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

Repetision: Electron Diffraction: Powder X-ray diffraction: Small wave length Large Ewald sphere -- Perfect crystals plane Very sensitive to changes in the crystal structure Small diffraction Strong intensity – short exposure time Directly observed on the viewing screen Obtained from very small crystals  Larger wave length Small Ewald sphere Ring pattern Larger driffrationangle Lower intensity Easier to interpret

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

The characteristic energy transitions Observable with EELS and EDS MENA 3100: Spectroscopy MENA 3100: Spectroscopy

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

1s 2s 2p 3s 3p 3d Empty states K shell L shell M shell K L1 L2,3 Kα1 Lα1 EELS Kβ1 EDS MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Energy dispersive X-ray spectroscopy M L Lα kα K kβ hν MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EELS Eo Eo K-edge (Si) – 1s orbital M L K L-edge (Si) – 2s and 2p orbital Filled bands Conduction band Eb(L)=Eo-Eb(cond.band-L) Eb(K)=Eo-E(cond.band-K)

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

EDS spectrum MENA 3100: Spectroscopy 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 MENA 3100: Spectroscopy

Quantification 𝐶 𝐴 𝐶 𝐵 = 𝐾 𝐴𝐵 𝐼 𝐴 𝐼 𝐵 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: 𝐶 𝐴 𝐶 𝐵 = 𝐾 𝐴𝐵 𝐼 𝐴 𝐼 𝐵 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 MENA 3100: Spectroscopy

The Detector MENA 3100: Spectroscopy MENA 3100: Spectroscopy Crystal (Si(Li)): Absorbs the energy of incoming x-rays by ionization, yielding free electrons in the crystal that become conductive and produce an electrical charge bias. The x-ray absorption thus converts the energy of individual x-rays into electrical voltages of proportional size; the electrical pulses correspond to the characteristic x-rays of the element. In older detector designs, the collection electrode is centrally located with an external FET (field effect transistor) to convert the current into a voltage and thus represents the first stage of amplification. Newer designs integrate the FET directly into the chip, which greatly improves energy resolution and throughput. This is due to the reduction of capacitance between anode and FET, which reduces electronic noise. MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Detection Mechanism MENA 3100: Spectroscopy MENA 3100: Spectroscopy When X-rays deposit energy in a semiconductor, electrons are transferred from the valence band to the conduction band, creating electron-hole pairs, as we saw back in Section 4.4. The energy required for this transfer in Si is 3.8 eV at liquid-N2 temperature. (This energy is a statistical quantity, so don’t try to link it directly to the band gap.) Since characteristic X-rays typically have energies well above 1 keV, 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 MENA 3100: Spectroscopy

MENA 3100: Spectroscopy MENA 3100: Spectroscopy The silicon drift detector (SDD) is fabricated from high-purity silicon with a large area contact on the entrance side. On the opposite side is a central small anode contact, which is surrounded by a number of concentric drift electrodes. When a bias is applied to the SDD detector chip and the detector is exposed to X-rays, it converts each X-ray detected to an electron cloud with a charge that is proportional to the characteristic energy of that X-ray. These electrons are then “drifted” down a field gradient applied between the drift rings and are collected at the anode Oxford MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EDS SEM TEM MENA 3100: Spectroscopy MENA 3100: Spectroscopy The weight percents of each element CA and CB can then be related to IA and IB thus K is the cliff-lorimer factor MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EDS mapping MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Recent advances MENA 3100: Spectroscopy MENA 3100: Spectroscopy SDD: 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 (Figure 32.8A–C). Applying a voltage from inside to outside the detector (rather than front to back as in a Si(Li)) permits collection of the electrons generated in the n-Si with a 4 lower voltage. Because the central anode in the middle of the p-doped rings has a much smaller capacitance than the large anode at the rear face of a Si(Li) detector (Figure 32.3A–C), a very high throughput of counts is possible, peaking at output rates of many hundred of kcps. MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Comparison Low Z element MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Comparison on resolution MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Artefacts in EDS Si escape peak: A small fraction of the energy is lost and not transformed into electronhole pairs 2. Sum peak: Two photons will enter the detector at exactly the same time. The analyzer then registers an energy corresponding to the sum of the two photons. Likely to occur if: - The input count rate is high. - The dead times are > 60%. - There are major characteristic peaks in the spectrum. Because the detector is not a perfect sink for all the X-ray energy, it is possible that a small fraction of the energy is lost and not transformed into electronhole 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 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. MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Thickness variations due to milling Contaminants and reaction products 3. Fluorescence: This is a characteristic peak from the Si (or Ge) in the detector dead layer. Sample preparation artefacts (ion milling , grids, reaction to solvent) Cu/Ni slot Thickness variations due to milling Contaminants and reaction products 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. As semiconductor detector design has improved and dead layers have decreased in thickness, the internal fluorescence peak artifact has shrunk but it has not yet disappeared entirely. So beware MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Electron Energy Loss Spectroscopy (EELS) MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Omega filter MENA 3100: Spectroscopy MENA 3100: Spectroscopy The filter is placed in the TEM column between the intermediate and projector lenses and consists of a set of magnetic prisms arranged in an O shape which disperses the electrons off axis, MENA 3100: Spectroscopy MENA 3100: Spectroscopy

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

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

MENA 3100: Spectroscopy MENA 3100: Spectroscopy

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

Energy Losses Zero Loss (includes quasi-elastic scattering) Intra-/Inter-band transitions (band gap) Cherenkov losses Bremsstrahlung Plasmon losses Core losses MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EEL Spectral background MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Low-Loss EELS Core-Loss EELS

Low-Loss EELS

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

The Zero Loss Peak (ZLP) MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Low-Loss EELS: Bulk plasmons 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 𝐸 𝑝 = ℎ 2𝜋 𝑁 𝑒 2 𝑚 𝑒 𝜀 0 h: Planck constant N: n/V : Valence electron density e: Elementary charge me: Electron mass εO: Permittivity of free space Plasmon peak

Low-Loss EELS: Surface plasmons Surface plasmon (Es): Vacuum/metal interface: Dielectric/metal boundary: Interface between two metals: 𝐸 𝑠 = 𝐸 𝑝 2 ZLP 𝐸 𝑠 = 𝐸 𝑝 1−ε 𝐸 𝑠 = 𝐸 𝑎 2 + 𝐸 𝑏 2 2 A. Thøgersen,et al.  Journal of Applied Physics 109, 084329 (2011). 

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

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

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

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

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

Eo Eo K-edge (Si) – 1s orbital M L K L-edge (Si) – 2s and 2p orbital Filled bands Conduction band Eb(L)=Eo-Eb(cond.band-L) Eb(K)=Eo-E(cond.band-K)

Core-Loss EELS: Peak shape Shape of the edge is a signature of the transition: K-edges: 1s states -- typical sawtooth profile L2,3-edges -- have a delayed maximum but can contain intense narrow peaks at the onset, known as “white lines”, corresponding to transitions to narrow d bands.

MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Microanalysis MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EFTEM: MENA 3100: Spectroscopy MENA 3100: Spectroscopy

MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Spectral Imaging (SI) STEM-SI EFTEM-SI MENA 3100: Spectroscopy B.Chaffer et al. Analytical and Bioanalytical Chemistry (2008) 390, Issue 6, pp 1439-1445 MENA 3100: Spectroscopy MENA 3100: Spectroscopy

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

MENA 3100: Spectroscopy MENA 3100: Spectroscopy

MENA 3100: Spectroscopy MENA 3100: Spectroscopy

EDS vs. EELS MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Applications MENA 3100: Spectroscopy MENA 3100: Spectroscopy