Lecture 6: Microscopy II
PHYS 430/603 material
Laszlo Takacs
UMBC Department of Physics
Useful sites:
Light microscopy
The principle and a
commercial scope
http://www.microscopyu.com/articles/optics/components.html
http://em-outreach.ucsd.edu/web-course/toccontents.html
Optical microscope (OM) and transmission electron microscope (TEM)
TEM modes:
Composite image, bright field image, dark field image, selected area diffraction
First image
Bright field
Transmitted and diffracted beams
Diffraction pattern
Bright field, transmitted electrons only
Dark field
Selected area diffraction pattern
Comparing
imaging and
diffraction in
a TEM.
Electron diffraction from a
monocrystal
polycrystal
glass
How do scanning microscopies work?
Measurement generates a value for every
location:
• Reflectivity of light (scanner)
0
2
1
2
2
1
1
1
• Ejected electrons (SEM)
1
9
7
2
1
8
7
0
• Current between tip and surface (STM)
2
8
6
1
2
9
8
1
0
8
8
1
8
7
0
1
1
9
7
9
6
0
1
1
1
7
8
8
7
1
1
0
2
9
8
8
6
1
2
1
1
8
9
2
9
7
1
1
0
8
7
1
1
8
3
1
2
1
1
0
2
1
0
1
Image = table of numbers
• Force between tip and surface (AFM)
• Any quantity of interest
Interpret numbers as intensities
for display or printer.
Digital image processing.
The principle of STM
What exactly is measured?
Tunneling current ~
probability of an electron
tunneling out from the
surface ~
electron density.
Largest where the atoms
are, thus we “see” the
atoms - kind of.
48 Fe atoms forming a “quantum collar” on a Cu (1 1 1)
surface. The inner rings are not from atoms at the location.
STM of Si (1 1 1) 7x7 reconstructed surface
Surface structure
•
•
•
Relaxation: change of distance between the first few lattice planes. Typically,
~10% change in the first, ~1% in the second distance.
Reconstruction: change of the arrangement/ symmetry of the arrangement of
atoms.
Reactive, usually picks up other atoms, e.g. O.
Notice that smaller circles
represent atoms farther from
the top layer.
Low Energy Electron Diffraction
Cu (1 0 0)
ZnO (1 0 -1 0)
Typical surface features after cleaving close to a lattice
plane or building up a crystal from the gas phase
FeSi2 on a Si (111) surface
Notice:
• Reconstruction of Si
• Relationship between the
orientations of substrate and
coating
• Steps and kinks in the FeSi2
layers
Contact-mode AFM: It is a miniature profilometer, directly measures
height. Best for larger-scale (at least a few nm) features.
The principle of field ion microscopy
Images taken by field ion microscope
Ir (1 1 0)
Pt (0 1 2)
E. W. Müller, Platinum Metal Rev. 9 (1965) 84-89
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Vacancies
Dislocations
on platinum tips