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 <i>in situ</i> (from 10^–8 to 0.1 mbar) and <i>ex situ</i> (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 <i>p</i>-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

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)₂Te₃ Crystal Doped with Mn

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone

Reactivity of Carbon in Lithium–Oxygen Battery Positive Electrodes

Unfortunately, the practical applications of Li–O₂ batteries are impeded by poor rechargeability. Here, for the first time we show that superoxide radicals generated at the cathode during discharge react with carbon that contains activated double bonds or aromatics to form epoxy groups and carbonates, which limits the rechargeability of Li–O₂ cells. Carbon materials with a low amount of functional groups and defects demonstrate better stability thus keeping the carbon will-o’-the-wisp lit for lithium–air batteries

Rapid Surface Oxidation of Sb₂Te₃ as Indication for a Universal Trend in the Chemical Reactivity of Tetradymite Topological Insulators

Within the past few years, topological insulators (TIs) have attracted a lot of interest due to their unique electronic structure with spin-polarized topological surface states (TSSs), which may pave the way for these materials to have a great potential in multiple applications. However, to enable consideration of TIs as building blocks for novel devices, stability of TSSs toward oxidation should be tested. Among the family of TIs with a tetradymite structure, Sb₂Te₃ is of <i>p</i>-type and appears to be the least explored material since its TSS is unoccupied in the ground state, a property that allows the use of optical excitations to generate spin currents relevant for spintronics. Here, we report relatively fast surface oxidation of Sb₂Te₃ under ambient conditions. We show that the clean surface reacts rapidly with molecular oxygen and slowly with water, and that humidity plays an important role during oxide layer growth. In humid air, we show that Sb₂Te₃ oxidizes on a time scale of minutes to hours, and much faster than other tetradymite TIs. The high surface reactivity revealed by our experiments is of critical importance and must be taken into account for the production and exploitation of novel TI-based devices using Sb₂Te₃ as a working material. Our results contribute to the comprehensive understanding of the universal trend underlying the chemical reactivity of tetradymite TIs

Laterally Selective Oxidation of Large-Scale Graphene with Atomic Oxygen

Using X-ray photoemission microscopy, we discovered that oxidation of commercial large-scale graphene on Cu foil, which typically has bilayer islands, by atomic oxygen proceeds with the formation of the specific structures: though relatively mobile epoxy groups are generated uniformly across the surface of single-layer graphene, their concentration is significantly lower for bilayer islands. More oxidized species like carbonyl and lactones are preferably located at the centers of these bilayer islands. Such structures are randomly distributed over the surface with a mean density of about 3× 10⁶ cm^–2 in our case. Using a set of advanced spectromicroscopy instruments including Raman microscopy, X-ray photoelectron spectroscopy (μ-XPS), Auger electron spectroscopy (nano-AES), and angle-resolved photoelectron spectroscopy (μ-ARPES), we found that the centers of the bilayer islands where the second layer nucleates have a high defect concentration and serve as the active sites for deep oxidation. This information can be potentially useful in developing lateral heterostructures for electronics and optoelectronics based on graphene/graphene oxide heterojunction