AFM Basics

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AFM Basics

• Xinyong Chen

2
Outline

• How AFM works
• Scanning
• Feedback control
• Contact mode and tapping mode
• Force measurements with AFM
• How AFM measures forces
• Calibrations

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3
How AFM works
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How AFM works

• Direct mechanical contact between the probe and
  the sampler surface
• Essential difference from traditional microscopy
• How AFM feels the surface topography?
• Optical level detection

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Optical level detection
Top-Bottom Signal (V) or Deflection (nm) or Force
(nN)
Quad photodiode
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How AFM works

• Direct mechanical contact between the probe and
  the sampler surface
• Essential difference from traditional microscopy
• How AFM feels the surface topography?
• Optical level detection
• Constant-height scan versus Constant-force scan

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Constant-height scan
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Click on graph to play animation (internet
connection required)
www.ntmdt.com
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Constant-height scan

• Advantages
• Simple structure (no feedback control)
• Fast response
• Disadvantages
• Limited vertical range (cantilever bending and
  detector dynamic range)
• Varied force

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Constant-force scan
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Click on graph to play animation (internet
connection required)
www.ntmdt.com
10
Optical level detection in constant-force mode
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Feedback control in constant-force mode
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Constant-force scan vs.constant-height scan
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Click on graph to play animation (internet
connection required)
www.ntmdt.com
13
Constant-force scan vs.constant-height scan

• Constant-force
• Advantages
• Large vertical range
• Constant force (can be optimized to the minimum)
• Disadvantages
• Requires feedback control
• Slow response

• Constant-height
• Advantages
• Simple structure (no feedback control)
• Fast response
• Disadvantages
• Limited vertical range (cantilever bending and
  detector dynamic range)
• Varied force

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14
How AFM works

• Direct mechanical contact between the probe and
  the sampler surface
• Essential difference from traditional microscopy
• How AFM feels the surface topography?
• Optical level detection
• Constant-height scan and constant-force scan
• Feedback control in constant-force scan

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15
Sample swept by AFM probes
1 mm
Self-assembly of octadecyl phosphonic acid (ODPA)
on single crystal alumina surface imaged in
ethanol with tapping mode. The central 1 mm 1
mm area was previously scanned in contact mode
with heavy loading force.
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Tapping mode AFM
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Click on graph to play animation
www.ntmdt.com
17
Feedback control in tapping mode
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Tapping mode AFM
PLA/PSA blend on Si imaged in air
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How AFM works

• Direct mechanical contact between the probe and
  the sampler surface
• Essential difference from traditional microscopy
• How AFM feels the surface topography?
• Optical level detection
• Constant-height scan and constant-force scan
• Feedback control in constant-force scan
• Contact mode and tapping mode

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Dimension AFM
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MultiMode AFM
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AFM Tips
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AFM sample preparation
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AFM in liquid environment
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Liquid AFM Images
Effect of DNase I enzyme on G4-DNA (0.51)
complex, the complex was immediately adsorbed
onto mica and imaged until stable images were
obtained, then the DNase I was introduced.
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Nucleic Acids Research, 2003, Vol. 31, No. 14
4001-4005
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Outline

• How AFM works
• Scanning and feedback control
• Contact mode and tapping mode
• Force measurements with AFM
• How AFM measures forces
• Calibrations

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Force measurements with AFM
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Experimental Force Curves
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Calibration of force measurements

• The Hookes law
• F -kx
• Detector sensitivity
• S Inverse of the contact slope measured on a
  hard surface (nm/V)
• Spring constant (N/m)
• Property of the cantilever and provided by the
  manufacturer
• Large variation due to difficulty in cantilever
  thickness control
• Should (and can) be experimentally measured for
  accuracy requirement
• Thermal fluctuation
• Resonance geometry
• Mass adding resonance
• Standard with known spring constant
• etc.

(V)
Deflection (nm)
Force (nN)
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Humidity affects the adhesion
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Environmental AFM
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Intermolecular interactions
Schematic of the forceextension characteristics
of DNA at 65 pN the molecule is overstretched to
about 1.7 times its contour length, at 150 pN the
double strand is separated into two single
strands, one of which remains attached between
tip and surface.
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33
Adhesion Force Imaging
Height
Adhesion
pH 7
Albumin
Polystyrene
PS
Si
Albumin
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Adhesion and Hardness Imaging
Height
Adhesion
Stiffness
PLMA/PmMl6 blend on Si imaged in water PLMA poly
(lauryl methacrylate) PmMl6 2-methacryloyloxyethy
l phosphorylcholine-co-lauryl methacrylate (16)
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Conclusions

• How AFM works
• Constant-height and constant-force scans (contact
  mode)
• Feedback control in constant-force mode
• Contact mode and tapping mode
• Force measurements with AFM
• Force curves contact part to measure hardness
  and adhesion to measure intermolecular
  interactions
• Calibrations
• Detector sensitivity (nm/V) Inverse of contact
  slope on a hard surface gt Convert the measured
  T-B signal (V) to cantilever deflection (nm)
• Spring constant (N/m) gt Convert the cantilever
  deflection to force (N) F-kx

End