AFM for Measuring Surface Topography and Forces

ENB 2007 07.03.2007 AFM for Measuring Surface Topography and Forces Andreas Fery Scanning Probe : What is it and why do we need it? AFM as a versatile tool for local analysis and manipulation

Dates Course today 2* 1h with 15 min break do it yourself AFM : Tomorrow, starting 9:00 in AFM Lab in PCII

Scanning Probe : What is it an why do we need it versus IBM

How sensitive is an AFM? AFM-sensitivity translated to the macro-scale Matterhorn = AFM tip Ping Pong ball = molecular dimensions Why should one try something so difficult?

optical microscopy: the fraunhofer limit a point source imaged by a lens of finite size transforms into an airy disc pattern due to diffraction this sets a natural limit to the resolution of far field wave optics devices x min λ 2

solving the problem 1. usage of shorter wavelength Problems: adsorption of the beam by glass for UV, the beam itself for x-ray 2. usage of high speed electrons Problems: electron source, samples can only be observed in UHV, oxidation of the sample 3. Scanning techniques Problems: long measurement time, UHV (for SEM), sample can be damaged due to probe sample interactions

Resolution of SPMs the resolution of SPMs is determined by: the spatial dimension of the probe the probe has to be small the distance probe-surface the probe has to be a point probe scanning probe microscopes are surface microscopes

The different types of SPM interaction the nature of the interaction determines the property of the sample that is investigated the strength of the interaction used in SPM determines whether one observes or modifies the sample: SPM as a microscope or a tool

The different types of SPM STM: scanning tunnelling microscopy SFM: scanning force microscopy SNOM: scanning near-field optical microscopy SEM: scanning electron microscopy SCM: scanning capacitance microscopy SThM: scanning thermal microscopy SICM: scanning ion conductance microscopy SAM: scanning acoustic microscopy

the right tool for the right problem the surface of a magnetooptic medium seen in AFM the same part of the sample imaged in MFM (Magnetic force microscopy)

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

AFM-Tip Approaching and Retracting Jump to Contact Deflection-Displacement Characteristics Hysteresis P Piezo Piezo Jump off contact Jump to Contact Jump off Contact Substrate

force-distance characteristics Deflection as a function of the distance Contact Region Stable Contact Non-Contact Region Unstable Very stable very damaging shear forces Too unstable but would be less damaging

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

principle Contact mode

Example DNA on a mica surface molecular resolution for a nonconducting sample in air

Convolution Effects One thing to keep in mind : convolution effect The smaller thing images the bigger thing The signal is always a convolution of sample topography And tip topography a bigger issue for AFM than for STM Tips should be as sharp as possible (10nm standard)

Artifacts DNA and debris PS spheres How can you check if this is an artifact??

Example : Surface Damage Molecules are swept out off the scanned Area

example : surface modification using the possibility to exert forces with the afm: nano- sculptures

recording friction friction properties can be recorded by monitoring the torsion of the cantilever

example LFM hydrophobic and hydrophilic regions can be clearly destinguished in LFM

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

Dynamic AFM Tip+cantilever can be described as harmonic oscillator 2 x m + kx = 2 t F ext x Here : F ext = A ext ( t) = A( ω ) sin( ωt + φ( ω )) sin ( ωt) A(w) : Amplitude F(w) : Phase Shift w R : resonance-frequency ω R = k m w R w

Vibrating Tip and Surface Forces 2 x m + kx = 2 t Simple picture : F ext 2 x m + kx + Fsurf = 2 t ( x) Fext Long-range attractive force Spring force Simplify force (linear approx.) A spring again, but k has decreased An attractive force gradient results in an effective decrease of the spring constant

Effect on Vibration Amplitude Phase-shift Excitation frequ. equals res. frequ. k effectively decreases ω R = k m w Ext = w R Free vibration (no surface force) w w R w Ext Surface influenced vibration Excitation frequency stays at w R, therefore : Phase changes A changes Amplitude can serve as Feedback parameter

example soft ultrathin pattern on silicon wafer (single molecular resolution under water)

Vibrating Tip Revisited: Phase Contrast Chemically heterogeneous surface Region 1 Region 2 Different surface forces F Long-range repulsive F Long-range attractive D D Different Phase Shift at same amplitude Phase 2 Phase 1 w R w R

Example Phase Contrast Phase Image (right) shows Material Contrast Here : film of Diblockcopolymer mixture (images courtesy M. Schneider / H. Schlaad)

ENB 2007 07.03.2007 AFM for Measuring Surface Topography and Forces II Andreas Fery Force Spectroscopy : AFM for measuring forces Modern trends in AFM

AFM-Tip Approaching and Retracting Jump to Contact Deflection-Displacement Characteristics Hysteresis P Piezo Piezo Jump off contact Jump to Contact Jump off Contact Substrate

Simulating a measured curve F Instability! Det. Deflection Piezo ext. Deflection unstable D Piezo ext. stable Rest position F Deflection Piezo ext. D

Impact of the spring constant on stability B. Cappella and G. Dietler, "Force-distance curves by atomic force microscopy", Surface Science Reports, 34, 1-+, (1999)

Impact of the spring constant on stability 2 B. Cappella and G. Dietler, "Force-distance curves by atomic force microscopy", Surface Science Reports, 34, 1-+, (1999)

From raw data to force-distance 1) Photodetector signal [V] Interaction force [N] 2) Piezo displacement [m] Distance probe-surface [m] 1) : a) get sensitivity from force-deflection characteristic of hard sample ( photodiode signal [V] -> deflection [nm] ) b) determine spring constant ( deflection [nm] -> force [N] ) 2) : a) use calibrated Piezo / measure displacement b) subtract/add cantilever deflection from piezo position

added mass effect J. P. Cleveland, S. Manne, D. Bocek and P. K. Hansma, Review of Scientific Instruments, 64, 403-405, (1993)

added mass effect 2 J. P. Cleveland, S. Manne, D. Bocek and P. K. Hansma, "A Nondestructive Method for Determining the Spring Constant of Cantilevers for Scanning Force Microscopy", Review of Scientific Instruments, 64, 403-405, (1993)

thermal noise spectrum J. L. Hutter and J. Bechhoefer, "Calibration of Atomic-force microscopy tips", Rev. Sci. Instrum., 64, 1868, (1993)

Two worlds I have measured that my AFM tip is attracted with 15 nn force at 10 nm distance I have calculated that 2 infinite half spaces of this material should have an interaction energy/area of 1J/m 2 at this distance Experimentalist Theoretician How can we build a bridge from one to the other??

A bridge 2R 1 2R 2 D Derjaguin relation : R R2 F( D) = 2π 1 W ( D ) R + R 1 valid for all forces, as long as R >> range of force 2 infinite half space D infinite half space F(D) : interaction Force W(D) : interaction energy per area

Typical AFM cantilevers Here : radius of tip R not much bigger than typical interaction ranges

Colloidal Probe AFM

A typical protein for single molecule experiments : Titin His-Tag to allow for easy purification Multiple repeats of TI I27 free SH allows for attachment to gold surfaces

The AFM Experiment

Pulling chains off the surface k sp v Force = k sp Deflection Stage Position = v Time

Pulling chains off the surface

statistical variation: to few unfolding events : not the full protein is stretched to many unfolding events, more than 1 protein

statistics over many pulls to compensate for that, statistics over many pulls is necessary from this the typical curve is obtained

what does the peak force mean? influence of additional coupling to spring :

the force is rate dependent Unfolding proteins by AFM is a kinetic measurement: average unfolding force depends on pulling speed. Average unfolding rates can be estimated by Monte-Carlo simulation or by extrapolation. Force (N) Force (N)

physically meaningful single molecule experiments * average over many experiments to extract the typical set of unfolding lengths and forces do this for different pulling speeds compare to molecular dynamics calculations

Real-life Cantilevers Real-life cantilevers are damped harmonic oscillators 2 x m + kx = 2 t F ext Ideal, no damping 2 x x m β + kx = 2 t t F ext Real, damping force proportional to speed Damping broadens the resonance peak, limiting sensitivity A ω Res ω ω 50% of max. ampl. Damping is characterized by the Q-Factor (Quality Factor) ω Q = R ω Typical Q-factors are between 100 and 1000

Q-switching : tuning the resonance 2 x x m β + kx = 2 t t F ext Usually with F ext = A ext sin ( ωt) Now, use additional external force coupled to speed, compensating damping F ext x = Aext sin( ωt) β Damping force compensated!! t A A Q>10000 possible ω ω In theory much better for imaging, in practise sometimes

Supertips carbon nanotubes as a try to get the perfect tip (no convolution problem)

Combination AFM - microscopy general problem in deformation measurements : only force (indentation) accessible for (mechanically) complex objects not suffcient methods that combine force-deformation with shape information!

Combination of force and deformation measurements combination RICM - AFM interferometry example : buckling process

Setup

Acknowledgement Marc Nolte H. Heinzelmann because they made so nice presentations concerning SPM where I could pinch graphics, animation,