⇦Chinese Physics Letters, 2017, Vol. 34, No. 9, Article code 096801 Fluorescence Enhancement of Metal-Capped Perovskite CH$_{3}$NH$_{3}$PbI$_{3}$ Thin Films * Peng Sun(孙鹏)1,2, Wei-Wei Yu(俞伟伟)2, Xiao-Hang Pan(潘晓航)2, Wei Wei(魏威)2, Yan Sun(孙艳)2**, Ning-Yi Yuan(袁宁一)1**, Jian-Ning Ding(丁建宁)1, Wen-Chao Zhao(赵文超)2, Xin Chen(陈鑫)2, Ning Dai(戴宁)2 Affiliations 1School of Materials Science and Engineering, Jiangsu Collaborative Innovation Center of Photovoltaic Science and Engineering, Jiangsu Province Cultivation Base for State Key Laboratory of Photovoltaic Science and Technology, Changzhou University, Changzhou 213164 2National Laboratory for Infrared Physics, Shanghai Institute of Technical Physics, Chinese Academy of Sciences, Shanghai 200083 Received 27 March 2017 *Supported by the Ministry of Science and Technology of China under Grant No 2016YFA0202201, the National Natural Science Foundation of China under Grant Nos 61290304, 11574335 and 61376016, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the 333 Project of Jiangsu province under Grant No BRA2017352.
**Corresponding author. Email: sunny@mail.sitp.ac.cn; nyyuan@cczu.edu.cn Citation Text: Sun P, Yu W W, Pan X H, Wei W and Sun Y et al 2017 Chin. Phys. Lett. 34 096801 Abstract We fabricate nano-structural metal films to improve photoluminescence of perovskite films. When the perovskite film is placed on an ammonia-treated alumina film, stronger photoluminescence is found due to local field enhancement effects. In addition, the oxide spacer layer between the metal (e.g., Al, Ag and Au) substrate and the perovskite film plays an important role. The simulations and experiments imply that the enhancement is related to surface plasmons of nano-structural metals. DOI:10.1088/0256-307X/34/9/096801 PACS:68.37.-d, 73.20.Mf, 78.55.-m © 2017 Chinese Physics Society Article Text Organic-inorganic hybrid perovskites have received extensive attention with long carrier lifetime and diffusion length,[1-5] and have been recently used in the field of photovoltaics. In addition to use in solar cells, lead halide perovskites are also ideal candidates for lasing attributing to wavelength tenability and high fluorescence yield.[5,6] Perovskite-based near-infrared lasing has also been demonstrated with low thresholds and wide mode-tunability at room temperature.[7-9] However, the experimental quantum yield is still too low for practical applications. Surface plasmons (SPs), one of the electromagnetic-field enhancement effects, are used to increase emission efficiency of fluorescence.[10-13] SPs have been prevalently applied in photodetectors[14,15] and solar cells. Zhang et al.[16] have demonstrated photocurrent and efficiency enhancement in meso-superstructured organometal halide perovskite solar cells incorporating core-shell Au@SiO$_{2}$ nanoparticles delivering a device efficiency of up to 11.4%. In the same way, Cui et al.[17] reported and illuminated the nanostructures initiated surface plasmons effect, in which significant increase in efficiency of perovskite cells was confirmed as the role of SPs. However, it has rarely been reported that the metal films inspired would affect the SPP effect on the perovskites. Near-field coupling like propagating surface plasmon or localized surface enhancements near metal nanoparticles plays a critical role on the surface enhanced fluorescence. Here we describe an alternative method for enhancing fluorescence of perovskites by near-field SPs of metal particles. The fluorescence enhancement and quenching depend on the alumina spacer layer between metal and perovskites. We find that the surface morphology, roughness and thickness of metal films affect the fluorescence enhancement, which have great potential to improve the quantum yield and quality factor of the perovskite nano-lasers. In this study, we coated perovskite (CH$_{3}$NH$_{3}$PbI$_{3}$) on different metal (e.g., Au, Ag and Al) films according to the construction displayed in Fig. 1(a). Especially, perovskite/Ag, perovskite/Au, perovskite/Al$_{2}$O$_{3}$/Ag, perovskite/Al$_{2}$O$_{3}$/Au, perovskite/Al$_{2}$O$_{3}$/Al, and perovskite/Al$_{2}$O$_{3}$/treated-Al were prepared and investigated by photoluminescence (PL) spectroscopy at room temperature. The construction geometry of Al/Al$_{2}$O$_{3}$/perovskite (before being ammonia-treated) is a three-layer stacking dielectric/metal/glass optical configuration, as shown in Fig. 1(a). From the bottom, all structures were constructed on quartz substrates coated with a dense and continuous metal (e.g., Au, Ag and Al) layer by physical vapor deposition (e.g., thermal vapor deposition or sputtering coating). Figure 1(b) shows a cross-sectional view SEM of the perovskite/Al(pretreated)/quartz. The as-fabricated Al films on the quartz are smooth and flat with 182 nm in thickness, as confirmed by SEM and AFM measurements in Figs. 1(c) and 1(d). The bottom metal layer eliminates the transmission and reflects most of the incident waves. Then, a dielectric oxide (e.g., Al$_{2}$O$_{3}$) layer with $\sim$5 nm in thickness was deposited on the metal layer (labeled as Al$_{2}$O$_{3}$ in Fig. 1(a)), by using atomic layer deposition (ALD). The fluorescent CH$_{3}$NH$_{3}$PbI$_{3}$ perovskites (labeled as perovskite in Fig. 1(a)) with about 200 nm thickness were grown on the top by spin-coating.[18] GBL and DMSO mixed solution of CH$_{3}$NH$_{3}$PbI$_{3}$ was then spin-coated on the metal layers. During a delay time, a second solvent (methylbenzene) was quickly added to the substrate. The role of the second solvent is to rapidly reduce the solubility of CH$_{3}$NH$_{3}$PbI$_{3}$ in the mixed solvent and thereby promoting fast nucleation and growth of the crystals in the film. A high-magnification SEM image of the surface of a CH$_{3}$NH$_{3}$PbI$_{3}$ film is shown in Fig. 2(a), in which CH$_{3}$NH$_{3}$PbI$_{3}$ exhibits full surface coverage and is composed of micron-sized grains. Intense diffraction peaks at 14.08$^{\circ}$, 28.40$^{\circ}$ and 31.86$^{\circ}$ can be respectively assigned to (110), (220) and (310) diffractions of the tetragonal CH$_{3}$NH$_{3}$PbI$_{3}$ phase, respectively. The peaks assigned to CH$_{3}$NH$_{3}$PbI$_{3}$ crystals are proven to have superior crystallization. In particular, CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite exhibits a stable crystallinity on a variety of substrates (as shown in Fig. S2). The layer of CH$_{3}$NH$_{3}$PbI$_{3}$ perovskites can provide missing momentum for the coupling of incident plane-wave light and SPP, which we will discuss in the following.
cpl-34-9-096801-fig1.png
Fig. 1. (a) Illustration of multilayered structure. (b) Cross-sectional view of the perovskite layer constructed on the metal di-layer structure. (c) The SEM morphological images of bare Al. (d) The AFM morphological images of bare Al.
Fluorescence spectra were used to investigate how perovskite fluorescence is affected by near-field surface plasmon resonance as illustrated in Fig. 3. The excitation wavelength 532 nm was used in PL measurements, and all measurements were performed at room temperature. A bare CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite sample (black line) shows an apparent PL peak located at $\sim$770 nm as in earlier reports.[19] However, the perovskite films on the metal layer show weaker PL. As illustrated in Fig. 3, the PL intensity of perovskites was largely enhanced when approximately 5 nm Al$_{2}$O$_{3}$ sandwiched between the metal and perovskite layers. Surface plasmon resonance should be physically essential behind this enhancement.[20] Under appropriate boundary conditions, the dispersion relation can be expressed as follows: $$\begin{align} \hbar k_{\rm sp} =\frac{\hbar \omega }{c}\sqrt {\frac{\varepsilon _{\rm m} \varepsilon _{\rm d} }{\varepsilon _{\rm m} +\varepsilon _{\rm d} }},~~ \tag {1} \end{align} $$ where $k_{\rm sp}$ is the wave vector of the SPPs, $\varepsilon _{\rm m}$ and $\varepsilon _{\rm d}$ are the dielectric constants of metal and dielectric medium and $\frac{\hbar \omega }{c}$ is the incident energy.[21-23] The dielectric constants of different metals and CH$_{3}$NH$_{3}$PbI$_{3}$ perovskite are obtained from Refs. [24,25]. Figure 4 illustrates the dispersion relations of metal/perovskite and perovskite. The SP energy of Au/perovskite was determined to be 2.25 eV (551 nm), which matches quite well with the incident illumination (532 nm), expecting a strong coupling in Au/Al$_{2}$O$_{3}$/perovskite. The resonances of the surface plasmon polaritons induce an enhanced electromagnetic field on the metal-dielectric interface, and then further improve the energy of the heat electrons in perovskite. Protected excitation process of electron-hole pairs by the thin Al$_{2}$O$_{3}$ passivation layer will be achieved in perovskite and then followed by the enhancement of fluorescence emission. There is no doubt that there is slight enhancement of PL although Ag/perovskite and Al/Al$_{2}$O$_{3}$/perovskite have the plasmon energies at 2.85 and 3.5 eV, which are far away from the emission of laser energy. The above discussion can reasonably explain the enhancement ratios of PL intensity, Au/Al$_{2}$O$_{3}$/perovskite (blue dotted line) is $\sim$7 while Ag/Al$_{2}$O$_{3}$/perovskite (green dotted line) is $\sim$1.67. We note that Ag/perovskite (green dot) and Au/perovskite (blue dot) samples show weaker PL peaks. This fluorescence quenching is attributed to the transfer of energy from the hot electrons in perovskite to the metallic surface[26] through the channel of non-radiative transition. The energy is ultimately transferred to electron-hole pairs of the metal, and dissipated in a lossy way leading to less emission of fluorescent photons.
cpl-34-9-096801-fig2.png
Fig. 2. (a) Top view SEM image of the as-prepared perovskite film morphology. (b) X-ray diffraction (XRD) spectra of a solution-processed perovskite film.
In addition to coupling with propagating surface plasmon, the surface-enhanced fluorescence is also significantly affected by the localized light near metal objects when the size is smaller than illumination wavelength.[27,28] In fact, any localization of electromagnetic intensity near metal discontinuity can increase the fluorescence by several orders of magnitude.[27,29] Figures 5(a) and 5(b) exhibit morphological images of the 182-nm-thick Al layer treated in an ammonia solution. Due to the chemical reaction between metallic Al with the ammonia solution, Al hemisphere particles with irregular grain sizes are randomly distributed on the metal layer forming an inhomogeneous rough surface. According to statistical analysis, the total surface coverage is about 64% for the Al hemisphere particles with size distribution centered at $\sim$20 nm (Fig. S4, according to the result of image acquisition calculation). In Fig. 5(c), we compare the experimental and simulated absorption coefficient ($A$) of the perovskite/Al$_{2}$O$_{3}$/treated-Al. Here the perovskite layer is of 200 nm, and Al$_{2}$O$_{3}$ with 5 nm in thickness. The experimental absorption (the black line) reaches about 97% at 780 nm, and more than 80% at 532 nm. The numerical simulations based on Lumerical FDTD solutions were performed by employing a uniform semispherical Al nanoparticle array with radius 20 nm on the Al substrate (182 nm), followed by a thin Al$_{2}$O$_{3}$ layer (5 nm) and about 200 nm perovskite layer. As we can see in Fig. 5(c), the simulated absorption spectra (the red line) are in agreement with the experimental results although there are some errors due to the arbitrary model and tolerance deviation of dielectric parameters.
cpl-34-9-096801-fig3.png
Fig. 3. The PL spectra of perovskite in various conditions (bare perovskite, Ag/perovskite, Au/perovskite, Ag/Al$_{2}$O$_{3}$/perovskite, Au/Al$_{2}$O$_{3}$/perovskite and Al(ammonia-treated)/perovskite). The enhancement ratio of integrated intensity on different substrates is shown in the inset.
To further investigate the mechanism of the enhanced absorption, Fig. 5(d) visually presents the cross-sectional views of the electric field distribution ($|E|^2$) evaluated for the sample of perovskite/Al$_{2}$O$_{3}$/treated-Al for the wavelength $\lambda =532$ nm, which are located at the absorption band. It is found that the electric field is strongly confined on the outer surface of the Al particles. The field strength is significantly improved by localized surface plasmon resonance, which benefits the enhancement of the fluorescence emission.[30] A 14-fold amplification of the fluorescence signal was obtained in the case of perovskite/Al$_{2}$O$_{3}$/treated-Al, as illustrated in Fig. 3. These results indicate that the strongest electromagnetic field enhancements are related with localized surface plasmon resonance. In addition, the enhanced electromagnetic field at the metal-dielectric interface greatly improves the spontaneous recombination rate in the semiconductor. It is important to note that the thin Al$_{2}$O$_{3}$ spacer layer between the rough metal and perovskite plays a non-negligible role in preventing excitons transferred into the metal, similar to PMMA as a spacer in Ag/perovskite structure.[29]
cpl-34-9-096801-fig4.png
Fig. 4. The surface plasmon dispersion relations of (a) perovskite, Ag/perovskite; (b) perovskite, Au/perovskite; and (c) perovskite, Al/perovskite.
cpl-34-9-096801-fig5.png
Fig. 5. (a) The SEM morphological images of Al (ammonia-treated) film. (b) The AFM morphological images of Al(ammonia-treated) film. (c) Absorption spectra of Al(ammonia-treated)/perovskite(red: simulation, black: experiment). (d) FDTD simulates the electric field distribution of two Al hemispheres.
To further investigate the effect of the nano-particle diameter distribution on PL intensity, we prepared perovskite/Al$_{2}$O$_{3}$/treated-Al structures by varying the pretreatment time. It is clear that a coarse layer formed on the surface of Al nanoparticles with the largest diameter when we treated the sample within 1 min, as exhibited in Fig. S5. We find that the PL intensity is absolutely in proportion to the roughness of the Al layer surface. The particles gradually disappear and furthermore tend to a quick decrement of the Al layer surface roughness as the treatment time increases, which leads to a lower PL intensity. Further, Fig. 6(b) implies that PL intensity is gradually improved by the increase of Al thickness although it is eventually saturated with a thickness of more than 150 nm. As we know, when the energy gap of a semiconductor is comparable with the energy at a metal/semiconductor interface, electron-hole pair recombination stimulates the resonance mode. This resonant coupling considerably enhances the spontaneous recombination rate in a semiconductor.[31] In our testing process, it is found that all fluorescence peaks are almost at the same location. The reason shown by Neogi et al.[32] is the Purcell enhancement factor, which is a function of the metal thickness.[33] In addition, when the thickness of metal coating becomes thicker, its extinction coefficient $k$ decreases, while the refractive index $n$ does not change.[34] The metal dielectric constant $\varepsilon _{\rm m}$ can be calculated by $$\begin{align} \varepsilon _{\rm m}=n^2-k^2+2nki.~~ \tag {2} \end{align} $$ From the real part of the formula, when the metal layer becomes thick enough (semi-infinite thickness), $\varepsilon _{\rm m}$ tends to a maximum constant value.[35] Combined with Eq. (1), the thickness of the metal layer has a significant effect on this system. Thicker Al film herein causes local field enhancement to adjust the energy of perovskite so that a better photoluminescence emission can be reached.
cpl-34-9-096801-fig6.png
Fig. 6. (a) The PL spectra of Al-capped perovskite as Al dipped in ammonia for different times. (b) The PL spectra of perovskite capped with different thicknesses of Al.
In summary, we have demonstrated the enhancement of perovskite fluorescence affected by near-field coupling. The essentials behind the fluorescence quenching and enhancement have been studied and concluded as follows: (1) Fluorescence quenches through non-radiative transition of the hot electrons in perovskite to the metallic surface. (2) Surface plasmon resonance propagating on the interface and the localized surface enhancements near metal nano-particles are the primary mechanism for the fluorescence enhancement. The thin spacer layer between metal and perovskites plays a key role in the emission process. Ongoing investigations open alternative routes in achieving fluorescence enhancements and potential application in nano-photonics, optical computing and nano-lasers. References Solvent engineering for high-performance inorganic–organic hybrid perovskite solar cellso -Methoxy Substituents in Spiro-OMeTAD for Efficient Inorganic–Organic Hybrid Perovskite Solar CellsCompositional engineering of perovskite materials for high-performance solar cellsHigh-performance photovoltaic perovskite layers fabricated through intramolecular exchangeHigh Photoluminescence Efficiency and Optically Pumped Lasing in Solution-Processed Mixed Halide Perovskite SemiconductorsBright light-emitting diodes based on organometal halide perovskiteLow-temperature solution-processed wavelength-tunable perovskites for lasingRoom-Temperature Near-Infrared High-Q Perovskite Whispering-Gallery Planar NanolasersVapor Phase Synthesis of Organometal Halide Perovskite Nanowires for Tunable Room-Temperature NanolasersA Comprehensive Model of Electric-Field-Enhanced Jumping-Droplet Condensation on Superhydrophobic SurfacesSurface plasmon photodetectors and their applicationsSurface plasmon enhanced photodetectors based on internal photoemissionInfluence of surface plasmon resonances of silver nanoparticles on optical and electrical properties of textured silicon solar cellSurface plasmon resonance enhanced highly efficient planar silicon solar cellEnhancement of Perovskite-Based Solar Cells Employing Core–Shell Metal NanoparticlesSurface Plasmon Resonance Effect in Inverted Perovskite Solar CellsA Fast Deposition-Crystallization Procedure for Highly Efficient Lead Iodide Perovskite Thin-Film Solar CellsInverted, Environmentally Stable Perovskite Solar Cell with a Novel Low-Cost and Water-Free PEDOT Hole-Extraction LayerSurface enhanced fluorescenceSurface-plasmon-mediated emission from metal-capped ZnO thin filmsEmission enhancement from metallodielectric-capped ZnO filmsTunable surface plasmon mediated emission from semiconductors by using metal alloysPlasmonic Films Can Easily Be Better: Rules and RecipesReversible Hydration of CH 3 NH 3 PbI 3 in Films, Single Crystals, and Solar CellsEfficient electron-blocking layer-free planar heterojunction perovskite solar cells with a high open-circuit voltageEnhanced localized fluorescence in plasmonic nanoantennaeVapor-deposited amorphous metamaterials as visible near-perfect absorbers with random non-prefabricated metal nanoparticlesLocalized surface plasmon for enhanced lasing performance in solution-processed perovskitesOptical invisibility through metasurfaces made of plasmonic nanoparticlesCoupling of InGaN quantum-well photoluminescence to silver surface plasmonsResonance Absorption by Nuclear Magnetic Moments in a SolidEnhancement of spontaneous recombination rate in a quantum well by resonant surface plasmon couplingInfluence of Metal Deposition on Exciton–Surface Plasmon Polariton Coupling in GaAs/AlAs/GaAs Core–Shell Nanowires Studied with Time-Resolved CathodoluminescenceTheory of coupling in dispersive photonic systems
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