Atomic Force Microscopy (AFM)

David Ji

Feb 21 06

AFM:

"Eyes of Nanotechnology"

A high-resolution imaging technique that can resolve features as small as an atomic lattice in the real space.

An fancier stylus-based instruments than record players

AFM image of A Digital Video Disc Surface

  AFM TEM SEM Optical

Resolution Atomic Atomic 1’s nm 100’s nm

Typical cost(x $1,000)

100 – 200 500 or higher

200 – 400 10 – 50

Imaging Environment

air, fluid, vacuum, special gas

vacuum vacuum air, fluid

In-situ Yes No No Yes

Sample preparation

Easy Difficult Easy Easy

Why people like AFM?

1. Laser 2. Mirror 3. Photodetector 4. Amplifier 5. Register 6. Sample7. Tip: sharp8. Cantilever: elastic

How AFM works?

The beam-bounce method

How AFM works?

Piezoelectric crystal: This crystal creates a voltage if pressure is applied, or in reverse, can create a pressure by expanding or contracting if a voltage is applied.

Sample under the AFM tip move

Force exerted by the tip on the sample is constant. The height changes of the scanner reflect the topography of the sample.

Varying force/constant height

Higher aspect ratio

About tips:

Tip end radius generally limits the resolution of AFM

Major Modes of Operation

Contact ModeTip scans the sample in close contact with the surface. Cantilever against the sample surface with a piezoelectric positioning element, it is up and down as it scans

Non-contact ModeTip hovers 50-150 Å above a sample surface. Attractive forces acting between the tip and the sample are detected.

Tapping Mode (intermittent contact) Tip in contact with the surface and then lifted off the surface to avoid dragging the tip across the surface.

the cantilever assembly oscillated at or near its resonant frequency using a piezoelectric crystal. The piezo motion causes the cantilever to oscillate with a high amplitude( typically greater than 20nm) when the tip is not in contact with the surface. The oscillating tip is then moved toward the surface until it begins to lightly touch, or tap.

Contact mode: Electrostatic and/or surface tension forces from the adsorbed gas layer will pull the scanning tip toward the surface. It can damage samples and distort image data. Contact mode imaging is heavily influenced by frictional and adhesive forces compared to non-contact or tapping mode.

Non-contact mode: Imaging generally provides low resolution and can also be hampered by the contaminant layer which can interfere with oscillation.

Tapping Mode: High resolution without inducing destructive frictional forces both in air and fluid. The very soft and fragile samples can be imaged successfully.

Summary of imaging modes

More operation modes

Lateral force mode  frictional forces exert a torque on the scanning cantilever 

Magnetic force mode

the magnetic field of the surface is imaged

Thermal scanning mode

the distribution of thermal conductivity is imaged

Thank you!