Using original optical microscopy techniques and cutting-edge electronic microscopy platforms, the laboratories of Université Paris-Saclay are leaving no stone unturned to push forward the limits of the observation and the characterization of nanomaterials, carried out on a scale of a few atoms or even at monoatomic level.
At the Laboratory of Innovation in Surface Chemistry and Nanosciences (LICSEN) at the Alternative Energies and Atomic Energy Commission’s research center at Saclay, Stephane Campidelli, Vincent Derycke and their colleagues explore the chemistry and electrochemistry of nanomaterials and bidimensional materials such as graphene and its derivatives or monolayer molybdenum disulfide (MoS2). In collaboration with the researchers at the Institute of Molecules and Materials in Le Mans (IMMM – CNRS/Universite du Maine), they have adapted a new optical microscopy technique to observe and characterize ultrathin, transparent materials in real time, especially during their chemical functionalization. Known as Backside Absorbing Layer Microscopy (BALM), this technique invented at IMMM is based on a simple optical principle, i.e. absorbing antireflection layers. Composed of a metal film a few nanometers thick, this substrate ensures that, in the absence of an object to be observed, no light is reflected. “The intercalation of a material, even ultra-thin, disturbs the antireflection effect and the material then shows up in very high contrast,” explains Stephane Campidelli.
High-contrast optical microscopy to study chemical reactivity
The technique has proven particularly powerful when applied to 2D materials. “The contrast obtained is striking and it is easy to observe even a monoatomic layer of a transparent material, such as graphene oxide,” adds Vincent Derycke. Thanks to the reverse geometry of the microscope, working with a solvent medium is also possible.“BALM offers tremendous benefits in that it’s very easy to implement and it’s versatile,” he notes.
The team is also investigating the kinetics involved in the adsorption and intercalation of small molecules. “We study in parallel and in real time the grafting of molecules onto monolayers, bilayers or trilayers of graphene oxide and its reduced form. While very close in chemical state, these two materials do not absorb light in the same way and their optical response is very different,” remarks Vincent Derycke. By analyzing each pixel individually, image by image, the researchers extract quantitative and local data, such as reaction times. “The efficiency of the chemical reaction is not homogeneous, because it depends on whether edge or bulk sites are involved. Thanks to this technique, we can identify which ones are more reactive and improve the design of the nanomaterials.”
Today, researchers are coupling the BALM technique with electrochemistry. “The antireflection substrate is a conductor, so it can serve as an electrode to set off electrochemical reactions,” explains Stephane Campidelli.
World-class equipment for investigating the physical properties of nano-objects
At the other end of these nanometric observations is TEMPOS, the microscopy and nanocharacterization platform that came onstream on the Saclay plateau at year-end 2018. Funded within the context of the “Equipex” call for projects bearing on next-generation equipment, this platform is intended for the study of nano- materials, from their growth to the most local measurement of their physical properties. It is the result of the combined efforts of Universite Paris-Sud, the French National Centre for Scientific Research (CNRS), Ecole Polytechnique and the French Alternative Energies and Atomic Energy Commission (CEA), with the assistance of two industrial firms, Saint-Gobain and Thales, to create a world-class center for transmission electron microscopy. It features two cutting-edge microscopes, Chromatem and Nanomax, as well as Nanotem, a more generalist electron microscopy facility that is complementary to the two others. Chromatem is used for experiments in photonic and electronic spectrometry, Nanomax to synthesize and observe nano-objects using imaging with high spatial resolution. “These evolutionary prototypes represent a world premier,” comments project coordinator Odile Stephan.
In a low-temperature environment, Chromatem reveals the particular optical properties of nano-objects at finer-than-atomic resolution. Its technical innovations include labdeveloped light injection and detection instrumentation that enables coupling the electron and photon beams used. By combining spatial and spectral resolution, “we are able to explore the elementary excitations (e.g. plasmons, excitons, phonons and magnons) that govern most of the original properties of nanomaterials, which is key to understanding how they work and developing new ones,” points out Odile Stephan. Mechanisms of metal-insulator phase transitions, new electronic states, optoelectronic properties of semiconductor nanostructures… The range of research possibilities is a broad one.
Observing and controlling crystalline growth
Nanomax, a high-resolution transmission electron microscope, is used to observe and characterize – in situ and at the atomic scale – the growth of semiconductor nanowires and carbon nanotubes. When matter is added and a vacuum maintained, it serves to synthesize varied nanostructures and control their growth kinetics. Matter is added using an original method, i.e. via a stream of atoms, either gas molecules or gaseous radicals. According to Odile Stephan, “the idea is to use labdesigned collimators to get highly concentrated matter as close as possible to the sample. The flow is very well controlled so that we can act on nanostructure growth rates.”
Like Chromatem, Nanomax has already yielded very good results. Recently, teams at the Center of Nanoscience and Nanotechnology (C2N – CNRS/Universite Paris-Sud) under Jean-Christophe Harmand and Gilles Patriarche and at the Laboratory of Physics of Interfaces and Thin Films (LPICM – CNRS/ Ecole Polytechnique) under Ileana Florea and Jean-Luc Maurice filmed a crystalline nanowire’s growth in real time and followed the formation of each of its atomic planes, step by step. “They showed that the nucleation sites were located at the triple point, i.e. at the intersection of the substrate, the drop of liquid used as a growth catalyst and the gas phase. They also saw that nanotube growth occurred in monoatomic steps, whose shape resulted from a reduction in the energy used by the system during its growth,”she adds.
Currently available for use by all nanoscience researchers on the Saclay plateau, TEMPOS will open up within a year to the community in France via the national network METSA, and to the international community via the european network ESTEEM3.
∙ Kévin Jaouen et al., Ideal optical contrast for 2D material observation using bi-layer antireflection absorbing substrates, Nanoscale (2019).
∙ M.Kociak et al., A spectromicroscope for nanophysics, Ultramicroscopy, 180, 81-92 (2017).
∙ Jean-Christophe Harmand et al., Atomic Step Flow on a Nanofacet, Phys. Rev. Lett. 121, 166101 (2018).
Portrait: Odile Stephan
“We ordered our dream machines. Not only did they meet our expectations, but they far surpassed them!”
Odile Stephan did a doctoral thesis on the latest developments in scanning transmission electron microscopes to study carbon nanotubes at the Solid State Physics Laboratory (LPS - CNRS/ Université Paris-Sud). She became the LPS electron microscope team director in 2009 and subsequently coordinated the TEMPOS Equipex project. Co-director of the LabEx NanoSaclay, she also serves as Vice-President for Research at the Physics Department at Université Paris-Sud. Highly involved in teaching, she sits on the relevant committee for the Physics Department and directs the Master 1 Physics and Applications course at Université Paris-Saclay.
By Véronique Meder.
The original version of this article was published in L'Edition #10.