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

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

Sample Sample Objective lens Image plane Image plane MENA 3100: TEM

Imaging sample Parallel incoming electron beam Sample Objective lens Diffraction plane (back focal plane) Image plane

Electron source / Filament Thermionic emission: 2. Field emission W or LaB6 Robust Relatively cheap Does not requite ultra high vacuum Cold FEG, ZrO/W or Schottky FEG High brightness Require high vacuum, MENA 3100: TEM

W Advantages: LaB6 advantages: FEG advantages: Rugged and easy to handle High brightness Extremely high brightness Requires only moderat vacuum High total beam current Long life time, more than 1000 h. Good long time stability Long life time (500-1000h) W disadvantages: LaB6 disadvantages: FEG disadvantages: Low brightness Fragile and delicate to handle Very fragile Limited life time (100 h) Requires better vacuum Current instabilities Long time instabilities Ultra high vacuum to remain stable MENA 3100: TEM

Lenses: MENA 3100: TEM

Objective lens (before and after specimen): Condenser lens: Converges the beam to a spot – spot size (beam diameter) and convergence angle Objective lens (before and after specimen):   Image formation (inverts image) and magnification Intermediate lens: Magnifies the initial image that is formed by the objective lens -- Image or diffraction mode The condenser lens current controls this initial spot size and is referred to as the spot size control: determine how the specimen is illuminated with electrons All the lenses below the specimen serve to magnify the image of the specimen The objective lens is the most important lens in the whole microscope. The objective lens forms an inverted initial image, which is subsequently magnified. http://www.matter.org.uk/tem/lenses/simulation_of_condenser_system.htm The first intermediate lens magnifies the initial image that is formed by the objective lens: The lens can be focused on: Initial image formed by the objective lens, or Diffraction pattern formed in the back focal plane of the objective lens. Magnification in the electron microscope can be varied from hundreds to several hundred thousands of times. This is done by varying the strength of the projector and intermediate lenses. Not all lenses will necessarily be used at lower magnifications. Projector lens: Controls the magnification MENA 3100: TEM

Apertures Condenser aperture: Objective aperture: Limits the number of electrons hitting the sample (reducing the intensity), Affecting the diameter of the discs in the convergent electron diffraction pattern. Objective aperture: Allows certain reflections to contribute to the image. Increases the contrast in the image. Bright field imaging (central beam, 000), Dark field imaging (one reflection, g), High resolution Images (several reflections from a zone axis). Selected area aperture: Allows only electrons going through an area on the sample that is limited by the SAD aperture to contribute to the diffraction pattern (SAD pattern). MENA 3100: TEM

Types of contrast: Amplitude Contrast Phase Contrast Mass Diffraction

Amplitude/Diffraction contrast Phase contrast HREM image BF image Objective aperture DF image Why is it called diffraction contrast? The contrast in the images are due to the fact that some regions in the specimen scatter the incident electron beam more than other parts of the specimen. - Where is the objective apperture located? Back focal plane MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Amplitude Contrast Two principal types Mass-density contrast Diffraction contrast -Primary contrast source in amorphous materials -In crystaline materials -Assume incoherent electron scattering (atoms scatter independently) -Coherent electron scattering (atoms does not scatter independently) Both types of contrasts are seen in BF and DF images -Can use any scattered electrons to form DF images showing mass-thickness contrast -Two beam to get strong contrast in both BF and DF images. MENA 3100: TEM

Mass-Density Contrast TEM variables that affect the contrast: -The objective aperture size (large -- bad). -The high tension of the TEM (small -- good) MENA 3100: TEM

Mass-Density Contrast TEM variables that affect the contrast: -The objective aperture size (large -- bad). -The high tension of the TEM (small -- good) MENA 3100: TEM

TEM STEM BF image Objective aperture DF image

Polymers

Thickness fringes Oscillations in I0 or Ig are known as thickness You will only see these fringes when the thickness of the specimen varies locally, otherwise the contrast will be a uniform gray

Bending contours Occur when a particular set of diffracting planes is not parallel everywhere; the planes rock into, and through, the Bragg condition. Remembering Bragg’s law, the (2h 2k 2l) planes diffract strongly when y has increased to 2θB. So we’ll see extra contours because of the higher-order diffraction. As θ increases, the planes rotate through the Bragg condition more quickly (within a small distance Δx) so the bend contours become much narrower for higher order reflections.

Energy Dispersive Spectroscopy M L Lα kα K kβ hν

Energy Dispersive Spectroscopy TEM SEM

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

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. 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 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 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

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 internal fluorescence peak: This is a characteristic fluoresce peak from the Si (or Ge) in the detector dead layer. 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. MENA 3100: Spectroscopy MENA 3100: Spectroscopy

Electron Energy Loss Spectroscopy (EELS) 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

MENA 3100: Spectroscopy MENA 3100: Spectroscopy