Search Results (5)

Natural IaA+B diamonds were exposed in their bulk by multiple 0.3 ps, 515 nm laser pulses focused by a 0.25 NA micro-objective, producing in the prefocal region (depth of 20–50 μm) a bulk array of photoluminescent nanostructured microtracks at variable laser exposures and pulse energies. These micromarks were characterized at room (25°) and liquid nitrogen cooling (−120 °C) temperatures through stationary 3D scanning confocal photoluminescence (PL) microspectroscopy at 405 and 532 nm excitation wavelengths. The acquired PL spectra exhibit a linearly increasing pulse-energy-dependent yield in the range of 575 to 750 nm (NV⁰, NV⁻ centers) at the expense of the simultaneous reductions in the blue–green (450–570 nm; N3a, H4, and H3 centers) and near-IR (741 nm; V⁰ center) PL yield. A detailed analysis indicates a low-energy rise in PL intensity for B2-related N3a, H4, and H3 centers, while at higher, above-threshold pulse energies it decreases for the H4, H3, and N3a centers, converting into NV centers, with the laser exposure effect demonstrating the same trend. The intrinsic and (especially) photo-generated vacancies were considered to drive their attachment as separate species to nitrogen centers at lower vacancy concentrations, while at high vacancy concentrations the concerted splitting of highly aggregated nitrogen centers by the surrounding vacancies could take place in favor of resulting NV centers. Full article

Ternary metal oxides (TMOs) with flexible band structures are of significant potential in the field of photocatalysis. The efficient utilization of renewable and green solar energy is of great importance to developing photocatalysts. To date, a wide range of TMOs systems has been developed as photocatalysts for water and air purification, but their practical applications in visible light-assisted chemical reactions are hindered mainly by its poor visible light absorption capacity. Introduction of N atoms into TMOs can narrow the band-gap energy to a lower value, enhance the absorption of visible light and suppress the recombination rate of photogenerated electrons and holes, thus improving the photocatalytic performance. This review summarizes the recent research on N-modified TMOs, including the influence of N doping amounts, N doping sites, and N-induced phase transformation. The introduced N greatly tuned the optical properties, electronic structure, and photocatalytic activity of the TMOs. The optimal N concentration and the influence of N doping sites are investigated. The substitutional N and interstitial N contributed differently to the band gap and electron transport. The introduced N can tune the vacancies in TMOs due to the charge compensation, which is vital for inducing different activity and selectivity. The topochemical ammonolysis process can convert TMOs to oxynitride with visible light absorption. By altering the band structures, these oxynitride materials showed enhanced photocatalytic activity. This review provides an overview of recent advances in N-doped TMOs and oxynitrides derived from TMOs as photocatalysts for environmental applications, as well as some relevant pointers for future burgeoning research development. Full article

Elongated photoluminescent micromarks were inscribed inside a IaAB-type natural diamond in laser filamentation regime by multiple 515 nm, 0.3 ps laser pulses tightly focused by a 0.25 NA micro-objective. The micromark length, diameter and photoluminescence contrast scaled as a function of laser pulse energy and exposure, coming to a saturation. Our Raman/photoluminescence confocal microscopy studies indicate no structural diamond damage in the micromarks, shown as the absent Raman intensity variation versus laser energy and exposition along the distance from the surface to the deep mark edge. In contrast, sTable 3NV (N3)-centers demonstrate the pronounced increase (up to 40%) in their 415 nm zero-phonon line photoluminescence yield within the micromarks, and an even higher—ten-fold—increase in NV⁰-center photoluminescence yield. Photogeneration of carbon Frenkel “interstitial–vacancy” (I–V) pairs and partial photolytic dissociation of the predominating 2N (A)-centers were suggested to explain the enhanced appearance of 3NV- and NV-centers, apparently via vacancy aggregation with the resulting N (C)-centers or, consequently, with 2N- and N-centers. Full article

Graphitic carbon nitride (g-C₃N₄) has attracted much attention because of its potential for application in solar energy conservation. However, the photocatalytic activity of g-C₃N₄ is limited by the rapidly photogenerated carrier recombination and insufficient solar adsorption. Herein, fluorinated g-C₃N₄ (F-g-CN) nanosheets are synthesized through the reaction with F₂/N₂ mixed gas directly. The structural characterizations and theoretical calculations reveal that fluorination introduces N vacancy defects, structural distortion and covalent C-F bonds in the interstitial space simultaneously, which lead to mesopore formation, vacancy generation and electronic structure modification. Therefore, the photocatalytic activity of F-g-CN for H₂ evolution under visible irradiation is 11.6 times higher than that of pristine g-C₃N₄ because of the enlarged specific area, enhanced light harvesting and accelerated photogenerated charge separation after fluorination. These results show that direct treatment with F₂ gas is a feasible and promising strategy for modulating the texture and configuration of g-C₃N₄-based semiconductors to drastically enhance the photocatalytic H₂ evolution process. Full article

Nickel oxide is one of the highly promising semiconducting materials, but its large band gap (3.7 to 4 eV) limits its use in practical applications. Here we report the effect of nickel/oxygen vacancies and interstitial defects on the near-band-edge (NBE) and deep-level-emission (DLE) in various sizes of nickel oxide (NiO) nanoparticles. The ultraviolet (UV) emission originated from excitonic recombination corresponding near-band-edge (NBE) transition of NiO, while deep-level-emission (DLE) in the visible region due to various structural defects such as oxygen vacancies and interstitial defects. We found that the NiO nanoparticles exhibit a strong green band emission around ~2.37 eV in all samples, covering 80% integrated intensity of PL spectra. This apparently anomalous phenomenon is attributed to photogenerated holes trapped in the deep level oxygen vacancy recombining with the electrons trapped in a shallow level located just below the conducting band. Full article