Electron Microscopes Flashcards
How do electron microscopes work?
Makes use of the particle nature of electrons - uses electric and magnetic fields to control electrons.
Makes use of wave nature to obtain detailed images. To form an image of an atom, the electrons need to have a de Broglie wavelength of 0.1nm (order of magnitude of diameter of an atom).
Outline the process of forming an image using a TEM.
A beam of electrons is directed at a thin sample in an evacuated tube.
As the electrons pass through the sample, some of them are scattered by the structures in the sample.
Electromagnetic coils (magnetic lenses) focus the scattered electrons onto a fluorescent screen at the end of the tube to form a magnified image of the sample structure.
How are electrons produced in a TEM?
An electron gun produces electrons by thermionic emission from a heated filament.
The electrons are accelerated through a hole in a metal anode at the constant potential difference relative to the filament and emerge through the hole at the same speed.
Which features use fields in a TEM and how are they used?
The magnetic condenser lens produces a magnetic field that forces the electrons into a parallel beam directed at the sample.
The objective lens deflects the scattered electrons so they form an enlarged inverted image of the sample.
The magnifier lens focuses the electrons from central are of the first image to form a magnified final image on the screen.
What affects the amount of detail in a TEM image?
The amount of detail is determined by the resolving power. Resolving power depends on how much diffraction occurs when the electrons pass through the objective lens.
The smaller the wavelength, the less diffraction so greater resolving power.
Increasing the anode potential difference also produces a larger and clearer image.
What are the limitations of a TEM?
Sample thickness: electrons passing through the sample suffer a slight loss of speed, increasing the de Broglie wavelength and reducing resolving power.
Lens aberrations: the magnetic field in the lens gap may focus electrons from a given point to a different position on the screen, causing the image to be blurred.
Electrons may also have different speeds due to thermionic emission and passing through different thicknesses of the sample so they would be focused differently on the screen.
Outline how a STM works.
A fine-tipped metal probe scans across a small area of a surface at a height of no more than about 1nm above the surface.
The probe is at a constant negative potential difference of about -1V relative to the surface to ensure the electrons only tunnel across the gap in one direction.
The tunnelling current increases if the gap decreases and decreases if the gap increases.
How is data recorded in constant height mode of a STM?
The tunnelling current is recorded as the top scans across the surface in a fixed plane.
How is data recorded in constant current mode of a STM?
The gap is kept constant by feeding back changes in the tunnelling current tip to the piezoelectric transducer that controls the tip height.
What does a STM record?
It shows the image or ‘map’ of a surface. It shows peaks and troughs in the surface due to the individual atoms or groups of atoms on the surface.
What are the limitations of a STM?
If the initial gap is too large, the tunnelling current will be negligible.
If the initial gap is too small, the tip might be damaged by the collisions with raised atoms on the surface.
What conditions ensure a STM work sufficiently?
The probe and surface are normally in a vacuum to prevent contamination of the surface.
The electrons have insufficient kinetic energy to overcome the potential barrier caused by the metal tip but their de Broglie wavelength is long enough to stretch across the narrow gap.
