NaLuF4:RE3+ (Dy3+, Tb3+, Eu3+, Tm3+): controllable morphology, multicolor light

A series of emission-tunable NaLuF4:RE3+ (RE = Dy, Tb, Eu, Tm) phosphors were firstly synthesized by a glycine assisted one-step hydrothermal process. The structure and morphology were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results revealed that the samples were Na5Lu9F32, LuF3 and NaLuF4 and the size and shape of the products could be tuned just by adjusting the pH values of the initial reaction solutions or by adjusting the RE3+/NaF ratio or RE3+/NaBF4 ratio. The morphologies for the products include irregular particles, irregular blocks, octahedral shapes, hexagons, or micron sized hexagonal prisms. Furthermore, the photoluminescence properties of NaLuF4:Dy3+,Tb3+, NaLuF4:Tb3+,Eu3+, NaLuF4:Dy3+,Eu3+ and NaLuF4:Tm3+,Dy3+ were investigated in detail. Additionally, when co-doping Dy3+ with Tb3+ or Eu3+ or co-doping Tb3+with Eu3+ or co-doping Tm3+ with Dy3+ ions in the single component, colorful emission can be obtained by giving abundant blue, green, yellow, orange, and especially white-light-emission. All these properties indicate that the developed phosphor may potentially be used as single-component multicolor-emitting phosphors.

Cryogenic enabled multicolor upconversion luminescence of KLa(MoO4)2:Yb3+/Ho3+ for dual-mode anti-counterfeiting

The rational development of multicolor upconversion (UC) luminescent materials is particularly promising for achieving high-tech anti-counterfeiting and security applications. Here, an Ho³⁺ and Yb³⁺ ion co-doped KLa(MoO₄)₂ material can achieve multicolored UC luminescence by thermally manipulating the electron transition process, which could be developed to execute advanced optical anti-counterfeiting applications. The emission color of this material turns from bright green to deep orange with the temperature controlled from 85 K to 240 K in a cryogenic environment. The maximum absolute sensitivity and relative sensitivity of this temperature-sensing material based on non-thermally coupled levels of Ho³⁺ ions reached 0.049 K^-1 and 4.6% K^-1. And utilizing the thermochromic luminescence properties and high sensitivity for low temperature of the KLa(MoO₄)₂:Yb³⁺/Ho³⁺ UC material, we created KLa(MoO₄)₂:Yb³⁺/Ho³⁺ fluorescent security inks and UC photonic barcodes to realize novel visual reading and digital recognition dual-mode anti-counterfeiting in a secure manner. These results may provide useful enlightenment for the design and modulation of high-sensitivity temperature-sensing materials for high-level anti-counterfeiting applications.

Efficient modulation of upconversion luminescence in NaErF4-based core–shell nanocrystals

Efficient modulation of upconversion luminescence in heavily-doped core–shell nanocrystals by the tuning of [F]/[RE] ratio during synthesis.

Enhanced Red Emission in Er3+-Sensitized NaLuF4 Upconversion Crystals via Energy Trapping

Luminescence efficiency of trivalent lanthanide-doped upconversion (UC) materials is significantly limited by luminescence concentration quenching. In this work, red UC emission is dramatically enhanced in Er³⁺-sensitized NaLuF₄ UC crystals through energy trapping under multiple excitation wavelengths. Cross-relaxation quenching and the energy migration to internal lattice defects are simultaneously suppressed by confining the excitation energy in the Er³⁺ activator after introducing the Tm³⁺ or Ho³⁺ energy trapping center. The enhanced red UC emission (Er³⁺: 660 nm) mainly comes from the effective excitation energy confinement by Tm³⁺ and Ho³⁺ trapping centers through an easy energy transfer between Er³⁺ and Tm³⁺/Ho³⁺: ⁴I_11/2 (Er³⁺) → ³H₅ (Tm³⁺) → ⁴I_13/2 (Er³⁺) and ⁴I_11/2 (Er³⁺) → ⁵I₆ (Ho³⁺) → ⁴I_13/2 (Er³⁺). It is found that the confining efficiency of excitation energy in Er³⁺-sensitized NaLuF₄ crystals is higher than that in Yb³⁺/Er³⁺ cosensitized NaLuF₄ crystals, and the luminescence efficiency of Er³⁺-sensitized NaLuF₄ crystals is much higher than that of Er³⁺-based host sensitization UC crystals (NaErF₄). Moreover, Er³⁺-sensitized UC particles can be efficiently excited by three different wavelengths (808, 980, and 1532 nm), indicating huge advantages for applications in bioimaging, anticounterfeiting, and solar cells.

A facile route to the controlled synthesis of β-NaLuF4:Ln3+ (Ln = Eu, Tb, Dy, Sm, Tm, Ho) phosphors and their tunable luminescence properties

Highly uniform and monodisperse β-NaLuF₄:Ln³⁺ (Ln = Eu, Tb, Dy, Sm, Tm, Ho) hexagonal prisms have been synthesized via a facile two-step hydrothermal method without any organic surfactants.

Facile synthesis and emission enhancement in NaLuF4 upconversion nano/micro-crystals via Y3+ doping

A series of Y³⁺-absent/doped NaLuF₄:Yb³⁺, Tm³⁺ nano/micro-crystals were prepared via a hydrothermal process with the assistance of citric acid. Cubic nanospheres, hexagonal microdisks, and hexagonal microprisms can be achieved by simply adjusting the reaction temperature. The effect of Y³⁺ doping on the morphology and upconversion (UC) emission of the as-prepared samples were systematically investigated. Compared to their Y³⁺-free counterpart, the integrated spectral intensities in the range of 445-495 nm from α-, β-, and α/β-mixed NaLuF₄:Yb³⁺, Tm³⁺ crystals with 40 mol% Y³⁺ doping are increased by 9.7, 4.4, and 24.3 times, respectively; red UC luminescence intensities in the range of 630-725 nm are enhanced by 4.6, 2.4, and 24.9 times, respectively. It is proposed that the increased UC emission intensity is mainly ascribed to the deformation of crystal lattice, due to the electron cloud distortion in host lattice after Y³⁺ doping. This paper provides a facile route to achieve nano/micro-structures with intense UC luminescence, which may have potential applications in optoelectronic devices.

Synthesis of Multicolor Core/Shell NaLuF₄:Yb3+/Ln3+@CaF₂ Upconversion Nanocrystals

The ability to synthesize high-quality hierarchical core/shell nanocrystals from an efficient host lattice is important to realize efficacious photon upconversion for applications ranging from bioimaging to solar cells. Here, we describe a strategy to fabricate multicolor core @ shell α-NaLuF₄:Yb³⁺/Ln³⁺@CaF₂ (Ln = Er, Ho, Tm) upconversion nanocrystals (UCNCs) based on the newly established host lattice of sodium lutetium fluoride (NaLuF₄). We exploited the liquid-solid-solution method to synthesize the NaLuF₄ core of pure cubic phase and the thermal decomposition approach to expitaxially grow the calcium fluoride (CaF₂) shell onto the core UCNCs, yielding cubic core/shell nanocrystals with a size of 15.6 ± 1.2 nm (the core ~9 ± 0.9 nm, the shell ~3.3 ± 0.3 nm). We showed that those core/shell UCNCs could emit activator-defined multicolor emissions up to about 772 times more efficient than the core nanocrystals due to effective suppression of surface-related quenching effects. Our results provide a new paradigm on heterogeneous core/shell structure for enhanced multicolor upconversion photoluminescence from colloidal nanocrystals.

Enhanced red upconversion emission and its mechanism in Yb3+–Er3+ codoped α-NaLuF4 nanoparticles

Red upconversion luminescence is greatly enhanced through manipulation of the initial solution pH.

Simultaneous spectra and dynamics processes tuning of a single upconversion microtube through Yb3+ doping concentration and excitation power

Doping and varying pump laser parameters are the widely applied technological processes for tuning spectra to yield desirable luminescence properties and functions.

Morphology evolution and pure red upconversion mechanism of β-NaLuF4 crystals

A series of β-NaLuF₄ crystals were synthesized via a hydrothermal method. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red upconversion (UC) luminescence can be observed in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ and Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺. Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the energy transfer UC from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2 and the cross-relaxation (CR) effect [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2] rather than the long-accepted mechanism [CR process among Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2]. In addition, compared to the Li⁺-free counterpart, the pure red UC luminescence in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping is enhanced by 13.7 times. This study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies.

Core–shell–shell heterostructures of α-NaLuF4:Yb/Er@NaLuF4:Yb@MF2 (M = Ca, Sr, Ba) with remarkably enhanced upconversion luminescence

Core–shell–shell heterostructures of α-NaLuF₄:Yb/Er@NaLuF₄:Yb@MF₂ (M = Ca, Sr, Ba) exhibit remarkably enhanced upconversion luminescence.

Ce(3+) /Tb(3+) non-/single-/co-doped K-Lu-F materials: synthesis, optical properties, and energy transfer

Using a hydrothermal method, Ce(3+) /Tb(3+) non-/single-/co-doped K-Lu-F materials have been synthesized. The X-ray diffraction (XRD) results suggest that the Ce(3+) and/or Tb(3+) doping had great effects on the crystalline phases of the final samples. The field emission scanning electron microscopy (FE-SEM) images indicated that the samples were in hexagonal disk or polyhedron morphologies in addition to some nanoparticles, which also indicated that the doping also had great effects on the sizes and the morphologies of the samples. The energy-dispersive spectroscopy (EDS) patterns illustrated the constituents of different samples. The enhanced emissions of Tb(3+) were observed in the Ce(3+) /Tb(3+) co-doped K-Lu-F materials. The energy transfer (ET) efficiency ηT were calculated based on the fluorescence yield. The ET mechanism from Ce(3+) to Tb(3+) was confirmed to be the dipole-quadrupole interaction inferred from the theoretical analysis and the experimental data. Copyright © 2015 John Wiley & Sons, Ltd.