Yixi Zhuang

Broadband downconversion based infrared quantum cutting by cooperative energy transfer from Eu2+ to Yb3+ in glasses

Near-infrared quantum cutting involving the emission of near-infrared photons by downconversion cooperative energy transfer from Eu2+ to Yb3+ in borate glasses was reported. Electron spin resonance spectra and absorption spectra were measured to prove the existence of Eu2+ in borate glasses. Excitation, emission, and fluorescence decay measurements were performed to examine the occurrence of cooperative energy transfer from Eu2+ to Yb3+ ions. Yb3+ concentration dependent quantum efficiency was calculated and the maximum efficiency approaches 164.19%.

Broadband conversion of visible light to near-infrared emission by Ce3+, Yb3+-codoped yttrium aluminum garnet

We show that Ce(3+) can be an efficient sensitizer for Yb(3+) in the host lattice of yttrium aluminum garnet (YAG). With blue-light excitation to induce the 4f-->5d transition of Ce(3+), characteristic near-IR emission of Yb(3+) due to transition of (2)F(5/2)-->(2)F(7/2) peaking at 1030 nm is generated as a result of energy transfer from Ce(3+) to Yb(3+). The result of spectral evolution with temperature indicates that the efficiency of energy transfer is enhanced owing to thermal effect. This evidence implies that the phonon-assisted process participates in the downconversion of YAG: Ce(3+), Yb(3+).

Enhancement of Red Persistent Luminescence in Cr3+-Doped ZnGa2O4 Phosphors by Bi2O3 Codoping

Bi2O3 was proved to be an effective codopant to enhance red persistent luminescence in Cr3+-doped ZnGa2O4 spinel. The Cr–Bi-codoped ZnGa2O4 phosphors showed about 10 times higher persistent luminescence intensity than the Cr-singly-doped phosphors. The radiance (in mW sr-1m-2) of persistent luminescence in ZnGa2O4:Cr,Bi phosphors was comparable to that in commercialized SrAl2O4:Eu,Dy phosphors. Increases of Cr3+ absorption and photoluminescence were also observed in the Cr–Br-codoped ZnGa2O4 sample. The obtained results suggest that Bi2O3 may play a critical role in stabilizing Cr3+ in ZnGa2O4 spinel.

Tunable trap depth in Zn(Ga1−xAlx)2O4:Cr,Bi red persistent phosphors: considerations of high-temperature persistent luminescence and photostimulated persistent luminescence

Thermoluminescence peaks for Cr3+ emissions shift to higher temperature due to the partial substitution of Al for Ga sites in ZnGa2O4:Cr,Bi phosphors, which indicates that the electron traps responsible for the red persistent luminescence are tuned to deeper levels by Al-doping. Al-containing phosphors with deeper traps showed a lower persistent luminescence intensity than Al-free samples at 20 °C because the ambient thermal energy is too low to release trapped electrons in deeper levels; however, the Al-containing persistent phosphors perform better at 80 °C with greater thermal energy. At 20 °C, the trapped electrons in the Al-containing phosphors can be efficiently released with the assistance of additional NIR photostimulation. NIR photostimulated red persistent luminescence (NIR-to-red mode) as presented in the Al-containing ZnGa2O4:Cr,Bi phosphors could be a promising technique for new in vivo imaging systems.

BCNO-Based Long-Persistent Phosphor

A BCNO-based long-persistent phosphor prepared by a low-temperature liquid-phase route is investigated. It shows a phosphorescence peak centered at 520 nm and its afterglow decay curve fits t -0.93 . Electron paramagnetic resonance confirms the presence of a paramagnetic center, the signal of which exhibits a synchronized decay with the afterglow intensity. We assume that the phosphorescence arises from electrons that are thermally released from nitrogen vacancies and then fall into the carbon-related defects, giving the greenish emission. The BCNO material is a competitive long-persistent phosphor because it is derived from cheap raw materials.

Spectroscopic investigation on BCNO-based phosphor: photoluminescence and long persistent phosphorescence

BCNO phosphor derived from a low-temperature urea route is an efficient and environmental-friendly phosphor with emission covering the full visible and the near ultraviolet region. Systematic characterization of the BCNO phosphor was carried out by means of steady state and time-resolved spectroscopy in order to unveil the origins of different luminescence centres. Two emission bands located at 335 and 410 nm together with a changeable band located ranging from 380 to 580 nm are identified and related to the presence of carbon defects as inferred from electron paramagnetic resonance spectroscopy. The emission of the changeable band exhibits a long persistent character, which depends on the emission energy of the specimens and originates from the tunnelling recombination of electrons with the emission centres. The spectroscopic properties of the BCNO phosphor are discussed in detail and a configurational coordinate model is constructed for the explanation of the photoluminescence as well as the afterglow behaviour.

Photochromism and white long-lasting persistent luminescence in Bi 3+ -doped ZnGa 2 O 4 ceramics

White long-lasting persistent luminescence covering the whole visible region in Bi3+-doped ZnGa2O4 ceramics is reported. The afterglow luminescence can be observed for several tens of minutes after 360 nm or 280 nm excitation. Photochromism is also observed during ultra-violet excitation. The persistent luminescence and photochromism are considered to originate from electron trapping by defect centers in the ZnGa2O4 crystals. The Bi3+-doped ZnGa2O4 ceramics are expected to be potential white-color afterglow phosphors.

Band-gap variation and a self-redox effect induced by compositional deviation in ZnxGa2O3+x:Cr3+ persistent phosphors

We made a systematic investigation on the optical properties of non-doped and Cr-doped ZnxGa2O3+x (0.98 ≤ x ≤ 1.02) spinel crystals. Absorption, photoluminescence excitation, and persistent luminescence excitation spectra indicated that the bottom of the conduction band (CB) was affected (i) by compositional deviation from stoichiometry or (ii) by atmosphere control in a similar way. By constructing an energy level diagram, a broad distribution of trap depth in a Cr3+-doped ZnxGa2O3+x sample with a composition of excess Zn or in one prepared in an O2 atmosphere was attributed to lowering of the CB bottom, which resulted in faster decay of persistent luminescence in these two samples. Defect chemical reactions revealed that the anti-site species of Zn′Ga may be the reason for lowering of the CB bottom. A self-redox effect of Cr ions induced by the compositional deviation was found and explained by the defect reactions. The investigation indicated that the composition with slight Zn deficiency is important to obtain a narrow distribution of trap depths and longer persistent luminescence, however the self-reduction effect of Cr ions in the Zn deficient composition also needs to be considered.

Y3Al5−xGaxO12:Cr3+: A novel red persistent phosphor with high brightness

We report on red persistent phosphors of Y3Al5−xGaxO12:Cr3+ garnet (YAGG:Cr3+, x from 0 to 5). In these materials, Cr ions act as both emission centers and electron traps. The trap depth, which is regarded as the energy gap between the bottom of the conduction band and the electron trap, can be optimized by modifying the Ga3+ substitution content (x). After ceasing UV illumination, the persistent luminescence of the YAGG:Cr3+ (x = 3) phosphor was nearly 5 times higher than that of the widely used ZnGa2O4:Cr3+ red persistent phosphor. Such novel red persistent phosphors have great potential for an improved in vivo bioimaging application.

Bandwidth broadening of near-infrared emission through nanocrystallization in Bi/Ni co-doped glass.

We demonstrated an effective way to broaden the bandwidth of near-infrared (NIR) emission from Bi/Ni codoped 58SiO₂₋21ZnO-13Al₂₋O₃₋5TiO₂₋3Ga₂O₃ glass through nanocrystallization. The nanocrystallized glass shows ultra-wide NIR luminescence with a full width at half maximum (FWHM) of 350 nm and long lifetime up to 476 µs. The observed broadband NIR emission, attributed to energy transfer suppression between Ni and Bi active centers, was realized by a separation process with Ni²⁺ ions selectively incorporated into nanocrystals. This bandwidth engineering through nanocrystallization inside glass suggests a promising approach for enhancement of glass functionality and construction of broadband light sources.