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History and Applications of Atomic Force Microscopy

Gregory JamesPhD 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.eduMilad 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 TodayAFMs 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

https://www.brukerafmprobes.com/Product.aspx?ProductID=3444

http://www.asylumresearch.com/Products/Cypher/CypherProduct.shtml

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How AFM Works

Sample

Laser Beam

Cantilever

Photo DetectorPiezo Drive

x

x

y

y

z

Camera

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AFM CantileversThe 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.aspx?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

Custom Applications• Dual cantilever/ correlation force AFM• Lubrication Force AFM• Nanofabrication

There are also a variety of specialty techniques used for specific systems.

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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

Sample

Laser Beam

Cantilever

Photo Detector

Piezo Drive

z

xy

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 z-position needed to keep the applied force (deflection) constant

F

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Contact Mode Imaging: Examples

Silica sphere

http://www.nanoandmore.com/USA/afm-gallery--the-curved-surface-of-a-human-hair-35-micron-scan-taken-with-tap300al-afm-probe-image-900.html

Human Hair

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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

Sample

Piezo Drive

z

xy

1. Tune and drive the cantilever at its resonance frequency

2. Define a known change in amplitude to the resonance of the cantilever

3. Move the sample (or cantilever) in the x and y planes

4. Measure the change in z-position needed to keep cantilever amplitude constant

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Tapping Mode Imaging: Examples

Nanobubbles on a hydrophobic surfaceCTAB 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

Hot Plate

X-Y-Z stage

Computer connected optics

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CP-AFM: How it works

Sample

Laser Beam

Cantilever

Photo Detector

Piezo Drive

Scan Rate

Probe particle and cantilever are driven toward and away from the sample at a known rate

Deflection vs. drive distance is measured and later converted into force vs. separation

F

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Region 1 Region 2 Region 3

Def

lect

ion/

Volts

Peizo Drive Distance/nm

CP-AFM: Processing the Data

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

Region 3: Zero force region. Used to define zero deflection (i.e. zero force)

Contact

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CP-AFM: Example Force Curves

Separation/nm

0 10 20 30 40 50

Forc

e/nN

-0.2

-0.1

0.0

0.1

0.2

0.31 cmc5 cmc30 cmc

Interaction of a 5 μm sphere and a flat plate in a solution of micelles

Separation/nm0 500 1000 1500 2000

Forc

e/nN

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0WithdrawalApproach

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

Laser Beam

Cantilever

Photo Detector

Piezo Drive

The interaction of two cantilevers, one driven and one static, is measured across an medium.

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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Laser Beam

Resonating Cantilever

Photo Detector

Piezo Drive

h

AFM: Custom Application

A lubrication Force AFM is also being developed in Dr. Duckers lab

Piezo Drive

The resonating cantilever is modeled as a simple harmonic oscillator

Controlling the separation distance allows for determination of the lubrication properties of the solution

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Where are the VT AFMsDepartment 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?