Part II: Sample Preparation for AFM Particle Characterization

Scanning Probe Microscopy has been routinely employed as a surface characterization technique for nearly 20 years. Atomic Force Microscopy is the most widely used subset of SPM, which can be used in ambient conditions with minimum sample preparation. AFM is able to measure three-dimensional topography information from the angstrom level to the micron scale with unprecedented resolution. This paper reviews the most common methods of sample preparation that are used for imaging nanoparticles with an AFM. AFM is well suited to individual particle characterization. The standard set of measured parameters includes: volume, height, size, shape, aspect ratio and particle surface morphology. As a single-particle technique, physical parameters for each particle in an image can be recorded and the data set can be processed to generate a statistical distribution for an entire set of particles (i.e. ensemble-like information). Speeding up the process of obtaining data is critical for many reasons and definitely makes AFM more attractive given its ability of individual particle imaging. In general, the AFM particle characterization is both cost and time effective as well as easier to use than electron microscopy. The resolution of AFM is greater or comparable to that of SEM/TEM, and strong advantages of AFM for particle characterization include direct measurements of height, volume and 3D display. Part II: Sample Preparation for AFM Particle Characterization 2 Revision.1/16/06.A For particle characterization, there is no single instrument that is the “right tool for every job”. In fact, more than 400 different techniques exist for particle counting, sizing, analyzing, and characterizing. Typically, instrumentation is chosen by engineers and researchers through consideration of what measurements need to be made, and in what environment the measurements need to be made. There are two primary considerations for selecting instrumentation: a) single particle versus ensemble, and b) the environment the measurement is made in – air, liquid, vacuum. Table 1 shows a comparison of the most common material classifications and environmental media for particle analysis. 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. As presented in Figure 2, there are three methods of characterizing nanoscale particles -SEM, TEM, and AFM . TEM and SEM are examples where an established singleparticle characterization technique is combined with image processing to measure and analyze particles. The emergence of the application of AFM to nanoparticle characterization has developed over the past 10 years. AFM is well suited to individual particle characterization including: parameters, volume, size, shape, and particle surface morphology. With single-particle techniques, physical parameters for each particle in a set of particles can be recorded and the data set can be processed to generate a statistical distribution (i.e. ensemble-like information) for the entire set of particles. A major factor that contributes to microscope single particle analysis (and ultimately the accuracy of measurements) is image quality. The AFM is similar to electron microscope techniques, SEM and TEM, where proper sample preparation is the key to measuring high quality data. The art of sample preparation is in fact a simple procedure of critical-path steps, where every single step makes a difference. TEM is well known for very time-consuming and complicated sample preparation8,9. SEM samples are easier to prepare, however the requirement for conductivity adds some difficulty. AFM samples do not have to be conductive, which makes sample preparation easier for the user, Figure 3. However, there are very important criteria to be met in order to do AFM imaging. Figure 2: Particle analysis can vary depending on the air or liquid borne or ensemble or single-particle. Liquid-borne methods Air-borne methods