By Christopher Moore
Donnie and I have finally managed to get a stable tunneling current with our STM. On Thursday we ran a couple of traces in the x-direction, and as long as we didn’t touch the table or the air handler wasn’t on we saw minimal historesis.
Next week we will set everything up on stone block/foam sandwiches for vibration isolation, put in a gold sample, and try to get a decent XY scan on the oscilloscope. I’ll post pictures then.
By Christopher Moore
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.
By Christopher Moore
I am a scanning tunneling microscope (STM) guy. I completed my M.S. thesis using a commercial instrument and I have always wanted to build one. Until recently, I haven’t had the time or the money. About 1 month ago, a student here at Longwood University approached me about doing a project. The first thing that came to my mind was “Let’s build an STM!” The student agreed, and off we went. Now building an STM is part of my job!
Commercial research STM systems cost up to $250k or more. Less powerful “student” systems run around $15,000 but offer little to the researcher. The STM that my student Donald and I are building will end up costing ~$100 in parts. More important, the parts can be found at the local hardware store or ordered online.
Over the next few months Donald and I will chronicle the construction of our STM, showing you what parts we have used, how we are constructing it, and how it works. We plan to have a fully-functional analog STM up and running by the end of this semester (early December 2007). Next semester begining in January, we will chronicle the build of our second STM which will be digitally controlled via a computer. The budget for our computer controlled model is less than $200, and the microprocessor and control board are being designed by an undergraduate at Virginia Commonwealth University.
We have already begun. To learn more about STM, this specific project, or current progress, and what’s next, continue reading.
(more…)
