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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.

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Presentation on theme: "AFM Basics Xinyong Chen. Outline How AFM works –Scanning –Feedback control –Contact mode and tapping mode Force measurements with AFM –How AFM measures."— Presentation transcript:

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 Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Photodiode Laser Scanner Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe Photodiode Laser Scanner Cantilever + Sharp probe During scanning, the sample surface may lift the cantilever up, resulting in corresponding move up of the optical reflection spot on the photodiode. However, this single photodiode couldnt detect small position change of the spot. (Click for the next) Lets split the photodiode into two – the top and the bottom. Assume that the optical reflection spot originally locates in the exactly middle of this split photodiode, resulting in the exactly same voltage output from the two photodiodes. So, the difference between the top (T) and the bottom (B) is zero. (Click for the next) Optical level detection Photodiode Laser Scanner Cantilever + Sharp probe Voltage Difference Between Top & Bottom Photodiodes Photodiode Laser Scanner Cantilever + Sharp probe Top-Bottom Signal (V) or Deflection (nm) or Force (nN) Quad photodiode to detect Both vertical and horizontal Movements of the light spot. Click for the Next With this split photodiode, any slight vertical movement of the reflection spot position is detected by checking the difference between the top and the bottom photodiode dutputs (the T-B signal). (Click for the next)

6 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 How AFM works Click for the Next

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

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

10 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 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.

11 Horizontal Feedback control in constant-force mode P.I.D. Control Click for the Next In constant-force mode, the cantilevers 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. Vertical

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

13 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

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 Self-assembly of octadecyl phosphonic acid (ODPA) on single crystal alumina surface imaged in ethanol with tapping mode. The central 1 m × 1 m area was previously scanned in contact mode with heavy loading force. 1 m Click for the Next 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.

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

17 Feedback control in tapping mode P.I.D. Control Click for the Next In tapping mode, the system uses the same feedback control as that used in constant-force contact mode. However, it usually uses the cantilevers oscillation amplitude (the AC signal) instead of its DC component (the Deflection) as the input signal.

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

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

23 AFM sample preparation Click for the Next

24 AFM in liquid environment Click for the Next 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.

25 19 Liquid AFM Images 4145485660 70 nm t=0 min202212 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. Nucleic Acids Research, 2003, Vol. 31, No. 14 4001-4005 Click for the Next

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 Force measurements with AFM Z Displacement Deflection A B C D (A+B)-(C+D) A+B+C+D Defl= P.I.D. Control Click for the Next When an AFM works in force measurement mode, the feedback loop is temporarily cut off. The cantilever deflection (the T-B signal) is then recorded while the AFM probe is vertically ramped towards/backwards the sample surface. (Click step-by-step to see how this is done.)

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

29 The Hookes 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. Calibration of force measurements T-B Signal Z Displacement (nm) (V) x Slope = D / Z (V/nm) x Z D Deflection (nm) Force (nN) Click for the Next

30 Humidity affects the adhesion AFM probe Salbutamol Measure ment of particle- particle interacti on Lactose 1µm Force (nN) 0 200 400 600 800 1000 1200 <10%22%44%65% Nanoscale contact Macroscale contact Click for the Next

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

32 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. MFP Click for the Next MFP is specially designed for force measurement purpose

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

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

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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