History and Applications of
Atomic Force Microscopy
Gregory James
PhD Candidate
Department of Chemical Engineering
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Who are we?
• PhD students working for Dr. John Walz and Dr. William Ducker in the
Department of Chemical Engineering
• AFM is the major technique used in each of our research projects
Points of Contact:
Dr. William Ducker – PI Surface Science Lab – wducker@vt.edu
Milad Radiom – President of Colloids and Surfaces VT – mradiom@vt.edu
One Final Note:
The Colloidal Dispersion Lab is going away. Dr. Walz is now Dean of
Engineering at the University of Kentucky. His research group will be
“graduating out” by summer 2014.
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Key Reference:
Butt, H.-J.; Cappella, B.; Kappl, M. Force measurement
with the atomic force microscope: Technique,
interpretation and applications Surf. Sci. Rep. 2005, 59,
1-152.
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Need for AFM
Limitations of other imaging techniques:
• Scanning electron microscopy (SEM) – Conducting Samples
• Transmission electron microscopy (TEM) – Very Thin Samples
• Can’t image in environment
• Limited ability to work with
soft samples or reuse samples
• Expensive!
http://en.wikipedia.org/wiki/File:Misc_pollen.jpg
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Development of AFM
First AFM was developed by Binnig et al. in 1985 (published in 1986)
Binnig, G.; Quate, C. F. Phys. Rev. Lett. 1986, 56, 930-933.
Lateral resolution of 3 nm and vertical resolution less than 0.1 nm
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AFM Today
AFMs are now a commercial product available from several different
manufacturers.
They are relatively inexpensive (Avg. cost ~ $100K)
Variety of suppliers of AFM Cantilevers and AFM Probes
Lateral and Vertical Resolution on the order of 1 Angstrom
http://www.asylumresearch.com/Products/Cypher/Cy
pherProduct.shtml
https://www.brukerafmprobes.com/Product.aspx?
ProductID=3444
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How AFM Works
Camera
Piezo Drive
Photo Detector
y
x
z
Laser Beam
Cantilever
Sample
y
x
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AFM Cantilevers
The key piece of any AFM measurement is the cantilever!
Materials include: Carbon, Silicon, Silicon Nitride, or Platinum
Spring Constants: 0.005 – 60 N/m (application specific selection)
Tips have variable characteristics: height, shape, radius, etc.
https://www.brukerafmprobes.com/Product.asp
x?ProductID=3256
https://www.brukerafmprobes.com/Product.aspx?ProductID=3436
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How AFM Works
4 quadrant Photo Detector
A
B
C
D
Cantilever deflection can be measured in both the x and y direction
Vertical Deflection is measured as (A+B) – (C+D)
Lateral Deflection is measured as (B+D) – (A+C)
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AFM Applications
Imaging
• Contact Mode
• Tapping Mode
Force Measurements
• Colloid Probe AFM (CP-AFM)
• Nanoindentation
• Single Molecule experiments
• Lateral Force Measurements
There are also a variety of
specialty techniques used
for specific systems.
Custom Applications
• Dual cantilever/ correlation force AFM
• Lubrication Force AFM
• Nanofabrication
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Contact Mode Imaging
• As the name suggests, the cantilever is in direct contact
with the sample
• Requires a “hard” sample that will not be damaged by the
motion or force applied by the tip
• Requires a sample that will not create an attractive force
with the cantilever or hinder its motion
• Typically uses relatively low spring constant cantilevers
• Can be preformed in air or liquid
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Contact Mode Imaging: How it works
1. Apply a known force
(known deflection) to the
sample with the cantilever
2. Move the sample (or
cantilever) in the x and y
planes
3. Measure the change in zposition needed to keep
the applied force
(deflection) constant
Piezo Drive
Photo
Detector
z
F
Laser
Beam
Cantilever
Sample
y
x
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Contact Mode Imaging: Examples
Human Hair
Silica sphere
http://www.nanoandmore.com/USA/afm-gallery--the-curvedsurface-of-a-human-hair-35-micron-scan-taken-with-tap300al-afmprobe-image-900.html
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Contact Mode Imaging: Limitations
• Limited to “hard” samples
• Sample and tip cannot interact
• Static
• Noise
• Limited scan range (~30 μm2)
• Limited feature height (~ 1 μm)
• Trade off between scan area and image detail
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Tapping Mode Imaging: Noncontact
Imaging
• The cantilever now only “taps” the sample surface
• Can be used with “soft” and “hard” sample as this is a
nondestructive technique
• Requires a sample that will not create an attractive force
with the cantilever or hinder its motion
• Typically uses relatively high spring constant cantilevers
• Can be preformed in air or liquid
• Requires cantilevers that can tuned at their resonance
frequency
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Tapping Mode Imaging: How it works
1. Tune and drive the
cantilever at its resonance
frequency
2. Define a known change in
amplitude to the
resonance of the
cantilever
Piezo Drive
z
3. Move the sample (or
cantilever) in the x and y
planes
4. Measure the change in zposition needed to keep
cantilever amplitude
constant
Sample
y
x
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Tapping Mode Imaging: Examples
Nanobubbles on a hydrophobic surface
CTAB micelles on graphite
http://www.asylumresearch.com/Gallery/Cypher/Cypher4.shtml
Nature Methods 6, 792 (2009)
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Tapping Mode Imaging: Limitations
• Sample and tip cannot interact
• Noise
• Limited scan range (~30 μm2)
• Limited feature height (~ 500 nm)
• Trade off between scan area and image detail
• Image quality varies based on numerous factors (i.e. drive
speed, drive amplitude, and set point)
• Limited applications in liquid
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Colloid-Probe AFM
Technique developed simultaneously by Ducker et al. and Butt et al. in
1991
Arose due to issues measuring force interactions with an AFM tip
In this technique, a probe particle is attached to the end of an AFM
cantilever, allowing for the forces applied to the particle to be
measured
Allows for the measurement of Force versus Separation Distance
Allows for the measurement of a variety of interactions:
• Surface interaction forces
• Depletion and structural forces
• Hydrodynamic interactions
Ducker, W.; Senden, T.; Pashley, R. Nature 1991, 353, 239.
Butt, H.-J. Biophys. J. 1991, 60, 1438-1444.
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Mounting the Probe Particle
Probe particles are mounted to
AFM cantilever using a variety of
glues (i.e. heat setting, epoxy,
etc.)
Many custom set ups exist for
mounting. In some cases the AFM
itself is used
Computer
connected optics
X-Y-Z stage
Hot Plate
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CP-AFM: How it works
Probe particle and
cantilever are driven
toward and away from the
sample at a known rate
Piezo Drive
Photo
Detector
Scan
Rate
Laser Beam
Deflection vs. drive
distance is measured and
later converted into force
vs. separation
Cantilever
F
Sample
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CP-AFM: Processing the Data
Region 2
Deflection/Volts
Region 1
Region 3
Region 1: Known as the Constant
Compliance region. Defines zero
separation and allows for the
conversion of deflection voltage to
deflection distance
Region 2: Region of interaction.
Where the forces are. Region of
interest
Peizo Drive Distance/nm
Region 3: Zero force region. Used
to define zero deflection (i.e. zero
force)
Contact
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CP-AFM: Example Force Curves
0.3
2.0
1 cmc
5 cmc
30 cmc
0.2
Withdrawal
Approach
1.5
1.0
Force/nN
Force/nN
0.1
0.0
0.5
0.0
-0.5
-0.1
-1.0
-0.2
-1.5
0
0
10
20
30
40
50
500
1000
1500
2000
Separation/nm
Separation/nm
Interaction of a 5 μm sphere and
a flat plate in a solution of
micelles
Hydrodynamic interaction of a 30
μm sphere with a flat plate
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AFM: Custom Application
A dual cantilever AFM, also known as a correlation force AFM, is
currently being developed in the Department of Chemical Engineering
by Dr. Ducker’s group
Piezo
Drive
The interaction of two
cantilevers, one driven and
one static, is measured
across an medium.
Photo
Detector
Cantilever
Laser
Beam
The cantilever interaction
gives information about the
medium.
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AFM: Custom Application
As a secondary application of the dual cantilever AFM, a single
molecule is stretched between the tips. This allows for the measuring
of forces on the molecule
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AFM: Custom Application
A lubrication Force AFM is also being developed in Dr. Duckers lab
Piezo Drive
Photo
Detector
The resonating cantilever is
modeled as a simple
harmonic oscillator
Controlling the separation
distance allows for
determination of the
lubrication properties of the
solution
Laser Beam
Resonating
Cantilever
h
Piezo Drive
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Where are the VT AFMs
Department of Chemical Engineering (x3):
• Asylum Research Cypher
• Asylum Research MFP-3D
• Custom Dual Cantilever Set-up
Nano Characterization and Fabrication Laboratory (NCFL):
• Veeco BioScope II
Department of Environmental Engineering:
• Dr. Vikeland’s Lab
Department of Chemistry
• Dr. Esker’s Lab
Department of Physics:
• Dr. Tao’s Lab
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Thank you
Any Questions?
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