AFM Basics Xinyong Chen Outline How AFM works

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 • 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

Voltage Difference Between Top & Bottom Photodiodes Optical level detection With this “split photodiode”, any slight vertical movement Top-Bottom Signalof(V) Let’sthe split the photodiode into two – the “top” the “bottom”. reflection spot position is detected by checking the or and Deflection (nm) Assume that the optical the reflection spotthe originally locates difference between “top” and “bottom” photodiode or Force (n. N)in the exactly middle of“T-B this split photodiode, resulting in the exactly During scanning, thetosample surface may dutputs (the signal”). Quad photodiode detect same voltage output the two So, the difference lift. Both the from cantilever up, photodiodes. resulting in corresponding (Click for the next) vertical and horizontal between the “top” (T)Photodiode and “bottom” (B) is zero. Photodiode move up of the optical reflection spot on the Photodiode Photodiode Movements of the light spot. Photodiode (Click for the next) photodiode. However, this single photodiode Laser couldn’t detect small. Laser position change of the spot. Laser (Click for the next) Scanner Scanner Scanner Cantilever + Sharp probe Cantilever + Sharp probe +probe Sharp probe Cantilever +Cantilever Sharp probe Cantilever + Sharp 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 Constantforce 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: – 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 constantforce mode Photodiode Laser In constant-force mode, whenever the sample surface topography would result in the cantilever deflection change, the other end of cantilever would be accordingly adjusted so that the cantilever deflection angle, and hence the contact force, would keep constant. Z scanner Cantilever + Sharp probe Click for the Next

Feedback control in constant-force mode P. I. D. Control In constant-force mode, the cantilever’s vertical position is adjusted by an electronic feedback loop, with the T-B signal as the input and the vertical scanner voltage as the output. Horizontal Vertical Click for the Next

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

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 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 The constant AFM probe contact with the sample surface may cause damage of the sample, typically shown as “sweeping”. One of the techniques to avoid such a problem is the “tapping mode”. 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 In tapping mode, the system uses the same feedback control as that used in constant-force contact mode. However, it usually uses the cantilever’s oscillation amplitude (the “AC” signal) instead of its DC component (the “Deflection”) as the input signal. Click for the Next

Tapping mode AFM PLA/PSA blend on Si imaged in air In addition to the normal topographic image, tapping mode AFM can also provide simultaneously a “phase image” map, which results from variation in interactions between the AFM probe and the various sample surfaces. 1 mm Height Phase 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

Multi. Mode AFM Click for the Next

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

AFM sample preparation Click for the Next

AFM in liquid environment One extraordinary feature of AFM is to work in liquid environment. A key point for liquid AFM is a transparent solid (usually glass) surface, which, together with the solid sample surface, retains the liquid environment whilst maintains stable optical paths for the laser beams. An optional O-ring can be used to form a sealed liquid cell. Otherwise, the system can also work in an “open cell” fashion. Click for the Next

Liquid AFM Images 70 nm t=0 min 12 41 45 19 20 48 56 22 60 Effect of DNase I enzyme on G 4 -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 – 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 P. I. D. Control Deflection (A+B)-(C+D) Defl= When an AFM works in force measurement A+B+C+D mode, the feedback loop is temporarily “cut off”. The cantilever A B deflection (the “T-B signal”) is then recorded while the C D AFM probe is vertically “ramped” towards/backwards the sample surface. (Click step-by-step to see how this is done. ) Z Displacement 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 • Detector sensitivity S = Inverse of the contact slope measured on a hard surface x (nm/V) • Spring constant (N/m) – Property of the cantilever and provided by the manufacturer DD • 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 Deflection Force (n. N) (nm) (V) F = -kx DZ x Z Displacement (nm) Click for the Next

Humidity affects the adhesion AFM probe Salbutamol 1200 Force (n. N) 1000 800 600 1µm Measure ment of particle Lactose interactio n 400 200 0 <10% 22% ‘Macroscale’ contact 44% 65% ‘Nanoscale’ contact Click for the Next

Environmental AFM Both temperature and humidity can be controlled in this environmental chamber. Click for the Next

Intermolecular interactions MFP is specially designed force measurement purpose MFP Schematic of the force–extension characteristics of DNA: at 65 p. N the molecule is overstretched to about 1. 7 times its contour length, at 150 p. N 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 p. H 7 Albumin Polystyrene 5 mm Albumin PS Si Click for the Next

Adhesion and Hardness Imaging Height Adhesion Stiffness 1 mm PLMA/Pm. Ml 6 blend on Si imaged in water PLMA: poly (lauryl methacrylate) Pm. Ml 6: 2 -methacryloyloxyethyl phosphorylcholine-co-lauryl methacrylate (1: 6) Simultaneous Height, Adhesion and Stiffness maps are obtained with “Pulsed-Force” AFM technique. Click for the Next

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