Upconversion Emission Color Research Articles

Chromatic tuning of upconversion emission upon dual infrared wavelength co-excitation in Er3+ doped Ba3Lu4O9 phosphor

Emission color change of the upconversion luminescence materials is often required for applications in optical anti-counterfeiting, laser lighting, and 3D displays. In this work, we developed a new route for chromatic tuning of upconversion emission upon two wavelength co-excitation by changing excitation power density. The proposed route was used for Er3+ single-doped Ba3Lu4O9 phosphor which was derived from a traditional solid-state reaction. The crystal structure of the obtained Ba3Lu4O9:Er3+ phosphor was characterized by X-ray diffraction and Rietveld refinement modeling. Under individual excitation of 980 or 1550 nm, it was found that the luminescence intensity ratio of red to green changes slightly with respectively increasing excitation power density (working current of the laser) of 980 or 1550 nm laser. However, upon both 980 and 1550 nm co-excitation at the same time and by adjusting the excitation power density of one laser amongst the two lasers, the luminescence intensity ratio of red to green changes greatly with increasing excitation power density. The change of luminescence intensity ratio of red to green implies the upconversion emission color tuning. The mechanism for the color tuning was discovered. It was found that with changing excitation power density the ratio of red to green changed greatly when the phosphor was irradiated by 980 and 1550 together, but the ratio of red to green changed slightly when the phosphor was irradiated individually by 980 or 1550. It is proved that it is feasible to turn color by the method of two infrared wavelengths excitation.

Tunable multimode emissions of Yb3+/Er3+-doped rare-earth-based Cs2NaYCl6 double perovskite crystals for versatile applications

Lead-free halide double perovskite materials are gaining recognition because of their excellent optoelectronic performance. However, achieving multimode luminescence with tunable emission colors in single-component double perovskites remains a significant challenge. Understanding the mechanism of trivalent lanthanide-ion doping could provide a valuable solution. Herein, we report the synthesis of rare-earth-based Cs2NaYCl6 double perovskites using a hydrothermal method. Efficient visible to near-infrared down-shifting (DS) photoluminescence and green up-conversion (UC) emissions were realized by incorporating Er3+ ions with abundant energy levels into the hosts. Notably, the DS and UC emission colors could both be adjusted by further introducing Yb3+ ions that acted as sensitizers, and the photoluminescence was significantly enhanced due to the strong absorption at 980 nm. Spectral analysis revealed that the high compatibility of lanthanide ions with the rare-earth-based double perovskites and cross-relaxation processes played a key role in achieving modulated emission and multiple excitation modes. This study aimed to understand the photophysical mechanism of lanthanide-ion-doped double perovskites, as well as to highlight their ability to modulate emissions. Furthermore, this study expands the versatile applications of lanthanide-ion-doped double perovskites in the fields of high-security anti-counterfeiting, tissue imaging, and night-vision systems.

In situ growth of lanthanides-doped nanoparticles inside zeolites with enhanced upconversion emission for gallic acid detection

The combination of upconversion nanoparticles (UCNPs) with porous zeolites could enable the development of multifunctional composites and extend their optical applications in sensing, detection and biomedical monitoring. Herein, a series of high luminescent UCNPs@Zeolites nano-micro composites were constructed via the in situ growth strategy, by taking the low phonon-energy fluoride nanoparticles of NaLnF4 (Ln = Y, Gd) as doping hosts for Er3+/Yb3+, desilicated FAUY and ZSM-5 as the target zeolites. Benefiting from the formation of tightly combined interfaces between the UCNPs and the target zeolites that effectively passive the surface defects of UCNPs, three orders of magnitude of improved upconversion emission in maximum was obtained under 980 nm excitation through afterward heat treatment at 400 ℃. Moreover, the pre-exchange of Yb3+ into target zeolites before the in situ growth of UCNPs is another feasible approach to drastically improve the upconversion emission intensity of the UCNPs@Zeolites nano-micro composites. By taking NaGdF4:Yb,Er@DSZSM-5/HT as an example probe, the detection of GA was demonstrated and the detection ability of which is super than that of the corresponding bare NaGdF4:Yb,Er UCNPs. This research provided a universal approach to construct the UCNPs@Zeolites nano-micro composites with varied upconversion emission colors simply by choosing activator ions, which therefore indicates wide potential applications.

Synthesis and Luminescence Properties of Green-to-Red Color-Tunable Upconverting K2Gd(PO4)(WO4):Yb3+,Tb3+,Eu3+ Phosphors

Scientists are increasingly interested in new inorganic luminescence materials that could be excited with near-infrared (NIR) radiation. These materials can be used as luminescent thermometers, bio-imaging agents, anti-counterfeiting pigments, etc. In this manuscript, we report the synthesis and investigation of optical properties of two series of K2Gd(PO4)(WO4):20%Tb3+ (KGPW): the first, KGPW:20%Tb3+ doped with 1–20% Eu3+, and the second, KGPW:10%Yb3+,20%Tb3+ doped with 1–20% Eu3+. The phase-pure specimens were prepared using a solid-state synthesis method. Down-shifting and upconversion luminescence studies have been performed using 340 and 980 nm excitation, respectively. For upconversion emission luminescence, Yb3+ ions were used as sensitizers in the KGPW phosphors. In these phosphors, Yb3+ ions absorb the 980 nm radiation and transfer the energy to Tb3+ ions. At his point, Tb3+ ions either emit themselves or transfer part or all of their energy to Eu3+ ions. It was observed that the emission color of the synthesized phosphors could be successfully tuned from the green to red by varying the Tb/Eu concentration ratio regardless of the 340 or 980 nm excitation. Such color change proves that one luminescent material (KGPW) can provide three colors (i.e., green, orange, and red). Herein, the optical properties, such as reflection, down-shifting excitation and emission spectra, upconversion emission spectra, fluorescence lifetime, thermal quenching, color coordinates, and quantum efficiency, were studied using steady-state and kinetic spectroscopy.

Upconversion luminescence thermal enhancement and emission color modulation of LiYGeO4:Er3+/Yb3+ phosphors

A series of LiYGeO4:Er3+/Yb3+ phosphors were synthesized by solid-phase sintering method, and the upconversion luminescence (UCL) thermal enhancement and emission color modulation were investigated under 980 and 1550 nm laser excitations. When Er3+ doping concentration is kept at 0.02, the UCL intensity reaches the maximum at the Yb3+ doping concentration with 0.20. With temperature increasing from 303 to 723 K, the green UCL intensities of samples reach maximum enhancement as high as 4.56 and 11.54 times under 980 and 1550 nm laser excitations, respectively. The optical temperature sensing performance of these samples was evaluated by luminescence intensity ratio (LIR) technology, and the maximum absolute sensitivities in LiYGeO4:0.02Er3+/0.40Yb3+ are 0.01020 K−1 at 625 K and 0.01026 K−1 at 655 K under 980 and 1550 nm laser excitations, respectively. In addition, the modulation of UCL color can be realized by changing laser excitation power, temperature and excitation wavelength. These results show that LiYGeO4:Er3+/Yb3+ phosphors can be used as high-temperature optical thermometer and anti-counterfeiting.

White light upconversion through precise manipulation of energy transfer channels in a core-shell-shell nanostructure

Photon upconversion of lanthanides is an important approach for white light emission, showing great potential in displays, nanophosphors, white light lasers and so on. However, the white light upconversion in previous reports is usually based on the codoping schemes with conventional lanthanide emitters such as Er3+/Tm3+ or Ho3+/Tm3+, of which the performance is limited by the serious quenchable interactions between dopants (e.g., cross relaxation). Here, we report a new mechanistic approach to the white light upconversion by constructing the LiYbF4:Tb@LiGdF4:Yb/Tm@LiYF4:Eu core-shell-shell nanostructure. The use of lithium lanthanide fluoride matrix can improve the doping content of Yb3+ into the host lattice easily with resultant enhancement in the absorption of the excitation energy and subsequent upconversion luminescence. Through combining both interfacial energy transfer (IET) and cooperative sensitization upconversion (CSU), the white light upconverting emission has been realized under 980 nm excitation. Also, the upconversion emission has different lifetime values providing a temporal way to manipulate the upconversion emission colors. Our results offer a new experimental strategy for white light upconversion of lanthanide-doped materials, showing great promise in applications such as anti-counterfeiting.

Primary thermometers based on sol–gel upconverting Er3+/Yb3+ co-doped yttrium tantalates with high upconversion quantum yield and emission color tunability

Primary thermometers based on sol–gel upconverting Er3+/Yb3+ co-doped yttrium tantalates with high upconversion quantum yield and emission color tunability

Host dependent upconversion, color tunability and laser induced optical heating via NIR excitation in Ho3+, Yb3+ doped YXO4 (X = V, Nb, Ta) phosphor materials: Application as a security tool and optical heater

This work reports a comparative upconcversion (UC) emission study of the Ho3+, Yb3+co-doped in pure YVO4, YTaO4, YNbO4 and in their mixed (YTa1-xVxO4, YTa1-yNbyO4, YNb1-zVzO4, where x, y, z= 0, 0.25, 0.50 and 0.75, 1 mol) phosphor materials synthesized via high temperature solid state reaction method at 1473 K. The pure and the mixed phosphors have been structurally analysed through X-ray diffraction technique. The phonon frequencies and the surface morphologies are investigated through Fourier transform Infrared (FTIR) and scanning electron microscope (SEM) techniques for pure and mixed compositions. Under NIR (980 nm) excitation, YNbO4:Ho3+,Yb3+ and YTaO4:Ho3+,Yb3+phosphors emit intense green alongwith weak red and very weak NIR UC emissions, whereas the YVO4:Ho3+,Yb3+ phosphor exhibits strong red emission with weak green and NIR UC emissions. The UC emissions in the three cases is explained using energy level diagram and the phonon frequency of the host matrix. The pure YNbO4 host shows comparatively efficient UC emission than the other two due to its large particle size and low phonon frequency. Therefore, the YNbO4:Ho3+,Yb3+ serves as an excellent infrared to green upconverter and useful in display devices.On the other hand, the color tunability from green to orange/red UC emissions is observed on varying the vanadate (V) concentration in the mixed compositions (in YTa1-xVxO4 and YNb1-zVzO4). The variation of ‘V’ in YTa1-xVxO4:Ho3+,Yb3+ and in YNb1-zVzO4:Ho3+,Yb3+intensify the vibrational bands which promotes the non-radiative relaxation and hence tune the upconversion emission color as well as the intensity. The UC emission intensity in YTa1-xVxO4:Ho3+,Yb3+is maximum for x= 0.25. For higher concentrations of V, the green to orange/red color tunable emission is observed. Thus, the mixing of ‘V’ acts as sensitizer as well as modulater in YTa1-xVxO4:Ho3+,Yb3+ phosphor. The colortunable emission in mixed compositions with high color purity makes it promising for security and display applications. The mixing of different compositions of NbO4 and VO4 give different proportions of red and green emissions. On the other hand, mixing of NbO4 and TaO4 give intense green color. Such type of materials are also very useful in security ink applications.The laser induced optical heating is found maximum in case of pure YVO4:Ho3+,Yb3+ phosphor. The temperature of sample YVO4:Ho3+,Yb3+ increases upto 353 K at power density 107.59 W/cm2. Therefore, the YVO4:Ho3+,Yb3+ can be used as an optical heater in medicals such as physiotherapy, cancer and wound treatments etc.

Controllable multi‐color upconversion in glass ceramics through engineering crystal lattice distortion

AbstractLanthanide‐doped up‐conversion (UC) materials with color tunable property show promising application in many fields, such as biological imaging, optical multiplexing, and information security. Although great efforts have been devoted to regulate UC emission color via modifying chemical composition, crystal structure, or external stimuli condition, it is remain a daunting challenge to develop a general strategy that probably suitable for various lanthanide‐doped nano‐systems. Herein, we verify the cross‐relaxation possibility between sensitizer Yb3+ and activator Er3+ as well as Er3+‐ Er3+ pairs can be well modified by engineering the crystal lattice distortion of matrix, leading to the evident UC emission color variations. Specifically, with incorporating Al3+ or Ca2+ ions into the Yb/Er co‐doped BaF2 nanocrystals (NCs) contained glass ceramics (GCs), the contraction of the crystal lattice lead to the change of UC emission color from green to red. On the contrary, the high laser excitation power results in the lattice expansion of the Al3+‐doped BaF2:Yb/Er GCs at different Yb3+ concentration as well as Al3+ doped CaF2 GCs, which leads to the greatly increased green‐to‐red ratio. These multi‐color UC GCs show potential application in anti‐counterfeiting filed. Our results open up a general avenue for the control of UC emission color via engineering crystal lattice distortion.

Up-conversion luminescence enhancement and tunable emission color of Y2O3: Tm/Yb@Y2O3: Yb@Y2O3: Er/Yb multilayer thin films

The Y2O3: Tm/Yb, Y2O3: Tm/Yb@Y2O3: Yb and Y2O3: Tm/Yb@Y2O3: Yb@Y2O3: Er/Yb thin films are successfully prepared via spin-coating method. The surface grain size of the thin films is changed by multiple calcinations. The up-conversion luminescence emission of the thin films under 980 nm excitation is improved through two methods, including the increase of surface grain size and the structure of Y2O3: Tm/Yb@Y2O3: Yb bilayer. When the emission intensity of the Y2O3: Tm/Yb@Y2O3: Yb thin films are maximized by adjusting the Yb3+ concentration of the coating layer, the bilayer thin film continually undergoes multiple calcinations, thus further increasing the enhancement effect of each band emission. Besides, the Y2O3: Tm/Yb@Y2O3: Yb@Y2O3: Er/Yb multilayer thin films are designed to realize the modulation of up-conversion emission color, meaning that its luminescence emission color can change from blue to white and finally to orange by adjusting the thickness of the top Y2O3: Er/Yb luminescence layer and the number of calcination. With the capabilities of up-conversion emission enhancement and multicolor luminescence modulation, this multilayer thin film can promote the application of rare earth doped materials in the fields of fluorescence detection and color displaying.

Control of transient states for upconversion emission color adjustment

Lanthanide ions doped upconversion luminescence materials have attracted great attention due to their potential applications in anti-counterfeiting, volumetric display, optical multiplexing, and bioimaging. Previous studies showed that it is possible to control transient states of lanthanide ions, enabling a large luminescence color gamut in one fixed composition upconversion material. However, the influences of material characteristics and excitation conditions on transient states have not been fully understood. Here we experimentally demonstrated that three factors, namely, doping concentration, excitation pulse width, and excitation pulse peak intensity, can be utilized to effectively control the transient states of NaYF4:20%Yb/0.5%Er and NaYF4:90%Yb/0.5%Er microrods for upconversion emission color adjustment and theoretically studied their temporal responses. Moreover, we demonstrated a proof of concept of a fluorescence anti-counterfeiting application based on the distinguishable transient-state properties of the upconversion microrods.

Influence of Yb doping on optical and upconversion photoluminescence properties of Yb-, Er-co-doped Y2O3 transparent ceramics prepared by SPS

x% Yb-, 0.5% Er-co-doped Y2O3 transparent ceramics (x = 0, 0.1, 0.5, 1.0, 5.0 and 10) were prepared by the spark plasma sintering, and then we evaluated the effect of Yb doping on optical and upconversion (UC) photoluminescence (PL) properties. Under an excitation at 380 nm, the sharp emission peaks appeared around 550 and 660 nm, and the quantum yield was approximately 2%. The PL decay curves monitoring around 550 nm were fitted by a single exponential decay function. The decay time constants were 0.03–0.11 ms, and the values were typical for the 4f → 4f transitions of Er3+. Regarding the UC PL properties, the sharp UC emission peaks around 550 and 660 nm were observed, and the UC emission intensity of 5.0% Yb-, 0.5% Er-co-doped Y2O3 transparent ceramic sample was the highest among the prepared samples. With increasing x, the chromaticity of UC emission color was tuned from green to red region. Regarding the UC PL decay curves, the rise time became slower as increasing x. This behavior is one of the evidence of energy transfer from Yb3+ to Er3+.

Emission enhancement and color modulation of Tm(Ho)/Yb codoped Gd2(MoO4)3 thin films via the use of multilayered structure

The multilayered structure thin films (Gd2(MoO4)3:Ho(Tm)/Yb@Gd2(MoO4)3:Yb)) were prepared through sol–gel and spin-coating method, while the average thinness was nearly 140 nm. We investigated the up-conversion luminescence of Gd2(MoO4)3:Ho(Tm)/Yb@Gd2(MoO4)3:Yb thin films. The results show that the double-layer structured thin films are able to increase the emission intensity. The fluorescence enhancement factors of the luminescence from Gd2(MoO4)3:Ho/Yb@Gd2(MoO4)3:Yb thin films, located at 540 and 662 nm, reach 7.5 and 4.3, respectively. And the enhancement factors of emissions located at 450, 475 and 650 nm (Gd2(MoO4)3:Tm/Yb@Gd2(MoO4)3:Yb)) reach 9, 2 and 2, respectively. The considerable enhancement is due to the suppression of surface quenching and energy harvesting via the Yb ions in the outer shell. In addition, the emission color of thin films can be modulated from yellow to blue via tuning the number of Gd2(MoO4)3:Ho/Yb and Gd2(MoO4)3:Tm/Yb layers, which provides a feasible strategy to tune the up-conversion emission color.

Color thermal stability of white up-conversion luminescent emission in Eu3+/Yb3+ co-doped YOF phosphor

The Eu3+/Yb3+ co-doped YOF phosphor is synthesized via the hydrothermal method. The intense blue, green and red up-conversion emission of the phosphor can be radiated at 980 nm excitation. The up-conversion emission color adjustment from yellow to blue is realized through cooperative sensitization up-conversion process by altering the excitation power and pulse width of 980 nm pulse laser. Especially, the white up-conversion emission of the phosphor can be generated under 980 nm continuous laser and pulse laser irradiation. The corresponding energy transfer mechanism of Eu3+/Yb3+ co-doped YOF phosphor has been proposed in this work. The influence of temperature on up-conversion luminescent emission color is explored under the condition that can generate optimal white emission. The excellent color thermal stability of white up-conversion emission can be found at continuous high temperature. Strong white emission at high temperature and multicolor emissions of the phosphor make the phosphor suitable for using in high temperature environment for white light illuminating and color displaying application.

Visualized thermal history sensor based on the rare-earth-doped zirconia.

Temperature is one of the most fundamental parameters, and its accurate measurement is critically important for thermal systems. Despite substantial progress in temperature measuring techniques, design and fabrication of a reliable thermal history sensor, which can remember thermal events, still remain a significant challenge. In this Letter, we propose and experimentally demonstrate a new thermal history sensor based on the rare-earth-activated and yttria-stabilized zirconia (YSZ). This material candidate exhibits strong heat-treatment-dependent upconversion emission color. The structure and optical characterizations indicate that the phenomenon originates from the cooperative effects of multiple physical parameters, including crystallinity, crystal size, and the quantity of residual OH. This allows us to achieve excellent linear relationship between the heat-treatment temperature and the intensity ratio of the green and red emission band over a wide temperature range from 800°C to 1350°C. Thus, the thermal history information can be directly judged based on the emission color. These results provide a major step forward in expanding the scope of thermal history sensor materials.

Simultaneous phase and morphology control of Ba2YbF7: Er3+ upconversion nanocrystals through La3+ doping

Lanthanide doped upconversion nanocrystals are widely applied in biological imaging, photonics, photovoltaics and therapeutics. Highly monodispersed and uniform products with controllable optical modulation of upconversion fluorescence really matters their promising applications. Here, Er3+ doped Ba2YbF7 nanocrystals with phase transformation, controllable morphology and tunable upconversion emission color is investigated via varying the concentration of La3+. Accompanied with tetragonal Ba2YbF7 transformed to be cubic phase, nanocubes appear instead of nanorods, and a distinct upconversion color changing from yellow to green is obtained. Crystal structure management of Ba2YbF7 contributing to a suppressive back energy transfer process, which facilitates a controllable upconversion color in monodispersed nanocrystals with a uniform morphology, is demonstrated.

Cross-relaxation and non-steady-state processes induced up-conversion color modulation in Gd2O3: Ln3+/Er3+ (Ln = Tm, Ho)

Two efficient approaches based on cross-relaxation and non-steady-state upconversion processes were used to regulate the upconversion emission colors of Er3+ in Gd2O3 host, in which the cross-relaxation induced tunable up-conversion color of Er3+ was implemented via adjusting the doping concentration of Er3+ and co-doping Tm3+ and Ho3+. The corresponding energy transfer mechanism was elaborately investigated through down-conversion emission spectra and decay lifetimes. In addition, the influences of the excitation pulse-width of 980 nm laser on up-conversion emission color of Er3+ were also investigated on the basis of non-steady-state up-conversion processes. Results provide new insights into the influence of cross-relaxation and non-steady-state processes on the up-conversion luminescence, which endows up-conversion phosphor with the more opportunities to satisfy diverse applications by controlling its color output.

Spectroscopic and pump power dependent color-tunable upconversion studies of Yb3+/Er3+-doped Sr2ScF7

Er3+ and/or Yb3+ doped Sr2ScF7 up-conversion (UC) phosphors were synthesized, and the optimum doping concentrations of Yb3+ and Er3+ in the Sr2ScF7 host were found to be 20% mol and 7% mol, respectively. Under excitation of 980nm laser, the UC spectra of the samples is composed of three green emission bands from 510 to 570nm centered at 525, 543 and 551nm and two red emission band from 640 to 690nm with two peaks at 657 and 671nm, which is attributed to the 2H11/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions of Er3+, respectively. The UC emission color of Er3+ can be tuned by adjusting the intensity ratio of red to green emission through manipulating the population of red and green emitting states.

Upconversion Modulation through Pulsed Laser Excitation for Anti-counterfeiting

Lanthanide-doped upconversion nanomaterials are emerging as promising candidates in optoelectronics, volumetric display, anti-counterfeiting as well as biological imaging and therapy. Typical modulations of upconversion through chemical methods, such as controlling phase, composition, morphology and size enable us to rationally manipulate emission profiles and lifetimes of lanthanide ions by using continuous-wave laser excitation. Here we demonstrate that under pulsed laser excitation the emission color of NaYF4:Er/Tm (2/0.5%)@NaYF4 core-shell nanoparticles has an obvious transformation from green to red colors. Moreover, both pulse duration and repetition frequency are responsible for manipulating the upconversion emission color. The mechanism of the phenomena may be that the pulsed laser sequence triggers the emission levels to non-steady upconversion states first, and then cuts off the unfinished population process within the pulse duration. This pump source dependent and resultant tunable fluorescence emission enables NaYF4:Er/Tm (2/0.5%)@NaYF4 nanoparticles as a promising fluorophore in the transparent anti-fake printing.

Up‐Conversion Fluorescent Labels for Plastic Recycling: A Review

The demand for more efficient and complete sorting techniques for plastic waste is growing, and one possible solution isbased on fluorescent labeling. Novel fluorescent labels based on trivalent lanthanide (Ln3+) activated inorganic up‐conversion (UC) materials offer a promising technological solution for plastic recycling. UC is a nonlinear, anti‐Stokes process of combining two or more low energy near‐infrared (NIR) photons to obtain the emission of a single higher energy photon. While Ln3+ based UC materials possess one key disadvantage –low quantum yield, they also exhibit many unique features, such as high signal/noise ratio, tailored emission color, long photo‐luminescent lifetime, and low toxicity. These unique features endear them for a diverse range of applications and offer many new opportunities. Herein, we review the recent advances in the Ln3+ activated inorganic micro‐sized UC materials from the perspective of tailoring UC emission color and intensity for plastic recycling applications.