Chapter 5 : Surface Analytical Instrumentation Technique
Atomic Force Microscopy (AFM) is a popular method used by many industrial R&D sectors to measure surface topography and material properties.
It is used to measure the material properties with spatial resolutions of 5-20 nm and height dimensions in the range from 100 nm down to approx. 5 nm and exceptionally to 0.2 nm.
AFM works by bringing an atomically sharp tip close to a surface.
There is an attractive force between the tip and the surface and this force is kept the same throughout the experiment.
The force is detected by a deflection of a spring, usually a cantilever.
Forces between the probe tip and the sample are sensed to control the distance between the tip and the sample.
There are two modes:
Topics covered in this snack-sized chapter:
- Contact Mode (Repulsive force)
At short probe-sample distances, the forces are repulsive.
- Non-contact Mode (Attractive force)
At large probe-sample distances, the forces are attractive.
The AFM cantilever can be used to measure both attractive force and repulsive forces.
The cantilever is designed with a very low spring constant (easy to bend) so it is very sensitive to force.
Since all this is going on at a very small scale, we can't watch the tip directly.
A laser is pointed at the tip and is reflect off the cantilever and onto the sensor.
As the tip goes up and down the laser hits different parts of the sensor.
With the information the sensor collects, an image of the surface can be recreated.
Tip moves across a sample; as the tip moves up and down over the surface, a laser detector records the height.
The tip passes back and forth in a straight line across the sample.
A topographic image is built up by the computer by recording the vertical position as the tip is raster across the sample.
An Electron Microscope uses a particle beam of electrons to illuminate the specimen and produce a magnified image.
Electron Microscope has a greater resolving power than a light-powered
This is due to electron have wavelengths about 100,000 times shorter than visible light (photons) and can achieve better than 0.2nm resolution and magnification of up to 2,000,000x.
Transmission electron microscope,
Scanning electron microscope.
Transmission Electron Microscopy is an imaging technique whereby a beam of electrons is focused onto a specimen causing an enlarged version to appear on a fluorescent screen or layer of photographic film.
A schematic diagram of Transmission electron microscope is shown in the figure below:
In TEM electrons are emitted by an electron gun.
The electron beam is accelerated by an anode, focused by electrostatic and electromagnetic lenses, and transmitted through the specimen.
When it emerges from the specimen,
the electron beam carries information about the structure of the specimen that
is magnified by the objective lens
system of the microscope.
Series of electromagnetic lenses (Objective, Intermediate, and Projector) act to illuminate the specimen and focus specimen on the fluorescent screen.
In SEM, images are produced by probing the specimen with a focused electron beam that is scanned across a rectangular area of the specimen.
SEM is done by scanning electron microscope which is shown in the figure below:
The virtual source at the top represents the electron gun, producing a stream of monochromatic electrons.
The stream is condensed by the first condenser lens.
This lens is used for both forms the beam and limit the amount of current in the beam.
It works in conjunction with the condenser aperture to eliminate the high-angle electrons from the beam.
The beam is then constricted by the condenser aperture, eliminating some high-angle electrons.
The second condenser lens forms the electrons into a thin, tight, coherent beam and is usually controlled by the fine probe current knob.
A user selectable objective aperture further eliminates high-angle electrons from the beam.
A set of coils then scans or sweep the beam in a grid fashion dwelling on points for a period of time determined by the scan speed.
The final lens, the objective, focuses the scanning beam onto the part of the specimen desired.
When the beam strikes the sample interactions occur inside the sample and are detected with various instruments.
Before the beam moves to its next dwell points, these instruments count the number of electron interactions and display a pixel on a CRT whose intensity is determined by the number.
This process is repeated until the grid scan is finished and then repeated, the entire pattern can be scanned 30 times/sec.