Deprotecting thioacetyl-terminated terphenyldithiol for assembly on gallium arsenide

We characterize the assembly of terphenyldithiol (TPDT) on gallium arsenide (GaAs) from ethanol (EtOH) and tetrahydrofuran (THF) as a function of ammonium hydroxide (NH₄OH) concentration. NH₄OH 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 NH₄OH concentration. In the presence of low concentrations of NH₄OH (1 mM), TPDT assemblies from EtOH are oriented upright. The same assemblies are less upright when adsorption is carried out at higher NH₄OH concentrations. In THF, TPDT does not adsorb significantly on GaAs at low NH₄OH concentrations. The surface coverage and structural organization of these assemblies improve with increasing NH₄OH 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.