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Topological insulators containing single atoms of magnetic metals are useful for studying the effects of magnetism on the topological properties of matter. Controlling the distribution of magnetic atoms remains a major issue during the synthesis of these materials, as the formation of clusters and/or unwanted precipitates competes energetically with the formation of dilute phases. Here, we report the synthesis of large Bi2Se3 single crystals with rhombohedral structure and extremely low concentration (0.15 at%) of substitutional Cr ions using the Bridgman method. After exfoliation, the crystals exhibit a topological surface state with an energy-momentum dispersion different from that of pristine Bi2Se3, due to the presence of an apparent energy gap at the Dirac point. The properties of this state are discussed in comparison with previous spectroscopic measurements of Cr-doped Bi2Se3 films. [ABSTRACT FROM AUTHOR]

Localized surface plasmon resonance (LSPR) is the cause of the photo-thermal effect observed in topological insulator (TI) bismuth selenide (Bi2Se3) nanoparticles. These plasmonic properties, which are thought to be caused by its particular topological surface state (TSS), make the material interesting for application in the field of medical diagnosis and therapy. However, to be applied, the nanoparticles have to be coated with a protective surface layer, which prevents agglomeration and dissolution in the physiological medium. In this work, we investigated the possibility of using silica as a biocompatible coating for Bi2Se3 nanoparticles, instead of the commonly used ethylene-glycol, which, as is presented in this work, is not biocompatible and alters/masks the optical properties of TI. We successfully prepared Bi2Se3 nanoparticles coated with different silica layer thicknesses. Such nanoparticles, except those with a thick, ≈200 nm silica layer, retained their optical properties. Compared to ethylene-glycol coated nanoparticles, these silica coated nanoparticles displayed an improved photo-thermal conversion, which increased with the increasing thickness of the silica layer. To reach the desired temperatures, a 10–100 times lower concentration of photo-thermal nanoparticles was needed. In vitro experiments on erythrocytes and HeLa cells showed that, unlike ethylene glycol coated nanoparticles, silica coated nanoparticles are biocompatible. [ABSTRACT FROM AUTHOR]

We explored the properties of the quasi-binary Bi2Se3–Bi2S3 system over a wide compositional range. X-ray diffraction analysis demonstrates that rhombohedral crystals can be synthesized within the solid solution interval 0–22 mol% Bi2S3, while at 33 mol% Bi2S3 only orthorhombic crystals are obtained. Core level photoemission spectroscopy reveals the presence of Bi3+, Se2− and S2− species and the absence of metallic species, thus indicating that S incorporation into Bi2Se3 proceeds prevalently through the substitution of Se with S. Spin- and angle-resolved photoemission spectroscopy shows that topological surface states develop on the surfaces of the Bi2Se3−ySy (y ≤0.66) rhombohedral crystals, in close analogy with the prototypical case of Bi2Se3, while the orthorhombic crystals with higher S content turn out to be trivial semiconductors. Our results connect unambiguously the phase diagram and electronic properties of the Bi2Se3–Bi2S3 system. [ABSTRACT FROM AUTHOR]

Understanding the chemical processes at the interface between a metal and topological insulator (TI) is important when it comes to designing devices that exploit the peculiar topological surface states or studying the properties of TI heterostructures. In this paper we show that the interface between Ti and Bi2Se3 is unstable at RT and results in the formation of interfacial phases of titanium selenides and metallic Bi. The reaction has shown significant kinetics already at cryogenic temperatures, which is very surprising for a solid-state redox reaction. This can be explained with the possibility of electrons in the topological surface states playing a role in enhancing the Bi2Se3 surface reactivity due to the electron-bath effect. For the Ti coverage above 40 nm, the interfacial processes cause compressive stress that triggers the morphological change (buckling) of the deposited film. The observed interface reaction, with all of its consequences, has to be considered not only in the design of devices, where the Ti adhesion layer is often used for contacts, but also for possible engineering of 2D TI heterostructures. [ABSTRACT FROM AUTHOR]

A sustainable nexus between renewable energy production and plastic abatement is imperative for overall sustainable development. In this regard, this study aims to develop a cheaper and environmentally friendly nexus between plastic waste management, wastewater treatment, and renewable hydrogen production. Fluorescent carbon (FCs) were synthesized from commonly used LDPE (low-density polyethylene) by a facile hydrothermal approach. Optical absorption study revealed an absorption edge around 300 nm and two emission bands at 430 and 470 nm. The morphological analysis showed two different patterns of FCs, a thin sheet with 2D morphology and elongated particles. The sheet-shaped particles are 0.5 μm in size, while as for elongated structures, the size varies from 0.5 to 1 μm. The as-synthesized FCs were used for the detection of metal ions (reference as Cu2+ ions) in water. The fluorescence intensity of FCs versus Cu2+ ions depicts its upright analytical ability with a limit of detection (LOD) reaching 86.5 nM, which is considerably lesser than earlier reported fluorescence probes derived from waste. After the sensing of Cu2+, the as-obtained FCs@Cu2+ was mixed with TiO2 to form a ternary FCs@CuO@TiO2 composite. This ternary composite was utilized for photocatalytic hydrogen production from water under 1.5 AM solar light irradiation. The H2 evolution rate was found to be ~1800 μmolg−1, which is many folds compared to the bare FCs. Moreover, the optimized FCs@CuO@TiO2 ternary composite showed a photocurrent density of ~2.40 mA/cm2 at 1 V vs. Ag/AgCl, in 1 M Na2SO4 solution under the illumination of simulated solar light. The achieved photocurrent density corresponds to the solar-to-hydrogen (STH) efficiency of ~0.95%. The efficiency is due to the fluorescence nature of FCs and the synergistic effect of CuO embedded in TiO2, which enhances the optical absorption of the composite by reaching the bandgap of 2.44 eV, apparently reducing the recombination rate, which was confirmed by optoelectronic, structural, and spectroscopic characterizations. [ABSTRACT FROM AUTHOR]

In multicomponent thin films, properties and functionalities related to post-deposition annealing treatments, such as thermal stability, optical absorption and surface morphology are typically rationalized, neglecting the role of the substrate. Here, we show the role of the substrate in determining the temperature dependent behaviour of a paradigmatic two-component nanogranular thin film (Ag/TiO2) deposited by gas phase supersonic cluster beam deposition (SCBD) on silica and sapphire. Up to 600 °C, no TiO2 grain growth nor crystallization is observed, likely inhibited by the Zener pinning pressure exerted by the Ag nanoparticles on the TiO2 grain boundaries. Above 600 °C, grain coalescence, formation of However, the two substrates steer the evolution of the film morphology and optical properties in two different directions. anatase and rutile phases and drastic modification of the optical absorption are observed. On silica, Ag is still present as NPs distributed into the TiO2 matrix, while on sapphire, hundreds of nm wide Ag aggregates appear on the film surface. Moreover, the silica-deposited film shows a broad absorption band in the visible range while the sapphire-deposited film becomes almost transparent for wavelengths above 380 nm. We discuss this result in terms of substrate differences in thermal conductivity, thermal expansion coefficient and Ag diffusivity. The study of the substrate role during annealing is possible since SCBD allows the synthesis of the same film independently of the substrate, and suggests new perspectives on the thermodynamics and physical exchanges between thin films and their substrates during heat treatments. [ABSTRACT FROM AUTHOR]

Truly alloyed PbSxSe1-x (x = 0-1) nanocrystals (5 nm in size) have been prepared, and their resulting optical properties are red-shifted systematically as the sulfur content of the materials increases. Their optical properties are discussed using a modified Vegard's approach and the bowing parameter for these nanoalloys is reported for the first time. The alloyed structure of the nanocrystals is supported by the energy-filtered transmission electron microscope images of the samples, which show a homogeneous distribution of sulfur and selenium within the nanocrystals. X-ray photoelectron spectroscopy studies on ligand-exchanged nanocrystals confirmed the expected stoichiometry and various oxidized species, [ABSTRACT FROM AUTHOR]