AFM Basics Xinyong Chen

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

How AFM works Click for the Next

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 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

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 Click for the Next Click on graph to play animation (internet connection required) www.ntmdt.com

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

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 Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Photodiode Laser Z scanner Cantilever + Sharp probe Click for the Next

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

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) www.ntmdt.com

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

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 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

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

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

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

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 Click for the Next

MultiMode AFM Click for the Next

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

AFM sample preparation Click for the Next

AFM in liquid environment Click for the Next

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. 14 4001-4005

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

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

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

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

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

Environmental AFM Click for the Next

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

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

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

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