Friday, January 25, 2019

"Seeing" atoms

The power of modern transmission electron microscopy (TEM) is very impressive.  Often in TEM images at high magnification, you can see what looks like the atomic lattice, but that can be a bit illusory.  Because the scattering effects of individual atoms, especially light ones like carbon, can be very slight, often those images are looking at the result from scattering off columns of atoms, with the crystalline structure of the material helping greatly to produce a clean image.  With state of the art instrumentation and processing, however, it is possible to resolve single atoms, even in atomically thin, light materials like graphene.  This image, from a new ACS Nano paper by Lee et al. from Oxford University, is a great example of what is now possible, showing the reconfiguration of carbon bonds as a nanoscale graphene constriction is modified.  Pretty eye-popping.

7 comments:

Anonymous said...

Amazing images Doug.

If anyone finds a non-subscriber link to the paper please post it here. Thanks in advance!

DanM said...

On the other hand, if you use a picosecond-duration transient to induce tunneling through an STM junction, then you can push a peak current of more than 100 micro-amps through a single silicon atom. See here:
https://www.nature.com/articles/nphys4047

Anonymous said...

What do you mean on the other hand? Not seeing the connection between the two besides atomic resolution, unless I'm missing something...

DanM said...

Ok, not 'on the other hand'. Rather, substitute "Also amazingly cool:"

Anonymous said...

Do you have any comment on the hi-res cryo-EM, compared to the TEM? Thanks.

Jon Barnard said...

The stuff that Jamie Warner's group does is good, fun stuff. The 2D materials boon has really given him a good platform for playing with high-resolution imaging that Oxford does so well. However, the real question, and it is one that is becoming increasingly asked, is how does the electron beam affect what you're seeing? Beam damage is now *the* topic of interest for most electron microscopy conferences I go to. The practice now is to quote your dose (electrons per square Angstrom) or dose rate, i.e. electron flux. These are now important metrics and quite rightly so.

We had an interesting episode in 2003. We published an APL showing that electron beam damage in InGaN quantum wells was creating the localized strain contrast, creating the impression that indium was locally agglomerating to create local 'quasi electric fields'* that confine electrons and holes for recombination. Showing that the electron microscope caused problems gave us no end of grief. We were attacked by some really big people in the field, even shouting at us in the audience of conference talks, accusing us of using bad sample preparation, for example. However, once the paper was out, everyone was seeing the same effect and a crisis in the TEM community emerged. It called into question how to observe atoms in alloys reliably. Fortunately, the 3D atom probe technique (APT) was available and, with the development of laser-assisted evaporation, semiconductors could be looked at. So, within an couple of years everyone knew about the 'false indium clustering' problem and things moved on. We are now a lot more primed about how to use the electron dose 'budget' effectively and we are continuing to understand how to maximize the information content of our signals. It was certainly a tumultuous time, but we were a good group with an high international reputation, but I do wonder how hard it is for scientists in smaller, less visible institutions to call out that the Emperor has no clothes. Isn't science great!

* See Herbert Kroemer's Nobel lecture paper for an explanation.

@Anonymous - cryoEM = cryo (transmission) electron microscope = TEM + cold sample

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