Efficient Upconversion Emission Research Articles

Asymmetric BODIPY Dyes Enabling Triplet-Triplet Annihilation Upconversion.

The construction of triplet-triplet annihilation upconversion (TTA-UC) systems with upconversion (UC) emission efficiency at low power densities is still under continuing exploration. From an environmental point of view, the utilization of purely organic pairs is more beneficial than the involvement of transition-metal complexes. In this context, 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) dyes, which can be found in a wide range of applications, have been previously used as suitable sensitizers in TTA-UC systems. The versatility of these scaffolds makes them magnificent objectives for designing and synthesizing potential entities with different target abilities. Herein, we prepared several asymmetric BODIPY dyes with excellent optical properties to be applied to a bimolecular TTA-UC system. In the presence of 2,5,8,11-tetra-tert-butylperylene (TBPe) as a suitable annihilator, a green-to-blue light conversion was clearly observed by means of detailed spectroscopic investigations. The results revealed a high UC emission efficiency (ηUC) of ∼8%, together with a low threshold intensity (I th) of ∼40-50 mW/cm2. All data indicated that these asymmetric BODIPY dyes were ideal sensitizers for TTA-UC, providing a particular design for further investigations.

Photoluminescence properties and Mn4+ → Tm3+ energy transfer of La0.557Li0.33TiO3: Mn4+, Tm3+ for thermometry and NIR-LED applications

Over the span of years, Mn4+-doped red phosphors have been utilized to meaningful optoelectronic devices due to their outstanding optical properties, and Tm3+ usually is used for activator ion because of its efficient up-conversion emission, but the down-conversion processes are rarely used. Herein, we have successfully synthesized a series of novel La0.557Li0.33Ti1-xO3:xMn4+ (x = 0.1–7 %) phosphors. The photoluminescence properties have been analysis in detailed. Using Mn4+-Tm3+ co-doped strategy, we successfully synthesis La0.557-yLi0.33Ti0.993O3:0.7 %Mn4+, yTm3+ phosphors. Upon excitation with 360 nm light, this phosphor exhibits two distinct emission peaks at 729 nm and 806 nm. The emission intensity of Tm3+ is greatly improved after Mn4+ doping, which is attributed to the energy transfer of Mn4+ to Tm3+. In terms of potential applications, utilizing fluorescence intensity ratio technology, La0.551Li0.33Ti0.993O3:0.7 %Mn4+, 0.6 %Tm3+ exhibits maximum absolute and relative sensitivities of 3.25 % K−1 at 523 K and 3.49 % K−1 at 298 K, which demonstrating its potential for application in optical thermometry. Furthermore, The NIR-LED using phosphor and 365 nm chips reveals potential applications of the La0.551Li0.33Ti0.993O3:Mn4+, Tm3+ phosphor in NIR night vision and biosensing.

Efficient green upconversion emission in transparent Teo2-Geo2 glass-ceramic obtained by billet extrusion

Transparent Yb3+/Er3+glass-ceramic was successfully obtained by the extrusion method. The extrusion of oxyfluoride tellurite-germanate glass co-doped with Yb3+and Er3+ions at 520°C resulted in the formation of Ba0:75Er0:25F2:25 nanocrystals, leading to an increase in the upconversion (UC) emission intensity of 35 times in glass-ceramic with respect to the glass. The glass to glass-ceramic transition was confirmed by X-ray diffraction (XRD) and Transmission electron microscope (TEM). Also, the structural changes that occurred during crystallization were assessed using Fourier-transform infrared (FTIR) spectroscopy. Furthermore, the pump power and temperature UC emission dependence of glass and glass-ceramic under 976 nm laser excitation were investigated in detail. The assessments showed that i) two-phonons are involved in the UC process and ii) the temperature has a significant influence over it. The Yb3+/Er3+ codoped glass-ceramic shows relatively high Sa and Sr values in a wide temperature range from 300 to 573 K, presenting the maximal Sa value of 3:50 x 10–3 at 573 K and the maximal Sr value of 6:30 x 10–3at 364 K. These results suggest that the glass-ceramic is a good candidate for optical applications such as luminescent thermometry.

Highly efficient upconversion luminescence in narrow-bandgap Y2Mo4O15.

Lanthanide-doped upconversion (UC) materials have been extensively investigated for their unique capability to convert low-energy excitation into high-energy emission. Contrary to previous reports suggesting that efficient UC luminescence (UCL) is exclusively observed in materials with a wide bandgap, we have discovered in this study that Y2Mo4O15:Yb3+/Tm3+ microcrystals, a narrowband material, exhibit highly efficient UC emission. Remarkably, these microcrystals do not display any four- or five-photon UC emission bands. This particular optical phenomenon is independent of the variation in doping ion concentration, temperature, phonon energy, and excitation power density. Combining theoretical calculations and experimental results, we attribute the vanishing emission bands to the strong interaction between the bandgap of the Y2Mo4O15 host matrix (3.37 eV) and the high-energy levels (1I6 and 1D2) of Tm3+ ions. This interaction can effectively catalyze the UC emission process of Tm3+ ions, which leads to Y2Mo4O15:Yb3+/Tm3+ microcrystals possessing very strong UCL intensity. The brightness of these microcrystals outshines commercial UC NaYF4:Yb3+,Er3+ green phosphors by a factor of 10 and is 1.4 times greater than that of UC NaYF4:Yb3+,Tm3+ blue phosphors. Ultimately, Y2Mo4O15:Yb3+/Tm3+ microcrystals, with their distinctive optical characteristics, are being tailored for sophisticated anti-counterfeiting and information encryption applications.

Efficient Spin Interconversion by Molecular Conformation Dynamics of a Triplet Pair for Photon Up-Conversion in an Amorphous Solid.

Solid-state materials with improved light-to-energy conversions in organic photovoltaics and in optoelectronics are expected to be developed by realizing efficient triplet-triplet annihilation (TTA) by manipulating the spin conversion processes to the singlet state. In this study, we elucidate the spin conversion mechanism for delayed fluorescence by TTA from a microscopic view of the molecular conformations. We examine the time evolution of the electron spin polarization of the triplet-pair state (TT state) in an amorphous solid-state system exhibiting highly efficient up-conversion emission by using time-resolved electron paramagnetic resonance. We clarified that the spin-state population of the singlet TT increased through the spin interconversion from triplet and quintet TT states during exciton diffusion with random orientation dynamics between the two triplets for the modulation of the exchange interaction, achieving a high quantum yield of up-conversion emission. This understanding provides us with a guide for the development of efficient light-to-energy conversion devices utilizing TTA.

Pressure modified upconversion luminescence of Yb3+ and Er3+-doped Bi3TeBO9 ceramics

Yb3+ and Er3+-doped Bi3TeBO9 ceramics exhibit efficient upconversion luminescence in the visible spectral range upon laser diode excitation at 975 nm. Bi3TeBO9:Yb3+/Er3+ powder was prepared by means of the modified Pechini method. A series of Bi3TeBO9:Yb3+/Er3+ ceramic samples was fabricated using the high-pressure low-temperature technique, by sintering Bi3TeBO9:Yb3+/Er3+ powder under different pressures (2, 4, 6 or 8 GPa). The structure and morphology of the above-mentioned samples were analyzed using XRD, SEM and EDX measurements. The phonon energy of Bi3TeBO9 matrix associated with the vibrations of TeO6 and BO3 molecular groups was revealed using μ-Raman spectroscopy. The low phonon energy of Bi3TeBO9 matrix allows the effective energy transfer between Yb3+ and Er3+ ions as well as the 2H11/2 → 4I15/2, 4S3/2 → 4I15/2 and 4F9/2 → 4I15/2 transitions from the excited to ground states of Er3+ ions, resulting in efficient upconversion emission at 523, 540 and 653 nm, respectively. The decay times determined for all analyzed emissions were longer for Bi3TeBO9:Yb3+/Er3+ ceramics than for the powder sample. The highest values of decay times were detected for the ceramics obtained under a pressure of 8 GPa and were equal to 174, 172 and 175 μs for the emissions at 523, 540 and 653 nm, respectively. The obtained results suggest that Bi3TeBO9:Yb3+/Er3+ ceramics can be proposed as efficient spectral converters in the new type solar cells enhancing the efficiency of the photovoltaic effect.

Multipath optical thermometry realized by electronic levels and Stark sublevels of Er3+

High-performance optical thermometer that can operate in the long-wavelength range of the second near infrared (1500 nm–1700 nm, NIR-IIb) biological window is in desperate need for bio-medical treatment. In this paper, a multipath temperature sensor Y3NbO7: Yb3+/Er3+ with pure cubic phase is synthesized by the high temperature solid state method. Under the excitation of 980 nm wavelength, efficient green upconversion emission attributed to thermally coupled Er3+: 2H11/2/4S3/2 levels are found, through which an excellent temperature sensing properties are realized with the maximal absolute and relative sensitivity of approximately 0.54 % K−1 and 1.37 % K−1 as well as the minimal temperature resolution of about 0.024 K. More importantly, the sample exhibits remarkable temperature detection capacity in deep tissues, which is implemented by the thermally coupled Stark sublevels of Er3+: 4I13/2 locating just in the NIR-IIb sub-window. Thanks to this appropriate response wavelength range, the maximal penetration depth of the NIR-IIb optical thermometer in the biological tissues can reach up to 8 mm. All the findings indicate that the present sample is a supernormal multipath thermometric probe with detectability in the deep biological tissues.

Enhanced Triplet–Triplet Annihilation Upconversion Luminescence through Conformational Restriction Based on Donor–Acceptor–Heavy Atom Molecules

AbstractDesigning optimized systems for triplet‐triplet annihilation upconversion (TTA‐UC) with high UC emission efficiency at low power densities remains challenging. To improve the lifetime of triplet photosensitizers for TTA‐UC, donor‐acceptor‐heavy atoms were used to synthesize rotationally restricted PY‐MBD‐BR and unrestricted PY‐BD‐BR photosensitizers. The singlet oxygen quantum yields of both photosensitizers was ≈0.60 due to their structural similarity, except for the degree of rotation between the donor and acceptor. The triplet lifetime of PY‐BD‐BR and PY‐MBD‐BR was 352 μs and 1,503 μs, respectively. The latter may be among the highest reported for BODIPY photosensitizers. The prolonged triplet lifetime enabled excellent green‐to‐blue upconversion emission of PY‐MBD‐BR, with a high upconversion emission efficiency (ηUC) of 8.8 % and low threshold intensity (Ith) of 90 mW/cm2 compared to PY‐BD‐Br (low ηUC of 4.0 % and high Ith of 381 mW/cm2). The rigid molecular design of triplet photosensitizers based on donor‐acceptor‐heavy atom systems has potential for efficient TTA‐UC.

Efficient down-shifting and up-conversion emission dual-mode in RE3+-doped NaTbF4-based nano-glass ceramics

Intense green and red emissions, visible to the naked eye, coming from Tb3+ to Eu3+ ions, respectively, were observed by down-shifting (from UV-blue excitation) and by cooperative up-conversion (from 980 nm excitation) mechanisms.

Enhancement of up-conversion luminescence efficiency and temperature sensing properties in BaWO4: Ho3+/Yb3+ phosphor via Gd3+ incorporation

In this work, tungstate BaWO 4 phosphors doped Ho 3+ -Yb 3+ -Gd 3+ ions are successfully synthesized through solid-state reaction method. Under the excitation of a 980 nm laser, the up-conversion (UC) luminescence properties are investigated simultaneously. The blue (465 nm, 489 nm), green (541 nm) and red (665 nm) emission intensities of BaWO 4 : Ho 3+ /Yb 3+ /Gd 3+ samples are dramatically enhanced by 20.59, 9.83, 7.37, and 27.59 times more than those without Gd 3+ ions. The possible reason is discussed, which attributes to the energy transfer efficiency enhancement from Yb 3+ to Ho 3+ due to the lattice expansion induced by Gd 3+ ions. The temperature-sensing properties of BaWO 4 : Ho 3+ /Yb 3+ /Gd 3+ samples are also discussed based on the fluorescence intensity ratio (FIR) technology. The sensitivity of temperature measurement for BaWO 4 : Ho 3+ /Yb 3+ /Gd 3+ sample is higher than that of BaWO 4 : Ho 3+ /Yb 3+ . It proves that the codoped Gd 3+ ions can also increase the temperature sensitivity because the higher UC emission efficiency will lead to a high signal-to-noise ratio. The result provides an effective way to design UC luminescence materials with high temperature sensing properties.

Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3+

The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we develop a series of Er3+ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. β-NaYS2:Er3+ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (4I9/2, 9.24 ms; 4I13/2, 30.27 ms) of excited state levels of Er3+, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS2:Er3+ phosphors are further developed for high-performance underwater communication and narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

Yb3+ induced highly efficient green up-conversion emission of NaY(WO4)2:Sm3+ phosphors

Yb3+ induced highly efficient green up-conversion emission of NaY(WO4)2:Sm3+ phosphors

Extraordinary upconversion enhancement in hybrid metasurface by simultaneous excitation/emission band match

Lanthanide doped upconversion nanoparticles (UCNPs) suffers a significant challenge of low quantum efficiency, which seriously limited their applications in photovoltaic device and bioimaging. In this work, we proposed a plasmonic pattern/UCNPs/metal film hybrid metasurface with ultrahigh upconversion enhancement factor. The plasmonic absorption bands can be precisely controlled to simultaneously match both the excitation and the emission wavelength of UCNPs. By constructing the efficient coupling between the local field and the UCNPs, the upconversion fluorescence can be enhanced by 4–6 orders of magnitude. It’s worth noting that such strategy almost covered the two major types of lanthanide doped UCNPs, which were sensitized by Yb3+ (excited under 980 nm) and Nd3+ (excited under 808 nm). The emission enhancement factor was found to be sensitive to the structural parameters and refractive index of the surroundings. Based on this effect, an upconversion intensity and spectra shift dual-mode sensing to environment refractive index can also be achieved. The proposed hybrid metasurface provided an effective way to achieve highly efficient upconversion emission, and the results can also be helpful in solar cells and biological detection.

Highly efficient green up-conversion emission from fluoroindate glass nanoparticles functionalized with a biocompatible polymer.

Up-conversion nanoparticles have garnered lots of attention due to their ability to transform low energy light (near-infrared) into high-energy (visible) light, enabling their potential use as remote visible light nano-transducers. However, their low efficiency restricts their full potential. To overcome this disadvantage, fluoroindate glasses (InF3) doped at different molar concentrations of Yb3+ and Er3+ were obtained using the melting–quenching technique, reaching the highest green emission at 1.4Yb and 1.75Er (mol%), which corresponds to the 4S3/2 → 4I15/2 (540–552 nm) transition. The particles possess the amorphous nature of the glass and have a high thermostability, as corroborated by thermogravimetric assay. Furthermore, the spectral decay curve analysis showed efficient energy transfer as the rare-earth ions varied. This was corroborated with the absolute quantum yield (QY) obtained (85%) upon excitation at 385 nm with QYEr = 17% and QYYb = 68%. Additionally, InF3–1.4Yb–1.75Er was milled and functionalized using poly(ethylene glycol) to impart biocompatibility, which is essential for biomedical applications. Such functionalization was verified using FTIR, TG/DSC, and XRD.

Efficient second-order nonlinear response and upconversion emission from a wide-bandgap quasi-1D lead bromide perovskite

In this work, a new quasi-1D lead bromide perovskite with efficient second-harmonic generation and unexpectedly anomalous green PL phenomenon is developed. The second-order NLO coefficient is about 0.1 pm V−1, 1/4 times that of the commercial crystal KH2PO4 (KDP).

Synthesis, structure, and upconversion emission of Er3+ and Yb3+ co-doped YBO3

We investigated the upconversion (UC) properties of YBO3 co-doped with Er3+ and Yb3+ ions. The YBO3 samples were synthesized by the solid state reaction method. The Er3+ and Yb3+ were doped at various ratios into YBO3 to investigate the effect of concentration of the Er3+/Yb3+ ions on the UC emission. Rietveld refinement was performed on the X-ray diffraction patterns to obtain the crystal structural parameters. Strong red and green UC emissions were observed from Er3+ and Yb3+-codoped YBO3 under 980 nm diode laser excitation. The highest UC emission efficiency was achieved at 5 mol% concentration of both the Er3+ and Yb3+ ions. The power-dependent UC emission spectra of our samples indicate that the red UC emission of Er3+ should originate from a two-photon absorption process.

Tuning broadband up-conversion by precipitation of perovskite KMgF3 nanocrystals in fluorosilicate glasses

Co-doping of Yb3+ with Mn2+ is a promising way to realize broadband up-conversion (UC) luminescence. The tunable broadband emission has been widely demonstrated in crystal materials, but the research on Yb3+/Mn2+ co-doped transparent optical glass has been scare, especially in fluoride-based transparent glass ceramics (GC) that can realize the uniform ions incorporation and efficient UC emission. Herein, a series of Yb3+/Mn2+ activated transparent fluorosilicate GCs containing perovskite KMgF3 nanocrystals were successfully prepared, and the effect of heat-treatment process and/or Mn2+ ion content on the UC luminescence modulation has been investigated. Our results show that the spontaneous crystallized precursor sample exhibits only a green emission at ~538 nm, while additional orange-emitting band centered at ~613 nm is noticeable in the GCs with the gradual precipitation of KMgF3 nanocrystals after heat-treatment. The controllable luminescence can be assigned to the formation of Yb3+-Mn2+ dimers that are located in multiple coordination environments provided by the glass matrix and crystalized nanocrystals. In addition, the prepared GC samples exhibit excellent reversible emission characteristics during heating and cooling cycles. This research not only provides a new option to design desirable coordination environment for Yb3+-Mn2+ dimers, also helps to recognize the UC process in the transparent matrix in detail.

Efficient upconversion emission and high-sensitivity thermometry of BaIn2O4:Yb3+/Tm3+/RE3+ (RE = Er3+, Ho3+) phosphor.

BaIn2O4:Yb3+/Tm3+/RE3+ (RE = Er3+, Ho3+) upconversion (UC) phosphors were synthesized via the sol-gel method. Rietveld refinement based on XRD data proved that In3+ ions were replaced by rare earth (RE) ions. Under 980 nm excitation, UC and optical temperature-sensing properties were investigated, and the results indicated that the samples demonstrated high UC emission efficiencies and bright emission visible to the naked eye. The luminescence intensity ratio (LIR) technology based on thermally and non-thermally coupled levels was applied to study the optical temperature-sensing properties of the RE-doped BaIn2O4 phosphors. For the BaIn2O4:Yb3+/Tm3+/Er3+ phosphor, the absolute sensitivity reached a maximum value of 0.1433 K-1 at 473 K, which was based on the non-thermally coupled levels (non-TCLs) of 1G4 (blue luminescence at 480 nm of Tm3+ ion) and 2H11/2 (green luminescence at 525 nm of Er3+ ion). For the BaIn2O4:Yb3+/Tm3+/Ho3+ phosphor, the maximum absolute sensitivity was 0.2545 K-1, which came from the non-TCLs of 3H4 (infrared luminescence at 795 nm of Tm3+ ion) and 1G4 (blue luminescence at 480 nm of Tm3+ ion). BaIn2O4:Yb3+/Tm3+/RE3+ (RE = Er3+, Ho3+) samples were proven to have excellent optical temperature-sensing properties and could be applied to design optical thermometry.

Enhancing multiphoton upconversion emissions through confined energy migration in lanthanide-doped Cs2NaYF6 nanoplatelets.

Lanthanide (Ln3+)-doped upconversion (UC) nanocrystals have drawn tremendous attention because of their intriguing optical properties. Currently, it is highly desired but remains challenging to achieve efficient multiphoton UC emissions. Herein, we report the controlled synthesis of a new class of UC nanocrystals based on Cs2NaYF6:Yb/Tm nanoplatelets (NPs), which can effectively convert the 980 nm light to five-photon and four-photon UC emissions of Tm3+ without the fabrication of a complicated core/multishell structure required in traditional nanocrystals. Particularly, the as-prepared Cs2NaYF6:Yb/Tm NPs exhibit a maximal UV-to-NIR emission intensity ratio of 1.2, which is the highest among Tm3+-doped core-only UC nanocrystals. We reveal that the enhanced multiphoton UC emissions may benefit from the confined energy migration of Ln3+ dopants in the unique two-dimensional-like structure of Cs2NaYF6 NPs. As such, intense red and green UC emissions of Eu3+ and Tb3+ can further be generated via the cascade sensitization of Tm3+ and Gd3+ in Cs2NaYF6:Yb/Tm/Gd/Eu and Cs2NaYF6:Yb/Tm/Gd/Tb NPs, respectively. These results validate the superiority of Cs2NaYF6 for the future design of efficient UC nanocrystals towards versatile applications.

Non-Linear Optical Properties of Er3+-Yb3+-Doped NaGdF4 Nanostructured Glass-Ceramics.

Transparent oxyfluoride glass–ceramics containing NaGdF4 nanocrystals were prepared by melt-quenching and doped with Er3+ (0.5 mol%) and different amounts of Yb3+ (0–2 mol%). The selected dopant concentration the crystallization thermal treatments were chosen to obtain the most efficient visible up-conversion emissions, together with near infrared emissions. The crystal size increased with dopant content and treatment time. NaGdF4 NCs with a size ranging 9–30 nm were obtained after heat treatments at Tg + 20–80 °C as confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Energy dispersive X-ray analysis shows the incorporation of rare earth ions into the NaGdF4 nanocrystals. Near-infrared emission spectra, together with the up-conversion emissions were measured. The optical characterization of the glass–ceramics clearly shows that Er3+ and Yb3+ ions are incorporated in the crystalline phase. Moreover, visible up-conversion emissions could be tuned by controlling the nanocrystals size through appropriated heat treatment, making possible a correlation between structural and optical properties.