Measuring & Characterizing Nanoparticle Size – TEM vs SEM

SEM image of smartphone - Image Credits: Phenom World BV

Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) are widely considered the gold standard for nanoparticle characterization. However, choosing which to employ is a complex process as both techniques provide similar but distinct analysis. Investing in either system requires significant capital expenditure, so understanding their respective strengths and limitations is essential to determine which will deliver the greatest value. This article reviews the difference between TEM and SEM to support scientists in making the most informed purchasing decisions.

Linking Nanoparticle Size to Performance

Nanotechnology is at the forefront of novel materials development, from colloidal gold nanoparticles for targeted cancer treatment through to carbon nanotubes in renewable energy capture. Despite the diversity in application, the performance of all nanoparticle technology is defined by the same physical properties, including particle size, size distribution, shape, and surface features.

Nanoparticle size characterization therefore forms an important step in nanotechnology R&D and QC. Within the development toolkit, electron microscopy is one of the most powerful methods for determining these critical performance defining attributes. This is reflected by the US Food and Drug Administration’s (FDA) recommendation of the technique in identifying and demonstrating the efficacy and safety of innovator and generic drug submissions. Microscopy also plays an important role in validating the reliability of other routine particle sizing techniques, such as laser diffraction or dynamic light scattering.  

Electron microscopy is responsible for some of the most detailed nano and microscopic images ever produced. Beyond their aesthetic appeal, this advanced particulate insight has helped researchers to accelerate development in areas as diverse as cement composition to forensic sciences. Numerous microscopy techniques are commercially available, however Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM) are arguably the most popular for nanoparticle analysis. The similarities in operation and data output means that these techniques are often referred to interchangeably. In reality, however, the applications to which they are best suited vary substantially.  

The Not-So Microscopic Differences

Electron microscopy works by bombarding a sample with a stream of electrons and monitoring either the resulting transmission (TEM) or scattering (SEM) effects. These electrons are detected and converted into magnified images of particles in the sample dispersion. Image analysis software uses this information to generate particle size data for individual particles, number based size distributions for the entire dispersion and various shape and morphological parameters.

The primary difference in data output between the two techniques is the way in which the nanoparticle images are resolved. SEM produces accurate 3D images of particles in the dispersion while TEM produces 2D images that require further interpretation. However, although the images rendered are two dimensional, TEM systems are capable of delivering much greater resolution. A premium SEM instrument from manufacturer Jeol ltd, for instance, secures resolution down to 1.2 nm. By contrast, a TEM device from the same supplier will resolve images down to 0.17 nm. By monitoring electrons as they transmit from the sample, TEM also derives internal composition details, such as a particle’s crystallinity and lattice structure. SEM also provides this information, but is well suited to looking at samples’ surface characteristics.

TEM may therefore seem the more powerful technique; however this would be an over simplification. In some applications where a particle’s surface features and their coordination respective to one another impacts functionality, SEM may be better suited. Moreover, sample preparation in TEM generally takes longer than with SEM, as particles need to be thinly sliced. Furthermore, TEM covers only a very small sample selection. This narrow range combined with longer sample preparation times may result in the presence of aggregates during analysis. Aggregated particulates may result in size distribution measurements being generated from single runs that are unrepresented of the bulk sample. SEM enables a larger amount of sample to be measured at one time, which can improve both the statistical reliability and efficiency of nanoparticle size and shape distribution measurements.

Practical Choices  

Nanoparticle development benefits from an orthogonal approach to analysis that builds a comprehensive picture of nanoparticle performance. For those with the resource to accommodate both techniques, TEM and SEM offer a powerful combination of size, shape, surface area, crystal structure and morphological data.

Phenom World BV is a world leader in microscopy and offers a range of SEM systems for scientists and researchers. The Phenom XL SEM system, for example is a compact benchtop SEM that offers a range of features including: fast analysis, high throughput, a compact motorized stage for full scans of sample areas and a single-shot optical navigation camera to allow users to move to any spot on the sample in seconds.

Phenom XL - Image Credits: Phenom World BV

SEM may offer better performance for surface and shape analysis, particularly in applications such as quality control of colloidal nano-precipitates or for measuring surfaces and microstructures of nano sized powdered materials. For many nanotechnology developers looking at fundamental size and shape properties, SEM may offer a more productive path to high quality analysis than its TEM counterpart.

FEI combines hardware and software expertise in electron, ion, and light microscopy with extensive application knowledge in the materials science, life sci­ences, electronics, and natural resources markets. The company offers a range of TEM solutions including the Titan Themis STEM which combines proven optics and excellent XEDS performance with a powerful software platform, 16 megapixel CMOS camera, and enhanced piezo stage which delivers fast atomic-scale information.

As TEM can provide a higher resolution it may be better suited for particle size analysis in the nano to sub-nano region. Biological sciences, such as virology, may benefit from this technique. The ability to probe internal structure and crystallography means that TEM is arguably more powerful in the development of solid state materials such as semiconductors or nanotechnology. For example, TEM is eminently suited to observing dislocations or structural defects in real materials, such as solid state semiconductors.

Conclusion

SEM and TEM both offer unique benefits for nanoparticle characterization. SEM and TEM offer scientist and researchers very detailed images of specimens at microscopic and nano scale. The main difference in data output between the two techniques is the way in which the nanoparticle images are resolved. It is important to fully understand the benefits each technique offers before deciding which technique to use.

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