Upconversion nanoparticles capable of strongly absorbing photons in a wide spectral range are highly desired for practical applications. In this work, IR-806 dye was used to increase the light absorptivity of Nd3+/Yb3+/Er3+ tri-doped core/shell nanoparticles and then to enhance their upconversion luminescence under ∼800 nm excitation. The IR-806 dye exhibited more efficient energy transfer to Nd3+ ions than to Yb3+ ions for subsequent upconversion emission due to the increased spectral overlap between the dye emission and Nd3+ absorption. The influence of the Nd3+ concentration in the shell and the dye/nanoparticle ratio on the dye-sensitization effect was also investigated. A maximum 28-fold overall enhancement in the emission intensity was achieved for NaYF4:Yb3+/Er3+@NaYF4:Yb3+/Nd3+ core/shell nanoparticles using dye sensitization. The dye-sensitized NaYF4:Yb3+/Er3+@NaYF4:Yb3+/Nd3+ core/shell nanoparticles also exhibited increased photothermal conversion capabilities and excellent temperature sensing properties, enabling their potential application in photothermal nanoheaters with real-time temperature monitoring under 808 nm single beam excitation.

•The K2SiF4:Mn4+ phosphor shows excellent thermal stability between 20 and 150 °C.•The abnormal PL enhancement at elevated temperatures was found and discussed.•The K2SiF4:Mn4+ phosphor shows potential applications for temperature sensing.Investigating and understanding temperature-dependent photoluminescence (PL) properties of inorganic phosphors are of vital importance to optimize their performance and expand their application. Recently, K2SiF4:Mn4+ phosphors have attracted intense attention as a red phosphor candidate for white LED applications because of their extremely narrow emission peaks in the red spectral region. Here, temperature-dependent (20–200 °C) diffuse reflection spectra and steady and transient PL spectra of K2SiF4:Mn4+ were systematically investigated. Both the emission intensities and spectral characteristics of the phosphor showed excellent thermal stability at temperatures from 20 to 150 °C, ensuring its great potential for practical white LED applications. An abnormal PL enhancement was found at elevated temperatures that differed from the thermal quenching behavior of common phosphors and could be attributed to enhanced phonon-assisted radiative transitions at higher temperatures. We further successfully tested and verified that the intensity ratio of anti-Stokes and Stokes emission peaks of Mn4+ ions was sensitive to temperature and followed a Boltzmann-type distribution function very well, making the K2SiF4:Mn4+ phosphor a good candidate for temperature sensing, especially for investigating the thermal distribution of white LEDs.

Ultrasmall (10.8 nm) and uniform hexagonal NaYF4:Yb3+,Er3+ upconversion nanocrystals have been successfully synthesized through a temperature oscillation technique, and the nucleation and growth processes of nanocrystals were effectively separated and controlled by simply regulating the heating procedures.

Upconversion nanoparticles (UCNPs) are an excellent choice to construct security features against counterfeiting, owing to their unique NIR-to-VIS upconversion luminescence (UCL) characteristics. However, the application of upconversion materials is limited, due to their single and invariant emission colors. Herein, the temperature-dependent UCL properties of NaGdF4:Yb/Ho (or Tm) UCNPs in the solid state have been investigated. An anomalous UCL enhancement at higher temperatures has been demonstrated for these small-sized (<10 nm) UCNPs and the underlying mechanism is discussed herein. Meanwhile, effective UCL with tunable multicolor emissions has been realized by the rational incorporation of Ho3+ and Tm3+ emitters into a single nanostructure. The emission colors of these Ho/Tm co-doped Na(Gd,Yb)F4 UCNPs can be tuned by changing the laser power or temperature, due to the different spectral sensitivities of the Tm3+ and Ho3+ emitters to the excitation power density and temperature. The power- and temperature-responsive color shifts of these Ho/Tm co-doped UCNPs are favorable for immediate recognition by the naked eye, but are hard to copy, offering the possibility of designing more secure anti-counterfeiting patterns.

Hexagonal phase (β-) NaYF4:Yb3+,Er3+ upconversion nanocrystals have attracted great attention due to their potential applications in a variety of fields, such as biomedical imaging, labeling, and photodynamic therapy. However, the precise adjustment of the particle size of pure β-NaYF4:Yb3+,Er3+ nanocrystals (without doping), especially below 20 nm, is still a great challenge, which is critical for their biomedical utilization. In this paper, a facile methodology for regulating the size of β-NaYF4:Yb3+,Er3+ upconversion nanoparticles (UCNPs) is demonstrated by varying the relative ratio of oleic acid to rare-earth acetates. The roles of oleic acid and 1-octadecene in the synthesis of UCNPs are studied in detail and their internal mechanism for the nucleation and growth of the hexagonal UCNPs is also proposed.

NaYF4:Yb3+,Er3+@SiO2@Au heterogeneous nanostructures, with the ability of integrating upconversion luminescence (UCL) and photothermal therapy as well as real-time temperature monitoring, have been successfully prepared. The UCL intensities of NaYF4:Yb3+,Er3+ nanocrystals showed an obvious reduction after gold nanoparticle attachment and gold shell coating, but it was found that they were independent of the SiO2 spacer layer thicknesses. The presence of gold nanostructures can significantly improve the photothermal conversion properties of NaYF4:Yb3+,Er3+ nanocrystals with an increased temperature up to 39 °C, while retaining their temperature sensing properties, manifesting their great potential as diagnostic and therapeutic tools for use in biomedical fields.

Small-sized (∼11.86 nm) and monodisperse hexagonal NaYF4:Yb3+,Er3+ upconversion (UC) nanocrystals have been successfully synthesized by simultaneously controlling the nucleation and growth process with a relatively high oleic acid to precursor ratio.

Hexagonal-phase NaYF4:Yb3+,Er3+ upconversion nanoparticles (UCNPs) with a uniform size distribution were synthesized using rare-earth acetates as precursors. The effects of reaction temperature and time on the phase transition process of the UCNPs were systematically studied. Based on the evolution of particle morphology and phase with temperature and time, it could be concluded that the transition from cubic phase to hexagonal phase for NaYF4:Yb3+,Er3+ UCNPs was consistent with a dissolution/recrystallization process. In addition, the shape and size of the UCNPs could be controlled by adjusting the solvent ratio and the precursor ratio, respectively.Phase-, shape- and size-controlled synthesis of NaYF4:Yb3+,Er3+ nanoparticles

Size-dependent quantum confinement has important effects on the energy transfer and radiative and nonradiative transitions in nanophosphors. For lanthanide-doped nanoparticles, the confinement effect is induced mostly via electron–phonon interaction, and analysis of temperature-dependent spectroscopic properties provides an effective method for disclosing its underlying mechanism. Herein, an intriguing and unprecedented enhancement of the upconversion luminescence (UCL) at higher temperatures in hexagonal-phase NaYF4:Yb3+, Er3+ upconversion nanoparticles (UCNPs) is reported. Moreover, this anomalous UCL enhancement shows a strong dependence on the particle size and becomes more significant for UCNPs with a smaller size. This anomalous thermal behavior is interpreted on the basis of phonon-assisted energy transfer and phonon confinement effect. The findings are relevant to the engineering of the nanostructures of UCNPs and to the further understanding of the UCL mechanism.

The effects of the excitation wavelength, Ce3+ concentration and chemical substitution on the thermal quenching of Y3Al5O12:Ce3+ (YAG:Ce3+) phosphors were investigated over a temperature range from 30 to 250 °C. The quenching behavior exhibits a complex dependence on the excitation wavelength and Ce3+ concentration, which can be attributed to temperature-dependent absorption strength of the different f–d absorption bands and thermally activated concentration quenching with or without energy migrations between Ce3+ ions, respectively. With increasing Lu3+content the luminescence of (Y, Lu)3Al5O12:Ce3+ phosphors shows a pronounced blueshift, and simultaneously the temperature quenching is obviously improved due to a decrease in Stokes shift.Research highlights► Excitation wavelength and Ce3+ content can affect the thermal quenching of YAG:Ce3+. ► Ce3+ concentration effects can be explained by thermally activated concentration quenching with or without energy migrations between Ce3+ ions. ► Lu3+ addition can improve the thermal quenching behavior of YAG:Ce3+.

Gd3+-doped, La3+-doped and Gd3+/La3+ co-doped Y3Al5O12:Ce3+ (YAG:Ce3+) phosphors were prepared by high-temperature solid-state reaction. Their crystal structure and photoluminescence properties at various temperatures (25–200 °C) were investigated. By increasing the substitution concentration of Gd3+ in YAG:Ce3+ phosphor, the Ce3+emission band shifts toward longer wavelength but shows a stronger thermal quenching. Addition with small amount of La3+ can improve the thermal stability of YAG:Ce3+ phosphors without leading to evident shifts of Ce3+ emission band. The La3+ and Gd3+ doping can complement each other. The Gd3+/La3+ co-doped YAG:Ce3+ phosphors have longer wavelength emission and furthermore exhibit good thermal stability, which is especially favorable for high power white LEDs aiming at general illumination.

Sodium europium double tungstate [NaEu(WO4)2] phosphor was prepared by the solid-state reaction method. Its crystal structure, photoluminescence properties and thermal quenching characteristics were investigated aiming at the potential application in the field of white light-emitting diodes (LEDs). The influences of Sm doping on the photoluminescence properties of this phosphor were also studied. It is found that this phosphor can be effectively excited by 394 or 464 nm light, which nicely match the output wavelengths of near-ultraviolet (UV) or blue LED chips. Under 394 or 464 nm light excitation, this phosphor exhibits stronger emission intensity than the Y2O2S:Eu3+ or Eu2+-activated sulfide phosphor. The introduction of Sm3+ ions can broaden the excitation peaks at 394 and 464 nm of the NaEu(WO4)2 phosphor and significantly enhance its relative luminance under 400 and 460 nm LEDs excitation. Furthermore, the relative luminance of NaEu(WO4)2 phosphor shows a superior thermal stability compared with the commercially used sulfide or oxysulfide phosphor, and make it a promising red phosphor for solid-state lighting devices based on near-UV or blue LED chips.

Small-sized (∼11.86 nm) and monodisperse hexagonal NaYF4:Yb3+,Er3+ upconversion (UC) nanocrystals have been successfully synthesized by simultaneously controlling the nucleation and growth process with a relatively high oleic acid to precursor ratio.

Ultrasmall (10.8 nm) and uniform hexagonal NaYF4:Yb3+,Er3+ upconversion nanocrystals have been successfully synthesized through a temperature oscillation technique, and the nucleation and growth processes of nanocrystals were effectively separated and controlled by simply regulating the heating procedures.