Structural, chemical and electronic characterization of materials at the nanoscale by electron microscopy and spectroscopy techniques

M.S. MORENO

Valdivia

Congreso; 3rd International Conference on Materials Science; 2015

Manipulation of matter at the nanoscale creates new challenges for quantitative characterization and modellingsince many physical and chemical properties are highly dependent on the size and shape. A transmission electronmicroscope offers, in addition to the well known high spatial resolution, several imaging modes and spectroscopies that takeadvantage of the different interactions between the electron beam and the specimen.Electron energy-loss spectroscopy (EELS) is a technique whereby materials are studied through the energy lossescaused by inelastic interactions with the electron beam. These losses are interpreted in terms of the interaction involvedincluding phonon excitation, intra and interband transitions, plasmon excitation, ionization, etc. Thus the EEL spectrumcontains local structural, chemical and electronic information. The different absorption edges in the spectrum correspond toexcitations of electrons from core levels of particular atoms in the sample. In particular the fine structure seen above thoseedges is highly sensitive to the nature of chemical bonds and to the local coordination around the excited atom, providinginformation on the chemical composition and chemical bonding state (valence). This fine-structure is also related to theelectronic structure in terms of the site- and symmetry-projected density of unoccupied states [1]. The appearance of theseedges allows the elements present in the specimen be identified and the measurement of their intensities enables theelemental concentrations to be estimated. Chemical information can be obtained in the form of spectra or as images formedwith electrons that suffered an especific energy loss (energy-filtered TEM, EFTEM). The images obtained for a specificabsorption edge provide the spatial distribution of that element (elemental map). It is important to note that thisspectroscopy is especially suitable for the detection of light elements such as lithium. The example below shows an exampleof the combined use of these different signals in catalysis (Fig.1, ref. [2]).The combination of this wealth of information of images and spectroscopy with high spatial and energy resolutionshows the power of the current TEM as a characterization tool, which allows to establish a correlation betweenmicrostructure, electronic properties and nanochemistry with sub-nanometer or atomic resolution.