Controlling Spontaneous Emission from Perovskite Nanocrystals with Metal-Emitter-Metal Nanostructures

: We show the enhancement in the intensity and emission rate of perovskite cesium lead bromide (CsPbBr 3 ) and formamidinium lead bromide (FAPbBr 3 ) nanocrystals in the presence of single and double gold layer cavities, which we refer to as Metal-Emitter (ME) and Metal-Emitter-Metal (MEM) nanostructures. Up to 1.9-fold photoluminescence intensity and up to 5.4-fold emission rate enhancements were obtained for FAPbBr 3 nanocrystals confined by double gold layers, which are attributed to plasmonic confinement from the gold layers. The experimentally obtained values are validated by analytical calculations and electromagnetic simulations. Such an effective method of manipulation of the spontaneous emission rate by simple plasmonic nanostructures can be utilized in sensing and detection applications.


Introduction
Metal halide perovskites for quantum emitter have gained interest over the years due to their low cost and solution-processable characteristics [1]. Importantly, the defect-tolerant characteristics of halide perovskites are responsible for their high absorption coefficients and high photoluminescence quantum yield, enabling potential large-scale applications in photovoltaic (PV) devices [2], scintillators [3][4][5][6], light-emitting diodes (LEDs), lasers, photodetectors, and field-effect transistors (FETs) [7]. Moderate exciton binding energy in the several meV range, large oscillator strength, and tuneable bandgap achieved via tailoring of chemical composition and structural diversity are the other advantages of halide perovskite, notably in nonlinear optics applications [8] and strong light-matter interaction between quantum emitter and optical cavity.
Coupling electromagnetic modes with quantum emitter [9,10] results in the emission rate modification known as the Purcell effect, which is relevant for realizing single-photon nano-emitters [11]. The light-confining structures can be realized via different platforms, such as photonic crystals [12,13], disorder [14,15], and plasmonic nanostructures [16]. In this work, we investigate the spontaneous emission rate modification of halide perovskite nanocrystals coupled with simple plasmonic cavities. The quantum emitters investigated in this work are cesium lead bromide (CsPbBr3) and formamidinium lead bromide (FAPbBr3) nanocrystals (NC), while the plasmonic cavity is defined in metal-emitter (ME) and metal-emitter-metal (MEM) structures in which the nanocrystals are sandwiched between single and double gold films, respectively. We obtained 1.2fold and 1.9-fold intensity enhancement for CsPbBr3 and FAPbBr3 nanocrystals respectively; whereas the emission rate enhancements are 4.3-fold and 5.4-fold for CsPbBr3 and FAPbBr3 nanocrystals respectively. Manipulation of the spontaneous emission rate by simple plasmonic nanostructures can be utilized in sensing and detection applications [17,18]. Furthermore, such simple structures with large emission enhancement factors will improve the brightness for light-emitting devices (LED) applications and improve the signal modulations in telecommunication technologies.

Materials and Methods
The schematic of the ME and MEM structures are shown in Figure 1(a). The 100-nm thick bottom gold layer was physically deposited on glass substrate by electron beam evaporation. To avoid nonradiative losses, a thin polystyrene (PS) layer was deposited in between gold and perovskite layer. Prior to polystyrene deposition, the gold surface was treated by oxygen plasma with low power and pressure of 450mBar for 30-50s. PS layer was spin coated from PS in toluene solution (1.5% w/v) at 8000rpm to thickness of 15nm. Cesium lead bromide (CsPbBr3) NC was synthesized according to a hot-injection protocol described in the literature [5,19]. In a typical synthesis, 0.7 g lead (II) bromide (PbBr2), oleic acid (0.5 mL), oleylamine (0.5 mL) and octadecene (5 mL) were loaded into a 50 mL flask and dried under vacuum at 100 °C for 0.5 h. Subsequently, nitrogen (N2) was introduced and the mixture was heated up to 170 °C until PbBr2 was completely dissolved. A hot Cs-oleate precursor (0.5 mL) was quickly injected into the flask under vigorous stirring. After 10 s of reaction, the flask was quickly immersed into an ice bath to quench the reaction. The CsPbBr3 NC precipitate was collected by centrifugation at 8000 rpm for 10 min and subsequently spin coated on the PS surface to thicknesses of 50nm and 100nm. Formamidinium lead bromide (FAPbBr3) NC was synthesized and spin coated by methods described in Chin et al [20]. was excited via a 10 MHz picosecond pulsed diode laser (Master Oscillator Fibre Amplifier, Picoquant, excitation wavelength at 355 nm, pulse width 50 ps, and power of 40 µW). PL spectra were detected using AvaSpec-HERO spectrometer. The emission was then filtered by a linear variable filter at 505 and 545 nm and detected by a single-photon avalanche photodiode connected to a timecorrelated single-photon counting acquisition module (Edinburgh Instruments, TCC900). The TRPL decay curves were fitted with two exponential functions. The average lifetime was calculated from the decay time components and amplitudes of the fitted TRPL decay curves.
For double gold structure, another gold layer was evaporated on another PS layer on top of the perovskite NC layer to 100nm thickness. The cross-sectional scanning electron micrograph (SEM) of the double gold structure is shown in Figure 1 Figure 2 presents the photoluminescence (PL) spectra of CsPbBr3 and FAPbBr3 nanocrystals in different scenarios. We varied the thickness of the NC layer to 50nm and 100 nm for the two nanocrystals and normalized the PL spectra to those of the reference NC layer on glass substrate (dashed black curves). The emission wavelengths are found at 505nm (for CsPbBr3 NC) and 545 nm (for FAPbBr3 NC). The normalized PL spectra of NC in metal-emitter situation are shown in red curves, showing intensity enhancement of ~1.13 and ~1.41 times for CsPbBr3 and FAPbBr3 nanocrystals of 50-nm thickness, respectively. Here, 15-nm thick PS layer is introduced between the gold film and the NC layer to minimize non-radiative loss. Stronger resonance is expected when a second gold layer is introduced. Normalized PL spectra of NC in metal-emitter-metal situation are shown in blue curves, showing PL intensity enhancements of ~1.23 (for CsPbBr3 NC) and ~1.91 (for FAPbBr3 NC) of 50-nm thickness. As both the gold film(s) and the NC layer form the plasmonic cavity, with light confinement within the NC layer, the role of the NC layer is thus expected on the resonance condition. This is Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 6 November 2020 doi:10.20944/preprints202011.0235.v1

Results and Discussion
illustrated when the NC layer thickness is increased to 100 nm, presented in Figure 2(b) and Figure  2(d) for CsPbBr3 and FAPbBr3 nanocrystals, respectively. While the PL intensity enhancement remains relatively unchanged for CsPbBr3 nanocrystals, it decreases from ~1.41 to ~1.21 (for metalemitter configuration) and from ~1.91 to ~1.42 (for metal-emitter-metal configuration) for FAPbBr3 nanocrystals. By the same reasoning, the resonance condition is also dependent on the thickness of the PS spacer layer. This means that the PS layer serves not only to minimize non-radiative loss, but also to modify the resonance condition of the plasmonic cavity. The PL characteristics of NC layer without PS spacer layer is shown in Figure S1 (Supplementary Information), indicating strong decrease in emission intensity, which has also been observed before [21]. Time-resolved photoluminescence characteristics for all scenarios are presented in Figure 3, showing spontaneous emission rate modification as the nanocrystals are coupled in metal-emitter or metal-emitter-metal configurations. Since both intensity and emission rate are enhanced, as shown in Table 1, this indicates Purcell effect taking place in both types of nanocrystals. To verity the role of PS spacer layer in Purcell effect, we present in Figure S2 (Supplementary Information) the time-resolved PL measurements of 50-nm thick CsPbBr3 NC on gold film without PS spacer layer. We found that the fast decay component in this situation is very close to the instrument response function (IRF) of about 0.3 ns [22]. Coupled with the intensity decrease in Figure S1, it can be seen that the fast decay rate is not attributed to Purcell effect, but to non-radiative loss instead. Therefore, while there is a reduction of non-radiative recombination by PS, the plasmonic/dielectric cavity are still the key driving force which enhances the emission rate. To further understand the experimental results, we turn to analytical calculation and then validate it with finite element simulation (FEM) in COMSOL. The metal-emitter and metal-emittermetal configurations can be modelled as a set of thin-film stacks with different optical permittivities and the nanocrystals as dipoles within the NC layer, as denoted by the red dots, as shown in Figure  4(a). The orientation of the electric dipole is such that it gives emission in the XY plane. Using the Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 6 November 2020 doi:10.20944/preprints202011.0235.v1 Fresnel reflections, the spontaneous emission rate modification of a dipole in metal-emitter configuration can be calculated as [23][24][25][26] where is the spontaneous emission rate enhancement factor, is the quantum efficiency, is the dipole distance from the interface of metal layer, is the thickness of the metal layer, is the propagation constant, = − / − , , , is the total p-polarized reflection from the thin film stack , , ( , ) = , + ( , . ) with , = denoting p-polarized Fresnel reflection between mth and nth layer. The subscripts 1, 2, and 3 refers to perovskite nanocrystal layer, gold layer, and glass substrate, respectively. For an electric dipole in metal-emitter-metal configuration, the emission rate modification is where the subscripts 1, 2, and 3 refers to perovskite nanocrystal layer, gold layer, and air, respectively. The graphs next to the schematics in Figure 4(a) illustrate the change in spontaneous emission rate enhancement over dipole distance from metal surface. At the centre of the perovskite film (d = 0.5L), the spontaneous emission rate enhancement factor (FP) for double gold layer structure is approximately two times of that of the single gold case, in range of L ≲ 0.5λ. The quantum efficiency of the perovskite nanocrystals is assumed to be = 0.4 [19,20], while their permittivities were obtained from spectroscopic ellipsometry, as shown in Figure 4(b).
In the finite element simulation [22,27], the emission rate enhancement is calculated as the dipole power ratio between that within the plasmonic layer structures and that in the perovskite film only. The calculated |E|-field mappings at resonances for ME and MEM structures with CsPbBr3 NC film are shown in Figure 4(c) and Figure 4(d), respectively. The NC layer is denoted by the green dashed lines, while the gold films are indicated by the yellow dashed lines. The optical modes are confined differently in the two configurations. The E-field for the ME case is localized at the vicinity of the electric dipole, while the E-field for the MEM case is rather extended in the transverse direction, mimicking that of a waveguide. This translates to stronger light-matter interaction in the MEM case, which gives higher emission rate modification. The comparison among the calculated, simulated, and measured emission rates for the two nanocrystals with 50-nm and 100-nm thicknesses is presented in Figure 5. For the calculation and FEM simulation in this work, the emission rate is averaged over more than 10 equally spaced points over the entire perovskite NC layer thickness. We found that our analytical results and FEM simulations are in good agreement with our experimental results. The difference is due to the boundary problem between PS and NC layers in both simulations. For FAPbBr3 NC, up to 1.9-fold PL intensity and up to 5.4-fold emission rate enhancements were observed for the case of 50nm thick NC layer in double gold layer structure. We note that the intensity enhancement of FAPbBr3 nanocrystals is higher than that of CsPbBr3 nanocrystals. This can be attributed to the interband transition of gold at 539 nm [28,29], suggesting that the gold layer behaves more as dielectric at wavelengths shorter than its interband transition. Indeed, this appears to be the case for CsPbBr3 nanocrystals whose emission wavelength is at 505 nm (< 539 nm). However, the light is still confined by virtue of total internal reflection. This is because the real refractive index of gold is 0.887 at 505 nm [28]. Considering the real refractive index of CsPbBr3 NC is 2.3 at 505nm, the index contrast of ~1.4 is large enough for ensuring optical confinement within the NC layer. Taking into account that the emission linewidth of CsPbBr3 NC of ~20nm, the enhancement is attributed more to dielectric confinement rather than plasmonic confinement. This is in contrast with FAPbBr3 nanocrystals, whose emission wavelength (545 nm) is longer than the interband transition of gold. Considering its emission linewidth of ~30 nm, the light confinement is dominated more by plasmonic mechanism rather than by total internal reflection. This also agrees with more pronounced enhancements in FAPbBr3 nanocrystals compared to CsPbBr3 nanocrystals.

Conclusions
The intensity and emission rate characteristics of CsPbBr3 and FAPbBr3 nanocrystals have been investigated in two metal-emitter configurations. Owing to their emission wavelengths with respect to the interband transition of gold, the enhancements in the two nanocrystals are attributed to different mechanisms. For CsPbBr3 NC, whose emission wavelength is shorter than gold interband transition, the gold behaves more as dielectric and the light is confined by virtue of strong index contrast. For FAPbBr3 nanocrystals, whose emission wavelength is longer than gold interband transition, the light is confined by plasmonic mechanisms. These confinement mechanisms are the reason for the big differences in the intensity and emission enhancements of the two nanocrystals. By treating the metal-emitter and metal-emitter-metal configurations as thin film stacks, we arrived at the analytical formulation to elucidate the emission enhancement, which was also validated by our FEM simulation. We found good agreement between our calculation and our experimental results. In addition, we have shown that a simple plasmonic cavity based on single or double gold films can be used as a platform to study light-matter interaction, particularly in solution-processable perovskite NC, which can be used for sensing application [17,18].
Supplementary Materials: Figure S1: Photoluminescence (PL) spectra of CsPbBr3 NC-Au layers (without PS) at room temperature (RT); Figure S2: Time-resolved emission of CsPbBr3 NC film with Au, Table S1: Key parameters of time resolved PL spectrum.