Plasmon-enhanced up-conversion luminescence in multiple Cu2-xS@SiO2-embedded Er(OH)CO3 composites
Introduction
Rare earth up-conversion (UC) materials, characterized by the ability to convert near-infrared light into the visible, have drawn enormously research interests [[1], [2], [3], [4], [5]]. Due to the inherent advantages such as great anti-stokes shift, fluorescence resistance to bleaching and low toxicity to organisms [[6], [7], [8], [9]], rare earth UC materials could be applied to many fields, such as photo-thermal therapy, bio-fluorescence imaging, commercial phosphors, as well as solar cells [[10], [11], [12], [13], [14]]. However, because of its low absorption cross-section and low quantum efficiency, the luminescence performance of the rare earth material is limited and needs to be improved [15,16]. Currently, several methods such as doping of ytterbium or manganese ion, utilization of dye-sensitized nanostructures and synthesis of multi-layer core-shell structures to reduce the quenching effects, have been applied to resolve the existed problem [[17], [18], [19], [20]]. Additionally, surface enhanced luminescence of nano-emitters with plasmonic structures is a simple and effective approach to largely promote UC fluorescence of rare earth ion doped nanophosphors [[21], [22], [23], [24]]. Since gold nanoparticles were confirmed to have the ability to enhance UC luminescence of rare earth materials [25], Ag nanocrystals, silver thin films, triple structure of metal-UC nanocrystal-metal, as well as Au nanorods were reported for improving UC luminescence of rare earth ion doped materials [[26], [27], [28], [29]]. These common noble metals have free electron densities in the range 1022–1023 cm−3 with corresponding localized surface plasmon resonances (LSPRs) in the visible region [30,31]. Recently, Cu2-xS nanocrystals have emerged as an appealing plasmonic material due to its ability to sustain a LSPR similar to noble metal nanomaterials, and its synergetic natures and utilization potentials in biological and catalytic technologies [32,33]. Cu2-xS is a class of non-stoichiometric compounds, therefore, controllable modulation of plasmon resonance peak of Cu2-xS nanocrystals can be achieved by changing the ratio of Cu to S, which is different from noble metals [34,35]. Cu2-xS nanocrystals exhibit highly plasmon absorption in the near-infrared region, which suggests it has a great potential applications in wide areas, including photo-catalysis, optics, photo-thermal therapy and heavy metal ion detection [[36], [37], [38], [39]]. Especially, Cu2-xS nanoparticles are proved to be a promising candidate for increasing UC luminescence of rare earth based materials. Zhou et al. have applied Cu9S5 nanocrystals for enhancing UC luminescence of Y2O3:Yb3+, Er3+ nanophosphors. They reported that the UC enhancement factor was up to 35-fold [40]. However, the absorption peak position of Cu9S5 nanocrystals is too far from the UC excitation wavelength (980 nm). Moreover, the thermal stability of Cu9S5 nanomaterials is poor. Actually, the structure of Cu9S5 will be changed by annealing at high temperature [41]. Our group prepared Cu8S5@SiO2@Er2O3 nanocomposites for UC luminescence. The results showed that Cu8S5 nanoparticles had a strong resonance peak at ∼1000 nm, which suggested that Cu8S5 nanoparticles showed more promising in enhancing UC luminescence of rare earth materials. However, the Er2O3 layer of Cu8S5@SiO2@Er2O3 composites was too thick, which led to the up-conversion luminous enhancement factor was only 1.14 times [42]. Therefore, the semiconductor based plasmonic hybrid structure for UC luminescence should be more optimized. A crucial advantage of employing nano-semiconductors for plasmonics is that their free carrier concentrations can be tailored by doping, temperature, and phase transitions, granting the engineering of LSPRs and optimizing of the thermal stability. However, up to now, the reports for the synthesis of well-defined and efficient semiconductor based plasmonic hybrid structures for altering light-matter interactions and potential applications in enhancing UC processes are limited, and most of them are based on cation exchange reactions [43].
Among different kinds of rare earth emitters, trivalent lanthanide doped ions, such as Er3+, are the typical activator ions for an UC process to proceed due to their plentiful energy levels [44]. Currently, spherical alkaline rare-earth carbonates nanoparticles have been reported via an uncomplicated hydrothermal process. The spherical carbonate powders are fabricated under hydrothermal conditions, supplying a simple and green approach for synthesizing a regular spherical rare-earth doped nanophosphor without using additional organic solvent and surfactant. For example, monodisperse core-shell structured UC Yb(OH)CO3@YbPO4:Er3+ hollow spheres were prepared and studied as drug carriers. The results showed that the core-shell Er3+doped Yb(OH)CO3@YbPO4 hollow spheres had the potential to load and deliver drugs to cancer cells, thereby inducing cell death [45]. Spherical Lu(OH) CO3:Ln3+/LuBO3:Ln3+ (Ln = Eu, Tb) nanoparticles were also reported by Wang et al. They demonstrated that the classic urea-based homogeneous precipitation was not only simple, but also did not require additional organic solvents and catalysts [46].
Herein, we reported a semiconductor-based plasma hybrid structure that Er(OH)CO3 were embedded with multiple Cu2-xS@SiO2 nanoparticles for enhancing UC luminescent processes. The results of UV–Vis–Infrared spectra showed that Cu2-xS nanocrystals had a strong absorption of near-infrared photons. Cu8S5 nanoparticles had a blue shift relative to Cu9S5 nanocrystals, which was owing to the increase of the carrier concentration in the former. Compared with the case of Cu9S5 nanoparticles, the LSPR position of Cu8S5 nanoparticles was closer to the excitation wavelength of 980 nm, forming more efficient semiconductor based plasmonic hybrid structures for enhancing the photon couple with the excitation light and thus, reinforcing the UC emissions. As a result, the UC luminescence spectra of multiple Cu8S5@SiO2-embedded Er(OH)CO3 and multiple Cu9S5@SiO2-embedded Er(OH)CO3 composites were collected. The UC luminescence intensity of multiple Cu8S5@SiO2-embedded Er(OH)CO3 was about 45.0-fold higher than that of SiO2@Er(OH)CO3, while the UC enhancement factor was about 15.2 for multiple Cu9S5@SiO2-embedded Er(OH)CO3 composites. Theoretical-calculation data not only supported the experimental results, but also demonstrated that hotspots among multiple Cu2-xS@SiO2 nanoparticles played an important role in enhancing UC luminescence of Er(OH)CO3. The UC enhancement mechanisms from Cu2-xS based plasmonic semiconductors were discussed.
Section snippets
Materials
Sulfur (99.9%), oleylamine (OLA, 70%), oleic acid (OA, 90%), erbium nitrate hexahydrate were purchased from Sigma-Aldrich. Copper (I) chloride (98%), 1-octadecene (ODE, 90%), polyoxyethylene (5) nonylphenyl ether (Igepal co-520, Mn = 441) were purchased from Alfa Aesar. Tetraethyl orthosilicate (TEOS, 99.9%), acetone (99.9%), cyclohexane (99.9%), urea (98%) and ammonia (25%∼28%) were all acquired from Shanghai Reagent Company. All the chemicals were used as received.
Preparation of Cu2-xS@SiO2 nanoparticles
Fabrication of Cu2-xS
Phase, structure, optical properties of multiple Cu2-xS@SiO2-embedded Er(OH)CO3 composites
Fig. 1 outlined a schematic illustration for preparation of multiple Cu2-xS@SiO2-embedded Er(OH)CO3 composites. Firstly, Cu8S5 nanoparticles were synthesized by the hot-injection method without any gas protection, Fig. 2a showed the TEM image of as-synthesized nanoparticles, exhibiting that the nanoparticles were monodisperse and well-assembled. The average diameter of nanoparticles was about 14 nm. Fig. 2b presented the high resolution TEM image of Cu8S5 nanoparticles. As shown in this figure,
Conclusions
In summary, we have provided direct and unambiguous experimental evidence of significant plasmon-enhancement of the UC luminescence properties in lanthanide doped phosphors by engineering LSPRs of Cu2-xS based semiconductors to form more efficient semiconductor based plasmonic hybrid structures. In order to do so, semiconductor based plasmonic hybrid structures with hotspots was synthesized by embedding multiple Cu2-xS@SiO2 nanoparticles into Er(OH)CO3. According to the UC luminescence spectra
CRediT authorship contribution statement
Meidong Yu: Conceptualization, Data curation, Writing - original draft. Peng Tang: Methodology, Software. Zhenjie Zhao: Writing - review & editing. Sumei Huang: Visualization, Investigation, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by Natural Science Foundation of Shanghai (Nos. 18ZR1411900, 18ZR1411000) and National Natural Science Foundation of China (Nos. 11774091, 11704122 and 11574084).
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