Characterization of thio compounds for a surface-controlled electroless deposition method in the preparation of silver nanoparticles on germanium for surface-enhanced infrared absorption measurements.

An electroless re-growing method has been developed to prepare silver nanoparticles (Ag-NPs) on substrates for surface-enhanced infrared absorption (SEIRA) measurements. To understand the roles of specific molecules in controlling the properties of Ag-NPs on germanium substrates, thio compounds bearing different functional groups were examined systematically. Four classes of thio compounds were examined in this work, including aromatic thio compounds with second functional group, alkyl thio compounds with second functional group, structural isomers of aromatic thio compounds, and alkyl thio compounds with different alkyl chain lengths. By correlating SEIRA signals with the observed morphologies of the Ag-NPs, it was found that thio compounds bearing amine polar functional groups produced Ag-NPs that were the most regular in shape and size. Also, aromatic thio compounds provided better performance than did alkyl thio compounds in terms of producing suitable shapes and sizes for SEIRA. The Ag-NPs prepared in this work were generally around 50 nm in diameter and round in shape. To examine the prepared substrates for chemical sensing purposes, the enhancement factors were determined by deposition of para-nitrothiophenol (pNTP) on the Ag-NP substrates produced by this surface-controlled electroless deposition method. Results indicated that the surface of the newly formed Ag-NPs actively interacted with pNTP, and the enhancement factors obtained ranged from 50 to 150.