Anders Mikkelsen Department of Synchrotron Radiation Research e-mail: anders.mikkelsen@sljus.lu.se - PowerPoint PPT Presentation

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Anders Mikkelsen Department of Synchrotron Radiation Research e-mail: anders.mikkelsen@sljus.lu.se

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Title: Anders Mikkelsen Department of Synchrotron Radiation Research e-mail: anders.mikkelsen@sljus.lu.se


1
Anders MikkelsenDepartment of Synchrotron
Radiation Researche-mail anders.mikkelsen_at_sljus.
lu.se
  • Scanning Probe methods (STM, AFM, MFM,...)

2
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Scanning Tunneling Spectroscopy (STS)
  • Scanning Tunneling Luminesence (STL)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Capacitance Microscopy (CM)
  • Scanning Nearfield Optical Microscopy(SNOM)
  • ...

3
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Resolve individual atoms, measure electrical
    properties, induce photo luminesence.
  • Only conducting samples.
  • Atomic Force Microscopy (AFM)
  • Resolution limit STM, but more difficult to
    achieve.
  • Any sample type.
  • Magnetic Force Microscopy (MFM)
  • Measure magnetic domains combine with
    topography

4
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Scanning Tunneling Spectroscopy (STS)
  • Scanning Tunneling Luminesence (STL)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Capacitance Microscopy (CM)
  • Scanning Nearfield Optical Microscopy(SNOM)
  • ...

5
The scale of things
Natural
Artificial
6
Nanomanipulation and electron density waves by
STM Quantum Corrals (Don Eigler IBM)
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8
The Scanning Tunneling Microscope
Two ways of forming the STM image a) is almost
always used!
The expermental setup
a) I constant, feedback loop active, Z
variation measured
b) Z constant, feedback loop idle, I
variation measured
9
LEED vs. STM of Pt(110)-(1x2)
STM image
LEED image
Real space model from LEED
10
Experimental issues
STM stage with magnetic damping
STM tips made by electrochemical etching
11
Tripod STM with magnetic damping
STM stage side view
STM top view
  1. Tripod scanner
  2. Sample stage
  3. Magnetic damping
  4. Current amplifier

12
Scanner design
SPM scanners are made from a piezoelectric
material PZT, lead zirconium titanate. PZT
expands and contracts proportionally to an
applied voltage.
In tripod design three piezos are used to move
in XYZ
Piezoelectric scanner based on images of known
surfaces calibrate the piezoelectric response to
convert Volts to nm the response is
continuous down to atomic lengths scales
unfortunately, the response is highly non linear
(hysteresis)
13
Scanner designs
Precise coarse movement with piezos
Piezo tube can also be used as scanner
14
Electron tunneling in 1D from I to III
Time independent Schrödinger equation
1D model of tunneling from one side (I) to the
other (III)
Y(x) oscillate in region I and III Y(x) decay
exponentially in region II outgoing
current density incomming current density
T
Texp(-2ka), k2(V0-E)
T falls of exponentially with barrier width
15
Tunneling between two metals
In STM a bias is now added to get an actual
current running, and in the time dependent
solution waves on either side can transfer
across the barrier. However the result turn out
to be quite similar to the previous slide.
Tunneling current is proportional to exp(-2Kd)
K(2mf)/h F is the average workfunction of the
two metals
16
Sensitivity of STM
Tunneling current is proportional to exp(-2Kd)
K(2mf)/h For average work function f of 4eV,
K1Å-1 gt Changing d 0.1 nm will change current
by an order of magnitude! Typical currents
0.1-1nA Typical voltages 0.1-4V
17
What are we imaging in STM?
Tersoff Hamann showed that in a simple model
the tunneling current is proportional to the
Local Density of States (LDOS) r(r,E) near the
fermi level (EEfermi) . Local density of
states r(r, E) is the density of allowed
electron states in the energy range E to EdE at
a point r in the unit cell. More complex models
show that the Tersoff Hamann result is
qualitatively correct. LDOS can be calculated
directly theoretically using a full quantum
mechanical description gt theory can be used to
help interpret STM images.
More complex wave function description
18
What are we imaging in STM?
Some general concepts can also be
derived Metals High density of states at
atoms gt atoms appear as bright
protrusions Insulators No conduction possible
gt we crash Semiconductors and thin oxides
complex electronic structure at fermi level gt
be carefull!
19
Measuring empty or filled states by bias switching
Filled state imaging
Empty state imaging
Tip
Sample
Sample
Tip
Negative Bias
Negative Bias
Positive Bias
Positive Bias
FT
FS
FS
e-
e-
FT
1eV
Filled Electronic Valence Band States
d
d
Potential Barrier
Potential Barrier
20
GaAs(110) an example of different filled /
empty state imaging
LEED pattern
High density of states in conduction band above
Ga atoms High density of states in valence
band above As atoms We can change bias to image
different species.
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But the special advantage of XSTM is the direct
resolution of atomic scale structure....
25
In-Plane XSTM on GaAs nanowires
Carbon Dopant
26
Out-of-Plane XSTM on GaAs nanowires
27
SEM image
P.S. We know they are free of major defects from
TEM
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29
2D oxide on Pd(111) imaged by STM
(Edvin Lundgren Synchrotron Radiation Research)
Oxide
Reduced oxide
very clear STM images but how should they be
interpreted??? upon heating surface is reduced,
and large 1 ML deep holes are observed from that
the number of Pd atoms can be estimated in the
add layer 5 Pd atoms
30
Surface Crystallography in the Virtual Lab
  • common practice trial and error modeling
  • set up models and compare with experiment
  • STM
  • vibrational spectroscopy (EELS)
  • core-level binding energies
  • LEED
  • set up a number of reasonable models and
    calculate their energythe one with the lowest
    energy is most stable(ab initio
    thermodynamics)
  • Ab-initio theoretical method assumptions
  • Quantum mechanics with a few simplifying
    approximations
  • Basic structural model (density, type and
    symmetry of atoms)
  • Thermodynamc equilbrium
  • S-wave STM tip

31
Favorable model from theory
  • 7 Pd5O4 cells on undistorted 3 layer substrate
    (6x848 Pd atoms per layer)

Three fold Oxygen Four fold Oxygen
4 fold Pd atoms are above Pd-Pd rows
32
Comparison of STM images
Theoretically simulated STM image
Experimental STM image
33
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Scanning Tunneling Spectroscopy (STS)
  • Scanning Tunneling Luminesence (STL)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Capacitance Microscopy (CM)
  • Scanning Nearfield Optical Microscopy(SNOM)
  • ...

34
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36
Contact! Biorecognition experiment.
  • The tip is approached to the surface until
    contact occurs.
  • Retracting the cantilever stretches the
    connection of the single biomolecule to the
    surfaces. When the force reaches the unbinding
    force of the complex, the biological interaction
    is ruptured and
  • The cantilever is available for a new force
    distance curve.
  • (B) Loading rate dependence of the unbinding
    forces of the avidin-biotin system under
    physiological conditions
  • ( Single Mol. 1 (2000) 285.)

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38
High resolution AFM in non contact mode
Si(111)-(7x7) AFM vs. STM
AFM
STM
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40
Full AFM experimental setup
Quite simmilar to our STM experimental setup
41
AFM tips
Three common types of AFM tips
Commercial, from silicon or silicon
nitride -Standard chip size 1.6 x 3.6 x 0.4 mm
(more than 1000 probes from aSiwafer) -High
reflective Au coating (reflective property is 3
times better in comparison with uncoated
cantilevers) -Typical curvature radius of a tip
10 nm -Cantilever length 100 -200 µm -Cantilever
width 10 -40 µm -Cantilever thickness 0,3 -2
µm -Available for NONCONTACT, SEMICONTACT and
CONTACT modes -Triangular (V shaped) and
rectangular cantilevers -Available with
conductive TiN, W2C, Pt, Au and magnetic Co
coatings
Different cantilever shapes
42
Tip Artifacts
Overestimate object size
AFM image with Double tip
Underestimate object size
Double tip
Tip shape imaged
43
AFM image of blood clotting
  • Polymerization of fibrin catalyzed by the
    presence of thrombin
  • Mechanism for blood clotting

B. Drake et al., Science 243, 1586 (1989)
44
AFM image of biomolecules
190 nm-long DNA strands
Human chromosone
45
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Scanning Tunneling Spectroscopy (STS)
  • Scanning Tunneling Luminesence (STL)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Capacitance Microscopy (CM)
  • Scanning Nearfield Optical Microscopy(SNOM)
  • ...

46
MFM Magnetic Force Microscope
AFM with magnetic probe
laser
photodiode
magnetic tip
piezo-element
47
Magnetic Force Microscopy (MFM)
  • Special probes are used for MFM. These are
    magnetically sensitized by coating with a
    ferromagnetic material.
  • The tip is oscillated 10s to 100s of nm above
    the surface
  • Gradients in the magnetic forces on the tip shift
    the resonant frequency of the cantilever .
  • Monitoring this shift, or related changes in
    oscillation amplitude or phase, produces a
    magnetic force image.
  • Many applications for data storage technology

48
MFM of cobalt islands
49
Some Scanning Probe Techniques
  • Scanning Tunneling Microscopy (STM)
  • Scanning Tunneling Spectroscopy (STS)
  • Scanning Tunneling Luminesence (STL)
  • Atomic Force Microscopy (AFM)
  • Magnetic Force Microscopy (MFM)
  • Capacitance Microscopy (CM)
  • Scanning Nearfield Optical Microscopy(SNOM)
  • ...

50
Scanning Tunneling Spectroscopy
Tunneling I(V) curves at the specified points
A,B,C
Si(111)-(7x7)
I
V
We can study LDOS as a function of energy by
varying voltage on tip
51
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