ИСТИНА

Understanding how topologically protected surface states behave at surfaces and interfaces requires knowledge of the atomic structure. The long-term stability of functional properties of topological insulator materials is crucial
 for the operation of future topological insulator based devices. These two issues are considered for second generation of topological insulators and topological crystal insulators. Bi2Se3, Bi2Te3 and Sb2Te3 present an iconic example of topological insulators with layered structure of tetradimite. Employing photoelectron diffraction and holography, we find bulklike chalcogen termination with a very small surface relaxation (<1%) in agreement with density functional theory simulations. Water and oxygen are to be
 the main sources of surface deterioration by chemical reactions. In the present work, we investigate the
 behavior of the topological surface states on Bi2X3 (X = Se, Te), Sb2Te3 and mixed crystals by valence-band and core level 
photoemission in a wide range of water and oxygen pressures. We find that no chemical reactions occur in pure oxygen and in pure water for Bi2X3. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. The surface reactivity is found to be negligible, which allows expanding the acceptable ranges of conditions for preparation, handling and operation of future Bi2X3-based devices. Sb2Te3 and mixed crystals containing Sb react with water and show much higher reactivity towards oxygen and water. The underlying mechanisms are discussed. SnTe, (Pb,Sn)Se and (Pb,Sn)Te are topological crystalline insulator with rocksalt structure. Two surface orientations are typically can be obtained for such materials as (001) and (111). In contrast to (111) for (001) surfaces no surface reconstruction, no trivial surface states and small SCLS of -0.3 eV were found. This class of materials is much more unstable chemically toward oxidation by atmospheric or residual oxygen, that may restrict essentially their applications. The comparative reactivity is discussed.