Rare-earth Doped Nanocrystals Research Articles

Crystallization processes of rare-earth doped GdF3 nanocrystals in silicate glass matrix: Dimorphism and photoluminescence properties

Rare-earth doped GdF3 nanocrystals embedded in silica glassy matrix have been prepared by controlled crystallization of the xerogel; the influence of rare-earth ion (Pr, Sm, Eu, Tb, Dy, Er, Yb) and additional Li-codopant on structural and optical properties was discussed. The precipitation of RE-doped GdF3 nanocrystalline phase is the result of Gd-trifluoracetate thermolysis revealed as a strong exothermic peak at around 300 °C. Structural analysis of RE-doped SiO2-GdF3 glass-ceramic sample calcined at 525 °C showed the occurrence of GdF3 nanocrystals of about 25 nm size, showing hexagonal or orthorombic structure depending on the RE-ion. Under UV-light excitation at 273 nm of Gd3+ ions (8S7/2 → 6I13/2,15/2 transition), photoluminescence spectra showed characteristic RE3+ luminescence due to the non-radiative energy transfer between the excited Gd3+ and acceptor RE ions; the highest energy transfer (≅ 80 %) was observed for Tb3+ and lowest (≅ 21 %) for Sm3+.

Optical line broadening mechanisms in rare-earth doped oxide nanocrystals

Rare-earth ions in nanocrystals are promising solid-state platforms for quantum light-matter interfaces, combining millisecond-long nuclear spin T2 with kHz-range optical homogeneous linewidths. Coupled to optical micro-cavities these materials can enable scalable quantum processors, nanoscale quantum memories and single photon sources. Here, we report on optical dephasing in Eu3+:Y2O3 nanocrystals of 100 nm diameter through photon echo and spectral hole burning techniques as a function of temperature and magnetic field. We identify two-level systems (TLS) and electric noise as major line broadening mechanisms below 2 K and demonstrate that material-processing strategies such as high-power oxygen plasma can partially counteract these effects. Through modeling we show that these results are compatible with a reduction in the number of oxygen vacancy pairs in the nanocrystals by the oxygen plasma processing. These pairs, even at very low concentrations (hundreds of ppb), appear detrimental to optical coherence due to their ability to generate fluctuating electric fields.

Metasurface-Controlled Photonic Rashba Effect for Upconversion Photoluminescence.

We demonstrate the effect of spin-momentum locking of upconversion photoluminescence emitted from rare-earth doped nanocrystals coupled to a phase-gradient dielectric metasurface. We observe different directionalities for left and right circular polarized light and associate this experimental observation with the photonic Rashba effect realized for upconverted photoluminescence that is manifested in the spin-dependent splitting of emitted light in the momentum space.

Bright Tm3+-based downshifting luminescence nanoprobe operating around 1800 nm for NIR-IIb and c bioimaging

Fluorescence bioimaging based on rare-earth-doped nanocrystals (RENCs) in the shortwave infrared (SWIR, 1000–3000 nm) region has aroused intense interest due to deeper penetration depth and clarity. However, their downshifting emission rarely shows sufficient brightness beyond 1600 nm, especially in NIR-IIc. Here, we present a class of thulium (Tm) self-sensitized RENC fluorescence probes that exhibit bright downshifting luminescence at 1600–2100 nm (NIR-IIb/c) for in vivo bioimaging. An inert shell coating minimizes surface quenching and combines strong cross-relaxation, allowing LiTmF4@LiYF4 NPs to emit these intense downshifting emissions by absorbing NIR photons at 800 nm (large Stokes shift ~1000 nm with a absolute quantum yield of ~14.16%) or 1208 nm (NIR-IIin and NIR-IIout). Furthermore, doping with Er3+ for energy trapping achieves four-wavelength NIR irradiation and bright NIR-IIb/c emission. Our results show that Tm-based NPs, as NIR-IIb/c nanoprobes with high signal-to-background ratio and clarity, open new opportunities for future applications and translation into diverse fields.

Nanoantennas Patterned by Colloidal Lithography for Enhanced Nanophosphor Light Emission.

Transparent coatings made of rare-earth doped nanocrystals, also known as nanophosphors, feature efficient photoluminescence and excellent thermal and optical stability. Herein, we demonstrate that the optical antennas prepared by colloidal lithography render thin nanophosphor films with a brighter emission. In particular, we fabricate gold nanostructures in the proximity of GdVO4:Eu3+ nanophosphors by metal evaporation using a mask made of a monolayer of polymer beads arranged in a triangular lattice. Optical modes supported by the antennas can be controlled by tuning the diameter of the polymer spheres in the colloidal mask, which determines the shape of the gold nanostructure, as confirmed by numerical simulations. Confocal microscopy reveals that metallic antennas induce brighter photoluminescence at specific spatial regions of the nanophosphor film at targeted frequencies as a result of the coupling between gold nanostructures and nanophosphors. Patterning of nanophosphor thin layers with arrays of metallic antennas offers an inexpensive nanophotonic solution to develop bright emitting coatings of interest for color conversion, labeling, or anti-counterfeiting.

Rare earth doped double perovskite nanocrystals with controllable emission wavelength and model for high-level anti-counterfeiting

Fluorescent anti-counterfeiting technology has become one of the most commonly used anti-counterfeiting technologies because of its simple preparation, low cost and high safety. However, there are still some key technical problems to be solved in developing multi-mode and polychromatic fluorescent materials for high-level anti-counterfeiting. Herein, Yb3+/Er3+ codoped Cs2AgIn0.99Bi0.01Cl6 double perovskite nanocrystals (DPNCs) with dual-mode polychromatic emission were developed. Under the irradiation of 365 nm ultraviolet (UV) light, the DPNCs emitted a bright downshifting (DS) yellow light, which comes from the self-trapping excitons (STEs) recombination. While the DPNCs produced upconversion (UC) green fluorescence under the 980 nm laser excitation, which is attributed to the transitions of 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+ ions. In view of its dual-mode and polychromatic fluorescence characteristics, a high-level anti-counterfeiting based on this fluorescent material is designed. Moreover, the brightness of the anti-counterfeiting pattern based on this material decreases very little after three months. These results indicated that the dual-mode and polychromatic DPNCs reported have potential application in information encryption and identity recognition.

Measuring the Magnetic Dipole Transition of Single Nanorods by Spectroscopy and Fourier Microscopy

Rare-earth doped nanocrystals possess optical transitions with significant either electric or magnetic dipole characters. They are of considerable interest for understanding and engineering light-matter interactions at the nanoscale with numerous applications in nanophotonics. Here, we study the ${}^{5}{D}_{0}{\ensuremath{\rightarrow}}^{7}{F}_{1}$ transition dipole vector in individual ${\mathrm{NaYF}}_{4}:{\mathrm{Eu}}^{3+}$ nanorod crystals by Fourier and confocal microscopies. A single-crystal host matrix leads to narrow emission lines at room temperature that permit separation of the Stark sublevels resulting from the crystal-field splitting. We observe a fully magnetic transition and low variability of the transition dipole orientation over several single nanorods. We estimate the proportion of the dipole transitions for the Stark sublevels. We also determine an effective altitude of the rod with respect to the substrate. The narrow emission lines characteristic of ${\mathrm{NaYF}}_{4}:{\mathrm{Eu}}^{3+}$ ensure well-defined electric or magnetic transitions, and are thus instrumental for probing locally their electromagnetic environment by standard confocal microscopy.

Upconversion ladder enabled super-sensitive narrowband near-infrared photodetectors based on rare earth doped florine perovskite nanocrystals

Abstract Filterless narrowband near-infrared photodetectors (NIRPDs) offer significantly reducing cost and the system volume for various applications, but limited by the poor photosensitivity. Herein, we theoretically and experimentally demonstrate a novel class of sensitive filterless NIRPDs in CsPbF3: Zn2+-Yb3+- Tm3+ (or Er3+) perovskite nanocrystals (PeNCs)/Au nanorods (NRs) array hybrid via direct photon-electric upconverion (UC) from ground state of Yb3+- Tm3+ (or Er3+) to conduction band of PeNCs. It shows the selectively spectral photodetection at 980-nm NIR region with all full-width at half-maximum of 20 nm, high responsivity of 106 A/W, detectivity of 1.52 × 1012 Jones, and external quantum efficiency of 135%. The excellent performance can be mainly ascribed to (1) direct photon-electric UC mechanism; (2) the reduced trapping-center density, high carrier mobility, and increased extraction and separation efficient of election-hole pairs of PeNCs; (3) localized surface plasmon enhancement. Our findings open the exciting potential for developing the next-generation narrow NIRPDs.

Study of optical and dielectric properties of alkali metal cation (Li+, Na+, K+) codoped Eu3+ activated gadolinium aluminate nanoparticles

In the present work, solution combustion method is used to synthesize GdAlO3 nanophosphors, doped with Eu3+(1 mol%) and Co-doped with alkali metals such as Li+, Na+, K+ (1 mol%) using ODH (oxalyldihydrazide) as fuel. The morphological details have been investigated using various analytical techniques. X-ray diffraction patterns reveal the formation of pure, orthorhombic structured nanoparticles in the Pbnm phase group having an average crystallite size of about 45–60 nm. The functional groups and chemical interactions were probed using the FTIR spectrum. The agglomerated spherical shaped nanoparticles were evidenced by SEM micrographs. The optical energy gap Urbach energy (ΔE) and refractive index (n) were determined by UV spectroscopy from the direct allowed transitions. Due to the highly stable physical and chemical properties, rare earth doped nanocrystals have been widely employed miniaturized dielectric devices. The dielectric and electrical properties are measured in the frequency ranging from 10 Hz to 8 MHz at RT using an LCR meter. The synthesized nanoparticles have a high dielectric constant in the low-frequency region and at the higher frequency region, the low dielectric loss was observed. Thus, these nanomaterials find potential utility for high-frequency applications.

Rare Earth Hydroxide as a Precursor for Controlled Fabrication of Uniform β-NaYF₄ Nanoparticles: A Novel, Low Cost, and Facile Method.

In recent years, rare earth doped upconversion nanocrystals have been widely used in different fields owing to their unique merits. Although rare earth chlorides and trifluoroacetates are commonly used precursors for the synthesis of nanocrystals, they have certain disadvantages. For example, rare earth chlorides are expensive and rare earth trifluoroacetates produce toxic gases during the reaction. To overcome these drawbacks, we use the less expensive rare earth hydroxide as a precursor to synthesize β-NaYF4 nanoparticles with multiform shapes and sizes. Small-sized nanocrystals (15 nm) can be obtained by precisely controlling the synthesis conditions. Compared with the previous methods, the current method is more facile and has lower cost. In addition, the defects of the nanocrystal surface are reduced through constructing core–shell structures, resulting in enhanced upconversion luminescence intensity.

Thermally enhanced NIR-NIR anti-Stokes emission in rare earth doped nanocrystals.

Nanoparticles with anti-Stokes emissions have enabled many sensing applications, but their efficiencies are considerably low. The key to enable the process of anti-Stokes emissions is to create phonons that assist the excited photons to be pumped from a lower energy state onto a higher one. Increasing the temperature will generate more phonons, but it unavoidably quenches the luminescence. Here by quantifying the number of phonons being generated from the host crystal and those at the surface of Yb3+/Nd3+ co-doped nanoparticles, we systematically investigated mechanisms towards the large enhancements of the phonon-assisted anti-Stokes emissions from 980 nm to 750 nm and 803 nm. Moreover, we provided direct evidence that moisture release from the nanoparticle surface at high temperature was not the main reason. We further demonstrated that the brightness of 10 nm nanoparticles was enhanced by more than two orders of magnitude, in stark contrast to the thermal quenching effect.

Evidencing early pyrochlore formation in rare-earth doped TiO2 nanocrystals: Structure sensing via VIS and NIR Er3+ light emission

Er3+ doping of TiO2 colloidal nanocrystals enhances their performance for photo-induced applications. Such doping is known to delay the anatase to rutile transformation under thermal treatment; nonetheless relevant information on the Er3+ light emission and location within the TiO2 structures is still incomplete. Er3+ photoluminescence emission both in the visible (upconverted) and infrared photoluminescence is used for the first time to probe the ions location within the different TiO2 structures. The results show that Er3+ ions in the as-prepared xerogels are not embedded in the anatase crystallites, and only upon thermal treatment Er3+ diffusion is induced into crystal interstitial positions to form a solid solution. At higher temperatures rutile is formed inducing Er3+ segregation and giving rise to the formation of pyrochlore (Er2Ti2O7), as shown by a distinct emission in the infrared spectrum due to the Er3+ ions located within the pyrochlore compound. Although pyrochlore is usually a high temperature phase, analysis of the photoluminescence allows its labeling at temperatures as low as 600–700 °C for small Er3+ concentrations (1 mol %). Increasing Er3+ concentration promotes its enrichment at the nanocrystallites surface accomplished by the anatase-to-rutile phase transformation, suggesting that Er3+ ions control the TiO2 nanocrystals surface properties.

Efficient up-conversion red emission from TiO2:Yb,Er nanocrystals.

Usually, up-conversion (UC) green emission is easily observed by using rare-earth doped fluoride nanocrystals. However, preferential red emission is desired for some actual applications especially in biological field. Here, we demonstrated that the dominant UC red emission can be realized by preparing TiO2:Yb,Er nanocrystals under 980 nm exciation. By controlling the crystal symmetry and size via the annealing temperature and Yb3+ ions concentration, the enhanced UC red emission is achieved. The multi-photon relaxation and cross-relaxation mechanisms may be responsible for the energy transform process and in turn the UC emission.

Sol-gel synthesis of biocompatible Eu3+/Gd3+ co-doped calcium phosphate nanocrystals for cell bioimaging

Herein, a polyacrylic acid sol-gel method was presented to synthesize rare earth doped calcium phosphate nanocrystals (RE-CaP) as cell bioimaging agent. The effects of calcination temperature, calcination time and PAA concentration on phase composition, morphology, size and luminescence were discussed. In addition, the hemolysis, cytotoxicity, cell uptake and cell imaging of resulting RE-CaP nanocrystals were investigated. Results show that the increases in calcination temperature and time promote the transformation of product from impure crystalline phase (CaCO3, CaO) to pure CaP phase. The rise of calcination temperature results in significant enhancement of luminescence intensity. PAA concentration does not influence the crystalline phase composition and its increase leads to a little increase in luminescence intensity. In addition, the co-doping of Gd3+ improves the luminescence intensity of Eu3+ emissions. This method achieves the doping of Eu3+ ions in the crystallographic Ca2+ I and Ca2+ II sites of HAP with a preferable occupation at Ca2+ II site. The resulting RE-CaP nanocrystals (PAA: 0.6mg/mL, Eu/Gd = 2:1.5, 600°C, 3h) show good biocompatibility and can be easily uptaken by cancer cells. The internalized nanocrystals show stable and strong red luminescence under visible wavelengths excitation. In summary, PAA sol-gel method can be used to synthesize biocompatible RE-CaP luminescent nanomaterials for cell bioimaging.

Effects of surface functionalization of hydrophilic NaYF4 nanocrystals doped with Eu3+ on glutamate and GABA transport in brain synaptosomes

Specific rare earth doped nanocrystals (NCs), a recent class of nanoparticles with fluorescent features, have great bioanalytical potential. Neuroactive properties of NaYF4 nanocrystals doped with Eu3+ were assessed based on the analysis of their effects on glutamate- and γ-aminobutyric acid (GABA) transport process in nerve terminals isolated from rat brain (synaptosomes). Two types of hydrophilic NCs were examined in this work: (i) coated by polyethylene glycol (PEG) and (ii) with OH groups at the surface. It was found that NaYF4:Eu3+-PEG and NaYF4:Eu3+-OH within the concentration range of 0.5–3.5 and 0.5–1.5 mg/ml, respectively, did not influence Na+-dependent transporter-dependent l-[14C]glutamate and [3H]GABA uptake and the ambient level of the neurotransmitters in the synaptosomes. An increase in NaYF4:Eu3+-PEG and NaYF4:Eu3+-OH concentrations up to 7.5 and 3.5 mg/ml, respectively, led to the (1) attenuation of the initial velocity of uptake of l-[14C]glutamate and [3H]GABA and (2) elevation of ambient neurotransmitters in the suspension of nerve terminals. In the mentioned concentrations, nanocrystals did not influence acidification of synaptic vesicles that was shown with pH-sensitive fluorescent dye acridine orange, however, decreased the potential of the plasma membrane of synaptosomes. In comparison with other nanoparticles studied with similar methodological approach, NCs start to exhibit their effects on neurotransmitter transport at concentrations several times higher than those shown for carbon dots, detonation nanodiamonds and an iron storage protein ferritin, whose activity can be registered at 0.08, 0.5 and 0.08 mg/ml, respectively. Therefore, NCs can be considered lesser neurotoxic as compared to above nanoparticles.

Physical Background for Luminescence Thermometry Sensors Based on Pr3+:LaF3 Crystalline Particles

The main goal of this study was creating multifunctional nanoparticles based on rare-earth doped LaF3 nanocrystals, which can be used as fluorescence thermal sensors operating over the 80–320 K temperature range including physiological temperature range (10–50°C). The Pr3+:LaF3 (CPr = 1%) microcrystalline powder and the Pr3+:LaF3 (CPr = 12%, 20%) nanoparticles were studied. It was proved that all the samples were capable of thermal sensing into the temperature range from 80 to 320 K. It was revealed that the mechanisms of temperature sensitivity for the microcrystalline powder and the nanoparticles are different. In the powder, the 3P1 and 3P0 states of Pr3+ ion share their electronic populations according to the Boltzmann and thermalization of the 3P1 state takes place. In the nanoparticles, two temperature dependent mechanisms were suggested: energy migration within 3P0 state in the temperature range from 80 K to 200 K followed by quenching of 3P0 state by OH groups at higher temperatures. The values of the relative sensitivities for the Pr3+:LaF3 (CPr = 1%) microcrystalline powder and the Pr3+:LaF3 (CPr = 12%, 20%) nanoparticles into the physiological temperature range (at 45°C) were 1, 0.5, and 0.3% °C−1, respectively.

Visualization of melanoma tumor with lectin-conjugated rare-earth doped fluoride nanocrystals.

AimTo develop specific fluorescent markers for melanoma tumor visualization, which would provide high selectivity and reversible binding pattern, by the use of carbohydrate-recognizing proteins, lectins, combined with the physical ability for imaging deep in the living tissues by utilizing red and near infrared fluorescent properties of specific rare-earth doped nanocrystals (NC).MethodsB10F16 melanoma cells were inoculated to C57BL/6 mice for inducing experimental melanoma tumor. Tumors were removed and analyzed by lectin-histochemistry using LABA, PFA, PNA, HPA, SNA, GNA, and NPL lectins and stained with hematoxylin and eosin. NPL lectin was conjugated to fluorescent NaGdF4:Eu3+-COOH nanoparticles (5 nm) via zero length cross-linking reaction, and the conjugates were purified from unbound substances and then used for further visualization of histological samples. Fluorescent microscopy was used to visualize NPL-NaGdF4:Eu3+ with the fluorescent emission at 600-720 nm range.ResultsNPL lectin selectively recognized regions of undifferentiated melanoblasts surrounding neoangiogenic foci inside melanoma tumor, PNA lectin recognized differentiated melanoblasts, and LCA and WGA were bound to tumor stroma regions. NPL-NaGdF4:Eu3+ conjugated NC were efficiently detecting newly formed regions of melanoma tumor, confirmed by fluorescent microscopy in visible and near infrared mode. These conjugates possessed high photostability and were compatible with convenient xylene-based mounting systems and preserved intensive fluorescent signal at samples storage for at least 6 months.ConclusionNPL lectin-NaGdF4:Eu3+ conjugated NC permitted distinct identification of contours of the melanoma tissue on histological sections using red excitation at 590-610 nm and near infrared emission of 700-720 nm. These data are of potential practical significance for development of glycans-conjugated nanoparticles to be used for in vivo visualization of melanoma tumor.

Synthesis and luminescence properties of transparent nanocrystalline GdF3:Tb glass-ceramic scintillator

Transparent glass-ceramic containing rare-earth doped halide nanocrystals exhibits enhanced luminescence performance. In this study, a glass-ceramic with Tb doped gadolinium fluoride nanocrystals embedded in an aluminosilicate glass matrix is investigated for X-ray imaging applications. The nanocrystalline glass-ceramic scintillator was prepared by a melt-quench method followed by an anneal. The GdF3:Tb nanocrystals precipitated within the oxide glass matrix during the processing and their luminescence and scintillation properties were investigated. In this nanocomposite scintillator system, the incorporation of high atomic number Gd compound into the glass matrix increases the X-ray stopping power of the glass scintillator, and effective energy transfer between Gd3+ and Tb3+ ions in the nanocrystals enhances the scintillation efficiency.

Narrow Optical Homogeneous Linewidths in Rare Earth Doped Nanocrystals

A homogeneous linewidth of 85.6±4.4 kHz is reported in 60 nm Eu3+ doped Y2O3 nanocrystals at 1.3 K. This linewidth was measured by two-pulse photon echoes on highly scattering powders using heterodyne detection. Spectral diffusion was also investigated by three-pulse photon echoes and resulted in a limited broadening of the homogenous linewidth of about 250 kHz over 120 μs. Compared to achievable Rabi frequencies, in the range of several MHz, these values show that rare earth doped nanocrystals can be useful for applications in optical quantum information processing.

Enhanced Luminescent Properties of Solution Combustion Synthesized Nanocrystalline Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>:Eu<sup>3+</sup> Phosphors

Nanocrystalline Y3Al5O12:Eu 3+ phosphors with particle size about 20-40 nm have been synthesized by a facile solution com- bustion method. XRD and TEM studies show that Y3Al5O12 nanocrystals can directly form at a low temperature of 825 � and highly crystalline at 850 �. With the increase of annealing temperature, the charge transfer band shows a blue shift due to the dielectric con- finement effect. The color purity of Y3Al5O12:Eu 3+ phosphors can be improved by decreasing the grain size of nanocrystals. Furthermore, the high concentration doping of Eu 3+ can be realized in Y3Al5O12 nanocrystals, which will enhance the luminescent intensity. This work demonstrated that solution combustion is a feasible method to synthesize europium rare earth doped Y3Al5O12 nanocrystals with en- hanced optical properties.