Minghao Fang

New Yellow-Emitting Whitlockite-type Structure Sr1.75Ca1.25(PO4)2:Eu2+ Phosphor for Near-UV Pumped White Light-Emitting Devices

New compound discovery is of interest in the field of inorganic solid-state chemistry. In this work, a whitlockite-type structure Sr1.75Ca1.25(PO4)2 newly found by composition design in the Sr3(PO4)2-Ca3(PO4)2 join was reported. Crystal structure and luminescence properties of Sr1.75Ca1.25(PO4)2:Eu(2+) were investigated, and the yellow-emitting phosphor was further employed in fabricating near-ultraviolet-pumped white light-emitting diodes (w-LEDs). The structure and crystallographic site occupancy of Eu(2+) in the host were identified via X-ray powder diffraction refinement using Rietveld method. The Sr1.75Ca1.25(PO4)2:Eu(2+) phosphors absorb in the UV-vis spectral region of 250-430 nm and exhibit an intense asymmetric broadband emission peaking at 518 nm under λex = 365 nm which is ascribed to the 5d-4f allowed transition of Eu(2+). The luminescence properties and mechanism are also investigated as a function of Eu(2+) concentration. A white LED device which is obtained by combining a 370 nm UV chip with commercial blue phosphor and the present yellow phosphor has been fabricated and exhibit good application properties.

Crystal structure and Temperature-Dependent Luminescence Characteristics of KMg4(PO4)3:Eu2+ phosphor for White Light-emitting diodes

The KMg4(PO4)3:Eu2+ phosphor was prepared by the conventional high temperature solid-state reaction. The crystal structure, luminescence and reflectance spectra, thermal stability, quantum efficiency and the application for N-UV LED were studied respectively. The phase formation and crystal structure of KMg4(PO4)3:Eu2+ were confirmed from the powder X-ray diffraction and the Rietveld refinement. The concentration quenching of Eu2+ in the KMg4(PO4)3 host was determined to be 1mol% and the quenching mechanism was certified to be the dipole–dipole interaction. The energy transfer critical distance of as-prepared phosphor was calculated to be about 35.84Å. Furthermore, the phosphor exhibited good thermal stability and the corresponding activation energy ΔE was reckoned to be 0.24eV. Upon excitation at 365nm, the internal quantum efficiency of the optimized KMg4(PO4)3:Eu2+ was estimated to be 50.44%. The white N-UV LEDs was fabricated via KMg4(PO4)3:Eu2+, green-emitting (Ba,Sr)2SiO4:Eu2+, and red-emitting CaAlSiN3:Eu2+ phosphors with a near-UV chip. The excellent color rendering index (Ra = 96) at a correlated color temperature (5227.08K) with CIE coordinates of x = 0.34, y = 0.35 of the WLED device indicates that KMg4(PO4)3:Eu2+ is a promising blue-emitting phosphor for white N-UV light emitting diodes (LEDs).

Luminescence properties and energy transfer of Eu/Mn-coactivated Mg2Al4Si5O18 as a potential phosphor for white-light LEDs.

A series of blue-to-white emitting Mg2Al4Si5O18: Eu(2+), Mn(2+) phosphors were synthesized via high-temperature solid-state method, and their luminescence properties were investigated in detail. Under near-ultraviolet (UV) light excitation of 365 nm, Eu(2+)-doped Mg2Al4Si5O18 exhibits a broad blue emission band peaked at 469 nm, and Mn(2+)-doped Mg2Al4Si5O18 shows a broad orange-red emission band near 600 nm. The energy transfer from Eu(2+) to Mn(2+) in Mg2Al4Si5O18 host matrix can be found, and the resonant type is demonstrated by a dipole-quadrupole mechanism. The emission hue can be tuned from blue (0.17, 0.17) to bluish green (0.22, 0.29) and finally to white (0.31, 0.33) by properly varying the ratio of Eu(2+)/Mn(2+). The thermal quenching property of the sample was investigated, and the activation energy ΔE was estimated to be 0.30 eV. Additionally, the energy transfer critical distance between Eu(2+) and Mn(2+) was calculated. With appropriate tuning of activator content, the Mg2Al4Si5O18: Eu(2+), Mn(2+) phosphor may have potential application for UV light-emitting diodes.

Energy Transfer from Sm3+ to Eu3+ in Red-Emitting Phosphor LaMgAl11O19:Sm3+, Eu3+ for Solar Cells and Near-Ultraviolet White Light-Emitting Diodes

The red-emitting phosphor LaMgAl11O19:Sm(3+), Eu(3+) was prepared by solid-state reaction at 1600 °C for 4 h. The phase formation, luminescence properties, and energy transfer from Sm(3+) to Eu(3+) were studied. With the addition of 5 mol % Sm(3+) as the sensitizer, the excitation wavelength of LaMgAl11O19:Eu(3+) phosphor was extended from 464 to 403 nm, and the emission intensity under the excitation at 403 nm was also enhanced. The host material LaMgAl11O19 could contain the high doping content of Eu(3+) (20 mol %) without concentration quenching. This energy transfer from Sm(3+) to Eu(3+) was confirmed by the decay times of energy donor Sm(3+). The mechanism of energy transfer (Sm(3+) → Eu(3+)) was proved to be quadrupole-quadrupole interaction. Under the 403 nm excitation at 150 °C, the emission intensities of the characteristic peaks of Sm(3+) and Eu(3+) in LaMgAl11O19:0.05Sm(3+), 0.2Eu(3+) phosphor were decreased to 65% and 56% of the initial intensities at room temperature, and the relatively high activation energy proved that this phosphor had a good thermal stability. The CIE coordinate was calculated to be (x = 0.601, y = 0.390). The LaMgAl11O19:0.05Sm(3+), 0.2Eu(3+) phosphor is a candidate for copper phthalocyanine-based solar cells and white light-emitting diodes.

Enhanced thermal properties of novel shape-stabilized PEG composite phase change materials with radial mesoporous silica sphere for thermal energy storage.

Radial mesoporous silica (RMS) sphere was tailor-made for further applications in producing shape-stabilized composite phase change materials (ss-CPCMs) through a facile self-assembly process using CTAB as the main template and TEOS as SiO2 precursor. Novel ss-CPCMs composed of polyethylene glycol (PEG) and RMS were prepared through vacuum impregnating method. Various techniques were employed to characterize the structural and thermal properties of the ss-CPCMs. The DSC results indicated that the PEG/RMS ss-CPCM was a promising candidate for building thermal energy storage applications due to its large latent heat, suitable phase change temperature, good thermal reliability, as well as the excellent chemical compatibility and thermal stability. Importantly, the possible formation mechanisms of both RMS sphere and PEG/RMS composite have also been proposed. The results also indicated that the properties of the PEG/RMS ss-CPCMs are influenced by the adsorption limitation of the PEG molecule from RMS sphere with mesoporous structure and the effect of RMS, as the impurities, on the perfect crystallization of PEG.

Fe-catalyzed growth of one-dimensional α-Si3N4 nanostructures and their cathodoluminescence properties.

Preparation of nanomaterials with various morphologies and exploiting their novel physical properties are of vital importance in nanoscientific field. Similarly to the III-N compound semiconductors, Si3N4 nanostructures also could be potentially used for making optoelectronic devices. In this paper, we report on an improved Fe-catalyzed chemical vapour deposition method for synthesizing ultra-long α-Si3N4 nanobelts along with a few nanowires and nanobranches on a carbon felt substrate. The ultra-long α-Si3N4 nanobelts grew via a combined VLS-base and nanobranches via a combined double-stage VLS-base and VS-tip mechanism, as well as nanowires via VLS-tip mechanism. The three individual nanostructures showed variant optical properties as revealed by a cathodoluminescence spectroscopy. A single α-Si3N4 nanobelt or nanobranch gave a strong UV-blue emission band as well as a broad red emission, whereas a single α-Si3N4 nanowire exhibited only a broad UV-blue emission. The results reported would be useful in developing new photoelectric nanodevices with tailorable or tunable properties.

Emission red shift and energy transfer behavior of color-tunable KMg4(PO4)3:Eu2+,Mn2+ phosphors

Eu2+- and Mn2+-co-doped KMg4(PO4)3 phosphors were prepared via conventional high temperature solid-state reactions. Their crystal structures, luminescence properties, emission red shifts, and energy transfer between Eu2+ and Mn2+ were investigated systematically. Under excitation at 365 nm, KMg4(PO4)3:Eu2+,Mn2+ phosphors exhibited a broad excitation band ranging from 250 to 425 nm and two broad emission bands that peaked at 450 nm and 625 nm, which were ascribed to the 4f–5d transition of Eu2+ and the 4T1 → 6A1 transition of Mn2+ ions, respectively. Three emission bands of Mn2+ were observed in KMg4(PO4)3: Eu2+,Mn2+, which can be attributed to the disordering of Mn2+ in the Mg2+ sites to form different luminescence centers. The energy transfer between the Eu2+ and Mn2+ ions is of a resonant type via a dipole–quadrupole mechanism. The emission red shift that takes place with increasing Mn2+ concentration and operating temperature are discussed in relation to the crystal structure and energy transfer in KMg4(PO4)3:Eu2+,Mn2+. Utilizing the redshift and the energy transfer from Eu2+ to Mn2+, KMg4(PO4)3:Eu2+,Mn2+ phosphors can be tuned from blue to pink by appropriate adjustment of the Mn2+ content and may have potential application for white light-emitting diodes and plantlet culturing.

Growth of α-Si3N4 nanobelts via Ni-catalyzed thermal chemical vapour deposition and their violet-blue luminescent properties

α-Si3N4 nanobelts were grown on a graphitic carbon felt via an improved Ni-catalyzed chemical vapor deposition (CVD) process. The as-prepared nanobelts were up to several millimeters long and 300–1200 nm wide exhibiting a unimodal diameter distribution with peak range at 500–600 nm. Ni originally mixed with Si partially evaporated and then condensed on the carbon felt surface, forming catalytically active centers which absorbed gaseous Si and N and accelerated the growth of α-Si3N4 nanobelts. The formation process is considered to be co-dominated by a vapour–liquid–solid (VLS) base-growth mechanism and a vapour–solid (VS) tip-growth mechanism. The former was responsible for the initial nucleation and the proto-nanobelt formation and successive base-growth along the [101] direction of α-Si3N4, and the latter additionally contributed the growth at tips. The formation of α-Si3N4 nanobelts instead of nanowires is attributed to the anisotropic growth in the width and thickness directions, dictated by the liquid Ni catalyst droplets, in particular, in the initial proto-nanobelt formation stage. The room-temperature photoluminescence spectrum showed that the as-synthesized α-Si3N4 nanobelts had a strong emission with two maximum peaks at 416 nm (2.98 eV) and 436 nm (2.84 eV) located in the violet-blue spectral range, making it a potential material for applications in LED and optoelectronic nanodevices.

Ca6La4(SiO4)2(PO4)4O2:Eu2+: a novel apatite green-emitting phosphor for near-ultraviolet excited w-LEDs

A novel apatite phosphor Ca6La4(SiO4)2(PO4)4O2:Eu2+ was prepared by conventional high-temperature solid-state reaction. Phase purity was examined by XRD and XPS analysis. The crystal structure information, such as space group, cell parameters and atomic coordinates, were refined by the Rietveld method, revealing that Eu2+ occupied the sites of Ca2+ ions. Moreover, low-temperature experiments, including low-temperature PL spectra and low-temperature decay curve, were used to prove the existence of two luminescence centers in Ca6La4(SiO4)2(PO4)4O2:Eu2+. With the increase in doping concentration of Eu2+, the emission wavelength shows a red shift from 498 nm to 510 nm, which is mainly caused by the increase in crystal-field splitting by Eu2+. The optimized concentration of Eu2+ was confirmed to be 0.01, the Rc was calculated to be 20.09 A and the energy transfer between Eu2+ was demonstrated to be by exchange interaction. Moreover, good thermal stability has been proved by a temperature-dependence experiment; it shows that the phosphor can maintain 55% of emitting intensity at 150 °C compared to that at room temperature. Finally, the Ca6La4(SiO4)2(PO4)4O2:Eu2+ phosphor was fabricated with commercial red (CaAlSiN3:Eu2+) and blue (BAM:Eu2+) phosphor coating on a n-UV chip. This proves that this green phosphor has the potential to be used in a w-LED lamp.

Thermal evaporation synthesis of SiC/SiOx nanochain heterojunctions and their photoluminescence properties

Core–shell SiC/SiOx nanochain heterojunctions have been successfully synthesized on silicon substrate via a simplified thermal evaporation method at 1500 °C without using catalyst, template or flowing gases (Ar, CH4, N2, etc.). X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy combined with energy-dispersive X-ray spectroscopy, scanning transmission electron microscopy and Fourier-transform infrared spectroscopy are used to characterize the phase composition, morphology, and microstructure of the as-synthesized nanostructures. A combined vapor–solid growth and modulation procedure is proposed for the growth mode of the as-grown SiC/SiOx nanochains. The formation of SiOx beads not only relates to the Rayleigh instability and the poor wettability between SiC and SiOx, but also to the existence of a high density of stacking faults within SiC-core nanowires. The photoluminescence spectrum of the nanochains exhibits a significant blue shift, which can be highly valuable for future potential applications in blue-green emitting devices.