Polymorph Analysis by Scanning Electron Microscopy (SEM)
Scanning Electron Microscopy has two distinct advantages over common optical microscopy - namely, much higher magnification and greater depth of field (DOF). While optical microscopy is limited to a magnification of about 1,000 X with almost no DOF, modern SEMs can attain magnifications of 500,000 X or more with excellent DOF. Additionally, SEMs also offer in situ elemental chemical analysis and mapping capabilities. For all these reasons, the SEM is an important tool often used for studies of particle morphology, size distribution analysis, tendency for particle agglomeration, and analysis of chemical homogeneity.
SEM Facilities
We utilize two SEMs - one for routine work at magnifications of less than 20,000 X and a second one for specialized work at the highest magnifications on beam sensitive materials. The second instrument is a Zeiss Field Emission SEM (FE-SEM) that uses a low voltage emitter to minimize sample damage and has enhanced imaging and analytical capabilities. Both instruments can be used for examining particle morphology and size, although the FE-SEM is the preferred instrument for examining nanoscale or sub-micron powders..
SEM Capabilities
In addition to obtaining images at high magnification, the SEM is frequently used to analyze particle morphology, particle size and its distribution. An example is shown in the image to the left where several particles are shown, many with markers showing their individual diameters. By analyzing an image such as this, the average particle size, aspect ratio and distributions can be determined.