Spectrometer-free electron probe of ultrafast thermal dynamics in optically excited samples

Cutting edge electron microscopes allow to probe the optical excitations in nanostructures with subangstrom spatial and femtosecond temporal resolution. In this study we discuss the prospects of probing the ultrafast out-of-equilibrium dynamics of plasmonic excitations in graphene and graphite samples. We show that, the plasmon excitation dynamics in nanostructured graphene and graphite can be traced through energy integrated and momentum resolved spectra of inelastically scattered electrons in contrast to more traditional energy-resolved spectroscopy approach, where highly-monochromatized electron beams and electron spectrometers with sufficient energy resolutions are essential. (C) 2021 The Author(s)

Electron Spin Resonance Study of Graphene

Luka Ciric

Graphene, a honeycomb lattice of a single atom thick plane of carbon atoms has captivated the attention of physicists, materials scientists, and engineers because of its extraordinary physical properties. These include exceedingly high charge carrier mobility, current-carrying capacity, thermal conductivity, and mechanical strength. These properties qualify it as an excellent candidate for new electronic technologies both within and beyond metal oxide semiconductors. The attractive properties of graphene for future applications are observed only in high quality graphene produced by mechanical exfoliation of graphite by using the so-called "scotch tape method". However, the yield is low and it satisfies only the needs of basic research. For applications one has to have material which is produced on large scale but preserves the quality of graphene. There are many production routes proposed in the scientific community and their number is still growing. One of the tasks of this PhD work was to test by Electron Spin Resonance (ESR) the quality of graphene produced by various methods. ESR is a suitable technique for looking for ferromagnetic interactions in graphene predicted by theory, and to determine the intrinsic spin relaxation time? These features are important for integration of graphene in spintronics applications. The main results are obtained on a large assembly of three differently prepared samples: 1. Mechanically exfoliated graphite (MEG), 2. Reduced graphene oxide (RGO) and 3. Liquid phase exfoliated (LPE) graphite. Attempts were done to study graphene synthesized by chemical vapor deposition (CVD) and by epitaxial growth on SiC. The mother compound graphite, as a reference sample was also thoroughly investigated by ESR. The LPG sample, obtained by heavy sonication of graphitic powder in NMP solvent, contains more nanographitic particles than graphene. The overall response resembles that of graphite with the exception of detecting a strong magnetic interaction below 26 K. This unambiguously shows the possibility of creating a strong magnetic response in carbon based material, but the challenge to inducing it graphene still remains. The RGO sample, prepared by chemical methods provides a large quantity of sample, suffers from the harsh oxidative and reduction steps involved in the synthesis. Both steps can be incomplete, leaving behind a large number of defects which Anderson-localize the extended states necessary for electronic applications. The best characteristics are obtained on MEG graphene. The temperature dependence of the magnetic susceptibility follows that predicted by theory, and it is distinctly different from that of graphite. Furthermore, the 10-8 s spin lifetime deduced from the ESR linewidth is by two orders of magnitude longer than that obtained in spin-valve experiments and it is encouraging for spintronics applications.

EPFL2011

Toxicity study of nanostructures

Lenke Horváth

The great achievement of nanotechnology is the controlled synthesis of a large variety of nanometer-size materials like needle-formed nanotubes, nanowires and two-dimensional graphene flakes. Due to their unique physico-chemical properties, these nanostructures are considered to be of great benefit for many applications in engineering, electronics, alternative energies and nanomedicine. Since the expectations for the improvement of our everyday life through engineered nanomaterials are high, there is rapid expansion of their manufacturing, which makes it likely that intentional and unintentional human and environmental exposure will increase in the near future. Consequently, the concern grows, related to their possible health hazards, as some of them strongly resemble asbestos. Motivated by this issue, we have investigated the in vitro acute cellular toxicity associated with four model nanomaterials: carbon nanotubes, boron nitride nanotubes, titanium dioxide nanofilaments and graphene oxide. This study focused on the toxic effect these nanomaterials had on cell types found in the respiratory system, where exposure to these materials is most prominent. These are lung epithelial cells, macrophages, and fibroblasts, but also other cell types like kidney cells were tested. The cytotoxicity was assessed by using MTT, DNA and FMCA assays, which measure different endpoints such as metabolic activity, cell proliferation and viable cell number. The cell death was determined by the Annexin V assay. We employed various microscopic techniques: light microscopy to reveal the morphological alterations associated with the nanomaterial toxicity at the cellular level; scanning electron microscopy to study the cell-nanomaterial interactions on the surface of the cell membrane; transmission electron microscopy to examine the uptake and subsequent localization of the nanomaterials within the cytosol and cell organelles. In addition, the generation of intracellular reactive oxygen species induced by graphene oxide was detected by the DCF assay. Last but not least, the pro-inflammatory potential and the biochemical perturbations in cells exposed to boron nitride nanotubes were investigated by Western blot and Synchrotron Infrared Microspectroscopy (SIRMS), respectively. All these techniques point to the adverse effects of the investigated nanostructures: i) The toxic effect of carbon nanotubes and carbon nanoparticles showed a time- and dose-dependent impairment in the metabolic activity of the cells characteristic for each cell type. Moreover, distinct morphological alterations typical for cell death were particularly apparent in macrophages. ii) The toxic potential of boron nitride nanotubes exhibited more pronounced adverse effects than carbon nanotubes. This was demonstrated by cell viability assays combined with cytopathological and biochemical analyses, showing induction of serious morphological changes, particularly in macrophages and fibroblasts, and biochemical processes characteristic for cell death. A higher acute toxicity was determined for boron nitride nanotubes when compared to crocidolite asbestos and their pro-inflammatory potential was demonstrated by the secretion of mature IL-1β cytokine in macrophages. iii) Titanium dioxide nanofilaments were also shown to impair the metabolic activity of the studied cells and induce morphological changes pointing to cell insult. iv) Graphene oxide exhibited a mild cytotoxic action in comparison to carbon nanotubes on epithelial cells and macrophages. The interaction of the nanomaterial with the cell surface generated reactive oxygen species during the initial phase of the exposure and transmission electron microscopy showed that graphene oxide flakes are taken up via the endocytic pathway. In summary, our findings highlight important physico-chemical parameters, which are important in relation to the toxic effect of nanomaterials. These are: i) the chemical composition; ii) the surface modification including functionalized groups and structural defects; iii) the geometry: length, diameter and tortuosity. In addition to the identified nanomaterial characteristics, we pinpoint that the target cells ́ response depends on their type, which is likely to be linked to their physiological function.

EPFL2012

Angular Resolved low loss EELS for Materials Characterization

Simon Schneider

Electron Energy Loss Spectrometry (EELS) in Transmission Electron Microscopy (TEM) is a powerful tool for the investigation of the electronic structure of materials. In the low loss regime, one can access the optical properties which are governed by the dielectric function ε(ω), which may be retrieved thanks to the so-called "loss function". The energy range covered by EELS goes from the infrared (less than 1 eV) to the hard X-rays (about 3 keV), each region having its own interest. The low loss region (0–100 eV) serves to investigate the valence electron structure and the inter-band transition behavior. Using angular resolved EELS gives further information about the dispersion behavior of the investigated material. This work is a part of a large project aiming a closer agreement between theory and experiment in angular resolved EELS by improving the state of the art in both fields. The purpose of this work is to set up a reliable experimental technique to retrieve the momentum transfer resolved single scattering distribution at very low energy with good accuracy and resolutions. One of the main challenges was to use the newly installed JEOL 2200 FS transmission electron microscope that is equipped with an in-column Ω filter for retrieval of the loss function in diffraction energy filtered TEM. Issues such as the angular resolution, contamination, image-coupling in diffraction mode and plural scattering have been solved. Several optical alignments and operational modes of the JEOL 2200 FS have been tested, and finally nano-beam diffraction was found to be the optimal running mode. This work provides for the first time a full 3D energy loss data cube in diffraction mode with energy and angular resolutions of 1 eV and 0.5 nm−1 respectively. After processing, the energy and angular relevant data span an energy loss range from 2.75 eV to 40.25 eV and the angular momentum transfer up to 12.5 nm−1. Although theoretically, without CCD camera binning and the maximum available camera length of the JEOL (250 cm), a momentum transfer resolution of 0.04 nm−1 would have been possible, the effective angular resolution was limited by the smallest obtainable beam convergence as well as by the remaining aberrations of the microscope. The presented technique can be reproduced on any transmission electron microscope equipped with an in-column energy filter. Furthermore, it is demonstrated that the proposed method provides working ranges in energy, spatial and angular resolutions limited only by the perfor- mance of the microscope. Previous techniques did not provide experimental results showing an energy resolution independent of the spatial and angular resolutions.

EPFL2013