Scanning Probe Microscopies: STM and AFM

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Scanning Probe Microscopies STM and AFM

• CH20016 Lecture 10

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Introduction

• Scanning Tunnelling Microscopy (STM) is a method
  of imaging a surface with a resolution as high as
  single atoms. A fine tip with a bias voltage
  applied, is scanned over a conducting surface and
  the tunnelling current measured. Gives
  topographic information.
• Atomic Force Microscopy (AFM) is where a fine tip
  is scanned across a surface the tip-surface force
  measured. Gives topographic, frictional and
  adhesion information.

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In 1986 the Nobel Prize for Physics was awarded
to Gert Binnig and Heinrich Röher at IBM Zürich
for their work on developing the Scanning
Tunnelling Microscope (STM).
Scanning Tunnelling Microscopy
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STM How does it work?
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Experimental setup
The ultra-sharp tip is controlled by means of a
piezoelectric tube. The extremely low tunnelling
currents (ca. 10-10 A) are amplified and
measured as a function of piezoelectric
position of the tip. The tunnelling current
measured is a function of the gap (W) between
the tip and substrate I(W) Cexp(-Wvf) C is
a constant, f is the sample work function. (work
function energy barrier to electron transfer
can be controlled by changing the potential on
the tip or substrate.
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Effect of applied Potential
Ultra-sharp tip to within approximately 1 nm of a
substrate and applying a bias potential between
the tip and substrate. Wavefunction of an
electron is non-zero at finite distances from the
atom. If another conducting surface is
introduced close to the surface, with a lower
energy, there is a possibility that electrons can
tunnel across the energy gap between the two
surfaces and hence a current can flow.

In the above diagram, the Fermi level of the
substrate is higher than the Fermi level of the
tip, so electrons can tunnel across the vacuum
gap to the Fermi level of the tip. Note Vbias
EF, sample EF, tip
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Modes of operation

• Constant height, variable tunnelling current
• Constant current by varying height to match
  surface topography

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Images
Famous IBM logo
Image of chromium growing on crystalline iron Fe
(100).

• One of the principal disadvantages of STM is that
  it requires a conducting substrate. Since a great
  many materials found in nature are not
  conducting, another method for studying surfaces
  with atomic resolution has been developed the
  atomic force microscope (AFM).

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Atomic (Scanning) Force Microscopy

• Principle of operation
• An ultra-sharp tip on a spring cantilever is
  brought into contact with a surface and rastered
  across the sample. The change in forces
  attractive and repulsive on the tip are measured
  as a function of tip position on the surface.

tip (white bar 1 mm)
The position of the cantilever is measured by
reflecting a laser off its backside and onto a 4
quadrant photodiode. Hence both up-down and
sideways forces on the tip can be measured.
Similarly to STM, the tip forces or tip height
can be kept constant depending on the mode of
operation.
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Forces experienced by the tip

• The exact origin of the forces that operate on
  the tip will very much depend on
• the substrate being studied,
• the conditions around the tip (e.g. water, air or
  vacuum)
• the nature of the chemical functionality of the
  surface (e.g. hydrophilic / hydrophobic)
• softness / hardness of the surface
• Tip raster velocity
• As a tip is introduced to the surface, close to
  the substrate a conventional Morse curve is
  measured, a sum of the attractive and repulsive
  forces measured as a function of tip surface
  position.

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Approach and retraction of tip at fixed point
The numbers refer to the tip positions
illustrated in the last cartoon.
However, since adhesive forces can operate on the
tip, the conventional Morse curve seen above is
uni-directional that is only seen when
bringing the tip to the surface not always when
retracting it due to hysteriasis
This gives a force distance curve of this type
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Information from AFM 1. Topography

• By operating in constant height mode on a
  chemically homogenous surface one can obtain
  topographical information.
• Measure change in tip force as tip-sample
  distance varies as tip rastered across surface.
• Example (1) Contact mode AFM, height imaging of
  a patterned Self-assembled Monolayer SAM.
• Au S-CH2-CH2OH
• Au-(CH2)17CH3

Au-(CH2)17CH3
Au S-CH2-CH2OH
Example (1) Contact mode AFM, height imaging of
a patterned Self-assembled Monolayer SAM.
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Information from AFM (2) Friction
Example (2) Frictional force image of patterned
Self-assembled Monolayer
Note, now measuring friction NOT height. See high
friction on OH terminal surface due to capillary
force of surface adsorbed water on tip.
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Tapping mode AFM

• Instead of dragging tip over surface, oscillate
  tip over surface, tapping. Change in amplitude as
  average tip-surface distance changes

Makes contact mode AFM unsuited to studying soft
materials, especially biological
materials. Reducing the contact force and
ossilating the tip allows soft samples to be
imaged. Tapping frequency 50 500 KHz.
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Examples of Tapping mode results
Final Example
Example 1 DNA on mica

Imaging of H-ATPase within a lipid membrane on
micropatterned Self-assembled monolayers.
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Further Reading

• A.T.A. Jenkins, H. Schönherr , S.D. Evans, ,
  R.E. Miles and S.D. Ogier, G.J.Vancso. Retention
  of Protein Activity in Biomimetic Lipid Bilayers
  Supported by Microcontact Printed Lipophilic
  Self-Assembled Monolayers. J. Am. Chem. Soc. 121,
  5274-5280, (1999).