Surface-Controlled Electroless Deposition Method in the Preparation of Stacked Silver Nanoparticles on Germanium for Surface-Enhanced Infrared Absorption Measurements

A new method to prepare highly sensitive sensing elements for surface-enhanced infrared absorption (SEIRA) measurements was investigated. A surface-controlled procedure was employed to grow round and stacked silver nanoparticles (Ag-NPs) on germanium substrates. In this method, an initial layer of Ag-NPs was prepared using a common method of electroless deposition. After subsequently placing a controlled layer of p-aminothiophenol (pATP) on the surface of the initial layer of Ag-NPs, the substrates were placed in a silver nitrate solution to grow a second layer of Ag-NPs. By repeating these growing procedures, multi-layers of stacked Ag-NPs can be obtained. To examine the influence of morphology of the formed Ag-NPs on the resulting SEIRA signals, the factors affecting the reactions were systematically examined. These factors included the concentrations of silver nitrates, the reaction times to prepare both the initial layer and the second layer of Ag-NPs, and the coverage of pATP. Results indicate that the Ag-NPs making up the second layer were round in shape and much more densely distributed than those in the initial layer. The observed SEIRA spectra did not show derivative-shaped absorption bands for pATP on the Ag-NPs after re-growth, indicating that pATP was sandwiched between the two layers of Ag-NPs, preventing the nanoparticles from coming into direct contact with one another. Also, the SEIRA signals of the controlled molecules between the particles were found to be two to five times more intense than the signals before growing another layer of Ag-NPs. The reaction conditions can be adjusted to vary the morphology and thickness of the Ag-NP layers, and, by repeating the growing procedures, a thick layer of stacked Ag-NPs with suitable size for SEIRA measurements can be obtained that is highly suited to chemical sensing applications.