Search

This article represents the first foray into investigating the consequences of various material combinations on the short-wave infrared (SWIR, 1000-2000 nm) performance of Tm-based core-shell nanocrystals (NCs) above 1600 nm. In total, six different material combinations involving two different types of SWIR-emitting core NCs (α-NaTmF 4 and LiTmF 4 ) combined with three different protecting shell materials (α-NaYF 4 , CaF 2 , and LiYF 4 ) have been synthesized. All corresponding homo- and heterostructured NCs have been meticulously characterized by powder X-ray diffraction and electron microscopy techniques. The latter revealed that out of the six investigated combinations, only one led to the formation of a true core-shell structure with well-segregated core and shell domains. The direct correlation between the downshifting performance and the spatial localization of Tm 3+ ions within the final homo- and heterostructured NCs is established. Interestingly, to achieve the best SWIR performance, the formation of an abrupt interface is not a prerequisite, while the existence of a pure (even thin) protective shell is vital. Remarkably, although all homo- and heterostructured NCs have been synthesized under the exact same experimental conditions, Tm 3+ SWIR emission is either fully quenched or highly efficient depending on the type of material combination. The most efficient combination (LiTmF 4 /LiYF 4 ) achieved a high photoluminescence quantum yield of 39% for SWIR emission above 1600 nm (excitation power density in the range 0.5-3 W/cm 2 ) despite significant intermixing. From now on, highly efficient SWIR-emitting probes with an emission above 1600 nm are within reach to unlock the full potential of in vivo SWIR imaging.

More complete recycling of plastic waste is possible only if new technologies that go beyond state-of-the-art near-infrared (NIR) sorting are developed. For example, tracer-based sorting is a new technology that explores the upconversion or down-shift luminescence of special tracers based on inorganic materials codoped with lanthanide ions. Specifically, down-shift tracers emit in the shortwave infrared (SWIR) spectral range and can be detected using a SWIR camera preinstalled in a state-of-the-art sorting machine for NIR sorting. In this study, we synthesized a very efficient SWIR tracer by codoping Li 3 Ba 2 Gd 3 (MoO 4 ) 8 with Yb 3+ and Er 3+ , where Yb 3+ is a synthesizer ion (excited near 976 nm) and Er 3+ emits near 1550 nm. Fine-tuning of the doping concentration resulted in a tracer (Li 3 Ba 2 Gd (3- x-y ) (MoO 4 ) 8 : x Yb 3+ , yEr 3+ , where x = 0.2 and y = 0.4) with a high photoluminescence quantum yield for 1550 nm emission of 70% (using 976 nm excitation). This tracer was used to mark plastic objects. When the object was illuminated by a halogen lamp and a 976 nm laser, the three parts could be easily distinguished based on reflectance and luminescence spectra in the SWIR range: a plastic bottle made of polyethylene terephthalate, a bottle cap made of high-density polyethylene, and a label made of the tracer Li 3 Ba 2 Gd 3 (MoO 4 ) 8 :Yb 3+ , Er 3+ . Importantly, the use of the tracer in sorting may require only the installation of a 976 nm laser in a state-of-the-art NIR sorting system.

Lanthanide-based upconversion (UC) allows harvesting sub-bandgap near-infrared photons in photovoltaics. In this work, we investigate UC in perovskite solar cells by implementing UC single crystal BaF 2 :Yb 3+ , Er 3+ at the rear of the solar cell. Upon illumination with high-intensity sub-bandgap photons at 980 nm, the BaF 2 :Yb 3+ , Er 3+ crystal emits upconverted photons in the spectral range between 520 and 700 nm. When tested under terrestrial sunlight representing one sun above the perovskite's bandgap and sub-bandgap illumination at 980 nm, upconverted photons contribute a 0.38 mA/cm 2 enhancement in the short-circuit current density at lower intensity. The current enhancement scales non-linearly with the incident intensity of sub-bandgap illumination, and at higher intensity, 2.09 mA/cm 2 enhancement in current was observed. Hence, our study shows that using a fluoride single crystal like BaF 2 :Yb 3+ , Er 3+ for UC is a suitable method to extend the response of perovskite solar cells to near-infrared illumination at 980 nm with a subsequent enhancement in current for very high incident intensity.

Opportunities for enhancing solar energy harvesting using photon upconversion are reviewed. The increasing prominence of bifacial solar cells is an enabling factor for the implementation of upconversion, however, when the realistic constraints of current best-performing silicon devices are considered, many challenges remain before silicon photovoltaics operating under nonconcentrated sunlight can be enhanced via lanthanide-based upconversion. A photophysical model reveals that >1-2 orders of magnitude increase in the intermediate state lifetime, energy transfer rate, or generation rate would be needed before such solar upconversion could start to become efficient. Methods to increase the generation rate such as the use of cosensitizers to expand the absorption range and the use of plasmonics or photonic structures are reviewed. The opportunities and challenges for these approaches (or combinations thereof) to achieve efficient solar upconversion are discussed. The opportunity for enhancing the performance of technologies such as luminescent solar concentrators by combining upconversion together with micro-optics is also reviewed. Triplet-triplet annihilation-based upconversion is progressing steadily toward being relevant to lower-bandgap solar cells. Looking toward photocatalysis, photophysical modeling indicates that current blue-to-ultraviolet lanthanide upconversion systems are very inefficient. However, hope remains in this direction for organic upconversion enhancing the performance of visible-light-active photocatalysts.

The efficiency of photon upconversion via triplet-triplet annihilation is characterized by an upconversion quantum yield (Φ UC ); however, uncertainties remain for its determination. Here, we present a new approach for the relative measurement of Φ UC for green-to-blue upconversion using BODIPY-pyrene donor-acceptor dyad (BD1) as a heavy-atom-free triplet sensitizer. This new approach exploits broad fluorescence from a charge-transfer (CT) state of BD1, which possesses (i) a significant Stokes shift of 181 nm in dichloromethane and (ii) a comparably high CT-fluorescence quantum yield (Φ ref = 7.0 ± 0.2%), which is independent from oxygen presence and emitter (perylene) concentration while also exhibiting a linear intensity dependence. On the basis of this, we developed an upconversion reference using the BD1 sensitizer mixed with perylene (1 × 10 -5 M/1 × 10 -4 M) in dichloromethane. With this reference system, we investigated the performance of three BODIPY donor-acceptor dyads in the upconversion process and achieved one of the highest Φ UC of 6.9 ± 0.2% observed for heavy-atom-free sensitizers to date.

The upconversion of near-infrared (NIR) to visible (vis) photons is of interest for display technologies and energy conversion. Although triplet-triplet annihilation (TTA) offers a mechanism for upconversion that works efficiently at low incident irradiance flux densities, current strategies for NIR-vis upconversion based on TTA have fundamental limitations. Herein, we report a strategy for NIR-vis TTA based on lanthanide-containing complexes to sensitize the upconversion. We demonstrate a β-diketonate complex of Yb 3+ paired with rubrene that emits yellow (λ em = 559 nm) under NIR excitation (λ exc = 980 nm). This corresponds to an exceptional anti-Stokes shift of just less than 1 eV. Thus, lanthanide complexes could unlock high-performance NIR-vis upconversion, with lanthanide sensitizers overcoming the energy loss, reabsorption, and short triplet lifetime that fundamentally limit porphyrin, nanocrystals, and direct S 0 -T 1 sensitizers.

A novel combination of a poly(vinylidene fluoride) (PVDF) membrane with pore size 0.2 μm and a photosensitizer 5,10,15,20-tetrakis (pentafluorophenyl)-21 H ,23 H -porphine palladium(II) (PdTFPP) makes a promising hybrid material for the generation of singlet oxygen ( 1 O 2 ) and, thus, water treatment applications. The fabricated photocatalytic membrane exhibits permeability of 4280 ± 250 L·m -2 ·h -1 ·bar -1 and stable photocatalytic degradation performance over a 90 h period, when illuminated with green light (528 ± 20 nm) and operated in a dead-end, single-pass configuration. Methylene blue (MB) degradation of 83% was achieved for MB concentration of 1 mg·L -1 under the flow rate of 0.1 × 10 -3 L·min -1 (flux of 30 L·m -2 ·h -1 ), light intensity of 21 mW·cm -2 , and PdTFPP loading of 25 μmol·g -1 . Due to an enhanced mass transfer, the reaction rate of MB removal (with apparent rate constant of k app = 6.52 min -1 ) results in an efficient photodegradation of MB inside the PdTFPP-PVDF membrane. The influence of experimental parameters such as catalyst loading, flow rate, light intensity, and solute concentration on MB removal was investigated. This research enables the application of photocatalytic PdTFPP-PVDF membranes as a potential technology for water decontamination under visible-light illumination.

In photon upconversion (UC) based on triplet-triplet annihilation, the upconversion photoluminescent quantum yield (UC-PLQY) depends on the excitation power density in a way that can be described by a single figure of merit. This figure of merit, the threshold value, allows the excitation power density required for efficient UC-PLQY to be compared between different triplet-triplet annihilation systems. Here, we investigate the excitation power density dependence of two-photon UC processes in a series of four lanthanide-doped inorganic host materials (oxides, fluorides, and chlorides) all doped with 18 mol % Yb 3+ sensitizer ions and 2 mol % Er 3+ activator ions. We demonstrate that an analogous figure of merit, which we call the critical power density (CPD), accurately describes the UC power dependence of these samples. Better CPD values are obtained when the lifetime of the intermediate states is long. The UC-PLQY at the CPD is linked to the saturation UC-PLQY. Thus, a measurement of the UC-PLQY at this low power density can be used to estimate the theoretical saturation UC-PLQY in the absence of deleterious effects such as laser-induced heating. This is compared to another method to estimate the saturation based on the CPD model, namely, taking half of the level's PLQY under direct excitation. Our careful analysis of the upconversion spectra as a function of excitation power density gives several insights into the differing upconversion pathways in the hosts and proves to be a useful tool for their comparison.

Efficient photon-harvesting materials require easy-to-deposit materials exhibiting good absorption and excited-state transport properties. We demonstrate an organic thin-film material system, a palladium-porphyrin-based surface-anchored metal-organic framework (SURMOF) thin film that meets these requirements. Systematic investigations using transient absorption spectroscopy confirm that triplets are very mobile within single crystalline domains; a detailed analysis reveals a triplet transfer rate on the order of 10 10 s -1 . The crystalline nature of the SURMOFs also allows a thorough theoretical analysis using the density functional theory. The theoretical results reveal that the intermolecular exciton transfer can be described by a Dexter electron exchange mechanism that is considerably enhanced by virtual charge-transfer exciton intermediates. On the basis of the photophysical results, we predict exciton diffusion lengths on the order of several micrometers in perfectly ordered, single-crystalline SURMOFs. In the presently available samples, strong interactions of excitons with domain boundaries present in these metal-organic thin films limit the diffusion length to the diameter of these two-dimensional grains, which amount to about 100 nm. Our results demonstrate high potential of SURMOFs for light-harvesting applications.

Efficient single-band NIR-to-NIR upconversion (UC) emission is strongly desired for many applications such as fluorescent markers, plastic recycling, and biological imaging. Herein, we report highly efficient single-band NIR-to-NIR UC emission in La 2 O 3 :Yb 3+ ,Tm 3+ (LYT) microcrystals. Under 980 nm laser excitation, LYT exhibits a NIR UC emission at ∼795 nm (Tm 3+ : 3 H 4 → 3 H 6 ) and blue UC emission at ∼476 nm; the NIR UC emission is dominant, with the intensity ratio of the NIR to blue I NIR / I blue > 100. Remarkably, a high absolute UC quantum yield (UCQY) of 3.4% is obtained for the single-band NIR UC emission of LYT at a relatively low excitation power density of 7.6 W/cm 2 . This value is much higher than the reported values of a single-band NIR UC for rare-earth-based UC materials in literature, such as the well-known benchmark UC materials of β-NaYF 4 :Yb 3+ ,Er 3+ (∼0.9%, with a excitation power density of 9 W/cm 2 ) and Gd 2 O 2 S:Yb 3+ ,Er 3+ (∼1.9%, with a excitation power density of 20 W/cm 2 ). The high absolute UCQY of single-band NIR UC emission combined with their facile preparation hints at their potential application in anti-counterfeiting, verified by the proof-of-concept demonstration of fluorescent labeling of a transparent IMT pattern.

Triplet-triplet annihilation up-conversion (TTA-UC) is a developing technology that can enable spectral conversion under sunlight. Previously, it was found that efficient TTA-UC can be realized in polymer hosts for temperatures above the polymer's glass transition (T > T g ). In contrast, TTA-UC with high quantum yield for temperatures below T g is rarely reported. In this article, we report new polymer hosts in which efficient TTA-UC is observed well below T g , when the polymer is in a fully solid state. The four poly(olefin sulfone) hosts were loaded with upconversion dyes, and absolute quantum yields of TTA-UC (η TTA-UC ) were measured. The highest value of η TTA-UC = 2.1% was measured for poly(1-dodecene sulfone). Importantly, this value was the same in vacuum and at ambient conditions, indicating that the host material acts as a good oxygen barrier. We performed time-resolved luminescence experiments in order to elucidate the impact of elementary steps of TTA-UC. In addition to optical characterization, we used magic angle spinning solid-state NMR experiments to estimate the T2 transverse relaxation time. Relatively long T2 times measured for poly(olefin sulfone)s indicate an enhanced nanoscale fluidity in the studied (co)polymers, which unexpectedly coexists with a rigidity on the macroscale. This would explain the exceptional triplet energy transfer between the guest molecules, despite the macroscopic rigidity.

The development of solid materials which are able to upconvert optical radiation into photons of higher energy is attractive for many applications such as photocatalytic cells and photovoltaic devices. However, to fully exploit triplet-triplet annihilation photon energy upconversion (TTA-UC), oxygen protection is imperative because molecular oxygen is an ultimate quencher of the photon upconversion process. So far, reported solid TTA-UC materials have focused mainly on elastomeric matrices with low barrier properties because the TTA-UC efficiency generally drops significantly in glassy and semicrystalline matrices. To overcome this limit, for example, combine effective and sustainable annihilation upconversion with exhaustive oxygen protection of dyes, we prepare a sustainable solid-state-like material based on nanocellulose. Inspired by the structural buildup of leaves in Nature, we compartmentalize the dyes in the liquid core of nanocellulose-based capsules which are then further embedded in a cellulose nanofibers (NFC) matrix. Using pristine cellulose nanofibers, a sustainable and environmentally friendly functional nanomaterial with ultrahigh barrier properties is achieved. Also, an ensemble of sensitizers and emitter compounds are encapsulated, which allow harvesting of the energy of the whole deep-red sunlight region. The films demonstrate excellent lifetime in synthetic air (20.5/79.5, O2/N2)-even after 1 h operation, the intensity of the TTA-UC signal decreased only 7.8% for the film with 8.8 μm thick NFC coating. The lifetime can be further modulated by the thickness of the protective NFC coating. For comparison, the lifetime of TTA-UC in liquids exposed to air is on the level of seconds to minutes due to fast oxygen quenching.

We present a method for the fabrication of ultralight upconverting mats consisting of rigid polymer nanofibers. The mats are prepared by simultaneously electrospinning an aqueous solution of a polymer with pronounced oxygen-barrier properties and functional nanocapsules containing a sensitizer/emitter couple optimized for triplet-triplet annihilation photon upconversion. The optical functionality of the nanocapsules is preserved during the electrospinning process. The nanofibers demonstrate efficient upconversion fluorescence centered at λ max = 550 nm under low intensity excitation with a continuous wave laser (λ = 635 nm, power = 5 mW). The pronounced oxygen-barrier property of the polymer matrix may efficiently prevent the oxygen penetration so upconversion fluorescence is registered in ambient atmosphere. The demonstrated method can be used for the production of upconverting ultralight porous coatings for sensors or upconverting membranes with freely variable thickness for solar cells.