Scanning probe microscopies (SPM)

TFY4255 Materials Physics 2007

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Scanning probe microscopies (SPM)

Principle: A nano-sized probe is brought close to a sample surface and scanned along the surface while some physical property is recorded

Techniques:

• STM (Scanning Tunneling Microscopy)

• AFM (Atomic Force Microscopy)

• SFM (Scanning Force Microscopy)

• DFM (Differential Force Microscopy)

• SNOM (Scanning Near-field Optical Microscopy)


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STM (Scanning Tunneling Microscopy)Scanning microscopy:• 1970’s: Young developed the first non-contact “feeler” microscope• 1980’s: Binnig and Rohrer developed the first stable STM. They earned the Noble price in 1986


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In classical physics an electron cannot penetrate into or across a potential barrier if its energy E is smaller than the potential Φ within the barrier

• A quantum mechanic treatment predicts an exponential decaying solution for the electron wave function in the barrier. And for a rectangular barrier we get:

• The probability of finding an electron behind the barrier of width s is now:

• And the transition probability T can be expressed:

( )( ) ( ) ( ) ( ) ( )

2

2 2

2 0 0 2 /zd z m E z z e where m Edz

−κΨ− Φ− Ψ = → Ψ = Ψ κ= Φ−

zs

( ) ( ) ( )2 2 20 sP s s e− κ= Ψ = Ψ

( ) ( )22 2

16 2 with the approximation s 1 and sE V E m V E

T eV

− κ− −= κ κ=

Φ is the workfunction, often approximated to Φ = ½ (Φsample + Φtip)


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• The current is proportional to the probability of electrons to tunnel through the barrier:

• With 5 eV as a typical example for a workfunction value, a change in 1 Å in distance between tip and sample (s increases by 1 Å) causes a change of nearly one order of magnitude in current! This high sensitivity facilitates a high vertical resolution!

( )2 20

F

n F

Es

nE E eV

I e− κ

= −

∝ Ψ∑

Iron on cupper

STM can only be used to study conducting materials

(and to some degree semiconductors)


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Modes of operation

Constant-Current Mode Constant-Height Mode

• The distance between tip and sample is constant and a x-y-scan gives a topographic image of the surface

• Better vertical resolution

• Slower scanning can give drift in the x-y-scan

• Used for surfaces that aren’t atomically flat

• The tip height is kept constant and tunneling current is monitored

• Lower vertical resolution

• Very fast scans minimal image distortion due to drift

• Allows studies of dynamical processes


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Bardeen Approach• Another way of describing electron tunneling comes from Bardeen’s approach which makes use of the time dependent perturbation theory. The probability of an electron in the state Ψ at EΨ to tunnel into a state χ at Eχ is given by Fermis’s Golden Rule:

• The tunneling matrix element is given by an integral over a surface in the barrier region laying between the tip and the sample:

• δ(EΨ - Eχ) means that an electron can only tunnel if there is an unoccupied state with the same energy in the other electrode (thus inelastic tunneling is not treated).

• In case of a negative potential on the sample the occupied states generate the current, whereas in case of a positive bias the unoccupied states of the sample are of importance. Therefore, by altering the voltage, a complete different image can be detected as other states contribute to the tunneling current. This is used in tunneling spectroscopy.

( )22P M E EΨ χ

π= δ −

2d dM dS

m dz dz

∗∗

⎛ ⎞Ψ χ ⎟⎜ ⎟= χ −Ψ⎜ ⎟⎜ ⎟⎜⎝ ⎠∫∫


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The occupied states of (00-1) SiC

The unoccupied states of (00-1) SiC


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AFM (Atomic Force Microscopy)

A feedback system to control the vertical position of the tip

A computer system that drives the scanner, measures data and converts the data into an image

A piezoelectric scanner which moves the sample under the tip (or the tip over the sample) in a raster pattern

Means of sensing the vertical position of the tip

A coarse positioning system to bring the tip into the general vicinity of the sample


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A computer system that drives the scanner, measures data and converts the data into an image

A piezoelectric scanner which moves the sample under the tip (or the tip over the sample) in a raster pattern

Definition of a piezoelectric material: A material that generates an electric charge when mechanically deformed. Conversely, when an external electric field is applied to piezoelectric materials they mechanically deform

Photodiode


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The cantilever/tip system

The tip is usually a non-conductingmaterial like Si3N4, SiO2 or Si


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AFM monitors interfacial forcesF α (1/d)x

F = interfacial forced = tip-sample separation

Interfacial forces include:• repulsive forces (contact AFM)• van der Waals forces (non-contact AFM)• electrostatic forces (EFM)• magnetic forces (MFM)• chemical forces (CFM)

The interfacial forces are typically in the range pN - nN


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The Force-Distance curve


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Modes of operation:

• Contact mode• Non-contact mode• Tapping mode

Non-contact mode

Contact mode

Contact mode:

• Tip in contact with sample surface

• Monitor cantilever deflection

• Monitor lateral forces

Non-contact mode:

• Tip oscillates just above sample surface

• Monitor Van der Waals forces between tip and sample

• Lower resolution than contact mode

• Lateral forces on the sample are reduced


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Contact and Lateral force AFM

Langmuir-Blodgett film (1 µm scan). (a) Topography image and (b) LFM image.

(a) (a)(b) (b)

Mica surface (3 nm scan). (a) Topography image and (b) LFM image.


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Tapping mode AFM• Tip oscillates while scanning across the sample surface

• Resolution similar to contact mode

• Lateral forces on the sample are reduced because of the tapping motion

The cantilever amplitude is monitored

(Light colors are higher)

(Light color correspond to higher surface

modulus/stiffness)


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Magnetic Force Microscopy (MFM)• Tip cantilever tip is now coated with a soft-magnet material

• The cantilever deflects when is scans over the magnetized domains

• MFM images locally magnetized domains

• Magnitude of deflection (up/down) is proportional to sample magnetization

• MFM is used to determine the local magnetization variation

MFM image showing the bits of a hard drive (30 x 30 µm)


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Electrostatic Force Microscopy (EFM)• Voltage is applied between tip and sample while cantilever hovers above sample

• The cantilever deflects when is scans over static charges

• EFM plots locally charged domains similar to MFM plots of magnetic domains

• Magnitude of deflection is proportional to the charge density

• EFM is used to determine the local charge density variation


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Parameter SEM AFM

Operating environment

Depth of field

Resolution (x,y)

Resolution (z)

Magnification range

Sample preparation

Vacuum

Large

~ 5 nm

10 - 106

Soft materials: Freeze

drying, coating

Ambient, liquid, vacuum

Medium

0.1 – 0.3 nm

0.01 nm

5·102 - 108

None

Comparison between SEM and AFM:


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Scanning Near-field Optical Microprobe

(SNOM)or

Near-field ScanningOptical Microprobe

(NSOM)• Diffraction imposes a natural limit to available resolution with conventional optics. The best confocal microscopes can obtain ~ 250 nm resolution.

• The idea of SNOM is to use an optic fiber, tapered at the exit end and having an exit diameter only a few fractions of the wavelength of the light in the beam

• The distance between the radiation source (tapered exit) and the sample surface must then also be short compared to the radiation wavelength

Two fundamental differences between near-field (NF) and far-field (FF) (conventional) optical microscopy:

1. In NF microscopy the NF interaction region is much smaller than the FF interaction area for conventional microscopy

2. NF microscopy has a sub-wavelength distance between the radiation source and the sample while FF microscopy has much larger distance


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SNOM/NSOM today is very similar to standard scanning probe microscopy, but with an optical channel

Mechanical:

• Translation stage, piezoelectric scanner

• Feedback control (z-distance)

• Anti-vibration optical table

Electrical:

• Scanning drivers for piezoelectric scanner

• Z-distance control

• Amplifiers, signal processors

• Software and computer

Optical:

• Light source (lasers), fiber, mirrors, lenses, objectives

• Photon detectors (photon multiplier tupe, charge coupled devices (CCD)

• Probe to give the window to the near-field


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Commercial NSOM:

Nanonics MultiView 2000


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Two different probe types are being used:

1. Aperture type• Taped fiber, multiple-taped fibre

• Cantilevered AFM/NSOM tips (Si3N4, SiO2)

• Other kinds, such as tetrahedral tip, fluorescent tip

• Metal coating optional

2. Apertureless type• Dielectrics, semiconductors or metals

• Other kinds, such as nano-particle attached tip


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What can we do with NSOM/SNOM?1. Ultrahigh resolution OPTICAL Imaging/Plasmonic studies

2. Spectroscopy

• Near-field Surface Enhanced Raman Spectroscopy

• Local Spectroscopy of Semiconductor Devices

• Near-field Broadband Spectroscopy

3. Modification of surfaces

• Sub-wavelength photolithography (write patterns into a photoresist)

• Ultra High Density data storage (write and read data on magneto-optical materials)

• Laser Ablation (nano-lithography, photo mask repair)

4. Near-field femtosecond studies


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High resolution:

A resolution of 25 nm (or one-twentieth of the 488 nm radiation wavelength) have been demonstrated by the IBM group by utilizing a test specimen consisting of a fine metal line grating

Limitations of near-field optical microscopy:

• Practically zero working distance and an extremely small depth of field

• Extremely long scan times for high resolution images or large specimen areas.

• Very low transmissivity of apertures smaller than the incident light wavelength.

• Only features at the surface of specimens can be studied

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Scanning probe microscopies (SPM)

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