2D Materials, sometimes referred to as single layer materials, are crystalline materials consisting of a single layer of atoms. Since the isolation of graphene, a single-layer of graphite, in 2004, a large amount of research has been directed at isolating other 2D materials due to their unusual characteristics and for use in applications such as photovoltaics, semiconductors, electrodes, catalysis and water purification.
Carbon exists is many forms and is a constituent of the majority of materials. Carbon materials have risen specific interest in the nano-materials sector, following the discovery of carbon nanotubes (Iijima) and the demonstration of graphene (Geim and Novoselov), with promising applications in materials strengthening and conductivity enhancement, as components in batteries, as antennae and filters, to name a few. Electron microscopy has been and carries on being instrumental in the research and development of these materials, for which knowledge of their structure at the atomic level is an utmost requirement.
Nanomaterial constituents are of the size of a few up to several hundreds of nanometers. They encompass single or poly-crystalline materials, as well as pieces of composite, amorphous and organic materials. Nanotechnology is a fast and globally expanding area, enabling immense developments not only in the ICT, but also in environmental, medical and generally all sectors impacting on daily life. Since the fundamentals shaping the properties of nano-materials differ from those of 3-D materials, the former assume new and unknown forms and shapes; hence TEM is indispensable in establishing and understand these properties and enable the development of nano-materials.
Most materials research, including in-organic and organic materials requires structural imaging. Imaging via electron microscopy, down to the atomic level, is carried out on a variety of materials, including nano- and 2D-materials, composites, ceramics, semiconductors, organic crystals and pharmaceuticals, to assess structural perfection/imperfections, boundaries and interfaces. This is done in stationary TEM (diffraction contrast and phase contrast) imaging mode as well as in scanning TEM (STEM) mode. The aberration correctors (for the condenser and objective lenses) and the monochromator allow for high angle annular dark field (HAADF) STEM imaging -also called Z-contrast imaging- to be carried out with atomic resolution. Using the latter method, the dependence of the intensity on the atomic number (as ~Z2) of the elements in the sample then enables determination of the elemental nature of the material under investigation down to the single atom level. In 2-D materials this has proven extremely useful, since elemental occupancy of lattice sites, including elemental nature and site of individual of dopant and impurity atoms, can be directly determined.
Electron Energy Loss Spectroscopy (EELS) is an absorption spectroscopy technique, where the energy, absorbed from the incoming electron beam when passing through the sample, is measured with high energy and spatial resolution. The achievable energy resolution is <100 meV (for an electron beams with energies from ranging from 80 to 300 keV), and the spatial resolution down to the Angstrom level. The energy losses are characteristic for the elements in a material (-> characteristic absorption edges, similar to EDX absorption peaks); in the low loss regime they reflect the electronic bandstructure (-> bandgap energies and density of states) of the material. EELS can be carried out in energy-filtered imaging mode (EFTEM), where an image is taken with an electron which has lost a certain amount of energy due to interaction with the material, revealing, e.g., elemental maps. It can also be carried out in spectrum imaging mode, where an entire energy loss spectrum is taken in each pixel, when the electron beam is scanned across the sample. The method is applied to a variety of materials, including nano- and 2D-materials, complex oxides, alloys, semiconductors…. to reveal composition and elemental distributions down to the atomic level. EELS can furthermore reveal bonding states, energy level distributions and phenomena related to optical properties. It has been extensively employed to investigate elemental nature, local bonding and effects of defects on the electronic structure, as well as to reveal plasmonic properties in semiconductors, nano- and especially 2D-materials.
Plasmons are collective excitations in materials exhibiting metallic and semiconducting properties, and, depending on the material, occur over an energy range from tens of eV down to the meV regime. Based on their collective nature, their confinement and enhancement of the optical near field with sub-wavelength dimensions, they have immense potential for applications in the field of nano-photonics and plasmonics, and in the development of nano-photonic devices. Research into the plasmonic properties of materials together with assessment of the related nano-structural properties can be carried out via electron energy loss spectroscopy in energy-filtered or scanning spectrum imaging mode.
In diffraction tomography, a series of diffraction patterns of the same object is taken at varying angles. Data reconstruction then enables the determination of complex nanocrystal- and cell-structures and of the disorder, e.g., polycrystallinity, twinning, within. Due to the short, one-shot exposures, it can also be used for unit cell determination of organic crystals, enabling characterisation of crystals as small as 10nm.
Knowledge of the atomic-scale structure of materials in all application sectors, especially concerning the impact of structural perfection/defects on the strength and on interfacing of materials, as well as on their performance in the electronics and nano-material development sector is of paramount importance. The methods of high resolution (phase contrast) imaging (HREM) as well as atomic resolution high angle annular dark field imaging in scanning mode (STEM) have proven to be instrumental and unique methods for these investigations.
Dissolution and reactivity of nanomaterials in an aqueous environment is vitally important to a wide range of disciplines including pharmaceutical chemistry, geochemistry, battery science etc. In the image above AlOOH mineral nanoparticles suspended in water within a liquid cell are imaged by TEM at 200kV. Here the imaging of the dissolution of boehmite reveals it is in fact a much more complex multi-step process than previously inferred. The electron beam irradiation destabilises the hydrogen bonding mineral inter-layer network resulting in internal delamination of the particle forming 2D exfoliated boehmite nanosheets prior to the complete dissolution.