Self-assembled thin films of thiocyanate and selenocyanate bithiophenes

Abstract

Presented herein is the organic synthesis and thin film (TF) assembly of six alkyl-substituted thiocyante and selenocyante bithiophene molecules: NCSe-T2-Hx, NCS-T2-Hx, NCS-Hx-T2-Hx, NCSe- Hx-T2-Hx, NCS-Hx-T2-Hx-SCN and NCSe-Hx-T2-Hx-SeCN. TFs assembled from these thio and selenocyanates were characterized by six different techniques: electrochemical capacitance (Cp) measurements, scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), atomic force microscopy (AFM), x-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Cp experiments were used to confirm the development of TFs on an amorphous Au surface using ethanol or hexanes as a reaction solvent. A sputtered Au surface was observed by SEM/EDX to be an inappropriate surface for TF assembly. TFs assembled onto epitaxially grown Au were characterized by multiple techniques. Large crystallites were observed on TFs assembled from NCSe-T2-Hx and NCSe-Hx-T2-Hx-SeCN. Chemical maps of these crystallites were obtained through SEM/EDX. The presence of a covalent Au-S or Au-Se bond on the surface was confirmed by XPS signals from the S 2p3/2 and Se 3d5/2/3d3/2 regions respectively. XPS signals of ~ 399 eV in the N 1s region were observed in all assemblies characterized by this technique, indicating the presence of residual gold-bound cyanide (Au(CN)ads) on the surface. ToF-SIMs chemical intensity maps confirmed the presence of expected TF fragments on the surface of all TFs characterized by this technique. ToF-SIMS results suggested that TFs connected to the Au surface through an aromatically bound S or Se atom produced TFs with higher surface coverage.