Shixun Lian

Towards understanding performance differences between approximate density functionals for spin states of iron complexes

Density functional theory has been widely used to investigate the structural and electronic properties of heme-containing proteins such as cytochrome P450. Nevertheless, recent studies have shown that approximate exchange-correlation energy density functionals can incorrectly predict the stability order of spin states in, for instance, iron-containing pyridine and imidazole systems. This raises questions about the validity of earlier theoretical studies. In this work, we systematically investigate a few typical inorganic and organic iron-containing complexes and try to understand the performance difference of various density functionals. Two oxidation states of iron, Fe(II) and Fe(III), with different spin states and both adiabatic and vertical structures are considered. A different description of the outmost molecular orbital is found to play the crucial role. Local density and generalized gradient based functionals bias the lower spin state and produce a more localized frontier orbital that is higher in energy than the hybrid functionals. Energy component analysis has been performed, together with comparison of numerous structural and electronic properties. Implications of the present work to the theoretical study of heme-containing biological molecules and other spin-related systems are discussed.

Changing Ce3+ Content and Codoping Mn2+ Induced Tunable Emission and Energy Transfer in Ca2.5Sr0.5Al2O6:Ce3+,Mn2+

A series of color-tunable Ce3+ single-doped and Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors were synthesized by a high-temperature solid-state reaction. The crystal structure, luminescent properties, and energy transfer were studied. For Ca2.5Sr0.5Al2O6:Ce3+ phosphors obtained with Al(OH)3 as the raw material, three emission profiles were observed. The peak of photoluminescence (PL) spectra excited at ∼360 nm shifts from 470 to 420 nm, while that of the PL spectra excited at 305 nm stays unchanged at 470 nm with the increase of Ce3+ content. Furthermore, the peak of PL spectra is situated at 500 nm under excitation at ∼400 nm. The relationship between the luminescent properties and crystal structure was studied in detail. Ce3+, Mn2+ codoped Ca2.5Sr0.5Al2O6 phosphors also showed interesting luminescent properties when focused on the PL spectra excited at 365 nm. Obvious different decreasing trends of blue and cyan emission components were observed in Ca2.5Sr0.5Al2O6:0.11Ce3+,xMn2+ phosphors with the increase in Mn2+ content, suggesting different energy transfer efficiencies from blue- and cyan-emitting Ce3+ to Mn2+. Phosphors with high color-rendering index (CRI) values are realized by adjusting the doping content of both Ce3+ and Mn2+. Studies suggest that the Ca2.5Sr0.5Al2O6:Ce3+,Mn2+ phosphor is a promising candidate for near UV-excited w-LEDs.

Composition Screening in Blue-Emitting Li4Sr1+xCa0.97–x(SiO4)2:Ce3+ Phosphors for High Quantum Efficiency and Thermally Stable Photoluminescence

Photoluminescence quantum efficiency (QE) and thermal stability are important for phosphors used in phosphor-converted light-emitting diodes (pc-LEDs). Li4Sr1+xCa0.97-x(SiO4)2:0.03Ce3+ (-0.7 ≤ x ≤ 1.0) phosphors were designed from the initial model of Li4SrCa(SiO4)2:Ce3+, and their single-phased crystal structures were found to be located in the composition range of -0.4 ≤ x ≤ 0.7. Depending on the substitution of Sr2+ for Ca2+ ions, the absolute QE value of blue-emitting composition-optimized Li4Sr1.4Ca0.57(SiO4)2:0.03Ce3+ reaches ∼94%, and the emission intensity at 200 °C remains 95% of that at room temperature. Rietveld refinements and Raman spectral analyses suggest the increase of crystal rigidity, increase of force constant in CeO6, and decrease of vibrational frequency by increasing Sr2+ content, which are responsible for the enhanced quantum efficiency and thermal stability. The present study points to a new strategy for future development of the pc-LEDs phosphors based on local structures correlation via composition screening.

Color-tunable emission in Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphor

In this paper, Ce3+ doped and Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors were synthesized by a high-temperature solid-state reaction. Upon excitation at 360 nm, the emission spectra of Ce3+ doped phosphors exhibit a broad emission band peaking at 392 nm, which originates from the 5d to 4f transition of Ce3+. The Ce3+, Tb3+ co-doped phosphors show strong energy transfer from Ce3+ to Tb3+, and the emission color can be tuned from purplish blue to green by changing the Tb3+ content. The excitation band in the 300–400 nm region broadens when monitored at 541 nm compared to that monitored at 392 nm. Furthermore, the co-doping of Tb3+ facilitates the appearance of green emitting Ce3+, which originates from Ce3+ on the Ca site other than that for purplish-blue Ce3+. The relationship between the luminescence properties of Ce3+ and its coordination environments, namely, different Ca sites, is discussed based on the calculations of centroid shift and crystal field splitting of 5d energy levels of Ce3+. Results suggest that Ce3+, Tb3+ co-doped Ca5(BO3)3F phosphors may be a candidate for near-UV chip based white light-emitting diodes.

Tricolor emitting and energy transfer in the phosphor Ba2ZnSi2O7:Ce3+,Eu3+,Eu2+ for white-LED based near-UV chips

Abstract The red, green and blue (R/G/B) tricolor emitting phosphors Ba 2 ZnSi 2 O 7 co-doped with Ce 3+ and Eu 3+ were synthesized in air atmosphere by a conventional high temperature solid-state reaction technique. All of the excitation spectrum of the phosphor Ba 2 ZnSi 2 O 7 :Ce, Eu showed a strong broad band absorption in the n-UV region whenever monitored by red (630 nm)-emitting or by green (500 nm)- and blue (402 nm)-emitting. Under the excitation of 330 nm, the emission spectrum containing a blue-violet emission band, a green emission and four sharp lines originated from the characteristic transitions of Ce 3+ , Eu 2+ and Eu 3+ ions, of which the relative intensities of the three emission bands could be controlled by the doping concentration of Ce 3+ . The ca. CIE chromaticity coordinates ( x =0.317, y =0.309) of the phosphor Ba 1.94 ZnSi 2 O 7 :0.03Eu,0.01Ce was very close to the standard white ( x =0.33, y =0.33), which suggested that it was a novel single-phased white-light emitting phosphor for LED-based near-UV chip. The mechanisms of energy transfer from Eu 2+ to Eu 3+ via Ce 3+ was also discussed.

A green approach to green-conversion material and green-agriculture: alkaline-earth metal sulfide phosphors

In this paper, a novel intermediate phase transition metathesis strategy is adopted to prepare Eu2+-doped CaS-based materials on the basis of green chemistry. ZnS acts as a vulcanizing agent and contributes to the formation of CaS from CaO through thermal decomposition of an intermediate state of CaZnOS. The recycling of zinc content is realized by collections of zinc evaporation during the process at high temperature. Few atoms would be wasted and no hazardous auxiliaries are used or produced. Meanwhile, solid solubilities of Mg, Sr and Ba in the CaS compound are verified quite different from the previous results in this case. The effect of Sr/Ca ratios on the luminescence properties of (Ca,Sr)S:Eu2+ phosphors is also investigated. A solar energy conversion laminated glass is promoted to overcome the drawback of decomposition of phosphors to moisture and recycling of agriculture films, which provides a promising permanent functional green-to-red sunlight spectrum conversion greenhouse device for green agriculture.

The structural evolution and spectral blue shift of solid solution phosphors Sr3−mCamB2O6:Eu2+

An unexpected spectral blue shift of the emission from yellow to green is observed for the solid solution phosphors Sr3−mCamB2O6:Eu2+ (m = 0–3), when larger Sr2+ ions are substituted by smaller Ca2+ ions in Sr3B2O6 lattices. The powder X-ray patterns of all the phases indicate a structural similarity to the end compounds and show a smooth variation of the structural parameters with composition in the full range. The linear structural evolution of the iso-structural Sr3−mCamB2O6 solid solutions obeying Vegards rule has also been examined and verified by Rietveld refinement, high resolution transmission electron microscopy and Fourier transform infrared spectroscopy. The average bond lengths of the central cations to the coordinating anions and the distortion indexes of the coordination polyhedra are investigated. As a result, the symmetry of the coordination crystal field is found to be higher with increasing Ca content in the solid solutions phosphors, which should be primarily responsible for the continuous blue shift of the emission spectra of Sr2.96−mCamB2O6:0.04Eu2+ with the growth of the m value.

Synthesis, crystal structure and luminescence of a near ultraviolet-green to red spectral converter BaY2S4:Eu2+, Er3+

Based on the idea that modification of light quality could realize the modulation of morphogenesis and growth rate of plants, we fabricate a near ultraviolet (NUV)-green to red spectral converting BaY2S4:Eu2+, Er3+ phosphor by a solid state reaction route. We herein investigated the crystal structure and photoluminescence properties of the compounds in details. The incorporation of Eu2+ ions does not only enhance the red emission of Er3+ tenfold, but also broadens the absorption region of phosphor. Photoluminescence and decay curves confirm the energy transfer from the host lattices and Eu2+ ions to Er3+ ions. The optimized compositions have strong absorption in the NUV-green region and show intense luminescence at 600 and 663 nm, which matches well with the absorption spectrum (red region) of chlorophyll a. On the basis of the experiment results, a possible luminescent mechanism for the red-emitting BaY2S4:Eu2+, Er3+ phosphor is proposed. We believe this new NUV-green to red spectral converter may open a new route to the design of advanced phosphors for agricultural applications.

Near-UV-to-red light conversion through energy transfer in Ca2Sr(PO4)2:Ce3+,Mn2+ for plant growth

A series of Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors were synthesized by a high-temperature solid-state reaction. Under ∼320 nm excitation, the Ce3+ and Mn2+ co-doped phosphors exhibit two emission bands peaking at 370 and 645 nm, which originate from 5d–4f and 3d–3d transitions of Ce3+ and Mn2+, respectively. Luminescence properties, diffuse reflectance spectra, and decay curves indicate that energy transfer (ET) occurs from Ce3+ to Mn2+, and the ET efficiency reaches its maximum (91%) at an Mn2+ content of 0.35. The diffuse reflectance spectrum shows that the co-doped phosphors have strong absorption around 320 nm, which is good for the anti-aging ability of an agricultural film. The absolute quantum efficiency of the Ca1.6Sr(PO4)2:0.15Ce3+,0.10Mn2+,0.15Na+ phosphor is ∼94%. And the co-doped phosphors exhibit good stability in water. Therefore, Ca2Sr(PO4)2:Ce3+,Mn2+,Na+ phosphors have potential application as a light conversion material in agricultural films.

Preparation and Characterization of Red Luminescent Ca0.8Zn0.2TiO3: Pr3+, Na+ Nanophosphor

Abstract Nanophosphor with the nominal composition of Ca 0.8 Zn 0.2 TiO 3 :Pr 3+ , Na + (CZTOPN) was synthesized at relatively low temperature by the sol-gel method. Metal ions were dispersed by citric acid in ethylene glycol solvent and then react with Ti(OC 4 H 9 ) 4 to form sol and gel. The decomposition process of the precursor, and crystallization and particle size of CZTOPN were examined by thermal analysis (TG-DSC), powder X-ray diffraction (XRD), and scan election microscopy (SEM). Results of TG-DSC and XRD reveal that the composition of Ca 0.8 Zn 0.2 TiO 3 :Pr 3+ , Na + changes with the sintering temperature. SEM data indicate that the diameter of particles is under 50 nm even if the sintering temperature increases to 1000 °C. In contrast to a solid state reaction, the excitation spectra of samples synthesized by the sol-gel method shift blue about 10 nm and the emission intensity at 617 nm increases significantly.