Synthesis, electronic structure and optical properties of PbBr1.2I0.8

Physicochemical interactions in the quasi-binary system PbBr2-PbI2 were studied by X-ray phase and differential thermal analysis methods. The boundaries of the solid solution ranges of the system components were determined. The solid solution range of PbBr2 extends up to ∼20 mol% PbI2, while that of PbI2 is less than 5 mol% PbBr2. For the first time, measurement results are presented as well as technology of growth of high-quality single crystal PbBr1.2I0.8 possessing centimeter dimensions which enable its use in optoelectronic devices. The structure parameters of the PbBr1.2I0.8 compound are ascertained. It crystallizes in the orthorhombic space group Pnma with the unit cell parameters a = 8.6332(5), b = 4.4236(3), c = 10.4508(7) Å. X-ray photoelectron spectroscopy (XPS) was employed to measure for the PbBr1.2I0.8 compound the binding energies of the constituent element core electrons as well as peculiarities of the energy distribution of the electronic states within the valence-band region. The XPS core-level and valence band spectra were measured for both pristine and Ar+ ion-bombarded PbBr1.2I0.8 single crystal surfaces. The studies have revealed the sensitivity of the PbBr1.2I0.8 single crystal surface with respect to Ar+ ion-bombardment: such treatment causes partial transformation of lead ions from the formal valence +2 to the lower valence state, namely Pb0. The XPS data reveal comparatively low hygroscopicity for the pristine PbBr1.2I0.8 surface. Comparison on a common energy scale of the XPS valence-band spectrum and the X-ray emission band representing the energy distribution of the Br 4p states indicates that, the main contributions of these states occur in the upper portion of the valence band with also their substantial contributions in other valence-band portions. Energy band gap of the PbBr1.2I0.8 compound, as estimated by measurements of the fundamental absorption edge, is equal to 2.77 eV at room temperature and it increases up to 2.83 eV with decreasing temperature to 100 K.