Download Scanning Probe Microscopy

Scanning Probe Microscopy
Binnig & Rohrer
Nobel prize 1986
SPM Principles
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All of the techniques are based
upon scanning a probe just above
a surface whilst monitoring some
interaction between the probe and
the surface.
The interaction that is monitored
in:
STM - is the tunnelling current
between a metallic tip and a
conducting substrate which are
in very close proximity but not
actually in physical contact.
AFM - is the van der Waals force
between the tip and the surface;
this may be either the short
range repulsive force (in
contact-mode) or the longer
range attractive force (in noncontact mode).
SPM
•
For the techniques to provide
information on the surface
structure at the atomic level:
the position of the tip with respect
to the surface must be very
accurately controlled (to within
about 0.1 Å) by moving either the
surface or the tip.
It is possible to accurately control
the relative positions of tip and
surface by ensuring good
vibrational isolation of the
microscope and using sensitive
piezoelectric positioning devices.
•
the tip must be very sharp - ideally
terminating in just a single atom at
its closest point of approach to the
surface.
STM
If the tip is biased with respect to
the surface, electrons can tunnel
between the two, provided the
separation of the tip and surface is
sufficiently small - this gives rise to
a tunnelling current.
• The direction of current flow is
determined by the polarity of the
bias.
If the sample is biased -ve with
respect to the tip, then electrons
will flow from the surface to the tip,
whilst if the sample is biased +ve
with respect to the tip, then
electrons will flow from the tip to
the surface.
STM
Quantum mechanical
tunnelling permits
particles to tunnel
through a potential
barrier which they could
not surmount according
to the classical laws of
physics.
The probability of
tunnelling is
exponentiallydependent upon the
distance of separation
between the tip and
surface : the tunnelling
current is therefore a
very sensitive probe of
this separation.
STM Modes
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Imaging of the surface topology
may then be carried out in one of
two ways:
in constant height mode
A plot of the tunnelling current v's
tip position therefore shows a
periodic variation which matches
that of the surface structure.
in constant current mode
Normal way of imaging the
surface is to maintain the current
constant whilst the tip is scanned
across the surface by adjusting
the tip's height above the surface
so that the tunnelling current does
not vary with the lateral tip
position.
The image is then formed by
plotting the tip height (strictly, the
voltage applied to the z-piezo) v's
the lateral tip position.
STM Links & Images
• Scanning Tunneling
Microscopy Tutorial
• IBM STM Gallery
• Veeco Nanotheatre
Nickel (110)
Xenon on Nickel (110)
When you look through the hole in the xenon
atom you see a nickel atom located directly
beneath. Evidently, xenon binds to the on-top
site.
More
Xenon on Nickel (110)
HOPG
AFM
1. Laser
2. Mirror
3. Photodetector
4. Amplifier
5. Register
6. Sample
7. Probe
8. Cantilever
An atomically sharp tip is scanned over a surface with feedback mechanisms to
maintain the tip at a constant force (to obtain height information), or height (to obtain
force information) above the sample. Tips are typically made from Si3N4 or Si, and
extended down from the end of a cantilever. A diode laser is focused onto the back of
the reflective cantilever. As the tip scans the surface of the sample, moving up and
down with the contour of the surface, the laser beam is deflected into a dual element
photodiode which measures the difference in light intensities between the upper and
lower photodetectors, and then converts to voltage.
AFM
• Resolution depends on the tip sharpness
AFM Modes
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In contact AFM electrostatic and/or surface
tension forces from the adsorbed gas layer
pull the scanning tip toward the surface. It
can damage samples and distort image
data. Therefore, contact mode imaging is
heavily influenced by frictional and adhesive
forces compared to non-contact or tapping
mode.
Non-contact imaging generally provides low
resolution and can also be hampered by the
contaminant layer which can interfere with
oscillation.
TappingMode AFM was developed as a
method to achieve high resolution without
inducing destructive frictional forces both in
air and fluid. With the TappingMode
technique, the very soft and fragile samples
can be imaged successfully. Also,
incorporated with Phase Imaging, the
tapping mode AFM can be used to analyze
the components of the membrane.
Images
• Phase imaging of a
polymer blend
AFM Images
AFM image of short DNA fragment with RNA
polymerase molecule bound to transcription
recognition site. 238nm scan size. Courtesy of
Bustamante Lab, Chemistry Department, University of
Oregon, Eugene OR
Images
• 10 µm scan Tapping Mode image of a CD
(left) and a DVD (right) surface disc.