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Traditional fluorescent materials used in anti-counterfeiting field usually exhibit monochromatic luminescence at a single-wavelength excitation, which is easily be forged by sophisticated counterfeiters. In this work, Zn2GeO4:Mn, x%Li (x=0 and 20), Zn2GeO4­NaLiGe4O9:Mn, x%Li (x=50 and 70) and NaLiGe4O9:Mn micro-phosphors with the multi-chromatic and multi-mode luminescence have been successfully synthesized via a hydrothermal approach followed by an annealing treatment. As expected that these Li+ doped Zn2GeO4:Mn and NaLiGe4O9:Mn phosphors exhibit a double peak emission including a long green afterglow (~540 nm) and a red photoluminescence (∼668 nm). By tuning Li+ doping concentrations, a gradual colour output and a tuneable afterglow duration are achieved. Particularly, the Zn2GeO4:Mn,Li and NaLiGe4O9:Mn phosphors exhibit excellent performance as security inks for printing luminescent numbers and anti-counterfeiting patterns, which shows an afterglow time-dependent or excitation wavelength-dependent luminescence colour evolution. This work proves the feasibility of Li+ doping strategy in emission tuning, which can stimulate further studies on multi-mode luminescent materials in anti-counterfeiting applications.

For anti-counterfeiting and full-color display applications, adjusting the Yb3+ concentrations or excitation conditions is the basic and simple way to control luminescence color and intensity. However, this approach is significantly limited because the luminescence color is not very sensitive to the response of doped Yb3+ concentrations or excitation conditions. Herein, we have demonstrated that the as-synthesized NaLuF4:Yb/Er and their annealed counterpart LuOF:Yb/Er microrods exhibit upconversion/downshifting double-mode luminescence with complementary and rich colors from green, yellow to red under 980/365/488 nm excitation. The anti-counterfeiting patterns printed with these microrods inks have the ability of the color response to the excitation wavelength, excitation power and laser scanning speed, which make them be hard to duplicate. This study indicates that Yb/Er doping NaLuF4 and their complementary LuOF microrods have great potential in multilevel anti-counterfeiting application and display. In addition, heat treatment way also brings novel ideas to the synthesis of high efficient luminescence materials.

The development of infrared stealth clothing technology and materials has been widely studied. However, the research of near-infrared stealth clothing still faces some challenges including complex preparation processes, narrow spectral ranges, and poor antidetection efficiency. To solve these questions, a CoGaZnSe multilayer film used for anti-near-infrared detection is designed and prepared by pulse laser deposition (PLD) at different pressures from 2 to 12 Pa. Microstructures of the film can transform from amorphous to crystal at different Ar atmospheres demonstrated by X-ray diffraction, Raman spectroscopy, and theoretical calculations. The films with different transmittances from 10 to 95% are combined with multilayer films for reducing the infrared reflection, which results in the lowest energy loss per unit thickness of 1.01 × 10-11 dB/cm. The light propagation in the multilayer is calculated by finite-difference time domain, revealing the regular sinusoidal propagation and absorption in multilayers. The multilayer films are coated on the surface of common clothing materials and tested using an infrared detector in the range of 400-6000 nm. The results prove that the quantum efficiency of infrared detection can be effectively reduced by CoGaZnSe multilayer films, especially for wool (83% in the range of 400-1800 nm and 86% in the range of 2000-6000 nm). Finally, a physical model is established to discover the mechanism of the antireflection and infrared effect and a theory of anti-infrared radiation. This research provides a new idea and method supporting the stealth technology and materials, which improves the efficiency of anti-infrared detection and makes it feasible to be applied in military, detection, and stealth technology.

Heterostructures are an important class of materials with desirable size, morphology, and functionality because of their promising applications in biosensor and photonic communication device. However, no such technology of synthesizing heterojunctions is as convenient as electric welding. Particularly, synthesizing heterojunctions with large lattice‐mismatch has been subjected to a great challenge. Herein, the hybrid heterojunction structures are obtained by in situ chemical transformation from fluoride to oxyfluoride via a laser irradiation‐welding approach (LIWA). Single NaYbF4:Ho3+@YbOF:Ho3+ heterojunctions with unique morphologies including hollow spheres, lollipops and sponge‐like plates, are successfully prepared by deliberate control over precursor particles and light welding parameters. These unique luminescent heterojunctions can not only integrate distinctive properties of fluoride and oxide, but also exhibit the unprecedented convenience for color tunability. This work provides an ultrafast LIWA for the fabrication of lattice‐mismatched heterojunctions in seconds, and LIWA allows tunable laser parameters to control the microstructures and characteristics of heterojunctions for widespread applications.

Traditional luminescent inks used in anti-counterfeiting applications usually consist of single-mode photoluminescence (PL) or up-conversion luminescence (UCL) materials, which are mainly operated in single-level, monochromatic anti-counterfeiting and can be easily counterfeited. Here we report the achievement of quintuple-mode, multi-colour dynamic anti-counterfeiting using multi-mode Zn3Ga2GeO8:Cr3+,Yb3+,Er3+ and Zn1.6Li0.4GeO4:Mn2+ persistent phosphors as the luminescent inks. The Zn3Ga2GeO8:Cr3+,Yb3+,Er3+ phosphors exhibit quintuple luminescence modes of PL, persistent luminescence (PersL), photostimulated luminescence (PSL), photostimulated persistent luminescence (PSPL) and UCL with rich emission colours, while the Zn1.6Li0.4GeO4:Mn2+ phosphors possess quadruple luminescence modes of PL, PersL, PSL and PSPL with a long perceptible green afterglow. A ‘dragonfly-on-lotus’ pattern printed using these multi-mode persistent phosphors shows dynamic evolution of emission colours and brightness under different luminescence modes. The design strategy reported in this study may lead to the design and development of more sophisticated optical anti-counterfeiting technology.

The majority of the fabrication procedures of lanthanide-doped materials involve thermal treatment that often results in crystallite regrowth, stabilizing the specific crystal structure and resulting in luminescence enhancement. The efficiency and intensity of up-conversion luminescence are closely related to the structure and synthesis process of the materials. Herein, well-crystallized and pure tetragonal NaY(MoO4)2 microcrystals with a uniform octahedral shape have been successfully synthesized via an environmentally friendly hydrothermal method, followed by annealing treatment. The phases, structures, morphologies, and compositions of the synthesized products annealed at 500–1000 °C remain unchanged, indicating high thermal stability. Furthermore, the NaY(MoO4)2:Yb3+/Er3+ microcrystals exhibit strong green emission when irradiated using infrared (980 nm) or ultraviolet (378 nm) wavelengths. Upon 980 nm excitation, up to 37-fold luminescence enhancement is achieved when the samples are annealed at about 700 °C. Interestingly, the high colour purity of the strong green emission is not only independent of the dopant concentration and heat treatment temperature, but it is also independent of the excitation conditions, including power and wavelength, and this makes it particularly suitable as a green safety signal light and luminescent security ink in paintings. As-prepared safety inks with NaY(MoO4)2:Yb3+/Er3+ microcrystals were used for visual fingerprint recognition printed on A4 paper with three-level fingerprint security features, significantly increasing the difficulty of illegal imitation and enhancing the levels of anti-counterfeiting.

Rare earth-doped upconversion (UC) micro/nanocrystals have important applications in varying fields including multilevel anti-counterfeiting, photonics, deep-tissue imaging, and therapeutics. However, one of the major hurdles is the weak luminescence associated with the constraints of conventional multi-peak emissions in achieving photoluminescence imaging with high spatial resolution and sensitivity. Herein, a technique for the heat treatment of a NaYF4 maternal lattice is presented for achieving porous one-dimensional NaYF4:Yb/Er@YOF:Yb/Er hybrid structures with excellent optical performance through in situ ion-exchange. In the course of detailed studies, an irreversible spatially confined recrystallization of maternal crystals to different phases is found, which preserves the crystal's original morphology and size. The enhanced single-band dark red luminescence emission has been achieved and the responding mechanisms are proposed based on the control of the UC path by phonon mediation. Particularly, these porous one-dimensional NaYF4:Yb/Er@YOF:Yb/Er hybrid structures have been used as potential security ink for writing, indicating immensely promising results for direct anti-counterfeiting applications. Our results demonstrate that the heat treatment technology opens the door for the synthesis of highly efficient single-band red phosphors with expanded applications.

Traditional fluorescent materials used in the anti-counterfeiting field usually exhibit monochromatic luminescence at a single-wavelength excitation, which is easily forged by sophisticated counterfeiters. In this work, Zn

Upconversion (UC) micro/nanomaterials have enabled many promising applications owing to their unique near-infrared to visible emission characteristics. However, some practical applications are limited by the static and invariant emission colours and patterns of UC materials. Here, we demonstrated a novel dynamic UC luminescence pattern and luminescent heterogeneous spatial distribution mediated by laser irradiation. Interestingly, the distinct red dumbbell pattern was submerged gradually by moving the green stripes from the excitation position at one end of a microtube to the other end along the length direction when elevating the power density of excitation light. The underlying reason for the dynamic luminescence patterns obtained by power density controlling was explored, and a mechanism based on an in situ oxidation reaction on NaYbF4:Ho3+ UC microcrystals was revealed. The oxidation reaction between O2 and NaYbF4:Ho3+ microcrystals was induced by increasing excitation power density and irradiation duration leading to a lattice-mismatched NaYbF4 and YbOF heterojunction. This unique heterojunction architecture not only modifies the profiles and colours of the luminescent patterns, but also separates the processes of light–light conversion and light–heat conversion in time and space. These studies have provided a deeper insight into the local in situ oxidation mechanisms of fluoride crystals and offered a novel route toward the tuning of UC luminescence using a heterojunction.

The majority of the fabrication procedures of lanthanide-doped materials involve thermal treatment that often results in crystallite regrowth, stabilizing the specific crystal structure and resulting in luminescence enhancement. The efficiency and intensity of up-conversion luminescence are closely related to the structure and synthesis process of the materials. Herein, well-crystallized and pure tetragonal NaY(MoO4)2 microcrystals with a uniform octahedral shape have been successfully synthesized via an environmentally friendly hydrothermal method, followed by annealing treatment. The phases, structures, morphologies, and compositions of the synthesized products annealed at 500-1000 °C remain unchanged, indicating high thermal stability. Furthermore, the NaY(MoO4)2:Yb3+/Er3+ microcrystals exhibit strong green emission when irradiated using infrared (980 nm) or ultraviolet (378 nm) wavelengths. Upon 980 nm excitation, up to 37-fold luminescence enhancement is achieved when the samples are annealed at about 700 °C. Interestingly, the high colour purity of the strong green emission is not only independent of the dopant concentration and heat treatment temperature, but it is also independent of the excitation conditions, including power and wavelength, and this makes it particularly suitable as a green safety signal light and luminescent security ink in paintings. As-prepared safety inks with NaY(MoO4)2:Yb3+/Er3+ microcrystals were used for visual fingerprint recognition printed on A4 paper with three-level fingerprint security features, significantly increasing the difficulty of illegal imitation and enhancing the levels of anti-counterfeiting.

Optical hysteresis behavior (OHB) is of great importance for technological applications including optical data switching and manipulation. In this study, the power-dependent luminescent property of a single one-dimensional NaYbF4:X3+ (X = Er, Ho, and Tm) microcrystal was systematically investigated. We observed the interesting OHB that a system exhibits two different optical responses for the same laser input. A series of real colour luminescence patterns with information-rich upconversion cryptography were also obtained, which offers great opportunities for optical barcoding at the single particle level. The intrinsic OHB strongly depends on excitation approach, excitation power density range, Yb3+ concentration and sample crystallinity rather than active center. The underlying mechanisms of the intrinsic OHB and luminescence loss were explored, and a mechanism based on permanent local structural changes and intrinsic defect induced luminescence quenching is proposed. The laser thermal effect was confirmed by decreased luminescence intensity ratio of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2 transitions of Er3+ as the excitation power increased. The luminescent loss includes both irreversible and reversible processes, and these two processes can be distinguished via excitation power cycling experiments. Reversible luminescence loss contributed to defect quenching due to energy migration from Yb3+ to the native defects, and irreversible luminescence quenching originated from the thermally induced permanent local structural changes of microcrystals. This work not only enables a deeper understanding of power-dependent OHB in related systems, but also provides the new dimensions for special applications such as multilevel anti-counterfeiting and intelligent memory storage.

Lanthanide (Ln)-doped upconversion (UC) micro/nanomaterials have attracted significant attention in various applications. However, spatial control of UC emission from micro/nanomaterials has not been developed to date due to the ineffective quantum yield and weak luminescence intensity. In this study, the fascinating luminescent patterns, including a dual-colour dumbbell, sunflower, rod-shaped candy, dichroic plate, and coloured ring from a series of single NaYF4:Yb/Ln (Ln = Er, Ho, and Tm) microcrystals, that exhibit different distinctive distributions on luminescence intensity and colour in space are obtained by varying the excitation modes. The overall integral UC intensity of a single microtube by waveguiding-excitation is invariably stronger relative to that in the case of the spot-excitation approach, and the enhancement factor increases 10 times. We explained the underlying physical mechanisms behind the changes in the colour and intensity of the luminescent pattern based on the native diffraction and interference effect of microcrystals. These findings provide a new way to improve UC luminescence and facilitate a basic understanding of the use of UC micro/nanocrystals as natural interference devices, which will result in new applications in displays, optical waveguides, and anti-counterfeiting.

Applications of rare-earth (Re)-doped upconversion (UC) micro/nanocrystals in varying areas have sparked a growing, constant demand for deliberate control over the luminescent profile and lifetime. A general strategy for adjusting UC luminescence is to control the dopant Re3+ concentration. However, the progress of elevating dopant concentration is significantly limited owing to frequently encountering severe concentration quenching. Herein, by designing UC system using Tm3+ with super long intermediate energy level lifetime and Yb3+ with a simple two-level structure as a activator/sensitizer combination, we have achieved the unusually intense deep ultraviolet (UV) UC emissions at 290 nm and 345 nm, near UV emission at 361 nm, visible blue emissions at 452 nm and 478 nm, and near infrared (NIR) emission at 803 nm in NaYF4:Yb/Tm microtubes under 980 nm excitation. The luminescence intensity reaches about 56 times and 8 times enhancement for the UV and blue emission, respectively, with the increase of dopant Yb3+ contents from 20 to 80 mol%. While luminescence lifetimes for all emissions decrease, particularly, for NIR emission at 803 nm, from 1507 μs down to 384 μs. Importantly, upon 980 nm irradiation, these UC NaYF4:Yb/Tm microtubes exhibit superior optical performance as a promising candidate of advanced anti-counterfeiting applications and other applications including UV light triggered reactions and multiplexing information storage.

The structural, energetic and electronic properties of germanene adsorbed with small nitrogen-based molecules, including N 2 , NH 3 , NO 2 and NO, have been investigated by using first-principles calculations. The results show that all nitrogen-based molecules considered bind much stronger to germanene than to graphene due to the hybridized sp 2 -sp 3 bonding of Ge atoms. The N 2 , NO and NO 2 molecules all act as an acceptor, while the NH 3 molecule donates electrons to germanene. We also found sizable band gaps (2–158 meV) are opened at the Dirac point of germanene through N 2 , NH 3 , and NO 2 adsorptions, but with only slightly destroying its Dirac cone shape. The NO 2 molecule also shows a heavy p-type doping character and makes germanene to be metallic. Moreover, when adsorbed by NO molecule, the germanene can change to be a ferromagnetic half-metal with 100% spin-polarization at the Fermi level. Overall, the different adsorption behaviors of small nitrogen-based gas molecules on germanene provide a feasible way to exploit chemically modified germanene for a wide range of practical applications, such as field-effect transistors, gas sensors and spintronic devices.

Doping and varying pump laser parameters are the widely applied technological processes for tuning spectra to yield desirable luminescence properties and functions. For micro/nanocrystalline materials, doping is of fundamental importance in modifying electronic properties, modulating magnetism, as well as tuning the red-to-green luminescence ratio. Here we describe a tunable upconversion (UC) emission process in single NaYF4:Yb/Er microtubes excitated with a focused laser. We show that the emission colours from single NaYF4:Yb/Er microtubes can be rationally tuned in the red-to-green luminescence ratio and dynamics process by elevating Yb3+ ion concentration or pump power density. The underlying mechanism of spectral tuning is explored by using the power dependent UC luminescence, downconversion spectra and the temporal evolutions of UC emission from a series of single NaYF4:Yb/Er microtubes. A mechanism of the red luminescence enhancement based on mediating electronic energy transfer channels by inducing three-photon processes is proposed for single microtubes. The natural decays of the luminescence levels are modified in the UC process relative to downconversion, which could be interpreted by using rate equations. Here, an insight into UC processes by use of unconventional focused experimental and theoretical techniques indicates the bidirectional feature of the electron transition of the interexcited-state by linear decays and UC processes via controlling external experimental parameters.

The dependencies of EPR g-factors on the local structural parameter and intrinsic crystal-field parameters for a 4f3 configuration ion Nd3+ in Bi4Ge3O12 crystal have been studied by diagonalising 364 × 364 complete energy matrices. The results indicate that the local structure of (NdO6)9− cluster exhibits an oblate distortion in Bi4Ge3O12 crystal, where the ligand ions O2− undergo an outward departure (Δθ1 ≈ 0.92°, Δθ2 ≈ 1.27°) away from the C3 axis. An inflexion with six-order intrinsic parameter Ā6 ≈ 10 cm−1 was found that may well account for previous abnormal g-factors evaluation. Furthermore, our studies show that a stronger covalence between the ligand ions O2− and the central Nd3+ ions may exist in the (NdO6)9− cluster. Also, the high-order mixing effects on EPR parameters have been evaluated. It is shown that the dominant high-order contribution to EPR g-factors is from the J-J mixture between the terms 4I9/2 and 2H9/2, which accounts for about 3.3%.

The strain effect on the electronic and optical properties of Germanene/MoS2 heterobilayer have been investigated by using first principles calculations with vdW corrections. Different stacking patterns of Germanene/MoS2 heterobilayer have been designed, which all show similar electronic and optical properties. Germanene is found to interact with MoS2 monolayer via weak vdW interactions, only leading to a tiny band gap opened with its Dirac cone well preserved. This implies MoS2 monolayer can be a suitable semiconducting substrate for germanene adhension. Through various in-plane external strains, the opened Dirac gap of heterobilayer can be further modulated and a semiconductor-metal transition even occurs. In addition, the Germanene/MoS2 heterobilayer can exhibit much wider optical adsorption range together with a red-shift in adsorption edge compared to individual layers. The position and intensity of optical adsorption peaks in ultraviolet and visible light regions also can be effectively tuned by applying external strains. With the strain controllable electronic and optical properties, Gemanene/MoS2 heterobilayer is expected to be a candidate for applications in new functional nanoelectronic and optoelectronic devices.

We present a facile and highly effective method to tailor upconversion (UC) emission from LaF 3 : Tm 3+ nanoparticles (NPs) by adjusting ambient temperature from 20 K to 400 K accompanied with the pulse laser excitation. Spectral tuning mechanism controlled by ambient temperature at pulse laser excitation is revealed, and a mechanism based on the modification on multi-phonon relaxation rates for the rapid population of intermediate level 3 H 4 and multi-phonon-assisted excited state absorption is proposed. Based on multi-phonon relaxation theory and time-resolved photoluminescence studies, it is reasonable that UC luminescence under short-pulse laser excitation mainly originates from the ions at/near the surface of NPs. These exciting findings in ambient temperature accompanied with the short-pulse excitation dependent UC selectivity offer a general approach to tailoring lanthanide related UC emissions, which will benefit multicolor displays and imaging.

The structures, energies, spontaneous polarized properties and charge densities of 180° domain walls with Mn doping at different lattice positions were investigated in ferroelectric PbTiO3 (PT) using the first-principles calculations. The results show that Mn ion prefers to enter the bulk region rather than the domain wall in terms of energy minimization whether it substitutes for an A-site Pb ion or a B-site Ti ion of PT. In addition the domain wall energy has a minimum value of 124.52 mJ/m2 when Mn ion substitutes for Pb ion in the bulk region. And the spontaneous polarization along z axis increasing and the charge density in the domain wall region reducing could be the essential reasons for improving PT's performance after Mn doping at A site.

Lanthanide doped β-NaYF4 micro/nanocrystals have attracted extensive attentions for various applications by taking advantage of their ability to convert near-infrared into high-energy visible or ultraviolet emission. However, to date, achieving efficient photon upconversion (UC) and distinct far-field luminescent patterns of emission light in UC phosphors is still a fundamental challenge. Here, we report the utilization of the microrod cavity waveguide arrays with the concave top/bottom surfaces to manipulate energy flux and significantly enhance UC processes. By enhancing incident light harvesting and changing Yb3+ concentration, directional luminescence amplification and chrysanthemum-shaped luminescent pattern with various colors can be achieved relative to its disorderly microrod counterpart stackings. This design delineates a strategy of energy flux manipulation to converge a low-power incident light beam into a photonic hotspot of high field intensity and amplify the UC processes resulting in directional luminescence and stereoscopic luminescent patterns.

Hexagonal-phase NaYbF4:Er3+ upconversion nanoparticles (UCNPs) have been synthesized via a co-precipitation method in high-boiling-point solvents, and remarkably enhanced upconversion luminescence, particularly in red emission bands (650–670 nm) in NaYbF4:Er3+ UCNPs, has been achieved by Mn2+ doping. The underlying reason for luminescence enhancement by Mn2+ doping is explored by a series of controlled experiments, and a mechanism of enhancement based on the decrease of Na+-vacancies and organic adsorption is proposed. The Mn2+ substitution disturbs the equilibrium of the charge and crystal lattice in the hexagonal-phase NaYbF4:Er3+ UCNPs, which makes the Na+-vacancies that quenched luminescence become filled with Na+ or Mn2+ to offset the imbalance of the charge and electron cloud distortion. In addition, the Mn2+ doping at the surface of UCNPs could reduce the organic adsorption on the surface of the UCNPs by an extra F− ion on the grain surface resulting in luminescence enhancement. Therefore, the Mn2+-doping approach provides a facile strategy for improvement of luminescence, which will impact on the field of bioimaging based on UCNP nanoprobes.

The pure tetragonal crystal phase LaOF:Eu3+ (2 mol%) nanocrystals (NCs) are obtained by annealing ultrasmall-sized LaF3:Eu3+ (2 mol%) NCs assisted with the surfactant oleic acid. Spectroscopic characterization of Eu3+ in various host materials, including hexagonal-phase LaF3, NaYF4 and NaLaF4, cube-phase NaYF4, tetragonal-phase LiYF4 and LaOF crystals, is performed to tune the luminescence by controlling the host matrices with different compositions or different phase structures of the same inorganic compound. The excitation and emission spectra arising from LaOF:Eu3+ (2 mol%) clearly show that the excitation and emission profiles are different from those of the other matrices. By combining the equation of the spectral conversion with the emission and excitation spectra, it is evident that the efficiency of the spectral conversion in the LaOF host material is superior to that in others, indicating a potential application in solar cells. The underlying reason for the enhanced efficiency of the spectral conversion in the LaOF host material is explored, and a mechanism based on the low local symmetry induced by the shortened bond distance and the asymmetric spatial distribution of the chemical bonds of anions coordinating the rare-earth ions is proposed.

Pr3+/Yb3+ co-doped hexagonal NaYF4(beta-NaYF4) microprisms were synthesized by the hydrothermal method, and ethylenediaminetetraacetic acid (EDTA) was introduced to control the size of the microcrystal samples. Bright upconverted fluorescence emission was observed when the samples were excited with an infrared (IR) laser at 976.4 nm. The emission was found to originate from the transitions of 3P0-3F2, 3P0-3H6 or 1G4-3H4, 3P1-3H6, 3P0-3H5, 3P1-3H5, and 3P0-3H4 of Pr3+ ions. Possible mechanisms for upconversion fluorescence and concentration dependence as well as the crystal structure and its formation of NaYF4:Yb3+/Pr3+ microprisms were explored and discussed based on the experimental observations.

A facile hydrothermal route at designated pH values has been developed to synthesize a series of well-dispersed LaF3 colloidal nanocrystals (NCs) with a rich variety of morphologies, including nanoparticles, hexagonal nanoplates and fullerene-like nanoparticles. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, and photoluminescence spectroscopy were used to characterize the samples. It is found that the formation of monodispersed fluoride NCs not only closely correlates with the pH values of the mother solutions, but also depends on the basicity of the base employed to adjust the pH value. The strong alkaline solution in the absence of any organic additive in this system was found to be a prerequisite for producing hexagonal fullerene-like LaF3 nanodisks. A mechanism for the formation of the fullerene-like LaF3via the local Ostwald ripening process has been proposed based on observations of time-dependent experiments. The multicolor upconversion (UC) emission was successfully realized in a series of Yb3+/Er3+ doped LaF3 NCs by excitation in the near-infrared region. The UC emission ratios of red to green for a series of LaF3 NCs can be tuned by adjusting the pH values of the mother liquids, and an UC mechanism activated by high-energy phonons inherent in the hollow LaF3 nanoplates is proposed. It is expected that these rare-earth fluoride NCs might have potential applications in photocatalysis, biolabelling and drug-delivery.

In this work, the uniform bundle-shaped microtubes composed of six half-pipes are synthesized for the first time in hydrothermal solutions via an intentional delayed phase transition pathway induced by Mn(2+) doping. The structural and kinetic factors that govern the phase and shape evolution of NaYF4 microcrystals have been carefully studied, and the influences of Mn(2+) to RE(3+) ratio, the amount of trisodium citrate, and the pH value in conjunction with the intrinsic character of RE(3+) ions on the phase and shape evolution are systematically discussed. It is found that the proper Mn(2+) to RE(3+) ratio is mainly responsible for delayed phase transition process and induces interior density gradient of solid aggregate for creating hollow bundle-shaped microtubes. While the amount of trisodium citrate and the pH value are the keys for the shape control of the NaYF4 microcrystals such as prismatic microtubes, prismatic short rods, thin plates, and particles. The up and downconversion emissions were obtained independent of whether α- or β-NaYF4:Er(3+)/Yb(3+) samples doped with Mn(2+), but the significant tuning of output color was only obtained in cube NaYF4 nanoparticles rather than in hexagonal microtubes via adjusting the amount of Mn(2+) ions. These spectral measurements and EDX analyses indicate that the distribution or concentration of Mn(2+) in hexagonal phase solid solution has changed, which supports Ostwald ripening growth mechanism and rules out agglomeration or oriented attachment growth mechanism. We designed crystal growth mode by simply addition of dopant may provide a versatile approach for fabricating a wide range of hollow nano/microcrystals and thus bring us a clearer understanding on the interaction between the dopant reagents and the nano/microcrystals.

A series of β-NaYF 4 microrods doped with Yb 3+ /Er 3+ have been successfully synthesized through a facile hydrothermal method. The fluoride microcrystals have been characterized by XRD, SEM, and the photoluminescence spectra method. The upconversion spectra show that the output light color have been tuned from green to red under near infrared excitation at 977 nm when replacing Y 3+ ions with a different amount of Yb 3+ in NaYF 4 :Yb/Er microcrystals. The effect of the intrinsic structural feature of fluoride compounds and four external factors, namely the concentration of Yb 3+ , the concentration of Er 3+ , the particle size and the use of an organic additive (EDTA), on the ultimate output light color of samples are investigated detailedly. The results show that the output light color is more sensitive to the concentration of Yb 3+ than that of Er 3+ . When elevated the concentration of dopants (Er 3+ or Yb 3+ ) in the NaYF 4 host lattice, the cross-relaxation process derived from Er 3+ promoted the red emission and quenched the green emission.

Luminescence-based waveguide is widely investigated as a promising alternative to conquer the difficulties of efficiently coupling light into a waveguide. But applications have been still limited due to employing blue or ultraviolet light as excitation source with the lower penetration depth leading to a weak guided light. Here, we show a quasi-one-dimensional propagation of luminescence and then resulting in a strong luminescence output from the top end of a single NaYF4:Yb3+/Er3+ microtube under near infrared light excitation. The mechanism of upconversion propagation, based on the optical waveguide effect accompanied with energy migration, is proposed. The efficiency of luminescence output is highly dependent on the concentration of dopant ions, excitation power, morphology and crystallinity of tube as an indirect evidence of the existence of the optical actived waveguide effect. These findings provide the possibility for the construction of upconversion fiber laser.

Europium doped lanthanide fluoride (LaF3) nanoparticles were prepared through a hydrothermal method with different PH values of precursor complex solution. The influence of PH value on the luminescence properties is investigated. It was found that the local symmetry of doped ions reduced with the increase of PH value, leading to the increase of the inversion symmetry ratio. The fluorescence quenching was observed for small nanoparticles, which was attributed to the large amount of OH groups absorbed to the surface of the nanoparticles.

Fluorescence emission of Ln3+ doped nanoparticles is influenced by many factors. Enhancement of fluorescence emission of Ln3+ doped nanoparticles can be achieved either through an external approach or an internal adjustment. The external approaches mainly include application of core/shell configuration and metal or semiconductor nanoparticles. Varying the crystal phase of the nanoparticle or introducing a codopant sensitizer are the internal approaches frequently used to obtain fluorescence enhancement, which are through inner adjustments to change the local structure, local symmetry and interaction with active ions. In the current work, efforts to obtain luminescence enhancement in Ln3+ doped nanoparticles through external or internal approaches are reviewed. The important roles of local symmetry and asymmetric interactions with optically active ions are extensively discussed.

Efficient up and down frequency conversions in Tm(3+) and Ho(3+) doped LaOF tetragonal nanocrystals have been investigated. Bright fluorescence emissions are obtained in co-doped Tm(3+)/Ho(3+):LaOF tetragonal nanocrystals through UV and infrared excitation. Green florescence from doped Ho(3+) ions, which can be clearly seen with bared eyes, is obtained when Tm(3+) ion is excited. Specific mechanism of the cross relaxation between doped ions is explored through spectroscopic measurements in time and frequency domains. About 90% energy transfer efficiency is obtained when the weak radiative and nonradiative relaxations are neglected.

Optical dephasing of Pr3+ and Tm3+ ions doped in transparent oxyfluoride glass ceramics was studied with the two-pulse photon echo technique. It was found that the dephasing time of rare earth ions is dramatically less in nanocrystals embedded in a glass matrix than in bulk crystals. Aquasi-linear temperature dependence obtained at low temperatures proved that the long-range interaction of the ions inside the nanocrystals with the two level systems of the glass matrix dominates the optical dephasing. The local thermal effect in glass ceramics containing nanocrystals elevates the local temperature, which results in the reduction of optical dephasing time. For Tm3+-doped glass ceramics, the elevation of local temperature induced by the irradiation of excitation laser even quenched the photon echo signals in the experimental study.

We demonstrate a versatile approach to fine-tuning the ratio of blue to near-infrared emission intensity from Tm3+ ions in LaF3 nanocrystals by adjusting the pulse widths and excitation wavelengths of red laser beams. The mechanism of color-tunable Tm3+ emission by pulse widths is explored, and a mechanism based on promoting the population of some luminescence levels and cutting off the population of others by suitably adjusting pulse duration is proposed. The underlying reason of excitation wavelength-modulated emission is ascribed to tuning absorption probability ratio of ground state absorption to excited state absorption by tuning the matching degree between the energies of excitation wavelength and ground (excited) state absorption of Tm3+. The ability of our LaF3:Tm3+ nanocrystals to emit variable emissions on demand in response to pulse width and excitation wavelength provides keen insights into controlling the population processes of luminescent levels and offers a versatile approach for tuning the spectral output.

Hexagonal-phase NaYbF4:Er(3+) upconversion nanoparticles (UCNPs) have been synthesized via a co-precipitation method in high-boiling-point solvents, and remarkably enhanced upconversion luminescence, particularly in red emission bands (650-670 nm) in NaYbF4:Er(3+) UCNPs, has been achieved by Mn(2+) doping. The underlying reason for luminescence enhancement by Mn(2+) doping is explored by a series of controlled experiments, and a mechanism of enhancement based on the decrease of Na(+)-vacancies and organic adsorption is proposed. The Mn(2+) substitution disturbs the equilibrium of the charge and crystal lattice in the hexagonal-phase NaYbF4:Er(3+) UCNPs, which makes the Na(+)-vacancies that quenched luminescence become filled with Na(+) or Mn(2+) to offset the imbalance of the charge and electron cloud distortion. In addition, the Mn(2+) doping at the surface of UCNPs could reduce the organic adsorption on the surface of the UCNPs by an extra F(-) ion on the grain surface resulting in luminescence enhancement. Therefore, the Mn(2+)-doping approach provides a facile strategy for improvement of luminescence, which will impact on the field of bioimaging based on UCNP nanoprobes.

Pr3+/Yb3+ co-doped hexagonal NaYF4(beta-NaYF4) microprisms were synthesized by the hydrothermal method, and ethylenediaminetetraacetic acid (EDTA) was introduced to control the size of the microcrystal samples. Bright upconverted fluorescence emission was observed when the samples were excited with an infrared (IR) laser at 976.4 nm. The emission was found to originate from the transitions of 3P0-3F2, 3P0-3H6 or 1G4-3H4, 3P1-3H6, 3P0-3H5, 3P1-3H5, and 3P0-3H4 of Pr3+ ions. Possible mechanisms for upconversion fluorescence and concentration dependence as well as the crystal structure and its formation of NaYF4:Yb3+/Pr3+ microprisms were explored and discussed based on the experimental observations.