Week 9 Flashcards
(35 cards)
Briefly describe the SEM.
- Beating light diffraction with electrons to go sub-nm
- Electron-matter interactions
extracting information from matter - Imaging with SEM maximises resolution
What is equation for minimum feature resolvable by microscope?
2d = lambda/ n sintheta
What is an Elastic interaction?
No energy is transferred to the sample
What does the scattering angle tell you about the sample?
The scattering angle
increases with
the atomic number Z -
information about
composition and thickness
of the sample
What is Elastic Interactions?
Some energy is transferred to the sample.
What are examples of inelastic interactions?
Cathodoluminescence-Plasmons
X-Rays -Composition
How are x-rays used in SEM?
Measuring the wavelength of the X-rays, provides precise information about the material constituents
How is the image formed in SEM?
The electron beam goes thorough scanning coils and hits the sample on a scan raster. The detector detects the backscattered electrons and the signal goes to the amplifier and is displayed on a monitor.
How does an oblique edge correspond to the image brightness?
Volume from which
secondary electrons can escape is larger -
Edges appear brighter.
What is cathodoluminescence microscopy in terms on excitation?
Optical modes
can be excited with SEM - energy from the beam is deposited in the
electron bath (or e-h pairs in semiconductors).
Excites optical modes -decay into
photons which are detected (or creates e-h
pairs which decay into photons).
What is cathodoluminescence limited by?
Limited by the lateral size of the electron
beam (approximately 5 – 10 nm).
How does cathodoluminescence work?
*Electron beam scanned over the sample through a hole in a parabolic reflector.
*Emitted light imaged on the detector with a spectrum recorded for each position of the electron beam.
How is the emitted light intensity related to the EM field intensity at a given point?
Emitted light intensity at a given wavelength is proportional to the local density of photonic states (LDOS).
What do secondary electrons tell us?
Provide information about
the topography - with the signal
enhanced at edges/discontinuities.
What do back scatter electrons tell us?
Sensitive to the atomic
number - provide compositional contrast in images.
What do X-rays tell us?
Identify atomic
species based on their quantised electron energy levels.
How does AFM work?
*Scanning a sharp probe over the surface of a sample in a raster pattern.
- By monitoring the movement of the probe, a 3-D image of
the surface can be
constructed.
How does the cantilever mechanism work?
Cantilever comprised of silicon
nitride or silicon, undergoes deflection.
Deflects a laser which whose position is measured using a quadrant
photo-diode.
What are VDW?
*Longer
range attractive forces, which may be felt at
separations of up to 10 nm or more. They arise
due to temporary fluctuating dipoles.
What are the tip-sample forces in AFM?
*Van der Waals interactions
- Coulombic Interactions
What is the Coulombic interaction?
This strong, short
range repulsive force arises from electrostatic
repulsion by the electron clouds of the tip and
sample. This repulsion increases as the
separation decreases.
As the tip is moved, what are the forces?
*Tip close to sample - VDW cause attraction, increasing
closer to the surface.
*At small separations the repulsive
coulombic forces become dominant.
Describe instrumentation/probe issues? How would you minimise these issues?
*Tip may pick up loose debris from the
sample surface.
Reduced by cleaning the sample with compressed air or
N2 before use.
*Tip can be damaged
during scanning, which degrades the images.
What is an application of AFM beyond surface imaging?
Pizeoresponse
