Photoluminescence and structural characteristics of double tungstates A(M1−X PrX)W2O8 (A = Li,Cs, M = Al,Sc, La)

In this article, photoluminescence of Pr³⁺ ions in the double tungstate A(M_1?X Pr_X)W₂O₈ (A = Li, Cs, M = Al, Sc, La; 0.0 ≤ X ≤ 0.1) are characterised. By varying ion radius in A and M sites the crystal structure was modified and even in crystals with similar structural characteristics three distinctive types of luminescence are observed. When the substitution ions in both A and M sites are relatively small the host lattice exhibits luminescence dominantly. With the small A site ion (Li⁺) and the large M site ion (La³⁺, 1.03 Å) the Pr³⁺ ion exhibits prominent luminescence. With the very large A site ion (Cs⁺, 1.67 Å) and relatively small M site ion (Sc³⁺, 0.75 Å) the Pr³⁺ exhibits both the 4f²–4f5d excitation and the ³P_J manifold excitations in the absorption spectrum. These excitation levels lead to two strong emissions from the Pr³⁺. PL characteristics are discussed with respect to crystal structural criteria.     

Photoluminescence properties and energy transfer between activators at different crystallographic sites in Ce3+ doped Sr2MgAl22O36

In this paper, a series of Ce3+ doped Sr₂MgAl₂₂O₃₆ (SMA) phosphors have been prepared by high temperature solid-state reaction method. The phase structure of prepared samples was checked by the powder X-ray diffraction (XRD). The morphology of the samples was inspected using a field-emission scanning electron microscope (SEM). Under different UV radiation, this phosphor exhibits different emission bands due to the Ce³⁺ ions located at different lattice sites. The corresponding luminescence and energy transfer mechanisms have been proposed in detail. The phosphor exhibits different concentration quenching mechanisms because the Ce³⁺ ions substitute two different crystallographic sites in the host. Moreover, the temperature dependent emission properties of SMA:Ce³⁺ were conducted from 30 °C to 200 °C, as much as 72.96% of the room-temperature emission intensity is retained at 150 °C. The SMA:Ce³⁺ phosphor exhibits bright blue emission with CIE coordinates (x=0.16, y=0.12) under UV excitation. The results indicate that SMA:Ce³⁺ phosphor has great potential applications in UV-pumped light emitting diodes.     

Improved radiation damage tolerance of titanium nitride ceramics by introduction of vacancy defects

TiN and TiN_0.7 were irradiated using a 100 keV Ar-ion beam at 600 °C to target doses of 3 × 10¹⁷ ions cm⁻². SRIM estimation, GIXRD and fluorescence analysis have been performed to evaluate the effect of pre-existing vacancy defect on the radiation tolerance. The lattice parameter of TiN increased after irradiation due to interstitial atoms and vacancies in as-irradiated TiN. In contrary, the lattice parameter decreased for as-irradiated TiN_0.7, which indicates that the nitrogen atom vacancies in TiN_0.7 acted as sinks for displacement atoms generated by irradiation to limit interstitial atoms existing. The intensity of peaks in fluorescence spectrum of as-irradiated TiN was higher than that of as-irradiated TiN_0.7. That attributed to the presence of color centers formed by Frenkel defects in as-irradiated TiN. All of the results indicate that introducing vacancy defect in materials would offer capability to realize self-heal of irradiation damage.     

The influence of fluorine doping on the structural and electrical properties of the LiFePO4 powder

Low intrinsic electronic conductivity is the main disadvantage of LiFePO₄ when used as a cathode material in lithium ion batteries. The paper offers experimental proofs of the theoretical prediction that fluorine doping of LiFePO₄ can enhance its electrical conductivity. The LiFePO₄ and fluorine-doped LiFePO₄ olivine type, carbon-free powders are synthesized and examined. The crystal structure refinements in the Pnma space group reveal that doping with fluorine ions preserves the olivine structure, while reducing both the lattice parameters and the antisite defect, and increasing the crystallite size. A small amount of incorporated fluorine enhances the electrical conductivity from 4.6×10⁻⁷ S cm⁻¹ to 2.3×10⁻⁶ S cm⁻¹ and has a positive impact on the electrochemical performance. Several spectroscopy techniques (Mössbauer, FTIR, and Raman) reveal differences between the two powders and additionally support the findings of both the Rietveld refinement and the conductivity measurements.     

Characterization of free-standing single-crystal diamond prepared by hot-filament chemical vapor deposition

Hot filament chemical vapor deposition (HFCVD) possesses a large potential to scale-up for the growth of single-crystal diamond (SCD). This study investigates the crystalline qualities of SCD fabricated by HFCVD. A tungsten impurity level of 10¹⁸ cm^− 3 was detected with secondary ion mass spectroscopy. The full-width at half maximum (FWHM) of the rocking curve (004) was 39.5 arcsec, where that of seed substrates was 42.8 arcsec. A clear free-exciton recombination radiation was observed in cathodoluminescence spectra. The Raman spectra presented a single peak centered at 1333.2 cm^− 1 with a FWHM value of 1.9 cm^− 1. These results indicate that the HFCVD-grown SCD has good crystalline quality comparable to the seed substrates.     

Mn2+ activated MgAlON transparent ceramic: A new green-emitting transparent ceramic phosphor for high-power white LED

A novel Mn²⁺ activated green-emitting MgAlON transparent ceramic phosphor was synthesized from Mg_0.21Al_2.57O_3.80N_0.20:0.03Mn²⁺ (MgAlON:Mn) phosphor powder by pressureless sintering combining with hot isostatic pressing. By crystalline structure refinement and cathodoluminescence (CL) characterization, it is demonstrated that Mn²⁺ was dissolved in the spinel lattice and occupied the tetrahedral site. The ceramic, retaining high transmittance in UV–vis region (up to 82% at 800 nm) and excellent thermal-mechanical properties of MgAlON transparent ceramic-matrix, shows a strong green emission at 513 nm under 445 nm light excitation. Compared with its powder counterpart, the ceramic phosphor exhibits higher green color purity, higher internal quantum efficiency (47%) and lower thermal quenching. It is suggested that this novel green solid phosphor could be applied in high color rendering and high-power white light-emitting diodes when combined with a red solid phosphor and a blue LED chip.     

The effects of surface oxidation on luminescence of nano diamonds

In this work we quantify and characterise the effects of air-oxidation on nitrogen-vacancy defect luminescence in both high-temperature-high-pressure and detonation synthesized nanodiamonds using Raman and luminescence spectroscopies. We find that oxidation treatments result in an increased nitrogen-vacancy centre excited state lifetime from 13 ns to 25 ns and in 5-nm diamonds the intensity of this luminescence increases by at least 5-fold. At the same time, in 5-nm diamonds, short lived surface-defect related luminescence is reduced by 10-fold. Furthermore we find that air oxidation reduces the sp² and disordered carbon fraction of nanodiamonds by up to 5-fold in 5-nm nanodiamonds. Based on these results, the authors suggest that the disordered-carbon and graphite shell of 5-nm nanodiamonds quenches nitrogen-vacancy luminescence, and that this quenching can be partially reduced by surface oxidation. These findings provide useful insights into the role of the graphite and disordered carbon shell in quenching luminescence, and have implications for the applicability of 5-nm nanodiamonds to bio- and quantum physics applications.     

Formation of a heavily B doped diamond layer using an ion implantation technique

In this report, we present a study on lattice and electronic structures of B doped layers formed using B implantation into diamond. Boron layers were produced using the multiple-energy B ion implantation (total dose: 2.1 × 10¹⁵ to 1.7 × 10¹⁷ cm^− 2) into type IIa diamond at ~ 400 °C. Optical absorption and Hall effects were measured in the range of 80−1000 K for investigating the change of the lattice and electronic structures with the B concentration in diamond. The p-type carrier conduction was observed at 80−1000 K in all the samples. While a lightly B doped sample displays typical semiconductive, temperature-dependent valence-band conduction, heavily B doped samples have the very weak or almost zero temperature dependence of the carrier concentrations, resistivity and Hall mobility in this temperature region, suggesting characteristics of a p-type degenerate semiconductor. In such heavily doped samples, broad optical absorption bands, most likely corresponding to Drude absorption originating from free holes, were observed. The minimum resistivity and the sheet resistance at room temperature among the samples were 1.4 mΩcm and 56 Ω/□, respectively. These results indicate that very low-resistive p-type degenerate semiconducting layers were produced, preserving diamond lattice (preventing graphitization), despite high-dose ion irradiation.     

Ion implantation and annealing of diamond studied by emission channeling and cathodoluminescence

We review emission channeling (EC) studies to determine the lattice sites of radioactive impurities in IIa diamond after low dose room temperature implantation and subsequent annealing. The recovery of implantation damage was studied after implantation of ¹¹¹In and ⁷³As. Two distinct annealing stages, one in the temperature interval 300–600 K and the other at 1200 K were observed. The lattice sites of the potential donor ⁸Li, ³³P and ⁷³As atoms were studied. Li occupies mainly tetrahedral interstitials sites, however, a smaller substitutional fraction is presumably due to replacement collisions occurring during implantation. Significant Li diffusion does not take place up to 900 K. EC measurements reveal high substitutional fractions for P and As of 70(10)% and 55(5)%, respectively. Complementary to the EC studies, cathodoluminescence (CL) studies of the optical properties of implanted diamond are especially suited to determine the electrical activation of implanted impurities. This is demonstrated with CL measurements of B and P implanted diamond. Finally, the feasibility of EC studies using the potential donor atoms Na, S, and Se is discussed.     

An electron spin resonance and photoluminescence investigation of the effect of annealing temperature on Gd-doped La2Zr2O7 nano-ceramics

Lanthanum zirconate pyrochlore (La₂Zr₂O₇=LZO) is considered as a potential ceramic host matrix for nuclear waste immobilization. In this context, incorporation of Gd ions, a surrogate for trivalent minor actinides, in the LZO host was investigated. Undoped and Gd doped LZO samples were synthesized using a low temperature gel-combustion route and characterized using X-ray diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS), electron spin resonance (ESR) and photoluminescence (PL) spectroscopic techniques. It was observed that even at 500 °C, a single phase pyrochlore could be stabilized without any impurity phase. The average particle size of the synthesized product was estimated to be 85–90 nm. At lower temperatures, it was observed that, the Gd ions were present mostly on the surface of the LZO host which diffused into the lattice and replaced the La³⁺ ions after annealing at 700 °C for 5 h. Due to difference in the ionic radii, the environment around the Gd ion was highly asymmetric. This was further confirmed by PL investigations wherein upon excitation with UV radiation, the nano ceramics exhibited strong and sharp transition at 310 nm suggesting the existence of Gd³⁺ions in a D_2d geometry. Upon annealing, agglomeration of the particles was taking place reducing the surface defects and increasing the PL intensity as well as life time values.     

Dielectric and thermal properties of AlN ceramics

The effects of slow-cooling and annealing conditions on dielectric loss, thermal conductivity and microstructure of AlN ceramics were investigated. Y₂O₃ from 0.5 to 1.25 mol% at 0.25% increments was added as a sintering additive to AlN powder and pressureless sintering was carried out at 1900 °C for 2 h in a nitrogen flowing atmosphere. To improve the properties, AlN samples were slow-cooled at a rate of 1 °C min⁻¹ from 1900 to 1750 °C, subsequently cooled to 970 °C at a rate of 10 °C min⁻¹ and then annealed at the same temperature for 4 h. AlN and YAG (5Al₂O₃/3Y₂O₃) were the only identified phases from XRD. AlN doped with 0.5 and 0.75 mol% Y₂O₃ had a low loss of <2.0 × 10⁻³ and a high thermal conductivity of >160 W m⁻¹ °C⁻¹.     

Terbium-biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate framework with sulfonate sites for luminescent sensing of Cr3 + ion

A 3D terbium-organic framework [TbK(BPDSDC)(DMF)(H₂O)₂]_n (1) with the bifunctional carboxylate-sulfonate ligand of biphenyl-3,3′-disulfonyl-4,4′-dicarboxylate (BPDSDC^4 −) was synthesized and characterized. The Tb^3 + and K⁺ ions are briged by the carboxylate and sulfonate groups to give a 2D layer, which is linked by the biphenyl moieties to form the 3D pillared framework with the sulfonate groups pointing at the pores. The sulfonate groups on the pores act as the basic sites for the Cr^3 + ion sensing by luminescence quenching, highlighting the potential for recognition of Cr^3 + ion for compound 1 with the characteristic bright green color.     

Annealing effect on dielectric property of AlN ceramics

Effects of slow-cooling at high temperatures and annealing at intermediate temperatures on dielectric loss tangent of AlN ceramics were explored. Y₂O₃ was added as a sintering additive to AlN powders, and the powders were pressureless-sintered at 1900 °C for 2 h in a nitrogen flow atmosphere. In succession to the sintering, AlN samples were slow-cooled at a rate of 1 °C/min from 1900 to 1750 °C and/or annealed at 970 °C for 4 h. Al₅Y₃O₁₂ was detected in the AlN ceramics obtained by the slow-cooling and AlYO₃ was found in the ceramics cooled at a rate of 30 °C/min. AlN ceramics with a relative density of 0.986 were obtained by the slow-cooling method. On the other hand, very low tan δ values between 2.6 and 4.6 × 10⁻⁴ were obtained when the AlN ceramics were annealed at 970 °C for 4 h.     

Spectroscopic evaluation of out-of-plane surface vibration bands from surface functionalization of graphite oxide by fluorination

The fluorination of graphite/graphite oxide (GO) and their derivatives has been widely investigated for how fluorine interacts with sp²/sp³ carbon; however, the mechanism of this interaction has not yet been elucidated. Fluorination of GO (FGO) at either 10 or 15 psi for 24 h, produced two new absorption bands at ∼743 cm⁻¹ and 482 cm⁻¹, and are attributed to the presence of out-of-plane surface fluorine bonds in FGO (absent in fluorographite – FG). IR studies confirmed the stability of the formed C–F bonds and defect formation due to the introduction of oxyfluorinated species into the graphitic carbon through fluorination of epoxides. Fluorination of GO resulted in ∼4–5 times more fluorine incorporation in bulk as compared to FG. (4.57 vs. 0.8 at.% and 6.64 vs. 1.4 at.% at 10 and 15 psi, respectively). PXRD analyses also showed that the interlayer spacing of FGO expanded in the presence of intercalated C–F species and a defect formation was observed with the evidence of increase of the I_D/I_G ratio from Raman spectra. To this end, understanding the origin of surface C–F bonds and structural changes in FGO therefore leads to new applications such as implementation of FGO for sensing, nano-electronics and energy storage.     

Radioluminescence,thermoluminescence and dosimetric properties of ZnO ceramics

Two types of ZnO ceramics were fabricated and characterized by XRD, SEM methods. The radioluminescence spectra were measured within the 300–550 K range. The defect luminescence band peaking at ~2.35 eV is the dominant one in radioluminescence spectra in both of the fabricated ceramics. The thermostimulated luminescence (TSL) glow-curves were measured after X-ray irradiation at 300 K. It was concluded that the complex overlapping peak within the 320–450 K temperature range consists of two components (~360–375 K and 400–420 K). The ratio of component intensities differs in both ceramics. The positions of high temperature TSL components (480–520 K) also differ in both samples; therefore not only sintering conditions but also the properties of the initial powder are very important for characteristics of TSL. A linear dependence of peak intensity on irradiation dose was observed up to ~3 kGy for ceramic 1 and up to 9 kGy for ceramic 2.     

Generation of a low temperature NO linkage isomer in a transition metal complex of group 9: K[IrCl5NO]

The [IrCl₅NO]^− 1 ion in the potassium salt can be partially transformed into a metastable state by light irradiation in the violet-near UV region at low temperature (77 K). The excitation process was followed by infrared spectroscopy, comparing the spectra before and after irradiation.A new band grew at 1812 cm^− 1 after irradiation with light in the 308–420 nm spectral region. The metastable state behavior is verified as this band decays upon heating the samples above 90 K or by subsequent irradiation using 450–680 nm light. The band at 1812 cm^− 1 was assigned to ν(NO) of metastable state 1 (or an IrON linkage isomer) for comparison with the band position reported for other transition metal nitrosyls. To our knowledge, this is the first generation of a M-ON linkage isomer reported for a nitrosyl of transition metal of group 9. Its behavior is similar to that observed in other nitrosyl complexes of group 8.     

Structural,thermal and mechanical properties of aluminum nitride ceramics with CeO2 as a sintering aid

AlN ceramics have been prepared with CeO₂ as a sintering aid at a sintering temperature of 1900 °C. The effect of CeO₂ contents on the microstructure, density, thermal conductivity and hardness was investigated. Addition of CeO₂ exerted a significant effect on the densification of AlN ceramics and hence on the microstructure. Thermal conductivity of AlN ceramics increased with CeO₂ content and was greater than that of Y₂O₃-doped AlN ceramics at a similar sintering temperature. The resulting AlN ceramics with 1.50 wt% of CeO₂ had the highest relative density of 99.94%, thermal conductivity of 156 W m⁻¹ K⁻¹ and hardness of 72.46 kg/mm².     

Design,synthesis, crystal structure and photophysical studies of an emissive,terbium based sensor for zinc

The synthesis, crystal structure and photophysical studies of the probe complex Tb · L are described. Tb · L is a mononuclear complex with a 46-membered macrocyclic rings constructed by intermolecular hydrogen bond. The mononuclear complex Tb · L shows Laporte-forbidden ⁵D₄ → ⁷F_J f–f transitions and exhibits strong green luminescence emission bands in the solid state at ambient temperature which is characteristic of terbium ion. New selective luminescent lanthanide chemosensor for Zn(II) ion based on Tb · L is also described, suggesting host–guest complexation signaling transduction mechanism.     

A new estimation method for the intrinsic thermal conductivity of nonmetallic compounds: A case study for MgSiN2, AlN and β-Si3N4 ceramics

A new method for estimating the maximum achievable thermal conductivity of non-electrically conducting materials is presented. The method is based on temperature dependent thermal diffusivity data using a linear extrapolation method enabling discrimination between phonon-phonon and phonon-defect scattering. The thermal conductivities estimated in this way for MgSiN₂, AlN and β-Si₃N₄ ceramics at 300 K equal 28, 200 and 105 W m⁻¹ K⁻¹, respectively in favourable agreement with the highest experimental values of 23, 266 and 106–122 W m⁻¹ K⁻¹. This suggests the general applicability of the proposed estimation method for non-metallic compounds. It is expected that when optic phonons contribute to the heat conduction (as is the case for AlN) the intrinsic thermal conductivity at lower temperatures (e.g. 300 K) is underestimated. However, the reliability and accuracy of the presented ‘easy to use’ estimation method seems to be much better than several other estimation methods. Furthermore the needed input for this method can provide information about which processing parameters should be optimised to obtain the highest thermal conductivity.     

A double-perovskite Sr2ZnWO6:Mn4+ deep red phosphor: Synthesis and luminescence properties

Novel double-perovskite Sr₂ZnWO₆:Mn⁴⁺(SZW:Mn⁴⁺) phosphor is synthesized by high-temperature solid-state reaction method in air. SZW:Mn⁴⁺ phosphor with excitation at 325 and 526 nm emits deep-red light, the chromaticity coordinate is (0.7315,0.2685), and the emission band peaking at ~702 nm within the range 640–760 nm is assigned to the ²E→⁴A₂ transition of Mn⁴⁺ ion. The influences of “Mn⁴⁺- ligand” bonding and crystal field strength to emission properties of Mn⁴⁺ ion are analyzed. The optimal Mn⁴⁺ ion concentration in SZW:Mn⁴⁺ phosphor is ~0.8 mol%. Lifetime of SZW:Mn⁴⁺ phosphor decreases from 554.77 to 401.35 μs with increasing Mn⁴⁺ ion concentration in the range of 0.2–1.0 mol%. The lifetime data and decay curves indicate that there is only a single type of Mn⁴⁺ ion luminescent center in SZW:Mn⁴⁺ phosphor. The luminous mechanism of SZW:Mn⁴⁺ phosphor is analyzed by Tanabe-Sugano energy level diagram of Mn⁴⁺ in the octahedron together with the simple energy level diagram. The experimental results are helpful to research the influences of the neighboring coordination environment around Mn⁴⁺ and host crystal structure to the luminescence properties of Mn⁴⁺ ion and to deeply understand other Mn⁴⁺-dopedmaterials.