Yonghu Chen

Efficient near-infrared quantum cutting in NaYF 4 : Ho 3+ , Yb 3+ for solar photovoltaics

Quantum cutting converting a ultraviolet photon into two near-infrared photons has been demonstrated by spectroscopic measurements in NaYF4:Ho3+,Yb3+ synthesized by hydrothermal method. Evidence is provided to confirm the occurrence of quantum cutting. Upon excitation of Ho3+ 5G4 level, near-infrared quantum cutting could occur through a two-step resonance energy transfer from Ho3+ to Yb3+ by cross relaxation, with a maximum quantum efficiency of 155.2%. This result reveals the possibility of violet to near-infrared quantum cutting with a quantum efficiency larger than 100% in Ho3+/Yb3+ codoped fluorides, suggesting the possible application in modifying the solar spectrum to enhance the efficiency of silicon solar cells.

Visible quantum cutting in BaF2:Gd, Eu via downconversion

The visible quantum cutting in BaF2: Gd, Eu via Downconversion has been observed. In the quantum cutting process, one VUV photon absorbed by Gd3+ can be split into two visible photons emitting by Eu3+ through cross-relaxation between Gd3+ and Eu3+. According to the calculation from the emission spectra under different wavelength excitation, we can obtain the two-step energy transfer process with a visible quantum efficiency up to 194%

Efficient red-emitting phosphor for near-ultraviolet-based solid-state lighting.

Eu(3+) ion activated Y(2)MoO(6) synthesized by the sol-gel method had been investigated as an alternative red-emitting phosphor for solid-state lighting. Excitation spectra, emission spectra, and decay curves were measured to characterize the luminescent properties. The phosphor shows efficient absorption of near-ultraviolet (NUV) light, and it also exhibits excellent performance in emission intensity and color purity compared with the commercial phosphor in current use. This red-emitting material may be applied as a promising red component for the NUV excited white diode.

Temperature sensor based on ladder-level assisted thermal coupling and thermal-enhanced luminescence in NaYF 4 : Nd 3+

NaYF4: Nd³⁺ microprisms were synthesized by a hydrothermal method. The bands of near-infrared (NIR) luminescence originating from the 4F3/2, 4F5/2 and 4F7/2 levels of Nd³⁺ ions in NaYF4: Nd³⁺ microcrystals were measured under 574.8 nm excitation at various temperatures from 323 to 673 K. The fluorescence intensity ratios (FIRs) between any two of the three bands change monotonically with temperature and agree with the prediction assuming thermal couplings. A large relative temperature sensitivity of 1.12% K⁻¹ at 500K is reached with the FIR of 4F7/2 to 4F3/2 levels. In addition, anti-Stokes fluorescence from 4F5/2 level (740 nm) and 4F5/2,7/2 levels (740 nm and 803 nm) of Nd³⁺ ions was studied meticulously under 793.8 nm and 864.2 nm excitations, respectively. The intensities were shown to be greatly enhanced as temperature increases, and the 740 nm band from 4F7/2 level at 458 K increases in intensity by 170 fold relative to that at 298 K under the 793.8 nm excitation.

Strategy for thermometry via Tm 3+ -doped NaYF 4 core-shell nanoparticles

Optical thermometers usually make use of the fluorescence intensity ratio of two thermally coupled energy levels, with the relative sensitivity constrained by the limited energy gap. Here we develop a strategy by using the upconversion (UC) emissions originating from two multiplets with opposite temperature dependences to achieve higher relative temperature sensitivity. We show that the intensity ratio of the two UC emissions, ³F(2,3) and ¹G₄, of Tm³⁺ in β-NaYF₄:20%Yb³⁺, 0.5%Tm³⁺/NaYF₄:1%Pr³⁺ core-shell nanoparticles under 980 nm laser excitation exhibits high relative temperature sensitivity between 350 and 510 K, with a maximum of 1.53%  K⁻¹ at 417 K. This demonstrates the validity of the strategy, and that the studied material has the potential for high-performance optical thermometry.

Control of the Metal-Insulator Transition in VO2 Epitaxial Film by Modifying Carrier Density

External controlling the phase transition behavior of vanadium dioxide is important to realize its practical applications as energy-efficient electronic devices. Because of its relatively high phase transition temperature of 68 °C, the central challenge for VO2-based electronics, lies in finding an energy efficient way, to modulate the phase transition in a reversible and reproducible manner. In this work, we report an experimental realization of p-n heterojunctions by growing VO2 film on p-type GaN substrate. By adding the bias voltage on the p-n junction, the metal-insulator transition behavior of VO2 film can be changed continuously. It is demonstrated that the phase transition of VO2 film is closely associated with the carrier distribution within the space charge region, which can be directly controlled by the bias voltage. Our findings offer novel opportunities for modulating the phase transition of VO2 film in a reversible way as well as extending the concept of electric-field modulation on other phase transition materials.

Luminescent properties of Gd2SiO5 powder doped with Eu3+ under VUV?UV excitation

The luminescent properties of Gd2SiO5 powder crystals doped with Eu3+ were investigated using synchrotron radiation and a VUV laser (157.6?nm) as excitation sources. The excitation spectra in the range of 160?330?nm monitoring the red emission from Eu3+ ions reveal bands corresponding to intraconfigurational 4f?4f transitions of Gd3+ and charge transfer states (CTS) of Eu3+?O2?, as well as interband transitions of the Gd2SiO5 host, indicating an efficient energy transfer process from the host or directly from Gd3+ to Eu3+ ions. The inspection of emission lines from Eu3+ ions suggests that there are three inequivalent sites for Eu3+ in this host. The excitation band around 215?nm and the unique emission features associated with this excitation band were attributed to one of the inequivalent Eu3+ sites, which exhibits an unusually weak coupling with host lattice. The energy levels of Eu3+ in the Gd2SiO5 host were tentatively assigned according to the laser excited emission spectra.

Temperature dependence of GdVO4:Eu3+ luminescence

Abstract The temperature dependence of the GdVO4:Eu3+ luminescence from the Eu3+ 5D0→7FJ transition above room temperature has been studied. The intensity of the luminescence when excited at 313 and 365 nm increases rapidly as temperature rises up to 600 K while the emission intensity at 254 nm only changes a little. The origin of this novel phenomenon is analyzed and discussed by energy transfer from host to Eu3+ in terms of a configurational-coordinate model.

Luminescence properties of Er3+-doped transparent NaYb2F7 glass-ceramics for optical thermometry and spectral conversion

Novel Er3+-doped transparent NaYb2F7 glass-ceramics (GCs) were successfully fabricated for the first time by a conventional melt-quenching technique with subsequent heat treatment. The formation of NaYb2F7 nanocrystals (NCs) was confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), selected-area electron diffraction (SAED), and photoluminescence emission spectra. Moreover, the appearance of Stark level splitting of Er3+ emission bands and the variation of the decay curves demonstrate the accumulation of active centers into the NaYb2F7 NCs lattice. Hence, the photoluminescence emission intensities of Er3+ doped GC680 are greatly enhanced relative to those in precursor glass. Furthermore, the temperature dependent fluorescence intensity ratio (FIR) of thermally coupled emitting states (4S3/2, 2H11/2) in Er3+ doped GCs was studied under 980 nm laser excitation with a very low power density of 13 mW mm−2 to avoid the possible laser induced heating. A high temperature sensitivity of FIR of 1.36% K−1 is obtained at 300 K and the corresponding effective energy difference (ΔE) is 852 cm−1. Besides, laser induced heating at several excitation power densities was measured to evaluate the laser induced heating effect and the accuracy of temperature sensing for our sample. The GCs with relatively high sensitivity under low excitation power density are promising for temperature sensing. Moreover, the study on down-conversion (DC) spectra of the GC samples shows their ability to convert a high energy photon into two low energy photons, implying that they may also have important application as DC materials.

794 nm excited core–shell upconversion nanoparticles for optical temperature sensing

Hexagonal core–shell NaYF4 upconversion nanoparticles (UNCPs) based on Nd3+ sensitization for optical temperature sensing were successfully synthesized by a solvothermal method using oleic acid and octadecene as coordinating solvents. Compared to the conventional Yb3+ sensitized UNCPs, the usage of Nd3+ as the sensitizers can shift the excitation wavelength from 975 nm to 794 nm where the optical absorption of water decreased dramatically, and thus make UNCPs more suitable for biological application. The upconversion (UC) luminescence intensity of the 794 nm-excitation UNCPs is comparable to that of the conventional 975 nm excitation, showing that Nd3+ sensitized UNCPs are efficient. The efficiently successive Nd3+ → Yb3+ → Er3+ energy transfer processes in this UNCP were demonstrated by excitation spectra and time-resolved spectra. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green emissions (525 nm and 545 nm) from the thermally coupled levels of Er3+ was studied in the temperature range from 25 to 60 °C under 808 nm excitation, and the temperature mapping of a device was acquired according to this technique. These indicate that Nd3+ sensitized core–shell UNCPs are promising candidates for application in optical temperature sensors.