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).

Review Last Week about EFM

EFM is used for electrical failure analysis, detecting trapped charges, mapping electric polarization, and performing electrical read/write, among other applications.

The application of EFM

Electrostatic Interactions is long range interaction

EFM/MFM

Conductive Domains

Magnetic tip, instead of only a conductive tip

Piezoelectric materials

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.

Resonance frequency

Phase line

Driving frequency

Normal TM AFM: ver der Waal Force gradientEFM: lift mode: Electrostatic Force gradient

Mechanical Driven Mode

V piezo = Vac sin(t+ 1)

Free cantilever: A1 (d) sin(dt)+ 1)Interacted cantilever: A2(d) sin( d t) + 2)

Constant drive frequency

Normal TM AFM: ver der Waal Force gradientEFM: 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

Conductive Domains

Magnetic domains

Magnetic tip, instead of only a conductive tip

Piezoelectric materials

Comparison: Phase imaging and the phase image in EFM

• 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

Noncontact techniques, phase images are obtained in lift mode And the phase changes results from Electrostatic Force gradient

Surface Potential Detectionor 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 Detectionor 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

V1 V2

Sample surface

but the conducting regions should not be passivated.

• Let = fo

• Potential on surface will shift phase of the cantilever

• Detect phase change, use potential feedback loop to

keep phase constant

Surface Potential Detection

Vac= 0 - 12 Vpp

V1 V2

Sample surface

Electrostatic Force in a Capacitor

22

it charge to done workthe is capacitor a in Energy

Vz

C

z

CV

z

UF

z

WF

zFW

2Vd

C

d

VqqEF sample

Energy Treatment

Classical Treatment

Surface Potential Detection

First Harmonic Force Second Harmonic Force

• Let = Vdriving

• 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.

Not force gradient

( Deflection Amplitude caused by electrical force)

Comparison:

EFM and Surface Potential Microscopy

• Phase image in EFMNoncontact techniques, phase images are obtained in lift mode And the phase changes results from Electrostatic 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 first

harmonic response to zero.

• Potential image in Surface potential Microscope

(Amplitude modulation ( AM-KPFM)

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)-5-methyloxy]-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