TEM stands for transmission electron microscopy, a technique in which a beam of accelerated electrons is transmitted through an ultrathin specimen to form an image with resolution far beyond that of light microscopy, revealing internal structures at the nanometre scale.
Explanation
Transmission electron microscopy uses the wave-like properties of electrons to visualise structures that are too small to resolve with visible light. In a TEM instrument, electrons emitted from a heated filament are accelerated to 60–300 kilovolts and focused by electromagnetic lenses into a coherent beam. When this beam passes through a specimen that is typically less than 100 nanometres thick, some electrons are scattered or absorbed while others are transmitted. The transmitted electrons are projected through additional lenses to form a magnified image on a fluorescent screen or a digital camera. Contrast is generated by differences in electron density, which are enhanced by staining specimens with heavy metals such as osmium tetroxide, uranyl acetate or lead citrate. Because electrons have very short wavelengths, TEM can resolve features at sub-nanometre scales, enabling detailed examination of cellular ultrastructure, viruses, macromolecular complexes and nanomaterials. Sample preparation requires fixation, dehydration, embedding in resin and ultramicrotomy to produce thin sections; cryogenic techniques (cryo‑TEM) allow imaging of vitreous samples without staining, preserving native conformation.
Techniques and Applications
TEM has been instrumental in elucidating the internal organisation of cells, revealing organelles such as mitochondria, endoplasmic reticulum and ribosomes. Negative staining and shadow casting enable visualisation of viruses, flagella and filamentous proteins. Electron tomography combines multiple tilted TEM images to reconstruct three-dimensional structures of bacteria and organelles. In materials science, TEM is used to study crystal defects, nanoparticles and semiconductor devices. The high resolving power and ability to image internal features make TEM a versatile tool across biology, physics and engineering.
TEM continues to advance with the development of direct electron detectors and computational methods. Its ability to visualise ultrastructure at atomic or near-atomic detail has made it indispensable for modern research.
Related Terms: Electron microscopy, SEM, Cryo-EM, Ultrastructure, Thin section