Upconversion Emission Intensity Research Articles

Improvement in the dual-mode multicolor luminescence of the NaAlSiO4:Er3+, Yb3+ phosphors and the mechanistic investigations

Na0.90-yAlSiO4:0.1Er3+,yYb3+ (0 ≤ y ≤ 0.007) phosphors were prepared by the solid state reaction method and their dual mode multicolor luminescence properties were systematically investigated. The phosphors were crystallized into the carnegieite polymorph of NaAlSiO4 at 1300 °C. The synthesized phosphors were characterized by using powder XRD technique, FT-IR and UV-VIS-NIR diffuse reflectance spectra. Upon 380 nm excitation, the phosphors emitted visible and NIR downconversion emission, which follows the quantum cutting process. The upconversion emission spectra, irrespective of the laser excitation wavelength (980 nm, 808 nm, or 1550 nm), show characteristics sharp green, and red emissions, from 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+, respectively. Intense green (552 nm), red (664 nm) and NIR (809 nm) emissions were observed at 980, 808 and 1550 nm laser excitation, respectively. The dual mode emission of the Na0.90-yAlSiO4:0.1Er3+,yYb3+ (0 ≤ y ≤ 0.007) phosphors is enhanced upon Yb3+ co-doping. By analyzing the dependence of upconversion emission intensities on the laser excitation power, a possible upconversion emission mechanism is proposed.

Simultaneous influence of Mg2+ and Sc3+ co-doping on upconversion luminescence and optical thermometry in β-NaYF4: Yb3+/Ho3+ microphosphor

A series of xMg2+/ySc3+ co-doped β-NaYF4:0.2Yb3+/0.02Ho3+ (x = y = 0 −0.2) upconversion (UC) green phosphors are synthesized using EDTA-assisted hydrothermal method. The structure, morphology and elemental compositions are investigated in detail using different experimental techniques. The lengthwise suppression of crystal due to co-doping is associated to the kinetics involved in the crystal growth in presence of Mg2+/Sc3+. The effect of Mg2+/Sc3+ co-doping on the UC green emission of β-NaYF4:0.2Yb3+/0.02Ho3+ microphosphors is studied and about 31-fold enhancement in the green emission (5F4,5S2→5I8) is observed for the 0.08Mg2+/0.12Sc3+ co-doped phosphor. The downconversion properties of the phosphors is also discussed to showcase the dual mode luminescence. The temperature sensing performance is investigated using luminescence intensity ratio (LIR) technique for non-thermally coupled levels (5F5/5F4,5S2) of Ho3+ ions and maximum relative sensitivity of 0.258% K−1 at 374 K is obtained for the optimized phosphor. The microphosphor can be sustained at high temperature as the UC emission intensity retains about 65% and 36% at 423 K and 574 K. From chromaticity diagram, the maximum color purity of 91.37% is obtained for 0.08Mg2+/0.12Sc3+ doping concentration. The estimated thermal parameters and color purity of Mg2+/Sc3+ co-doped β-NaYF4:0.2Yb3+/0.02Ho3+ UC phosphors suggest their applicability in fabricating temperature sensors and LEDs.

Upconversion luminescence studies in (Er3+, Yb3+, and Mo6+) tri-doped SrWO4 ceramics

Augmented sensitization in (Er3+, Yb3+, and Mo6+) tri-doped SrWO4 ceramic compositions is investigated, and the possible role of [Yb-MoO4]2- dimer formation is probed. Structural distortion in SrO8 deltahedron and WO4 tetrahedral units in SrWO4 due to Er–Yb ion substitution for Sr2+, and Mo6+ for W6+ is confirmed by Rietveld refinement of X-ray diffraction data. Evolution of defect induced phase depending on Mo6+ dopant concentration, and the formation of [Yb-MoO4]2- dimer is envisaged by Raman and infrared spectroscopy. Selective enhancement in the green emission band (530 nm) by ∼ 58 times is supported by [Yb-MoO4]2- dimer development, and the enhancement of up-conversion emission intensity is confirmed using excited state decay life time measurements.

Fe/Mn Bimetal-Doped ZIF-8-Coated Luminescent Nanoparticles with Up/Downconversion Dual-Mode Emission for Tumor Self-Enhanced NIR-II Imaging and Catalytic Therapy.

ZIF-8, as an important photoresponsive metal-organic framework (MOF), holds great promise in the field of cancer theranostics owing to its versatile physiochemical properties. However, its photocatalytic anticancer application is still restricted because of the wide bandgap and specific response to ultraviolet light. Herein, we developed lanthanide-doped nanoparticles (LDNPs) coated with Fe/Mn bimetal-doped ZIF-8 (LDNPs@Fe/Mn-ZIF-8) for second near-infrared (NIR-II) imaging-guided synergistic photodynamic/chemodynamic therapy (PDT/CDT). The LDNPs were synthesized by encapsulating an optimal Yb3+/Ce3+-doped active shell on the NaErF4:Tm core to achieve dual-mode red upconversion (UC) and NIR-II downconversion (DC) emission upon NIR laser irradiation. At the optimal doping concentration, the UC and DC NIR-II emission intensities of LDNPs were increased 30.2- and 13.2-fold above those of core nanoparticles, which endowed LDNPs@Fe/Mn-ZIF-8 with an outstanding capability to carry out UC-mediated PDT and NIR-II optical imaging. In addition, the dual doping of Fe2+/Mn2+ markedly decreased the bandgap of the ZIF-8 photosensitizer from 5.1 to 1.7 eV, expanding the excitation threshold of ZIF-8 to the visible light region (∼650 nm), which enabled Fe/Mn-ZIF-8 to be efficiently excited by UC photons to achieve photocatalytic-driven PDT. Furthermore, Fe2+/Mn2+ ions could be responsively released in the tumor microenvironment through degradation of Fe/Mn-ZIF-8, thereby producing hydroxyl radicals (·OH) by Fenton/Fenton-like reactions to realize CDT. Meanwhile, the degradation of Fe/Mn-ZIF-8 endowed the nanosystems with tumor self-enhanced NIR-II imaging function, providing precise guidance for CDT/PDT.

Novel insights on energy transfer processes in [Ce4+/Ce3+]-Er3+-doped tellurite glass

Previous works have been already reported on multiphonon-assisted non-resonant energy transfer in Ce3+-Er3+-doped tellurite glasses. However, it is not clear the mechanism of the radiative emission of Er3+ centered at 1530 nm. In this paper, we reported a better understanding of the mechanism of interactions between those two rare-earth ions via a systematic study. For that, we will explore the energy transitions between Ce4+/Ce3+ and Er3+ ions in a tungsten-tellurite glass to both emission (in the near infrared) and upconversion (in the visible) spectrum. Here, Ce4+ and Ce3+ were obtained in an Er3+-doped tellurite glasses via the addition of different concentrations of CeO2 as part of the composition of the samples. Emission spectrum, under a 980 nm excitation, giving rise to a series of interactions between Ce3+↔Er3+ resulting in: (i) a subtle increase of the Er3+ emission intensity in the near-infrared region for 0.1 mol% of CeO2, and then a decrease in the emission for higher CeO2 concentration in both cases without any significant increase in the bandwidth, and (ii) a decrease of the visible upconversion emission intensity with the addition of CeO2. Such interactions are achieved via a coupling yielding and energy transfer from both rare-earth ions.

Anomalous upconversion behavior and high-temperature spectral properties of Yb/Ho-SiAlON ceramics

Traditionally, NaYF4 and glass-based materials are considered the most efficient upconversion materials. However, those materials may show signs of thermal warping or risks of fracture during long-term service life in extreme environments. In contrast, SiAlON ceramics preserve their spectral and physical integrity during continuous use due to their ultra-low thermal expansion coefficient (∼3.0 × 10−6/K) even at extreme temperatures. Here, we investigate the upconversion photoluminescence properties of Ho and Yb co-doped SiAlON (Ho/Yb-SiAlON) ceramics, prepared by hot-press method, at room temperature and at high temperature (298–1023 K). At room temperature, Ho/Yb-SiAlON ceramics show intense red upconversion emission under 980 nm excitation. At the high-temperature range, the spectral shift is absent in Ho/Yb-SiAlON indicating that the SiAlON ceramics have high-temperature spectral stability, which may be attributed to the low thermal expansion coefficient of SiAlON ceramics. Ho and Yb concentration-dependent spectra show an anomalous upconversion emission behavior. It is found that a higher sensitizer and low activator concentration combination is not always an ideal choice for sensitized luminescence in SiAlON ceramics which is in stark contrast to the common understanding of sensitized luminescence. The mechanism involving the dominant cross-relaxation process has been proposed to explain the observed anomalous upconversion behavior.

Enhanced upconversion luminescence and temperature sensing performance in Er3+/Yb3+ -codoped K3ScF6 phosphors induced by tridoping with Bi3+ ions

Herein, K3ScF6 upconversion luminescence (UCL) phosphors codoped with fixed Er3+, Yb3+ and various Bi3+-concentrations were prepared by solid-state reaction method. Phase and structure analysis indicate that the prepared samples belong to a cubic system and exhibit the shift in peaks position due to Bi3+ ions doping. The upconversion (UC) emission properties of K3ScF6:Er3+/Yb3+/Bi3+ are investigated in detail. The UC emission intensity can be enhanced via tridoping with Bi3+ ions under 980 nm laser diode (LD) excitation. Changes in Bi3+ concentration and power density generate color tunability from green to red. Similarly, the CCT value changes from cool light to warm light. The fluorescence intensity ratio (FIR) technique based on Er3+ thermal coupling level (TCLs) was used to study the temperature sensing performance, the relative sensitivity (SR) of K3ScF6:0.05Er3+/0.15Yb3+/0.10Bi3+ reaches a maximum of 0.0174 K−1 at 298 K. This indicates that K3ScF6:Er3+/Yb3+/Bi3+ phosphors have potential applications in non-contact optical temperature sensors and color tunable devices.

(INVITED)Judd-Ofelt analysis, visible to NIR photoluminescence emission under 450 nm and 976 nm excitations and energy transfer of barium fluorotellurite glasses doping with Ho3+, Yb3+, Ho3+:Yb3+

Barium fluorotellurite glasses doping with Ho3+, Yb3+, and Ho3+:Yb3 were fabricated using the conventional melt and quenching method, and their optical properties were studied using different optical spectroscopic techniques. The FITR absorption band at 3000 cm−1 was used to investigate OH- concentrations of the prepared glasses, which increased with Yb3+ content. However, it decreased by substituting a portion of BaF2 content with Ba2CO3 concentration. The electric dipole line strengths and Judd-Ofelt parameters were estimated using UV–visible near-infrared (NIR) optical absorption spectra to gain information on the rare earth ions doped glasses for laser applications. The various radiative parameters such as line strengths, optical intensity, and radiative rates from the higher energy levels, including 5F4+5S2, 5F5, 5I4,5I5,5I6 and 5I7, to the ground state, 5I8 have been reported and discussed. The influence of the Yb3+ and Ba2CO3 concentrations on the upconversion and downconversion photoluminescence emission intensities of the glasses prepared have been investigated by pumping with 450 nm and 976 nm excitation sources. Under 450 nm and 976 nm, the NIR photoluminescence emissions reveal a band ranging from 980 nm to 1220 nm attributing to an overlap between Yb3+ and Ho3+ transitions. The influence of Yb3+ concentration on photoluminescence lifetime at the 2F5/2→2F7/2, 5F4+5S2 →5I8 and 5I6 →5I8 transitions under 976 nm laser excitation has been studied and reported. The results indicate that lifetime decreases with increasing Yb3+ concentration attributing to an efficient energy transfer from Yb3+ to Ho3+ ions. The energy transfer efficiency from Yb3+ → Ho3+ has been examined, which rises from 30% to 47% upon increasing Yb3+ content. Nevertheless, it decreases with the mixing of BaF2 and Ba2CO3 lattice modifier contents. The results demonstrated that the 1200 nm (5I6 →5I8 transition) has remarkable potential for developing fiber lasers.

Pump power-responsive optical thermometry of titanate phosphors via TCLs and NTCLs based LIR techniques

A new strategy for modulating the optical thermometric performance of the Er3+-Yb3+-Mo6+ doped/co-doped CaBi4Ti4O15 phosphors with laser excitation power has been demonstrated in this paper. The structural, functional, optical and thermometric properties of the phosphors prepared at 1100 ºC using the solid-state reaction method have been studied thoroughly. The influence of Yb3+-MoO42- complex on the upconversion emission intensity of the Mo6+ ion co-doped phosphors have been explained with a suitable energy level diagram. Despite having the massive reputation of the luminescence intensity ratio (LIR) technique in estimating the temperature sensitivity, the limitations and further improvement using non-thermally coupled levels (NTCLs) have been discussed. The dependence of maximum sensitivities and thermal quenching ratios on the laser excitation power has been noticed. The maximum relative sensitivities of ∼0.0120 K−1 at 303 K and 0.0104 K−1 at 498 K corresponds to the TCLs and NTCL based LIR technique, respectively at 1.56 W/cm2 confirms the optical temperature measurement in a wide temperature range.

Multifunctional CaF2: Yb3+, Ho3+, Gd3+ Nanocrystals: Insight into Crystal Growth and Properties of Upconversion Luminescence, Magnetic, and Temperature Sensing Properties.

The upconversion (UC) emission intensity of Ln3+-doped CaF2 nanomaterials is not ideal, which limits their application in some advanced scientific fields. Hence, it is extremely imperative to enhance the emission intensity of UC nanocrystals. In this work, an ionic-liquid-assisted hydrothermal method based on an ethylene glycol (EG) and ionic liquid (IL) two-phase system was used to synthesize CaF2 doped with Yb3+ and Ho3+. The influence of the amount of IL and the reaction time as well as the concentration of Gd3+ doping on morphology and size was studied in detail, and the growth mechanism was proposed. Green UC luminescence materials were obtained through co-doping Yb3+ and Ho3+ ions. Furthermore, the luminescence of UC was increased monotonically with the introduction of Gd3+ ions. The effect mechanism of Gd3+ doping on the UC luminescence was put forward, which might provide a new method for the promotion of UC luminescence. In addition, the temperature sensing of CaF2: Yb3+/Ho3+/Gd3+ was investigated, which demonstrated that the phosphor has a potential application prospect in thermal sensing. Meanwhile, CaF2: Yb3+/Ho3+/Gd3+ also exhibited a paramagnetic property at room temperature. Therefore, these multifunctional nanocrystals may have prospective applications in optical bioimaging, magnetic resonance imaging, and temperature sensing.

Investigation on the determinant of upconversion luminescence efficiency and temperature measurement sensitivity based on AWO4 (Ba, Sr, Ca, and Mg): Er3+/Yb3+ microcrystals

In this work, a comparative study of the upconversion (UC) luminescence properties of E r 3 + − Y b 3 + -co-doped A W O 4 ( A = B a , Sr, and Ca) scheelite phosphors and E r 3 + − Y b 3 + -co-doped M g W O 4 wolframite phosphors is reported. Under 980 nm excitation, all the samples emit green and red emissions. The E r 3 + − Y b 3 + -doped M g W O 4 phosphor has a higher UC emission intensity. Furthermore, the optical temperature sensing properties in E r 3 + − Y b 3 + -co-doped A W O 4 ( A = B a , Sr, Ca, and Mg) phosphor samples are investigated. The relative sensitivity ( S r ) based on thermally coupled levels (TCLs) increases gradually with an increase in matrix phonon energy. The absolute sensitivity ( S a ) values of the samples are accurately predicted by the chemical bonding parameter ( f c ). The results of the correlation between the UC luminescence properties and the microscopic crystal structure, and the relationship of the temperature sensitivity and the host parameters, provide us with a guiding insight for the selection of a host material with ideal temperature measurement capability.

Luminescence and temperature sensing properties of KLu(MoO4)2:Ln3+ phosphors

A series of phosphors based on KLu(MoO4)2 are synthesized by the citric acid assisted hydrothermal method combined with subsequent calcination in this paper. By doping Yb3+/Er3+, Yb3+/Ho3+ and Yb3+/Tm3+ into KLu(MoO4)2 host, the green, red and blue emissions are realized, respectively. Moreover, the amount of Yb3+/Er3+ doping can affect the fluorescence upconversion emission intensity. The temperature sensing property of KLu(MoO4)2: 20%Yb3+,0.8%Er3+ is explored in the range of 323–573 K. In the heating process, the maximum absolute sensitivity is 0.01187 K−1 at 498 K. In addition, by constructing the cycle process of heating and cooling, the maximum sensitivity relative deviation of the two processes is only −0.72%, which means the KLu(MoO4)2:Yb3+/Er3+ phosphor has excellent stability in temperature measurement. Our research demonstrates that the KLu(MoO4)2:Ln3+ phosphors have potential application prospects in the fields of temperature sensor and multicolor fluorescence.

Near-infrared rechargeable glass-based composites for green persistent luminescence

The fabrication of Yb3+, Tm3+ co-doped oxyfluorophosphate glass-based composites, with green persistent luminescence after being charged with near-infrared light, is demonstrated. The mechanism responsible for the green afterglow after near-infrared illumination is unveiled. The composite is prepared using a modified melting process to limit the evaporation of fluorine during melting. Intense (blue and ultraviolet) up-conversion emission is obtained by optimizing the Yb2O3 and Tm2O3 concentrations. A heat treatment promotes volume precipitation of Yb3+, Tm3+ co-doped CaF2 crystals. Although the intensity of the blue up-conversion emission from the Tm3+1G4 level is lower in the highly Yb3+-concentrated glass-ceramic due to reverse energy transfer from Tm3+ to Yb3+, the heat treatment leads to an increase of the intensity of the emissions around 346 nm, 361 nm nm and 450 nm coming from the Tm3+1I6 and 1D2 levels. By combining the Yb3+ and Tm3+ ions with SrAl2O4:Eu2+,Dy3+crystals, green afterglow can be obtained after charging with near-infrared light.

Intense green photoluminescence and upconversion-based intrinsic optical bistability in Ho3+/Yb3+ codoped CaSc2O4 and upconversion in Ho3+/Yb3+ codoped CaY2O4 phosphors

The present study details the downshifting, frequency upconversion and intrinsic optical bistability in CaSc2O4:Ho3+/Yb3+ and upconversion in CaY2O4:Ho3+/Yb3+ phosphors synthesized by a complex-based precursor solution method. The X-ray powder diffraction confirms the formation of highly crystalline phosphors with pure orthorhombic phase. Fourier transform infrared studies of synthesized phosphors show vibrational bands due to the presence of Ca–O, Sc–O and Y–O groups. The diffuse reflectance spectra show a number of bands in the UV–vis–NIR regions due to presence of Ho3+ and Yb3+ ions. The values of optical band gap (Eg) are found to be 5.69 eV and 5.58 eV for the Ho3+/Yb3+ codoped CaSc2O4 and CaY2O4 phosphors, respectively. The Ho3+/Yb3+ codoped CaSc2O4 phosphors display intense green downshifting emission upon 454 nm excitations. The lifetime of green luminescence of CaSc2O4:Ho3+/Yb3+ phosphors with varied Yb3+ concentrations have also been measured. The decay curves show a non-exponential nature fitted to the Inokuti-Hirayama model and the energy transfer microscopic parameters have been also calculated. The Ho3+/Yb3+ co-doped CaSc2O4 and CaY2O4 phosphors display intense green alongwith weak blue, red, and near-infrared upconversion (UC) emissions upon 980 nm excitations. The spectral color purity (Sgr) of CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor is calculated to be 0.78. Importantly, the variation of pump power generates intrinsic optical bistability (IOB) in the CaSc2O4:Ho3+(1%)/Yb3+(5%) phosphor for the green emission, which is not observed in the CaY2O4:Ho3+(1%)/Yb3+(5%) phosphor. Therefore, the Ho3+/Yb3+ co-doped CaSc2O4 phosphor could potentially be used in UC-based devices, optical memory devices, optical gates, optical transistors and switching devices for optical communications.

Dual luminescence optimization of Ho3+/Yb3+/Eu3+-doped Gd2O3 phosphor prepared by spray pyrolysis for anti-counterfeiting application

Ho3+/Yb3+/Eu3+-doped Gd2O3 spherical particles with upconversion (UC) and down-conversion (DC) emission were synthesized by spray pyrolysis. The resulting phosphor showed intense DC red and UC green emissions. The Eu3+, Ho3+ and Yb3+ contents were optimized to simultaneously achieve the highest UC and DC emission. Resultantly, the optimal Eu3+, Ho3+ and Yb3+ doping concentrations were found as 0.2%, 3.0% and 0.2%, respectively. The green UC emission resulted from a typical two-photon process, and the UC and DC emission intensity showed a linear relationship with crystallite size. The microstructure of Gd2O3:Ho3+/Yb3+/Eu3+ was controlled by introducing organic additives into the spray solution. The use of organic additives made it possible to improve crystallinity having a dense morphology, which led to largely enhance the DC and UC intensity. The prepared Gd2O3:Ho3+/Yb3+/Eu3+ spherical particles showed excellent dual luminescence suitable as anti-counterfeiting security materials.

Enhanced red up-conversion of β-NaYF4:Er3+,Tm3+ microcrystals for bio-imaging applications

Pure red fluorescence generated by near infrared (NIR) excitation is desired for bio-imaging with high biological penetration, but without cell damage or multi-photon photo-toxicity. In this work, β-NaYF4:Er3+ and β-NaYF4:Er3+,Tm3+ microcrystals were synthesized using a novel hydrothermal method. β-NaYF4:Er3+ microcrystals exhibited intense green and red up-conversion emissions under 976 nm laser diode excitation. The introduction of Tm3+ ions into β-NaYF4:Er3+ resulted in strong inhibition of green up-conversion emission (Er3+:2H11/2 /4S3/2→4I15/2) and, consequently, red up-conversion emission (Er3+:4F9/2→4I15/2) was significantly promoted. To simultaneously restrain green up-conversion emissions and enhance red up-conversion emissions, the optimal concentrations of Er3+ and Tm3+ ions were determined to be 20 mol% and 2.0 mol%, respectively. The red/green up-conversion emission intensity ratio increased from ~1.80 (β-NaYF4:20Er3+, mol%) to ~78 (β-NaYF4:20Er3+,2Tm3+, mol%) after the addition of 2 mol% Tm3+ ions to β-NaYF4:20Er3+ (mol%). The red up-conversion emission intensity of β-NaYF4:20Er3+,2Tm3+ was stronger than that for commercial red up-conversion phosphors (NaYF4 and ZnF2) under the same experimental conditions. Based on the results of the emission spectra, luminescence rise, and decay profiles, the mechanism of enhanced red up-conversion emission by Tm3+ ion co-doping in β-NaYF4:20Er3+,2Tm3+ (mol%) was proposed. The bio-fluorescence experiment under NIR excitation proves that β-NaYF4:Er3+,Tm3+ microcrystals have potential applications in bio-imaging.

Spectroscopic characterization of rare events in colloidal particle stochastic thermodynamics.

Given the remarkable developments in synthetic control over chemical and physical properties of colloidal particles, it is interesting to see how stochastic thermodynamics studies may be performed with new, surrogate, or hybrid model systems. In the present work, we apply stochastic dynamics and nonlinear optical light-matter interaction simulations to study nonequilibrium trajectories of individual Yb (III):Er (III) colloidal particles driven by two-dimensional dynamic optical traps. In addition, we characterize the role of fluctuations at the single-particle level by analyzing position trajectories and time-dependent upconversion emission intensities. By integrating these two complementary perspectives, we show how the methods developed here can be used to characterize rare events.

The effect of crosslinker contents on the up-conversion luminescence properties of CdNb2O6: Er3+ powders embedded in the polyethylmethacrylate (PEMA) networks

In this work, CdNb2O6:Er2O3(1.5 mol% Er3+) powders doped with Er3+ were embedded in bulk linear polyethylmethacrylate (PEMA) and also as a novelty embedded in bulk PEMA crosslinked networks (gels) with four different crosslinker contents by free-radical crosslinking polymerization with 0.1 EMA (weight %) at 60 °C. The Er3+ ions structures embedded in bulk linear PEMA and PEMA gels were investigated by the X-ray diffraction (XRD) technique. The crystalline particle size was found using Pielazsek particle distribution and the Scherrer equation. The average of the crystalline particle sizes was found to be decreasing from 75 nm to 12.50 nm by increasing the crosslinker contents therefore, it was shown that the distribution became more uniform. The morphology of CdNb2O6: Er3+ embedded in PEMA gel samples was monitored by scanning electron microscopy (SEM). The functional groups on linear PEMA and PEMA gels samples were monitored by Fourier transform infrared (FT-IR) spectra, and the results were consistent with XRD results. The optical properties of polymer samples were investigated by measuring luminescence spectra at room temperature. The doped Er3+ ion excited with a 975 nm diode laser achieved the up-conversion (UC) emissions by involving two or three-photon absorptions in the visible region. The effects of crosslinker contents on the UC emission, color tuning, and color quality parameters were observed. As the crosslinker contents increased, the UC emission intensities increased due to increasing the distance between Er3+- Er3+ ions. The change in color coordinates was also observed by increasing crosslinker content. The chromaticity diagram over the increasing power of the excitation source visualized the tunability of the emitted color. The improvement of the color purity was observed as a function of crosslinker concentration.

Surface Plasmon Enhanced Upconversion Fluorescence in Short-Wave Infrared for In Vivo Imaging of Ovarian Cancer.

Short-wave infrared (SWIR; 850-1700 nm) upconversion fluorescence enables "autofluorescence-free" imaging with minimal tissue scattering, yet it is rarely explored due to the lack of strongly emissive SWIR upconversion fluorophores. In this work, we apply SWIR upconversion fluorescence for in vivo imaging with exceptional image contrast. Gold nanorods (AuNRs) are used to enhance the SWIR upconversion emission of small organic dyes, forming a AuNR-dye nanocomposite (NC). A maximal enhancement factor of ∼1320, contributed by both excitation and radiative decay rate enhancement, is achieved by varying the dye-to-AuNR ratio. In addition, the upconversion emission intensity of both free dyes and AuNR-dye NCs depends linearly on the excitation power, indicating that the upconversion emission mechanism remains unchanged upon enhancement, and it involves one-photon absorption. Moreover, the SWIR upconversion emission shows a significantly higher signal contrast than downconversion emission in the same emission window in a nonscattering medium. Finally, we apply the surface plasmon enhanced SWIR upconversion fluorescence for in vivo imaging of ovarian cancer, demonstrating high image contrast and low required dosage due to the suppressed autofluorescence.

Manipulating Upconversion Emission and Thermochromic Properties of Ho3+/Yb3+-Codoped Al2Mo3O12 Microparticles by Negative Lattice Expansion for Multimode Visual Optical Thermometry.

To ameliorate the inherent thermal quenching behaviors of upconverting materials, a series of Ho3+/Yb3+-codoped Al2Mo3O12 (i.e., Al2Mo3O12:Ho3+/2xYb3+) microparticles were developed. Upon excitation at 980 nm, intense upconversion (i.e., UC) emissions arising from Ho3+ are observed, and their optimal states occur at x = 0.09. Besides, the UC mechanisms of these generated emissions from 5F4/5S2 and 5F5 levels all pertain to a two-photon absorption process. Furthermore, modified thermal quenching performances are realized in the resultant microparticles, in which the intensities of the UC emissions arising from 5F4/5S2 levels decrease as the temperature increases, while that of the UC emission from the 5F5 level increases and then decreases with the increase of temperature. The coexistence of nonradiative transition promoted crossrelaxation, and energy transfer routes can be responsible for the above phenomenon. By studying the diverse UC emission characteristics at high temperatures, we revealed the thermometric properties of Al2Mo3O12:Ho3+/2xYb3+ microparticles, where their sensitivities can be regulated by selecting the spectral mode and dopant contents. According to the intensity ratio of the UC emissions originating from 5F5 → 5I8 to (5F4,5S2) → 5I7 transitions at different temperatures, one obtains that the relative and absolute sensitivities of the developed compounds reach up to 0.464% and 0.1739 K-1, respectively. Additionally, by the analysis of the thermochromic performances of final products, their thermometric characteristics were also investigated. Note that the environmental temperature is able to be facilely read out by distinguishing the emitting color. These results verify that the Al2Mo3O12:Ho3+/2xYb3+ microparticles are promising luminescent materials for multimode visual optical thermometry.