We report the synthesis of high-quality single crystals of ReS2 and ReSe2 transition metal dichalcogenides using a modified Bridgman method that avoids the use of a halogen transport agent. Comprehensive structural characterization using X-ray diffraction and electron microscopy confirm a distorted triclinic 1T′ structure for both crystals and reveal a lack of Bernal stacking in ReS2. Photoluminescence (PL) measurements on ReS2 show a layer-independent bandgap of 1.51 eV, with increased PL intensity from thicker flakes, confirming interlayer coupling to be negligible in this material. For ReSe2, the bandgap is weakly layer-dependent and decreases from 1.31 eV for thin layers to 1.29 eV in thick flakes. Both chalcogenides show feature-rich Raman spectra whose excitation energy dependence was studied. The lower background doping inherent to our crystal growth process results in high field-effect mobility values of 79 and 0.8 cm2/(V s) for ReS2 and ReSe2, respectively, as extracted from FET structures fabricated from exfoliated flakes. Our work shows ReX2 chalcogenides to be promising 2D materials candidates, especially for optoelectronic devices, without the requirement of having monolayer thin flakes to achieve a direct bandgap.

In this work we report the design and synthesis of high-surface-area photocatalysts by coating TiO2 on fibrous nanosilica (KCC-1) using atomic layer deposition (ALD). Our developed catalyst showed enhanced photocatalytic activity, better than that of the well-known MCM-41- and SBA-15-supported TiO2 catalysts using ALD as well as that of other silica-supported TiO2 catalysts reported in the literature to date. This work shows how one can tune the photocatalytic activity of supported TiO2 catalysts by simply tuning the morphology of the support. In addition to extensive characterization of materials using various techniques, comprehensive mechanistic insight into ALD TiO2 coating on KCC-1 fibers was gained using solid-state NMR and UV-DRS. For the first time, we also observed the formation of small and monodispersed TiO2 nanoparticles after heat treatment of these ALD-coated samples of KCC-1. Notably, we observed size quantization effects in these TiO2 nanoparticles, which was confirmed by band gap shift measurements and the Brus effective mass approximation method. We believe that the combination of the unique textural properties and morphology of KCC-1 and TiO2 nanoparticle formation and their size quantization is the reason behind the enhanced photocatalytic activity of KCC-1/TiO2 catalysts.

Despite the numerous reports on the metal-catalyzed growth of GaN nanowires, the mechanism of growth is not well understood. Our study of the nickel-assisted growth of GaN nanowires using metalorganic chemical vapor deposition provides key insights into this process. From a comprehensive study of over 130 nanowires, we observe that as a function of thickness, the length of the nanowires initially increases and then decreases. We attribute this to an interplay between the Gibbs-Thomson effect dominant in very thin nanowires and a diffusion induced growth mode at larger thickness. We also investigate the alloy composition of the Ni-Ga catalyst particle for over 60 nanowires using energy dispersive X-ray spectroscopy, which along with data from electron energy loss spectroscopy and high resolution transmission electron microscopy suggests the composition to be NiGa. At the nanowire growth temperature, this alloy cannot be a liquid, even taking into account melting point depression in nanoparticles. We hence conclude that Ni-assisted GaN nanowire growth proceeds via a vapor-solid-solid mechanism instead of the conventional vapor-liquid-solid mechanism.

We report on the single crystal growth and anisotropic physical properties of CeAgAs2. The compound crystallizes as on ordered variant of the HfCuSi2-type crystal structure and adopts the orthorhombic space group P mca (#57) with two symmetry inequivalent cerium atomic positions in the unit cell. The orthorhombic crystal structure of our single crystal was confirmed from the powder x-ray diffraction and from electron diffraction patterns obtained from the transmission electron microscope. The anisotropic physical properties have been investigated on a good quality single crystal by measuring the magnetic susceptibility, isothermal magnetization, electrical transport and heat capacity. The magnetic susceptibility and magnetization measurements revealed that this compound orders antiferromagnetically with two closely spaced magnetic transitions at TN1 = 6 K and TN2 = 4.9 K. Magnetization studies have revealed a large magnetocrystalline anisotropy due to the crystalline electric field (CEF) with an easy axis of magnetization along the [010] direction. The magnetic susceptibility measured along the [001] direction exhibited a broad hump in the temperature range 50 to 250 K, while typical Curie-Weiss behaviour was observed along the other two orthogonal directions. The electrical resistivity and the heat capacity measurements revealed that CeAgAs2 is a Kondo lattice system with a magnetic ground state.

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