Electronic and structural properties of Bi2Se3 and its extension to copper doped Bi2Se3:Cu were studied using combined ab initio simulations and transmission electron microscopy based techniques, including electron energy loss spectroscopy, energy filtered transmission electron microscopy, and energy dispersive x-ray spectroscopy. The stability of the mixed phases was investigated for substitutional and intercalation changes of basic Bi2Se3 structure. Four systems were compared: Bi2Se3, structures obtaining by Cu intercalation of the van der Waals gap, by substitution of Bi by Cu in quintuple layers, and Cu2Se. The structures were identified and their electronic properties were obtained. Transmission electron microscopy measurements of Bi2Se3 and the Bi2Se3:Cu system identified the first structure as uniform and the second as composite, consisting of a nonuniform lower-Cu-content matrix and randomly distributed high-Cu-concentration precipitates. Critical comparison of the ab initio and experimental data identified the matrix as having a Bi2Se3 dominant part with randomly distributed Cu-intercalated regions having 1Cu-Bi2Se3 structure. The precipitates were determined to have 3Cu-Bi2Se3 structure.
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