P. S. Ghosh

Understanding the local environment of Sm3+ in doped SrZrO3 and energy transfer mechanism using time-resolved luminescence: a combined theoretical and experimental approach

A combined experimental and theoretical study on the photoluminescence (PL) properties of strontium zirconate (SZ) and Sm3+ doped SZ nanostructures is presented in this work. SZ and Sm3+ doped SZ is synthesized by a gel-combustion route and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence (PL) spectroscopy, and electron paramagnetic resonance (EPR) experimental techniques. PL studies on nanocrystalline SZ show strong violet-blue and weak orange-red emission under excitation wavelength at 243 nm. An EPR study shows the presence of oxygen vacancy in SZ nanocrystals. Combined emission, EPR studies and theoretical calculations brings out the possible reason for multicolor emission in SZ nanocrystals. The results of the PL spectroscopy measurement imply that the Sm3+ emissions, which originated from the 4G5/2 → 6HJ (J = 5/2, 7/2, 9/2, and 11/2) intra-4f transitions of Sm3+ ions, are due to the indirect excitation of the Sm3+ ions through an energy transfer process from electron–hole pairs generated in the SZ hosts. Based on combined experimental and theoretical studies, a possible mechanism for PL of undoped and Sm3+-doped SZ is proposed.

Luminescence Properties of SrZrO3/Tb3+ Perovskite: Host-Dopant Energy-Transfer Dynamics and Local Structure of Tb3+

SrZrO3 perovskite (SZP) was synthesized using gel-combustion route and characterized systematically using X-ray diffraction and time-resolved photoluminescence techniques. A detailed analysis of the optical properties of Tb(3+) ions in SrZrO3 was performed to correlate them with the local environment of the lanthanide ions in this perovskite. Photoluminescence (PL) spectroscopy showed that emission spectrum consists of host as well as Tb(3+) emission indicating the absence of complete host-dopant energy transfer. On the basis of emission spectrum and PL decay study it was also observed that Tb(3+) is not homogeneously distributed in SrZrO3 perovskite; rather, it is occupying two different sites. It is corroborated using extended X-ray absorption fine structure studies that Tb(3+) is stabilized on both six-coordinated Zr(4+) and eight-coordinated Sr(2) site. The energies calculated using density functional theory (DFT) indicates that Tb occupation in Sr site is energetically more favorable than Zr site. The analysis of valence charge distribution also substantiated our structural stability analysis of site-selective Tb doping in SrZrO3. Time-resolved emission spectroscopy is employed to elucidate the difference in the spectral feature of Tb(3+) ion at Sr(2+) and Zr(4+) site. DFT-calculated density of states analysis showed that energy mismatch of Tb-d states with Zr-d and O-p states of SZP makes the energy transfer from host SZP to Tb(3+) ion difficult.

Intense red emitting monoclinic LaPO4:Eu3+ nanoparticles: host–dopant energy transfer dynamics and photoluminescence properties

LaPO4 nanoparticles were synthesized using complex polymerization method and characterized systematically using X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. Varied concentration of europium ion is doped in the LaPO4 lattice and optical properties and Judd–Ofelt analysis were investigated. It is observed that LaPO4 nanoparticles give violet–blue emission when irradiated with UV light. On doping europium ion the band gap of LaPO4 decreases. Based on DFT calculations it is proposed that energy range over which d and f states of Eu are distributed is coincident with the valence band of LaPO4 so causing an efficient energy transfer from LaPO4 to europium ion. The actual site symmetry for europium ion in lanthanum orthophosphate was also evaluated as D2d based on the Stark splitting pattern although it is C1 for La3+ in LaPO4. The critical energy-transfer distance for the Eu3+ ions was evaluated, based on which the quenching mechanism was verified to be an electric multipolar interaction. It is also observed that the red emission intensity of LaPO4:Eu3+ (2.0 mol%) is almost 85% of a commercial red phosphor, which clearly demonstrates that the as-prepared samples are promising red phosphors under near-UV for use in white-light emitting diodes.

Origin of blue emission in ThO2 nanorods: exploring it as a host for photoluminescence of Eu3+, Tb3+ and Dy3+

Nanorods of ThO2 were synthesized in a reverse micelle technique using cetyl trimethyl ammonium bromide as a surfactant and characterized by X-ray diffraction and transmission electron microscopy. Doping with europium, terbium and dysprosium was performed at a 1.0 mol% level and was confirmed using XRD. Photoluminescence study showed that nanorods of thoria emit blue color on UV-excitation. Lifetime and electron paramagnetic resonance (EPR) spectroscopy showed that this blue emission is because of oxygen vacancy. First principle calculations using projector augmented wave potentials and generalized gradient approximations predicted that the structural relaxation due to neutral and positively charged oxygen defects in bulk thoria leads to symmetric distortion around the vacancy sites and this prediction was experimentally complemented by measuring highly symmetric isotropic signal with g = 1.959 in EPR studies. Density of states analysis showed the presence of defects states mainly attributed to Th d and f states near the conduction band minima for the double positively charged oxygen vacancy. The radiative transitions from these states qualitatively explain blue emission in thoria. On the basis of the time resolved emission data (TRES), it has been inferred that two different types of Ln3+ ions were present in the thoria nanorods: A long lived species (τ ∼ 1.5 ms) was present at cubic site with Oh symmetry and the other was a short lived species (τ ∼ 300 μs) present at the non-cubic site with C3v symmetry. Most of Eu and Tb are distributed in cubic site with Oh symmetry whereas Dy is mainly localized at the non-cubic site with C3v symmetry. Under UV light excitation, ThO2:Ln3+ nanorods show the characteristic f–f transitions of Ln3+ (Eu, Tb and Dy) ions and give bright red, green, bluish-yellow emission, respectively. In addition, multicoloured luminescence containing white emission has been successfully achieved for tri-doped ThO2:Ln3+ phosphors by simultaneous doping with all the three lanthanides ions and adjusting their doping concentrations for the simultaneous luminescence of Ln3+ in the ThO2 host.

Probing local site environments and distribution of manganese in SrZrO3:Mn; PL and EPR spectroscopy complimented by DFT calculations

In order to understand the local environment, valence state and cationic distribution of manganese ions in gel-combustion derived SrZrO3 (SZO), a combined experimental and theoretical approach was formulated based on photoluminescence (PL), electron paramagnetic resonance (EPR) and density functional theory (DFT) calculations. An attempt was also made to investigate the same as a function of manganese ion concentration. The phase identification of the samples was confirmed using powder X-ray diffraction technique (PXRD). In all the doped compounds, manganese was found to be stabilized as divalent Mn2+ and preferentially occupying the 8-coordinated Sr2+ ion site. However, the proportion of manganese ions residing at zirconium sites was enhanced at higher concentrations. The cohesive energies from DFT calculations explained the stability of Mn2+ ions at different sites. It was also observed from the density of states (DOS) that the substitution of manganese at strontium sites leads to the generation of shallow defect states, whereas that at zirconium site generates both shallow and deep defect states within the band gap of the material. A change in the host emission due to these defect states with varied concentrations of Mn2+ was also observed, which further supported the observed cationic distributions trend. The decrease in the band gap energy explained the red shift of the emission spectra. PL decay study also suggested the existence of shallow and deep trap states. The intensity of the EPR signal at g ≈ 1.976, due to paramagnetic oxygen vacancies, was found to increase at higher Mn-concentration because of more substitution at Zr4+ sites. Two additional EPR sextets with g ≈ 1.993 and 2.013 in Mn doped SZO compounds were attributed to lattice and surface bound Mn2+ ions, respectively, which disappeared at higher Mn-concentrations, giving a broad signal.

Luminescence of undoped and Eu3+ doped nanocrystalline SrWO4 scheelite: time resolved fluorescence complimented by DFT and positron annihilation spectroscopic studies

SrWO4 (SWO) and Eu3+ doped SrWO4 (SEWO) scheelite samples were synthesized using a polyol method. Crystallite sizes of the as prepared samples annealed at 300 and 500 °C are in a similar range (<20 nm) whereas those annealed at 700 and 900 °C are about ∼40–50 nm and 80–100 nm, respectively. Photoluminescence (PL) spectra of SWO samples show a broad peak corresponding to the oxygen to tungsten charge transfer transition. Along with the enhancement in emission intensity in the samples annealed at 700 and 900 °C, there is a blue shift in peak maxima. Our first principles quantum mechanical calculations showed that the break in symmetry of the unit cell of SrWO4 creates inherent defects in the lattice which are responsible for the reduction of the electronic band gap in the SrWO4 sample with decrease in size. On europium doping; energy transfer from the O2− → W6+ charge transfer band to Eu3+ takes place and the reason behind this is explained using density functional theory (DFT) calculations. Based on time resolved PL measurements, it is suggested that Eu3+ ions occupy two sites in SWO; a regular symmetric Sr2+ site and an asymmetric site (closer to charge compensating defects). With increase in annealing temperature, emission intensity as well as asymmetry around europium increased. The changes in asymmetry around europium and defect densities as determined from positron annihilation lifetime spectroscopy (PALS) suggest that though the overall vacancy concentration is reduced with an increase in annealing temperature, it is likely that vacancies closer to europium are slowly annealed out than the others.

Nature of defects in blue light emitting CaZrO3: spectroscopic and theoretical study

The optical behaviour of a perovskite ceramic CaZrO3 is investigated. The orthorhombic CaZrO3 was obtained by gel combustion synthesis which yielded phase pure product at temperatures as low as 600°C. Transmission electron microscopy shows the formation of highly monodisperse nanospheres of calcium zirconate. Despite the absence of any activator, undoped CaZrO3 showed distinct excitation and emission spectra attributed to the presence of local defects in the perovskite phase. Photoluminescence decay and EPR spectroscopy shows the presence of oxygen vacancies which is responsible for intense violet blue emission in the CaZrO3 nanospheres. The presence of oxygen vacancies was further confirmed by comparing the intensity of emission and the EPR spectrum of the sample annealed in completely reducing and completely oxidizing atmospheres with that of the as prepared sample. To explain the PL emission in the blue region, a distortion model is proposed. Our DFT based hybrid functional calculations show distortion in the Ca network causes less disorder in the unit-cell compared to the Zr network. DFT calculations also show distortion in the Ca network comprising complex clusters generates shallow defect states very close to valence band maxima leading to PL emission in the blue region.

Experimental and theoretical approach to account for green luminescence from Gd2Zr2O7 pyrochlore: exploring the site occupancy and origin of host-dopant energy transfer in Gd2Zr2O7:Eu3+

Pure and Eu3+-doped Gd2Zr2O7 pyrochlore was synthesized using a gel combustion method using citric acid as a fuel. Samples of Gd2Zr2O7 pyrochlore were characterized systematically using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and time-resolved photoluminescence spectroscopy (TRPLS). On irradiating the undoped Gd2Zr2O7 pyrochlore with Ultraviolet (UV) light, an intense green emission was observed. Photoluminescence lifetime measurements and X-ray photoelectron spectroscopy (XPS) showed the presence of oxygen vacancies in the pyrochlore sample, which were responsible for the intense green emission in Gd2Zr2O7. Calculations based on density functional theory (DFT+U) of the electronic density of states in the presence of charged oxygen defects qualitatively explained the origin of the green emission in undoped Gd2Zr2O7. The emission spectrum of Eu3+ revealed that it was distributed at both Gd3+ and Zr4+ sites in Gd2Zr2O7, which was also confirmed using lifetime measurements. DFT-based calculations of cohesive energy also showed that doping with Eu is almost equally favorable at Gd and Zr sites. Based on DFT calculations, it is proposed that the distribution of f and d states of Eu3+ atoms matches well with the total density of states (DOS) of ordered Gd2Zr2O7 (o-Gd2Zr2O7), which signifies efficient transfer of photon energy from the host to the Eu3+ dopant. The actual site symmetry of europium ions in gadolinium zirconate was also determined based on the Stark splitting pattern and was found to be D2d, although it is Oh for Gd3+ in Gd2Zr2O7. Calculations of the Judd–Ofelt parameters revealed that Ω2 > Ω4, which indicates high covalency and low symmetry around Eu3+, which is in agreement with the results of emission spectroscopy. The high intensity of the red emission corresponding to the 5D0 → 7F2 transition and good fluorescence yields (51%) highlight the unexplored potential of Gd2Zr2O7:Eu3+ as a promising red phosphor.

Role of various defects in the photoluminescence characteristics of nanocrystalline Nd2Zr2O7: an investigation through spectroscopic and DFT calculations

For the first time, visible photoluminescence could be seen in the blue and green regions in Nd2Zr2O7 nanocrystals synthesized by gel combustion at 800 °C. The nanocrystalline nature of samples was confirmed using X-ray diffraction (XRD), and transmission electron microscopy (TEM). The samples were further characterized using Fourier transform infrared spectroscopy (FTIR), diffuse reflectance spectroscopy (DRS). The photoluminescence emissions pointed to the presence of defects related to oxygen vacancies. DFT-based calculations showed that electronic transitions between defect states (which arise due to V1+O and V2+O defects) and the conduction band, as well as impurity states at the bottom of the CB, can lead to emissions in the green and blue regions. Samples were further annealed at higher temperatures of up to 1200 °C to observe the evolution of defects and its implications for the photophysical characteristics of Nd2Zr2O7. The emission intensity was found to increase with an increase in temperature. The increase in intensity upon annealing at a higher temperature was attributed to a reduction in the concentration of surface defects and cation vacancies as confirmed using positron annihilation lifetime spectroscopy (PALS). Based on positron annihilation gamma-ray coincidence Doppler broadening (CDB) measurements, it was observed that the nature of the defects probed by positrons did not change on annealing but their concentrations significantly changed. This was also reflected in the emission spectra, in which the spectral features remained the same but the intensity increased as the annealing temperature was increased. The value of the direct optical band did not change much either as a function of the annealing temperature, which further supports the trend in the emission spectra. In brief, it can be said that the characteristic emission in Nd2Zr2O7 samples is due to oxygen vacancies, whereas the increase in the emission intensity with temperature is due to a decrease in the concentration of cation vacancies and surface defects, which serve as alternative non-radiative paths.

Defects induced changes in the electronic structures of MgO and their correlation with the optical properties: a special case of electron–hole recombination from the conduction band

A detailed investigation on different defects from induced emission characteristics in MgO, which are responsible for the multicolor emissions and lasing property of that material, is presented in this report. The color centers are characterized by absorption spectroscopy, decay kinetics, and a TRES study. Various defect centers such as oxygen vacancies (e.g., F, F+, , , ), cationic vacancies (, , ), interstitial oxygen (, , ), Schottky defect , etc., create different electronic states inside the wide band gap. Density Functional Theory (DFT) based calculation was performed for these defect centers to characterize their ground electronic states inside the band-gap. In MgO, a photo ionization process of the F center is involved at an excitation wavelength of 250 nm, followed by the equation F + hν ↔ F+ + e. The released electron in this process may prompt into the conduction band and thereby behaves as a free carrier. Being free, the electron may recombine with different types of positively charged defect centers in addition to the newly formed F+ centers. Thus, different electronic transitions from the conduction band (CB) to the empty ground electronic states of positively charged F- and F2-type centers can be correlated with their observed emission components. Recombination of a hole in the valence band (VB) with a filled electron in the electronic states may also be responsible for some emission behaviors. Thus, an understanding about all the emitting color components due to various defect centers in MgO might be possible by considering those special recombination processes and may also help to remove the long standing contradiction regarding their origin.