Techniques EDX, EELS et HAADF en TEM: possibilités d analyse et applications Thomas Neisius Université Paul Cézanne
Plan Imaging modes HAADF Example: supported Pt nanoparticles Electron sample interaction EELS Example: AlN/Si interface EFTEM Example: embedded Al nanoparticles EDX in S/TEM Préparé par: T. Neisius 30 novembre 2011 2
Imaging modes Scanning Transmission Electron Microscope STEM k 0 k -g k g Specimen Objective lens Back focal plane Objective aperture Detector plane
Imaging modes Scanning Transimission Electron Microscope STEM High Angle Annular Dark Field At large collection angles the signal is mainly due to incoherent scattering. Z- contrast Illumination aperture Objective lens pre-field Si (110) Specimen Detector plane BF ADF HAADF
HAADF Example: NOx conversion catalyst Pt particles supported by LaCo 1-y Fe y O 3 - Perovskite C. Lancelot UCCS Lille TEM 300kV Exposure time: 0.5 s low particle contrast beam damage Difficult to identify the Pt-particles
HAADF Example: NOx conversion catalyst Pt particles supported by LaCo 1-y Fe y O 3 - Perovskite C. Lancelot UCCS Lille STEM HAADF 300kV Exposure time: 60 s chemical contrast reduced beam damage during scan Easy to identify the Pt-particles
HAADF Example: NOx conversion catalyst Pt particles supported by LaCo 1-y Fe y O 3 - Perovskite C. Lancelot UCCS Lille
Electron Sample Interaction incoming electrons E in thin specimen
Electron Sample Interaction Elastic Interaction No energy transfer incoming electrons E in thin specimen: mostly elastic Scattering events elastically scattered electrons E out = E in Ideal for imaging: high coherence between the scattered electrons
Inelastic Interaction Energy transfer from electron to the sample Electron Sample Interaction incoming electrons thin specimen E in elastically scattered electrons E out < E in
Inelastic Interaction Energy transfer from electron to the sample Electron Sample Interaction incoming electrons thin specimen E in
Inelastic Interaction Energy transfer from electron to the sample Electron Sample Interaction incoming electrons Collective Excitations E in Phonon: E ~ 0.01eV Plasmon: E ~ 10eV
Inelastic Interaction Energy transfer from electron to the sample Electron Sample Interaction Core level Excitation E ~ 50eV 10keV
Inelastic Interaction Energy transfer from electron to the sample Electron Sample Interaction «Knock-on» effect E ~ E in C Si Cu Au E 0 [kev] 53 148 449 1310
Spectroscopy: how to exploit the inelastic electron sample interaction measuring the electron energy behind the sample ( EELS ) measuring the energy release during the sample relaxation ( EDX, Auger )
«Primary» Spectroscopy: Electron Energy Loss Spectroscopy EELS: Intensity of a certain energy loss has be measured I( E) is detemined by electronic structure
EELS Scheme from Botton 2007
EELS Spectrum from Sauer
Making the sample as thin as possible is the most important part of EELS
Making the sample as thin as possible is the most important part of EELS
Instrumentation GIF
AlN/ structure and chemistry of the interface G. Radtke IM2NP AlN buffer grown by MOVPE @ 735 C (Cambridge University) AlN <11-20> Si <110>
AlN buffer grown by MOVPE @ 1040 C (Cambridge University) AlN <11-20> Si <110>
EFTEM- Al nanoparticles embedded in Al 2 O 3 M. Cheynet SIMAP Grenoble
EFTEM- Al nanoparticles embedded in Al 2 O 3 M. Cheynet SIMAP Grenoble
EFTEM- Al nanoparticles embedded in Al 2 O 3 M. Cheynet SIMAP Grenoble
«Secondary» Spectroscopy: X-ray fluorescence ( EDS, WDS ) For a certain moment system stays in an excited state (electron hole pair).
«Secondary» Spectroscopy: X-ray fluorescence ( EDS, WDS ) hν Relaxation of the sample via emission of a characteristic photon
EDX in SEM versus TEM from Fulz et al. 2002 Beam broadening (empirical): b ~ ZE -1 t 3/2 broadening of the beam less important loss of signal easier to correct
Typical geometry for EDX in TEM Only 1% of the signal detected Sample damage Drift problems Ω = 0.13sr from Otten 1998 The way to the optimal EDX in TEM Best lateral resolution at shortest acquisition times Thin sample Maximum solid angle Brighter electron source
Newest geometry for EDX in TEM FEI ChemiSTEM probe corrected high brilliance optics 4 symmetrical arranged SDD detectors Ω = 0.9sr Fe Au Pt Acquisition time < 4min