Atomic Force Microscope: Fundamental Principles
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A first principle study of the atomic force microscope focussing on the forces being measured and their implication in experimentation.
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Atomic Force Microscope: Fundamental Principles
- 1. Atomic Force Microscope Fundamental Principles -Joy Bhattacharjee IIT Kanpur. Co-Founder & Director, Kanopy Techno Solutions
- 2. Microscopes Optical Simple Compound Stereoscopic Confocal X-Ray STXM TXM XPEEM Electron TEM SEM STEM REM Probe STM EC-STM MFM CFM EFM AFM C-AFM Acoustic SAM C-SAM Neutron Invented by IBM Scientists in 1986 Gerd Binnig and Heinrich Rohrer
- 3. AFM The AFM (center) has inspired a variety of other scanning probe techniques. Originally the AFM was used to image the topography of surfaces, but by modifying the tip it is possible to measure other quantities (for example, electric and magnetic properties, chemical potentials, friction and so on), and also to perform various types of spectroscopy and analysis. (Image: Christoph Gerber; copyright Nature Publishing Group)
- 5. Working of AFM: Block Diagram
- 6. Significant Forces Capillary Van Der Waals Chemical Bond Electro- Static Magnetic
- 8. Operation Signal for Post-Processing and Feedback Non- Contact Tapping Contact
- 9. Contact Mode In contact mode the tip contacts the surface through the adsorbed fluid layer on the sample surface. The feedback circuit adjusts the probe height to try and maintain a constant force and deflection on the cantilever. This is known as the deflection setpoint. F = − k x (F = force, k = spring constant, x = cantilever deflection) Advantage Disadvantage Contact Mode • High scan speeds • Rough samples with extreme changes in vertical topography can sometimes be scanned more easily • Lateral (shear) forces may distort features in the image • In ambient conditions may get strong capillary forces due to adsorbed fluid layer • Combination of lateral and strong normal forces reduce resolution and mean that the tip may damage the sample, or vice versa
- 10. Feedback Without Feedback With Feedback Contact Mode
- 11. Tapping Mode In tapping mode the cantilever oscillates at or slightly below its resonant frequency. The resonant frequency of the cantilever is dependent on this separation. The oscillation is also damped when the tip is closer to the surface. The feedback circuit adjusts the probe height to try and maintain a constant amplitude of oscillation i.e. the amplitude setpoint. Advantage Disadvantage Tapping Mode • Lateral forces almost eliminated • Higher lateral resolution on most samples • Lower forces so less damage to soft samples or tips • Slower scan speed than in contact mode
- 12. Non-Contact Mode • In non-contact mode the cantilever oscillates near the surface of the sample, but does not contact it. The oscillation is at slightly above the resonant frequency. • In ambient conditions the adsorbed fluid layer is often significantly thicker than the region where van der Waals forces are significant. Therefore non-contact mode AFM works best under ultra-high vacuum conditions. Advantage Disadvantage Non-Contact Mode • Both normal and lateral forces are minimized, so good for measurement of very soft samples • Can get atomic resolution in a UHV environment • In ambient conditions the adsorbed fluid layer may be too thick for effective measurements • Slower scan speed than tapping and contact modes to avoid contacting the adsorbed fluid layer
- 13. Effect of Shape of Tip
- 14. Details of Parts of AFM Property Typical Value Desired Quality Material Silicon, Silicon Nitride, Silicon Oxide Hard, Unreactive Tip Radius < 10 nm Small Tip Height 15-20 µm Mechanically stable Cantilever Length 100-250 µm Appropriate reach Mean Width 20 – 70 µm Mechanically stable Half Cone Angle 25° Sample dependent Base Shape configurable Sample dependent Apex Shape configurable Sample dependent Resonant Frequency Several kHz, depends on shape Matching piezo’s resonant frequency Coating None, Gold, Platinum, Diamond Experiment dependent Cantilever and Tip
- 15. Details of Parts of AFM Shapes of AFM Tip
- 16. Details of Parts of AFM Shapes of AFM Tip Protruding from the Very End Positioned at the Very End Square-Based Pyramid Rectangular- based Pyramid Circular Symmetric Spike
- 17. Details of Parts of AFM High Aspect Ratio Spike AFM Tips Focused Ion Beam Electron Beam Deposited Carbon Nanotube Plateau Rounded Sphere Critical Dimension
- 18. Details of Parts of AFM Scanner In most AFMs piezoelectric materials are used to achieve this. These change dimensions with an applied voltage. The diagram below shows a typical scanner arrangement.
- 19. Details of Parts of AFM Scanner The presence of electrical resonances and anti-resonances make the piezoelectric impedance unique. The resonances result from the electrical input signal exciting a mechanical resonance in the piezo element. Equivalent Circuit Model
- 20. Details of Parts of AFM Feedback The feedback system is affected by three main parameters: 1. Setpoint 2. Feedback gains 3. Scan rate
- 21. Optical AFM • Advanced Surface Topography technique avoids cantilever mechanism by use of optical fiber based tips and using Fabry–Pérot Interferometry (or Etalon): There is only one limitation of such an approach: surface of the sample should be smooth enough and homogeneously reflecting.
- 22. Artefacts in AFM Scanner Related Hysteresis The piezoelectric’s response to an applied voltage is not linear. This gives rise to hysteresis.
- 23. Artefacts in AFM Scanner Related Scanner creep If the applied voltage suddenly changes, then the piezo-scanner’s response is not all at once. It moves the majority of the distance quickly, then the last part of the movement is slower. This slow movement will cause distortion, known as creep. Change in x-offset Change in y-offset Change in size
- 24. Artefacts in AFM Scanner Related Bow and Tilt Because of the construction of the piezo-scanner, the tip does not move in a perfectly flat plane. Instead its movement is in a parabolic arc (scanner bow). Also the scanner and sample planes may not be perfectly parallel (tilt). Both of these artefacts can be removed by using post-processing software.
- 25. Artefacts in AFM Tip Related Blunt tip: Use Feedback Mode Tip picks up debris: Cleaning the sample with compressed air or N2 before use
- 26. Artefacts in AFM Feedback Related Poor tracking due to high scan rate Gains are set too high, then the feedback circuit can begin to oscillate. This causes high frequency noise
- 27. Artefacts in AFM Vibration Related AFMs are very sensitive to external mechanical vibrations, which generally show up as horizontal bands in the image. These can be minimised by the use of a vibrational isolation table, and locating the AFM on a ground floor or below. Acoustic noise such as people talking can also cause image artefacts, as can drafts of air. An acoustic hood can be used to minimise the effects of both of these.
- 28. Beyond just surface Seeing the atomic orbital
- 29. Beyond just surface Seeing the atomic orbital Ref: Minghuang Huang, Martin Cuma, and Feng Liu. (27 June, 2003). Seeing the Atomic Orbital: First- Principles Study of the Effect of Tip Termination on Atomic Force Microscopy. Physical Review Letters. Volume 90, Number 25.
- 30. Beyond just surface Seeing the reaction Work done by Franz J. Giessibl at the Department of Physics, University of Regensburg have been success to image chemical reaction using AFM by having a carbon monoxide molecule at the tip to obtain high spatial resolution. Ref: Science Vol 340, 21 JUNE 2013