Peifen Zhu

Intense ultraviolet upconversion luminescence from hexagonal NaYF 4 :Yb 3+ /Tm 3+ microcrystals

Under 980 nm excitation, unusual 3P2-->3H6 (approximately 264 nm) and 3P2-->3F4 (approximately 309 nm) emissions from Tm3+ ions were observed in hexagonal NaYF4:Yb3+ (20%)/Tm3+ (1.5%) microcrystals. In comparison with the strong emissions from 1D2 and 1I6, the emissions from 1G4 and 3H4 almost vanished due to the efficient cross-relaxation of 1G4 + 3H4-->3F4 + 1D2(Tm3+). Double logarithmic plots of the upconversion emission intensity versus the excitation power are neither straight lines nor typical saturation curves. Theoretical analysis indicated that the complicated dependent relationships were mainly caused by phonon-assisted energy transfers and nonradiative relaxation.

FDTD Analysis on Extraction Efficiency of GaN Light-Emitting Diodes With Microsphere Arrays

The improvement of light extraction efficiency of InGaN light-emitting diodes (LEDs) using microsphere arrays with various refractive indices was analyzed. Finite-difference time-domain (FDTD) simulations show that the use of microsphere (dmicrosphere = 500 nm) arrays with refractive indices of 1.8 and 2.5 led to increase in light extraction efficiency of InGaN LEDs by 1.9 times and 2.2 times, respectively. The enhancement in light extraction efficiency is attributed to the decrease in the Fresnel reflection and increase in effective photon escape cone due to graded refractive index and curvature formed between microsphere and free space. The maximum enhancement of light extraction efficiency of InGaN quantum well LEDs was achieved by employing the refractive index matched anatase-TiO2 microsphere arrays. The effects of microsphere diameters on the light extraction efficiency were also investigated and 2.4 times enhancement was achieved by employing 400-nm refractive index matched TiO2 sphere arrays.

Ultraviolet upconversion emissions of Gd3

Under 980 nm excitation, upconversion (UC) emissions in the UV range of 270-320 nm were observed in nanocrystals Y(0.795-x)Gd(x)Yb(0.2)Tm(0.005)F(3) (x=0, 0.1, 0.2, 0.5, and 0.795), which were synthesized through a hydrothermal method. These UC emissions can be assigned to the transitions of (6)I(J), (6)P(J)-->(8)S(7/2)(Gd(3+)), and (3)P(0)/(1)I(6)-->(3)H(6) (Tm(3+)). The energy transfer from Tm(3+) to Gd(3+) plays a crucial role in populating the excited states of Gd(3+). The shortest wavelength of upconverted emission converted from the infrared region was demonstrated here.

Ultraviolet upconversion fluorescence from 6 D J of Gd 3+ induced by 980 nm excitation

Ultraviolet upconversion emissions of 246.2 and 252.8 nm from (6)D(J) levels of Gd(3+) ions were observed in GdF(3): 10% Yb(3+), 0.7% Tm(3+) nanocrystals under 980 nm excitation from a laser diode. The (6)D(J) levels of Gd(3+) ions can be efficiently populated by energy transfer processes of Yb-->Tm-->Gd and Yb-->Gd. A six-photon upconversion process was confirmed by the dependence of 252.8 nm emission intensity on the pumping power. The upconversion mechanism in the six-photon process was discussed based on excited state absorption of Gd(3+) ions, cross relaxation energy transfer between two excited Gd(3+) ions, and energy transfer between Gd(3+) and Yb(3+) or Tm(3+) ions.

Synthesis and upconversion luminescence properties of NaYF4:Yb3+/Er3+ microspheres

Abstract Cubic NaYF 4 :Yb 3+ (20%)/Er 3+ (1%) microspheres were synthesized by EDTA-assisted hydrothermal method. Under 980 nm excitation, ultraviolet ( 4 G 11/2 → 4 I 15/2 ), violet ( 2 H 9/2 → 4 I 15/2 ), green ( 4 F 7/2 → 4 I 15/2 , 2 H 11/2 → 4 I 15/2 , and 4 S 3/2 → 4 I 15/2 ), and red ( 4 F 9/2 → 4 I 15/2 ) upconversion fluorescence were observed. The number of laser photons absorbed in one upconversion excitation process, n , was determined to be 3.89, 1.61, 2.55, and 1.09 for the ultraviolet, violet, green, and red emissions, respectively. Obviously, n =3.89 indicated that a four-photon process was involved in populating the 4 G 11/2 state, and n =2.55 indicated that a three-photon process was involved in populating the 4 F 7/2 / 2 H 11/2 / 4 S 3/2 levels. For the violet and red emissions, the population of the states 2 H 9/2 and 4 F 9/2 separately came from three-photon and two-photon processes. The decrease of n was well explained by the mechanism of competition between linear decay and upconversion processes for the depletion of the intermediate excited states.

Photoluminescence studies of Y2O3:Eu3+ under high pressure

The Eu-doped yttria (Y2O3:Eu3+) has been investigated by the in situ high-pressure angle dispersive synchrotron X-ray diffraction (XRD) and the photoluminescence (PL) spectroscopy. The red shift and intensity ratio variation of emissions with increasing pressure were observed and elucidated. It was found that the red shift of emissions is related to the expansion of the f orbit of the Eu3+ and the intensity ratio variation of emissions is ascribed to the change of the crystal field under high pressure. The pressure-induced changes in spectrum are related to the phase transition, which was confirmed by XRD pattern. The two high pressure phases were identified as the monoclinic (C2/m) phase and hexagonal (P-3m1) phase by the Rietveld refinement.

Enhanced ultraviolet upconversion in YF3:Yb3+/Tm3+ nanocrystals

Abstract Unusual intense infrared-to-ultraviolet upconversion luminescence was observed in YF 3 :Yb 3+ (20%)/Tm 3+ (1%) nanocrystals under 980 nm excitation. The intense ultraviolet emissions ( 1 I 6 → 3 H 6 , 1 I 6 → 3 F 4 , and 1 D 2 → 3 H 6 ) were affirmed arising from the excitation processes of five-photon and four-photon. In comparison with the bulk sample with the same chemical compositions, ultraviolet upconversion luminescence of the nanocrystals was markedly enhanced. Spectral analysis indicated that the enhancement was attributed to the decrease of Judd-Ofelt parameter Ω 2 , which precluded the transition rate from 3 F 2 to 3 F 4 , enhanced the energy transfer process and populated the 1 D 2 level: 3 F 2 → 3 H 6 (Tm 3+ ): 3 H 4 → 1 D 2 (Tm 3+ ).

Effect of packing density and packing geometry on light extraction of III-nitride light-emitting diodes with microsphere arrays

The finite-difference time-domain method was employed to calculate light extraction efficiency of thin-film flip-chip InGaN/GaN quantum well light-emitting diodes (LEDs) with TiO2 microsphere arrays. The extraction efficiency for LEDs with microsphere arrays was investigated by focusing on the effect of the packing density, packing configuration, and diameter-to-period ratio. The comparison studies revealed the importance of having a hexagonal and close-packed monolayer microsphere array configuration for achieving optimum extraction efficiency, which translated into a 3.6-fold enhancement in light extraction compared to that for a planar LED. This improvement is attributed to the reduced Fresnel reflection and enlarged light escape cone. The engineering of the far-field radiation patterns was also demonstrated by tuning the packing density and packing configuration of the microsphere arrays.

Resonant cavity effect optimization of III-nitride thin-film flip-chip light-emitting diodes with microsphere arrays

Comprehensive studies were carried out to investigate the light extraction efficiency of thin-film flip-chip (TFFC) light-emitting diodes (LEDs) with anatase TiO(2) microsphere arrays by employing the finite-difference time-domain method. The quantum well position and the resonant cavity effect were studied to obtain optimum light extraction for the planar TFFC LED. Further enhancement in light extraction was achieved by depositing microsphere arrays on the TFFC LED. The calculation results showed that the sphere diameter, packing density, and packing configuration have significant effects on the light extraction efficiency. A maximum light extraction efficiency of 75% in TFFC LEDs with microsphere arrays has been achieved. This study demonstrates the importance of optimizing the quantum well position, cavity thickness, sphere diameter, sphere packing density, and packing configuration for enhancing the light extraction efficiency of TFFC LEDs with microsphere arrays.

Narrow-linewidth red-emission Eu3+-doped TiO2 spheres for light-emitting diodes

In this work, the amorphous Eu3+-doped TiO2 spheres were synthesized by low cost mixed-solvent method, while the anatase and rutile spheres can be obtained by annealing the as-synthesized amorphous TiO2 spheres at elevated temperatures. The optical properties of Eu3+-doped TiO2 spheres were also investigated, and strong red emission (centered at 610 nm) with narrow line-width of 30 nm was observed under 465 nm or 394 nm excitations for the Eu3+-doped anatase TiO2 spheres. Our findings indicate the potential of using Eu3+-doped TiO2 spheres to achieve red emission with InGaN blue light emitting diodes (LEDs). Owing to the high light extraction efficiency in the GaN-based LEDs using anatase TiO2 spheres as demonstrated in our previous works, this work shows the strong potential of Eu3+-doped TiO2 spheres as the red phosphor material for high efficiency GaN-based white light-emitting diode.