In our previous study, we have reported the cooperative luminescence of Yb3+-trimers and cooperatively sensitized Gd3+ luminescence by Yb3+-tetramers in a doped CaF2 host. In this study, we experimentally observed an unusual luminescent phenomenon of Gd3+ in CaF2:Yb3+/Gd3+. Upon excitation with a 980 nm laser, the upconversion luminescence of Gd3+ first increases and then decreases in the Gd3+ concentration range of 0–0.9 mol%; this is different from the monotonic increase of Gd3+ luminescence observed in the downconversion spectra via the direct excitation of Gd3+. This special luminescent behavior was indicated to be related to the energy transfer from the Yb3+-tetramers to Gd3+ and the destruction of Yb3+-clusters. Herein, we proposed a new luminescence quenching mechanism, Yb3+-cluster destructive quenching, which was verified by fluorescence dynamic analysis and an optically inactive rare-earth ion-doping experiment.

•A new type of NIR photocatalyst was designed and prepared.•Only a one-step energy transfer was involved for NIR photocatalysis.•The present photocatalyst exhibited a higher photocatalytic activity than that of our previously reported photocatalyst.•This new type of photocatalyst showed good photochemical stability.It is a challenge to develop efficient photocatalyst that can be activated by photons with long wavelength. In our previous work, a near infrared (NIR) driven photocatalyst (e.g., NaYF4:Yb,Tm@TiO2) was designed, for which, however, a two-step energy transfer was involved for photocatalysis so that the ultimate photocatalytic capability is low. In this work, we designed and prepared a new type of NIR photocatalyst composed of cooperative luminescence agent CaF2:Yb and semiconductor BiVO4, in which a one-step energy transfer occurs to realize NIR photocatalysis. Steady-state and dynamic fluorescence spectroscopy analysis revealed that cooperative energy transfer of a Yb3+-dimer to BiVO4 leads to the indirect excitation of semiconductor BiVO4 by NIR light. The degradation of methylene blue (MB) compound by CaF2:1%Yb3+@BiVO4 particles upon NIR radiation and its corresponding controlled experiments demonstrated the NIR light responsive photocatalytic capability. It is noteworthy that nearly 80% degradation ratio of MB was achieved with 7 h of NIR light irradiation, indicating a relatively high photocatalytic activity, compared with that of our previously reported system NaYF4:Yb,Tm@TiO2, for which ∼65% of degradation ratio of MB was obtained within 14 h of NIR irradiation. Recycling experiments of photocatalysis indicate that our present CaF2:Yb@BiVO4 composite has good photochemical stability in spite of a long time of NIR irradiation.Download high-res image (145KB)Download full-size image

•We propose a new fluorescence quenching mechanism---structure-destroyed fluorescence quenching.•This fluorescence quenching belongs to the static quenching. In other words, the doped Al3+or Y3+ions induce a new fluorescence quenching that comes from the destruction of Yb3+-trimer and Yb3+-dimer structures.•The doped Al3+or Y3+ ions destroyed Yb3+clusters, which cause the intensity of TCL and DCL decreased gradually.•On the other hand, the reason is also that the structural destruction of Yb3+clusters reduced two depopulation channels of excited Yb3+ ions when increase of doped Al3+concentration.•Therefore, the decay time of Yb3+-trimers and Yb3+-dimers increased a little.The cooperative luminescence of Yb3+-trimers and Yb3+-dimers were observed in Yb3+-doped CaF2 under excitation of near infrared (NIR) light at room temperature. With the increase of doped Al3+ or La3+ concentration, the cooperative luminescence from 3-Yb3+ trimers and 2-Yb2+ dimers decreased gradually and until to extinguished. In order to explain the phenomenon, in this paper, we propose a new fluorescence quenching mechanism, structure-destroyed fluorescence quenching. This new fluorescence quenching belongs to the static quenching.

•We report the first experimental observation of upconversion (UV) triplet cooperative luminescence (TCL) peaked at 343 nm from Yb3+-trimer in SrF2.•The UV TCL intensity in the CaF2 and SrF2 are compared at room temperature.•We discuss why the cooperative luminescence intensities of CaF2: 1%Yb3+ in the ultraviolet and visible region is much stronger than the SrF2: 1%Yb3+.The triplet cooperative luminescence (TCL) of Yb3+- doped AF2 (A=Ca, Sr) was studied under excitation of near infrared (NIR) light. First, the TCL from Yb3+ ions in the SrF2: Yb3+ was studied for the first time under NIR excitation. Next, the TCL and dimer cooperative luminescence (DCL) intensities in the CaF2 and SrF2 are compared at room temperature. Finally, we discuss why the cooperative luminescence intensities of CaF2: 1%Yb3+ in the ultraviolet (UV) and visible region is much stronger than the SrF2: 1%Yb3+.

•Tm3+ doped β-NaYbF4 with different aspect ratios has been synthesized by a facile hydrothermal method.•β-NaYbF4 microrods with 35 μm in length and 7.5 μm in diameter were prepared when NaF/Re molar ratio is 6.•Intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from as-prepared microrods under 980 nm excitation.Tm3+ doped β-NaYbF4 with different aspect ratios has been synthesized by a facile hydrothermal method. A different morphology and size of β-NaYbF4 microrods were obtained by changing the NaF/RE (RE = Yb,Tm, 99.5% Yb3+, 0.5% Tm3+) molar ratio. The morphology and size of products were explored by SEM, and the influences of the NaF/RE molar ratio on the microrod size were evidenced by XRD. It is shown that the size and morphology of the products can be precisely controlled by modulation of the NaF/RE molar ratio. β-NaYbF4 microrods with 35 μm in length and 7.5 μm in diameter were prepared when NaF/RE molar ratio is 6. Intense ultraviolet emissions at 290 nm, 345, and 361 nm and visible emissions at 451 and 494 nm were observed in Tm3+ doped NaYbF4 microrods with an excitation of a 980 nm diode laser. The effect of NaF/RE ratio on growth rate has been proposed.Tm3+ doped β-NaYbF4 with different aspect ratios has been synthesized by a facile hydrothermal method. Under 980 nm excitation, intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from as-prepared microrods.Download high-res image (232KB)Download full-size image

•Single-layer MnO2 nanosheets synthesized by a template-free and one-step method.•Rapid response for colorimetric detection of glutathione (GSH).•Low detection limit of 0.5 μM.•Selective response toward GSH.The rapid, sensitive and selective detection of glutathione (GSH) is of great importance in the biological systems. In this work, a template-free and one-step method was used to synthesize the single-layer MnO2 nanosheets via a redox reaction. The resulting product was characterized by XRD, TEM, FTIR, XPS and UV–vis absorption. The addition of GSH results in the change of solution color depth owing to the occurrence of a redox reaction between MnO2 and GSH, enabling colorimetric detection of GSH. At a pH of 3.6, the proposed sensor gives a linear calibration over a GSH concentration range of 10–100 μM, with a rapid response of less than 2 min and a low detection limit of 0.5 μM. The relative standard deviation for seven repeated determinations of GSH is lower than 5.6%. Furthermore, the chemical response of the synthesized MnO2 nanosheets toward GSH is selective. Owing to the advantages with good water solubility, rapid response, high sensitivity, good biocompatibility and operation simplicity, this two-dimensional MnO2-based sensing material might be potential for detecting GSH in biological applications.

Under 978 nm near infrared (NIR) excitation, ultraviolet (UV) upconversion (UC) emissions from Pb2+ ions in CaF2:Pb2+,Yb3+ were first observed at room temperature. The UC emission centered at ∼383 nm was ascribed to the 3P0 → 1A1g (1S0) transition of Pb2+ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process: three excited Yb3+ ions simultaneously transfer their energy to one Pb2+ ion. With the increase in Pb2+ concentration, the cooperative luminescence from 3-Yb3+ clusters decreased gradually until it was extinguished. The cooperative sensitization to one Pb2+ ion comes from three excited Yb3+ ions, which is a four-ion process and exhibits a third power dependence on the NIR pumping power.

Herein, we report the phenomenon of upconversion luminescence from Sm2+ ions, which demonstrates that changeable valence lanthanides can serve as ions for optical frequency transformation. Upon excitation with a 980 nm diode laser, the doped Sm2+ ions in the hybrid material BaFCl0.5Br0.5:1%Sm2+–CaF2:1%Yb3+ emit red upconversion fluorescence peaks at 631 nm, 644 nm, 665 nm, 689 nm, 704 nm, and 729 nm from the 5D0,1 → 7F0,1,2 transitions. By transient dynamic analysis, we attributed the excitation of Sm2+ ions to the cooperation energy transfer process, in which two excited Yb3+ ions simultaneously transfer their energy to one Sm2+ ion.

Sub-25 nm β-NaLuF4 nanocrystals (NCs) with high homogeneity, monodispersity, and good crystallinity were synthesized through thermal decomposition at different temperatures (285, 295, 305, and 315 °C), using an automatic nanomaterial synthesizer. The phase transition of NaLuF4 NCs from α-phase to β-phase suggests that the entire growth process consists of four stages: a cubic phase, the coexistence of cubic and hexagonal phases, a homogeneous hexagonal phase, and an inhomogeneous hexagonal phase, caused by Ostwald ripening. Based on this observation, we obtained the growth phase diagram of NaLuF4:Yb3+/Tm3+/Gd3+ NCs, relating to the reaction time, temperature, crystal size, and crystal phase. From spectral analysis, the products prepared in the third stage were shown to exhibit the characteristics of having small size (sub-25 nm), uniformity, and high upconversion luminescence (UCL) intensity. On the other hand, the luminescence properties of NaLuF4:Yb3+,Tm3+ were studied via incorporating different concentrations of Gd3+ ions. These results indicated that 10% Gd3+ was the best doping concentration for obtaining NaLuF4 NCs with relatively small size and high brightness. Further increasing the Gd3+ concentration induced a drastic decrease in UCL intensity.

We demonstrate a passively Q-switched Yb3+/Er3+ co-doped fiber (YEDF) laser using a filmy sodium carboxymethylcellulose (NaCMC)-based Cu1.8S nanocrystals (NCs) saturable absorber (SA). Cu1.8S NCs SA exhibit a broad absorption band from 600 nm to more than 2500 nm. By placing Cu1.8S NCs SA into a YEDF laser cavity, stable passively Q-switched laser with a central wavelength of ∼1567.2 nm was achieved at a threshold pump power of ∼1.4 W. On gradually increasing the pump power from 1.4 W to 5.6 W, the repetition rate of Q-switched laser increases from 16.6 kHz to 51.14 kHz and the pulse duration decreases from 8.7 μs to 2 μs. Particularly, we measure the output power of Q-switched lasers based on two types of plasmonic materials, Cu1.8S NCs and gold nanorods (GNRs). The maximum output power of the Q-switched laser based on Cu1.8S NCs SA is 3–4 times higher than that based on GNRs SA owing to weak photothermal effect of Cu1.8S NCs. These results show that Cu1.8S NCs are promising SAs for constructing high power pulse lasers.

Despite showing great promise in bioapplications, lanthanide-doped upconversion nanoparticles (UCNPs) are still restricted by low upconversion efficiency. In this work, ultra-small, monodisperse and uniform β-NaYbF4:0.5% Tm NPs were successfully prepared through a solvothermal route, and remarkably enhanced ultraviolet upconversion luminescence (UV UCL) had been achieved via Fe3+ doping. The UV UCL enhancement in NaYbF4:0.5% Tm NPs was investigated in detail. It was found that the UCL intensities of the UV and blue emissions of NPs doped with 5 mol% Fe3+ ions were enhanced by 10 and 6 times, respectively, compared to that of the Fe3+-free sample. The enhancement of UV UCL is mainly ascribed to the high population density of excited Yb3+ and the energy transfer from |2F7/2, 4T1g > (Yb3+–Fe3+ dimer) to 3F2,3 (Tm3+) states. Moreover, through dynamic analysis, we further demonstrate the proposed mechanisms of UC processes. The results suggest that ultra-small UCNPs with enhanced UV UCL may have potential applications in the biomedical field.

In response to the comment by L. Marciniak et al. (J. Mater. Chem. C), we maintain the determination of sensor sensitivity by using the equation of because all the levels of RE3+ ions can be thermally coupled according to Boltzmann distribution.

Under 978 nm near-infrared (NIR) excitation, blue upconversion (UC) emissions from CaF2:Cu2+,Yb3+ were first observed at ∼420 nm. It was ascribed to the 3d84s1 → 3d9 transition of Cu2+ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process in which three excited Yb3+ ions simultaneously transfer their energy to one Cu2+ ion. The influence of Cu2+ concentration and temperature on the UC emission as well as the Jahn–Teller effect was also investigated.

White upconversion luminescence (UCL) was achieved under 980 nm excitation in the CaF2:Yb3+/Eu3+ material using Y3+ to adjust the luminescence performance. In this luminescent system, Yb3+ not only plays the role of a sensitizer of Eu3+, but also generates green fluorescence from Yb3+ dimers (2-Yb3+) by cooperative transitions in the CaF2 matrix. One of the primary colors of green corresponds to the 2-Yb3+ cooperative emission exactly. Eu3+ acts as an activator for emitting red and blue fluorescence simultaneously. Interestingly, the color of the UCL can be controlled by adjusting the doping concentration of Y3+ ions, and white UCL was realized when the concentration of Y3+ was 1%.

Mesoporous-silica-coated Gd2O3:Eu/silica nanoparticles were synthesized by a multistep chemical process and characterized by XRD, TEM and N2 adsorption/desorption isotherms in terms of size, morphology and porosity. The core Gd2O3:Eu obtained by this method was highly luminescent upon excitation, giving the function of cell imaging upon incubation with the human cervical carcinoma (HeLa) cells. The outer porous silica shell is able to load the anticancer drug with a relatively high loading efficiency and release the loaded drugs at a sustained rate. The HeLa cells can be killed effectively on incubation with the core–shell porous particles loaded with the anticancer drug DOX. Meanwhile, the accumulation of mesoporous nanoparticles loaded with drugs in the target location could be monitored via fluorescence imaging. Therefore, the core–shell hybrid nanoparticles presented in this work are potential multifunctional biomaterials for smart detection or diagnosis and therapy in future biomedical engineering.

A facile float zone method to grow ZnO microrods with a hexagonal crystal structure is described. It was found that the crystal-growth mechanism was different from the well-known vapor–liquid–solid (VLS) growth mechanism. However, the one-dimensional growth morphologies occurring in the vapor phase are similar to those of ZnO grown using conventional VLS processes. The free-exciton, bound-exciton, acceptor-exciton, two-electron satellite emission, and their phonon replicas, obtained from the 15 K photoluminescence (PL) spectra of the ZnO microrods, were recorded. The PL spectra of the ZnO microrods at temperatures between 15 and 150 K show that the bound-exciton peak dissociates into a free-exciton peak and that the free-exciton emission and its phonon replicas dominate at temperatures above 120 K. Following the approach of Viswanath for measuring the intensity ratio of PL peaks, we found that the strongest bound-exciton peak at 3.3615 eV had a thermal activation energy of 15.9 meV, consistent with the value expected for the exciton-defect binding energy. This bound-exciton peak was not observed at temperatures above 120 K.

The most common materials used to generate near-infrared-driven photocatalysis occur by 980 nm laser excitation of composites that are a combination of a semiconductor and upconverting luminescence particles. The challenge remains to increase the light harvesting efficiency, and thus, it is necessary to extend the absorption spectra of photocatalysts. In this work, NaYF4:Yb,Er/CdSe composites were prepared by depositing CdSe nanocrystals onto the surface of NaYF4:Yb,Er microcrystals. UV and visible emission of light resulted from multiphoton upconverting processes in Er3+ under 1560 nm laser irradiation, which, in turn, activated the CdSe catalyst. The energy transfer between NaYF4:Yb,Er and CdSe was investigated by steady-state and dynamic fluorescence spectroscopy. The photocatalytic performance was investigated by the degradation of methylene blue in aqueous solution. These results show that Er coupled with semiconductor heterojunctions provides a photocatalyst that operates in the extended near-infrared range.

•We demonstrated a wet-chemical synthesis of NaYF4:20%Yb, 0.5%Tm/CdSe nanoheterostructures.•Greatly enhanced NIR emission and suppressed blue & UV emissions was observed.•The nanoheterostructures exhibit high-efficiency ET between NaYF4:Yb,Tm and CdSe.NaYF4:Yb3+,Tm3+/CdSe nanoheterostructures were synthesized by an improved two-step wet-chemical route. Upon the excitation from a 980-nm near-infrared (NIR) laser, an intense 797 nm NIR emission from Tm3+ ions was observed in the nanoheterostructures while neither ultraviolet (UV) nor blue emissions of Tm3+ ions appeared in the upconversion (UC) spectrum. It indicated that the energy transfer (ET) occurred between NaYF4:Yb3+,Tm3+ and CdSe. Under 980 nm irradiation, the excitation energy absorbed by Yb3+ ions transferred to Tm3+ ions successively, then transferred to CdSe partially from the 1G4 state of Tm3+ ions, and finally the excited CdSe returns the energy back to the 3F2,3 levels of Tm3+ ions. Both the excitation (980 nm) and the emission (797 nm) are located in the NIR spectral range (700–1100 nm) that is referred as the “optical window” of biological tissues. Therefore, the nanoheterostructures could have great potential in applications of biology and biomedicine.

•A simple and sensitive reaction system for colorimetric detection of H2O2 and glucose.•Low detection limit: 0.3 μM and 0.4 μM for H2O2 and glucose, respectively.•High stability and reproducibility.•Highly specific response toward glucose.The detection for H2O2 is essential in many areas, including life activity, medical diagnosis, industry and agriculture production and environmental monitoring, etc. This work developed a simple and sensitive two-step reaction system Ce(OH)CO3/H2O2/TMB for H2O2 determination. Upon sequential addition of H2O2 and TMB to Ce(OH)CO3 powders, a typical color reaction occurred quickly, producing a characteristic blue color in a slightly acidic aqueous solution. The underlying reaction mechanism was proposed based on the color reaction catalyzed by mimetic enzyme. The dependence of the color depth on H2O2 concentration enabled the colorimetric determination of H2O2. This reaction system responds linearly and quickly in a wide H2O2 concentration range of 0–80 μM, and achieves a detection limit of 0.3 μM H2O2 and a relative standard deviation lower than 5.1%. This H2O2 sensing system was modified to allow for the detection of glucose since H2O2 is one of the main products in the oxidation reaction of glucose catalyzed by oxidase enzymes. In addition to a wide linear response, a low detection limit and a high reproducibility, our present reaction system for glucose determination showed a highly specific response to glucose due to the specificity of glucose oxidase to glucose.

A strategy for enhancing the sensitivity of optical thermometers is developed herein by using non-thermally coupled levels of Er3+. Under the excitation of a 980 nm laser, the temperature dependence of 244 nm and 256 nm upconversion luminescences (UCLs) of Er3+ was studied. The corresponding 2I11/2 and 4D7/2 levels were confirmed to be non-thermally coupled levels. By using the fluorescence intensity ratio (FIR) technique and by investigating different thermal population behaviors of 2I11/2 and 4D7/2 levels, the optical temperature sensing performance based on the non-thermally coupled levels of Er3+ was fulfilled here for the first time. The obtained maximum sensor sensitivity is 0.106 K−1 at 525 K, which is much higher than those of all other RE3+ doped optical thermometers using the thermally coupled level-based FIR technique. This suggests that the use of the FIR from neighboring non-thermally coupled levels of RE3+ is a promising approach for enhancing the sensor sensitivity of optical thermometers.

We demonstrated optical amplification at 650 nm in KMnF3:Yb3+,Er3+@KMnF3:Yb3+ active-core–active-shell nanoparticle (NP) doped polymer waveguides pumped by a 976 nm laser diode for the first time. KMnF3:Yb3+,Er3+ NPs were synthesized via a solvothermal method. With the excitation of a 976 nm laser diode, bright red upconversion (UC) fluorescence was observed from KMnF3:Yb3+,Er3+ NPs owing to the existence of efficient energy transfer between Er3+ and Mn2+:2H11/2,4S3/2 + 6A1 → 4I15/2 + 4T1,2H9/2 + 6A1 → 4I13/2 + 4T1 and 4I15/2 + 4T1 → 4F9/2 + 6A1. The red UC emissions originated from the 4F9/2 → 4I15/2 transition of Er3+. Furthermore, the red UC emissions of KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs were enhanced by 7.5 times compared to that of KMnF3:18 mol% Yb3+,1 mol% Er3+ core-only NPs after coating an active shell containing Yb3+ ions on the core-only NPs. The above results showed that the active-shell could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shell. By using KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs as the gain medium and doping NPs into a polymer waveguide, we constructed polymer-based waveguide amplifiers. For an input signal power of 7.4 mW and a pump power of 45.2 mW, a relative optical gain of ∼3.5 dB was obtained at 650 nm in a 17 mm-long waveguide.

The demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water-soluble dual mode emission core–shell rare earth nanoprobes (∼30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions co-doped heterogeneous NaYF4/NaLuF4 nanocrystals as cores and amorphous SiO2 embedded with europium (Eu) or terbium (Tb) complexes as shells. Excited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions, which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple upconversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.

Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications. However, it is still a challenge to synthesize ultra-small NPs with high brightness owing to non-radiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands absorbed on the NPs. In this paper, we reported a strategy to improve up- and down-conversion luminescence of ultra-small BaLuF5:Yb3+,Er3+ NPs by using multi-layer active-shells (containing Yb3+). Sub-10 nm BaLuF5:Yb3+,Er3+@(X-shell, X = 1–5)BaLuF5:Yb3+ NPs were synthesized via a high boiling solvent process through a layer-by-layer strategy. Up- and down-conversion fluorescence spectra of the NPs were recorded and analyzed by using a 980 nm laser diode as the excitation source. In comparison with optical properties of BaLuF5:Yb3+,Er3+ NPs, the intensities of up- (∼545 nm) and down-conversion (∼1530 nm) fluorescence were enhanced by 52 and 9.8 times after coating 5-layer active-shells (BaLuF5:Yb3+) on the BaLuF5:Yb3+,Er3+ NPs, respectively. In addition, the intensities of up- and down-conversion fluorescence of the BaLuF5:Yb3+,Er3+ NPs with multi-layer active-shells were 1.3 and 1.1 times larger than those of the BaLuF5:Yb3+,Er3+ NPs with a one thick-layer active shell, respectively. These results showed that multi-layer active-shells could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shells.

Flying particles with visible fluorescence were observed when a target, ZnF2:Tm3+/Yb3+, was irradiated by using a 953.6 nm pulsed infrared laser. Narrow blue beams showed the ejecting particles with upconversion emissions. The changing brightness along fluorescence traces exhibited the energy transfer from Yb3+ ions to Tm3+ ions and the depopulation of excited Tm3+ ions. Parameters of microexplosions induced by the pulsed laser were estimated by analyzing these flying particles. Compared with the target, these particles have strong infrared-to-ultraviolet upconversion ability. These results not only suggest a way to explore microexplosions induced by pulsed lasers but also provide a feasible scheme for developing ultraviolet upconversion materials.

•The Ag3PO4 microcrystals can be controlled by regulating the ratio of water and EG.•The photocatalytic activities of Ag3PO4 with uniform morphology have been improved.•The size and surface area of Ag3PO4 played a key role for DeNOx.Silver phosphates (Ag3PO4) with varied morphology were prepared via an ammonia-mediate method. The as-prepared Ag3PO4 has a cubic-type structure and the morphologies are controlled under different ratio of water and ethylene glycol. The decomposition test of RhB indicated that the uniform morphology of Ag3PO4 exhibit much higher photocatalytic activities than that of not uniform ones under visible light irradiation. The DeNOx experiments revealed that the micronsize Ag3PO4 microcrystals possessed photocatalytic ability lower than that of the commercial Degussa P25 under ultraviolet and visible light irradiation, which suggested that the size of Ag3PO4 is a key role for DeNOx.Graphical abstractThe as-prepared Ag3PO4 has a cubic-type structure and the morphologies are controlled under different ratio of water and ethylene glycol. The decomposition test of RhB indicated that the uniform morphology of Ag3PO4 exhibit much higher photocatalytic activities than that of not uniform ones. The DeNOx experiments revealed that the Ag3PO4 microcrystals possessed photocatalytic ability lower than that of the commercial Degussa P25 under ultraviolet and visible light irradiation.

•β-NaLuF4:Yb3+, Er3+ NCs were synthesized via thermal decomposition.•Polymer waveguides were fabricated by using SU-8 doped with β-NaLuF4:Yb3+, Er3+ NCs.•The optical gain of 2.413 dB was achieved when the NCs size was 8 nm.Hexagonal structured NaLuF4 (β-type) nanocrystals (NCs) with controllable particle size were prepared by thermal decomposition approach. The as-prepared NCs possessed good dispersibility in organic solvents. Under a 980 nm laser excited, these Er3+ and Yb3+ co-doped β-NaLuF4 NCs demonstrated emission around 1530 nm. These β-NaLuF4:Yb3+, Er3+ NCs were dispersed into SU-8 polymer to fabricate polymer waveguides, and the results showed that their crystal size influenced greatly on the performance of polymer waveguides. The relative optical gain of 2.413 dB, 1.653 dB and 0.481 dB at 1530 nm was achieved when the particle size was about 8 nm, 15 nm, and 23 nm, respectively. These results show that β-NaLuF4:Yb3+, Er3+ NCs with small particle size were promising materials for building Er3+-doped polymer-based optical waveguide amplifiers.Hexagonal structured NaLuF4 (β-type) nanocrystals (NCs) with controllable particle size were prepared by thermal decomposition approach. The as-prepared NCs possessed good dispersibility in organic solvents. Under a 980 nm laser excited, these Er3+ and Yb3+ co-doped β-NaLuF4 NCs demonstrated emission around 1530 nm. These β-NaLuF4:Yb3+, Er3+ NCs were dispersed into SU-8 polymer to fabricate polymer waveguides, and the results showed that their crystal size influenced greatly on the performance of polymer waveguides. The relative optical gain of 2.413 dB, 1.653 dB and 0.481 dB at 1530 nm was achieved when the particle size was about 8 nm, 15 nm, and 23 nm, respectively. These results show that β-NaLuF4:Yb3+, Er3+ NCs with small particle size were promising materials for building Er3+-doped polymer-based optical waveguide amplifiers.

•The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method.•With the shell amount increasing, the crystals shape changed from hexagonal plate to hexagonal prism gradually.•The emissions from 1I6 and 1D2 of core–shell crystals were enhanced by several times compared with core-only crystals.•When the shell thickness increased to a certain extent, it was unfavorable for the high-order upconversion luminescence.High-order upconversion luminescence (ultraviolet (UV), violet and blue emissions) of rare earth nanocrystals is quite desirable for photodynamic therapy and near-infrared photocatalysis applications. The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method. With the amount of the shell precursor increasing, the crystals shape changed from hexagonal plate to hexagonal prism gradually, with the particles size increased. After coating a homogeneous inert NaYF4 shell, the high-order upconversion luminescence (1I6 and 1D2 radiative transitions) of all core–shell crystals were enhanced by several times compared with core-only crystals. Impressively, when the amount of the shell precursor was 0.1 mmol, the increased ratio of the emissions from 1I6 and 1D2 were 7.44 and 5.63 times, respectively. With further increasing the amount of the shell precursor, the increased ratio displayed a slight decline. The further photoluminescence decay results were consistent with the upconversion luminescence spectra. We demonstrated that the enhancement of high-order upconversion luminescence was attributed to the well-known surface passivation effect of the homogeneous shell.The hexagonal phase NaYF4:Yb3+,Tm3+/x mmol NaYF4 (x = 0, 0.1, 0.5, and 1) crystals were prepared by a hydrothermal method. After coating a homogeneous shell with the precursor amount of 0.1 mmol, the increased ratio of high-order upconversion luminescence (the emissions from 1I6 and 1D2) were 7.44 and 5.63 times, respectively. With further increasing the amount of the shell precursor, the increased ratio displayed a slight decline. We demonstrated that the enhancement of high-order upconversion luminescence was attributed to the well-known surface passivation effect of the homogeneous shell.

•α/β-­NaYF4:Yb3+, Tm3+/ZnO NCs, a near-infrared (NIR) light photocatalyst, were successfully fabricated by a high temperature decomposition reaction method.•The photocatalytic activities of as-prepared α/β-NYT/ZnO NCs were studied upon NIR irradiation.•The α-NYT/ZnO NCs exhibited higher photocatalytic activity than that of β-NYT/ZnO NCs for the degradation of RhB under NIR irradiation.In this work, the effects of NaYF4:Yb3+, Tm3+ (NYT) structure on the near-infrared light (NIR) photocatalysis performance of ZnO were investigated. α-NYT/ZnO nanocomposites (NCs) and β-NYT/ZnO NCs were fabricated by a high temperature decomposition reaction method. The prepared NCs were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), UV–vis spectroscopy, Raman spectroscopy, and photoluminescence spectra (PL). The NIR photocatalytic activities of the synthesized α- and β-NYT/ZnO NCs were studied by the degradation of Rhodamine B. The differential photocatalytic activities between of α- and β-NYT/ZnO NCs are believed to be correlated to the effects of α- and β-NYT on the oxygen vacancies and exposed crystal face of ZnO.

Here we show that a near-infrared (NIR) diode laser is capable of generating vacuum ultraviolet (VUV) emissions in fluoride nanocrystals through photon upconversion (UC) processes. By using Yb3+ and Tm3+ as sensitizers, we successfully obtained the VUV photons with the energy exceeding 6 eV in YF3: Yb, Tm, and Gd nanocrystals. The seven photon UC fluorescence from the 6GJ → 8S7/2 transitions of Gd3+ ions and the possible VUV UC mechanism were reported along with the calculation of the branching ratio under different pumping power excitation. Practically, it offers a promising solution for VUV light generation without cryogens and expensive instrumentations. Fundamentally, the extremely high-order UC processes will intrigue great interest in exploring unusual high-energy radiative transitions in rare earth ions.

Ultrasmall upconversion (UC) nanoparticles with intense luminescence are quite desirable for in vitro and in vivo bioimaging. Herein, we report the successful synthesis of sub-10 nm hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals via a high boiling solvent process. The sizes and phases of the crystals can be finely tailored by tuning the amounts of the ligands used (octadecylamine and sodium oleate) and the temperature. The intensity of the UC emission of the nanocrystals increases as the the relative content of Yb3+ ions increases from 20% to 98%, owing to improved near-infrared absorption and energy-transfer from Yb3+ to Tm3+. The experimental results indicate that hexagonal NaYbF4 is the best host for ultrasmall Tm3+-doped UC nanoparticles and shows great potential in biomedical applications.

Hydrogen peroxide (H2O2) is an essential molecule in intracellular signaling transduction and normal cell functions. It is critical to be able to detect H2O2 quantitatively in cellular processes for getting useful physiological information. Herein, we developed a novel fluorescent probe for H2O2 sensing, CePO4:Tb colloidal solution. Upon addition of H2O2, the luminescence of the colloidal CePO4:Tb solution responds linearly in a wide H2O2 concentration range of 0–200 μM, allowing for quantitative detection of H2O2. The H2O2 sensing by this method exhibits a rapid response within several minutes, a detection limit of 1.03 μM H2O2, and a relative standard deviation lower than 3.1%. This sensing material for H2O2 is also suitable for the detection of glucose since H2O2 is generated via the catalytic oxidation of glucose by oxidase enzymes. In addition to a wide linear response, a low detection limit and a high reproducibility, our present method for glucose sensing shows a highly specific response to glucose in a mixed carbohydrate solution due to the specificity of glucose oxidase to glucose. This lanthanide-based fluorescent sensing material might have potential for detecting H2O2 and glucose in biological applications.

The present work reports a self-sacrificing template strategy to synthesize porous α-NaYF4 microspheres via the reaction of as-prepared Y(OH)CO3·H2O@SiO2 with NH4F and NaNO3 solutions. XRD, SEM, TEM and N2 adsorption–desorption measurements were used to characterize the resulting product. The surface SiO2 shell was shown to play a vital role in size and shape control and porosity formation. A possible reaction mechanism was explored in terms of a surface-protected etching and ion-exchange reaction process. To explore their application potential, the storage and release behavior of Rhodamine 640 dye in the porous α-NaYF4 microspheres was investigated, showing a relatively high loading efficiency and a sustained release ratio. Under near-infrared (NIR) irradiation, porous α-NaYF4 microspheres doped with lanthanide ions showed typical upconverting luminescence characteristics that can convert NIR photons to ultraviolet/visible photons. The above features and properties indicate that our present porous upconverting luminescence particles are promising in biological applications as luminescence imaging agents and drug carriers.

Lu2O3:Yb3+/Er3+ nanocrystals with various sizes and shapes (nano-aggregates, sub-micrometer wires, and nanospheres) have been synthesized by the soft chemistry coprecipitation route. By regulating the reactant ratio of rare earth to urea precipitant ([RE3+]/(NH2)2CO), uniform spherical Lu2O3:Yb3+/Er3+ nanoparticles with the sizes of 45, 100, 165, 200, and 250 nm were obtained in the experiments. The phases, morphologies, as well as the luminescence properties of as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and upconversion luminescence (UCL) spectra, respectively. Under the excitation of a 980 nm diode laser, the Lu2O3:Yb3+/Er3+ samples showed size-dependent upconversion luminescence property. From the results of Fourier transform infrared (FTIR) spectra and dynamical analysis, it was confirmed that the surface effect is dominant in influencing the UCL of Er3+ in the spherical Lu2O3:Yb3+/Er3+ samples. By coating Lu2O3:Yb3+/Er3+ nanospheres with different shell layers, UCLs of Er3+ were changed greatly in these core–shell samples. The possible physical mechanism involved in size/surface-dependent upconversion processes was discussed in detail.

A heterogenous core/shell strategy has been introduced to induce the growth of hexagonal phase NaREF4 shells. During the core-mediated hetero-shell growth process, the cores significantly affect the crystalline phase of shells. Using heterogeneous cubic nanocrystals as cores, the resulting sub-30 nm hybrid nanoparticles possessed both hexagonal phase shells and good water solubility.

•The synthesis of YbF3 and NaYbF4 crystals was successfully fulfilled by hydrothermal method.•The phase and morphology of products were adjusted by changing the hydrothermal conditions.•Relatively enhanced ultraviolet upconversion emissions were observed in YbF3 nanocrystals.•The crystalline phase impact on the upconversion luminescence was systematically studied.The synthesis of YbF3 and NaYbF4 crystals was successfully fulfilled by a facial hydrothermal method. The phase and morphology of the products were adjusted by changing the surfactant additive and fluorine source and tuning the pH value of the initial solution. The products with various morphologies range from octahedral nanoparticles, corn-like nanobundles, nanospheres, microrods, and hollow microprisms were prepared at different conditions. The growth mechanism of these products has been systematically studied. Impressively, relatively enhanced high order ultraviolet (UV) upconversion (UC) luminescence was observed in Tm3+ (Er3+) ions doped YbF3 nanocrystals (NCs) compared with NaYbF4 microcrystals under the excitation of 980 nm infrared laser. The investigation results reveal that the crystal symmetry of matrix has significant effect on the spectra and lifetimes of the doping lanthanide ions. The simply synthesized water soluble YbF3 NCs with efficient UV UC luminescence may find potential application in biochemistry.Graphical abstract

Cooperative luminescence (CL) occurs in spectral regions in which single ions do not have energy levels. It was first observed more than 40 years ago, and all results reported so far are from a pair of ions. In this work, upconverted CL of three Yb3+ ions was observed in the ultraviolet (UV) region under near-infrared (NIR) excitation. The UV CL intensity showed a cubic dependence on the NIR pump power, whereas the luminescence lifetime was nearly one-third the luminescence lifetime of single Yb3+ ions. The triplet CL (TCL) has a clear spectral structure, in which most emission peaks are consistent with the self-convoluted spectra from single Yb3+ ions. Blue shifts were observed for certain peaks, indicating complex interactions among the excited Yb3+ ions. The probability of the TCL process versus the average distances among three Yb3+ ions was derived via the first- and second-order corrections to the wave functions of lanthanide ions, indicating that the formation of Yb3+ clusters containing closely spaced ions favors the occurrence of the multi-ion interaction processes. Furthermore, the cooperative sensitization of one Gd3+ ion by four excited Yb3+ ions (Yb3+-tetramer) was demonstrated experimentally, which exhibited a novel upconversion mechanism—cluster sensitization. Our results are intriguing for further exploring quantum transitions that simultaneously involve multiple ions.

•The CdS nanoparticles (NPs) have been embedded among TiO2 NPs to form nanospheres (NSs).•CdS/TiO2 NSs with a size of ∼150 nm have good dispersion.•The prepared CdS/TiO2 hybrid NSs extended to the visible light region at about 470 nm.•CdS/TiO2 NSs showed obviously improved photocatalytic activity compared to pure TiO2 or CdS.A novel CdS/TiO2 hybrid for visible photocatalysis was prepared by solvent thermal processes. The prepared samples were characterized by X-ray diffraction (XRD), UV–vis absorption spectroscopy (UV–vis), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). It was found that the cubic phase CdS nanoparticles (NPs) have been embedded and dispersed among anatase TiO2 NPs to form the uniform spheres with a size of ∼150 nm. In comparison with pure TiO2, UV–vis absorption spectroscopy confirmed that the photoresponse of the prepared CdS/TiO2 hybrid NSs extended to the visible light region at about 470 nm. Meanwhile, the CdS may be reducing the contact with the external environment due to CdS embedded in the TiO2. As a result, compared to pure TiO2, CdS and the mixture of TiO2 and CdS, the CdS/TiO2 hybrid nanospheres (NSs) exhibited obviously enhanced photocatalytic activity in the decomposition of methyl orange under visible light irradiation. The mechanism of photocatalysis on CdS/TiO2 NSs under visible light is also discussed.The CdS nanoparticles (NPs) have been embedded among TiO2 NPs to form the uniform spheres with a size of ∼150 nm by solvent thermal processes. Compared to pure TiO2, CdS and the mixture of TiO2 and CdS, the CdS/TiO2 hybrid NSs exhibited obviously enhanced photocatalytic activity in the decomposition of methyl orange under visible light irradiation.

We report on a europium-complex-grafted polymer for preparing stable nanoparticle probes with high luminescence brightness, narrow emission bandwidth, and long luminescence lifetimes. A Eu complex bearing an amino group was used to react with a functional copolymer poly(styrene-co-maleic anhydride) by the spontaneous amidation reaction, producing the polymer grafted with Eu complexes in the side chains. The Eu-complex-grafted polymer was further used to prepare Eu-complex-grafted polymer dots (Pdots) and Eu-complex-blended poly(9-vinylcarbazole) composite Pdots, which showed improved colloidal stability as compared to those directly doped with Eu-complex molecules. Both types of Pdots can be efficiently quenched by a nile blue dye, exhibiting much lower detection limit and higher quenching sensitivity as compared to free Eu-complex molecules. Steady-state spectroscopy and time-resolved decay dynamics suggest the quenching mechanism is via efficient fluorescence resonance energy transfer from the Eu complex inside a Pdot to surface dye molecules. The amplified quenching in Eu-complex Pdots, together with efficient cell uptake and specific cell surface labeling observed in mammalian cells, suggests their potential applications in time-resolved bioassays and cellular imaging.

Core–shell structured nanoparticles for near-infrared (NIR) photocatalysis were synthesized by a two-step wet-chemical route. The core is composed of upconversion luminescence NaYF4:Yb,Tm prepared by a solvothermal process, and the shell is anatase TiO2 nanocrystals around NaYF4 particles formed via a method similar to a Stöber process. Methylene blue compound as a model pollutant was used to investigate the photocatalytic activity of NaYF4:Yb,Tm@TiO2 composites under NIR irradiation. To understand the nature of NIR-responsive photocatalysis of NaYF4:Yb,Tm@TiO2, we investigated the energy transfer process between NaYF4:Yb,Tm and TiO2 and the origin of the degradation of organic pollutants under NIR radiation. Results indicate that the energy transfer route between NaYF4:Yb,Tm and TiO2 is an important factor that influences the photocatalytic activity significantly and that the degradation of organic pollutants under NIR irradiation is caused mostly by the oxidation of reactive oxygen species generated in the photocatalytic reaction, rather than by the thermal energy generated by NIR irradiation. The understanding of NIR-responsive photocatalytic mechanism helps to improve the structural design and functionality of this new type of catalytic material.Keywords: energy transfer; near-infrared; photocatalysis; TiO2; upconversion

The shortest ever recorded and enhanced 204.5 nm ultraviolet (UV) upconversion (UC) emission from 6G7/2 levels of Gd3+ ions was observed in NaLuF4:20%Yb3+,2%Ho3+,20%Gd3+ microcrystals (MCs) under 980 nm near infrared (NIR) excitation for the first time. The 6GJ levels of Gd3+ ions can be efficiently populated by the energy transfer (ET) processes of Yb → Ho → Gd and Yb → Gd. The effect of the pH value on the morphology of the MCs was explored and the UC mechanisms in the tri-doped NaLuF4 samples were analyzed in detail based on the emission spectra and the dependence of the luminescence intensity on the laser pumping power where a 6-photon UC process of the 204.5 nm emission is observed. The contrast spectra of UC luminescence suggest that β-NaLuF4 MCs are a better UC host material than the β-NaYF4 counterpart. Furthermore, this paper also provides an effective and facile approach to modulating the sizes of β-NaLuF4 MCs by manipulating the pH values of the initial reaction solutions and the doping of Gd3+ ions.

Optical temperature sensing characteristics based on the ultraviolet (UV) upconversion luminescence (UCL) of Gd3+ ions are reported here for the first time. Under 980 nm excitation, the temperature dependence five-photon UV UCL from the 6PJ and 6IJ levels of Gd3+ ions in NaLuF4:Yb3+, Tm3+, Gd3+ microcrystals were investigated systematically. The fluorescence intensity ratios (FIR) of two pairs of thermally coupled levels (6P5/2, 6P7/2 and 6I9/2, 6I7/2) were studied as a function of temperature in a range of 298–523 K. The maximum sensor sensitivities were found to be about 0.0004 K−1 (333 K) and 0.0029 K−1 (298 K) by exploiting the UC emissions from the 6PJ and 6IJ levels, respectively. This suggests that the Gd3+-based UCL materials are promising prototypes for application as multi-mode probes for use in bio-separation, MRI imaging, optical thermometers, etc.

Sub-10 nm BaYF5:Yb3+,Er3+ core–shell nanoparticles (NPs) were synthesized by a high boiling solvent process. Under a 980 nm laser excitation, up- and down-conversion fluorescence spectra from BaYF5:Yb3+,Er3+ NPs were recorded and analyzed. After coating the active-shell (containing Yb3+ ions) around the BaYF5:Yb3+,Er3+ NPs, the intensity of the 1.53 μm fluorescent band was enhanced by 11 times owing to surface passivation effect and efficient energy transfer between Yb3+ ions in active-shell and Er3+ ions. Furthermore, the NPs showed excellent redispersibility in common nonpolar solvents. By dispersing BaYF5:Yb3+,Er3+ active-core–active-shell NPs into an SU-8 2005 polymer, we constructed polymer-based optical waveguide amplifiers and measured their performance. For an input signal power of 0.1 mW and a pump power of 200 mW, a relative optical gain of ∼6.3 dB was obtained at 1535 nm in a 13 mm-long waveguide.

We report a ratiometric fluorescent sensor based on semiconducting polymer dots chelated with terbium ions to detect bacterial spores in aqueous solution. Fluorescent polyfluorene (PFO) dots serve as a scaffold to coordinate with lanthanide ions that can be sensitized by calcium dipicolinate (CaDPA), an important biomarker of bacterial spores. The absorption band of PFO dots extends to deep UV region, allowing both the reference and the sensitizer can be excited with a single wavelength (∼275 nm). The fluorescence of PFO remains constant as a reference, while the Tb3+ ions exhibit enhanced luminescence upon binding with DPA. The sharp fluorescence peaks of β-phase PFO dots and the narrow-band emissions of Tb3+ ions enable ratiometric and sensitive CaDPA detection with a linear response over nanomolar concentration and a detection limit of ∼0.2 nM. The Pdots based sensor also show excellent selectivity to CaDPA over other aromatic ligands. Our results indicate that the Tb3+ chelated Pdots sensor is promising for sensitive and rapid detection of bacterial spores.

A series of Yb3+–Er3+ and Yb3+–Er3+–Tm3+ codoped LaOF nanocrystals were synthesized via a modified sol–gel Pechini method. The phases and morphologies as well as the luminescence properties of the as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and upconversion luminescence (UCL) spectra, respectively. Under 980 nm near infrared (NIR) excitation, the UCL properties and the corresponding luminescent colors of the samples could be precisely modulated by changing the annealing temperatures, the dopant concentrations and the pump densities. By regulating the intensities of blue, green and red emissions in Yb3+–Er3+–Tm3+ codoped LaOF nanocrystals, an ideal white upconversion luminescence with chromaticity coordinates of (0.331, 0.368) is obtained in La0.885Yb0.1Er0.01Tm0.005OF nanocrystals for the first time. The blue, green and red components of the white light come from the 1G4 → 3H6 transition of Tm3+, 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ and 4F9/2 → 4I15/2 transition of the Er3+ ions, respectively. The possible physical mechanisms involved in the different upconversion processes were discussed in detail.

The effect of alkali ions (Li+, Na+, and K+) on controllable synthesis of rare earth (RE) fluorides was investigated by introducing corresponding nitrates into hydrothermal systems. It was found that the products transformed from YF3 truncated octahedra to α-NaYF4 spheres and then to β-NaYF4 microrods as the amount of NaNO3 increased. With the increasing amount, the additive Na+ had the effect on controlling the size of α-NaYF4 particles to decrease. Different alkali ions have distinct abilities of forming corresponding alkali RE fluorides based on their different ionic radii. Possible mechanisms of the alkali cation effect were proposed. By doping Yb3+/Tm3+ ions, upconversion luminescent properties were also investigated.

A novel near-infrared (NIR)-responsive photocatalyst, β-NaYF4:Yb3+,Tm3+@ZnO composites, was prepared by a two-step high temperature thermolysis method. In the NIR-responsive photocatalysis, β-NaYF4:Yb,Tm served as a NIR-to-UV upconverter and provided “UV light” or “necessary energy” to the ZnO catalyst. The energy transfer in the composites and the mixtures of β-NaYF4:Yb,Tm and ZnO was studied by using steady-state and dynamic fluorescence spectroscopy. The NIR photocatalytic activities were investigated by the decomposition of Rhodamine B. It was found that the energy transfer processes dominated the overall photocatalytic activities, and the generation of hydroxyl radicals was the origin of organic pollutant decomposition under NIR irradiation.

•The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed.•The crystal phase conversion was observed by adjusting the NaF/Re3+ ratio.•Intense ultraviolet and weak violet upconversion emissions were obtained in the hexagonal NaYF4 crystals.The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980 nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+, 0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.The selective synthesis of cubic and hexagonal NaYF4 crystals was successfully performed by a facile citric acid assisted hydrothermal method. The crystal phase conversion was observed through tuning the added amount of fluoride. A possible growth mechanism was proposed for the formation of hexagonal NaYF4 microcrystals (MCs). Under 980 nm excitation, intense ultraviolet (UV), blue, and weak violet upconversion (UC) emissions were obtained in the hexagonal NaYF4:20%Yb3+,0.5%Tm3+ MCs. The 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. The enhancement of UV UC emissions was attributed to higher crystallization degree and less luminescence quenching centers.

Rare earth (RE) fluoride fibers were synthesized by polymer-supported electrospinning combining two subsequent thermal treatments. One-dimensional (1D) structure was maintained after the fibers were heated. The treated fibers, which were usually several millimeters long, consisted of closely-packed crystalline grains. Pumped with a 980 nm laser, Yb3+ and Tm3+ codoped YF3 fibers exhibited intense upconversion (UC) fluorescence.

Tm3+ and Yb3+ ions co-doped α-NaYF4 nanocrystals (NCs) were prepared by a one step hydrothermal method with citric acid as the organic additive. The morphology of the as-prepared materials maintains a spherical shape when the surfactant amount, hydrothermal time, and hydrothermal temperature were varied. Under 980 nm excitation, intense ultraviolet (UV) and blue upconversion (UC) emissions were observed from the α-NaYF4 NCs and their intensities were much higher than those of the β-NaYF4 NCs modified by trisodium citrate. The intensities of the 5-photon UC emissions from the 1I6 level of Tm3+ ions were much stronger than those of the 4-photon UC emissions from the 1D2 level and the 3-photon UC emissions from the 1G4 level. It is for the first time, to our knowledge, that such intense UV UC emissions were obtained with α-NaYF4 NCs. The intense UV UC emissions and the relatively long fluorescent lifetimes for the 1I6, 1D2, and 1G4 levels of Tm3+ ions in α-NaYF4 NCs indicate that they could be one of the promising candidates for potential application in short-wavelength solid-state lasers.

Mesophase silica thin film doped with in-situ formed binary terbium (Tb) complex was synthesized through a simple one-step evaporation-induced self-assembly method. In this process, the precursors of rare earth complex and surfactant were added into hydrolyzed tetramethoxy-silane (TMOS) together and the inorganic/organic mesophase thin film was formed after spin coating. The mesophase structure was characterized as a 2D-hexagonal structure by X-ray diffraction (XRD) analysis. The excitation spectra (λem = 544 nm) and emission spectra (λex = 315 nm) indicated that the binary complex, Tb(SA)3, formed in-situ during the formation of the film. Under the UV excitation, the mesophase silica thin film showed bright and consistent green luminescence. The luminescence quantum efficiency of the hybrid thin film was confirmed to be 35.2%.Graphical abstract.Highlights► Mesophase silica thin film doped with in-situ formed binary terbium complex with bright blue luminescence was synthesized. ► The 2D-hexagonal structure was formed in such mesophase hybrid thin film. ► The luminescence decay process was indicative of a single average site distribution for Tb3+ ions in the hybrid thin film. ► The hybrid thin film exhibits much high quantum efficiency of 35.2%.

Fe3O4/NaYF4 submicro-rods were synthesized via a facile hydrothermal process with the presence of EDTA and Fe3O4 NPs. The structure of Fe3O4/NaYF4 submicro-rods is pictured as a large number of Fe3O4 NPs firmly attached on the surface of NaYF4 submicro-rods, generating a hetero structure. It is the first time to synthesize this structured magnetic upconversion luminescent composites via a hydrothermal method. Furthermore, these Fe3O4/NaYF4 hetero-submicro-rods with a high saturated magnetization value of 9.4 emu g−1 can emit bright green luminescence under 980 nm laser, which demonstrates these Fe3O4/NaYF4 submicro-rods possess better overall bifunctional properties compared with other reported works.Graphical abstractFe3O4/NaYF4 submicro-rods were synthesized via a facile hydrothermal process with the presence of EDTA and Fe3O4 NPs. A large number of Fe3O4 NPs were firmly attached on the surface of NaYF4 submicro-rods to form a hetero structure. These Fe3O4/NaYF4 hetero-submicro-rods performed obviously bifunctional superparamagnetic and upconversion luminescent properties with a saturated magnetization at 9.4 emu g−1 and a bright green luminescence under 980 excitation.Highlights► One effective method for synthesizing hydrophilic hetero-structured Fe3O4/NaYF4:Yb,Er submicro-rods. ► Oleic modified Fe3O4 NPs were directly used to frabricate Fe3O4/NaYF4:Yb,Er submicro-rods. ► A good overall property of magnetism and upconversion luminescence was found.

CaF2:Yb,Tm nano- and micro-crystals with different sizes were synthesized by a simple hydrothermal method. All the nanocrystals present intense ultraviolet (UV), visible, and near infrared (NIR) upconversion (UC) luminescence. The relationship between the UC luminescence intensities and the sample sizes from 50 nm to 1200 nm was investigated. The UV UC luminescence intensity of the sample with the particle size around 600 nm was the highest among all the samples, but the NIR UC luminescence intensity of the sample was the lowest. Our experimental results indicated that there was a size between 50 nm and microscale at which the UV emission intensity was the highest for CaF2 matrix. The size-dependent UC luminescence was proposed and analyzed based on experimental data.CaF2:Yb,Tm nano- and micro-crystals with different sizes were synthesized by a simple hydrothermal method. All the nanocrystals present intense ultraviolet (UV), visible, and near infrared (NIR) upconversion (UC) luminescence. The relationship between the UC luminescence intensities and the sample sizes from 50 nm to 1200 nm was investigated. The UV UC luminescence intensity of the sample with the particle size around 600 nm was the highest among all the samples, but the NIR UC luminescence intensity of the sample was the lowest. Our experimental results indicated that there was a size between 50 nm and microscale at which the UV emission intensity was the highest for CaF2 matrix. The size-dependent UC luminescence was proposed and analyzed based on experimental data.Highlights► CaF2 crystals with different sizes were synthesized by hydrothermal method. ► UC luminescence intensities exhibited size dependent characteristic. ► The sample with the size 600 nm had the highest UV UC luminescence intensity.

Monodisperse β-NaYF4:Yb,Tm nanocrystals (nanospheres and nanoplates) with a uniform size were successfully synthesized. By tuning the ratios of solvent, reaction temperature, and reaction time, we manipulated their phase, shape, and size in the range of 30–400 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixtures and played a key role in fine-tuning the nanocrystals. Possible mechanisms were proposed for forming the products with various architectures. Spectral analysis showed that the β-NaYF4:Yb,Tm nanocrystals were excellent materials for intense ultraviolet and blue upconversion luminescence. Here, we demonstrate that, under 980 nm excitation, the intense 5-photon upconversion fluorescence (290 nm and 345 nm) from the 1I6 level of Tm3+ ions is much stronger than the 4-photon upconversion fluorescence (361 nm and 451 nm) from the 1D2 level and the 3-photon upconversion fluorescence (474 nm) from the 1G4 level. The analysis of the temporal evolution of UC luminescence suggests that long lifetimes benefit the intense ultraviolet upconversion luminescence.

Unselectively enhanced multicolour upconversion (UC) emissions and low pumping threshold were achieved in Au@β-NaYF4:Yb,Tm hybrid nanostructures. This demonstrates that the plasmon field enhancement effect is the main reason for the improved UC emission efficiency.

Monodisperse, hexagonal phase NaLuF4:Yb,Tm nanoplates (NPs) with uniform size have been successfully synthesized by a novel solution-based method. The nanoplates have a perfect hexagonal shape with a diameter of ∼180 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixture and played a key role in fine-tuning the nanoplates. A possible growth mechanism was proposed for the formation of β-NaLuF4 nanoplates. Spectral analysis showed that the β-NaLuF4:Yb,Tm nanoplates were excellent materials for intense ultraviolet and blue upconversion luminescence. To our best knowledge, it is the first time such intense 5-photon upconversion fluorescence from the 1I6 level of Tm3+ ions, which is much stronger than the 4-photon upconversion fluorescence from the 1D2 level and the 3-photon upconversion fluorescence from the 1G4 level, has been demonstrated. The analysis on temporal evolutions of UC luminescence suggests that β-NaLuF4 nanocrystals might be a better kind of upconversion material than their β-NaYF4 counterpart. This powerfully demonstrates that β-NaLuF4 is an excellent host lattice for upconversion luminescence materials. Due to the unique luminescence, these β-NaLuF4 nanoplates may be promising for further fundamental research and applications in color displays and solid-state lasers.

Under 1560 nm excitation, ultraviolet (UV) upconversion (UC) emissions of Gd3+ and Er3+ ions were observed in β-NaYF4:Yb3+/Gd3+/Er3+ microcrystals, which were synthesized through a facile EDTA-assisted hydrothermal method. Experimental analysis exhibited that these UV emissions came from high-order UC processes. In the Yb3+–Gd3+–Er3+ codoped system, ground state absorption (GSA) and excited state absorption (ESA) of Er3+, ESA of Gd3+, energy transfers (ETs) from Yb3+ to Er3+ and from Er3+ to Gd3+ ions worked simultaneously in populating these high-energy states of Er3+ and Gd3+ ions.Graphical abstractUnder 1560 nm excitation, ultraviolet (UV) upconversion (UC) emissions of Gd3+ and Er3+ were observed in β-NaYF4:Yb3+/Gd3+/Er3+ microcrystals. In the complex frequency UC processes, energy transfers (ETs) from Er3+ to Gd3+ play crucial roles in populating the high-energy excited states of Gd3+ ions.Research highlights► UV UC emissions of Gd3+ were observed in NaYF4:Yb3+/Gd3+/Er3+ microcrystals under IR excitation. ► Er3+ were used as bridging ion to populate the excited states of Gd3+ in the codoped sample. ► Experiments on concentration variation demonstrated the ET processes from Er3+ to Gd3+ ions. ► Under IR excitation, UV emissions of Gd3+ came from eight- and nine-photon UC processes.

Yb3+/Er3+ codoped β-NaYF4 microcrystals were synthesized through a facile EDTA-assisted hydrothermal method. Under 980 nm excitation, 244, 256, and 276 nm upconversion (UC) emissions were observed in NaYF4:Yb3+/Er3+ microcrystals, which were assigned to the 2I11/2 → 4I15/2, 4D7/2 → 4I15/2, and 4G9/2 → 4I15/2 transitions of Er3+ ions, respectively. Successive energy transfers (ETs) from Yb3+ to Er3+ played crucial roles in populating the high-energy states of Er3+ ions. Power dependence analysis exhibited that 244 and 256 nm UC emissions came from six-photon processes. Temperature-dependent UC emissions of 4D7/2 → 4I15/2 and 2I11/2 → 4I15/2 transitions of Er3+ were discussed and the nonradiative relaxation (NR) process of 2I11/2 → 4D7/2 was confirmed.Graphical abstractUnder 980 nm excitation, 256 and 244 nm UC emissions were observed in β-NaYF4:Yb3+/Er3+ microcrystals and assigned to six-photon processes. These two emissions exhibited temperature dependent characteristic, which is attributed to the multiphonon relaxation process of 2I11/2 → 4D7/2.Research highlights▶ UV UC emissions were observed in β-NaYF4:Yb3+/Er3+ microcrystals. ▶ 256 and 244 nm UC emissions of Er3+ ions came from six-phonon processes. ▶ 256–244 nm UC emissions of Er3+ exhibited temperature dependent characteristic. ▶ This material provides a possible candidate for building UV solid-state lasers.

A novel upconversion luminescence nanocrystals Yb3+,Tm3+:Ba2YF7 were synthesized via the hydrothermal method. They have uniform morphology with a mean size of 30 nm even if annealed at 600 °C. Pumped by 980 nm laser diode the as-synthesized powers emit ultraviolet/blue light, which is in the range of the specific upconversion luminescent spectra of Tm3+ ions. After post-annealing at 600 °C in an argon atmosphere for 2 h, their upconversion luminescence intensity is 5 multiple improved and the ultraviolet/blue light can even be seen by the naked eyes under a low excitation power of 20 mW. This indicates that Ba2YF7 is a very effective luminescent host material. Excitation power dependences of individual upconversion emission intensity are plotted, which partly uncover the upconversion luminescence mechanism of Tm3+ ions.

Silica-coated NaYF4:Yb/Er(Tm)/Eu nanocrystals (NCs) with a mean size of 35 nm were prepared and characterized. Each of the core/shell NCs can be dispersed in ethanol and water to form stable colloidal solutions and emit bright visible light of two colors (blue and red, green and red) by up- and down-converting excitation modes. As we know, this is the first time to obtain the distinct dual-color photos of NaYF4:Yb/Er(Tm)/Eu NCs which were dispersed in deionized water. In particular, the ability to optically manipulate luminescence color of NCs doped with RE ions opens the door to multiplexed detection for high precision in more complex biotic environment.

Yb3+, Er3+, and Tm3+-codoped Gd2O3 nanotubes were synthesized via a simple wet-chemical route at low temperature and ambient pressure followed by a subsequent heat treatment at 800 °C. X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), upconversion (UC) emission spectra, and kinetic decay were used to characterize the samples. Bright white UC luminescence in the nanotubes was observed under diode laser excitation of 980 nm. The white light consists of the blue (1G4 → 3H6 of Tm3+), green (2H11/2/4S3/2 → 4I15/2 of Er3+), and red (4F9/2 → 4I15/2 of Er3+) UC emissions. As the excitation power density changed in the range of 20–200 W cm−2, the calculated CIE color coordinates shift only slightly and fall well within the white region. This material may be a potential candidate for applications of color displays, lighting and photonics.

Novel SiO2 hierarchical nanostructure has been grown on SiC nanowires using thermal decomposition of a mixture of ethanol and titanium tetrachloride. Novel nanostructure was realized in one synthesis route. Based on SEM and TEM observations, the hierarchical nanostructure consists of core-shell SiC–SiO2 main stem nanowire and a lot of SiO2 nanorod branches grown on the main stem. A mean diameter of SiC central cores was about 40 nm and their lengths reach about 100 μm. The lengths and diameters of SiO2 nanorod-like branches were ranged in 400–800 nm and 30–120 nm, respectively. The growth of core-shell SiC–SiO2 nanowires obeyed vapor–liquid–solid mechanism and the SiO2 nanorod-like branches grew via vapor–solid mechanism. The infrared absorption and photoluminescence properties of the grown nanostructure were investigated.

SiC/SiO2 one-dimensional nanochains and SiC/SiO2 two-dimensional X-junction and Y-junction nanochains were synthesized by using a simple and low-cost microwave method. Structural, morphological, and elemental analysis revealed that the SiC/SiO2 nanochains consisted of 3C-SiC strings with diameters of 20−80 nm and periodic SiO2 beads with diameters of 100−400 nm. Spectral analysis indicated that both SiC strings and SiO2 beads produced significant photoluminescence, and the presence of SiO2 beads enhanced the emissions from SiC strings. On the basis of experimental characterizations, a two-step growth mechanism of the nanochains was proposed to elucidate the growth process.

A facile hydrothermal method is used for the preparation of Tm3+/Yb3+ codoped fluoride microphosphors. The effect of crystal structure and ions concentration on the spectra and lifetimes of the radiative levels of Tm3+ ions in the different fluoride microcrystals is studied in detail. XRD analysis of Tm3+/Yb3+ codoped LaF3 microcrystals shows that 20% Yb3+ doping is sufficient for hexagonal LaF3 microparticles to crystallize completely in the orthorhombic phase. And lifetime analysis suggests that the average lifetimes of the radiative levels of Tm3+ ions increased when the matrix phase structure changing from orthorhombic phase to hexagonal phase with ytterbium dopant concentration changing.XRD patterns of samples (a) LaF3:10Yb3+, 0.5Tm3+; LaF3:20Yb3+, 0.5Tm3+; (b) YF3:10Yb3+, 0.5Tm3+ MCs annealed at 600 °C.

One-dimensional BaSiF6:Yb3+(20%)/Tm3+(1.2%) nanorods were synthesized by a facile microemulsion method for the first time. X-ray topographic analysis found that the nanorods have a pure rhombohedral structure. Under 980 nm excitation, bright-blue upconversion luminescence was presented in the nanorods, indicating that BaSiF6 is a new host material for producing desirable upconversion luminescence.(a) TEM image of BaSiF6 nanorods before aging. (b) Magnified TEM image and electron diffraction pattern of BaSiF6 nanorods before aging. (c) TEM image and electron diffraction pattern of BaSiF6 nanorods aged for 7 days.

YF3:Yb3+(20%)/Tm3+(2%) octahedral nanocrystals were synthesized by a microemulsion method with NH4HF2. Pumped with a 980-nm diode laser, the nanocrystals emitted weak blue and intense ultraviolet light. Especially, unusual 3P2 → 3H6 (∼265 nm) and 3P2 → 3F4 (∼309 nm) emissions, coming from a five-photon excitation process, were observed. The emissions from 1D2 and 1I6 were much stronger than those from 1G4 and 3H4. The upconversion mechanism was discussed in detail.UC luminescence spectrum of YF3:Yb3+(20%)/Tm3+(2%) nanocrystals under 980-nm excitation. Inset: magnification of the spectrum in the range of 475–850 nm.

Large-scale SiC–SiO2 core–shell nanowires decorated with carbon nanoparticles have been synthesized on Si substrate using thermal decomposition of ethanol. The amorphous carbon nanoparticles on SiO2 shell are of hemispherical configuration hiving a mean diameter of about 20 nm. The average size of carbon nanoparticles estimated from Raman measurement is 23.4 nm close to that evaluated from transmission electron microscopic observation. Strong enhancement of blue emission band and appearance of a new yellow emission band were observed in SiC–SiO2 core–shell nanowires decorated with carbon nanoparticles and their origins were discussed. A possible synthesis mechanism of SiC–SiO2 core–shell nanowires decorated with carbon nanoparticles was proposed.PL spectra of nanowires synthesized under different Ar flow rates.

Low-dimension semiconductor nanostructures were successfully synthesized by a fast, simple, and low-cost microwave method. By heating raw materials under microwave irradiation and controlled conditions, diverse nanostructures for semiconducting oxides and carbides were synthesized without extra metal catalysts. In this paper, flower-like and net structural oxide and carbide semiconductors in nanoscale have been studied in detail. Structural, morphological, and elemental analysis revealed that the as-synthesized nanostructures were highly pure and structurally uniform. The possible growth mechanisms of these nanostructures were preliminarily discussed. The temperature and the gas-phase supersaturation in their growing processes have important effects on their morphologies. The unique synthesis method may open a new way for the fabrication of self-assembled multidimensional structures, which are expected to find a wide range of important applications in nanodevices and nanocomposites.

Pumped with a 980-nm diode laser, violet/ultraviolet upconversion fluorescence was presented in Y0.83Yb0.15Er0.02F3 nanocrystals. Observed emissions at 318 nm and 379 nm were affirmed coming from a four-photon excitation process. In comparison with a bulk sample having the same chemical compositions, the nanocrystals had a markedly enhanced ability of emitting violet/ultraviolet upconversion fluorescence. By employing Tm3+ ions as structural probes in the samples, we found that the enhancement could be attributed to the decrease of Judd–Ofelt parameter Ω2. A model for revealing the four-photon excitation process was proposed based on spectral analysis.

Eu3+-doped Gd3PO7 nanospheres with an average diameter of ∼300 nm and a narrow size distribution have been prepared by a facile combustion method and structurally characterized by X-ray diffraction and field emission scanning electron microscopy. The luminescent properties were systemically studied by the measurement of excitation/emission spectra, and emission spectra under different temperatures, as well as by photostability. The strong red-emission intensity peaking at 614 nm originates the 5D0→7F2 transition and is observed under 254-nm irradiation, indicating that Eu3+ ions in Gd3PO7 mainly occupied non-centrosymmetry sites. The CIE1931 XY chromaticity coordinates of Gd3PO7:Eu3+ nanospheres are (x=0.654, y=0.345) in the red area, which is near the National Television Standard Committee standard chromaticity coordinates for red. Thus, Gd3PO7:Eu3+ nanospheres may be potential red-emitting phosphors for PDP and Xe-based mercury-free lamps.SEM image of as-prepared Gd3PO7:Eu3+ nanospheres.

Tm3+/Yb3+ codoped rod-like YF3 nanocrystals were synthesized through a facile hydrothermal method. After annealing in an argon atmosphere, the nanocrystals emitted bright blue and intense ultraviolet (UV) light under a 980-nm continuous wave diode laser excitation. Up-conversion emissions centered at ∼291 nm (1I6 → 3H6), ∼347 nm (1I6 → 3F4), ∼362 nm (1D2 → 3H6), ∼452 nm (1D2 → 3F4), ∼476 nm (1G4 → 3H6), ∼642 nm (1G4 → 3F4), and ∼805 nm (3H4 → 3H6) were recorded using a fluorescence spectrophotometer. Especially, enhanced UV emissions were studied by changing Yb3+/Tm3+ doping concentrations, the annealing temperatures, and the excitation power densities. A possible mechanism, energy transfer–cross relaxation–energy transfer (ET–CR–ET), was proposed based on a simple rate-equation model to elucidate the process of the enhanced UV emissions.Tm3+/Yb3+ codoped rod-like YF3 nanocrystals were synthesized through hydrothermal method. After annealing, the nanocrystals emitted bright blue and intense ultraviolet (UV) light under 980-nm excitation. Enhanced UV emissions were studied. A mechanism of energy transfer–cross relaxation–energy transfer (ET–CR–ET) was proposed and elucidated based on a rate-equation model.

YF3:Eu3+ nanobundles were synthesized by a facile microemulsion method. Analysis of X-ray diffraction, scanning electron microscope, and transmission electron microscopy reveals that each nanobundle consists of numerous nanowhiskers with a mean length of ∼500 nm and a mean diameter of ∼2 nm. Under 393-nm excitation, the luminescence was dominated by 5D0 → 7F1 transition, indicating the inversion symmetry of Eu3+ site. The luminescence intensity increased with increasing Eu3+ concentration, up to about 30 mol%, and then decreased abruptly. The peak positions and spectral shapes of emissions were independent of Eu3+ concentration. Finally, the critical distance of energy transfer was calculated.(a) SEM image of YF3:Eu3+(5%) nanobundles. (b) TEM image of nanobundles. (c) A crossed nanobundle; Inset: electron diffraction patterns.

SrF2:Eu3+ nanospheres with homogeneous diameter have been synthesized by a microemulsion-mediated hydrothermal method for the first time, in which quaternary microemulsion of CTAB/water/cyclohexane/n-pentanol was used. The possible reaction mechanism and the luminescent properties of SrF2:Eu3+ nanospheres were also investigated in this paper. The morphology and grain sizes of final products were characterized by field emission scanning electron microscopy and transmission electron microscopy, indicating that most of the products were nanospheres with an average diameter of ∼50 nm. Room-temperature emission spectra, recorded under 394-nm excitation, showed that the transition of 5D0 → 7F1 emission be dominating in SrF2:Eu3+ nanospheres. From the dependence of the luminescence intensity on the concentration of Eu3+ ions, the optimal dopant concentration is 2 mol%.(a) Scanning electron microscopy (SEM) image of SrF2:Eu3+ nanospheres; (b) transmission electron microscopy (TEM) image of SrF2:Eu3+ nanospheres; inset: select area electron diffractions (SAEDs) recorded on individual nanospheres.

Tm3+/Er3+/Yb3+ tri-doped CaF2 phosphors were synthesized using a hydrothermal method. The phosphors were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and up-conversion (UC) emission spectra. After annealing, the phosphors emitted white light under a 980 nm continuous wave diode laser (CW LD 2 W) excitation. As the excitation power density changed in the range of 20–260 W/cm2, the chromaticity coordinates of the UC light of the phosphor Ca0.885Tm0.005Er0.01Yb0.1F2 fell well in the white region of the 1931 CIE diagram. For the proportion of red, green and blue (RGB) in white light is strict, key factors for achieving UC white light, such as host materials, rare earth ions doping concentrations, annealing temperatures, as well as the excitation power densities, were investigated and discussed.

Water soluble YVO4:Ln3+ and YVO4:Ln3+/Ba2+ (Ln = Ce, Dy, Eu, and Sm) nanocrystals were synthesized by a polyvinylpyrrolidone-assisted hydrothermal method. Under the excitation of the host absorption, phosphors can emit blue light for YVO4:Ce3+/Ba2+, yellow light for YVO4:Dy3+/Ba2+, red light for YVO4:Eu3+/Ba2+, and reddish orange light for YVO4:Sm3+/Ba2+. In comparison with those of YVO4:Ln3+ nanocrystals, the emissions of YVO4:Ln3+/Ba2+ are greatly enhanced. Furthermore, the excitation spectra of YVO4:Eu3+/Ba2+ and YVO4:Dy3+/Ba2+ show the similar features, which are different from those of YVO4:Sm3+/Ba2+ and YVO4:Ce3+/Ba2+.

A growth mechanism was proposed for YF3 nanocrystals forming in the quaternary reverse microemulsion system with water/cetyltrimethylammonium bromide/cyclohexane/1-pentanol. In the proposed mechanism, primary particles are formed by constant collision, fusion, and fission of micelles. These primary particles can further self-organize to one-dimensional nanostructure at room temperature. Results of X-ray diffraction and transmission electron microscopy reveal that each nanobundle consists of numerous nanowhiskers with a mean length of ∼700 nm and a mean diameter of ∼2 nm. Under 980 nm excitation, blue (1G4 → 3H6 and 1D2 → 3F4) and ultraviolet (1D2 → 3H6 and 1I6 → 3F4/3H6) upconversion fluorescence emitted from the YF3−Yb3+/Tm3+ nanobundles. The relative intensity of the ultraviolet to the blue emission increases with decreasing the size of nanowhiskers. Spectral analysis indicates that the enhancement is attributed to the decrease of Judd−Ofelt parameter Ω2, which precludes the transition from 3F2 to 3F4 and enhances the cross relaxation of 3F2 + 3H4 → 3H6 + 1D2.

Water-soluble PVP-stabilized hexagonal-phase La0.78Yb0.20Er0.02F3 nanocrystals (NCs) were synthesized by hydrothermal method. The NCs were coated with a very thin silica shell, and amino groups were introduced to the surface of silica shells by copolymerization of 3-aminopropyl(triethoxy)silane. The core/shell NCs can be dispersed in ethanol and water to form stable colloidal solution. The transmission electron microscopy (TEM), selected area electron diffraction (SAED), powder X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FT-IR) were used to characterize the core/shell materials. In addition, the green up-conversion fluorescence mechanism of La0.78Yb0.20Er0.02F3/SiO2 NCs was studied with a 980-nm diode laser as excitation source. The water solubility, small core/shell particles size, and well colloidal stability mean the green up-conversion fluorescence NCs have potential applications in bioassay.Colloidal La0.78Yb0.20Er0.02F3/SiO2 Core/Shell nanocrystals (NCs) were synthesized and the free amino groups were introduced to the surface of silica shells by copolymerization 3-aminopropyl(triethoxy)silane. The NCs can be dispersed in ethanol and water to form stable colloidal solution. In addition, the NCs exhibit green up-conversion fluorescence under 980-nm excitation.

Eu(DBM)3Phen-embedded silica nanoparticles were synthesized in water-in-oil (W/O) microemulsion containing aqueous phase of Eu(DBM)3Phen, surfactant Triton X-100, cosurfactant octanol and oil-phase cyclohexane. The size and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM). The low-temperature time-resolved emission spectra indicate that the Eu complex in the silica nanoparticles have longer lifetime than that of the pure complex. Under 355 nm continuous excitation, the nanoparticles show high resistance to photobleaching. The free amino groups were attached to silica surfaces by copolymerization of 3-aminopropyl(triethoxy)silane. Preliminary results demonstrated that the silica-coated Eu complex nanoparticles can be a probe in the detection of biomolecular interactions.

Single phase cubic CaF2 microcrystals co-doped with Eu3+ and Yb3+ ions were synthesized through a coprecipitation process followed by annealing at 1200 °C. Under the excitation of 980 nm laser, the intensive emission at ~614 nm (5D0 → 7F2) in the red region was observed at room temperature. The color coordinate of CaF2:0.1%Yb3+/0.1%Eu3+ was calculated with the Commission Internationale de I’Eclairage (CIE) chromaticity coordinate of (x=0.572, y=0.265). The excitation of Eu3+ ions came from the energy transfer of Yb3+-dimers through a cooperation sensitization process. Furthermore, not only the emissions from the 5Di (i=0, 1, 2, 3)→7Fj (j=0, 1, 2, 3) transitions but also the rarely reported emission (~398 nm) from the 5L6 →7F0 transition was recorded. The cooperation sensitization process was so effective in the Yb3+/Eu3+ co-doped system that all the cooperative luminescence (CL) from Yb3+-clusters were quenched and only the red upconversion luminescence of Eu3+ ions could be seen.(Left) Schematic energy level diagrams of Eu3+ and Yb3+ showing the mechanism for UC population and emission processes under the excitation of 980 nm.(Right) CIE chromaticity coordinates for UCL of 5Di →7Fj in CaF2:0.1%Yb3+/0.1%Eu3+ under 980 nm laser excitation. Inset represents the luminescence photo of the sample.Download high-res image (215KB)Download full-size image

Eu(DBM)3Phen-embedded silica nanoparticles were synthesized in water-in-oil (W/O) microemulsion containing aqueous phase of Eu(DBM)3Phen, surfactant Triton X-100, cosurfactant octanol and oil-phase cyclohexane. The size and morphology of the nanoparticles were characterized by transmission electron microscopy (TEM). The low-temperature time-resolved emission spectra indicate that the Eu complex in the silica nanoparticles have longer lifetime than that of the pure complex. Under 355 nm continuous excitation, the nanoparticles show high resistance to photobleaching. The free amino groups were attached to silica surfaces by copolymerization of 3-aminopropyl(triethoxy)silane. Preliminary results demonstrated that the silica-coated Eu complex nanoparticles can be a probe in the detection of biomolecular interactions.

•Ho3+/Yb3+ co-doped TeO2-BaF2-Y2O3 glasses were prepared.•Intense ∼1.2 μm fluorescence was observed in the TeO2-BaF2-Y2O3 glasses.•Microstructured fibers based on the TeO2-BaF2-Y2O3 glasses were fabricated.•A relative positive gain of 9.42 dB at 1175.3 nm was obtained in a 5 cm long fiber.Intense ∼1.2 μm fluorescence is observed in Ho3+/Yb3+ co-doped TeO2-BaF2-Y2O3 glasses under 915 nm laser diode excitation. The 1.2 μm emission can be ascribed to the transition 5I6→5I8 of Ho3+. With the introducing of BaF2, the content of OH in the glasses drops markedly, and the 1.2 μm emission intensity increases gradually as increasing the concentration percentage of BaF2. Furthermore, microstructured fibers based on the TeO2-BaF2-Y2O3 glasses are fabricated by using a rod-in-tube method, and a relative positive gain of ∼9.42 dB at 1175.3 nm is obtained in a 5 cm long fiber.

Large-scale SiC nanocables were synthesized on a Ni(NO3)2-catalyzed Si substrate by using a simple and cheap method based on thermal decomposition of methanol. Based on X-ray diffraction and high-magnification transmission electron microscopy, the as-grown nanocables consisted of crystalline SiC cores and amorphous SiO2 shells. The diameters of SiC cores were 5.7–10 nm and the thicknesses of SiO2 shells were 9–20 nm. Dividing of nanocables was observed and its origin was investigated. An asymmetric feature of SiC TO band with a shoulder at the high-frequency side was attributed to the contribution of SiC TO mode. The nanocables displayed strong violet–blue emission. A possible growth mechanism was proposed.

Semiconducting polymer dots (Pdots) represent a new class of fluorescent nanoparticles for biological applications. In this study, we investigated their size-dependent fluorescence and cellular labeling properties. We demonstrate that the polymer conformation in solution phase largely affects the polymer folding and packing during the nanoparticle preparation process, resulting in solution-phase control over the fluorescence properties of semiconducting polymer nanoparticles. The resulting Pdots exhibit apparent size dependent absorption and emission, a characteristic feature of different chain packing behaviors due to the preparation conditions. Single-particle fluorescence imaging was employed to perform a side-by-side comparison on the Pdot brightness, indicating a quadratic dependence of single-particle brightness on particle size. Upon introducing a positively charged dye Nile blue, all the three type of Pdots were quenched very efficiently (Ksv > 1 × 107 M–1) in an applied quenching process at low dye concentrations, but exhibit apparent difference in quenching efficiency with increasing dye concentration. Furthermore, Pdots of different sizes were used for cell uptake and cellular labeling involving biotin–streptavidin interactions. Fluorescence imaging together with flow cytometry studies clearly showed size dependent labeling brightness. Small-sized Pdots appear to be more effective for immunolabeling of cell surface, whereas medium-sized Pdots exhibit the highest uptake efficiency. This study provides a concrete guidance for selecting appropriate particle size for biological imaging and sensing applications.

In our previous study, we have reported the cooperative luminescence of Yb3+-trimers and cooperatively sensitized Gd3+ luminescence by Yb3+-tetramers in a doped CaF2 host. In this study, we experimentally observed an unusual luminescent phenomenon of Gd3+ in CaF2:Yb3+/Gd3+. Upon excitation with a 980 nm laser, the upconversion luminescence of Gd3+ first increases and then decreases in the Gd3+ concentration range of 0–0.9 mol%; this is different from the monotonic increase of Gd3+ luminescence observed in the downconversion spectra via the direct excitation of Gd3+. This special luminescent behavior was indicated to be related to the energy transfer from the Yb3+-tetramers to Gd3+ and the destruction of Yb3+-clusters. Herein, we proposed a new luminescence quenching mechanism, Yb3+-cluster destructive quenching, which was verified by fluorescence dynamic analysis and an optically inactive rare-earth ion-doping experiment.

Mesoporous-silica-coated Gd2O3:Eu/silica nanoparticles were synthesized by a multistep chemical process and characterized by XRD, TEM and N2 adsorption/desorption isotherms in terms of size, morphology and porosity. The core Gd2O3:Eu obtained by this method was highly luminescent upon excitation, giving the function of cell imaging upon incubation with the human cervical carcinoma (HeLa) cells. The outer porous silica shell is able to load the anticancer drug with a relatively high loading efficiency and release the loaded drugs at a sustained rate. The HeLa cells can be killed effectively on incubation with the core–shell porous particles loaded with the anticancer drug DOX. Meanwhile, the accumulation of mesoporous nanoparticles loaded with drugs in the target location could be monitored via fluorescence imaging. Therefore, the core–shell hybrid nanoparticles presented in this work are potential multifunctional biomaterials for smart detection or diagnosis and therapy in future biomedical engineering.

Optical temperature sensing characteristics based on the ultraviolet (UV) upconversion luminescence (UCL) of Gd3+ ions are reported here for the first time. Under 980 nm excitation, the temperature dependence five-photon UV UCL from the 6PJ and 6IJ levels of Gd3+ ions in NaLuF4:Yb3+, Tm3+, Gd3+ microcrystals were investigated systematically. The fluorescence intensity ratios (FIR) of two pairs of thermally coupled levels (6P5/2, 6P7/2 and 6I9/2, 6I7/2) were studied as a function of temperature in a range of 298–523 K. The maximum sensor sensitivities were found to be about 0.0004 K−1 (333 K) and 0.0029 K−1 (298 K) by exploiting the UC emissions from the 6PJ and 6IJ levels, respectively. This suggests that the Gd3+-based UCL materials are promising prototypes for application as multi-mode probes for use in bio-separation, MRI imaging, optical thermometers, etc.

We demonstrated optical amplification at 650 nm in KMnF3:Yb3+,Er3+@KMnF3:Yb3+ active-core–active-shell nanoparticle (NP) doped polymer waveguides pumped by a 976 nm laser diode for the first time. KMnF3:Yb3+,Er3+ NPs were synthesized via a solvothermal method. With the excitation of a 976 nm laser diode, bright red upconversion (UC) fluorescence was observed from KMnF3:Yb3+,Er3+ NPs owing to the existence of efficient energy transfer between Er3+ and Mn2+:2H11/2,4S3/2 + 6A1 → 4I15/2 + 4T1,2H9/2 + 6A1 → 4I13/2 + 4T1 and 4I15/2 + 4T1 → 4F9/2 + 6A1. The red UC emissions originated from the 4F9/2 → 4I15/2 transition of Er3+. Furthermore, the red UC emissions of KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs were enhanced by 7.5 times compared to that of KMnF3:18 mol% Yb3+,1 mol% Er3+ core-only NPs after coating an active shell containing Yb3+ ions on the core-only NPs. The above results showed that the active-shell could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shell. By using KMnF3:18 mol% Yb3+,1 mol% Er3+@KMnF3:2 mol% Yb3+ NPs as the gain medium and doping NPs into a polymer waveguide, we constructed polymer-based waveguide amplifiers. For an input signal power of 7.4 mW and a pump power of 45.2 mW, a relative optical gain of ∼3.5 dB was obtained at 650 nm in a 17 mm-long waveguide.

Monodisperse β-NaYF4:Yb,Tm nanocrystals (nanospheres and nanoplates) with a uniform size were successfully synthesized. By tuning the ratios of solvent, reaction temperature, and reaction time, we manipulated their phase, shape, and size in the range of 30–400 nm. As a chelating agent and shape modifier, oleic acid (OA) was introduced into the reaction mixtures and played a key role in fine-tuning the nanocrystals. Possible mechanisms were proposed for forming the products with various architectures. Spectral analysis showed that the β-NaYF4:Yb,Tm nanocrystals were excellent materials for intense ultraviolet and blue upconversion luminescence. Here, we demonstrate that, under 980 nm excitation, the intense 5-photon upconversion fluorescence (290 nm and 345 nm) from the 1I6 level of Tm3+ ions is much stronger than the 4-photon upconversion fluorescence (361 nm and 451 nm) from the 1D2 level and the 3-photon upconversion fluorescence (474 nm) from the 1G4 level. The analysis of the temporal evolution of UC luminescence suggests that long lifetimes benefit the intense ultraviolet upconversion luminescence.

We report the novel near-infrared (NIR) photocatalysis of YF3:Yb3+,Tm3+/TiO2 core/shell nanoparticles. The core/shell nanoparticles show photocatalytic activity under the NIR irradiation. This study demonstrates that the NIR energy can be used as the driving source for photocatalysis besides the UV and visible energy.

Yb3+, Er3+, and Tm3+-codoped Gd2O3 nanotubes were synthesized via a simple wet-chemical route at low temperature and ambient pressure followed by a subsequent heat treatment at 800 °C. X-Ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), thermogravimetric analysis (TGA), upconversion (UC) emission spectra, and kinetic decay were used to characterize the samples. Bright white UC luminescence in the nanotubes was observed under diode laser excitation of 980 nm. The white light consists of the blue (1G4 → 3H6 of Tm3+), green (2H11/2/4S3/2 → 4I15/2 of Er3+), and red (4F9/2 → 4I15/2 of Er3+) UC emissions. As the excitation power density changed in the range of 20–200 W cm−2, the calculated CIE color coordinates shift only slightly and fall well within the white region. This material may be a potential candidate for applications of color displays, lighting and photonics.

A novel near-infrared (NIR)-responsive photocatalyst, β-NaYF4:Yb3+,Tm3+@ZnO composites, was prepared by a two-step high temperature thermolysis method. In the NIR-responsive photocatalysis, β-NaYF4:Yb,Tm served as a NIR-to-UV upconverter and provided “UV light” or “necessary energy” to the ZnO catalyst. The energy transfer in the composites and the mixtures of β-NaYF4:Yb,Tm and ZnO was studied by using steady-state and dynamic fluorescence spectroscopy. The NIR photocatalytic activities were investigated by the decomposition of Rhodamine B. It was found that the energy transfer processes dominated the overall photocatalytic activities, and the generation of hydroxyl radicals was the origin of organic pollutant decomposition under NIR irradiation.

White upconversion luminescence (UCL) was achieved under 980 nm excitation in the CaF2:Yb3+/Eu3+ material using Y3+ to adjust the luminescence performance. In this luminescent system, Yb3+ not only plays the role of a sensitizer of Eu3+, but also generates green fluorescence from Yb3+ dimers (2-Yb3+) by cooperative transitions in the CaF2 matrix. One of the primary colors of green corresponds to the 2-Yb3+ cooperative emission exactly. Eu3+ acts as an activator for emitting red and blue fluorescence simultaneously. Interestingly, the color of the UCL can be controlled by adjusting the doping concentration of Y3+ ions, and white UCL was realized when the concentration of Y3+ was 1%.

The shortest ever recorded and enhanced 204.5 nm ultraviolet (UV) upconversion (UC) emission from 6G7/2 levels of Gd3+ ions was observed in NaLuF4:20%Yb3+,2%Ho3+,20%Gd3+ microcrystals (MCs) under 980 nm near infrared (NIR) excitation for the first time. The 6GJ levels of Gd3+ ions can be efficiently populated by the energy transfer (ET) processes of Yb → Ho → Gd and Yb → Gd. The effect of the pH value on the morphology of the MCs was explored and the UC mechanisms in the tri-doped NaLuF4 samples were analyzed in detail based on the emission spectra and the dependence of the luminescence intensity on the laser pumping power where a 6-photon UC process of the 204.5 nm emission is observed. The contrast spectra of UC luminescence suggest that β-NaLuF4 MCs are a better UC host material than the β-NaYF4 counterpart. Furthermore, this paper also provides an effective and facile approach to modulating the sizes of β-NaLuF4 MCs by manipulating the pH values of the initial reaction solutions and the doping of Gd3+ ions.

Sub-10 nm BaYF5:Yb3+,Er3+ core–shell nanoparticles (NPs) were synthesized by a high boiling solvent process. Under a 980 nm laser excitation, up- and down-conversion fluorescence spectra from BaYF5:Yb3+,Er3+ NPs were recorded and analyzed. After coating the active-shell (containing Yb3+ ions) around the BaYF5:Yb3+,Er3+ NPs, the intensity of the 1.53 μm fluorescent band was enhanced by 11 times owing to surface passivation effect and efficient energy transfer between Yb3+ ions in active-shell and Er3+ ions. Furthermore, the NPs showed excellent redispersibility in common nonpolar solvents. By dispersing BaYF5:Yb3+,Er3+ active-core–active-shell NPs into an SU-8 2005 polymer, we constructed polymer-based optical waveguide amplifiers and measured their performance. For an input signal power of 0.1 mW and a pump power of 200 mW, a relative optical gain of ∼6.3 dB was obtained at 1535 nm in a 13 mm-long waveguide.

A strategy for enhancing the sensitivity of optical thermometers is developed herein by using non-thermally coupled levels of Er3+. Under the excitation of a 980 nm laser, the temperature dependence of 244 nm and 256 nm upconversion luminescences (UCLs) of Er3+ was studied. The corresponding 2I11/2 and 4D7/2 levels were confirmed to be non-thermally coupled levels. By using the fluorescence intensity ratio (FIR) technique and by investigating different thermal population behaviors of 2I11/2 and 4D7/2 levels, the optical temperature sensing performance based on the non-thermally coupled levels of Er3+ was fulfilled here for the first time. The obtained maximum sensor sensitivity is 0.106 K−1 at 525 K, which is much higher than those of all other RE3+ doped optical thermometers using the thermally coupled level-based FIR technique. This suggests that the use of the FIR from neighboring non-thermally coupled levels of RE3+ is a promising approach for enhancing the sensor sensitivity of optical thermometers.

Ultrasmall upconversion (UC) nanoparticles with intense luminescence are quite desirable for in vitro and in vivo bioimaging. Herein, we report the successful synthesis of sub-10 nm hexagonal NaTm0.02Lu0.98−xYbxF4 nanocrystals via a high boiling solvent process. The sizes and phases of the crystals can be finely tailored by tuning the amounts of the ligands used (octadecylamine and sodium oleate) and the temperature. The intensity of the UC emission of the nanocrystals increases as the the relative content of Yb3+ ions increases from 20% to 98%, owing to improved near-infrared absorption and energy-transfer from Yb3+ to Tm3+. The experimental results indicate that hexagonal NaYbF4 is the best host for ultrasmall Tm3+-doped UC nanoparticles and shows great potential in biomedical applications.

The demands for multiplexed biological detection have driven the development of fluorescence encoding nanoprobes. We have synthesized water-soluble dual mode emission core–shell rare earth nanoprobes (∼30 nm) for fluorescence encoding. The nanoprobes were composed of ytterbium (Yb), erbium (Er) and/or thulium (Tm) ions co-doped heterogeneous NaYF4/NaLuF4 nanocrystals as cores and amorphous SiO2 embedded with europium (Eu) or terbium (Tb) complexes as shells. Excited by both infrared light and ultraviolet light, the nanoprobes exhibited dual characteristic emissions, which enable a novel spectral encoding strategy. The core nanocrystals exhibited tunable up-conversion emissions through various lanthanide ions doping. Combining these multiple upconversion emissions of core nanocrystals with downconversion emissions of shell containing rare earth complexes, a large amount of distinct fluorescence codes can be generated.

Ultra-small luminescent nanoparticles (NPs) are quite desirable for optoelectronic and biomedical applications. However, it is still a challenge to synthesize ultra-small NPs with high brightness owing to non-radiative energy losses caused by the surface defects as well as from vibrational deactivation ascribed to solvent molecules and ligands absorbed on the NPs. In this paper, we reported a strategy to improve up- and down-conversion luminescence of ultra-small BaLuF5:Yb3+,Er3+ NPs by using multi-layer active-shells (containing Yb3+). Sub-10 nm BaLuF5:Yb3+,Er3+@(X-shell, X = 1–5)BaLuF5:Yb3+ NPs were synthesized via a high boiling solvent process through a layer-by-layer strategy. Up- and down-conversion fluorescence spectra of the NPs were recorded and analyzed by using a 980 nm laser diode as the excitation source. In comparison with optical properties of BaLuF5:Yb3+,Er3+ NPs, the intensities of up- (∼545 nm) and down-conversion (∼1530 nm) fluorescence were enhanced by 52 and 9.8 times after coating 5-layer active-shells (BaLuF5:Yb3+) on the BaLuF5:Yb3+,Er3+ NPs, respectively. In addition, the intensities of up- and down-conversion fluorescence of the BaLuF5:Yb3+,Er3+ NPs with multi-layer active-shells were 1.3 and 1.1 times larger than those of the BaLuF5:Yb3+,Er3+ NPs with a one thick-layer active shell, respectively. These results showed that multi-layer active-shells could be used to not only suppress surface quenching but also transfer the pump light to the core region efficiently through Yb3+ ions inside the active-shells.

In response to the comment by L. Marciniak et al. (J. Mater. Chem. C), we maintain the determination of sensor sensitivity by using the equation of because all the levels of RE3+ ions can be thermally coupled according to Boltzmann distribution.

Under 978 nm near-infrared (NIR) excitation, blue upconversion (UC) emissions from CaF2:Cu2+,Yb3+ were first observed at ∼420 nm. It was ascribed to the 3d84s1 → 3d9 transition of Cu2+ ions. From transient measurements, the UC process was found to be dominated by the energy transfer process in which three excited Yb3+ ions simultaneously transfer their energy to one Cu2+ ion. The influence of Cu2+ concentration and temperature on the UC emission as well as the Jahn–Teller effect was also investigated.

A series of Yb3+–Er3+ and Yb3+–Er3+–Tm3+ codoped LaOF nanocrystals were synthesized via a modified sol–gel Pechini method. The phases and morphologies as well as the luminescence properties of the as-prepared samples were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and upconversion luminescence (UCL) spectra, respectively. Under 980 nm near infrared (NIR) excitation, the UCL properties and the corresponding luminescent colors of the samples could be precisely modulated by changing the annealing temperatures, the dopant concentrations and the pump densities. By regulating the intensities of blue, green and red emissions in Yb3+–Er3+–Tm3+ codoped LaOF nanocrystals, an ideal white upconversion luminescence with chromaticity coordinates of (0.331, 0.368) is obtained in La0.885Yb0.1Er0.01Tm0.005OF nanocrystals for the first time. The blue, green and red components of the white light come from the 1G4 → 3H6 transition of Tm3+, 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ and 4F9/2 → 4I15/2 transition of the Er3+ ions, respectively. The possible physical mechanisms involved in the different upconversion processes were discussed in detail.

The present work reports a self-sacrificing template strategy to synthesize porous α-NaYF4 microspheres via the reaction of as-prepared Y(OH)CO3·H2O@SiO2 with NH4F and NaNO3 solutions. XRD, SEM, TEM and N2 adsorption–desorption measurements were used to characterize the resulting product. The surface SiO2 shell was shown to play a vital role in size and shape control and porosity formation. A possible reaction mechanism was explored in terms of a surface-protected etching and ion-exchange reaction process. To explore their application potential, the storage and release behavior of Rhodamine 640 dye in the porous α-NaYF4 microspheres was investigated, showing a relatively high loading efficiency and a sustained release ratio. Under near-infrared (NIR) irradiation, porous α-NaYF4 microspheres doped with lanthanide ions showed typical upconverting luminescence characteristics that can convert NIR photons to ultraviolet/visible photons. The above features and properties indicate that our present porous upconverting luminescence particles are promising in biological applications as luminescence imaging agents and drug carriers.

Unselectively enhanced multicolour upconversion (UC) emissions and low pumping threshold were achieved in Au@β-NaYF4:Yb,Tm hybrid nanostructures. This demonstrates that the plasmon field enhancement effect is the main reason for the improved UC emission efficiency.

We demonstrate a passively Q-switched Yb3+/Er3+ co-doped fiber (YEDF) laser using a filmy sodium carboxymethylcellulose (NaCMC)-based Cu1.8S nanocrystals (NCs) saturable absorber (SA). Cu1.8S NCs SA exhibit a broad absorption band from 600 nm to more than 2500 nm. By placing Cu1.8S NCs SA into a YEDF laser cavity, stable passively Q-switched laser with a central wavelength of ∼1567.2 nm was achieved at a threshold pump power of ∼1.4 W. On gradually increasing the pump power from 1.4 W to 5.6 W, the repetition rate of Q-switched laser increases from 16.6 kHz to 51.14 kHz and the pulse duration decreases from 8.7 μs to 2 μs. Particularly, we measure the output power of Q-switched lasers based on two types of plasmonic materials, Cu1.8S NCs and gold nanorods (GNRs). The maximum output power of the Q-switched laser based on Cu1.8S NCs SA is 3–4 times higher than that based on GNRs SA owing to weak photothermal effect of Cu1.8S NCs. These results show that Cu1.8S NCs are promising SAs for constructing high power pulse lasers.