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Scanning Probe Microscopy (SPM)

- Fundamental principles of SPM

- Scanning tunnelling microscopy (STM)

- Atomic force microscopy (AFM)

Markus Ahlskog

Nanoscience Center

University of Jyväskylä

Main types of microscopy

Optical microscopy Electron microscopy(TEM,SEM)

Scanning probemicroscopy (SPM)

1 µm – 1 cm1 Å – 1 mm 1 Å – 10 µm

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Historical developments

1600 1700 1800 1900 2000

Hansson & Janssen,1597, Holland, 20-30xVan Leeuwenhoek,1673, Holland, 270x

1934 Electron microscopy

Scanning probe microscopySTM 1981, AFM 1986

The optical (compound)microscope

The father of microscopy, Anton

Van Leeuwenhoek of Holland (1632-1723)

Present day instruments, changed but little, give magnifications up to 1250x with ordinary light and up to 5000 with blue light

A light microscope, even one with perfect lenses and perfect illumination, simply cannot be used to distinguish objects that are smaller than half the wavelength of light

Any two lines that are closer together than 0.275 micrometers will be seen as a single line, and any object with a diameter smaller than 0.275 micrometers will be invisible or, at best, show up as a blur

magnifications up to 270x

Optical Microscopy

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Switzerland Federal Republic of Germany

Federal Republic of Germany

1/4 of the prize

1/4 of the prize

1/2 of the prize

Heinrich Rohrer

Gerd BinnigErnst Ruska

The Nobel Prize in Physics 1986

"for their design of the scanning tunneling microscope"

"for his fundamental work in electron optics, and for the design of the first electron microscope"

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Electron microscope image of yeast cell

Kuva 1.9

The founders Scanning Probe Microscopy are Binnig and Rohrer . Patent for Scanning Tunneling Microscope was issued Aug. 10, 1982 (Priority Sept. 20, 1979)

Founding Fathers of Scanning Probe Microscopy

Microscopy for Nanotechnologists

Most electron microscopes can "see" down to about 10 angstroms--an incredible feat, for although this does not make atoms visible, it does allow to distinguish individual molecules. In effect, it can magnify objects up to 1 million times.

The STM can image atomic details as tiny as 1/25th the diameter of a typical atom, which corresponds to a resolution several orders of magnitude better than the best electron microscope.

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Scanning Probe Microscopy (SPM) basics I

1) A very sharp tip moves alonga surface contour with velocity v.

2) The distance between the tip andthe surface is kept constant byadjusting the tip height z.

3) The variation of tip height (z(x)) isrecorded which gives the data foran image of the surface:

SPM basics: Image formation

An image is created by

collecting data (z) point-

by-point (pixels) from a

rectangular area of the

surface.

Area size: 10 nm -100 µm

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SPM: Effect of finite width of tip I

Any tip then is characterized by a finite “tip-radius” Rt. If an imaged objecthas a radius RS, then the minimum width of the object in the image will be:

st RRw 4=

SPM: Effect of finite width of tip II

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SPM basics II

For SPM we need to:

1) …enable highly accurate motion of the tip. Solution: Piezoelements

2) …measure the distance between the tip and the surfacevia some interaction between them (feedback signal).Solution: Many (STM, AFM, SNOM)

3) …record the motion of the tip in the z-direction, to createthe image. Solution: from 1 & 2

Piezoelements for SPM

Tube scanner:

Controlled movement with resolution ~ 1 Å (0.1 nm) routinely possible

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SPM basics III

2)

1) 3)

Types of Scanning Probe Microscopy (SPM)

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SPM modes

Tapping mode AFM

accounts for most of

the use among the

different types of SPM

SPM basics: Control of scanning

)()( teKtu rPP =

∫=t

rII dtteKtu0

)()(

Feedback via PI-controller:

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Scanning Tunnelling Microscopy (STM)

Feedback signal obtainedfrom tunnelling current be-tween tip and conductingsurface.

STM image (20 x 20 nm2) of Si(111) surface

LT STM stage

Specifications

Lowest temperature at the sample: < 5 KInitial cool down time to 5 K: < 6 hTime between LHe refills: > 15 hCoarse movement: X/Y/Z = 5 x 5 x 10 mmScan range (and offset range) : X/Y/Z = 10x10x1 µm at 300 KX/Y/Z = 1.8x1.8x0.2 µm at 5 KZ-resolution: < 0.01 nmGap Voltage: ± 0.5 mV to ± 10 VTunneling current setpoint: 50 pA... 50 nABakeout temperature: up to 150°CVacuum achievable: 10-11 mbar range

“Multiprobe LT” - ultrahigh-vacuum variable temperature STM system from “Omicron”;

440 kEuro

Katholieke Universiteit Leuven

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1989: Atomic manipulation by STMIBM logo – 35 Xenon atoms

IBM Almaden Research Center, San Jose

The enabling tool – STM

Can be used not only to image a surface with atomic resolution, but also to manipulate individual atoms and molecules.

Atomic force microscope (AFM)

Feedback signal obtainedfrom bending of flexiblecantilever, due to tip-surfaceforce.

Better name is:Scanning Force Microscope(SFM) [but much less used].

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AFM cantilever and tip

1 µµ µµm

Tip motion detection in AFM

Bending of cantilever gives

change in A-B signal from

photodetector (PSPD)

Detection of Z-motion

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SPM AutoProbe M5Stage:Z - travel range: 35 mmX, Y -travel range: 200 x 200 mmResolution: 1.0 µmMetrology Scanner:x, y 100 µmz 7.5 µmResolution: x, y 1 nm; z 0.1 nm

Contact mode AFM

http://www.ntmdt.ru/

Problems:

- Tip wear

- Tip may push looseobject, rather thanmove over them.

The tip is in direct mechanicalcontact with the surface.

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Non-contact atomic force microscopy

http://www.ntmdt.ru/

- The tip oscillates a small distanceabove the surface (1-10 nm).

- Feedback signal from change inamplitude (for example) with changing

tip-surface distance.

- Avoids the main problems with

contact-mode.

Tapping mode : tip slightly touches (”taps”)at bottom of each oscillation cycle

AFM: Cantilever dynamics

m

k=0ω

2

02220

0

)(

/)(

+−

=

Q

mFA

ωωωω

ω

Resonant frequency:

Amplitude vs. frequency (driving forceF0sin(ωt)):

In non-contact/Tapping mode AFM, cantilever is oscillated at a frequencyclose to the resonant frequency.

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AFM: Tapping mode of Au islands

SEM image

AFM: indentation

Error mode images taken 5minutes apart of an indentedcell undergoing a slowrecovery.(University of Texas, Austin)

F-Z curve as tip is lowered andpresses on the surface (indentation)

F

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MFM, how it works

Magnetic force microscopy

Electrostatic force microscopy

http://www.ntmdt.ru/