Influence of Mn, Fe, Co, and Cu Doping on the Photoelectric Properties of 1T HfS2 Crystals

Doping plays a vital role in the application of transition-metal dichalcogenides (TMDCs) because it can increase the functionality of TMDCs by tuning their native characteristics. In this study, the influence of Mn, Fe, Co, and Cu doping on the photoelectric properties of HfS2 was investigated. Pristine, Mn-, Fe-, Co-, and Cu-doped HfS2 crystals were grown using the chemical vapor transport method. Scanning electron microscopy images showed that the crystals were layered and transmission electron microscopy, X-ray diffraction, and Raman spectroscopy measurements confirmed that the crystals were in the 1T-phase with a CdI2-like structure. The bandgap of pristine HfS2 obtained from the absorption and photoconductivity spectra was approximately 1.99 eV. As the dopant changed from Mn, Fe, and Co, to Cu, the bandgap gradually increased. The activation energies of the samples were determined using temperature-dependent current-voltage curves. After doping, the activation energy decreased, and the Co-doped HfS2 exhibited the smallest activation energy. Time-resolved photoresponse measurements showed that doping improved the response of HfS2 to light; the Co-doped HfS2 exhibited the best response. The photoresponsivity of HfS2 as a function of the laser power and bias voltage was measured. After doping, the photoresponsivity increased markedly; the Co-doped HfS2 exhibited the highest photoresponsivity. All the experimental results indicated that doping with Mn, Fe, Co, and Cu significantly improved the photoresponsive performance of HfS2, of which Co-doped HfS2 had the best performance.

Single crystals of SnTe3O8 in the millimetre range grown by chemical vapor transport reactions

Tin(IV) trioxidotellurate(IV), SnTe3O8, is a member of the isotypic M IVTeIV 3O8 (M = Ti, Zr, Hf, Sn) series crystallizing with eight formula units per unit cell in space group Ia\overline{3}. In comparison with the previous crystal structure model of SnTe3O8 based on powder X-ray diffraction data [Meunier & Galy (1971). Acta Cryst. B27, 602–608], the current model based on single-crystal X-ray data is improved in terms of precision and accuracy. Nearly regular [SnO6] octahedra (Sn site symmetry .\overline{3}.) are situated in the voids of an oxidotellurate(IV) framework built up by corner-sharing [TeO4] bisphenoids (Te site symmetry 2..). A quantitative structural comparison revealed a very high degree of similarity for the structures with M = Ti, Zr, Sn in the M IVTe3O8 series.

A Study of the Possibility of Obtaining Deposited Coatings based on Intermetallic Titanium and Aluminum Compounds using the Chemical Vapor Transport Method

Theoretical and experimental investigations of processes of titanium-aluminum coating formation on refractory wire substrates by the Chemical Vapor Transport Reactions (CVT) method were carried out. Modelling of CVT was based on the thermodynamic analysis of an equilibrium iodide system implying the presence of one of titanium aluminides in the condensed phase, titanium and aluminum iodides in the gas phase, as well as atomic and molecular iodine. The fact that the volatility and stability of various metals are strongly interrelated was considered as a working hypothesis, which made it possible to obtain a number of alloys with the simultaneous decomposition of metal iodides constituting an alloy. Experiments on the deposition of titanium-aluminum coatings were conducted in a quasi-closed reactor, which allowed us to obtain deposited coatings on long-length refractory substrates

Deterministic Transfer of Large-Scale β-Phase Arsenic on Fiber End Cap for Near-Infrared Ultrafast Pulse Generation

Gray arsenic (β-phase) has aroused great attention in photonics and electronics applications, as a novel family member of two-dimensional (2D) elemental crystals of group-VA. Here, β-phase arsenic (β-As) bulk crystals were synthesized via the chemical vapor transport (CVT) method. Meanwhile, large-scale β-As nanoflake was transformed using the polydimethylsiloxane (PDMS)-assisted dry transfer method and was placed on the end cap of optical fiber with high coverage over the core area. Moreover, the β-As was used as a saturable absorber in ytterbium-doped fiber ring cavity resonance, and we demonstrated near-infrared ultrafast pulse fiber laser with the central wavelength, repetition rate, and signal-to-noise ratio (SNR) of 1,037.3 nm, 0.6 MHz, and 67.7 dB, respectively. This research demonstrates a 2D material small area deterministic transfer method and promotes the potential application of group-VA crystals in near-infrared ultrafast laser generation.

Anisotropic Optical Response of WTe2 Single Crystals Studied by Ellipsometric Analysis

In this paper we report the crystal growth conditions and optical anisotropy properties of Tungsten ditelluride (WTe2) single crystals. The chemical vapor transport (CVT) method was used for the synthesis of large WTe2 crystals with high crystallinity and surface quality. These were structurally and morphologically characterized by means of X-ray diffraction, optical profilometry and Raman spectroscopy. Through spectroscopic ellipsometry analysis, based on the Tauc–Lorentz model, we identified a high refractive index value (~4) and distinct tri-axial anisotropic behavior of the optical constants, which opens prospects for surface plasmon activity, revealed by the dielectric function. The anisotropic physical nature of WTe2 shows practical potential for low-loss light modulation at the 2D nanoscale level.

Asymmetric misfit nanotubes: Chemical affinity outwits the entropy at high-temperature solid-state reactions

Asymmetric two-dimensional (2D) structures (often named Janus), like SeMoS and their nanotubes, have tremendous scope in material chemistry, nanophotonics, and nanoelectronics due to a lack of inversion symmetry and time-reversal symmetry. The synthesis of these structures is fundamentally difficult owing to the entropy-driven randomized distribution of chalcogens. Indeed, no Janus nanotubes were experimentally prepared, so far. Serendipitously, a family of asymmetric misfit layer superstructures (tubes and flakes), including LaX-TaX2 (where X = S/Se), were synthesized by high-temperature chemical vapor transport reaction in which the Se binds exclusively to the Ta atoms and La binds to S atoms rather than the anticipated random distribution. With increasing Se concentration, the LaS-TaX2 misfit structure gradually transformed into a new LaS-TaSe2-TaSe2 superstructure. No misfit structures were found for xSe = 1. These counterintuitive results shed light on the chemical selectivity and stability of misfit compounds and 2D alloys, in general. The lack of inversion symmetry in these asymmetric compounds induces very large local electrical dipoles. The loss of inversion and time-reversal symmetries in the chiral nanotubes offers intriguing physical observations and applications.

Tuning Magnetic and Transport Properties in Quasi-2D (Mn1−xNix)2P2S6 Single Crystals

We report an optimized chemical vapor transport method to grow single crystals of (Mn1−xNix)2P2S6 where x = 0, 0.3, 0.5, 0.7, and 1. Single crystals up to 4 mm × 3 mm × 200 μm were obtained by this method. As-grown crystals are characterized by means of scanning electron microscopy and powder X-ray diffraction measurements. The structural characterization shows that all crystals crystallize in monoclinic symmetry with the space group C2/m (No. 12). We have further investigated the magnetic properties of this series of single crystals. The magnetic measurements of the all as-grown single crystals show long-range antiferromagnetic order along all principal crystallographic axes. Overall, the Néel temperature TN is non-monotonous; with increasing Ni2+ doping, the temperature of the antiferromagnetic phase transition first decreases from 80 K for pristine Mn2P2S6 (x = 0) up to x = 0.5 and then increases again to 155 K for pure Ni2P2S6 (x = 1). The magnetic anisotropy switches from out-of-plane to in-plane as a function of composition in (Mn1−xNix)2P2S6 series. Transport studies under hydrostatic pressure on the parent compound Mn2P2S6 evidence an insulator-metal transition at an applied critical pressure of ~22 GPa.