What is Atomic Force Microscopy?
An atomic force microscope (AFM) is not like a normal optical microscope: an AFM doesn’t have any lenses. Instead of “looking” at the sample, an AFM uses a tiny probe tip that scans across the surface of the sample. An AFM “feels” the sample, kind of like how a record player feels the grooves of a vinyl record.
By feeling the surface of the sample, instead of taking a picture, AFM provides 3-dimensional data of the surface. And AFM can be used to investigate features down to the nanoscale.
How does it work?
The animation below shows how AFM works: the sharp probe tip scans across the surface of the sample, made up of red cubes on a blue plate.
The probe tip scans along one entire row before moving to the next row in a pattern called a raster scan. As the tip approaches the red cubes, it moves up and over them.
3-dimensional data, down to the nanoscale
The position of the tip in X, Y, and Z coordinates is recorded throughout the scan. When the scan is complete, a 3-dimensional reconstruction of the surface is created. This means that unlike an optical microscope or an electron microscope, which provide information in two dimensions, AFM provides information in three dimensions.
This 3-d data is also known as the topography of the surface. The images the AFM are called topography images. The images below show what a butterfly wing and what a DVD look like on the nanoscale, captured by an nGauge AFM.
These images are contrast images—they are not the real colour of the surface. The bright, gold areas show tall regions and the dark areas show the deepest regions of the scan. The scale on the right size of each image shows that the brightest spot in the butterfly image is 207 nm tall and the darkest is 207 nm deep. The scale bar on the bottom right corner shows that that length is 1 µm (micrometer) across. (To put that into perspective, the face of a grain of sand is 500 µm across!)
And here’s the nGauge in action, showing how the data comes in line-by-line:
How does the nGauge work?
The probe tip gets very close to the surface of the sample — so close that the intermolecular forces between the tip and the surface cause the tip to deflect. That’s where the force part of Atomic Force Microscopy comes from.
In a conventional AFM, a laser is reflected off the cantilever (the beam that the tip is attached to) and reflected to a detector. As the cantilever is deflected by repulsive or attractive forces from the sample, the laser is deflected, and the precise change in height can be determined. Before using the AFM, the laser must be aligned so that it shines directly onto the cantilever. A conventional AFM also must be placed on a vibration isolation table to reduce errors from building vibrations.
ICSPI has integrated all of the components of a traditional AFM onto a single 1 mm x 1 mm chip to create the world’s first single-chip AFM. That means that 250 AFMs can fit on the face of a penny. The nGauge AFM uses MicroElectroMechanical Systems (MEMS) to control the X, Y and Z position of the tip. This means that no alignment of lasers is required.
The nGauge is the smallest AFM in the world. Because of its small size, the nGauge rejects building vibrations, so no vibration isolation table is needed, which makes the nGauge a true benchtop or desktop AFM. For more information about small AFMs, check out our blog post that describes the reasons why small AFMs are superior to conventional AFMs.
What can I use AFM for?
AFM provides topographical data of a surface. That means that you can look at the shape and size of individual features, such as the pits on a DVD, or look at the particle density, such as the number of nanoparticles in an area.
The nGauge AFM can be used to investigate surfaces where the features are up to 10 µm tall. It's tricky to pinpoint a lower limit, but the RMS noise in the vertical (z) direction of the nGauge is 1 nanometre (nm). So, depending on your requirements, features as small as 5–10 nm can be imaged with the nGauge with acceptable accuracy.
To put what a nanometer is into perspective, a quarter (25-cent coin) is about 2 cm wide. An E. coli bacterium cell is 2 µm wide (10,000× smaller than a quarter). And the diameter of a DNA helix is 2 nm (1,000× smaller than a bacterium).
Take a look at the Applications of AFM for more information.
What surfaces can I investigate?
AFM can be used on a very wide variety of surfaces: carbon nanotubes, thin films, coatings, graphene, corrosion, particles, or biological samples, AFM is an indispensable tool at the nanoscale. Take a look at our Gallery to see all the possibilities!
Why should I use AFM instead of SEM?
In fact, AFM was developed because its predecessor, scanning tunnelling microscopy (STM), could only be used on conductive surfaces. AFM finally allowed researchers to look at features on the nanoscale that were non-conductive, like polymers and biological materials.
Many types of microscopes require tedious sample preparation. For scanning electron microscopy, the sample must be coated in a metallic film prior to imaging. For tunneling electron microscopy, the sample must be sectioned into very thin sections.
AFM is a nondestructive technique, meaning that your sample is not damaged during imaging. AFM also does not need to be operated under vacuum and can be operated at room temperature.
Many AFMs require that the sample be "cut to size" to fit into the sample holder. The nGauge can image samples that are 100 mm x 50 mm x 18 mm (LxWxH) without having to be cut. Depending on their weight, the nGauge can accommodate even larger samples. Please contact us at firstname.lastname@example.org if you are interested in imaging larger samples.