Atomic Force Microscopy

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Atomic Force Microscopy

• Lecture 7 Outline 1. Introduction to Atomic
  Forces2. AFM Modes of operation 3. Case
  study 11 Information from AFM

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• Atomic Force Microscopy
• Both STM and atomic force microscopy (AFM) are
  part of the scanning probe microscope family.
• STM uses the electronic properties between the
  tip and the sample.
• AFM uses Forces between the sample and a tip on
  the end of a cantilever. These forces change as
  the tip gets closer to the sample.
• So what are these forces???

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Atomic Forces
Force versus distance
Force
Repulsive force
Tip-to-sample separation d
1/d8
1/d7
attractive force
Fig. 7.1 Force - distance curve
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• Let us analyze what is going on in this curve
• 1. As the atoms are gradually brought together,
  they first weakly attract each other (1/d8).
• 2. Attraction increases (1/d7) until the atoms
  are so close together that their electron clouds
  begin to repel each other electrostatically.

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• 3 The force goes to zero when the distance
  between the atoms reaches a couple of angstroms,
• about the length of a chemical bond.
• When the total van der Waals force becomes
  positive (repulsive), the atoms are in contact.

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4. The slope of the Force curve is very steep in
the repulsive or contact regime. ? As a
result, the repulsive force balances almost any
force that attempts to push the atoms closer
together. ? In AFM this means that when the
cantilever pushes the tip against the sample, the
cantilever bends rather than forcing the tip
atoms closer to the sample atoms.
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Fig. 7.2 AFM cantilever
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• In practice
• An atomically sharp tip is scanned over a surface
  with feedback mechanisms that enable the
  piezoelectric scanners to maintain the tip at
  either
• (i) a constant force (to obtain height
  information),
• (ii) or height (to obtain force information)
  above the sample surface.
• Tips are typically made from Si3N4 or Si, and
  extended down from the end of a cantilever.

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• Modes of operation. There are 3 modes of AFM
  operation
• Contact mode
• Non-contact mode
• Tapping mode

Contact mode
Non-contact mode
Tapping mode
Fig. 7.3 Modes
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• Contact mode
• The tip is moved over the surface by the
  scanning system.
• A value of the cantilever deflection, for
  example, is selected and then the feedback system
  adjusts the height of the cantilever base to keep
  this deflection constant as the tip moves over
  the surface.

• Non-contact mode
• The cantilever oscillates close to the sample
  surface, but without making contact with the
  surface.
• The capillary force makes this particularly
  difficult to control in ambient conditions. Very
  stiff cantilevers are needed.

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• Tapping mode
• The cantilever oscillates and the tip makes
  repulsive contact with the surface of the sample
  at the lowest point of the oscillation.
• Tapping mode is useful for imaging soft samples
  such as biology or polymers.
• Oscillation ? resonance condition important

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• The cantilever is usually driven close to a
  resonance of the system.
• The phase of the cantilever oscillation can
  give information about the sample properties,
  such as stiffness and mechanical information or
  adhesion.
• Resonant frequency of the cantilever depends on
  its mass and spring constant stiffer cantilevers
  have higher resonant frequencies.

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Resonance frequency
Phase Amplitude
Frequency
Question Where would you want to operate system
at??? At resonance?
Fig. 7.4
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Cantilever spring constant k
Cantilever deflection s
s
F k s
Force F
Fig. 7.5
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• Used to measure long range attractive or
  repulsive forces between the probe tip and the
  sample surface.
• Force curves( force-versus-distance curve)
  typically show the deflection of the free end of
  the AFM cantilever as the fixed end of the
  cantilever is brought vertically towards and then
  away from the sample surface.
• The deflection of the free end of the cantilever
  is measured and plotted at many points as the
  z-axis scanner extends the cantilever towards the
  surface and then retracts it again..

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Force measurements. The AFM can record the amount
of force felt by the cantilever as the probe tip
is brought close to - and even indented into - a
sample surface and then pulled away
Fig. 7.6 Force distance
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Consider a cantilever in air approaching a hard,
incompressible surface such as glass or mica.
AB As the cantilever approaches the surface,
initially the forces are too small to give a
measurable deflection of the cantilever, and the
cantilever remains in its undisturbed position.
BC At some point, the attractive forces
(usually Van der Waals, and capillary forces)
overcome the cantilever spring constant and the
tip jumps into contact with the surface.
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CD Once the tip is in contact with the sample,
it remains on the surface as the separation
between the base and the sample decreases
further, causing a deflection of the tip and an
increase in the repulsive contact force. DF,
FG As the cantilever is retracted from the
surface, often the tip remains in contact with
the surface due to some adhesion and the
cantilever is deflected downwards. GH At some
point the force from the cantilever will be
enough to overcome the adhesion, and the tip will
break free.
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Applications Molecular interactions
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Modern AFM imaging phase imaging. In tapping
mode, we vibrate the cantilever and we have
resonance frequency. Nornally we ignore any
thing to do with the phase, however there is now
a lot of research in using phase imaging to go
beyond simple topographical mapping to detect
variations in composition, adhesion, friction,
viscoelasticity.
Fig. 7.7 Phase imaging
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Some examples of phase imaging.
Fig. 7.9 Bond pad on an integrated circuit imaged
by TappingMode (left) and phase (right). Pad
contaminated with polyimide produce light
contrast with phase shifts of over 120 deg. 1.5
µm scan
Fig. 7.10 Tapping Mode (L) and phase images (R)
of a composite polymer embedded in a uniform
matrix
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• AFM vs STM
• Resolution of STM is better than AFM because of
  the exponential dependence of the tunneling
  current on distance.
• STM is generally applicable only to conducting
  samples while AFM is applied to both conductors
  and insulators.
• AFM offers the advantage that the writing
  voltage and tip-to-substrate spacing can be
  controlled independently, whereas with STM the
  two parameters are integrally linked.

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Other types of SPM MFM Magnetic Force
Microscopy Fig. 7.11 The dots are made of
permalloy and are 400 nm apart. The MFM image
gives the field distribution for a non polarized
sample, and a few different configuration are
observed.
EFM Electric Force Microscopy
Field of view 10µm x 10µm
Fig. 7.12 Different types of material are
deposited on Si wafers during processing.
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Scanning Capacitance Microscopy SCM Fig. 7.13
shows two sets of AFM measurements (topography
and SCM) for a correctly aligned mask(left) and
for a misaligned mask (right). The large
pictures are a combination of both the topography
(grey) and the SCM image (orange). From these
images the amount and direction of the
misalignment can be observed.
Field of view 30µm x 30µm
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Using AFMs as tools I Dip-pen Nanolithography

• In the year 2000, , they will wonder why it was
  not until the year 1960 that anybody began
  seriously to move in this direction
  (miniaturization)."

Richard Feynman
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Dip-pen Nanolithography
Fig. 7.14 DPN
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http//www.chem.northwestern.edu/mkngrp/dpn.htm
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Using AFMs as tools II Nanomanipulation
"NanoMan and Best Friend" DNA strand
400 x 480 nm scans showing carbon nanotube
manipulation
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• Summary
• AFM uses force to probe the surface topology and
  properties of materials.
• Tapping modes vs Contact mode
• Phase imaging is a modern way to prove extra
  information.
• Dip- pen nanolithography is a way to perform
  serial writing.