Hello Christian, thank you for accepting this interview. We wanted to take advantage of the 10th anniversary of your microscope to have an update with you about your lab and your use of the ARM200F.
Let’s start with a presentation of your lab and your team.
Presentation of the team
In 2000, I created the Advanced Electron Microscopy and NanoStructures team (Me−ANS) within the newly created research federation Materials and Quantum Phenomena. Initially composed of Emmanuel Fort and myself, I was able to expand the team by recruiting a new lecturer, Cyril Langlois in 2004, the year I was appointed Professor at the University of Paris Diderot. From 2004 to 2009, we developed our own research themes in materials science thanks to the development of an ultra-high vacuum chamber for materials growth while continuing to have a sustained activity in quantitative electron microscopy. The period 2009-2016 saw a complete overhaul of the team. Cyril Langlois and Emmanuel Fort having joined other laboratories: Emmanuel went to ESPCI to create his own team and Cyril joined INSA of Lyon. Then Jasyen Nelayah joined the team as a lecturer in 2009, Damien Alloyeau as a CR at CNRS in 2010 and Guillaume Wang as a research engineer at Paris Diderot University in 2010. We permanently welcome two or three PhD students per year and one or two long-term visitors (post-doc or senior visitor).
What are the main themes you work on?
We work on different topics, but all are related to materials properties and their understanding:
- Structures and thermodynamic properties of nanoalloys
- Reactivity of nano-alloys by operando microscopy
- Growth in liquid media by operando microscopy
Recently we have extended our field of expertise to biology by studying materials at the Physics/Biology Interface.
What aspects of your job are you most the passionate about?
Unquestionably, the observations of unexpected phenomena or phenomena that no one has ever seen before.
There are also the interactions with collaborators who explain to you what they want to try to understand and the resulting discussions to determine which experimental strategy is going to set up on the microscope to see / highlight the sought out phenomenon. Often, it takes a lot of daring and ingenuity to observe the phenomena in question.
Finally, there is everything that concerns the setting up of projects and networks around a given scientific theme which allows us to animate a group that will work with the same objective. When this teamwork works optimally, it is the key to success.
What is your best memory with this microscope?
The first images of single atoms on a surface with the possibility, via a quantitative analysis of the image contrast, to determine their chemical nature. This type of imaging was previously impossible on conventional TEM because, in high resolution, the focusing conditions were too far from zero focus, which implied a strong contribution of the support to the image contrast. With an aberration corrector of the objective lens, we work almost at zero focus, which implies that the contrast of a very thin amorphous carbon support is very low and especially uniform over the whole image. We can then observe very fine details, related to small variations in contrast, on this surface.
Secondly, the coupling of a UHR (Ultra High Resolution) lens, a very coherent electron source (cold cathode) and the aberration corrector allowed to realize high resolution images with an exceptional contrast and spatial resolution. This ensemble is certainly the best configuration today for ultra-high resolution TEM imaging. In 2010, the point resolution is 75 pm at 200 kV and 100 pm at 80 kV. The ultimate resolution achievable at 200 kV is less than 50 pm.
According to you, what are the two most outstanding researches realized on the ARM of your laboratory?
It is very difficult to summarize all our work by citing only two publications as our research is so varied and has led to several leading publications at the international level. But if I had to quote only two, it would be the following ones:
Unravelling Kinetic and Thermodynamic Effects on the Growth of Gold Nanoplates by Liquid Transmission Electron Microscopy
D. Alloyeau, W. Dachraoui, Y. Javed, H. Belkahla, G. Wang, H. Lecoq, S. Ammar, O. Ersen, A. Wisnet, F. Gazeau and C. Ricolleau
Nano Letters, 15 (4), 2574-2581 (2015)
The growth of colloidal nanoparticles is simultaneously governed by kinetic and thermodynamic effects that are difficult to distinguish. To unravel the mechanisms that influence the formation of nuclei and the morphology of nanoparticles, we have developed scanning transmission electron microscopy experiments in liquid medium. These experiments, completely original in France, allowed us to study the growth of anisotropic gold nanostructures by radiolysis of water. The electron dose provides a direct control of the growth rate which allows to quantify the kinetic effects on the nucleation/growth mechanism of the two-dimensional nanoparticles (nano-platelets). Indeed, we have shown that the reaction rate per unit area has the same behavior as a function of the dose rate as the concentration of reducing agents in the liquid cell. We also determined that there is a critical rate of gold monomers, corresponding to three monolayers per second, to be formed to induce nanoparticle faceting above which nanoplatelet formation is not possible because growth is then dominated by kinetic effects. At low electron doses, growth is thermodynamically driven and nanoplatelet growth is directly related to the formation of twin planes during growth.
Reshaping Dynamics of Gold Nanoparticles under H2 and O2 at Atmospheric Pressure
A. Chmielewski, J. Meng, B. Zhu, Y. Gao, H. Guesmi, H. Prunier, D. Alloyeau, G. Wang, C. Louis, L. Delannoy, P. Afanasiev, C. Ricolleau, J. Nelayah
ACS Nano, 13, 2024-2033 (2019)
The nature of the active sites for the adsorption and dissociation of O2 and H2 by supported gold nanoparticles (NPs) remains an unsolved problem in heterogeneous catalysis. This is due to the lack of a clear view of the structural evolution of gold NPs under near-reaction conditions, i.e. at high pressures and temperatures. In this work, we studied the evolution of equilibrium forms of gold NPs supported on rutile under O2 and H2 at atmospheric pressure using environmental transmission electron microscopy under gas. In situ observations show instantaneous changes in the equilibrium shape of Au NPs during cooling under O2 from 400 °C to room temperature. In comparison, no instantaneous changes in the equilibrium shape are observed under H2. To interpret these results, the equilibrium shape of Au NPs under O2, atomic oxygen and H2 was calculated using a multiscale structure reconstruction model in collaboration with Hazar Guesmi (Charles Gerhardt Institute, Montpellier, France) and Beien Zhu (Academy of Sciences, Beijing, China). The excellent agreement between microscopy observations and theoretical modeling of Au NPs under O2 provided strong evidence for molecular adsorption of oxygen onto gold NPs at 120 °C on specific facets. In the case of H2 , modeling predicts no interaction with gold atoms, which explains the high morphological stability of NPs under this gas. This paper provides atomic structural information for the fundamental understanding of the adsorption properties of O2 and H2 on Au NPs under real conditions and proposes a way to identify the active sites of heterogeneous nano-catalysts under reaction conditions by studying the structure reconstruction.
Recently, your ARM200F received a rejuvenation treatment. To summarize, your ARM200F with TEM corrector has been upgraded to NEOARM, i.e. double corrected with the last generation of ASCOR STEM corrector, and last generation of EDS detector. All of this was done on site in your lab. What are your feedbacks about this upgrade ?
This is an excellent operation for us because we absolutely needed to move to the high-resolution STEM mode for all our recent developments on the physics of nanoalloys and also to answer the growing demand for this mode of microscopy from the users of the advanced microscopy platform of the University of Paris and METSA.
The operation lasted a year, since everything was done on site and the sanitary crisis has passed. We benefited from the sustained work of Jean-Paul Derouet and Matsuzaki-San who both worked in a very good symbiosis so that the installation went well.
What I really appreciated was the phase of work to prepare the rooms (the microscope room and the technical room) which was entirely taken care of by JEOL, which was therefore in charge of the project. The work was carried out by professionals, who also provide a very good after-sales service, which means that the technical environment of the machine is excellent. We took the opportunity to have the latest generation of JEOL active magnetic field corrector installed, thus greatly improving the working environment of the microscope.
What do you think of the new ASCOR corrector?
It is a very efficient corrector both at 200 kV and at 80 kV which is easily aligned. It also has the advantage of being stable over time, which means that if you do not want to go to the limits of the device, it is not useful to repeat the corrector from one experiment to another.
How many people have you trained on your ARM?
About twenty since 2012. There have been 11 PhD students trained in the team and about ten others trained during collaborations with other groups either internal to the University of Paris or at the national level.
I remember the TEM school that you organized, do you plan a new edition ?
Yes we are planning to repeat the event with a part on operando microcopy and a part on data analysis with artificial intelligence. For the later, we are in the process of learning these new techniques so for the moment no date is planned.
Last May you celebrated the 10th anniversary of your ARM installation. You were the first to have a Cold FEG, what are your feedbacks on this gun?
Nothing to say, this gun is excellent for its simplicity of use, its stability and its performance. We currently have a tip that will soon be 10 years old , and which works like the first day with a single Flash per day and provides as good results as when the gun was installed.
Why did you choose a Cold FEG?
Initially, it was to have a good quality STEM while having optimized TEM imaging with the corrector. Afterwards, it turned out that the coupling of the UHR lens with a very low chromatic aberration and the very high coherence of the Cold FEG allowed us to image at ultra high resolution with a contrast equaled by no other machine currently on the market. With this first prototype instrument (2010), we were able to reach ultimate resolutions of 0.26 eV at 200 kV and 0.23 eV at 80 kV with a probe current of 0.5 µA.
What significant technological changes in the scientific world have you seen during your career?
The most important change I’ve seen since the beginning of my career was the development and commercialization of aberration correctors for microscopes. I had the chance to start my career in a privileged laboratory (LEM, CNRS/ONERA joint unit) since it was one of the only two French laboratories to be equipped with a 400kV microscope. Although this instrument allowed me to obtain good results for my work, it was limited in terms of resolution and signal-to-noise ratio of high-resolution images. The arrival of aberration correctors was a real revolution in the field of electron microscopy since we could see better and farther, opening the way to new researches that we could not imagine until then because of the lack of adequate instruments. Today the point resolution at 200 kV is around 75 pm for the best configurations and for the moment this performance has been sufficient to solve all the questions I have been working on recently. The future will tell us if we need to go even further in terms of spatial resolution. There is of course also the considerable progress that has been made in electron sources, both in terms of their brightness and their energy dispersion. On the ARM, when we place ourselves in the right conditions for electron energy loss spectroscopy, we can routinely reach a resolution of 0.3 eV, which allows us for example to study the plasmonic properties of individual nanostructures.
And the future…
This is probably the most delicate question of this interview because we have just been equipped with a new machine, finally, with its two aberration correctors. No one would understand if I say what my needs are for the future. Besides, I have always proceeded in this way for the other projects I have set up. From a given instrument, you have to do a lot of experiments to realize the limitations, what is missing to go further and imagine what would be the future microscope that you would like to acquire. So we will proceed in this way with the Super TEM 2. Let’s work on it for a few years, pushing it to its most extreme limits to see where it is lacking and what we can’t achieve with it. Then we will be able to imagine the future of electron microscopy development at the University of Paris.
Ok, let’s meet again in 5 years.