Transcript Dental Enamel Structures

EEW508
Scanning probe microscopy
Scanning Tunneling Microscopy and
Atomic Force Microscopy
EEW508
Scanning 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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Scanning probe microscopy
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) surface
Phys Rev Lett (1983)
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Scanning probe microscopy
Principle of Scanning Tunneling Microscopy
I
STM tip
I ~ e –2d
A
(d: tip-sample
separation, K is the
constant)
V
Sample
surface
Tunneling current (I)
d
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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Scanning probe microscopy
Principle of Scanning Tunneling Microscopy
t
s
eV
tip
vacuum
sample
Because the density of state of the sample
contributes the tunneling current, STM is
effective technique for the conductive
surface (semiconductor or metallic surface).
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Scanning probe microscopy
Schematic of Scanning Tunneling Microscopy
The instrument basically consists of
a very sharp tip which position is
controlled by piezoelectric elements
(converting voltage in mechanical
deformation)
Figure: Michael Schmid, TU Wien
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Scanning probe microscopy
Imaging modes of Scanning Tunneling Microscopy
STM tip
A
Constant height mode
Feedback off
Constant current mode
Feedback on
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 probe microscopy
Scanning Tunneling Spectroscopy
Silicon (100) (2x1) dimer row
reconstruction structure
dI/dV/(I/V)
6
5
4
3
2
1
-3
(M. Crommie group)
-2
-1
0
1
2
sample voltage(V)
3
Tunneling spectroscopy reveals the
bandgap of 0.7 eV due to the
presence of surface states
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Scanning probe microscopy
STM instrumentation
Beetle type walker
Commercial system
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Scanning probe microscopy
Examples of STM studies
1. Atomic manipulation (Don Eigler, IBM)
A node in the electron standing wave
Fe
atoms
Xe atoms on Ni (100) at 8K
assembled by atomic manipulation
Quantum corral (D. Eigler)
Iron on Copper (111)
assembled by atomic manipulation
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Scanning probe microscopy
Examples of STM studies (Dynamics of molecules)
Water molecules on Pd(111) surface
T. Mitsui et al. Science (2002)
Water dimers diffuse much faster
than monomer and trimer
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Scanning probe microscopy
High pressure STM reaction studies
(Somorjai group)
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Scanning probe microscopy
Examples of STM studies
– Correlating the atomic structure with electronic properties
(n,m) nanotube, if n − m is a multiple of 3,
then the nanotube is metallic, otherwise the
nanotube is a semiconductor.
STM image and spectroscopy of
single walled carbon nanotube
(C. M. Lieber group)
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Scanning probe microscopy
Examples of STM studies – revealing periodicity and aperiodicity
Fibonacci sequence
A progression of numbers which are
sums of the previous two terms
f(n+1) = f(n) + f (n-1),
F(n)
n
Golden
string
1
0
S
1
1
L
2
2
LS
3
3
LSL
5
4
LSLLS
8
5
LSLLSLSL
13
6
LSLLSLSLLSL
LS
21
7
LSLLSLSLLSL
LSLSLLSLSL
STM image of two-fold surface of
Al-Ni-Co decagonal quasicrystal surface
J. Y. Park et al. Science (2005)
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Scanning probe microscopy
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.
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).
J. Y. Park, R. J. Phaneuf, and E. D. Williams,
Surf. Sci. 470, L69 (2000).
It has been also demonstrated the
modification of existing nanodots,
via the application of additional,
larger voltage pulses of both
polarities.
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Scanning probe microscopy
Atomic Force Microscopy
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Scanning probe microscopy
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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Scanning probe microscopy
Principle of Atomic Force Microscopy
Position sensitive
Photodiode array
Laser beam
Bending
Torsion
FRICTION
Forces:
Van der Waals force
electrostatic force
Magnetic force
Chemical force
Pauli repulsive force
LOAD
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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Scanning probe microscopy
Constant height and force mode AFM
laser
detection
cantilever
Constant height mode
(force feedback off)
Constant force mode
(force feedback on)
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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Scanning probe microscopy
micromotor
AFM instrumentation
Beetle type
walker
cantilever
photodiode
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Scanning probe microscopy
Cantilevers in atomic force microscopy
Tip
Coating
(TiN)
back
Coating
(Au)
for laser
Cantilevers can be seen as springs.
the extension of springs can be described by Hooke's Law F = - k * s. reflection
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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Scanning probe microscopy
Imaging mode in atomic force microscopy
Feedback : lever deflection
the 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 amplitude
The 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 amplitude
Feedback : lever deflection
the cantilever oscillates close to the sample surface,
but without making contact with the surface.
Electrostatic / magnetic force microscopy
the tip does not leave the surface at all during the
oscillation cycle. (interfacial force microscopy)
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Scanning probe microscopy
Friction force microscopy
laser
A
xyz
actuator
C
B
D
4 quadrant
photodiode
cantilever
sample
AFM topography
friction
n type
silicon
C16
silane
AFM topographical and friction images
of C16 silane self-assembled
monolayer on silicon surface revealing
lower friction of molecule layers
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Scanning probe microscopy
Measurement of adhesion force between tip and
sample with force-distance curve
Normal force (nN)
100
0
B
-100
B
-200
-300
A
-400
-500
-100
0
100
A
200
distance (nm)
At the point A, the tensile load is the
same with the adhesion force (FAB
corresponds to the adhesion force)
300
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Scanning probe microscopy
Force Volume Mapping
•
Three dimensional mapping the adhesion force and Young’s modulus
Conductive
AFM
Vs
A
tetrapod
Au(111) or Si
35
28nN
30
25
20
18nN
15
10
CdSe tetrapod
5
topography
Adhesion
mapping
0
5
10 15 20 25 30
Adhesion force (nN)
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Scanning probe microscopy
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 stickslip process
Contact mode topographical
(up) and friction images
(bottom) of polymer
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Scanning probe microscopy
Nanoscale material properties is different from
macroscopic properties – for example, friction
Friction at the
Single
Macroscopic scale
asperity
Real contact
AFM
co nta ct a re a A [n m 2 ]
F   F
f
n
6000
Friction at the
single asperity
JKR
rm
i n te
e d ia
DMT
4000
2000
-M
He
-1 5 0
-1 0 0
-5 0
0
50
e x te rn a lly a p p lie d lo a d F [n N ]
l
te
rtz
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Scanning probe microscopy
Perspective of SPM
1981 : First STM results in the lab
1985 : Invention of AFM in Stanford (Quate group)
1986 : Nobel Prize for Rusk , Binnig & Rohrer
1987: 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
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Scanning probe microscopy
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