1. Introduction

In recent years, perovskite materials have become outstanding semiconductor materials for solid-state photovoltaic and light emitting devices due to their excellent photoelectric properties [1–5], such as long electronic carrier diffusion lengths, high optical absorption coefficients and impressive photovoltaic device performance [6–13]. As a member of this material category, the hybrid organic-inorganic perovskites (abbreviate to HOIPs) were in the spotlight of researching, because of its several unique characters, including efficient photoluminescence (abbreviate to PL) and color tuning, flexible preparation of bulk to 2D-dimension [14,15].

Unfortunately, HOIPs show an unstable luminance performance in atmospheric environment. When they are exposed to the air, generally, it is easy to decompose into other components [16,17]. A few factors can cause their luminous efficiency to be reduced rapid or even completely quenched, which severely restricts the practical application of perovskite [18]. Recently, it has been proved that mesoporous structures embedding perovskite nanocrystals are an effective method to maintain perovskites in atmospheric environment, by which many devices can be fabricated including solar cells, photodetectors, etc [19].

For further optimizing the luminescence performance of perovskites, the local surface electromagnetic field generated by surface plasmons (abbreviate to SPs) can be a promising method [20–22]. The effect of local electromagnetic field enhancement will improve the probability of radiative transition during luminescence emission of the nearby perovskite, which will significantly enhance the luminescence emission intensity of perovskite [23]. Previous reports have suggested that the luminescence enhancement of perovskites have been achieved by the combination of precious metal nanoparticles [24–27]. Among many morphological metal nanoparticles, silver nanowires with large aspect ratios have unique advantages, which exhibit a long-distance energy transport and a broad plasmonic resonance profile, for example, the silver with several-micron-transport and a resonance profile extending from 400 nm up to 1 μm [28,29].

In this work, we designed a nano-composite structure of a silver nanowire wrapped with mesoporous silica (mSiO₂), which was formed into a core/shell nanostructure as an effective means to enhance photonics phenomena [30]. Methyl ammonium lead bromide (CH₃NH₃PbBr₃, abbreviate to MAPbBr₃) is one of HOIPs, which was embedded in the mesopores of the composite structure as a light-emitting material. Thus, the Ag@mSiO₂ (silver core wrapped with mSiO₂ layer) nanowire composite structure can not only use to form perovskite quantum dots in mesopores of mSiO₂, but also can control the size of perovskite quantum dots by mesopores, in which quantum confinement can play a part to manipulate light [31]. More than that, the mesoporous structures also can be expected to reduce the reaction between the perovskite and factors in atmospheric environment. By this nano-composite perovskite, significantly enhanced stability and amplified spontaneous emissions (ASE) performance of MAPbBr₃ have been demonstrated. The silver core in the composite structure can provide a plasmon-enhanced effect, which results to enhance the PL and reduce the threshold of ASE [32]. All results indicate that the nano-composite of Ag@mSiO₂/perovskites is an effective strategy to improve the stability and emission performance of MAPbBr₃ simultaneously, which provides a new insight into the practical applications of MAPbBr₃.

2. Experiment

The fabrication procedure of the novel nanocomposites was divided into two parts (Fig. 7). The first part was the synthesis of Ag@mSiO₂ core-shell nanowires. The silver nanowires were wrapped in a mSiO₂ shell through a surfactant-templating sol-gel approach by using CTAB as a template [33]. The second part was embedding MAPbBr₃ in mSiO₂ shells [19]. In order to understand the enhancement effect of the silver core on MAPbBr₃ luminescence, we prepared the sample without silver core for comparison. The silver-free core structure was prepared by immersing Ag@mSiO₂ nanowires in a low-concentration nitric acid solution (2.3 mol/L). Under the situation, the action rates of Ag and SiO₂ are estimated to be ∼100 mg/h and ∼0.00037 mg/h, respectively [34,35]. The silver-removal processing lasted 30 min for ensuring silver-free structure preparation, which can hardly induce morphological change of mSiO₂.

The structure, morphology, the element distribution on the surface of the Ag and Ag@mSiO₂ nanowires were analyzed by means of a scanning electron microscope (SEM, Zeiss Gemini Ultra-55), and a transmission electron microscopy (TEM, JEM-1200EX, 123 kV). To further exploring the morphology of the composite, the pore size of mSiO₂ was measured by accelerated surface area and porosimetry system (Micromeritics ASAP2460).

In order to study the reasons for the enhanced luminescence performance of MAPbBr₃ embedded in the mSiO₂ layer containing silver core, the samples with and without silver core were further studied. A home-built optical density (OD) spectroscope with a halogen lamp and a spectrometer (Andor, resolution of 0.01 nm) was used for exploring the optical properties of samples. Beyond that, the power-dependent PL spectrum measurements were conducted by using an amplified femtosecond laser system (Spectra-Physics Spitfire Ace with Mai Tai SP, a repetition rate of 1 kHz, wavelength: 800 nm) as an optical excitation source. And the 400 nm excitation was generated by frequency-doubling of 800 nm pulses from laser system through β-barium borate (BBO) crystal.

Consideration of the possible mechanism of luminescence enhancement, the structure of the samples was reconstructed in finite-difference time-domain method (abbreviate to FDTD) commercial software (Lumerical 2018a). Field distributions were simulated based on the samples with and without silver cores.

3. Results and discussion

3.1 Nano-structure characterization

According to the SEM and TEM images of Ag and Ag@mSiO₂ nanowires [ Figs. 1(a) and 1(b)], diameters of the nanowires are 120 nm and 300 nm, respectively. This indicates that the thickness of mSiO₂ is ∼90 nm. Energy Dispersive Spectrometer (EDS) spectrum demonstrated that the ratio of Si and O elements was approximately 1:2, as shown in Fig. 8, which was in accordance with the elemental composition ratio of SiO₂. As well the EDS mappings (Fig. 8 inset) characterized the elements of silver, silicon and oxygen and their distribution, which were shown that the Ag nanowire core has been successfully wrapped by the mesoporous silica shell. Figure 9(a) include two dark-field images of Ag@mSiO₂ and Ag nanowires, that character scattering light in same light intensity. These results indicate that mSiO₂ is successfully and fully coated around the silver cores. To further find out the mesoporous structure of the Ag@mSiO₂ sample, the mesopore size was characterized by N₂ adsorption-desorption isotherms [Fig. 9(b)]. After value calculation, the average mesopore size is ∼2.35 nm, which suggests that the mesoporous have been formed and are ready for embedding MAPbBr₃.

 

Fig. 1. SEM images of the (a) Ag@mSiO₂ and (inset) Ag nanowires. TEM (b) images of the magnified interface of Ag and mSiO₂, inset is the magnified Ag@mSiO₂ nanowire viewed in hundred-nanometer-scale.

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3.2 Optical characterization

To investigate the optical properties of samples, we measured OD and PL spectra of prepared samples. Figure 2(a) exhibits OD spectra of pristine Ag nanowire, Ag@mSiO₂, and Ag@mSiO₂/MAPbBr₃ samples. The black line of Ag nanowires presents a typical silver nanowire scattering spectrum of the maximum scattering peak from 352 nm extending to 394 nm, which corresponds to the transverse and longitudinal scattering of the wire-shape. By coated mSiO₂ (orange line), the OD peak of the sample was red-shifted to 405 nm. The red-shift is induced by longitudinal absorbed change, because the silver nanowires have been wrapped by mSiO₂. Changes of structure and medium made the effective refractive index of nanowires have changed, that the spectral peak moved to the direction of increasing wavelength [36,37]. After embedding MAPbBr₃, the characteristic absorption peaks of Ag@mSiO₂/MAPbBr₃ composites continue to red-shift, but the peak shift amplitude is small, and the peak appears near 410 nm. It is noticed that the green curve has a dip around 540 nm. This is because samples will luminesce under the halogen lamp stimulated. We use a 400 nm femtosecond laser to excite both samples, and the excitation power is 2.80 μJ/pulse. The corresponding luminance peaks of both samples were around 538 nm [Fig. 2(b)]. The results correspond to Fig. 2(a), in which the dip of OD spectrum comes from compensation of the luminescence peak. The plasmon resonance band of silver nanowires overlaps with the luminous wave band, and also overlaps with the band of excitation wave light, which makes the luminance intensity of the sample with silver core stronger. This phenomenon indicates that the silver core has a non-negligible enhancement in the perovskite luminescence.

 

Fig. 2. (a) Normalized optical density spectrum of different composite materials. (b) PL spectrum of different composite materials under 400 nm pumped emission.

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For further studying the luminescence performance of composites with and without silver cores, we used different excitation powers to excite the composites [ Fig. 3(a) and 3(c)]. A 400 nm femtosecond laser was used to excite the sample at low pump light intensity and a broad spontaneous emission (SE) was obtained with the peak at 538 nm. With the gradual increase of the excitation power, when the incident pump power exceeds about 0.46 μJ/pulse, a peak located at 549 nm emerges and quickly becomes dominant, and the luminous intensity of the composite materials also increases sharply. By the two-step linear fitting, the threshold intensity was estimated to be ∼0.46 μJ/pulse as well. When the incident pump exceeds the threshold, the FWHM of the emission spectra narrows sharply, which signifies the transition from SE process into ASE regime. The peak at 549 nm shows a narrow linewidth of approximately 5.5∼7.5 nm. Combined with the situation of strong intensity laser exciting, this PL peak can be defined to be ASE instead of trapped exciton or carrier effects [38]. As the pump power increased, the emission peak became narrower due to the preferential amplification at frequencies close to the maximum of the gain spectrum. The initial FWHM of the emission spectrum was about 45 nm, but sharply narrowed to about 6.6 nm at this power. Similarly, we excited a silver-free sample with a 400 nm femtosecond laser, also obtained a luminescence peak at 538 nm. As the same processing of Fig. 3(b), the threshold of SE to ASE regime was estimated to be ∼1.2 μJ/pulse. When the incident pump power exceeds the threshold, the peak appeared and quickly dominated. Comparing the value of the maximum integrated intensity in Figs. 3(b) and 3(d), both are quite close (approximately ∼3500). The FWHM of both samples also exhibit a similar trend of narrowing from the original 50 nm to 9 nm. All evidences clearly point to the process of change from SE to ASE. Under the pump power of 2.20 μJ/pulse [ Fig. 4(a)], it can be understood that the ASE luminous intensity of the silver core is ∼43.5% higher than that of the non-silver core. The laser threshold is reduced by 62%, and the FWHM is also significantly reduced. This indicates that silver core can assist in enhancing the spontaneous emission of perovskite due to SP enhancement of metal core in the structure [30].

 

Fig. 3. The measured spectroscopy of (a) Ag@mSiO₂/MAPbBr₃ NWs and (c) mSiO₂/MAPbBr₃ NWs films; The ASE threshold and FWHM behaviors of (b) Ag@mSiO₂/MAPbBr₃ NWs and (d) mSiO₂/MAPbBr₃ NWs films.

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Fig. 4. Emission spectra of samples with/without Ag nanowire cores under (a) 400 nm and (b) 800 nm femtosecond laser (2.2 μJ/pulse and 3.4 μJ/pulse pumped emission respectively).

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For further study the performance of two-photon luminescence of composite structures, we employed an 800 nm femtosecond laser to excite both samples (Fig. 10). Similar to the case of 400nm excitation, a wide SE was obtained at the peak center of 541 nm when the sample is excited with 800 nm femtosecond laser under low pump light intensity. As the excitation power gradually increased, the peak appears at 546 nm and quickly dominates, and the luminous intensity of the composite also increases sharply. Similarly, we excited the sample without silver core with 800 nm femtosecond laser and obtained a luminescence peak at 540 nm. The luminous intensity also gradually increases with excitation power increasing. Comparing the luminescence of both samples under the pump power of 3.40 μJ/pulse [Fig. 4(b)], it can be seen that luminous intensity of the sample with silver core is 46.4% higher than that of the sample without silver core. The laser threshold dropped from 2.8 μJ/pulse to 1.9 μJ/pulse, a 32% decrease. These similar results indicate that the silver core can play a part in enhancement of two-photon luminescence processing as well.

3.3 Simulation results

To understand the mechanism of luminescence enhancement, the field distribution of two structures were simulated based on the FDTD. The effect of the localized SP of silver nanowires on the luminescence characteristics of the perovskite material was analyzed. In the simulation models, we considered the two restructured configurations as shown in Fig. 5, which were (a) silver core coated with mSiO₂ and (b) silver-free and hollow SiO₂ coated with mSiO₂. As the calculated field intensity distributions, enhanced local electric field occurs in Ag@mSiO₂ composite structure, giving a thirty-time localized light amplitude compared to coreless@mSiO₂ composite structure.

 

Fig. 5. The field intensity distributions of cross-section of the two structures at 400 nm (a) with and (b) without silver cores, respectively.

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The significant field enhancement may result from the induced localized SP. In the localized SP resonance state, the free electrons oscillate with the maximum amplitude, exhibiting intense and spatially non-homogeneous oscillating electrical fields in the vicinity of the plasmonic nanostructures or the structures with sharp features. Therefore, the energy of the incident light irradiation is transferred and localized in the near-field region of the nanostructures. It is also found that the induced plasmonic fields can enhance the charge carrier generation of perovskites with an overlapping of the localized SP and the bandgap energies. Under such a circumstance, a strong electric field is expected to generate between the silver nanowires and the perovskites, whose intensity can be enhanced up to several times larger than that of the far-field incident light. Thus, with an introduction of silver nanowires, the required excitation power for lasing action in perovskite nanostructure may be greatly reduced due to the local enhanced electric field.

3.4 Luminance stability detection

The photostability of the Ag@mSiO₂/MAPbBr₃ composite material under continuous femtosecond laser pumping is shown in Fig. 6. The two-photon pump laser intensity is set as 2.80 μJ/pulse. Ag@mSiO₂/MAPbBr₃ composite material has slight fluctuations around the mean intensity during 7.8 × 10⁵ continuous laser shots. While the intensity of the sample keeps stable under the same measured condition. Thus, we can conclude that the perovskite nanocrystals embedded in mesoporous silica have good photostability.

 

Fig. 6. Photostability of PL intensity of the Ag@mSiO₂/MAPbBr₃ nanowires under 400 nm pumped emission (2.8 μJ/pulse).

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Compared with some reported works, this structural design shows a comparable balance between protection of photostability and active enhancements. In the ASE regime, benefitted from thirty-time confined field intensity with silver cores over without cores, clear enhancements can be observed, of which the level can be comparable to previous composites of aluminous and gold nanoparticles [39,40]. Under a certain power of ultrafast laser pumping, the nanocomposite sample exhibits quite luminance stability, which can match results of inorganic samples, for instance of CsPbBr₃ [41].

4. Conclusion

In conclusion, embedding perovskite in mesoporous silica around silver nanowires can greatly improve the ASE performance of perovskite. Meanwhile, the composite structure can also reduce the threshold excitation power and FWHM. The presence of silver core not only enhances the single-photon emission of MAPbBr₃ but also enhances two-photon emission. By comparing the experimental results and simulations, strong exciton-plasmon coupling occurs which is relative to the core-shell structure of silver and perovskite nanocrystals due to metal induced localized field enhancement. The simulated results revealed the position and intensity of localized SP field. Benefit from nanostructure, up to thirty-time field intensity of pump light can be localized and interacted with perovskite nanocrystals. In experimental results, it exhibited that the threshold the composite can be effectively reduced and the ASE efficiency can be improved obviously. Beside structural enhancement, the framework structure of mSiO₂ can be used to effectively prevent the perovskite from decomposing, especially when it illuminated by ultrafast laser for a long time. The near invariance of the output intensity and spectrum bears testimony to the excellent optical stability of these composite materials. The result can demonstrate that the nanocomposite of perovskite embedded in core-shell structure of mSiO₂ layer and metal core can be potentially applied in compact photonic device by its excellent performance of ASE.

Appendix

A1. Fabrication of MAPbBr₃ nano-composite

In the preparing procedure of mSiO₂ and perovskites, ammonia aqueous solution (28 wt%), Acetone, NH₄NO₃, Tetraethyl orthosilicate (TEOS) and Hexadecyltrimethylammonium bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co. (China); lead bromide (PbBr₂, ≥ 98.0%), methylammonium bromide (MABr, ≥ 98.0%), and N,N-dimethylformamide (DMF, ≥ 99.7%) from Sigma Aldrich. The silver nanowire (20 mg/ml) was mature commercial product purchased from XFNANO nanomaterials technology co. LTD (China). Deionized water was used in all experiments. The process of the samples is divided into the following two steps (Fig. 7).

 

Fig. 7. A schematic representation of preparation process of the Ag@mSiO₂/MAPbBr₃ composite structure.

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Step 1: Synthesis of Ag@mSiO₂ core-shell nanowires. As shown in Fig. 7, (a) certain amount of CTAB was added into the solution containing water, ethanol and ammonia aqueous solution (28 wt %). Then an ethanol solution of the silver nanowires was added to the mixture with stirring. It was followed by the addition of 120 μL of TEOS was added dropwise with continuous stirring for about 10 seconds and the reaction was continued for 6 h. The nanowires were collected by centrifugation and washed with ethanol and water, respectively. The CTAB surfactant was removed by using 60 mL of NH₄NO₃/ethanol solution and stirring for 2 h. After washing with ethanol and DI water for 3 times, the Ag@mSiO₂ core-shell nanowires were obtained. In order to obtain a silver-free core structure, we immersed the sample in a low-concentration nitric acid solution. After the sample became colorless and transparent, it was washed with deionized water until the pH was neutral.

Step 2: Embedding of MAPbBr₃ in mSiO₂ shells. Take appropriate amount of composite structure powder, as shown in the second step in Fig. 7, two 20 μL of MAPbBr₃ precursors was added dropwise to the powder followed by vortex mixing for 30 min. The powders were then dried on a hot plate at 95 °C for 30 min before further characterization.

A2. Nano-structure characterization

Dark field images of the Ag and Ag@mSiO₂ nanowires are shown in Fig. 9(a). In Fig. 9(a) inset of the dark-field image, the nanowire profile cannot be seen in only silver nanowires without mSiO₂ coverage, with the exception of scatters light of particles partially. After coating mSiO₂, profiles and shapes of Ag@mSiO₂ were able to clearly catch, and Ag@mSiO₂ nanowires exhibited bright and characteristic scattered light in the same light intensity, which revealed the roughness of these two samples were great different.

To explore the size of mesopores, nitrogen sorption isotherms were employed to measure with a facility of Micromeritics ASAP 2460 (USA). The Brunauer-Emmett-Teller (BET) method was utilized to calculate the specific surface areas (SBET), using adsorption data in a relative pressure range from 0.05 to 0.25. The pore volume and pore size distributions were derived from the adsorption branches of isotherms using Barrett-Joyner-Halenda (BJH) model. The total pore volume, Vt, was estimated from the amount adsorbed at a relative pressure P/P₀ of 0.95. N₂ adsorption-desorption isotherms displayed typical type-IV curves [Fig. 9(b) inset]. After value calculation, the average mesopore size is ∼2.35 nm, BET surface area and the total pore volume are 325.29 m²/g and 0.19 cm³/g, respectively.

 

Fig. 8. EDS spectrum and EDS element mapping images (inset) of Ag@mSiO₂ nanowires.

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Fig. 9. (a) Dark field images of the Ag@mSiO₂ nanowires and (inset) the Ag nanowires. (b) Pore size distribution and N₂.

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A3. Supplementary spectra

Figure 10 shows the luminous emission of the two structures at different excitation powers. Figure 10(a) is the emission spectrum of structure with silver core, while Fig. 10(b) is the spectrum of structure without silver core.

 

Fig. 10. The measured spectra of nano composites (a) with and (b) without silver cores under 800 nm pumped emission, respectively.

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Funding

Key Technology Research and Development Program of Shandong (2019GGX102048); National Natural Science Foundation of China (11904212); China Postdoctoral Science Foundation (2019M662455).

Disclosures

The authors declare no conflicts of interest.

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