41
Overview
History and background of AFM
AFM basic components
Basic imaging modes
Advantages and disadvantages of AFM
Recognising artifacts
Other SPM techniques
I. Jurewicz et. al. Advanced Functional
Materials 2014
Scanning Probe Microscopy (SPM) Family
www.nisenet.org
- type of microscopy that forms images of
surfaces using a physical probe that scans
the specimen
- allows to image and measure surfaces
down to the level of molecules and atoms
Introduction to Atomic Force Microscopy Theory Practice Applications, P.E. West, 2006
Atomic Force Microscope (AFM)
invented in 1986 by Binnig, Quate and
Gerber and earned them the Nobel
Prize for Physics.
(Phys. Rev. Letters, 1986, Vol. 56, p
930)
In 1987, Wickramsinghe et al.
developed an AFM setup with a vibrating
cantilever technique, which used the
light-lever mechanism.
(J. Appl. Phys. 1987, Vol. 61, p 4723)
Brief History of AFM
World’s first Atomic Force Microscope in Science Museum London
The extensions of the Scanning Tunnelling Microscope
(STM) technique such as AFM have enabled the
investigation of electrically non-conductive materials
(polymers and biological materials)
How does the AFM work?
the principle of operation is very similar with that of a stylus profilometer
a sharp cantilever tip interacts with the sample surface sensing the local forces between the molecules of the tip and sample surface
ultra-small forces (less than 1 nN) between the probe tip and the sample are measured
Tip atoms
Surface atoms
Interaction Forces
Basic components of AFM
force sensor based on a
micro-cantilever with a sharp
probe
light-lever detection based on
laser and photodiode array
feedback control
piezoelectric scanner
The essential components of an AFM
Sharper tip – higher resolution
NT-MDT.com
NT
-MD
T.co
m
Force sensor
are considered a disposable component of the AFM
most often fabricated out of silicon or silicon nitride
can be supplied with Au or Al back-side reflective coating as well as with magnetic
or conductive coating
tip should have a radius of curvature less than 20-50 nm (smaller is better) a cone
angle between 10-20 degrees
the geometry of the probe is critical to the quality of images measured
Basic components of AFM
force sensor based on a
micro-cantilever with a sharp
probe
light-lever detection based on
laser and photodiode array
feedback control
piezoelectric scanner
The essential components of an AFM
Piezoelectric Scanner
scanners are made of piezoelectric materials that change dimensions in
response to an applied voltage and conversely, they develop an electrical
potential in response to mechanical pressure
constructed by stacking three independent piezoelectric crystals, each of
them responsible for movements on one axis
the scanner controls the movement of the tip or sample in the x,y, and z-
directions by expanding in some directions and contracting in others
some challenges exist due to scanner nonlinearities
Piezoelectric material (lead
zirconium titanate), PZT changes
dimensions when a voltage is applied
http://www.doitpoms.ac.uk/tlplib/afm/scanner.php
Force vs Distance curve
When the tip is brought close to the sample, a
number of forces may operate.
Typically the forces contributing most to the
movement of an AFM cantilever are the
coulombic and van der Waals interactions.
Coulombic Interaction: This strong,
short range repulsive force arises
from electrostatic repulsion by the
electron clouds of the tip and
sample. This repulsion increases as
the separation decreases.
Van der Waals interactions: These are longer
range attractive forces, which may be felt at
separations of up to 10 nm or more. They arise
due to temporary fluctuating dipoles. http://www.doitpoms.ac.uk
Main AFM modes:
Contact mode
Intermittent contact mode (tapping mode)
Non-contact mode
Contact Mode
The tip apex is in direct contact with the surface – repulsive regime
The AFM probe is scanned at a constant force or constant height between the probe and
the sample surface
In Contact mode of operation the cantilever deflection under scanning reflects repulsive
force acting upon the tip (Hooke’s Law)
Tip
Spring
The amount of force between the probe and sample is
dependent on the spring constant (stiffness of the
cantilever) and the distance between the probe and the
sample according to Hooke’s law:
F=-kx F = Force
k = spring constant
x = cantilever deflection
(displacement)
Contact Mode
The tip apex is in direct contact with the surface – repulsive regime
The AFM probe is scanned at a constant force or constant height between the probe and
the sample surface
In Contact mode of operation the cantilever deflection under scanning reflects repulsive
force acting upon the tip (Hooke’s Law)
Advantages:
- used to image hard surfaces when the
presence of lateral forces is not
expected to modify the morphological
features
- high resolution (lateral resolution of <1
nm and height resolution of <1 Å can
be obtained)
- minimal sample preparation
- operates in air and fluids environments
- provides information about physical
properties such as: elasticity, adhesion,
hardness, friction, etc.
Disadvantage:
- can damage fragile surfaces
- lateral forces experienced by both
probe and sample
- limitations in tip’s sharpness
Other contact techniques:
Lateral Force Mode, Spreading
Resistance Imaging Mode, Force
Modulation Mode and Contact Error
Mode.
Contact Mode
The probe cantilever is oscillated at
or near its resonant frequency ()
given by
The oscillating probe tip is then
scanned at a height where it barely
touches or “taps” the sample surface
The system monitors the probe
position and vibrational amplitude
to obtain topographical and other
property information Other semi-contact techniques: phase
detection mode, magnetic domains,
and local electric fields.
Tapping Mode
m
k
2
1
k – spring constant
m – mass of the cantilever
Add resonance curve
Tapping Mode
Advantages:
- optimum resolution is 50 Å lateral and
<1 Å height.
- low tip-sample shear forces generated
during scanning
- allows high resolution of samples that
are easily damaged and/or loosely held
to a surface (ideal for biological
samples)
Disadvantage:
- more challenging to image in liquids
- slower scan speeds needed
- lower resolution than contact-mode
AFM height image of live CHO cells
in PBS obtained in a liquid cell
(image curtesy of Dr. E.W. Brunner)
Non-contact Mode
The probe does not contact the
sample surface
The cantilever is oscillated near its
resonant frequency with an
amplitude of a few nanometers
(<10nm)
Advantages:
- soft materials can be imaged with very
low force (10-12) exerted on the sample
(absence of repulsive forces)
- no limitation in tip’s sharpness
- extended probe lifetime
Disadvantage:
- low lateral resolution because of the
long range forces
• Non destructive (unlike TEM)
• True vertical height measurements (with
a sub nm resolution)
• 3D surface profiles
• Can be used to characterize various
properties of materials: topography,
adhesion, hardness, friction, etc.
AFM Capabilities (Advantages)
• Samples do not need to be conducting (unlike SEM or STM)
• Simple sample preparation procedure (no metal coatings required that
would reversibly change or damage the sample)
• Works in air or even a liquid environment
Disadvantages of AFM
• The single scan image size(unlike SEM)
• Possibility of imaging artifacts
• Maximum scanning area of 150 x 150 mm (limited by PZ scanners)
• The sample cannot be scanned if it is rougher than the maximum
vertical range of the piezo (≈ 8 μm)
• For a smaller scanner (10 μm square lateral range), the vertical
range is 2 μm.
• Very sticky and adhesives cannot be scanned as it makes the tip stick
to the sample surface (tip crash)
• Slow speed of scanning
• AFM images can be affected by hysteresis of the piezoelectric
material and cross-talk between the x, y, z axes that may require
software enhancement and filtering
Recognising and avoiding artifacts
Primary sources of artifacts in images measured with AFM:
Probes
Scanners
Image processing
Vibrations
Artifacts - features that are not present in the sample in reality , but
are a direct result of the measurement itself
60 nm
Probe Artifacts
Left: AFM image of an 1.8 nm
diameter carbon nanotube. The
line profile of the image shows a
diameter of 60 nm and a height of
1.8 nm. The broadening is caused
by the shape of the probe used .
The height of the feature when
measured by line profile is
correct.
To avoid probe artifacts:
- use the optimal probe for the application
- probe should be much smaller than the
features of the image being measured
Features appear too large
Probe Artifacts
If the probe needs to go into a feature that is below
the surface, the size of the feature can appear too
small.
The line profile is established by the geometry of the
probe and not the geometry of the sample
Features appear too small
Strangely shaped objects
Strangely shaped object may appear if the probe iss
broken or chipped
Chipped AFM probe follows the geometry of the
sample surface and creates an image with a substantial
artifact
Probe Artifacts
Repetitive Abnormal Patterns in an Image
Chipped/broken tip
Common reasons:
scanning a rough surface,
using a high scan speed,
fast tip approach,
normal wear and tear over several scans
Errors Introduced by the PZT scanner
Scanner Drift – Creep
Due to thermal drift in the piezoelectric scanner
AFM can be susceptible to external temperature changes
most commonly occurs at the beginning of a scan of a zoomed-in region of an
the continued motion of the scanner after a rapid change in voltage, for
example, when moving the scanning position
image
http://www.ammrf.org.au/myscope/spm/introduction/
• can happen for flat surface
• the laser beam reflected off of the cantilever and some part of the surface may
interfere
• the period of these oscillations is typically close to the wavelength of the laser light
• such artifacts can be eliminated by moving the tip to a different location on the
surface
Interference
Errors Introduced by the PZT scanner
Nonlinearity
- the sensitivity of the scanner is not a linear function of the applied
voltage
- will not reduce when the same region is scanned multiple times
- AFM calibration required
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Large “tilt” can be
observed if the
probe/sample angle is not
perpendicular
“Bow” occurs when the tip
does not move in a flat
plane but in a parabolic arc
Both artifacts can be
removed by using post-
processing software
Errors Introduced by the PZT scanner
Tilt and Bow
http://www.doitpoms.ac.uk/tlplib/afm/scanner_related.php
Other AFM imaging modes
Lateral Force Microscopy
http://en.wikibooks.org/wiki/Nanotechnology/AFM
Left Right
LFM measures lateral deflections (twisting) of the cantilever that arise from
forces on the cantilever parallel to the plane of the sample surface
performed in contact mode
example: greater lateral force hysteresis on hydrophilic surface s than
hydrophobic ones due to capillary forces exerted on the tip
Snap-in
Snap-back
Sample Sample
Water layer Water layer
Probe
Probe
Force-distance spectroscopy
force vs. distance curves are used to measure the vertical force that the tip applies to
the surface while a contact-AFM image is being taken
force vs. distance curve is a plot of the deflection of the cantilever versus the
extension of the piezoelectric scanner
measured using a position-sensitive photodetector
for investigation of local variations in the elastic properties of the surface at the
nanoscale, analysis of surface contaminants’ viscosity, lubrication thickness
Shahin, V. et al. J Cell Sci 2006;19:23-30
Slope: Elastic Properties
Force-distance spectroscopy
Phase angle
Ph
ase
an
gle
Height
Phase
a secondary imaging technique for tapping mode
used for differentiating multiple components of composite
materials
used to map variations in surface properties such as elasticity,
adhesion, and friction
based on monitoring of the phase lag between the signal that
drives the cantilever oscillation and the cantilever oscillation
output signal reflecting changes in the mechanical properties
of the surface
topography and material properties can be collected
simultaneously
Phase Imaging
Right: AFM height and
phase images of
polymer wrapped
carbon nanotubes on
glass substrate
Phase Imaging – other examples
Left: AFM height and
phase images of
scratch resistant paint
Left: AFM height and
phase images of
polymer latex with
carbon nanotubes
under strain
Roughness analysis by AFM
AFM is essential for studying surface
roughness at the nanoscale
Resolution far exceeding that of other
stylus and optical based methods
AFM in tapping mode should be used
to minimize the effects of friction and
other lateral forces
Factors affecting the resolution of the
surface roughness measurement:
1) AFM instrument noise – limits the
vertical resolution
2) Tip radius - limits the spatial
resolution
AFM height profile of
smooth mica surface
Scanning Spreading Resistance Microscopy
A conductive AFM-probe is scanned at a high
force (μN) in contact mode (constant force
mode) while a DC bias is applied between the
probe and the sample
The high force is necessary to reduce the probe
contact resistance and noise level
The spreading resistance is the dominant one in
the total measured resistance and is related to
the local resistivity
𝑅 = 𝜌/4𝑎 R – resistance
- local resistivity
a – tip radius Spreading Resistance – for a contact
with a small area (point contact), the
resistance that does not lie strickly along
the path between electrodes.
K. Schroder, Semiconductor Material and Device Characterization, Wiley
Various resistance components
involved in SSRM measurement
Scanning Probe Microscopy: Electrical and Electromechanical
Phenomena at the Nanoscale, Volume 1, S.V. Kalinin et al.
2007,Springer Science & Business Media
Scanning Spreading Resistance Microscopy
(a) AFM topography and corresponding (b) SSRM surface conductivity map of low-density silver
nanowires network. (c) AFM topography and corresponding (d) SSRM surface conductivity map of
low-density silver nanowires network with graphene deposited on top
I. Jurewicz et. al. Advanced Functional Materials 2014
Scanning Spreading Resistance Microscopy
(a) A SSRM image across a laser scribe in a
silver nanowire film, where no electrical
connection can be seen to the laser ablated
region. (b) A current profile across the laser
scribe shown by the yellow line in a. (c) Contact
mode AFM topography image showing the same
region
KPFM provides a measurement of the contact potential difference (CPD) between a
conducting AFM probe and a sample with high spatial resolution
For metals and semiconductors, the CPD is determined, which is related to the
sample’s work function, while for insulators information about local charges is
obtained
The CPD (VCPD) measured between the probe and the sample is defined as:
KPFM is performed in non-contact mode
eV
sampleprobe
CPD
sample and probe - work functions of the sample and tip
e - electronic charge
Kelvin Probe Force Microscopy (KPFM)
Kelvin Probe Force Microscopy
If the AFM probe is separated from the sample surface by a distance d1, the system is not
connected electrically; the Fermi levels are of different energies relative to the vacuum level
When the AFM probe is brought into close proximity to the sample surface, upon electrical
connection, the Fermi levels align. Both the probe and the surface of material are now charged
(by the formation of an electrcic double layer) Due to the charging of the probe and the sample
surface, an electrostatic force develops due to the VCPD
This force can then be nullified by applying an external bias (VDC) between the probe and the
sample. The magnitude of this bias is a direct measurement of the CPD
Probe
Sam
ple
Probe
Sam
ple
Probe
Sam
ple
Kelvin Probe Force Microscopy (KPFM)
Kelvin probe force microscopy (KPFM) maps electrostatic potential at sample
surfaces to provide information about:
electronic structure
doping variations
trapped charges
chemical identity
in applications ranging from organic photovoltaics research to silicon and wide band-
gap semiconductor characterization.
Other SPM Techniques
Electrostatic Force Microscopy (EFM) - plots the locally charged domains of the sample
surface. The EFM applies a voltage between the tip and the sample while the cantilever scans
the surface in non-contact mode. The cantilever deflects when it scans over static charges.
Force Modulation Microscopy (FMM) – characterises the mechanical properties of a sample
in contact mode. The system generates a force modulation image, which is a map of the
sample's elastic properties, from the changes in the amplitude of cantilever modulation.
Magnetic Force Microscopy (MFM) - images the spatial variation of magnetic forces on a
sample surface. The system operates in non-contact mode, detecting changes in the resonant
frequency of the cantilever induced by the magnetic field’s dependence on tip-to-sample
separation.
Scanning Capacitance Microscopy (SCM) - images spatial variations in capacitance. SCM
induces a voltage between the tip and the sample. The cantilever operates in non-contact,
constant-height mode.
Summary
SEM/TEM AFM
Samples Must be conductive Insulating/Conductive
Magnification 2 Dimensional 3 Dimensional
Environment Vacuum Vacuum/Air/Liquid
Imaging time 0.1-1 minute 1-5 minutes
(or longer)
Horizontal resolution 0.2 nm (TEM)
5 nm (SEM)
0.2 nm
Vertical resolution n/a 0.05 nm
