Dysprosium site occupancy in SrZnO2 nanophosphors probed through XANES.

Doping-assisted lattice site engineering is widely practiced to obtain a tailor made response, which subsequently poses a need for an efficient probe of the local electronic structure of the system. This study presents a detailed analysis of the local electronic structure around the host cations (Zn2+ and Sr2+) and dopant (Dy3+) through combined experimental and simulated X-ray absorption near edge structure. The real space full multiple scattering-based simulations of the Zn K-edge are done by substituting Dy at cationic sites in the second coordination shell around Zn, in various combinations along with and/or without oxygen vacancies in the system. The results revealed that Dy tends to substitute the less symmetric Sr2+ site at low doping concentration, whereas it starts substituting the relatively more symmetric Zn2+ lattice site with an increase in doping concentration, consequently affirming the origin of cold white emission upon charge transfer in the system (Manju, M. Jain, P. Vashishtha, G. Gupta, A. Sharma, S. O. Won, A. Vij and A. Thakur, J. Phys.: Condens. Matter, 2020, 33, 035703). The effect of Zn site occupancy is seen as bifurcation of the single peaked Dy L3 absorption edge, which is usually reported as the sole indication of the existence of a mixed valence state. Thus, the combined analyses decipher the effect of lattice site occupancy on the local electronic structure of host as well as dopant atoms.

Intense green light emission and thermoluminescence glow curve analysis of Tb3+ -activated CaY2 O4 phosphor.

Here, the synthesis and luminescence analysis of the Tb3+ -activated phosphor were reported. The CaY2 O4 phosphors were synthesized using a modified solid-state reaction method with a variable doping concentration of Tb3+ ion (0.1-2.5 mol%). As synthesized, the phosphor was characterized using Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction analysis techniques for the optimized concentration of doping ions. The prepared phosphor showed a cubic structure, and FTIR analysis confirmed functional group analysis. It was discovered that the intensity of 1.5 mol% was higher than at other concentrations after the photoluminescence (PL) excitation and emission spectra were recorded for different concentrations of doping ions. The excitation was monitored at 542 nm, and the emission was monitored at 237 nm. At 237 nm excitation, the emission peaks were found at 620 nm (5 D4 →7 F3 ), 582 nm (5 D4 →7 F4 ), 542 nm (5 D4 →7 F5 ), and 484 nm (5 D4 →7 F6 ). The 1931 CIE (x, y) chromaticity coordinates showed the distribution of the spectral region calculated from the PL emission spectra. The values of (x = 0.34 and y = 0.60) were very close to dark green emission. Therefore, the produced phosphor would be very useful for light-emitting diode (green component) applications. Thermoluminescence glow curve analysis for various concentrations of doping ions and various ultraviolet (UV) exposure times was carried out, and a single broad peak was found at 252°C. The computerized glow curve deconvolution method was used to obtain the related kinetic parameters. The prepared phosphor exhibited an excellent response to UV dose and could be useful for UV ray dosimetry.

Electronic structure and defect-induced luminescence study of phase-stabilized t-ZrO2 nanocrystals.

Luminescent tetragonal-ZrO2 (t-ZrO2 ) nanocrystals were synthesized using an optimized combustion method without post-synthesis annealing and characterized using X-ray diffraction, electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-Vis. spectroscopy, photoluminescence spectroscopy, thermoluminescence (TL), and vibrating sample magnetometry. The as-synthesized t-ZrO2 nanocrystals have a bandgap of 4.65 eV and exhibit defect-assisted blue emission (Commission Internationale de I'Elcairage coordinates 0.2294, 0.1984) when excited at 270 nm. The defect states were qualitatively and quantitatively analyzed using TL after irradiating nanocrystals with γ- and UV radiations at various doses. The TL glow curves show intense emission in the high-temperature region from 523 to 673 K for both UV- and γ-irradiated samples; however, another less-intense TL peak was also observed in the low-temperature region from 333 to 453 K with γ irradiation at higher doses, indicating the formation of shallow trapping states. The activation energies, frequency factor, and order of kinetics were estimated using the computerized glow curve deconvolution method for the shallow and deep traps for γ- and UV-irradiated samples. The present study shows that phase-stabilized t-ZrO2 nanocrystals are potential candidates for luminescence-based applications.

UV/blue/green converted efficient red-NIR photoluminescence in Cr incorporated MgAl2O4nanocrystals: Site selective emission tailored through cation inversion and intrinsic defects.

The UV/Visible activated near-infrared (NIR) phosphors have many applications in solid state lighting, night vision devices and bio-imaging. The early research reported the red-NIR emitting phosphors doped with Cr3+centers upon visible light excitation. Here, in this work the intense red-NIR emission and color tuning is achieved for broad excitation range (UV/blue/green) through Cr dopant induced defect centers and cation inversionWe present the infuence of Cr dopant induced defect centers and cation inversion in Mg1-xCrxAl2O4(x= 0.5, 1, 3, 5 and 10 mol%) nanocrystals. The Cr3+doped MgAl2O4nanocrystals were synthesized by combustion method through stoichiometric substitution of Mg by Cr, while most of the Cr3+ions occupied the octahedral sites of spinel host with the formation of antisite defects, Cr3+clusters, magnesium and oxygen vacancies. These defect centers were probed through Rietveld refinement, PL, X-ray photoelectron and nuclear magnetic resonance spectra analyses. At UV excitation, the intrinsic defects played an interesting role in exhibiting the blue-violet emission attributed to host lattice defects and red-NIR emission attributed to strong/weak ligand field octahedral Cr3+sites, via charge transfer to Cr3+ions. The PL spectra evinced the enhanced red-NIR emission intensity upon 266 nm excitation than upon blue and green light excitation. Further, the weak ligand field site emission is found to be dominating with increase in doping concentration. Thus, Cr doped MgAl2O4nanocrystals showed their potency of exhibiting the intense red-NIR emission and color tuning (from red purple to bluish purple and then to red color) upon UV/blue/green excitation.

Effect of Eu doping on the thermoluminescence of UV and gamma irradiated Mg2 B2 O5 nanophosphors.

This article presents the effect of europium (Eu) doping on the thermoluminescence (TL) of ultraviolet (UV-254 nm) and gamma irradiated triclinic Mg2 B2 O5 nanophosphors. The diffuse reflectance predicts slight decrease in band gap from 5.18 to 4.99 eV with increasing Eu (1%, 3%, and 5%) content in Mg2 B2 O5 . The TL glow curves of UV irradiated samples comprised of a main peak around 500 K with weak intensity peak/shoulders in low temperature region. Interestingly Eu (3%) doped Mg2 B2 O5 shows maximum TL intensity with suppression of low temperature shoulder peaks and almost linear UV dose dependent TL response. However, in the case of gamma irradiated Eu (1% and 3% doped) samples, TL glow curve comprises of a main peak around 425-445 K and closely lying peak around 500-515 K with relatively lesser intensity. In case of Eu (5%) doped samples, TL peak around 508 K starts dominating over peak around 425 K. TL of both UV and gamma irradiated samples showed the presence of various deep and shallow defect states within the bandgap of materials having different kinetic parameters, which were determined using TLanal software based on Kiti's general order equation. The present study shows that Eu doped Mg2 B2 O5 nanophosphors can be tuned for UV and gamma dosimetry.

Structural, diffuse reflectance and luminescence study of t-Mg2B2O5 nanostructures.

We report here the structural, reflectance, photoluminescence and thermoluminescence study of t-Mg2B2O5 nanostructures synthesized using optimized combustion method relatively at much lower temperature. The rietveld refinement of X-ray diffraction data confirms single-phase triclinic crystal structure of Mg2B2O5 nanoparticles. The direct band gap determined using diffuse reflectance spectra (DRS) was 5.23 eV, which is contrary to earlier reports quoting Mg2B2O5 as indirect band gap material. To elucidate the nature of band gap in Mg2B2O5, we performed first principle calculations based on full potential linearized augmented plane-wave (FPLAPW) method, predicting the direct band gap of 5.10 eV in t-Mg2B2O5 which is in good agreement with our experimental results. The t-Mg2B2O5 nanoparticles were found to exhibit yellow-reddish photoluminescence peaking at 588 nm, attributed to various defects states. The combustion synthesized Mg2B2O5 nanocrystals exhibited ultraviolet (254 nm) responsive thermoluminescence (TL). TL glow curve of Mg2B2O5 comprises of one dominant peak around 417-428 K and less intense shoulder around 573-589 K which arouse possibility of various trapping sites or defects present in the sample. The TL analysis using general order Kitti's equations was performed to estimate the activation energies of trapping states. Owing to already well-known mechanical and thermal properties, the direct wide band gap nature and UV responsive thermoluminescence of combustion synthesized t-Mg2B2O5 nanostructures can pave way for its use in luminescence-based applications and UV dosimetry. As an additional application of Mg2B2O5, anti-biofilms activity of Mg2B2O5 nanoparticles using pseudomonas aeruginosa bacterial cells was also performed which revealed 91 ± 2.7% inhibition of biofilms formed by P. aeruginosa, respectively, at 100 µg/ml after 24 h of treatment.

Mechanistic insights into defect generation and tuning of optical properties in Zn1-xFexAl2O4(0.01 ≤ x ≤ 0.40) nanocrystals.

The correlation of several defects and optical and magnetic properties with Fe content in Zn1-xFexAl2O4 (0.01 ≤ x ≤ 0.40) nanocrystals has been scrutinized through X-ray diffraction, O K-edge X-ray absorption near-edge structure, FT-IR, diffuse reflectance, photoluminescence and electron spin-resonance spectroscopies, and vibrating sample magnetometry. Increasing Fe content causes elongation in the octahedral units of the lattice, accompanied by distortion in the octahedral coordination. Fe introduces non-radiative centres in the forbidden gap, thereby tuning the band gap from 4.37 to 3.88 eV and eliminating emission in the visible region. Zn vacancies are found to tail off, while {\rm Fe}_i^{\bullet \bullet \bullet}, {\rm Al}_{\rm Zn}^\bullet and FeAl× antisite defects increase in concentration with increasing Fe content. Inhomogeneous broadening of spin-resonance signals infers strong spin-lattice interactions of Fe3+ ions at distorted octahedral and non-symmetric tetrahedral sites. A transition is observed from paramagnetism to superparamagnetism at higher Fe concentrations. A visual colour change from pearly white to orange-brown is observed in Zn1-xFexAl2O4 nanocrystals with increasing Fe content, revealing its potential candidature for pigments in the paint and dye industries.

Oxygen vacancies induced photoluminescence in [Formula: see text] nanophosphors probed by theoretical and experimental analysis.

We report, for the first time, the influence of oxygen vacancies on band structure and local electronic structure of [Formula: see text] (SZO) nanophosphors by combined first principle calculations based on density functional theory and full multiple scattering theory, correlated with experimental results obtained from X-ray absorption and photoluminescence spectroscopies. The band structure analysis from density functional theory revealed the formation of new energy states in the forbidden gap due to introduction of oxygen vacancies in the system, thereby causing disruption in intrinsic symmetry and altering bond lengths in SZO system. These defect states are anticipated as origin of observed photoluminescence in SZO nanophosphors. The experimental X-ray absorption near edge structure (XANES) at Zn and Sr K-edges were successfully imitated by simulated XANES obtained after removing oxygen atoms around Zn and Sr cores, which affirmed the presence and signature of oxygen vacancies on near edge structure.

Switchable cool and cold white emission from dysprosium doped SrZnO2.

In presented work, excitation selective novel cool and cold white emission is reported from dysprosium (Dy) doped SrZnO2nanophosphors, synthesized by combustion technique. The host lattice provided selective excitation routes for Dy3+levels and intrinsic defects levels via charge transfer (270 nm) and host defects absorption bands (375 nm), respectively. The emission due to Dy3+levels was found to be exhibiting cool white emission and that from intrinsic defects was cold white emission, as characterized from correlated color temperature. UV irradiated glow curve analysis complemented the results by exhibiting signal due to Dy assisted traps on near UV exposure (254 nm) and that of host related traps at far UV exposure (365 nm). The luminescence phenomenon is comprehended through proposed band model. The obtained results proclaimed SrZnO2:Dy as a potential member among white emitting phosphors to be used as standard daylight sources in commercial and aesthetic lighting.

Color modulation by selective excitation activated defects and complex cation distribution in Zn1-xMgxAl2O4 nanocrystals.

Complex cation distribution in spinel solid solution, fosters defect generation and permutates the optical properties. To scrutinize the effect on structural properties, viz. the cation distribution and defect states upon substitution of Zn with Mg; and to tune the emission properties, Zn1-xMgxAl2O4 (0.01 ≤ x ≤ 0.30) nanocrystals are synthesized. The nanocrystals show increased inversion and generation of multiple defects, namely zinc vacancies, zinc interstitials, oxygen vacancies and antisite defects with increasing Mg content, which thereby impacts the optical band gap. Pentahedral coordination in addition to tetra- and octahedral coordination of Al has been observed, which infers the presence of oxygen vacancies and dangling bonds. Moire fringes formation has intimated the presence of two or more crystal lattices with higher Mg substitution. Band-to-band and defect-assisted photoluminescence shows the role of multiple defects, especially defect clusters, in deciphering the properties of the resulting crystals. Color change from bluish-white to pink has been achieved depending upon the excitation wavelength and emission mechanism, as proposed through a band model schematic. The presented study may be beneficial for designing the Zn1-xMgxAl2O4 nanocrystals with optimized emission properties.

A Novel Synthesis of the Graphene Oxide-Silver (GO-Ag) Nanocomposite for Unique Physiochemical Applications.

Graphene oxide-silver nanocomposite (GO-Ag) was fabricated via the sonochemical method, which shows unique physiochemical properties. Graphene oxide (GO) and silver nanoparticles (AgNPs) were synthesized by modified Hummer's and Chemical reduction methods, respectively. The synthesized nanocomposite was characterized using powder X-ray diffraction, Raman spectroscopy, and Fourier-transform infrared spectroscopy. The surface morphology of synthesized nanoparticles was studied using scanning electron microscopy and transmission electron microscopy. The thermoluminescence property of the nanocomposite was analyzed by irradiating the samples in gamma radiation at 1 kGy. Electrochemical reversibility of the GO-Ag nanocomposite was examined by cyclic voltammetry. The photocatalytic application of the nanocomposite was studied using degradation of methylene blue dye. Results reveal that doping of AgNPs on the GO surface not only improves its dye degradation property but also enhances its thermoluminescence property. This knowledge will be helpful in determining the antibacterial property of the GO-Ag nanocomposite in the future.

Defect states and kinetic parameter analysis of ZnAl2O4 nanocrystals by X-ray photoelectron spectroscopy and thermoluminescence.

Defect states in ZnAl2O4 have a significant role in its applicability as a luminescent material. To understand the nature and distribution of defects in its crystal lattice, thermoluminescence (TL) study has been carried out. Excellent TL response is observed from γ- and ultraviolet-irradiated samples at different doses and exposure durations, respectively. Different type of fuels employed in combustion synthesis show a remarkable effect on the trap distribution and hence luminescence properties. Shallow and deep traps are observed in crystals attributed to O- vacancies and F+ centers. The mechanism of trapping, retrapping and recombination have been depicted through schematic band model diagram. X-ray photoelectron spectroscopy indicated the presence of various types of defects specifically AlZn antisite defect, oxygen and zinc vacancies which are further upheld by photoluminescence and Raman spectroscopy. All results when summed up, predict ZnAl2O4 to be a quality material for dosimetry.

Nanostructured Boron Nitride With High Water Dispersibility For Boron Neutron Capture Therapy.

Highly water dispersible boron based compounds are innovative and advanced materials which can be used in Boron Neutron Capture Therapy for cancer treatment (BNCT). Present study deals with the synthesis of highly water dispersible nanostructured Boron Nitride (BN). Unique and relatively low temperature synthesis route is the soul of present study. The morphological examinations (Scanning/transmission electron microscopy) of synthesized nanostructures showed that they are in transient phase from two dimensional hexagonal sheets to nanotubes. It is also supported by dual energy band gap of these materials calculated from UV- visible spectrum of the material. The theoretically calculated band gap also supports the same (calculated by virtual nano lab Software). X-ray diffraction (XRD) analysis shows that the synthesized material has deformed structure which is further supported by Raman spectroscopy. The structural aspect of high water disperse ability of BN is also studied. The ultra-high disperse ability which is a result of structural deformation make these nanostructures very useful in BNCT. Cytotoxicity studies on various cell lines (Hela(cervical cancer), human embryonic kidney (HEK-293) and human breast adenocarcinoma (MCF-7)) show that the synthesized nanostructures can be used for BNCT.