Marina E. Tamm

Negligible Surface Reactivity of Topological Insulators Bi2Se3 and Bi2Te3 towards Oxygen and Water

The long-term stability of functional properties of topological insulator materials is crucial for the operation of future topological insulator based devices. Water and oxygen have been reported 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) by valence-band and core level photoemission in a wide range of water and oxygen pressures both in situ (from 10(-8) to 0.1 mbar) and ex situ (at 1 bar). We find that no chemical reactions occur in pure oxygen and in pure water. Water itself does not chemically react with both Bi2Se3 and Bi2Te3 surfaces and only leads to slight p-doping. In dry air, the oxidation of the Bi2Te3 surface occurs on the time scale of months, in the case of Bi2Se3 surface of cleaved crystal, not even on the time scale of years. The presence of water, however, promotes the oxidation in air, and we suggest the underlying reactions supported by density functional calculations. All in all, 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.

Synthesis and electrocatalytic activity of platinum nanoparticle/carbon nanotube composites

Pt/CNT nanocomposite materials with an average platinum particle size of 3–5 nm and platinum content of 13–28 wt % have been prepared by reducing chloroplatinic acid, H2PtCl6, in the presence of conical carbon nanotubes. The effect of synthesis conditions on the average platinum particle size, total platinum content, and surface composition of the nanocomposites has been studied using X-ray photoelectron spectroscopy, IR spectroscopy, electron microscopy, X-ray diffraction, and thermogravimetry. The materials have been tested as catalysts for hydrogen oxidation and oxygen reduction. Their performance has been assessed by cyclic and steady-state voltammetric techniques. The structure and composition effects on the electrocatalytic properties of the nanocomposites are discussed.

Phase Equilibria in Ternary Reciprocal Systems Based on IV-VI Compounds

Using the experimental data obtained in this study, literature data, and a unified thermodynamic model, we have calculated the T − x − y phase diagrams of all the pseudoternary reciprocal systems formed by the Group 14 metal chalcogenides: Sn, Pb ‖ S, Se; Sn, Pb ‖ S, Te; and Sn, Pb ‖ Se, Te.

Manifestation of topological surface electron states in the photoelectromagnetic effect induced by terahertz laser radiation

We demonstrate that measurements of the photoelectromagnetic effect using terahertz laser radiation may provide a unique opportunity to discriminate between the topological surface states and other highly conductive surface electron states. We performed a case study of mixed (BiIn x )2Se3 crystals undergoing a topological phase transformation due to the transition from the inverse to the direct electron energy spectrum in the crystal bulk at variation of the composition x. We show that for the topological insulator phase, the photoelectromagnetic effect amplitude is defined by the number of incident radiation quanta, whereas for the trivial insulator phase, it depends on the power in a laser pulse irrespective of its wavelength. We assume that such behavior is attributed to a strong damping of the electron–electron interaction in the topological insulator phase compared to the trivial insulator.

Fabrication of nanocomposites based on carbon nanotubes containing Pt nanoparticles and TiO2

Using conical multiwalled carbon nanotubes (CNTs), we have prepared Pt/CNT and Pt/TiO2/CNT nanocomposites with an average platinum particle size of 3–5 nm, Pt/Ti molar ratio on the surface in the range 3.5–4, and C/Pt = 21–22. Titania was deposited onto the CNTs through titanium tetrachloride (TiCl4) hydrolysis. Platinum particles were produced by reducing chloroplatinic acid (H2PtCl6) with sodium borohydride (NaBH4) in the presence of CNTs. The composition and structure of the composites have been studied using X-ray photoelectron spectroscopy, electron microscopy, X-ray diffraction, and thermogravimetry. The materials have been tested as catalysts for hydrogen oxidation and oxygen reduction. The results demonstrate that the modification of Pt/CNT with titania enhances the catalytic activity of the material.

Can surface reactivity of mixed crystals be predicted from their counterparts? A case study of (Bi1−xSbx)2Te3 topological insulators

The behavior of ternary mixed crystals or solid solutions and its correlation with the properties of their binary constituents is of fundamental interest. Due to their unique potential for application in future information technology, mixed crystals of topological insulators with the spin-locked, gapless states on their surfaces attract huge attention of physicists, chemists and material scientists. (Bi1−xSbx)2Te3 solid solutions are among the best candidates for spintronic applications since the bulk carrier concentration can be tuned by varying x to obtain truly bulk-insulating samples, where the topological surface states largely contribute to the transport and the realization of the surface quantum Hall effect. As this ternary compound will be evidently used in the form of thin-film devices its chemical stability is an important practical issue. Based on the atomic resolution HAADF-TEM and EDX data together with the XPS results obtained both ex situ and in situ, we propose an atomistic picture of the mixed crystal reactivity compared to that of its binary constituents. We find that the surface reactivity is determined by the probability of oxygen attack on the Te–Sb bonds, which is directly proportional to the number of Te atoms bonded to at least one Sb atom. The oxidation mechanism includes formation of an amorphous antimony oxide at the very surface due to Sb diffusion from the first two quintuple layers, electron tunneling from the Fermi level of the crystal to oxygen, oxygen ion diffusion to the crystal, and finally, slow Te oxidation to the +4 oxidation state. The oxide layer thickness is limited by the electron transport, and the overall process resembles the Cabrera–Mott mechanism in metals. These observations are critical not only for current understanding of the chemical reactivity of complex crystals, but also to improve the performance of future spintronic devices based on topological materials.

Experimental and Computational Insight into the Chemical Bonding and Electronic Structure of Clathrate Compounds in the Sn-In-As-I System.

Inorganic clathrate materials are of great fundamental interest and potential practical use for application as thermoelectric materials in freon-free refrigerators, waste-heat converters, direct solar thermal energy converters, and many others. Experimental studies of their electronic structure and bonding have been, however, strongly restricted by (i) the crystal size and (ii) essential difficulties linked with the clean surface preparation. Overcoming these handicaps, we present for the first time a comprehensive picture of the electronic band structure and the chemical bonding for the Sn(24-x-δ)InxAs(22-y)I8 clathrates obtained by means of photoelectron spectroscopy and complementary quantum modeling.