Transcript Lec 2014 09 23

TAPPINGMODE™ IMAGING
APPLICATIONS AND TECHNOLOGY
TappingMode imaging is implemented in ambient air (or liquid)
by oscillating the cantilever assembly at or near the cantilever’s
resonant frequency using a piezoelectric crystal. The piezo motion
causes the cantilever to oscillate with a high amplitude (the “free
air” amplitude, typically greater than 20nm) when the tip is not in
contact with the surface. The oscillating tip is then moved toward
the surface until it begins to lightly touch, or “tap” the surface.
During scanning, the vertically oscillating tip alternately contacts
the surface and lifts off, generally at a frequency of 50,000 to
500,000 cycles per second. As the oscillating cantilever begins to
intermittently contact the surface, the cantilever oscillation is
necessarily reduced due to energy loss caused by the tip
contacting the surface. The reduction in oscillation amplitude is
used to identify and measure surface features.
TM AFM: Constant Force Gradient: taking advantage
of the vibrational characteristics of the cantilever
ACantilever = Acantilever sin(t+ 1)
Resonant
Applied electronic Signal to the Piezo
V piezo = Vac sin(t+ 1)
Produced a sine wave oscillation
with the same frequency and phase angle
A piezo = Aac sin(t+ 1)
Tapping cantilever on free air
Energy Lost
Tapping cantilever on sample surface
ACantilever = Acantilever sin( t+ 1)
1
ACantilever(scanning) = Acantilever(Scanning) sin(2t+ 2)
TappingMode Cantilever oscillation amplitude in free air and
during scanning.
Force gradient changes the effective resonance
frequency of the cantilever
In this method, the cantilever is vibrated by a small
piezoelectric element near its resonant frequency. The
cantilever’s resonant frequency changes in response to any
additional force gradient. Attractive forces make the
cantilever effectively “softer,” reducing the cantilever
resonant frequency. Conversely, repulsive forces make the
cantilever effectively “stiffer,” increasing the resonant
frequency.
Force gradient changes the vibrational amplitude
at the driving frequency of the cantilever
A (xy) is a function of
Position on the sample.
Driving frequency
A
Phase line

Resonance frequency
TM AFM: ver der Waal Force gradient
Phase Imaging: Beyond Topography
By mapping the phase of the cantilever oscillation during
the TappingMode scan, phase imaging goes beyond simple
topographical mapping to detect variations in composition,
adhesion, friction, viscoelasticity, and perhaps other
properties.
Applications include identification of contaminants,
mapping of different components in composite materials,
and differentiating regions of high and low surface adhesion
or hardness. In many cases phase imaging complements
lateralforce microscopy (LFM) and force modulation
techniques. often providing additional information more
rapidly and with higher resolution. Phase imaging is as fast
and easy to use as TappingMode AFM — with all its
benefits for imaging soft, adhesive, easily damaged or
loosely bound samples
Force gradient changes the phase of the cantilever
Phase imaging ( Elastic properties of the surface)
ACantilever(scanning) = Acantilever(Scanning) sin(2t+ 2)
x2y2, x2y2)
x1y1, x1y1)
Driving frequency
Phase line


Resonance frequency
TM AFM: ver der Waal Force gradient
Condensation of Plasmid DNA into
nanoparticles with Gold Nanoparticles
modified with PPI G-3
Free Plasmid
DNA
Phase shift: 6.1
+/- 1.1 nm
B
A
Phase shift:
0.8-1.7 nm
Height image
Phase image
Summary
In TappingMode AFM, the cantilever is excited into resonance
oscillation with a piezoelectric driver. The oscillation amplitude is
used as a feedback signal to measure topographic variations of
the sample. In phase imaging, the phase lag of the cantilever
oscillation, relative to the signal sent to the cantilever’s piezo
driver, is simultaneously monitored. The phase lag is very
sensitive to variations in material properties such as adhesion and
viscoelasticity.
Need Imaging in Fluid to Enhance Resolution or In-situ
Monitoring Biological or Chemical Processes
Electric Force Microscopy (EFM)
EFM is used to map the vertical (z) and near-vertical gradient of
the electric field between the tip and the sample versus the in-plane
coordinates x and y. This is done using LiftModeTM. The field
due to trapped charges—on or beneath the sample surface—is
often sufficiently large to generate contrast in an EFM image.
Otherwise, a field can be induced by applying a voltage between
the tip and the sample. The voltage may be applied directly from
the microscope’s electronics under AFM software control, or from
an external power supply with appropriate current-limiting
elements in place. EFM is performed in one of three modes:
amplitude detection, phase detection, or frequency modulation
(FM).
Electrostatic Interactions is long range interaction
EFM/MFM
Piezoelectric materials
Conductive Domains
Magnetic tip, instead of only a conductive tip
In this method, the cantilever is vibrated by a small
piezoelectric element near its resonant frequency. The
cantilever’s resonant frequency changes in response to any
additional force gradient. Attractive forces make the
cantilever effectively “softer,” reducing the cantilever
resonant frequency. Conversely, repulsive forces make the
cantilever effectively “stiffer,” increasing the resonant
frequency.
Driving frequency
Phase line


Resonance frequency
Normal TM AFM: ver der Waal Force gradient
EFM: lift mode: Electrostatic Force gradient
Mechanical Driven Mode
V piezo = Vac sin(t+ 1)
Constant drive frequency
Free cantilever: A1 (d) sin(dt)+ 1)
Interacted cantilever: A2(d) sin( d t) + 2)
Normal TM AFM: ver der Waal Force gradient
EFM: lift mode: Electrostatic Force gradient
Electric Force Microscopy (EFM)
•
•
•
•
Mechanical vibration given to tip
Electric field will produce a change in phase of tip
Measure phase lag relative to piezo drive signal.
In most cases, it is necessary to apply a voltage
across the tip or sample to achieve a high-quality
image.
• Even if a layer of insulating material covered the
conductive domains, they still can be detected.
EFM/MFM
Piezoelectric materials
Magnetic domains
Conductive Domains
Magnetic tip, instead of only a conductive tip