We interviewed Sara Bals, head of the Electron Microscopy group at the University of Antwerp (EMAT). We talked about her passion for electron microscopy, her team and the importance of tomography when creating 3D images that reveal the real structures of nanoparticles and clusters. This technique can lead to the development of novel materials and improvements in application fields such as catalysts.
“The idea that we can see what nobody else can see, that today might be the day when we discover something novel, that is really what keeps me motivated and what keeps me going.”
Where does your passion for Electron Microscopy come from?
When I had to choose a topic for my master’s thesis, I decided to start working here at EMAT with Professor van Tendeloo and he introduced me to the basic principles of transmission electron microscopy. During my master’s thesis I used this technique to investigate superconducting thin films and tapes, and I continued this research during my PhD. After my PhD, I went to the National Center for Electron Microscopy in Berkeley and there, I discovered the possibility of investigating nanomaterials in three dimensions using electron tomography.
Fig. 1. DENSsolutions Wildfire Tomography holder
Tomography is based on the acquisition of a tilt series of two-dimensional projection images. You use those images as an input for a three-dimensional reconstruction. I became really passionate about this technique because without this every image that you acquire using a transmission electron microscope is only a two-dimensional projection of a three-dimensional object and that can be very misleading.
What I like about electron tomography is that it is a very visual technique. Once the reconstruction is obtained it is rather straightforward to interpret the results. And the idea that we can see what nobody else can see, that today might be the day when we discover something novel, that is really what keeps me motivated and what keeps me going.
Can you give an example of one of these discoveries?
In 2018 We received samples from the group led by Professor Sara Skrabalak at Indiana University. She sent us samples where, from two-dimensional projections, we could see that there was some sort of structure. However, when we performed the electron tomography you could see that these particles were extremely symmetrically shaped octapods1. So the idea that you can visualize that, and that you can give feedback to the growers of these materials showing them what they have made, that is really very nice.
Also, something that we have been investigating quite a lot recently are assemblies of nanoparticles. These are groups of nanoparticles of which from a two-dimensional projection image you can not say how many particles you have in the assembly or how they are organized. So we did tomography on one of these assemblies from the group of Professor Luiz Liz-Marzán who is the director of the CIC biomaGUNE Institute in San Sebastian. And we could see that this assembly was almost a perfect three-dimensional puzzle of what is called gold nano dumbbells (Fig. 2.).
Developing electron tomography at high temperatures2was one of the aims within the EUSMI project where EMAT, Denssolutions and CIC BiomaGUNE are partners. It is great to see these groups joining forces.
What is the promise of this research into gold nanoparticles?
They can be used for several applications; they can be used as sensors or they can be used for medical applications or it can be used for catalysis. So there is a broad variety of applications but very often the properties of these nanomaterials; the catalytic properties, the optical properties, depend on the three-dimensional shape. That’s why it’s important to investigate the shape and the three-dimensional structure using electron tomography and we want to do that with atomic resolution because you can consider nanoparticles to be agglomerates of individual atoms.
So if we are able to determine the positions of the individual atoms, together with their chemical nature and the bonding between them, then we can provide the necessary input to predict their properties through simulations. In this manner we may even guide the synthesis of novel nanomaterials.
Fig. 3. The research team of Sara Bals at EMAT
What makes your research group, EMAT, unique in the world of Electron Microscopy?
EMAT is the electron microscopy group at the University of Antwerp. We are quite a large group. We have about 60 researchers. At the moment we have six transmission electron microscopes of which two are aberration-corrected. But I think it’s not the instruments that make us unique. It’s really the team and I often say to new people or people I’m talking with: ‘team’ has the same letters as EMAT because we work together as a team.
At EMAT we have different principal investigators. They all have their own expertise but we work together and the expertise is very complementary. That is what I enjoy most about my work; that we can share knowledge and combine knowledge and that we can create bigger results than what we would be able to do on our own. So the research that we are doing is a good mixture of fundamental, applied and industrial research. But in any case, our main goal is to solve relevant problems in materials science.
Can you give us some examples of these relevant problems that you want to solve?
For example to really understand the connection between the properties and the structure of materials. So we’re not just trying to improve the record of resolution in a well-known material. Let’s say silicon. We want to investigate real-life materials that are sometimes difficult to image because they’re electron beam sensitive or they have many different types of elements. Those are the kinds of materials that we want to investigate. I also think about metallic nanoparticles with the applications that I’ve already mentioned, or maybe some of the organic perovskites.
We’re developing novel techniques in collaboration with people making these samples and we try to understand very well what the questions are that they would like to answer. This work is more challenging but more meaningful and it really pushes the boundaries of technology. Projects that have this combination are my favourites.
How do the in situ stimuli provided by the DENSsolutions systems, like gas and heating, contribute to your research?
Recently I realized that all of the experiments we’ve done so far are done under the conventional conditions of an electron microscope meaning room temperature and ultra-high vacuum. And those conditions are no longer sufficient if you want to understand the behaviour of these nanoparticles and their applications. So that’s why we started thinking; can we visualize the three-dimensional changes that these nanomaterials undergo when they’re exposed to high temperatures or high pressures? So that is what our main goal is within my ERC consolidator grant REALNANO where we are trying to combine the principles of electron tomography with in situ electron microscopy.
So far we got some preliminary results using the Climate gas system, investigating platinum nanoparticles. These are of importance for catalytic applications. But we know that the surface structure might change in a gaseous environment. So this is what we wanted to investigate.
Again we want to perform these measurements in three dimensions and not just based on a two-dimensional projection of a three-dimensional object which can be very misleading. So by combining the expertise of several of the principal investigators here within the EMAT group and with the help of DENSsolutions, we were able to do some first experiments where we acquired high-resolution images of these platinum nanoparticles.
From these images, we’re going to quantify how many atoms we have in a given atomic column and these counting results serve as an input for molecular dynamic simulations that enable us to obtain a three-dimensional model.
Fig. 4. DENSsolutions Climate Gas supply system
Fig. 5. DENSsolutions Climate Gas analyzer
Now using this methodology we investigated a given nanoparticle. We looked at that nanoparticle in a vacuum, in hydrogen and in oxygen. Using the DENSsolutions Climate gas supply system we kept on switching between hydrogen and oxygen. What we saw is that in hydrogen clear facets are present whereas in oxygen a more round structure is observed. And so these surface dynamics are very important if later want to understand what happens to these particles during catalytic reactions. So we have been developing the methodology and we really look forward to investigating these systems under real catalytic conditions.
Thanks to a recent ERC consolidator grant we were able to acquire our own Climate system. Using this system, also in combination with the included Gas Analyzer, we are planning on expanding this type of research.
Can you elaborate on the future investigation under real catalytic conditions?
We want to start investigating different sorts of catalytic nanoparticles, not only model-like systems. For instance; supported nanoparticles or hetero nanostructures rather than one type of element. There are plenty of experiments and different catalytic reactions we can think of plenty of so we’re also collaborating with a lot of groups in Europe to figure out what would be the most important tests that we could perform in this with the Climate system. And of course, we’re also trying to continuously improve our methodologies to visualize the three-dimensional structure.
This is also why we’re really happy that there is currently a heating tomography holder available which tilts across a range of plus-minus 70 or 80 degrees. This is the holder that we have used to investigate the thermal stability of gold stars and octapods in the past.
EMAT also recently acquired a Stream, liquid and biasing, system from DENSsolutions. What are your plans with this?
My colleague Professor Joke Haderman is investigating battery materials under realistic, in situ, conditions.
We also would like to investigate assemblies of nanoparticles in a liquid state. Because these assemblies are formed in a liquid, creating a three-dimensional structure. Then if we would put this structure on a conventional TEM grid there is a possibility that the shape will deform. So far we have been investigating all of them in three dimensions but in a dried state and what we could try and figure out now is how different this dried state is from the native state in a liquid. So we have been doing some very preliminary testing on that and I have a postdoc who is going to apply for a project in order to obtain funding to really push this research.
Next to your current projects, is there any other research at the moment that excites you?
What I am really interested in is the fact that all of these nanoparticles, or at least the nanoparticles that I’ve been investigating most: colloidal nanoparticles, are covered by surface ligands and most of the time when using an electron microscope we completely ignore these. Surface ligands maintain the shape of the nanoparticle but they also form the interface with the environment so they are extremely important.
We do not visualise them because they contain very light elements such as carbon. This makes imaging more complicated because sometimes they form a carbon shell. But I started to understand that it is very important to visualize them because they will interact with the environment first. So we have been looking into how to visualize these surface ligands using more advanced electron microscopy techniques. Also by exploiting the single electron detectors that nowadays became available and also by looking into the support that we are using in the transmission electron microscope. So using graphene type supports rather than the conventional carbon supports. And so when I heard that also DENSsolutions is thinking in that direction I was also very excited about the idea of replacing the silicon nitride by the graphene and I think that again there will be a lot of new possibilities through such chips.
Fig. 6. Graphene – artist impression
I think the graphene support would make a lot of people happy and increasing the tilt capabilities of a Climate system would also open up a lot of possibilities for new experiments, especially in the field of catalysis. Think about supports loaded with catalytic nanoparticles where you really want to understand the three dimensional structure and understand for example degradation mechanisms in three dimensions.
How do you experience your collaboration with DENSsolutions?
Well, something that I really appreciate is that for example at the moment one of my students is performing an internship at DENSsolutions. This is in the framework of a Marie Curie training network called Mummering. So I think it’s really great that DENSsolutions is offering him the possibility to do a secondment there because I believe that once he understands how the chips are made and what the ideas behind the approach are that this will enable him to perform electron tomography experiments at high temperature in a much more efficient thought through manner. So this is really something that I appreciate that DENSsolutions also wants to put the effort into training students like that.
I also appreciate that when we are doing an experiment where we think: OK this is it, now everything is coming together, we have the right samples and we know what we want to aim for, that during the experiments that we did, which later ended up in the Nanoletters paper3, that people from DENSsolutions came over to help and we could do the experiments together. This really accelerated the experiment which is important because of microscopy time is valuable. So it was really nice that it was a group effort into getting those results.
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1 “Thermal Stability of Gold/Palladium Octopods Studied in Situ in 3D: Understanding Design Rules for Thermally Stable Metal Nanoparticles” Albrecht W, Bladt E, Vanrompay H, Smith J.D,Skrabalak S.E, Bals, S, ACS Nano 13, (2019) 6522-6530
2 “3D characterization of heat-induced morphological changes of Au nanostars by fast in situ electron tomography”. Vanrompay H, Bladt E, Albrecht W, Béché A, Zakhozheva M, Sánchez-Iglesias A, Liz-Marzán LM, Bals S, Nanoscale 10, 22792 (2018)
3 “Three-Dimensional Quantification of the Facet Evolution of Pt Nanoparticles in a Variable Gaseous Environment”. Altantzis T, Lobato I, De Backer A, Béché A, Zhang Y, Basak S, Porcu M, Xu Q, Sánchez-Iglesias A, Liz-Marzán LM, Van Tendeloo G, Van Aert S, Bals S, Nano Letters 19, 477 (2019).