It's roughly the probability of an electron undergoing a state change in the vicinity. It took a lot of measurements to make those images. It also took a very sensitive instrument, very good control of high energy particles tuned to the target, and a very stable local environment (cold, dark, and quiet).
Edit: reviewing awgl's comment: I just want to clarify which electrons we're talking about. The target has electrons. The electron beam is also, by definition, electrons. So you're shooting electrons at A) electrons, and B) nuclei.
What are the odds of a negative, 100 KeV (medium-high energy) electron in the beam interacting with a heavy, relatively stationary, positively charged nucleus? High.
What are the odds of a negative, high energy electron interacting with a low-energy electron that may, occasionally, be in the area? Low.
But we've been shooting high energy electrons at dense targets since the 1920's or 1930's. The joke in accelerators is that most of the particles you fire miss, implying you can't hit the broad side of a barn. (1)
What's far more impressive is the ability to focus the beam down to sub-angstrom scale (1^-10 m) and then scan at equal or higher resolution! And then detect at the same scale of resolution! How? Almost certainly the beam is steered electromagnetically. I'm interested in the detector. I'm guessing these are reconstructed using a combination of side-scatter and forward-scatter information. Not entirely sure how though.
> And, honestly, I think I was conflating the STEM in this article with Scanning Tunneling Microscopy
It's easy to do. TEM/STEM vs SEM vs STM. All completely different things. This is what happens when scientists name things :P
For those confused:
TEM/STEM: An electron beam is transmitted through your sample. Good for atomic-scale imaging.
SEM: An electron beam is scanned across your sample, but none are transmitted through. Good for topography/surface features (the interaction volume of the beam is too large for atomic resolution).
STM: No electron beam. Instead think of a vinyl record player, and physically scanning a very sample tip across the surface of your sample. Good for atomic-scale imaging of a surface.
A good STM image and a good STEM image can, at first glance, look quite similar (especially for a 2D material like graphene), but they're very different techniques.
> What are the odds of a negative, 100 KeV (medium-high energy) electron in the beam interacting with a heavy, relatively stationary, positively charged nucleus? High.
> What are the odds of a negative, high energy electron interacting with a low-energy electron that may, occasionally, be in the area? Low.
Just to clarify this, so people don't get the wrong idea, electron-electron interactions are also incredibly important in an electron microscope.
> and then scan at equal or higher resolution!
Note that while this is what's happening here, you can get similar atomic-resolution images without scanning at all, by just illuminating a sample with a broad electron beam (see 'TEM' (or 'conventional TEM') vs 'STEM'). Strictly speaking, STEM and TEM are equivalent techniques (by something called the reciprocity theorem), but in practice you'd almost certainly prefer one over the other depending on your sample and one technique is not necessarily better than the other.
> I'm interested in the detector. I'm guessing these are reconstructed using a combination of side-scatter and forward-scatter information.
For some of the STEM-HAADF images in the article, a 'high angle annular' (the 'HAA' part) detector is used, which collects electrons that are just scattered at high-angles in the forward direction, because this results in images that are much easier to interpret (intensity is just proportional to mass and thickness). The detector itself is commonly a scintillator/photomultiplier tube combo.
There are tonnes of other detectors used though. Until recently, it was common to just expose TEM images onto film (I have some of my own samples actually). An operator would find the region of the specimen they wanted to take an image of using a live view which was the electron beam projector onto a phosphor screen and, when ready, move the screen aside and expose it to film. More recently, the vast majority of TEM micrographs are taken using normal CCD tech.
> just to clarify this, so people don't get the wrong idea, electron-electron interactions are also incredibly important
Thanks. Yeah, no doubt. For the graphene image, for example, the beam appears to be interacting with the pi bonds.
> There are tonnes of other detectors used though.
Yes, I recall entire section of my nuclear physics professor dedicating a whole week, after we understood the basics of scintillators and photomultipliers, going through a multitude of detectors.
In brief, quantum particles act like waves sometimes (think ripples of water) and act like particles (think tiny billiard balls) at other times. The consequence of that, and a major tenet of quantum mechanics, is the 'wavefunction' of a quantum particle. A wavefunction of a particle, instead of just being a single point in space, amounts to a probability density of position and momentum.
So now that we know the above, in these images, what they are actually measuring is the spatial probability density of the electrons. The lighter values correspond to high density of electrons. The darker values correspond to less electron density. The high density occurs around the nuclei of the atoms. Thus, atomic resolution. However, note that individual electrons are not resolved in these images.
Finally, I want to recommend against thinking of atoms as electron planets orbiting a nucleus sun full of empty space in between. That thinking ignores quantum mechanics. The truth is much more fascinating, which is that electrons are wave-particles that have probabilistic densities.
P.S. Protons and neutrons are themselves made of up more elementary quantum particles: quarks!
Does the microscope care about probabilistic densities? Aren't these images rendering interference/difference between electrons sent out and electrons received?
Those 'particles' supposedly were in the space and interacted with the electron beams. They were or weren't in a place at a time.
In wave particle-duality, how are we not just suggesting particles are there because it creates an estimate model that helps model behaviors observed?
In the facetted nano-diamond void, why, in the void, do we see apparent 'ghosts' of the lattice in the void? Are there particles there or not? If there are, why are they dim?
Right. Well, given your comment and niels_olson's, I feel I have misspoken about this STEM experiment. Not that was I said about quantum mechanics was wrong, just that its relevance to the measurement in this experiment is misguided.
As niels_olson points out, the primary interaction here is between the electron beam and the atomic nuclei.
In case anyone is interested we've been able to do this for quite some time for TEM and SEM. One big breakthrough for EM lately was the ability to use direct detectors to look at frozen samples for tomography. With these techniques we can now reconstruct things like ribosomes all the way down to their atomic structure [1]. We used to have to use crystallography for this.
> In case anyone is interested we've been able to do this for quite some time for TEM and SEM.
Well, not with SEM :) Do you have any more links to other significant cryo-EM papers btw? Looks incredibly fascinating.
On the non-bio side, the recent major advances have been in aberration-correction technology. The big limiting factor in electron microscopy has been aberrations introduced by the electromagnetic lenses used to focus the beam (and the quality of the electron source itself). We're starting to get past that, which is good.
The next step seems to be creating a lot more specialised electron microscopes, rather than just adding multiple detectors to a single microscope. To use an example, X-rays are also generated due to how the electron interacts with the sample, and if you detect the X-rays it can tell us what elements are present. Trouble is, X-rays are emitted in all directions, and in older microscopes there'd only be room for a single X-ray detector off to the side, because it also had a bunch of other detectors in there too, so it'd only capture a fraction of the X-rays emitted. So now people are realising that if they really want to push the limits of the technique, they have to create a electron microscope dedicated to, say, detecting these X-rays, so they can cram as many X-ray detectors around the sample as they can. This can make the difference between detecting an unambiguous signal to not detecting one at all, which is incredibly important when looking at, say, dopants or impurities in semiconductors which exist at very very low concentrations.
This is just a single example, but there have been many many electron microscopy-based techniques that have been successfully validated in practice and can now be taken to the next level by building dedicated, specialised microscopes.
While in college, I put in dozens of hours on an SEM and got pretty good at it. A TEM is a whole 'nother story... I would have needed an order of magnitude (or two!) more training on it to be able to use it proficiently.
> While in college, I put in dozens of hours on an SEM and got pretty good at it. A TEM is a whole 'nother story...
For the benefit of anyone who doesn't know, a SEM (scanning electron microscopy) is an electron microscope that builds an image by scanning an electron beam across a surface and detecting the scattered electrons. A TEM (transmission electron microscopy) passes electrons through the material to detect them on the other side. (Not to be confused is STEM and SEM; STEM is a scanning variant of TEM, and uses similar equipment, and is equally unlike SEM.)
The big difference between the techniques is that, once you want to pass the electron through the sample, a lot changes. First, your electron beam has to be much higher energy, and your electron source much better quality. This makes the microscope a lot larger and more expensive. The big thing though is that suddenly you have to take extreme care preparing your samples. They need to be thin enough to pass an electron through! So sample preparation is typically much, much more difficult and painstaking.
TEM is also more advanced in that the interpretation of your images is non-trivial. With SEM, your image is typically topographic and easier to interpret (you see what's there), but in TEM which is at much higher resolution, the many many different ways the electron beam interacts with your sample on the quantum/atomic-level becomes important, and the contrast you see in your image doesn't necessarily nicely correspond to a real object. You can also use your layers of atoms as a diffraction grating and gather information from the diffraction pattern formed by the electron beam too. So there's a lot going on.
So if I read this right, this is 2d only? Could a theoretic 3d scanner with this resolution be used to e.g. duplicate matter like in a Trek transporter?
Good question. I assume it could only scan the surface though unless you scrapped the object away layer by layer. So a complete scan would be destructive.
It's not just the surface. It depends on the technique and the microscope, but all the atoms between where the beam enters and leaves the sample will contribute in some way (though you can have a narrower depth of focus that basically images a few layers at a time, and you can image that way).
> unless you scrapped the object away layer by layer. So a complete scan would be destructive.
There are other techniques that almost do exactly that! e.g. a SEM (not to be confused with a (S)TEM) can be equipped with a focused ion beam to selectively remove material from your sample. Sadly it's not possible inside a (S)TEM though, at least not today. But that doesn't mean your (S)TEM imaging is non-destructive. The electron beam itself can knock-off lighter atoms, and will seriously damage your sample if you leave it there long enough (and not in a controlled way, so you couldn't use this to purposefully remove layers from your sample). This is especially problematic for anything biological.
You can get down a layer two but that's about it. There is no method for capturing a full 3D model in experiment, although we can get projections from X-Ray diffraction and other scattering methods. Also there are single photon X-Ray methods being developed that will get us extremely close to complete 3D models. But that aside you also have to take into account that these materials are often not at 0K so they're vibrating or even moving at the atomic scale. This is a huge problem for example, when studying Proteins.
> There is no method for capturing a full 3D model in experiment
3D atom probe is getting there[1], though that's not at all appropriate for biological samples. This is where you get a microscopic conical section of your sample and remove material one layer of atoms at a time using a laser and then detect the ions removed using a mass spectrometer. This gives a 3D reconstruction of your sample, which is incredibly useful for crystallography.
Of course, you can also do tomography with STEM/TEM too, and reconstruct a 3D model that way. I've seen people do this with analytical EM so you get a 3D model colour-coded by composition, though this hasn't been at atomic resolution.
> there are single photon X-Ray methods being developed that will get us extremely close to complete 3D models
Do you have a link to some key papers? Sounds fascinating!
26 comments
[ 3.4 ms ] story [ 35.8 ms ] threadEdit: reviewing awgl's comment: I just want to clarify which electrons we're talking about. The target has electrons. The electron beam is also, by definition, electrons. So you're shooting electrons at A) electrons, and B) nuclei.
What are the odds of a negative, 100 KeV (medium-high energy) electron in the beam interacting with a heavy, relatively stationary, positively charged nucleus? High.
What are the odds of a negative, high energy electron interacting with a low-energy electron that may, occasionally, be in the area? Low.
But we've been shooting high energy electrons at dense targets since the 1920's or 1930's. The joke in accelerators is that most of the particles you fire miss, implying you can't hit the broad side of a barn. (1)
What's far more impressive is the ability to focus the beam down to sub-angstrom scale (1^-10 m) and then scan at equal or higher resolution! And then detect at the same scale of resolution! How? Almost certainly the beam is steered electromagnetically. I'm interested in the detector. I'm guessing these are reconstructed using a combination of side-scatter and forward-scatter information. Not entirely sure how though.
(1) http://en.wikipedia.org/wiki/Barn_(unit)
So, yeah, my comment is not entirely accurate about the electron densities of the atoms. If you feel it is too misguided, I'll remove it.
This is what happens when you ask a theoretical chemist to explain an experiment. ;)
It's easy to do. TEM/STEM vs SEM vs STM. All completely different things. This is what happens when scientists name things :P
For those confused:
TEM/STEM: An electron beam is transmitted through your sample. Good for atomic-scale imaging.
SEM: An electron beam is scanned across your sample, but none are transmitted through. Good for topography/surface features (the interaction volume of the beam is too large for atomic resolution).
STM: No electron beam. Instead think of a vinyl record player, and physically scanning a very sample tip across the surface of your sample. Good for atomic-scale imaging of a surface.
A good STM image and a good STEM image can, at first glance, look quite similar (especially for a 2D material like graphene), but they're very different techniques.
Just to clarify this, so people don't get the wrong idea, electron-electron interactions are also incredibly important in an electron microscope.
> and then scan at equal or higher resolution!
Note that while this is what's happening here, you can get similar atomic-resolution images without scanning at all, by just illuminating a sample with a broad electron beam (see 'TEM' (or 'conventional TEM') vs 'STEM'). Strictly speaking, STEM and TEM are equivalent techniques (by something called the reciprocity theorem), but in practice you'd almost certainly prefer one over the other depending on your sample and one technique is not necessarily better than the other.
> I'm interested in the detector. I'm guessing these are reconstructed using a combination of side-scatter and forward-scatter information.
For some of the STEM-HAADF images in the article, a 'high angle annular' (the 'HAA' part) detector is used, which collects electrons that are just scattered at high-angles in the forward direction, because this results in images that are much easier to interpret (intensity is just proportional to mass and thickness). The detector itself is commonly a scintillator/photomultiplier tube combo.
There are tonnes of other detectors used though. Until recently, it was common to just expose TEM images onto film (I have some of my own samples actually). An operator would find the region of the specimen they wanted to take an image of using a live view which was the electron beam projector onto a phosphor screen and, when ready, move the screen aside and expose it to film. More recently, the vast majority of TEM micrographs are taken using normal CCD tech.
Thanks. Yeah, no doubt. For the graphene image, for example, the beam appears to be interacting with the pi bonds.
> There are tonnes of other detectors used though.
Yes, I recall entire section of my nuclear physics professor dedicating a whole week, after we understood the basics of scintillators and photomultipliers, going through a multitude of detectors.
Overall, Osmium, thanks for your comments here!
While the electrons and nucleus (i.e. protons + neutrons) of atoms are indeed 'particles', they are exhibit wave-particle duality: http://en.wikipedia.org/wiki/Wave%E2%80%93particle_duality
In brief, quantum particles act like waves sometimes (think ripples of water) and act like particles (think tiny billiard balls) at other times. The consequence of that, and a major tenet of quantum mechanics, is the 'wavefunction' of a quantum particle. A wavefunction of a particle, instead of just being a single point in space, amounts to a probability density of position and momentum.
So now that we know the above, in these images, what they are actually measuring is the spatial probability density of the electrons. The lighter values correspond to high density of electrons. The darker values correspond to less electron density. The high density occurs around the nuclei of the atoms. Thus, atomic resolution. However, note that individual electrons are not resolved in these images.
Finally, I want to recommend against thinking of atoms as electron planets orbiting a nucleus sun full of empty space in between. That thinking ignores quantum mechanics. The truth is much more fascinating, which is that electrons are wave-particles that have probabilistic densities.
P.S. Protons and neutrons are themselves made of up more elementary quantum particles: quarks!
Those 'particles' supposedly were in the space and interacted with the electron beams. They were or weren't in a place at a time.
In wave particle-duality, how are we not just suggesting particles are there because it creates an estimate model that helps model behaviors observed?
In the facetted nano-diamond void, why, in the void, do we see apparent 'ghosts' of the lattice in the void? Are there particles there or not? If there are, why are they dim?
As niels_olson points out, the primary interaction here is between the electron beam and the atomic nuclei.
I would assume at the void it's dimly picking up the atoms at the bottom.
It's that same "stuff" that keeps your hand from passing through the table that the electrons "see" to create the STEM pictures.
What exactly is "empty space"?
1. http://elifesciences.org/content/2/e00461
Well, not with SEM :) Do you have any more links to other significant cryo-EM papers btw? Looks incredibly fascinating.
On the non-bio side, the recent major advances have been in aberration-correction technology. The big limiting factor in electron microscopy has been aberrations introduced by the electromagnetic lenses used to focus the beam (and the quality of the electron source itself). We're starting to get past that, which is good.
The next step seems to be creating a lot more specialised electron microscopes, rather than just adding multiple detectors to a single microscope. To use an example, X-rays are also generated due to how the electron interacts with the sample, and if you detect the X-rays it can tell us what elements are present. Trouble is, X-rays are emitted in all directions, and in older microscopes there'd only be room for a single X-ray detector off to the side, because it also had a bunch of other detectors in there too, so it'd only capture a fraction of the X-rays emitted. So now people are realising that if they really want to push the limits of the technique, they have to create a electron microscope dedicated to, say, detecting these X-rays, so they can cram as many X-ray detectors around the sample as they can. This can make the difference between detecting an unambiguous signal to not detecting one at all, which is incredibly important when looking at, say, dopants or impurities in semiconductors which exist at very very low concentrations.
This is just a single example, but there have been many many electron microscopy-based techniques that have been successfully validated in practice and can now be taken to the next level by building dedicated, specialised microscopes.
IIRC there are people working on element detection with EELS.
For the benefit of anyone who doesn't know, a SEM (scanning electron microscopy) is an electron microscope that builds an image by scanning an electron beam across a surface and detecting the scattered electrons. A TEM (transmission electron microscopy) passes electrons through the material to detect them on the other side. (Not to be confused is STEM and SEM; STEM is a scanning variant of TEM, and uses similar equipment, and is equally unlike SEM.)
The big difference between the techniques is that, once you want to pass the electron through the sample, a lot changes. First, your electron beam has to be much higher energy, and your electron source much better quality. This makes the microscope a lot larger and more expensive. The big thing though is that suddenly you have to take extreme care preparing your samples. They need to be thin enough to pass an electron through! So sample preparation is typically much, much more difficult and painstaking.
TEM is also more advanced in that the interpretation of your images is non-trivial. With SEM, your image is typically topographic and easier to interpret (you see what's there), but in TEM which is at much higher resolution, the many many different ways the electron beam interacts with your sample on the quantum/atomic-level becomes important, and the contrast you see in your image doesn't necessarily nicely correspond to a real object. You can also use your layers of atoms as a diffraction grating and gather information from the diffraction pattern formed by the electron beam too. So there's a lot going on.
It's not just the surface. It depends on the technique and the microscope, but all the atoms between where the beam enters and leaves the sample will contribute in some way (though you can have a narrower depth of focus that basically images a few layers at a time, and you can image that way).
> unless you scrapped the object away layer by layer. So a complete scan would be destructive.
There are other techniques that almost do exactly that! e.g. a SEM (not to be confused with a (S)TEM) can be equipped with a focused ion beam to selectively remove material from your sample. Sadly it's not possible inside a (S)TEM though, at least not today. But that doesn't mean your (S)TEM imaging is non-destructive. The electron beam itself can knock-off lighter atoms, and will seriously damage your sample if you leave it there long enough (and not in a controlled way, so you couldn't use this to purposefully remove layers from your sample). This is especially problematic for anything biological.
3D atom probe is getting there[1], though that's not at all appropriate for biological samples. This is where you get a microscopic conical section of your sample and remove material one layer of atoms at a time using a laser and then detect the ions removed using a mass spectrometer. This gives a 3D reconstruction of your sample, which is incredibly useful for crystallography.
[1] http://www.cameca.com/instruments-for-research/atom-probe.as...
Of course, you can also do tomography with STEM/TEM too, and reconstruct a 3D model that way. I've seen people do this with analytical EM so you get a 3D model colour-coded by composition, though this hasn't been at atomic resolution.
> there are single photon X-Ray methods being developed that will get us extremely close to complete 3D models
Do you have a link to some key papers? Sounds fascinating!