Low Energy Electron Diffraction (LEED) consists of a low energy mono-energetic electron beam in the range of 20-500 eV which is elastically backscattered from the surface of the sample to a phosphor screen maintained at +6kV, see the figure below. From this set-up, a clear diffraction pattern can be observed in the case of well ordered surface and which represents the projection of the reciprocal surface with a magnification factor that is determined by the incident beam energy. In most cases, LEED is used to confirm the surface periodicity of crystal samples after cleaning.

The LEED can also be used to obtain the lattice constant of the crystal if one knows the exact geometry during the LEED measurement. The equation relates the lattice constant a of a simple cubic crystal structure with the different parameters shown in the figure above.

,

where h and k are the miller indices, me is the mass of an electron, h is the Planck constant over 2Π, x is the distance between the spots of miller indices (0,0) and (h,k), and d is the distance between the sample and the phosphor screen. NiAl forms a bcc crystal structure with Ni atoms at the corners the cube and an Al atom in the center. The (110) face has a rectangular unit mesh with a length a and b. In the case shown here, the LEED pattern of the NiAl(110) crystal was obtained at a distance of d = 86±1 mm between the sample and the fluorescent screen with a beam energy of E = 60.0±0.1 eV. This gives a = 2.8±0.02 Å and b = 3.8±0.04 Å for the unit mesh.

## Recent Comments