Episode: 1931
Title: HPR1931: Atomic force microscopy
Source: https://hub.hackerpublicradio.org/ccdn.php?filename=/eps/hpr1931/hpr1931.mp3
Transcribed: 2025-10-18 11:18:22

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This is HPR episode 1931 titled, Atomic Force Microscopy.
It is hosted by Amunib and is about 26 minutes long.
The summary is general view of the Anokale tools.
Special interest with Atomic Force Microscopes AFM.
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Get 15% discount on all shared hosting with the offer code HPR15.
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Hello, this is Andres Mnyev at A-P.
And I'm here to talk to you a bit of science.
Now I'm going to go into a bit of nanotechnology or the more like the instruments used for nanotechnology.
And specifically I'd like to delve into a tool called Atomic Force Microscopes.
So let me start by, this is basically something that I go over with a lot of people.
So hopefully won't sound too rehearsed.
So the main problem with small features.
By the way, nano is a dimensional unit.
And if you take a meter, divide that by 1,000, you've got a micrometer.
If you take 100 micrometers, that's about the thickness of a hair.
Then one micrometer between one and five micrometers.
That's about the size of a hair or something like that.
And then if you keep going down, if you take a micrometer and divide it in a thousand parts, then you get nanometer.
And nanometers, if I think if you take 0.1 nanometers, that's classical unit called Armstrong that is used as the space between atoms.
Organic molecules, such as the ones that you can find on your organic light-emitting dials on some phones and OLED dials,
they can be maybe 100 nanometers in size.
So it's several atoms bundled up together to make these carbon-based molecules.
So nanometer, the nanoworld is according to international, the ISO standard.
I believe is anything that has one of its significant dimensions below 100 nanometer.
So for example, if something called a nanotube is called a nanotube, even though it can be several microns and even millimeters in length,
just as long as the diameter of the tube is less than 100 nanometers.
And then again we have things that the new stuff called, for example, graphene or single layered materials,
which is basically one atom thick sheets of stuff.
In the case of graphene, it would be carbon.
And so, yeah, I think, oh, and then lastly, the original nanostuff was the fluorine ball, which is basically a little ball that is made out of carbon, looks like a football.
And it can be less than 100 nanometers in size.
So anyway, but that's like a sphere. So anyway, that's defining the scale.
Now, the main problem, when I started doing this straight as I said, I'm ashamed to say that,
sadly, straight out of university, I would say, yeah, you want to see something really, really small.
You just use clever objects.
And, you know, you put the lens one behind each other, and hopefully, you know, at one point, you'll start seeing really small stuff.
Now, the problem with that is that it seems that the light travels in waves.
And anything contained within that wave, which can be 500 nanometers, you don't see.
So, you know, there's waves around it. You can basically put several waves together and align them in such a way.
And, you know, they're slightly out of phase, and you'll be able to find out things that are smaller than 300 nanometers or whatever the wavelength of that particular light is.
But what's normally, and I think that's called white light interferometry and whatnot.
So, anyway, so that's one technique.
Another technique you can use is doing something called scanning electron microscopes.
So, what you do is instead of using light, you basically use electrons.
And you float these electrons through lenses, which are basically kind of like coils that generate a magnetic field and move them on.
And since electrons basically tend to move in a straighter line, then light, it seems.
They hit the surface, and they bounce back, and there's this little detector thing that fans find out how much energy that electron bounce back.
And that's where you get those really cool images that you see sometimes on the web of fleas and mosquitoes looking in a gray scale and black and white.
That's because color as well loses its definition at that small scale, because there's no light.
So, yeah, so that's scanning electron microscopy.
Others, another tool that you can do.
But the limitation of that is that since you're using electrons, you need to have the sample that you're looking at.
You need to be fairly, you know, have some sort of conductivity.
So, those fleas and little mosquitoes, and you know that you can actually see the little hairs in the mosquitoes.
Those images, what they actually had to do is that they actually had to coat, you know, first they had to kill the animal, the insect.
And second, they had to coat it with some sort of carbon-based stuff.
So, at least they have something called spudger coating where they cover it with gold.
You know, they basically metalize the insects so that you can actually see it.
Now, another thing you can do is that if you, something's called, that's called transmission electron microscope.
So, transmission electron microscopes, the way they work, this is very, very overview of all the stuff that I'm talking about.
It's very overview.
But the way I understand it is that you take a material and you cut it really, really thin.
And so, you know, you have only several atomic layers, you know, maybe, you know, it can be something like a hundred nanometers thick layer.
And you put it into this huge microscope, and you basically look at this, and you shine electrons through it.
And what you look at actually is some sort of projection of your crystal structure, of your material.
So, you basically shine the electrons through, and the electrons go through the material.
And depending on how close the electrons go gets to the atoms or the crystals.
More precisely, the crystals in the, in the, in the relief in material, they will kind of change directions, like this.
So, they will be going on a straight line, and then as they not hit, but go through the material.
I think I'm maybe, maybe it's tunneling through the material, I don't know what the term is.
But anyway, as it goes through the material, it slightly deviates, and it hits the, and it hits some sort of recorder.
Which, before, used to be kind of like these picture, you know, oh geez, these things that, you know, that exposed things.
And then you have to go into like a photographic room, and expose things, and put it through the chemistry.
But now, now they have like these really cool, which would say, like, little cameras, electronic cameras that would detect the electrons.
And they, and there you can actually see the crystal structure of material.
But of course, then the, this, this involves like a huge sample preparation.
So, you know, you need to, you need to get, basically, get something called a microtome, or some other precise piece of kit that, that will be able to slice your material to really thin, so you can actually see through it.
Now, the last set, which is the one I'm gonna, I mean, I understand better, is, which doesn't mean that I understand it completely.
But, is the, a range of equipment called scanning probe microscopes. Now, scanning probe microscopes are, are a different way of approaching, seeing things in a very small scale.
So, instead of actually trying to use slides or electrons, what you do is to just, as a colleague, I said, well, today I've been poking things with the stick.
So, that's basically what you do. You have this very, very fine stick, and you poke the, the surface.
And this, this, this, this probe can be, you know, incredibly small, about 100 nanometres maybe, maybe, maybe smaller, maybe larger, but depending on how large the probe is, you know, you, you'll be able to see smaller or bigger features.
Now, there's several ways of using this probe. One, one of the methods, one of the methods that, the way it started scanning probe microscopy, was with them, something called scanning, tunerling microscopy.
And there's a guy, he's a noble Laureate now, I think, for, for, for embedding this method.
And what this basically is, is that you take a tungsten wire. So, tungsten is, you know, in the old light bulbs you have, you have tungsten there.
And tungsten is a very brittle material, very brittle metal. And you can basically, if you, you cut it, you can, since it's brittle, you can kind of ensure that, that it's going to be very, very sharp.
So, you take that material and you go into a vacuum chamber. You take something that is electrically conductive, say, you know, a piece of pure silicon, for example, or, or graphite.
And you put a voltage bias to it. So, you put, you say, you say, you put your sample at one volt.
And then you, you approach the probe, this very probe, very close to the surface, but without touching.
And what you measure is the current that suddenly jumps through from the sample to your tip, or the other way around, I'm not sure, depending on what side is grounded.
And this, this current, it's very small. It's a nano amps, and it's considered a nano and pico amps, and it's called a tunneling current.
And that goes into tunneling effect that you can go into quantum mechanics about it. But basically, what it is is that, if you, if the gap is small enough, electrons suddenly jump from one place to another.
I mean, that's as far as I understand it. And so, you measure this current, and you can imagine if you get slightly farther away, that current is going to be reduced, because, you know, the electrons can jump that far.
And if you get closer to the sample, then the electrons will flow more, so you will, you know, you will get more, more current.
So the idea is that you say, okay, I want to be a certain distance away, so I'll say, you know, one nano amp, and you tell the machine to scan, to move the probe, ensuring that the distance is always the same.
And by doing that, you're saying that the current is going to be a certain fixed value in nano amps.
So this distance doesn't change, but for example, if you have a material, at some point, you might have something that has a different material.
So different materials conduct electricity in a different way. So your probe might get closer or might be farther away.
So with this technique, you know, there's clever ways to get around it, so you'll be able to find out what one material is and what the other material is not.
So, but yeah, it needs to be the problem with STM scanning to limit cross-covy, which is what I've been rambling on for a bit, is that this, the sample needs to be conductive.
So, in comes another technique called atomic force microscopy. Now, within atomic force microscopy, there's, there's lots of different sub, you know, all the other microscope methods also have a lot of sub methods.
But this atomic force microscopy is really interesting because you can find out more things apart from the shape of the surface. You can find out if something is magnetic, you can find out if something is more conductive or less conductive by using this different modes.
So, just a backtrack, a quick explanation that I'd like to make about atomic force microscope is imagine you're in the nanoscale, so you are effectively blind, you can't see.
So, what do blind people do? They, they, they, you know, they use several tools, but one of the tools that, you know, that I, I, I classically see in the street is a, is a, is a stick.
So, as you're walking, you can poke things around and you, you, you find out that, you know, there's a step there and there's a certain height and, you know, that there's a wall and you can see where there's a sign or a lamp post so you can, you can avoid it.
So, you know, you can, you can basically feel your way around the surface. Now, let's imagine that you, you, you are the same person.
And instead of a walking stick, you will be using a metal detector, you know, one of those metal detectors that you maybe you see people using it for, for, for scavenger hunts and to, to find coins in the beach or whatever.
So, this, this, this blind person will have this, this metal detector and, you know, it's slightly bulkier, it's heavier.
So, you know, he won't be able to find out smaller part, smaller features as in size, but at least he, he or she now, now has more, more information regarding the conductivity of the materials.
So, you know, pause there. So, you, this, this blind person, you know, might not have, you know, cannot see the small holes in, in the sidewalk, but now we'll, we'll be able to know that, you know, there's a coin on the floor or that there's one of these metal lids of, you know, one of the, are they called manholes? Well, anyway, one of these, one of these holes in the street.
So, you know, the, he gets more information. So, that's the advantage of, of, of using a probe. So, now, going back to the, our nanoscale, the, the way this, this, this works, the way it was invented was, it's a little more complicated to explain, but in short, in short, it was, you, you had this probe that didn't have to be, now didn't have to be conductive in any way to, to feel the surface.
Because it just had to, let's say, physically bend upwards as it was going up. And what they did was they put an STM. So, the, the process before, you know, using the tunneling method on the back of this probe.
So, it was actually feeling as the tip was bending upwards and downwards, by measuring that nano current coming, coming through it.
But it was actually being, the probe below it was able to see smaller things. So, let me just start by describing the probe, which will be, which will make, put things in, in place.
So, the probes used in these nanoscale, you know, you buy them off the shelf. They can be anything between, I don't know, maybe 10, 10, 10 to 20, 15 euro each to about 500 euro each for the more complex ones.
So, the 500 euro ones are actually, that I've seen are scanning thermal microscopy tubes that actually are basically little thermometers going, going over the surface.
So, but anyway, let's go to the, to the basic one. So, the basic one is basically silicon, made out of, you know, the stuff that sand is made out of without the oxygen.
And they, they, you know, like silicon chips, you know, that your processors are made. So, there's, you know, there's a good industry behind it. So, you can buy a box of 10 of these chips.
So, these chips contain a tip, sorry, a cancer lever. A cancer lever, I sometimes use cancer lever and most people know it in English.
So, it's basically, if you imagine a trampoline. So, you know, something that's held only from one side, but not held on the other side.
So, that's what's called, that's what a trampoline is. So, if you met, sorry, that's what a cancer lever is, a trampoline, of course, you know, but you probably do know what it is.
So, this, this, this cancer lever is, if you met, if you imagine you're, you're a trampoline and you, you, it has a certain width, the width of the, of the trampoline in, in most, in the case I've seen, can be 15 microns thick.
Y, sorry, and it can be one or two microns thick, and then the length of it can be maybe a hundred microns in length.
So, you know, a hundred microns, I say, as if it's very long, but that's, as I said before, that's about the thickness of a human hair.
So, this, and then at the end of it, say, you know, if you imagine you're on the trampoline, at the end of it, on the bottom,
there's this kind of like cone shaped feature coming out of the end, and that is what's called the tip.
Now, the tip is, it can be a cone, well normally, since it's silicon, it has to, and the way it's fabricated, it breaks through the crystal structure, which I believe has a 70 degree tilt.
But anyway, it, it, it breaks through the crystal structure, and it can be quite sharp. So, at the tip end, it can be maybe less 50 nanometers in, in the rate in the tip radius at the end, but the shape of it in general is, is kind of like a pyramid.
And it can, it can come off the edge of the, let's say the cantilever, it comes down to maybe two microns, and some, you know, depending on how deep features you want to find out, they can be tens of microns in that.
So, we can go over the units, but you know, go to the beginning of the, of the cast and find out what microns and nanometers are, but hopefully you're in the loop.
Anyway, so, the, what we have is a chip that holds the cantilever, and the cantilever has a tip at the end. So, it's, you know, you can, you can probably do a web search for,
AFM tips or AFM cantilevers or AFM tips, and there you would get scanning electron microscope images, remember those black and white images I talked about before, that show you the, you know, more or less the size of it, so you get an idea of how, how small these things are.
So, the way, now, how, how it doesn't really help you to know that, you know, you're scanning this, you're moving this probe across the surface, line by line, you know, just, from left to right, as if you were lawn, you know, using a lawn mower over your, your backyard or your lawn or whatever.
So, it goes back, forwards, and then it did that line, and then it moved down one line, and goes across another time, and back and forwards. Now, the, the probe is flexing up and downwards, and by flexing up and downwards, you know, the, the height of the features.
Now, these, how do you know that, that really small thing is actually moving up and down. So, at the beginning, as I said, if they, they used an STM probe on the back of it, so they can measure the current going up and down from it, but that quite limited the range, so you couldn't, you couldn't really, maybe that wasn't a limitation, I'm guessing that's a limitation that, you know, it could only see features that were,
a few nanometers in size, and the people wanted to see things that were in microns in size. So, what, what came up was a method using a shining a laser light on the back of the cantilever.
So, if you shine a laser light on the back of the cantilever, and that reflects into a photo detector, which is basically a solar system, let's say, and that photo detector has two parts, a part on the top and a part on the bottom.
So, normally the laser spot would be slightly in the middle, so the, the voltage that you get from the top minus the voltage that you get from the part on the bottom would be zero.
Now, if the, if the laser spot shining on the cantilever, the cantilever bends upwards, then the laser spot will move upwards on the photo detector, hence you will have the, the top part will be larger than the bottom part, so you'll have a positive voltage.
If the cantilever finds a hole, it will bend downwards, so you will have more voltage going into the B side, and the, yeah, so the voltage will be negative. Hence, you know, you, you can then feed that voltage back into the instruments and requests and ask the, the tip to move up and down.
Now, how do you move? Well, this is getting really in depth. So, how do you move something up and down by several nanometers?
So, for that, you use a material called Pietzo electric material. Now, Pietzo electric materials are really interesting, because you can put a voltage into it, and they actually either DC voltage, and they actually either shrink or, or grow depending on the signal of, of the sign of the voltage.
And they, you know, you can calibrate them to make sure that they are, they grow via certain amount.
So, this, these, these, these, these, these Pietzo materials, maybe, maybe I'll go into it in a different, or cast, if people want. But anyway, so you have like this, this basically little material that acts like a motor moving the thing up and down.
And actually, there's another one using it to move left and right.
Now, so you're scanning along, and, you know, that thing goes up and down, and the detector tells you where it is on, on, on height. And then what you get is this map of, let's say, 1,512 pixels by 500 and 12 pixels of, of values in, in Z.
And with that, you put it into a software analysis program. I personally like Guardian, and I would highly recommend it. I've used several other data analysis programs for, for this atomic force microscopy.
And this one is the best, and it's a free software. It's in freeism, freedom, GPA, or licensed. And the reason I like it, if you listen to my previous podcast, is that it is maintained by the Czech National Measurement Institute, the CMI.
And it can open almost all, if not all file formats, that all the different makes and models of atomic force microscopes do.
So I've used five different types of atomic force microscopes from five different companies, and those five different companies have their, for some reason, decide to do their own data analysis software.
And, of course, each data analysis software then only opens their type of file format, and doesn't open the, you know, this, again, open document formats.
We can, you know, this is where free software foundation, your open free software foundation, quite insist on having open formats.
But anyway, that's not, that's not deviate. And, uh, and, uh, William opens all of them. So, uh, it is a great piece of software. Uh, it is scriptable as well, which is great when you are recording a lot of, a lot of data.
And I highly recommend it if you want to get into atomic force microscopy. There are, um, you know, instructions, even papers and, and stuff that you can download and find, get instructions on how to build your own atomic force.
I would recommend doing, you know, maybe the one, there's one about Lego, uh, that's really interesting. So, uh, I think that's, that's about it. Um, hopefully, uh, I have a grand belong for too long.
And, uh, I can, you know, depending on how I feel like, uh, I'll, I'll try to get more podcasts and, I think so.
Uh, I used audacity here. It was highly complicated. Uh, the previous podcast, I'm not sure how I, how I did. Uh, but, uh, this one, I just decided to record everything in one track. I don't know how it's going to come, come out.
Uh, and hopefully this will be saved because I've been going on for about 25 minutes. If it's all lost, I'll be really upset.
But anyway, thank you very much. And, uh, see you next time. Bye.
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