I synthesized ZnO nanowires on a glass substrate via thermal evaporation method. The nanowires have successfully grown vertically and well-alligned on the substrate. 

However, I faced a problem to get the IV curve (using cAFM) at different temperatures (by cooling down my ZnO nanowires sample). The IV curve appeared ok at room temperature, but the problem occurred when I vacuuming the chamber and start the cooling process. Please see the attached pictures (1st picture: the IV curve at room T, 2nd picturet: IV graph after my sample was cooled-down to below 290K).

Note that, I have also silver-pasted my sample (ZnO nano wires) to ensure it's conductive and have also tried different spots from my nanowires sample but the same problem still occur.

Plus, I tried to change the substrate to silicon (ZnO on silicon substrate), and managed to get the right IV curve both at room T and after the cooling process. Is it because my substrate is from glass that makes me unable to get the right IV curve?

Have anyone experienced the same problem with me? or perhaps anyone have any related paper for suggestion?

Thank you.