Surface acoustic waves in graphene structures: Response to ambient humidity

We report on the response to the ambient air humidity of a surface acoustic wave (SAW) in YZ LiNbO₃ substrate with the 200 nm-thick multi-layer graphene sheets deposited by chemical reduction from graphene oxide. With increasing relative humidity (RH), the SAW velocity decreases and the SAW attenuation increases. At humidity values below 60-65%, the SAW velocity linearly depends on RH with the slope proportional to the SAW frequency. This is attributed to mass loading of the SAW propagation surface due to moisture adsorption by the graphene layer. The normalized fractional change in SAW velocity is 4.23·10^-2 ppm/(MHz-%RH). At higher humidity values, the velocity decrease becomes super-linear due to the change in elastic graphene properties. An exponential growth of the humidity-induced SAW attenuation by the graphene film with frequency (from 1 dB/cm at 57 MHz to 7 dB/cm at 286 MHz) has been observed. The time response to abrupt humidity changes is different for the transmitted SAW phase velocity and attenuation pointing out to different physical mechanisms responsible for humidity-induced SAW attenuation and velocity variations.