Introduction to Nanoparticles

Qualitative Analysis

Using the AFM, individual particles and groups of particles can be resolved. Microscope images are essential in research and development projects and can be critical when troubleshooting quality control issues.

The AFM offers visualization in three dimensions. Resolution in the vertical, or Z, axis is limited by the vibration environment of the instrument: whereas resolution in the horizontal, or X-Y, axis is limited by the diameter of tip utilized for scanning.

Typically, AFM instruments have vertical resolutions of less than 0.1 nm and X-Y resolutions of around 1 nm.

Figure 2

In Figure 2, 73nm NIST traceable microspheres are shown in both perspective view and top view. 3D information is incorporated in both views. In the perspective view, the 3D nature of the image is obvious. In the top view, the intensity of the color reflects the height of the particle. In material sensing mode, the AFM can distinguish between different materials, providing spatial distribution information on composite materials with otherwise uninformative topographies. In Figure 3, material inhomogeneity can be seen on a topographically flat organic film. Similarly, nanocomposites can be analyzed for dispersion of particulate matter.

Quantitative Analysis

Software-based image processing of AFM data can generate quantitative information from individual nanoparticles and between groups of nanoparticles. For individual particles, size information (length, width, and height) and other physical properties (such as morphology and surface texture) can be measured. In Figure 4, surface roughness data generated from a scan of a wood fiber is shown.

Figure 3

Figure 4

Statistics on groups of particles can also be measured through image analysis and data processing. Commonly desired ensemble statistics include particle counts, particle size distribution, surface area distribution and volume distribution. With knowledge of the material density, mass distribution can be easily calculated. Image processing of an AFM image is shown in Figure 5.

Figure 5

Whenever data from single-particle techniques is processed to provide statistical information, the concern over statistical significance exists. It is easy to attain greater statistical significance in AFM by combining data from multiple scans to obtain information on the larger population.

Experimental Media

AFM can be performed in liquid or gas mediums. This capability can be very advantageous for nanoparticle characterization. For example, with combustion-generated nanoparticles, a major component of the particles are volatile components that are only present in ambient conditions.
Dry particles can be scanned in both ambient air and in controlled environments, such as nitrogen or argon gas. Liquid dispersions of particles can also be scanned, provided the dispersant is not corrosive to the probe tip and can be anchored to the substrate.

Figure 6

Particles dispersed in a solid matrix can also be analyzed by topographical or material sensing scans of cross-sections of the composite material. Such a technique is useful for investigating spatial nanocomposites. Precipitates in a nickel aluminum distribution are shown in Figure 6.


Size

In many industries, the ability to scan from the nanometer range into the micron range is important. With AFM, particles anywhere from 1nm to 5μm in height can be measured in a single scan. It is important to note that AFM scanning is done with a physical probe in either direct contact or near contact. Therefore, particles must be anchored to the sample surface during the scan. With the Nano-Rp™, the maximum 2D scan range for a single scan is 80μm x 80μm. Multiple scans can be performed, however, to provide greater statistical accuracy.


Sample Preparation For AFM Nanoparticle Characterization

Nanoparticles typically fall into one of two categories when it comes to sample preparation. The first category is nanoparticles rigidly attached to a solid structure. The second category is nanoparticles with weak adhesion to the substrate, such as dispersions of nanoparticles in liquid or dry mediums. A good example of the first category is nanoparticles imbedded a solid matrix, as in the case of nanocomposites or nanoprecipitates3. In such cases, typically a cross-section of the composite material is scanned to determine such properties as average particle size and spatial distribution.

Examples of nanoparticles in the second category are quantum dots, diesel soot particles, carbon black, and colloidal suspensions. Sample preparation for the second category of nanoparticles involves the stable attachment of particles onto the substrate. Because the AFM works by scanning a mechanical probe across the sample surface, any structure being imaged must have greater affinity to the flat surface than to probe tip. When nanoparticles do accidentally attach to the probe, the resulting images typically show reduced resolution. Streaking will occur in the images if nanoparticles are not rigidly attached to the flat surface while scanning in contact mode. To avoid such artifacts, close contact mode (near contact) or CFM (crystal sensing) is strongly recommended for such samples.

Figure 7

In certain cases, it is necessary to affix nanoparticles to a sticky substrate within liquid or dry mediums. A cheap and easy way to do this is the use of double-sided sticky tape or other similar methods commonly used by microscopists. More refined techniques include the use of mica, “Tacky Dot” slides, and TempFix. Calcium phosphate nanocrystals mounted on TempFix can be seen in Figure 7. Information on mica use is readily available in technical literature. Use of “Tacky Dot” slides and TempFix are well described in PNI’s Application Note “Atomic Force Microscopy for Nanostructures.

Ensemble vs. Single-Particle Techniques

Particle analysis techniques can generally be classified as ensemble or single-particle techniques.
With ensemble techniques, individual particles cannot be isolated. Instead, ensemble techniques measure the response from statistically significant numbers of particles simultaneously. Laser light diffraction is a commonly employed ensemble technique.

In contrast with ensemble techniques, single-particle techniques isolate and identify data from individual particles. Statistical information from groups of particles can be obtained by processing the combined measurements of many different individual particles. A common example of a single-particle technique is optical imaging combined with image processing to measure and analyze particles.

In general, morphological information, such as shape and aspect ratio, as well as surface information, such as texture and roughness parameters, cannot be obtained using ensemble techniques. Only single-particle techniques, which look at individual particles, can supply such information. Physical parameters for each particle within a set of particles are recorded to generate a statistical distribution for the entire set of particles.


Which Technique is “The Best”?

Obviously, there is not one single “best technique” for all situations. Determining the best technique for a particular situation requires knowledge of the particles being analyzed, the ultimate application of the particles, and the limitations of techniques being considered.

Depending on the application of interest, a number of techniques can be used to analyze and characterize nanoparticles. In industries where aerosols play an important role, tools such as the Differential Mobility Analyzer (DMA) are commonplace. With fine powders, light scattering techniques are common. Table 4 describes some common particle analysis techniques and their benefits and drawbacks in comparison with the AFM.

Comparison of AFM with SEM / TEM

AFM has several advantages over SEM/TEM for characterizing nanoparticles. Images from an AFM represent data in three dimensions, so that it is possible to measure the height of the nanoparticles quantitatively. With an SEM/TEM, the images measured are only two dimensional, Figure 8, 9. With an AFM, images can be measured in all environments; ambient air, liquids and vacuums.

The AFM scans more slowly than an SEM. However, a complete measurement session that includes sample preparation, acquiring an image, and then analyzing the image takes much less time with an AFM. In fact, typically it takes about 1/4 of the time to get data from an AFM than with an SEM/TEM.

An AFM is a very cost effective microscope for nanoscale imaging. In general, an AFM with the comparable resolution to a SEM/TEM costs much less than the SEM/TEM. Further, the AFM requires substantially less laboratory space than an SEM/TEM; only a desk or possibly vibration table is required for an AFM. And finally, the AFM is much simpler to operate than the SEM/TEM so the AFM does not require a specially trained operator.

Figure 8

Figure 9