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AFM Basics. Xinyong Chen. Outline. How AFM works Scanning Feedback control Contact mode and tapping mode Force measurements with AFM How AFM measures forces Calibrations. Click for the Next. How AFM works. Click for the Next. How AFM works.

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Afm basics

AFM Basics

Xinyong Chen


Outline
Outline

  • How AFM works

    • Scanning

    • Feedback control

    • Contact mode and tapping mode

  • Force measurements with AFM

    • How AFM measures forces

    • Calibrations

Click for

the Next


How AFM works

Click for

the Next


How afm works
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

Click for

the Next


Optical level detection

Voltage

Difference

Between

Top & Bottom

Photodiodes

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Photodiode

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Laser

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Z scanner

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Optical level detection

Top-Bottom Signal (V)

or Deflection (nm)

or Force (nN)

Quad photodiode

Click for

the Next


How afm works1
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

Click for

the Next


Constant height scan
Constant-height scan

Click for

the Next

Click on graph to play animation (internet connection required)

www.ntmdt.com


Constant height scan1
Constant-height scan

  • Advantages:

    • Simple structure (no feedback control)

    • Fast response

  • Disadvantages:

    • Limited vertical range (cantilever bending and detector dynamic range)

    • Varied force

Click for

the Next


Constant force scan
Constant-force scan

Click for

the Next

Click on graph to play animation (internet connection required)

www.ntmdt.com


Optical level detection in constant force mode

Photodiode

Photodiode

Photodiode

Laser

Laser

Laser

Z scanner

Z scanner

Z scanner

Cantilever + Sharp probe

Cantilever + Sharp probe

Cantilever + Sharp probe

Optical level detection in constant-force mode

Click for

the Next


Feedback control in constant force mode

P.I.D. Control

Feedback control in constant-force mode

Click for

the Next


Constant force scan vs constant height scan

Constant-height mode

Constant-force mode

Constant-force scan vs.constant-height scan

Click for

the Next

Click on graph to play animation (internet connection required)

www.ntmdt.com


Constant force scan vs constant height scan1

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

Constant-force scan vs.constant-height scan

Click for

the Next


How afm works2
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

Click for

the Next


Sample swept by afm probes
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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the Next


Tapping mode afm
Tapping mode AFM

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the Next

Click on graph to play animation

www.ntmdt.com


Feedback control in tapping mode

P.I.D. Control

Feedback control in tapping mode

Click for

the Next


Tapping mode afm1

1 mm

Height

Phase

Tapping mode AFM

PLA/PSA blend on Si imaged in air

Click for

the Next


How afm works3
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

Click for

the Next


Dimension afm
Dimension AFM

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the Next


Multimode afm
MultiMode AFM

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the Next


Afm tips

20 mm

35 mm

125 mm

80 – 320 mm

AFM Tips

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the Next


Afm sample preparation
AFM sample preparation

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the Next


Afm in liquid environment
AFM in liquid environment

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the Next


Liquid afm images

t=0 min

12

19

20

22

70 nm

41

45

48

56

60

Liquid AFM Images

Effect of DNase I enzyme on G4-DNA (0.5:1) complex, the complex was immediately adsorbed onto mica and imaged until stable images were obtained, then the DNase I was introduced.

Click for

the Next

Nucleic Acids Research, 2003, Vol. 31, No. 14 4001-4005


Outline1
Outline

  • How AFM works

    • Scanning and feedback control

    • Contact mode and tapping mode

  • Force measurements with AFM

    • How AFM measures forces

    • Calibrations

Click for

the Next


Force measurements with afm

(A+B)-(C+D)

A+B+C+D

B

A

Defl=

D

C

P.I.D. Control

Deflection

Z Displacement

Force measurements with AFM

Click for

the Next


Experimental force curves

Contact slope to study hardness

Adhesion to study intermolecular interactions

Experimental Force Curves

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the Next


Calibration of force measurements

Slope = DD / DZ (V/nm)

T-B Signal

DD

DZ

x

x

Z Displacement (nm)

Calibration of force measurements

  • The Hooke’s 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)

Click for

the Next


Humidity affects the adhesion

AFM probe

1200

Salbutamol

Force (nN)

Measurement of particle-particle interaction

1000

800

600

400

Lactose

200

1µm

0

<10%

22%

44%

65%

‘Nanoscale’ contact

‘Macroscale’ contact

Humidity affects the adhesion

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the Next


Environmental afm
Environmental AFM

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the Next


Intermolecular interactions

MFP

Intermolecular interactions

Schematic of the force–extension 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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the Next


Adhesion force imaging

5 mm

Adhesion Force Imaging

Height

Adhesion

pH 7

Albumin

Polystyrene

PS

Si

Albumin

Click for

the Next


Adhesion and hardness imaging

1 mm

Adhesion and Hardness Imaging

Height

Adhesion

Stiffness

PLMA/PmMl6 blend on Si imaged in water

PLMA: poly (lauryl methacrylate)

PmMl6: 2-methacryloyloxyethyl phosphorylcholine-co-lauryl methacrylate (1:6)

Click for

the Next


Conclusions
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 => Convert the measured T-B signal (V) to cantilever deflection (nm)

      • Spring constant (N/m) => Convert the cantilever deflection to force (N) [F=-kx]

End


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