This project is my attempt to build a low-cost scanning tunneling microscope (STM) capable of atomic resolution imaging in air. The piezo scanners typically used in STM typically cost at least hundreds of dollars. Some time ago I came across John Alexander’s simple STM project, in which he used a cheap piezoelectric buzzer element with one of the electrodes cut into quadrants to enable full XYZ motion. This type of scanner is less rigid than what is usually used for STM, but I decided to give it a try and see how far I could get with it. Turns out I was able to image highly-oriented pyrolytic graphite (HOPG) with my STM with atomic resolution!
It's been a while since I've posted any updates, so here are a bunch of images taken with the microscope. I added an automated coarse approach mechanism some time ago using the very cheap 28BYJ-48 stepper motor. By driving the rear fine-pitch screw of the STM with this motor, which takes 2048 full steps/rev, I get a tip-sample step size of about ~6 nm. This (ideally) results in a crash-free approach, and much better quality images. You can read more about it on my blog: https://dberard.com/2015/11/08/coarse-approach/
Still no luck getting atomic resolution on gold, which I believe is due to some combination of acoustic noise/vibrations and DAC glitches, which are actually quite large and cannot be completely filtered out. You can see the "noise" in some of the images below. More on this later...
I'm also testing out a new PCB, which is very similar to the previous one but has the power supply, Teensy, and analog stuff integrated onto one board. I'm also working on a higher performance version, with better DACs, HV amplifiers, an FPGA, and hardware for tuning-fork AFM!
Here's a picture of the microscope with coarse approach motor:
A gold surface with a mirror finish looks quite rough at the nanoscale: (I really need to find some other samples to image...)
A smaller scan of the same area, shown as a 3D surface to emphasize the edges of the atomic planes:
Smaller scan of the same area:An attempt at imaging beta-mercaptoethanol (BME) bound to gold. The surface clearly looks modified (covered with a layer (or several) of BME molecules). I tried to take smaller scans to image the molecules, but the coating seemed to be swept away by the tip... maybe a crash? Going to have to try this again... :Zooming in on the above surface, most of the BME has disappeared.
I now have a PCB for the data converters and analog electronics! Full schematics and gerber files are available on my website. I’m now using digital feedback and linearizing the tunneling current data in software using a logarithmic lookup table. This really stabilized the feedback loop and I’m now able to get atomic resolution in constant current mode, something which I just couldn’t get to work with the previous analog feedback loop. I’m also using sigma-delta modulation to increase the resolution of the scanner drive signals from 16-bit to 20-bit.
Here's a picture of the analog PCB:
I'm still using a Teensy 3.1 but now that It's running a PI loop and sigma-delta, it's slowing things down a bit. I've put the Teensy on a separate piece of prototype board and connected it to the main analog PCB with a ribbon cable. This way, I can easily upgrade the digital electronics later on. I'm also working on an AFM, which will use an FPGA, and may integrate the STM digital electronics into the same FPGA. I've left plenty of room in the box for it :)
Here are some images acquired with the new system:
Take a close look at this image of gold. If you zoom in on the image, you can see the atomic layers!
I got some single-walled carbon nanotubes (SWNT) the other day. They're 5 nm in diameter and are functionalized with a detergent, which coats the SWNTs so that they can be dispersed in water. Here's a 492 X 492 nm image I took with the STM after placing a drop of SWNT solution on HOPG and allowing it dry:
Those lines in the top-right have a ~5 nm pitch, so I suspect it's a bundle of SWNTs. The HOPG surface looks really "noisy", probably due to detergent left on the surface. I definitely crashed the tip at least once, so that might have something to do with it! Looks like the tip changes in some way about a third of the way down the image. I'll try baking off the detergent to see if it changes anything, and try a few more scans to see if I can find some individual SWNTS.