Chapter 5 - Characterization Techniques Flashcards
What is the basic principal of characterization techniques of nanomaterials?
Fire a beam at a surface and read the beam that leaves
*beam can be electrons, x-rays, photons, ions, or neutrons
**may be reading different type of beam than incident beam
Which techniques are used to read chemical structure?
XPS
Which techniques are used to read chemical structure?
XPS
Which techniques are used to read physical topography?
SEM/TEM
What is X-ray diffraction (XRD) used for?
To fine:
- phase determination (identify crystalline phases)
- relative composition of mixed phases
- calculate lattice parameters (and examine structural variations under different conditions)
- estimate size of crystalline domain and disorder through size and strain
- structure solution (complete structure refinement of unknown phases)
What are the 3 crystalline materials from long-range to short-range order?
- single crystal (periodic across whole volume)
- polycrystal (periodic across each grain)
- amorphous (not periodic)
What is a unit cell?
Contains a single atom or an arrangement of atoms that makes up a material
It is an infinitely repeating box - must be infinitely stackable using only translation
How is a unit cell described?
With the 3 smallest non-coplanar vectors: a, b, and c
*a is x axis (in/out of page)
*b is y axis (length wise of page)
*c is z axis (up/down of page)
How are projections of a unit cell found?
Start by drawing a plane through where the arrow ends
Their a, b, and c are found in proportion to one another than reduced to common integers and put in square brackets
What is a family of directions?
parallel vectors ([100] and [1^00])+ similar directions ([100], [010])
written in <>
ex. [100], [1^00], [010], [01^0], [001], [001^] = <100>
what is Miller indices?
reciprocal lattice used to describe a plant in a crystal. Any parallel planes have identical Miller indices
How to find Miller indices?
- construct a parallel plan in the unit cell or select an appropriate origin
- determine where plane intercepts axes (if plane is parallel, use inf)
- take reciprocals of intercepts (reciprocal of inf is 0)
- multiply or divide to clean fractions
- enclose values in brackets (hkl) = [1/a, 1/b, 1/c]
FCC exists in which materials?
Pt, Rh, Pd, Ir, Cu, Ag
What is FCC (100)
A 4-fold symmetry
- cut fcc parallel to front surface of unit cell
What is FCC (110)
A 2-fold symmetry
- cut fcc that intersects the x and y axes but not the z-axis
*the atoms in the underlying second layer are to some extent exposed at the surface
What is FCC (111)?
6-fold symmetry (hexagonal)
- cut fcc that intersects the x, y, and z axes at the same value
What is a family of planes?
Miller indices that share the same atomic arrangements.
(100), (1^00), (010), (01^0), (001), (001^) = {100}
*only in cubic systems are directions perpendicular to planes with the same indices
What is the principal that X-Ray diffraction works off of?
The law
Bragg’s Law
x-rays are diffracted off atoms and either constructively or destructively interfere from layers of atoms depending on interplanar spacing (d) and angle of incident beam (theta) and wavelength (lambda)
d is the perpendicular distance between planes
What can X-Ray diffraction study?
phases and material types
- cannot study gases - see nothing
- liquids get broad peaks
- get broad peaks for amorphous
- crystalline is what we can see:
1. single crystal is well measured (no overlap of reflections)
2. polycrystallines can get overlapping peaks
What are the particles/waves with wavelengths equivalent to interatomic distances?
- x-rays
- electrons
- neutrons
*wavelength on order of angstroms
When were x-rays discovered?
1895 by Roentgen
How are x-rays produced for x-ray diffraction?
x-rays are produced when electrons strike a metal target. The electrons are liberated from a heated filament and accelerated by a high voltage towards the metal target
The x-rays are produced when the electrons collide with the atoms and nuclei of the metal target
To produce an x-ray, the electron is desired to hit an electron in the atom’s K shell (nearest shell) and eject that electron (energy is dispersed as an x-ray)
Where do x-rays come from?
- fire beam of electrons at a metal target
- accelerate electrons in a particle accelerator
How does firing a beam of electrons at a metal target produce x-rays?
causes ionization of inner shell electrons, which results in the formation of an ‘electron hole’. The energy difference between shells results in the form of x-rays being released of a specific wavelengths (alpha, the one used is from K shell to L shell)
- commonly uses Cu (with Ka of 1.5418A)
**very inefficient since most energy dissipates as heat (and requires permanent cooling)
How does electron acceleration produce x-rays?
Electrons are accelerated at relativistic velocities in circular orbits. As the velocity approaches the speed of light, they emit electromagnetic radiation in the x-ray region.
*produces a wide range of wavelengths and results in high flux of x-rays
What are the 3 parts of an x-ray diffraction result?
- signal (what you want)
- background (want to remove)
- noise (also want to remove)
What does the peak position, height, area, wide, and shape of an x-ray diffraction tell you?
peak position = d spacing
height = approximate peak intensity
area = crystal structure and phase amount (for a mixture)
shape = crystalline size, defects
width = crystallite size, defects, integral breadth (less dependent on peak profile)
What happens to x-ray diffraction as the NPs size decreases?
The peaks become less distinct (start to overlap into one another more as the broaden)
What are the 2 types of electron microscopy?
- tunnelling electron microscopy (TEM)
- scanning electron microscopy (SEM)
Why is electron microscopy used over light?
electrons can investigate smaller objects
- light down to 0.2 micrometers
- sem 1.5 nm
- tem 0.15 nm
*as the wavelength decreases the do increases, allowing greater minimum resolvable separation
How to electron microscopes work?
An electron is created, it is then condensed, with a series of lens to hit the target
- SEM: detector detects the secondary and backscattered electrons
-TEM: detects the transmitted electrons that penetrated the system
What can electron microscopy find?
- morphology (size, shape, ect.)
- topography
- composition (elemental analysis)
- crystollagraphic
Why do electron microscopes requires a vacuum?
- electrons are inelastically scattered by gas molecules
- don’t want surface oxidation
- want the column to be clean
