AFM Lecture

8/16/2019 AFM Lecture

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Introduction in Atomic Force Microscopy

• How can we “see” very small things?

T

image (geometrical representation)

shape

size

color

- light

- electronsT

surface information

11:25 Introduction in AFM


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• Scanning probe microscopy techniques resemble the way

of how blind people get images about things. They explore

the thing surfaces by touch.

•In scanning probe microscopy a sensitive tip explores thesurface of a micro or nano object in the same way as a

stylus profilometer get the profile of a sample surface.

image

tip

surface

What is scanning probe microscopy? How SPM help us to “see” very small things



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How a tip probes the surface of a sample ? FIELD EMISSION EFFECT (1972

R.D. Young, J. Ward, F. Scire,Rev. Sci. Instrum. 43 (1972) 999.

)exp( d c I

O

s u r f a c e

d

STM

TUNNELING ELECTRON CURRENT INTENSITY １９８２

G. Binnig, H. Rohrer, C. Gerber, E. Weibel

Surface Studies by Scanning Tunneling Microscopy

Phys. Rev. Lett. 49 (1982) 57.

d

t i p

R

sample



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How a tip probes the surface of a sample ?

O

s u r f a c e

d

2d

AR F VdV

nd F 1

AFM

ATOMIC AND MOLECULAR FORCES １９８６

G. Binnig, C.F. Quate, C. Gerber Atomic Force Microscope

Phys. Rev. Lett. 56 (1986) 930.

d

t i p

R

sample



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How a tip probes the surface of a sample ?

INTENSITY OF REFLECTED LIGHT (1984)SCANNING NEAR FIELD OPTICAL MICROSCOPY

D. W. Pohl、W. Denk, M. Lanz, Appl. Phys. Lett. 44 (1984) 651

O

s u r f a c e

d

4

1

d I

SNOM

d

t i p

R

sample



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Principle of the STM operation

+

-

U

feedback

controlunit

(x, y, z)

piezoelectricactuator

I t

sample

z

y

x (x, y) scan

controlunit

samplestage

10 mV to 1V

0.2 to 10 nA

•The tip is approached to sample

surface until the tunneling current

reaches certain preset value.

•Then, the tunneling current is is

kept constant during the scan by a

feedback unit that controls the tip

height, z , through a piezoelectric

actuator. The sample surface is

raster scanned in a (x, y) plan parallel to the sample surface.



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Principle of the STM operation

sample

tipU

I t = const.

d

z = variable

sample

tipU

I t= variable

d

z = const.

z(x, y)

y x

I t(x, y)

y x

Depending on the feedback gain,

the STM may operate in one of

the either constant-current mode

or constant-height mode.

High feedback gain

I t = const. mode

Low feedback gain

z = const. mode



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+

-U

STM

feedback

control unit

X,Y,Z scan

STMZ PZT

I t

sample

STM

tip

AFM

tip modulating

piezo

Principle of the AFM operation G. Binnig, C.F. Quate, C. Gerber, Atomic Force Microscope,

Phys. Rev. Lett. 56 (1986) 930.

3

4)(

l

hb E

l z

F k N N

h

b

l

FORCE SENSOR

k N = 50 N/m



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+

-

normal force

signalOA

laser

cantilever base

cantilever

photodiode

Detection of cantilever deflection system(optical lever)

w / 2

l l

l eff

3

2/122 4/w2

l

hb E k N

k N = 0.1 - 1 N/m

force sensor



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Detection of cantilever deflection system(piezoelectric sensor and tuning fork)



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Silicon microfabrication

S. Akamine, R. C. Barrett, M. J. Zdeblick, and C. F. Quate,

A Planar Process for Microfabrication of a Scanning

Tunneling Microscope,

Sensors and Actuators A21-23 (1990) 964.

S. Akamine, R. C. Barett, and C. F. Quate,

Improved AFM images using microcantilevers with sharp tips,

Appl. Phys. Lett. 57 (1990) 316.

Manufacturing the AFM probes

0.5mm

3.5mm

Pyrexglass

Si3N

4

Au

0.1-0.2 mm

35o

3-4 m

(111)

35o (110)

100)

•Low effective mass

•high resonant frequency (> 10KHz)

•small elasticity constant (0.1-1 N/m)

•high quality factor ( 104 in UHV)

•good light reflectivity

•sharp tips (10-50 nm)



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Commercial AFM probes

silicon triangular pyramidal tip

triangular single-beam



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AFM tip characteristics

TEM image of a carbon nanotubeattached to the AFM tip



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Scan system: the piezolelectric tube lead zirconate titanate cylindrical

tube with one inner electrode andfour outer electrodes

P I E Z O

-x +x

-y

-x +x

-y

+y

z

l

d

t

+V x

-V x

+V y

-V y

d t

l d V z y x z y x

2

31,,,,

x

c o n t r a c t e d

e l o n g a t e d

fixed base

-Vx

+Vx



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Block diagram of the AFM

force detector (F )

x

y z

z

-y

-x +

x

+

y

preset force

value

(F 0 )

subtraction

stage

(F -F 0 )

error

signalPC

z s i g n a l

y s c a n

x s c a n

high voltageamplifier

A D C

DAC

+

y

sample

P I E Z O

-x +

x -y

The force signal from the force detector isfed into the feedback loop consisting of

subtr action stage that yields the error

signal, which is the difference between the

preset force and the detected force. The

error signal is integrated to remove high

frequency noise and is fed to a correctionblock to set the voltage that has to be

applied to the z actuator in order to keep

constant the tip-sample interaction force.

feedback loop -digital

-analog



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forwardbackward

Y

X

fast scan direction

s l ow

s c an d i r e c t i on

(a)

forwardbackward

Y

X

fast scan direction

s l ow

s c an d i r e c t i on

(b)

How an AFM image is acquired？

j

j + 1i i+1y

x

z

z i, j

y x

11,

N

Y

N

X y x

0 200 400 600 800 1000

-10

0

10

z(x)

x [ nm ]



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Contact mode of AFM operation

sample

sample

sample

(a)

(b)

(c)

20 0 -20 -40 -60 -80 -100

-20

-15

-10

-5

0

5

10

15

20

f

e

c

b

a

working point

(z)

tip

a t t r a c t i v

e

r e p u l s i v e

jump out of contact

jump into contact

approach

retract

t i p - s a m p l e i n t e r a c t i o n f o r c e

[ n N

]

sample height [ nm ]

k N = 0.57 N/m

0 20 40 60 80 100

-20

-15

-10

-5

0

510

15

20

a t t r

a c t i v

e

r e p u l s i v e

jump out of contact

jump into contact

approach

retract


[ n N

]

tip-sample distance [ nm ]



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20 0 -20 -40 -60 -80 -100

-20

-15

-10

-5

0

5

10

1520

f

e

c

b

a

working point

(z)

tip

a t t r

a c t i v

e

r e p u l s i v e

jump out of contact

jump into contact

approach

retract


[ n N

]


k N = 0.57 N/m

20 0 -20 -40 -60 -80-40

-20

0

20

40

60

80

100

120

140

f

d

c

b

a

jump into contact

approach

retract

t i p - s a m p l e i n t e r a c t i o n f o r c

e

[ n N ]


k = 15 N/m

Role of the cantilever stiffness. Capillary

condensation

sample

sample

stiff

soft



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F z F x

F z

F x

recedeadvance

Lateral force microscopy (LFM)

0 500 1000 1500 2000

-0.11

-0.10

-0.09

-0.08

-0.07

Ffr (recede)

Ffr (advance)

null lateral force line

advance

recede

l a t e r a l s i g n a l [ V ]

advancin or receeding distance [ nm ]

d

A F

fr

fr 2



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vertical

deflectionlateral

deflection

torsion

vertical

deflection

laser

beamvertical

deflectionlateral

deflectionvertical

signal

lateral

signal

+

+

-

-

How is measured the lateral force?



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Dependence of the friction force on the

sample chemical composition

-CH3

-CH3

-COOH

-COOH

tip

tip couvered by

functional

molecules

terminated with

CH3 group

high friction

-CH3

-CH3

-COOH

-COOH

tip

tip couvered byfunctional

molecules

terminated with

COOHgroup

low friction

C. D. Frisbie, L. F. Rozsnyai, A. Noy,

M. Wrighton, C. M. Lieber

Science 265 (1994) 2071.

Chemical Force Microscopy



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Dependence of the friction force on the

sample chemical composition?

20

30

40RH 40

z [ n m ]

0 200 400 600 800 1000 1200

0

200

400

600

l a t e r a l s i g n a l [ m V ]

Topography image Friction force image

Au Si(100)

Au

Si(100)



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•Working in contact mode may deform or even

destroy soft surfaces as those of polymers or biological samples.

•To investigate the topography of soft samples

a non-contact AFM mode, which employs the

long-range tip-sample interaction forces to

determine the sample height, should be used.

•The non-contact AFM modes use a vibrating

AFM tip to explore the sample surface. When

such a vibrating tip approaches a sample

surface, the amplitude, frequency and the

phase of the oscillations change and this

changes are used by a feedback loop to

determine the sample surface height.

• This technique is called dynamic force

microscopy (DFM).

Dynamic force Microscopy

F(d)

d

s a

m p l e

excitation

F(d)

d

s a

m p l e

excitation

A A

Tapping

intermittent contact mode Non-contact mode


O i d A l i h h


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0.8 0.9 1.0 1.1 1.20

20

40

60

80

100

/ 0

a m p l i t u d e [ a . u .

]

no external force

attractive

repulsive

z z eff k z F //10

eff

z

mk 0

O

s u r f a c e

d

2d

AR F VdV

nd

F 1

AFM

0,0

z

F

z

V z z 0

z

V z

Operation modes. Analogy with the

harmonic oscillator

vacuum: Q = 104

air: Q = 50-200

liquid: Q = 2-50

repulsive

Intermittentcontact

atractive

Non-contact11:25 Introduction in AFM

O i d i i


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Operation modes. intermittent contact

and non-contact modes

intermittent non-contact



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vacuum: Q = 104

air: Q = 50-200

liquid: Q = 2-50

Cantilever quality factor


W ki i li id i Th l


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Working in liquid environment. Thermal

noise frequency power spectrum



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0 -20 -40 -60 -80 -100

13.0

13.5

14.0

14.5

15.0

intermittent-contact

approach

retract

o s c i l l a t i o n a m p l i t u d

e [ n m ]


Amplitude curve

Typical amplitude curve

in air.

The sample surface isdetected by the decrease of

the tip oscillation

amplitude


D d f th h l


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0.96 0.98 1.00 1.02 1.040

20

40

60

80

100

120

140

160

180

500

100

180

90

0

/ 0

p h a s e l a g [

d e g .

]

Q = 100

Q = 500

Dependence of the phase lag on energy

loss: case of the harmonic oscillator

22

0

0 /)()tan(

Q

0

022/

eff Q

l

eff W

W Q 02


D d f th h l


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Dependence of the phase lag on

surface chemical composition

Topography image

Phase lag image

Au

Si

SiAu


D d f th h l


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Dependence of the phase lag on

surface structure and topography

Topography effect

Variations of the

phase lag occur

mainly at the grain

borders

Effect of the

crystal structure

The contrast in the

phase lag is due to

composite crystal

structure of the

surface

(dark -rutile TiO2)

(light -amorphous TiO2)


Ch i th i ht til


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Choosing the right cantilever



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Force curve mapping •All important information on sample

surface properties and forces is contained

by the tip-sample force curves.•If force curve data are digitally acquired

for a number of points homogeneously

distributed on a sample surface, then these

data can be digitally processed to extract

the relevant information on the samplesurface properties. This technique is called

force curve mapping (FCM)

•FCM provide simultaneously imaging of

sample topography along with other

important sample properties, as surfacestiffness (elasticity), viscosity, adhesion

force, shear force, chemical composition ,

etc., at the atomic or nano scale.

x y

z

LASER PHD

Z piezodriv er

PC

approach

retract

sample

X, Y piezo drivers

Memory = 128 x 128 x (2 x 128) x 2 bytes



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Acoustic mode AFM


AFM as a biologic sensor: shift in


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AFM as a biologic sensor: shift in

resonant frequency



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Plane correction. Image flattening

Surface tilted in both x and y directions

xy

Surface after correction along ydirection, still tilted in x direction

y x

Surface after correction along x and ydirections

y

x



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Widening small objects: lateral versus normal resolutio



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Lateral resolution



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Effect of tip shape: double tip



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correct feedback

slow feedback

too fast feedback

Effect of feedback on the topography image



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Further reading

11 25 I d i i AFM

Documents

AFM Lecture