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Vibrational sum frequency generation can provide valuable structural information at surfaces and buried interfaces. Relating the measured spectra to the complex-valued second-order susceptibility χ(2) is at the heart of the technique and a requisite step in nearly all subsequent analyses. The magnitude and phase of χ(2) as a function of frequency reveal important information about molecules and materials in regions where centrosymmetry is broken. In this tutorial-style perspective, the origins of the χ(2) phase are first described, followed by the utility of phase determination. Finally, some practical methods of phase extraction are discussed. [ABSTRACT FROM AUTHOR]

Within cell plasma membranes, unsaturated lipids are asymmetrically distributed over the inner and outer leaflets, offering an attractive local structural feature. However, the mechanism to keep lipid transmembrane asymmetry and the closely related transmembrane movement (flip-flop) for unsaturated lipids remain poorly understood. Here, we applied sum frequency generation vibrational spectroscopy to investigate this lipid transmembrane asymmetry upon mimicking the cell membrane homeostatic processes. On the one hand, unsaturated lipids were found to hinder the flip-flop process and preserve lipid transmembrane asymmetry in model cell membranes, owing to the steric hindrance caused by their bent tails. On the other hand, local unsaturated lipids in the mixed unsaturated/saturated lipid bilayer were conducive to the formation of the local asymmetry. Therefore, lipid unsaturation can be recognized as an intrinsic key factor to form and maintain lipid transmembrane asymmetry in cell membranes. [ABSTRACT FROM AUTHOR]

Surface-specific sum frequency generation vibrational spectroscopy is applied to study the molecular configuration of short-chain n-alkanethiol self-assembled monolayers (SAMs with n = 2–6) on the Au surface. For monolayers with n ≥ 3, the alkanethiols are upright-oriented, with the CH3 tilt angle varying between ∼33° and ∼46° in clear even–odd dependency. The ethanethiol monolayer (n = 2) is, however, found to exhibit a distinct lying-down configuration with a larger methyl tilt angle (67°–79°) and a smaller CH2 tilt angle (56°–68°). Such a unique configurational transition from n = 2 to n ≥ 3 discloses the steric effect owing to chain–chain interaction among neighboring molecules. Through density functional theory calculations, the transition is further confirmed to be energetically favorable for thiols on a defective reconstructed Au(111) surface but not on the pristine one. Our study highlights the roles of the chain–chain interaction and the substrate surface atomic structure when organizing SAMs, offering a strategic pathway for exploiting their applications. [ABSTRACT FROM AUTHOR]

Lanthanide-doped upconversion (UC) luminescent materials display multicolor emissions, making them ideal for a variety of applications, such as multi-channel biological imaging, fluorescence encryption, anti-counterfeiting, and 3D display. Manipulating the UC emissions of the luminescent materials with a fixed composition is crucial for their applications. Herein, we propose a facile strategy to achieve pulse-width-dependent multicolor UC emissions in NaYF4:Yb/Er/Tm nanocrystals. Upon excitation with a 980 nm continuous-wave laser diode, Er3+ ions in NaYF4:20%Yb,15%Er,1%Tm nanocrystals exhibited UC emissions with a red-to-green (R/G) ratio of 11.3. Nevertheless, by employing a 980 nm pulse laser with pulse widths from 0.1 to 10 ms, the UC R/G ratio can be easily adjusted from 0.9 to 11.3, resulting in continuous and remarkable color transformation from green, yellow, orange, to red. By virtue of the dynamic luminescence color variation of these NaYF4:20%Yb,15%Er,1%Tm nanocrystals, we demonstrated their potential applications in the areas of anti-counterfeiting and information encryption. These findings provide deep insights into the excited-state dynamics and energy transfer of Er3+ in NaYF4:Yb/Er/Tm nanocrystals upon 980 nm pulse excitation, which may pave the way for designing multicolor UC materials toward versatile applications. [ABSTRACT FROM AUTHOR]

The intriguing network of antibody–antigen (Ab–Ag) interactions is highly governed by environmental perturbations and the nature of biomolecular interaction. Protein–protein interactions (PPIs) have potential applications in developing protein-adsorption-based sensors and nano-scale materials. Therefore, characterizing PPIs in the presence of a nanomaterial at the molecular level becomes imperative. The present work involves the investigation of antiferritin–ferritin (Ab–Ag) protein interactions under the influence of tungsten disulfide quantum dots (WS2 QDs). Isothermal calorimetry and contact angle measurements validated the strong influence of WS2 QDs on Ab–Ag interactions. The interfacial signatures of nano–bio-interactions were evaluated using sum frequency generation vibration spectroscopy (SFG-VS) at the air–water interface. Our SFG results reveal a variation in the tilt angle of methyl groups by ∼12° ± 2° for the Ab–Ag system in the presence of WS2 QDs. The results illustrated an enhanced ordering of water molecules in the presence of QDs, which underpins the active role of interfacial water molecules during nano–bio-interactions. We have also witnessed a differential impact of QDs on Ab–Ag by raising the concentration of the Ab–Ag combination, which showcased an increased inter-molecular interaction among the Ab and Ag molecules and a minimal influence on the methyl tilt angle. These findings suggest the formation of stronger and ordered Ab–Ag complexes upon introducing WS2 QDs in the aqueous medium and signify the potentiality of WS2 QDs relevant to protein-based sensing assays. [ABSTRACT FROM AUTHOR]

Graphene oxide (GO) is a two-dimensional, mechanically strong, and chemically tunable material for separations. Elucidating GO–ion–water interactions at the molecular scale is highly important for predictive understanding of separation systems. However, direct observations of the nanometer region by GO surfaces under operando conditions are not trivial. Therefore, thin films of GO at the air/water interface can be used as model systems. With this approach, we study the effects of alkali metal ions on water organization near graphene oxide films at the air/water interface using vibrational sum frequency generation (SFG) spectroscopy. We also use an arachidic acid Langmuir monolayer as a benchmark for a pure carboxylic acid surface. Theoretical modeling of the concentration-dependent sum frequency signal from graphene oxide and arachidic acid surfaces reveals that the adsorption of monovalent ions is mainly controlled by the carboxylic acid groups on graphene oxide. An in-depth analysis of sum frequency spectra reveals at least three distinct water populations with different hydrogen bonding strengths. The origin of each population can be identified from concentration dependent variations of their SFG signal. Interestingly, an interfacial water structure seemed mostly insensitive to the character of the alkali cation, in contrast to similar studies conducted at the silica/water interface. However, we observed an ion-specific effect with lithium, whose strong hydration prevented direct interactions with the graphene oxide film. [ABSTRACT FROM AUTHOR]

Chirality-selective vibrational sum frequency generation (chiral SFG) spectroscopy has emerged as a powerful technique for the study of biomolecular hydration water due to its sensitivity to the induced chirality of the first hydration shell. Thus far, water O–H vibrational bands in phase-resolved heterodyne chiral SFG spectra have been fit using one Lorentzian function per vibrational band, and the resulting fit has been used to infer the underlying frequency distribution. Here, we show that this approach may not correctly reveal the structure and dynamics of hydration water. Our analysis illustrates that the chiral SFG responses of symmetric and asymmetric O–H stretch modes of water have opposite phase and equal magnitude and are separated in energy by intramolecular vibrational coupling and a heterogeneous environment. The sum of the symmetric and asymmetric responses implies that an O–H stretch in a heterodyne chiral SFG spectrum should appear as two peaks with opposite phase and equal amplitude. Using pairs of Lorentzian functions to fit water O–H stretch vibrational bands, we improve spectral fitting of previously acquired experimental spectra of model β-sheet proteins and reduce the number of free parameters. The fitting allows us to estimate the vibrational frequency distribution and thus reveals the molecular interactions of water in hydration shells of biomolecules directly from chiral SFG spectra. [ABSTRACT FROM AUTHOR]

Doubly resonant sum frequency generation (DR-SFG) serves as a potent characteristic technique for probing the electronic spectra and vibronic coupling of molecules on surfaces. In this study, we successfully developed a novel infrared (IR)–white light (WL) DR-SFG spectroscopy based on narrowband IR and tunable broadband WL. This novel method was employed to explore the excitation spectrum and vibronic couplings of sub-monolayer Rhodamine 6G molecules. Our findings elucidate that the xanthene skeleton vibrational modes exhibit strong coupling with the S0–S1 electronic transition. Notably, we observed not only the 0–0 transition of the S0–S1 electronic continuum but also the 0–1 transition, a first time observation in the realm of DR-SFG spectroscopy. This advanced DR-SFG spectroscopy methodology facilitates a more sensitive examination of electronic spectra and the coupling between electronic transitions and vibrational modes, heralding a significant advancement in the understanding of molecular interactions on surfaces. [ABSTRACT FROM AUTHOR]

Polymer chains at a buried interface with an inorganic solid play a critical role in the performance of polymer nanocomposites and adhesives. Sum frequency generation (SFG) vibrational spectroscopy with a sub-nanometer depth resolution provides valuable information regarding the orientation angle of functional groups at interfaces. However, in the case of conventional SFG, since the signal intensity is proportional to the square of the second-order nonlinear optical susceptibility and thereby loses phase information, it cannot be unambiguously determined whether the functional groups face upward or downward. This problem can be solved by phase-sensitive SFG (ps-SFG). We here applied ps-SFG to poly(methyl methacrylate) (PMMA) chains in direct contact with a quartz surface, shedding light on the local conformation of chains adsorbed onto the solid surface. The measurements made it possible to determine the absolute orientation of the ester methyl groups of PMMA, which were oriented toward the quartz interface. Combining ps-SFG with all-atomistic molecular dynamics simulation, the distribution of the local conformation and the driving force are also discussed. [ABSTRACT FROM AUTHOR]

Sum frequency generation (SFG) spectroscopy is an interface-selective spectroscopic technique that enables us to selectively observe the vibrational or electronic resonances of molecules within a very thin interface layer. The interfacial properties probed by SFG are contained in a complex quantity called the second-order nonlinear susceptibility ( χ 2 ). It is usually believed that the imaginary part of χ 2 (Im χ 2 ) exhibits the resonant responses of the system, whereas the nonresonant responses appear solely in the real part of χ 2 (Re χ 2 ). However, it was recently theoretically pointed out that a portion of the nonresonant responses actually contributes to the observed Im χ 2 spectra when the finite thickness of the interface layer is taken into account. In this study, by considering a simple air/liquid interface without any solutes as a model system, we theoretically evaluate the nonresonant contribution to experimentally accessible Im χ 2 as well as to Re χ 2 , from which the complex phase of the nonresonant background is estimated. It is shown that the deviation of the complex phase from 0° or 180° is less than 1° even if the thickness of the interface layer is taken into account. This means that the nonresonant contribution to Im χ 2 is practically negligible, and it is a very good approximation to think that the nonresonant background appears solely in Re χ 2 in the case of air/liquid interfaces. This result implies that Im χ 2 practically contains only the resonant responses of the system, and molecular resonances at the interface can be conveniently studied using Im χ 2 spectra at such interfaces. [ABSTRACT FROM AUTHOR]

Nonlinear optical methods, such as vibrational sum frequency generation (vSFG) and second harmonic generation (SHG), are powerful techniques to study elusive structures at charged buried interfaces. However, for the separation and determination of the Stern and diffuse layer spectra at these charged interfaces, complex vSFG spectra and, hence, the absolute phase need to be retrieved. The maximum entropy method is a useful tool for the retrieval of complex spectra from the intensity spectra; however, one caveat is that an understanding of the error phase is required. Here, for the first time, we provide a physically motivated understanding of the error phase. Determining the error phase from simulated spectra of oscillators with a spectral overlap, we show that for broadband vSFG spectra, such as for the silica/water interface, the diffuse and Stern layers' spectral overlap within the O–H stretching window results in a correlation between the error phase and the phase shift between the responses of these layers. This correlation makes the error phase sensitive to changes in Debye length from varying the ionic strength among other variations at the interface. Furthermore, the change in the magnitude of the error phase can be related to the absolute SHG phase, permitting the use of an error phase model that can utilize the SHG phase to predict the error phase and, hence, the complex vSFG spectra. Finally, we highlight limitations of this model for vSFG spectra with a poor overlap between the diffuse and Stern layer spectra (silica/HOD in D2O system). [ABSTRACT FROM AUTHOR]

In view of the current situation in which non-polarizable rigid water models have been scarcely examined against surface-specific properties, we appraise TIP4P-type models at the liquid water surface on the basis of heterodyne-detected sum frequency generation (HD-SFG) spectroscopy. We find in the HD-SFG spectrum of the water surface that the peak frequency of the hydrogen-bonded OH band, the half width at half maximum of the hydrogen-bonded OH band, and the full width at half maximum of the free OH band are best reproduced by TIP4P, TIP4P/Ew, and TIP4P/Ice, respectively, whereas it is already well known that TIP4P/2005 best reproduces the surface tension. These TIP4P-type models perform better at the water surface in terms of the present appraisal items than some polarizable models in the literature. [ABSTRACT FROM AUTHOR]

Recent vibrational sum frequency generation spectroscopic experiments [Sengupta et al., J. Phys. Chem. Lett. 13, 11391–11397 (2022)] demonstrated synergistic interfacial adsorption effects between the anionic dodecyl sulfate (DS−) and the polar, but charge-neutral hexaethylene glycol monododecyl ether (C12E6), surfactants. In this study, the interfacial adsorption thermodynamics underlying these synergistic effects are analyzed through free energy decompositions. A general decomposition method utilizing alchemical intermediate states is outlined. Combining free energy decompositions with the potential distribution theorem illuminates the statistical interpretations of correlated effects between different system components. This approach allows for the identification of the physical effects leading to synergistic adsorption thermodynamics of DS− binding to the air-C12E6-water interface. The binding properties are found to result from a combination of effects predominantly including energetic van der Waals stabilization between DS− and C12E6, as well as competing energetic and entropic effects due to changes in the interfacial water structure as a result of introducing a C12E6 monolayer into the bare air–water interface. [ABSTRACT FROM AUTHOR]

Upconversion luminescence presents obvious advantages in optical temperature sensing, nevertheless the application in complex scenarios is limited by luminescence thermal quenching. Herein, NaYS 2 :Yb3+,Nd3+ phosphors are synthesized by the solid-gas reaction method and used for temperature sensing. A series of emissions for Nd3+ from visible to near-infrared region are achieved based on energy transfer from Yb3+ to Nd3+ under 980 nm excitation. With elevating the temperature, significant thermal enhancement effect of nearly three orders of magnitude is detected in near-infrared emission of 4F 7/2 → 4I 9/2 , which is attributed to phonon-assisted energy transfer of Yb3+→Nd3+ and the corresponding thermal population effect. The thermal behaviors of thermally coupled energy levels for 4G 7/2 /2G 9/2 and 4F 7/2 /4F 5/2 in NaYS 2 :Yb3+,Nd3+ are evaluated by luminescence intensity ratio technique. Remarkably, the optical thermometer based on the enhanced emission of 4F 7/2 /4F 5/2 shows excellent temperature measurement performance, which is expected to be applied in wide-temperature-range and highly-sensitive temperature sensing. These results not only provide a pathway to realize high performance temperature sensing, but also heighten the understanding of UC emission thermal enhancement. [ABSTRACT FROM AUTHOR]

Extremely intense blue upconversion luminescence (UCL) is got in Tm3+/Yb3+ co-doped BiTa 7 O 19 (BTO) phosphors, which are synthesized by solid phase sintering. The doping concentration of Tm3+ is fixed at 7 mol%, and the concentration of Yb3+ is adjusted. When the concentration of Yb3+ is 80 mol%, the strongest blue UCL can be obtained under 980 nm laser excitation. The blue UCL integral intensity under 808 + 980 nm laser co-excitation is 1.54 times of the sum of intensity under 808 and 980 nm single excitation, respectively. Blue UCL thermal enhancement behavior was found for all the samples under 808 or 785 nm laser excitation, and blue UCL thermal quenching happened under 660, 660 + 808, and 359 nm excitations. It is demonstrated that UCL thermal enhancement behavior is caused by oxygen defects. The possible electron transition pathways are proposed under excitation with different laser wavelengths. Using the luminescence intensity branching ratio (LIR) technique, the maximum relative temperature sensitivity range is calculated to be from 0.00610 to 0.00732 K−1 at 303 K. The experimental results show that BTO:Tm3+/Yb3+ has excellent blue UCL with thermal enhancement, which can have some applications in the field of UCL display and temperature sensing in high temperature environment. [ABSTRACT FROM AUTHOR]

The KYb(MoO 4) 2 : Er3+, Y3+ phosphors were synthesized by high temperature solid state reaction method. By further doping Y3+ ions, the near-infrared (NIR, 804 nm, 4I 9/2 → 4I 15/2) luminescence of KYb(MoO 4) 2 : 0.1 % Er3+ phosphor is greatly enhanced. Based on the upconversion luminescence (UCL) spectra, the optimal doping concentration of Y3+ ions were 7.5 %. Its near-infrared UCL intensity is about 26.9 times that of undoped Y3+ ions. Through a series of characterization methods, we find that the enhancement of NIR upconversion emission is due to the generation of defect bands. It plays a crucial role in facilitating the transfer of energy from green light level (2H 11/2 , 4S 3/2) to red light level (4F 9/2) and near-infrared level. To explain the luminescence mechanism, the power dependence, UV-VI-NIR absorption spectra, excitation spectra at 804 nm and emission spectra at 380 nm were studied. In addition, the 804 nm single near-infrared UCL intensity of KYb (MoO 4) 2 : Er3+, Y3+ shows a linear variation with temperature in the temperature range of 298 K–673 K. The maximum sensitivity is 0.13 % K−1. This study provides a new method and theoretical support for the application of near-infrared UCL in optical temperature measurement. [ABSTRACT FROM AUTHOR]

The perovskite based phosphor materials are widely used to increase the efficiency of solar cells. In this work, Er3+ doped and Er3+/Yb3+ co-doped CaTiO 3 perovskite phosphor samples have been synthesized by solid state reaction technique at 1473 K. The phosphor samples show orthorhombic phase with Pnma (62) space group. The average crystallites and particles size of CaTiO 3 are increased in presence of Er3+ and Yb3+ ions. Er3+ doped CaTiO 3 phosphor samples give downshifting emission under 379 nm excitation. Though upconversion emission is seen in Er3+ under 980 nm excitation without Yb3+ ions in this host. The emission intensity of Er3+ ion is enhanced by 46 and 16 times for green and red emissions, respectively in presence of Yb3+. An intense quantum cutting (QC) emission is observed at 980 nm in presence of Yb3+ in CaTiO 3 :0.5Er3+ phosphor under 379 nm excitation. The QC efficiency has been found to be 119 % for CaTiO 3 :0.5Er3+/5 Yb3+ phosphor. Further an interesting phenomenon of back energy transfer (BEnT) from Yb3+ to Er3+ giving an intense NIR emission from Er3+ at 1002 and 1550 nm have been observed. The phosphor sample also shows an intrinsic optical bistablity (IOB) by upconversion. Thus, the prepared phosphor samples may be useful to increase the efficiency of c-Si solar cell, NIR emission for communication, bistable material and green light emitting source. [Display omitted] [ABSTRACT FROM AUTHOR]

The internal modification with rare earth ion doping proved to be a very effective strategy for improving the luminescent properties of NaYF 4 -based upconversion materials. However, greatly enhancing the luminescence efficiency of NaYF 4 :Yb3+/Er3+ remains a major challenge. Herein, the effects of Sn4+ doping on the structure and luminescent performance of such material were explored. The hydrothermal molten-salt method was applied to synthesize the Sn4+-doped NaYF 4 :Yb3+/Er3+ upconversion materials, and their crystal structures, morphologies, surface chemical composition and element states, and luminescence performance were characterized. It was found that Sn4+ doping can significantly enhance the luminescence intensity and tailor the chromaticity of NaYF 4 :Yb3+/Er3+. In particular, the green (G) and red (R) luminescence intensity levels of the 30 mol% Sn4+ doped material were increased by factors of 26.97 and 38.91, respectively. The R/G ratio was incremented from 0.34 for the undoped material to 0.83 for the 40 mol% Sn4+ doped counterpart. The Sn4+ doping led to the change of lattice distortion and crystal growth pattern of NaYF 4 :Yb3+/Er3+. The mechanism for Sn4+ doping to affect the luminescence properties of the prepared upconversion material was also explored. The changes in luminescence intensity levels and R/G ratios could be attributed to the highly asymmetric distorted lattice and crystal field resulting from Sn4+ doping. [ABSTRACT FROM AUTHOR]

Ho3+-Yb3+ co-doped glass ceramics containing LiGd(WO 4) 2 were prepared by melt crystallization method. By using the methods of differential scanning calorimetry (DSC), X-ray diffraction (XRD), transmittance, and scanning electron microscopy (SEM), the optimal heat treatment condition for the samples was identified to be 580 °C/140 min. In these conditions, the transmittance of glass ceramic is about 75 % in the 380–780 nm range. In the upconversion emission spectra, strong green emission (543 nm) and red emission (650 nm) due to 5F 4 /5S 2 , 5F 5 →5I 8 transitions are observed at 980 nm excitation. Additionally, the optimum doping concentrations of Ho3+ and Yb3+ were 0.12 % and 0.4 %. The crystal field changes around dopant ions were analyzed using J-O theory and Eu3+ probe to explain the up-conversion luminescence enhancement of glass ceramics containing LiGd(WO 4) 2. The chrominance coordinates of 0.12 % Ho3+-0.4 % Yb3+ co-doped glass ceramics are located in the green light region, and the color purity is 95.6 %. In summary, Ho3+-Yb3+ co-doped transparent glass ceramics containing LiGd(WO 4) 2 have potential applications in infrared detection and green solid-state lasers. [ABSTRACT FROM AUTHOR]

Femtosecond fluorescence upconversion experiments were combined with CASPT2 and time dependent DFT calculations to characterize the excited state dynamics of the mutagenic etheno adduct 1,N2‐etheno‐2'‐deoxyguanosine (ϵdG). This endogenously formed lesion is attracting great interest because of its ubiquity in human tissues and its highly mutagenic properties. The ϵdG fluorescence is strongly modified with respect to that of the canonical nucleoside dG, notably by an about 6‐fold increase in fluorescence lifetime and quantum yield at neutral pH. In addition, femtosecond fluorescence upconversion experiments reveal the presence of two emission bands with maxima at 335 nm for the shorter‐lived and 425 nm for the longer‐lived. Quantum mechanical calculations rationalize these findings and provide absorption and fluorescence spectral shapes similar to the experimental ones. Two different bright minima are located on the potential energy surface of the lowest energy singlet excited state. One planar minimum, slightly more stable, is associated with the emission at 335 nm, whereas the other one, with a bent etheno ring, is associated with the red‐shifted emission. [ABSTRACT FROM AUTHOR]

The photoelectrochemical (PEC) detection method, as a potential strategy for Cu2+ detection, has garnered widespread attention. In this paper, we present a PEC sensing platform using upconversion nanoparticles (UCNPs) as the conversion light source and CdTe quantum dots (QDs) as the photoactive material for the detection of Cu2+ in solution. When irradiated with a 980 nm light source, the UCNPs will absorb the 980 nm laser and emit fluorescence around 550 nm, which is then absorbed by the CdTe QDs. This absorption leads to electron-hole separation, with electrons transferring through the multi-walled carbon nanotubes (MWCNTs) into the indium tin oxide (ITO) electrode. In the presence of Cu2+, the Cu2+ will be reduced to Cu+ by the electrons generated by the CdTe QDs, thereby hindering the transfer of electrons from the CdTe QDs to the ITO electrode and resulting in a reduction in current. The photocurrent continuously decreases with increasing Cu2+ concentration and shows a good linear relationship with Cu2+ concentration in the range of 1 µM to 25 µM. The lowest detection limit is 0.5 µM. [ABSTRACT FROM AUTHOR]

[Display omitted] Hypothesis : Understanding the rules that control the assembly of nanostructured soft materials at interfaces is central to many applications. We hypothesize that electrolytes can be used to alter the hydration shell of amphiphilic oligomers at the air-aqueous interface of Langmuir films, thereby providing a means to control the formation of emergent nanostructures. Experiments : Three representative salts – (NaF, NaCl, NaSCN) were studied for mediating the self-assembly of oligodimethylsiloxane methylimidazolium (ODMS-MIM+) amphiphiles in Langmuir films. The effects of the different salts on the nanostructure assembly of these films were probed using vibrational sum frequency generation (SFG) spectroscopy and Langmuir trough techniques. Experimental data were supported by atomistic molecular dynamic simulations. Findings : Langmuir trough surface pressure – area isotherms suggested a surprising effect on oligomer assembly, whereby the presence of anions affects the stability of the interfacial layer irrespective of their surface propensities. In contrast, SFG results implied a strong anion effect that parallels the surface activity of anions. These seemingly contradictory trends are explained by anion driven tail dehydration resulting in increasingly heterogeneous systems with entangled ODMS tails and appreciable anion penetration into the complex interfacial layer comprised of headgroups, tails, and interfacial water molecules. These findings provide physical and chemical insight for tuning a wide range of interfacial assemblies. [ABSTRACT FROM AUTHOR]

Reducing the size of upconversion nanoparticles (UCNPs) down to a few nm yields luminescent materials containing a very small number of emitters. Considering the bottom limit of one activator per particle ultrasmall UCNPs offer an unprecedented platform to study the contributions of the different energy transfers at play in upconversion luminescence. Maintaining detectable emission despite the limited number of emitting ions and the high surface‐to‐volume ratio requires suitable particle architectures. Na(Gd‐Yb)F4:Tm3+ emissive sub‐3 nm diameter β‐phase UCNPs are prepared using a gadolinium‐rich composition in situ mixing of the precursors and a microwave high‐temperature cycling sequence allowing precise control of the particle size and dispersity. These cores are coated with a NaGdF4 inert shell to minimize the deleterious influence of surface quenching (SQ). Time‐resolved luminescence measurements combining standard NIR excitation of the Yb3+ sensitizer and direct UV excitation of the Tm3+ activator are performed to quantify cross relaxation and surface quenching processes. The fine tuning of the number of activators per particle via an optimized synthesis pathway along with the use of an appropriate excitation scheme enabled to provide an accurate analysis of the different mechanisms at play in these model nanoparticles and to characterize the structure of the core‐shell architecture. [ABSTRACT FROM AUTHOR]

Chemodynamic therapy (CDT) has emerged as a novel approach to overcome cancer resistance and enhance anticancer efficacy. Despite the considerable effort devoted to current chemodynamic therapeutic agents, developing efficient delivery systems to induce ferroptosis remains demanding due to their limited efficacy and lack of selectivity. Herein, an iron‐based single‐atom upconversion photocatalyst (UmFe‐OA@hPM) mimicking natural horseradish peroxidases has been developed. This nanoformulation not only targets tumors via the existence of a hybrid platelet membrane (hPM) coating but also generates excessive hydroxyl radicals in response to both tumor microenvironment and external laser irradiation. This nanoenzyme overcomes the low tissue penetration of UV light, which sensitizes the iron‐doped graphitic carbon nitride network, attributed to the unique anti‐Stokes shift from infrared to UV displayed by upconversion nanoparticles. Together with an increase in intracellular polyunsaturated fatty acid accumulation induced by oleanolic acid (OA), lipid peroxidation is significantly elevated, leading to the enhancement of CDT. UmFe‐OA@hPM is demonstrated to induce significant ferroptosis in vitro, superior antitumor efficacy in breast cancer mouse models, and suppression of metastasis status when incorporated with an immune checkpoint blockade. These findings provide a potential strategy for developing a precisely controlled CDT to deal with aggressive cancers, especially in combination with immunotherapy. [ABSTRACT FROM AUTHOR]

The commonly used automotive head‐up display (HUD) technology is confronted with the problems of poor stability in organic resin counterpart and the unsatisfactory display effect of projection lighting source. To solve these issues, developing state‐of‐the‐art materials and designing new HUD schemes with a wide color gamut is of necessity. Herein, a series of pure red/green/blue (R/G/B) upconverting (UC) nano‐glass‐ceramics (GCs) containing lanthanide‐doped NaLuF4 nanocrystals (NCs) are synthesized in a cost‐effective, environment‐friendly, and scalable manner, simultaneously achieving high crystallinity (≈45.44%) and high transparency (>80%). Yb3+ is chosen as the sensitizer to Tm3+ (blue UC)/Ho3+ (green UC), due to its large absorption cross section from ground state 2F7/2 to excited state 2F5/2; meanwhile Er3+, Tm3+ co‐doped into NCs can obtain pure red UC emission via promoting the population of Er3+ red‐emitting 4F9/2 state. Consequently, the as‐prepared materials exhibit high R/G/B color purity of > 80% owing to the energy transfer between different lanthanide ions (Yb3+, Tm3+, Ho3+, and Er3+) upon 980 nm near‐infrared (NIR) laser excitation, and possess superior thermal reversibility/water resistivity. Remarkably, two unprecedented HUD prototypes are constructed to demonstrate the application feasibility of developed pure R/G/B UC GCs. [ABSTRACT FROM AUTHOR]

Infrared upconversion devices (UCDs) enable NIR imaging without array and readout circuits, making them desirable for portable sensor, imaging and monitoring. However, the exorbitant cost and difficulties in fabrication associated with vacuum‐deposited materials, which are usually employed in high‐performance UCDs, restrict their application in flexible‐stretchable systems. Here, a solution‐processed upconversion device (s‐UCD), which is composed of detector and emitter, with high conversion efficiency and low turn‐on voltage achieved by device structure design and interlayer engineering is reported. The role of the electron blocking layer is investigated in s‐UCDs, and a peak luminance of 5,500 cd m−2 @7 V and a luminance on‐off ratio of 110000 @5.25 V are achieved. The s‐UCDs exhibit high resolution, microsecond response time and are compatible with flexible substrates. With the high‐performance large‐area s‐UCDs, direct non‐invasive transmission‐based bioimaging applications with high quality of bioimaging are further performed. It is believed that the s‐UCD imaging system offers potential applications for portable low‐cost non‐invasive tissue analysis, disease diagnosis, and virtual reality. [ABSTRACT FROM AUTHOR]

Molecular scaffolds are ideal for investigating upconversion (UC) at the highest spatial resolution and to create precisely controllable luminescent materials. Such control may be the key to overcoming the limitations of brightness and reproducibility found in UC micro‐ and nanoparticles. Cooperative UC can significantly increase luminescence brightness and bulk studies showed that highest efficiencies can be obtained by sensitizer‐to‐activator ion ratios ≥ 2, that is, via high probabilities of sensitizing the emitting lanthanide ion. Using nonanuclear molecular complexes, the authors demonstrate both experimentally and theoretically that interion distances are more relevant and that the highest UC efficiencies are actually attained for sensitizer‐to‐activator ion ratios around 1. By modeling accretive and cooperative sensitization UC, energy migration, and fitting experimental data, it is revealed that cooperative sensitization is predominant for the determination of UC luminescence intensities, whereas energy migration defines UC luminescence kinetics. The implementation of interion distances and different energy transfer mechanisms into advanced modeling of experimental UC data will be paramount for designing brighter and better UC materials. [ABSTRACT FROM AUTHOR]

Visible‐to‐ultraviolet (UV) upconversion provides a fascinating strategy to achieve deep UV emission through readily accessible visible light. However, the intensity of deep UV emission obtained through visible‐to‐UV upconversion progress is still far from satisfactory, severely constraining its practical applications. Herein, a novel class of praseodymium ion (Pr3+)‐doped rare‐earth oxyhalides (YOCl, YOBr, and LuOBr) to achieve efficient upconverted deep UV emission in the spectral range of 250–350 nm is developed. The upconverted UV emission intensity of LuOBr:Pr3+ is determined to be 56.7 times stronger than that of the well‐established Lu7O6F9:Pr3+. When employed as a photon‐converter to activate photocatalytic water splitting reactions, upconverted deep UV emission enables H2 generation under visible light (λ > 420 nm) excitation from a xenon lamp. The efficient deep UV upconversion stems from tuning 4f15d1 state of Pr3+ by oxyhalide constituent which both facilitates the absorption of excitation photons in long‐lived intermediate 4f2 states and suppress the probability of nonradiative relaxation from 4f15d1 state. These findings not only provide new insights into a mechanistic understanding of the host effect on upconversion process but also make a breakthrough in developing efficient deep upconversion materials that will expand their further applications. [ABSTRACT FROM AUTHOR]

Organic upconverters made by integrating an infrared‐sensitive photodetector with a light‐emitting diode offer a low‐cost route to visualize images taken in the infrared. However, making such devices sufficiently efficient is challenging. Here, upconversion devices are demonstrated with an efficiency of 13.9% for converting infrared photons (980 nm, 5 mW cm−2) to visible photons (575 nm). Infrared photons are detected with a photomultiplication photodetector that includes a copper thiocyanate electron‐blocking/injection layer and an infrared‐sensitive squaraine dye dispersed (3 wt−%) in a fullerene matrix. At turn‐on, the detector achieves an external quantum efficiency of 1200% (at 1020 nm, −10 V, 44 µW cm−2). Photomultiplication occurs via hole trap‐induced injection of electrons. In the upconverter, these electrons are driven into the emitter and recombine with holes under visible light emission. During operation the photodetector current increases because, presumably, rearranging mobile ions in copper thiocyanate narrows the injection barrier. Thereby, the upconverter photoconversion efficiency gradually increases to 18.7%. The performance of the present upconverter is limited by the not‐yet‐ideal charge‐blocking/injection layer, which is too thick and blocks electrons in the dark insufficiently. With thin and compact charge‐blocking layers at hand, the device concept paves the way for widespread use in sensitive infrared imaging. [ABSTRACT FROM AUTHOR]

A series of phosphine oxide‐containing gold(III) complexes with tunable emission colors spanning from sky‐blue to near‐infrared region is reported. This is accomplished by the switching of the excited state characters from intraligand to ligand‐to‐ligand charge transfer through the replacement of the auxiliary ligand from aryl to nitrogen‐based ligands. In addition to high photoluminescence quantum yields in both solution and solid‐state thin films, these complexes exhibit large radiative decay rate constants of the order of 106 s−1, much larger than those commonly found for other gold(III) complexes. The origin of such enhanced performance is believed to be arising from the occurrence of both thermally activated delayed fluorescence and thermally stimulated delayed phosphorescence processes within the emitters. This is probed by ns‐ and fs‐transient absorption spectroscopy, time‐resolved, and temperature‐dependent emission spectroscopy. In particular, the direct observation of the upconversion processes and the determination of the activation barriers are achieved in the variable‐temperature fs‐transient absorption spectroscopic studies. Solution‐processed organic light‐emitting devices with satisfactory external quantum efficiencies of up to 15.2% are achieved, which could be ascribed to the presence of thermally activated delayed fluorescence and/or thermally stimulated delayed phosphorescence processes. [ABSTRACT FROM AUTHOR]

Transparent glass with photochromic and luminescent properties has recently garnered considerable interest in optical storage, while the photochromic transparent glass system is still limited. Herein, it is proposed that combining transparent glass matrix with photochromic nanocrystals is an efficient strategy to develop a broader range of photochromic luminescent glass systems. The Yb3+/Tb3+ co‐doped BaMgSiO4 glass ceramics are successfully prepared by a two‐step melt‐quenching approach followed by a thermal treatment in a reducing atmosphere. The glass ceramics exhibit high transparency and distinct coloration, showing a color change between colorless and pink by alternating irradiation between 365 nm ultraviolet light and 473 nm laser. The formation of color centers in the BaMgSiO4 nanoparticles drives the coloration of such glass ceramics. Based on their photochromic and photobleaching behavior, the efficient reversible upconversion luminescence modulation can be implemented with the maximum luminescence modulation rate of 79.83%. The excellent reproducibility and anti‐fatigue of such upconversion luminescence modulation demonstrate the promising application of Yb3+/Tb3+ co‐doped BaMgSiO4 glass ceramics as an optical storage medium. [ABSTRACT FROM AUTHOR]

Yb3+ doped CsPbCl3 metal halide perovskite photodetectors (PDs) in the structure of CsPbCl3(50 nm)/YbCl3(x nm)/CsPbCl3(50 nm), in which x ranges from 10 to 40 nm corresponding to the molar ratio from 6.3% to 25.2%, are fabricated by thermal evaporation on Si/SiO2 substrate. Photoresponse from 350 to 980 nm have been achieved with the optimal responsivity (R) of 3959, 5425, 955 A W−1 for the case of 20 nm YbCl3 at the wavelength (λ) of 420, 680 and 980 nm, respectively. A series of photophysical and electrical characterization has been performed and it is found that the remarkably improved photoresponse originates from the combining effects of upconversion and defects passivation from Yb3+. Moreover, the optimal YbCl3 thickness of 20 nm can be ascribed to the balance between upconversion and concentration quenching of Yb3+. The influence of the YbCl3 doping on the CsPbCl3 electronic structure is investigated and downshifting and stabilization of valence band maximum (VBM) can be attributed to the p-type doping and counteracting effect of Yb3+ and Cl−, respectively. [ABSTRACT FROM AUTHOR]

To investigate a near-infrared-to-blue luminescence upconversion curing method for polymerizing resin cements under zirconia discs. Lava zirconia discs of different thicknesses (0.5–2.0 mm) were manufactured. First, the transmittances of the NIR and two blue lights (BLs) (LED and halogen lights) through these discs were measured. Second, NaYF 4 :Yb3+/Tm3+ upconversion phosphor (UP) powder was milled into 0.5-μm particle sizes. A light-curable resin cement VariolinkII base was chosen as the control (UP0), and an experimental cement (UP5) was prepared by adding 5 % UPs. These two cements were examined using multiphoton excitation microscopy for particle distribution. UP5 and UP0 were polymerized with or without zirconia shielding then subjected to a microhardness test. A multifold analysis was performed to examine the effects of zirconia thickness, curing protocols (pure BL or combined BL and NIR curing), and cement type. The transmittance of NIR was superior to that of BL through zirconia discs of all thicknesses. UP particles were homogeneously distributed in UP5 and emitted blue luminescence under 980-nm NIR excitation. UP5 showed higher microhardness values than UP0 under any curing protocol or zirconia shielding condition. The combination of 20-s BL and 40-s NIR curing yielded the highest microhardness in uncovered UP5. However, combining 40-s BL and 20-s NIR curing surpassed the other groups when the zirconia discs were thicker than 0.5 mm. NIR exhibits higher transmission through zirconia than BL. UP particles work as strengthen fillers and photosensitizers in cements. NIR upconversion curing could be a new strategy for polymerizing resin cements under thick zirconia restorations. • An NIR activated upconversion reaction was used to induce blue light (BL) emission. • The transmittance of NIR through zirconia discs is higher than conventional BL. • Phosphor particles work as photosensitizers and strengthen fillers in cements. • Combining BL and NIR effectively enhanced polymerization of cements under zirconia. [ABSTRACT FROM AUTHOR]

An upconversion fluorescence sensing platform was developed with upconversion nanoparticles (UCNPs) as energy donors and gold nanoparticles (AuNPs) as energy acceptors, based on the FRET principle. They were used for quantitative detection of uranyl ions (UO22+) by amplifying the signal of the hybrid chain reaction (HCR). When UO22+ are introduced, the FRET between AuNPs and UCNPs can be modulated through a HCR in the presence of high concentrations of sodium chloride. This platform provides exceptional sensitivity, with a detection limit as low as 68 pM for UO22+ recognition. We have successfully validated the reliability of this method by analyzing authentic water samples, achieving satisfactory recoveries (89.00%–112.50%) that are comparable to those of ICP-MS. These results indicate that the developed sensing platform has the capability to identify trace UO22+ in complex environmental samples. [ABSTRACT FROM AUTHOR]

Photoactivatable carbon monoxide‐releasing molecules (CORMs), typically based on transition‐metal carbonyl complexes, have reliance on activation by UV or visible light that restricts their biomedical applications. To address this limitation, a near‐infrared (NIR)‐responsive nanoplatform is presented based on upconversion nanoparticles (UCNPs) loading with manganese carbonyl complex Mn2(CO)10 that concurrently releases CO and manganese ion (Mn2+). With the UCNPs, the more tissue‐penetrable NIR is used to locally generate UV light for photodecomposition of Mn2(CO)10 into CO and manganese oxide (MnOX), after which MnOX is reduced to Mn2+ by the overexpressed glutathione in cancer cells. Moreover, the released Mn2+ can serve as a magnetic resonance imaging contrast agent to monitor the NIR‐controlled corelease of CO and Mn2+ in real time. Therefore, this nanoplatform can provide a potential strategy for NIR‐enabled spatiotemporally release of CO and Mn2+, enhancing the controlled delivery and biomedical application of CORMs. [ABSTRACT FROM AUTHOR]

Real-time monitoring of temperature changes in substation equipment is crucial for preventing failures and ensuring a stable power supply. Currently, temperature monitoring of substation equipment primarily relies on manual infrared thermometry, which has limitations such as strong operational dependence, susceptibility to interference, and difficulty in detecting internal faults. The luminous intensity ratio (LIR) is a stable optical parameter unaffected by factors such as spectral loss and environmental influences, making it suitable for temperature detection. The multi-emission characteristics of rare-earth-doped upconversion (UC) materials are highly compatible with LIR technology, demonstrating their potential in high-precision temperature monitoring. Here we introduces a non-contact temperature monitoring method for substation equipment based on NaErF4@ NaYF4 core-shell UC materials and LIR technique. Experiments show that the method has high accuracy and sensitivity within temperature range of 25 °C to 225 °C, with a sensitivity as high as 35 x 10-3 °C-1, effectively monitoring temperature changes both inside and outside the equipment. This provides a new technical solution for temperature monitoring of substation equipment. [ABSTRACT FROM AUTHOR]

Weak luminescence of the small‐sized lanthanide‐doped nanoparticles has limited their applications long. In recent years, there is a growing interest in luminescence thermal enhancement in small‐sized upconversion nanoparticles (UCNPs). The mitigation of surface quenching effects and the improvement of energy transfer (ET) are two convincing explanations for this phenomenon. A systematical investigation on the luminescence dynamics of Yb3+ is implemented. ET rate and then the proportion of ET increases greatly with rising temperatures in Yb3+/Tm3+ codoped system, while an alternative trend can be summarized in Yb3+/Er3+ one. Based on these findings, therefore it can be concluded that the surface quenching mitigation related to the desorption of water molecules on the surface is a common mechanism of the luminescence thermal enhancement. But ET related to the thermally‐activated surface phonons is a special case, it does play a positive role in Yb3+/Tm3+ system. These findings not only explain the upconversion luminescence thermal enhancement in Yb3+/Tm3+ system is more significant but also help to understand thermal enhancement and benefit highly‐sensitive temperature probe design. [ABSTRACT FROM AUTHOR]

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr‐ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge‐transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr‐ESFG especially sensitive to the trion formation dynamics. The presence of charge‐transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers. [ABSTRACT FROM AUTHOR]

Although interfacial solvation plays an important role in determining carbon dioxide reduction (CO2R) kinetics, present understanding of the potential dependent properties of the electrochemical double layer under conditions relevant for CO2R remains limited. This article summarizes the development and recent applications of plasmon‐enhanced vibrational sum frequency generation (VSFG) spectroscopy to study the effects of cation hydration and interfacial solvation on CO2R using CO as a vibrational Stark reporter. Results show that electrolyte cations retain their entire solvation shell upon adsorption to inactive sites, while active sites retain only a single water layer between the gold surface and the cation. Measurements also show that the total interfacial electric field can be separated into two contributions: one from the electrochemical double layer (Stern field) and another from the polar solvation environment (Onsager field). Surprisingly, correlating VSFG spectra with reaction kinetics reveals that it is the solvation‐mediated Onsager field that governs the chemical reactivity at the electrode/electrolyte interface. Measuring the interfacial water spectra during electrocatalysis also provides evidence for the proton source during H2 evolution, which competes with CO2R in aqueous electrolyte. These findings highlight the importance of directly probing cation hydration and interfacial solvation, which mediates reaction kinetics at electrochemical interfaces. [ABSTRACT FROM AUTHOR]

In this work, we investigate trion dynamics occurring at the heterojunction between organometallic molecules and a monolayer transition metal dichalcogenide (TMD) with transient electronic sum frequency generation (tr‐ESFG) spectroscopy. By pumping at 2.4 eV with laser pulses, we have observed an ultrafast hole transfer, succeeded by the emergence of charge‐transfer trions. This observation is facilitated by the cancellation of ground state bleach and stimulated emission signals due to their opposite phases, making tr‐ESFG especially sensitive to the trion formation dynamics. The presence of charge‐transfer trion at molecular functionalized TMD monolayers suggests the potential for engineering the local electronic structures and dynamics of specific locations on TMDs and offers a potential for transferring unique electronic attributes of TMD to the molecular layers. [ABSTRACT FROM AUTHOR]

The document is a contents list for the journal CrystEngComm, which focuses on the design and understanding of solid-state and crystalline materials. It includes a range of papers on various topics, such as photon upconversion, catalytic activity, structural diversity, and crystal stability. The journal is published by The Royal Society of Chemistry, a leading chemistry community. The document provides a list of papers and their authors, allowing library patrons to quickly identify articles of interest for their research. [Extracted from the article]

808 nm excited Nd3+ doped upconversion nanoparticles (UCNPs) have become a new research hotspot due to their reduced thermal effect when applied in a water-rich system. Different from Tm3+-mediated energy migration upconversion of Tb3+, in this work, NaYF4:Yb/Tb@NaYF4:Yb@NaNdF4:Yb@NaYF4 core–shell–shell–shell nanoparticles were designed based on the Yb3+ sensitized cooperative energy-transfer process to realize photon upconversion. NaYF4:Yb served as the migration layer, separating the activator Tb3+ from the sensitizer Nd3+ and minimizing the deleterious cross-relaxation between them. At 40% Yb3+ doping concentration in the NaYF4:Yb layer, bright green luminescence was achieved under 808 nm excitation. Meanwhile, the presence of Yb3+ in the sensitization layer (NaNdF4:Yb) reduced Nd3+ cross-relaxation, leading to improved energy migration efficiency of Nd3+ → Yb3+. Finally, the incorporation of an inert NaYF4 shell resulted in a significant luminescence boost. These nanoparticles can be dispersed in water after surface modification and could be effectively excited after laser penetration of 3 cm of water, indicating the enormous potential of the designed UCNPs in the biological system. [ABSTRACT FROM AUTHOR]

Early-stage diagnosis of prostatic carcinoma is essential for successful treatment and, thus, significant prognosis improvement. In laboratory practice, the standard non-invasive diagnostic approach is the immunochemical detection of the associated biomarker, prostate-specific antigen (PSA). Ultrasensitive detection of PSA is essential for both diagnostic and recurrence monitoring purposes. To achieve exceptional sensitivity, we have developed a microfluidic device with a flow-through cell for single-molecule analysis using photon-upconversion nanoparticles (UCNPs) as a detection label. For this purpose, magnetic microparticles (MBs) were first optimized for the capture and preconcentration of PSA and then used to implement a bead-based upconversion-linked immunoassay (ULISA) in the microfluidic device. The digital readout based on counting single nanoparticle-labeled PSA molecules on MBs enabled a detection limit of 1.04 pg mL−1 (36 fM) in 50% fetal bovine serum, which is an 11-fold improvement over the respective analog MB-based ULISA. The microfluidic technique conferred several other advantages, such as easy implementation and the potential for achieving high-throughput analysis. Finally, it was proven that the microfluidic setup is suitable for clinical sample analysis, showing a good correlation with a reference electrochemiluminescence assay (recovery rates between 97% and 105%). [ABSTRACT FROM AUTHOR]

A method of the transfer of NaYF4:Yb3+Tm3+/NaYF4 upconversion core/shell nanoparticles with an average size of 30 nm via laser-induced forward transfer is proposed. The method provides a high spatial resolution by creating a "sandwich" structure on the donor substrate: for reliable fixation, nanoparticles are located between gold layers 50 and 20 nm thick. The transfer of upconversion nanoparticles is implemented by focusing nanosecond laser radiation into a 30-μm-diameter spot and at optimal pulse energies of 8.5–25 μJ. It has been shown that, despite large temperature, K, and pressure, MPa, fluctuations upconversion nanoparticles fully retain their photoluminescent characteristics. [ABSTRACT FROM AUTHOR]

A near‐infrared luminescent complex Er(DBTTA)3(DBFDPO) [where DBTTA = dibenzotetrathienoacene; DBFDPO = 4,6‐bis (diphenylphosphoryl) dibenzofuran] is synthesized. Based on the intramolecular energy transfer between organic ligands and Er3+ ions, optical gains at 1535 nm are demonstrated in Er(DBTTA)3(DBFDPO)‐doped polymer waveguides under the excitation of light‐emitting diodes (LEDs) instead of laser pumping. Relative gains of 6.4, 8.2, and 10.6 dB are obtained in 1 cm long waveguides with cross‐sectional dimensions of 6 × 4, 4 × 4, and 2 × 3 µm2 respectively, using the vertical top‐pumping mode of a 365 nm LED with 462 mW. Incorporating an ≈100 nm thick aluminum reflector grown under the lower cladding, enhanced the optical gain to 11.6 dB cm−1 in a waveguide with a cross‐section of 4 × 4 µm2, and an internal gain of ≈7.4 dB cm−1 is achieved. By relying on the intramolecular energy transfer and LED top‐pumping technology, the upconversion luminescence of Er3+ ions and thermal damage to polymer waveguides caused by the high power density of laser pumping can be effectively reduced. The complex Er(DBTTA)3(DBFDPO)‐doped polymer can be spin‐coated on different types of waveguides to compensate for optical losses at 1.5 µm and is expected to have a critical role in planar photonic integration. [ABSTRACT FROM AUTHOR]

A novel force sensor exploiting the interaction between plasmonic nanostructures and upconversion nanoparticles (UCNPs) is reported. The nanosensor is composed of a gold nanodisk and UCNPs separated by a flexible polymer layer. The gold nanodisk is designed to exhibit a plasmon resonance that selectively enhances one of the emission bands of the UCNPs while leaving the other ones largely unaffected. As the nanosensor is compressed or stretched by an external force, the polymer layer thickness changes, modulating the plasmon‐UCNP coupling. The resulting changes in the luminescence intensity provide the basis for sensing. Furthermore, the nanosensor employs ratiometric sensing, which makes it highly robust against any environmental variations. The nanosensors exhibit two orders of magnitude higher responsivity than previously reported UCNP‐based force sensors. They can be prepared as an on‐chip sensor array or in a colloidal solution, making them suitable for a variety of applications in biology and robotics. [ABSTRACT FROM AUTHOR]

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:Sr 5 (PO 4) 3 F (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. • The xEr:S-FAP (x=0–0.05) transparent nanostructured ceramics with highly optical quality were prepared by hot-pressed sintering for the first time. • The microstructural characteristics, upconversion luminescence property, and mechanism of as-synthesized xEr:S-FAP transparent ceramics were investigated for the first time. • The wide visible light spectrum from red light to green light was achieved by changing the doping amount of Er3+ from 0.5 to 5 at%. [ABSTRACT FROM AUTHOR]

Lanthanide‐doped upconversion nanoparticles (UCNPs) have attractive emission properties but suffer from weak light‐absorbing capacities and thereby relatively low brightnesses. This motivates using strongly absorbing dye molecules as antennas and sensitizers. However, despite much effort, understanding of this dye‐UCNP interplay is still limited. Major sensitization mechanisms are still under discussion, largely because there is a lack of effective means to observe key factors such as dark state transitions within the dyes. Here, a combined spectroscopic procedure is established to systematically investigate the photophysics behind the dye‐UCNP interaction, embracing fluorescence‐based transient‐state excitation‐modulation, lifetime and correlation spectroscopy, and spectrofluorometry/spectrophotometry. With this procedure the heptamethine cyanine dye IR806, a typical UCNP sensitizer is studied, its photophysical model is established, its photophysics in UCL‐sensitization‐related environments is deciphered, and the energy transfer from the IR806 singlet excited state to Yb3+ (UCNP sensitizer ion) can be identified as the dominant sensitization mechanism. These studies suggest that IR806 can form non‐emissive H‐aggregates at the nanoparticle surfaces, which can be dissociated after certain light excitation duration (typically>100 µs). Moreover, buildup of a non‐fluorescent, photo‐redox state of IR806 after longer irradiation times (10–100 ms) can deleteriously affect its UCL sensitization effect, inferring an optimal excitation duration for dye‐sensitized UCNPs, relevant for, e.g., optical imaging applications. [ABSTRACT FROM AUTHOR]