Synergistic Upconversion Enhancement Induced by Multiple Physical

Article pubs.acs.org/cm

Synergistic Upconversion Enhancement Induced by Multiple Physical Effects and an Angle-Dependent Anticounterfeit Application Donglei Zhou,† Dali Liu,† Wen Xu,*,†,‡ Xu Chen,† Ze Yin,† Xue Bai,† Biao Dong,† Lin Xu,† and Hongwei Song*,† †

State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China ‡ Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan S Supporting Information *

ABSTRACT: 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 angledependent 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.

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INTRODUCTION Recently, semiconductor plasmons have attracted increasing attention because of their unique double character as a semiconductor and metal. Among them, Cu2−xS, a kind of heavily doped semiconductor nanoparticle, exhibits a broadband localized surface plasmon resonance (LSPR) in the nearinfrared region,1,2 which has attracted considerable interest because of its ability to fundamentally alter light−matter interactions and its potential applications in enhanced spectroscopies,3 sensing,4 photocatalysis,5,6 and optical devices.7,8 In addition to the LSPR effect, Cu2−xS NPs have a direct bandgap of ∼2.5 eV,1,9−11 which varies with structure and size and would cause a two-photon effect under a high excitation power. This unique semiconductor/metal double character of Cu2−xS NPs makes them significant in optoelectronic devices. Lanthanide (Ln)-doped upconversion nanoparticles (UCNPs) are currently attracting extensive interest because of their potential applications in solar cells,12 infrared detectors,13,14 biolabels,15,16 and biosensors.17,18 However, the lower luminescent efficiency and the smaller absorption cross section of Ln-doped UCNPs have seriously limited their applications. Extensive effort has been devoted to enhancing their upconversion luminescence (UCL).17,19−21 In particular, localized field modulation is the most effective method to enhance the UCL, and enhancements from several to a © 2017 American Chemical Society

thousand times have been achieved thus far via the modulation of noble metals and/or PCs.22,23 However, a large enhancement is usually obtained under the illumination of a relatively low excitation power density because the hybrids of UCNPs/ metals lead to a metal-induced thermal effect, resulting in serious thermal quenching and a saturation effect of UCNPs, particularly under high-power-density excitation. However, the absolute intensity of enhanced UCNPs remains low and insufficient for practical applications. The need for a novel mechanism/material to realize effective UC enhancement at a high power density is urgent. Semiconductor plasmon NPs, such as Cu2−xS, not only demonstrate the LSPR effect but also display a nonlinear absorption effect under excitation by intense near-infrared light, which is a novel strategy for solving this problem. Three-dimensional photonic crystals (PCs), as a type of material with a periodically varying index,24,25 can also be used to modulate localized electric fields and have been successfully used to enhance the luminescent intensity of UCNPs.26 The combination of the PC and LSPR effects can further enhance the localized electromagnetic field and UCL. However, the Received: May 2, 2017 Revised: July 26, 2017 Published: July 26, 2017 6799

DOI: 10.1021/acs.chemmater.7b01783 Chem. Mater. 2017, 29, 6799−6809

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Chemistry of Materials

Figure 1. (a) Synthetic process of the PMMA OPC/Cu2−xS (x = 1, 0.2)/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid. (b) Scanning electron microscope (SEM) image of PMMA OPC. (c,d) SEM images of OPC/Cu2−xS hybrid with low and high resolution, respectively. (e) SEM image of OPC/Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid. (f) Transmission electron microscope (TEM) images of CuS NDs with different sizes: 20.1 (f1), 16.5 (f2), 14.3 (f3), and 11.5 nm (f4). (g) TEM images of Cu1.8S NSs with different sizes of 7.5 (g1), 8.8 (g2), 9.8 (g3), and 10.5 nm (g4). (h) Absorption spectra of CuS NDs and Cu1.8S NSs. (i) Density of free carriers and copper deficiency (x) of CuS NDs and Cu1.8S NSs with different sizes.

photonic stop band (PSB) of approximately 980 nm was selfassembled on the glass substrate (the location of the PSB was controllable through adjustment of the diameter of the PMMA beads). The OPC yielded a strong preference for cubic close packing over hexagonal and 111 planes and was parallel to the substrate with a lattice constant of 330 nm (Figure 1b). The OPC was then immersed into two kinds of Cu2−xS solution (CuS nanodisks (NDs) and Cu1.8S nanospheres (NSs)). As the solvent evaporated, Cu2−xS NPs permeated onto the surface voids of the opal template via capillary force and padded regularly on the hexagonal corners of the PMMA PCs, forming a two-dimensional orderly structure (Figure 1c and d). The Cu2−xS NPs mainly distributed on the top two layers of the PMMA beads (Figure S1). The driving force of the deposition could be attributed to an electrostatic attraction between the PMMA and Cu2−xS NPs, which induced the Cu2−xS NPs to occupy a potential minimum of the PMMA plane. Afterward, the MoO3 layer was deposited onto the surface via a vapor deposition method at a typical thickness of 8 nm (Figure S2a). Finally, NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ NPs of a size of 10

combination of opal photonic crystals (OPCs) with a plasmon semiconductor has not been reported. In this work, we first demonstrated the structure and size dependence of the LSPR effect and two-photon effect in Cu2−xS NPs (CuS and Cu1.8S NPs) and its interaction with NaYF4:Yb3+,Er3+ UCNPs. We then prepared poly(methyl methacrylate) (PMMA) OPC/Cu 2 − x S/MoO 3 /NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrids and obtained a 3 orders of magnitude UC enhancement under a high excitation power density (∼1.25 W/mm2) by combining the LSPR, PC, and two-photon effects. Furthermore, the enhanced UC hybrids were applied in a flat panel display for twodimensional-code patterned storage and for angle-dependent anticounterfeiting.

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RESULTS AND DISCUSSION Structure, LSPR, and Two-Photon Effect of the Cu2−xS NPs. The synthetic process of the PMMA OPC/Cu2−xS (x = 1, 0.2)/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid structure is shown in Figure 1a. First, the PMMA OPC with a typical 6800


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Figure 2. (a) Size-dependent bandgaps of CuS NDs and Cu1.8S NSs. (b) Dependence of photoconductivity of the annealed Cu1.8S film (consisting of Cu1.8S NSs with a diameter of 10.5 nm) and CuS film (consisting of Cu1.8S NDs with a diameter of 11.5 nm) on the irradiation power density of 980 nm light. (c) Schematic of the energy band of Cu1.8S and CuS in comparison to the energy of 808, 980, and 1540 nm light.

Figure 3. (a) XRD patterns of NaYF4:Yb3+,Er3+ NPs and NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ NPs. (b) Emission spectra of Cu2−xS/MoO3/ NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ under excitation of 808, 980, and 1540 nm light. (c) Enhancement factor of CuS/MoO3/NaYF4:Yb3+,Er3+@ NaYF4:Yb3+,Nd3+ hybrid under 808 nm excitation and Cu1.8S/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid under 980 nm excitation with different thicknesses of the MoO3 spacer. (d) Enhancement factor depends on the sizes of Cu2−xS NPs as well as the locations of LSPR at 0.15 and 1.25 W/mm2 power density under irradiation of 980 nm light. (e) Power dependent enhancement factors of Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@ NaYF4:Yb3+,Nd3+ hybrid excited by 980 nm laser. (f) Enhancement factor of Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid with different thickness of NaYF4 layer under irradiation of 980 nm light. (g) Enhancement factors of Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid irradiated by 808 nm, 980 and 1540 nm laser, respectively.

deficiencies in the valence band, which depend strongly on the structure and particle size of Cu2−xS (Figure 1h) and on the refractive index of the solvent in which they are dispersed, indicating their active plasmonic nature (Figure S6). Interestingly, for the CuS NDs, the LSPR band gradually shifted toward red as the average diameter increased from 11.5 to 20.1 nm; by contrast, in the case of Cu1.8S NSs, the LSPR peak gradually shifted toward blue with an increase in particle size (Figure 1h). As is known, the LSPR behavior of Cu2−xS NPs arises from collective oscillations of excess free carriers, which is associated with constitutional copper deficiency (x) in the lattice. On the basis of Figure 1h and the modified Mie−Drude theory,1,27 the density of free charge carriers (Nh) in different Cu2−xS NPs was deduced. As the structure and size of the Cu2−xS NPs change, Nh varies in the range from 3.65 × 1021 to 4.70 × 1021 cm−3. Naturally, the variation of Nh is related to the copper deficiency (x) in the Cu2−xS NPs. The copper

nm (Figure S1b−d) were deposited on top of the PMMA OPC/Cu2−xS (x = 0, 0.2)/MoO3 at a thickness of 50 nm (Figure 1e and S2b). Notably, the colloidal CuS NDs and Cu1.8S NSs were both prepared through the hot-injection method. The CuS NDs yielded average basal-plane diameters of 20.1, 16.5, 14.3, and 11.5 nm and an average height of 4 nm (Figure 1f). The Cu1.8S NSs displayed average diameters of 10.5, 9.8, 8.8, and 7.5 nm (Figure 1g). The CuS (covellite) was in the hexagonal phase, and the Cu1.8S (digenite) was in the rhombohedral phase (Figure S3). The copper deficiency (x) of Cu2−xS was estimated via energy-dispersive X-ray spectroscopy (EDX) analysis and X-ray photoelectron spectroscopy (XPS) measurements (Figure S4 and S5) to be approximately x = 1 for CuS NDs and x = 0.2 for Cu1.8S NSs. As shown, all of the Cu2−xS NPs for both CuS NDs and Cu1.8S NSs exhibited a broad absorption band in the NIR region because of excess holes resulting from copper 6801


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The UCL enhancement of Cu 2 − x S/MoO 3 /NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ (including CuS NDs and Cu1.8S NSs) hybrids was studied. In the hybrids, a suitable thickness for the MoO3 spacer is important (Figure 3c) because this spacer can effectively prevent nonradiative ET from NaYF4 to Cu2−xS; however, an excessively thick spacer adversely affects the interaction of the LSPR with the emitters. After optimization, the thickness of the MoO3 layer was fixed at 8 nm. In addition, the EFs depend on the size and structure of Cu2−xS, the LSPR wavelength of the Cu2−xS NPs, and the power density of the excitation light. Generally, the EF increased as the LSPR peak shifted to the excitation wavelength for both the CuS NDs and the Cu1.8S NSs (Figure 3d). This result definitely reveals that the upconversion enhancement is related to the LSPR effect at low excitation power and that the exact matching of the excitation light with the LSPR leads to a greater improvement in the excitation field. In the Cu2−xS/ MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid, the UCL enhancement depends strongly on the occurrence of twophoton band-to-band excitation processes in Cu2−xS. When the two-photon effect is allowed to occur (Cu1.8S, the energy of double 980 nm photons is larger than the bandgap), the EF in the hybrid increases with increasing excitation power density (Figure 3e). Because the energy of double 980 nm photons is less than the bandgap of the CuS NDs, the two-photon effect in the CuS/MoO3/NaYF4 hybrid could not occur, and the EFs decreased with increasing power density because of thermal quenching. The thickness of the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ layer also strongly affected the upconversion enhancement induced by the two-photon effect. The decreased thickness of the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ layer resulted in a gradual improvement in the EFs, which is attributed to the effective interaction distance between the UCNPs and the Cu2−xS plasmon NPs (Figure 3f). The EFs in Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@NaYF 4:Yb3+,Nd3+ hybrid were obtained via excitation by 808, 980, and 1540 nm wavelengths (Figure 3g). At a lower excitation power (0.15 W/ mm2), the EFs were approximately 10−30-fold under the excitation at different wavelengths because the excitation wavelengths were all similar to the wavelength of the LSPR peak. At higher excitation power (1.25 W/mm2), the EFs varied greatly with excitation wavelength, depending on the occurrence of a two-photon effect. When the two-photon processes were allowed to occur, the EFs increased significantly to hundreds of fold (for CuS under 808 nm excitation and Cu1.8S under 808 and 980 nm excitation). Otherwise, EFs decreased considerably compared with those excited at a lower density. Overall, the two-photon effect inhibited the thermalquenching effect/saturation effect to some extent and was significant for realizing an effective UC enhancement at high excitation power. This effect is important for developing photonic devices in the domain of a strong electromagnetic field. Three-Order UCL Enhancement in OPC/Cu2−xS/MoO3/ NaYF4 Hybrids. PCs, as a type of material with a periodically varying index, are another effective method to modulate localized electric fields and has been successfully explored to enhance the UCL of NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ UCNPs.28−30 The hybrids of three-dimensional PCs with Cu2−xS plasmonic semiconductor NPs may exhibit a combined PC, LSPR, and two-photon effect, synergistically contributing to the upconversion enhancement. According to our previous results (Figure S10), matching of the PSB with the excitation

deficiency (x) in different Cu2−xS NPs was determined via EDX and XPS spectra (Figure 1i, Figures S4 and S5). As shown in Figure 1i, the density of free carriers for both the CuS NDs and the Cu1.8S NSs has a positive relationship with the copper deficiency. The shift in the LSPR peak with a change in size was determined by the copper deficiency. The opposite trend for the copper deficiency with the sizes of CuS NDs and Cu1.8S NSs can be ascribed to different adding orders of the Cu and S precursors during the synthetic process. In addition to the LSPR effect, Cu2−xS NPs demonstrate a bandgap-related nonlinear optical effect. On the basis of the absorption spectra, the optical band gaps of Cu2−xS NPs were further deduced (Figure 2a and Figure S7). The bandgap of the CuS NDs ranged from 2.60 to 2.67 eV and that of Cu1.8S NSs ranged from 2.32 to 2.41 eV.1,9−11 The bandgap decreased with increasing size of the Cu2−xS NPs because of the quantum confinement effect (Figure 2a). Upon irradiation with 808 nm (1.534 eV), 980 nm (1.256 eV), and 1540 nm (0.805 eV) light, the power dependence of the photoconductivity was studied in the annealed CuS NDs (D = 11.5 nm, bandgap = 2.67 eV) and Cu1.8S NSs (D = 10.5 nm, bandgap = 2.32 eV) films with a 30 MPa pressure treatment, as shown in Figure 2b. After annealing (at 400 °C for 2 h), the ligands on the surface of the CuS NDs and Cu1.8S NSs that hinder the electronic transmission disappeared completely (Figure S8). Thus, when the double energies of the pumping lights are larger than the bandgaps of the CuS NDs or Cu1.8S NSs and the power densities were sufficiently strong (∼1 W/mm2), two-photon excitation processes occurred, inducing a rapid increase in the conductivity with a slope of approximately 2.0 in the ln−ln plot. This result corresponds to a process in which electrons within the valence band of the Cu2−xS film are pumped into the direct conduction band. The two-photon process occurs only when the double energies of the pumping lights are larger than the bandgaps (Figure 2c). UCL Enhancement Induced by the Cu2−xS NPs under Multiwavelength Pumping. Given their unique structure and the size dependence of the LSPR and two-photon effect, Cu2−xS NPs were used to improve the UCL of NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ NPs under the excitation of 808, 980, and 1540 nm continuous laser diodes. The size of NaYF4:Yb3+,Er3+ NPs increased from 7 to 10 nm after coating with the NaYF4:Yb3+,Nd3+ shell (Figure S1), both of which appeared to have a cubic phase (Figure 3a). In all of the measurements, a reflection optical path was adopted (Figure S9). Notably, the core−shell-structured NaYF4:Yb3+(20%),Er3+(2%)@NaYF4:Yb3+(10%),Nd3+(10%) NPs, which can be excited by multiwavelength lights, including 808 nm, were used in the experiments; these NPs are more suitable for in vivo imaging and detection than traditional NaYF4:Yb3+,Er3+ NPs. Figure 3b shows the blue 2H9/2−4I15/2, green 2 H11/2/4S3/2−4I15/2, and red 4F9/2−4I15/2 transitions in NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ under 808, 980, and 1540 nm excitation, respectively. The corresponding populating process is shown in Figure S9. The emission spectra under the excitation of different lights rarely change except for the variation in intensity ratio for different transitions. To better understand the upconversion enhancement in the hybrids of UCNPs and in the Cu2−xS NPs, we defined the enhancement factor (EF) as the intensity ratio of Er3+ in Cu2−xS-based hybrids to that in the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ films, which are both measured strictly under the same conditions (see Note 1 in the Supporting Information). 6802


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Figure 4. (a) Schematic illustration of OPC/Cu2−xS/MoO3/NaYF4:Yb3+(20%),Er3+(2%)@NaYF4:Yb3+(10%),Nd3+(10%) excited by 808, 980, and 1540 nm, respectively. (b) Enhancement factors of the green, red, and whole emission for OPC/CuS/MoO3/NaYF4 hybrids (808 nm light at 1.25 W/mm2, 980 and 1540 nm light at 0.15 W/mm2) and OPC/Cu1.8S/MoO3/NaYF4 hybrids (808 and 980 nm light at 1.25 W/mm2, 1540 nm light at 0.15 W/mm2). (c) Simulated electric field intensity distribution of OPC/CuS/MoO3/NaYF4 and OPC/Cu1.8S/MoO3/NaYF4 hybrids. The incident light wavelengths are 808, 980, and 1540 nm, respectively. |E|2 /|E0|2 represents the averaged electric field enhancement factor.

high enhancement and brightness for the UCL is attributed to the amplification of the electric field induced by the PC effect, LSPR effect, and energy transfer from the conduction band of Cu2−xS to NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ due to the twophoton absorption of Cu2−xS NPs. The PC and LSPR effects contributed to the amplification in all of the samples. For the Cu2−xS NPs, the two-photon effect requires a bandgap that is smaller than double the energy of the pumping light and a high excitation power density. Because energy from the Cu2−xS NPs are transferred to the green 2H11/2/4S3/2−4I15/2 transitions because of energy-level matching, the increase in intensity of the green emissions is much greater than that of the red emissions when the two-photon effect occurs. Consistent with two-photon absorption of the CuS NDs and Cu1.8S NSs (Figure 2), for the OPC/CuS/MoO3/NaYF4 hybrid, the UC enhancement is greatest under 808 nm excitation, and for the OPC/Cu1.8S/MoO3/NaYF4 hybrid, it is greatest under 980 nm excitation. Compared with 808 and 980 nm excitation, UC enhancement at 1540 nm excitation is relatively low because its energy is not sufficient to pump the electrons to the conduction band via two-photon absorption for both the CuS NDs and Cu1.8S NSs. To precisely understand the enhancement of the electric field intensity on the hybrid structure, we simulated the |E|2 surrounding isolated CuS NDs, Cu1.8S NSs, OPCs CuS/ OPCs, and Cu1.8S/OPCs at different wavelengths (808, 980,

light is important to utilize the PSB effect and obtain a substantially improved upconversion enhancement. Therefore, in the experiments with the hybrids of the Cu2−xS NPs and PMMA OPCs, the locations of the PSBs were controlled to be the same with different excitation wavelengths. After the introduction of the PMMA OPC layers, the EFs were further improved compared with those of Cu2−xS/MoO3/NaYF4. In the OPC/Cu2−xS/MoO3/NaYF4 hybrids, because of the extra contribution of the PC effect, hundreds to thousands of fold enhancements in the upconversion were achieved (Figure 4a and b), which depended strongly on the composition of the Cu2−xS NPs and the excitation wavelength. Generally, the Cu1.8S hybrid yielded a stronger enhancement than that of the CuS hybrid, and the optimum enhancement was observed at an excitation of 980 nm. For the overall UCL, the optimized enhancement factor for the OPC/Cu 1.8 S/MoO 3 /NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrids excited by the 980 nm laser was nearly 1500-fold, to which a green enhancement of 2000-fold and a red enhancement of 1200-fold contributed. This enhancement factor was obtained at a relatively high power density (1.25 W/mm2). The absolute brightness for the OPC/Cu1.8S/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrids (consisting of 10 nm UCNPs, 50 nm in thickness) was measured to be 1282 cd/m2 under excitation by a 1.25 W/mm2 980 nm laser diode, whereas it was only 147 cd/m2 for the metal sample OPC/Au nanorod/NaYF4:Yb3+,Er3+.31 Such a 6803


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Figure 5. (a−f) Power density dependence of integral UCL intensity of OPC/Cu2−xS/MoO3/NaYF4, Cu2−xS/MoO3/NaYF4, and pure NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ under irradiation of 808, 980, and 1540 nm light, respectively.

and 1540 nm) via the finite-difference time domain (FDTD) solutions, as shown in Figure 4c and Figure S11. The |E|2 of an isolated CuS ND or Cu1.8S NS clearly exhibits a maximum intensity increase of 15-fold at the hot spots. After formation of an aggregated Cu2−xS film, the electric field distribution is quite random with a maximum intensity increase of 25 at the hot spots. The electric field on the surface layer of the OPCs is concentrated periodically on the corners with a maximum intensity increase of 7-fold. After random aggregated CuS NDs or Cu1.8S NSs were placed on the surface void of the OPCs, a maximum electric field intensity increase of approximately 80− 115 was obtained at the hot spots. The averaged electric field enhancement factors obtained by calculating the average value of the electric field intensity enhancement in the mesh range were very consistent with the experimental results. As known, the UC process is a multiphoton (n) process, and the enhancement factor of the UCL is the nth-power of the averaged electric field enhancement factor. The simulated |E|2 indicates that the coupling of Cu2−xS NPs and OPCs can substantially magnify the field intensity over the separated Cu2−xS NPs or OPCs. Under 808 or 980 nm excitation, because the UCL in the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ NPs is mainly a two-photon process at low excitation power, the enhancement factor of the UCL is the square of the averaged electric field |E|2. The enhancement induced by the LSPR effect at low excitation power should be 100−400-fold. However, at high excitation power the UC probability does not track with the two photon process in a linear fashion. Therefore, the enhancement induced by the LSPR effect at high power should be only 10−20-fold, and thus the contribution from the two-photon effect at this high power should be approximately 100-fold. Under 1540 nm excitation, the UC enhancement should be attributable entirely to the local electric field amplification instead of the two-photon effect. In general, the LSPR-induced luminescent enhancement in the vicinity of plasmon NPs can be classified into two aspects: the interaction of the LSPR with the emission electromagnetic field, resulting in an improved radiative transition rate for the emitters, and the interaction of the LSPR with the excitation electromagnetic field, leading to an enhancement in the localized excitation field strength. To clarify the origin of the

LSPR-induced UC enhancement in the Cu2−xS/MoO3/NaYF4 and OPC/Cu2−xS/MoO3/NaYF4 hybrids, the UCL dynamics of 2H11/2, 4S3/2−4I15/2, and 4F9/2−4I15/2 transitions in different samples were compared (Figure S12). The results indicate that, in the hybrids, the total spontaneous transition rates, including radiative and nonradiative ones for these transitions, increased by only approximately 20%, whereas the steady-state UCL for Er3+ increased by 3 orders of magnitude. This result indicates that, in the hybrids, the UCL enhancement mainly results from the interaction of the LSPR with the excitation electromagnetic field rather than with the emission electromagnetic field, which is the Purcell effect.32 The occurrence of the two-photon effect in Cu2−xS NPs is clearly observed in the figure that shows the power dependence on the emission intensity. The power dependence of the green 2 H11/2/4S3/2−4I15/2 transitions in the Cu2−xS/MoO3/NaYF4 and OPC/Cu2−xS/MoO3/NaYF4 hybrids under excitations of 808, 980, and 1540 nm were systemically studied (Figure 5) and compared with the NaYF 4 :Yb 3+ ,Er 3+ @NaYF 4 :Yb 3+ ,Nd 3+ UCNPs assembled on glass substrates. In the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ assemblies, as evident in the ln−ln plot (IUCL ∝ Pn), the slope n for the green emission was approximately 2 under an excitation of 808 or 980 nm; thus, two photons were required to populate the green level. Under excitation by 1540 nm light, n was approximately 3, revealing that three photons were required. In the Cu2−xS/MoO3/NaYF4 and OPC/Cu2−xS/MoO3/NaYF4 hybrids, a decreased slope n was observed when the two-photon effect in the Cu2−xS NPs did not occur. This decreased slope is attributed to the cointeraction of the saturation effect and local thermal effect.32,33 When the two-photon effect in the Cu2−xS NPs occurred (for the CuS NDs, λex = 808 nm; for the Cu1.8S NSs, λex = 808, 980 nm), the slope n increased promptly when the excitation power density was sufficiently strong and substantially surpassed the required photon number, similar to the photon avalanche effect. The two-photon effect in Cu2−xS NPs results in a considerably improved luminescence with an increase in excitation power density, which is a unique behavior for the interaction of semiconductor NPs and the UCNPs and is important for obtaining an extremely intense UCL. 6804


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Figure 6. (a) Schematic illustration of the energy transfer mechanism in OPC/Cu2−xS/MoO3/NaYF4 hybrids. (b) Time-resolved spectra of Cu1.8S and Cu1.8S-NaYF4 films. Themporal dynamics of the relative differential transmission (ΔT/T) were probed at 520 nm, pump by 490 nm light. (c) Dependence of 1/EF and R6. R: thickness of NaYF4 spacer.

The enhancement factors and slopes of the Cu2−xS/MoO3/ NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid increased with decreasing thickness of NaYF4 (Figure S13). The energy transfer process from Cu2−xS to NaYF4 should be Förster resonance energy transfer (FRET) or electron diffusion. Considering that NaYF4 and PMMA microspheres are poor conductors, electron diffusion would be difficult. The FRET efficiency is traditionally described as

According to our studies, we propose a schematic for the UCL enhancement of Er3+ in the OPC/Cu2−xS/MoO3/ NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrids. In the present work, the NaYF4 layer was only approximately 50 nm, and its absorption to the excitation light (808, 980, and 1540 nm) was quite small considering the smaller absorption coefficient of NaYF4 (see Note S2 in the Supporting Information). After illumination with the excitation wavelength, most of the excitation photons could pass through the NaYF4 layer and irradiate the Cu2−xS and PC layers. Some of the excitation light scattered or reflected back to the NaYF4 layer via the LSPR and PC effects, leading to extra absorption of NaYF4:Yb3+,Er3+@ NaYF4:Yb3+,Nd3+ (Nd3+ for 808 nm laser, Yb3+ for 980 nm laser, Er3+ for 1540 nm) and improvement of the UCL. The other light was absorbed by the CuS or Cu1.8S layer, resulting in a photothermal effect and an increase in local temperature of the hybrids, which led the EF to gradually decrease with excitation power density, similar to the interaction of NaYF4:Yb3+,Er3+ UCNPs with metal NPs. The photothermal effect also results in a change in the populating channel of Er3+, inducing an increase of the red (4F9/2−4I15/2)-to-green (2H11/2/4S3/2−4I15/2) intensity ratio. When the photon energy of the excitation light is double the energy of the CuS or Cu1.8S band gap and the excitation power density is sufficiently strong, two-photon band-to-band excitation substantially occurs. Free electrons accumulated in the conduction band of Cu1.8S NSs or CuS NDs should transfer energy to Er3+ ions by pumping the Er3+ ions from 4I15/2 to 2 H11/2/4S3/2 states because of exact energy-gap matching. This matching results in a larger luminescent enhancement of the green than the red and in the extraordinary power-dependent behavior of the UCL (slope of n > 2). To demonstrate this point, the decay dynamics of the Cu1.8S film and Cu1.8S/NaYF4 hybrid film were compared. In the hybrid, the decay time constant of the Cu1.8S significantly decreased, indicating the occurrence of nonradiative energy transfer from the Cu1.8S to the NaYF4 layer (Figure 6b).

E = R 0 6/(R 0 6 + R6)

(1)

where R is the distance between donor and acceptor and R0 is the Förster distance. We prepared Cu2−xS/MoO3/NaYF4:Yb3+/ NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid films with tunable thicknesses (R) of undoped NaYF4 and measured the EF in these hybrid films. EF is positively correlated with energy transfer efficiency (E). Figure 6c shows that 1/EF displayed a linear relationship with R6, indicating that the interaction between Cu2−xS and NaYF4 NPs should be a FRET process. Angle-Dependent Infrared Anticounterfeit Imaging. The high UC enhancement and exceptional nanocrystal brightness provide compelling advantages for a wide range of fields, including imaging,34 document security,35−37 and anticounterfeit fields.38 Anticounterfeit technologies have found widespread applications37,39−41 in identity cards, currency, tags, luxury items, and important documents, whereas traditional anticounterfeit technologies are easily circumvented. The design and realization of an effective UC anticounterfeit technology that is difficult to duplicate, easy to prepare, convenient to recognize, and inexpensive are urgently needed. To demonstrate the extraordinary utility of the OPC/ Cu2−xS/MoO3/NaYF4 hybrids, we introduced the flexible OPCs into a nanoprinting technique to achieve two-dimensional-code patterned storage and angle anticounterfeiting. For the nanoprinting application, in contrast to other techniques, the pattern, combined with the fluorescence of the UCNPs, can remedy the background interference from the surface, thus offering excellent optical contrast and allowing high detection 6805


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Figure 7. (a) Schematic diagram of fabricating the anticounterfeiting hybrids. Photograph of the flexible OPC and hybrid. (b) Photograph of the two-dimensional code pattern on the bended flexible OPC. Scale bar: 0.5 cm. (c) Schematic diagram of angle-dependent anticounterfeiting. (d) d1− d3: Photographs of fluorescence of a two-dimensional code pattern on the paper invoice taken at 90°, 60°, and 30° angles under irradiance of 980 nm light, respectively; (e) e1−e3: Photographs of fluorescence of a school badge pattern on the hybrid taken at 90°, 60°, and 30° angles, respectively. (f) f1−f3: Photographs of fluorescence patterns on the hybrids taken at 90°, 60°, and 30° angles under irradiance of 980 nm light, respectively. (g) g1−g3: Photographs of fluorescence patterns on the hybrids taken at 90°, 60°, and 30° angles under irradiance of 980 nm light, respectively. (h) h1−h3: Emission spectra of anticounterfeiting hybrids measured at 90°, 60°, and 30° angles, respectively.

sensitivity.37 However, the brightness of smaller-sized UCNPs is quite weak, and the local field modulation induced by the enhanced UC device can dramatically enhance the brightness of the smaller-sized UCNPs. Therefore, we explored the nanoprinting technique for fabricating the Cu2−xS NPs and UCNPs on a flexible OPC film, which yielded good reproducibility and high recognition using a 980 nm excitation laser. The PMMA OPCs were first fabricated on the PET. Afterward, the Cu2−xS NPs and UCNPs were formulated into inks and printed onto the OPCs in accordance with a preset pattern. Figure 7a shows a photograph of the flexible OPC film before printing. A beautiful, gradient structural color is observed under normal indoor illumination when the OPC film is bent. After printing of the Cu2−xS NPs and UCNPs, the two-dimensional code pattern was stored in the hybrid film. The pattern observed under the irradiation of a 980 nm diode laser has a very high resolution and is not affected by the bending shown in Figure 7b. The reflectivity and photonic bandgap of the OPCs change with the observation angle (Figure S14). On the basis of this point, angle anticounterfeiting was achieved, as shown in Figure 7c. We printed the OPC/Cu2−xS/MoO3/NaYF4 hybrid with the OPC bandgap at 550 nm using NaYF4:Yb3+(40%),Er3+(5%) @NaYF4:Yb3+(10%),Nd3+(10%) NPs in which the emissions of the UCNPs were strongly enhanced by the synergy among the PC, LSPR, and two-photon effects. Figure 7d shows the thin and flexible hybrid pasted onto the paper invoice in which the bright and clear two-dimensional code pattern can be viewed only under the irradiance of infrared light. Because the OPC bandgap is at 550 nm, the proportion of green emission in the overall emission spectra of the UCNPs increased compared with that of pure UCNPs, resulting in a clear pattern with green

color observed vertically in the hybrid (Figure 7d1). When changing the angle of observation to 60°, the bandgap of OPC broadened and moved to 515 nm, resulting in a bright pattern with orange color (Figure 7d2). Further observing at 30° angle, the bandgap of OPC moved to 465 nm, which was far away from the green luminesce center wavelength. The green light can not be sufficiently enhanced, and the red color became the dominating color in the pattern (Figure 7d3). The changes of colors were observed in the spectra of hybrids measured at 90°, 60°, and 30° angles (Figure 7h). The angle-dependent anticounterfeiting based on the OPC/Cu2−xS structure can be used in many fields, such as in products for high security levels and antifraud and confidential documents. Panels e−g in Figure 7 show the applications of angle-dependent anticounterfeiting in different situations. The flexible structural design allows the production of complicated security patterns, and changes can be made rapidly using software-based printer-control systems, which offer tremendous potential for mass production. This approach is particularly advantageous for applications that require high display quality, such as security features and microdisplays.

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CONCLUSIONS In this work, we studied the size and structural dependence of LSPR and the two-photon absorption properties of Cu2−xS NPs (Cu1.8S NSs and CuS NDs) and the effect of their NIR antennas on the well-known upconversion nanophosphors, NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ NPs. First, the Cu2−xS NPs display a strong, broad, and tunable LSPR in the nearinfrared range, which should enhance the UCL of rare-earth ions at multiple wavelengths. This behavior is helpful for solar 6806


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colloidal Cu2−xS NPs were dissolved in a cyclohexane solution. A glass substrate was dropped vertically into the solution. Because of the slow volatilization of cyclohexane, the Cu2−xS NPs were self-organized onto the glass surface. A thin layer of MoO3 was subsequently deposited using an organic vapor-phase vacuum vapor deposition apparatus. This deposition method provides an accurate thickness for the dielectric spacer on the nanometer scale, which enables precise control over the distance between the UCNPs and the semiconductor NPs in our structure. Next, a large monolayer area for the UCNPs was spin-coated on top of the oxide layer with a thickness of approximately 50 nm. The PMMA OPCs on the glass substrate were prepared using a selfassembly method. Then, a certain thickness of Cu2−xS NPs was suspended on the PMMA OPCs uniformly by immersing the OPC in the Cu2−xS NP solution for 5 s. A thin layer of MoO3 was subsequently deposited using the organic vapor-phase vacuum vapor deposition apparatus. Afterward, the PMMA opal template was dropped vertically into the cyclohexane solution of dissolved NaYF4:Yb3+,Er3+@ NaYF4:Yb3+,Nd3+ UCNPs (at a concentration of 0.5 mg/mL) and placed in an oven at 35 °C for over 4 h. Because of the slow volatilization of cyclohexane, the NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ UCNPs were weakly self-organized onto the void space of the PMMA OPCs or deposited directly onto the surface of the glass substrate, which was driven by the capillary force of the liquid during the evaporating process. For the flexible nanoparticle-print device, the hydrophilic surface of a PET film was first irradiated with an ultraviolet lamp for 10 min. The OPC/Cu2−xS film was then prepared. The nanoparticle (NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+) solutions were injected into cartridges printed on the surface of the OPC/Cu2−xS film using an HP Deskjet 1010 inkjet printer. Measurement and Characterization. The morphologies of the products were recorded with a Hitachi H-8100IV transmission electron microscope operated at an acceleration voltage of 200 kV. The phase structure and purity of the as-prepared samples were characterized via X-ray power diffraction (XRD) conducted on a Rigaku D/max 2550 X-ray diffractometer equipped with a monochromatized Cu target radiation source (λ = 1.54 Å). AFM was performed using a DI Innova atomic force microscope (Bruker) in light tapping mode. UV/vis−NIR absorption spectra were collected using a Shimadzu UV-3600PC UV/vis−NIR scanning spectrophotometer in the wavelength range from 300 to 2000 nm. For the powerdependent UCL spectral measurements, a continuous 980 nm laser was used to pump the samples. In the luminescent dynamics measurements, the samples were pumped using a laser system consisting of an Nd:YAG pumping laser (1064 nm), a third-order harmonic generator (355 nm), and a tunable optical parameter oscillator (OPO, Continuum Precision II 8000) with a pulse duration of 10 ns, a repetition frequency of 10 Hz, and a line width of 4−7 cm−1. A visible photomultiplier (350−850 nm) combined with a double grating monochromator were used for spectral collection. The photoconduction curves were collected via a source measurement unit under the illumination of a continuous 980 nm diode laser. XPS was performed with an ESCALab220i-XL electron spectrometer from VG Scientific. Monochromatic Al Kα X-rays (hν = 1486.6 eV) were used for the analysis at an incident angle of 30° with respect to the surface normal. FTIR measurements in the 4000−400 cm−1 spectral range were conducted in transmission mode at a resolution of 4 cm−1 using a Bruker Equinox 70 spectrometer. Raman spectra were collected using an excitation wavelength of 532 nm on a Horiba Jobin Yvon Xplora. The laser spot size was 1 μm. Samples were prepared by depositing Cu2−xS powder onto glass wafers. All of the photographs were taken by an iPhone 5s. The brightness of the samples was measured using a PR655 spectroscan spectrometer.

spectral conversion, which requires the conversion of whole NIR light into visible light to improve the photoelectric conversion efficiency of solar cells. Second, in contrast to metal NPs, Cu2−xS NPs not only demonstrate tunable LSPR but also a strong two-photon effect, which can induce a substantial upconversion enhancement based on an energy transfer mechanism under a higher excitation power density (∼1.25 W/mm2). This approach overcomes the shortage in absolute strength and thermal quenching of UCNPs. Third, in hybrid Cu2−xS NPs and PMMA OPCs, a synergistic effect of the LSPR, two-photon, and PC effects occurs, which induces a dramatic UCL enhancement of up to 3 orders of magnitude. In addition, the enhanced UC devices are applicable to angledependent anticounterfeiting, where they display good performance and show strong potential for applications in security and antifraud areas. Overall, this work opens a new train of thought for overcoming the shortage of UCNPs as well as for broadening the novel applications of plasmon semiconductor NPs in photonics devices.

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EXPERIMENTAL SECTION

Synthesis of Cu2−xS NPs. The synthesis of the Cu2−xS NPs was accomplished via a typical hot-injection method by injecting a copper precursor into a sulfur precursor.42 The entire synthesis was performed under air-free conditions. A typical synthesis was carried out as follows. The copper precursor was prepared by mixing CuCl (0.01 mol) with a mixture of 4 mL of oleic acid (OA) and 5 mL of oleylamine (OAm) at 130 °C under continuous mechanical stirring. The as-prepared copper precursor was then cooled to room temperature. The sulfur precursor was prepared in a three-necked bottle by dissolving sulfur powder (0.01 mol) into 40 mL of ODE at 200 °C under mechanical stirring. The sulfur solution was subsequently heated to 180 °C, followed by a swift injection of the copper precursor; the resulting mixture was maintained at 180 °C for 5−15 min. Afterward, the heating metal was removed and cooled to room temperature. The obtained colloidal solution was then precipitated using excess acetone and recovered by centrifugation of the suspension; the supernatant was discarded. The precipitate was redispersed in an organic solvent such as carbon tetrachloride, trichloromethane, or cyclohexane. The precipitation and redispersion steps were repeated twice. Synthesis of the NaYF 4 :Yb 3 + (20%),Er 3 + (2%)@NaYF4:Yb3+(10%),Nd3+(10%) UCNPs. NaYF4:Yb3+,Er3+ UCNPs (6 nm) were synthesized according to a typical solvothermal method43 at a reaction temperature of 290 °C. The as-prepared products were collected and washed with absolute ethanol−cyclohexane (1:1 v/v) three times, and the final products were dissolved in the cyclohexane solution for the subsequent experiments. NaYF4:Yb3+(20%),Er3+(2%) @NaYF4:Yb3+(10%),Nd3+(10%) UCNPs (10 nm) were synthesized via a general procedure. The growth of the shell layer was carried out via similar procedures. The NaYF4:Yb,Nd shell growth process was as follows: 2 mL of cyclohexane solution containing the as-obtained core nanocrystals was added to a mixed solution of 10 mL of OA and 10 mL of ODE in a 100 mL four-neck round-bottom reaction vessel followed by heating at 120 °C under vacuum and magnetic stirring for 30 min to completely remove residual water and oxygen. Next, the solution was heated to 290 °C under a N2 atmosphere. During the process of heating, the shell precursors, including 0.5 mmol RECl3 (RE = 80% Y + 10% Yb + 10% Nd) and 0.5 mmol of NaOH and NH4F were dissolved in 4 mL of OA/ODE (v:v = 1:1); they were then injected into the aforementioned reaction solution dropwise when the temperature reached 290 °C. The reaction was maintained at this temperature for 1.5 h. Finally, the as-obtained transparent yellowish reaction mixture was cooled to 60 °C, precipitated by the addition of sufficient ethanol, collected via centrifugation, and redispersed in 20 mL of cyclohexane. Fabrication of the Cu 2−xS/MoO3/NaYF4:Yb3+,Er 3+@NaYF4:Yb3+,Nd3+ Hybrid Structure. For the Cu2−xS/MoO3/NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ hybrid structure to be obtained, the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01783. 6807


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TEM and AFM images, FITR and Raman spectra, UC mechanism of upconversion, simulated electric field intensity distribution by FDTD, and angle-changing transmittance spectra of OPC (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Lin Xu: 0000-0001-5831-430X Hongwei Song: 0000-0003-3897-5789 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Major State Basic Research Development Program of China (973 Program) (2014CB643506), the National Key Research and Development Program (2016YFC0207101), the National Natural Science Foundation of China (61674067, 11304118, 11504131, 11374127, 81201738, 11674127), the Jilin Province Natural Science Foundation of China (20160418055FG), Jilin Provincial Economic Structure Strategic Adjustment Fund Special Projects (2014Y082), Jilin Province Natural Science Foundation of China (20150520090JH, 20170101170JC), and the Jilin Province Science Fund for Excellent Young Scholars (20170520129JH, 20170520111JH).

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Synergistic Upconversion Enhancement Induced by Multiple Physical

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