Scanning electron microscope

SEM excels at surface imaging and is widely used for both material and biological samples where surface details are important.

• Key hardware components

  • Electron gun (source): Generates the primary electron beam used for scanning the sample.

  • Electromagnetic lenses: Focus and control the electron beam as it travels down the column toward the specimen.

  • Scanning coils: Deflect the focused electron beam in a raster pattern over the sample surface.

  • Specimen stage: Holds and allows precise positioning of the sample under the electron beam.

  • Detectors: Capture secondary electrons, backscattered electrons, or X-rays emitted from the sample to form images or analyze composition

  • Vacuum system: Maintains a high vacuum environment to prevent electron scattering by air molecules and ensure accurate imaging.

  • Control and imaging electronics: Manage beam parameters, data acquisition, and image display.

• Sample preparation milestones

  Conductivity

  Gold sputter coating is a sample preparation technique for scanning electron microscopy (SEM) in which a very thin layer of gold (typically a few nanometers thick) is deposited onto the surface of a specimen to make it electrically conductive. This conductive coating helps to minimize charging effects and improve image quality when imaging non-conductive samples, as the gold layer allows electrons to dissipate rather than accumulate on the sample surface.

  

  Water-free biological samples

  If air-drying is not sufficient, several techniques for removing water from biological samples were developed.

  Critical point drying

  A critical point dryer for scanning electron microscopy (SEM) samples works by replacing the water in a biological specimen with a transitional fluid, usually liquid carbon dioxide, after dehydration. The sample is then brought to the critical point of the fluid—where liquid and gas phases are indistinguishable—allowing the fluid to be removed as a gas without passing through a liquid-gas interface. This process prevents surface tension forces that would otherwise cause delicate structures to collapse or distort during drying, thus preserving the specimen’s ultrastructure for high-resolution SEM imaging.

  

  Hexamethyldisilazane

  Hexamethyldisilazane (HMDS) is a chemical often used as an alternative to critical point drying (CPD) for preparing biological samples for scanning electron microscopy (SEM). It allows for efficient drying of samples while preserving surface details, potentially reducing artifacts and making the process faster and less expensive than CPD.
  HMDS has a low surface tension, which helps to minimize the collapse and distortion of tissues during drying, leading to better preservation of surface details. It is generally faster and does not require specialized equipment like CPD, which can be expensive. However, HMDS is flammable and volatile, so appropriate safety precautions should be taken when using it, such as using a fume hood and wearing protective equipment.

  

Focused ion beam scanning electron microscopy

• Key hardware components

  • Focused Ion Beam (FIB) Column : Generates and focuses a beam of ions (commonly gallium) used for site-specific milling, cross-sectioning, and sample modification.

  • Scanning Electron Microscope (SEM) Column : Provides high-resolution imaging of the sample surface using an electron beam.

  • Sample Stage : Precisely positions and moves the sample for both imaging and ion milling operations.

  • Detectors : Capture secondary electrons, backscattered electrons, and ion-induced signals for image formation and analysis.

  • Vacuum System : Maintains the high vacuum required for both ion and electron beam operations.

  • Gas Injection System: Delivers precursor gases for processes like deposition or enhanced etching during FIB operations.

  • Control and Automation Electronics : Coordinate beam parameters, stage movement, data acquisition, and automated workflows, such as site-specific sample preparation

• Sample preparation milestones

  The sample preparation follows the same procinples asdetailed in SEM imaging above.

Transmission electron microscope

TEM provides unparalleled resolution for internal structures, making it indispensable for both nanomaterials and cellular ultrastructure studies.

• Key Hardware Components
  • Electron gun (source): Produces a beam of electrons that will be transmitted through the sample.
  • Condenser lenses: Focus and control the electron beam before it reaches the specimen.
  • Specimen holder and stage: Securely holds the ultra-thin sample and allows precise positioning and movement.
  • Objective lens: Focuses the electrons that have passed through the specimen to form an initial image.
  • Intermediate and projector lenses: Further magnify and project the image onto the viewing screen or camera.
  • Imaging system (fluorescent screen, digital camera, or photographic film): Captures and displays the final image.
  • Vacuum system: Maintains a high vacuum throughout the column to prevent electron scattering by air molecules.
  • Control and alignment electronics: Manage beam parameters, lens currents, and image acquisition.
• Sample preparation

  Drop-casting

  The dropcasting technique for preparing nanoparticles for transmission electron microscopy (TEM) involves depositing a small droplet of a nanoparticle suspension onto a TEM grid, usually coated with a thin carbon film. After allowing the droplet to air dry, the nanoparticles adhere to the grid surface, forming a thin layer suitable for electron transmission and imaging. This straightforward method enables rapid sample preparation for morphological and size analysis of nanoparticles, although care must be taken to avoid aggregation or uneven distribution during drying.

  Epoxy embedding and ultramicrotomy

  Ultramicrotomy is a sample preparation technique for transmission electron microscopy (TEM) in which an ultramicrotome—a precision instrument equipped with a diamond or glass knife—is used to cut ultra-thin sections (typically 50–100 nanometers thick) from a specimen embedded in a suitable resin (typically epoxys). These thin sections are required because electrons must be able to transmit through the sample for imaging in TEM.

  Ultramicrotomy is widely used for biological samples such as cells and tissues, as well as soft materials like polymers, composites, and certain layered or coated materials. It can also be applied to some inorganic and composite materials, although very hard or brittle samples may present challenges and sometimes require alternative preparation methods. This technique is valued for producing large, uniform thin sections with minimal beam or chemical damage, making it ideal for high-resolution ultrastructural studies in especially life sciences.

  

  Cryoplunging (CryoTEM)

  Cryoplunging is a sample preparation technique for transmission electron microscopy (TEM) in which a thin film of the sample, often a biological macromolecule, nanoparticle suspension or liposomal preparation, is rapidly plunged into a cryogen such as liquid ethane or propane cooled by liquid nitrogen. It is paramount that the medium is water and that the liquid is not viscous.

  This ultra-rapid freezing process, known as vitrification, prevents the formation of ice crystals and preserves the sample in a close-to-native, hydrated state, making it ideal for high-resolution structural studies of proteins, viruses, organelles, and other sensitive biological or soft matter samples. Cryoplunging is especially valuable for studying structures that would be damaged or altered by chemical fixation or drying, and is a foundational step in cryo-electron microscopy (cryo-EM) workflows

  

The above-mentioned knowledge and technology led to the development and implementation of numerous microscopy techniques and different setups. In the table below, you will find a list of well-established microscopy techniques. Not all of these techniques are available through the core facility or a particular laboratory at our university. However, the microscopy core facilities in Switzerland are well networked. If a particular instrument is not available on campus, our staff will direct you to the closest core facility that has the instrument in question available.