Opposite luminescence thermal behavior of upconversion core/shell nanocrystals for anticounterfeiting.

Inorganic luminescent materials generally suffer from thermal quenching due to accelerated nonradiative transitions at high temperature, whereas Yb3+ sensitized core nanocrystals with small sizes (<30 nm) exhibit a temperature-dependent upconversion luminescence (UCL) enhancement. The related mechanism of the anomalous UCL thermal behavior is still under debate. In this work, we find that the UCL of NaGdF4:Yb/Tm@NaGdF4 inert-shell nanocrystals declines at elevated temperature, while that of NaGdF4:Yb/Ho@NaGdF4:Yb active-shell ones is enhanced. The thermally-induced UCL enhancement of active-shell nanocrystals is attributed to a gradually attenuated surface quenching effect. The initiators of the surface quenching are H2O molecules, which mainly attenuate Yb3+ excited states through an overtone energy transfer. The energy transfer is a coupling effect between ion dipoles of Yb3+ and atomic dipoles of H2O. Utilizing the opposite UCL temperature-dependence of active- and inert-shell nanocrystals, we designed their hybrids, which exhibit temperature-responsive multicolor emissions. The color-tunable hybrids are demonstrated to be excellent candidates for producing anticounterfeiting patterns with high security but simple recognition methods.

Single-Mode-Tuned Tricolor Emissions of Upconversion/Afterglow Hybrids for Anticounterfeiting Applications.

This work presents a highly secure anticounterfeiting strategy based on upconversion/afterglow hybrids with tricolor emissions tuned by a single 975 nm laser. The hybrids are composed of NaYF4:Yb/Tm and NaYF4:Yb/Er microrods and CaS:Eu2+ afterglow phosphors. Under 975 nm excitation, the hybrids exhibit multicolor emissions from green to white by adjusting laser power and then emit red afterglow light when the 975 nm laser is off. Under synergistic excitation of the blue-green light emitted by Tm/Er microrods, the red afterglow emission not only has a strong initial intensity but also lasts for 3 s. Obvious trichromatic changes from green to white to red can be observed by the naked eye. A pattern printed by the hybrid ink exhibits tricolor emissions by laser adjustment and switch. This proves that upconversion/afterglow hybrids are an excellent candidate for anticounterfeiting applications with high-level security but a simple recognition method.

Molten Salt Synthesis of Broad-Band Near-Infrared InBO3:Cr3+ Submicron Phosphor and Its Luminescent Enhancement by Lanthanide Ion Codoping.

Phosphor materials with small particle sizes and high luminescent efficiency are desired for the fabrication of phosphor-converted light-emitting diodes (pc-LEDs). Near-infrared (NIR) pc-LED light sources have great application potential in the food industry and medical fields, which stimulate the extensive exploration of NIR phosphors. In this work, broad-band NIR-emitting InBO3:Cr3+ phosphors with submicron size and spherical morphology are successfully synthesized via the molten salt method. The InBO3:Cr3+ phosphor exhibits a broad emission band covering 700-1000 nm and peaking at ∼820 nm. The maximum emission intensity is obtained for InBO3:0.02Cr3+ with an internal quantum yield (IQY) of ∼62%, which is higher than that of microsized counterparts derived from solid-state reaction. Furthermore, the absorption and emission enhancements are achieved by codoping lanthanide ions into InBO3:Cr3+ submicron phosphors. The codoping of inert La3+ ions can increase the absorption efficiency of InBO3:Cr3+, due to the increased octahedral distortion of Cr3+ sites. The codoping of active Yb3+ ions can significantly enhance the NIR emissions of InBO3:Cr3+ between 950 and 1100 nm. Meanwhile, the increased IQY of ∼73% is achieved for InBO3:0.02Cr3+,0.005Yb3+ simultaneously with suppressed thermal quenching, originating from the effective energy transfer from Cr3+ to Yb3+ ions.

Enhancing the temperature sensitivity of Cr3+ emissions by modification of the host's composition for fluorescence thermometry applications.

The fluorescence intensity ratio (FIR) technique is widely adopted in thermometric phosphor materials, but the improvement of relative sensitivity is normally limited by the fixed energy gap between two thermally-coupled emitting levels of luminescent ions. Herein, LnAl3(BO3)4:Cr3+ (LnAB:Cr3+, Ln = Gd, Y, Lu) phosphors are found to simultaneously show 4T2 and 2E emissions of Cr3+, and their FIR is sensitive to temperature and suitable for fluorescence thermometric applications. Moreover, the energy gap between the 4T2 and 2E levels of Cr3+ is tunable and the relative sensitivity can be greatly improved by modifying the host's composition. Structural analysis and spectroscopic data confirm that the enhanced crystal-field of the Cr3+/Al3+ sites caused by incorporating smaller Ln3+ ions into the host contributes to the improvement of relative sensitivity. This work would provide new insights into the development of novel FIR thermometric materials with high-sensitivity.

Broadband near-infrared light source derived from Cr3+-doped phosphors and a blue LED chip.

NIR-emitting YAl3(BO3)4:Cr3+/Yb3+ (YAB:Cr/Yb) and NaScSi2O6:Cr3+ (BSSO:Cr) phosphors were demonstrated as luminescent converters for broadband NIR phosphor-converted LEDs (pc-LEDs). YAB:Cr/Yb phosphors show emissions in 670-800 nm (Cr3+ emission) and 950-1050 nm (Yb3+ emission) upon excitation at 450 nm. In the BSSO host, Cr3+ ions occupy Sc3+ sites with relatively weak crystal field, and thus a broadband Cr3+ emission at longer wavelengths of 750-950 nm is found for BSSO:Cr phosphors. Moreover, temperature-dependent spectral studies indicate that both YAB:Cr/Yb and BSSO:Cr phosphors exhibit good thermal stability, and more than 80% of the initial emission intensities can be sustained at 150°C. A NIR pc-LED prototype was fabricated by integrating these two phosphors with a blue LED chip (∼450 nm), which generated a broadband emission in the NIR spectral range from 780 to 1050 nm. A NIR light output power of ∼26 mW was achieved at the injection current of 100 mA, with the corresponding energy conversion efficiency of ∼8.6%.

Bidirectional Photoswitching via Alternating NIR and UV Irradiation on a Core-Shell UCNP-SCO Nanosphere.

Bidirectional photoswitching of molecular materials under ambient condition is of significant importance. Herein, we present for the first time that a core-shell UCNP-SCO nanosphere (UCNP = upconversion nanophosphor, SCO = spin crossover), which was composed of a UCNP core (NaYF4: 20 mol % Yb3+, 1 mol % Er3+) and an SCO iron(II) shell ([Fe(H2Bpz)2(bipy-COOH)], H2Bpz = dihydrobis(1-pyrazolyl)borate, bipy-COOH = 4,4'-dicarboxy-2,2'-bipyridine), can be reversibly photoswitched between the high-spin and low-spin states at room temperature in the solid state, via alternating irradiation with near-infrared (λ = 980 nm) and ultraviolet (λ = 310 nm) light. What's more, this reversible spin-state switching was accompanied by a variation of fluorescent spectrum and dielectric constants. The strategy here, that is, integrating the SCO iron(II) complex into a UCNP-SCO nanosphere for molecular photoswitching, may open a new area in the development of photocontrolled molecular devices.

Photoluminescence properties of a ScBO3:Cr3+ phosphor and its applications for broadband near-infrared LEDs.

The rapid extension of solid state lighting technologies offers the possibility to develop broadband near-infrared (NIR) phosphor-converted LEDs (pc-LEDs) as novel NIR light sources. In this paper, a new NIR-emitting phosphor ScBO3:Cr3+ was synthesized by a high temperature solid state reaction method. Phase structure, spectroscopic properties, luminescent lifetime, quantum yield, emitter concentration influences and thermal quenching behavior of ScBO3:Cr3+, as well as its applications for NIR pc-LEDs, were systematically investigated. ScBO3:Cr3+ phosphors exhibit a broad absorption band ranging from 400 to 530 nm, which matches well with the characteristic emission of the blue LED chip. Moreover, Cr3+ ions occupy the Sc3+ sites with relatively low crystal field strength in the ScBO3 host, and therefore ScBO3:Cr3+ phosphors show intense broadband emission peaking at ∼800 nm upon excitation at 460 nm, originating from spin-allowed 4T2 → 4A2 transition of Cr3+ ions. The optimum Cr3+ concentration was determined to be ∼2 mol% with a quantum yield of ∼65%. A broadband NIR pc-LED prototype device was fabricated by the combination of ScBO3:Cr3+ phosphors and a blue LED chip, which showed a maximum NIR light output power of ∼26 mW and a corresponding energy conversion efficiency of ∼7%. The results indicate the great potential of ScBO3:Cr3+ phosphors for applications in broadband NIR pc-LEDs.

Multifunctional Lanthanide-Doped Core/Shell Nanoparticles: Integration of Upconversion Luminescence, Temperature Sensing, and Photothermal Conversion Properties.

Multifunctional integration on single upconversion nanoparticles (UCNPs), such as the simultaneous achievement of imaging, sensing, and therapy, will be extremely attractive in various application fields. Herein, we demonstrated that single core/shell NaGdF4:Yb/Er-based UCNPs (<10 nm) with a highly Yb3+ or Nd3+ doped shell simultaneously exhibited good upconversion luminescence (UCL), temperature sensing, and photothermal conversion properties under 980 or 808 nm excitation, respectively. The spatial separation between the emission/sensing core and the heating shell was able to tailor the competition between the light and heat generation processes, and hence higher UCL efficiency and enhanced heating capability were achieved by introducing the rational core/shell design. Especially, Nd3+-sensitized core/shell nanoparticles were excitable to the laser at a more biocompatible wavelength of 808 nm, and hence the heating effect of water was greatly minimized. The heating and sensing capabilities of Nd3+-sensitized core/shell UCNPs with smaller sizes (<10 nm) were confirmed in aqueous environment under single 808 nm laser excitation, implying their promising applications in imaging-guided and temperature-monitored photothermal treatments.

Emission color tuning of core/shell upconversion nanoparticles through modulation of laser power or temperature.

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.

Enhancing the upconversion luminescence and photothermal conversion properties of ∼800nm excitable core/shell nanoparticles by dye molecule sensitization.

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 ∼800nm 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 808nm single beam excitation.

Multifunctional nanoheater based on NaGdF4:Yb3+, Er3+ upconversion nanoparticles.

Upconversion nanoparticles (UCNPs) provide an ideal platform for achieving multifunction, such as multimodal imaging, sensing, therapy, etc., mainly by combining with other nanomaterials to construct complicated heterogeneous nanostructures. Multifunctional integration on a simple single-phase structure still is an open question and poses a big challenge. Here we show that small-sized NaGdF(4):Yb(3), Er(3+) UCNPs (~7.5 nm) can simultaneously possess upconversion luminescence (UCL), temperature sensing, paramagnetic and photothermal conversion properties, endowing them great potential for photothermal treatments with real-time imaging and temperature monitoring. Effects of Yb(3+) concentrations, nanoparticle sizes and core/shell structures on the light-to-heat conversion capability of UCNPs were also investigated, and the results were discussed on the basis of the variation in absorption rates and non-radiative relaxation probabilities of UCNPs. There is a competition between UCL and light-to-heat conversion processes. Higher UCL efficiency and enhanced photothermal conversion properties can be realized on UCNPs with the active-core/active-shell structure due to enhanced absorption rates.

A facile synthesis of small-sized and monodisperse hexagonal NaYF4:Yb3+,Er3+ nanocrystals.

Small-sized (~11.86 nm) and monodisperse hexagonal NaYF4:Yb(3+),Er(3+) upconversion (UC) nanocrystals have been successfully synthesized by simultaneously controlling the nucleation and growth process with a relatively high oleic acid to precursor ratio.