Synthesis of Pr3+-doped WO3 particles: correlation between photoluminescent and photocatalytic properties

The WO3 and WO3:Pr3+ particles were successfully synthesized by the co-precipitation method. The XRD analysis with Rietveld refinement revealed the formation of a monoclinic phase for WO3 and for doped samples, this result was later confirmed by Raman spectroscopy studies. The presence of Pr3+ in the WO3 crystalline lattice induced structural and optical changes in the particles, increasing the crystallite size, distorting the clusters (shortening of the W-O bonds), favoring the crystallinity and changing the optical gap. The predominant morphology of the particles of WO3 and WO3:Pr3+ obtained was nanocubes constituted by the superposition of plates of nanometric thicknesses. The photoluminescence of WO3 and WO3:Pr3+ was produced by the existence of surface defects in the particles. The increase in the concentration of Pr3+ promoted an increase in the intensity of PL, due to the increase in the rate of recombination of electron/hole charges. The WO3 sample exhibited emission in the white region due to the adjustment of simultaneous electronic transitions in the blue, green and red regions, characteristic of the broadband spectrum. The interval of the 2.65 eV gap band and the high efficiency in the separation of the photogenerated charges (e-/h+) with the low recombination rate contributed to the photocatalytic degradation of Crystal Violet (CV) by the catalyst. The WO3:4% Pr3+ sample showed the best photocatalytic efficiency, degrading 73% of the CV dye in 80 minutes. This result was associated with a reduction in particle size and density of oxygen vacancies on the material surface.

Revealing the impact of strain in the optical properties of bubbles in monolayer MoSe2

Strain plays an important role for the optical properties of monolayer transition metal dichalcogenides (TMDCs). Here, we investigate strain effects in a monolayer MoSe₂ sample with a large bubble region using μ-Raman, second harmonic generation (SHG), μ-photoluminescence and magneto μ-photoluminescence at low temperature. Remarkably, our results reveal the presence of a non-uniform strain field and the observation of emission peaks at lower energies which are the signatures of exciton and trion quasiparticles red-shifted by strain effects in the bubble region, in agreement with our theoretical predictions. Furthermore, we have observed that the emission in the strained region decreases the trion binding energy and enhances the valley g-factors as compared to non-strained regions. Considering uniform biaxial strain effects within the unit cell of the TMDC monolayer (ML), our first principles calculations predict the observed enhancement of the exciton valley Zeeman effect. In addition, our results suggest that the exciton-trion fine structure plays an important role for the optical properties of strained TMDC ML. In summary, our study provides fundamental insights on the behaviour of excitons and trions in strained monolayer MoSe₂ which are particularly relevant to properly characterize and understand the fine structure of excitonic complexes in strained TMDC systems/devices.

Diffusion of Photoexcited Holes in a Viscous Electron Fluid

The diffusion of photogenerated holes is studied in a high-mobility mesoscopic GaAs channel where electrons exhibit hydrodynamic properties. It is shown that the injection of holes into such an electron system leads to the formation of a hydrodynamic three-component mixture consisting of electrons and photogenerated heavy and light holes. The obtained results are analyzed within the framework of ambipolar diffusion, which reveals characteristics of a viscous flow. Both hole types exhibit similar hydrodynamic characteristics. In such a way the diffusion lengths, ambipolar diffusion coefficient, and the effective viscosity of the electron-hole system are determined.

Correction: Synthesis and evaluation of photocatalytic and photoluminescent properties of Zn2+-doped Bi2WO6

Correction for ‘Synthesis and evaluation of photocatalytic and photoluminescent properties of Zn2+-doped Bi2WO6’ by L. X. Lovisa, et al., Dalton Trans., 2022, https://doi.org/10.1039/d2dt03175b.

Photocatalytic degradation of methylene blue and dye mixture using indium-doped CaWO4 synthesized by sonochemical and microwave-assisted hydrothermal methods

Synthesis methods and characterization of indium-doped calcium tungstate particles.

Electron-phonon coupling enhancement and displacive magnetostructural transition in SrCr2 As2 under magneto-Raman spectroscopy

A Raman spectroscopy study on high quality single crystals of SrCr₂ As₂ (SCA) in the temperature T range 4 K < T < 300 K and high applied magnetic fields up to H = 9 T is presented. The chromium B _1g phonon analysis reveals two anomalous shifts in the frequency, the first below T = 250 K at H = 0 T in the saturated AFM G-type order likely due to an enhanced electron-phonon coupling by the magnetic order, whereas the second anomaly occurs above H = 4 T at T = 4 K likely as a consequence of a magnetostructural displacive transition. Renormalization of the electronic Raman spectra in both studies reveals a decrease in the electronic density of states with decreasing T and increasing H, respectively, with consequent changes in the Fermi surface, which are intrinsically related to the observed anomalies.

Growth and formation mechanism of shape-selective preparation of ZnO structures: correlation of structural, vibrational and optical properties

A shape-selective preparation method was used to obtain highly crystalline rod-, needle-, nut-, and doughnut-like ZnO morphologies with distinct particle sizes and surface areas.

Optical and transport properties correlation driven by amorphous/crystalline disorder in InP nanowires

Indium phosphide nanowires with a single crystalline zinc-blend core and polycrystalline/amorphous shell were grown from a reliable route without the use of hazardous precursors. The nanowires are composed by a crystalline core covered by a polycrystalline shell, presenting typical lengths larger than 10 μm and diameters of 80-90 nm. Raman spectra taken from as-grown nanowires exhibited asymmetric line shapes with broadening towards higher wave numbers which can be attributed to phonon localization effects. It was found that optical phonons in the nanowires are localized in regions with average size of 3 nm, which seems to have the same order of magnitude of grain sizes in the polycrystalline shell. Regardless of the fact that the nanowires exhibit a crystalline core, any considerable degree of disorder can lead to a localized behaviour of carriers. In consequence, the variable range hopping was observed as the main transport instead of the usual thermal excitation mechanisms. Furthermore the hopping length was ten times smaller than nanowire cross-sections, confirming that the nanostructures do behave as a 3D system. Accordingly, the V-shape observed in PL spectra clearly demonstrates a very strong influence of the potential fluctuations on the exciton optical recombination. Such fluctuations can still be observed at low temperature regime, confirming that the amorphous/polycrystalline shell of the nanowires affects the exciton recombination in every laser power regime tested.

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In0.4Ga0.6 As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Aharonov-Bohm interference in neutral excitons: effects of built-in electric fields

We report a comprehensive discussion of quantum interference effects due to the finite structure of neutral excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. The signatures of built-in electric fields and temperature on quantum interference are demonstrated by theoretical models that describe the modulation of the interference pattern and confirmed by complementary experimental procedures.