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Transcript of AFM Lecture
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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
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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
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
]
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
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
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 ]
sample height [ nm ]
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
11:25 Introduction in AFM
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
11:25 Introduction in AFM
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 ]
sample height [ nm ]
Amplitude curve
Typical amplitude curve
in air.
The sample surface isdetected by the decrease of
the tip oscillation
amplitude
11:25 Introduction in AFM
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
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Dependence of the phase lag on
surface chemical composition
Topography image
Phase lag image
Au
Si
SiAu
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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)
11:25 Introduction in AFM
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
11:25 Introduction in 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