3 AFM Lecture

8/9/2019 3 AFM Lecture

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Microscopy

1


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2


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Scanning Probe Microscopy (SPM)

y x

Moni tor the interactions between a probe and a sample surface

Types of SPM


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Scanning Probe Microscopy

History: The Scanning Tunneling Microscope (STM)was invented by G. Binnig and H. Rohrer, for whichthey were awarded the Nobel Prize in 1984

A few years later, the first Atomic Force Microscope

(AFM) was developed by G. Binnig, Ch. Gerber, andC. Quate at Stanford University by gluing a tinyshard of diamond onto one end of a tiny strip of goldfoil

TYPES OF SPM ?


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• Atomic Force Microscopy (AFM)-Monitors the forcesof attraction and repulsion between a probe and asample surface

• Scanning Tunneling Microscopy (STM). Tunneling ofelectrons through air between probe and surface

• Lateral Force Microscopy (LFM) Frictional forcesmeasured by twisting or “sideways” forces on

cantilever.• Magnetic Force Microscopy (MFM)-Magnetic tip

detects magnetic fields/measures magnetic propertiesof the sample.

• Electrostatic Force Microscopy (EFM)-Electricallycharged Pt tip detects electric fields/measuresdielectric and electrostatic properties of the sample


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•Chemical Force Microscopy (CFM)-Chemically

functionalized tip can interact with molecules onthe surface – giving info on bond strengths, etc.•Near Field Scanning Optical Microscopy (NSOM)-Optical technique in which a very small aperture is

scanned very close to sample, Probe is a quartzfiber pulled to a sharp point and coated withaluminum to give a sub-wavelength aperture (~100nm), A brief introduction of few techniques is given

below


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STM: scanning tunneling microscope

nA

R

piezo-

element

e- e-

e-

e- e-

e-

e-

e-

e-

< 1nm

tunneling of electrons through

air between probe and surface

only conducting material

probe

x-y stage


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Scanning Tunneling Microscopes (STMs)

Monitors the electrontunneling current between a probe and asample surface

What is electrontunneling?

Classical versus

quantum mechanicalmodel

Occurs over veryshort distances

Scanning Probe

Tip and surface and electron tunneling


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STM Tips

Tunneling current

depends on the

distance between

the STM probe andthe sample Tip

Surface

Tunneling current depends on distance between tip and surface


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x

10 6 x 10 8 x 10 8

STM Tips

How do you

make an

STM tip

“one atom”

sharp?

Let’s Zoom In!

e-


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Putting It All Together

The human hand

cannot precisely

manipulate at the

nanoscale level

Therefore,

specialized

materials are used

to control the

movement of thetip


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AFM Tips

The size of an

AFM tip must

be carefully

chosen

Interatomic interaction for STM

(top) and AFM (bottom).Shading shows interaction

strength.

STM tip

AFM tip


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STM: scanning tunneling microscope

nA

I control

I tip

∆I

R

∆V

piezo-element (changes

length at different

voltages)


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MFM: magnetic force microscope

AFM with magnetic probe

e.g. hard disc, tape

magnetic tip

laser photodiode

piezo-element


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SNOM: scanning near-field optical microscope

fiber tunneling of photons between

probe and surface

shows the amount of lightthat is

absorbed/transmitted for

different colors

sample

lens

detector

filter

e.g. fluorescent molecules

metal-coated

fiber tip


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Atomic Force Microscopy


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General Applications

Materials Investigated: Thin and thick filmcoatings, ceramics, composites, glasses,synthetic and biological membranes,metals, polymers, and semiconductors.

Used to study phenomena of: Abrasion,adhesion, cleaning, corrosion, etching,friction, lubricating, plating, and polishing.

AFM can image surface of material inatomic resolution and also measure forceat the nano-Newton scale.


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How Does AFM Work?


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Parts of AFM

1. Laser –

deflected offcantilever

2. Mirror – reflects laser beamto photodetector

3. Photodetector – dualelement photodiode that

measures differences in lightintensity and converts to voltage

4. Amplifier

5. Register

6. Sample

7. Probe –

tip that scanssample made of Si

8. Cantilever – moves asscanned over sample anddeflects laser beam


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Necessary Components

Indirect detection of force

Vibration Isolation

Flexibility of Cantilever


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Modes of AFM

(1) Contact Mode,

Prob –surface separation< 0.5 nm

(2) Non-Contact Mode

Prob – surface separation< 0.5-2 nm

(3)Tapping Mode (Intermittent contact),

Prob –surface separation< 0.1-10 nm)


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Force Measurement

The cantilever is designed witha very low spring constant (easy

to bend) so it is very sensitive to

force.

The laser is focused to reflect offthe cantilever and onto the

sensor

The position of the beam in the

sensor measures the deflectionof the cantilever and in turn the

force between the tip and the

sample.


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Contact Mode

Measures repulsion between tip and sample

Force of tip against sample remains constant

Feedback regulation keeps cantileverdeflection constant

Voltage required indicates height of sample

Problems: excessive tracking forces appliedby probe to sample


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Contact Mode

Contact mode operates in the repulsive regime

of the van der Waals curve

Tip attached to cantilever with low springconstant (lower than effective spring constant

binding the atoms of the sample together).

In ambient conditions there is also a capillaryforce exerted by the thin water layer present

(2-50 nm thick).


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How It Works

Three common types of AFM tip. (a) normal tip (3

µm tall); (b) supertip; (c) Ultralever (also 3 µm tall)


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Non-Contact Mode

Uses attractive forces to interact surface with tipOperates within the van der Waal radii of theatoms

Oscillates cantilever near its resonant frequency

(~ 200 kHz) to improve sensitivity Advantages over contact: no lateral forces,non-destructive/no contamination to sample, etc.


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Tapping (Intermittent-Contact) Mode

Tip vertically oscillates between contactingsample surface and lifting of at frequency of50,000 to 500,000 cycles/sec.

Oscillation amplitude reduced as probecontacts surface due to loss of energy causedby tip contacting surface

Advantages: overcomes problems associatedwith friction, adhesion, electrostatic forces

More effective for larger scan sizes


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Figures of Merit

Can measure surface features with

dimensions ranging from inter-atomic

spacing to 0.1mmResolution limited by size of tip (2-3 nm)

Resolution of imaging 5 nm lateral and

0.01nm vertical


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Advantages of AFM

AFM versus STM (scanning tunnelingmicroscope): both conductors and

insulators AFM versus SEM (scanning electronmicroscope): greater topographiccontrast

AFM versus TEM (transmission electronmicroscope): no expensive sampleprep.


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Biological Applications

Used to analyze DNA, RNA, protein-nucleic acidcomplexes, chromosomes, cell membranes, proteinsand peptides, molecular crystals, polymers,

biomaterials, ligand-receptor bindingLittle sample prep required

Nanometer resolved images of nucleic acids

Imaging of cells

Quantification of molecular interactions in biologicalsystems

Quantification of electrical surface charge


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P l f difi ti


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Polymer surface modification


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Raster the Tip: Generating an Image

The tip passes back and forth ina straight line across the sample(think old typewriter or CRT)

In the typical imaging mode, thetip-sample force is held constant

by adjusting the vertical positionof the tip (feedback).

A topographic image is built upby the computer by recording thevertical position as the tip is

rastered across the sample.

S c a n n i n g T i p

R a s t e r M o t i o n


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Scanning the Sample

Tip brought within nanometers ofthe sample (van der Waals)

Radius of tip limits the accuracy ofanalysis/ resolution

Stiffer cantilevers protect againstsample damage because theydeflect less in response to a smallforce

This means a more sensitivedetection scheme is needed

measure change in resonancefrequency and amplitude ofoscillation


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Some of Pictures

2D topographical image of Atomic Step 3D Image

Screw dislocations on InSb grown by MBE


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The Bad Examples

Histogram shows level surface, butscan is very streaky

Typically the sample will have aslight tilt with respect to the AFM.

The AFM can compensate for this

tilt.

The horizontal lines are due to tip hops –

where the tip picks up or loses a small

“nanodust”

In this image the tilt have not

yet been removed.


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So What Do We See?

Nickel from an STM ZnO from an AFM


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Teeny little dust mites, ultra tiny dust mites

about 2,000 in the average bed

http://www.micropix.demon.co.uk/sem/dustmite/article/12664_2.gif


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Surface Roughness

Roughness typically measured as root mean squared (RMS)


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Carbon Nanotube Tips


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Carbon Nanotube Tips

Well defined shape and composition.

High aspect ratio and small radius of curvature.

Mechanically robust. Chemical functionalization at tip.

DNA

CNT Tips


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SPM Lithography


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Electrochemistry: carbon nanotube used as a conducting AFM tip for localoxidation of Si.

SPM L ithography

M il li pede Memory


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Millipede is a non-volatile computer memory stored on nanoscopic pits burned into

the surface of a thin polymer layer, read and written

by a M icroelectromechani cal systems (MEMS)-based probe.

Mil l ipede Memory


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Mil l ipede Memory

Canti lever Gas Sensors (Noses)


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Canti lever Gas Sensors (Noses)

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3 AFM Lecture