Detection of organic vapors on sputtered and annealed thin Au films

Introduction Thanks to a high ratio of free volume the atoms forming thin films are susceptible to surface diffusion over substrates and the films undergo a structural change after annealing at relatively low temperatures.... [ view full abstract ]

Introduction

Thanks to a high ratio of free volume the atoms forming thin films are susceptible to surface diffusion over substrates and the films undergo a structural change after annealing at relatively low temperatures. This transformation process called solid state dewetting leads to evolution of an island-like structure. Isolated metal islands show interesting properties, namely localised surface plasmon resonance (LSPR), which is observed as a distinct absorption band in UV-Vis spectrum, which is highly influenced by refractive index of surrounding medium. This phenomenon can be used for construction of sensitive devices for detection of gases in the atmosphere around the metal nanostructure. 

Methods 

Au films of 2-60 nm thickness were prepared by cathode sputtering at 20 mA for 10-300 s on glass slips. Annealing of the thin Au films was performed in FED 23 oven at 300 °C for 1 h. Surface structure was observed by atomic force microscopy (AFM, CP II Bruker, tapping mode), optical properties were determined by UV-Vis spectroscopy (Lambda 25, Perkin Elmer). The prepared Au nanostructures were tested for their optical response to introduction of vapors of aceton, water, ethanol and toluene in an experimental arrangement (Fig. 1). 

Results

AFM measurements showed dramatic structural change occuring after annealing of the thin Au films. The as prepared continuous films are transformed into structure of separated Au nanoislands. This phenomenon is readily observable on the thicker Au layers (Fig. 2). Impact of the structural change on optical properties of the material is expressed in the UV-Vis spectra as evolution of strong absorption bands associated with LSPR of the fine metal nanostructure (Fig. 3). Measurements of response of the nanostructure to organic vapors showed the highest sensitivity in the case of 4 nm thick films and the highest response was found to be to aceton vapors (Fig. 4).

Discussion

The optical properties of the Au nanostructures were found to be definitively dependent on the presence of organic vapors. However, selectivity of such a simple sensitive system is low. This problem could, however, be solved in further research by e.g. addition of a polymer layer with affinity to specific analyte.