XPS and EELS study of the bismuth selenide

A Bi 2 Se 3 crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss Spectroscopy). Al Kα radiation and an electron beam energy in the range of 0.1 to 2 keV were used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi 2 Se 3 . The spectra in the valence band region suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p -orbitals and that the band arising from the valence s -orbitals occurs about 12 eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5 d 3 and Bi5 d 5 interband transitions. Considerations of the energies of the Bi5 d transitions as measured by XPS and EELS indicate that the bottom of the conduction band of the compound is 1.2 eV above the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon.