INELASTIC SCATTERING AND BEAM DAMAGE In. D.B Williams, C. B. Carter. 2009. Transmission Electron Microscopy. Springer. CHEM E5225 Electron microscopy 13.9.2016

WHAT IS INELASTIC SCATTERING? Inelastic scattering process means energy-loss process Transfers energy to specimen Conventionally TEM uses only elastic signals (direct beam and diffracted beams) Vast amount of information from inelastic scatter is often thrown away Can be used in TEM Allows more complete characterization of the specimen http://wiley-vch.e-bookshelf.de/products/reading-epub/product-id/752992/title/Low%2BVoltage%2BElectron%2BMicroscopy.html?lang=en

WHAT CAUSES INELASTIC SCATTERING/ENERGY-LOSS? When electrons from the beam are focused on a specimen, they penetrate and interact with atoms The deeper the penetration, the grater the energy that may be lost Produces range of scattering angles: X-ray Other secondary electrons Collective interactions with many atoms or electrons These three processes are discussed further in the following slides

1. X-ray emission signal The most important of the three inelastic processes Gives information what elements of the specimen interacts with beam and their quantification Principle: The electrons in the electron cloud have a set of energy levels = electron shells From closest to the nucleus outwards: K, L, M, N, O, P, Q K shell has the highest ionization energy (more energy needed to remove an electron from this shell) Characteristic X-rays are produced by electron transitions between the electron shells http://www.ammrf.org.au/myscope/analysis/eds/xraygeneration/characteristic/

Production of X-rays Electron is removed from one of the inner shells of the atom by an electron from the beam The atom regains stability when electron from an outer shell fills the inner shell ”hole” X-ray photon is emitted 3 2 1 http://www.ammrf.org.au/myscope/analysis/eds/xraygeneration/characteristic/

2. Secondary-electron emission 4 Electrons within the specimen that are ejected by the beam electron Far less likely process than all other inelastic processes ”Free electrons” – not associated with specific atoms – no specific elemental information Can only escape if they are near the specimen surface (standard signal in SEM) Can provide high resolution topographig images of the surface (STEM) 3 2 1 1: electron removed by beam 2: L1 electron fills the hole in K shell 3: Energy released is transferred in the L2,3 shell 4: Ejection as Auger electron (no x-ray emerges from the atom)

3. Collective interactions with many atoms or electrons Transferred energy is usually small Collisions Cathodoluminenscence: electrons and holes will recombine and give off light Plasmons and phonons: collective oscillations of free electrons in gas or atoms in crystal lattice

BEAM DAMAGE Inelastic collisions cause electron-beam damage – it’s the drawback! Once your thin specimen is damaged, its structure is changed and is not representative But beam damage can be used purposely for in-situ transformations Three forms of damage: Radiolysis: inelastic scattering breaks chemical bonds Knock-on damage or sputtering: displacement of atoms from crystal lattice Heating: phonons heat specimen and damages polymers and biological tissue Be avare of the dangers