After a few days, I cannot find an answer to this question:
Besides mechanical stress, is there another way to induce a negative charged surface on quartz?
Note: several scholarly articles kept referring to photons/light but never gave an example or explanation. This is one example of not finding adequate material. Of course, I found other discussions which usually went into silicon and semiconductors. Not what I am looking for.
According to the triboelectric series, rub a Teflon rod with a piece of silk, then separate them and touch the rod to the surface of the quartz, and some charge will be transferred to the quartz surface.
Besides mechanical stress, one way to induce a voltage across a quartz-crystal is a change in temperature, since quartz is also a pyroelectrica. The crystal structure changes in response to a different temperature, and induces polarization.
This is not always the case for piezoelectrica, since not all piezoelectrica are pyroelectrica:
The generated voltage will gradually disappear due to leakage current.
