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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 s

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. (1) http://en.wikipedia.org/wiki/Barn_(unit) http://en.wikipedia.org/wiki/Barn_(unit)

awgl 12y ago
I'll readily admit that I'm not a STEM expert. And, honestly, I think I was conflating the STEM in this article with Scanning Tunneling Microscopy (http://en.wikipedia.org/wiki/Scanning_tunneling_microscope http://en.wikipedia.org/wiki/Scanning_tunneling_microscope). 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. ;)
- Osmium 12y ago
  > 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.
Osmium 12y ago
> 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.
- niels_olson 12y ago
  > 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. Overall, Osmium, thanks for your comments here!

awgl 12y ago

I'll readily admit that I'm not a STEM expert. And, honestly, I think I was conflating the STEM in this article with Scanning Tunneling Microscopy (http://en.wikipedia.org/wiki/Scanning_tunneling_microscope http://en.wikipedia.org/wiki/Scanning_tunneling_microscope). 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. ;)

Osmium 12y ago
> 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.

Osmium 12y ago

> 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.

niels_olson 12y ago
> 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. Overall, Osmium, thanks for your comments here!