Xiangfu Wang

Optical temperature sensing of rare-earth ion doped phosphors

Accurate and reliable temperature measurement of many special inaccessible objects is a challenging task. Optical temperature sensing is a promising method to achieve it. The current status of optical thermometry of rare-earth ion doped phosphors is reviewed in detail. Based on the mechanisms of optical temperature sensing of different phosphors, temperature dependent luminescence spectra, the fluorescence intensity ratio technique in the data fitting process, and errors of the energy difference between thermally coupled levels, we describe the recent developments in the use of optical thermometry materials. The most important results obtained in each case are summarized, and the main challenges that we need to overcome are discussed. Research in the field of phosphor sensors has shown that they have significant advantages compared to conventional sensors in terms of their properties like greater sensitivity, freedom from electromagnetic interference, long path monitoring, and independence of compatibility with electronic devices.

Influence of Doping and Excitation Powers on Optical Thermometry in Yb 3+ -Er 3+ doped CaWO 4

Optical thermometry has been widely studied to achieve an inaccessible temperature measurement in submicron scale and it has been reported that the temperature sensitivity depends mainly on host types. In this work, we propose a new method to improve the optical temperature sensitivity of Yb3+-Er3+ co-doped CaWO4 phosphors by doping with Li+, Sr2+, and Mg2+ ions and by controlling excitation powers of 980 nm laser. It is found that the thermometric parameters such as upconversion emission intensity, intensity ratio of green-to-red emission, fluorescence color, emission intensity ratios of thermally coupled levels (2H11/2/4S3/2), and relative and absolute temperature sensitivity can be effectively controlled by doping with Li+, Sr2+, and Mg2+ ions in the Yb3+-Er3+ co-doped CaWO4 system. Moreover, the relative sensitivity SR and the absolute sensitivity SA are proved to be dependent on the pump power of 980 nm laser. The sensitivities of SR and SA in Yb3+-Er3+ co-doped CaWO4 increase about 31.5% and 12%, respectively, by doping with 1 mol% Sr2+.

Detecting the origin of luminescence in Er 3+ -doped hexagonal Na 1.5 Gd 1.5 F 6 phosphors

Understanding site-selective fluorescence is one of valuable importance for spectrum modulation. In this Letter, we observed the existence of two non-equivalent Gd-activated crystallographic sites in an Er3+-doped hexagonal Na1.5Gd1.5F6 phosphor. It is proved that two green emissions from the S3/24 level separately originate from the Gd1 (540 nm) and Na2/Gd2 (550-555 nm) crystallographic sites, and the 657 nm red emission from the F9/24 level only originates from Na2/Gd2 site through using the time-resolved luminescence spectra. The 142.2% absolute enhancement of the red emission is realized through the synergistic effect of ultraviolet downconversion and infrared upconversion induced by the 370 nm and 1.54 μm dual-mode excitation.

Tunable electronic properties of GeSe/phosphorene heterostructure from first-principles study

Vertical integration of two-dimensional materials has recently emerged as an exciting method for the design of electronic and optoelectronic devices. In this letter, first principles calculations are employed to explore the structural and electronic properties of the GeSe/phosphorene van der Waals (vdW) p-n heterostructure. Our results suggest that this heterostructure has an intrinsic type-II band alignment and indirect band gap. Moreover, we also find that an intriguing indirect-direct and insulator-metal transition can be induced by strain. In addition, spontaneous electron-hole charge separation is expected to occur, implying that the GeSe/phosphorene heterostructure is a good candidate for applications in optoelectronics. These results provide a route for applications of the GeSe/phosphorene vdW heterostructure in future flexible electronics, optoelectronics, and semiconductor devices.

Metalized B40 fullerene as a novel material for storage and optical detection of hydrogen: a first-principles study

The first experimentally observed all-boron fullerene (B40) has potential applications in hydrogen storage [Nat. Chem., 6, 727 (2014)]. The surface of B40 fullerene contains 4 heptagonal rings and 2 hexagonal rings, which may facilitate metal atom adsorption. Based on (van der Waals corrected) density functional theory, we show that alkali metal (AM) adsorbed on B40 can serve as a promising candidate not only for hydrogen storage, but also for hydrogen detection. The binding energy of AM atoms on B40 strengthens, due to the energetically favorable B 2p-AM p orbital hybridization. Thus, AM atoms are not likely to form clusters on B40, indicating a good reversible hydrogen storage. The (AM)6B40 can reach a gravimetric capacity of ∼8 wt% hydrogen with an optimal adsorption energy that is intermediate between the physisorbed and chemisorbed state. Interestingly, the number of adsorbed H2 molecules per AM atom depends on the position of the AM atoms adsorbed on the B40 surface. Furthermore, the optical absorption spectra of (AM)6B40 before and after hydrogen molecule adsorption are computed and a clear red (blue) shift is observed. These results can serve as a guide in the design of promising hydrogen storage materials and optical sensors based on the B40 fullerene.

Two dimensional WS2 lateral heterojunctions by strain modulation

“Strain engineering” has been widely used to tailor the physical properties of layered materials, like graphene, black phosphorus, and transition-metal dichalcogenides. Here, we exploit thermal strain engineering to construct two dimensional (2D) WS2 in-plane heterojunctions. Kelvin probe force microscopy is used to investigate the surface potentials and work functions of few-layer WS2 flakes, which are grown on SiO2/Si substrates by chemical vapor deposition, followed by a fast cooling process. In the interior regions of strained WS2 flakes, work functions are found to be much larger than that of the unstrained regions. The difference in work functions, together with the variation of band gaps, endows the formation of heterojunctions in the boundaries between inner and outer domains of WS2 flakes. This result reveals that the existence of strain offers a unique opportunity to modulate the electronic properties of 2D materials and construct 2D lateral heterojunctions.

Giant enhancement of upconversion emission in NaYF4:Er3+@NaYF4:Yb3+ active-core/active-shell nanoparticles

Varying the type and the doping concentration of rare-earth ions was reported as a conventional method to tune upconversion emissions of rare-earth ions doped nanoparticles (NPs). In this work, an effective method to enhance greatly the infrared-visible upconversion emissions of Er3+ ions is demonstrated through coating an optimized active NaYF4:Yb3+ shell around NaYF4:Er3+ core nanoparticles. Phase, shape, and size of the resulting core–shell NPs are studied by X-ray diffraction and high-resolution transmission electron microscopy. Compared with the core-only NaYF4:Er3+ NPs, a maximum 97.56-fold overall enhancement in the emission intensity of Er3+ ions in the core–shell NPs is achieved under 980 nm infrared excitation, and a maximum 38.6-fold overall enhancement is achieved under 1540 nm infrared excitation. The luminescence enhancement effect and color output exhibits a strong dependence on the doping concentrations of Yb3+ in the NaYF4:Yb3+ active-shell. By analyzing the decay lifetimes of the emission bands, the giant luminescence enhancement is due to the significant increase in the near-infrared absorption and efficient energy transfer from Yb3+ primary-sensitizers to Er3+ activators via Yb3+ bridging sensitizers.

Upconversion emission of SrYbF5:Er3+ nanosheets modified by Tm3+ ions

Abstract Through a hydrothermal route, the Er3+ and Tm3+ co-doped SrYbF5 nanosheets were synthesized. The resulting samples were characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and luminescence spectra. Under the excitation of 980 nm laser irradiation, the upconversion emissions of Tm3+ ions centered at 474 nm (1G4→3H6), 679 nm (3F2→3H6), 699 nm (3F3→3H6), 803 nm (3H4→3H6) and emissions of Er3+ ions centered at 522 nm (2H11/2→4I15/2), 543 nm (4S3/2→ 4I15/2), 654 nm (4F9/2→4I15/2) were observed. The upconversion emissions of Er3+ ions were adjusted by the concentration of Tm3+ ions. The energy transfer mechanisms among Er3+-Yb3+-Tm3+ in SrYbF5 nanosheets were discussed.