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Scanning Tunneling Microscopy and Atomic Force Microscopy

EEW508Scanning probe microscopy

Scanning Tunneling Microscopy (STM) - History - Principle of STM - Operation modes – constant current mode, constant height mode,

conductance mapping, tunneling spectroscopy - Examples of STM studies – atomic structures, dynamics, STM manipulation

Atomic Force Microscopy (AFM) - History - Principle of AFM - Operation modes – contact, non-contact, intermittent modes - Variation of AFM – friction force microscopy, conductive probe AFM,

electrostatic force microscopy, etc. - Examples of AFM studies – atomic stick-slip, friction, adhesion properties of

surfaces

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Beginning of Scanning Probe Microscopy

• Invention of scanning tunneling microscopy (1982)

• Gerd Binnig & Heine Rohrer, IBM Zurich (nobel prize in 1986)

First STM image of Si (7x7)Reconstruction on Si (111) surfacePhys Rev Lett (1983)

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Sample surface

Tunneling current (I)

d

Principle of Scanning Tunneling Microscopy

STM tip

I ~ e –2d

(d: tip-sample separation, K is the

constant)

A

V

I

The key process in STM is the quantum tunneling of electrons through a thin potential barrier separating two electrodes. By applying a voltage (V) between the tip and a metallic or semiconducting sample, a current can flow (I) between these electrodes when their distance is reduced to a few atomic diameters.

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Principle of Scanning Tunneling Microscopy

Because the density of state of the sample contributes the tunneling current, STM is effective technique for the conductive surface (semiconductor or metallic surface).

tip samplevacuum

eV

t s

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Schematic of Scanning Tunneling Microscopy

Figure: Michael Schmid, TU Wien

The instrument basically consists of a very sharp tip which position is controlled by piezoelectric elements (converting voltage in mechanical deformation)

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Constant height modeFeedback off

A

Constant current modeFeedback on

STM tip

Imaging modes of Scanning Tunneling Microscopy

STM topographical imaging (constant current mode)The tip is moved over the surface (x direction), while the current, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

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Scanning Tunneling Spectroscopy

(M. Crommie group)

Silicon (100) (2x1) dimer row reconstruction structure

1

2

3

4

5

6

-3 -2 -1 0 1 2 3sample voltage(V)

dI/d

V/(

I/V)

Tunneling spectroscopy reveals the bandgap of 0.7 eV due to the presence of surface states

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STM instrumentation

                                                                                                                     

Beetle type walker Commercial system

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Examples of STM studies1. Atomic manipulation (Don Eigler, IBM)

Quantum corral (D. Eigler)

Xe atoms on Ni (100) at 8Kassembled by atomic manipulation

Iron on Copper (111) assembled by atomic manipulation

A node in the electron standing wave

Fe atoms

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Examples of STM studies (Dynamics of molecules)

Water molecules on Pd(111) surface Water dimers diffuse much faster than monomer and trimer

T. Mitsui et al. Science (2002)

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(Somorjai group) High pressure STM reaction studies

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Examples of STM studies – Correlating the atomic structure with electronic properties

STM image and spectroscopy of single walled carbon nanotube (C. M. Lieber group)

(n,m) nanotube, if n − m is a multiple of 3, then the nanotube is metallic, otherwise the nanotube is a semiconductor.

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Examples of STM studies – revealing periodicity and aperiodicity

J. Y. Park et al. Science (2005)

LSLLSLSLLSLLSLSLLSLSL

721

LSLLSLSLLSLLS

613

LSLLSLSL58

LSLLS45

LSL33

LS22

L11

S01

Golden string

nF(n)

LSLLSLSLLSLLSLSLLSLSL

721

LSLLSLSLLSLLS

613

LSLLSLSL58

LSLLS45

LSL33

LS22

L11

S01

Golden string

nF(n)

Fibonacci sequenceA progression of numbers which are

sums of the previous two termsf(n+1) = f(n) + f (n-1),

STM image of two-fold surface of Al-Ni-Co decagonal quasicrystal surface

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STM Fabrication and Characterization of Nanodots on Silicon Surfaces

This involves field evaporation from either an Al- or Au-coated tungsten STM tip. This has the advantage of allowing imaging of the structures subsequent to fabrication, with the same tip.

Application of a short voltage pulse to a tip held in close proximity to the surface produces nanodots with a probability and dot size which depend on the size and polarity of the pulse.

It has been also demonstrated the modification of existing nanodots, via the application of additional, larger voltage pulses of both polarities. J. Y. Park, R. J. Phaneuf, and E. D. Williams,

Surf. Sci. 470, L69 (2000).

Left: STM image of Au dots (approx. 10 nm dia. x 1.2 nm ht.) deposited on oxidized Si(100) by application of -8V, 10 msec pulses to the tip. Right: Same Au dots after modification by application of +10 v pulse (left, dot erased) and –10 v pulse (right, dot enlarged). 

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Atomic Force Microscopy

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History of Atomic Force Microscopy

• Invention of atomic force microscopy (1985)• Binnig, Quate, Gerber at IBM and Stanford

Binnig et al. PRL (1985)

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Position sensitive Photodiode array Laser beam

Bending

Torsion

LOAD

FRICTION

Principle of Atomic Force Microscopy

When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law.

This deflection is characterized by sensing the reflected laser light from the backside of cantilever with the position sensitive photodiode.

Because force signal (including Van der Waals force, electrostatic force, Pauli repulsive force) is measured, various samples including insulator can be imaged in AFM.

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Forces:

Van der Waals forceelectrostatic forceMagnetic forceChemical forcePauli repulsive force

Constant height mode(force feedback off)

Constant force mode(force feedback on)

laserdetection

cantilever

Constant height and force mode AFMConstant height and force mode AFM

AFM topographical imaging (constant force mode)The tip is moved over the surface (x direction), while the force, and consequently the distance between the tip and the sample are kept constant. In order to do so, the vertical (z) position of the tip is adjusted by a feedback loop. Thus reading the z position of the tip, one obtains real-space imaging of the sample surface.

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Ion pump / TSP

RHK AFM headXyz manipulator /heater stage

Sputter gun

RGA

Loadlock for sample transfer

LEED/Auger

Wobble stick10 “ flange forSample stage

micromotor

photodiode

Beetle type walker

cantilever

AFM instrumentation

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Tip Coating(TiN)

back Coating(Au)for laser reflection

Cantilevers in atomic force microscopy

Cantilevers can be seen as springs.the extension of springs can be described by Hooke's Law F = - k * s.

This means: The force F you need to extend the spring depends in linear manner on the range s by which you extend it. Derived from Hooke's law, you can allocate a spring constant k to any spring. Damping spring of wheel in the car : 10000 N/m, spring in the ball point pencil : 1000 N/m, spring constant of commercial cantilever :0.01 – 100 N/m

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Imaging mode in atomic force microscopy

Feedback : lever deflectionthe feedback system adjusts the height of the cantilever base to keep this deflection constant as the tip moves over the surface(friction force microscopy, conductive probe AFM)

Feedback : oscillation amplitudeThe cantilever oscillates and the tip makes repulsive contact with the surface of the sample at the lowest point of the oscillation (Tapping mode AFM)

Feedback : oscillation amplitudethe cantilever oscillates close to the sample surface, but without making contact with the surface. Electrostatic / magnetic force microscopy

Feedback : lever deflectionthe tip does not leave the surface at all during the oscillation cycle. (interfacial force microscopy)

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AB

DC

AB

DC

AB

DC

xyz actuator

4 quadrant photodiode

laser

cantilever

sample V(A+C)-(B+D)

Lateral distance

quadrantphotodiode

Friction signal

A B

C D

A B

C D

Friction force microscopy

AFM topography friction

C16 silane

n typesilicon

AFM topographical and friction images of C16 silane self-assembled monolayer on silicon surface revealing lower friction of molecule layers

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Measurement of adhesion force between tip and sample with force-distance curve

-500

-400

-300

-200

-100

0

100

-100 0 100 200 300

distance (nm)

No

rmal

for

ce (

nN)

A

B

A

B

At the point A, the tensile load is the same with the adhesion force (FAB corresponds to the adhesion force)

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Force Volume Mapping• Three dimensional mapping the adhesion force and Young’s

modulus

CdSe tetrapod

Adhesion force (nN)

35

30

25

20

15

10

5

0

30252015105

18nN

28nN

ConductiveAFM

Au(111) or Si

A

Vs

A

Vs

tetrapod

topography Adhesion mapping

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AFM images of various materials

Contact mode AFM topography (left), friction (right) images of graphite surface

Contact mode friction image (left) and its line profile of mica surface which show atomic stick-slip process

Contact mode topographical (up) and friction images (bottom) of polymer

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Nanoscale material properties is different from macroscopic properties – for example, friction

Singleasperity

Real contact

AFM cont

act

are

a

[nm

]

A2

-150

externally applied load [nN]Fl

-100 -50 0 50

2000

4000

6000

Friction at the single asperity

Friction at the Macroscopic scale

Ff

Fn

F

L

Elementary mechanisms

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1981 : First STM results in the lab1985 : Invention of AFM in Stanford (Quate group)1986 : Nobel Prize for Rusk , Binnig & Rohrer1987: First commercial instruments (Park Scientific

Instrumentation from Stanford, Digital Instrumentation from Paul Hansma)

1991: first year > 1000 STM papers published

2005 : Over 2000 STM and 6500 AFM papers published

Scanning Probe Microscopy is one of major tools to characterize and control nanoscale objects

Perspective of SPM

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Summary

Scanning tunneling microscopy (STM): tunneling current between the sharp tip and conductive surface is detected and used to acquire STM images.

Atomic force microscopy (AFM): Force between the cantilever and the surface is measured and used for AFM imaging Both insulating and conductive materials can be imaged in AFM

EEW508Scanning probe microscopy