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Photocatalytic technology exhibits promising prospects in environmental remediation owing to its sustainable and environmentally friendly advantages. Among the bismuth-rich halide oxides, Bi₄O₅I₂ has demonstrated remarkable efficacy in dye degradation due to its favorable valence band and conduction band positions. In this study, we successfully synthesized Bi₄O₅I₂/NaYF₄:Yb,Tm composites through a solvothermal method. When exposed to visible light, the Bi₄O₅I₂/NaYF₄:Yb,Tm composite achieved an impressive degradation rate of 86.7% for RhB solution after 60min of light-induced reaction. Moreover, the incorporation of NaYF₄:Yb,Tm into Bi₄O₅I₂ extended the utilization of near-infrared (NIR) spectroscopy. Under 980nm NIR light irradiation, the degradation rate of Rhodamine B (RhB) solution by the Bi₄O₅I₂/NaYF₄:Yb,Tm composite reached 43.0% after 240min of light reaction. Free radical capture experiments confirmed that h${}^{+}$ and • O${}^{2-}$ played a significant role as the primary active species in the degradation process of RhB by the Bi₄O₅I₂/NaYF₄:Yb,Tm composites. Furthermore, we explored the mechanism behind the photocatalytic degradation of RhB solution using the Bi₄O₅I₂/NaYF₄:Yb,Tm composites. Bi₄O₅I₂/NaYF₄:Yb,Tm holds great potential as a promising candidate for utilization of NIR light for photocatalytic reactions.

Zr:Nd:LiNbO₃ crystals codoped with 0.1 mol% of Nd₂O₃ and three concentrations of ZrO₂ (0, 2 and 4 mol%) were grown by the Czochralski method from the congruent melt. The X-ray diffraction (XRD) patterns, UV–visible absorption and infrared (IR) spectra were measured to analyze the crystal composition and defect structure. The blue and green upconversion emissions of Nd³⁺ ions under 598 nm excitation were observed. The intensity of upconversion emissions was increased by the introduction of 2 mol% zirconium ion (Zr⁴⁺) and decreased by the introduction of 4 mol% Zr⁴⁺ ions. The luminescence decay measurement indicated that the ⁴G_7/2 state of Nd³⁺ ion was mainly populated by excited state absorption process. It was proposed that Nd:LiNbO₃ crystals doped with approximately 2 mol% Zr⁴⁺ ions could be applied as laser materials at 522/535 nm.

Size-dependent upconversion properties for Er³⁺-doped Y₂O₃ nano-crystal have been investigated. The investigation shows that the reduction of the particle size intensifies the hypersensitive ²H_11/2 → ⁴I_15/2 transition, decreasing the intensity ratio of the ⁴S_3/2 → ⁴I_15/2 transition to the ²H_11/2 → ⁴I_15/2 one. The enhanced non-radiative decay probability and the intensified energy transfers in the small particles also enhance the population of the ⁴F_9/2 level, leading to the increase of the intensity ratio of the red to the green one. It is believed that the absorbed hydroxyl and carbonate groups on the surface as well as the lowered symmetry of the local surroundings of the doped Er³⁺ ions near the surface result in the size-dependent luminescence properties in the nano-crystal.

We have further developed widely-tunable monochromatic THz sources. These sources are based on difference-frequency generation (DFG) in GaSe and GaP crystals. Using a 47 mm long GaSe crystal the output wavelength was tuned in the range from 66.5 to 5664 μm (from 150 to 1.77 cm^-1) with the peak powers reaching 389 W. This record-high power corresponds to a conversion efficiency of ~0.1%. On the other hand, using a 20 mm long GaP crystal the output wavelength was tuned in the range 71.1–2830 μm whereas the highest peak power was 15.6 W. The advantage of using GaP over GaSe is obvious: crystal rotation is no longer required for wavelength tuning. Instead, one just needs to tune the wavelength of one mixing beam within the bandwidth of as narrow as 15.3 nm. Most recently, we implemented a new scheme for detecting THz waves based on upconversion at room temperature, i.e. by mixing the THz wave with an infrared laser beam, we observed the upconverted signal at a wavelength just slightly longer than that of the infrared laser. To date the detectable THz power is just an order of magnitude higher than that for a bolometer. This scheme allows us to measure the pulse energy density, wavelength, linewidth, and pulse width of a THz beam at room temperature. Using our widely-tunable monochromatic THz beam, we directly measured the absorption spectra of three different families of the homologues of the chemical vapors.

The upconversion (UC) fluorescences from 300 nm to 500 nm of NaYF₄:La³⁺(Er³⁺, Tb³⁺) nanocrystals under 532 nm laser excitation were investigated at room temperature. These UC luminescences have potential applications in UC lasers.

NaYF₄:Er³⁺, Yb³⁺ crystals were prepared by simple synthetic method. Under 980 nm laser excitation, 408 nm, 539 nm and 655 nm upconversion emissions were recorded. Laser power and signal intensities of the upconverted emissions were obtained to understand the upconversion mechanisms.

We report the strong bistability effect of multiphoton upconversion, observed at room temperature, in Yb${}^{3+}$-sensitized Tm${}^{3+}$:ZrO₂ condensed-phase nanophosphors via the efficient upconversion excitation. The sizable bistability of upconversion from a collection of excited Tm${}^{3+}$ ions exhibit good controllability in polarity only by varying the density of Yb${}^{3+}$ sensitizer. The polarity of bistability converts regularly from clockwise into anti-clockwise cycles as the dopant density grows from 2% to 10% Yb${}^{3+}$. The results suggest a practicable way to control bistable emission of solids at room temperature.

A novel 5,10,15,20-tetrakis-(4-(triazol-1-yl)phenyl)porphyridine compound, namely, Zn[5,10,15,20-tetrakis-(4-(triazol-1-yl)phenyl)porphyridine] (hereafter tagged as 1) was synthesized through a solvothermal reaction with mixed solvents at 413 K. The X-ray single-crystal structure of compound 1 is featured as a two-dimensional (2D) layer-like structure with the zinc ion located at the center of the 5,10,15,20-tetrakis-(4-(triazol-1-yl)phenyl)porphyridine. The macrocycle of the 5,10,15,20-tetrakis-(4-(triazol-1-yl)phenyl)porphyridine is coplanar. The zinc ion has six coordination and coordinates with three porphyridines. The photoluminescence spectra of compound 1 with DMF solution reveal that it shows upconversion red photoluminescence. The time-dependent density functional theory (TDDFT) calculation confirms that this upconversion red photoluminescence originated from the MLCT process (metal to ligand charge transfer). The CCT (Correlated Color Temperature) is 2200 K and the CIE (Commission Internationale de I’Éclairage) chromaticity coordinate is (0.6311, 0.3595) for compound 1. The UV-vis diffuse reflectance curve measured with a solid state sample reveals that compound 1 possesses a 2.75 eV band gap.

The oleate capped NaYF₄:Yb/Er upconversion nanoparticles (OA-UCNPs) were firstly synthesized by the thermal decomposition method and subsequently converted to water-dispersible, ligand-free UCNPs (LF-UCNPs) via an easy acid treatment process. The interaction of LF-UCNPs with bovine serum albumin (BSA) was then investigated. The binding of LF-UCNPs with BSA was confirmed by the static quenching of intrinsic fluorescence of BSA upon addition of LF-UCNPs. The spectra of synchronous fluorescence and circular dichroism (CD) of BSA in the absence and presence of LF-UCNPs revealed that the presence of LF-UCNPs led to a slight conformational change of BSA, suggesting a minor nanotoxicity of LF-UCNPs toward BSA.

In most of the reports on nanocrystal (NC) luminescent, the power depedence of luminescent intensity is studied using a continuous-wave laser with low power density, and the slopes of those different emission bands keep unchanged. In this paper, the up/down conversion of monodisperse Yb/Er: NaGdF₄@Yb: NaGdF₄@Yb: NaNdF₄ NCs under the excitation of 800nm femtosecond laser was detected simultaneously by a home-built optical measurement setup. The power-dependent optical properties under the excitation of femtosecond laser are quite different from the normal situation, which is due to their high peak power and energy transfer processes: Nd${}^{3+}\to {\mathrm{\text{Yb}}}^{3+}\to {\mathrm{\text{Er}}}^{3+}$.

We prepared a series of Gd₂O₃:Yb,Er,Ca (3/1/$x$mol.%, $x$ represents the nominal concentration of Ca element including 0, 2, 3, 4, 5, 7, 10) upconversion nanoparticles (UCNPs) with regular morphology via a wet-chemical route. We observed a simultaneous enhancement of upconversion luminescence (UCL) emission of Gd₂O₃:Yb,Er after Ca${}^{2+}$ ions were doped. When the doping level of Ca${}^{2+}$ reaches its optimal concentrate at 5mol.%, the red and green emissions increased by 6.3 and 11 times, respectively. The potential application of Gd₂O₃:Yb,Er,Ca material as a noninvasion optical thermometry based on FIR technique was investigated. The sensing of temperature at both high and room temperatures was realized by choosing different parameters. The absolute temperature sensitivity ($S_a^{\prime }$) of Gd₂O₃:Yb,Er nanoparticles at 293K reached 0.0875K${}^{-1}$, whereas Gd₂O₃:Yb,Er, 5mol.%Ca was chosen as sensor of high temperature due to its considerable Sa of 0.0060K${}^{-1}$ at 573K. The resultant UCNPs provided a new way for sensitive thermal detection at various target temperatures.

Upconverting lanthanide nanoparticles overcome many of the problems associated with more traditionally used luminescent contrast agents, such as photobleaching, autofluorescence, cytotoxicity and phototoxicity. For this reason, they are an attractive choice for biomedical imaging applications, particularly for imaging in living tissues. The last decade has seen numerous improvements to these nanocrystals, but a comprehensive guide to the synthesis of upconverting lanthanide nanoparticles has not yet been written. Methods vary from paper to paper and from group to group, and results vary between research groups for each method. For this reason, development of these nanoparticles remains a significant endeavor for any research group interested in joining the field. In this review, we look at the varying synthetic methods employed over the last decade and detail methodology for a select few that have been favored in the field.

A series of Ho³⁺/Yb³⁺/Tm³⁺ tridoped GdF₃ nanorods with different dopant concentrations were synthesized by a hydrothermal method. Transmission electron microscope (TEM) images indicate that the length and diameter of the nanorods is about 90 nm and 31 nm, respectively on average. No bright white upconversion light was observed from the samples with different Yb³⁺, Ho³⁺ or Tm³⁺ concentrations. Unexpectedly, the emission color coordinates of the samples after heat treatment move toward the central white region of the chromaticity diagram, and among these samples, the color coordinate (0.349, 0.329) of GdF₃:15% Yb³⁺, 0.1% Ho³⁺, 0.8% Tm³⁺ is the most close to the standard white light (0.333, 0.333). This is unlike previous reports in which white light was achieved via tuning dopant concentration or excitation power. The reasons for the above phenomenon are presented by means of FT-IR spectra and the energy level diagram of dopants.

In this paper, we demonstrate multifunctional upconversion nanoparticles with intense near-infrared emission and unique magnetic properties for dual-modal upconversion luminescent bioimaging and T₂-weighted magnetic resonance imaging. High-quality BaYbF₅:Tm³⁺ nanoparticles are synthesized via a hydrophobic method and then converted to be hydrophilic via a hydrochloric acid treatment. The as-synthesized nanoparticles are cubic phase and about 6 nm in diameter with narrow size distribution. The intense near-infrared emission makes these nanoparticles can be acted as bio-probes in upconversion luminescent bioimaging with deep tissue penetration. Besides, these nanoparticles can also be used as T₂-weighted contrast agents in magnetic resonance imaging due to the high value of relaxation rate (r₂ = 4.05) in 0.55 T. This finding may have further bio-applications in the future due to the high performance of these BaYbF₅:Tm³⁺ nanoparticles in dual-modal bioimaging.

In this study, Yb${}^{3+}$/Ho${}^{3+}$, Yb${}^{3+}$/Tb${}^{3+}$ co-doped, and Yb${}^{3+}$/Ho${}^{3+}$/Tb${}^{3+}$tri-doped phosphate glasses have been prepared by the high-temperature melting method using P₂O₅ as the base material, and these phosphate glasses are characterized as non-crystalline structures by X-ray diffraction. Under 980 nm excitation, highly efficient blue (489 nm), green (545 nm), and red (661 nm) upconversion luminescences can be observed in this glass system, which are attributed to Tb${}^{3+}$: ⁵D₄$\to$⁷F₆, Tb${}^{3+}$: ⁵D₄$\to$⁷F₅, and Ho${}^{3+}$: ⁵F₅$\to$⁵I₈ radiative transitions, respectively. The upconversion emission intensity and excitation power dependence analysis reveal that the blue and green light emissions are three-photon processes, while red light emission is a two-photon process. White light emission can be achieved by adjusting the doping concentration of rare-earth ions in the Yb${}^{3+}$/Ho${}^{3+}$/Tb${}^{3+}$tri-doped phosphate glasses. The CIEchromaticity coordinates of 2Yb${}^{3+}$/0.1Ho${}^{3+}$/0.4Tb${}^{3+}$ tri-doped phosphate glass (0.39, 0.39) under 980 nm excitation are relatively close to those of the standard light source (0.33, 0.33), indicating the potential application of this material in the field of illumination, such asemissive displays, fluorescent lamps and fiber lasers.

Erbium-doped barium titanate (BaTiO₃:Er) xerogel film with a thickness of about 500 nm was formed on the porous strontium titanate (${\text{SrTiO}}_3)$ xerogel film on Si substrate after annealing at 800${}^{\circ }$C or 900${}^{\circ }$C. The elaborated structures show room temperature upconversion luminescence under 980 nm excitation with the photoluminescence (PL) bands at 523, 546, 658, 800 and 830 nm corresponding to ²${\text{H}}_{11/2}\to$⁴${\text{I}}_{15/2}$, ⁴${\text{S}}_{3/2}{\to }^4$${\text{I}}_{15/2}$, ⁴${\text{F}}_{9/2}{\to }^4$${\text{I}}_{15/2}$ and ⁴${\text{I}}_{9/2}\to$⁴${\text{I}}_{15/2}$ transitions of trivalent erbium. Raman and X-ray diffraction (XRD) analysis of BaTiO₃:Er\porous SrTiO₃\Si structure showed the presence of perovskite phases. Its excellent up-conversion optical performance will greatly broaden its applications in perovskite solar cells and high-end anti-counterfeiting technologies.

In this study, Yb³⁺/Ho³⁺, Yb³⁺/Tb³⁺ co-doped, and Yb³⁺/Ho³⁺/Tb³⁺ tridoped phosphate glasses have been prepared by the high-temperature melting method using P2O5 as the base material, and these phosphate glasses are characterized as non-crystalline structures by X-ray diffraction. Under 980 nm excitation, highly efficient blue (489 nm), green (545 nm), and red (661 nm) upconversion luminescences can be observed in this glass system, which are attributed to ${\text{Tb}}^{3+}:{}^5\text{D}{}_4\to {}^7\text{F}{}_6,{\text{Tb}}^{3+}:{}^5\text{D}{}_4\to {}^7\text{F}{}_5$, and ${\text{Ho}}^{3+}:{}^5\text{F}{}_5\to {}^5\text{I}{}_8$ radiative transitions, respectively. The upconversion emission intensity and excitation power dependence analysis reveal that the blue and green light emissions are three-photon processes, while red light emission is a two-photon process. White light emission can be achieved by adjusting the doping concentration of rare-earth ions in the Yb³⁺/Ho³⁺/Tb³⁺ tri-doped phosphate glasses. The CIE chromaticity coordinates of 2Yb³⁺/0.1Ho³⁺/0.4Tb³⁺ tri-doped phosphate glass (0.39, 0.39) under 980 nm excitation are relatively close to those of the standard light source (0.33, 0.33), indicating the potential application of this material in the field of illumination, such as emissive displays, fluorescent lamps and fiber lasers.