Shift Of Charge Transfer Band Research Articles

The coloration of smalt produced at Ust-Ruditsa glass factory from 1753 to 1768

The Ust-Ruditsa glass factory, founded in 1753 by the Russian scientist Mikhail Lomonosov has produced a variety of smalt glasses for mosaics and bijouterie. Lomonosov, which was the glass-technologist of the factory, by using a limited number of glass colorants elaborated the glass receipts with the aim to obtain colored glasses. At that site several hundred colored glasses were prepared, investigated and employed. We report and discuss here the use of copper as colorant in the preparation of glasses having the same composition of selected Ust-Ruditsa glasses. A series of K2O-PbO-SiO2 glasses (KPS) doped with copper 9.5K2O·xPbO·(90-x)SiO2·0.5CuO (x = 10.1, 19.0, 27.0, 38.0, 47.3, 52.1, 56.8, 61.4 % in mass) has been prepared by melt-quenching technique. The Brillouin and Rayleigh scattering was measured with a 532 nm laser by using the calculated glasses refraction indexes. We found that the color changes due to the bathochromic shift of charge transfer band with increasing lead concentration. The copper center coordination environment is always a distorted octahedron. Upon reduction of copper by carbon the melt glass changes to an opaque orange smalt coloration due to the presence of Cu2O dendrimers. The calculation of Landau-Placzek ratio shows a non-monotonic change with increasing lead concentration. The nano-heterogeneous structure of glasses, characterized by an elevated Landau-Placzek ratio, is confirmed by TEM.

Electronic structure engineering of Gd2.97Tb0.03Ga5−xAlxO12 persistent luminescence phosphors

Gd2.97Tb0.03Ga5−xAlxO12 powders with different Ga:Al ratio were prepared using modified Pechini method. Changing Ga:Al ratio opens the way to modulate the energy gap, and so, to modify the spectroscopic properties of the phosphors. Al3+ ions stabilize the metastable Gd3Ga5O12 garnet structure widening the energy gap. With increasing Ga content in the garnet structure, the energy gap between an excited 5d level of Tb3+ ions and the conduction band (CB) decreases, which leads to an overlap of the conduction band with the 5d band suppressing the energy transfer between the 5d band and the 4f levels of Tb3+ ions. The band gap changes lead to the blue shift of charge transfer band due to an expansion of the unit cell and decreased interaction between oxygen ligands and luminescent ions. The collected spectroscopic data show significant effect of crystal field strength of the host matrix on both conventional and persistent luminescence. From the resultant data the influence of Ga:Al ratio on the luminescence properties and trapping/releasing of electrons is discussed. The mechanism of Tb3+ persistent luminescence in Gd2.97Tb0.03Ga5−xAlxO12 is proposed.

Photoluminescence, optical transition properties and temperature-induced shift of charge transfer band and temperature sensing property of GdNbTiO6: Sm3+ phosphors

GdNbTiO6: Sm3+ phosphors with various Sm3+ concentrations were prepared via a high temperature solid-state reaction method. The crystal structure of the samples was characterized by means of X-ray diffraction (XRD) and the as-prepared samples were confirmed to be orthorhombic phase GdNbTiO6. Photoluminescence properties were investigated by measuring the concentration- and temperature-dependent photoluminescence spectra. Concentration-dependent luminescence quenching and luminescent thermal quenching behaviors were observed and they were respectively ascribed to the electric dipole-dipole interaction between Sm3+ ions and the cooperation of energy transfer and crossover process. The chromatic characteristics were found to be dependent on the excitation wavelength and Sm3+ concentration. In addition, temperature-induced redshift of charge transfer band of GdNbTiO6 host was found in temperature-dependent excitation spectra and the opposite variations of different excitation peaks were utilized for optical thermometry. Finally, the optical transition property was studied on the basis of the diffuse reflectance spectra and Judd-Ofelt (J-O) theory, meanwhile, its accuracy was evaluated by the result of emission spectra.

Molybdenum substitution simultaneously induced band structure modulation and luminescence enhancement in LiLaMg(W, Mo)O6: Eu3+ red-emitting phosphor for near ultraviolet excited white light diodes

In this study, LiLaMgW1-xMoxO6: Eu3+ phosphors have been synthesized by high-temperature solid-state reaction method. By changing the WO charge transfer (CT) band through Mo6+ substitution, the excitation band of obtained phosphors had been widened largely and could match well with the InGaN-based LED chips. The crystal structure, diffuse reflection spectra, photoluminescent properties of the LiLaMgW1-xMoxO6: 0.1Eu3+ phosphors were investigated as a function of W/Mo ratio. The red shift of charge transfer band (CTB) was observed with the increase of Mo6+ concentration. Density functional theory (DFT) were performed to calculate the band structure in order to support the excitation band modulation. Moreover, LiLa1-yMgW0.7Mo0.3O6:yEu3+ phosphors exhibit strong red emission. The excitation, emission spectra, decay dynamics and thermal stability of the phosphors had been measured and investigated in detail. The quenching concentration of Eu3+ is up to 20 mol% and the concentration quenching is attributed to the dipole-dipole interaction. Finally, a white-LED device was fabricated by integrating a mixture of optimal red phosphor, commercial green- and blue-emitting phosphors with a near-ultraviolet (UV) InGaN-based LED chip. The results suggest that the Eu3+-activated LiLaMgW0.7Mo0.3O6 phosphor is a promising red-emitting phosphor for white-LEDs.

Luminescence properties of Eu3+ doped La3Ga5GeO14 and effect of Bi3+ co-doping

Red-emitting phosphors La3Ga5GeO14: Eu3+, Bi3+ were prepared by high temperature solid state reactions. The structure and luminescence properties of these phosphors were investigated in detail. The X-ray diffraction study shows the phase formation of La3Ga5GeO14: Eu3+, Bi3+ phosphors. Scanning electronic microscope observation proves that the size of obtained samples is approximately 1–2µm. The band gap of La3Ga5GeO14 is calculated to be 5.234eV based on obtained diffuse reflectance spectrum. Under the near ultraviolet excitation, Eu3+ doped samples can emit bright red light corresponding to the electric dipole transition (5D0→7F2 transition). The dipole–dipole interaction is dominating mechanism for the concentration quenching. The introduction of Bi3+ enhances the luminescence intensity of Eu3+. It also results in the red shift of charge transfer band of O2-→Eu3+. The effect of the co-doping of Bi3+ on the luminescence properties of Eu3+ is discussed in detail. The CIE chromaticity coordinates of typical La3Ga5GeO14: Eu3+, Bi3+ phosphor is close to red-emitting phosphor Y2O2S: Eu3+. Present research indicates that La3Ga5GeO14: Eu3+, Bi3+ is a promising candidate of red-emitting phosphor for white light-emitting diodes.

Luminescence properties of AgTb(WO4)2 doped with Ce3+: Experimental determination of the Stern-Volmer quenching constant

Scheelite-like AgTb(WO4)2 doped Ce3+ were synthesized by ceramic method. Efficient characteristic bands of Tb3+ were observed in AgTb(WO4)2 compounds. However, the presence of Ce3+ quenched the emission band of Tb3+ which can be caused by metal-to-metal charge transfer between Tb3+/Ce3+. The quenching is more efficient at high temperature which can be attributed to more overlaps of wave functions of Ce3+-W6+-Tb3+ at high temperature. The experimental Stern-Volmer quenching rate constant at room temperature and liquid nitrogen temperature were measured as (376±4)·10−3μs−1 (x content/mol)−1 and (45±4)·10−3μs−1 (x content/mol)−1 respectively. In excitation spectra, evident blue shift of charge transfer band in the case of liquid nitrogen temperature was observed. In addition, the blue shift of CT band also occurred with increasing concentration of Ce3+. The shifting of charge transfer band can be interesting to the design of solar cells and tunable excitation sources and detectors.

Luminescence dynamics and concentration quenching in Gd2−xEuxO3 nanophosphor

Gd2−xEuxO3 nanophosphor with enhanced luminescence has been synthesized by solution combustion method in diethylene glycol medium. The effect of Eu3+ content on the structure, morphology and luminescence dynamics has been investigated. The observed red shift in charge transfer band of the nanophosphor with increase in Eu3+content is attributed to the increase in covalency and Eu–O bond length. Excess europium content leads to reduction in intensity of photoemission as a result of concentration quenching. Transient characteristics exhibit single exponential behaviour and the fitted life time values get shortened with increase of Eu3+ content, due to increase in non-radiative transition rate. The observed quenching of luminescence in the phosphor is found to be in agreement with the energy transfer theory proposed by Dexter and Schulman and found that dipole–dipole interaction between Eu3+ ions is the key mechanism responsible for quenching. Critical energy transfer distance between Eu3+ ions in Gd2−xEuxO3 is determined to be 1.148nm. Judd–Ofelt intensity parameters and the radiative parameters of the phosphors were evaluated from the emission spectrum to analyse the crystal field environment and the degree of covalency between the rare-earth ion and the surrounding ligands. The value of Ω2 intensity parameter confirmed the hypersensitive nature of the 5D0–7F2 transition with larger asymmetry around Eu3+ ions.

Synthesis and luminescence of LaPO4:Eu nanoparticles from different phosphate sources

LaPO4:Eu nanoparticles were synthesized from different phosphate sources by the method of precipitation. The influences of phosphate sources on properties of LaPO4:Eu nanoparticles were studied. The LaPO4:Eu sample synthesized from (NH4)2HPO4 has the uniform morphology and good dispersion. The excitation spectra show the Eu–O charge-transfer band and f–f orbital transitions of Eu3+. The strong absorption of charge transfer band (CTB) can be measured when H3PO4 is used as the phosphate source. However, there is a red shift of CTB when (NH4)2HPO4 and Na5P3O10 are used as phosphate sources, which indicates the increase in O2−–Eu3+ distance and the decrease in charge transfer energy. The emission spectra show different emission transitions originating from 5D0 state (5D0 → 7Fj) of Eu3+. The life times of the Eu3+ ions are determined to be 2.176, 4.576, and 5.608 ms for the samples synthesized from H3PO4, Na5P3O10 and (NH4)2HPO4, respectively.

Synthesis and optical properties of Ce0.95Pr0.05−xMxO2 (M = Mn, Si) as potential ecological red pigments for coloration of plastics

Ecological red pigments Ce0.95Pr0.05−xMxO2 (M=Mn, Si) have been synthesized by conventional solid-state route and characterized by X-ray diffractometer, scanning electron microscope and UV–vis spectroscopy. Mn4+/Si4+ was incorporated into the CeO2–PrO2 system to tune the color properties of the pigments by shifting the optical absorption edge. Si4+ substitution blue shifts the absorption edge of Pr-doped ceria and shows bright reddish brown color. Mn4+ substitution stabilizes the absorption edge and exhibits dark brown hue. The coloring mechanism is based on the shift of charge transfer band of CeO2 to higher wavelength by co-substitution of Pr4+ and tetravalent metal ions in ceria. Si co-doped pigments possess smaller particles and hence exhibit more lightness compared to Mn co-doped samples. The reddish brown pigments exhibit very good coloring performance in polymer matrix. These Ce0.95Pr0.05−xMxO2 (M=Mn, Si) pigments have potential to be used as ecological red pigments for coloration of plastics.

Low Temperature Combustion Synthesis and Characterization of Pr-CeO<sub>2</sub> Doped with Zr Red Ceramic Pigments

New inorganic pigments Ce0.95ZrxPr0.05-xO2(x = 0, 0.01, 0.02, 0.03, 0.04, 0.05) based on CeO2-ZrO2-Pr6O11solid solutions were synthesized by low-temperature combustion synthesis (LCS) method using citric acid as a reductant and metal nitrate as an oxidant. The pigments are CeO2solid solutions doped with praseodymium and zirconium ions, and display colors ranging from dark brown via brick red to bright cream. The prepared pigments were characterized by X-ray powder diffraction, transmission electron microscopy, ultraviolet-visible absorption spectroscopy and colorimetrical measurements. Results show that cubic fluorite structure solid solution of 100 nm Ce0.95ZrxPr0.05-xO2particles can be obtained at 1000 °C. There was an effective absorption of visible blue and green in 600 nm, which is originated from the shift of charge transfer band in CeO2by doping of zirconium and praseodymium ions. Effective red hue was obtained for Ce0.95Zr0.02Pr0.03O2composition, which exhibited better L*, a*,and b* values (57.64, 23.83 and 23.78). The coloring mechanism is based on the introduction of an additional electronic energy level of unpaired 4f electron of the lanthanide ion into the cerianite forbidden band. Furthermore, the doping of Zr in Pr-CeO2system raises the slope of the absorption edge with a blue shift resulting in enhanced lightness. The prepared pigments were prospective to be alternatives for toxic pigments used in coloration of plastics.

Environment-friendly red pigments from CeO2–Fe2O3–Pr6O11 solid solutions

New inorganic pigments based on CeO2–Fe2O3–Pr6O11 solid solutions were synthesized and their color properties were assessed from the view point of possible environmentally benign nontoxic red pigments. A wide range of compositions, Ce0.9FexPr0.1−xO2−δ (x ranges from 0.02 to 0.095) and Ce0.8FexPr0.2−xO2−δ (x ranges from 0.04 to 0.19) were prepared and characterized by powder X-ray diffraction, lattice parameter measurements, electron microscopy (SEM/EDAX), UV–vis spectroscopy and L*, a*, b* color measurements. The prepared solid solutions absorb the visible blue and green under 600 nm efficiently, which is originated from the shift of charge transfer band in CeO2 by co-substitution of iron and praseodymium for cerium. The optical absorption edge of these pigments depends on the Pr and Fe content and was found to shift their absorption edge towards the lower energy or higher energy side as the co-substitution of praseodymium increases or iron increases for cerium with the band gap changing from 1.81 to 1.97 eV as a result the pigments show brick red to dark brown. Effective red hue was obtained for Ce0.9Fe0.04Pr0.06O1.95 and Ce0.8Fe0.08Pr0.12O1.9 compositions, which exhibit better L*, a*, and b* values (55, 18.30, and 21.55) and (43, 18.67, and 17.40), respectively. In addition, they present a much lower toxicity than cadmium sulfoselenide based red pigments, because they are composed of non-toxic and safe elements.

The photoluminescence properties of Ba 0.95− xM xB 8O 13:Eu 3+ (M = Ca, Sr, 0 ≤ x ≤ 0.1) in 100–400 nm regions

The phosphors Ba 0.95 − x M x B 8O 13:Eu 3+ (M = Ca, Sr, 0 ≤ x ≤ 0.1) were synthesized by solid-state reaction. Their luminescent properties were studied under ultraviolet (UV) and vacuum ultraviolet (VUV) regions. The excitation spectrum of Ba 0.95Eu 0.05B 8O 13 showed that the charge transfer (CT) excitation band of Eu 3+ at about 246 nm was dominated in the UV region, while two weak broad bands of the host lattice absorption with maxima at 157 and 183 nm were observed in the VUV region. The band of Ba–O absorption was overlapped with the CT band of Eu 3+, which is indicated that the energy of the host lattice absorption can be efficiently transfer to the Eu 3+. With the incorporation of Sr 2+ into BaB 8O 13:Eu 3+ lattice, the CT band of Eu 3+ as well as the band absorption of the host lattice were enhanced. For Ca 2+ co-doped BaB 8O 13:Eu 3+, the excitation intensity was decreased, as well as the CT band showed a little blue-shift. The intense red emission located at 612 nm and the improved luminescent intensity of Eu 3+ were observed when Ba 2+ was replaced by Sr 2+ partially in the BaB 8O 13:Eu 3+ lattice, while the incorporation of Ca 2+ ions do not show efficient luminescence and the main emission line shift to 588 nm. The shift in CT band of Eu 3+ could be explained by the cation structures of borate have an influence on the electron transfer and the energy of the CT band of Eu 3+ in the host lattice.

Interaction of DNA with [Cr(Schiff base)(H 2O) 2]ClO 4

The binding of Schiff base complexes of chromium(III) of the type [Cr(salen)(H 2O) 2] + and [Cr(salprn)(H 2O) 2] +, where salen denotes 1,2-bis(salicylideneamino)ethane and salprn denotes 1,3-bis(salicylideneamino)propane to calf thymus DNA has been investigated by absorption, emission, circular dichroism, melting temperature and viscosity measurements. These chromium(III) complexes showed absorption hyperchromicity accompanied by red shift in charge transfer band, fluorescence enhancement, increase in melting temperature, some structural changes in CD spectra and changes in specific viscosity when bound to calf thymus DNA. The binding constant K b has been determined from absorption measurements for both the complexes and found to be (2.5±0.4)×10 3 M −1 for [Cr(salen)(H 2O) 2] + and (1.7±0.3)×10 4 M −1 for [Cr(salprn)(H 2O) 2] +. From the binding stoichiometry of DNA-[Cr(salprn)(H 2O) 2] +, the number of binding site size has been determined and found to be ten base pairs per bound complex molecule. The chromium(III) complexes also bring about single strand cleavage in plasmid DNA. The experimental results show that the chromium(III) complexes bind to DNA by non-intercalative mode. Major groove binding is the preferred mode of interaction for these Schiff base complexes of chromium(III).