HOW TO CHOOSE BY: RESOLUTION
Suppose there are three AFM probes available for imaging: a former Long Scanning with R < 30 nm, a General Purpose AFM probe with R < 10 nm and a High Resolution AFM probe with R ~ 1 nm. Which of the three AFM probes should be chosen to get enough resolution?
One way of describing resolution is to consider whether two adjacent objects can be resolved by a particular AFM probe. If we have two rigid spikes and a 0.1 nm detector sensitivity in the z-direction, then the minimum diameter between these spikes for the AFM probe to distinguish between them is d=(0.8R)1/2, where d is the distance between the spikes and R is the radius of the AFM tip.
The amount of lateral resolution required should also be considered. Is it necessary only to know that there are two objects present? Or is it important to have the accurate lateral dimensions of these objects? The larger the radius of the AFM tip and the larger the opening angle of the AFM tip, the greater convolution will be present in the lateral dimensions. This effect has a greater impact on the accuracy of the dimensions of smaller objects than larger ones. For two spheres, the distance separating them must be d=4(Rr)1/2, where r is the radius of the spheres, in order for the AFM tip to fully probe between them. δr = (d - r) is also the lateral distance that will be "added" by the size of the tip to the sphere's topography.
Step size, which is the ratio of the scan size and the number of sampling points, should be also taken into account.Let's consider the case of imaging the topography of very small spheres 4 nm in diameter. The choice of the AFM tip and the scan size will determine if the spheres can be resolved. There are different possibilities:
LS AFM probes were mostly used for accurate measurement of vertical dimensions of rigid samples, lateral information for large features, or for when accurate lateral measurements are not important. These AFM probes were especially durable, allowing many more scans to be taken without a loss of resolution or damage to the AFM tip.
Genegal Purpose AFM probes provide additional resolution in the lateral dimension and are good for getting good quality everyday images of large and small features.
Because the Hi'Res-C AFM tips have a very sharp AFM tip and a high aspect ratio geometry, the convolution in the lateral dimension is minimized. Hi'Res-C AFM probes should be used when the best lateral resolution is necessary or when the feature size is exceptionally small.
Rc < 30 nm
Scan size 3µm. Diameter of spherical
molecules 150 nm. Click to enlarge. Height
image of polystyrene latex spheres.
/Courtesy of S. Magonov/
Rc < 10 nm
Scan size 800 nm. Diameter of spherical
molecules about 40 nm. Click to enlarge.
Composite film of poly(p-xylylene) matrix
and nanoparticles of MoO3 and TiO2.
/Courtesy of R. Gaynutdinov/
Rc < 1 nm
Scan size 250 nm. Diameter of
spherical molecules 9 nm. Click to enlarge.
Carbosilane dendrimers in a dense film.
/Courtesy of D. Ivanov/
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Lateral resolution below 1 nm. For scanning small areas below 250 nm at 512 points.
1 nm radius
AFM probes with sharp carbon AFM tips
Lateral resolution down to 5 nm for scan size below 1 μm.
8 nm radius
AFM probes with silicon tips
Accurate resolution of surface features larger than 10 nm in diameter. Good for scan sizes above 3 μm at 512 points.
20 nm radius
AFM probes with DLC wear-resistant coating