concentrations. The surface coverage and structural organization of these assemblies improve with increasing NH 4 OH concentrations, although these assemblies are never as organized as those from EtOH. The difference in the final structure of TPDT assemblies is attributed to differences in the thiolate fraction in the assembly solution at the point of substrate immersion. Introduction The drive to create nanoscale electronics 1 with active components composed of single molecules or molecular layers has prompted strong interest in molecular assembly. Thiols, in particular, have been shown to chemically attach to metals (Au, 2-5 Ag, 3,5,6 Ni, 7 Pt, 8 etc.) and semiconductors (GaAs 3,9-13 ). Con- jugated thiol chemistries with rigid backbones have thus played a central role as model systems for molecular wires, 14 switches, 15 rectifiers, 16 and diodes. 17 To this end, charge transport in these systems is governed not only by the chemical structure of the molecule but also by the way the molecules are organized and attached to the electrode surface. 18,19 High-quality and well- characterized self-assembled monolayer (SAM) structures are thus prerequisites for reliable charge transport studies. 18 Much of the recent attention has focused on the assembly of thiols on GaAs, 12,13,20,21 including the use of dithiols to form GaAs- molecule-metal junctions. 22-24 The understanding of how conjugated dithiols assemble on GaAs surfaces, however, is limited. In light of this, we have chosen to investigate the assembly of a model conjugated dithiol molecule, thioacetyl-protected terphenyldithiol (TPDT), on GaAs. As-synthesized, conjugated dithiols are usually protected with thioacetyl groups because free thiols have a tendency to oxidize 14 and dimerize. 14,25 The thioacetyl groups are converted in situ to thiolates with the addition of ammonium hydroxide (NH 4 OH) during assembly. 14,26 The assembly of such systems on GaAs is nontrivial and is highly sensitive to the conditions at which the molecules adsorb. For instance, we have shown that the assembly of terphenyl- and quaterphenyldithiols on GaAs is strongly affected by the solvent quality. 20 Here, we report how the concentration of the deprotecting agent, NH 4 OH, in the assembly solution affects the final structure of TPDT assembled from EtOH and THF on GaAs. 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Science 2001, 292, 2303. 851 Langmuir 2008, 24, 851-856 10.1021/la702430j CCC: $40.75 © 2008 American Chemical Society Published on Web 12/29/2007