Use of atomic layer deposition to improve the stability of silver substrates for in situ, high temperature SERS measurements

A method to stabilize silver surface-enhanced Raman spectroscopy (SERS) substrates for in situ, high-temperature applications is demonstrated. Silver island films grown by thermal evaporation were coated with a thin layer (from 2.5 to 5 nm) of alumina by atomic layer deposition (ALD), which protects and stabilizes the SERS-active substrate without eliminating the Raman enhancement. The temporal stability of the alumina-coated silver island films was examined by measurement of the Raman intensity of rhodamine 6G molecules deposited onto bare and alumina-coated silver substrates over the course of 34 days. The coated substrates showed almost no change in SERS enhancement, while the uncoated substrates exhibited a significant decrease in Raman intensity. To demonstrate the feasibility of the alumina-coated silver substrate as a probe of adsorbates and reactions at elevated temperatures, an in situ SERS measurement of calcium nitrate tetrahydrate on bare and alumina-coated silver was performed at temperatures ranging from 25 to 400 degrees C. ALD deposition of an ultrathin alumina layer significantly improved the thermal stability of the SERS substrate, thus enabling in situ detection of the dehydration of the calcium nitrate tetrahydrate at an elevated temperature. Despite some loss of Raman signal, the coated substrate exhibited greater thermal stability compared to the uncoated substrate. These experiments show that ALD can be used to synthesize stable SERS substrates capable of measuring adsorbates and processes at high temperature. Copyright (C) 2009 John Wiley & Sons, Ltd.