Download SEM, TEM, Bragg Law

Bragg’s Law &
SEM/TEM
By Nathan Brummel
Objectives
• Bragg’s Law
• X-ray Emission
• Powder Diffraction
• Diffractometer
• The Electron Microscope
Bragg’s Law
In 1913 William Lawrence
Bragg & William Henry
Bragg came up with
Bragg’s Law
Nobel Prize in physics in
1914
Bragg’s Law is simple &
Elegant
Confirmed the existence
of real particles at the
atomic scale
Sir William Bragg
William Henry Bragg
nλ = 2dsinθ
nλ = 2dsinθ
X-ray Emission
The X-ray Tube
• Breaking radiation “Bremsstrahlung” is produced from
the decelerating incident-beam particle
• Results in the production of x-rays
Classification of
Materials
• Incoming x-ray is sufficiently large • Characteristic x-rays can be discharged
• Leads to characteristic peaks
• Peaks are from inter-orbital transitions
within the target atom as the electron jumps
from a higher energy band to a lower energy
band to fill vacancy left due to the
characteristic x-ray that is emitted
Powder Diffraction
X-ray Diffraction
The Problem & Solution
• Problem:
• Polychromatic radiation is perfect for
mono-crystalline structures
• Solution:
• Monochromatic radiation
Debye & Scherrer/Hall
•
Diffraction is inevitable in
powder
•
•
orientations present
chance for constructive
interference is
maximized
Diffractometer
High precision
Can determine lattice parameter Small ∆es in composition can be detected
The Electron Microscope
•For complex/smaller dimension materials
•Much higher potential •Higher energy
•Lower Vacuum
•SEM
•Scanning Electron Microsope
•Versatility by way of electronic circuitry
•TEM
•Transsmission Electron Microscope
•Enhanced for high spatial resolution
SEM
• High voltage source is used to accelerate
electrons • Electromagnetic lenses are used to focus
the electrons
• An aperture adjust focal length
• Vacuum is required
• Bulk materals can be used
•Backscattering at high angles
•Produces topographical
contrast and undulation in the
surface
•Efficiently differentiate between
regions of high and low atomic
concentration
•Bad for amorphous samples
TEM
TEM
• Similar to SEM
• Needs higher vacuum and voltage potential
• Needs extremely thin samples
• a few atoms think
Lens System
• System consists of anywhere from
100 to 1000 lenses
• They are coupled into groups
• Objective lens
•high spatial resolution images
and diffraction
•Condenser lens controls
illumination and magnification
•Causes electrons to travel in a
helical trajector
Modes
• Image mode
• focusing on the front plane producing a
magnified image
• Diffractino mode
• focusing on the back focal plane
• diffraction pattern that correctly
corresponds to a family of
crystallographic planes within the sample
Bright Field vs. Dark Field
Preparation of Samples
• Slice very thin ( mm in thickness)
• Place into a micro lathe to file down the
center point, at which, it is almost trasparent
• Place into an ion mill until a hole starts to
appear
• Area around hole is thin enough for use in a
TEM
Conclusion
• Bragg’s Law
• X-ray Emission
• Powder Diffraction
• Diffractometer
• The Electron Microscope