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Abstract
All-inorganic halide perovskite quantum dots (QDs) have recently received much attention due to their excellent optoelectronic properties. And their emission properties still need to be improved for further applications. Here, we demonstrated a remarkable emission enhancement of the CsPbBr3 QDs based on an Ag nanoparticle-Ag film plasmonic coupling structure. Through precise control of the gap distance between Ag nanoparticle and Ag film, the localized surface plasmon resonance (LSPR) peak was tuned to match the emission wavelength of the CsPbBr3 QDs. We achieved a 30-fold fluorescence intensity enhancement and a lower lasing threshold, which is 25% of that of the CsPbBr3 QDs without plasmonic coupling structure. It is attributed to that the plasmonic coupling structure exhibits an extremely strong local electric field owing to the coupling between LSPR of Ag nanoparticle and surface plasmon polariton of Ag film. This work provides an effective way to enhance the optical emission of perovskite QDs and promotes the further exploration of on-chip light source.
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1. Introduction
In recent years, metal halide perovskites have attracted extensive attention due to their remarkable optoelectronic properties. In particular, all-inorganic halide perovskite CsPbX3 (X = Cl, Br and I) quantum dots (QDs) exhibit a high photoluminescence quantum yield, narrow emission bandwidth, tunable band gap, and high gain [1,2]. These properties lead to a significant progress in the applications of CsPbX3 QDs in photodetector [3–5], light-emitting diodes [6–11], and lasers [12–17]. Furthermore, CsPbX3 QDs are promising candidate materials for single-photon sources and have the potential applications in ultra-small integrated circuits for quantum communications and computing. To adapt to a wider range of applications, the emission properties of CsPbX3 perovskite QDs still need to be improved.
Metal plasmons exhibit great advantages in improving emission characteristics of luminescent materials due to their near-field enhancement properties [18–24]. To date, there are many researchers who used localized surface plasmon (LSP) of metal nanoparticles (NPs) or surface plasmon polariton (SPP) of metal films to enhance the emission properties of perovskite QDs. For example, in 2019, Shakuri et al. observed a fluorescence enhancement of CH3NH3PbBr3 quantum dot solution by doping with Au NPs [25]. Yang et al. reported a lasing threshold reduction of CsPbBr3 perovskite nanocubes by depositing CsPbBr3 nanocubes on the Au NPs [26]. In 2020, Zhang et al. showed an enhancement of fluorescence intensity by embedding Ag NPs into CsPbBr3 QDs film [27]. In 2021, Zhao et al. observed a fluorescence enhancement by depositing CsPbBr3 QDs onto the SiO2/ Ag substrate [28]. Xing et al. achieved a lasing threshold reduction of CsPbBr3 QDs film based on Ag nanowires [29]. In 2022, Xiao et al. proposed a configuration of SiO2-coated CsPbBr3 QDs deposited on Au nanorods, and the lasing threshold was reduced [30]. In 2023, Su et al. showed a fluorescence enhancement of CsPbBr3 QDs by depositing them onto collapsed Ag nanofingers [31]. According to these studies, although researchers have made fruitful achievements in enhancing optical properties of perovskite quantum dots based on independent metal NPs or metal film, it is still urgent to further enhance the optical properties of CsPbX3 QDs to develop applications. Comparing to the independent metal NPs or metal film, the metal nanoparticle-metal film plasmonic coupling structure can effectively confine light to sub-wavelength range and generate a stronger electric field. Therefore, it is highly desirable to investigate the effect of the metal nanoparticle-metal film plasmonic coupling structure on the optical properties of CsPbX3 QDs.
In this work, we proposed an Ag nanoparticle-Ag film plasmonic coupling structure to improve the emission performance of the CsPbBr3 QDs. By regulating the distance between Ag nanoparticle and Ag film, the localized surface plasmon resonance (LSPR) peak was tuned to match the emission wavelength of the CsPbBr3 QDs. A 30-fold fluorescence enhancement was achieved when the CsPbBr3 QDs were deposited within the 20 nm-thick nanogap between Ag nanoparticle and Ag film. At the same time, the lasing threshold of the CsPbBr3 QDs film was significantly reduced by introducing the Ag NPs-Ag film coupling structure. This method holds a great potential for advancing the applications of perovskite QDs.
2. Experiments section
2.1 Chemicals
Cesium carbonate (Cs2CO3, 99.99%) was purchased from Macklin. Lead bromide (PbBr2, 99.999%), Tetraoctylammonium bromide (TOAB, 98%), silver nitrate (AgNO3, 99%) and trisodium citrate (analytical grade) were obtained from Aladdin Reagent Company. Propionic acid (PA, ≥ 99.5%), oleylamine (OAM, 80-90%), N, N-dimethylformamide (DMF, 99.9%), n-hexane (HEX, 99.5%), and Isopropyl Propanol (IPrOH, 99.5%) were purchased from Sigma-Aldrich. Toluene (analytical grade) was purchased from Sinopharm Chemical Reagent Co. Ltd. Polymethyl methacrylate (PMMA, 99%) was obtained from MERYER Company. All the chemicals were used without further purification.
2.2 Synthesis of the CsPbBr3 QDs
CsPbBr3 QDs were synthesized using a ligand-assisted precipitation method. The details of the process were as follows: firstly, 0.18 mmol of Cs2CO3 was dissolved in 100 µL of PA solution to form a Cs precursor solution. 0.42 mmol of PbBr2 was dissolved in 750 mL of a mixture solvent composed of 250 µL of DMF, 250 µL of PA, and 250 µL of OAM, to form a Pb precursor solution. The Cs and Pb precursor solutions were stirred at room temperature until the solid powders were completely dissolved. Then, 10 µL of Cs precursor solution and 200 µL of Pb precursor solution were in turn added into 6 mL mixture solvent composed of 3 mL of n-hexane and 3 mL of isopropyl alcohol, and the mixture was stirred for 15 seconds. After that, the CsPbBr3 QDs solution was centrifuged at 6000 rpm for 4 min, and the precipitated CsPbBr3 QDs was added with 300 µL of the prepared ligand passivation solution composed of PbBr2 and TOAB to form a stable quantum dots solution.
2.3 Synthesis of the Ag nanoparticles
Ag NPs were synthesized by reducing AgNO3 with trisodium citrate [32]. The preparation process was as follows, firstly, 100 mg of trisodium citrate was dissolved in 10 mL of deionized water. 40 mg of AgNO3 was added into 200 mL of deionized water, and AgNO3 solution was heated to 60 °C for 10 min. Then, 7 mL of trisodium citrate solution was added into the AgNO3 solution. The mixture solution was heated to 90°C, and kept at 90°C for 1 h to form the Ag NPs. Finally, the Ag NPs colloidal solution was centrifuged at 6000 rpm for 10 min to obtain the final precipitate of Ag NPs. Figure 1(a) shows the scanning electron microscope (SEM) image of the Ag NPs. It can be seen that the particle size is uniform and the average diameter of Ag NPs is approximately 80 nm. Figure 1(b) presents the absorption spectrum of the Ag NPs. It shows that the absorption peak is located at 445 nm.
2.4 Preparation of the CsPbBr3-doped PMMA film based on plasmonic coupling structure
In order to study the effect of the Ag nanoparticle-Ag film plasmonic coupling structure on the fluorescence properties of the CsPbBr3 QDs, the CsPbBr3-doped PMMA film was deposited between Ag nanoparticle and Ag film. For comparison, the samples without Ag film were also prepared. The structures of two kinds of devices are shown in Fig. 2. The devices were prepared as follows: firstly, 100 nm of Ag film was evaporated onto the glass substrate by electron beam evaporation under a vacuum of 5 x10−3 Pa at the deposition rate of 2 Å/s. Then, the mixture toluene solutions of PMMA and CsPbBr3 QDs with different concentration ratios were spin-coated on the Ag film at a speed of 3000 rpm for 30 s to form CsPbBr3-doped PMMA films of different thicknesses, in which CsPbBr3 QDs concentration was 0.1 mg/mL, and PMMA concentration varied from 0.002 to 2 mg/mL. The spin-coated films were annealed at 110 °C for 10 min. Finally, the Ag nanoparticles solution with a diameter of 80 nm was spin-coated on the CsPbBr3-doped PMMA films at a speed of 1000 rpm for 10 s. Figure 3 shows the surface topography image of the sample. We can find that the film is uniform, and Ag nanoparticles are uniformly dispersed on the CsPbBr3-doped PMMA film.
Fig. 2. Schematic illustration of the devices of (a) Glass/ Ag film (100 nm)/ CsPbBr3-doped PMMA film (d nm)/ Ag nanoparticle, and (b) Glass/ CsPbBr3-doped PMMA film (d nm)/ Ag nanoparticle.
Fig. 3. Surface topography image of the device of Glass/ Ag film/ CsPbBr3-doped PMMA film/ Ag nanoparticles.
2.5 Measurement
The structure and morphology of the CsPbBr3 QDs and Ag NPs were characterized using a transmission electron microscopy (TEM, JEM-2100, JEOL, Japan) and a scanning electron microscopy (SEM, JSM-7000F, JEOL, Japan). The surface topography image of the sample was characterized using a scanning electron microscopy (SEM, JSM-7610FPlus, JEOL, Japan). The thicknesses of polymer films were obtained by an atomic force microscopy (AFM, SPM-9700HT, Japan). The crystalline structure of the CsPbBr3 QDs was analyzed by an X-ray powder diffractometer (XRD, D8 Advance Bruker, USA). The absorption and photoluminescence (PL) spectra were obtained by an UV-Vis spectrometer (Cary 5000, Agilent, Japan) and a fluorescence spectrometer (FLS 980, Edinburgh, England), respectively. The dark field scattering spectra were obtained by using a laser confocal microscope system equipped with a white light source from a halogen lamp (50 mW, Olympus). The white light was obliquely illuminated on the sample. Then, an objective (50x, NA = 0.9, Olympus) was used to collect the scattered light from the sample, and the scattered light was directed to a spectrometer (Horiba, LabRam HR evolution) [33]. The micro-region PL spectra was obtained with a laser confocal Raman spectrometer (Horiba, LabRam HR evolution). The devices were excited with a pulsed Nd:YAG laser (355 nm/10 Hz/5.5 ns), and the fiber spectrometer (USB2000) was used to record amplified spontaneous emission (ASE) and lasing spectra.
3. Results and discussion
3.1 Characterization of the CsPbBr3 QDs
In order to clarify the physical properties of the CsPbBr3 QDs, we characterized the crystal structure, absorption and fluorescence spectra of the CsPbBr3 QDs. Figure 4(a) shows a TEM image of the CsPbBr3 QDs. It can be observed that the CsPbBr3 QDs exhibit a cubic phase morphology. The high-resolution TEM (HRTEM) image of the CsPbBr3 QDs (inset in the Fig. 4(a)) shows that the lattice spacing is 0.41 nm, which is consistent with the (110) plane of the CsPbBr3 QDs. Figure 4(b) exhibits a size distribution of the CsPbBr3 QDs, it shows a narrow size distribution with an average diameter of 6.1 nm. Figure 4(c) shows the X-ray diffraction spectrum of the CsPbBr3 QDs, it exhibits the main peaks at 2θ values of 15.124°, 21.456°, 30.268°, 34.224°, 37.605° and 43.671°, coinciding with diffraction patterns from (001), (110), (002), (120), (121), (202) crystalline planes of monoclinic CsPbBr3 (JCPDS PDF #18-0364) [34]. Figure 4(d) illustrates the absorption and PL spectra of the CsPbBr3 QDs. The absorption band edge of the CsPbBr3 QDs is located at 505 nm and the PL peak is observed at 515 nm, exhibiting bright green fluorescence under UV light irradiation.
Fig. 4. (a) Transmission electron microscopy (TEM) images, and (b) size distribution of the CsPbBr3 QDs. (c) X-ray diffraction (XRD) spectrum of the CsPbBr3 QDs, and standard XRD profile of JCPDS PDF #18-0364. (d) Absorption (Abs) and PL spectra of the CsPbBr3 QDs, and the inset is the optical image of CsPbBr3 QDs under UV light irradiation.
3.2 Study on the characteristics of the plasmonic coupling structure
It is well known that the distance between metal nanoparticle and metal film could greatly affect the plasmonic characteristics of the coupling structure [35–38]. To investigate the effect of the gap distance on the plasmonic properties of coupling structure, we prepared the PMMA films with different thicknesses by changing the concentrations of PMMA solutions to regulate the gap distance between Ag nanoparticle and Ag film, the corresponding thicknesses of the PMMA films were measured by scratch experiment [37–39]. Figure 5 shows the AFM diagrams and the corresponding cross-sectional height profile curves of the PMMA films. The results show that the PMMA film thicknesses are 7, 12, 20, 30, and 65 nm, when the PMMA concentrations are 0.002, 0.05, 0.2, 0.5, and 2 mg/mL, respectively.
Fig. 5. AFM images of the scratched polymer films corresponding to PMMA concentrations of (a) 0.002, (b) 0.05, (c) 0.2, (d) 0.5, and (e) 2 mg/mL, respectively. (f)-(j) AFM cross-sectional height curves marked with blue lines in the graphs (a)-(e).
To study the plasmonic properties of the coupling structures with different gap distances between Ag nanoparticle and Ag film, we performed a dark field scattering test of coupling structures with different PMMA thicknesses between Ag nanoparticle and Ag film. Figure 6(a) shows the dark field scattering images of the plasmonic coupling structures with different PMMA thicknesses of 7, 12, 20, 30, and 65 nm, respectively. We can observe that the color of the scattered light gradually changes from red to blue with the increases of the PMMA spacer thickness from 7 to 65 nm. Then, we investigated the dark field scattering spectra of the Ag nanoparticle-Ag film coupling structures with different PMMA spacer thicknesses, as shown in Fig. 6(b). When the PMMA concentration increases from 0.002 to 2 mg/mL, the spacer thickness increases from 7 to 65 nm, the scattering peak wavelength blue shifts from 614 to 476 nm.
Fig. 6. (a) Dark field scattering images, and (b) normalized scattering spectra of the Ag particle-Ag film coupling structures with different PMMA spacer thicknesses. (I∼V present the coupling structures with the PMMA thicknesses of 65, 30, 20, 12, and 7 nm).
To further confirm the plasmonic properties of the Ag nanoparticle-Ag film coupling structure, we calculated the scattering spectra and electric field distributions of the coupling structure through the finite element method using the COMSOL software. The incident plane wave propagated along the z-axis and the polarization direction was along the x-axis, and the permittivity of silver was taken from the experimental data of Johnson and Christy [40]. The mesh size was set to 2 nm. Figure 7(a) shows the calculated scattering spectra of the Ag nanoparticle-Ag film plasmonic coupling structure with different thicknesses of PMMA spacer. It shows that with the increases of the PMMA thickness from 7 to 65 nm, the scattering peak exhibits an obvious blue shift from 610 to 462 nm, which verifies the experimental results shown in Fig. 6(b). Figure 7(b) and (c) show the electric field distributions of the Ag nanoparticle-Ag film plasmonic coupling structure and the independent Ag nanoparticle, respectively. The enhanced electric field of the plasmonic coupling structure is much stronger than that of the independent Ag nanoparticle, which is owing to the coupling between LSPR of Ag nanoparticle and SPP of Ag film.
Fig. 7. (a) Simulated scattering spectra of the plasmonic coupling structures with different PMMA spacer thicknesses. Electric field distributions of the (b) Ag nanoparticle-Ag film plasmonic coupling structure with the PMMA thickness of 7 nm and (c) independent Ag nanoparticle.
3.3 Fluorescence enhancement of the CsPbBr3 QDs based on plasmonic coupling structure
To study the influence of the plasmonic coupling structure on the fluorescence properties of the CsPbBr3 QDs, different thicknesses CsPbBr3-doped PMMA films between Ag nanoparticle and Ag film of plasmonic coupling structures were prepared shown in Fig. 2(a). Moreover, in order to clarify the advantages of the plasmonic coupling structure, the neat CsPbBr3-doped PMMA film and CsPbBr3-doped PMMA film with Ag nanoparticle shown in Fig. 2(b) were also prepared for comparison. The micro-area fluorescence of CsPbBr3 QDs based on single Ag nanoparticle or Ag nanoparticle-Ag film coupling structure were measured by using a laser confocal Raman spectrometer. Figure 8(a)-(e) present the micro-area fluorescence spectra of the CsPbBr3-doped PMMA films, CsPbBr3-doped PMMA films based on single Ag nanoparticle and the Ag nanoparticle-Ag film plasmonic coupling structure, and the corresponding CsPbBr3-doped PMMA film thicknesses are 7, 12, 20, 30, and 65 nm, respectively. For each thickness of the CsPbBr3-doped PMMA film, it shows that the fluorescence intensities of the CsPbBr3 QDs based on Ag nanoparticle, or the Ag nanoparticle-Ag film coupling structure are stronger than that of the neat CsPbBr3-doped PMMA film, while the Ag nanoparticle-Ag film plasmonic coupling structure can enhance fluorescence to a greater extent than the Ag nanoparticle. Figure 8(f) shows the fluorescence intensity enhancement factor (EF) as a function of the CsPbBr3-doped PMMA film thickness for the CsPbBr3 QDs based on the Ag nanoparticle-Ag film coupling structure and Ag nanoparticle. Here, the fluorescence enhancement factor is defined as EF = I/I0, where I and I0 are the fluorescence peak intensity from the samples with and without plasmonic structure. It shows that for the sample with Ag nanoparticle-Ag film coupling structure, with the increase of the CsPbBr3-doped PMMA film thickness, the enhancement factor exhibits an initial increase following by a subsequent decrease, and the maximum fluorescence enhancement of 30 times occurs when the CsPbBr3-doped PMMA film thickness is 20 nm. It is because that the plasmon resonance peak of the Ag nanoparticle-Ag film with 20 nm gap distance is located at 521 nm, which is closest to the PL wavelength of 514 nm, exhibiting the largest overlap with the PL spectra of CsPbBr3 QDs.
Fig. 8. Fluorescence spectra of the CsPbBr3-doped PMMA films (black line), CsPbBr3-doped PMMA films based on Ag nanoparticle (NP) (red line) and plasmonic coupling structure (Ag NP-Ag film) (blue line) with the CsPbBr3-doped PMMA film thicknesses of (a) 7 nm, (b) 12 nm, (c) 20 nm, (d) 30 nm, and (e) 65 nm, respectively. (f) Fluorescence enhancement factors (EF) as a function of the CsPbBr3-doped PMMA film thickness for samples based on Ag nanoparticle (red line) or plasmonic coupling structure (blue line).
3.4 Lasing emission properties of the CsPbBr3 QDs based on plasmonic coupling structure
To further demonstrate the advantage of the plasmonic coupling structure, the influence of the Ag nanoparticle-Ag film structure on the lasing emission properties of the CsPbBr3 QDs was investigated. We fabricated the devices of the CsPbBr3-doped PMMA film deposited on glass, Ag NPs, and Ag film/ SiO2 spacer/ Ag NPs. The CsPbBr3-doped PMMA film was prepared by spin-coating the CsPbBr3-doped PMMA solution with a speed of 3000 rpm (the mixture solution was composed of 100 µL 10 mg/ml PMMA solution and 300 µL CsPbBr3 QDs solution). Figure 9(a) shows the dependence of the emission spectra of the CsPbBr3-doped PMMA film on the pump intensity. It exhibits an obvious ASE behavior. When the sample was pumped at low energy, a broad spontaneous emission spectrum is observed at 514 nm. Once the pump energy turns large enough (larger than 10.8 µJ/cm2), the emission spectrum derives a narrower emission at 528 nm. Figure 9(b) and (c) show the emission characteristics of the CsPbBr3-doped PMMA films deposited on the Ag NPs and Ag NPs-Ag film coupling structure with a gap distance of 20 nm. Figure 9(d) illustrates the emission intensity as a function of pump energy for CsPbBr3-doped PMMA film deposited on different substrates. It shows that the plasmonic coupling structure can reduce the lasing threshold to a greater extent compared to the independent Ag NPs. And the lasing threshold of the CsPbBr3-doped PMMA film deposited on the Ag NPs-Ag film plasmonic structure is 2.7 µJ/cm2, which is 25% of that of the CsPbBr3-doped PMMA film deposited on glass.
Fig. 9. Emission spectra of the devices of (a) Glass/ CsPbBr3-doped PMMA film, (b) Glass/ Ag NPs/ CsPbBr3-doped PMMA film, and (c) Glass/ Ag film/ SiO2 spacer (20 nm)/ Ag NPs/ CsPbBr3-doped PMMA film. (d) Dependences of the emission intensities on the pump energy intensities for samples with different structures.
4. Conclusion
In summary, we demonstrated that the optical emission of the CsPbBr3 QDs can be significantly enhanced by introducing an Ag nanoparticle-Ag film plasmonic coupling structure. By regulating the gap distance between Ag nanoparticle and Ag film, the plasmon resonance wavelength can be tuned to match the emission wavelength of the CsPbBr3 QDs emitter. When the gap distance between Ag nanoparticle and Ag film was 20 nm, a 30-fold enhancement of fluorescence intensity was achieved and the lasing threshold was down to 25% of the CsPbBr3-doped PMMA film deposited on glass. It is due to that the plasmonic coupling structure exhibits a strong local electric field by the coupling between LSPR of Ag nanoparticle and SPP of Ag film. This work provides an effective method to enhance the optical emission of perovskite QDs and promotes the further exploration of on-chip light sources.
Funding
National Natural Science Foundation of China (62375163, 52177225); Key Research and Development Projects of Shaanxi Province (2023-YBSF-126); China Postdoctoral Science Foundation (2019M653635); Natural Science Basic Research Program of Shaanxi Province (2022JM-346).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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