Chandramouli Kulshreshtha

Effect of Local Structures on the Luminescence of Li2 ( Sr , Ca , Ba ) SiO4 : Eu2 +

The baricenter and crystal field splitting of the 5d transition of the Eu 2+ activator were approximated from the experimental spectrum data for Li 2 SrSiO 4 , Li 2 CaSiO 4 , and Li 2 BaSiO 4 hosts and examined in conjunction with the ionic radii of neighboring alkali earth ions and the local structure around the divalent europium activator. In addition, Li2(Sr,Ca,Ba)SiO 4 :Eu 2+ ternary phosphors were screened in terms of photoluminescence (PL) intensity and color chromaticity at an excitation of 400 nm to determine their suitability for use in white-light-emitting diodes. As a result, several intermediate compounds (mixtures or solid solutions) were pinpointed in a specific composition range in the Li 2 (Sr,Ca,Ba)SiO 4 :Eu 2+ ternary composition library. In particular, Li2(Ba 1-x Sr x )SiO 4 :Eu 2+ (0.3 < x < 0.6) showed the best performance in terms of both PL intensity and color chromaticity.

Photoluminescent and decay behaviors of Mn2+ and Ce3+ coactivated MgSiN2 phosphors for use in light-emitting-diode applications.

We examined the photoluminescent behaviors of MgSiN2:Mn2+ and MgSiN2:Ce3+,Mn2+ phosphors for use in white-light-emitting diodes. The red emission from MgSiN2:Mn2+ phosphors consisted of two Gaussian components, P1 from a single Mn2+ ion and P2 from either Mn2+ pairs or clusters. Decay analysis based on the Yokota and Tanimoto equation identified long decay for P1 and fast decay for P2. Most importantly, Ce3+ codoping enhanced Mn2+ emission intensity; in particular, emission at 460 nm excitations was promoted by the Ce3+ codoping.

Decay Behavior of Li2 ( Sr , Ba , Ca ) SiO4 : Eu2 + Phosphors

The decay behavior of Li 2 SrSiO 4 :Eu 2+ , Li 2 CaSiO 4 :Eu 2+ , and Li 2 BaSiO 4 :Eu 2+ phosphors, which exhibit similar stoichiometries but different structures, differed significantly, even at the same activator concentration. Such differences in decay behavior cannot be elucidated by the traditional energy-transfer model based on homogeneous distribution of activators in the host. In this regard, we examined the decay curves of Li 2 SrSiO 4 :Eu 2+ , Li 2 CaSiO 4 :Eu 2+ , and Li 2 BaSiO 4 :Eu 2+ phosphors based on the angular class model. It was found that the decay behavior was closely related to the local structure around the activator, indicating that the higher the site symmetry the more active the concentration quenching.

Deep red color emission in an Sm2+-doped SrB4O7 phosphor

The main objective of the present investigation was to increase the photoluminescence and color chromaticity of Sm 2+ -doped Sr-borate phosphors by adding co-dopants. Among the co-dopants used, which included Ce, Mn, Eu, Mg, Zn, Ca, and Ba, the co-doping of europium and samarium was effective in improving the luminescence and color chromaticity of Sr 1-x Sm x B 4 O 7 under excitation at 254 nm. The photoluminescence spectrum of the Sr 1-x Sm x B 4 O 7 phosphor showed a peak corresponding to Sm 3+ ions at 595 nm, and another for Sm 2+ ions at 686 nm under 254 nm excitation even after a strong reduction process. Eu 2+ ion co-doping increased the intensity of the Sm 2+ peak and eliminated the Sm 3+ peak. Thus, the europium co-doping shifted the luminescence to a deep red region. In addition, energy transfer between Eu 2+ and Sm 2+ was also investigated in detail.

Search for new red phosphors using genetic algorithm-assisted combinatorial chemistry.

A genetic algorithm was employed in association with high-throughput synthesis and characterization in an attempt to search for red phosphors with high photoluminescent intensity. A tetravalent manganese-doped alkali earth germanium oxide system, with an emission color close to a desirable deep red, was screened with the assistance of a genetic algorithm to pinpoint the phosphor exhibiting the highest photoluminescence. As the genetic algorithm was in progress, the PL intensity increased and maximized in the fourth generation. The highest and the average PL intensity of the fourth generation improved by 23 and 120%, respectively, compared with that of the first generation.

Lithium barium silicate, Li2BaSiO4, from synchrotron powder data

The structure of lithium barium silicate, Li(2)BaSiO(4), has been determined from synchrotron radiation powder data. The title compound was synthesized by high-temperature solid-state reaction and crystallizes in the hexagonal space group P6(3)cm. It contains two Li atoms, one Ba atom (both site symmetry ..m on special position 6c), two Si atoms [on special positions 4b (site symmetry 3..) and 2a (site symmetry 3.m)] and four O atoms (one on general position 12d, and three on special positions 6c, 4b and 2a). The basic units of the structure are (Li(6)SiO(13))(5-) units, each comprising seven tetrahedra sharing edges and vertices. These basic units are connected by sharing corners parallel to [001] and through sharing (SiO(4))(4-) tetrahedra in (001). The relationship between the structures and luminescence properties of Li(2)SrSiO(4), Li(2)CaSiO(4) and the title compound is discussed.

Systematic Control of Experimental Inconsistency in Combinatorial Materials Science

We developed a method to systematically control experimental inconsistency, which is one of the most troublesome and difficult problems in high-throughput combinatorial experiments. The topic of experimental inconsistency is never addressed, even though all scientists in the field of combinatorial materials science face this very serious problem. Experimental inconsistency and material property were selected as dual objective functions that were simultaneously optimized. Specifically, in an attempt to search for promising phosphors with high reproducibility, photoluminescence (PL) intensity was maximized, and experimental inconsistency was minimized by employing a multiobjective evolutionary optimization-assisted combinatorial materials search (MOEO combinatorial material search) strategy. A tetravalent manganese-doped alkali earth germanium/titanium oxide system was used as a model system to be screened using MOEO combinatorial materials search. As a result of MOEO reiteration, we identified a halide-detached deep red phosphor with improved PL intensity and reliable reproducibility.

Pulsed Laser Deposition Growth of CaMgSi2O6 : Eu2 + Thin Film Phosphors

CaMgSi 2 O 6 :Eu 2+ (CMS) phosphors, which are comparable to BaMgAl 10 O 17 :Eu 2+ phosphors, were prepared in the form of thin films deposited on quartz glass using a pulsed laser deposition technique. The luminescent and structural properties of the CMS thin film phosphors were monitored as a function of key processing parameters such as, the partial pressure of oxygen inside the deposition chamber, laser energy density, and the type of post-deposition treatments used. By optimizing these processing parameters, CMS thin films with large homogenous areas and a high photoluminescence could be produced.

Preparation of blue emitting BaMgAl10O17:Eu2+ thin-film phosphors by pulsed laser deposition

BaMgAl 10 O 17 :Eu 2+ (BAM) phosphors in the form of thin films have not yet been realized by physical vapor deposition methodologies due to technical difficulties. We report here on the preparation of thin-film-type BAM phosphors using a pulsed laser deposition technique. The luminescent and structural properties of BAM thin-film phosphors were monitored as a function of key processing parameters such as oxygen partial pressure, deposition time, laser energy density, and the type of postdeposition treatments used. By optimizing these processing parameters, BAM thin films with large homogenous areas (∼ 150 mm 2 ) and an acceptable photoluminescence could be produced.