Scanning probe microscopy

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Scanning probe microscopy

• Freshman Seminar
• Nanoscience and Nanotechnology
• February 28, 2006

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Scanning Probe Schematic
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Scanning Probe Schematic
Scanner
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Scanner
What makes the SPM work is the scanner, which is
made out of a piezoelectric element A
piezoelectric is a material that extends or
contracts when a voltage is applied to it The
reverse also applies when a piezoelectric is
deformed, a voltage is developed (this is the way
a gas lighter works)
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Piezoelectric materials and scanners
The extension or contraction of a piezoelectric
element is small For example, for a 5 cm long
piezoelectric element, a voltage of 100 V will
result in an extension of 1 micron Since
voltages can be controlled on the level of at
least 10 mV, this gives a resolution of 0.1 nm or
1 Angstrom
One can put three of them together to form a
scanner in three dimensions
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Tube scanners
Another favorite design of the scanner is the
tube scanner. In this geometry, the electrodes
on the scanner are cut into four quadrants
Applying opposite voltages on two X or Y
electrodes with to the inner electrode will bend
the tube one way or the other. Applying the same
voltage to all the 4 electrodes will extend or
contract the tube in the Z direction
Either tip or sample is attached to scanner
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Scanning Probe Schematic
Coarse approach mechanism
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Coarse Approach Mechanism
Since the scanner can move either tip or sample
only by a few microns, we need a mechanism to
bring the tip and sample within striking
distance of each other. The simplest coarse
approach mechanism is a threaded mechanism of
some type driven by a stepper motor How small a
step size can we achieve with this? The smallest
typical screw that one can buy readily has an
0-80 thread80 threads per inch. A typical
stepper motor will have about 200 steps per
revolution Resolution 25.4 mm / 80 / 200 1.5
microns With a finer resolution stepper (500
steps per revolution) 0.6 microns
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Coarse Approach Mechanism
For finer control, one can use a slip-stick
mechanism
Support
Piezo
Contact
Apply a sawtooth waveform voltage to the piezo
Scan tube
slip
Slowly extend piezo
Voltage
time
Rapidly contract piezo
Can move in small steps (50 nm) and very rapidly
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Slip-stick Coarse Approach Mechanism
Some examples from the Mesoscopic Physics Lab
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Coarse Approach Procedure
It is clear that one does not want to crash the
tip into the sample, so we need to establish a
procedure to get within striking distance of
the sample. This is done as in the following
flow diagram
Use Z piezo to check for feedback
In feedback?
No
Advance one step
Yes
Start scanning
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Scanning Probe Schematic
Tip-sample interaction
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Tip-sample interaction Tunneling
The first scanning probe microscope to be
invented was the scanning tunneling microscope,
or STM. This depends on a tip-sample interaction
that involves tunneling of electrons from the
metallic tip to the substrate. The
tunneling current depends exponentially on the
tip-substrate distance, making it a very
sensitive instrument. It gives information not
only about topography, but also about the spatial
variation of the density of electrons. It can be
used to image atoms and electron orbits.
However, a major drawback is that the sample
substrate must be conducting, restricting its
use.
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Tip-sample interaction Force microscopy
The force microscope was invented after the
scanning tunneling microscope. The simplest
force microscope depends on the van der Waals
(or fluctuating dipole) attractive interaction
between the tip and the sample
Coulomb repulsion
Force
distance
van der Waals interaction
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Force microscopy optical detection
In a conventional cantilever AFM, the interaction
between the tip and the surface results in a
deflection of the cantilever holding the tip, or
a change in the resonant frequency of the
cantilever. This can be detected by many
techniques, the most widely used in commercial
systems being optical detection
mirror
Laser
4-quadrant photodetector
The signal in the detector is used to keep the
deflection or the resonant frequency of the
cantilever constant by moving the z piezo
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Contact mode AFM
The simplest mode of operation is contact mode
AFM. In this mode, the tip is essentially placed
in contact with the surface, and either the
deflection of the cantilever or the movement in
the z piezo required to keep the deflection is
recorded as a function of the x-y displacement
Force
Coulomb repulsion
Force constants of commercial cantilevers are
typically 0.1 N/m. Hence a displacement of 1 nm
corresponds to a force 0.1 nN
distance
van der Waals interaction
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Non-contact mode AFM
A more complicated technique of using AFM is
non-contact AFM. In this mode, the cantilever is
set into oscillation at its natural frequency,
usually with another piezo. As the tip
approaches the surface, the attractive force
tip-sample force changes the frequency, amplitude
and phase of oscillation. Any of these
parameters can be used in the feedback circuit.
Force
Coulomb repulsion
distance
van der Waals interaction
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Tapping mode AFM
Contact mode AFM has high resolution, but wears
out the tip, and may modify the surface
especially if it is soft. Non contact mode does
not have as good resolution. Intermittent
contact or tapping mode AFM, where the tip
touches the surface each half-cycle of an
oscillation, has advantages of both.
Coulomb repulsion
Force
distance
van der Waals interaction
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Force microscopy other detection techniques
Sometimes it is not desirable or practical to use
optical techniques for measuring cantilever
deflection. Other transducers can be used to
measure the force between between tip and sample.
One favorite is the piezoresistive cantilever,
where a resistor is embedded in the cantilever,
and deflection of the cantilever produces a
change in the resistance. For use in non-contact
mode, the cantilever still has to be excited
(actuated) by a second piezo (the actuation
piezo)
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Force microscopy tuning-fork transducers
Another favorite force transducer is a watch
crystal, which is in the form of a quartz tuning
fork. This has the advantage of being
self-actuated and self-detected. The actuation
is provided by applying a voltage (usually at a
frequency corresponding to the resonant frequency
of the tuning fork) and detection is accomplished
by measuring the resulting current, which is
proportional to the deflection.
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Other force microscopies
Any other force interaction between tip and
sample can be used to image. A good example is
magnetic force microscopy (MFM), where the tip is
coated with a magnetic material (e.g., CoPt).
This will interact with a magnetic sample via a
magnetic dipolar interaction, that is, like two
bar magnets interacting.
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Lift-mode MFM
However, at short distances, it is difficult to
separate the magnetic interactions from van der
Waals interactions. Taking advantage of the
fact that van der Waals interactions are short
range while magnetic interactions are long range,
one uses the lift-mode technique take an
image of the sample at short distances to obtain
primarily topographic information, then use this
information to keep the tip a fixed height above
the sample, following the topography, and thereby
obtain a (almost) purely magnetic image.
AFM and MFM images of an array of elliptical
permalloy (ferromagnetic) particles
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Other microscopies (a partial list)
Electrostatic force microscopy (EFM) Lateral
force microscopy (LFM) Kelvin force
microscopy Scanning capacitance microscopy
(SCM) Scanning gate microscopy Scanning SET
microscopy Scanning SQUID microscopy Scanning
thermal microscopy Magnetic resonance force
microscopy (MRFM) Scanning Hall probe
microscopy Near field scanning optical microscopy
(NSOM)
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Other uses of scanned probe techniques
Lithography (electric field, DPN,
atom-by-atom) Local voltage probes Biomolecular
force detection Nanomanipulation .