Unveiling the role of Gd3+ and Mg2+ co-doping on morphology and upconversion luminescence in NaYbF4: Er3+ particles

Lanthanide-based upconversion nanoparticles (UCNPs) offer new strategies for luminescence-based sensing and imaging. One of the best studied materials are hexagonal 𝛽-NaYF4 UCNPs doped with 20 % Yb3+ and 2 % Er3+, which efficiently convert 976 nm light to photons emitted at 540 nm, 655 nm, and 845 nm, respectively, reveal long luminescence lifetimes (> 100 μs), and are very photostable and chemically inert.[1,2] The properties of their upconversion (UC) luminescence (UCL) are, however, strongly influenced by particle size, concentration and spatial arrangement of dopant ions, surface chemistry, and microenvironment.[3,4] In addition, the multiphotonic absorption processes responsible for UCL render UCL dependent on excitation power density (𝑃). The rational design of brighter UCNPs particle architectures encouraged us to assess systematically the influence of these parameters on UCL for differently doped UCNPs relying on the commonly used -NaYF4 matrix using steady state and time resolved fluorometry as well as integrating sphere spectroscopy for P varied over almost three orders of magnitude. This includes comprehensive studies of the influence of size and shell, Yb3+ and Er3+ dopand concentrations, and energy transfer processes from UCNPs to surface-bound organic dyes or vice versa [5]. Our results underline the need for really quantitative luminescence studies for mechanistic insights, the potential of high P to compensate for UCL surface quenching, and the matrix- and P-dependence of the optimum dopand concentration. Key words: upconverting nanoparticles, size, FRET, fluorescence, absolute fluorescence quantum yield, fluorescence decay kinetics, power density dependence References. [1] Haase, M.; Schafer, H., Angew. Chem. Int. Ed. 2011, 50, 5808-5829. [2] Liu, G. K., Chem. Soc. Rev. 2015, 44, 1635-1652. [3] Wurth, C.; Kaiser, M.; Wilhelm, S.; Grauel, B.; Hirsch, T., Resch-Genger, U.; Nanoscale 2017, 9, 4283-4294. [4] Kaiser, M.; Wurth, C.; Hyppanen I.; Soukka T;, Resch-Genger, Nanoscale 2017, on line (10.1039/C7NR02449E). [5] Kraft, M.; Wurth, C.; Muhr, V.; Hirsch, T.; Resch-Genger, U., Nanoscale, submitted.

The spatially resolvable multicolored microrods have potential applications in many fields. However, achieving spatially resolved multicolor luminescence tuning on the microrod with a fixed composition remains a daunting challenge. Herein, a strategy is proposed that allows for the tuning of spatially resolved, multicolored upconversion (UC) luminescence (UCL) along a 1D heterogeneous microrod by modifying the pulse width of an external laser. NaYbF4:1 % Ho is identified as an UCL color-adjustable material, exhibiting pulse width-dependent multicolored UCL, resulting in a significant regulation of the red/green (R/G) ratio from 0.1 to 10.3 as the pulse width is varied from 0.1 to 10 ms. Such variability can be ascribed to differences in the number of photons incident upon the microrod throughout the period necessary for the UC process to occur. Additionally, NaYbF4:1 %Tm and NaYF4:20 %Yb,1 %Ho are employed as materials that emit blue and green light, respectively, with their UCL colors largely unaffected by changes in the pulse width. Subsequently, a tip-modified epitaxial growth method is utilized to integrate both UCL color-adjustable and non-adjustable segments within the same microrod. Comparing with single-color or fixed multicolor microrods, our developed multisegmented emissive color adjustable 1D heterogeneous microrods have unique optical characteristics and can carry more optical information.

Ho‐based lead‐free double perovskite microcrystals (MCs), Cs2NaHo1‐xYbxCl6, are synthesized and demonstrated to be promising optical temperature sensing materials by understanding the concentration‐dependent upconversion (UC) luminescence mechanisms. By optimizing the dopant concentration, a dominant emission shifting from red to green is successfully achieved in an activator‐rich system. During this process, the red to green emission intensity ratio (R/G) can be changed from 12.5 (pure red light) to 0.14 (pure green light). It is clarified that the cross‐relaxation (CR) process between activator and the energy transfer process between Yb3+ and Ho3+ are the key mechanisms of UC luminescence intensity and R/G ratio variation. Moreover, the intensity of red and green UC emission shows strong dependence on temperature, resulting from the competition between CR and energy transfer in the emission levels. The high‐sensitivity Ho‐based thermometer can be obtained by Cs2NaHo0.99Yb0.01Cl6 MCs with bright red UC luminescence, whose fluorescence intensity ratio thermometer gains maximum relative sensitivities reaching 0.68% K−1 at 400 K. The multicolor UC luminescence quantitative modeling mentioned here is crucial for understanding the concentration‐dependence UC luminescence characteristics and to optimize the design of the Ho‐based thermometer. This study can be an extension of the application range for perovskite UC materials.

Green ${\mathrm{Tb}}^{3+}{}^{5}{\stackrel{\ensuremath{\rightarrow}}{{\mathrm{D}}_{4}}}^{7}{\mathrm{F}}_{\mathrm{J}\mathrm{}}$ luminescence visible by eye is observed under near-infrared laser excitation. Optical spectroscopic techniques including absorption, luminescence, and excitation spectroscopy are used to characterize this upconversion (UC) luminescence. The ${\mathrm{Tb}}^{3+}$ UC luminescence is present for all temperatures within a range from 10 to 300 K, and gains intensity by three orders of magnitude between 10 and 300 K. For $T>~100\mathrm{K}$ the dominant upconversion mechanism is the cooperative sensitization of ${\mathrm{Tb}}^{3+}$ by two ${\mathrm{Yb}}^{3+}$ ions. In this temperature regime the ${\mathrm{Tb}}^{3+}$ UC luminescence dominates the visible (VIS) spectrum for all near-infrared (NIR) excitations, resulting in the characteristic green luminescence. At 10 K, the color of the luminescence changes from green to blue, depending on the excitation wavelength corresponding to the dominance of ${\mathrm{Tb}}^{3+}$ UC luminescence or the ${\mathrm{Yb}}^{3+}\ensuremath{-}{\mathrm{Yb}}^{3+}$ cooperative pair luminescence. Two color excitation spectroscopy is performed to directly observe an excited state absorption (ESA) step in the ${\mathrm{Tb}}^{3+}$ UC luminescence excitation spectrum at 10 K. This allows the unambiguous assignment of a type of ground state absorption/excited state absorption (GSA/ESA) mechanism responsible for the upconversion in this system at 10 K. We explain this cooperative interaction in the framework of an ${\mathrm{Yb}}^{3+}\ensuremath{-}{\mathrm{Tb}}^{3+}$ exchange-coupled dimer. An energy level diagram for this dimer is presented. Excitation into dimer levels around 12000--14500 ${\mathrm{cm}}^{\ensuremath{-}1},$ where neither ${\mathrm{Yb}}^{3+}$ nor ${\mathrm{Tb}}^{3+}$ single ions have levels, leads to ${\mathrm{Yb}}^{3+}$ luminescence at 10 K. For laser excitation, 53 $\mathrm{W}/{\mathrm{mm}}^{2},$ resonant with an ESA transition a VIS/NIR photon ratio of ${2.7(10)}^{\ensuremath{-}5}$ is found at 10 K.

This paper presents a study on the enhanced red upconversion (UC) luminescence via efficient energy transfer (ET) between Er3+ and Tm3+ in Er-Tm codoped NaYF4 microtubes. Er doped and Er-Tm codoped NaYF4 UC hollow microtubes have been synthesized using a hydrothermal method. Under 1560 nm excitation from a diode laser, the Er doped NaYF4 microtubes emitted dominant green UC luminescence while the Er-Tm codoped NaYF4 microtubes emitted dominant red UC luminescence, which implies the energy transfer between Er3+ and Tm3+ plays a key role in the enhanced red UC emissions. The red UC luminescence is significantly enhanced compared with the green UC luminescence with the increase of Tm3+ doping concentration. In addition, our experimental results show that the UC luminescence properties under 980 nm excitation are almost identical with that under 1560 nm excitation. Furthermore, the possible ET mechanism was proposed on the basis of our experimental results.

In this study, intense single-band red-emitting upconversion nanophosphors (UCNPs) excited with 800 nm near-infrared (NIR) light are reported. When a NaYF4:Nd,Yb active-shell is formed on the 12.7 nm sized NaGdF4:Yb,Ho,Ce UCNP core, the core/shell (C/S) UCNPs show tunable emission from green to red, depending on the Ce3+ concentration under excitation with 800 nm NIR light. Ce3+-doped C/S UCNPs (30 mol %) exhibit single-band red emission peaking at 644 nm via a 5F5 → 5I8 transition of Ho3+. A high Nd3+ concentration in the shell results in strong absorption at around 800 nm NIR light, even though the shell thickness is not large, and small-sized C/S UCNPs (16.3 nm) emit bright red light under 800 nm excitation. The formation of a thin NaGdF4 shell on the C/S UCNPs further enhances the upconversion (UC) luminescence and sub-20 nm sized core/double-shell (C/D-S) UCNPs exhibit 2.8 times stronger UC luminescence compared with C/S UCNPs. Owing to the strong UC luminescence intensity and Gd3+ ions on the surface of nanocrystals, they can be applied as a UC luminescence imaging agent and a T1 contrast agent for magnetic resonance (MR) imaging. In vivo UC luminescence and high-contrast MR images are successfully obtained by utilizing the red-emitting C/D-S UCNPs.

Upconversion nanophosphors (UCNPs) are lanthanide ion-doped nanocrystals that exhibit anti-Stokes shift luminescence. Typically, UCNPs such as NaYF4:Yb,Er and NaYF4:Yb,Tm emit green and blue light under 980 nm near-infrared (NIR) light excitation. The emission color of lanthanide ion-doped UCNPs depends on the dopant ions in the host nanocrystals. Thus, one can adjust the dopant ions to control the upconversion (UC) luminescence color. For example, UC luminescence color can be tuned by co-doping activator ions into a single host nanocrystal and adjusting the ratio of two different activator ions. In addition, UC luminescence color can be tuned through energy migration UC process in core/shell-structured nanoparticles and by controlling the pulse of an NIR laser. Recently, orthogonal UC luminescence has been reported. In this study, UC luminescence color tuning is shown from the orthogonal UC luminescent UCNPs. To achieve orthogonal UC luminescence, core/multishell-structured UCNPs are synthesized. Depending on the excitation wavelength of NIR light, the UC luminescence color changes, and the core/multishell-structured UCNPs exhibit various UC luminescence colors by adjusting the excitation conditions. Finally, the feasibility of applying core/multishell-structured UCNPs in transparent displays is investigated.

Concentration dependence of visible up-conversion luminescence in the laser crystal Gd 3Ga 5O 12 doped with erbium

Facile synthesis of NaYF4:Ln/NaYF4:Eu composite with up-conversion and down-shifting luminescence

Transparent oxyfluoride nano-glass-ceramics with highly efficient up-conversion and adjustable color luminescence were developed in the 50SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -15AlF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -10BaF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -15PbF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -(9.3-x)GdF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -0.5YbF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -0.2TmF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -xHoF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> (x = 0.02, 0.05, 0.10, 0.50)composition in mol%. X-ray diffraction measurements revealed that heat treatments of the oxyfluoride glasses cause the precipitation of rare-earth ions co-doped fluorite-type of β-PbF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the glass matrix. Under 980 nm laser excitation, intense red, green and blue up-conversion luminescences were simultaneously observed in these transparent nanoglass-ceramics owing to the successive energy transfer from Yb <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> to Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> and Tm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions. Various colors of luminescence, including bright perfect white light, can be tuned by adjusting the concentrations of the Ho <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> ions in the material. A possible energy transfer process and up-conversion luminescence mechanism in the nano-glass-ceramics are proposed and discussed.

Luminescent 2D metal-organic frameworks (MOFs) are a class of metal-organic framework materials expanded in a 2D plane, which have a wide range of applications in fields of optoelectronic devices, sensors, and information storage due to their unique layered structure and excellent optical properties. Currently, research on luminescent 2D MOFs mainly focuses on down-shifting luminescence, while the exploration of upconversion luminescence remains in its early stages. Herein, a novel 2D Yb-PMA MOFs were synthesized, and red upconversion luminescence at 660 nm under 980nm excitation was successfully achieved by introducing Ho3+ as the luminescence center and using Yb3+ as the sensitizer. In addition, multimode emitting MOFs with both upconversion and down-shifting luminescence were further constructed by codoping Tb3+ or Eu3+. Inspired by the layered structure of 2D materials, multilayer stacked 2D MOFs composites were prepared by ultrasonic exfoliation method and upconversion luminescence was realized for the first time by interfacial energy transfer between different components. This strategy not only expands the optical modulation of luminescent 2D MOFs, but also provides new ideas for the construction of multifunctional luminescent materials. The upconversion/down-shifting luminescent lanthanide-based 2D MOFs designed in this work show good potential for application in the field of information encryption.

Effects of Er3+ concentration on upconversion luminescence and temperature sensing properties in Bi4Ge3O12 crystal

The authors report on upconversion nanocrystals (NCs) based on a fluoroapatite (FAp) support that was engineered to enable multimodal imaging by fluorescence imaging (FI), magnetic resonance imaging (MRI), and upconversion luminescence imaging. A fluorescein based fluorophore (FITC) was incorporated into the FAp nanocrystals and then doped with Yb(III) and Ho(III) by microwave-assisted solution combustion synthesis. The hexagonal phase nanocrystals (FITC-FAp:Yb/Ho) exhibit spindle like morphology with an average diameter and length of 15 nm and 196 nm, respectively. The doping concentration of the Yb (5 %) and Ho (0.6 %) was determined by ICP-MS. The nanocrystals exhibit upconversion luminescence when irradiated with NIR light of wavelength 980 nm. The emission spectrum consists of two bands centered at 542 nm (green emission) and 654 nm (red emission) corresponding to two transitions of Ho(III). The pump power dependence of upconversion luminescence intensity confirmed the 2-photon process. The presence of FITC in the nanocrystal imparts green fluorescence (peaking at 521 nm) by a conventional downconversion process. The presence of Ho(III) endows the NCs with paramagnetism. The magnetization is 21.063 emu∙g−1 at room temperature. The NCs exhibit a longitudinal relaxivity (r1) of 0.12 s−1∙mM−1, and a transverse relaxivity (r2) of 29 s−1∙mM−1, which makes the system suitable for developing T2 MRI contrast agents. The nanocrystals are surface aminized using polyethyleneimine (PEI) and covalently conjugated to folic acid (FA) in order to target the folate receptors that are overexpressed in many cancer cells. The FA-conjugated nanocrystals have been tested for their applicability in fluorescence imaging of HeLa cells. Their biocompatibility, upconversion and downconversion luminescence, and magnetism render these NCs potentially powerful nanoprobes for trimodal imaging.

Bright red upconversion luminescence from Er3+ and Yb3+ co-doped ZnO-TiO2 composite phosphor powder

Upconversion luminescence properties of Er3+/Yb3+ in transparent α-Sialon ceramics