“…Consequently in the gas sensor field, it is difficult to use directly the piezoelectric crystal device to detect gas features, etc., (Lynnworth, 1965). One may turn to the medium of slower density, like P(VDF/TrFE) (Takahashi, Ohigashi, 2009), or other technologies, i.e., adding the chemical reaction film (Wang et al, 2021), the transition layer (Surappa et al, 2018), or a resonance cavity (Dong et al, 2003) for the specific kind of gas.…”