Upconversion Luminescence Process Research Articles

Modification of a desktop FFF printer via NIR laser addition for upconversion 3D printing

Traditional photopolymer-based 3D printing methods require sequential printing of thin layers, due to short penetration depths of UV or blue light sources used by these techniques. In contrast, upconversion 3D printing circumvents the layer-by-layer limitation by taking advantage of upconversion luminescence processes and the high penetration depths offered by near-infrared (NIR) lasers, allowing for selective crosslinking of voxels at any depth or position within the resin container. The implementation of this technique required the construction of a 3D printer with the ability of focusing the laser on any point of the space. For this, a low-cost fused filament fabrication (FFF) printer was modified by incorporating a 980 nm laser and laser control circuit. The total cost of the parts required for modification was £180. With enhanced penetration depths up to 5.8 cm, this method also allows for printing inside or through existing 3D printed parts. This opens doors for restoration of broken items, in situ bioprinting, 3D-circuitry, and notably, 3D printing inside cavities of a different material, illustrating numerous opportunities for practical applications.

Fabrication, microstructure and upconversion luminescence properties of Er:Sr5(PO4)3F transparent nanostructured ceramics

Fluoride-upconversion transparent ceramic materials doped with Er ions have attracted extensive interest due to their higher stability, lower phonon energy, and higher upconversion efficiency. xEr:Sr5(PO4)3F (S-FAP) (x=0.5–5 at%) transparent ceramics have been synthesized by hot pressing sintering in this work, and the maximum optical transmittance of the sample reached 74.52 and 82.45% at 500 and 1000 nm, respectively. The XRD results of the powder and ceramic samples were consistent with the diffraction characteristics of the hexagonal S-FAP crystal structure. The SEM and TEM images of powder revealed that the morphology of the sample was composed of mostly rice grains with a diameter of around 20 nm. The XPS and EDS results confirmed the presence of Er3+ and the uniform distribution of Er elements. The thermally etched surface and cross section of ceramics revealed residual pores as the most important scattering source, and the SEM images of the ceramics show that Er doping can obviously promote the growth of the average grain size of S-FAP transparent ceramics. Furthermore, the upconversion luminescence characteristics and fluorescence lifespan of S-FAP transparent ceramics with varied Er doping concentrations were examined, as well as the upconversion luminescence process. The findings in this work indicate that Er:S-FAP transparent ceramics can be employed as a potential upconversion fluorescent material in display technology, biomedicine, temperature sensing, and other applications.

Down- and up-conversion luminescence processes in olivine-type ceramic phosphors Li2AGeO4:Er3+ (A = Zn, Mg)

Luminescence properties of olivine-type germanate phosphors with general formula Li2AGeO4:0.5Er3+ (A = Zn, Mg) have been studied. Olivine-type materials prepared via conventional solid-state reaction method were examined for down- and up-conversion luminescence processes. In the down-conversion process after 381 nm excitation, two strong green emission bands due to 2H11/2/4S3/2 → 4I15/2 transitions of Er3+ were registered, with a change in the intensity ratio depending on the ceramic matrix. Weak red emission bands related to 4F9/2 → 4I15/2 transition were also recorded. In contrast, luminescence bands behaved completely differently for the up-conversion process after infrared excitation at λexc = 980 nm. Red emission bands were much more significant than green emission bands. These effects are stronger for Li2MgGeO4:0.5Er3+ than Li2ZnGeO4:0.5Er3+. In the red and green spectral range, slope values for both matrices Li2AGeO4:0.5Er3+ (A = Zn, Mg) confirmed two-photon process.

Experimental optimization design synthesis and up-conversion luminescence properties of YNbO4:Ho3+/Yb3+

In order to obtain the maximum green up-conversion luminescence intensity of Ho3+/Yb3+ co-doped YNbO4, a uniform design and a quadratic general rotation combination design were applied to optimize the doping concentration. The concentration of the rare earth corresponding to the strongest green luminescence intensity was determined to be 10% Ho3+/34.9% Yb3+. Maximum luminous intensity of green light tested under 980 nm excitation was 157373.266, which was close to the theoretical calculated integral intensity value of 157290.825. The variation of the up-conversion luminescence spectra at different power density of the 980 nm laser was characterized. According to the formula fitting, the up-conversion luminescence process is a two-photon process. Meanwhile, the temperature sensing characteristic of the samples has been discussed. Finally, the CIE coordinates were analyzed and calculated to be (0.297, 0.692).

Air-tolerant upconverted circularly polarized luminescence enabled by confined space of chiral micelle.

Circularly polarized luminescence (CPL) has been widely demonstrated that the circular polarization in excited state can be significantly amplified through the triplet-triplet annihilation-based upconversion (TTA-UC) luminescence process in various chiral nano-assemblies. However, constructing such an upconverted circularly polarized luminescence (UC-CPL) system in the aqueous phase remains a challenge. In this work, a kind of amphiphilic chiral cationic gemini surfactant is utilized to construct chiral spherical micelle in the aqueous phase, whose internal chiral cavity can provide a hydrophobic and deoxygenated environment for air-sensitive TTA-UC system. In addition, due to the co-assembly process between the emitters and chiral micelles, achiral emitters of upconversion pairs exhibit induced chiroptical properties. More importantly, the luminescence dissymmetry factor (glum ) can be amplified by one order of magnitude through TTA-UC process. This work provides an effective and useful strategy for realizing UC-CPL in aqueous phase.

Multi-shell structured nanocarriers with enhanced multiphoton upconversion luminescence efficiency for NIR-mediated targeted photodynamic therapy

Recently, upconversion nanoparticle (UCNP)-based photodynamic therapy (PDT) has attracted attention as an anticancer treatment that can address the limitations of light penetration depth to activate commercial photosensitizers (PSs) using the upconversion luminescence (UCL) process. However, the low UCL intensity of UCNPs and the overheating effect by the 980 nm NIR laser significantly reduced the therapeutic efficacy. To overcome these problems, it is essential to form UCNP with a multi-shell structure by varying the feed composition of Yb3+, which can improve the UCL intensity. To generate highly efficient theranostic systems, we prepared a multi-shell structured UCNP (UCNC60S3) system with enhanced UCL efficiency in this study that can activate two types of PSs with one laser irradiation and have tumor targeting properties. UCNC60S3 capable of emitting multiple wavelengths with enhanced photoluminescence efficiency (PL) were synthesized by a thermal decomposition method with two kinds of activators in a single nanoparticle. The UCNC60S3 with a multi-shell structure showed 267.6 and 387.3 fold enhancements in UCL efficiency than the core UCNP at wavelengths of 545 nm and 660 nm, respectively. In addition, multi-shell structured UCNPs combined with dual PS and tumor-targeted ligands (F-UCNC60S3-RC) exhibited a significant improvement in the therapeutic effect on cancer cells by near-infrared (NIR) irradiation. When an 808 nm laser at 2 W/cm2 intensity was irradiated for 5 min, F-UCNC60S3-RC showed 10 times higher phototoxicity to cancer cells than free PS. Therefore, the NIR-induced F-UCNC60S3-RC nanocarriers with a multi-shell structure and dual PS binding will be able to provide multiple emissions for bio-imaging as well as improved targeted cancer therapy.

Regulative control and enhancement of multi-color upconversion luminescence with DBR cavities

Upconversion luminescence (UCL) of lanthanide-doped materials (e.g. NaYF4:Yb3+/Er3+) involves multi-step, multi-channel transitions (corresponding to multi-color emissions) in a multi-level system, and is a low-efficiency nonlinear process. Usually, the UCL is enhanced by local enhancement of the excited field or one of its multi-color emissions is promoted by matching with a resonance cavity mode based on Purcell effect. Here, we propose to regulatively control and enhance the UCL by fostering one color of the UCL emissions with a resonance mode and inhibiting the other with an anti-resonance or nonresonance mode in forbidden band in an optical cavity, so that excited-state ions (e.g. Er3+) transit to the ground state more via the fostered UCL emission channel, rather than the inhibited one. As such, high-contrast single-color UCL emission can be achieved with an enhancement beyond that of Purcell effect. For the purpose, superior mode properties of distributed Bragg reflector (DBR) cavities can be applied, whose forbidden band of the DBR layers and resonance cavity mode in the forbidden band can be independently tuned in positions to match with the UCL emissions for them to be inhibited and fostered respectively. In experimental implementation, multi-color UCL (e.g. red and green) of NaYF4:Yb3+/Er3+ nanoparticles embedded in such DBR cavities are studied. And high-contrast single-color UCL emissions are demonstrated with enhancements factors beyond Purcell factors calculated in numerical simulations. A hypothesis on modifications of intermediate transitions in the UCL processes, as feedbacks to influences of mode characteristics in the DBR cavities for regulative control, is also proposed to explain the phenomena. The work suggests a way to regulatively control multi-channel photon emissions in multi-level systems for enhanced single-channel photon emission.

Surface crystallization enhanced upconversion luminescence in NaGdF4:Yb,Er@NaYF4 core/shell nanocrystals

Coating upconversion nanocrystals with an inert shell for removing surface defects is confirmed to be an efficient way for remarkably enhancing upconversion luminescence. However, it is not clear whether surface atomic structures on the outer inert shell affect upconversion luminescence of active cores. Here, we improved the surface crystallinity of NaGdF4:Yb,Er@NaYF4 active-core/inert-shell nanocrystals using ultrasonic oscillation. It was observed that surface crystallization of the inert shell could lead to an obvious enhancement in upconversion luminescence. The improved surface crystallinity has no obvious influence on the electronic transition in upconversion luminescence processes, but decrease the scattering and absorption to excitation and emission photons. This result clearly indicates that surface crystallization of the inert shell is a powerful route for further enhancing upconversion luminescence besides of the concept of core/shell structure.

Light–Heat and Light–Light Dual-Controlled Upconversion Luminescence and Photochromic Processes in Er-Activated Bi7Ti4NbO21 for Optical Information Storage

Light–Heat and Light–Light Dual-Controlled Upconversion Luminescence and Photochromic Processes in Er-Activated Bi₇Ti₄NbO₂₁ for Optical Information Storage

Efficient near infrared to visible light upconversion from Er/Yb codoped PVDF fibrous mats synthesized using a direct polymer doping technique

Upconverting-materials are capable of converting near infra-red (NIR) excitation into lower wavelengths. These materials could have a myriad of promising potential applications. However, in some areas, its use has been hindered by its low upconversion luminescence (UCL) efficiency and processing/handling disadvantages from luminescent materials in powder form. This work presents a step towards overcoming this limitation, by developing polymer-based fiber membranes without adding external luminescent powders. Herein, we report on the synthesis of Yb3+ and Er3+ codoped polyvinylidene difluoride (PVDF) and its development into fiber membranes using the Forcespinning® technique. X-ray diffraction (XRD), Fourier transformed infrared (FTIR) spectroscopy were performed to confirm the phase formation, crystallinity and the presence of vibrational bands of the corresponding PVDF matrix. Scanning Electron microscopy (SEM) and thermogravimetric analysis were conducted to investigate the morphological and thermal properties of the codoped nanofibers while the X-ray photoelectron spectroscopy (XPS) showed efficient doping of the lanthanides in the PVDF fiber. Under exposure to 980 nm NIR illumination, the PVDF:Yb3+,Er3+ fibrous mats exhibited efficient UCL emission in the visible wavelength region (523, 540 and 656 nm). The observed results show a NIR to visible UCL process where the Yb3+ ions act as the sensitizer for the generation of visible upconversion emission from Er3+ ions through an excited state absorption (ESA) and two photon energy transfer (ET) mechanisms. The developed material further opens potential lighting and imaging related applications given the ease of preparation, handling, and flexibility of the developed membranes.

Efficient dual-mode emissions of high-concentration erbium ions doped lead-free halide double perovskite single crystals

Lead-free halide double perovskites have attracted great attention due to the intriguing optoelectronic properties. However, the related researches mainly focus on their down-shifting (DS) luminescence process，while the up-conversion (UC) luminescence process is still rarely studied. Herein, Er3+ ions doped Cs2Ag0.6Na0.4InCl6:0.3%Bi double perovskite single crystals were prepared by an improved hydrothermal method, and the efficient UC and DS emission of Er3+ ions were realized by high-level doping concentration of Er3+ ions. Interesting, the UC green emission of Er3+ ions can be all obtained under 808，980 and 1540 nm excitation, and the optimal photoluminescence quantum yield (PLQY) is 0.17%. To be emphasized, it is the UC luminescence of rare earth ions doped semiconductor materials, which is more advantages in the application of optoelectronic devices than the UC luminescence of rare earth ions doped insulator materials (e.g, NaYF4). Moreover, with the increase of Er3+ ions doping concentration, the DS luminescence color can be controlled from yellow to green. And the near-infrared (NIR) light with the peak at 1540 nm is simultaneously emitted, and the highest PLQY reaches 38.36%, which is suitable for optical communication. Notably, the NIR light (1540 nm) can be also obtained under 808 and 980 nm excitation, which has positive significance for biological applications. This work demonstrates that rare earth ions doping has the potential to impart new functions to double perovskites.

A Synergy Approach to Enhance Upconversion Luminescence Emission of Rare Earth Nanophosphors with Million-Fold Enhancement Factor

Lanthanide (Ln3+)–doped upconversion nanoparticles (UCNPs) offer an ennormous future for a broad range of biological applications over the conventional downconversion fluorescent probes such as organic dyes or quantum dots. Unfortunately, the efficiency of the anti−Stokes upconversion luminescence (UCL) process is typically much weaker than that of the Stokes downconversion emission. Albeit recent development in the synthesis of UCNPs, it is still a major challenge to produce a high−efficiency UCL, meeting the urgent need for practical applications of enhanced markers in biology. The poor quantum yield efficiency of UCL of UCNPs is mainly due to the fol-lowing reasons: (i) the low absorption coefficient of Ln3+ dopants, the specific Ln3+ used here being ytterbium (Yb3+), (ii) UCL quenching by high−energy oscillators due to surface defects, impurities, ligands, and solvent molecules, and (iii) the insufficient local excitation intensity in broad-field il-lumination to generate a highly efficient UCL. In order to tackle the problem of low absorption cross-section of Ln3+ ions, we first incorporate a new type of neodymium (Nd3+) sensitizer into UCNPs to promote their absorption cross-section at 793 nm. To minimize the UCL quenching induced by surface defects and surface ligands, the Nd3+-sensitized UCNPs are then coated with an inactive shell of NaYF4. Finally, the excitation light intensity in the vicinity of UCNPs can be greatly enhanced using a waveguide grating structure thanks to the guided mode resonance. Through the synergy of these three approaches, we show that the UCL intensity of UCNPs can be boosted by a million−fold compared with conventional Yb3+–doped UCNPs.

Up-conversion luminescence properties and temperature sensing behavior of Nd3+/Yb3+/Tm3+ triply doped LaNbO4 phosphors under 808 nm and 980 nm excitations

LaNbO4: Nd3+/Yb3+/Tm3+ phosphors were synthesized via a solid-state reaction method. Under both 808 nm and 980 nm excitations, the effects of Tm3+ concentration and temperature on the up-conversion luminescence properties of Tm3+ were investigated. From the excitation power densities dependent up-conversion emission intensities, it was confirmed that the up-conversion luminescence process of Tm3+ was insensitive to sample temperature within the temperature range we studied. Under the excitations of 808 nm and 980 nm, both blue and red up-conversion emissions of Tm3+ kept two-photon and three-photon processes at various temperatures. And the near-infrared (NIR) up-conversion emission of Tm3+ was two-photon process when excited at 980 nm. Variation of the fluorescence intensity ratio (FIR) based upon the two thermally coupled energy levels 3F2,3 and 3H4 of Tm3+ with temperature was investigated and the temperature sensing behavior of Tm3+ was studied under 980 nm excitation. In the temperature range from room temperature to 663 K, the absolute temperature sensitivity of Tm3+ increases monotonously with the elevation of temperature, and the maximum value was obtained to be 3.702 × 10-4 K-1 at 663 K.

Nonlinear Optical Thermometry—A Novel Temperature Sensing Strategy via Second Harmonic Generation (SHG) and Upconversion Luminescence in BaTiO3:Ho3+,Yb3+ Perovskite

AbstractNonlinear optical spectroscopy may be a powerful tool for sensing of various intrinsic properties of materials and different state functions of the system. This is due to the strong dependence of nonlinear phenomena on numerous physicochemical factors. The feasibility of simultaneously employing the second harmonic generation (SHG) and upconversion luminescence (UCL) processes in BaTiO3:Ho3+,Yb3+ for optical temperature sensing is demonstrated for the first time. Under 976 nm laser excitation, the evolution of the SHG and UCL band intensity ratio is correlated with temperature and calibrated within the temperature range of 25–305 °C. The band intensity ratio between SHG and UCL exhibits a sigmoidal dependence on temperature, and, hence, it can allow the detection of phase transitions from non‐centrosymmetric to centrosymmetric systems, and vice versa. Most importantly, from the perspective of optical temperature sensing, this work provides a novel and effective strategy for nonlinear optical thermometry, with high sensitivity of up to 2.78% °C–1.

Er3+/Yb3+ co-doped SiO2-Al2O3-CaO-CaF2 glass: Structure, J-O analysis and fluorescent properties

The fluorosilicate glass was prepared by a conventional melt-quenching method, namely Er3+/Yb3+: SACF. The structure of Er3+/Yb3+: SACF glass was characterized by XRD, FTIR, and Raman spectra. According to the absorption spectra and Judd-Ofelt (J-O) theory, the spectral intensity parameter (Ωλ) were obtained, Ω2 = 4.16 × 10−20 cm2, Ω4 = 1.13 × 10−20 cm2, Ω6 = 0.86 × 10−20 cm2, and compared to the parameters in other glass host matrix. The radiative properties including spontaneous transition probability, fluorescence branching ratio, and a radiative lifetime of Er3+ ion in Er3+/Yb3+: SACF glass were calculated by J-O theory. And the excitation and emission spectra were recorded to characterize the luminescence properties in the visible region. In the near-infrared region, the emission spectra were obtained excited by a 980 nm laser. Also, under the excitation of a 980 nm laser with different pumping power, the up-conversion emission spectra of Er3+/Yb3+: SACF glass was tested, and a possible up-conversion luminescence process in the Er3+/Yb3+ system was analyzed. Finally, the up-conversion luminescence color and chromaticity coordinates were depicted by the CIE1931 chromaticity diagram.

Effects of Bi3+ on down-/up-conversion luminescence, temperature sensing and optical transition properties of Bi3+/Er3+ co-doped YNbO4 phosphors

A series of YNbO4:Bi3+ and YNbO4:Bi3+/Er3+ phosphors were prepared by a conventional high temperature solid–state reaction method. The results of XRD and Rietveld refinement confirm that monoclinic phase YNbO4 samples are achieved. The down-/up-conversion luminescence of Er3+ ions was investigated under the excitation of ultraviolet light (327 nm) and near infrared light (980 nm). Under 327 nm excitation, broad visible emission band from Bi3+ ions and characteristic green emission peaks from Er3+ ions are simultaneously observed, while only strong green emissions from Er3+ ions are detected upon excitation of 980 nm. Remarkable emission enhancement is observed in down-/up-conversion luminescence processes by introducing Bi3+ ions into Er3+-doped YNbO4 phosphors. Pumped current versus up-conversion emission intensity study shows that two-photon processes are responsible for both the green and the red up-conversion emissions of Er3+ ion. Through the study of the temperature sensing property of Er3+ ion, it is affirmed that the temperature sensitivity is sensitive to the doping concentration of Bi3+ ions. By comparing the experimental values of the radiative transition rate ratio of the two green emission levels of Er3+ ions and the theoretical values calculated by Judd-Ofelt (J-O) theory, it is concluded that the temperature sensing property of Er3+ ions is greatly affected by the energy level splitting.

Synthesis and up-conversion properties of Er3+ doped ZnTiO3-Zn2TiO4 composite phosphor

The conventional solid state reaction method was used to prepare Er3+ activated ZnTiO3-Zn2TiO4 composite phosphor. The structure, particle morphology, absorption, and photoluminescent properties were analyzed using x-ray diffraction (XRD), scanning electron microscopy (SEM), UV-Vis-NIR absorption spectrometer, and photoluminescence (PL) spectroscopy, respectively. The XRD confirmed the crystallization of mixed hexagonal ZnTiO3 and cubic spinel Zn2TiO4 phases. The SEM data showed that the prepared phosphor was made up of faceted hexagons. Furthermore, the phosphor was excited in the near-infrared (NIR) region using a laser diode with a wavelength of 980 nm, and the corresponding up-conversion emission lines were observed in the visible range at 543, 553, and 650–670 nm, associated with the 2H11/2 → 4I15/2, 4S1/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions of Er3+ ions, respectively. The interaction mechanisms involved in the up-conversion process of ZnTiO3-Zn2TiO4:Er3+ phosphor are discussed with the help of an energy-level schematic diagram, and the number of the photons involved in the up-conversion luminescence process was estimated.

Lanthanide-Doped Upconversion Nanomaterials: Recent Advances and Applications

Lanthanide-doped upconversion nanomaterials are well known for converting near-infrared (NIR) excitation photons into visible, ultraviolet, and NIR emission photons. Excitation of NIR light offers low autofluorescence background and high penetration depth in biological environments due to the reduced light scattering. Consequently, these have attracted considerable interest because their exceptional optical properties have led to applications in diverse areas such as bioassays, biomedical imaging, and forensics; these properties are, namely: large anti-Stokes shifts, sharp emission spectra and long excited-state lifetimes. The nanomaterials can be utilized for a variety of applications including detection of various analytes, bioimaging, therapies, energy conversion, and security. In this review, we briefly describe the upconversion luminescence process, effective synthetic approaches, and recent literature that elucidate the exceptional properties of these materials and its applications.

Luminous and thermal characteristics of Er3+/Yb3+ co-doped WO3: a heat-sensitive micro-particle

Er3+/Yb3+ co-doped WO3 micro-particles were prepared by sol-gel method. The green upconversion (UC) luminescence and the photothermal characteristics have been investigated. The center wavelength of the green luminescence shifted to short wavelength with the increase of pumping power. For the W2 sample (98.9W2O3-0.1Er2O3-1Yb2O3), the laser-induced temperature can rise up to ∼85 °C under 975 nm excitation. These results indicated that an optical heating of the nanoparticles can be realized, and the temperature rise of the nanoparticles can be investigated via the UC luminescence process. These researches will be meaningful to explore high-quality photothermal materials.

High-purity green emission in Yb3+/Ho3+ Co-doped BaIn6Y2O13 up-conversion phosphor

Yb3+-Ho3+ pairs doped BaIn6Y2O13 (BIY) phosphors were successfully prepared through sol-gel process, and the crystal structure and optical properties were investigated in detail. As-prepared samples exhibited strong green and weak red emission peaked at 549 nm and 652 nm originated from Ho3+ transitions of 5S2, 5F4 → 5I8 and 5F5 → 5I8 with 980 nm excitation, respectively. The dopant contents, pumping power and pulse width of excitation laser dependent up-conversion luminescence (UCL) spectra of samples were modulated to acquire the intense and high-purity green emissions. Moreover, possible dynamic UCL process from non-steady-state to steady-state between Yb3+ and Ho3+ was schematically proposed in detail. These results indicate that BaIn6Y2O13: Yb3+/Ho3+ phosphors with strong and pure green UC emissions are potential candidates in display and lighting technology.