High brightness of upconversion luminescence (UCL) for a thinner layer of UC nanoparticles is significant for routine applications of effective trademark anticounterfeiting technology. In this work, efficient UCL of NaYF4:Yb3+,Er3+/Tm3+ was realized by combining a Ta2O5 dielectric layer on the cyclical island silver films supported by poly(methyl methacrylate) opal photonic crystals (PCs). The synergistic modulation of localized surface plasmon resonance and PC effect results in a significant improvement of the local electromagnetic field and an optimum UC enhancement of 145 folds. Furthermore, colorful pattern nanoprinting has been applied to this composite and used for trademark anticounterfeiting. The combination of angle-dependent PC effect and infrared-to-visible UCL represents a more advanced anticounterfeiting technique.Keywords: dielectric; nanoparticles; periodic silver; photonic crystal; surface plasmon; upconversion;

The ability to modulate the intensity of electromagnetic field by semiconductor plasmon nanoparticles is becoming attractive due to its unique doping-induced local surface plasmon resonance (LSPR) effect that is different from metals. Herein, we synthesized mCu2–xS@SiO2@Y2O3:Yb3+/Er3+ core–shell composites and experimentally and theoretically studied the semiconductor plasmon induced up-conversion enhancement and obtained 30-fold up-conversion enhancement compared with that of SiO2@Y2O3:Yb3+/Er3+ composites. The up-conversion enhancement was induced by the synthetic effect: the amplification of the excitation field and the increase of resonance energy transfer (ET) rate from Yb3+ ions to Er3+ ions. The experimental results were analyzed in the light of finite-difference time-domain (FDTD) calculations, confirming the effect of the amplification of the excitation field. In addition, up-conversion luminescence (UCL) spectra, up-conversion enhancement, and dynamics dependent on concentration (Yb3+ and Er3+ ions) were investigated, and it was found that the resonance ET rate from Yb3+ ions to Er3+ ions increased ∼25% in the effect of LSPR waves. Finally, the power dependence of fingerprint identification was successfully performed based on the mCu2–xS@SiO2@Y2O3:Yb3+/Er3+ core–shell composites, the color of which can change from green to orange with excitation power increasing. Our work opens up a new concept to design and fabricate the up-conversion core–shell structure based on semiconductor plasmon nanoparticles (NPs) and provides applications for up-conversion nanocrystals (UCNPs) and semiconductor plasmon NPs in photonics.Keywords: fingerprint identification; local surface plasmon resonance; semiconductor plasmon; up-conversion enhancement; up-conversion nanoparticles;

Cesium lead halide (CsPbX3) perovskite nanocrystals (NCs) have demonstrated extremely excellent optical properties and great application potentials in various optoelectronic devices. However, because of the anion exchange, it is difficult to achieve white-light and multicolor emission for practical applications. Herein, we present the successful doping of various lanthanide ions (Ce3+, Sm3+, Eu3+, Tb3+, Dy3+, Er3+, and Yb3+) into the lattices of CsPbCl3 perovskite NCs through a modified hot-injection method. For the lanthanide ions doped perovskite NCs, high photoluminescence quantum yield (QY) and stable and widely tunable multicolor emissions spanning from visible to near-infrared (NIR) regions are successfully obtained. This work indicates that the doped perovskite NCs will inherit most of the unique optical properties of lanthanide ions and deliver them to the perovskite NC host, thus endowing the family of perovskite materials with excellent optical, electric, or magnetic properties.Keywords: lanthanide; multicolor emissions; near-infrared emissions; Perovskite nanocrystal;

Uniform, monodispersed, and size-controllable YVO4:Eu3+ nano- and microspheres ranging of 20−1200 nm in diameter were successfully synthesized under the solvothermal condition with N,N-dimethylformamide as the solvent, poly(vinyl pyrrolidone) (PVP) and cetyltrimethyl ammonium bromide (CTAB) as surfactants, meanwhile, introducing HCl acid in the reaction. They were characterized by various techniques, including field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray power diffraction (XRD), and Fourier transform infrared (FTIR) spectra. The results indicate that the assistant of PVP played an important role in the formation of uniform and monodispersed YVO4:Eu3+ spheres, which effectively prevented aggregation of crystallites. The amount of acid can adjust particle size, while the coassistant of CTAB is helpful of forming spherelike particles. A possible formation mechanism of YVO4:Eu3+spheres was proposed. Furthermore, size-dependent luminescent properties were studied. It is interesting to observe that the excitation bands originating from VO43− groups became narrow on the red side with the decreasing particle size, which is attributed to the size confinement effect to energy transfer. As a consequence, the intensity ratio of 5D0−7FJ to 5D1−7FJ increased and the initial population ratio of 5D0−5D1 increased. A detailed model was proposed to explain the size-dependent photoluminescence.

Semiconductor plasmon nanoparticles are currently attracting extensive interest because of their unique double character as a semiconductor and metal. In this work, we report that Cu2–xS nanoparticles (NPs) demonstrate not only tunable localized surface plasmon resonance (LSPR) but also a two-photon absorption effect under infrared light pumping, which depends strongly on the size, composition, and band gap of the NPs. This interaction with the upconversion nanoparticles NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ was systemically studied under excitation of multiwavelengths 808, 980, and 1540 nm. For the localized electromagnetic field to be enhanced further, the Cu2–xS NPs were inlayed into the surfactant apertures of three-dimensional poly(methyl methacrylate) opals. On the basis of the synergistic interaction of the LSPR effect, the nonlinear effect and the photonic crystal effect of the hybrids, an upconversion enhancement of up to 1500-fold was achieved with an absolute brightness of 1282 cd/m2 under excitation by 1.25 W/mm2 980 nm light. The experimental results were analyzed through comparison with finite-difference time-domain calculations. Finally, on the basis of the hybrids, novel angle-dependent infrared anticounterfeiting was successfully performed, which is extremely difficult to simulate. Our discovery provides a new concept for designing and optimizing luminescent materials and highlights the novel application of plasmonic semiconductor NPs in photonics.

Efficient targeting is a major challenge in practical photodynamic therapy (PDT). Though the “enhanced permeability and retention” (EPR) effect is a widely used tumor targeting method, magnetic targeting strategy is more promising considering the issue of high targeting efficiency and reducing concentration-dependent toxicity. Herein, magnetic targeting and highly effective Fe3O4@Ce6/C6@silane NPs are reported as a class of precisely controlled photosensitizers (PS) for PDT. On the basis of the amphiphilic silane encapsulation, PS chlorin e6 (Ce6) and Coumarin 6 (C6) as well as Fe3O4 NPs were coloaded into the inside hydrophobic environment of amphiphilic silane, forming a theranostic agent for dual-mode imaging guided and magnetic targeting enhanced in vivo PDT agent. To solve the problem of over-irradiation, the coloaded design of C6 and Ce6 molecules can afford the real time PDT monitoring by ratio emissions with same excitation wavelength. When Fe3O4@Ce6/C6@silane and Ce6/C6@silane NPs are compared in in vitro and in vivo experiments, the introduction of Fe3O4 in the composite does not affect the PDT efficiency, whereas, in contrast, it brings MRI imaging and magnetic targeting functions. Fe3O4@Ce6/C6@silane injection followed with magnetic field (MF) and light irradiation is important in generating an effective PDT process, showing great potential in tumor therapy.Keywords: amphiphilic silane; iron oxide; magnetic resonance imaging; magnetic targeting; photodynamic therapy;

Perovskite solar cells (PSCs) with high efficiency have recently received tremendous attention, but the stability under light irradiation, namely, photostability, of PSCs still represents a major obstacle that must be overcome before their practical applications can be used. The degeneration of perovskite under ultraviolet irradiation from sunlight is a major impacting factor. To solve this problem, in this work we introduce fluorescent carbon dots (CDs), which could effectively convert ultraviolet to blue light in the mesoporous TiO2 (m-TiO2) layer of the traditional PSCs. As a result, CD-based devices exhibit an improved power conversion efficiency (PCE) of 16.4% on average compared to 14.6% for bare devices, and the light stability of CD-based devices is highly enhanced. These devices can maintain nearly 70% of the initial efficiency after 12 h of full sunlight illumination, while the bare devices maintain only 20% of the initial efficiency. This work indicates that fluorescent down conversion based on CDs is a novel and effective approach to improve the performance and photostability of PSCs.Keywords: carbon dots; fluorescent down conversion; perovskite solar cells; photostability; photovoltaic performance;

Currently, perovskite solar cells (PSCs) are attracting extensive interest due to their potential for overcoming the energy crisis. However, increasing the power conversion efficiency (PCE) of PSCs to realize outdoor applications is still one of the crucial issues in PSC research. In this study, metallic nanostructures including Au, Ag and Au–Ag nanoalloy with different sizes and morphology were synthesized via a chemical solution method and embedded into an electron collecting TiO2 layer in PSCs based on the physical deposition method, which allows control over the incorporation of the metallic-nanostructures. The best improvement in the PCE was obtained using the Au–Ag nanoalloy, exhibiting an efficiency of 14.8%, which increases 17.5% compared to the bare PSCs. We believe that the increased device performance originates from increased light harvesting due to the increased optical path length caused by the light scattering of the metallic nanostructures. This strategy demonstrates a novel way to enhance the performance of photovoltaic devices.

Carbon dots (CDs) have emerged as novel fluorescent probes due to their remarkable optical properties; however, red emission is still rare, has a relatively low efficiency, and its mechanism remains ambiguous. Herein, relatively efficient red-emission CDs based on p-phenylenediamine were prepared through various solvothermal means, where the highest quantum yield approached 41.1% in n-amyl alcohol, which was the most efficient quantum yield reported to date. Various structural characterizations were performed and confirmed that the red emission originated from the molecular states consisting of a nitrogen-containing organic fluorophore. The CDs were dispersed in different organic solvents and showed tunable emission, evolving from green to orange-red in aprotic solvents and a red emission in protic solvents. Further solvent correlation studies indicated that the hydrogen bond effect between the CDs and solvents was the main mechanism leading to the spectral shift. Accordingly, solid-state luminescent CDs–polymers were fabricated, which also demonstrated continuously tunable emission properties. This work opens a new window for recognizing the generation of tunable and red-emission CDs.

AbstractQuantum cutting can realize the emission of multiple near-infrared photons for each ultraviolet/visible photon absorbed, and has potential to significantly improve the photoelectric conversion efficiency (PCE) of solar cells. However, due to the lack of an ideal downconversion material, it has merely served as a principle in the laboratory until now. Here, the fabrication of a novel type of quantum cutting material, CsPbCl1.5Br1.5:Yb3+, Ce3+ nanocrystals is presented. Benefiting from the larger absorption cross-section, weaker electron–phonon coupling, and higher inner luminescent quantum yield (146%), the doped perovskite nanocrystals are successfully explored as a downconverter of commercial silicon solar cells (SSCs). Noticeably, the PCE of the SSCs is improved from 18.1% to 21.5%, with a relative enhancement of 18.8%. This work exhibits a cheap, convenient, and effective way to enhance the PCE of SSCs, which may be commercially popularized in the future.

•A specific 3DIO nanostructure ZnO-Fe3O4 was obtained which can not only lead to largely increase of the surface area, but also the interaction between ZnO and Fe3O4 could effectively increase the sensing performances.•The effect of 3DIO structure and ZnO-Fe3O4 composite structure on response enhancement was analyzed through the comparison between samples of ZnO nanoparticles/3DIO ZnO and 3DIO ZnO/3DIO ZnO-Fe3O4 composite structure.•The 3DIO ZnO-Fe3O4 sensor exhibited high response as 47 to 50 ppm acetone at the optimal temperature. And the detection limit as low as 100 ppb was obtained, indicating great potential in ppb-level acetone sensing application, especially for diabetes biomarker gas.Three-dimensional inverse opal (3DIO) ZnO-Fe3O4 composite structure was prepared by a template method and the gas sensing property was investigated toward trace acetone which is an important biomarker of diabetes. The prepared 3DIO composite sensors have large surface area due to both macropores (larger than 100 nm) and mesoporous structure in skeleton, which make the inner surface accessible and provide more active sites to adsorb more oxygen species and target molecules. 3DIO ZnO-Fe3O4 composite sensor exhibits much higher response toward acetone relative to pure ZnO sensor. The optimal 3DIO sensor exhibited a detection limit as low as 100 ppb acetone and showed 40% response increase from 0.9 ppm (healthy humans) to 1.8 ppm (diabetics) which allow reliable diagnosis of diabetic patients by acetone monitoring.

Detection and isolation of circulating tumor cells (CTCs) play a pivotal role in the diagnosis and prognosis of cancer, while the high capture efficiency and purity of CTCs are difficult to achieve simultaneously among the various isolation methods. In this work, we designed an inverted microchip integrating silicon nanowires (SiNWs) and multifunctional magnetic nanocomposites (Fe3O4@C6/Ce6@silane, Coumarin 6 (C6), Chlorin e6 (Ce6)) for enhanced capture efficiency and purity of CTCs. The Fe3O4@C6/Ce6@silane conjugated with antibody can label the CTCs and pull them to the upside SiNWs capture surface by the upward magnetic field with high purity. This inverted structure was also featured with real-time detection and photodynamic therapy (PDT) of CTCs with the confocal laser scanning microscope (CLSM). The results indicate the important role of the composites labels and the magnetic field, which greatly improves the capture purity of the CTCs to 90%. Meanwhile, capture efficiency of CTCs achieve to 90.3% in culture medium and 82% in blood with 2 mL/h flow rate, respectively. Based on the structure of the device and composites, the captured CTCs could be directly inactivated by the in situ photodynamic therapy in the capture process which holds positive impact to block cancer spread.

Fluorescent organic nanoparticles (FONs) based on polydopamine (PDA) have recently emerged as a novel fluorescent probe due to its facile synthesis procedure, good water solubility, and excellent biocompatibility. However, previously reported PDA-FONs show low monodispersity and efficiency, which largely limit their application. In this study, we report a new type of FONs that has been prepared using carbon dots (CDs) as seeds and assembled via the self-polymerization of dopamine molecules. The prepared FONs showed high efficiency and monodispersity; moreover, via controlling the time of the polymerization reaction, different FONs could be obtained, which demonstrated similar structures but with tunable emission properties, and the emission gradually evolved from blue to green with the increasing reaction time. The mechanism of the prepared FONs was confirmed to be via the Förster resonance energy transfer (FRET) effect occuring between CDs and polymerized dopamine, leading to high efficiency and tunable emission. The FONs were also explored for cell imaging and cytotoxicity experiments, and they showed excellent biocompatibility and good prospects in biotechnological applications.

Fabrication of near-infrared light-triggered photoelectrochemical (PEC) sensors based on the upconversion nanophosphors (UCNPs) is a novel approach, which exhibits the advantages of low background signal and non-damage to the biological substance as well as high sensitivity and improved electric detection in PEC sensors. Herein we demonstrate the preparation of novel and high-quality ZnO inverse opal photonic crystals (IOPCs)/Ag/NaYF4:Yb,Tm hybrid films by different advanced film techniques, including colloidal self-assembling, vapor phase deposition and pulsed laser deposition and its application to sensitive detection of alpha-fetoprotein (AFP). In the complex device, ZnO IOPCs and surface plasmon resonance (SPR) of silver had ability to largely enhance local excitation electromagnetic field of NaYF4:Yb,Tm, resulting in efficient near-infrared to visible/ultraviolet upconversion luminescence (UCL). The ultraviolet light emitted by NaYF4:Yb,Tm could b further reabsorbed by ZnO, generating PEC responses. Furthermore, because of the high specific surface area of ZnO IOPCs and the conductivity of Ag films, ZnO IOPCs/Ag/NaYF4:Yb,Tm hybrid films based near-infrared light-triggered PEC sensors showed ultrasensitive detection of AFP with a linear range from 0.05 ng mL−1 to 100 ng mL−1 and a low detection limit of ∼0.04 ng mL−1 (40 pg mL−1). Such an advanced device also shows promise in detection of other cancer markers in clinical and biological analysis.

Fabrication of near-infrared light-triggered photoelectrochemical (PEC) sensors based on the upconversion nanophosphors (UCNPs) is a novel approach, which exhibits the advantages of low background signal and non-damage to the biological substance as well as high sensitivity and improved electric detection in PEC sensors. Herein we demonstrate the preparation of novel and high-quality ZnO inverse opal photonic crystals (IOPCs)/Ag/NaYF4:Yb,Tm hybrid films by different advanced film techniques, including colloidal self-assembling, vapor phase deposition and pulsed laser deposition and its application to sensitive detection of alpha-fetoprotein (AFP). In the complex device, ZnO IOPCs and surface plasmon resonance (SPR) of silver had ability to largely enhance local excitation electromagnetic field of NaYF4:Yb,Tm, resulting in efficient near-infrared to visible/ultraviolet upconversion luminescence (UCL). The ultraviolet light emitted by NaYF4:Yb,Tm could b further reabsorbed by ZnO, generating PEC responses. Furthermore, because of the high specific surface area of ZnO IOPCs and the conductivity of Ag films, ZnO IOPCs/Ag/NaYF4:Yb,Tm hybrid films based near-infrared light-triggered PEC sensors showed ultrasensitive detection of AFP with a linear range from 0.05 ng mL−1 to 100 ng mL−1 and a low detection limit of ∼0.04 ng mL−1 (40 pg mL−1). Such an advanced device also shows promise in detection of other cancer markers in clinical and biological analysis.

Near-infrared-downconversion-near-infrared (NIR-DC-NIR) bioimaging based on lanthanide doped upconversion nanoparticles (UCNPs) has received much attention due to its deeper penetration, and higher contrast imaging and signal-to-noise ratio in biological tissues. The size of UCNPs determines the mechanism and rate of cell uptake of the nanoparticles and their ability to permeate through biological tissues. Herein, we experimentally and theoretically demonstrate downconversion-near-infrared (DC-NIR) emission behavior in different sized UCNPs ranging from 5–150 nm. Interestingly, 15–40 nm UCNPs have more effective DC-NIR emissions than 150 nm UCNPs and an extremely high excitation threshold, which is entirely different from the size-dependent upconversion-visible (UC-VIS) emissions usually observed in UCNPs. We also observed that the intensity ratio of the DC-NIR emission to the UC-VIS emission decreases with the increase of the particle size and the excitation power, attributed to the more efficient upconversion (UC) process. Finally, we further confirmed that the competition process between the UC population and non-radiative relaxation to the DC-NIR level plays a key role in size-independent DC-NIR emissions. Our discovery would provide guidance for optimizing and designing NIR-DC-NIR NPs for bioimaging applications.

Carbon dots (CDs) exhibit excellent ultraviolet (UV) absorption and tunable photoluminescence over the full visible light range, which endows CDs with huge potential to be designed as efficient full-color emitting phosphors for UV to white light conversion. However, the low quantum yield (QY) for white light emission and solid-state quenching dramatically limit their optoelectronic applications. We proposed an effective strategy for modulating the emitting states of colloidal CDs by introducing hexadecyltrimethyl ammonium bromide. Consequently, white light emission with tunable correlated color temperature from 8121 K to 3623 K was realized. Furthermore, we dispersed CDs in a PVP matrix for solid-state films, where the solid-state quenching was effectively avoided. A white light-emitting QY of 38.7% was thus achieved through the inhibition of non-radiative electron–hole recombination as well as the cooperation between the intrinsic state of the carbogenic cores and the surface-related state of the organic ligands. The white light emitting QY is much higher than that of other reported CDs (ca. 15% in the soluble state and not reported in the solid-state) and is comparable to that of the nanophosphors with the highest UV pumped single-component white light emissions reported in the literature.

Photon upconversion (UC) is an attractive strategy to substantially enhance the power conversion efficiency (PCE) of solar cells via upconverting unavailable near-infrared sunlight to available visible light. However, to date, it is almost infeasible to achieve effective PCE improvement of solar cells with the assistance of UC materials, limited by their poor UC efficiency and extremely weak and narrowband near-infrared absorption. Here, we demonstrate the efficient photon energy UC in semiconductor plasmon mCu2−xS@SiO2@Er2O3 (mCSE) nanocomposites, where the broadband semiconductor plasmon (800–1600 nm) of mCu2−xS serves as an antenna to sensitize UC of Er2O3 nanoparticles. The overall upconversion luminescence (UCL) of the composites was dramatically enhanced by a factor of ∼1000, with a maximal inner quantum efficiency of 14.3%. The excitation range was expanded, ranging from 800 to 1600 nm. As a proof-of-concept, the highly efficient mCSE nanocomposites were utilized to improve the PCE of perovskite solar cells (PSCs). The expansion of the near-infrared response (800–1000 nm) and considerable improvement of the PCE were obtained, with an optimum PCE of 17.8%. The mCSE composites in PSCs enhanced the photocurrent via electron transfer from oxygen defects to the conduction band of TiO2 under irradiation of one sunlight. Under irradiation of 15 suns, the electron transfer and reabsorption of UCL both contributed to the enhancement of PCE. Our work can provide an insightful thought on boosting UC efficiency as well as broadening the PCE of PSCs.

Carbon dots (CDs) have emerged as a promising new type of fluorescent nanomaterial, although one of their main problems is the tuning of the emission wavelength toward the long wavelength region. In this work, the influence of reaction solvents to emission of CDs was systematically studied using four groups of classical precursors (citric acid individually mixed with four nitrogenous organic compounds). Water and toluene were selected to represent the hydrophobic and hydrophilic reaction medium, respectively. It is interesting to observe that in toluene solvent, all the CD products yield two emission bands of blue and yellow light, and the relative intensity of the yellow to blue can be finely tuned by the precursors. In contrast, the CDs formed in water only demonstrate blue emission. Systematic studies indicate that the spectral change results from the content control of carbogenic/defect states to surface states and the energy transfer from the carbogenic/defect states to the surface states. Moreover, solid-state luminescent CD-polymers were fabricated that also demonstrated continuously tunable emission properties. This work provides a new strategy for recognizing the generation of long wavelength-emitting CDs.

•The generation principle and interaction rules of localized electromagnetic field induced upconversion enhancement.•Experimental progresses of upconversion enhancement induced by metal/semiconductor plasmon, and photonic crystal effect.•Various applications developed by local field enhanced upconversion.Rare earth doped upconversion nanocrystals (RE-UCNCs) have attracted extensive interests owing to their unique physical properties and great potential applications in bio-application, photonic and photoelectric devices etc. Although UCNCs open doors to a wide range of new opportunities, they are confronting with some difficulties and one of the fatal problems is their low upconvsersion luminescent strength/efficiency. To date, various methods have been explored to solving this significant issue. Totally to say, the methods can be classified into two aspects, the traditional size, structure, surface and crystal field controls of the UCNCs, and the novel local electromagnetic field modulation surrounding the UCNCs. The local electromagnetic field modulation on UCNCs is a powerful strategy to enhance the strength/efficiency of UCNCs, reporting enhancement from several times up to four orders in short times. The timely and concise summary on the previous literatures is significant for more rapid and formulated development of this field. This review is aimed at offering a comprehensive framework for metal/semiconductor plasmon-induced and photonic crystal effect induced upconversion enhancement. Differing from the other review articles, we first introduced the generation principle of localized electromagnetic field in metal/semiconductor nanostructure/photonic crystals, and their general interaction rules with various emitters. Then, we summed up the recent published works on the local field modulation-induced upconversion enhancement, on emphasis we did our best to discover the generality of obtaining highly improved photoluminescence for any emitters and the personality of realizing highly improved upconversion enhancement. We further prospected the future development in this attractive field based on the previous theoretical and experimental results and the requirement of application. The marriage of upconversion with nanophotonic could explore a novel frontier in photonics that potentially spawn many exciting new fields.Download high-res image (153KB)Download full-size image

AbstractRecently, considerable progress is achieved in lab prototype perovskite solar cells (PSCs); however, the stability of outdoor applications of PSCs remains a challenge due to the high sensitivity of perovskite material under moist and ultraviolet (UV) light conditions. In this work, the UV photostability of PSC devices is improved by incorporating a photon downshifting layer—SrAl2O4: Eu2+, Dy3+ (SAED)—prepared using the pulsed laser deposition approach. Light-induced deep trap states in the photoactive layer are depressed, and UV light-induced device degradation is inhibited after the SAED modification. Optimized power conversion efficiency (PCE) of 17.8% is obtained through the enhanced light harvesting and reduced carrier recombination provided by SAED. More importantly, a solar energy storage effect due to the long-persistent luminescence of SAED is obtained after light illumination is turned off. The introduction of downconverting material with long-persistent luminescence in PSCs not only represents a new strategy to improve PCE and light stability by photoconversion from UV to visible light but also provides a new paradigm for solar energy storage.

Lanthanide-doped upconversion nanoparticles (UCNPs) are attracting extensive attention due to their unique physical properties and great application potential. However, the lower luminescence quantum yield/strength is still an obstacle for real application. Local field modulation is a promising method to highly enhance the upconversion luminescence (UCL) of the UCNPs. In this work, a novel kind of two-dimensional photonic crystal (2D-PC), anodic aluminum oxides (AAOs), was explored to improve the UCL of NaYF4:Yb3+,Er3+ nanoplates (NPs). An optimum enhancement factor (EF) of 65-fold was obtained for the overall intensity of Er3+ under 980 nm excitation, and 130-fold for the red emission. Systematic studies indicate that UCL enhancement mainly originates from the enlargement of the excitation field by scattering and reflection of AAO PCs. It should also be highlighted that the modulation of 2D-PC on the UCL of NaYF4:Yb3+,Er3+ NPs demonstrates weak size-dependent and thickness-dependent behavior, which is well consistent with the stimulated electromagnetic field distribution by the finite difference time domain (FDTD) method.

A novel solvent-thermal Y2O3 template method was explored to synthesise NaYF4:Yb3+,Tm3+ inverse opal photonic crystals (IOPCs), which show highly improved up-conversion luminescence properties.

Luminescent upconversion is a promising way to harvest near-infrared (NIR) sunlight and transforms it into visible light that can be directly absorbed by active materials of solar cells and improve their power conversion efficiency (PCE). However, it is still a great challenge to effectively improve the PCE of solar cells with the assistance of upconversion. In this work, we demonstrate the application of the transparent LiYF4:Yb3+, Er3+ single crystal as an independent luminescent upconverter to improve the PCE of perovskite solar cells. The LiYF4:Yb3+, Er3+ single crystal is prepared by an improved Bridgman method, and its internal quantum efficiency approached to 5.72% under 6.2 W cm–2 980 nm excitation. The power-dependent upconversion luminescence indicated that under the excitation of simulated sunlight the 4F9/2–4I15/2 red emission originally results from the cooperation of a 1540 nm photon and a 980 nm photon. Furthermore, when the single crystal is placed in front of the perovskite solar cells, the PCE is enhanced by 7.9% under the irradiation of simulated sunlight by 7–8 solar constants. This work implies the upconverter not only can serve as proof of principle for improving PCE of solar cells but also is helpful to practical application.Keywords: internal quantum efficiency; perovskite solar cell; power conversion efficiency; simulated sunlight; upconversion single crystal;

Upconversion nanophosphor is attracting worldwide interests owing to its unique optical properties and great application potentials. However, it is still a great challenge to effectively improve the efficiency/strength of upconversion nanophosphor. Plasmonic modulation is a promising way to solve this bottleneck. In this work, we present a simple yet versatile concept on magnifying upconversion luminescence of NaYF4:Yb3+, Er3+ nanocrystals through local field manipulation of surface plasmon. Gold nanorods were directionally assembled into a vertically aligned monolayer supercrystals over large areas. The FDTD simulation indicates that the electromagnetic field strength |E|2 can be improved about 113 folds at the hot spots of monolayer supercrystals. After optimization, on the surface of the vertically aligned monolayer supercrystals, the overall upconversion luminescence intensity of NaYF4:Yb3+, Er3+ under 980 nm excitation was improved more than 35 fold.Keywords: monolayer supercrystal; nanoparticles; plasmonically enhanced luminescence; surface plasmon; upconversion

A facile method was developed to synthesize fluorescent carbon-dot–Eu3+ hybrid composites (CD–Eu–HCs) by one-pot hydrothermal methods. The prepared composites demonstrate unique dual fluorescence which originates from the blue emission of the CDs and intrinsic photoluminescence of the Eu3+ ions, respectively. Moreover, such dual fluorescent characteristics show quite different responses for different pH value environments and have been developed in a ratiometric pH sensor. Lastly, they can realize white light emission by co-doping of Tb3+ and the color temperature becomes tunable by adjusting the relative proportion of Eu3+.

Specific three-dimensional inverse opal (3DIO) In2O3–CuO architecture with additional via-holes was first prepared by a simple sacrificial template method. Such specific nanostructures enable fast transport of gas molecules to the entire thin-walled sensing layers, which is very helpful for improving the sensing performance. Moreover, the mole ratio of Cu/In was controlled, ranging from 0–38.1% to adjust the hetero-contact amounts in the In2O3–CuO composites. The gas sensing properties of the as-prepared 3DIO In2O3–CuO samples were evaluated toward trace acetone, which is an important biomarker of diabetes in exhaled breath. The response of the 3DIO In2O3–CuO gas sensor with the best performance (with a mole ratio of Cu/In = 16.4%) was ∼14 to 5 ppm acetone, and had a calculated low detection limit of ∼30 ppb at 370 °C when Ra/Rg ≥ 1.2 was used as the criterion for reliable gas sensing. Besides, it also showed good selectivity, fast response (τres) and recovery (τrec) times, and stability. The enhanced gas sensing performance could be attributed to the hetero-contact effects between the different components and the specific 3DIO structure with the via-holes which provided a larger effective surface area for gas adsorption. It is believed that the as-prepared 3DIO sensor can be a promising ppb-level acetone sensor in various areas.

Metal nanoclusters (NCs) have attracted plenty of attention because of their unique properties and great application potentials. In this work, DNA scaffold Ag–Au alloy nanoclusters (Ag–Au ANCs) were fabricated by a one pot wet-chemical strategy and characterized by various techniques, including TEM, XPS and mass spectrometery (MS). The results indicate that owing to the strong interaction between DNA and Ag+, the silver NCs were formed first, then bundled with Au shells. In the Ag–Au ANCs, some of the Au is in an oxidized state as Au(I), which can largely modify the optical properties of the silver NCs. The Ag–Au ANCs demonstrate tunable emissions from green to red with highly improved stability. The fluorescence of Ag–Au ANCs was explored to detect Hg2+ in contrast to Ag NCs. The detection using Ag–Au ANCs demonstrated highly improved and excellent linearity and selectivity, which could effectively avoid the disturbance of Cu2+ and was promising for applications.

Localized surface plasmon resonances (LSPRs) are achieved in heavily doped semiconductor nanoparticles (NPs) with appreciable free carrier concentrations. In this paper, we present the photonic, electric, and photoelectric properties of plasmonic Cu2–xS NPs/films and the utilization of LSPRs generated from semiconductor NPs as near-infrared antennas to enhance the upconversion luminescence (UCL) of NaYF4:Yb3+,Er3+ NPs. Our results suggest that the LSPRs in Cu2–xS NPs originate from ligand-confined carriers and that a heat treatment resulted in the decomposition of ligands and oxidation of Cu2–xS NPs; these effects led to a decrease of the Cu2+/Cu+ ratio, which in turn resulted in the broadening, decrease in intensity, and red-shift of the LSPRs. In the presence of a MoO3 spacer, the UCL intensity of NaYF4:Yb3+,Er3+ NPs was substantially improved and exhibited extraordinary power-dependent behavior because of the energy band structure of the Cu2–xS semiconductor. These findings provide insights into the nature of LSPR in semiconductors and their interaction with nearby emitters and highlight the possible application of LSPR in photonic and photoelectric devices.Keywords: broadening; decrease and red-shift of LSPRs; electron injection; semiconductor plasmon nanoparticles; upconversion luminescence enhancement

Analyzing the volatile organic compounds (VOCs) in exhaled breath effectively is crucial to medical treatment, which can provide a fast and noninvasive way to diagnose disease. Well-designed materials with controlled structures have great influence on the sensing performance. In this work, the ordered three dimensional inverse opal (3DIO) macroporous In2O3 films with additional via-hole architectures were fabricated and different amounts of gold nanoparticles (Au NPs) were loaded on the In2O3 films aiming at enhancing their electrical responses. The gas sensing to acetone toward diabetes diagnosis in exhaled breath was performed with different Au/In2O3 electrodes. Representatively, the best 3DIO Au/In2O3 sensor can detect acetone effectively at 340 °C with response of 42.4 to 5 ppm, the actual detection limit is as low as 20 ppb, and it holds a dynamic response of 11 s and a good selectivity. Moreover, clinical tests proved that the as-prepared 3DIO Au/In2O3 IO sensor could distinguish acetone biomarkers in human breath clearly. The excellent gas sensing properties of the Au/In2O3 electrodes were attributed to the “spillover effects” between Au and In2O3 and the special 3DIO structure. This work indicates that 3DIO Au/In2O3 composite is a promising electrode material for actual application in the monitoring and detection of diabetes through exhaled breath.

Here, we report the wavelength-dependent and angle-dependent upconversion luminescence (UCL) enhancement of NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+ core–shell nanocrystals (NCs) resulting from Ag grating structures, which provides a novel insight for improving UCL.

A sensitive, label-free immunosensor based on iridium oxide (IrOx, 0 ≤ x ≤ 2) nanofibers, which were synthesized through a simple one-spinneret electrospinning method, was first developed for immunoassay of the cancer biomarker α-fetoprotein (AFP). The specific wire-in-tube nanostructure could be obtained and the composition of IrOx nanofibers also could be controlled through changing the annealing temperature. The unique structure and properties of IrOx nanofibers obtained at 500 °C not only led to increased electrode surface area and accelerated electron transfer kinetics but also could provide a highly stable matrix for the convenient conjugation of biomolecules together with chitosan (CS). The good electrochemical properties of the IrOx-nanofiber-modified immunosensor allowed one to detect AFP over a wide concentration range from 0.05 to 150 ng/mL, with a detection limit of 20 pg/mL. The proposed immunosensor also has been used to determine AFP in human serum with satisfactory results. The present protocol was shown to be quite promising for clinical screening of cancer biomarkers and point-of-care diagnostics applications.Keywords: electrochemical immunosensor; electrospinning; IrOx; label-free; wire-in-tube nanostructure; α-fetoprotein

SrTiO3 has the potential to be used as the photoanode for quantum dot sensitized solar cells (QDSSCs) since it has very similar band structures to TiO2 that exhibits the best performance in QDSSCs. In this work, SrTiO3 nanoparticles (NPs) with a cubic crystal structure were prepared by a hydrothermal method. 120, 70 and 30 nm SrTiO3 NPs were obtained by using different starting materials. CdS quantum dots (QDs) were coated on three different sized SrTiO3 NPs using the successive ion layer adsorption and reaction (SILAR) method. QDSSCs based on three different sized SrTiO3 NPs were fabricated and investigated. Due to the higher surface area compared to larger NPs, the device based on 30 nm SrTiO3 NPs shows the best open-circuit voltage (Voc) of 0.76 V and fill factor (FF) of 67% which are relatively high for Voc and FF reported for CdS-QDSSCs. Comparison studies of light-harvesting efficiency, electron transfer processes and band diagrams were performed to analyze the device performance in the SrTiO3-QDSSCs and TiO2-QDSSCs. The 30 nm SrTiO3 NPs can be combined with TiO2 NPs (P25) to improve the performance of the CdS-QDSSCs with an increasing percentage of 12.5%, which represents an alternative way to realize high efficiencies in QDSSCs.

•The photoelectrochemical immunosensor based on the ZnO IOs with signal amplification of CdS-QDs have been prepared successfully.•The detection of AFP possesses largely linear detection range of 0.1–500 ng/ml with a detection limit of 0.01 ng/ml.•The photoelectrochemical sensor has been successfully applied in the analysis of AFP in serum sample.In this work, ZnO inverse opals structure (IOs) based photoelectrochemical (PEC) electrode was fabricated for alpha-fetoprotein (AFP) detection. Then, the uniform CdS quantum dots (QDs) were hydrothermally synthesized, which allowed the binding of AFP and glucose oxidase (GOD) on CdS QDs, forming the AFP–CdS–GOD composite. The competitive immunosensor of AFP and the AFP–CdS–GOD composite with anti-AFP antibodies (Ab) immobilized on FTO (fluorine-doped tin oxide) /ZnO IOs electrode was successfully applied to the detection of AFP. GOD could catalyze glucose to produce hydrogen peroxide (H2O2) acting as an electron donor to scavenge photogenerated holes in the valence band of CdS QDs, reducing the recombination of electrons and holes of CdS QDs. Also the effective energy level matching between the conduction bands of CdS QDs and ZnO widened the range of light absorption, allowing for electron injection from excited CdS QDs to ZnO upon visible light irradiation, which enhanced the photocurrent. The results show that the immunosensor of AFP possesses a large linear detection range of 0.1–500 ng/ml with a detection limit of 0.01 ng/ml. It also exhibits excellent anti-interference property and acceptable stability. This work provides a promising method for achieving excellent photoelectrochemical biosensor detection of other proteins.

There has been great progress in the development of fluorescence biosensors based on quantum dots (QDs) for the detection of lead ions. However, most methods are detecting lead ions in aqueous solution rather than in human serum due to the influence of protein autofluorescence in serum excited by visible light. Thus, we developed a novel fluorescence resonance energy transfer (FRET) biosensor by choosing the upconversion NaYF4:Yb3+/Tm3+ nanoparticles as the energy donor and the CdTe QDs as the energy acceptor for lead ion detection. It is the first near infrared (NIR)-excited fluorescent probe for determination of lead ions in serum that is capable of overcoming self-luminescence from serum excitation with visible light. The sensor also shows high selectivity, a low detection limit (80 nm) and good linear Stern–Volmer characteristics (R = 0.996), both in the buffer and serum. This biosensor has great potential for versatile applications in lead ion detection in biological and analytical fields.

Upconversion luminescence (UCL) detection based on rare-earth doped upconversion nanocrystals (UCNCs) as probes has been proved to exhibit a large anti-Stokes shift, no autofluorescence from biological samples, and no photobleaching. However, it is still a challenge to achieve a stable, reproducible solid-based UCL biosensor because of ineffective UCL of the UCNCs. In this work, we fabricated TiO2 inverse opal photonic crystals (IOPCs)/NaYF4:Yb3+,Tm3+ (Er3+) UCNC composite films, which can tremendously improve the overall UCL of Tm3+ as high as 43-fold. Based on the fluorescence resonance energy transfer (FRET) and the specific interaction between biotin and avidin, a novel solid-based UC biosensor is presented for sensing avidin. This solid-based detection system is convenient for detection, and also can offer two parameters for detecting trace amounts of avidin, namely, the emission intensity and the fluorescence decay time. The sensor has a high sensitivity of 34 pmol−1, a good linear relationship of 0.996 and a low detection limit of 48 pmol. It also exhibits excellent long-time photostability, and the absence of autofluorescence, and thus may have great potential for versatile applications in biodetection.

In the present work, a novel strategy for the synthesis of Nd2O3/Au nanocomposites was successfully applied by a co-precipitation process, in which the upconversion luminescence (UCL) of Nd2O3 was a white broadband emission under 780–980 nm excitation. UCL enhancement in Nd2O3/Au nanocomposites with different doped concentrations of Au nanorods (NRs) under 980 nm and 808 nm excitation was systemically studied. It was observed that by the interaction of Nd2O3 with surface plasmon (SP) of Au NRs, the threshold power for generating broadbands was largely suppressed in contrast to the Nd2O3 nanoparticles (NPs). Further, it was interesting to observe that the enhancement was strongly dependent on the doped concentration of Au NRs and the excitation power of 980 nm and 808 nm laser diodes. The optimum UCL enhancement was 11-fold and 9-fold at 980 nm and 808 nm excitation, respectively. In addition, the upconversion (UC) broadband emission and UCL enhancement mechanism of Nd2O3/Au nanocomposites were proposed.

The competition between 4I13/2–4I15/2 near-infrared downconversion emissions and visible upconversion emissions of 2H11/2/4S3/2/4F9/2–4I15/2 is a vital factor for obtaining efficient upconversion luminescence (UCL) in Er3+ and Yb3+ co-activated oxides. In this paper, we systemically studied the entire emission spectra of Er3+ ranging from 400–1700 nm in Er3+ and Yb3+ co-activated cubic Y2O3, tetragonal GdVO4 and tetragonal NaGd(WO4)2 micro-sized powders, prepared by the same sol–gel method, which have different phonon thresholds, 600 cm−1, 880 cm−1 and 1000 cm−1, respectively. It is interesting to observe that in these phosphors, the visible UCL of Er3+ relative to near-infrared emissions increase remarkably with the increase of phonon thresholds. In comparison with the UCL in the Y2O3 host, the UCL in YVO4 and tetragonal NaGd(WO4)2 was tremendously improved by 9.5- and 120-fold, respectively. The UCL intensity of the NaGd(WO4)2:Yb3+/Er3+ was comparable to that of the famous UCP NaYF4:Yb3+/Er3+. A model was proposed to explain the phonon-modulated UC process. This work is of great significance in the search for a novel, efficient UCP.

In this work, novel upconversion lanthanide oxyfluoride (LnOF:Yb3+, Er3+, Ln = La, Y, Gd) inverse opal photonic crystals (IOPCs) were successfully fabricated by the sol–gel method combined with polymethylmethacrylate (PMMA) template technique and the modulation of the photonic stop band (PSB) on the green emissions 2H11/2/4S3/2 → 4I15/2 for Er3+ ions were systemically studied under 980 nm excitation. The results showed that the LaOF IOPCs (annealed at 500 °C) were of cubic phase while GdOF and YOF matrices were of rhombohedral phase, and the LaOF IOPCs demonstrated more efficient upconversion luminescence (UCL) than GdOF and YOF due to the phase transition. In contrast to the ground reference (REF) samples, strong suppression of UCL was observed in the IOPCs while the PSB overlapped with the 2H11/2/4S3/2 → 4I15/2 lines. Furthermore, the spontaneous decay rates (SDRs) of 2H11/2/4S3/2 → 4I15/2 were suppressed in the IOPCs, independent of the location of the PSB. In LaOF IOPCs, the decay time constants of 4S3/2 → 4I15/2 were increased by as much as 9 times in contrast to the corresponding REFs. It was also significant to observe that in the IOPCs the local thermal effect was greatly suppressed. In addition, broadband UCL extending into the visible range was observed in LnOF:Yb3+, Er3+ REF samples under high excitation power, and the origin was identified by electron paramagnetic resonance (EPR) spectroscopy.

Three-dimensional (3D) inverse opal photonic crystals can not only modulate the emissions of the inserted emitters, they also have the advantage of a large surface to volume ratio, which permits their use in noninvasive fluorescence detection. In this work, novel NaY(MoO4)2:Eu3+ and NaY(MoO4)2:Tb3+, Eu3+ inverse opals were synthesized using a polymethylmethacrylate (PMMA) template by the sol–gel method. It was observed that the photoluminescence (PL) intensity and spontaneous decay rates (SDRs) of the inverse opals were suppressed, in contrast to the corresponding ground reference (REF) samples, due to the modulation of the effective refractive index (neff). The concentration quenching of Eu3+ and energy transfer from Tb3+ to Eu3+ were suppressed, due to the periodic empty cavity structure of the inverse opals.

Oleic acid-capped NaYF4:Yb3+/Er3+ upconversion nanocrystals (UCNCs) with different sizes and crystalline phases were prepared, and their temperature-dependent upconversion luminescence (UCL) and dynamics were studied. It is interesting to observe that the temperature-dependent behavior of UCL for the β-phase (25 nm, 45 nm and bulk) and α-phase (<10 nm) UCNCs is quite different. The UCL intensity of Er3+ ions of the β-phase NaYF4 demonstrates a maximum around 100 K, while the intensity of the α-phase quenches monotonously with elevated temperature (10–400 K). The intensity ratio of 2H11/2–4I15/2 to 4S3/2–4I15/2, RHS, increases solely with temperature for β-phase NaYF4, while for the α-phase sample, it demonstrates a complex and indeterminate variation as a function of temperature. In the β-phase samples, rising processes were observed in the dynamics of Er3+ ions, while in the α-phase sample, no rising process was observed and the decay processes of Er3+ ions were bi-exponential. The rationale for these different temperature-dependent UCL properties was explained carefully.

•NaYF4:Yb,Tm and GNRs are as NIR energy donor and quenching acceptor for FRET.•Linkage between biotin and streptavidin make the distance between the donors and the acceptors short enough for FRET.•The FRET system in this work was applicable for the detection of streptavidin.•The donor and acceptor NPs can be modified by proper molecules for other biological molecules detection.We represent a fluorescence resonance energy transfer (FRET) system using upconversion nanoparticles (UCNPs) and the gold nanorods (GNRs) as the energy donor–acceptor pair for directly determining streptavidin in near-infrared (NIR) region. NaYF4:Yb,Tm UCNPs, which had a strong emission at 800 nm under 980-nm excitation, were adopted as the energy donor. The GNRs, which demonstrated strong surface plasmon absorption around 800 nm, were chosen as acceptor to quench the 800 nm emissions of the UCNPs. There had the spectral overlap between the emission of the donor nanoparticles (UCNPs) and the absorption of the acceptor nanoparticles (GNRs). This UCNP-based FRET system was then used to determine the amount of streptavidin. In this system, NaYF4:Yb,Tm UCNPs conjugated with biotin, while GNRs conjugated with streptavidin. When added GNRs into UCNPs, the streptavidin were preferred to bind with biotin and decreased spacing between the donor and acceptor NPs. Consequently, FRET occurred and a linear relationship between the luminescence quenching efficiency and the concentration of streptavidin was obtained. Owing to the aforementioned merits of UCNPs as an energy donor and the strong quenching ability of GNRs, satisfactory analytical performances have been acquired.

•The Nafion/glucose oxidase/ZnO IOPCs modified FTO electrodes were fabricated.•The sensitivity of ZnO IOPCs modified electrode was 18 times than thin film.•The sensitivity of photoelectrochemical was four times than electrochemical.The ZnO inverse opal photonic crystals (IOPCs) were synthesized by the sol–gel method using the polymethylmethacrylate (PMMA) as a template. For glucose detection, glucose oxidase (GOD) was further immobilized on the inwall and surface of the IOPCs. The biosensing properties toward glucose of the Nafion/GOD/ZnO IOPCs modified FTO electrodes were carefully studied and the results indicated that the sensitivity of ZnO IOPCs modified electrode was 18 times than reference electrode due to the large surface area and uniform porous structure of ZnO IOPCs. Moreover, photoelectrochemical detection for glucose using the electrode was realized and the sensitivity approached to 52.4 µA mM−1 cm−2, which was about four times to electrochemical detection (14.1 µA mM−1 cm−2). It indicated that photoelectrochemical detection can highly improve the sensor performance than conventional electrochemical method. It also exhibited an excellent anti-interference property and a good stability at the same time. This work provides a promising approach for realizing excellent photoelectrochemical biosensor of similar semiconductor photoelectric material.

Luminescent upconversion (UC) is a promising way to harvest near-infrared (NIR) sunlight and improve the power conversion efficiency (PCE) of solar cells. However, most of efficient upconversion phosphors (UCPs) are based on 4f–4f transitions of rare earth (RE) ions, which have only a narrower excitation band matching with sun spectrum. To solve this significant problem, we designed and fabricated a novel kind of efficient UC nanocomposites, Yb2O3/Au, in which the upconversion luminescence (UCL) of Yb2O3 was a white broad band that originated from electron–hole recombination, and the excitation bands were expanded at least a range of 770–980 nm through the energy transfer (ET) from anisotropic gold nanoparticles (GNPs) to the Yb2O3 host. To our knowledge, the direct ET from the noble metal to the lanthanide phosphors has never been evidenced. Exploring Yb2O3/Au as the upconverter of a dye-sensitized solar cell (DSSC), NIR photovoltaic response was successfully demonstrated as proof-of-concept.

CuO nanoparticles (NPs) based graphene oxide (CuO/GO) composites with different CuO NPs loading amount as well as pure CuO NPs with different hydrothermal temperatures were synthesized using a hydrothermal method. Transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy were employed to characterize the morphology and structures of our samples. The influence of hydrothermal temperature, GO sheet, and loading amount of CuO on particle size and structure of CuO was systemically investigated. The nonenzymatic biosensing properties of CuO/GO composites and CuO NPs toward glucose were studied based on glassy carbon electrode (GCE). The sensing properties of CuO NPs were improved after loading on GO sheets. The CuO/GO composites with saturated loading of the CuO NPs exhibited the best nonenzymatic biosensing behavior. It exhibited a sensitivity of 262.52 μA mM–1 cm–2 to glucose with a 0.69 μM detection limit (S/N = 3) and a linear range from 2.79 μM to 2.03 mM under a working potential of +0.7 V. It also showed outstanding long term stability, good reproducibility, excellent selectivity, and accurate measurement in real serum sample. It is believed that CuO/GO composites show good promise for further application on nonenzymatic glucose biosensors.Keywords: composite; CuO; glucose detection; graphene; hydrothermal procedure;

In this study, Lu2O3:Eu3+ inverse-opal-photonic crystals (IOPCs) with controllable lattice constants were fabricated using a polymethylmethacrylate (PMMA) template. The modification effect of PC on the 5D0–7FJ and 5D1–7FJ (J = 1–6) transitions were systemically studied by emission spectra, luminescent dynamics and the temperature-dependence. It is significant to observe that the increase of 5D0–7FJ radiative lifetime for Eu3+ ions (30%) in contrast to the reference sample was mainly due to modulation of the effective refractive index, rather than the density of optical states. The spontaneous decay rate in 5D1 increased linearly with the decreasing lattice constants, which was due to the change in 5D1–5D0 nonradiative relaxation of the IOPC samples. The temperature quenching of Eu3+ ions in the IOPCs could be suppressed considerably more than the reference. On this basis, dual functional refractive index detection with infiltrated solutions was realized by monitoring the variation in the photonic stop band (PSB) and the lifetime of 5D0–7F2 transition of the Lu2O3:Eu3+ IOPCs. This work shows that the Lu2O3:Eu3+ IOPCs present highly modified photoluminescence properties and are promising candidates for dual-functional refractive index sensing application.

Inverse opals (IO), as a special kind of macroporous material with large surface to volume ratio and ordered layer structure, have great application potential in highly sensitive gas sensing area. In this paper, macroporous SnO2 IO were synthesized by a simply PMMA template method and their porous sizes were controlled ranging of 140–400 nm. The performance of the SnO2 IO sensors to formaldehyde (HCHO) gas was systemically studied. The results indicated that the sensing properties of the SnO2 IO sensors to HCHO were highly improved than the traditional sensors, depending strongly on the porous sizes. The response of the SnO2 IO sensors increased gradually with the increase of porous size. The response of the optimum SnO2 sensor was as high as 629 for 100 ppm HCHO gas detection and the practical detection limit was as low as 10 ppb, which was one of the best levels for the detection of HCHO gas. And more, they also demonstrated fast response dynamics and long time stability.

•The modified PSCs exhibited a long-term stability over 2400 h aging.•PLD was firstly introduced to compact Nb-doped TiOx films preparation in the PSCs.•The PSCs showed an improved PCE of 16.8% due to the reduced carrier recombination.•PSCs with large area (225 mm2) and flexibility were also fabricated.•Best water contact angle (115.4°) proved the hydrophobic property of ZnPc films.Recently, much progress has been achieved in the lab prototype perovskite solar cells (PSCs), however, the challenging toward the commercial production of PSCs still remains, which is their low stability due to the inherent moisture sensitivity of the perovskite photoactive layer. In this work, a hydrophobic hole modification layer, zinc phthalocyanine (ZnPc), is introduced to the typical planer hetero-junction (PHJ) structure to improve the device performance and prevent moisture. The ZnPc based devices show an improved power conversion efficiency (PCE) of 16.8% compared to 15.1% for the unmodified devices, and highly enhanced long-term stability (91% of initial PCE remains after 2400 h), which is the best report for PSCs in ambient environment without sealing. In addition, pulsed laser deposition (PLD) technique is used to prepare electron collection layer Nb-doped TiOx in PSC fabrication process. Devices with large area (225 mm2) and flexibility were also fabricated in this study to demonstrate the potential practical application of this strategy. This work paves a way for the developments of PSCs with long-term stability and demonstrates a strategy toward practical applications for PSCs.In this work, Nd-doped TiOx by pulsed laser deposition and hydrophobic ZnPc by rotary vacuum thermal evaporation were firstly employed as electron and hole transport layers to improve the PSC performance. Such improvement lends crucial support to the enhanced injection and extraction of photo-generated carriers at the electron transport layer/perovskite interface and hole transport layer/perovskite interface, which sequentially trigger remarkable increases in Jsc and Voc. More importantly, the hole modification layer ZnPc could also prevent moisture permeation into the perovskite photoactive layer, improving the long-term stability of PSCs. After optimization, the device exhibits an optimum PCE of 16.8% and the long-term stability (91% of initial PCE remains after 2400 h), which is the best report for PSCs in ambient environment without any sealing until now. Furthermore, large area device (225 mm2) and flexible devices were also fabricated for demonstrating the potential practical application prospects. This work offers novel insights on the PSC devices with respect to high permanence and long-term stability.

Currently, perovskite solar cells (PSCs) are attracting extensive interest due to their potential for overcoming the energy crisis. However, increasing the power conversion efficiency (PCE) of PSCs to realize outdoor applications is still one of the crucial issues in PSC research. In this study, metallic nanostructures including Au, Ag and Au–Ag nanoalloy with different sizes and morphology were synthesized via a chemical solution method and embedded into an electron collecting TiO2 layer in PSCs based on the physical deposition method, which allows control over the incorporation of the metallic-nanostructures. The best improvement in the PCE was obtained using the Au–Ag nanoalloy, exhibiting an efficiency of 14.8%, which increases 17.5% compared to the bare PSCs. We believe that the increased device performance originates from increased light harvesting due to the increased optical path length caused by the light scattering of the metallic nanostructures. This strategy demonstrates a novel way to enhance the performance of photovoltaic devices.

Near-infrared-downconversion-near-infrared (NIR-DC-NIR) bioimaging based on lanthanide doped upconversion nanoparticles (UCNPs) has received much attention due to its deeper penetration, and higher contrast imaging and signal-to-noise ratio in biological tissues. The size of UCNPs determines the mechanism and rate of cell uptake of the nanoparticles and their ability to permeate through biological tissues. Herein, we experimentally and theoretically demonstrate downconversion-near-infrared (DC-NIR) emission behavior in different sized UCNPs ranging from 5–150 nm. Interestingly, 15–40 nm UCNPs have more effective DC-NIR emissions than 150 nm UCNPs and an extremely high excitation threshold, which is entirely different from the size-dependent upconversion-visible (UC-VIS) emissions usually observed in UCNPs. We also observed that the intensity ratio of the DC-NIR emission to the UC-VIS emission decreases with the increase of the particle size and the excitation power, attributed to the more efficient upconversion (UC) process. Finally, we further confirmed that the competition process between the UC population and non-radiative relaxation to the DC-NIR level plays a key role in size-independent DC-NIR emissions. Our discovery would provide guidance for optimizing and designing NIR-DC-NIR NPs for bioimaging applications.

The competition between 4I13/2–4I15/2 near-infrared downconversion emissions and visible upconversion emissions of 2H11/2/4S3/2/4F9/2–4I15/2 is a vital factor for obtaining efficient upconversion luminescence (UCL) in Er3+ and Yb3+ co-activated oxides. In this paper, we systemically studied the entire emission spectra of Er3+ ranging from 400–1700 nm in Er3+ and Yb3+ co-activated cubic Y2O3, tetragonal GdVO4 and tetragonal NaGd(WO4)2 micro-sized powders, prepared by the same sol–gel method, which have different phonon thresholds, 600 cm−1, 880 cm−1 and 1000 cm−1, respectively. It is interesting to observe that in these phosphors, the visible UCL of Er3+ relative to near-infrared emissions increase remarkably with the increase of phonon thresholds. In comparison with the UCL in the Y2O3 host, the UCL in YVO4 and tetragonal NaGd(WO4)2 was tremendously improved by 9.5- and 120-fold, respectively. The UCL intensity of the NaGd(WO4)2:Yb3+/Er3+ was comparable to that of the famous UCP NaYF4:Yb3+/Er3+. A model was proposed to explain the phonon-modulated UC process. This work is of great significance in the search for a novel, efficient UCP.

In the present work, a novel strategy for the synthesis of Nd2O3/Au nanocomposites was successfully applied by a co-precipitation process, in which the upconversion luminescence (UCL) of Nd2O3 was a white broadband emission under 780–980 nm excitation. UCL enhancement in Nd2O3/Au nanocomposites with different doped concentrations of Au nanorods (NRs) under 980 nm and 808 nm excitation was systemically studied. It was observed that by the interaction of Nd2O3 with surface plasmon (SP) of Au NRs, the threshold power for generating broadbands was largely suppressed in contrast to the Nd2O3 nanoparticles (NPs). Further, it was interesting to observe that the enhancement was strongly dependent on the doped concentration of Au NRs and the excitation power of 980 nm and 808 nm laser diodes. The optimum UCL enhancement was 11-fold and 9-fold at 980 nm and 808 nm excitation, respectively. In addition, the upconversion (UC) broadband emission and UCL enhancement mechanism of Nd2O3/Au nanocomposites were proposed.

In this work, novel upconversion lanthanide oxyfluoride (LnOF:Yb3+, Er3+, Ln = La, Y, Gd) inverse opal photonic crystals (IOPCs) were successfully fabricated by the sol–gel method combined with polymethylmethacrylate (PMMA) template technique and the modulation of the photonic stop band (PSB) on the green emissions 2H11/2/4S3/2 → 4I15/2 for Er3+ ions were systemically studied under 980 nm excitation. The results showed that the LaOF IOPCs (annealed at 500 °C) were of cubic phase while GdOF and YOF matrices were of rhombohedral phase, and the LaOF IOPCs demonstrated more efficient upconversion luminescence (UCL) than GdOF and YOF due to the phase transition. In contrast to the ground reference (REF) samples, strong suppression of UCL was observed in the IOPCs while the PSB overlapped with the 2H11/2/4S3/2 → 4I15/2 lines. Furthermore, the spontaneous decay rates (SDRs) of 2H11/2/4S3/2 → 4I15/2 were suppressed in the IOPCs, independent of the location of the PSB. In LaOF IOPCs, the decay time constants of 4S3/2 → 4I15/2 were increased by as much as 9 times in contrast to the corresponding REFs. It was also significant to observe that in the IOPCs the local thermal effect was greatly suppressed. In addition, broadband UCL extending into the visible range was observed in LnOF:Yb3+, Er3+ REF samples under high excitation power, and the origin was identified by electron paramagnetic resonance (EPR) spectroscopy.

A novel solvent-thermal Y2O3 template method was explored to synthesise NaYF4:Yb3+,Tm3+ inverse opal photonic crystals (IOPCs), which show highly improved up-conversion luminescence properties.

Three-dimensional (3D) inverse opal photonic crystals can not only modulate the emissions of the inserted emitters, they also have the advantage of a large surface to volume ratio, which permits their use in noninvasive fluorescence detection. In this work, novel NaY(MoO4)2:Eu3+ and NaY(MoO4)2:Tb3+, Eu3+ inverse opals were synthesized using a polymethylmethacrylate (PMMA) template by the sol–gel method. It was observed that the photoluminescence (PL) intensity and spontaneous decay rates (SDRs) of the inverse opals were suppressed, in contrast to the corresponding ground reference (REF) samples, due to the modulation of the effective refractive index (neff). The concentration quenching of Eu3+ and energy transfer from Tb3+ to Eu3+ were suppressed, due to the periodic empty cavity structure of the inverse opals.

In this study, Lu2O3:Eu3+ inverse-opal-photonic crystals (IOPCs) with controllable lattice constants were fabricated using a polymethylmethacrylate (PMMA) template. The modification effect of PC on the 5D0–7FJ and 5D1–7FJ (J = 1–6) transitions were systemically studied by emission spectra, luminescent dynamics and the temperature-dependence. It is significant to observe that the increase of 5D0–7FJ radiative lifetime for Eu3+ ions (30%) in contrast to the reference sample was mainly due to modulation of the effective refractive index, rather than the density of optical states. The spontaneous decay rate in 5D1 increased linearly with the decreasing lattice constants, which was due to the change in 5D1–5D0 nonradiative relaxation of the IOPC samples. The temperature quenching of Eu3+ ions in the IOPCs could be suppressed considerably more than the reference. On this basis, dual functional refractive index detection with infiltrated solutions was realized by monitoring the variation in the photonic stop band (PSB) and the lifetime of 5D0–7F2 transition of the Lu2O3:Eu3+ IOPCs. This work shows that the Lu2O3:Eu3+ IOPCs present highly modified photoluminescence properties and are promising candidates for dual-functional refractive index sensing application.

Oleic acid-capped NaYF4:Yb3+/Er3+ upconversion nanocrystals (UCNCs) with different sizes and crystalline phases were prepared, and their temperature-dependent upconversion luminescence (UCL) and dynamics were studied. It is interesting to observe that the temperature-dependent behavior of UCL for the β-phase (25 nm, 45 nm and bulk) and α-phase (<10 nm) UCNCs is quite different. The UCL intensity of Er3+ ions of the β-phase NaYF4 demonstrates a maximum around 100 K, while the intensity of the α-phase quenches monotonously with elevated temperature (10–400 K). The intensity ratio of 2H11/2–4I15/2 to 4S3/2–4I15/2, RHS, increases solely with temperature for β-phase NaYF4, while for the α-phase sample, it demonstrates a complex and indeterminate variation as a function of temperature. In the β-phase samples, rising processes were observed in the dynamics of Er3+ ions, while in the α-phase sample, no rising process was observed and the decay processes of Er3+ ions were bi-exponential. The rationale for these different temperature-dependent UCL properties was explained carefully.

Here, we report the wavelength-dependent and angle-dependent upconversion luminescence (UCL) enhancement of NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+ core–shell nanocrystals (NCs) resulting from Ag grating structures, which provides a novel insight for improving UCL.