Transcript Lecture 8-
Electron Energy Loss Spectroscopy (EELS) Suggested Reading: Used electron spectrometer Used TEM Review of theory for elastic + inelastic scattering Examples for graphite and gold Problem: How can we gain information on unoccupied electronic states of the system IPES—good, but low count rates, few labs do this. Auger spectroscopy—common, high count rates XPS-- not bad either. Can we use such instrumentation and excitation methods? Yes! (Well, sort of…) Electron Energy Loss—basic process Losses in XPS spectra Electron Energy Loss Spectroscopy—electron excitation *TEM+EELS analyzer *Electron energy analyzer Near Edge X-ray Absorption Fine Structure (NEXAFS)—synchrotron based photon absorption measurement. e- EK = hv-EB- Φanal. - ΔE hv Evac CB VB ΔE EFermi E field Promotion of an electron from VB to CB Core Transit of an electron through a solid (ionization of core electron shown) induces a strong, local electric field (E) This electric field can induce excitations of higher lying electrons: (a) From ground to higher vibrational states (HREELS) (b) From filled to empty electronic states (EELS) –considered here. EELS effects observed in XPS, Auger, etc. as well as from reflected/transmitted electrons C(1s)XPS from ~ 3ML graphene /Co3O4/Co (Zhou, et al., JPCM 24 (2012) 72201 Loss spectra from backscattered electrons, few layer graphene/SiC Using high resolution electron spectrometer Can also use TEM with electron energy loss analyzer for this purpose (Can get EELS spectra from VERY localized area) EELS in TEM mode United States Patent 7067805 From uspatentsonline.com Double pass Cylindrical Mirror Analyzer (CMA) with co-axial gun (STAIB instruments) --Coaxial gun, reflection geometry --Large angular acceptance. --Field free region between analyzer and sample EELS can be used to monitor evolution of surface vs. bulk plasmons in deposited nanoparticles. (Plasmon collective excitation of electron gas in, e.g., metals) note, surface plasmon typically distinct in energy from bulk) Used double pass CMA Langer, et al. EP 60 One can get some idea of energy band dispersion by collecting at well defined angles (CMA not good for this), but note effects on resolution of varying the primary energy: ΔE/E = const. for electrons from excitation source. Also better surface sensitivity Energy and Emission Angle Effects 1. Surface excitations (surface plasmons) enhanced at higher emission angles. 2. Excitation cross section for specific loss energy EL increases as ΔE = Ep – EL increases. REELS of Si Note: Surface plasmon intensity increases with emission angle of reflected electron Bulk plasmon Surface Plasmon Note relative intensities of surface and bulk plasmons at 500 vs 2000 eV primary energy—due to both cross section and imfp effects Ask to borrow your Mom and Dad’s synchrotron! Can get sampe phenomena during X-ray absorption One can monitor photo yield, or electron yield Absorption vs. wavelength From Grant Bunker, NEXAFS overview Near Edge and pre-edge region EXAFS region, excitation into continuum Excitation of Core level to unoccupied bound state e- O 1s (or N, or C NEXAFS (XANES) Core unoccupied bound states, like EELS but different, it is a direct absorption measurement