Observation of Considerable Upconversion Enhancement Induced by

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Observation of Considerable Upconversion Enhancement Induced by Cu S Plasmon Nanoparticles 2-x

Donglei Zhou, Dali Liu, Wen Xu, Ze Yin, Xu Chen, Pingwei Zhou, Shaobo Cui, Zhanguo Chen, and Hongwei Song ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00649 • Publication Date (Web): 05 May 2016 Downloaded from http://pubs.acs.org on May 6, 2016

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Observation of Considerable Upconversion Enhancement Induced by Cu2-xS Plasmon Nanoparticles Donglei Zhou, Dali Liu, Wen Xu*, Ze Yin, Xu Chen, Pingwei Zhou, Shaobo Cui, Zhanguo Chen, Hongwei Song* Dr. D. L. Zhou, Prof. D. L. Liu, Dr. W. Xu, Dr. Z. Yin, Dr. X. Chen, Dr. P. W. Zhou, Prof. Z. G. Chen, Prof. H. W. Song State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China. Dr. W. Xu, School of Chemical and Biomedical Engineering, Nanyang Technological University, 70 Nanyang Drive, Singapore 637457. Dr. S. B. Cui, College of Physics, Jilin University, 2699 Qianjin Street, Changchun, 130012, P. R. China.

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ABSTRACT

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 an 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: Semiconductor Plasmon Nanoparticles; Upconversion Luminescence Enhancement; Broadening, decrease and red-shift of LSPRs; Electron Injection.

Localized surface plasmon resonance (LSPR) in heavily doped semiconductor nanoparticles (NPs, 5-11 nm) and in quantum dots (QDs, 2-4 nm) has attracted considerable interest since its discovery because of the unique optical properties and application potential of these materials in various photonics devices.1-3 In NPs composed of a noble metal such as gold or silver, LSPR originates from collective oscillations of excess free carriers, resulting in enhanced and tunable absorption and scattering resonances.4,5 The plasmonic behavior of semiconductor NPs is


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commonly considered to arise from collective oscillations of excess free carriers associated with constitutional vacancies or ionized dopant impurities in the lattice, leading to intense extinction bands at near-infrared (NIR) wavelengths.6,7 The LSPR in a semiconductor NP is unique because the LSPR energy can be tuned via doping or stoichiometry of the NP, providing an additional means of tuning the optical properties that is not as readily available in metals.8 Achievement of LSPR by free-carrier doping of semiconductor NPs would enable active on-chip control of LSPR responses. Furthermore, NPs enable the realization of LSPR and quantum-confined excitons within the same structure, opening the possibility of strong coupling of photonic to electronic modes, with implications for light harvesting, nonlinear optics, and quantum information processing.9,10 However, the basic features of the relationships between the structures, compositions, electronic structures, and optical properties of semiconductor NPs remain poorly understood. In this regime, Cu2-xE (E = S, Se, or Te) NPs are p-type semiconductors because of their copper deficiency, resulting in a large number of highly mobile hole carriers that support a strong LSPR in the near-infrared.1,11,12

At resonance, the electric field near the surface of a plasmon nanoparticle is strongly enhanced, resulting in molecules in the vicinity exhibiting altered light harvesting and emission.13,14 LSPR-induced luminescence enhancement based on various metal nanostructures has been observed in various emitters (including dye molecules, quantum dots and rare-earth (RE) ions) and has been applied to various photonic and photoelectric devices and to singlemolecule detection.15,16 Recently, researchers have exploited the LSPR of some noble-metal nanostructures to enhance the near-infrared-to-visible upconversion luminescence (UCL) of REdoped upconversion nanoparticles (UCNPs); the results of these studies demonstrate unique nonlinear emission features and considerable potential applications (ranging from bio-


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applications to photoelectric devices) of these materials.17,18 The challenge of effectively improving the UCL strength/efficiency19 of RE-doped UCNPs has been a bottleneck in the implementation of various applications of UCL. The discovery of LSPR in heavily doped semiconductor NPs has opened a new regime in plasmonics1,20-22 and could provide an approach for improving the UCL of RE-doped UCNPs.

In this paper, we report our observation of considerably enhanced UCL in an semiconductor-based LSPR device, Cu2-xS-MoO3-NaYF4:Yb3+,Er3+, and discuss the physical characteristics of LSPR in Cu2-xS NPs and the nature of the interaction of LSPRs with the NaYF4:Yb3+,Er3+ UCNPs.

Results

Characterization of the structure, morphology and composition

A schematic of the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid structure is shown in Figure 1a. To prepare these hybrids, we prepared colloidal p-type Cu2-xS NPs with tunable LSPRs via the hot-injection method.8,23,24 We then spin-coated them onto a glass surface to serve as NIR nanoantennas; the thickness of the NP layer was fixed at approximately 900 nm. In an attempt to prevent the possible energy transfer (ET) from NaYF4:Yb3+,Er3+ UCNPs to Cu2-xS NPs and provide an appropriate interaction distance, a tunable MoO3 spacer (0-16 nm) was deposited onto the Cu2-xS layer using the vacuum vapor deposition method. Finally, an NaYF4 layer with an approximate thickness of 150 nm was deposited on the top of the MoO3 (see Figure S1). Figures 1b-1i show the structure, morphology and composition of various Cu2-xS NPs and Cu2-xS-MoO3NaYF4:Yb3+,Er3+ hybrid films. Four X-ray diffraction (XRD) peaks were observed in the XRD


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patterns of the Cu2-xS NPs, located at 27.8, 32.2, 46.2, and 54.7°. These peaks were indexed to the (0,0,15), (1,0,10), (0,1,20), and (1,1,15) planes, respectively, of rhombohedral Cu2-xS (digenite) with space group R3̅m (JCPDS No. 47-1748). In the XRD pattern of the Cu2-xS-MoO3NaYF4:Yb3+,Er3+ hybrid film, peaks originating from both rhombohedral Cu2-xS and cubic NaYF4 are evident, indicating the formation of a hybrid structure. Note that in the pattern of the hybrid film, no peaks attributable to MoO3 were observed because of its low thickness (see Figure 1b). The TEM images imply that the Cu2-xS NP products are monodispersed, homogeneous and sphere-like. The average sizes of the NPs are 6.5, 7.8, 8.8, 9.8 and 10.8 nm. The HR-TEM image of Cu2-xS NPs in Figure 1h clearly shows that in a given particle, the lattice is arranged along a single direction and that the lattice fringes have an interplanar spacing of 0.32 nm, which corresponds to the (0,0,15) spacing of rhombohedral-phase Cu2-xS. To obtain the elemental compositions of the resulting samples, we analyzed various Cu2-xS NPs using quantitative energy-dispersive X-ray spectroscopy (EDX). As shown in Figure 1i, the atomic ratio of Cu to S increases linearly in the range from 1.75 to 1.91 with decreasing Cu2-xS particle size. XPS spectra were also recorded to confirm the elemental compositions of the resulting samples (see Figure S2). The results indicate the co-existence of Cu+ and Cu2+ in the Cu2-xS NPs. The atomic ratios of Cu to S were determined to be 1.87, 1.83 and 1.71 for 6.5 nm, 8.8 nm and 10.8 nm Cu2-xS NPs, respectively; these results match well with the EDX analysis results (2% error).

Optical, electric, photothermal and photoelectronic properties.

Before photoluminescence measurements, the optical, electric, photothermal and photoelectronic properties of the Cu2-xS NPs and Cu2-xS-MoO3-NaYF4 hybrids were systemically


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studied to deeply understand the essence of LSPRs in semiconductor Cu2-xS NPs and its interaction with NaYF4:Yb3+,Er3+ UCNPs. Figure 2a shows the LSPR absorption spectra of Cu2xS

NPs of different sizes (6.5-10.8 nm) dispersed in cyclohexane solutions. A broad band

extending from the visible to the NIR range was observed in the spectra of all the Cu2-xS NPs, and the peak wavelength gradually shifted toward the red with decreasing Cu2-xS particle size; this behavior is attributed to the decrease of the carrier density. In the case of the spectrum of the 10.8 nm Cu2-xS NPs, the LSPR peak is located at approximately 1000 nm, matching well the wavelength of the pumping light of UCL. Note that in the spectra of the Cu2-xS films or Cu2-xSMoO3-NaYF4 films, the location of the LSPR peak was shifted approximately 40 nm toward the red (Figure S3), which we attributed to the coupling of different LSPR modes of Cu2-xS NPs.

Based on the modified Mie-Drude theory by Luther et al.,1 the LSPR absorption frequency ωsp depends on the density of charge carriers (Nh), which can be expressed as

=

(1)

,

where εm is the dielectric constant of the environmental medium; e is the elemental charge; ε0 is the free-space permittivity; mh is the hole effective mass, which is approximated to be 0.8 m0 where m0 is the electron mass;22 and γ is the full-width at half-maximum of the LSPR-related absorption peak. Nh is the density of the free carriers. On the basis of Eq. 1, the values of Nh were determined to be 3.1 × 1021, 3.8 × 1021, 4.1 × 1021, 4.7 × 1021 and 5.3 × 1021 cm−3 for the 6.5, 7.8, 8.8, 9.8 and 10.8 nm Cu2-xS NPs, respectively. For comparison, Ne in gold is 5.9 × 1022 cm−3. Next, the Cu-to-S ratios for the 6.5, 7.8, 8.8, 9.8, and 10.8 nm Cu2-xS NPs were estimated to be 1.86, 1.83, 1.82, 1.79, and 1.76, respectively (see Note S1), which matched well with the EDX


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analysis and the XPS measurement (2.7% in error). To determine the free carrier density of Cu2xS

NPs, we performed Hall effect measurements (Figure S4). The results show that the Cu2-xS

NPs were p-type semiconductor materials with a free carrier density of 1.55 × 1018 cm-3, which is three-orders lower than the charge density required for generating LSPR (1021 cm-3). To further investigate this point, the collected FTIR and Raman spectra of the samples (Figure S5). The results indicate that the as-prepared Cu2-xS NPs were wrapped with extra oleylamine (OAm) as surface ligands, which functioned as a surfactant solvent during the synthesis process. In fact, the insulated OAm ligands on the surface of Cu2-xS NPs confined the migration of charge carriers from one Cu2-xS NP to another, leading to the lower carrier mobility. Therefore, the difference between the calculated carrier density and that determined by Hall measurements was mainly caused by the insulated ligands on the surface of the particles, which resulted in a low carrier mobility. The Hall effect measurement indicated that the charge carriers for generating the LSPR were confined inside the Cu2-xS nanocrystals by the OAm ligands, which could not move freely among the particles.

Figure 2b shows the temperature-dependent LSPR of the Cu2-xS film consisting of 10.8-nm NPs after the sample was heated for approximately 2 min to reach the target temperature and then measured immediately. The results show that with increasing temperature, the LSPR of the Cu2-xS NPs gradually red-shifted and broadened and that the absorption strength gradually decreased. The broadening of the LSPR for Cu2-xS NPs is attributed to the enhancement of electron-phonon coupling. To demonstrate this point, we studied the temporal dynamics of the relative differential transmission (∆T/T) probed at 900 nm with a pump pulse wavelength of 1200 nm (Figure S6). In the dynamics measurements, the fast decay of the differential transmission that occurs within a few picoseconds is related to the coupling of electrons and


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phonons. The damping time induced by the coupling of the electrons and phonons decreases with increasing temperature, indicating that the broadening of plasmonic resonance is mainly induced by the coupling of electrons and phonons. A similar result has been reported for Cu2-xSe plasmon NPs.25 The redshift of LSPR peak with temperature is attributable to two possible causes: the variation of ε with temperature and the density change of charge carriers. According to the theoretical calculations reported by Scotognella et al.,25 the variation of LSPR induced by the change of ε is negligible in the studied temperature range.25 The density change of charge carriers may originate from two factors: lattice expansion with increasing temperature and a decrease of the number of holes, which is dominated by Cu2+/Cu+ ratio. The lattice expansion coefficient for Cu2S is approximately 28.6 × 10-6 K-1.26,27 When the temperature varied from room temperature to 200°C, the lattice expansion was only approximately 0.5%; lattice expansion therefore only slightly influenced the position of the LSPR peak. It is suggested that the chemical reaction on the surface of NPs is the main mechanism leading to the significant decrease of the hole density with increasing temperature. As previously discussed, the Cu2-xS nanoparticles were packed by oxygen-containing organic groups. When the temperature was increased the organic groups decomposed, generating O2, which could react with sulfur on the surface of the particles, resulting in the loss of sulfur and a decrease of Cu2+/Cu+.

To further investigate the reaction on the surface of the NPs, we subjected the Cu2-xS films to thermal treatments. In the experiments, each Cu2-xS film was heated for 2 h at a certain temperature under the protection of nitrogen and was then cooled to room temperature for its LSPR spectrum to be collected again. The LSPR properties of the Cu2-xS films after the thermal treatment are displayed in Figure 2c. After the annealing process, the LSPR bands gradually shifted to red and decreased substantially in intensity. When the films were annealed at


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temperatures greater than 200°C, the LSPR bands disappeared completely. To evaluate the structural change, the XRD patterns of the annealed Cu2-xS NPs were collected; these patterns revealed a shift of 0.3-0.5° toward smaller angles, indicating that slight lattice expansion occurred (Figure S7). The XPS and EDX spectra were also collected; the results indicate that after annealing, the proportion of Cu2+/Cu+ decreased. Meanwhile, a peak associated with lattice oxygen was observed in the XPS spectra of annealed Cu2-xS, whereas only adsorbed oxygen was detected in the case of as-prepared Cu2-xS. This appearance of lattice oxygen indicates that oxidation indeed occurred during annealing, resulting in oxygen into the crystal lattice of Cu2-xS (Figure S8). Therefore, we conclude that oxidation occurred during the annealing process, leading to the observed decrease of the Cu2+/Cu+ ratio and to the redshift of the peaks in the LSPR spectra of the annealed Cu2-xS nanoparticles. Notably, self-purification might also occur in Cu2-xS nanoparticles, which would further contribute to the decrease of the Cu2+/Cu+ ratio.28,29

Figure 2d shows the temperature dependence of the conductivity σ of the Cu2-xS film. In the measurement, the sample temperature was increased quickly; thus, the removal of ligands in the Cu2-xS film could be neglected. As evident in Figure 2d, the conductivity increases dramatically and exponentially with increasing temperature over the studied temperature range. As is well known, = , where e is the charge quantity, µ is the charge mobility, and n is the charge density. In the high-temperature region, the variation of carrier mobility with temperature can be ignored in contrast to the exponential increase of charge density with temperature.28 Therefore, the dependence of the conductivity of the Cu2-xS film on temperature can be expressed as follows:

(2)

σT = − /"# ,


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where σ(T) is the conductivity at any temperature, σ0 is the conductivity at 0 K, ΔE is the energy gap between the valence band and the indirect bandgap of Cu2-xS NPs, k is the Boltzmann constant, and T is the absolute temperature. On the basis of Eq. 2, we deduced the values of ΔE to be 1.31, 1.29, 1.23, 1.18 and 1.14 eV, corresponding to the 6.5, 7.8, 8.8, 9.8 and 10.8 nm Cu2xS

samples, respectively. These deduced values are generally consistent with the indirect

bandgap of Cu2S estimated by optical measurements, and the gradual increase of ΔE with decreasing particle size is attributable to the quantum confinement effect.29,30 To evaluate the influence of the organic groups on the conductivity of Cu2-xS particles, we annealed the Cu2-xS films at 400°C for 2 h under a nitrogen atmosphere and then measured the temperature dependence of their conductivity. The results indicated that the conductivity of Cu2xS

films after the thermal treatment increased by approximately 104 times compared to the

conductivity of the as-prepared Cu2-xS films, whereas the slopes in the temperature-conductivity plots changed only slightly (Figure S9). Therefore, we concluded that the surface ligands strongly influenced the carrier mobility and σ0, but only slightly affected the temperature dependence of σ. After the annealing process, ΔE was almost independent of the Cu2-xS NP size, which we attributed to the disappearance of the confinement effect due to the removal of ligands. To further elucidate the relationship between the optical and electric properties of the Cu2-xS plasmon NPs, the 980 nm light-irradiation-induced LSPR spectral change and conductivity change were investigated, as shown in Figure 2e and Figure 2f, respectively. Figure 2e shows that upon irradiation with 980 nm light, the position of the LSPR absorption band gradually shifted to the red and the absorption intensity decreased. This observation is attributed to the temperature increase of the Cu2-xS films under irradiation with 980 nm light, resulting in oxidation at the surface of Cu2-xS NPs and a decrease of the Cu2+/Cu+ ratio.


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Figure 2f displays the dependence of photoconductivity of the annealed Cu2-xS films under 30 MPa pressure on the irradiation power density of 980 nm light. Note that after the Cu2-xS NPs were annealed at 400°C for 2 h, the ligands on their surface disappeared completely. As shown in Figure 2f, when the power density was less than 0.6 W/mm2, the conductivity slowly increased with increasing excitation power density (slope = 0.06). When the power density exceeded 0.6 W/mm2, the conductivity increased quickly, with a slope of 2.21 in the ln-ln plot, indicating that the conductivity was approximately proportional to the square of power density. This result provides evidence that at a high excitation power density of 980 nm light (1.26 eV), the electrons within the valence band of a Cu2-xS film can be pumped into the direct conduction band via a two-photon process. The direct bandgap of 10.8 nm Cu2-xS NPs determined by optical measurement is approximately 2.4 eV, matching well with twice the energy of a 980 nm photon.31 In the presence of a large amount of ligands, although the band-to-band excitation via the two-photon process could still occur, the carriers would not be allowed to hop freely across the ligand barriers from one particle to the next. However, under irradiation by strong 980 nm light, the temperature of the Cu2-xS film would increase considerably (Figure S10), leading to the removal of some of the ligands on the surface of the Cu2-xS NPs; therefore, the migration of carriers is likely to occur from one particle to another.

UCL enhancement of Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid films We were excited to observe that in the presence of a suitable amount of MoO3, the UCL of Er3+ in a Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid film was greatly improved compared to that in the corresponding NaYF4:Yb3+,Er3+ film on a glass substrate. Moreover, we compared the Cu2xS-induced

UCL enhancement with metal-plasmon-nanostructure (Au-MoO3-NaYF4:Yb3+,Er3+)-


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induced enhancement; the results show that the interaction of NaYF4:Yb3+,Er3+ UCNPs with Cu2xS

NPs differed dramatically from that with the metal nanostructure. The preparation and

characterization of Au nanorods and Au film are shown in Figure S11. Note that the comparison was performed under strict conditions, including the same laser source, the same excitation power density and the same optical circuit (Figure 3a). In addition, the density of NaYF4:Yb3+,Er3+ UCNPs in the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid film and in the NaYF4:Yb3+,Er3+ film are approximately the same, as determined by the optical properties of the NaYF4:Yb3+,Er3+ layer (Figure S1). Figure 3b shows a comparison of the normalized UCL spectra for a NaYF4:Yb3+,Er3+ film, a Cu2-xS-MoO3 (8 nm)-NaYF4:Yb3+,Er3+ hybrid film and a Cu2-xS-MoO3(8-nm)- NaYF4:Yb3+,Er3+ hybrid film. In the spectra, blue 2H9/2-4I15/2, green 2H11/2/4S3/2-4I15/2 and red 4F9/2-4I15/2 transitions are observed. The populating of the blue 2H9/2 level is a three-photon energy transfer (ET) process, whereas those for the green 2H11/2/4S3/2 and red 4F9/2 levels are both two-photon ET processes.32 In the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ and Au-MoO3-NaYF4: Yb3+,Er3+ hybrids, the intensity ratios of

2

H11/2-4I15/2 to

4

S3/2-4I15/2 both increase in comparison to that in the

NaYF4:Yb3+,Er3+ nanofilm, indicating an increased temperature in the two samples.19,33,34 This increased temperature is due to the extra LSPR absorption and photothermal effect induced by the Cu2-xS and Au nanofilms. Interestingly, the variation of the intensity ratio of green 2H11/2, 4

S3/2-4I15/2 to red 4F9/2-4I15/2 (G/R) in the two hybrid samples is opposite. The G/R in the Au-

MoO3-NaYF4:Yb3+,Er3+ hybrid decreases, whereas that in the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid increases compared to that in the NaYF4:Yb3+,Er3+ film. In the Cu2-xS-MoO3NaYF4:Yb3+,Er3+ and Au-MoO3-NaYF4:Yb3+,Er3+ hybrids, the overall UCL intensity and the UCL intensities of different transitions all increase in contrast to those in the NaYF4:Yb3+,Er3+


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film. Figure 3c shows a comparison of the UCL enhancement factors (EFs) of different transitions between Au-MoO3-NaYF4:Yb3+,Er3+ and Cu2-xS-MoO3-NaYF4:Yb3+,Er3+. Here, EF is defined as the UCL intensity ratio of Cu2-xS-based or Au-based hybrid films to that of the NaYF4:Yb3+,Er3+ film under the same conditions. On the basis of Figure 3b, we concluded that (1) the EF of the red emission is higher than that of the green emission for the Au-based hybrid, whereas the EF of the green is higher than that of the red for the Cu2-xS-based hybrid; and (2) under relatively weak excitation power density, the EF for the Cu2-xS-based hybrid is smaller, whereas at relatively high excitation power density, EF for the Cu2-xS-based hybrid is larger. To further study these phenomena, we compared the UCL intensity as a function of excitation power in different samples, as shown in Figure 3d (green emissions) and Figure S12 (blue and red emissions). As evident in the ln-ln plot(IUCL∝Pn), the slope n for the green emission in the NaYF4:Yb3+,Er3+ film is approximately 2, i.e., two photons are required to populate the green level. In the case of the Au-based hybrid, in the starting range the slope deviates from 2, which is attributed to the saturation effect and the local thermal effect.35 The saturation effect depends on the competition between linear decay and UC processes for the depletion of the intermediate excited states, resulting in the deviation of n from the required photon number. The local thermal effect originates from the sample absorption of excitation light and the subsequent photothermal conversion, leading to a local temperature increase of the irradiated sample and luminescence quenching. As the excitation exceeds 0.7 W/mm2, the UCL even decreases with increasing excitation power due to temperature quenching. Similar to the Au-based hybrid, the slope n in the case of the Cu2-xS-based hybrid also deviates from 2 in the starting power range. Surprisingly, as the power density exceeds 0.7 W/mm2, the slope dramatically increases to 7.0, similar to the photon avalanche phenomenon.36 This increase in slope results in an unusual power-dependence


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of EF (Figure 3e). For the Au-based hybrid, similar to the results of previous literature reports, the EF decreases solely with increasing temperature, which is mainly attributed to the influence of the saturation effect and a local thermal effect.35 In the case of the Cu2-xS-based hybrid, the EF initially decreases with increasing temperature and approaches a minimum at approximately 0.6 W/mm2. Beyond this value, the EF gradually increases with increasing excitation power. This result suggests that the enhancement mechanism for the Cu2-xS-based hybrids may differ from that for the Au-based hybrid, as will be further discussed in the following text. We note that in the aforementioned experiments, an 8-nm MoO3 spacing layer was used in the Au- and Cu2-xS-based hybrids to effectively enhance the UCL of NaYF4:Yb3+,Er3+. In fact, in the case of the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrids, the EF also depends strongly on the thickness of the MoO3 spacer (Figure 3f). For the sample without an MoO3 layer, a slight decrease in UCL was observed, which might be related to the nonradiative ET from NaYF4:Yb3+,Er3+ to Cu2-xS. An enhancement of the UCL was observed with the assistance of the MoO3 spacer, and the optimal enhancement occurred when the thickness of the MoO3 layer was 8 nm; the enhancements were 82-fold at 1.23 W/mm2 and 8-fold at 0.15 W/mm2 for the Au- and Cu2-xS-based hybrids, respectively. A further increase of the MoO3 thickness led to a decrease of the EF. This result indicates that the effective interaction distance of plasmon Cu2-xS NPs is approximately 8 nm. To further demonstrate the essential interaction between the plasmon Cu2-xS NPs and NaYF4 Yb3+,Er3+ UCNPs, the dependences of EF on the particle size of Cu2-xS NPs and on the position of the LSPR band are shown in Figure 3g. At a power density of 0.15 W/mm2, the EF increases rapidly with increasing size of the Cu2-xS NPs and the LSPR band shifting toward the excitation wavelength. However, at 1.23 W/mm2, the EF exhibits only a slight increase with


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increasing size of the Cu2-xS NPs and blue-shifting of the LSPR band. This result definitely reveals that at relatively low power density, the UCL enhancement originates mainly from the coupling of UCNPs with the LSPR of Cu2-xS NPs. At high excitation power, the UCL enhancement might be independent of the interaction of UCNPs with the LSPR. To demonstrate this point, the UCL enhancement in the annealed Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid was studied. After the Cu2-xS NPs were annealed at 400°C for 2 h, the LSPR of Cu2-xS NPs disappeared completely with the removal of the ligands. In this case, the UCL enhancement nearly disappeared at lower excitation power densities, whereas it remained almost unchanged at higher power densities (Figure S13). These results confirm that the UCL enhancement mechanism for Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid is power-dependent. UCL dynamics of Au- and Cu2-xS-based hybrid samples In general, the LSPR-induced luminescence enhancement in the vicinity of plasmon NPs can be classified into two aspects: (1) the interaction of the LSPR with the emission electromagnetic field, resulting in the improved radiative transition rate of emitters; (2) the interaction of the LSPR with the excitation electromagnetic field, leading to an enhancement of the localized excitation field strength. Figure 4 shows the UCL dynamics of 2H9/2, 4S3/2 and 4F9/2– 4

I15/2 transitions of different samples. In the Cu2-xS-based and Au-based hybrids, the luminescent

lifetimes for these transitions decrease in comparison to those in the NaYF4:Yb3+,Er3+ nanofilm, which implies an improvement of the total spontaneous transition rates for these transitions, including radiative and nonradiative ones. However, although the steady-state UCL for Er3+ increases to several ten times, the spontaneous rate for the corresponding transition increases by only up to 15%. This result indicates that in the Cu2-xS-based and Au-based hybrids, the UCL


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enhancement results mainly from the interaction of LSPR with the excitation electromagnetic field, rather than from the interaction of LSPR with the emission electromagnetic field. In the vicinity of metal nanostructures, a significant increase of the radiative rate is frequently observed in dye molecules,37,38 whereas in the RE-doped UCNPs, the variation of the radiative rate was typically unnoticeable. This result is attributable to the following two facts. First, the interaction distance between dye molecules and plasmon nanostructure is easier to control than that between the UCNPs and plasmon nanostructure. In the UCNPs, some luminescence centers are located within the effective distance (< 10 nm) in the plasmon nanostructure (metal or semiconductor), whereas the others are located outside of the effective distance. The rate deduced here is the average for all the luminescent centers in the space and thus should be affected by the contributions of the distant centers. In the case of the other luminescence centers, the excited-state lifetime for dye molecules is close to that of vibrational excited states of the metal nanostructures; thus, its coupling with the plasmon during the excitation is constant. Second, the lifetime for 4f excited states of RE ions is much longer than that of the vibrational excited states for the metal nanostructures; thus, its coupling time with the plasmon is too short in comparison to its excited-state lifetime. On average, the coupling contributes little to the dynamics for the whole time range.

Discussion On the basis of the structural, optical, electric and photoelectric properties of the Cu2-xS plasmon NPs and their interaction with NaYF4:Yb3+,Er3+ under 980 nm light, we can construct a schematic for the power-dependent UCL enhancement of Er3+ in the Cu2-xS-MoO3NaYF4:Yb3+,Er3+ hybrids (Figure 5). The bandgap of MoO3 is 3 eV,39,40 and the valence band of MoO3 is lower than that of Cu2-xS, which has a direct bandgap of 2.4 eV and an indirect bandgap


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of 1.1-1.2 eV. In the present work, the NaYF4:Yb3+,Er3+ layer is only approximately 150 nm thick, and its absorption of the excitation light is quite low (~10-4) given the smaller absorption coefficient of NaYF4:Yb3+ (20%),Er3+ (2%) (see Note S2). After illumination with 980 nm light, most of the 980 nm photons pass through the NaYF4:Yb3+,Er3+ layer and the MoO3 layer (which is nearly transparent at 980 nm) and irradiate the Cu2-xS layer. Some of the 980 nm light is scattered back to the NaYF4:Yb3+,Er3+ layer via the LSPR effect, leading to the extra absorption of NaYF4:Yb3+,Er3+ (Yb3+) and to the improvement of the UCL. The remaining light is absorbed by the Cu2-xS layer. In the case of the plasmonic NPs, the scattering coefficient of LSPR increases with increasing particle size; thus, the UCL enhancement factor increases with increasing size of the Cu2-xS NPs (Figure S14). The UCL enhancement is mainly induced by the scattering of plasmon Cu2-xS NPs when the excitation power is less than 0.7 W/mm2 (Figure 5a). Differently, when the 980 nm excitation power is greater than 0.7 W/mm2, the LSPR in a Cu2-xS film decreases and its contribution to the enhancement is degraded. In this case, the electrons arising from the valence band of Cu2-xS NPs are accessibly excited into the direct conduction band of Cu2-xS through a two-photon process, as previously evidenced. The free electrons gathered in the conduction band of Cu2-xS NPs pass through the MoO3 layer to the NaYF4:Yb3+,Er3+ layer through diffusion. In the NaYF4:Yb3+,Er3+ layer, the free electrons are captured by Er3+ ions on the 2H11/2/4S3/2 states because of the exact energy matching between the bandgap of Cu2-xS and the emission transitions of 2H11/2/4S3/2-4I15/2. This process coincides with the fact that the enhancement for the green 2H11/2, 4S3/2-4I15/2 transitions is greater than that for the red in Figure 3c and with the extraordinary power-dependent behavior in Figure 3d.


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To demonstrate the occurrence of electron diffusion from the Cu2-xS layer to the NaYF4:Yb3+,Er3+ layer, we investigated the decay dynamics of the Cu2-xS film and the Cu2-xSMoO3-NaYF4:Yb3+,Er3+ hybrid film. The results demonstrate that in the hybrid film, the decay time constant of the Cu2-xS decreased from 10.20 ps to 2.21 ps. This result indicates the occurrence of nonradiative energy migration from Cu2-xS toward the NaYF4:Yb3+,Er3+ layer (Figure S15). To further elucidate the behavior of electron diffusion, we prepared Cu2-xS-MoO3NaYF4:Yb3+,Er3+ hybrid films with different thicknesses of NaYF4 (Figures S16a-f) and investigated the power-dependent UCL in these hybrid films (Figure 6a). The results indicate that as the thickness of NaYF4:Yb3+,Er3+ was varied over the range from 29 to 430 nm, the slope n decreased gradually from 9.73 to 4.67 in the studied power density range, implying the suppressed interaction of the Cu2-xS film with the NaYF4:Yb3+,Er3+ UCNPs. All the slopes were much greater than 2.0, suggesting that given the average effect in the whole NaYF4:Yb3+,Er3+ film, the Cu2-xS strongly and constantly affected the NaYF4:Yb3+,Er3+ layer, even when its thickness approached 430 nm. The decreased thickness of the NaYF4:Yb3+,Er3+ layer also resulted in improved EF, as shown in Figure 6b: in the hybrid film with a 29 nm thick layer of NaYF4:Yb3+,Er3+, the EF was as high as 200-fold at a power density of 1.3 W/mm2. To determine the electron diffusion distance, we also prepared Cu2-xS-MoO3-NaYF4:Yb3+-NaYF4:Yb3+,Er3+ hybrid films, as shown in the inset of Figure 6c. The NaYF4:Yb3+ intermediate layer was controlled to be 20, 28, 37, and 50 nm, respectively (Figure S16). The NaYF4:Yb3+ layer did not possess an energy level (Er3+) that matched that of Cu2-xS and could not accept the electrons diffused from the Cu2-xS layer.


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Figure 6c shows the power-dependent UCL in the Cu2-xS-MoO3-NaYF4:Yb3+NaYF4:Yb3+,Er3+ hybrid films. As the thickness of the NaYF4:Yb3+ intermediate layer was increased from 20 nm, the slope decreased gradually, indicating a weakened interaction between Cu2-xS and NaYF4:Yb3+,Er3+. As the thickness of the NaYF4:Yb3+ layer approached 50 nm, the slope n decreased to 1.58, i.e., lower than that of the pure NaYF4:Yb3+,Er3+ film, indicating that electron diffusion had nearly disappeared. This observation suggests that the distance of electron diffusion was less than 50 nm. When the thickness of the NaYF4:Yb3+ layer was increased from 0 to 20 nm, the slope decreased dramatically from 7.52 to 4.38, indicating that the strong interaction distance was within 20 nm. Similar results were also obtained for the Cu2-xS-MoO3NaYF4 (undoped)-NaYF4:Yb3+,Er3+ hybrid films with NaYF4 (undoped) layers of different thicknesses (Figure S17), indicating that the Yb3+ ions contributed little to the electron diffusion. The UCL of a Cu2-xS-MoO3-NaYF4:Yb3+,Tm3+ hybrid film was also studied; the results showed that Er3+ ions were substituted by Tm3+ ions (Figure S18) and that a UCL enhancement was not obvious under high excitation power because the energy mismatch between the direct bandgap of Cu2-xS and the emission transitions of Tm3+.

Conclusions In this paper, we demonstrated the distinctive photonic and photoelectric properties of Cu2xS

semiconductor plasmon NPs and its unique interaction with NaYF4:Yb3+,Er3+ UCNPs. First,

we systematically studied the photonic, electric and photoelectric properties of Cu2-xS plasmon NPs/nanofilms and discovered that the LSPRs in Cu2-xS NPs originated from ligand-confined carriers. The oxidation reaction was observed to substantially occur on the surface of Cu2-xS NPs and to depend strongly on temperature, leading to the broadening, red-shift, decrease in intensity and even disappearance of the LSPRs. Second, we prepared a Cu2-xS-MoO3-NaYF4 hybrid film


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and achieved a highly improved UCL of NaYF4:Yb3+,Er3+ in the presence of an MoO3 spacer of suitable thickness (8 nm), which demonstrated extraordinary power-dependent behavior. Lastly, we proposed a model to explain the interaction mechanism between Cu2-xS plasmon NPs and NaYF4:Yb3+,Er3+ UCNPs, that is, the plasmon scattering effect at lower excitation power density and electron diffusion via a two-photon interband transition of Cu2-xS at higher excitation power. The diffusion distance was estimated to be less than 50 nm. Notably, the electron diffusion at high excitation power is associated with the unique energy band structure of semiconducting Cu2-xS, which is completely different from that of noble metals and may open a new regime in plasmonics. This work not only provides a new approach to effectively enhance the NIR to visible UCL of RE-doped UCNPs but also provides new insights into the nature of LSPR in semiconductor NPs.

MATERIALS AND METHODS Synthesis of Cu2-xS NPs, NaYF4: Yb3+,Er3+ NPs and the fabrication process of Cu2-xSMoO3-NaYF4:Yb3+,Er3+ hybrids. The Cu2-xS NPs were synthesized via a typical hot-injection method by injecting a copper precursor into a sulfur precursor. All the syntheses were conducted under air-free conditions. A typical synthesis process is described as follows. The copper precursor was prepared by mixing CuCl (0.01 mol) with a mixture of 4 mL oleic acid (OA) and 5 mL OAm at 130°C under continuous mechanical stirring. Next, the as-prepared copper precursor was cooled to room temperature. Sulfur precursor was prepared in a three-neck bottle by dissolving sulfur powder (0.01 mol) into 40 mL octadecene (ODE) at 200°C under mechanical stirring. The sulfur solution was then heated to 180°C, followed by swift injection of the copper precursor; the resulting solution was maintained at 180°C for 5–15 min. Next, the heat source


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was removed and the solution was allowed to cool 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 then redispersed in an organic solvent such as carbon tetrachloride, trichloromethane, or cyclohexane. The precipitation and redispersion was repeated twice. Next, NaYF4:Yb3+,Er3+ UCNPs (8 nm) were synthesized according to a typical solvothermal method41 at 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. To prepare the Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid structure, we dissolved the colloidal Cu2-xS NCs in the cyclohexane solution containing the NaYF4:Yb3+,Er3+ UCNPs. The glass substrate was dipped vertically into the solution. With the slow volatilization of cyclohexane, the Cu2-xS NCs self-organized onto the surface of the glass (Figure S1a). A thin layer of MoO3 was subsequently deposited using an organic vapor phase vacuum vapor deposition apparatus. This method resulted in the deposition of dielectric spacer with an accurate thickness on the nanometer scale, which enabled precise control over the distance between the UCNPs and the semiconductor NCs in our structure. Next, a large-area monolayer of UCNPs was spin-coated on top of the oxide layer to a thickness of approximately 150 nm (Figure S1i). Structural, physical and optical characterization. The morphology of the products was recorded with a Hitachi H-8100IV transmission electron microscope (TEM) under an acceleration voltage of 200 kV. The phase structure and purity of the as-prepared samples were characterized by X-ray power diffraction (XRD) with a Rigaku D/max 2550 X-ray diffractometer, using a monochromatized Cu target radiation resource (λ=1.54 Å ). AFM was


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performed using a DI Innova AFM (Bruker) in light tapping mode. UV/Vis–NIR absorption spectra were measured with a Shimadzu UV-3600PC UV/Vis–NIR scanning spectrophotometer in the range from 300 to 2000 nm. In the measurements of power-dependent UCL spectra, a continuous 980 nm was used to pump the samples. In the measurements of luminescent dynamics, the samples were pumped using a laser-system consisting of a 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 4–7cm-1. A visible photomultiplier (350–850 nm) combined with a double-grating monochromator were used for spectral collection. The photoconduction curves were collected by a source-measurement unit under the illumination of continuous 980 nm diode laser. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALab220i-XL electron spectrometer from VG Scientific. Monochromatic Al Kα X-ray (hν = 1486.6 eV) was employed for analysis with an incident angle of 30° with respect to surface normal. FITR measurements in the 4000-400 cm-1 spectral range were carried out in transmission mode at a resolution of 4 cm-1 using a Bruker Equinox 70 spectrometer. The Raman spectra were taken using an excitation of wavelength of 532 nm on Horiba Jobin Yvon Xplora. The laser spot size is 1µm. Samples were prepared by depositing Cu2-xS power on glass wafers. Conflict of Interest: The authors declare no competing financial interest. Supporting Information Available: Synthesis methods and morphology characterization of Cu2-xS NPs, NaYF4: Yb3+, Er3+ NPs and Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrids, XPS spectra, absorption spectra, Hall effect measurement, FITR spectra and Raman spectra, transient absorbance spectra, EDX and XPS spectra, conductance of the Cu2-xS-MoO3-NaYF4 hybrids with 980 nm light-irradiation or not at the same temperature, characterization of Au nanorods


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and Au film sample, diffuse reflection spectra for Cu2-xS NPs, temperature of Cu2-xS-MoO3NaYF4:Yb3+, Er3+ hybrids under the irradiation of 980 nm laser with different power density, time-resolved spectra of Cu2-xS film and Cu2-xS-NaYF4. This material is available free of charge via the Internet at http://pubs.acs.org. Author Information Corresponding Author: Hongwei Song E-mail: [email protected] Author Contributions. D.Z., W.X. and H.S. conceived the project and designed the experiments. D.Z. synthesized copper sulphide nanoparticles and NaYF4 nanoparticles, performed optical characterization and the spectra measurement. D.Z., Z.Y., X.C. and S.C. collected the data and performed the UCL dynamics. D.Z., D.L.,W.X., P.Z., Z.C. and H.S. analyzed the data and discussed the result. D.Z., W.X. and H.S. were primarily responsible for supporting information. D.Z. prepared figures. H.S. and D.Z. wrote the main manuscript text. All authors contributed to data analysis, discussions and manuscript preparation. ACKNOWLEDGMENT This work was supported by the Major State Basic Research Development Program of China (973 Program) (no.2014CB643506), the National Natural Science Foundation of China (grant no. 11504131, 11374127, 11304118, 61204015, 81201738, 81301289, 61179055, 61177042, and 11174111), and Program for Chang Jiang Scholars and Innovative Research Team in University (no. IRT13018), the Jilin Province Natural Science Foundation of China (No. 20140101171JC), the Open Research Fund of State Key Laboratory of Bioelectronics, Southeast University, Jilin Provincial Economic Structure Strategic Adjustment Fund Special Projects (No. 2014Y082). REFERENCES


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2013, 135, 7098-7101. 24. Liu, L. G.; Zhong, H. Z.; Bai, Z. L.; Zhang, T.; Fu, W. P.; Shi, L. J.; Xie, H. Y.; Deng, L. G.; Zou, B. S. Controllable Transformation from Rhombohedral Cu1.8S Nanocrystals to Hexagonal CuS Clusters: Phase- and Composition-Dependent Plasmonic Properties. Chem. Mat. 2013, 25, 4828-4834. 25. Scotognella, F.; Della Valle, G.; Srimath Kandada, A. R.; Dorfs, D.; Zavelani-Rossi, M.; Conforti, M.; Miszta, K.; Comin, A.; Korobchevskaya, K.; Lanzani, G.; Manna, L.; Tassone, F. Plasmon Dynamics in Colloidal Cu2-xSe Nanocrystals. Nano Lett. 2011, 11, 4711-4717. 26. Selivanov E. N.; Gulyaeva R. I.; Vershinin A. D.; Thermal Expansion and Phase Transformations of Copper Sulfides. Inorg. Mater. 2007, 43, 573-578. 27. Trots, D. M.; Senyshyn, A.; Mikhailova, D. A.; Knapp, M.; Baehtz, C.; Hoelzel, M.; Fuess, H. High-Temperature Thermal Expansion and Structural Behaviour of Stromeyerite, AgCuS. J.Phys. Condens. Matter. 2007, 19, 136204. 28. Stern, F. Calculated Temperature Dependence of Mobility in Silicon Inversion Layers. Phys. Rev. Lett. 1980, 44, 1469-1472. 29. Yu J. G.; Zhang J.; Liu S. W.; Ion-Exchange Synthesis and Enhanced Visible-Light Photoactivity of CuS/ZnS Nanocomposite Hollow Spheres. J. Phys. Chem. C 2010, 114, 13642-13649. 30. Klimov, V.; Haring Bolivar, P.; Kurz, H.; Karavanskii, V.; Krasovskii, V.; Korkishko, Y. Linear and Nonlinear Transmission of CuxS Quantum Dots. Appl. Phys. Lett. 1995, 67, 653655. 31. Bhuse, V. M.; Hankare, P. P.; Garadkar, K. M.; Khomane, A. S. A Simple, Convenient, Low Temperature Route to Grow Polycrystalline Copper Selenide Thin Films. Mater. Chem.


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Phys. 2003, 82, 711-717. 32. Boyer, J.-C., Vetrone, F., Cuccia, L. A. & Capobianco, J. A. Synthesis of Colloidal Upconverting NaYF4 Nanocrystals Doped with Er3+, Yb3+ and Tm3+, Yb3+ via Thermal Decomposition of Lanthanide Trifluoroacetate Precursors. J. Am. Chem. Soc. 2006, 128, 7444-7445. 33. Huang, P.; Zheng, W.; Zhou, S. Y.; Tu, D. T.; Chen, Z.; Zhu, H. M.; Li, R. F.; Ma, E.; Huang, M. D.; Chen, X. Y. Lanthanide-Doped LiLuF4 Upconversion Nanoprobes for the Detection of Disease Biomarkers. Angew. Chem.-Int. Edit. 2014, 53, 1252-1257. 34. Zheng, Q. D.; Zhu, H. M.; Chen, S. C.; Tang, C. Q.; Ma, E.; Chen, X. Y. FrequencyUpconverted Stimulated Emission by Simultaneous Five-Photon Absorption. Nat. Photon. 2013, 7, 234-239. 35. Pollnau, M.; Gamelin, D. R.; Luthi, S. R.; Gudel, H. U.; Hehlen, M. P. Power Dependence of Upconversion Luminescence in Lanthanide and Transition-Metal-Ion Systems. Phys. Rev. B 2000, 61, 3337-3346. 36. Xu, W.; Min, X. L.; Chen, X.; Zhu, Y. S.; Zhou, P. W.; Cui, S. B.; Xu, S.; Tao, L.; Song, H. W. Ag-SiO2-Er2O3 Nanocomposites: Highly Effective Upconversion Luminescence at High Power Excitation and High Temperature. Sci. Rep. 2014, 4, 9. 37. Lu, D.; Kan, J. J.; Fullerton, E. E.; Liu, Z. W. Enhancing Spontaneous Emission Rates of Molecules Using Nanopatterned Multilayer Hyperbolic Metamaterials. Nat. Nanotechnol. 2014, 9, 48-53. 38. Tan, C. L.; Qi, X. Y.; Huang, X.; Yang, J.; Zheng, B.; An, Z. F.; Chen, R. F.; Wei, J.; Tang, B. Z.; Huang, W.; Zhang, H. Single-Layer Transition Metal Dichalcogenide NanosheetAssisted Assembly of Aggregation-Induced Emission Molecules to Form Organic


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Nanosheets with Enhanced Fluorescence. Adv. Mater. 2014, 26, 1735-1739. 39. Brown, P. R.; Lunt, R. R.; Zhao, N.; Osedach, T. P.; Wanger, D. D.; Chang, L. Y.; Bawendi, M. G.; Bulovic, V. Improved Current Extraction from ZnO/PbS Quantum Dot Heterojunction Photovoltaics Using a MoO3 Interfacial Layer. Nano Lett. 2011, 11, 29552961. 40. Murase, S.; Yang, Y. Solution Processed MoO3 Interfacial Layer for Organic Photovoltaics Prepared by a Facile Synthesis Method. Adv. Mater. 2012, 24, 2459-2462. 41. Mai, H. X.; Zhang, Y. W.; Si, R.; Yan, Z. G.; Sun, L. D.; You, L. P.; Yan, C. H. High-Quality Sodium Rare-Earth Fluoride Nanocrystals: Controlled Synthesis and Optical Properties. J. Am. Chem. Soc. 2006, 128, 6426-6436.


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Figure 1. Schematic illustration and characterization on structure, morphology and composition of various Cu2-xS NPs and Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid film. (a) Schematic illustration of the Cu2-xS-MoO3-NaYF4 hybrid structure. (b) XRD patterns of Cu2-xS film and Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ hybrid film. (c-g) TEM images of Cu2-xS NPs with different sizes. Scale bar: 50 nm. (h) HR-TEM image of Cu2-xS NPs. (i) EDX spectra of Cu2-xS NPs with different sizes.


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Figure 2. Optical, electric, photothermal and photoelectronic properties of the Cu2-xS NPs and Cu2-xS-MoO3-NaYF4 hybrids. (a) LSPR absorption spectra of Cu2-xS NPs with different sizes (6.5-10.8nm) as dispersed in cyclohexane solutions. Dashed lines show trends in changes in LSPR peak values. (b) Temperature-dependent optical properties: LSPR absorption spectra of Cu2-xS film at different temperature. When the sample was heated for about 2 minutes to reach the target temperature, the absorption spectrum was measured immediately. (c) LSPR absorption spectra of Cu2-xS film after 2 hours thermal treatment under different temperatures. Insert: calculated density of free carriers depend on the temperature. (d) Electric properties of Cu2-xS film: the temperature-dependent conductance. (e) Power-dependent LSPR absorption: LSPR absorption spectra of Cu2-xS film with 980 nm light-irradiation at various power densities. Insert: Schematic

of

power-dependent

LSPR

absorption

measurement.

(f)

Dependence

of

photoconductivity of the annealed Cu2-xS film on the irradiation power density of 980 nm light. The Cu2-xS film was annealed at 400oC for 2 hours. Insert: Schematic of conductivity measurement.


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Figure 3. Comparison for the UCL enhancement between Au and semiconductor samples. (a) Schematic illustration of the fluorescence measurement methods. (b) Emission spectra excitated with 0.15 W/mm2 and 1.23 W/mm2 power respectively. (c) A comparison for the UCL enhancement factors (EFs) of different transitions between Au-MoO3-NaYF4:Yb3+, Er3+ and Cu2xS-MoO3-NaYF4:Yb

NaYF4:Yb3+,

Er3+,

3+

, Er3+. (d) The power density dependence of integral UCL intensity of Au-MoO3-NaYF4:Yb3+,

Er3+

and

Cu2-xS-MoO3-NaYF4:Yb3+,

Er3+

(4S3/2/2H11/2–4I15/2 transitions). (e) Enhancement factors of Cu2-xS-MoO3-NaYF4 and Au-MoO3NaYF4 film as a function of excitation power density. (f) Enhancement factor for 8nm size for different thickness of MoO3 at 0.15 and 1.23 W/mm2 excitation power respectively. (g) The dependence of EF on particle size of Cu2-xS NPs as well as the location of LSPR band.


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Figure 4. UCL dynamics of Au and semiconductor samples. (a) The decay time of 2

H9/2,4S3/ 2and 4F9/2–4I15/2 transitions of N aYF4:Yb3+, Er3+ NCNPs under 980 nm excitation

separated from Cu2-xS NPs as a function of MoO3 spacer layer. (b) The decay time of 2

H9/2,4S3/ 2and 4F9/2–4I15/2 transitions of NaYF4:Yb3+, Er3+ NCNPs on different substrates.


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b


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Figure 5. Schematic illustration and scheme of energy levels for the UCL enhancement. (a) Schematic illustration for the UCL enhancement at high excitation power and low excitation power in the Cu2-xS-MoO3-NaYF4:Yb3+, Er3+ hybrids. (b) Schematic illustration of electron injection process for Cu2-xS-MoO3-NaYF4:Yb3+, Er3+ hybrids.


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Figure 6 (a) The power density dependence of integral UCL intensity of Cu2-xS-MoO3NaYF4:Yb3+, Er3+ with various thickness of NaYF4:Yb3+,Er3+. (b) The power density dependence of UCL enhancement factor of Cu2-xS-MoO3-NaYF4:Yb3+,Er3+ with various thickness of NaYF4:Yb3+,Er3+. (c) The power density dependence of integral UCL intensity of Cu2-xS-MoO3NaYF4:Yb3+-NaYF4:Yb3+,Er3+ hybrid with various thickness of NaYF4:Yb3+. Intert: Schematic illustration of Cu2-xS-MoO3-NaYF4:Yb3+-NaYF4:Yb3+,Er3+ hybrid. (d) The power density dependence of UCL enhancement factor of Cu2-xS-MoO3-NaYF4:Yb3+-NaYF4:Yb3+,Er3+ with various thickness of NaYF4:Yb3+.


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The Table of Contents (TOC) and figure abstract

Cu2-xS-MoO3-NaYF4:Yb3+,Er3+

hybrid

film

exhibited

highly

improved

upconversion

luminescence in the presence of an MoO3 spacer of suitable thickness (8 nm). The enhancement mechanism was that the plasmon scattering effect at lower excitation power density and electron diffusion via a two-photon interband transition of Cu2-xS at higher excitation power. The electron diffusion distance at high excitation power was approximate 50 nm.


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Observation of Considerable Upconversion Enhancement Induced by

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