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AFM Basics

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

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  1. 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 Click for the Next

  3. How AFM works Click for the Next

  4. 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

  5. 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

  6. 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

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

  8. 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

  9. Constant-force scan Click for the Next Click on graph to play animation (internet connection required) www.ntmdt.com

  10. 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

  11. P.I.D. Control Feedback control in constant-force mode Click for the Next

  12. 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

  13. 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

  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 Click for the Next

  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. Click for the Next

  16. Tapping mode AFM Click for the Next Click on graph to play animation www.ntmdt.com

  17. P.I.D. Control Feedback control in tapping mode Click for the Next

  18. 1 mm Height Phase Tapping mode AFM PLA/PSA blend on Si imaged in air Click for the Next

  19. 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

  20. Dimension AFM Click for the Next

  21. MultiMode AFM Click for the Next

  22. 20 mm 35 mm 125 mm 80 – 320 mm AFM Tips Click for the Next

  23. AFM sample preparation Click for the Next

  24. AFM in liquid environment Click for the Next

  25. 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

  26. 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

  27. (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

  28. Contact slope to study hardness Adhesion to study intermolecular interactions Experimental Force Curves Click for the Next

  29. 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

  30. 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 Click for the Next

  31. Environmental AFM Click for the Next

  32. 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. Click for the Next

  33. 5 mm Adhesion Force Imaging Height Adhesion pH 7 Albumin Polystyrene PS Si Albumin Click for the Next

  34. 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

  35. 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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