Physics · MU Sofia

29. Electron microscope

Explains how electron microscopes work, contrasting transmission and scanning types. It starts with de Broglie waves and the roughly 3 nm resolution they allow, then describes the electron gun and lenses of the TEM, its very thin sample requirement and heavy-metal staining. The SEM section covers raster scanning, surface imaging and conductive coating of non-conductive samples. It closes with how biological specimens are prepared for each method, including drying and sputter-coating for SEM and resin embedding and microtome sectioning for TEM.

Inside: 1 section · 3 labelled pictures · 9 flashcards · 3 min

Topic 30 of 42 in the MU Sofia syllabus for Physics (Medicine, Year 1). The free AceMed note covers it in 1 section, about 363 words (3 min of reading), with 9 flashcards.

In this note

  1. Introduction

Introduction

Summary

  • The notes detail the principles and methodologies of electron microscopy, particularly focusing on transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Electron microscopes utilize electron beams, which due to their shorter wavelengths compared to visible light, allow for significantly higher resolution imaging. The principle of de Broglie waves explains that particles such as electrons can exhibit wave-like properties, permitting electron microscopy to achieve resolutions around 3 nm. Sample preparation varies between SEM and TEM, with SEM requiring conductive coatings and TEM needing ultrathin sectioning.

Principles of Corpuscular Microscopy

  • De Broglie Waves: The concept that particles such as electrons can be described by specific wavelengths, which are inversely proportional to their momentum. This wave-particle duality is fundamental to understanding how electron microscopy achieves its high resolution.
  • Resolution Capability: Electron microscopes can resolve features as small as approximately 3 nm due to the shorter wavelengths of electron beams compared to visible light.

Transmission Electron Microscope (TEM)

  • Principal Diagram: Involves an electron gun producing a beam that is focused onto the sample by electromagnetic lenses.
  • Image Formation: Electrons transmitted through the sample are affected by their interactions with the sample's atomic structure, creating varying contrast based on material density and thickness.
  • Sample Preparation:
  • Thickness Requirement: Samples must be extremely thin (10-100 nm) to allow electrons to pass through.
  • Staining: Heavy metals may be used to stain biological samples to enhance electron scatter, improving image contrast.

Scanning Electron Microscope (SEM)

  • Principal Diagram: Uses a focused beam of electrons scanned across the surface of a sample in a raster pattern.
  • Image Formation: Electrons reflected from or knocked off the sample surface are collected and analyzed to produce an image, capturing surface topology and composition.
  • Sample Preparation:
  • Conductivity: Non-conductive samples are coated with a thin layer of conductive material (e.g., gold, platinum) to prevent charging under the electron beam.

Preparation of Biological Samples for Electron Microscopy

  • SEM Preparation: Involves drying the sample, mounting on a suitable stub, and sputter-coating with a conductive metal to enhance electron reflection and image quality.
  • TEM Preparation: Requires embedding biological samples in resin, sectioning into ultrathin slices with a microtome, and sometimes staining with heavy metals to distinguish different cellular components clearly.

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