Friction Force Microscopy: Seeking new understanding of friction from a nanoscale perspective

1. Friction Force Microscopy: Seeking new understanding of friction from a nanoscale perspective Sarah A. Burke McGill University

3. What is friction? Dissipation of energy between two surfaces in relative motion Often expressed as a force opposing motion: Macroscopic view: ��force needed to plastically deform interlocking asperities of surfaces in relative motion�

4. Zoom in�

5. Atomic Friction So what does friction mean on the atomic scale? Still have dissipation of energy on atomic scale No plastic deformation of asperities ? atomically flat, or single asperity contacts Tomlinson, 1929 proposes �plucking of atoms� Atoms in the lattice are pulled slightly out of equilibrium ? like plucking a string

6. Why is friction important? Macroscopic friction is crucial in our everyday lives Eg: 20% of gasoline consumed in a typical automobile is used to counteract friction in the engine and drive train, but a car won�t go anywhere on a frictionless surface Still, friction is poorly understood, plastic deformation of interlocking asperities is only part of the story Microscopic friction is of increasing interest with the development of scanning probe techniques and nanoscience Friction influences AFM images and related techniques Crucial in describing manipulations of molecules

7. Manipulations of Molecules

8. Measuring friction What needs to be measured? Would like to obtain � ? need to measure both the normal force and a lateral force on a sliding contact Would also like to have a well defined contact area (most modern theories propose Ffriction is proportional to the contact area) Atomically clean surfaces

9. Tribometer Useful for: Measuring � Studying effects of lubrication Disadvantages: Contact is macrsocopic Cannot measure local variations in tribology Arrangement of strain gauges and/or cantilevers to measure normal force (load) and lateral force corresponding to friction

10. Is there something better? Need: microscopic contact with well defined normal force + ability to measure lateral force acting on contact AFM: 50-100 nm radius tip ? contact area of 30-600 nm2 Constant force contact imaging provides constant, well defined normal force Lateral force measured by torsion of a cantilever Scanning capability ? measure local variation in tribology

11. Friction and AFM Scanning: tip/surface move relative to one another Friction opposes motion of tip causing torsion of cantilever ? lateral deflection 2 possibilities: Maintains a constant lateral deflection Exhibits �stick-slip� motion

12. Typical AFM set-up Tip attached to cantilever Detector measures deflection (often optical) Sample is raster scanned by xyz piezo electric scanner Feedback from the deflection sensor controls z-direction for constant force measurements

13. Force Sensing Cantilever + interferometer Deflection of the cantilever is detected by interferometry Accurate and advantageous for UHV Alignment of optical fiber with cantilever difficult Cantilever + split/quadrant photodiode: Beam deflection system Deflection is detected by reflecting a laser beam off the back of the cantilever and measuring its change in position Commercially popular

14. Force Sensing (cont�d) Beam deflection system (cont�d) Alignment of beam with cantilever less difficult Can detect lateral deflection with quadrant photodiode Cantilever with built-in strain gauges Eliminates alignment problem and bulky optical systems Difficult to fabricate sufficiently sensitive device Can, in principle, be designed to detect lateral deflection

15. Quadrant Photodiode Beam deflection system with quadrant photodiode Difference signals give vertical and lateral deflection of cantilever Advantageous: can measure topography and lateral forces simultaneously, and maintain constant, known normal force

16. Considerations� UHV conditions: Want atomically flat and clean surface Atmospheric conditions: water layer, strong capilliary forces increase normal force (not well defined) Cantilever spring constants anisotropic Need to consider in interpretation of �friction images� Scan angle relative to cantilever may change image

17. Friction Force Microscope UHV AFM in contact mode Lateral deflection sensor (quadrant photodiode beam deflection system) Cantilevers with well defined spring constants (including torsion) Rectangular cantilevers, dimensions measured by SEM Note: NC-AFM dissipation imaging has been shown to contain a friction term resulting from the tip-surface interaction. Modeling shows coupling to phonons in the surface. (Kantorovich, 2001)

18. Typical Features of FFM Stick-slip motion is observed Dependence on scan angle (skewing due to anisotropy of cantilever) Preservation of translational symmetry Non-zero average lateral force Hysteresis loop (dependence on scan direction)

19. Interpretation

20. Tomlinson Model �plucking of atoms� Seems to suggest phonon mechanism for friction As tip moves across surface, the atoms are pulled by the interaction and the tip is pulled by the potential minima Class of models, many variations (model of tip, limiting cases, adiabatic)

21. Tomlinson Model (cont�d) General Results: �stick-slip� motion observed Critical value of cantilever spring constant (conservative, or frictionless phase) k>kcrit,: <Fx>=0, no hysteresis Scan direction hysteresis Affine distortion of critical curves (where �slip� occurs) Ffriction dependence on ln(v)

22. Tomlinson Model vs. Expt.

23. Dislocation Model Models motion of two surfaces in relative motion as a propagating dislocation Magnitude of frictional force on same order of Peierls force suggests validity of this approach For small contact radius: friction stress constant, of the order of the theoretical sheer stress ? agrees with AFM results Stick-slip motion ? dislocation moves jumpwise related to the lattice spacing

24. Dislocation: Acoustic waves Edge dislocation glide generates low frequency acoustic waves Special AFM set-up to measure (Rekhviahvilli, 2002) Again consistent with phonon mechanisms??

25. Right model? Which is the correct model? Difficult to distinguish: Both show major features of FFM images Both agree with concept of phonon mediated friction (energy is dissipated through lattice vibrations, or the creation of phonons) Combination of both models? Tomlinson model seems to agree well with FFM results, but dislocation model may lend insight to mechanism of �slip�

26. Friction: sticky business �Tribo�� borrowing from Greek, meaning �friction� Understanding friction very old problem, considered �messy physics� due to large number of contributing mechanisms Friction on the nanoscale still not well understood, but FFM can be used as a tool to both measure frictional properties as well as study the underlying mechanisms