Enhanced photoluminescence response of Ca²⁺/Ba²⁺ substituted solid solutions of SrS:Ce phosphors

Vij, Ankush,Kumar, Shalendra,Won, S.O.,Chae, K.H.,Dubey, Abhinav,Kumar, Ravi Elsevier 2018 Materials letters Vol.227 No.-

<P><B>Abstract</B></P> <P>The quest of improving luminescence efficiency in alkaline earth sulfide phosphors has been of great interest due to their applications in diverse areas. We synthesized SrS:Ce<SUB>0.05</SUB>, Sr<SUB>0.75</SUB>Ba<SUB>0.20</SUB>S:Ce<SUB>0.05</SUB>, Sr<SUB>0.75</SUB>Ca<SUB>0.20</SUB>S:Ce<SUB>0.05</SUB> and Sr<SUB>0.75</SUB>Ca<SUB>0.10</SUB>Ba<SUB>0.10</SUB>S:Ce<SUB>0.05</SUB> phosphors using solid state diffusion method. X-ray diffraction patterns of all the samples were found similar to the cubic SrS phase except minor shifts in angles. To ascertain the valence state of Ce, near edge X-ray absorption fine structure spectroscopy was employed using synchrotron radiations. We simulated Ce M<SUB>5,4</SUB>-edges of Ce<SUP>3+</SUP> ions using atomic multiplet calculations, which agrees well with the experimental data, clearly suggesting that Ce enters as Ce<SUP>3+</SUP> in all the samples. The photoluminescence spectra of synthesized phosphors exhibit blue-green emission assigned to the 5d-4f transitions in Ce<SUP>3+</SUP> levels, with minor shifts in wavelength. Interestingly, Ca<SUP>2+</SUP>/Ba<SUP>2+</SUP>substitution in SrS:Ce phosphors increases the PL intensity considerably. In comparison to SrS:Ce<SUB>0.05</SUB> , PL intensity of Sr<SUB>0.75</SUB>Ca<SUB>0.10</SUB>Ba<SUB>0.10</SUB>S:Ce<SUB>0.05</SUB> phosphors was enhanced by 2.7 times, which may be exploited in many optical applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Probed the effect of Ca<SUP>2+</SUP>/Ba<SUP>2+</SUP> substituted solid solutions of SrS:Ce<SUB>0.05</SUB> phosphors. </LI> <LI> NEXAFS spectra collected at Ce-M<SUB>5,4</SUB> edges confirms presence of Ce<SUP>3+</SUP>. </LI> <LI> PL intensity increases appreciably with Ca<SUP>2+</SUP>/Ba<SUP>2+</SUP> substitution in SrS:Ce. </LI> <LI> SrCaBaS:Ce exhibits maximum PL efficiency, 2.7 times as that of SrS:Ce phosphors. </LI> <LI> Enhanced PL emission in SrCaBaS:Ce can be exploited in many optical applications. </LI> </UL> </P>

Study of the Electronic Structure and Luminescence of Mn-doped SrS Phosphors

Ankush Vij,Sanjeev Gautam,채근화,R. Brajpuriya,S.P. Lochab 한국물리학회 2012 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.61 No.10

In this paper, the electronic structure and the luminescence properties of Mn (1 mole%)-doped SrS phosphors synthesized by using the solid state diffusion method are reported. The X-ray diffraction pattern reveals a single-phase rocksalt structure, and the average crystallite size calculated using a Williamson-Hall plot is 72 nm. The near-edge X-ray absorption fine structure (NEXAFS) collected at Mn L_3,2-edges confirms the presence of divalent Mn in SrS. A bright green photoluminescence (PL) at 547 nm is observed at an excitation wavelength of 355 nm, which may be attributed to the ⁴<i>T</i>₁−⁶<i>A</i>₁ inner atomic transition of the 3<i>d</i> electrons in the Mn²⁺ ion under the cubic crystal field of SrS. We have also studied the thermoluminescence (TL) of UV (254 nm)-irradiated Mn-doped SrS phosphors. The TL glow curve has a well-resolved single peak around 360 K, which shifts slightly to lower temperatures with increasing UV exposure time, confirming second-order kinetics. A kinetic analysis of the experimental TL glow curve has been carried out using Chen’s peak shape method to determine the trapping parameters.

Investigation of phase segregation in sol–gel derived ZnMgO thin films

Singh, Amanpal,Vij, Ankush,Kumar, Dinesh,Khanna, P K,Kumar, Mukesh,Gautam, Sanjeev,Chae, K H Institute of Physics 2013 Semiconductor science and technology Vol.28 No.2

<P>Highly c-axis-oriented Zn<SUB>1-x</SUB>Mg<SUB>x</SUB>O multilayered thin films have been deposited on p-type Si substrates with different concentration of Mg (x = 0.00−0.40) using a sol–gel spin-coating technique. The x-ray diffraction (XRD) shows that single-phase wurtzite thin films start showing phase segregation for a Mg content of x = 0.25 for the sol–gel-derived ZnMgO thin films. The element specific near edge x-ray fine structure (NEXAFS) collected at O K-edge also clearly evidence the phase segregation at x = 0.25. These results also show that films are deposited with wurtzite structure as dominant phase even after phase segregation. The NEXAFS spectra collected at Zn L<SUB>3</SUB>-edge rule out the presence of any Zn-related defect due to Mg doping. The atomic force microscopy (AFM) depicts the spherical shape of nanosized grains, and grain size varies slightly with Mg content. The single-phase ZnMgO thin films show a band gap tuning from 3.38 to 3.84 eV, which is also consistent with blue shifting of near-band edge PL emission. The electrical resistivity of thin films increases with Mg content before phase segregation. However, the optical band gap, photoluminescence and electrical resistivity show anomalous behavior at phase segregation limit which has been discussed and correlated with each other.</P>

Evidence of a moderate refrigerant capacity in cation disordered Ruddlesden-Popper compounds A1.4Sr1.6Mn2O7 (A = La, Pr, Nd) probed with various figures of merit

Kumar Akshay,Vij Ankush,Huh Seok Hwan,Kim Jong-Woo,Sharma Mohit K.,Kumari Kavita,Yadav Naveen,Akram Fazli,구본흔 한국물리학회 2023 Current Applied Physics Vol.49 No.-

We investigated the influence of structural disorders on the magnetic and magnetocaloric (MC) properties upon A-site rare-earth substitution in A1.4Sr1.6Mn2O7 (A = La, Pr, Nd) Ruddlesden-Popper (R–P) compounds. The samples were produced through the solid-state method, and the structural analysis indicated formation of R–P phase with the Jahn-Teller distortion parameter varying from highest in La1.4Sr1.6Mn2O7 (LSMO) to Nd1.4Sr1.6Mn2O7 (NSMO) and followed by Pr1.4Sr1.6Mn2O7 (PSMO). Hence, the magnetization value was increased in LSMO sample implying an enhanced interbilayer and interlayer spin correlations. It resulted in a high value of MC parameters like temperature average entropy change (TEC) and relative cooling power (RCP) of 4.21 J/kgK and 87 J/kg for LSMO, while lowest value of 0.53 J/kgK and 27 J/kg for PSMO respectively, at 2.5 T field. The room temperature MC response in PSMO, whereas a large TEC in LSMO and NSMO compounds advocate their candidacy in broad range of refrigeration applications.

Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO₂ thin films

Aljawfi, Rezq Naji,Vij, Ankush,Chae, K.H,Dalela, S.,Alvi, P.A.,AL-Maghrabi, M.A.,Kumar, Shalendra Elsevier 2018 APPLIED SURFACE SCIENCE - Vol.445 No.-

<P><B>Abstract</B></P> <P>In this report, nanostructure thin films of Ti<SUB>0.97</SUB> Mn<SUB>0.03</SUB> O<SUB>2−δ</SUB> were deposited on Si (001) substrate by pulsed laser deposition (PLD) technique, followed by rapid thermal annealing (RTA) in different ambient gases; O<SUB>2</SUB>, N<SUB>2</SUB> and Ar. The RTA treatment dramatically affected the surface defects, local environment, electronic structure and magnetic properties of the films, where, annealing of the film in O<SUB>2</SUB> gas recovered the dislocated atoms of oxygen vacancy (Vo) at the surfaces and induced a diamagnetic phase, whereas the annealing of film in N<SUB>2</SUB> gas introduced Vo, reduced the valence state of Ti<SUP>4+</SUP> (TiO<SUB>2</SUB>) into Ti<SUP>3+</SUP> (Ti<SUB>2</SUB>O<SUB>3</SUB>) and induced ferromagnetic (FM) signal. Therefore, the change in the local atomic defects of Vo at the surface texture is likely to be responsible for the magnetic response. The presence of oxygen vacancy has been traced by means of Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The NEXAFS spectra were carried out in synchrotron facility at Ti/Mn <I>L</I> <SUB>3,2</SUB> and O-<I>K</I> edges. The spectral feature at Ti <I>L</I> <SUB>3,2</SUB> edges revealed the rutile phase of TiO<SUB>2</SUB>. The asymmetry of <I>t</I> <SUB>2</SUB> <I> <SUB>g</SUB> </I> and <I>e<SUB>g</SUB> </I> bands at the O-<I>K</I> edge confirmed the formation of Vo and reflected the modification in the O co-ordination around Ti<SUP>4+</SUP>cations. The Mn <I>L</I> <SUB>3,2</SUB> edges revealed the incorporation of Mn ions in the TiO<SUB>2</SUB> network with Mn<SUP>2+</SUP> and Mn<SUP>3+</SUP> mixed valence states. This different valance states (Mn<SUP>2+</SUP> and Mn<SUP>3+</SUP>) is the basis for the ferromagnetism induced by Stoner spin-splitting of the local density of defects state at Fermi level (E<SUB>F</SUB>). The change in the magnetic moment of the films after RTA process is related to the change in the local density of defects band. Crystal field and charge transfer parameters were extracted roughly by using atomic multiplet calculations (theory).</P> <P><B>Highlights</B></P> <P> <UL> <LI> Ti<SUB>0.93</SUB>Mn<SUB>0.03</SUB>O<SUB>2</SUB> thin films were deposited on Si substrate (001) by PLD technique. </LI> <LI> The films have been treated via rapid thermal annealing (RTA) in Ar, N<SUB>2</SUB> and O<SUB>2</SUB>. </LI> <LI> A significant modification is induced by RTA in different ambient gas. </LI> <LI> RTA in O<SUB>2</SUB> decreases the oxygen vacancy (Vo) and optimizes the crystal structure. </LI> <LI> N<SUB>2</SUB> induces the Vo and reduces the valance state of TiO<SUB>2</SUB> (Ti<SUP>4+</SUP>) to be Ti<SUB>2</SUB>O<SUB>3</SUB> (Ti<SUP>3+</SUP>). </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Tuning the surface morphology and local atomic structure of Mn-TiO2 thin films using rapid thermal annealing

Aljawfi, Rezq Naji,Kumari, Kavita,Vij, Ankush,Hashim, Mohd.,Chae, K. H.,Alvi, P. A.,Kumar, Shalendra Springer-Verlag 2018 Journal of materials science Materials in electron Vol.29 No.7

<P>In this study, Ti0.95Mn0.05O2-delta nanostructured thin films were fabricated by pulsed laser deposition technique followed by rapid thermal annealing (RTA) in pure O-2 and N-2 atmospheres. The RTA process induced a substantial change in the surfaces morphology and local atomic structure around Ti4+ cation that have been studied by means of atomic force microscopy, Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. Raman spectra of the films were resembled to that of TiO2 rutile phase, and the change in the width of E-g (434 cm(-1)) Raman active modes has been attributed to oxygen non-stoichiometry. NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L (3,2) edges and O-K edge. The ligand-field splitting, estimated from the energy difference between t(2g) and e(g) features in O K-edge spectra were similar to 2.81 eV for pristine and annealed film, which is a characteristic of the TiO2 rutile structure, and the asymmetry of t(2g) and e(g) bands at the O-K edge has been ascribed to oxygen vacancy (Vo(2+)). The annealing of film in O-2 gas optimized the surface structure and healed the Vo(2+) bridging, while the RTA in N-2 gas introduced Vo and reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) that have been probed by comparing the NEXAFS spectra of N-2 annealed film with the reference spectra of Ti2O3. Experimental and atomic multiplet calculations revealed that the Mn ions exist in 2+ valence state.</P>

Synthesis and characterization of ZnGa2O4:Cr3+Ge3+ compounds with tunable photoluminescence properties

Kumar Pankaj,Sharma Aditya,Bishnoi Priyanka,Vij Ankush,Kumar Sandeep,Shin Hyun-Joon,Chae K. H.,Lee B. H.,Won S. O. 한국물리학회 2023 THE JOURNAL OF THE KOREAN PHYSICAL SOCIETY Vol.83 No.6

Pure and doped (Cr and Ge) ZnGa2O4 compounds are prepared using solid-state reaction method. Single-phase nature of pure and doped ZnGa2O4 compounds is investigated using X-ray difraction (XRD). XRD results indicate lower theta angle shifting, decrease in particle size and tensile strain in the doped samples. A granular morphology of amalgamated particles is seen in the SEM images. EDS results convey, practically, the same concentrations (atm%) of doped elements as used during the preparation. X-ray absorption near-edge structure (XANES) results helped to determine the valence state of elements. O K-edge XANES shows a signifcant orbital hybridization of O 2p shells with the d shells of doped atoms (i.e., Cr and Ge). Broad PL bands, of nearly the white-light emission, are observed in all of the samples. Modifcation in the PL intensity and emitted wavelengths, upon doping of Cr3+ and Ge3+ ions, are discussed through the energy transfer mechanisms and defects in the compounds.