Low-Loss and No-Loss Spectra Jani-Petteri Jylhä

Scattering Spectrum contains both inelastically and elastically scattered electrons Zero-loss peak contains primarily elastic electrons scattered forward Also some electrons, that have suffered small energy losses Forming images and DPs with zero-loss electrons have advantages especially with thicker specimen Low-loss (up to ~50 eV) region contains electrons, that have interacted with outer shell electrons of atoms Low-loss region can be used to reveal information about electronic structure of specimen

Spectrum Most understandable and more easily modelled at single scattering This requires very thin specimen and high acceleration voltages Usually sample is thicker causing plural-scattering Deconvolution routines is used to remove plural-scattering effects Deconvolution involves risk of introducing artifacts into part of the spectrum, where it is used

Typical energy losses EELS is used to study energy losses of principal scattering processes These consist of single-electron scattering Interband and intraband transitions at 2-20 eV Plasmon interactions at 5-30 eV Inner-shell ionizations at 50-2000 eV Phonon excitation also happens, but losses too small to be separated from ZLP Typical energy angles can be estimated as a function of energy loss as

Zero-loss peak (ZLP) Predominant feature of spectrum only if specimen is thin enough for EELS Consists primarily of electrons that have retained incindent-beam energy ZLP actually has electrons with energy loss below resolution limit Mostly those that excited phonons Phonon-scattered electron don’t have useful information but heat up the specimen ZLP can cause saturation of detector causing ghost peak in spectroscopy ZLP should be deflected from detector if it is not needed In microscopy ZLP is very useful when forming image or DP where most of energy-loss electrons have been excluded However, also forming images with ZLP filtered out and using selected energy-loss electrons is useful

Zero-loss images and DP If all energy-loss electrons are filtered, elastic image or DP is formed This removes chromatic-aberration effects and increases resolution of thick specimen and enhancing contrast Filtering has positive effect on all forms of TEM-image contrast, but especially for thick biological or polymeric specimen Inelastic scattering is stronger than elastic scattering in these Also diffraction contrast can be enhanced since inelastically scattered broaden the excitation error reducing diffraction contrast Also contrast tuning can be used Energy-loss window is selected and tuned to find best contrast Useful for both low- and high-loss regions of spectrum

Low-loss spectrum After ZLP, next major peak is caused by plasmon Spectrum is relatively featureless apart from plasmon peak Spectrum still contains useful data containing lot of counts Beam electrons have interacted with condution and/or valence bands Low-loss spectrum can be used to fingerprint materials Dielectric constant of specimen can be determined from low-loss spectrum Great interest of semiconductor industry For best low-loss spectroscopy FEG is required with high resolution and dispersion spectrometer and monochromator

Plasmons Plasmons are longitudinal wave-like oscillations occurring when a beam electron interacts with electrons in the conductance or valence band Plasmons have lifetime of about 10 -15 s Plasmons dominate in materials with free-electron structure Specimen is too thick for EELS if first plasmon peak is more than 1/10 of the zero-loss intensity Correlations between plasmon energy and elastic properties, hardness, valence-electron density, and cohesive energy exists Interest for this part of spectrum is rising for studing nanoscale materials

Single-electron excitations High energy beam electron can transfer enough energy to single electron in valence band to change its orbital state These single-electron interactions result in inter- and intraband transitions for valence electrons Energy losses for these go up to ~25 eV Intensity variation can be used to fingerprint particular phase Interactions with molecular orbitals produce characteristic peaks in spectrum, which may cause shifts in the plasmon peak causing intensity variations Valence band electron may be given sufficient energy to escape the nucleus making secondary electron (SE) These are used to form topographic images in SEM and STEM Typically SE requires < 20 eV to escape

The band gap If region between ZLP and intensity preceding plasmon peak has no interband transitions, forbitten-transition region can be seen This is band gap between valence and conduction band in semiconductors and insulators Need for sub-nanometer resolution imaging of the band gap will increase This is caused by advancing of sub-nanometer-scale semiconductor technology Low-loss EEL images are only way of visualizing this electronic property