Scanning Probe MicroscopyScanning Probe MicroscopyRagnar Erlandsson, Department of Physics Chemistry and Biology, Ragnar Erlandsson, Department of Physics Chemistry and Biology, LiULiU

Introduction to SPMIntroduction to SPMScanning Tunneling Microscopy (STM)Scanning Tunneling Microscopy (STM)Atomic Force Microscopy (AFM, SFM)Atomic Force Microscopy (AFM, SFM)Scanning Near Field Microscopy (SNOM)Scanning Near Field Microscopy (SNOM)

Primary operation modesPrimary operation modesArtifactsArtifacts

Application exampleApplication examplePiezoelectric writing and imaging of a polymerPiezoelectric writing and imaging of a polymer

Force spectroscopyForce spectroscopyInterpretation of force curvesInterpretation of force curvesExamples from literatureExamples from literature

Radiation inImage of

object

Tip shaped moving physical object

A new concept of microscopy

Before SPM

Optical microscope

TEM

SEM

Diffraction techniques

…

STM

AFM

SNOM

…

X = Tunneling current STM (1981)

X = Force interaction AFM (1986)

X = Near Field Optical Interaction SNOM

More than imaging

a) Topography b) Local interaction c) Manipulation

Atomic resolution changed everythingGerd Binnig, Heinrich Rohrer, 1981

Awarded the Nobel Prize 1986 together with Ernst Ruska (TEM)

Si(111) 7x7

Scanning Tunneling Microscopy (STM)Scanning Tunneling Microscopy (STM)

lengthdecay Inverse :

heightBarrier :

separation surface- tipis

1.12

)2exp(

12

κ

κ

κ

B

B

d

Åh

m

dI

Φ

≈Φ=

−∝

−

Constant current image of Si(111)7×7

Current increase approx. one order of magnitude for each Å of approach.

Ideal for high resolution mapping!

Inherent advantages of SPM techniques:Inherent advantages of SPM techniques:-- Can be used to manipulate nanoCan be used to manipulate nano--size objectssize objects

-- Can often operate in vacuum, air and liquidCan often operate in vacuum, air and liquid

Extreme example of manipulation. Atom corrals presented by Don Eigler

and coworkers, IBM Almaden Research Center.

The immediate recognition of STM combined with easy availability of the technical components led to an explosion of SPM-variations

Various interactions

• Tunneling current

• Forces of various types

• Optical interaction

• Capacitance

Various types of measurements

• Topography

• Interaction vs separation

• Friction forces• Capacitance

• Thermal conduction

• Ionic transport

• ………..

• Manipulation

• …………

Almost endless amount of combinations!

Acronymes…..Acronymes…..SPM Scanning Probe Microscopy

STM Scanning Tunneling Microscopy

STS Scanning Tunneling Spectroscopy

AFM Atomic Force Microscopy

SFM Scanning Force Microscopy

LFM Lateral Force Microscopy

FFM Frictional Force Microscopy

CAFM Conductive AFM

TUNA Tunneling AFM

ECAFM Electro Chemical AFM

AFAM Acoustic AFM

FFM Frictional Force Microscopy

FMM Force Modulation Microscopy

EFM Electrostatic Force Microscopy

MFM Magnetic Force Microscopy

SCM Scanning Capacitance Microscopy

SNOM Scanning Near-field Optical Microscopy

NSOM Near-field Scanning Optical Microscopy

SSRM Spreading Resistance Microscopy

Atomic Force Microscope, AFM (SFM)Atomic Force Microscope, AFM (SFM)

Force interaction is more complex than tunneling current, and have both repulsive and attractive components

InteractionInteraction SignSign RangeRange Sphere/flatSphere/flatexpressionexpression

CommentComment

Born forceBorn force repulsiverepulsive ~1Å ComplexComplex Always presentAlways present

Valence interactionValence interaction(”chemical forces”)(”chemical forces”)

repulsiverepulsiveattractiveattractive

~3Å ComplexComplex QuantumQuantummechanicmechanic

Van der WaalsVan der Waals(dispertion forces)(dispertion forces)

normallynormallyattractiveattractive

~100Å Dipole interactionDipole interactionalways presentalways present

ElectrostaticElectrostatic normallynormallyattractiveattractive

long long rangerange

Contact potentialContact potentialoften presentoften present

26d

HRF −=

d

RUF

20πε−=

MagneticMagnetic repulsiverepulsiveattractiveattractive

long long rangerange

ComplexComplex If tip and sampleIf tip and sampleare magnetizedare magnetized

Capillary forceCapillary force attractiveattractive ~100Å In humid atmosphereIn humid atmosphere

Double layer forceDouble layer force normallynormallyrepulsiverepulsive

~1000Å In liquid when In liquid when surfaces chargedsurfaces charged

Hydrophobic forceHydrophobic force repulsiverepulsiveattractiveattractive

long long rangerange

ComplexComplex Water mediatedWater mediated

Steric forceSteric force repulsiverepulsiveattractiveattractive

~50Å ComplexComplex In liquidsIn liquids

θγπ cos4 LRF −=

)exp(4 dWF κπ −−=

AFM cantilevers and tipsAFM cantilevers and tips

The cantilever-tip units are normally micro-machined of Si or Si3N4.

Characterized by:

- Tip radius (typ. 10 nm)

- Spring constant (0.1-100 N/m)

- Resonance frequency: (5-500 kHz)

Different coatings available

Magnetic cantilevers available

R

Tip shape and resolutionTip shape and resolution

If the interaction is short ranged like tunneling current, the resolution is determined by a micro-tip.

This makes it often easy to make STM tips that give atomic resolution on atomically flat terraces.

R If the sample structure is larger, the macroscopic tip geometry (radius an cone angle) is critical.

Artifacts to look out for in AFMArtifacts to look out for in AFM

Important classes of artifacts:

-Tip geometry dependent artifacts

-Force induced artifacts

-In ambient conditions, the sample surface is covered by a liquid layer, causing capillary forces.

Examples of artifactsExamples of artifacts

Palladium islands on SiO2

TEM image AFM imagebad tip

AFM imagegod tip

AFM image of plasmid DNA. The width of the double –helix is approx. 2 nm. In the image the width is approx. 15 nm

Primary operation modesPrimary operation modes

Large oscillationSmall

oscillation

Contact Mode

dc operation

F=k × X, where X is lever deflectionand k spring constant.

z = f(x,y) at const. F

Non-contact Mode

ac operationsmall amplitude

z = f(x,y) at const. F’

Used for long-rangeforces like electro-static and magnetic

Intermittent-contact Mode(”Tapping Mode”)

ac operationlarge amplitude

Block diagram of AFMBlock diagram of AFM

Early instruments used piezoelectric tripod scanners to move tip or sample.

Each orthogonal piezoelectric bar will extend or contract when a voltage is applied.

Modern instruments normally use compact tube scanners.

Scanner configurations for AFMScanner configurations for AFM

Older instruments often had the sample mounted on the scanner.

Here, the complete force head is mounted on the scanner, while the sample is stationary.

In a modern instrument only the tip and part of the optics move, while the sample is stationary.

Veeco Multimode SPM

SPM instruments at IFM

Veeco Multimode Veeco Dimension 3100

Veeco Enviroscope

Primary/Secondary imagePrimary/Secondary imageExample: Topography and Phase informationExample: Topography and Phase information

∆φ, Phase image

Scanner

Amplitudedetector

Feedback

PZT

z z, topography

Primary image

Secondary image

Lift modeLift mode

Two stage measurement:

-First the topography is recorded using tapping mode or contact mode.

Topography Magnetic force

1: Topography, tapping mode

2: Magnetic force, Non-contact

- System makes a retrace without feedback control at a pre-defined height during which the long-range force is measured using an ac non-contact method.

-Suitable for long range forces like electrostatic and magnetic

Scanning NearScanning Near--Filed Optical Microscopy, Filed Optical Microscopy, SNOM (NSOM)SNOM (NSOM)

In a SNOM, the tip is made from a tapered optical fiber with a small aperture at the end.

The surface is traced using an ac mode AFM method.ac mode AFM method.

As a ”secondary image” , the transmitted or reflected light can be recorded. The probe can also be used for light collection.

SNOMSNOMAs the optical near-field is generating the image, the Abbe limit that restricts the resolution of a normal optical microscope to λ/2 is defeated.

Comparison between

SNOM image Corresponding AFM image

Comparison between AFM and SNOM images of 30 nm gold spheres.

(Nanonics Imaging Ltd)

Application exampleApplication exampleUsing the AFM to Using the AFM to createcreate and and imageimageferroelectric domains in a polymerferroelectric domains in a polymer

-

By applying a voltage to the tip, a piezoelectric pattern is written.

E

Conducting substrate

V

Ferroelectric polymer

+- +-

+-

Field distribution around tip

Lock-in

5 kHz amplitude givepiezo-respons signal

Piezoelectric force microscopyPiezoelectric force microscopy

ac signal extracted Piezoresponse

By applying an ac voltage between tip and substrate, the film will vibrate where the piezoelectric pattern is written.

Scanner Feedback

Topography (primary) contact mode

Piezoelectric sample

Dc forpoling

5 kHz1 V

Conducting substrate

ac signal extracted by lockin

Piezoresponse

Force SpectroscopyForce Spectroscopy

The raw ”Force curve” that is measured by an AFM is really a Force vs. Sample positioncurve. See figure to the left.

1: Sample presses on the tip. If the contact is stiff a straight line results.

1.

2.2: The tip is pulled back but sticks to the surface. Adhesion.

3. The adhesion decrease stepwise until the tip finally is free

2.

3.

Interpretation of force curvesInterpretation of force curves

True Force vs. separation curve

AFM ”Force curve” Force vs. Sample pos .

Corrected exp. Force curve Force vs. separation

Snap-in

Cantilever spring

Slope of lines correspond to k

S

D

Z

0

correspond to k

Due to the snap-to-contact instability, it is normally not

possible to trace out the complete interaction curve.

The local pressure at the micro-tip will also make it

impossible to obtain atomic resolution in contact mode.

Cantilevermount plate

Glasswindow Membrane Force head

O-ringCell frame

Measurement in liquidMeasurement in liquid

1 cm

AlSteel

O-ring

Sampleholder

Tube scanner

BaseplateSample holdermount

(b)

BiotinBiotin--avedin interaction studied by AFMavedin interaction studied by AFMFlorin, Moy and Gaub Science, 264, p 415 (1994)

A: Avidin on tip, biotin on surface

B: Most interaction sites blocked by excess of avidin

C: As B but other scale

Histogram of height of last step in retract curve. A force quantum of 160 ± 20 nN was found from the statistics.

Agarose bead attached to AFM cantilever

F. Oesterhelt, D. Oesterhelt, M. Pfeiffer, A. Engel, H. E. Gaub, D. J. MullerScience 288 (2000)

Points are superposition of 17 force spectra.

Grey lines are from theoretical modelling

0 5 10 15 20-5-10 25

5

10

15

0

5

-5

0

-20

20

-10

Sample position [Å]

Def

lect

ion

[Å]

Am

plitu

de [Å

pp]

For

ce [n

N]

True atomic resolution in AFMTrue atomic resolution in AFMAFM, ac-mode

f res +50-50

Frequency shift [Hz]

5

10

15

2118

1512

0

Towardssample [Å]

Am

plitu

de [Å

pp]

STM, empty states STM, filled states Lever deflection curve andFrequency spectra vs separation

ConclusionsConclusions

�� Versatile techniques due to the Versatile techniques due to the multitude of multitude of interactionsinteractions that can be probedthat can be probed

�� Operates in Operates in most environmentsmost environments

�� Can Can imageimage various properties and various properties and manipulatemanipulate the the sample on the nanosample on the nano--scalescale

�� High resolution High resolution force measurementsforce measurements are important are important in many scientific fieldsin many scientific fields