Introduction

It is often said that:
“……a picture is worth a thousand words…….”

This can be especially true for Atomic Force Microscope images.

Although we often know that nanoparticles exist and we have a mental picture of what the nanoparticles look like, it is often helpful to be able to “see” the particles to verify that they are morphologically what we imagine they are. When the particles are less than 1 &gkmu;m in diameter, they cannot be directly visualised with an optical microscope. At best sub-micron particles appear as “diffraction spots” in an optical microscope. It is possible, however, to visualise nanoparticles with SEM or TEM. With these electronic beam methods, the particles may appear as 3-D objects when in fact the images are 2-D. With an SEM/TEM it is not possible to measure the depth of the particles directly.

Figure 1. Comparison between TEM (Left) and AFM (Right) images of 261nm latex spheres mounted on a TEM grid. Spacing of the grid is 463 nm.

With the AFM it is possible to directly visualise nanoparticles with sizes ranging from a nanometre up to 10,000 nm. Sample preparation for nanoparticles characterisation with the AFM is relatively simple. A clean, flat surface must be used and the nanoparticles must be dispersed on the sample’s surface. It is essential that the nanoparticles have a greater affinity for the surface than for the probe.

There are many properties of nanoparticles that can best be examined by directly visualising an AFM image of the particles. Examples of properties that are understood best by visualisation are shapes, size, dispersion of substrate and surface texture.

Shapes

When nanoparticles are fabricated they are often can be homogeneous or non-homogeneous in phase. As an example, Figure 2 shows nanoparticles of a material that were found to be either amorphous or crystalline. The two phases are immediately visualised in the image.

Figure 2. AFM image of crystalline and amorphous phase nanoparticles.

Dispersion

When nanoparticles are dispersed on a surface, they can often precipitate into large organised regions, or be a single particle on a surface. Visualising an AFM image of the nanoparticles dispersed on a surface immediately helps to understand the level of nanoparticle dispersion. Figure 3 shows an image of nanoparticles that were dispersed on a surface. Some of the nanoparticles are single and others are in an organised monolayer.

Figure 3. AFM image of 100 nm polymer spheres (Left) agglomerations, and (Right) monolayers.

Surface Texture

Although surface tension forces many of the surfaces of nanoparticles to be very smooth, there are certain processes that create nanoparticles with substantial texture. The texture could be surface roughness or it could be the facets of a crystalline phase. The surface texture must be substantially smaller than the diameter of the AFM probe to get meaningful measurements. Typically the diameter of the probe is 10 nm so it is not possible to measure surface texture on nanoparticles that are less than 100 nm in diameter.

Figure 4. Surface morphology: smooth versus rough particle. At the left is a 100nm polymer sphere and at the right is a milk powder nanoparticle, ~5 micron.

Relative sizes

The relative sizes of nanoparticles can typically be characterised quantitatively; regardless, it is often helpful to directly visualise the nanoparticles and observe the differences in particle sizes. Figure 4 is an AFM image of a specimen of nanoparticles having a few different diameters. The ratio of the numbers of particles at a given size is immediately apparent by visualising the image.

Figure 5. (Left) 2.64x2.64 micron image on powder nanoparticles, (Right) Image of inkjet particles on glossy photo paper, 78x78 microns.

Display formats

Atomic Force Microscope images are initially input into a computer as a three-dimensional array of numbers. There is a Z height value for each of the X,Y data points measured in the image. With visualisation software the images may be displayed in a number of different formats and colour schemes. Often it is helpful to visualise nanoparticle images with a variety of different display formats.

Figure 6. (Left) two dimensional and (Right) three dimensional image of 100 nm nanoparticles in monolayers on a surface.

Figure 7. Image of 100 nm nanoparticles showing different color palettes. (Left) Green color palette, (Right) purple blue light shaded color palette.

Conclusions

The atomic force microscope is very well suited for visualisation of both nano- and micro-size particles. Single particles, clusters, layers as well as the texture of the surface can be imaged directly and non-intrusively. Both 3D and 2D data displays comes in variety of colour palettes with additional lightening/shading options.

References

1. G. Binning, C.F. Quate, Ch. Gerber and E. Weibel, Phys. Rev. Lett. 49, 57–61 (1982).
2. G. Binning, C.F. Quate and Ch. Gerber, Phys. Rev. Lett. 56, 930–933 (1986).
3. P. Goodhew, J. Humpreys and R. Beanland, Electron Microscopy and Analysis. Taylor & Francis (2001).
4. P. West, N. Starostina, Advanced Materials & Processes 35–37 (February 2004).
5. N. Starostina and P. West, Proceedings of the 33rd Annual International Waterborne, High-Solids, and Powder Coating Symposium, pp. 307–320 (February 2006).

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