Sensitivity variation of the electrochemical quartz crystal microbalance in response to energy trapping

The spatial sensitivity of the electrochemical quartz crystal microbalance (EQCM) to deposited mass is known to vary, from a maximum at the electrode centre to a low value at its edge. For small mass loadings, the effects of such spatially non-uniform deposition can be described by simple convolution of the added mass and the spatial sensitivity function; the response is linear with added mass. However, when the added mass is large, the phenomenon of “energy trapping” is significant; the EQCM sensitivity is predicted to be non-linear with locally added mass. These deviations have been found experimentally (via local Cu deposition) to be significant. A model based on the Rayleigh principle of energy balance and energy trapping phenomena is able to describe the observed deviations. Heavy deposits change the shape of the resonator oscillations: their amplitude decreases exponentially from the edge of the deposit with a decay length that depends on the deposit mass. This effect allows one to vary the differential sensitivity of the EQCM in a desirable way while retaining conductivity of the complete electrode surface.