Structural design and synthesis of new MOO3-x interlayer bi-functional nanomaterials for enhanced up-conversion luminescence properties

A novel imaging materials based on bi-functional Fe₃O₄@M_OO_3-x@YF₃:Yb/Er nanoparticles (NPs) with strong up-conversion luminescence and magnetic properties was designed and synthesized by inlaying M_OO_3-x with localized surface plasmon resonance (LSPR) and ferromagnetic property in the bi-functional Fe₃O₄@YF₃:Yb/Er NPs. The morphology, structure and properties of Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), photoluminescence (PL) spectra, UV–Vis-NIR spectrophotometer and superconducting quantum interference devices (SQUID). It was found that the experimental conditions (reaction temperature, reaction time, the mass ratio of core:shell and oxygen pressure in heat treatment) have a certain effect on the oxygen defect concentration of the M_OO_3-x interlayer. XRD also showed the crystal structure of Fe₃O₄, Fe₃O₄@M_OO_3-x and Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs. The SEM and TEM results showed that the average grain size of the prepared NPs were 200 nm. The SQUID and PL results showed that the Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs have stronger magnetism (14.3 emu/g) and excellent up-conversion luminescence performance (the emission peak at 525 nm is nearly 20 times higher than the corresponding emission intensity of the Fe₃O₄@YF₃:Yb/Er). The Fe₃O₄@M_OO_3-x@YF₃:Yb/Er NPs can be easily used to magnetic resonance and fluorescence dual-mode image-guided visual delivery drug and also increase the accurate diagnosis and treatment effect of malignant tumor.     

Growth and optical properties of Er,Yb:YAl3(BO3)4 crystal

Single crystal of erbium, ytterbium-codoped yttrium aluminum tetraborate Er,Yb:YAl₃(BO₃)₄(Er,Yb:YAB) has been grown by the flux method. The absorption spectrum in the visible and NIR regions of Er,Yb:YAl₃(BO₃)₄ crystal are measured at room temperature and fluorescence spectrum of Er,Yb:YAl₃(BO₃)₄ crystal are also measured at room temperature, excited by 976 nm laser. Not only the strong NIR emission peaks located at 1548 nm was observed, but also the visible up-conversion luminescence has been found. The specific heat of the Er/Yb:YAB crystal at room temperature is 0.81 J/g °C.     

Luminescence properties and excitation process of a near-infrared to visible up-conversion color-tunable phosphor

Further investigation of previously reported color-tunable NaNbO₃-based up-conversion phosphor doped with Er³⁺, Yb³⁺, and Al³⁺ (NaNbO₃:Er,Yb,Al) was undertaken. Color tuning is achieved by variation of the relative intensity of the red/green emission, which is dependent on the pulse width of the near-infrared (980 nm) excitation. The chromaticity of emissions ranging from green to reddish-yellow with a change of pulse duration from 50 to 2400 μs is wider than that of the reference compound LaGaO₃:Er,Yb, and is not influenced by the excitation intensity. It was elucidated that the green and red emissions occur via different excitation routes, of which the latter is characterized by a much slower build-up of emission.     

Hydrothermal synthesis and luminescence properties of hierarchical SrF2 and SrF2:Ln3+ (Ln = Er,Nd, Yb,Eu, Tb) micro/nanocomposite architectures

Highly uniform SrF₂ and SrF₂:Ln³⁺ (Ln = Er, Nd, Yb, Eu, Tb) hierarchical microspheres assembled by 2D nanoplates have been successfully synthesized by a facile and friendly hydrothermal route. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and photoluminescence (PL) spectra were used to characterize the samples. The experimental results indicate that reaction time and chelating reagent play a key role in forming the hierarchical microspheres. The formation mechanism was proposed based on the evolution of this morphology as a function of hydrothermal time. The near-infrared luminescence of lanthanide ions (Er, Nd, and Yb) doped SrF₂ microspheres were discussed in detail. In addition, the as-obtained SrF₂:Eu³⁺ sample exhibits orange-red emission centered at 590 nm under excitation at 393 nm, while the SrF₂:Tb³⁺ exhibits a strong green emission at 540 nm. The as-synthesized SrF₂:Ln³⁺ luminescent microspheres might find some potential applications in areas of photoluminescence, telecommunication and laser emission.     

Photoluminescence properties of the Yb:Er co-doped Al2O3 thin film fabricated by microwave ECR plasma source enhanced RF magnetron sputtering

The Yb:Er co-doped Al₂O₃ thin film was deposited on oxidized silicon wafers by microwave ECR plasma source enhanced RF magnetron sputtering, and annealed from 800 °C to 1000 °C. The photoluminescence at 1.53 μm of thin film was obtained under room temperature. The mixture phase structure of γ and θ is observed by XRD, and the compositions of the thin film are investigated by EPMA. The maximum PL intensity was achieved with O₂:Ar at 1:1, annealing temperature at 900 °C, and experimental ratio of Yb:Er at 1:3.6. The energy transfer mechanism between Er and Yb ions is supported by theoretical analysis and experiment results.     

Luminescence and scintillation properties of rare-earth-doped LuF3 scintillation crystals

The Nd-doped and Er-doped LuF₃ single crystals were grown by the micro-pulling-down method to study their scintillation properties in the vacuum-ultraviolet (VUV) region. The doubly Nd–Er codoped single crystal was grown to study possibility of scintillation performance improvement by energy transfer from Er³⁺ to Nd³⁺ ions. The LiF flux was to avoid phase transition below melting temperature. The 1%Nd-doped sample showed the highest overall scintillation efficiency under X-ray excitation which was 7 times as high as that of the LaF₃:Nd 8% standard. The leading Nd³⁺ 5d–4f emission was situated at 176 nm, while the Er³⁺ 5d–4f emission for Er-doped samples was observed at 163 nm, which better matches the sensitivity of some VUV-sensitive photodetectors. The optimum Er concentration was determined to be around 1–3 mol%. No Er³⁺ 5d–4f emission was observed for the doubly Er,Nd-codoped sample due to energy transfer from the Er³⁺ to Nd³⁺ ions. Slight improvement of the light yield was observed in the doubly-doped sample with respect to the Nd-only doped one.     

Photoluminescence and thermal properties of erbium and ytterbium codoped poly(phthalazinone ether sulfone ketone) nanocomposites

Erbium and ytterbium codoped LaF₃ nanoparticles coated with organic ligands and Poly (phthalazine ether sulfone ketone) (PPESK) nanocomposites were prepared in this paper. The morphologies of nanoparticles were characterized by TEM. The thermal and photoluminescence properties of the PPESK nanocomposites were investigated by DSC, TGA and photoluminescence spectra. It was found that the nanoparticles with particle size of 20 nm can be homogeneously dispersed in PPESK-in-chloroform solutions, which allowed the transparent film to be facilely prepared. The photoluminescence spectra of PPESK nanocomposites film and solution in chloroform both reveal the typical features of the transition of Er³⁺ from ⁴I_13/2 to ⁴I_15/2 at 1530 nm. By adding nanoparticles to PPESK matrix, the T_g of PPESK has little change and the initial degradation temperature decreases.     

Neodymium‐Doped LaF3 Nanoparticles for Fluorescence Bioimaging in the Second Biological Window

The future perspective of fluorescence imaging for real in vivo application are based on novel efficient nanoparticles which is able to emit in the second biological window (1000–1400 nm). In this work, the potential application of Nd³⁺‐doped LaF₃ (Nd³⁺:LaF₃) nanoparticles is reported for fluorescence bioimaging in both the first and second biological windows based on their three main emission channels of Nd³⁺ ions: ⁴F_3/2→⁴I_9/2, ⁴F_3/2→⁴I_11/2 and ⁴F_3/2→⁴I_13/2 that lead to emissions at around 910, 1050, and 1330 nm, respectively. By systematically comparing the relative emission intensities, penetration depths and subtissue optical dispersion of each transition we propose that optimum subtissue images based on Nd³⁺:LaF₃ nanoparticles are obtained by using the ⁴F_3/2→⁴I_11/2 (1050 nm) emission band (lying in the second biological window) instead of the traditionally used ⁴F_3/2→⁴I_9/2 (910 nm, in the first biological window). After determining the optimum emission channel, it is used to obtain both in vitro and in vivo images by the controlled incorporation of Nd³⁺:LaF₃ nanoparticles in cancer cells and mice. Nd³⁺:LaF₃ nanoparticles thus emerge as very promising fluorescent nanoprobes for bioimaging in the second biological window.     

Upconversion luminescence of holmium and ytterbium co-doped yttrium oxysulfide phosphor

The upconversion optical characteristic and brightness of Y₂O₂S:Yb,Ho phosphors are investigated. It is shown that Y₂O₂S:Yb,Ho exhibits NIR, red, green, blue and even ultraviolet-blue emission bands under 980 nm pumping. Moreover, Y₂O₂S:Yb,Ho shows excellent upconversion luminescence and its upconversion brightness is 2.2 times to that of the commercial Y₂O₂S:Yb,Er phosphors. The upconversion mechanism is discussed: the green and NIR emissions are due to two consecutive energy transfer from Yb³⁺ to Ho³⁺, and the blue and red emissions can be explained as a loop-like mechanism.     

Synthesis of nanocrystalline LaF3 doped silica glasses by hydrofluoric acid catalyzed sol–gel process

Silica glasses doped with LaF₃ nanocrystals were prepared by HF-catalyzed sol–gel method. HF was used both as fluorine source and as catalyst of the sol–gel reaction, making it possible to shorten the processing time with reducing the concentration of SiOH groups to ～10¹⁸ cm^?3. The resultant glasses are transparent at visible spectral range, and the optical loss at the ultraviolet absorption edge is dominated by the Rayleigh scattering from LaF₃ crystallites. The size of LaF₃ crystallites increases with an increase in the sintering temperature and time, and is smaller than ～40 nm in samples showing good visible transparency. Green upconversion photoluminescence is observed in an Er³⁺-doped sample under excitation at 980 nm.     

The low-temperature sonochemical synthesis of up-converting β NaYF4:Yb,Er mesocrystals

In this paper, we report the mild sonochemical synthesis of hexagonal (β) NaYF₄:Yb,Er mesocrystals at low temperature of 40 °C. The nucleation and transformation of crystal structures is investigated in the course of time, and it is shown that the pure β phase is obtained after 2 h of pulsed sonication. The crystallization of orthorhombic YF₃:Yb,Er and cubic (α) NaYF₄:Yb,Er precedes the appearance of a thermodynamically stable β NaYF₄:Yb,Er phase. Based on the evolution of the nanoparticle morphology, it is concluded that the transition from α to β phase is consistent with the dissolution-recrystallization process, while the final shape of mesocrystals is a consequence of the oriented attachment growth in the newly formed β phase nanocrystallites along two planes. The structural properties of all compounds are analyzed and correlated with their thermal and optical characteristics. The pump power dependence of green and red emissions confirms that only two-photon process is involved in up-conversion, which is superior in β phase mesocrystals.     

The effect of In ion on the optical characteristics of Er in Er/Yb:LiNbO3 crystal

The effect of In³⁺ ion on the optical characteristics of Er³⁺ ion in Er/Yb:LiNbO₃ crystal under 980 nm excitation has been investigated. The Er and Yb contents in the crystals were measured by an inductively coupled plasma atomic emission spectrometer (ICP-AES). A significant enhancement of 1.54 μm emission was observed for Er/Yb:LiNbO₃ crystal doped with 1 mol% In₂O₃. The studies on the UV-vis absorption and the OH⁻ absorption spectra indicate that the threshold concentration of In³⁺ ion decreases with the Er/Yb doping in Er/Yb/In:LiNbO₃ crystal. The 1 mol% In₂O₃ doping results in the reduction of absorption cross section in the UV-vis region, meaning the formation of Er³⁺ cluster sites. The enhancement of 1.54 μm emission is attributed to the larger probabilities of the cross relaxation processes ⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2 (Er), ⁴S_3/2 + ⁴I_15/2 → ⁴I_13/2 + ⁴I_9/2 (Er) and ⁴I_9/2 + ⁴I_15/2 → ⁴I_13/2 + ⁴I_13/2 (Er) induced by Er³⁺ cluster sites.     

915 nm Light‐Triggered Photodynamic Therapy and MR/CT Dual‐Modal Imaging of Tumor Based on the Nonstoichiometric Na0.52YbF3.52:Er Upconversion Nanoprobes

Lanthanide (Ln³⁺)‐doped upconversion nanoparticles (UCNPs) as a new generation of multimodal bioprobes have attracted great interest for theranostic purpose. Herein, red emitting nonstoichiometric Na_0.52YbF_3.52:Er UCNPs of high luminescence intensity and color purity are synthesized via a facile solvothermal method. The red UC emission from the present nanophosphors is three times more intense than the well‐known green emission from the ≈30 nm sized hexagonal‐phase NaYF₄:Yb,Er UCNPs. By utilizing Na_0.52YbF_3.52:Er@SrF₂ UCNPs as multifunctional nanoplatforms, highly efficient in vitro and in vivo 915 nm light‐triggered photodynamic therapies are realized for the first time, with dramatically diminished overheating yet similar therapeutic effects in comparison to those triggered by 980 nm light. Moreover, by virtue of the high transverse relaxivity (r ₂) and the strong X‐ray attenuation ability of Yb³⁺ ions, these UCNPs also demonstrate good performances as contrast agents for high contrast magnetic resonance and X‐ray computed tomography dual‐modal imaging. Our research shows the great potential of the red emitting Na_0.52YbF_3.52:Er UCNPs for multimodal imaging‐guided photodynamic therapy of tumors.     

Upconversion and infrared luminescences in Er/Yb codoped Y2O3 and (Y0.9La0.1)2O3 transparent ceramics

Er³⁺ and Yb³⁺ codoped Y₂O₃ and (Y_0.9La_0.1)₂O₃ transparent ceramics were fabricated by the conventional ceramics processing with nanopowders. Compared to Er/Yb:Y₂O₃, Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics had higher transmittance. Intense upconversion (UC) and infrared emission (1543 nm) were observed under excitation of 980 nm. According to three intensity parameters Ω₂, Ω₄, and Ω₆ fitted by the Judd-Ofelt theory, the spectroscopic quality parameters (X), radiative lifetimes (τ_rad), and emission cross-sections (α_em) were determined. Er/Yb:(Y_0.9La_0.1)₂O₃ ceramics owned broader peaks and longer lifetime (12.3 ms) at 1548 nm due to the glass-like structure of (Y_0.9La_0.1)₂O₃ ceramics. The results showed Y₂O₃ and Y_1.8La_0.2O₃ transparent ceramics are promising gain media for developing the solid-state 1.5 μm optical amplifiers and tunable UC lasers.     

An Excitation Navigating Energy Migration of Lanthanide Ions in Upconversion Nanoparticles

Upconversion nanoparticles (UCNPs) doped with lanthanide ions that possess ladder-like energy levels can give out multiple emissions at specific ultra-violet or visible wavelengths irrespective of excitation light. However, precisely controlling energy migration processes between different energy levels of the same lanthanide ion to generate switchable emissions remains elusive. Herein, a novel dumbbell-shaped UCNP is reported with upconverted red emission switched to green emission when excitation wavelength changed from 980 to 808 nm. The sensitizer Yb ions are doped with activator Er ions and energy modulator Mn ions in NaYF₄ core nanocrystal coated with an inner NaYF₄:Yb shell to generate red emission after harvesting 980 nm excitation light, while an outer NaNdF₄:Yb shell is coated to form a dumbbell shape to generate green emission upon 808 nm excitation. Such specially designed UCNPs with switchable green and red emissions are further explored for imaging of latent fingerprint and detection of explosive residues in the fingerprint simultaneously. This work suggests a novel research interest in fine-tuning of upconversion emissions through precisely controlling energy migration processes of the same lanthanide activator ion. Furthermore, use of these nanoparticles in other applications such as simultaneous dual-color imaging or orthogonal bidirectional photoactivation can be explored.     

Facilely dispersible magnetic nanoparticles prepared by a surface-initiated atom transfer radical polymerization

Facilely dispersible magnetic nanoparticles (Fe₃O₄) prepared by a surface-initiated atom transfer radical polymerization (ATRP) of poly (ethylene glycol) methyl ether monomethacrylate (PEGMA) are reported. The initiator of 2-bromoisobutyrate (BIB) for ATRP was immobilized onto the surface of Fe₃O₄ nanoparticles by the reaction between 2-bromoisobutyryl bromide (BIBB) and the hydroxyl group on the nanoparticles. The results indicated that the poly(poly(ethylene glycol) monomethacrylate) (PPEGMA) was successfully grafted onto the surface of the magnetic nanoparticles. The core-shell nanoparticles with particle size of ≈ 20 nm in water (about 20 mg/mL) are facilely dispersible and can be easily captured by a magnet with magnetic field of 2000 G.     

Spectroscopic properties and energy transfer in Er–Tm co-doped bismuth silicate glass

In this paper, we investigate the spectroscopic properties of and energy transfer processes in Er–Tm co-doped bismuth silicate glass. The Judd–Ofelt parameters of Er³⁺ and Tm³⁺ are calculated, and the similar values indicate that the local environments of these two kinds of rare earth ions are almost the same. When the samples are pumped at 980 nm, the emission intensity ratio of Tm:³F₄ → ³H₆ to Er:⁴I_13/2 → ⁴I_15/2 increases with increased Er³⁺ and Tm³⁺ contents, indicating energy transfer from Er:⁴I_13/2 to Tm:³F₄. When the samples are pumped at 800 nm, the emission intensity ratio of Er:⁴I_13/2 → ⁴I_15/2 to Tm:³H₄ → ³F₄ increases with increased Tm₂O₃ concentration, indicating energy transfer from Tm:³H₄ to Er:⁴I_13/2. The rate equations are given to explain the variations. The microscopic and macroscopic energy transfer parameters are calculated, and the values of energy transfer from Er:⁴I_13/2 to Tm:³F₄ are found to be higher than those of the other processes. For the Tm singly-doped glass pumped at 800 nm and Er–Tm co-doped glass pumped at 980 nm, the pumping rate needed to realize population reversion is calculated. The result shows that when the Er₂O₃ doping level is high, pumping the co-doped glass by a 980 nm laser is an effective way of obtaining a low-threshold ∼2 μm gain.     

Enhancement of upconversion luminescence of Er and Yb co-doped Y2O3 nanoparticle by Ag half-shell

Upconversion photoluminescence (PL) properties of single Y₂O₃ nanoparticles doped with Yb and Er (Y₂O₃:Yb,Er) with a Ag over-layer is studied. We traced the PL and light scattering images of individual nanoparticles by changing the thickness of a Ag over-layer. When the Ag thickness is relatively small and only the upper part of a nanoparticle is covered by Ag (Ag half-shell), the PL is strongly enhanced. On the other hand, when the Ag thickness is increased and a continuous Ag over layer is formed, the enhancement factor decreases. From the correlation between the enhancement factors of the upconversion PL and scattering intensities as well as the change of the PL lifetime, the mechanism of the PL enhancement is discussed.     

Effects of Li+ ions on the enhancement of up-conversion emission in Ho3+-Yb3+ co-doped transparent glass–ceramics containing Ba2LaF7 nanocrystals

The up-conversion (UC) emission of Ho³⁺-Yb³⁺ and Li⁺ co-doped transparent glass ceramics 45SiO₂-15Al₂O₃-12Na₂CO₃-21BaF₂-7LaF₃-0.2HoF₃-1YbF₃-xLi₂CO₃ (x = 0, 0.5, 1, 2, 4 and 6 mol%) containing Ba₂LaF₇ nanocrystals were investigated. These glass ceramics samples were prepared using the conventional quenching techniques. The Ba₂LaF₇ nanocrystals precipitated from the glass matrix was confirmed by X-ray diffraction (XRD). Compared with the glass ceramics sample without Li⁺, the UC emission intensity of glass ceramics samples with Li⁺ were enhanced. It can be proved that the Li⁺ can affect the enhancement up-conversion (UC) emission. Particularly, the green UC emission intensity band centered at 546 nm was strongly increased twice with the concentration of Li⁺ increasing up to 4 mol%. Through the comparison and analysis of the energy graph, it was shown that the ⁵F₄/⁵S₂→⁵I₈ transition of Ho³⁺ ion obtained the green (546 nm) light. There are two weak fluorescences in the red (657 nm) region and near infrared (753 nm) region of spectrum, which is the ⁵F₄/⁵S₂→⁵I₇ and ⁵F₅→⁵I₈ transition of Ho³⁺. Therefore, the emission results showed that the incorporation of Li⁺ ions into the Ba₂LaF₇:Eu³⁺ lattice could induce a remarkable change of the emission intensity in red region (R = I_ED/I_MD) with 393 nm excitation wavelength. It was indicated that the symmetry of the lattice was destroyed by Li⁺ in glass ceramics. The possible mechanism responsible for the enhancement of UC emission in Ho-Yb co-doped was discussed.     

Citric acid-assisted growth of lanthanide ions co-doped one-dimensional upconversion microcrystals and their photovoltaic applications

We report for the first time the preparation of Yb³⁺–Er³⁺ co-doped β-phase sodium gadolinium fluoride (NaGdF₄:Yb/Er) microcrystals using citric acid (CA) as a shape modifier and the application of NaGdF₄:Yb/Er to amorphous silicon solar cells. The proposed hydrothermal method was facile and eco-friendly. It was found that CA played a critical role in the shape evolution of the final products. The related synthesis mechanisms are discussed. Under 980 nm laser excitation, the optimized NaGdF₄:Yb/Er crystals exhibited strong upconversion (UC) emissions at visible wavelengths. Amorphous silicon thin film solar cells were designed and fabricated for investigating different UC configurations. In combination with a metallic back reflector, the cell with the front side UC layer achieved a six-fold improvement of the photocurrent while the cell with the rear side UC layer exhibited a ten-fold enhancement of the photocurrent, under 980 nm light, compared with the cell without upconverter.