For W tips, we use the experimental set-up schematically shown in the figure. It consists of a filament and a stainless steel made spherical ball. This device is mounted on a Z-translator to approach it towards the tip. The spherical shape of the ball is to have a better vision of the tip during the approach. We perform in UHV several cycles of self-sputtering and annealing under high voltage field of the tip.
Device for tip preparation in UHV
To Anneal (under high voltage field), we approach a W wire (filament) to the tip until contact, and we let circulate a current (2-3 Amps) through it. The tip temperature is about 600ºC. In this situation, we connect the HV power supply to the sphere, and we apply to the sphere +2KV. Then, we increase the tip temperature (as a flash) up to 1000ºC (increasing the filament current), for about 1 sec.
After this procedure, we perform self-sputtering: We approach the tip towards the sphere in Ne atmosphere: between 1 and 7×10-5 mbar of Ne pressure. We place the tip as close as possible to the spherical ball. We invert the voltage sign with respect to the drawing. The field emission current increase up to about 300 nA . We maintain this conditions for 2-3 minutes.
We perform 3-4 cycles of annealing and self-sputtering previous to transfer the tip to the STM. We finish the cycles by annealing, to be sure that all Ne is removed.
Comments and further information: Gonzalo Otero