Introduction

Lanthanide-doped upconversion nanoparticles (UCNPs) have the remarkable ability to initiate the anti-Stokes process by combining two or more low-energy photons to generate a single high-energy photon, which enables a broad range of potential applications, ranging from high-efficiency photovoltaic cells to background-free bio-imaging and therapeutic probes [1]. Owing to the forbidden nature of the f–f transitions of trivalent lanthanide ions (Ln3+), however, these inorganic materials suffer from the small absorption cross section and low radiative decay rate/probability, leading to the fact that the corresponding upconversion luminescence remains inefficient for practical applications [2]. Hence, one major challenge in the field of UCNP research falls into the demand for vigorous enhancement in upconversion luminescence efficiency [3].

Considerable efforts have been devoted to modify the internal structure of UCNPs for efficiency improvement, including the host lattice manipulation [4], sensitizer/migrator/activator dopants design [5], and surface passivation [6], with focus on optimizing the energy transfer process as well as suppressing the competitive nonradiative quenching events [7]. In addition to these traditional approaches, tailoring the electromagnetic environment experienced by upconverting materials by means of plasmonic structures has emerged as an attractive strategy for dramatic upconversion emission enhancement [8]. In theory, when surface plasmons are resonantly excited, the energy of incident light can be concentrated within a small volume to induce enhanced local electric field (E) [9]. Such enhanced local electric field is capable of promoting the emission intensity of the nearby upconversion emitters, by either increasing the excitation rate when the plasmon resonances match to the absorption of sensitizer [10] or accelerating radiative decay rate (Purcell effect) when the plasmon resonances match to the emission of activator [11].

So far, the majority of plasmonic materials for upconversion luminescence enhancement (UCL) are noble metals, i.e., Au and Ag. The high fixed charge carrier concentration (on the order of 1022 cm−3) of these materials causes the natural resonance frequencies for LSPR to lie in the visible spectrum, allowing the matching to the emissions of activators such as Er3+ and Tm3+. For further expanding the LSPR peak of noble metals to near infrared (NIR) for matching the absorption of sensitizer, i.e., Yb3+ for most cases, elaborate shape/size tailoring and/or complex micro-/nanofabrication procedures are needed. As an alternative material system, the emerging plasmonic semiconductors provide additional opportunities for this purpose. For plasmonic semiconductor nanocrystals, the carrier concentration can be flexibly tunable (1019–1021 cm−3) via doping type/concentration/placement control or post-treatment, leading to the variation of resonance frequencies in broad range of NIR [12]. Considering the NIR excitation and visible emission nature of Ln-doped UCNPs, combining plasmonic noble metals with semiconductors, may be a promising way for increasing the excitation and emission rate simultaneously.

Last decade, several works have reported the colloidal synthesis of metal–semiconductor dual plasmonic nanostructures such as Au–Cu2−xS [13,14,15] and Au–Cu2−xSe [15,16,17,18]. Optical extinction spectra of these dual plasmonic nanostructures show two distinct peaks in visible and NIR, originating from the metal and semiconductor domains, respectively. These dual plasmonic nanostructures have been proposed to be excellent photothermal/photoacoustic agents [13, 15, 17, 18]. However, to our best knowledge, the employment of metal–semiconductor dual plasmonic antenna for UCL enhancement has not been reported.

Herein, we report that Au–CuS heterodimer nanocrystals with dual plasmon resonances both in visible and in NIR can function as a novel metal–semiconductor antenna for enhancing the UCL intensity of Yb/Er co-doped NaGdF4 UCNPs. CuS was selected as plasmonic semiconductor antenna for its high carrier (hole) density and the LSPR peak can locate at near 980 nm [19], matching the absorption of Yb3+. And Au was selected as the noble metal counterpart for its typical LSPR peak can locate at near 520 nm, matching the emissions of Er3+. Our results show that the Au–CuS heterodimers are able to enhance the UCL by 17.5-, 6.4-, and 7.4-fold at 520, 540, and 654 nm, respectively, which obviously outperform the single plasmonic component of Au or CuS. The Au–CuS heterodimer nanocrystals’ role in dual enhancement, i.e., simultaneous excitation and emission enhancement, is also demonstrated by the corresponding analysis of UCL decay dynamics.

Experimental section

Synthesis of Au nanocrystals

Au nanocrystals were synthesized via reacting HAuCl4·3H2O with OAM [20]. Typically, 0.3 mmol HAuCl4·3H2O was dissolved in 3 mL OAM to form a transparent complex and then injected into a 100-mL three-neck round-bottom flask containing 10 mL OAM at 150 °C under N2 flow. The solution was allowed to stir for 90 min. After cooled down to room temperature, the obtained nanocrystals were precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Synthesis of CuS nanocrystals

The plasmonic CuS nanocrystals were synthesized via reacting Cu(acac)2 with elemental sulfur. Typically, 0.2 mmol Cu(acac)2 was dissolved in 10 mL DCB with 2 mL OAM as surfactant and then heated up to 80 °C under N2 flow in a 100-mL three-neck round-bottom flask. At this point, 2 mL DCB containing 0.2 mmol dissolved elemental sulfur by ultrasound was injected swiftly and then reacted at 100 °C for 30 min. After cooled down to room temperature, the obtained nanocrystals were precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Synthesis of Au–CuS hybrid dimer nanocrystals

The plasmonic Au–CuS hybrid dimer nanocrystals were synthesized by a seeded growth method. Typically, 0.2 mmol Au seeds and 0.2 mmol Cu(acac)2 were dissolved in 10 mL DCB with 2 mL OAM as surfactant and then heated up to 80 °C under N2 flow in a 100-mL three-neck round-bottom flask. At this point, 2 mL DCB containing 0.2 mmol dissolved elemental sulfur by ultrasound was injected swiftly and then reacted at 100 °C for 30 min. After cooled down to room temperature, the obtained nanocrystals were precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Synthesis of NaGdF4:Yb3+, Er3+ core nanocrystals

The β-NaGdF4:Yb3+/Er3+ core-only nanocrystals were synthesized by a co-precipitation method. In a typical experiment, 0.8 mmol Ln(Ac)3·6H2O (Ln = Gd, Yb, Er) in an appropriate ratio was mixed with 8 mL OA and 12 mL ODE in a 100-mL three-neck round-bottom flask and then heated to 150 °C under the protection of N2 until a colorless homogenous Ln-OA complex coordinate solution is formed. After that, the coordinate solution was cooled down to 30–40 °C in a room-temperature water bath. A methanol solution (6 mL) containing 1 mmol NaOH and 3 mmol NH4F was added and stirred for 10 min. Then, the system was heated to 65 °C under N2 flow to remove the low-boiling methanol, and the clear solution was degassed and heated up to 290 °C for 1 h to obtain the nanocrystals. After cooled down to room temperature, the final clear solution was precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Synthesis of NaGdF4 sacrificial-shell nanocrystals

The synthesis of the NaGdF4 sacrificial-shell nanocrystals was analogous to the β-NaGdF4:Yb3+/Er3+ core-only nanocrystals, except 0.8 mmol Gd(Ac)3·6H2O was employed as the Ln(Ac)3·6H2O precursor. When the reaction temperature rises to 270 °C, the reaction was stopped by removing the heating mantle. The final clear solution was precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Synthesis of NaGdF4:Yb3+, Er3+/NaGdF4 core/shell nanocrystals

The NaGdF4:Yb3+, Er3+/NaGdF4 core/shell nanocrystals were synthesized by a ripening-mediated method. After the synthesis of β-NaGdF4:Yb3+/Er3+ core nanocrystals, the as-prepared NaGdF4 sacrificial-shell nanocrystals dispersed in 6 mL ODE were injected and repined 45 min (three times injection, 2 mL/time/15 min repining). After repining, the solution was cooled down to room temperature. The final clear solution was precipitated by addition of ethanol and collected by centrifugation. Cyclohexane/ethanol was employed to further purification of the nanocrystals.

Preparation of nanocrystals film

All of the nanocrystals films were prepared by spin-coating. Firstly, the glass substrate (10 mm × 10 mm) was cleaned by acetone via ultrasound. The substrate was then dried in an oven under air at 60 °C for 6 h for use. The hybrid film was prepared by further spin-coating the plasmonic nanocrystals and the NaGdF4:Yb3+, Er3+/NaGdF4 core/shell nanocrystals dispersed in cyclohexane layer by layer (1200 r/min, 15 s followed by 2000 r/min, 1 min). Before depositing the upper layer, the sample was dried in an oven under air at 60 °C for 6 h to ensure the complete removal of redundant solvent to enable the formation of homogenous nanocrystal film. The UCNP film was prepared analogous to the hybrid film, except the absent of plasmonic nanocrystals layer. Note that the concentration of NaGdF4:Yb3+, Er3+/NaGdF4 core/shell nanocrystals, the deposited volume of the UCNPs, and the parameter of spin-coating, including the rotating speed and the time, keep the same for the preparation of the UCNP film and the hybrid film.

Characterization

The crystalline phase structure of the as-prepared nanocrystals was measured by a diffractometer (Rigaku miniflex 600) using Cu-Kα (λ = 0.154 nm) radiation operating at 40 kV and 15 mA. The crystal morphology and structure were measured by transmission electron microscopy (TEM, JEM-2010) operating at 200 kV. UV–Vis–NIR spectra were recorded using a UV–Vis–NIR spectrophotometer (PerkinElmer Lambda 950), and the nanocrystals were dispersed in CCl4 with a low nanocrystal concentration.

Optical measurement

Upconversion luminescence (UCL) spectra were measured using a FLS 920 Spectrofluorimeter (Edinburgh) with an external 980 nm diode laser with power ranging from 0.02 to 2.7 W functions as the excitation source. The lifetime measurements were carried out on a customized UV to near-infrared steady state and phosphorescence lifetime spectrometer (FSP920-C, Edinburgh) equipped with a tunable mid-band OPO pulse laser as the excitation source (410–2400 nm, 10 Hz, pulse width ≤ 5 ns, Vibrant 355II, OPOTEK).

Results and Discussions

The Au–CuS heterodimer nanocrystals were synthesized by a seeded growth method. The crystalline phases of the product were examined by X-ray diffraction (XRD). The XRD pattern shows two series of Bragg reflections (Fig. 1a), which not only reserves the pattern of Au seeds but also arises the pattern of CuS (synthesized without the Au seeds) (Fig. S1). Accordingly, the crystalline phases of the product can be indexed as fcc Au (JCPDS No. 04-0784) and covellite CuS (JCPDS No. 06-0464). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) photographs were employed to characterize the as-synthesized samples and further confirm the crystalline phases. Nearly all of the particles observed by TEM are dimers which contain two domains with distinct contrast (Fig. 1b). The darker ones are determined to be Au domains, which can be confirmed by the lattice spacing measured to be 0.23 nm (Fig. 1c), in line with the d-spacing of (111) of fcc Au. Moreover, the lattice spacing of the brighter domain is measured to be 0.32 nm (Fig. 1c), which is in line with the d-spacing of (100) of covellite CuS. All the results indicate the successful synthesis of Au–CuS heterodimer nanocrystals.

Figure 1
figure 1

a–c XRD pattern, TEM, and HRTEM micrographs of the as-prepared Au–CuS heterodimer nanocrystals. d Optical absorption spectra of the as-prepared Au–CuS heterodimer nanocrystals (purple), CuS nanocrystals (dark cyan), and Au seed nanocrystals (red). Insert shows the corresponding photograph of these nanocrystals dispersed in CCl4. e TEM and f HRTEM micrographs of the NaGdF4:Yb/Er@NaGdF4 core–shell nanocrystals. Insert of f shows the corresponding FFT pattern

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The coupling of plasmonic Au with plasmonic CuS may endow the hybrid material distinct optical properties that are in stark contrast to both single-component counterparts. As observed by naked eyes, Au–CuS heterodimer nanocrystals, Au seed nanocrystals, and pure CuS nanocrystals dispersed in CCl4 solution appear in very different colors (inset of Fig. 1d), which indicates different extinction properties. UV–Vis–NIR extinction spectra of them were measured and compared (Fig. 1d). For the Au seeds, the extinction spectra show a typical LSPR band centered at 525 nm with narrow energy distribution [20], which overlaps very well with the 2H11/2 → 4I15/2 emission of Er3+. For the pure CuS nanocrystals, the extinction spectra show peak resulting from LSPR excitation at wavelengths spanning from 600 nm to 2000 nm, with the center wavelength of ~ 1030 nm, which was attributed to the in-plane dipole mode [21]. The steep feature beyond the wavelength of ~ 550 nm is attributed to excitonic transition absorption, in accordance with the direct band gap of about 2.2–2.4 eV as previously estimated for covellite [22]. For the Au–CuS heterodimer nanocrystals, the extinction spectra exhibit two peaks in visible and NIR, which can be assigned to the LSPR absorption of Au and CuS domain, respectively [13, 14]. Compared with Au seeds and pure CuS, redshifting and blueshifting of the corresponding plasmonic peaks, respectively, are observed. The redshift of Au LSPR (544 nm of Au–CuS hybrid dimer vs. 525 nm of Au seeds) can be attributed to the higher refractive index of CuS domain than the CCl4 solvent [14, 16, 17]. By contrast, the reason for the blueshift of CuS LSPR (983 nm of Au–CuS hybrid dimer vs. 1030 nm of pure CuS) can be more complex, since the significant difference in size and shape distribution of CuS domain (Fig. 1b) compared to that of plain CuS (Fig. S3) could add various influences to the position of LSPR peak, in addition to the change in refractive index [17,18,19]. It is noted that the broad LSPR absorption in NIR of Au–CuS heterodimer and pure CuS nanocrystals shows considerable spectral overlap with the narrow absorption of Yb3+ (centered about 980 nm) derived from the 2F7/2 → 2F5/2 transition, implying that resonant enhancement of UCL by these nanostructures is possible.

Yb/Er co-doped NaGdF4 was selected as the upconversion material for its relatively high efficiency of energy transfer upconversion (ETU) [5]. The oleic acid-capped NaGdF4:Yb/Er@NaGdF4 core–shell nanocrystals were synthesized by a two-step, ripening-mediated co-precipitation method [23]. TEM micrographs of the NaGdF4:Yb/Er (18/2 mmol%) core-only nanocrystals show that they are nearly monodispersed, with an average diameter of 14 nm (Fig. S4a). XRD pattern (Fig. S5), HRTEM photograph, and the corresponding fast Fourier transformation (FFT) pattern (Fig. S4b) reveal that the crystalline phase of the as-prepared NaGdF4:Yb/Er core-only nanocrystals was hexagonal (β), which is known as an asymmetric crystal field phase host with higher upconversion efficiency compared with the symmetric cubic phase (α) [4]. After an inert NaGdF4 shell growth, the crystalline phase remains unchanged (Fig. S5 and Fig. 1f) while the size of the final nanocrystal increases to ~ 30 nm (Fig. 1e, f), indicating a shell thickness around 8 nm. Such a thick epitaxial insert shell is expected to be capable to eliminate the surface defects of the core and suppress undesirable direct charge exchange, preventing significant luminescence quenching events. The successful synthesis of such NaGdF4:Yb/Er@NaGdF4 core–shell UCNPs allows us to probe plasmon-induced enhancement of UCL.

Multilayer films contain plasmonic nanocrystals (Au–CuS, plain Au, and CuS, respectively), and UCNPs (donated as Au–CuS–UCNP film, Au–UCNP film, and CuS–UCNP film, respectively) were constructed by layer-by-layer spin-coating (see Experimental and Fig. 2a) to couple the electric field induced by LSPR with the transition dipoles of Ln3+. Film containing only UCNPs (donated as UCNP film) was also prepared as a reference. Note that the concentration and deposited volume of UCNPs and all the parameters for spin-coating (i.e., the rotate speed and the coating time) keep the same for the preparation of all the film samples.

Figure 2
figure 2

a Schematic illustration of the multilayer film structure. b Diffuse reflection spectra of the films with different plasmonic nanostructures: Au–CuS heterodimer nanocrystals (purple), CuS nanocrystals (dark cyan), and Au seed nanocrystals (red). Insert shows the photograph of these plasmonic nanocrystal films, with plain glass as a reference

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The optical properties of these multilayer films were thereafter investigated. Diffuse reflection spectra of the Au–CuS film, CuS film, and Au film almost keep the same features as that of in dilute solution (Fig. 2b), with slight redshift and broadening (Fig. S6). This redshift and broadening should be ascribed to the strong coupling among adjacent nanoparticles [19]. Even so, there are still considerable spectra overlaps both between CuS domain LSPR and Yb3+ absorption, and between Au domain LSPR and Er3+ emissions, making it possible to couple the electric field of plasmonic antennas with the transition dipoles of Ln3+.

The UCL measurements of all samples are under the same condition, including the optical circuit (Fig. 3a), the laser resource, and the pump power. The enhancement of the UCL by plasmonic nanostructures can be clearly confirmed by comparing the typical emission spectra of the multilayer films with the plain UCNP film (Fig. 3b). All the spectra contain four characteristic emission peaks within visible region, which can be attributed to the 2H9/2 → 4I15/2 (409 nm), 2H11/2 → 4I15/2 (520 nm), 4S3/2 → 4I15/2 (540 nm), and 4F9/2 → 4I15/2 (654 nm) radiative transition of Er3+, respectively. Notably, the enhancement effect induced by Au–CuS hybrid dimer nanocrystals surpasses that by Au nanocrystals or CuS nanocrystals, manifesting the superiority of heterodimer formation. The enhancement factors (defined as the integrated UCL intensity ratio of the corresponding multilayer films to that of the UCNP film under the same conditions) of the multilayer films are also calculated (Fig. 3c). The enhancement factor by Au–CuS heterodimer reaches 17.5, 6.4, and 7.4 at 520, 540, and 654 nm, respectively, which is larger than that by Au (6.3, 4.1, and 5.0 at 520, 540, and 654 nm, respectively) and by CuS (3.5, 1.7, and 1.7 at 520, 540, and 654 nm, respectively). Note that the enhancement factors by CuS or Au alone in the present case may be smaller than the previously reported value [34], which should origin from the differences in size and shape of nanoparticles as well as the film details. The population processes of 2H11/2 and 4S3/2 excited states derive from two-photon ETU process (Fig. S7). The much larger enhancement factor of 2H11/2 → 4I15/2 than that of 4S3/2 → 4I15/2 for all the samples is attributed to local thermal effect which greatly influences the population distribution between the two thermal-couple levels with small energy gap (740 cm−1) according to Boltzmann distribution. The local thermal effect can actually be evaluated by monitoring the UCL intensity ratio originating from the two thermal-couple levels (i.e., the UCL integrated intensity ratio of 2H11/2 → 4I15/2 to 4S3/2 → 4I15/2, donates as RHS) [24]. The results show that, in the presence of plasmonic nanostructures, larger RHS are obtained (Fig. S8), indicating LSPR induces an increase in local temperature, and such thermal effect follows Au–CuS > CuS > Au.

Figure 3
figure 3

a The optical path for UCL measurements. b UCL spectra of the film samples: UCNP film (green), Au–UCNP film (red), CuS–UCNP film (dark cyan), and Au–CuS–UCNP film (purple). All the UCL spectra were measured under the excitation of 980 nm laser at a power density of 0.33 W/mm2. c Enhancement factor at different transitions with different plasmonic antennas. d Schematic illustration for the UCL enhancement by Au–CuS heterodimer

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In order to understand how the LSPR of different nanocrystals cast influence on the dynamic processes of the UCL emissions, the PL decay measurements for different radiative transitions were performed. The decay curves of three visible emissions (corresponding to 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions, respectively) for all samples are compared (Fig. 4a–c), and the corresponding values of lifetime are listed (Table S1). The results clearly manifest that the presence of plasmonic nanostructures reduces the lifetimes comparing to the plain UCNP film. Such effect for the three transitions always follows Au–CuS > CuS > Au. It is generally accepted that the influence of coupling between LSPR and emitters on the dynamics processes (i.e., absorption, radiative, and nonradiative decay of the emitters) depends largely on the spectral overlap [2]. The good matching between Au LSPR frequency with Er3+ emissions in the visible region is expected to enable the enhancement effect originated from Purcell effect [25], which increases the radiative rate by increasing the photonic density of states, leading to the decrease in corresponding emission lifetime [11, 26]. The relatively small lifetime decrease for all three emissions for Au–UCNP film compared with the primitive UCNP film infers that the Purcell effect may not be significant for the present case, probably due to the dominance of Au nanoparticles with size much smaller than 20 nm, since plasmon resonance scattering quantum yield is supposed to be pretty low in terms of such size distribution [27]. Theoretically, larger size of Au will contribute to larger scattering efficiency and local E-field, which is positive to the enhancement of upconversion emission, while small size of Au will increase absorption and decrease the local E-field, leading to UCL quenching. In addition, the position of LSPR peak is also dependent on the size of Au, which would show redshift when the size of Au becomes larger. Such redshift of LSPR peak could decrease the spectral overlaps between the green emissions and LSPR but increase the spectral overlaps between the red emission and LSPR, which would increase/decrease the green/red emissions as well as decrease the G/R ratio. We suggest that there would be an optimal size for the trade-off between local E-field and spectral overlap. Considering the large spectral overlap between plasmonic absorption of CuS and absorption of Yb3+, the excitation rate of Yb3+ will be increased by LSPR due to the enhanced local electric field E, which eventually leads to UCL enhancement of Er3+. Note that local field enhancement in absorption region is not supposed to obviously change the radiative decay rate, while the heat generation (Fig. S7) due to laser-induced photothermal effect is able to cause the accelerating depopulation of excited state via enhanced multiphonon-mediated nonradiative relaxation. In this regard, the decrease in UC luminescence lifetime for CuS–UCNP film sample can be ascribed to thermal quenching effect. Thus, the competition between local field enhancement and thermal effect decides the total UCL enhancement factor. Following this, the relative small enhancement factor of CuS should originate from the relative small local field enhancement and/or the relative large thermal effect. For Au–CuS–UCNP film, the shortest lifetime of Er3+ emissions, compared with Au–UCNP film and CuS–UCNP film, can be accounted for by the superposition of both Purcell effect and thermal effect. As such, the realization of spectral overlap between Yb3+ absorption and CuS LSPR as well as between Er3+ emissions and Au LSPR within Au–CuS–UCNP film enables intensified absorption and acculturating emission rate simultaneously (Fig. 3d), leading to the highest upconversion emission intensity (Fig. 3b).

Figure 4
figure 4

UCL decays of the film samples: UCNP film (green), Au–UCNP film (red), CuS–UCNP film (dark cyan), and Au–CuS–UCNP film (purple) from the (a–c) 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions by monitoring the Er3+ emissions at 520, 540, and 654 nm, respectively. d The rise edge of 4S3/2 → 4I15/2 transition

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It should be pointed out that tuning the size for both Au and CuS domains within the heterodimers to maximize the plasmonic properties may lead to a larger enhancement. However, it remains difficult for us to synthesize well-controlled Au–CuS heterodimer nanocrystals with larger and tunable size (e.g., 20–100 nm for Au and several tens to hundreds of nm for CuS), especially the CuS domain (because the synthesis of Au nanocrystals with tunable size in such size range have been reported previously), due to the intrinsic limitations (e.g., big steric hindrance) of colloidal synthesis method used in the present work. An aqueous synthesis method which could produce well-controlled Au–CuS heterodimer nanocrystals with larger and tunable size may be helpful to solve this problem. However, to our best knowledge, such a method has not been reported previously.

The population of excited states can be reflected in the rise edge of PL decay curve, which has been demonstrated by power-dependent profile evolution of rise edge in a way that larger excitation power leads to sharper rise edge [28,29,30]. Being analogous to increase in excitation power, near field enhancement upon excitation by LSPR is supposed to impose the similar influence on the population process [31]. Herein, we extract the rise edge of 4S3/2 → 4I15/2 PL decay curve to double-check the local field enhancement effect by different plasmonic antennas (Fig. 4d). It is shown that in all three cases (Au–UCNP film, CuS–UCNP film and Au–CuS–UCNP film), the sharpening of rise edge has been observed as compared to the UCNP film, verifying the local field enhancement effect upon excitation. The rise edge of CuS–UCNP film and Au–CuS–UCNP film almost coincides with each other, suggesting the very similar local field enhancement for both cases, which can be accounted for by considering that there is no spectral overlap between Au LSPR and 980 nm excitation. However, the sharpening of rise edge for Au–UCNP film with respect to that for UCNP film is surprising, which cannot be explained by LSPR enhancement effect.

The power-dependent UCL intensity and enhancement factor of Au–CuS–UCNP film were also measured (Fig. 5a) and calculated (Fig. 5b), respectively. The quenching of UCL under high power excitation is observed directly (Fig. 5a), which should result from the elevated temperature due to plasmon-to-heat transformation, in accordance with the trend of RHS. The enhancement factors for 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, and 4F9/2 → 4I15/2 transitions all exhibit a volcano relationship with power density (Fig. 5b). Note that the turning points for all the curves in Fig. 5b turn out to be at P ~ 0.33 W/mm2. This behavior can be understood by considering the competition between enhancement (local field enhancement and Purcell effect) and quenching factors (heat).

Figure 5
figure 5

Power-dependent a integral intensity and b enhancement factor comes from 2H11/2 → 4I15/2 (dark cyan), 4S3/2 → 4I15/2 (green), and 4F9/2 → 4I15/2 (red), respectively, of Au–CuS–UCNP film

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Rate equations are usually adopted for the theoretical understanding of the ETU mechanism, which have been presented in the Supporting Information according to the present Yb/Er co-doped system. The multiple processes involved in ETU that can be influenced by LSPR nanostructure mainly include absorption, radiative decay, and energy transfer between sensitizer and activator. When the plasmon resonance is coincident with excitation wavelength and weak excitation is adopted, according to the previous theoretical analysis, the corresponding enhancement factor for the green emission can be written as [33]

$$ F = \frac{{F_{d4} \cdot F_{a}^{2} }}{{F_{D10}^{2} }} $$
(1)

where Fd4 and Fa are enhancement factors for energy transfer and absorption processes, FD10 is the Purcell enhancement factor for D10 transition. For the present heterodimer LSPR nanostructure, the plasmon resonance covers both excitation and upconversion emission region, which means the Purcell enhancement factor (FA40) for green emission (A40 transition) should be introduced. Therefore, the total enhancement factor can be modified as

$$ F = \frac{{F_{d4} \cdot F_{a}^{2} }}{{F_{D10}^{2} }} \cdot F_{A40} $$
(2)

where both the excitation enhancement and the emission Purcell enhancement are combined to exhibit larger enhancement factor (FA40 ≥ 1).

Engineering the electromagnetic field near Ln-doped UCNPs emitters by means of plasmonic structures provides great opportunities for large UCL enhancement. The big stokes shift due to the NIR excitations and UV/visible emissions of Ln-doped UCNPs makes it possible to enhance the excitation and emission rate simultaneously by tuning the spectral overlaps between plasmon resonances and excitation/emission frequencies. In addition to the spectral overlaps, the magnitude of local field enhancement provided by plasmonic structures depends on the LSPR scattering efficiency, since only LSPR scattering, instead of LSPR absorption, contributes to the near field enhancement. In this regard, LSPR antenna with high LSPR scattering efficiency (high-quality factor) [32], which is closely associated with specific size distribution, is necessary to enable significant modification on the excitation and emission rate for UC enhancement. Although the good spectral overlaps between the NIR excitation region of UCNPs and CuS-related LSPR component as well as between visible emission region of UCNP and Au-related LSPR one have been realized simultaneously in the present system, the total UCL enhancement remains limited. We reason that the small size (< 20 nm) for both Au and CuS components within heterodimer leads to a relatively low LSPR scattering efficiency, which can be further modified in the future work so as to afford a more significant local field enhancement [26]. Finally, one may wonder the difference between the heterodimers and the mixture of the two components in terms of the LSPR-induced enhancement. The heterodimer nanostructure differs from the mixing of two components in structure characteristic, which is expected to lead to corresponding difference in various properties. As we have compared the extinction spectra for both heterodimer nanostructure and mixture of two nanoparticles, there do exhibit shift in peak positions. Therefore, in my opinion, there should exhibit some difference in LSPR enhanced upconversion properties for the above-mentioned comparison, which deserves further investigation in the future.

Conclusions

In conclusion, we have experimentally observed typical LSPR-induced UCL enhancement in a multilayer nanocrystals film consisting of dual plasmonic Au–CuS and Yb/Er co-doped NaGdF4. Our results show that the UCL enhancement by Au–CuS hybrid dimer reaches 17.5, 6.4, and 7.4 fold at 520, 540, and 654 nm, respectively, which is larger than that of Au and CuS. Further analysis of the UCL decay dynamics demonstrates the dual enhancement role, i.e., simultaneous excitation and emission enhancement, of Au–CuS heterodimer nanocrystals. This work provides the possibility to implement large enhancement of UCL by metal–semiconductor dual plasmonic antennas with well-controlled composition, size, and shape.