Outline Scanning Probe Microscope (SPM)

AFM

Outline Scanning Probe Microscope (SPM) A family of microscopy forms where a sharp probe is scanned across a surface and some tip/sample interactions are monitored Scanning Tunneling Microscopy (STM) Atomic Force Microscopy (AFM) contact mode non-contact mode tapping mode Other forms of SPM lateral force magnetic or electric force thermal scanning phase imaging

AFM Probe Construction Low spring constant (k - 10-2 to 10 2 N/m) Sharp protruding tip (r=5-50 nm) High resonance frequency 1 ω = 2 π k m Three common types of AFM tip normal supertip ultralever

Common types of cantilevers Si 3 N 4 Si Diamond

Fabrication of cantilevers

Calibration of cantilever Theoretical method Static method Dynamic method Measuring of thermal response of the cantilever Measuring of the change of resonance frequency caused by the addition of known masses E=300 GPa E=238 GPa ( Z ) ' Zt ks = Ztkc t

Superposition of two geometries

Reconvolution of the tip shape I II r d D=d real d D = D 2 4r

Deconvolution of the tip shape Tobacco Mosaic Virus (TMV) d~18 nm r-?

Calibration of the tip shape L 2r L h 2 L R = 4 r 2 L R = 2 h

Useful formulas 2 d z = 8R d 2. 8nm z = d > 3nm d 2 ( 4d + 8R ) d = 2R ( ) z + z + h

AFM Tip Artifacts We start off with an example of a good AFM image of 300 nm polystyrene spheres...

AFM Tip Artifacts Similar spheres imaged with a supposedly sharp tip

AFM Tip Artifacts This image should only contain images of large polysterene spheres

AFM Tip Artifacts Classical example of tip artifact, showing DNA and debris

AFM Tip Artifacts Here is a cute example of some serious artifacts. The obtained image looks like the three bears

Different types of forces relevant to AFM 4 3 F g = mg = πr ρg 3 3 ρ 1 g / cm r = 25 nm g = 9. 8 m / sec 2 9 F g 10 nn (a) A r F vdw = 2 6D 19 A = 10 J D = 0. 3 nm r = 25 nm F vdw ~ 5 nn F adh γ = 2 γ = 4πr γ ( γ γ ) 1/ 2 γ = 100 mj/m 2 s silica surface energy p s p = 25 mj/m 2 polymer surface energy F adh ~ 30 nn (d) Deformation forces (b) Capillary forces F cap = 4πRγ1 cos θ ~ 22nN F d = Ka R 3 ~ 6nN, a ~ 5 nm, the typical contact radius

Blind Reconstruction AFM profile of a single bump What does this single scan line tell us about the topography of the tip and sample? The tip geometry can be no bigger than the obtained profile

Blind Reconstruction Line scan having two bumps What does this tell us about the shape of the tip? Case 1: Tip with single apex

Case 2: double tip

True three-dimensional scanning? One of the drawbacks of typical AFM is that the images obtained are not truely threedimentional. No matter how sharp the tip, the data collected can never access the underside of the sample. Petticoat effect-all images of objects having steep walls or undercut regions appear to have flared sides

Method for imaging sidewalls by AFM Martin, Wickramasinghe, Appl. Phys Lett 1994, 64, 2498 Can we get a similar image using a typical AFM and the boot-shaped tip? No!

Titanium thin films 2 µm per 2 µm

The tip is not too bad, it just is not very sharp

A subtle example of a double tip

Here is an image taken by a multiple tip

This is an image of a triple tip

Sometimes you just can t figure ou what is wrong with the tip- it just doesn t look right!

The tip broke or had a large contaminant dislodged while scanning down

TGT spikes delta-like function: 1 µm high, 1.5 µm apart

TGT spikes delta-like function: 1 µm high, 1.5 µm apart A formation of B2 bombers? The return of space invaders A school of fish, maybe? The eyes are rounded tips about 100nm in radius

Normal tip vs NT-tip

Conclusion: Golden rule of AFM spectroscopy: Every time one measures one obtains an image Not every time one obtains an artifact

Oxide-Sharpened Tips increasing aspect ratio reducing tip radius SiO 2 HF etching Aspect ration- 10:1 Radius r~1nm

Electron beam deposition (EBD) High-aspect-ratio tips L=(1-5)µm R=(20-40)nm Carbon materials are deposed by the dissociation of background gases in the SEM vacuum chamber

Carbon Nanotube Tips Single-walled carbon nanotubes (SWNT), d=(0.7-3)nm Multiwalled carbon nanotubes (MWNT) (nested, concentrically arranged SWNT, d=(3-50)nm High-aspect-ration AFM probes Very stiff, E=10 12 Pa (the stiffest known materials) Buckled nanotubes Labor intensive Not amenable to mass production

Pick-up Tips d=0.9nm d=2.8nm

Chemical Vapor deposition (CVD) Direct grow nanotubes onto AFM tip Heating of nanocatalyst particle (r~3.5 nm) Presipitates carbon nucleates a grow of nanotube

Direct grow of nanotubes Alumina/iron/molybdenumpowdered catalyst 2 nm in diameter 2µm in length Labor intensive Not amenable to mass production

Modes of operation

The common AFM modes contact mode tapping mode Contact mode Non-contact mode Intermittent mode

Contact mode AFM A tip is scanned across the sample while a feedback loop maintains a constant cantilever deflection (and force) The force on the tip is repulsive ~ a few nn The tip senses lateral and normal forces The tip contacts the surface through the adsorbed fluid layer Forces range from nano to micro N in ambient conditions and even lover (0.1 nn or less) in liquid

Force curve

Contact mode C Force D B Static mode Dynamic mode Oscillation amplitude Probe-sample distance, z A F > k, z at "B"is F = kz, k = 0. 01 z = 0. 1nm F ~ 1pN 0. 1 N / m, jump into a contact to "C"

Problems of the contact mode Large deformation forces ~ 100 nn Capillary forces F cap = 4πRγ1 cos θ ~ 22nN To solve the problem operation in liquid Elimination of capillary forces Reduction of van der Waals forces

Problems of the contact mode Large lateral (shear) forces ~ 100 nn To solve the problem non-contact mode

Non-contact Mode AFM Highly unstable mode Ultra high vacuum at low temperature

Tapping mode AFM A cantilever with attached tip is oscillated at its resonant frequency and scanned across the sample surface A constant oscillation amplitude (and thus a constant tip-sample interaction) are maintained during scanning. Typical amplitudes are 20-100 nm Forces can be 200 pn or less The amplitue of the oscillations changes when the tip scans over bumps or depressions on a surface

Tapping mode AFM ω A 0 0 50 500kHz 10 100nm

Tapping mode AFM Force Static mode Dynamic mode Probe-sample distance, z A B C D Oscillation amplitude

Tapping mode AFM Three regimes of tapping mode: (i) Light tapping 0. 7 r 1 sp r sp = A sp A 0 (ii) Moderate tapping 0. 3 rsp 0. 7 (iii) Hard tapping 0. 01 rsp 0. 3

Tapping mode AFM Phase Imaging Driven force Actual responce Different characteristics of the sample different offset the phase

Examples of Phase Images AFM

Advantages and Disadvantages Contact Mode Advantages Disadvantages high scan speeds the only mode that can obtain atomic resolution images rough samples with extreme changes in topography can sometimes be scanned more easily lateral (shear) forces can distort features in the images the forces normal to the tip-sample interction can be high in air due to capillary forces from the adsorbed fluid layer on the sample surface the combination of lateral forces and high normal forces can result in reduced spatials resolution and may damage soft samples (i.e. biological samples, polymers) due to scraping

Advantages and Disadvantages Tapping mode Advantages higher lateral resolution on most samples (1 to 5 nm) lower forces and less damage to soft samples imaged in air lateral forces are virtually eliminated so there is no scraping Disadvantages slightly lower scan speed than contact mode AFM

Cantilevers used in contact and tapping modes k 0. 01 1 N / m k ~ 50 N / m

Contact vs Tapping modes

How to interpret height and phase TM-AFM images of a sample in terms of its physical and morphological properties?