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Prelims > Characterization > Flashcards

Characterization Flashcards

1
Q

What are the three methods of image formation?

A

Projection Image, Optical Image, Scanning Image

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2
Q

What is resolution and what factors impact it?

A

Resolution is the closest spacing between 2 points which can be seen through the microscope and separated completely. The higher the resolution, the smaller the r value in the Rayleigh criterion for resolution

r = 0.61 lambda / mu sin alpha
lambda = wavelength
mu = permeability
alpha = aperature

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3
Q

What is the depth of field?

A

range of positions for object h, where eye can’t detect change of sharpness in image.
h = r / tan(alpha)

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4
Q

How is the depth of field different in optical and scanning electron microscopes?

A

In SEM, the aperature is very small, so the depth of field is larger. In OM, the aperture is around 45 degrees, so the depth of field and resolution are similar

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5
Q

What are the three possible aberrations in optical microscopes?

A

Spherical aberration (lens field acting non-homogeneously on the off-axis rays)

Chromatic aberration - light deviates by amount depending on its wavelength

Astigmatism - rays travelling on different planes travel different distances

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6
Q

What are the two possible electron sources for EM and how do they work?

A

Thermoionic sources - supply heat so electrons overcome work function energy, usually heat up W or LaB6

Field Emission - electric fields are stronger at sharp tips, so the potential barrier reduces in height and becomes narrower, allowing electrons to tunnel through without thermal energy (Schottky effect). Needs ultra-high vacuum, but much brighter and can operate at lower temperatures

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7
Q

What is the difference between elastic, inelastic, coherent, and incoherent electron scattering?

A

Elastic - no loss in energy
Inelastic - some measurable loss in energy
Coherent - photons share frequency and wavelength, usually elastic
Incoherent - photons without same frequency and different wavelengths, usually inelastic

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8
Q

What is Rutherford Scattering? (Elastic Scattering)

A

electron that is travelling changes direction without changing energy due to coulomb interactions. Small angle scattering is much more probable.

Lower wavelengths and higher elements will scatter elastically more

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9
Q

How does electron diffraction work?

A

Atoms in crystal structure form 0th, 1st, and 2nd order waves due to constructive interference at different atoms

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10
Q

What are two main things that happen when electron inelastically scatters with an atom?

A
  1. Generates X-rays
  2. Generates Secondary Electrons
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11
Q

How are characteristic X-rays generated?

A

Electron beam knocks out K-shell electron, which is filled by a higher energy electron. While the electron transitions in energy, the energy is released via a characteristic X-ray.

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12
Q

What are the Laporte Rules (selection rules)?

A

s-s, p-p, d-d, f-f transitions are forbidden.

1s = K
2s = LI
2p = LII, LIII

Hence, LI to K transitions are forbidden

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13
Q

What is the critical ionization energy?

A

Energy of localized electron (binding energy) or absorption edge energy (Ec)

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14
Q

What are Bremsstrahlung X-rays?

A

Electrons that develerate by charge of nucleus emit X-rays, can only have energy up to E0 (electron beam energy).

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15
Q

What are secondary electrons and what is the difference between “slow secondary electrons” and “fast secondary electrons”?

A

Secondary electrons are electrons in the sample that are ejected by electrons of the beam.

Slow SEs are the electrons in conduction or valence bands, and it doesn’t take much energy to eject them (<50eV). These are usually free electrons and don’t give elemental information. Only escapes near specimen surface

Fast SEs are strongly bound inner electrons that are less readily ejected (~50% E0). They are able to escape from deeper areas in specimens since they are higher energy. Degrades spatial resolution - do not want them!!!

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16
Q

What are Auger electrons?

A

Outer shell electron that is ejected by a characteristic X-ray passing through. Favored in atoms with small binding energies, able to escape close to the surface. Can be used to analyze lighter elements

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17
Q

What is cathodoluminescence in EM?

A

Electron hits valence band electron that excites it over the bandgap to the conduction band. Electron relaxes back and emits with energy of band gap. Can be used to used to determine bandgaps

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18
Q

What are plasmons?

A

Collective oscillation of free electrons that occur when electron beam passes through “sea” of electrons. Creates regions of electron density, most common inelastic interactions, especially in metals

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19
Q

What are phonons?

A

Crystal lattice atoms vibrate collectively as specimen heating.

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20
Q

What is interaction volume and what variables impact it?

A

Electrons either stop in the specimen or leave the surface. Interaction volume is the volume within which 95% of primary electrons are brought to a rest.

Area analyzed&raquo_space; size of beam!!

Determined by Kayana-Okayama Equation

Larger interaction volume = larger atomic weight, larger beam energy, lower density, lower atomic number. Larger tilt angle, smaller interaction volume

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21
Q

What are the three major parameters of an SEM beam?

A

Probe diameter / spot size (1nm to 1 micron)
Probe current (pA to nA)
Convergence angle (0.001 to 0.1 rads)

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22
Q

What are backscattered electrons?

A

Multiple high-energy elastic scattering out of the sample

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23
Q

What kind of informations do BSEs tell us?

A

They are not dependent on applied voltage, and have higher yields with higher atomic numbers

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24
Q

Rank resolution of BSE, SE, and X-ray

A

SE > BSE > X-ray

SEs only come from surface
BSEs can come from deeper in the material
X-rays can come from very deep in the material

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25
Q

What are the energy distributions of SEs and BSEs?

A

SEs are less than 50 eV, BSEs have energy levels between 50 eV and the beam energy itself (E0)

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26
Q

How does sample tilt impact signal?

A

Larger tilts will appear brighter

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27
Q

How can we do chemical analysis with the SEM?

A

Use characteristic X-rays of a small portion of a large sample, non-destructive. Find which element is present & quantity of the element. Use X-ray EDS (electron dispersive spectroscopy)

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28
Q

How can we do chemical analysis in the TEM?

A

Do EELS or electron energy loss spectroscopy. When electron knocks out shell electrons, can measure the energy lost to identify elements

29
Q

Why are different peaks (K-alpha vs. K-beta) different intensities?

A

Different probabilities

30
Q

What range are X-rays absorbed by samples? What happens when the X-ray is absorbed?

A

Very low energies (0-5 keV), and 5keV above the photon binding energy. When the photon is absorbed, the inner electron is excited and then returns to ground state (fluorescence). This leads to either a characteristic X-ray or an Auger electron

31
Q

What are the two main points of Reciprocal Lattices?

A

Reciprocal lattice is a method that simplifies many planes in a unit cell into a grid
1. Reciprocal lattice vector is perpendicular to the hkl plane (direction of RL)
2. Interplanar distance is the reciprocal of the vector magnitude

32
Q

What is Laue Diffraction?

A

Use multiple wavelengths to cover a large ranger range of diffraction spots

33
Q

What is the structure factor?

A

Tells us the relative intensity of diffractive beam compared to the intensity beam. Related to scattering amplitude of a specific element, atomic coordinates of unit cell, and specific plane we are looking at.

34
Q

Structure Factor rules of BCC and FCC?

A

BCC: h+k+l = odd, F = 0; h+k+l=even, F = 2f
FCC: hkl mixed, F = 0; hkl unmixed, F = 4f

35
Q

What does kinematic theory tell us?

A

The bragg condition does not need to be perfectly satisfied. The thinner the cristal, the large possible deviation from the Bragg condition to still create a spot.

36
Q

If two materials are thinly grown on top of each other, what happens to its diffraction pattern?

A

Double diffraction. Diffracted beam from one crystal diffract the other. Creates extra reflections

37
Q

Where do Kikuchi Lines come from?

A

Electrons being inelastically scattered, losing energy, then elastically scattering. Larger thickness means more Kikuchi lines

38
Q

What are the three contrast mechanisms in TEM?

A

Thicker specimens and higher atomic numbers scatter more strongly, so will appear darker.

Diffraction of specimen will also give contrast

39
Q

What are the three different X-ray sources / monochromatic or not?

A
  1. X-ray Tube - Polychromatic background radiation, but can isolate for mostly monochromatic
  2. Syncatron radiation - electron experiences acceleration that creates radiation, can be both monochromatic and polychromatic
  3. Radioactive source - monochromatic
40
Q

How does an X-ray tube work?

A

Tungsten filament heated with current that emits electrons to hit negative voltage Cu target, which then emits X-rays through a Be window.

41
Q

In single electron scattering, what is the difference between coherent and incoherent scattering?

A

Coherent scattering is the classical Thompson Scattering, where X-ray waves accelerate electrons, which oscillates in phase with the X-ray. That emits electromagnetic radiation in all directions with the same wavelength.

Incoherent scattering is the Compton effect, in which a photon hits an electron, loses some energy, and is no longer in phase. This is more prevalent in lighter atoms

42
Q

What are factors that impact intensity in XRD?

A

Multiplicity Factor, Lorentz Factor, Crystallite size factor, temperature factor

43
Q

What is Bragg’s Law?

A

n\lambda = 2dsintheta
n relates to the higher order planes (111, 222, etc.)

44
Q

How do we create a monochromatic wavelength?

A

X-rays can be absorbed by materials depending on their Z. If we want to get only the K-alpha signal in a Cu source, then we can use a Ni filter which absorbs the K_beta wavelength. X-ray absorption is from the photoelectric effect, where an electron absorbs the X-ray and ejects an electron. Asbroption is highest at energies where this starts to happen

45
Q

What is the structure factor and what terms are relevant?

A

How the unit cell scatters X-rays. Depends on atomic scattering (amplitude of scattering from all electrons in single atom element versus amplitude of scattering from a single electron) and hkl plane and uvw atom coordinates.

46
Q

What are the different ways that X-rays can scatter?

A

Incident beam hits sample.

Get:
- transmitted beam
- fluorescent X-rays
- electrons
- compton recoil electrons
- Auger electrons
- photoeletrons
- Scattered X-rays
- unmodified (coherent)
- Compton modified (incoherent)

47
Q

What is Moseley’s Law?

A

square root of frequency of K or L lines is nearly proportional to Z of the excited element

sqrt(v) = a(Z-b)

48
Q

What are characteristic X-rays?

A

Characteristic X-rays are emitted by atoms when the inner-shell electrons are excited and subsequently drop down to fill the vacancies created in higher energy levels. These X-rays have energies characteristic of the elements involved and are unique to each element, making them useful for elemental identification and analysis. The energy of characteristic X-rays is determined by the energy difference between the initial and final electronic states involved in the transition. This energy is specific to each element, allowing for the identification of elements present in a sample through X-ray spectroscopic techniques such as X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS) in electron microscopy.

49
Q

Energy Dispersive Spectroscopy (EDS)

A

A technique used in conjunction with electron microscopy to analyze the elemental composition of materials. It detects characteristic X-rays emitted by a sample when it is bombarded with electrons, allowing for the identification and quantification of elements present.

50
Q

Wavelength Dispersive Spectroscopy (WDS)

A

A technique employed in electron microscopy for elemental analysis. Unlike EDS, which detects X-rays based on energy, WDS separates X-rays based on their wavelengths, allowing for more precise analysis of elemental composition.

51
Q

Atomic Force Microscopy (AFM)

A

A type of microscopy that uses a sharp probe to scan the surface of a sample. It measures the forces between the probe and the sample surface, generating high-resolution images and providing information about surface topography and mechanical properties at the nanoscale.

52
Q

Auger Electron Spectroscopy (AES)

A

A surface analysis technique that detects the energy of Auger electrons emitted by atoms when they undergo a transition following the removal of a core electron. It provides information about the elemental composition and chemical state of the surface layer of a material.

53
Q

X-ray Photoelectron Spectroscopy (XPS)

A

Also known as Electron Spectroscopy for Chemical Analysis (ESCA), XPS is a technique used to analyze the elemental composition and chemical state of the outermost atomic layers of a material. It measures the kinetic energy of photoelectrons emitted when a sample is irradiated with X-rays.

54
Q

Secondary Ion Mass Spectrometry (SIMS)

A

A surface analysis technique that bombards a sample with a primary ion beam, causing the ejection of secondary ions. The mass-to-charge ratios of these ions are then measured to determine the elemental and isotopic composition of the sample surface.

55
Q

X-ray fluorescence (XRF)

A

A non-destructive analytical technique used for elemental analysis. It involves irradiating a sample with X-rays, which cause the emission of characteristic fluorescent X-rays from the sample. The energy and intensity of these X-rays are used to identify and quantify elements present.

56
Q

Fourier Transform Infrared Spectroscopy (FTIR)

A

A spectroscopic technique that measures the absorption, emission, or reflection of infrared light by a sample. It provides information about the molecular structure, functional groups, and chemical bonds present in the sample.

57
Q

Raman Spectroscopy

A

A spectroscopic technique based on the inelastic scattering of monochromatic light (usually laser light) by a sample. It provides information about molecular vibrations, crystal structure, and chemical bonding within a material.

58
Q

Low Energy Electron Microscopy (LEEM)

A

A type of electron microscopy that uses low-energy electrons to image the surface of a sample with high spatial resolution. It is particularly useful for studying surface morphology and dynamics at the nanoscale.

59
Q

Low Energy Electron Diffraction (LEED)

A

A technique used to study the surface structure of crystalline materials. It involves directing a beam of low-energy electrons at a sample and analyzing the diffraction pattern produced, which provides information about surface crystallography.

60
Q

Glancing Angle X-ray Diffraction (GAXRD)

A

A variation of X-ray diffraction where the incident X-rays are directed at a shallow angle relative to the sample surface. This technique is often used to study thin films, surface layers, or interfaces.

61
Q

Thermal Mechanical Analysis (TMA)

A

A technique used to study the mechanical properties of materials as a function of temperature. It measures dimensional changes in a sample subjected to controlled heating or cooling, providing information about thermal expansion, softening, or phase transitions.

62
Q

Dynamic Mechanical Analysis (DMA)

A

A technique used to characterize the mechanical properties of materials as a function of temperature, frequency, or time under dynamic conditions. It measures the response of a sample to an oscillating force or deformation, providing information about viscoelastic behavior, stiffness, and damping.

63
Q

Differential Scanning Calorimetry (DSC)

A

A thermal analysis technique used to study the heat flow associated with phase transitions, chemical reactions, and other thermal events in materials. It measures the temperature difference between a sample and a reference material as they are subjected to controlled heating or cooling.

64
Q

What is the multiplicity factor?

A

Intensity of reflection is proportional to number of planes in a given family of planes.

Eg: {100} has 6 planes
{110} has 12 planes
{111} has 3 planes

65
Q

What is the Lorentz Factor?

A

Total energy of diffracted beam is the integrated intensity, and it depends on geometric factors and number of reflecting crystals. The higher the bragg-angle, the higher the deviation from the 2theta condition is possible. As the angle increases, the peak height and intensity decreases. As a result, peak heights taper off

66
Q

Crystallite Size Factor and Scherrer’s Equation

A

size of the crystal impacts the width of the peak
t = alpha*lambda/Bcostheta

t = average crystallite size or grain size
alpha = shape factor (dimensionless)
B = FWHM of diffraction peak
theta = bragg angles

decrease t, increase FWHM

more planes = destructive interference happens more readily

67
Q

Temperature Factor?

A

Increase T, increases unit cell size, increases plane spacing, so shifts to lower theta values (lambda = 2dcostheta)
intensity of the lines increases because of atomic vibrations at increased theta
more background scattering present

68
Q

What is Laue X-ray Diffraction?

A

Laue X-ray diffraction is a technique used to determine the crystal structure of single crystals.
It involves exposing a single crystal to a polychromatic (white) X-ray beam at various orientations.
The resulting diffraction pattern reveals information about the crystal structure, including lattice parameters, crystal symmetry, and orientation relationships between different crystal planes.
Laue X-ray diffraction is particularly useful for rapid crystal orientation determination and can be applied to both small and large single crystals.

69
Q

Rotating Crystal Method

A

The rotating crystal method is another technique used for single crystal X-ray diffraction analysis.
It involves rotating a single crystal specimen while exposing it to a monochromatic X-ray beam.
By rotating the crystal to different orientations, diffraction patterns are recorded at various crystallographic angles.
The resulting diffraction data can be used to determine the crystal structure, lattice parameters, and orientation relationships between different crystal planes.
The rotating crystal method offers high accuracy and precision in determining crystal structures, making it a valuable tool in crystallography and materials science research.

Prelims (5 decks)

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