A community of Physics Phans!

I realize I haven’t written an update on the scanning tunneling microscope (STM) in a while. I had mentioned about a month ago that the STM was working; however, Donnie left for Old Dominion University via our 3/2 engineering program and I haven’t yet recruited a student to pick up the pieces. I’m working out how we can computerize this thing while still keeping the total cost below $200. I’ve got some ideas!

Pictures after the jump!

Here is the zoomed out view of the tangly mess that is our STM. You can see the STM head, pre-amp, power supply, feedback electronics and scanner. We divided the electronics into sub-systems because it was easier to troubleshoot that way. It looks terrible, but it works. In the future, we’ll rebuild the electronics onto one board (with the exception of the pre-amp, which will be as close as possible to the tip to minimize resistance).

Here is a close up of the pre-amplifier. It is just a fairly simple current-to-voltage op-amp circuit.

The feedback electronics is a difference amplifier and an integrating amplifier. The difference amplifier is used to set the “current” (that comes from the pre-amp as about 300 mV per 1 nA) via the setpoint potentiometer. This is a constant current STM, so if the current drops, meaning the tip is further away from the surface, then the difference between what is seen coming out of the pre-amp and the setpoint voltage will be larger. This results in a voltage being applied onto the Z electrode on the piezo that moves the tip closer until there is no difference. This voltage is also read as the Z height.

The scanner is composed of four op-amp voltage followers, two of which are inverting. Two function generators (one for the X and one for the Y) control the XY motion of the tip. I use triangle wave functions with appropriate frequencies.

We needed to connect all of the various sub-systems together, so we built a power-supply board. The power supply is just two 9 V batteries.

I did not get a picture of a scan and I haven’t yet had time to get the STM running again so that I could. When the semester starts and I get another student involved, then his/her first order of business will be to get a good image on the oscilloscope and capture it with a digital camera. The next step is computerization! We have some LabVIEW and some nice data aquisition boards, so it should be easy. However, I want to come up with a solution that is under $200 total.