External-Field-Independent Thermometric Sensitivity and Green Emission of Upconversion Phosphor Sr2InF7: Yb3+, Er3.

Based on luminescence intensity ratio (LIR) technology, the noncontact upconversion (UC) optical temperature sensor has aroused a great deal of interest due to its great application prospects in some extreme environments. However, most of the studies focused on improving its sensitivity due to the fact that the sensitivity can be influenced by many external field factors, such as the power density and pulse width of pumping sources or temperature. Herein, a green-emitting UC phosphor Sr2InF7: Yb3+, Er3+ was developed as a potential thermometer, which retained bright green emission under 980 nm excitation with different pulse widths and power densities or at different temperatures; the possible mechanisms are discussed in detail. Its sensitivity almost remained constant when using both continuous wave (c.w.) and pulsed laser or different power densities, which meant the sensitivity of Sr2InF7: Yb3+, Er3+ was independent of the characteristics of pumping laser. A flexible thin-film thermometer composed of Sr2InF7: 2%Yb3+, 2%Er3+ was also fabricated to detect the temperature of microelectronic components, which can not only accurately measure the temperature of the working electronic circuit board but also exhibit excellent repeatability. The results indicated that the present noncontact UC temperature sensor showed stable green emission and thermometric sensitivity as well as the possibility of replacing the traditional thermometers.

Thermometry and up-conversion luminescence of Yb(3+)-Er(3+) co-doped Na2Ln2Ti3O10 (Ln = Gd, La) phosphors.

Yb(3+)/Er(3+)-ion co-doped Na2Ln2Ti3O10 (Ln = Gd, La) up-conversion (UC) phosphors were successfully synthesized by a sol-gel method, and their crystal structures were characterized by powder X-ray diffraction. Dazzling yellow-greenish light was emitted under the excitation of 980 nm near-infrared (NIR) light, composing green and red emission bands from the (2)H11/2/(4)S3/2→(4)I15/2 and (4)F9/2→(4)I15/2 transitions of Er(3+), respectively. The optimal composition and synthesis parameters were determined according to their UC emission intensity. The photon absorption and emission processes were illustrated based on the UC mechanism, in which energy transfer (ET) from Yb(3+) to Er(3+) plays a pivotal role and has been proved by the variation of green emission lifetime in Er(3+) singly and Yb(3+)/Er(3+) co-doped Na2Ln2Ti3O10 samples. The temperature-dependent fluorescence intensity ratios (FIR) of the two thermal coupled energy level (TCL) emission from (2)H11/2→(4)I15/2 (526 nm) and (4)S3/2→(4)I15/2 (549 nm) were calculated in the range of 290-490 K, and their sensitivity values were approximately 0.0058 K(-1) for Na2Gd2Ti3O10 at 490 K and 0.0061 K(-1) for Na2La2Ti3O10 at 470 K, as potential optical temperature sensor.

Near-infrared down-conversion and energy transfer mechanism in Yb(3+)-doped Ba2LaV3O11 phosphors.

Yb(3+)-doped Ba2LaV3O11 vanadate phosphors with near-infrared (NIR) emission are synthesized via the sol-gel method. The phase purity and structure of samples are characterized by X-ray diffraction (XRD). The electronic structure of the self-activated phosphor host Ba2LaV3O11 is estimated by density functional theory (DFT) calculation, and the host absorption is mainly ascribed to the charge transition from the O-2p to V-3d states. Photoluminescence emission (PL) and excitation (PLE) spectra, decay curves, absorption spectra and theoretical quantum yields of samples are also investigated. Results indicate that Ba2LaV3O11:Yb(3+) phosphors have strong broad band absorption and efficient NIR emission, which matches well with the spectral response of the Si-based solar cells. The energy transfer processes from [VO4](3-) to Yb(3+) and possible transfer models are proposed based on the concentration of Yb(3+) ions. Results demonstrate that Ba2LaV3O11:Yb(3+) phosphors might act as a promising NIR DC solar spectral converter to enhance the efficiency of the silicon solar cells by utilizing broad band absorption of the solar spectrum.

Quantum wells formed in transition-metal dichalcogenide nanosheet-superlattices: stability and electronic structures from first principles.

The possibility of forming quantum wells (QWs) in transition-metal dichalcogenide nanosheet assembled superlattices (SLs) was investigated by using the first principles calculations. The interfacial binding energies and electronic structures of MoS2/MX2 (MX2 = MoSe2, WS2, and WSe2) SLs were calculated. The interfacial binding energies show that all the SLs are stable, and the most stable atomic configuration is that where M atoms are located right above S atoms. By calculating the band offsets in the SLs, it was found that a QW with a depth of 0.17 eV can be formed in the MoS2 layer in MoS2/WSe2 SLs. The calculated band structure shows that this SL has an indirect band gap due to the tensile strained state of the MoS2 layer. The charge transfer between the two layers is very small, which is in favor of the QWs' formation. In particular, the depth of the QW in the SLs can be adjusted by strain engineering, which can be attributed to the different strain dependencies of the two materials' band gaps. These findings will guide the choice of future nanosheet assembled SLs to work on and suggest a new route to facilitate the design of QW based optoelectronic devices.

Up-conversion luminescence of Er(3+)-Yb3+ co-doped CaIn2O4.

Up-conversion emission spectra of Er(3+) singly and Er(3+)/Yb(3+) co-doped CaIn(2)O(4) were investigated under a 980 nm diode laser excitation. Double intense UC emission bands in green and red region are observed in Er(3+)/Yb(3+) co-doped CaIn(2)O(4), resulting in the tunable ratio of red to green and the chromaticity coordinates by adjusting the concentration of Er(3+) and Yb(3+). Based on the pump power dependence, the possible UC mechanism in Er(3+)/Yb(3+) co-doped CaIn(2)O(4) was proposed, and lifetime measurements were also carried out to support our proposal.

Multifunctional Antibacterial Nanoplatform Bi2WO6:Nd3+/Yb3+/Er3+@MoS2 with Self-Monitoring Photothermal and Photodynamic Treatment.

Synergistic therapy combining photothermal therapy and photodynamic therapy is considered to be a promising approach to treat cancer, but the precise temperature control of deep tissue remains a great challenge in achieving effective treatment. Herein, a two-dimensional Bi2WO6:Nd3+/Yb3+/Er3+@MoS2 nanoplatform with photothermal and photodynamic functions was constructed, where semiconductor MoS2 serves as both a photothermal agent and a photosensitizer. The photothermal conversion performance and the reactive oxygen species generation capacity of the nanoplatform were validated under the irradiation of 808 nm laser; meanwhile, the two sets of luminescence intensity ratios (IYb3+/INd3+ and IEr3+/INd3+) in the biological window region were selected as near-infrared temperature probes to monitor the heat generated during the photosynergistic process in real time. The feasibility of nanoplatform as an intratissue temperature probe and antibacterial agent was further assessed by vitro experiments, which provides an idea for designing multifunctional photosynergistic therapy nanoplatform.

Construction of core-shell structured WO3@SnS2 hetero-junction as a direct Z-scheme photo-catalyst.

An ideal photocatalyst not only offers high photo-generated electron-hole pairs separation ability, but also has suitable redox potential. Here, a direct Z-scheme core-shell structured WO3@SnS2 hetero-junction photo-catalyst was prepared via two-step hydrothermal method, in which the core-shell structure, rod morphology and micro-composition of hetero-junction were confirmed through X-ray diffraction (XRD) patterns, Fourier transform infrared (FTIR) spectra, field emission scanning electron microscope (FE-SEM), transmission electron microscope (FEI-TEM) and X-ray photoelectron spectra (XPS). Their enhanced photo-catalytic abilities were evaluated by photo-degradation of Rhodamine (RhB), photo-reduction of dichromate (Cr6+) solution and photo-catalytic H2 production through comparing with pure WO3, SnS2 or the mixture of WO3 and SnS2 (WO3/SnS2). The absorption spectra and electrochemical properties were used to estimate the band gap of samples, the expanded spectral absorption capacity and improved electron-hole separation ability, which are important factors for enhanced photocatalytic performance. Furthermore, the direct Z-scheme charge transfer mechanism of WO3@SnS2 hetero-junction was determined through the combination of theoretical calculation and experimental characterizations, which played a decisive role for retaining excellent redox potential and increasing photo-catalytic ability of WO3 and SnS2.

Upconverting LuVO4:Nd3+/Yb3+/Er3+@SiO2@Cu2S Hollow Nanoplatforms for Self-monitored Photothermal Ablation.

Self-monitored photothermal therapy (PTT) with minimal collateral damages has emerged as a challenging strategy for antibacterial and cancer treatments, which could be fulfilled via the rational integration of luminescent thermometry and photothermal ablation within a single upconverting (UC) nanoplatform. Herein, 808 nm light-driven dual-functional nanoplatforms LuVO4:Nd3+/Yb3+/Er3+@SiO2@Cu2S were successfully developed using olivelike LuVO4:Nd3+/Yb3+/Er3+ hollow nanoparticles as the thermal-sensing core and ultrasmall Cu2S nanoparticles as the photothermal satellite. Irradiated by 808 nm laser, thermal-sensing behaviors of samples were evaluated based on the high-purity Er3+ green emissions, while the surface-attached Cu2S exhibited superior photothermal effects due to the efficient absorption of incident laser and near-infrared emissions from the luminescent core. The feasibility of bifunctional samples acting as self-monitored photothermal agents in subtissues and antibacterial agents against drug-resistant bacteria was separately assessed. Results provide deeper insights into the desirable design of 808 nm-driven multifunctional nanoplatforms with intense UC emission, sensitive thermometry, and effective photothermal conversion toward self-monitored PTT with high therapeutic accuracy.

NIR-to-NIR Deep Penetrating Nanoplatforms Y2O3:Nd3+/Yb3+@SiO2@Cu2S toward Highly Efficient Photothermal Ablation.

A difunctional nano-photothermal therapy (PTT) platform with near-infrared excitation to near-infrared emission (NIR-to-NIR) was constructed through core-shell structures Y2O3:Nd3+/Yb3+@SiO2@Cu2S (YRSC), in which the core Y2O3:Nd3+/Yb3+ and shell Cu2S play the role of bioimaging and photothermal conversion function, respectively. The structure and composition of the present PTT agents (PTAs) were characterized by powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectra. The NIR emissions of samples in the biological window area were measured by photoluminescence spectra under the excitation of 808 nm laser; further, the penetration depth of NIR emission at different wavelengths in biological tissue was also demonstrated by comparing with visible (vis) emission from Y2O3:Yb3+/Er3+@SiO2@Cu2S and NIR emission from YRSC through different injection depths in pork muscle tissues. The photo-thermal conversion effects were achieved through the outer ultrasmall Cu2S nanoparticles simultaneously absorb NIR light emission from the core Y2O3:Nd3+/Yb3+ and the 808 nm excitation source to generate heat. Further, the heating effect of YRSC nanoparticles was confirmed by thermal imaging and ablation of YRSC to Escherichia coli and human hepatoma (HepG-2) cells. Results indicate that the YRSC has potential applications in PTT and NIR imaging in biological tissue.

Local symmetric distortion boosted photon up-conversion and thermometric sensitivity in lanthanum oxide nanospheres.

It is essential to simultaneously boost the luminescence intensity and thermometric sensitivity of up-converted optical thermometers towards potential biomedical sensing applications. Herein, the effects of local site symmetry on the up-conversion (UC) emission and thermal sensing ability in trigonal-phased La2O3:Er3+/Yb3+ nanospheres were qualitatively explored using cubic-phased Lu2O3 and Y2O3 with a similar shape and phonon energy as contrasts. Under 980 nm light excitation, much stronger UC emissions were detected in La2O3 samples than that in cubic Lu2O3 and Y2O3 samples, and the possible mechanisms were elaborately proposed using Eu3+ as a luminescent probe. Thermo-responsive emission intensity from 2H11/2/4S3/2 levels was monitored to evaluate the absolute sensitivity of three samples, which strongly depends on the dopant-induced local site symmetric distortions according to the Judd-Ofelt theory. The potentiality of La2O3:Er3+/Yb3+ for sub-tissue thermometry was also validated by ex vivo experiments. Results open a promising avenue for realizing highly sensitive thermometry with a large signal-to-noise ratio in sub-tissues via finely tailoring the local site symmetry.

808 nm Light-Triggered Thermometer-Heater Upconverting Platform Based on Nd3+-Sensitized Yolk-Shell GdOF@SiO2.

The realization of real-time and accurate temperature reading at subcutaneous level during the photothermal therapy (PTT) could maximally avoid the collateral damages induced by overheating effects, which remains a formidable challenge for biomedical applications. Herein, 808 nm light-driven yolk-shell GdOF:Nd3+/Yb3+/Er3+@SiO2 microcapsules were developed with thermal-sensing and heating bifunctions. Under 808 nm excitation, sensitive thermometry was implemented by monitoring thermoresponsive emission from 2H11/2/4S3/2 levels of Er3+; meanwhile, the addition of Nd3+ with rich metastable intermediate levels and the yolk-shell configuration with large specific surface area triggered efficient light-to-heat conversion via enhanced nonradiative channels. The potentiality of dual-functional samples for controlled subcutaneous photothermal treatment was validated through ex vivo experiments, and the antibacterial activity against Escherichia coli was also elaborately evaluated. Results open a general avenue for designing and developing upconverting platforms with sensitive thermal-sensing and efficient heating bifunctions, which makes a significant step toward the achievement of real-time controlled PTT.

Ab Initio Site Occupancy and Far-Red Emission of Mn4+ in Cubic-Phase La(MgTi)1/2O3 for Plant Cultivation.

Mn4+-activated oxide phosphors La(MgTi)1/2O3 (LMT) with far-red emitting were prepared via a sol-gel route. The structures of samples were determined by X-ray diffraction (XRD) and Reitveld refinement. The occupied sites of Mn4+ (d3 electronic configuration) in host La(MgTi)1/2O3 were confirmed by ab initio calculations in which the system has the lower formation energy, stable lattice structure, and strong bonding state as Mn4+ enters into Ti site. The luminescent properties of Mn4+-doped samples were investigated; the samples emit far-red light centered at 708 nm with ultraviolet light (345 nm) or blue light (487 nm) excitation. According to the photoluminescence (PL) and excitation (PLE) spectra, the crystal field strength of the Mn4+-occupied environment was estimated. The thermal stability of phosphor was also evaluated through temperature-dependent PL intensity in a heating and cooling cycle process. The emission band is well-matched with the absorption band of phytochrome PFR under the excitation of light in near-ultraviolet to blue, which suggests that the LMT: Mn4+ phosphor has great potential applications in light-emitting diodes (LEDs) for modulating plant growth.

Aluminate Red Phosphor in Light-Emitting Diodes: Theoretical Calculations, Charge Varieties, and High-Pressure Luminescence Analysis.

Searching for a non-rare-earth-based oxide red-emitting phosphor is crucial for phosphor-converted light-emitting diodes (LEDs). In this study, we optimized a blue and UV-light excited Sr4Al14O25:Mn phosphor exhibiting red emission peaked at ∼653 nm, which was successfully synthesized by solid-state reaction. The crystal structure, micromorphology, and luminescent properties of Sr4Al14O25:Mn phosphors were characterized by X-ray Rietveld refinement, high-resolution transmission electron microscopy, and photoluminescence spectra. The band gap and electronic structure of Sr4Al14O25 were analyzed by density functional theory calculations using the hybrid exchange-correlation functional. The crystal field environment effect of Al sites from introducing activator Mn ions was investigated with the aid of Raman 27Al nuclear magnetic resonance spectra and electron spin resonance. The pressure dependent luminescent properties and decay time of this compound were presented. The tricolor display spectrum by combining blue InGaN chips, commercial ß-SiAlON:Eu2+ green phosphor, and Sr4Al14O25:Mn red phosphor were evaluated for commercial applications: using the present Sr4Al14O25:Mn red phosphor converted LED as a backlighting source.

Mechanism and stability of spectrally pure green up-conversion emission in Yb(3+)/Ho(3+) co-doped Ba5Gd8Zn4O21 phosphors.

A series of green-emitting up-conversion (UC) phosphors Ba5Gd8Zn4O21:Yb(3+),Ho(3+) were prepared by a modified sol-gel method, and X-ray diffraction (XRD) patterns were measured to characterize the crystal structure. The obtained UC samples emit dazzling green light and their spectra are composed of strong green emission peaking at 544 nm and negligible red emission peaking at 666 nm with the excitation of a 980 nm near-infrared (NIR) laser diode, assigned to (5)F4/(5)S2 → (5)I8 and (5)F5 → (5)I8 transitions of Ho(3+), respectively. The dependence of UC spectra on dopant contents, temperature and pumping power was employed to analyze UC emission color stability. The possible UC mechanisms and processes were proposed based on dependence of the UC emission intensity on pump power, and the lifetimes of green emission ((5)F4/(5)S2 → (5)I8) were also investigated to better comprehend the energy transfer (ET) process. The origin of spectrally pure green-emitting was discussed in detail by analyzing UC and down-conversion (DC) spectra in both the visible and NIR region by comparing that of the UC phosphor CaIn2O4:Yb(3+)/Ho(3+) with highly efficient green emission. Results suggest that Ba5Gd8Zn4O21:Yb(3+),Ho(3+) phosphors with intense green emission and high color purity have potential applications in displays and illuminating technology.

Site-Dependent Luminescence and Thermal Stability of Eu(2+) Doped Fluorophosphate toward White LEDs for Plant Growth.

Eu(2+) activated fluorophosphate Ba3GdNa(PO4)3F (BGNPF) with blue and red double-color emitting samples were prepared via a solid-state method in a reductive atmosphere. Their crystal structure and cationic sites were identified in light of X-ray diffraction pattern Rietveld refinement. Three different Ba(2+) sites, coordinated by six O atoms referred to as Ba1, two F and five O atoms as Ba2, and two F and six O atoms as Ba3, were partially substituted by Eu(2+). Photoluminescence emission (PL) and excitation (PLE) spectra of phosphor BGNPF:Eu(2+) along with the lifetimes were characterized at the liquid helium temperature (LHT), which further confirm the existence of three Eu(2+) emitting centers resulting in 436, 480, and 640 nm emission from the 5d → 4f transitions of Eu(2+) in three different Ba(2+) crystallographic sites. These emissions overlap with the absorption spectra of carotenoids and chlorophylls from plants, which could directly promote the photosynthesis. Temperature-dependent PL spectra were used to investigate the thermal stability of phosphor, which indicates that the PL intensity of BGNPF:0.9% Eu(2+) with optimal composition at 150 °C still keeps 60% of its PL intensity at room temperature, in which blue emission has higher thermal-stability than the red emission. Furthermore, the approaching white LED devices have also been manufactured with a 365 nm n-UV LED chip and present phosphor, which make operators more comfortable than that of the plant growth purple emitting LEDs system composed of blue and red light. Results indicate that this phosphor is an attractive dual-responsive candidate phosphor in the application n-UV light-excited white LEDs for plant growth.

Sensitivity Modulation of Upconverting Thermometry through Engineering Phonon Energy of a Matrix.

Investigation of the unclear influential factors to thermal sensing capability is the only way to achieve highly sensitive thermometry, which is greatly needed to meet the growing demand for potential sensing applications. Here, the effect from the phonon energy of a matrix on the sensitivity of upconversion (UC) microthermometers is elaborately discussed using a controllable method. Uniform truncated octahedral YF3:Er3+/Yb3+ microcrystals were prepared by a hydrothermal approach, and phase transformation from YF3 to YOF and Y2O3 with nearly unchanged morphology and size was successfully realized by controlling the annealing temperature. The phonon energies of blank matrixes were determined by FT-IR spectra and Raman scattering. Upon 980 nm excitation, phonon energy-dependent UC emitting color was finely tuned from green to yellow for three samples, and the mechanisms were proposed. Thermal sensing behaviors based on the TCLs (2H11/2/4S3/2) were evaluated, and the sensitivities gradually grew with the increase in the matrix's phonon energy. According to chemical bond theory and first-principle calculations, the most intrinsic factors associated with thermometric ability were qualitatively demonstrated through analyzing the inner relation between the phonon energy and bond covalency. The exciting results provide guiding insights into employing appropriate host materials with desired thermometric ability while offering the possibility of highly accurate measurement of temperature.

Color tunable emission in Ce3+ and Tb3+ co-doped Ba2Ln(BO3)2Cl (Ln=Gd and Y) phosphors for white light-emitting diodes.

Ce(3+) and Tb(3+) co-doped Ba2Ln(BO3)2Cl (Ln=Y and Gd) green emitting phosphors were prepared by solid state reaction in reductive atmosphere. The emission and excitation spectra as well as luminescence decays were investigated, showing the occurrence of efficient energy transfer from Ce(3+) to Tb(3+) in this system. The phosphors exhibit both a blue emission from Ce(3+) and a green emission from Tb(3+) under near ultraviolet light excitation with 325-375 nm wavelength. Emission colors of phosphors could be tuned from deep blue through cyan to green by adjusting the Tb(3+) concentrations. The energy transfer efficiency and emission intensity of Ba2Y(BO3)2Cl:Ce(3+), Tb(3+) precede those of Ba2Gd(BO3)2Cl:Ce(3+), Tb(3+), and the sample Ba2Y(BO3)2Cl:0.03Ce(3+), 0.10Tb(3+) is the best candidate for n-UV LEDs.