Tip Reshaping

The most common method is to apply simple voltage pulses during tunneling. The absolute value of these pulses may vary from one tip and contamination to another but most tips get unstable and loose their contamination at sample bias above 4V. Sometimes it could also help to scan a higher bias (e.g. 9V) across a larger area for some time (>10min) to stabilize the tip. Another method is to use a high voltage to clean off the tip contaminations. In tunneling condition retract the tip by at least 50nm by applying a Zoffset. Disconnect the current preamplifier and connect a the high voltage line (+100VDC) with an in-line resistor (100kOhm to limit the current to max. 1mA) instead. Also connect the left or right sample contact via a current meter to the ground. This way the tip is connected to a positive bias in respect to the sample. Usually this is better, but you may also try the opposite way. Two things are important: a) the current preamp MUST be removed. It could be destroyed otherwise. b) the tunneling wire should not be loaded with more than 100V. It may survive a higher voltage, but if you burn it, you will have to open the STM for replacement. Once you have made these connections, you just decrease the distance to the surface - again by using the Zoffset potentiometer - until you get a stable (!) current of a few µA. If the current saturates (1mA), retract the tip and try again. It could happen that you need to change the tip position at the sample several times until you get it stable.