Role of the gold film nanostructure on the nanomechanical response of microcantilever sensors

Abstract

In this study, we have determined the relationship between the nanostructure of the gold film deposited on microcantilevers and the sensitivity and reproducibility of their static response to molecular adsorption. In order to tune the properties of the gold film, gold was deposited at different rates and thicknesses. The cantilever response to molecular adsorption was characterized by exposure of the cantilevers to mercaptohexanol in water. The morphology of the gold surface was characterized by atomic force microscopy, and the residual stress induced in the cantilevers was characterized by a profilometry technique based on the optical beam deflection method. We have found that the discontinuous morphology of the gold film for small thicknesses and low deposition rates gives rise to large values of residual tensile stress due to the formation of grain boundaries at the expense of strain energy. These cantilevers exhibit the highest sensitivity and reproducibility to molecular adsorption. However, larger thicknesses and higher deposition rates produce the coalescence of gold nanoislands via atom diffusion. This is characterized by a large relative decrease (increase) of the tensile (compressive) stress. These cantilevers exhibit small sensitivity and low reproducibility to molecular adsorption. We conclude that the control of the gold coating process is critical for the reliability of the measurements with nanomechanical sensors.