Absorption In near-UV Region Research Articles

Synthesis of a carboxylic acid-based ruthenium sensitizer and its applicability towards Dye-Sensitized Solar Cells

This work reports the synthesis of ruthenium based Ru(bpy)2(dcbpy)(ClO4)2[(bpy)2,2′-bipyridine;dcbpy = 4,4′-dicarboxy-2,2′-bipyridine] (RuC) dye and its application in solid and liquid state Dye-Sensitized Solar Cells (DSSCs). Synthesis resulted with high-pure orange coloured dye with a high yield percentage of 45%. The dye was characterized via Nuclear Magnetic Resonance (NMR) spectroscopy, Mass spectroscopy, Cyclic Voltammetry (CV) and UV–vis spectroscopy. The calculated bandgap (Eg) of 2.38 eV from absorbance spectra of pure RuC in ethanol solution showed best proximity with the data obtained from CV measurements. The RuC showed strong absorption in near UV region with the highest molar extinction coefficient (MEC) of 14,746 M−1cm−1 at 463 nm. Plateau of over 80% of External Quantum Efficiency (EQE) spectra reveals the efficient carrier generation of RuC in near UV region. Carboxylic acid groups of RuC provide the potential for enhanced electron transfer from TiO2 surface, and an increased electron density at the interface leads to higher current density. The RuC sensitized solid and liquid state DSSCs exhibited a short circuit current density (JSC) over 3.04 mA/cm2 and 5.82 mA/cm2, and power conversion efficiency (PCE) of 1.2% and 1.8% respectively under simulated Air Mass 1.5 irradiation (100 mWcm−2).

Luminescence enhancement of La4Ti9O24:Eu3+ red-emitting phosphors via cation-substitution of M (Ga, Nb, Ta, Zr, Al) ions

Eu3+- and M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+)-co-doped La4Ti9O24 phosphors were prepared by the solid-state reaction method. The structural phase formation, luminescence properties, and possible LED-lighting application were investigated. The undoped La4Ti9O24 shows an indirect transition with a band-energy of 2.88 ​eV La4Ti9O24 has a broad intrinsic emission band (350–700 ​nm) peaked at 530 ​nm (2.34 ​eV) at 300K. Eu3+-doped La4Ti9O24 presents red luminescence from 5D0→7FJ (J ​= ​0, 1, 2, 3, 4) transitions, which was obviously enhanced with the cation-substitution of M (Ga3+, Nb5+, Ta5+, Zr3+, Al3+) on Ti4+ sites in the lattices. The typical thermal quenching of La3.72Eu0.28Ti8.55M0.45O24 (M ​= ​Al) phosphor is ΔE ​= ​0.342 ​eV. Importantly, the optical excitation of the samples fit well with the commercial LED-chip because of its efficient absorption in near-UV region (395 ​nm) and blue (465 ​nm) wavelength region. The red-LED device packaging with the optimal phosphor was achieved, which has a luminous efficiency of 42 lm/W with a working current of 200 ​mA and a testing voltage of 3.2 ​V. The experiments suggest that the phosphor could be potential for solid-state LED-lighting and display.

Rise in UV and blue emission and reduction of surface roughness due to the presence of Ag and Al in monocrystalline ZnO films grown by sol-gel spin coating

ABSTRACT Highly transparent thin films of aluminium doped ZnO along with specific silver co-doping are deposited by sol-gel spin coating, which show preferred single orientation along (100) plane. Presence of Al dopant reduces, whereas co-doping with Ag improves crystallinity. Films show sharp absorption in near UV region and band gap observes minor shifts. UV and blue emission peaks are seen in the photoluminescence spectra. UV and blue emission peaks, respectively, demonstrate a fairly large i.e. 43% and 77% rise in the normalised intensity. Presence of Ag causes rise in UV peak because of enhanced excitonic recombination probability. Rise of blue emission intensity is ascribed to creation of Ag-Al interstitials. Surface roughness increases to 210.0 nm with Al increment into ZnO and drops drastically to 81.1 nm by Ag co-doping. Boundaries of nanostructures club together with rise in Al amount and thereafter Ag co-doping is seen to reduce the particle size.

Bi3+ induced broad NUV-excitation band in Eu3+-doped red phosphor with scheelite-related structure

The development of Eu3+-doped red phosphors with high absorption in near UV region is of great significance for application in warm white LEDs and remains a tremendous challenge owing to the special electronic structure of rare earth ions. In the present work, a novel BaBi2(MoO4)4:Eu3+ red phosphor were prepared, which exhibits high purity red emission with broad excitation band ranging from 200 to 400 nm, and have a high absorption. Raman analysis and DFT calculations certified that the broad excitation band originated from the characteristic transition absorption of Bi3+ ions. The optimum doping concentration of Eu3+ ions in BaBi2(MoO4)4 was determined to be as high as 50%. The luminescence intensity of BaBi2(MoO4)4:Eu3+ is much higher than that of commercial Y2O3:Eu3+ either under 365 nm or 395 nm excitation. More importantly, the covalence degree has a greater impact on the intensity ratio of electric dipole transition 5D0-7F2 and the magnetic dipole transition 5D0-7F1 (abbreviated as R/O) than the local symmetry around Eu3+ sites. This work presents a strategy to develop high-performance Eu3+-doped red phosphors with high absorption and brightness and provide a reference in exploring other rare earth doped phosphors.

Preparation, band-structure and luminescence of double perovskite Ba2MgMoO6:Eu3+ orange-red phosphor for white LEDs

Novel Eu3+-doped double perovskite Ba2MgMoO6 orange-red phosphors were successfully prepared using EDTA-citric acid complexing method at low calcination temperature for the first time. The XRD, FTIR, SEM, UV–VIS diffuse reflectance and photo-fluorescence spectroscopy were employed to study the structure and luminescence of powders. The first-principle calculations of state density and structure refinement were also performed to get the energy-band structure and structure information. At the low calcination temperature of 1000 °C, ions doped double perovskite powder with homogenous ordering of B-site was obtained. Its structure was an orthogonal perovskite in which BO6 octahedrons were periodically arranged and the space group with small tilting was P21/n. The strong and wide absorption in near-UV region was due to the charge transfer from the O 2p orbital to the Mo 4d orbital in the MoO6 octahedron. The orange-red (593 nm) resulted from the unforbidden magnetic dipole transition dominated the emission. However, the red light (615 nm) resulted from the electric dipole transition was dominated under the excitation of blue light (464 nm) due to the partial release of parity inhibition. With the increase of calcination temperature, the grain size, order degree of B site and particle size of the powder were all increased, and the powder calcined at 1200 °C exhibited the highest luminescence intensity.

Preparation and Luminescence Properties of Dy<sup>3+</sup> Doped Sr<sub>3</sub>Lu(PO<sub>4</sub>)<sub>3</sub> as a New Single Phase White Light Emitting Phosphor

The novel white light emitting phosphors Sr3Lu(1-x)Dyx(PO4)3 (x = 0.02, 0.04, 0.06, 0.08, 0.10) were synthesized via a conventional high-temperature solid-state reaction method at 1250°C. The excitation spectrum indicated that these phosphors had a strong absorption in near UV region in the range from 260 to 460 nm. And under 350 nm excitation, the blue emission at 483nm (4F9/2→6H15/2) and yellow emission at 576 nm (4F9/2→6H13/2) were observed in the phosphors, respectively. We studied the effect of different doping concentrations of Dy3+ activator on the luminescence properties and found that the optimum doping concentration is 6 mol%. According to the Dexter’s theory, this quenching behavior is ascribed to be electric d-d interaction. Furthermore, the chromaticity coordinate (x = 0.28, y = 0.32) of Sr3Lu (PO4)3:0.06Dy3+ phosphor was very close to the “ideal white” (x = 0.33, y = 0.33) in the chromaticity diagram. Our results indicate that the Sr3Lu (PO4)3:Dy3+ phosphor can serve as a promising candidate for single-component white light emitting phosphor under near UV excitation.

Studies on the photoluminescent properties of a single phase white light emitting phosphor CaLa1−xNbMoO8: x Dy3+ for pc-white LED applications

Dy3+ doped CaLa1−xNbMoO8 (x=0.02, 0.025, 0.03, 0.035, 0.05, 0.075) phosphors were synthesized using a high temperature solid state method. Powder XRD results confirm the tetragonal phase and detailed investigations on the photoluminescence properties indicated that Dy3+ doped CaLaNbMoO8 phosphors has a strong absorption in near UV region and two sharp emission peaks in the visible region. The yellow (575nm) and blue emissions (487nm) upon UV excitation resulted from the forced electric dipole transition of 4F9/2→6H13/2 and the magnetic dipole transition of 4F9/2→6H15/2 respectively. The emission colors of all the doped samples are located in the white light region. Therefore, Dy3+ doped CaLaNbMoO8 can serve as a promising candidate for single-phase white-light emitting phosphor under near UV excitation.

Luminescent properties and energy transfer of Sm3+ doped Sr2CaMo1-xWxO6 as a potential phosphor for white LEDs

A series of Sm3+ doped Sr2CaMo1-xWxO6 phosphors were synthesized through solid state reaction process and their luminescent properties have been investigated. They presented excitation bands in UV, near UV and blue regions, originating from charge transfer transitions within Mo(W)O6 or Sm3+ f–f transitions. Concentration quenching still occurs among MoO6 groups even when Mo concentration is very low, however, if there is no Mo content in Sr2CaMo1-xWxO6 phosphors, absorption in near UV region cannot happen. It was found there is a big difference between the calculated optical band gap of Sr2CaMo0.02W0.98O6: 2% Sm3+ and that of Sr2CaMo0.01W0.99O6: 2% Sm3+. Band structure and density of state of Sr2CaMo0.5W0.5O6 were examined to explore the origination of charge transfer transition. Site occupation of Sm3+ in host lattice was discussed based on Raman spectra and group theory analysis. There are two kinds of luminescence centers in Sm3+ doped Sr2CaMo1-xWxO6 (x > 0) phosphors. When 98% Mo were substituted by W, the excitation band at 350 nm by monitoring 608 nm emission in Sr2CaMo0.02W0.98O6: 2%Sm3+ is about 8.5 times stronger than that in Sr2CaMoO6: 2%Sm3+. Energy transfer process was also discussed considering the overlap between the emission spectrum of Mo(W)O6 groups and the absorption spectrum of Sm3+, as well as the atom arrays of Mo6+(W6+), O2− and Sm3+.

Blue to bluish-green tunable phosphor Sr2LiSiO4F:Ce3+,Tb3+ and efficient energy transfer for near-ultraviolet light-emitting diodes

Ce3+ and Tb3+ activated Sr2LiSiO4F phosphors were prepared by a solid state reaction technique at high temperature, and their ultraviolet (UV)-visible spectroscopic properties were investigated. Under ultraviolet light excitation, Ce3+-doped Sr2LiSiO4F phosphors emit blue light (420 nm), while Tb3+-doped phosphors show yellowish green emission. Efficient energy transfer from Ce3+ to Tb3+ ions in co-doped samples was confirmed in terms of corresponding excitation and emission spectra. The energy transfer mechanism between Ce3+ and Tb3+ was discussed and demonstrated to be dipole–dipole interaction in Sr2LiSiO4F:Ce3+,Tb3+ phosphors. Due to energy transfer from Ce3+ to Tb3+, Ce3+ and Tb3+ co-doped Sr2LiSiO4F phosphors show intense absorption in near-UV region, and present tunable emission from blue to bluish green under 360 nm light excitation. The results indicate that these phosphors can be considered as candidates for white LEDs pumped by n-UV chips.

A novel green-to-yellow emitting phosphor: BaSi2SN2.67:Eu2+ for potential application in UV-LEDs

A novel compound BaSi2SN2.67 (BSSN) has been obtained from BaS and Si3N4 by a facile solid state method at 1100°C in NH3 flow. X-ray powder diffraction pattern shows a pure phase of hexagonal BSSN. The lattice parameter a is 0.7504nm and c is 0.6480nm. Neither trace of raw materials or other compounds is presented in the pattern. A novel phosphor Eu2+ doped BSSN shows intense absorption in near UV region indicating potential application in UV-LEDs. It emits green-to-yellow photoluminescence (PL) with maximum at 523nm which shows a large red shift compared to the identified phosphor BaSi2O2N2:Eu2+ (499nm). Additionally, the full width high maximum (FWHM) of the emission band of BSSN:Eu2+ is 70nm that is much larger than that of BaSi2O2N2:Eu2+ (35nm). The dependence of PL intensity of Eu2+ on the doping concentration has been investigated.