This book (Practical Electron Microscopy and Database) is a reference for TEM and SEM students, operators, engineers, technicians, managers, and researchers.
 

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The TEM contrast decreases as the specimen thickness increases. For instance, the 1-nm He (helium) bubble in palladium tritides becomes hardly detectable when the thickness reaches 12 nm. [1] Moreover, the contrast is also affected by the position of the bubble in the specimen thickness. It increases when the bubble locates close to the lower surface of the specimen.

[1] S. Thiébaut, B. Décamps, J. M. Pénisson, B. Limacher, A. Percheron Guégan, TEM study of the aging of palladium-based alloys during tritium storage, Journal of Nuclear Materials 277 (2000) 217-225.