As known, in the SEM and TEM instruments, the desired materials are illuminated with high energy electron beams. In a SEM instrument, the beam is scanned over the sample, while in a TEM instrument, the beam passes through the sample. For this reason, a SEM image shows surface morphology and surface characteristics of the samples in form of powder or film, while a TEM image displays internal structure and real particle sizes because of operating at high working potential. The origin of the differences in the particles sizes obtained by SEM and TEM images can be understood from the step of sample supplying for these analyses. For an SEM analysis of powdered materials, some powders are usually utilized. The powder is stuck onto a SEM stub and it is coated by Au or other conductors for nonconductive materials. In the case of a TEM analysis, the desired material is usually suspended in a solvent by a high power ultrasonic instrument to spread the sample well. Then, the suspension put onto a TEM grid and let the solution air dry to obtain a thinly sliced sample. Therefore, the images obtained from TEM analysis has higher resolution and the agglomerations could be distinguished with this instrument, while in a SEM image, the agglomerated particles are usually observed with larger sizes. Hence, TEM analysis is the best technique to obtain the real sizes of materials in comparison with SEM analysis, which is difficult to study real particle sizes.
For example, in the enclosed file, we fabricated TiO2/carbon dots nanocomposites by using commercial TiO2 P25 (from Degussa with the average size of nearly 25 nm) to deposit various amounts of C‑Dots over its surface. As clearly seen in Fig. 2a, the nearly spherical particles have agglomerated and the boundaries are evidently observed. During the sample supplying for the TEM analysis, the operator sonicated the sample in a solvent to spread over the grid. That is why, the real particle sizes are observed in Figs. 2(b, c).
Aziz Habibi-Yangjeh
Useful discussion to have. A few minor points:
- Is either image representative of the whole? I’m trying to distinguish between particle size and particle size distribution
- What is the meaning of the third decimal place? What is its value?
- Shape is a 3-D issue, so how do we or the instrument decide where to place the ‘ruler’?
- How do we deal with aggregates of small particles when the actual transport properties reflect the larger entities? (Normal parlance would refer to the fused primary particles as aggregates - tightly, chemically bound - and looser clusters as agglomerates). In other words, how do we relate these microscopic properties to the entire system?
Aziz Habibi-Yangjeh
Generally speaking more techniques provide more information and it could be a dangerous game to play with the claim that one technique is better than another especially when we have a phrase 'real particle size' relating to irregular shaped particles. It all depends on the application (and availability) what one would select. Shape is a 3-D issue and many parameters (e.g. aspect ratio) are ratios of 1-D parameters (breadth/length) on a 2-D image. Typically speaking SEM allows a better feel of the 3-D aspects of a system and may look at more entities. But if we're looking at atomic planes and their spacing, then there's no substitute for TEM.
Dear Prof. Alan F Rawle
Many thanks for your valuable comments.
Regards
Aziz
Isha Arora
You need to open a separate question for your inquiry. This avoids confusing this thread which is about TEM/SEM...
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