We characterize the assembly of terphenyldithiol (TPDT) on gallium arsenide 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 not only sensitive 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 assembled 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.
Pub Type: Journals
IR spectroscopy, molecular electronics, NEXAFS, SAMS, surfaces