MENA 3100: Diff Analytical Transmissions Electron Microscopy (TEM) Part I: The microscope Sample preparation Imaging Part II: Diffraction Defects Part III Spectroscopy

Coherent incident high-kV beam Second electrons From within the specimen (SEM) Incoherent elastic backscattered electrons (SEM) Direct beam (imaging, diffraction, EELS) Coherent elastic scattered electrons (STEM, Diffraction, EELS) Incoherent elastic forward scattered Electrons (STEM, diffraction,EELS) Incoherent inelastic scattered electrons (EELS) Auger electrons (XPS) Characteristic X-rays (EDS) Visible light Bremsstrahlung X-rays (EDS) sample Sample Electron matter interactions

MENA 3100: Spectroscopy Energy Dispersive X-ray Spectroscopy (EDS)

MENA 3100: Spectroscopy K shell L shell M shell Valence electrons Empty states

MENA 3100: Spectroscopy 1s 2s 2p 3s 3p 3d Empty states K shell L shell M shell K α1 EDS K β1 L α1

MENA 3100: Spectroscopy Energy dispersive X-ray spectroscopy kαkα LαLα kβkβ hνhν K L M

MENA 3100: Spectroscopy EDS spectrum

MENA 3100: Spectroscopy X-ray Spectroscopy EDS: Energy Dispersive Spectroscopy EDXS: Energy Dispersive X-ray Spectroscopy X-EDS:X-ray Energy Dispersive Spectroscopy EDX: Energy Dispersive X-ray analysis

MENA 3100: Spectroscopy Quantification Peak intensities are proportional to concentration and specimen thickness. They removed the effects of variable specimen thickness by taking ratios of intensities for elemental peaks and introduced a “k-factor” to relate the intensity ratio to concentration ratio: Each pair of elements requires a different k-factor, which depends on detector efficiency, ionization cross-section and fluorescence yield of the two elements concerned.

MENA 3100: Spectroscopy The Detector

MENA 3100: Spectroscopy Detection Mechanism

MENA 3100: Spectroscopy Detection Mechanism The detector generates a charge pulse propotional to the X-ray energy X-ray creates electron-hole pairs Thousands of electron-hole pairs can be generated by a single X-ray The number of electrons or holes created is directly proportional to the energy of the X-ray photon. Even though all the X-ray energy is not, in fact, converted to electron-hole pairs, enough are created for us to collect sufficient signal to distinguish most elements in the periodic table, with good statistical precision.

MENA 3100: Spectroscopy Detection Mechanism Si contains acceptor impurities and exhibits p-type behavior : Compensate by ‘filling’ any recombination sites with Li, thus creating intrinsic Si in which the electrons and holes can be separated Cool the detector: - Thermal energy would activate electron-hole pairs -- noise - The Li atoms would diffuse under the bias - The noise level in the FET would mask signals from low-energy X-rays. Beryllium-window detectors: absorbs X-rays with energies < 1 keV. - Cannot detect Ka X-rays from elements below about Na (Z=11) in the periodic table

MENA 3100: Spectroscopy Silicon Drift Detector Basically a CCD consisting of concentric rings of p-doped Si implanted on a single crystal of n-Si across which a high voltage is applied to pick up the electrons generated as X-rays enter the side opposite the p-doped rings

MENA 3100: Spectroscopy Recent advances

MENA 3100: Spectroscopy EDS mapping

MENA 3100: Spectroscopy Comparison Low Z element

MENA 3100: Spectroscopy Comparison on resolution

MENA 3100: Spectroscopy Artifacts Escape peak: Because the detector is not a perfect sink for all X-ray energy, it is possible that a small fraction of the energy is lost and not transformed into electron-hole pairs. The easiest way for this to happen is if the incoming photon of energy E fluoresces a Si Ka X-ray (energy 1.74 keV) which escapes from the intrinsic region of the detector. The detector then registers an apparent X-ray energy of (E – 1.74) keV, as shown in Figure 33.1.

MENA 3100: Spectroscopy Artifacts The internal fluorescence peak: This is a characteristic peak from the Si (or Ge) in the detector dead layer. Incoming photons can fluoresce atoms in the dead layer and the resulting Si Ka or Ge K/L X-rays enter the intrinsic region of the detector which cannot distinguish their source and, therefore, register a small peak in the spectrum. The Si internal fluorescence peak in a spectrum from pure carbon obtained with a Be-window Si(Li) detector. The ideal spectrum is fitted as a continuous line that only shows the Si absorption edge

MENA 3100: Spectroscopy Artifacts Sum peak: The processing electronics are designed to switch off the detector while each pulse is analyzed and assigned to the correct energy channel. The sum peak arises when the count rate exceeds the electronics’ ability to discriminate all the individual pulses and so-called ‘pulse pile-up’ occurs. This is likely to occur when: The input count rate is high. The dead times are > 60%. There are major characteristic peaks in the spectrum. The Mg K sum (coincidence) peak occurs at twice the Mg Ka peak in this spectrum from a bulk specimen of (oxidized) pure Mg. The sum peak decreases change rapidly with decreasing dead times; upper trace 70%, middle trace 47%, lower trace 14% dead time. The sum-peak artifact is close to the background intensity at 14% dead time.

MENA 3100: Spectroscopy EDS SEMTEM

MENA 3100: Spectroscopy Electron Energy Loss Spectroscopy (EELS)

MENA 3100: Spectroscopy 1s 2s 2p 3s 3p 3d Empty states K shell L shell M shell K L1L1 L 2,3 K α1 EDS EELS K β1 L α1

KLM K-edge (Si) – 1s orbital Conduction band Filled bands L-edge (Si) – 2s and 2p orbital EoEo EoEo E b (K)=E o -E(cond.band-K)E b (L)=E o -E b (cond.band-L) EELS

MENA 3100: Spectroscopy Gatan Imaging Filter (GIF) Post column energy filter

Electron gun Condenser aperture Sample holderObjective aperture Objective lens Diffraction lens Intermediate aperture Intermediate lens Projector lenses Fluorescent screen Gatan Imaging Filter For EELS Microscope outline

MENA 3100: Spectroscopy

90 o magnetic prism Beam trap aperture Slit Multipole lenses Detector Projector crossover Viewing screen

Low-Loss EELS Core-Loss EELS

Low-Loss EELS

Elastic scattering: Coulomb attraction by nucleus Inelastic scattering: Coulomb repulsion (outer shell electrons) Zero Loss Peak Single electron outer shell excitation

MENA 3100: Spectroscopy The Zero Loss Peak (ZLP)

Low-Loss EELS: Bulk plasmons Plasmon peak h: Planck constant N: n/V : Valence electron density e: Elementary charge m e : Electron mass ε O : Permittivity of free space Outer-shell inelastic scattering involving many atoms of the solid. Collective effect is known as a plasma resonance An oscillation of the valence electron density

Low-Loss EELS: Energy filtering Kundmann M., Introduction to EELS in TEM, EELS course 2005 San Francisco

Low-Loss EELS: Energy filtering Kundmann M., Introduction to EELS in TEM, EELS course 2005 San Francisco

Low-Loss EELS: Energy filtering TEM image Si ITO EFTEM imaging of Si/aSi/ITO (Indium Tin Oxide) stack sample for REC

Low-Loss EELS: Energy filtering EFTEM (16 eV) EFTEM (23 eV) TEM image EFTEM imaging of Si/aSi/ITO (Indium Tin Oxide) stack sample for REC

Low-Loss EELS: Thickness t = thickness λ p = plasmon mean free path I p = Intensity of the plasmon peak I o = Intensity of the zero loss peak

Core-Loss EELS (Energy-Loss Near-Edge Structure)

KLM K-edge (Si) – 1s orbital Conduction band Filled bands L-edge (Si) – 2s and 2p orbital EoEo EoEo E b (K)=E o -E(cond.band-K)E b (L)=E o -E b (cond.band-L)

MENA 3100: Spectroscopy Microanalysis

MENA 3100: Spectroscopy EFTEM:

MENA 3100: Spectroscopy STEM-SIEFTEM-SI Spectral Imaging (SI) B.ChafferB.Chaffer et al. Analytical and Bioanalytical ChemistryAnalytical and Bioanalytical Chemistry (2008) 390, Issue 6, pp Issue 6

MENA 3100: Spectroscopy STEM-SIEFTEM-SI Spectral Imaging (SI)

MENA 3100: Spectroscopy

EDS vs. EELS

MENA 3100: Spectroscopy Applications