Low-energy electron microscopy

LEEM microscope

Since 2017 we have a LEEM microscope in our lab. This technique, about three decades old, is really neat for observing dynamics on crystalline surface. We have been using LEEMs starting in 2003. The lack of an instrument (until 2017) gave us the oportunity to develop fruitful collaborations with some LEEM owners, specially with our US colleagues, Kevin F. McCarty(Sandia Nat Labs, USA) who retired in 2015 and whose instrument is now handled by Farid El Gabaly, a former PhD student of our group, and Andreas K. Schmid (Berkeley Lab, USA), who is in charge of the Spin-polarized LEEM at the Nacional Center for Electron Microscopy, at Berkeley lab. If LEEMS in the world are less than a hundred, spin-polarized ones can be counted with one hand with spare fingers.

leem 06A LEEM is very similar in concept and design to a Transmission Electron Microscope but with reflected electrons: a beam of electrons at a few tens of keV is focused by several electromagnetic lenses, and slowed to energies of a few electron volts before reaching the sample. The reflected electrons are accelerated again to high energy (aberrations are already bad enough at higher energies), before their distribution is amplified by several lenses and imaged by a set of channel-plates and then a phosphor screen. The best thing is that this technique is very fast, as it is not a scanning technique. Videos of the evolution of the surface with nanometer resolution (ok, 10 nm) can be acquired in real time. You have not seen thin film growth until you have seen monolayer islands grow in real time!. A movie showing the growth of Co on Ru can be reached here. Surprises can arise with this kind of dynamic view of surfaces, such as  our reported serpentine growth of Pd on Ru.

Furthermore, this microscope allows you to perform low energy electron diffraction (LEED) in selected (micrometer sized or even smaller) areas of the surface. If you have ever taken a complex LEED pattern of a surface, you know that is it a pain to have all the patterns from every corner of the sample together. With a LEEM microscope, you can get only the patterns from a single terrace (check our publication on the LEED patterns of a single terrace on an hcp substrate). Or you can use diffraction contrast to image only the surface areas with a given diffraction spot (the so-called dark field imaging mode of TEM). We exploited this mode of work when understanding the evolution of two copper layers on ruthenium as presented a our Science paper.

But it gets better: you can use a source of spin-polarized electrons, and do Spin-Polarized Low Energy Electron Microscopy (SPLEEM). Then you can see the magnetization of a monolayer film in real tiem while you change the temperature. Or determine the three components of the magnetization vector in a ferromangetic film, one component at a time. For a good example of the type of research we have done with this, you can look at this Physical Review Focus.

If you have a synchrotron around, you can do Photoemission imaging with the same instrument. For that, we believe a good example is our work on "nanometer-thick magnetite", or our work on cobalt-iron oxides published in Advanced Materials.

We hope to have convinced you that LEEM is much, much better than having a traditional surface science system. So why doesn't everyone devoted to surface science (LEED, XPS,...) have one of this apparatus in their lab (starting with us)? Well, on one hand, it is "resolution challenged" when compared with the scanning tunneling microscope (STM). You get around 10nm resolution in commercial systems (two companies sell them, Specs and Elmitec). For higher resolution you need to do aberration correction (where you can get about 2 nm). And to be frank, it is expensive. Well, not that expensive, the price tag of entry systems is similar to a fully equipped STM one, but while for the latter we can cheat and build most of it ourselves (including the STM), building a LEEM is a bit outside our area of expertise.

Since 2017 we have the only LEEM in Spain devoted uniquely to imaging with electrons as illumination. Actually, there are only two LEEMs in Spain, so competition is not that great. The other one has been running since 2010 at the spanish synchrotron ALBA at the CIRCE beamline under Lucia Aballe and Michael Foerster, with whom we have an ongoing great collaboration. The microscope has been acquired through an infrastructure call of the Spanish Ministry of Economy (MINECO, CSIC15-EE-3056) and cofinanced by the ERDF (European Regional Development Fund) and the support of Adrian Quesada (ICV), Enrique G. Michel from the UAM and Arantzazu Mascaraque and Lucas Pérez from the UCM. You can see the instrument here:2017 09 29 all working

There are a few introductions by the several groups that use these instruments. These are a few of the most relevant ones (without pretending to have a complete list, but with a bit of shameless self-promotion):

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