Christopher Moore is an Assistant Professor of Physics at Coastal Carolina University. He lives in Myrtle Beach, SC with his wife and two children.My student and I are still working hard on our scanning tunneling microscope. We’ve had some issues with the solder-less breadboard we were using to prototype. Stuff was shorting on the breadboard, so we placed the components on a proto-board and soldered them together.
The pre-amp is now working exactly like it should. We supply a low current via a make-shift current source measured using a picoammeter and read correct voltage values on the other side. We’ve simulated the pre-amp using MultiSim and our circuit is behaving like the simulation.
The difference amplifier is another story! We’re still troubleshooting that, since it isn’t doing anything like what it should. With a +5 V signal and a set-point of +3 V, we should get a +2 V output. We get something different, though the something does scale correctly with variations in the set-point.
The scanner control electronics are working fine. They always have.
Over the next few weeks we expect to have all of the electronics properly placed on proto-boards, tested and functioning as they should. We have to go back and build a new approach mechanism for two reasons:
(1) The quasi-kinematic approach using machine screws is TERRIBLE! I have a micro-positioner thingy I salvaged from the junk heap that we plan on using for the fine approach. The coarse approach will still be made via machine screws. We’re also going to make the stage and lever much bigger. An STM that can fit in a pocket is no longer a goal — we’d rather have one that works!
(2) If you look back at the pictures of our piezo scanner, you’ll notice relatively large gauge wires and solder. I think the wires are too stiff, too thick and offer too much resistance. Also, it is possible that we’ve de-poled our piezo, though I’ve soldered piezo-electric ceramic before with fantastic results. We’re going to build the new scanner with smaller gauge wire and silver paste.
Concerning the piezo: I have a student using a Michelson interferometer to measure the piezo displacement with voltage for calibration purposes, and to convince myself that I haven’t de-poled the ceramic.
The department is purchasing licenses for LabVIEW and I have ordered some data acquisition boards. Once we’re done troubleshooting the STM via the oscilloscope, then the next student I can cajole into working on this project will be developing LabVIEW code to control the set-point, gains, and sample bias and pick-off the x,y,z voltages and convert them into pretty pictures. If we get really ambition, then maybe we’ll design an all digital feedback STM. That will require much faster DAQs, though, taking the project out of the couple-of-hundred-dollars range.
Update: Click the links below to navigate through the entire STM build series.
Building a scanning tunneling microscope for less than $100
STM update
STM is working!
Pictures of the STM
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