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Scanning Probe Microscopy HT12 1

Scanning Force Microscopy II

Measurement modes Magnetic force microscopy

Artifacts

Lars Johansson


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


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: FVdW = (HR)/(6D2) , for ideal tip-sample geometry H: Hamaker constant, material dependent •  VdW forces strongly dependent on medium between tip-sample

Electrostatic forces: •  long-range Coulomb interactions •  Localized surface - tip charges •  Tip-sample potential difference:

!

Fel = "#0RzUbias $Ucpd( )2


SFM operation modes

Dynamic modes – Q-factor



Contact - non-contact modes


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


Force curves

Distance

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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


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


Example: ”Lithography” on Au surface (Krister Svensson, Karlstads universitet)

Topography Lateral force


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


Adhesion measurements

Temperature-dependent adhesion on steels, A. Gåård, J. Appl. Phys. 103, 124301


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)


DFM theory How relate Δω to the force?

Damped harmonic oscillator approximation

Electrostatic force

Van der Waals force

Force curve can be derived from Δω vs. distance curves (”spectroscopy”)

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" freq =#ffkA3/2Reduced

frequency shift


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


Atomic resolution

•  Model system Si(111)7x7: dangling bonds, strong chemical force

Lantz et al., PRL 84, 2642

Non-contact SFM at 7.2 K


Artifacts in atomic resolution Monatomic step on Si(111)7x7

a) Topography (apparent inversion), b) Tunneling current

Interference of long-range forces


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)


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.


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


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


Topography vs. elastic properties

Topography Phase image Approach - retract curves

Kopp et al., Langmuir 16, 8432

Triblock copolymer - lamellar structure with glassy and rubbery domains

Apparent height difference – but purely mechanical origin


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

MFM operation modes

Tip-sample distance control: feedback control on magnetic forces impossible due to force sign changes, and very small forces => control on other interactions: •  Mixed van der Waals and magnetic forces: convoluted topographical and

magnetic information •  Constant tunneling current •  Tip-sample capacity •  Lift-off-techniques: measure topography, then lift up the tip, let the cantilever

follow the topography profile, in line by line scans.



MFM examples

a-b: magneto-optical disc,

Contact (a) and lift mode (b)

c-d: YBCO, 7 K,

Non-contact (c) and lift mode (d)


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


MFM contrast formation

•  Interaction magnetic tip - sample stray field: force on tip given by convolution

•  Necessary condition for quantitative measurements: 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 -  irreversible modification (hysteresis)

-  3x3=9 possible cases


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: tip-induced magnetic state changes

Separation of topography and magnetic signal

•  Problem: high sensitivity requires small tip-sample distance (few nm), but strong non-magnetic forces must be avoided or taken into account

•  Electrostatic forces: compensate with bias:


Fel =!C!z

Ubias "Ucpd( )2

•  Van der Waals forces: comparable to magnetic forces at the typical tip-sample distances.

•  Lift-mode measurements widely used, but risks for topography-induced artifacts at small lateral dimensions.

•  Example: Measure twice with opposite magnetizations of the tip, subtract the images

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



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 Al2O3(0001) with needle-shape structures

Tip curvature crucial parameter

Tip artifacts


R =h2 + w 2( )2

2h


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

Example of tip artifacts (T. Junno, Lund University)


Appl. Phys. Lett. 66, 3627 (1995)

Appl. Phys. Lett. 66, 3295 (1995)


Local inhomogeneities

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

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

Flateral = ±µFN +!s!xFN

Instrumental artifacts

•  Scanner-related: hysteresis, creep, non-linearities and calibration errors

•  Tip crashes •  Feedback oscillations •  Noise, thermal drift •  Vibrational noise •  Laser interference effects


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