Cross Relaxation Research Articles

Enhancement of 1532 nm emission in Ce3+, Er3+ co-doped Ca1.5Y1.5Al3.5Si1.5O12

Near-infrared emission material, C e 3 + , E r 3 + co-doped C a 1.5 Y 1.5 A l 3.5 S i 1.5 O 12 has been synthesized via a high-temperature solid-state reaction method. The luminescence intensity of 1532 nm emission excited with 455 nm increases 4.2 times after the introduction of C e 3 + ions because of C e 3 + → E r 3 + radiation-reabsorption energy transfer and nonradiative energy transfer sensitization as well as the 4 I 11 / 2 ( E r 3 + ) + 2 F 5 / 2 ( C e 3 + ) → 4 I 13 / 2 ( E r 3 + ) + C e 3 + ( 2 F 7 / 2 ) cross relaxation between C e 3 + and E r 3 + ions. The result shows that the developed material is convenient to obtain high-efficiency, near-infrared emission using a commercial blue GaN LED as a pumping source.

Red, Green, Blue and IR emitting zirconium Titanate nano composite co-doped with Er3+/Tm3+/Yb3+ synthesized by combustion synthesis

Activation of upconversion emission in visible region and emission in infrared region in tri-codoped (Er3+, Tm3+ and Yb3+ rare earth ions) zirconium titanate nano composites has been attained by synthesizing through solution combustion method. ZrTi1.4O4, serves as host matrix which offers lowest phonon energy at 870 cm−1. PL emission has been recorded by exciting the samples with wavelength of 980 nm in NIR region. X-Ray diffraction patterns confirm crystallinity of the composites. Morphological studies were performed by Field emission scanning electron microscopy. Primary colors, Blue (475 nm), green (520 nm, 530 nm, 540 nm and 550 nm) and red (650 nm and 670 nm) emissions in visible region along with IR emission peaks centered at 1531 nm, 1574 nm, 1646 nm, 1738 nm and 1793 nm were observed. Dominant red emission appearing in the PL spectra is regarded to be due to Multi phonon relaxation and cross relaxations.

Upconversion Visible Light Emission in Yb/Pr Co-Doped Yttria-Stabilized Zirconia (YSZ) Single Crystals

As a development on previous research on single crystals of Pr3+-doped yttria-stabilized zirconia (YSZ), we report here the preparation and optical properties of Yb/Pr co-doped YSZ single crystals with different Yb2O3 concentrations. Results from X-ray diffraction (XRD) and Raman spectroscopy indicated that all of the crystal samples had a cubic phase structure, and transmission was ≥88% in the 550–780 nm range. Photoluminescence (PL) under excitation with a 980 nm laser showed upconversion emission, and several peaks were observed centered on 448 nm, 508 nm, 525 nm, 542 nm, 617 nm and 656 nm. The effects of excited state absorption (ESA), energy transfer upconversion (ETU), cross relaxation (CR), and cooperative energy transfer (CET) on the upconversion luminescence and energy transition mechanism in YSZ crystals were further studied. The fluorescence lifetime of the 3P0 → 3H5 transition at 542 nm reached 207 μs, which shows that the samples are of potential use for laser and fluorescence output.

Trap Assisted Transition Energy Levels of SrF2:Pr3+-Yb3+ Nanophosphor in TiO2 Photoanode for Luminescence Tuning in Dye-Sensitized Photovoltaic Cells

Luminescent nanophosphors as spectral converters offer immense potential for dye-sensitized photovoltaics (DSPV) to harvest wide range of solar spectrum. Herein, we demonstrate a novel structural design of DSPV by employing downconverting ( dc ) nanophosphor layer in TiO2 photoanode for indoor (ambient) applications. Cubic SrF2:Pr3+-Yb3+ nanoparticles are synthesized by template-free hydrothermal technique. The dc nanophosphor absorbs photons of blue region leading to emission of broad luminescence band (green and red), which is well-matched with N719-dye absorption. For the first time, we have designed a unique Pr3+-Yb3+ co-doped dc system yielding tuned and intensified luminescence by effective cross-relaxation (CR) with back energy transfer (BET) mechanism and demonstrated efficient working of the dc nanophosphor layered DSPVs under indoor LED light (Warm-3200K; Day-5000K) conditions. The dc-DSPV achieved high PCE of 14.85 and 15.9% respectively under indoor low light conditions (1000 lux). This approach resulted in 63.44% increment in output power density for dc-DSPV compared with control-DSPV under LED-3200K irradiation. These findings suggested that this configuration of dc -layered DSPV can provide new strategy for future indoor electronic-operations under ambient light conditions. Keywords: DSPV, downconversion, SrF2:Pr3+-Yb3+, luminescence tuning, trap assisted transition energy levels, interfacial charge transport kinetics and indoor light condition. Acknowledgement: This work was supported by Brain Pool Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT, Korea. (Grant number: 2019H1D3A1A01071183). It was also supported by the Technology Development Program to Solve Climate Changes of the National Research Foundation, funded by the Ministry of Science, ICT & Future Planning (No: NRF-2016M1A2A2940912). Figure 1

Photoluminescence and energy transfer studies in Ce3+-Sm3+ co-doped phosphate glasses

0.5Ce3+-xSm3+ (x = 0.11, 0.68 and 1.05 mol%) co-doped phosphate glasses were successfully synthesized via melt-quenching method and analyzed using absorption, photoluminescence and decay curves. According to Judd-Ofelt formalism, the Ω2, Ω4 and Ω6 parameters were determined and used to obtain some radiative properties. With the increasing of Sm3+ concentration, it was noticed the decrease of emission peak intensity and decay curves of Ce3+ due to the energy transfer Ce3+→ Sm3+. Moreover, it was observed that the intensity of Sm3+ emission increases until 0.68 mol%, then decreases for the 1.05 mol% Sm3+ concentration. According to the decay curves of Sm3+, this decreasing can be explained by the cross relaxation between Sm3+ ions. The calculated lifetimes by Judd-Ofelt theory and the experimental lifetimes for the prepared phosphate glasses were obtained. The dependence of the emission intensity on the concentration of doping ions has been explained using energy transfer models in order to obtain the optimum Sm3+ concentration.

Magnetic field and angle-dependent photoluminescence of a fiber-coupled nitrogen vacancy rich diamond

Here, we investigate the magnetic field dependent photoluminescence (PL) of a fiber-coupled diamond single crystal with a high density of nitrogen vacancy (NV) centers. Angle-dependent magnetic field sweep measurements between 0 and 111 mT were performed using an oscillating illumination combined with lock-in techniques. Besides the expected superposed PL of differently oriented NV centers, a zoo of features in the PL are found. These features can be associated with level anti-crossings and cross relaxations. In particular, PL measurements allowed us to detect auto-cross relaxation between coupled NV centers. Moreover, the PL measurements at low magnetic fields show dips suggesting an interaction of NV centers with additional spin defects. The results presented here are not only a study for NV-based fiber-coupled sensors made of diamond, but also show a way to investigate with manageable effort and purely an optical multispin interaction with at least one NV center as a constituent.

Optical properties of amorphous [formula omitted] doped with lanthanum oxide and erbium oxide

The photoluminescence (PL) spectra of amorphous As2S3 samples are investigated in a wide temperature interval by using the optical and ellipsometric methods. PL spectra under radiation with excitation energy smaller than Tauc’s edge energy allows us to determine all defect states in the forbidden band of amorphous As2S3. The ellipsometric investigation of a−As2S3 on reflection indicates precise anomaly of real and imaginary parts of the dielectric function around the transition edge corresponding to the inter-band transition, determined from photoluminescence measurements. PL investigation of La2O3, Er2O3 and As2S3:(La2O3)(Er2O3) compounds from nearest ultra-violate up to infra-red (IR) energy interval shows a coincidence of many absorption lines of La2O3 and Er2O3, furthermore surviving of these lines in As2S3:(La2O3)(Er2O3). These facts allow us to conclude that S atoms in As2S3:(La2O3)(Er2O3) replace O atoms, and that the discrete PL lines observed as in La2O3, Er2O3 as well as in As2S3:(La2O3)(Er2O3) belong to La and Er atoms. Comparison of PL spectra lines of La2Os and As2S3:(La2O3)5% with those of Er2O3 and As2S3:(Er2O3)5% shows that the intensity of the former lines is 2–3 order stronger than those of the latter samples’ lines. Nevertheless this picture is reversed for amorphous As2S3 samples simultaneously doped with La2O3 and Er2O3, which confirms that a cross relaxation from excited states of La to Er takes place. It is worthy to note that the PL spectra in IR-region were firstly measured in this work in order to get a complete information on the localized states density in the forbidden band of a−As2S3.

Ultra-broadband 4.1 μm mid-infrared emission of Ho3+ realized by the introduction of Tm3+ and Ce3+

Ho3+/Tm3+/Ce3+ tri-doped tellurite glasses with lower phonon energy (761 cm−1) were prepared by melt quenching in this paper. Performing XRD, Raman, DSC performance characterization, it was concluded that the synthetic tellurite glass sample had a multi-element structure, and the ΔT of the prepared tellurite glass was 138 °C, which can be used for fiber drawing with good thermal stability. Under the 808 nm LD excitation, an ultra-broadband emission from 3800 to 4200 nm mid-infrared band with full-width at half-maximum (FWHM) of the prepared glass about 309 nm was achieved, which can be applied to mid-infrared ultra-wide tunable all-fiber laser. With the introduction of CeO2, the up-conversion emission were suppressed and strong mid-infrared emission were obtained, at the same time, the fluorescence lifetime of the sample was significantly prolonged, which are mainly due to the cross relaxation between Ho3+ and Ce3+, the energy transfer mechanism was further analyzed through the energy level transition process. The maximum emission cross section and gain coefficient of the tellurite glass were 1.01 × 10−20 cm2 and 1.13 cm−1, which were improved compared with the previously reported tellurite glasses. The results demonstrate that the Ho3+/Tm3+/Ce3+ tri-doped tellurite glass is a promising mid-infrared optical material.

Luminescence properties related to energy transfer process and cross relaxation process of Y2O3: Yb3+/Er3+ thin films doped with K+ ion

In this paper, K + ion-doped Y 2 O 3 : Yb 3+ /Er 3+ luminescent films were prepared using the sol-gel method. The structures of the films were studied using X-ray diffraction, scanning electron microscopy, Raman spectroscopy, and Fourier transform infrared spectroscopy. A series of high-quality thin films with good crystallization were prepared. The effect of K + ion doping on the light emission of the samples was investigated. With increasing K + doping concentration, initially, the lattice was compressed and then stretched. For the undoped samples, the main light emission bands were green and red emissions attributed to Er 3+ : 2 H 11/2 ( 4 S 3/2 ) → 4 I 15/2 and Er 3+ : 4 F 9/2 → 4 I 15/2 , respectively, both of which are two-photon processes, accompanied by weak purple emission attributed to Er 3+ : 2 G 7/2 → 4 I 15/2 , which is a three-photon process. Compared with the undoped sample, the emission attributed to three-photon process may be eliminated by using K + doping, and the green and red emission intensity may be increased by factors of 2.39 and 2.51 with increasing K + ion doping, respectively. With the increase in K + ion doping, the changes in active luminescent centers and distance between rare earth ions were analyzed, and the effect of the process energy transfer (ET) process and cross relaxation (CR) attributed to those on light emission was discussed. • K + enhanced both green and red luminescence intensity notably. • K + eliminated purple emission, retained green and red emission. • K + reduced lattice's symmetry and changed the distance between rare earth ions. • Cross relaxation played major role in low K + doping concentration. • Energy transfer played major role in high K + doping concentration.

Trap‐Assisted Transition Energy Levels of SrF2:Pr3+−Yb3+ Nanophosphor in TiO2 Photoanode for Luminescence Tuning in Dye‐Sensitized Photovoltaic Cells

Luminescent nanophosphors as spectral converters offer immense potential for dye‐sensitized photovoltaics (DSPV) to harvest a wide range of the solar spectrum. Herein, a novel structural design of DSPV using a downconversion (dc) nanophosphor layer in the TiO2 photoanode for both indoor (ambient) and outdoor applications is demonstrated. Cubic SrF2:Pr3+−Yb3+ nanoparticles are synthesized by a template‐free hydrothermal technique. The dc nanophosphor absorbs photons of the blue region, leading to emission of a broad luminescence band (green and red), which is well matched with N719‐dye absorption. The mixed‐valence state of Pr ions (Pr3+ and Pr4+) leads to trap‐assisted transition levels, which result in a broad visible emission. For the first time, a unique Pr3+−Yb3+ codoped dc system yielding tuned and intensified luminescence by effective crossrelaxation (CR) with a back energy transfer (BET) mechanism is designed and efficient working of the dc nanophosphor‐layered DSPVs under both outdoor 1 sun (AM 1.5 G) and indoor light (Warm‐3200 K; Day‐5000 K) conditions is demonstrated. Improved efficiency of 9.07% is attained in dc‐dye‐sensitized solar cells (DSSC) compared with a control‐DSSC (8.39%) at 1 sun intensity. Under indoor low‐light conditions (1000 lux), the dc‐DSPV achieves high power conversion efficiencies (PCEs) of 14.85 and 15.9%, respectively. This approach results in a 63.44% increment in output power density for dc‐DSPV compared with the control‐DSPV under LED 3200 K irradiation. These findings suggest that this configuration of dc‐layered DSPV can provide a new strategy for future indoor electronic operations under ambient light conditions.

Effect of Sm3+ concentration and excitation wavelength on spectroscopic properties of GdPO4:Sm3+ phosphor

Gd(1−x)SmxPO4 ((1, 2, 3 and 5 mol%) nanophosphors have been synthesized via combustion route. The phase structure and the morphology of the obtained compounds have been identified by X-ray powder diffraction analysis, FTIR spectroscopy and scanning electron microscopy (SEM). The luminescence properties of the phosphor have been investigated based on photoluminescence excitation and emission spectra, and decay curves. The photoluminescence excitation spectra of the phosphors show large number of excitation bands related to the charge transfer state and 4–4 f electronic transitions of Sm3+ and Gd3+ ions. The photoluminescence spectra registered under excitation in CTB, 6IJ level of Gd3+ and the 4F7/2 level of Sm3+ show different profiles and emission intensity trends as a function of Sm3+ concentrations. The decay time curves have double exponential profiles and the registered lifetimes depend on the excitation wavelength and the Sm3+ concentration. The possible cross relaxation channels have been discussed based on the near infrared emission of Sm3+ under excitation with 273 nm and 401 nm. The evaluation of emission intensities and decay times curves analysis converge to a dipole-dipole interaction under excitation with 273 nm, while under excitation with 401 nm, a nearest-neighbor ions energies transfer mechanism between Sm3+-Sm3+ ions is dominant. Cool white and pure orange emissions have been obtained upon excitation with 229 and 401 nm, respectively, which may be suitable candidates for displays and photonic devices.

A novel high-sensitive optical thermometer based on the multi-color emission in Pr3+ doped LiLaMgWO6 phosphors

Nowadays, optical thermometric materials with high sensing sensitivity in a wide temperature interval are urgently required due to the appealing prospect in the field of sensing technology. Herein, the double perovskite Pr3+: LiLaMgWO6 phosphors are successfully prepared via a high-temperature solid-state reaction method, and their concentration/temperature-dependent luminescence properties are discussed systematically. Upon UV excitation, the samples emit blue (3P0→3H4), green (3P0,1 → 3H5) and red emissions (1D2→3H4, 3P0→3F2) from Pr3+ ions. The different changing tendencies in relation to temperature are observed for the emissions from 3P0 and 1D2 multiplets, which are attributed to the effects of cross-over process to Pr3+-W6+ intervalence charge transfer (IVCT) state, cross relaxation process and multiphonon relaxation process. When the fluorescence intensity ratio (FIR) between 3P0→3F2 (654 nm) and 1D2→3H4 (616 nm) transitions is applied for temperature sensing, 0.5%Pr3+: LiLaMgWO6 displays a maximum absolute sensing sensitivity of 2421 × 10−4 K−1 at 573 K. Moreover, it can keep a comparatively high relative sensitivity (1%~3.25% K−1) and low temperature uncertainty (0.15–0.5 K) over a wide temperature range (~280 K), which are superior to those of many luminescence materials reported previously. The present study may provide useful design information to develop high-sensitive luminescent thermometers with a wide applicable temperature range.

Single‐Band Ratiometric Luminescent Thermometry Using Pr3+ Ions Emitting in Yellow and Red Spectral Ranges

The possibility of using Pr3+ ions to develop multicolor‐emission‐based luminescence thermometry in a single‐band‐ratiometric configuration is investigated for the first time. The thermally induced decrease in intensities of 3P0 → 3F2 and 1D2 → 3H4 emission bands is demonstrated using the excitation matching the ground‐state absorption, and an inverse relation is obtained using the optical excitation matching the excited‐state absorption. The strategy of modulation of the thermometric properties of the Pr3+‐based red‐ and yellow‐emitting single‐band ratiometric (SBR) luminescent thermometers by the activation of the cross‐relaxation (CR) phenomenon promoted by the dopant concentration is elaborated. Contrary to expectations, the {3P0,3H4} ↔ {1D2, 3H6} CR process is shown to cause the high and low dopant concentrations to be optimal for highly sensitive SBR luminescent thermometers operating in the red and yellow spectral ranges, respectively. In both cases, high relative sensitivities of the temperature readout are obtained. These results make Pr3+ ions promising candidates for a SBR thermometry approach.

Colour modulation and enhancement of upconversion emissions in K2NaScF6:Yb/Ln (Ln = Er, Ho, Tm) nanocrystals

Effective colour modulation of upconversion emissions in lanthanide-doped nanomaterials becomes even more important for fundamental and applied research. Herein, on the one hand, by raising the content of doped Yb3+ from 10 mol% to 50 mol%, a significant increase of the red/green emission ratio from 4.0 to 68.2 is observed in K2NaScF6:Yb/Er nanocrystals. This yellow to red colour change is attributed to the increased cross relaxation between Er3+ and Yb3+ caused by the increased Yb3+ amount, 4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+). On the other hand, the upconversion green and red emission of K2NaScF6:Yb/Er (20/2 mol%) nanocrystals are intensified 10.6 and 8.8 folds, respectively, after an active shell (K2NaScF6:Yb) is epitaxially grown, which are more effective than the 7.4- and 6.4-fold enhancement from an inert shell (K2NaScF6) growth. Moreover, the shell thickness from 2.85 to 9.5 nm through controlling the molar ratio of shell-precursor to core from 1:2 to 3:1 can be easily realized. This study will provide more opportunities for the application of K2NaScF6:Yb/Ln nanoparticles in varied fields such as theranostics, photovoltaics, and photocatalysis.

Exceptional Point and Cross-Relaxation Effect in a Hybrid Quantum System

Exceptional points (EPs) are exotic degeneracies of non-Hermitian systems, where the eigenvalues and the corresponding eigenvectors simultaneously coalesce in parameter space, and these degeneracies are sensitive to tiny perturbations on the system. Here we report an experimental observation of the EP in a hybrid quantum system consisting of dense nitrogen (P1) centers in diamond coupled to a coplanar-waveguide resonator. These P1 centers can be divided into three subensembles of spins, and cross relaxation occurs among them. As a new method to demonstrate this EP, we pump a given spin subensemble with a drive field to tune the magnon-photon coupling in a wide range. We observe the EP in the middle spin subensemble coupled to the resonator mode, irrespective of which spin subensemble is actually driven. This robustness of the EP against pumping reveals the key role of the cross relaxation in P1 centers. It offers a novel way to convincingly prove the existence of the cross-relaxation effect via the EP.

Effects of Er3+/Yb3+ Doping Concentration on the Crystal Structure and Luminescence Properties of Gd1.985Lu0.015O2S Up-Conversion Phosphors

A series of Er3+ and Yb3+ co doped Gd1.985Lu0.015O2S phosphors were prepared by the high-temperature solid state method. The crystal structure and luminescence characteristics of a series of samples were analyzed by X-ray diffraction (XRD), up-conversion luminescence spectra and decay curves. Under the excitation of 980 nm laser, the sample exhibited two green and one red emission, which belonged to the transitions of Er3+ ion 2H11/22 → 4I15/2,4S3/2 → 4I15/2 and 4F9/2 → 4I15/2, respectively. In addition, the energy transfer process between Er3+ and Yb3+ ions is described according to the energy level diagram. When the concentration of Er3+ and Yb3+ is too high, the concentration quenching effect occurs, which increases the reverse energy transfer from Er3+ to Yb3+ and reduces the luminescent brightness. A series of decay curves show that the strain fluorescence lifetimes at 548 nm and 670 nm decrease with the increase of Er3+ and Yb3+ concentration, which is due to the increase of Er3+ cross relaxation process and the back energy transfer process between the two ions. Subsequently, the introduction of Lu3+ ions was found to be beneficial to improve the temperature stability of the material by temperature dependent spectroscopy (300K–500K). This work provides useful ideas for the development of infrared detection materials with better temperature stability.

Enhanced dual mode luminescence via energy transfer in Er3+, Yb3+ co-doped β-spodumene

Fundamental understanding of the Stokes and anti-Stokes emissions in luminescence materials and their mechanism is very important for different technological applications. In this article, we report the dual mode, down/upconversion (DC/UC), luminescence in a LiAlSi2O6 (β-spodumene) single host along with its mechanistic investigations. A series of emission tunable Li1-xAlSi2O6:xEr3+ (LAS:xEr3+; 0 ≤ x ≤ 0.07) and Li0.99-yAlSi2O6:0.01Er3+,yYb3+ (LAS:0.01Er3+,yYb3+; 0.001 ≤ y ≤ 0.007) phosphor powders were prepared by solid state reaction method at 1300 °C. The formation of single-phase phosphors is verified by XRD technique and other structural properties are investigated by FT-IR, UV-Vis-NIR spectra. The phosphors produced NIR DC emission at 1535 nm (under 380 and 980 nm excitation) and an intense red UC emission at 655 nm (under both 980 and 808 nm excitations). The dual mode emission intensity of the LAS:0.01Er3+ is further improved by co-doping Yb3+ ions via energy transfer from Yb3+ to Er3+. The red UC emission intensity of LAS:0.01Er3+,0.005Yb3+ phosphor under 980 nm excitation is ~10 times stronger than the singly doped LAS:0.01Er3+ phosphor. In addition, the phosphors can also be efficiently excited by 808 nm laser and hence can be used for biological application since 808 nm NIR irradiation exhibits higher light penetration depth without inducing overheating. The mechanism of energy transfer from Yb3+ to Er3+ by considering different emission pathways viz: ground state absorption (GSA), excited state absorption (ESA), energy transfer upconversion (ETU), back energy transfer (BET) and cross relaxation (CR) are discussed in detail. Based on the dual mode emissions, the LAS:xEr3+ and LAS: xEr3+,yYb3+ (x = 0.1 and 0.001 ≤ y ≤ 0.007) phosphors could be a potential candidate not only for the biological application as bio-probe but also as energy convertors for solid-state lighting, displays, and solar cell applications.

A novel precursor route for Y2Mo4O15:Yb3+,Ho3+ phosphor and investigation of up-conversion luminescence

Hydrothermal reaction of Y(NO3)3·6H2O and (NH4)6Mo7O24·4H2O at 180 °C for 24 h was performed in this work via systematically varying the solution pH (4–10) and Mo/Y molar ratio (R, 2–5), which produced a NH4Y(MoO4)2·0.19H2O new compound that yielded phase-pure Y2Mo4O15 upon calcination at 500–700 °C. The pH/R window for the compound to crystallize was determined to be 6–7/3 and 6–9/4–5. The products were characterized in detail by XRD, FTIR, elemental analysis, SEM, TEM, STEM and TG to understand chemical composition and the courses of phase/morphology evolution. Applying the above synthesis strategy successfully produced Y2Mo4O15:xYb3+,yHo3+ phosphors that exhibit strong deep-red (630–670 nm, 5F5 → 5I8 transition) and weak green (530–560 nm, 5F4/5S2 → 5I8) emissions under 978 nm laser pumping, where the optimal Yb3+ and Ho3+ contents were determined to be x = 0.20 for y = 0.01 and y = 0.03 for x = 0.10, respectively. Spectral analysis indicated that the up-conversion luminescence occurred via a two-photon process, with the red/green intensity ratio and also the emission color dependent on Yb3+ and Ho3+ contents, which were explained by considering ground state excitation, energy transfer, non-radiative relaxation and cross relaxation.

Multiband emission from single \u03b2-NaYF4(Yb,Er) nanoparticles at high excitation power densities and comparison to ensemble studies

Ensemble and single particle studies of the excitation power density (P)-dependent upconversion luminescence (UCL) of core and core-shell β-NaYF4:Yb,Er upconversion nanoparticles (UCNPs) doped with 20% Yb3+ and 1% or 3% Er3+ performed over a P regime of 6 orders of magnitude reveal an increasing contribution of the emission from high energy Er3+ levels at P > 1 kW/cm2. This changes the overall emission color from initially green over yellow to white. While initially the green and with increasing P the red emission dominate in ensemble measurements at P < 1 kW/cm2, the increasing population of higher Er3+ energy levels by multiphotonic processes at higher P in single particle studies results in a multitude of emission bands in the ultraviolet/visible/near infrared (UV/vis/NIR) accompanied by a decreased contribution of the red luminescence. Based upon a thorough analysis of the P-dependence of UCL, the emission bands activated at high P were grouped and assigned to 2–3, 3–4, and 4 photonic processes involving energy transfer (ET), excited-state absorption (ESA), cross-relaxation (CR), back energy transfer (BET), and non-radiative relaxation processes (nRP). This underlines the P-tunability of UCNP brightness and color and highlights the potential of P-dependent measurements for mechanistic studies required to manifest the population pathways of the different Er3+ levels.

Multicolor luminescence of hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals with tunable morphology under 940 nm excitation for temperature-responsive anti-counterfeiting

In this study, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ microcrystals were synthesized controllably, and upconversion luminescence excited at 940 nm and its application in temperature-responsive anti-counterfeiting are reported. It is clarified that the Ln3+ (Ln = Y + Yb + Ho + Ce) density ratio of bottom plane to side plane in the unit cell can be regulated by Ce3+ doping. It is also proved that the energy transfer of Yb3+ to Ho3+ is responsible for the activation of Ho3+ under 940 nm excitation, while the cross relaxation between Ho3+ and Ce3+ participates in the redistribution of electron population of 5S2/5F4 and 5F5 levels. Both theory and experiment confirm that the intensity ratio of red to green emission (IR/IG) as a function of temperature as an independent variable has good linear characteristics in the temperature range of 300–500 K. Due to the good responsiveness of multicolor luminescence to temperature, the hexagonal NaYF4:Yb3+/Ho3+/Ce3+ with tunable morphology is a promising candidate for advanced temperature-responsive upconversion anti-counterfeiting. Our results provide a new pathway for the controllable synthesis of hexagonal NaYF4 microcrystals as well as the regulation of upconversion luminescence that is excited by wavelengths other than 980 nm and its application in anti-counterfeiting.