Santosh K. Gupta

Bio: Santosh K. Gupta is an academic researcher from Homi Bhabha National Institute. The author has contributed to research in topics: Photoluminescence & Luminescence. The author has an hindex of 35, co-authored 186 publications receiving 3465 citations. Previous affiliations of Santosh K. Gupta include Indian Institutes of Technology & University of Texas at Austin.

Energy transfer dynamics and luminescence properties of Eu3+ in CaMoO4 and SrMoO4

Abstract: Undoped and europium doped CaMoO4 and SrMoO4 scheelites are synthesized using a complex polymerization method. The phase purity of the sample is confirmed using powder X-ray diffraction (PXRD). X-ray photoelectron spectroscopy (XPS) was carried out to confirm the oxidation states of various constituents and dopant elements and also the presence of oxygen vacancies. Interestingly both CaMoO4 and SrMoO4 on irradiation with UV light give blue and green emission respectively. On europium doping, it was found that molybdate to Eu3+ ion energy transfer is more efficient in SrMoO4:Eu compared to CaMoO4:Eu. It is also justified using a luminescence lifetime study which shows biexponential decay in the case of CaMoO4:Eu corresponding to both the host and europium ion; whereas a single lifetime is observed in the case of SrMoO4:Eu. Anomalies in host–dopant energy transfer are suitably explained using density functional theory (DFT) calculations and XPS. The actual site symmetry for the europium ion in CaMoO4 and SrMoO4 was also evaluated based on a Stark splitting pattern which turns out to be D2 and C2v respectively although it is S4 for Ca/Ba2+ in AMoO4. This is also reflected in higher Ω2 values for SrMoO4:Eu than CaMoO4:Eu.

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

Abstract: 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.

An Insight into the Various Defects-Induced Emission in MgAl2O4 and Their Tunability with Phase Behavior: Combined Experimental and Theoretical Approach

Abstract: The present work describes various defects-induced tunable emission behavior of MgAl2O4 compounds obtained after annealing at different temperatures through a sol–gel combustion route. Multiple defect centers, such as F, F2, F+, and F22+ and different shallow and deep defects were found to be present inside the band gap, as confirmed by the lifetime and time-resolved emission spectroscopy (TRES) studies. The tunable emission characteristic at different annealing temperatures could be linked with the phase behavior of the spinel. Excitation wavelength variation suggested that a photoconversion process of F to F+ centers was involved with λex = 250 nm, followed by a trapping–de-trapping mechanism of the released electrons within different trap states. An exchange mechanism of electrons in between conduction band and shallow states was also observed at room temperature, which was absent at low temperature, as indicated by the emission profile. These observations render it to be a potential optical-based ther...

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

Abstract: 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.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

Hydrogen Sensors - A review

Abstract: Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.