Scanning Tunneling Microscopy (STM) is usually done in UHV (and sometimes in Air). Doing STM in UHV has many advantages but also imposes greater constraints. In some applications, possibly working in HV only could be more advantageous. So, I wonder exactly what one loses by performing STM in HV (apart from no having perfect surface cleanness). Would the current stability (and tip behaviour) be much worse? What drawbacks would high vacuum imply? Please indicate relevant literature on the subject.
I thing the response is 2-fold. First is the technical issue due to possible discharge as mentioned above. Note that STM head has many piezo which function with high voltages and which may be damage by discharge. You can use them without problem in air and in UHV but you have to be careful in between. This is because the distance for discharge to happen is minimal at a pressure between 10^-2 mBar and 10^-4mBar. Have a look here: http://en.wikipedia.org/wiki/Paschen%27s_law . Second is the kind of sample you want to study. STM is often use to study surface at atomic level. In order to do that you need the surface to stay clean. There the pressure will be related with the time the surface will stay clean. Look there: http://www.chem.qmul.ac.uk/surfaces/scc/scat4_2.htm
At 10^-10 mbar, you can work about 3 hours before the surface is cover by a monolayer of residual gas. At 10^-6, you have only 1 second to complete your image before 1 ML coverage !
Don't forget that if you use tungsten tips they oxidize faster in HV environments which will change your ability to image. Also attached is a file showing surface degradation of Si before and after six days of UHV (10-11 Torr) from my ancient dissertation which can be found at the attached link
http://wrap.warwick.ac.uk/2246/
- Badges
- Science topic
- Similar topics
- Engineering
- Instrumentation Engineering