Shaoshuai Zhou

Thermometric and optical heating bi-functional properties of upconversion phosphor Ba5Gd8Zn4O21:Yb3+/Tm3+

Yb3+/Tm3+ co-doped Ba5Gd8Zn4O21 upconversion (UC) phosphors with thermometric and optical heating properties were successfully prepared by a sol–gel process, and crystal structures of all samples were examined by X-ray diffraction (XRD). The phosphors show an intense near-infrared (NIR) and several weak visible emission peaks upon 980 nm excitation. The possible UC mechanisms and processes were proposed based on the power dependence of the upconversion luminescence (UCL) intensities, and the lifetimes of 1G4 → 3H6 blue emissions were also measured to confirm the occurrence of energy transfer (ET). Temperature sensing performances based on the Stark levels (1G4(1), 1G4(2)) of Tm3+ were evaluated by analyzing temperature-dependent UCL spectra in the range 300–510 K. The maximum sensitivity for phosphors with different UCL intensities was discussed in detail and approached approximately 0.0061 K−1 at 300 K. Furthermore, the heating effect produced by laser excitation was also measured, which caused the temperature of sample to rise from 278.8 to 321.8 K upon increasing the pump power from 638 to 1802 mW. The results indicate that Yb3+/Tm3+ co-doped Ba5Gd8Zn4O21 phosphors could be considered as potential candidates for temperature sensors and optical heaters.

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.

Griffiths phase and exchange bias in La1-xCaxMnO3 (x = 0.50, 0.67, and 0.75) nanoparticles

The magnetic properties of La1−xCaxMnO3 (x=0.50, 0.67, and 0.75) nanoparticles have been systemically investigated in this work. It is found that although their bulk counterparts have different magnetic and charge ordered states, the nanoparticles show no long-range charge ordered transition but very similar magnetic behaviors, i.e., a Griffiths phase appearing below ∼300 K and a ferromagnetic ordered state present below ∼270 K. The similarity suggests that the enhanced ferromagnetic behavior at low temperatures in the nanoparticles may originate from the development of the ferromagnetic correlations already present at high temperature upon cooling due to the suppression of the charge ordered state. Moreover, the field-cooling magnetic hysteresis loops reveal that exchange bias phenomena are present in the nanoparticles, which is of special interest for potential applications.

Synthesis and thermometric properties of shuttle-like Er3+/Yb3+ co-doped NaLa(MoO4)2 microstructures

Yb3+/Er3+ co-doped NaLa(MoO4)2 up-conversion (UC) monodisperse shuttle-like microcrystals were prepared by a hydrothermal method without any organic solvents or surfactants. The phase purity, structure and morphology of the samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The effects of the initial reaction solution pH values and reaction time on the morphologies of the final products were investigated. The temperature-dependent UC luminescence and temperature sensing properties of the samples were discussed according to the fluorescence intensity ratio (FIR) of green emissions from 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ at 530 and 550 nm in the range of 300–510 K under excitation of a 980 nm laser. After sintering at 600 °C for 1 h, the UC intensity of the sample increased about 1070 times that of the sample without calcination, and the maximum sensitivity of the samples with and without calcinations was approximately 0.0131 K−1 at 510 K and 0.0135 K−1 at 450 K, respectively. The results indicate that the sensitivity is hardly dependent on the UC luminescence intensities of the samples, and the present shuttle-like NaLa(MoO4)2 monodispersed microcrystals exhibited excellent temperature sensing properties.

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.

An abnormal fluorescence intensity ratio between two green emissions of Er3+ caused by heating effect of 980 nm excitation

Abstract An abnormal fluorescence intensity ratio (FIR) between two green emissions of Er 3+ at room temperature, which is larger than a normal value, emerged in many reported articles. However, up to now detailed work has seldom been done to clarify this abnormal phenomenon. In this paper, green upconversion luminescence of the β-NaLuF 4 :20%Yb 3+ ,2%Er 3+ powder sample was investigated under 980 nm excitation at different circumstances, different pump power densities and different temperatures as well as different air pressures. The corresponding local temperature calculated using FIR technique increased gradually with the enhancement of the pump power density. It was demonstrated that high pump power density of 980 nm laser led to the increase of local temperature of the luminescent material, which further gave the abnormal FIR.

Blue upconversion of Tm3+ using Yb3+ as energy transfer bridge under 1532 nm excitation in Er3+, Yb3+, Tm3+ tri-doped CaMoO4

Abstract Nano-sized CaMoO 4 phosphors tri-doped with Er 3+ , Yb 3+ and Tm 3+ ions were successfully synthesized by sol-gel method. Intense blue emission from Tm 3+ ions was observed upon excitation of 1532 nm infrared light in Er-Yb-Tm system, while this blue upconversion could not be achieved with the absence of Yb 3+ ions in Er-Tm co-doped sample. In order to understand this upconversion process, the upconversion spectra in these samples were investigated, and the possible mechanism was proposed based on experimental results. It showed that two different energy transfer from Er 3+ to Tm 3+ existed simultaneously in Er-Yb-Tm system, the one-step direct energy transfer from Er 3+ to Tm 3+ , and the two-step Er 3+ →Yb 3+ →Tm 3+ energy transfer. In particular, the 1 G 4 state of Tm 3+ could only be populated from the 3 H 4 state by cross-relaxation with an excited Yb 3+ ion, producing blue emission of Tm 3+ . In this upconversion process, Yb 3+ ions acted as an energy transfer bridge between Er 3+ and Tm 3+ , which also meant that the upconversion of other rare-earth ions under the excitation of 1532 nm was possible with the presence of Er 3+ and Yb 3+ .

A-site ion-size effect on the transport and magnetic properties of Ce doping Pr0.3Ce0.2CaxSr0.5―xMnO3 (0≤x≤0.25)

The effects of A-site ion-size 〈rA〉 on the crystal structures, transport and magnetic properties of the perovskite manganese oxide Pr0.3 Ce0.2Cax Sr0.5-xMnO (0 ≤ x ≤ 0.25) have been investigated. In those compounds, when 0≤ x≤0.125, the temperatures (Tmax) of the resistivity maximums below Curie Temperature TC are correlated with the Kondo-like scattering of Ce3+ and the onset of antiferromagnetic ordering of Ce3+ with respect to the Mn-sublattice moments. The decrease of 〈rA〉 causes the anomalous increase of lattice parameters b, c and unit cell volume, the decrease of the differences of TC and Tmax, the weakening of the Kondo-like scattering and magnetic order of Ce3+, and the enhancement of saturation magnetic moment, which give the evidences of the valence enhancing of Ce ions from + 3 toward + 4 with 〈rA〉 decreasing. Although the Ca doping is expected to drive the system toward the antiferromagnetic ground state, the increase of valence of Ce enhances the content of Mn3+ in the system, which drives t...

Optical properties of Er3+ doped KYbxF3x+1 (x=2, 3) upconverting nanoparticles

Abstract Well dispersed orthorhombic structure KYb 2 F 7 :2 mol.%Er 3+ and KYb 3 F 10 :2 mol.%Er 3+ nanoparticles were synthesized through solvothermal method. Measurements of X-ray diffraction (XRD), transmission electron microscopy (TEM) and upconversion spectra showed excellent morphology and optical properties of the as-prepared samples. The intensity ratio of the red emission (650 nm) to the green emission (545 nm) in these two materials was much stronger than that in other Er 3+ doped materials. The transition mechanism of the processes was investigated and discussed. Due to their strong red emission, the samples are promising for applications in display, color tuning, and biolabeling.

Size-dependent ferromagnetic phase transition in Sm0.5Sr0.5MnO3 nanoparticles

Magnetic properties of Sm0.5Sr0.5MnO3 nanoparticles with different particle sizes are investigated. It is found that all the nanoparticles show a first-order ferromagnetic transition under low magnetic fields, but a second-order one above a critical field HCR. As the particle size decreases, the ferromagnetic transition temperature, the thermal hysteresis width in the magnetizations, and HCR exhibit a significant decrease, which indicates that the ferromagnetism is weakened and the first-order transition is softened. A detailed analysis on the magnetic susceptibilities of these nanoparticles reveals that this weakening and softening are attributed to the weakened double-exchange interactions and the strongly suppressed charge-ordered antiferromagnetic state by the size reduction, respectively.