When the STM becomes unstable(large variations in the current), what procedures should I take in order to fix it? I am new to the STM, but after a few month`s experience I notice that one of the main problems is stabilizing it. When the tunneling current becomes unstable, I`ve tried to vary the bias voltage, current, gain, time constant, and applying voltage pulses. I find that applying pulses much higher than the bias voltage seems to be most effective. However, there are times when nothing works, and the topographic image lines can vary in orders of tenths of nanometers, with no similarity in the left and right scan directions. How much does sample preparation affect the STM measurement? I have to prepare my samples on HOPG in ambient conditions. To avoid contamination, I heat the sample after preparation of HOPG, then I dry in a vacuum dessicator. If there are more efficient methods, I`m willing to hear all advices.