Low-energy electron microscopy

The intensity distribution of the diffraction pattern will depend on the periodicity at the sample surface and is a direct result of the wave nature of the electrons.

Such phenomena include (but are not limited to): tomography, phase transitions, adsorption, reaction, segregation, thin film growth, etching, strain relief, sublimation, and magnetic microstructure.

It is assumed that each electron undergoes only one scattering event, and incident electron beam is described as a plane wave with the wavelength: Inverse space is used to describe the periodicity of the lattice and the interaction of the plane wave with the sample surface.

, respectively, and constructive interference occurs at the Laue condition: where (h,k,l) is a set of integers and is a vector of the reciprocal lattice.

Electromagnetic quadrupole electron lenses are used, the number of which depends on how much resolution and focusing flexibility the designer wishes.

The uniformity of the electrostatic field between the objective lens and specimen, limited by spherical and chromatic aberrations larger than those of any other lenses, ultimately determines the overall performance of the instrument.

In most electron microscopies, the contrast aperture is introduced into the back focal plan of the objective lens (where the actual diffraction plane lies).

It is worth noting that the spacing of diffracted beams does not increase with kinetic energy as for conventional LEED systems.

If the surface is not homogeneous, a diffraction pattern obtained from LEED experiment appears convoluted and is therefore hard to analyze.

In a microdiffraction experiment researchers may focus on a particular island, terrace, domain and so on, and retrieve a diffraction pattern composed solely of a single surface feature, making the technique extremely useful.

In the image planes after the contrast aperture it is then possible to observe where the electrons originate from in real space.

This technique allows scientists to study on which areas of a specimen a structure with a certain lattice vector (periodicity) exists.

This way of measuring (often called LEEM-IV) yields spectra for each diffraction spot or sample position.

PEEM was first developed in the early 1930s, using ultraviolet (UV) light to induce photoemission of (secondary) electrons.

However, since then, this technique has made many advances, the most important of which was the pairing of PEEM with a synchrotron light source, providing tunable, linear polarized, left and right circularized radiation in the soft x-ray range.

Such application allows scientist to retrieve topographical, elemental, chemical, and magnetic contrast of surfaces.

No LEED pattern is formed, because no scattering events have taken place, and therefore, reflected intensity is high.

The reflectivity coefficients of surfaces depend strongly on the energy of incident electrons and the nuclear charge, in a non-monotonic fashion.

A low-energy electron microscope used for surface science studies at University of Illinois' Frederick Seitz Materials Research Laboratory in Urbana, IL, USA.
Ewald's sphere construction for the case of normal incidence of the primary electron beam. Here, this is done in a typical LEED set-up, however, in LEEM, there are complicated electron optics that allow for imaging of the resultant diffraction patterns and hence, the surface of the sample.
A typical LEEM/LEED lens and ray diagram.
LEEM bright-field (step/phase contrast) image of Cr(100). Atomic steps, step bunches, islands and terraces are easily discernible from vertical diffraction contrast originating from the wave nature of electrons. Field of view is 5.6　μm.
The LEEM image corresponding to a sub-monolayer palladium film (dark contrast) grown on the (110) surface of a tungsten crystal (bright contrast). The barely visible islands are tungsten carbides due to carbon contamination. Diameter of the imaged area is 10 micrometres .
Graphene on SiC is composed of domains of different stacking order. (left) Bright-field LEEM micrograph of a samples of bilayer, trilayer, and four-layer intercalated graphene. (right) Dark-field images of the same area. Domains of alternating contrast are clearly visible, indicating areas of different stacking order. Adapted from [ 3 ]
Photoexcitation electron microscopy (PEEM) of Ag rods on Si. Here, an Hg lamp produces photons of energy just above the work function threshold of Ag, and the resultant secondary electron emission is imaged.
Low-energy electron holography of DNA molecules stretched over lacey carbon. Adapted from Latychevskaia et al. [ 7 ]