Nimai Pathak

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.

Photoluminescence and EPR studies on Fe3+ doped ZnAl2O4: an evidence for local site swapping of Fe3+ and formation of inverse and normal phase

Considering that ZnAl2O4 spinel has two different sites (octahedral and tetrahedral) and its properties change with dopant ion distribution among these two sites; ZnAl2O4 doped with varied concentrations of Fe(3+) was synthesized by a low temperature sol-gel combustion method. Phase purity and structural investigations were carried out using Rietveld refined X-ray diffraction which shows a decrease in the value of cell parameters at higher doping levels. Photoluminescence (PL) and electron paramagnetic resonance (EPR) studies have shown that on doping, Fe(3+) ions were distributed in both tetrahedral and octahedral sites. At octahedral sites, Fe(3+) exhibited a broad red emission around 745 nm while at tetrahedral sites it exhibited well-defined vibronic sidebands at 665, 674, 684 and 693 nm along with a broad blue band with a maxima at 445 nm at room temperature. EPR studies have shown a broad spectrum at g ≈ 2.2 which corresponds to the Fe(3+) in octahedral sites, while the broad signal at g ≈ 4.2 belongs to Fe(3+) in tetrahedral sites. It was also inferred from these studies that Fe(3+) prefers to occupy octahedral sites at higher concentrations and at higher annealing temperatures. The PL decay behavior of Fe(3+) in ZnAl2O4 has also shown that two different types of Fe(3+) ions were present in this matrix. The first type was a long lived species (τ ≈ 170 μs) present at octahedral sites and the other was a short lived species (τ ≈ 40 μs) present at the tetrahedral sites; the fraction of the long lived species predominate at higher concentrations. Thus the present work is mainly focused on understanding the tuning of local site occupancy of the dopant ion among those sites with varying concentration and annealing temperature, using the dopant ion itself as a spectroscopic probe, which further helps in understanding the phase (inverse and normal) of the spinel.

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.

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.

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.

Why host to dopant energy transfer is absent in the MgAl2O4:Eu3+ spinel? And exploring Eu3+ site distribution and local symmetry through its photoluminescence: interplay of experiment and theory

An undoped and Eu3+ doped magnesium aluminate spinel (MAS) was synthesized using a citric acid assisted combustion technique. MAS samples were characterized systematically using X-ray diffraction (XRD), time resolved photoluminescence spectroscopy (TRPLS) and ab initio calculations. On irradiating the undoped MAS with UV light; multicolor emission is observed. The blue emission peak was attributed to Mg2+ vacancies whereas the one in the green region was attributed to oxygen vacancies. Based on the emission spectrum it was inferred that the majority of europium ions are localized at the Mg2+ site which was also confirmed using lifetime measurements. DFT based cohesive energy calculations also showed Eu doping in the Mg position is energetically more favorable than doping in the Al position. Photoluminescence (PL) spectroscopy shows that the emission spectrum consists of host as well as Eu3+ emission indicating the absence of complete host–dopant energy transfer. DFT calculated density of states analysis shows that Eu states are solely localized in VB and CB regions and do not contribute in defects states. From the emission spectrum of the undoped MAS sample it was observed that photo-luminescence properties of the MgAl2O4 are dominantly governed by the defect states coming from the presence of cation and oxygen vacancies (neutral and charged). As a result photon energy transfer from host MAS to dopant Eu is difficult. The actual site symmetry for europium ions in MAS was also evaluated based on a stark splitting pattern which comes out to be C2v. Based on Judd–Ofelt analysis it was found that the Ω2 value is greater than Ω4; indicating high covalency and low symmetry around europium ions which is also observed in the emission spectrum. The high purity of the red emission coupled with good fluorescence quantum yields highlights the potential of this unexplored MAS as a promising phosphor.

New thorium–bismuth oxide solid solutions with oxygen vacancy induced tunable ferromagnetism

A combined X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS) spectroscopy, and magnetic and electron paramagnetic resonance (EPR) study on defect-induced magnetism in Th1−xBixO2−δ (0 ≤ x ≤ 0.3) solid solutions is presented in this paper. The solid solutions were prepared through a solid-state reaction route. The XRD patterns of the solid solutions suggest Bi dissolution up to 30 at% in the ThO2 matrix. The EXAFS study at the Bi and Th L3 edges indicates the formation of oxygen vacancies near Bi sites with an increasing trend with the Bi at% in the solid solutions. The magnetic measurements in field cooled (FC) and zero field cooled (ZFC) mode indicate an interesting observation involving ferromagnetic ordering with Curie temperature at around 70 K. The possibility of a spin glass-like phase was conclusively ruled out by a memory effect test experiment. The EPR studies revealed the presence of two different signals at g ∼ 2.05 and 2.25 due to the paramagnetic oxygen vacancy () and ferromagnetic resonance (FMR). The constant increase in intensity of the FMR signal with increasing doping level corroborates well with the increasing saturation magnetization value in magnetic measurements. By lowering the measurement temperature, this FMR signal showed an increase in its intensity along with continuous broadening and shift of the resonance field position to the lower field, which further signifies the presence of ferromagnetic ordering due to the paramagnetic oxygen vacancy in the Th1−xBixO2−δ solid solution.

Oxygen vacancy induced magnetism in (Th0.9Bi0.1)O1.95 solid solution

We present a magnetic study on Bi doped ThO2. EPR study revealed a clear signature of oxygen vacancies as a result of doping Bi. Interestingly, we also observe clear indication of a ferromagnetic behavior in the Bi doped ThO2. FC and ZFC measurements clearly indicate the ferromagnetic ordering at low temperature. The magnetism presumably originates from the oxygen vacancies created by doping lower valent cations into ThO2.