Simple chemical method for forming silver surfaces with controlled grain sizes for surface plasmon experiments

We report on how the mirror reaction method can be used to prepare silver films with well controlled grain sizes in the range that is known to be ideal for surface plasmon experiments in the visible region of the spectrum. We characterize the morphology of the silver films by atomic force microscopy and their optical properties by UV-vis absorption spectroscopy, comparing these results with the predictions of classical electromagnetism. The colloid particle size can easily be controlled to lie in the range 50-200 nm, and their shape is approximately spherical with aspect ratios (lateral dimensions to height) in the range 1:1 to 1:2. These dimensions and morphology are ideal for surface plasmon based techniques such as surface enhanced Raman spectroscopy (SERS) and surface plasmon resonance (SPR). The method also has the added advantage that it can be used in applications where conventional coating by evaporation can damage the substrate due to heating. We give one such example-the coating of an atomic force microscope probe for apertureless scanning near-field optical microscopy. We also report a simple, gentle, and highly effective method for cleaning these metal surfaces to completely eliminate the background contaminant signal that is typically present in SERS spectra from such surfaces.