Electron microscopes: the TEM and the scanning tunnelling microscope

The transmission electron microscope (TEM)

• A TEM uses a beam of electrons instead of light to form an image. Electrons are accelerated to high speeds (short de Broglie wavelength) and passed through a very thin specimen.
• Magnetic coils act as lenses to focus the electron beam. Three types of magnetic lens are used in sequence:
• Condenser lens: focuses the electron beam onto the specimen.
• Objective lens: produces a magnified image of the transmitted electrons.
• Projector lens: further magnifies the image and projects it onto a fluorescent screen or detector.
• Because electron wavelengths ($\sim 10^{-12}$ m at typical voltages) are much shorter than visible light wavelengths ($\sim 5 \times 10^{-7}$ m), the TEM can resolve details thousands of times smaller than an optical microscope.

Limitations of the TEM

• The specimen must be extremely thin (tens of nanometres) so that electrons can pass through it.
• Samples must be placed in a vacuum (electrons would be scattered by air molecules), which means living specimens cannot be observed.
• Sample preparation is complex and can introduce artefacts.

The scanning tunnelling microscope (STM)

• The STM does not use a beam of electrons in the same way. Instead, a very sharp conducting probe (ideally ending in a single atom) is brought extremely close to a surface, typically within about 1 nm.
• A voltage is applied between the probe and the surface. According to classical physics, no current should flow because there is a gap. However, due to quantum tunnellingThe quantum mechanical phenomenon where a particle passes through a potential energy barrier that it could not classically overcome. The probability of tunnelling decreases exponentially with barrier width., electrons can "tunnel" across the gap, producing a measurable tunnelling currentThe small electrical current that flows between the STM probe and sample surface due to quantum tunnelling of electrons across the nanometre-scale gap..
• This tunnelling current is extremely sensitive to the distance between probe and surface, changing exponentially with gap width.

STM operating modes

• Constant height mode: the probe scans across the surface at a fixed height. Variations in tunnelling current are recorded. This is faster but risks crashing the probe into surface features.
• Constant current mode: a feedback circuit adjusts the probe height to keep the tunnelling current constant. The adjustments in height map out the surface topography. This is slower but safer and gives a true height profile.
• The STM can image individual atoms and does not require a vacuum, so it can be used in air or even in liquids.