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AFM Basics Xinyong Chen
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Outline How AFM works Force measurements with AFM Scanning
Feedback control Contact mode and tapping mode Force measurements with AFM How AFM measures forces Calibrations Click for the Next
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How AFM works Click for the Next
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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 Click for the Next
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Optical level detection
Voltage Difference Between Top & Bottom Photodiodes Top-Bottom Signal (V) or Deflection (nm) or Force (nN) Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Quad photodiode Photodiode Laser Z scanner Photodiode Laser Z scanner Click for the Next
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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 Click for the Next
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Constant-height scan Click for the Next
Click on graph to play animation (internet connection required)
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Constant-height scan Advantages: Disadvantages:
Simple structure (no feedback control) Fast response Disadvantages: Limited vertical range (cantilever bending and detector dynamic range) Varied force Click for the Next
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Constant-force scan Click for the Next
Click on graph to play animation (internet connection required)
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Optical level detection in constant-force mode
Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Click for the Next
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Feedback control in constant-force mode
P.I.D. Control Click for the Next
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Constant-force scan vs. constant-height scan
Constant-height mode Constant-force mode Click for the Next Click on graph to play animation (internet connection required)
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Constant-force scan vs. constant-height scan
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 Click for the Next
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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 Click for the Next
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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
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Tapping mode AFM Click for the Next Click on graph to play animation
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Feedback control in tapping mode
P.I.D. Control Click for the Next
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Tapping mode AFM PLA/PSA blend on Si imaged in air Height Phase 1 mm
Click for the Next
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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 Click for the Next
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Dimension AFM Click for the Next
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MultiMode AFM Click for the Next
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AFM Tips 80 – 320 mm 20 mm 35 mm 125 mm Click for the Next
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AFM sample preparation
Click for the Next
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AFM in liquid environment
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Liquid AFM Images Click for the Next 70 nm
t=0 min 12 19 20 22 41 45 48 56 60 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
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Outline How AFM works Force measurements with AFM
Scanning and feedback control Contact mode and tapping mode Force measurements with AFM How AFM measures forces Calibrations Click for the Next
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Force measurements with AFM
B C D (A+B)-(C+D) A+B+C+D Defl= P.I.D. Control Z Displacement Deflection Click for the Next
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Experimental Force Curves
Contact slope to study hardness Adhesion to study intermolecular interactions Click for the Next
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Calibration of force measurements
Slope = DD / DZ (V/nm) 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. T-B Signal Z Displacement (nm) x (V) Deflection (nm) DD Force (nN) DZ x Click for the Next
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Humidity affects the adhesion
AFM probe Salbutamol Measurement of particle-particle interaction Lactose 1µm Force (nN) 200 400 600 800 1000 1200 <10% 22% 44% 65% ‘Nanoscale’ contact ‘Macroscale’ contact Click for the Next
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Environmental AFM Click for the Next
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Intermolecular interactions
MFP 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
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Adhesion Force Imaging
Height Adhesion pH 7 Albumin Polystyrene PS Albumin Si 5 mm Click for the Next
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Adhesion and Hardness Imaging
Height Adhesion Stiffness 1 mm 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
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Conclusions How AFM works Force measurements with AFM
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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