What Can an afm do?
ICSPI makes the nGauge—the world's smallest, simplest and most affordable atomic force microscope (AFM). In this post, we're going to discuss some of the information that can be extracted from AFM images: topography, line profiles, surface roughness, particle analysis and counting, thickness, grain size analysis and phase imaging.
An AFM produces a 3-dimensional representation of the surface that it scans over. That means that you can look at the shape and size of individual features, such as the pits on a DVD, or determine the particle density, such as the number of nanoparticles in a given area.
The AFM topography image on the left is of a calibration sample. The colour is not the real colour of the surface: the contrast is used to differentiate between high (light gold) and low (dark) points. The size of this scan is 10 µm x 10 µm: the rulers are on the top and left sides of the image. The scale bar on the right is for the vertical dimensions. It shows that the tallest feature is about 1.2 µm. The image on the right is a 3-dimensional representation of the surface, using the topography data from the image on the left.
The nGauge AFM can be used to investigate surfaces where the maximum step height is 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). That means that, depending on your requirements, features as small as 5–10 nm can be imaged with the nGauge with acceptable accuracy.
Line Profile — HEIGHT
The line profile is a very common and useful analysis technique of AFM images, which provides the height information (z) of the surface along a user-selected line.
On the left is an AFM image of DVD and on the right is a graph of the line profiles across the lines shown on the AFM image on the left. These line profiles were generated in Gwyddion The depth of the pits of a DVD is 120 nm. The line profiles show that the AFM topography data is in good agreement with the true value. Because the vertical resolution of an AFM is so high, the height of the features is highly accurate. The noise in the vertical direction of the nGauge is ~ 1 nm RMS. It's good to keep in mind that post-processing of the AFM topography image, with techniques such as levelling, will have a considerable effect on the final result.
LATERAL DISTANCE (WIDTH)
A DVD pit is 320 nm wide. In the line profile above, it is difficult to say where exactly the pit begins and ends. Fortunately, we can use the distance tool in Gwyddion to investigate the width of the DVD pit. We can also look at the distance between the pits, which we know is 740 nm.
Using the distance tool, we can see that the distance between the pits, also known as pitch, using Line 1 (741 nm) is in good agreement with the real pitch (740 nm). Similarly, the width of the pits in Lines 2 and 3 (322 nm) is in good agreement with the real width (320 nm).
For all scanning probe microscopes, including AFMs, the lateral (XY) resolution will depend largely on the tip sharpness or radius of curvature.
The tip radius of an nGauge AFM is ~80 nm. A nanoparticle with a 50-nm diameter will show up as ~175 nm-wide on an AFM image. This phenomenon is known as tip convolution artifact. There are a few ways to account for this:
(1) If the particles are spherical, the particle size can be determined from the height of the particle and the width can be ignored.
(2) If the tip radius is known, a correction can be made using post-processing software. More information about this is available in the Gwyddion article on Tip Convolution Artifacts.
nGauge specification: nGauge can measure surfaces with roughness between 10 nm and 4 µm (0.39 microinch to 150 microinch)
Surface roughness is a component of the texture of a surface: a higher value means that the surface is rougher. Surface roughness is also known as surface finish. The arithmetic roughness (Ra) and the root-mean-squared roughness (Rq) are common parameters used to describe roughness.
Many types of devices, such as profilometers or optical profilers, use data from a line scan to calculate surface roughness. Because AFM collects topographic data in two dimensions, the surface roughness can also be calculated from the entire 2-d scan area. The roughness parameters are area roughness parameters: arithmetic (Sa) and RMS (Sq).
The nGauge AFM can be used to accurately determine the roughness of a surface between 10 nm and 4 µm (0.39 microinch to 150 microinch). The surface roughness of a sample can easily be determined using the statistical quantities tool in Gwyddion.
[ mirror finish of a TiAl alloy ] [ surface roughness screenshot]
The maximum area scan size is 20 µm x 20 µm. To get a representative result of a large surface, it is best to take multiple scans of a surface and compute the average surface roughness from these scans.
To give an example application of surface roughness measurement, a study published in December 2017 by Prof. Richard Price's group in collaboration with Prof. Laurent Kreplak in the Journal of Esthetic and Restorative Dentistry used the nGauge AFM to determine the effect of tooth brushing on the surface roughness of dental composites.
Particle Analysis and Counting
Particle analysis is a very common application of AFM. A line profile can be used to look at individual particles, but for more than a few particles a particle segmentation routine can be used. A segmentation routine separates the particles from the flat surface that they are on. Software such as ImageJ can be used for segmentation.
- keep in mind that the lateral resolution of the AFM depends on the tip radius. The width of your feature will be approximately 4 * sqrt(Rtip*Rparticle).
- possible to correct this
AFM images can also be used for particle counting. It is possible to complement the topography images with the phase images to determine whether the particles are indeed separate and different from the substrate. Phase imaging is described in detail below.
Interested in learning more about AFM? Check out some of our videos on YouTube, including how to get started with the nGauge: https://youtu.be/OxVJjzfT00k