Scanning Force Microscopy II

Transcription

1 Scanning Force Microscopy II Measurement modes Magnetic force microscopy Artifacts Lars Johansson 1

2 SFM - Forces Chemical forces (short range) Van der Waals forces Electrostatic forces (long range) Capillary forces (in air) Magnetic forces (small) Many forces - can measure many properties, but complex measurements and analysis 2

3 Notes on forces Chemical forces: Due to wave function overlap, repulsive or attractive, very short-range (atomic resolution possible) Van der Waals forces: Induced dipole interactions, medium range Force: F VdW = (HR)/(6D 2 ), for tip-sample geometry H: Hamaker constant VdW forces strongly dependent on medium between tip-sample Electrostatic forces: long-range Coulomb interactions Localized surface - tip charges Tip-sample potential difference: R F el = πε 0 ( z U bias U ) 2 cpd 3

4 SFM operation modes 4

5 Dynamic modes Q-factor 5

6 Contact - non-contact modes 6

7 Contact force microscopy Most common topography imaging mode No atomic resolution (1-10 nm) Measurement of lateral forces possible - friction forces Not suitable for soft materials Equilibrium of attractive and repulsive forces - jump-to-contact instability Tip artifacts common Soft cantilevers 7

8 Force curves Distance Cantilever deflection 8

9 Friction force microscopy (Lateral force microscopy) Single contact friction (nano-tribology) different from macroscopic friction: non-linear dependence on normal force, F proportional to contact area, velocity dependence Langmuir-Blodgett film: Fluorocarbon and hydrocarbon areas Topography Lateral force 9

10 Friction at step edges Cu(111) surface with monatomic steps and scratch: Topography (a,b) identical in forward and backward scans. Lateral forces (c,d) inverted at step edges and scratch 10

11 Example: Lithography on Au surface (Krister Svensson, Karlstads universitet) Topography Lateral force 11

12 Atomic friction: slip-stick Atomic-scale features in lateral force measurements: slip-stick behaviour due to atomic interactions Example: NaCl(100) sawtooth curve follows the surface lattice 12

13 Adhesion measurements Temperature-dependent adhesion on steels, A. Gåård, J. Appl. Phys. 103,

14 Dynamic force microscopy (Non-contact mode) Cantilever oscillation excited at eigenfrequency - stiff cantilever to avoid contact - high Q-factor Frequency shift due to attractive force - Feedback via frequency shift or amplitude Stable operation more difficult Capable of atomic resolution Quantitative analysis of forces possible (with constant amplitude and freq. shift) 14

15 DFM theory How relate Δω to the force? Damped harmonic oscillator approximation Electrostatic force Van der Waals force Reduced frequency shift γ freq = Δf f ka 3/2 Force curve can be derived from Δω vs. distance curves ( spectroscopy ) 15

16 Force spectroscopy Frequency shift vs. distance curves Separate different force contributions Example: Electrostatic force minimized VdW force fitted to long-range part of curve and subtracted remaining short-range force 16

17 Atomic resolution Model system Si(111)7x7: dangling bonds, strong chemical force Non-contact SFM at 7.2 K Lantz et al., PRL 84,

18 Artifacts in atomic resolution Monatomic step on Si(111)7x7 a) Topography (apparent inversion), b) Tunneling current Interference of long-range forces 18

19 Quartz tuning fork sensor Franz Giessibl developed the qplus sensor based on a quartz tuning fork for non-contact SFM (Appl. Phys. Lett. 76, 1470) High stiffness (k=1800 N/m), low amplitude, ideal for short-range forces Self-sensing, simple electronics, uses quartz tuning forks for watches Example: sub-atomic resolution on Si(111)7x7 (Giessibl, Science 289, 422) 19

20 Dissipation force microscopy Non-conservative forces => damping = dissipation Internal damping due to internal friction Mechanisms: induced currents in sample, resistive losses, magnetic hysteresis loops, phonons, etc. 20

21 Tapping mode force microscopy (intermittent contact mode) Dynamic mode with intermittent contact in each cycle Strongly reduced lateral forces - ideal for soft materials (e.g. polymers, biological samples) Amplitude control parameter Resolution determined by tip shape Phase shift - measure of surface stiffness PMMA - PCL polymer blend 21

22 Non-linear effects in tapping mode F(z) is highly non-linear, especially for repulsive contact Several oscillation states - abrupt jumps in amplitude - topography artifacts Choose exp. parameters to avoid bistable regions 22

23 Topography vs. elastic properties Apparent height difference but purely mechanical origin Topography Phase image Approach - retract curves Triblock copolymer - lamellar structure with glassy and rubbery domains Kopp et al., Langmuir 16,

24 Magnetic force microscopy Large interest in magnetic thin films and nanostructures Spintronics Low-dimensional magnetism difficult topic Subtle mechanisms, structure dependent Large interest in MFM Problem I: magnetic forces very small, use non-contact modes Problem II: tip-sample distance control 24

25 MFM examples a-b: magneto-optical disc c-d: YBCO 25

26 Magnetic stray fields MFM measures stray fields outside the sample, not equivalent to magnetization inside Magnetic field inside the sample can not be uniquely determined 26

27 MFM contrast formation Interaction magnetic tip - sample stray field (or vice versa) Necessary condition: know the magnetization of the tip Calibration measurements, model structures Problem: Field of the sample may modify magnetization of the tip (and vice versa) three cases: - negligible modification - reversible modification - 3x3=9 possible cases - irreversible modification (hysteresis) 27

28 Reversible - irreversible modifications Barium ferrite crystal - imaging with magn. hard Co tip (a) and soft Ni tip (b). Soft tip magnetization is reversed when crossing a domain wall Permalloy nanoparticle: tipinduced magnetic state changes 28

29 Artifacts in SFM measurements Four classes Tip artifacts: most common: tip shape convoluted with sample topography Topography images influenced by local variations in properties like conductance, elasticity, adhesion, friction, etc. Local measurements influenced by local topography, e.g. SNOM, lateral force Instrumental artifacts 29

30 Artifacts in SFM measurements - Tip effects Tip artifacts most common: tip shape convoluted with sample topography - sample feature with high aspect ratio compared to tip => imaging of tip! DFM images of Al 2 O 3 (0001) with needle-shape structures 30

31 Tip artifacts Example: Nylon layer imaged with sharp and blunt (wedge-shaped) tips Reconstruction of topography - tip deconvolution - Requires knowledge of tip geometry - Disturbance from noise 31

32 Local inhomogeneities Local inhomogeneities can influence the topography image, e.g. friction, (contact mode), long-short-range forces (DFM) 32

33 Local measurements influenced by topography Typical artifact in SNOM (scanning near-field optical microscopy) measurements Lateral force measurements influenced by topography b) topography d-e) lateral force 33

34 Instrumental artifacts Scanner-related: hysteresis, creep, non-linearities and calibration errors Tip crashes Feedback oscillations Noise, thermal drift 34