Hello,
Thank you for kind words, I am happy you interested in AFM project.
I must admit I don’t follow what you mean by ‘large area AFM’.
The project has delays since I want to make shareable. It’s not enough to show some outdated schematic. I want it to be just like openFlexture, everyone could do it. For this I need to make it PCBs instead of breadboard. This transition taking much time but it important step.
The best place to see progress is still Hackaday, I sure there will be lots of progress soon.
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Cool project! I’m a bit late to the party. I did my PhD on AFM and STM, including building an STM from scratch and making custom AFM sensors, so I’d be happy to contribute some ideas.
First thoughts are:
- I like the scanning mechanism with the piezo disks
- I agree that the tuning fork is easier to fabricate and align than playing with a laser deflection system and silicone tips. The one issue you may have is they tend to be much stiffer. The cool thing is if you use conducting expoxy to attach the tip to the end, you can do combined STM/AFM. (STM electronics are much easier, and sometimes it is nice to start in STM, find something and then AFM it). Make sure the non-used tine of the fork is glued to a base, otherwise the mismatched modes of the two tines will eat all of your Q. (This is often called qPlus).
- How are you exciting the tuning fork? Normally we shake with an external oscillator, so you could try by driving your piezo disks, but as the natural frequency of the frame that holds the tip will be much lower than the tuning fork, you may struggle to transmit signal. If this happens, it might be worth adding an excitation peizo under the tuning fork.
- Have you considered Platinum-Iridium wire for the tip. It isn’t too expensive. At least for the very fine wire you need. You prepare with wire cutters at a very acute angle. Cut 80% of the way through and then pull apart to stretch and snap the very end. At lease for ambient STM the oxide on tungsten was always a problem, for AFM this shouldn’t be an issue, but I have never enjoyed Tungsten etching. It may actually be a bad idea for AFM though, as AFM is more sensitive to microscopic structure around the tip, than STM that really only uses the last atom.
- On the topic of tip etching. You don’t want the etch to go the whole way through, you really want it to stop etching and the gravity to take over. If you attach a weight to the wire then this helps. It also makes it easy to pick up what falls off, giving you two tips (lots of debate in pubs as been had on weather the top or the bottom tip is the best!). Also it is worth monitoring the current during the etch process, once you work out what the current is where the tip snaps, if you can make a system that monitors the current (or manually watch it) until it almost reaches this current, then switch off the etching. The thought is if you get to very close to the breaking point with the etch, and then leave it for a couple of hours, gravity should take over and stretch it into a really nice tip*
Anyway, cool project. Sorry if I am just repeating things you already know, or have already covered in your notes. I had a skim, but I struggle to read in dark mode, and so I’ll have to copy it out to somewhere and give it a read on another day.
Cool project, thanks for sharing
- As an aside we once put some really good STM tips and some awful ones (based on how they had imaged) into an SEM to look at the structure. The supposedly platinum-iridium tips with the pulling method actually create a crazy mess at the end, but if something atomically sharp and pointy is sticking out then great! For the Tungsten ones, one of the “bad” tips looked perfect, one of the good ones was actually curled up at the very end. Again this was pure STM, not combined AFM/STM so I don’t know how these ones would have performed with AFM.
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First thank you for being interested in this project, it’s encourage me to go on. I’ve read and re-read you post. There is a lot of new information, thank you for sharing.
You are not late to the party, you got there right on time 
Regarding quartz fork, I used AD9833 to generate frequency. It can provide frequency in 0.1Hz resolution
By the way, you said you built STM from scratch which electronics components you haved used to stabilize voltage?
I suppose on that side we cheated on the controller side as we used a commercial controller! We built the mechanics up from “scratch” using a combination of off the shelf components, custom machined parts and parts that were pre-machined from a previous design.
But I don’t think you will need anything better than a DC power supply, the voltages aren’t super tiny so need far less care than the tunnelling current. If the DC power supply is switch mode it may want a few smoothing capacitors on the output. Your I-V converter circuit should make a good virtual ground on for the tip (or sample depending on which one you bias).
The key bit of the electronics you need to get right to make sure your signal is not just noise is the tunnelling current (I-to-V) pre-amp which should be as close to the tunnelling junction as physically possible. The main thing is to get a decent op-amp, preferably one in a metal can. Rather than putting it on a circuit board you can solder large surface mount resistors (and capacitors for smoothing between the power rails) directly to the legs and then mount it directly behind either tip or sample.
I’ve managed to make quite stable mechanical stages and was able to measure van der vaals force using standard quartz fork. The measurements are quite stable.
So the only thing that I needed to finish the project was create those probes. I thought it would be simple, just glue Tungsteen wire to quartz fork and etch it in NaOH solution.
Here the troubles began, if I apply too much glue and the prong gets too much glue on it, it kills oscillation. Applying to few glue and Tungsteen wire gets loose. After few trials and errors I was able to make one without too much glue. But the mass of glue and wire kills oscillation. After wire felt off oscillation returned but frequency was 1.5KHz lower.
I’ve tried to glue one of the prongs to the board as @j.stirling suggested and ended up with frequency almost doubled of two prongs. I am not sure if the main frequency changes or this os the second lob of frequency response.
If someone knows how to fabricate or buy (cheaply) those probes I would be more than happy to know.
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Did you fix the other tine to the mount? A unbalanced tuning fork is going to damp out really quickly. I used to take pieces of macor and file them into the shape i needed for a mount, and then glue the components to them. You should also be able to read the tunnel current out of the tuning fork contacts if you use conducting epoxy:
This was an early qPlus sensor (from F. J. Giessibl, Rev. Mod. Phys. 75, 949 (2003))
The way I used to attach wires to quartz tuning forks was with electrically conducting epoxy. I would have a little jig holding my tuning fork mount. I would then hold a wire in a little clip, attached to a translation stage. Then using a pointed scalpel blade to put a tiny bit of epoxy on the end of the wire. I would then use the translation stage to move the wire into contact with the tuning fork. Then to cure the epoxy I would put tin foil over one side, and being in a big industrial spotlight on a gooseneck. I think this method is that it would melt a plastic translation stage, I used a cheap milling machine compound table (which you can get for about £30 on ebay). You don’t need super precision, but you want reasonable travel.
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Thank you for the quick response.
I’ve tried to glue on of the prongs to the base.
For some reason I didn’t get oscillation even without Tungsten wire.
I did managed to glue very thin Tungsten wire of 0.012mm (and make it oscillate), the problem is that it’s so thin, I don’t know how to etch it. (I need to try 0.25 or 0.5mm)
Light curing glue sounds like a great idea. I am using super glue, which is tricky, one mistake and you have to start all over again.
I really like the idea of electric conductive probe. It can turn AFM to high accuracy micro-probe.
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Saw this the other day on Youtube, it has some ideas that translate to AFM very well.
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It is a nice project. Looking on the schematic I probably should use new op-amp like he did. OPA2227p looks like a great idea.
Also the anti vibration system looks professional. I am using a box with rocks on bike tire… At least it cheap
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The isolation stage we used in our imaging lab for the AFM was made from plywood and bungee cords, using lead bricks for preload. Plenty of ways to solve that problem.
The big benefit of springs and eddy current dampers are that they are ultra high vacuum compatible. I have done the tyre methods, the bungee method, and the full springs and eddy current damper. All can work.
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I have been following your progress on Hackaday, and been daydreaming about designing and building one too.
I think there’s lots of space for recent improvements in rapid prototyping and fabrication to make a DIY AFM substantially cheaper and nicer than the ~2013-2015 generation of writeups online.
You’re doing really great work, and it feels like you might be really close to a full image, from the build log on Hackaday. I’ve got my fingers crossed.
(And I bought parts to build some OpenFlexure stuff, first. Although I’m no stranger to CAD and 3D printing, compliant/flexure stuff makes my brain boggle a bit, and I figure getting some hands-on experience is a good first step.)
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Thank you for your interest in this project, knowing that someone interested in it gives me the power to continue.
I am totally agree about rapid prototyping, now we can share not only schematics and PCBs but also layout and soldering. Basically using shareable PCBA is a simple and efficient way to share all electronics, coupled with STL files. We can reproduce project in matter of days.
Unfortunately, I got unexpected problem, I got some kind of creep. If I rise and lower the tip in the same place I don’t land with the same height. The height always changes. If it would change randomly with mean of zero I would say it’s some noise. But not, it always changes in one direction and with about constant velocity over time.
Although it’s a setback, it might provide interesting insight. If the creep is result of delta stage (I suspect in tension change of the rubber band) than we could have indication of change in nano scale of delta stage. If its something else I would be more than happy as it would probably be some minor mechanical problem I’ve missed. (And no big changes needed)
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Creep is a big problem with plastic, but it is also a big issues with piezos.
I have never used an AFM of STM where you can just return to the same position in software and assume it is the same place on the sample, or assume you won’t crash into the surface. There is always a feedback loop, you always have to do a staggered “approach” to the sample, and after large moves you have to scan for a while to let the creep settle to minimise image distortion.
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While I’m waiting for metallic micro positioner. I thought it would be a good idea to re try building qPlus sensor.
I’ve removed the metallic casing of standard quartz and glued one prong if the fork to the ground. I’ve even used different glues but the result is the same, no oscillation.
Is there someone here who can tell if there is something I’m missing?
Update:
I think I know what might be the problem, I glued the lower prong, all of it. Probably it interfering it’s oscillation and it’s looks like both prongs connected electrically. So I probably should glue from one side of qartz fork. Just like it described in the article @j.stirling have provided
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which micropositioner did you order?
Something cheap (about $30) on AliExpress but it didn’t help. Still have the creep issue.
In one of the tests it took 10 seconds to get from touching to not touching, while I didn’t change anything.
Now I suspect it’s my quartz fork, as I didn’t glue it to the base. Maybe it changes it’s position once fork touches/smashes the sample.
I must admit nano world is both weird and exciting
I still have to find algorithms not to smash quartz fork into the sample
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I realized I missed your December update on Hackaday. It’s looking good!
