Interview with Prof. María Varela del Arco, GFMC, Complutense University of Madrid
We interviewed Prof. María Varela del Arco, who is in charge of electron microscopy in the group GFMC at the Complutense University of Madrid. We talked about her research on, among other topics, magnetic materials and supercapacitors that was made possible by the DENSsolutions Lightning system, and interesting developments in big data mining. Her passion for tackling societal issues helps her bring the relevance of her research into a wider context than the nanoscale.
“In the end, all of the research we do is aimed at improving technology and energy efficiency. We want to find better and cheaper energy materials or cathode materials for batteries, for instance. ”
Who are the scientists that make up your research group?
Our research group is part of the School of Physics. We work in condensed matter and materials physics. I am part of a relatively large group including approximately 10 permanent staff members with different expertise. We use a number of techniques: growth of thin films, magnetometry, characterisation of physical and transport properties and of course electron microscopy, which I am in charge of working together with two other permanent professors.
We also count on a theorist who does simulations in order to help us with the interpretation of data and the design of new experiments. So it is a quite comprehensive way of addressing the understanding of multiple materials problems of relevance. In fact, a major strength of our group is that we can address any given problem in materials physics from many perspectives simultaneously.
Which fields do you tackle in your research?
We mainly work in magnetic superconducting materials link1 link2, functional materials link, ferroelectrics link and materials for energy like ionic conductors and such. We are trying to really understand the properties all the way down to the atom and the atomic configuration. Not just by looking at the static electron microscope image but also under different physical conditions. This way we can actually give an interpretation from many points of view simultaneously and really get to understand the whole physical mechanism responsible for any macroscopic behaviour of interest.
At the time being, we measure physical properties such as transport properties out of the microscope with a number of cryostats to measure all temperature properties and ionic conductivity, also at high temperatures. We also have lithography means so we can fabricate devices like electronic or spintronic devices. We do all these kinds of characterisation on the samples ex situ, but we would also like to do that in situ, inside the microscope.
How will your team implement In Situ capabilities in your lab?
We have a big expertise in the group on transport itself. We can measure the DC and AC transport properties. So it will be cool to bring our equipment into the microscope room, connect to the TEM holder, apply a bias and measure the properties while we’re watching. Maybe we’ll see a sort of electromigration, drifting vacancies like oxygen vacancies in oxides, field-effects across interfaces or injections of charge or spin going from one material to the other.
We are doing these kinds of things right now in the lab but we want to perform them inside the microscope while watching. That is our objective in the very near future. It’s not easy, it’s definitely very challenging, but we are really passionate about giving it a try.
Sample preparation. From left, Prof. María Varela del Arco and graduation student Gloria Orfila Rodriguez.
How did you get passionate about In Situ TEM research?
My early years of training was not in microscopy. On the contrary, I was mainly working in thin film growth link1 link2 and characterisation of transport, particularly in superconducting thin films. I’ve always been very interested in the atomistic properties underlying the macroscopic physical properties of materials systems. So, when I got into transmission electron microscopy I was able to combine this curiosity with my ex-situ expertise in order to really understand what may be going on in the material and how properties arise.
In situ TEM allowed me to add an additional dimension and measure nanoscale phenomena in real time. For example, think of device characterisation. In the lab, sometimes we broke an electric contact while measuring e.g. tunnel junctions. Maybe we applied too high a voltage bias or changed temperature way too fast and we never really knew why this was happening. Now in the TEM, by running in situ experiments, you can record the process and watch in real space actually how a contact breaks… or not!
Complex phenomena such as metal electromigration might happen, which would definitely cause device failure. Perhaps other chemical species or defects might migrate. One way or another, I always wanted to characterise such processes in a controlled way, to be able to watch phenomena at work way beyond looking at a static picture of a material or trying to infer mechanisms from blind measurement of resistivity or other transport properties.
Being able to study transport in real space and monitor carriers moving around, for example, in ionic conducting materials link1 link2, holds the key towards harnessing nanoscale functionality in these systems, opening up a massive universe of exciting possibilities.
Which new interesting things did you find using our In Situ TEM solutions?
We initially procured the DENSsolutions Lightning system mainly to electrically bias materials systems in situ. It was also capable of heating of course, which added potential applications although our research mostly takes place at lower temperatures – we often work with magnetic materials or superconductors which are functional systems that develop their properties well below room temperature. The initial stages of our in situ research consisted mainly in running experiments using the Lightning at different temperatures aiming at mastering the technique, the software, checking stability, etc. While doing so really cool phenomena started taking place in front of our eyes. This opened up a lot of questions which made us drift more into the heating part of the experiments. Heating experiments can also be a bit more simple from the point of view of sample preparation.
An example can be found in a recently published paper in collaboration with the University of Valencia, which has focused on harnessing magnetic nanomaterials used for supercapacitors. Nanocomposites made of iron, nickel and graphene were cycled hundreds of times up to 400 °C, under a magnetic field. A very strong segregation of iron and nickel was induced during the process. Iron gets oxidised which results in inhomogeneous core-shell systems. The behaviour observed was absolutely unexpected. Now, the resulting nickel – iron oxide interfaces exhibit a very high electrochemical activity, and the fact that the volume fraction of interface regions is massively increased enhances the capacitance of the system by hundreds and hundreds of times.
This is a very interesting finding because typically when electrode materials are cycled more often than not their properties are degraded. However, in this case, this high specific capacitance was actually increasing during cycling! The property of relevance actually gets better over use. This is completely the opposite of what usually happens in relevant applications such as batteries like the one in your smartphone. Their performance and behaviour gets increasingly worse the more you cycle them and in the end you need to either change the battery or buy a new phone. Well, it was exactly the opposite, and all of the understanding came could from the in situ TEM. Furthermore, this finding opened a whole new front of possibilities with different nanocomposites and materials that are sensitive to magnetic fields or other driving forces that can make the system segregate with varying temperatures in order to actually optimise any new properties. At the end of the day, it is these unexpected things that you run into that can sometimes be a lot more relevant and stir up a research field.
What are some of the bottlenecks you come across during your research?
One of the main bottlenecks we find is related to specimen preparation for electric biasing experiments. Particularly for the kind of samples that we grow here in the group: heterostructures or superlattices and thin films. Most of the functionality that we are interested in typically arises upon biasing in the form of applying a field across the interfaces. This might result in electroactive systems, colossal magnetoresistance, or real fancy phenomena and functionalities to exploit in a device. So in order to bias, measure and simultaneously observe a specific geometry is needed when it comes to sample preparation. For this you need not just a FIB system but also a very good FIB operator who can produce a very clean lamella, mount it and contact it on the chip in the right orientation and end up having a sample clean of contamination. When running transport properties surface contamination can be a killer. Electric conduction could place on the surface instead of through the device and measurements will be impossible to interpret correctly.
So at this point really refining a reliable method for this particular kind of preparation constitutes a major challenge and a major bottleneck for us at this moment. The Lightning system is really very flexible, the geometry is really easy to work with and it gives us the freedom to design many different kinds of measurements. The challenge really resides in the sample.
What do you expect from DENSsolutions in the future?
What I would really like to have for Christmas is new developments related to low-temperature capabilities. Like I explained before, many of the relevant functional materials that we are working with of interest for spintronics and oxide electronics, exhibit properties of interest at low temperature ranges, be it magnetism or be it superconductivity. So if we really want to study any sort of electronic phenomena like transport across an interface or ferroelectric polarization under bias it is highly desirable to do it within the relevant temperature range. Maybe liquid nitrogen would be enough to start with?
For example, many high-temperature superconductors are superconducting over 77 K. Yttrium barium copper oxide (YBCO) for example becomes superconducting at 92 K. It’s not so difficult to get a material to achieve the superconducting state in a microscope. Actually, the old cooling holders that we’ve had for years manage to get down to 90 degrees in liquid nitrogen rapidly. Of course, they have terrible spatial drift and stability so that would be the main issue to tackle but I’m sure that the DENSsolutions team can figure this out. The current wonderful thermal stability at high temperatures all the way up to 1000 °C allows watching nanoparticles evolve for minutes with a total lateral drift of less than a nanometer, so why not dream of doing the same thing at 100 K, or below?
How is your research connected to societal issues?
Addressing societal issues constitutes the most important drive for us, even if we work in basic research, which in general is not directly linked to an actual application. In the end, all of the research we do is aimed at improving technology and energy efficiency. We want to find better and cheaper energy materials or cathode materials for batteries, for instance. Also, we strive to develop materials for faster electronics and multi-functional devices capable of actually controlling a relatively large number of degrees of freedom with a relatively small amount of energy externally provided. You could have a faster computer that requires less power to run so energy would be saved worldwide or that the developing countries will have access to cheaper, reliable technology and therefore people will have access to better means and services, such as medical machines. Superconductors, for example, are an important part of MRI technology. And at the end of the day, I’d really like to think that these new materials we discover and the resulting advances end up making somebody’s life easier, cheaper and more secure
At the moment what other In Situ TEM research excites you outside of your own workfield?
Well, actually what really excites me outside my field at the moment is something closely related to in situ TEM research. The situation that we are running into now that we have all these wonderful holders, manipulators and samples combined under the electron beam, is that we are getting to the point that we are acquiring gigabytes and gigabytes of data in every session. Data which may become very difficult to analyse because these large volumes need to be quantified in order to extract any meaningful information, especially if you wish to extract statistically significant information from atomic-resolution electron microscopy.
Lack of statistics is a problem of high-resolution microscopy: we analyse really small volumes within our samples. You basically land on some position of your system and hope that this area is representative for the whole device. So in order to make sure that things in the end are representative you really need to analyse lots of regions in lots of samples and now with the in situ capabilities we are really up to the point that massive amounts of data will be generated, for example when you record movies or spectrum images.
So right now the newly emerging data processing techniques related to big data are very interesting, like applying artificial intelligence for example, to the quantitative analysis of microscopy data. Imagine we could end up realizing this dream of high throughput microscopy: recording movies, analysing process evolution under the electron beam and almost at the flick of a switch extracting meaningful information on the fly, without having to go into the office and wait for 2 months analysing noisy data. Imagine being able to extract the relevant physics out of the noise almost as you go while you are running an in situ experiment. That would be really cool.
Do you also collaborate with other TEM groups?
Yes, I was in the U.S. for over ten years before moving back to Spain and I did collaborate with lots of groups there, including not just microscopy, theorists and also growers, from different universities and national labs. I would say that is still a large part of my network of collaborators.
Now, in Europe, we work with a number of groups that we work with on a relatively regular basis. For instance the CNRS/Thales in Paris. They have a very strong spintronics group in particularly oxide-based spintronics which is where we mostly focus on. And we collaborate with groups in Italy, Switzerland, U. K. and others. But many of my European collaborators these days are excellent growers.
Perhaps I used to collaborate with microscopists a little bit more often in the past. It was back in the time, in the beginning of aberration correction when a lot of technique development was taking place and facilities were more scarce so it wasn’t easy for a given lab or group to have all of the equipment available. For example there was this collaboration with Robert Klie from the University of Illinois, in Chicago. He had a low T, liquid helium holder so we could actually run the first experiment testing the sensitivity of spectroscopic fine structure to spin by measuring the spin transitions while cooling down and warming up perovskite cobaltites link1 link2. Collaborations like this used to happen but now it seems like we can do a lot of the work in-house thanks to the local availability of advanced equipment.
Still, for example in Europe we also have joint projects with SuperSTEM at Daresbury since they have this wonderful monochromated Nion microscope, or with TU Darmstadt regarding in situ biasing. So we try to keep a strong network of international collaborators on all fronts.
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