Upconversion Color Research Articles

Synthesis and characterizations of upconverting luminescent Er3+/ Yb3+: Gd2O3 uniform nanospheres for biomedical applications

We present the synthesis of Er3+/ Yb3+ co-doped Gd2O3 nanospheres using the wet-chemical method followed by a synergistic step-by-step calcination process, a novel approach in the field. The nanospheres exhibited red color upconversion luminescence (UCL) after multi-step annealing at temperatures ranging from 650 °C to 1150 °C, achieving a fully spherical morphology. The diameter and surface morphology of the nanospheres were significantly altered by the annealing process, decreasing from 382.77 ± 1.72 nm with a smooth surface to 245.7 ± 1.31 nm with a rougher surface. The UCL intensity increased with the annealing temperature. The influence of Er3+/ Yb3+ co-doped Gd2O3 nanospheres, excited by a 975 nm laser, was investigated, and the decay time for UCL samples was analyzed. The luminescence peak at 1026 nm was attributed to the 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+) transition/ or 2F5/2 → 2F7/2 of the Yb3+ ions within the Gd2O3 matrix. Furthermore, the UCL properties of Er3+/ Yb3+ co-doped Gd2O3 nanophosphors were explored to detect the NTERA-2 cancer cells. These findings suggest that the rare earth co-doped Gd2O3 nanospheres might hold significant potential for biomedicine and imaging diagnostics applications, sparking new interest and possibilities in these fields.

Synthesis and upconversion color Tunability luminescence of K5Yb(MoO4)4 phosphor doped with Er3+and optical temperature sensing applications

A series of double molybdates with palmierite-related structure phosphors K5Yb1-x (MoO4)4:xEr3+ were synthesized by a solid-state method. It has been demonstrated that the K5Yb1-x (MoO4)4: xEr3+ phosphors are capable of exhibiting the characteristic green and red UC emission of Er3+ ions with excitation at 980 nm. Moreover, the green light emission is far more than the red light emission, so that the sample emits a powerful green light, which belongs to the two-photon absorption process. The upconversion efficiency is peaked when the content of Er3+ ions is 10 mol %. The optical thermometry properties of phosphors were explored by observing that the green UC emission intensity is temperature dependent. The maximum sensor sensitivity of the studied phosphor was discovered to be about 1.61%K−1 at the optimum doping concentration. And the phosphor is demonstrated to have high thermal stability by temperature cycling test. Eventually, a green light-emitting diode device was realized by using synthesized particles and a 980 nm near-infrared chip, which can emit a more obvious green light when the voltage reaches 1.80 V and the current can reach 800 mA, thus confirming its applicability in solid-state lighting. And the LED devices have been tested for life stability with little or no light degradation. These properties make the phosphors not only suitable for non-contact optical temperature measurement, but also able to be applied to solid state lighting.

Up-conversion color tunable and red luminescence of Ho3+/Yb3+ co-doped glass ceramics containing NaLaSiO4 crystals

By using the melt-annealing method, silicate precursor glasses were prepared. Optimal heat treatment conditions for glass ceramics at 610 °C for 90 min were established through a series of tests. Glass ceramics containing NaLaSiO4 crystals with β-K2SO4 structure were successfully synthesized after heat treatment. XRD patterns of glass ceramics were successfully fitted using Rietveld refinement, with rare earth ions entering the lattice by replacing sites of La resulting in a reduction in cell volume. A tunable emission from green light to red light was achieved by varying the Yb3+ doping concentration. By varying the concentration of Ho3+, the intensity of the up-conversion emission was significantly increased while maintaining high color purity in red light. The optimal concentration of Ho3+ was 0.4%, when the red light color purity was 98.9%. These results indicate that glass ceramics containing NaLaSiO4 crystals co-doped with Ho3+-Yb3+ could be used for up-conversion color-tunable emission and red luminescence.

Anomalous power-dependent color tuning of Er upconverting luminescence via 1530 nm excitation

The upconversion color tuning of Lanthanide ions usually requires complex core–shell structures and excitation configurations. This work presents simple Er3+ doped Yttria phosphors capable of color change through the manipulation of 1530 nm excitation intensity. Compared to the reported results of Er3+ color change from green to red with the increase in excitation intensity, the formed color-tunable phosphors show an inverse change of red to green. The color tuning mechanisms are attributed to the competition between the upward absorption and the downward decay at the Er3+ intermediate state. Finally, the potential applications of the formed color-tunable phosphors are demonstrated. This work provides a simple solution for high-performance upconversion color tuning and thus shows wide application prospects.

Intense up-conversion luminescence from Dy3+-doped multi-component telluroborate glass matrix: Role of CuO nanoparticles embedment

This paper reports an intense up-conversion luminescence from Dy3+-doped strontium-telluro-alumino-magnesium-borate glasses for the first time. The samples were made via the melt-quenching method and characterized to determine the influence of various CuO nanoparticles contents change on their up-conversion emission traits. Absorption spectral data were used to calculate the Judd-Ofelt intensity parameters. The sample without CuO nanoparticles revealed two intense photoluminescence up-conversion emission peaks at 478 and 570 nm. In addition, CuO nanoparticles-activated sample displayed about 1.4-fold up-conversion emission intensity improvement due to strong light absorption in the visible to the infrared region at 799 nm excitation. The stimulated emission cross-section of the CuO nanoparticles-activated glasses was increased from 102.4 × 10−23 to 1301.1 × 10−23 cm2 (nearly 10-fold amplification) while the branching ratio was reduced to 66.9%. Thus, CuO nanoparticles as an additive in the current glass matrix enhanced the up-conversion emission and strengthened the associated nonlinear optical properties. CIE 1931 color matching revealed the influence of CuO in modifying the up-conversion color coordinates, thereby improving the white color purity. The achieved up-conversion emission coupled with the color tunability of the proposed glasses may be advantageous for the up-conversion UV tunable laser making.

Elemental-Migration-Assisted Full-Color-Tunable Upconversion Nanoparticles for Video-Rate Three-Dimensional Volumetric Displays.

Herein, we demonstrate video-rate color three-dimensional (3D) volumetric displays using elemental-migration-assisted full-color-tunable upconversion nanoparticles (UCNPs). In the heavily doped NaErF4:Tm-based core@multishell UCNPs, erbium migration was observed. By tailoring this migration through adjustment of the intermediate shell thickness between the core and the sensitizer-doped second shell, red-green orthogonal upconversion luminescence (UCL) was achieved. Furthermore, highly efficient red-green-blue orthogonal UCL and full-color tunability were achieved in the UCNPs through a combination of elemental-migration-assisted color tuning and selective photon blocking. Finally, 3D volumetric displays were fabricated using a UCNP-polydimethylsiloxane composite. More specifically, 3D color images were created and motion pictures based on the expansion, rotation, and up/down movement of the displayed images were realized in the display matrix. Overall, our study provides new insights into upconversion color tuning and the achievement of motion pictures in the UCNP-polydimethylsiloxane composite is expected to accelerate the further development of solid-state full-color 3D volumetric displays.

Blue and red upconversion color tuning of Tm3+ via pulse excitation based on the thermal effect.

Color-tunable upconversion luminescence has wide prospects for anti-counterfeiting and disease diagnosis/treatment. To date, achieving high-quality tunable red and blue emissions using a single excitation wavelength remains a formidable challenge, due to the large energy difference between the red and blue photons. In this Letter, based on Tm3+ upconversion luminescence, blue dominant and red dominant emissions are generated upon 980-nm excitations using a short and long pulse, respectively. The corresponding color tuning mechanisms are investigated based on the spectral observations. The proposed color tuning strategy is particularly useful for in vivo applications as the red and blue lights play important roles in biological imaging and drug release, respectively.

Tuning Luminescence of Lanthanide-Doped Upconversion Nanoparticles through Simultaneous Binary Cation Exchange.

Dual-mode luminescent nanomaterials have outstanding performance in biosensing and multistage anticounterfeiting. Herein, we report the tuning of optical attributes of lanthanide-doped nanoparticles (NPs) via simultaneous binary cation exchange. We show that cation exchange of NaYF4:Yb/Er (18/2 mol %)@NaLnF4 (Ln = Y and Gd) NPs with a combination of Ce3+ and Tb3+ enables the resultant nanoparticles to exhibit both upconversion and downshifting emissions upon excitation at 980 and 254 nm, respectively. We find that in addition to introducing downshifting emission attributes, the use of Tb3+ ions allows conservation of the integrity of the parent core@shell NPs by decreasing the dissociation tendency caused by Ce3+ ions during the cation exchange. The upconversion color output can be tuned from green to red and blue by changing lanthanide combinations in the core NPs. This work not only provides an effective strategy for the optical tuning of lanthanide-doped NPs but also builds a platform for probing the difference in the reactivity nature of lanthanides.

Mechanisms of rare earth ion distribution in fluorosilicate glass containing KMnF<sub>3</sub> nanocrystal

Luminescent materials with an efficient single-(pure) color up-conversion luminescence (UCL) are expected to be applied to three-dimensional (3D) display, lighting, biological imaging, promoting plant growth and improving the photoelectric conversion efficiency of solar cells. In this work, perovskite-type KMnF<sub>3</sub> fluoride nanocrystals (NCs) are grown in situ in a fluorosilicate glass co-doped with rare earth (RE) ions Yb<sup>3+</sup>/Er<sup>3+</sup> by a controlled thermal treatment. Compared with precursor glass (PG), the nano-glass composites (also referred to as glass ceramics, or GC in short) thus obtained exhibit a significantly enhanced (by 6 times) red UCL emission. Although a weak green UCL emission can be also observed in the GC, the intensity ratio of the red UCL emission to green UCL emission is as high as 30, implying a good color purity. It is suggested that the dramatic enhancement of UCL emissions in the GCs is due to the doping of RE ions into the KMnF<sub>3</sub> NCs with a much lower phonon energy (330 cm<sup>–1</sup>) than that of the silica glass matrix about 1100 cm<sup>–1</sup>. However, the doping mechanisms of RE ions into KMnF<sub>3</sub> nano-glass composites are not yet conclusive, mainly because of the charge and ionic radius mismatch between RE ion dopants and cations of KMnF<sub>3</sub>. This work combines the high-resolution transmission electron microscopy (HR-TEM) analysis technology and the first principles calculation, to unravel the doping mechanism of RE ions in KMnF<sub>3</sub> nano-glass composites. First, the HR-TEM study provides straightforward evidence that RE ions are preferentially accumulated in KMnF<sub>3</sub> NCs embedded in the glass matrix. Then, through the first-principles calculation considering the charge balance, it is found that the formation energy of RE ions substituting for K<sup>+</sup> is lower than for Mn<sup>2+</sup> lattice sites in KMnF<sub>3</sub>, which is most likely related to the fact that the ionic radius of the eight-fold coordinated K<sup>+</sup> is larger than that of the six-fold coordinated Mn<sup>2+</sup> and thus is more conductive to accommodating the large size RE ions. The electronic densities of states at the top of the valence band and the bottom of the conduction band of KMnF<sub>3</sub> increase after doping the RE ions. It is inferred from the profile of partial density of state that RE ions have a strong bonding tendency with F<sup>–</sup> in the crystal. Benefiting from the efficient energy transfer between RE ions and Mn<sup>2+</sup> in KMnF<sub>3</sub>, the green UCL emission is dramatically quenched, and consequently, the GC is endowed with a highly pure red UCL emission. The present study is expected to deepen the understanding of RE ions doping mechanisms in NCs and facilitate the design of highly efficient UCL materials based on nano-glass composites.

Promising Yb3+-sensitized La2Mo2O9 phosphors for multi-color up-conversion luminescence and optical temperature sensing

In this paper, a series of Yb3+,Er3+/Tm3+/Ho3+ co-doped La2Mo2O9 with three-primary color up-conversion luminescence are synthesized using the solid state reaction method. The luminescence characteristics and mechanisms of La2Mo2O9:Yb3+,Er3+/Tm3+/Ho3+ phosphors excited by 980 nm are investigated in detail. The increased probability of 5I6→5I7 is an important cause of strong red emission in the Yb3+,Ho3+ co-doped La2Mo2O9. Furthermore, the optical temperature sensing performance are evaluated according to the typical thermally coupled and non-thermally coupled levels of Yb3+,Er3+/Tm3+/Ho3+ co-doped systems. The maximum relative sensitivity and absolute sensitivities are 1.15% K−1 and 10.03 × 10−3 K−1 for La2Mo2O9:Yb3+,Er3+; 2.29% K−1 and 0.284 × 10−3 K−1 for La2Mo2O9:Yb3+,Tm3+; and 0.30% K−1 and 20 × 10−3 K−1 for La2Mo2O9:Yb3+,Ho3+, respectively. The multi-color up-conversion luminescence phosphors with high temperature sensing sensitivities present broad application prospects in the fields of anti-counterfeiting, three-dimensional display, and non-contact temperature sensing.

Effects of inert shell on the upconversion intensity and color of Na(Er/Yb)F4 nanocrystals

Inert passivation can significantly enhance the upconversion intensity of lanthanide ion doped nanocrystals, especially in the heavily doped cases. In this paper, we revealed that, besides the well-known enhancement of intensity, the inert shell can also influence the upconversion color of Er3+/Yb3+ heavily doped nanocrystals in two opposite directions. The mechanisms of the effects of inert shell passivation are discussed based on the spectroscopic observations. The quantum yields of upconversion nanocrystals were estimated for the core-only and core–shell nanocrystals. This work provides guidelines for designing upconversion nanomaterials with bright emission and desired color and thus can further help them to be applied in multichannel imaging, display devices, and phototherapy.

Color tuning in a compact core-shell nanocrystal based on intense and high-purity green and red photon upconversion.

To date, color-tunable photon upconversion (UC) in a single nanocrystal (NC) still suffers from cumbersome structures. Herein, we prepared a compact two-layer NC with bright and high-purity red and green UC emission upon 980 and 1530 nm excitation, respectively. The effects of trace Tm3+ doping and inert-shell coating on the UC color and intensity were discussed. In addition, the color tuning via various dual-excitation configurations and the color stability with temperature and excitation intensity were demonstrated. The proposed UC NC, featuring compact structure and high-quality color tuning, can lower the synthesis time cost and difficulty of its kind and can find wide applications in multi-channel imaging, display devices, anti-counterfeiting, and so on.

High Color-Purity Red, Green, and Blue-Emissive Core-Shell Upconversion Nanoparticles Using Ternary Near-Infrared Quadrature Excitations.

Development of multicolor-emitting upconversion nanoparticles (UCNPs) is of significant importance for applications in optical encoding, anti-counterfeiting, display, and bioimaging. However, realizing the orthogonal three-primary color (TPC) upconversion luminescence in a single nanoparticle remains a huge challenge. Herein, we have rationally designed core-multishell-structured NaYF4 UCNPs through regulating the dopant concentration, composition of luminescent layers, and shell position and thickness, which are capable of emitting red, green, and blue luminescence with high color purity in response to ternary near-infrared quadrature excitations (1560/808/980 nm). Moreover, their high color purity is well retained with varying excitation power densities. This orthogonal TPC emissions property of such UCNPs endows them with great promise in the field of security. As a proof-of-concept, we have demonstrated the feasibility of combining such UCNPs with MnO2 nanosheets for information encryption and decryption. This work not only offers a new way to achieve TPC upconversion luminescence at a single nanoparticle level but also broadens the scope of application for security protection.

Bright constant color upconversion based on dual 980 and 1550 nm excitation of SrF2:Yb3+, Er3+ and β-NaYF4:Yb3+, Er3+ micropowders― considerations for persistence of vision displays

Abstract Upconversion phosphors are of interest for 2D head-up and 3D volumetric displays. These exploit persistence of vision; fast scanning of the near-infrared excitation means that the material at each emission point is only excited for a short fraction of the time it takes to trace the whole image. To achieve an average luminance on the order of 100 cd m−2 (necessary for the display to be visible in ambient indoor lighting), the luminance during excitation must be several orders of magnitude higher than its time-averaged value. For this purpose, efficient energy-transfer upconversion materials such as the benchmark β-NaYF4:Yb3+,Er3+ are of obvious interest. However, under 980 nm excitation the perceived color of their emission varies with their luminance, limiting their applicability for a grayscale display. We demonstrate that under dual 980 nm and 1550 nm excitation, a constant green color can be maintained at CIE coordinates (0.31, 0.66), as the luminance is varied. At moderate power densities of 100 W cm-2 at each excitation wavelength, a luminance of 8 × 106 cd m−2 can be achieved. Conservatively considering that the luminance will be reduced by 5 orders of magnitude in a laser scanning display, this material will still be bright enough to be viewed in indoor ambient light. We also investigate SrF2:Yb3+,Er3+, and find color tuning from the red to green is possible under dual wavelength excitation, but only at limited luminance, so color tuning in this manner would only be appropriate for displays viewed in a dark environment.

Up-conversion luminescence tuning induced by cross-relaxation processes in Gd2O3: Yb3+/Ho3+/Ce3+ nanoparticles

In this work, we reported the influences of Ce3+ doping on continuous and pulse 980 nm laser excited up-conversion luminescence of Gd2O3: Yb3+/Ho3+. Under continuous excitation at 980 nm, a 7.6-fold up-conversion red-to-green ratio enhancement was achieved by co-doping Ce3+ into Gd2O3: Yb3+/Ho3+, which could be attributed to two effective Ho3+-Ce3+ cross-relaxation processes. Besides, introducing Ce3+ also induced the more significant enhancement in the red to green intensity ratio with increasing the excitation pulse-widths of 980 nm laser in comparison to the sample without Ce3+. The results were contributed to better understand the influence of Ce3+ doping on the up-conversion luminescence of Ho3+, especially under the non-continuous excitation, endowing us with the more opportunities to control the up-conversion color output for satisfying diverse applications in a multitude of fields.

Change in the emission saturation and kinetics of upconversion nanoparticles under different light irradiations

Nd3+-sensitized upconversion nanoparticles (UCNPs) can be excited by both 980 and 808 nm light, which is regarded as a particularly advantageous property of these particles. In this work, we demonstrate that the nanoparticles can exhibit significantly different response when excited at these two excitation wavelengths, showing dependence on the intensity of the excitation light and the way it is distributed in time. Specifically, with 808 nm excitation saturation in the emitted luminescence is more readily reached with increasing excitation intensities than upon 980 nm excitation. This is accompanied by delayed upconversion luminescence (UCL) kinetics and weaker UCL intensities. The different luminescence response at 808 and 980 nm excitation reported in this work is relevant in a manifold of applications using UCNPs as labels and sensors. This could also open new possibilities for multi-wavelength excitable UCNPs for upconversion color display and in laser-scanning microscopy providing selective readouts and sub-sectioning of samples.

Tuning Long‐Lived Mn(II) Upconversion Luminescence through Alkaline‐Earth Metal Doping and Energy‐Level Tailoring

AbstractCrystal‐site engineering of hexagonal‐phase NaGdF4:Mn through alkaline‐earth metal (A2+) doping is used to tune the upconversion luminescence of the Mn2+ dopants. Experimental and calculated results show the ability of A2+ doping to alter the occupancy site of Mn2+ ion from Na+ to Gd3+, thus enabling an emission change from green (520 nm) to yellow (583 nm) upon excitation at 980 nm. The yellow emission of Mn2+ shows a long emission lifetime of 65 ms, more than three times longer than the green emission of Mn2+ (20 ms). The combination of tunable long‐lived Mn2+ emission with lanthanide emission at a single‐particle level provides a convenient route to triple transient upconversion color codes upon dynamic excitation, which offers an attractive optical feature particularly suitable for efficient document encoding.

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.

Low-temperature synthesis of thermally stable BaWO4:Yb3+:Ho3+ nanophosphors: Tuning visible emission by controlling activator ion concentration

Abstract Upconverting nanophosphors are a special class of luminescent nanomaterials where lanthanide dopant assisted anti-stokes emission is exploited to achieve a wide range of applications. Herein we report Yb3+/Ho3+ co-doped BaWO4 nanophosphor with a significant thermal and chemical stability synthesized by a low-temperature hydrothermal process with red (655 nm) and green (538 nm) double color upconversion (UC) under 980 nm near-infrared (NIR) excitation, resulting in yellow emission. The synthesized samples were characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), Vis–NIR Spectroscopy and Transmission Electron Microscopy (TEM) techniques. The dependence of emission behavior on various concentrations of activator ion (Ho3+) in BaWO4: Yb3+:Ho3+ phosphors were studied in detail. Temperature dependent emission spectra revealed that the phosphor possesses stable emission up to 200 °C and luminescence intensity drop of only 11.5% of the initial luminescence intensity was observed at 150 °C. Therefore, the luminescence behavior and the thermal stability of the phosphor suggest an effective pathway in the optoelectronic applications.

Upconversion color tunability and white light generation in Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals

Yb3+/Er3+/Tm3+ tri-doped CaF2 single crystals have been successfully synthesized via Temperature Gradient Technique (TGT) method, and represented by X-ray diffraction (XRD) examination, absorption and fluorescence spectrum, pump power dependence and lifetime analysis in details. Under the excitation of 980 nm laser diode (LD), the upconversion luminescence (UCL) spectra of Yb3+/Er3+/Tm3+/CaF2 as a function of Tm3+ concentration are made up of intense blue (Tm3+:1G4 → 3H6), green (Er3+: 4S3/2, 2H11/2 → 4I15/2), and red (Er3+:4F9/2 → 4I15/2) emissions, which are located at 478, 523, 539 and 656 nm, respectively. With increasing concentration of Tm, the color can be tunable from orange-yellow, white to blue light. And the optimal Yb3+/Er3+/Tm3+ doping concentrations for white light are determined to be 10%Yb, 0.1%Er and 0.05%Tm. The research indicates that the as-prepared materials may have potential applications in the fields of three dimensional displays, back lighting, white light sources, and so on.