Transcript group-talk-11-29-2007 - Northwestern University Mesoscopic

Tuning Fork Scanning Probe
Microscopy
Mesoscopic Group Meeting
November 29, 2007
Scanning Probe Schematic
Scanning Probe Schematic
Scanner
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)
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
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
Scanning Probe Schematic
Coarse approach
mechanism
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 thread=80 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
Coarse Approach Mechanism
For finer control, one can use a “slip-stick” mechanism
Voltage
Apply a sawtooth waveform
voltage to the piezo
slip
Support
Piezo
Scan tube
Slowly extend piezo
time
Can move in small steps (50
nm) and very rapidly
Rapidly contract piezo
Contact
Slip-stick Coarse Approach Mechanism
Some examples from the Mesoscopic Physics Lab
Coarse Approach Mechanism
Slip-stick mechanism can be very fickle--depends on friction
Walker mechanism (Pan design)
Scan tube
Scan tube
Scan tube
Scan tube
Scan tube
Scan tube
Typically use 6 shear piezos--high voltage pulses need to
be precisely timed
Coarse Approach Mechanism
Modified Pan design (Anjan Gupta)
Use a slotted scan tube
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
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?
Yes
Start scanning
No
Advance one step
Scanning Probe Schematic
Tip-sample
interaction
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.
I
The tunneling current depends exponentially on the tipsubstrate 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.
Tip-sample interaction: Force microscopy
The force microscope was invented after the scanning
tunneling microscope. The simplest force microscope
depends on the van der Waal’s (or fluctuating dipole)
attractive interaction between the tip and the sample
Coulomb repulsion
Force
distance
van der Waal’s interaction
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
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 constants
of commercial
cantilevers are
typically 0.1 N/m.
Hence a
displacement of 1
nm corresponds to
a force 0.1 nN
Force
Coulomb repulsion
distance
van der Waal’s interaction
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 Waal’s interaction
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.
Force
Coulomb repulsion
distance
van der Waal’s interaction
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.
V
I
Quartz Tuning Fork as a Force Sensor
Micro-machined Cantilever
-
optical deflection
laser diode
photo diode
optical alignment
addition actuator
Quartz Crystal Tuning fork
-
Self actuating
Self sensing
No light
No alignment
Force Sensitivity of Quartz Tuning
Fork
Cantilever
Tuning Fork
f ~ 10 - 100 kHz
k ~ 0.01 - 100 N/m
f ~ 32 - 100 kHz
k ~ 103 - 105 N/m
Q ~ 102
~ 10 nm dithering
Q ~ 104 (106 in vacuum)
< 1 nm dithering
Force sensitivity  (Qf/k) 1/2
• Low force sensitivity
• Low thermal noise due to high stiffness
• High resolution by small dithering amplitude
Force Sensitivity of Quartz Tuning
Fork
Spring constant is given by
Where the Young’s modulus for quartz is given by E=7.87 x 1010 N/m^2
Force sensitivity is proportional to
Putting a tip on a tuning fork
Most groups use thin etched wire tips
Problem is that mass of wire loads
tuning fork--changes quality factor
and frequency
Todorovic and Schultz (1998)
Putting a tip on a tuning fork
Our group--use commercial cantilever tips
Rozhok et al.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Putting a tip on a tuning fork
Our group--use commercial cantilever tips
Rozhok et al.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Putting a tip on a tuning fork
Our group--use commercial cantilever tips
Rozhok et al.
Surface of HOPG
graphite
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Measurements using a tuning fork
As the tip is brought close to the surface, the amplitude, phase
and frequency of the tuning force changes as a consequence of
the force interaction with the surface
f = 32.768 KHz
k = 1300 N/m
Q = 1300
L = 2.2 mm, t = 190 mm, w = 100 mm
Damped forced harmonic oscillator
On resonance, the response of the tuning fork is 90 degrees
out of phase with the drive
20
3.5
A( ) / f
15
Q2
Q  10
5
 ( )
2.5
2
Q  20
10

3
 /2
1.5
1
 / 0
Q2
Q  10
Q  20
 / 0
0.5
0
0
0
0.5
1
1.5
2
0
0.5
Exploit this to self-excite the tuning fork
1
1.5
2
Lift-mode MFM
However, at short distances, it is difficult to separate the
magnetic interactions from van der Waal’s interactions. Taking
advantage of the fact that van der Waal’s 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
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)
Other uses of scanned probe techniques
Lithography (electric field, DPN, atom-by-atom)
Local voltage probes
Biomolecular force detection
Nanomanipulation
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