A study of the optical joint density-of-states function

First principles study of strain effects on prospective 2D photocatalysts Sn2Se2X4 (X = P, As) with ultra-high charge carrier mobility

Ab initio calculations were employed to investigate the properties of Sn2Se2P4 and Sn2Se2As4, which are new semiconductors formed based on the 2D SnP3 structure. A comprehensive analysis was conducted to examine the structural characteristics and stability of both compounds. It was observed that both Sn2Se2P4 and Sn2Se2As4 exhibit notable toughness and ductility, characterized by a Poisson's ratio ranging from 0.16 to 0.20 and a Young's modulus ranging from 42.12 to 49.84 N m-1. The investigation focused on the examination of the electronic characteristics of the two compounds, as well as their correlation with optical properties, charge transport, and potential as photocatalysts. Being ductile semiconductors, the effects of strains on the properties of Sn2Se2P4 and Sn2Se2As4 were also investigated. The charge carrier mobility in the y-direction ranges from 103 to 104 cm2 V-1 s-1. Moreover, the electron-hole separation is expected to be very high as the difference in the mobilities of holes and electrons is really large. Moreover, it is worth noting that both Sn2Se2P4 and Sn2Se2As4 exhibit a significantly high absorption rate of 106 cm-1 in the visible region. The observed features of Sn2Se2P4 and Sn2Se2As4 indicate their potential as effective photocatalysts for the process of water splitting through the utilization of solar energy.

Photogalvanic Effect in Nitrogen-Doped Monolayer MoS2 from First Principles

We investigate the photogalvanic effect in nitrogen-doped monolayer molybdenum disulfide (MoS₂) under the perpendicular irradiation, using first-principles calculations combined with non-equilibrium Green function formalism. We provide a detailed analysis on the behavior of photoresponse based on the band structure and in particular the joint density of states. We thereby identify different mechanisms leading to the existence of zero points, where the photocurrent vanishes. In particular, while the zero point in the linear photovoltaic effect is due to forbidden transition, their appearance in the circular photovoltaic effect results from the identical intensity splitting of the valance band and the conduction band in the presence of Rashba and Dresslhaus spin-orbit coupling. Furthermore, our results reveal a strong circular photogalvanic effect of nitrogen-doped monolayer MoS₂, which is two orders of magnitude larger than that induced by the linearly polarized light.

Transmission electron microscopy of carbon-coated and iron-doped titania nanoparticles

We present a study on the properties of iron (Fe)-doped and carbon (C)-coated titania (TiO2) nanoparticles (NPs) which has been compiled by using x-ray diffraction (XRD), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS). These TiO2 NPs were prepared by using the flame synthesis method. This method allows the simultaneous C coating and Fe doping of TiO2 NPs. XRD investigations revealed that the phase of the prepared NPs was anatase TiO2. Conventional TEM analysis showed that the average size of the TiO2 NPs was about 65 nm and that the NPs were uniformly coated with the element C. Furthermore, from the x-ray energy dispersive spectrometry analysis, it was found that about 8 at.% Fe was present in the synthesized samples. High-resolution TEM (HRTEM) revealed the graphitized carbon structure of the layer surrounding the prepared TiO2 NPs. HRTEM analysis further revealed that the NPs possessed the crystalline structure of anatase titania. Energy-filtered TEM (EFTEM) analysis showed the C coating and Fe doping of the NPs. The ratio of L3 and L2 peaks for the Ti-L23 and Fe-L23 edges present in the core loss electron energy loss spectroscopy (EELS) revealed a +4 oxidation state for the Ti and a +3 oxidation state for the Fe. These EELS results were further confirmed with XPS analysis. The electronic properties of the samples were investigated by applying Kramers-Kronig analysis to the low-loss EELS spectra acquired from the prepared NPs. The presented results showed that the band gap energy of the TiO2 NPs decreased from an original value of 3.2 eV to about 2.2 eV, which is quite close to the ideal band gap energy of 1.65 eV for photocatalysis semiconductors. The observed decrease in band gap energy of the TiO2 NPs was attributed to the presence of Fe atoms at the lattice sites of the anatase TiO2 lattice. In short, C-coated and Fe-doped TiO2 NPs were synthesized with a rather cost-effective and comparatively easily scalable method. The presented analysis enables us to predict the excellent efficiency of these NPs for solar-cell and photo-catalysis applications.