Download Lec 2014 10 07

Review Last Week about EFM
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).
The application of EFM
EFM is used for electrical failure analysis, detecting
trapped charges, mapping electric polarization, and
performing electrical read/write, among other
applications.
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
Comparison:
Phase imaging and the phase image
in EFM
During phase imaging, tapping mode imaging the tip is
tapping the surface, the phase change due to the ver der Waal
force gradient of the sample
• Phase image in EFM
Noncontact techniques, phase images are obtained in lift mode
And the phase changes results from Electrostatic Force gradient
Surface Potential Detection
or Kelvin Probe Microscopy
Surface Potential
(SP) Imaging
SP imaging maps the electrostatic potential on the sample
surface with or without a voltage applied to the sample. SP
imaging is a nulling technique. As the tip travels above the
sample surface in LiftMode, the tip and the cantilever
experience a force wherever the potential on the surface is
different from the potential of the tip. The force is nullified
by varying the voltage of the tip so that the tip is at the same
potential as the region of the sample surface underneath it.
The voltage applied to the tip in nullifying the force is plotted
versus the in-plane coordinates, creating the surface potential
image.
Surface Potential Detection
or Kelvin Probe Microscopy
The piezo disengaged to stop mechanically driving the cantilever
to vibrate, a AC voltage directly applied to the tip
• Detects the potential of the surface
• AC voltage to the tip
• (no induced mechanical vibration)
Vac= 0 - 12 Vpp
Sample surface
V1
V2
but the conducting regions should not be passivated.
Surface Potential Detection
• Let  = fo
• Potential on surface will shift phase of the cantilever
• Detect phase change, use potential feedback loop to
keep phase constant
Vac= 0 - 12 Vpp
Sample surface
V1
V2
Electrostatic Force in a Capacitor
Energy Treatment
 W  F z
W
F
z
Energy in a capacitor is the work done to charge it

U  CV
F

z
z
2
2
z
Classical Treatment
F  qEsample
  C V
V   C  2
 q    V
d  d 
Surface Potential Detection
First Harmonic Force
Second Harmonic Force
( Deflection Amplitude caused by electrical force)
• Let  = Vdriving
Not force gradient
• The lock-in technique allows extraction of the first
harmonic of tip deflection proportional to F. A feedback
loop is employed to keep it equal to zero by adjusting
the Vdc on the tip.
• Vdc=Vsurf
• Therefore the surface potential is directly measurement
by adjusting the potential offest on the tip and keep the
fisrt harmonic response to zero.
Comparison:
EFM and Surface Potential Microscopy
• Phase image in EFM
Noncontact techniques, phase images are obtained in lift mode
And the phase changes results from Electrostatic Force gradient
• Potential image in Surface potential Microscope
(Amplitude modulation ( AM-KPFM)
• The lock-in technique allows extraction of the first harmonic of
tip deflection proportional to F. A feedback loop is employed to
keep it equal to zero by adjusting the Vdc on the tip.
• Vdc=Vsurf
• Therefore the surface potential is directly measurement by
adjusting the potential offest on the tip and keep the first
harmonic response to zero.
EFM and Frequency modulation KPFM
Kelvin Probe Force Microscopy Study on Conjugated
Polymer/Fullerene Bulk Heterojunction Organic Solar Cells
Nano Letters 2005, 5, 269
A breakthrough in power conversion efficiency in polymer
solar cells was achieved for bulk heterojunctions with the
MDMO-PPV/PCBM (poly-[2-(3,7-dimethyloctyloxy)-5methyloxy]-para-phenylene-vinylene/1-(3-methoxycarbonyl)
propyl-1-phenyl [6,6]C61) system.
The drastic increase in the power conversion efficiency was a
result of changing the spin casting solvent from toluene to
chlorobenzene, which accounted for a change in the film
nanomorphology.
Charge separation in bulk heterojunctions is based on the
Photoinduced charge transfer, and photoexcited excitons in
Organic materials have a very limited diffusion length on the
Order of 10 nm.
Not all photoexcitations will dissociate into separated charges carries.
Photoluminescence of PCBM was found for toluene cast blends.
This means that part of the adsorbed photons is not used for
Photocurrent generation.
Another study reported that hole mobility of prinstine MDMO-PPV
Films to decrease when cast from toluene as compared to chlorobezene.
In this study, Kelvin probe force microscope was used to
reveal that the electronic work functions of films prepared from
Different solvent.
sample - tip = q CP
tip=4.28 eV, PtIr coated Si cantilevers
CP: contact potential, q is the elementary charge