Atomic Force Microscopy imaging and beyond

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1 Atomic Force Microscopy imaging and beyond Arif Mumtaz Magnetism and Magnetic Materials Group Department of Physics, QAU Coworkers: Prof. Dr. S.K.Hasanain M. Tariq Khan Alam

2 Imaging and beyond Scanning Probe Microscopy Imaging Modes Beyond Imaging

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4 The first AFM

5 Surface imagistics and topography The most widespread techniques for surface imaging and morphological characterization

6 AFM schematically Photodiode AFM-Probe mounted on spring Spring deflection detection Laserdiode Mirror Sample - Probe displacement Sample Feedback Mechanism Feedback XYZ Piezo-Scanner

7 AFM Imaging Idea : Stay at the same separation by keeping tip-sample Interactions constant while scanning the sample z-position controlled by feedback xy-position is scanned Possible feedback parameters : Spring deflection : Contact Mode For vibrating tips : Amplitude of vibration

8 Contact Mode

9 forces between probe and surface Van der Waals force: always present, attractive, outer electrons, long distance contact force: repulsion, chemical, core electrons capillary force: attractive, water layer! electrostatic and magnetic force friction force forces in liquids

10 strong repulsive forces in long range forces: in air: nn in liquids: pn in UHV: nn contact These long range forces must be compensated through short range repulsive forces: risk of damage for delicate samples! reduce by lever bending: F short range = F long range -k z lever

11 Methods Using Vibrating Tips Feedback parameter : Amplitude Advantages : No permanent tip-sample contact No shear forces Non-contact imaging possible Tapping Mode, Intermittent Contact Mode And Non-Contact Mode are the most successful methods for pure imaging

12 Tapping Mode

13 Phase imaging

14 Example of phase imaging

15 AFM image of PZT-Nickel Ferrite composite 500 nm

16 Other SPM Techniques MFM Magnetic Force Microscopy EFM Electric Force Microscopy SCM Scanning Capacitance Microscopy Many more

17 Magnetic force Microscope Special probes are used for MFM. These are magnetically sensitized by sputter coating with a ferromagnetic material. The cantilever is oscillated near its resonant frequency (around 100 khz). The tip is oscillated 10 s to 100 s of nm above the surface Gradients in the magnetic forces on the tip shift the resonant frequency of the cantilever. Monitoring this shift, or related changes in oscillation amplitude or phase, produces a magnetic force image. Many applications for data storage technology

18 Magnetic force Microscope

19 MFM Probe

20 MFM of Hard Drive HEIGHT MFM IMAGE

21 AFM and MFM image of PZT-Nickel Ferrite composite

22 AFM Beyond imaging Force Spectroscopy Example: Phase segregated binary polymer blend AFM Electrostatic Nanolithography

23 Phase segregation in polymers Segregation at nano scale under certain conditions Interest: antireflection coatings, polymer brushes, nanopatterning and template formation Model System: Polystyrene (PS)--Poly(methylmethacrylate) (PMMA) Question: identification of structure and the matrix

24 Phase segregation in polymers Height image Phase image

25 PS/PMMA blend thin film vacuum annealed at 210 o C for 1 hour 2umX2mu image of 90PS/10PMMA 5 5 µm2 images of 80PS/20PMMA

26 50 50µm2 image of 40PS/60PMMA 50 50µm2 image of 60PS/40PMMA

27 Local probing of surface structure and mechanical properties (elastic modulus, frictional and adhesive forces, shear stress, etc.) with a submicron resolution became possible after the introduction of atomic force microscopy

28 Force distance measurements The approach curve is roughly divided in thee parts Approach Withdrawal Zero line No interaction between tip-sample Snap in Attractive force > stiffness of tip Contact line sample is further pushed against the tip after contact

29 Hertz Model

30 The Young modulus determination from Force-Distance curve

31 Force vs penetration depth: 80PS/20PMMA matrix structure Data: Data1_B Model: Hook's Law Equation: y = k*x Weighting: y No weighting Chi^2/DoF = R^2 = Data: Data1_B Model: Hook's Law Equation: y = k*x Weighting: y No weighting Chi^2/DoF = R^2 = k ± k ± Force (nn) Force (nn) Pentration Depth (nm) (a) Pentration Depth (nm) (b) Young Modulus = 2.6 GPa Young Modulus = 3.8 GPa Bulk Young Modulus PS = 2.8 GPa Bulk Young Modulus PMMA = 4.06 GPa

32 Heating Stage

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34 Electrostatic Nanolithography AFM Tip Electric field applied across polymer film between AFM tip and conductive back-plane T=T g Dielectric liquid T > T g Polymer film Electric field Vm -1 results in electronic breakdown through polymer film Localized Joule heating occurs in high field region from current flow Conductive film

35 Nanolithography

36 Conclussions Considerations for AFM imaging - a smooth sample surface is required - Particles must - be strongly attached to the substrate. - AFM a probe or a tool?

37 Thank You

38 Surface segregation of PMMA can be explained mainly in terms of two factors, conformational entropy and translational entropy of PS and PMMA components Note an amorphous polymer chain in the bulk takes random coil conformation, its conformation entropy increases with increase in molecular weight polymeric chains existing at film surface are compressed along the direction perpendicular to the film surface. Thus the conformational entropy of a chain at the film surface is fairly smaller than that in bulk because conformations break at film surface. Therefore there is conformational entropy penalty at the surface and this conformational entropic penalty decreases with decrease in molecular weight