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Abstract
We compared the photoluminescence (PL) of perovskite-polymer composite films on gold nanoparticle (AuNPs) substrates without and with a buffer layer of polymethyl methacrylate (PMMA). It is found that a 1.8-fold PL enhancement can be experienced due to surface plasmons with solely AuNPs substrates. With PMMA placed between the emissive layer and AuNPs, the plasmonic effect is reduced while reflectance and interface enhanced emission is increased, which results in a final PL increase of 2-fold. Such enhancements provide potential strategies to enhance the light-emitting properties of in-situ fabricated perovskite quantum dots films for light conversion applications such as LCD backlights, silicon solar cells, and photodetectors.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In-situ fabrication strategy offers a facile and well dimensional controlled growth of perovskite quantum dots (PQDs) towards photonic and optoelectronic applications [1–6]. Regarding the photoluminescence (PL) purpose for high-end displays, in-situ fabricated PQDs in a polymeric matrix composite film has been demonstrated, providing highly luminescent efficiency and high color purity [7–9]. Given the decent stability against humid, oxygen and heat of these composite films, they are further employed as color conversion films for improving the backlights of liquid crystal displays (LCDs) [10] as well as enhancing the ultraviolet {UV) response of silicon-based photodetectors and solar cells [11,12]. Very recently, continuous-wave pumped lasing has been achieved using a PQD-embedded polyacrylonitrile (PQDs-PAN) film as a gain medium [13].
In order to enable efficient nanophotonic devices based on the in-situ fabricated PQDs in a diverse range of applications, several challenges still exist, such as uncontrolled directional emission, low luminance efficiency at red and blue wavelengths, and low light extraction efficiency. One of the approaches to improve these parameters is to use the metal nanoparticles (MNPs) [14], which have resonant absorption and scattering at optical frequencies and offer unique ways of extracting light [15,16]. The impact of planar noble metal layers on the fluorescence intensity and lifetime has been comprehensively studied [17–22] and plasmon-enhanced PL has also been observed for the emitters near the colloidal MNPs [23–27]. Enhanced emission from dye-doped-polymer composite films has been achieved by depositing these films on gold (Au) and silver (Ag) NPs layers [22]. In the case of colloidal MNPs, however, the post-processing of these colloidal NPs onto targeted substrates or into aimed objects is complicated [28]. In this regard, instead of producing high-quality MNPs in the solution phase, sputtered MNPs films fabricated from controlled sputtering and annealing process are more suitable for device integrations [29,30].
Here in this work, we report on the improvement of light emission from the in-situ fabricated PQDs-PAN composite films on a substrate with 10 nm-30 nm sputtered AuNPs. The enhancement of the PL depends on the size and density of the plasmonic NPs layer, and a polymethyl methacrylate (PMMA) buffer layer introduced between the AuNPs and PQDs-PAN. We show that the buffer layer diminishes the influence of the plasmonic NPs on the radiative and non-radiative decay rates of the PQDs-PAN while saving the positive effect of the nanotexturization of the surface and, therefore, increases the reflection and absorption of the emissive layer. The highest achieved PL enhancement was 1.8-folds with solely AuNPs and a 2-folds enhancement was achieved by adding the PMMA layer. Observed enhanced PL of PQDs-PAN deposited on the AuNPs/PMMA substrates results in enhanced light extraction and can be applied in silicon solar cells, LCDs backlights, and photo-illuminated energy converting devices.
2. Samples and methods
2.1. Preparation of AuNPs films
The fabrication process of sputtered AuNPs films was similar to our previous work [30]. Au source disk was set 4 cm above the aimed substrate. The chamber was vacuumed to 10 Pa and the sputtering process was initiated at 10 mA current with various time lag of 10 s, 20 s, 30 s and 40 s, respectively. Then the sputtered Au films were annealed at 280 C for 20 min to separate the NPs and make them more spherical. A buffer layer was prepared on the glass substrate and on sputtered AuNPs substrates by spin coating of PMMA 2 wt. % solution in toluene.
2.2. In-situ fabrication of the PQDs-PAN composite films
The in-situ fabrication process of the MAPbBr3-based PQDs-embedded PAN films was used as reported previously [13] with little modifications. A precursor solution was prepared by adding 92 mg MABr, 143 mg PbBr2, and 1 g PAN powder in 8 mL dimethylformamide (DMF). The precursor solution was stirred vigorously for 10 h until all visible opaque matter disappeared. The well-sealed precursor solution was spin-coated on the glass substrate at 4000 rpm for 75 s and dried in a vacuum oven for 30 min at 45 C to remove the solution DMF. Under drying, the precursor films gradually changed into emissive composite films.
2.3. Characterizations
Scanning electron microscopy (SEM) images were taken on an S-4800 microscope (Hitachi, Ltd., Japan). Ultraviolet-visible (UV-Vis) extinction spectra were measured on a UV-6100 spectrophotometer and PL spectra were obtained using an F-380 fluorescence spectrometer. Atomic force microscope (AFM) measurements were performed using a solver scanning probe microscope in tapping mode. The measurement of the reflection spectrum of the sample was performed under the reflection detection method of ARM (Angle-resolved Spectrum System in Micro-region, Idea Optics, China). Where the incoming angle of the laser light source could vary from 0 to 60° (the specific value was related to the parameters which had been set during the measurement), and is also always related to the reception angle of the spectrometer.
2.4. Angular emission measurements
The 405 nm wavelength laser beam (Laser Inc., Ningbo, China) was used to excite the as fabricated composite films. The emitted light was received by a spectroradiometer (Spectra Scan Spectroradiometer PR-655, Photo Research, Inc.). By rotating the spectrometer, the emitted intensity at different angles could be obtained. The angular emission of the light from the device had been measured using a self-made lab setup, depicted in Fig. 1. In this setup, a 405 nm continuous wave laser beam was fallen on the device at a specific angle of incidence. The PR-655 spectroradiometer was used to detect the scattered light, which was placed on a goniometer and could be rotated to collect the scattered light at angles with 15° separation. The spot size of the laser beam was a few millimeters in diameter. This technique of light scattering measurements, which is a powerful tool for characterization of surface nano-texture, gives information about a surface area in a range broader than that of AFM analysis.
2.5. Simulation methods
Numerical simulations for power absorption and dipole orientation were performed with the help of commercial software from Lumerical, based on the finite difference time domain method. The absorbed power was calculated using the following formalism:
where ω is angular frequency, |E| is the modulus of the electric field magnitude and ɛ is the dielectric permittivity. For the layer without AuNPs, the integral power absorbed in a layer was calculated. The zone of interest was mesh-gridded by a mesh refinement of 1 nm. For layers containing AuNPs, firstly we retrieved the 3D-matrix of electric field. Then we separated the part of the matrix representing only the perovskite layer with the use of a refractive index data corresponding to each point of the mesh. Using the Eq. (1), we then calculated the power absorbed by perovskite layer deposited on AuNPs [31].The scattering simulation was done by taking into account the inter-particle interactions. Simulation of scattering for plasmon-enhanced perovskite-polymer composite films was done by solving the Maxwell equations using the COMSOL multiphysics (COMSOL Inc., Burlington, MA). We performed it for PQDs-PAN composite film when it was surrounded by a layer of PMMA, and/or AuNPs. At a wavelength of 405 nm, the dielectric constants of PAN, PMMA, and AuNPs are 1.56, 1.49 and -1.65 + 5.65j, respectively.
3. Results and discussion
Here we discuss the size and density of AuNPs sputtered on a glass substrate and their absorption and scattering properties. Then we analyze the PL enhancement for the device fabricated on these substrates. Important factors like reflectance/absorption in the presence of AuNPs/PMMA, which have a concrete impact on the PL enhancement from PQDs-PAN composite film, are further discussed in detail.
3.1. AuNPs substrates
Noble metal NPs have excellent scattering properties and resonantly absorb visible light due to the excitation of the localized surface plasmon resonances (LSPR) at optical frequencies [26,32,33]. The SEM micrographs of as-prepared sputtered substrates after thermal annealing are presented in Fig. 2(a-c) for sputtering times of 10 s, 20 s, 30 s and that for sputtering time of 40 s, comes in Fig. 9(a) of Appendix A, respectively. The average particle densities of these samples are approximated as Nd-A ∼6.3 AuNPs /100 nm2 (film A), Nd-B ∼7.3 AuNPs /100 nm2 (film B), and Nd-C ∼9.7 AuNPs /100 nm2 (film C) and Nd-D ∼3.5 AuNPs /100 nm2 (Film D).
Fig. 2. AuNPs size and density observations: (a-c) SEM micrographs for AuNPs densities Nd-A, Nd-B, Nd-C grown for various sputtering times, (d) Extinction spectra of AuNPs films, (e) Simulated scattering intensity of AuNPs for various NP spacing.
After Au sputtering, uniform and discrete AuNPs (or clusters) appear on the substrate surface on annealing. When the sputtering time increases, the surface coverage also increases, and the clusters get in close contact with each other. After the surface is fully covered, we observe vertical layer growth, while its horizontal expansion is controlled by cluster boundary motion. The average particle size in films A-C are 19 nm, 21 nm, and 26 nm, respectively. With the increase in size, the average inter-particle spacing among AuNPs of films A-C is simultaneously reduced, from 21-6 nm (Appendix B Fig. 10(d)). The number density (Nd) of AuNPs increases from 6.3 to 9.7 AuNPs per each 100 nm2, for Nd-A to Nd-C of the sputtered substrates, referring Nd-A as least dense and Nd-C as densest substrate for this study. It is observed that a sharp peak at 519 nm for the less dense film A, at 520 nm for the film B and at 525 nm for the film C are obtained. The effect of the varied size and Nd of sputtered AuNPs on the position and intensity of LSPR peak in the extinction spectra of the films is depicted in Fig. 2(d).
The extinction peaks of AuNPs red-shift with the increase in particle size, exhibiting a size dependence of the LSPR [34–36]. The extinction peak positions for films A-C are in range with the emission wavelength of the PQDs-PAN, indicating strongly coupled LSPR effect. However, it is noted that the average particle size in film D remarkably increased to 41 nm, leading to obvious red-shift of the extinction peak, as shown in Fig. 9(b) of Appendix A, which ultimately gives rise to weaker coupling for the LSPR effect.
Comparative analysis of SEM micrographs of films A-C shows that roughly spherical shaped AuNPs are uniformly formed over the entire surface of the glass as the sputtering time increases, meanwhile their density (Nd) increases rapidly along with a slight increase in the size. Besides, this increase in Nd of AuNPs also increases the thickness and roughness of the Au layer (Appendix B Fig. 10(a-c) depicts this phenomenon).
Based on the measured average inter-particle spacing among AuNPs (Appendix B Fig. 10(d)), the scattering effect of the AuNP substrates is investigated by applying electrical scattering intensity simulations at the angle range from -90 to 90 for AuNPs with different NP spacing closer to our experimental range (0 nm–20 nm). As shown in Fig. 2(e), without an inter-particle spacing, the scattering effect of AuNPs is highest and it decreases rapidly as particles get apart by 10 nm-20 nm space.
3.2. Analysis of PQDs-PAN composite films
3.2.1. Influence of PMMA buffer layer
Figure 3(a) depicts the schematic interpretation of composite films without & with PMMA buffer layer fabricated on Au sputtered substrates (SEM cross-section image of the later has been shown in Appendix C), which explains different processes occurring in the system upon light excitation. Here, significant terms are defined as a small fraction of incident light intensity reflected from PQDs-PAN surface (Ro), from the interface between PQDs-PAN and PMMA (R1), from the interface between PMMA and AuNPs (Io), interference between PMMA & PQDs-PAN (I1) and a surface plasmon-exciton coupling (Co).
Fig. 3. (a) Schematic model of the devices without (on the left side) and with PMMA (on the right side), (b-c) PL enhancement of the devices without and with PMMA respectively.
Without the PMMA buffer layer, more AuNPs are immersed in the top film of PQDs-PAN, where the scattering from these particles enhances pathways of interacting light into PQDs-PAN film. As a result, more excitons are generated to fasten Co process, to cause their rapid spontaneous decay. The PQDs-PAN integrated among AuNPs are excited rapidly and decay instantly because of the strong electric field among AuNPs. Along with scattering, diffused reflection from AuNPs is significant when the density and size of AuNPs are increased.
Utilizing a PMMA buffer, magnitude of Co is reduced from its maximum to minimum as the Nd of AuNPs increases from Nd-A to Nd-C and it verifies the dependence of the plasmonic influence on the separation between emissive and plasmonic material as discussed earlier by many researchers [37–39]. The scattering/reflectance (R1) effect of the PMMA layer results in the PQDs-PAN fluorescence enhancement surrounded by PAN. Hence further increase in the density of AuNPs to Nd-D has not given the continual enhancement of PL as shown in Fig. 12 of Appendix D. Also when the Nd of AuNPs increases the boundaries per unit area are increased and penetration depth of incident light decreases, leading to an increase in its reflectance and decrease in transmittance (To) (see Fig. 13 Appendix D for the details).
Figure 3(b-c) show PL emission enhancement for devices without and with PMMA. One may see that although the PL enhancement for the film without PMMA is higher (1.42 times for film A, 1.61 times for film B and 1.8 times for the film C), the absolute PL intensity is higher for the case with PMMA. The PL intensity from the PQDs-PAN on the PMMA film is higher by 1.13 times than without PMMA, which can be connected with the surface properties of the PMMA layer and Au, respectively.
The increase in intensity in our case may be attributed to several near-field and far-field effects. When the PQDs-PAN is deposited directly to the AuNPs, PL intensity may benefit from the local field enhancement, however, took disadvantage of the increase in non-radiative decay rate. We then examined the role of reflection, absorption, and reabsorption in the PL enhancement for both systems with and without the buffer layer.
3.2.2. Reflectance analysis
To evaluate the contribution of the reflectance in PL emission enhancement, we performed the reflectance test of the devices (without and with PMMA) for various Nd and sizes of AuNPs. Without PMMA the reflectance raises as Nd of AuNPs is increased and goes up from 22% without Au to 28%, 33% and 37% (Fig. 4(a)). While with PMMA reflectance increases from 25% to 31%, 37% & 44% respectively, as shown in Fig. 4(b).
Fig. 4. (a-b) Experimental reflectance from the devices with AuNPs (NdA - NdC) without and with PMMA, respectively, (c-d) Represents experimental reflectance vs theoretical reflecting intensity at peak emission for the respective devices.
Theoretically approximated intensity reflectance is compared with the reflectance percentage of the samples and plotted in Fig. 4(c-d). Here we find the linear-like trend of reflecting intensity enhancement and is more dominant for Nd-C along with PMMA and has less effect for the system without PMMA. Thus for Nd-C with PMMA, reflected power is strong enough to cause excitations in the emissive layer, it can work as back striking exciting energy, which contributes to the enhanced PL emission.
3.2.3. Absorption analysis
The reflectance of incident light helps to recover the absorbed incident power in the emissive layer by structuration of the substrate. To evaluate how much power is absorbed by the emissive layer in the presence of AuNPs, we have calculated the power absorption (PA) percentage at the excitation wavelength (405 nm) by the PQDs-PAN film which has been explained in Fig. 5.
Fig. 5. (a-b) Theoretical PA of PQDs-PAN films without and with PMMA, respectively, (c) PA of PQDs-PAN film on flat and structured (with AuNPs) substrates.
Figure 5(a-b) shows the cases when the PQDs-PAN is coated on the glass substrate and on various Nd of specific sized AuNPs without or with PMMA layer. The calculated percentage represents the PA only by PQDs-PAN, and the PA of AuNPs is excluded. We note that by varying Nd of AuNPs the PA is enhanced from 42% (without Au) to 56%. This enhancement may be dedicated to increased absorption of the layer due to the structurization of the surface and scattering from AuNPs [38]. The increase is not attributed to the near field enhancement as the distance between PQDs-PAN and AuNPs is increased due to the PMMA and is too far to have near field enhancement especially for non-resonant conditions. To evaluate the impact of the nanostructurization we calculated the PA of the PQDs-PAN coated flat substrate and substrate with a topology of the Au film (Fig. 5(c)). The structured substrate can absorb more light, for example, due to an increase in the multiple scattering.
We clearly see that the PA of structured PQDs-PAN (43% to 54%, Fig. 5(c)) is lower to the case of AuNPs/PMMA/PQDs-PAN (Fig. 5(b)) but higher than that of AuNPs/PQDs-PAN (Fig. 5(a)). Hence, when the PQDs-PAN layer is deposited directly on the AuNPs film, we benefit from the structuration and at the same time lose the energy on AuNPs due to inter-band absorption, which can be recovered by the PMMA buffer layer. Based on these enhanced properties of the PQDs-PAN film quantum efficiency (QE) of the device with PMMA is significantly enhanced (see Appendix E for the details). It makes this structure applicable to photodetectors and other photo-illuminated energy converting devices.
3.3 Angular emission investigations
The angular emission from the films in the angular range from -90 to 90 was further tested. Theoretical angular scattering properties and angular dipole orientation from the Au sputtered substrates were carried out to evaluate the angular PL emission of the films.
3.3.1. Angular PL emission
Figure 6(a-b) show the enhanced angular PL emission for samples without and with PMMA in the angular range of -90 to 90 respectively. For devices without PMMA the enhanced PL of 0.8-fold is attributed to Co and is more inclined to the direction of 0 (shown in Fig. 8). It gradually weakens for emission in the higher angles. But the maximum emission is squeezed in the range of -25 to 25 due to orientation of dipoles in the 0 and magnitude of Co enhances in this direction effectively. While for devices with PMMA, a higher enhanced PL of 1.26-fold in the direction of 0 is attributed to the reflectance from PMMA as previously discussed. Under the influence of reflectance and scatterings, the enhanced emission width is also enlarged in the arc of angles from -30 to 30 .
Fig. 6. Experimental angular PL intensity for the devices without (a) and with PMMA (b) respectively.
3.3.2. Theoretical analysis of angular emission
Theoretical analysis has been done to understand trend of experimental angular PL emission. One can see from Fig. 7, that scattering of the emitted light rises in the direction of the 0 after AuNPs deposition. Then the direction of scattered light is more aligned to the detector at this angle, as observed in Fig. 7(a), and scattering intensity is increased from 11-13 units in 0 , 9.25-10.8 in 15 . The effect of the AuNPs on the angular PL emission may originate from several processes which not only scatter the incident light but also enhance the PL emission by the impact of Co. The system with a PMMA layer provides the increased scattering intensity from 12-14 units in 0 , 9.9-11.8 units in 15 , 6-6.5 units in 30 (Fig. 7(b)) which may contribute to the enhanced experimental PL emission in the direction of 0 and in the enlarged arc of angles from -30 to 30 . Overall scatterings to the incident light are increased by only 2-units for densest AuNPs with Nd ∼Nd-C in the direction of 0 detection and there is a unit enhancement to overall scatterings when we added PMMA over Nd-C.
Fig. 7. Simulated scattering intensity for the emitted light at 520 nm for devices (a) without PMMA and (b) with PMMA.
Theoretical evaluation for samples reveals that for the densest AuNPs (Nd-C) such enhancement in the emission is dedicated to the two factors 1) increase of scatterings/reflectance from the interface and 2) enhancement of the spontaneous emission decay without PMMA. In the presence of PMMA the latter factor is replaced by the reflectance of the layer. Therefore, for the device with PMMA, higher number of excitons are generated in the emissive layer due to reflectance from the layer below and Co contribution, then more emission is recorded in 0 as well as in other closer angles.
When the emitter is placed in the vicinity of the AuNPs the change in the emission rate and also in the emission directivity happens due to the antenna effect. A critical parameter for this effect is the orientation of the emitter transition dipole relative to the surface of the MNP.
Simulation results reveal that the strongest emission is observed for a dipole oriented parallel to the substrate interface whilst normally to the AuNPs surface (Fig. 8(a)), however in the experiment we have the averaged situation depicted in Fig. 8(b). The emission patterns of the dipoles oriented parallel to the substrate are similar and only have the intensity modulation due to the AuNPs in the vicinity.
Fig. 8. (a) Simulated far-field angular emission for the different dipole orientations near the AuNPs (Nd-B), (b) Simulated far-field angular emission for the averaged dipole orientation from various AuNPs sputtered substrates.
In the studies of the MNP’s influence on the emission rates of the dipoles, its orientation distribution is assumed to be isotropic or to have an optimal orientation [22,40,41]. In averaged case the emission intensity increases in 1.78 times in the front direction (Fig. 8(b)). As a result, incorporation of such structure in energy conversion devices may increase energy absorption of the active layer and can boost the power conversion efficiency.
4. Conclusion
We have observed that the PL emission of PQDs-PAN polymer composite films can be enhanced by using plasmonic NPs and PMMA buffer layers. We explored that scatterings from sputtered substrates can be tuned by changing the density along with slight increase in the size of AuNPs. These sputtered substrates acting as a metallic structurization for enhanced light extraction, while the presence of PMMA as a back reflecting agent serves to further boost it up. The combined effect of scatterings/reflectance from AuNPs/PMMA enhances PL emission by 2-folds. It is further demonstrated that the PMMA layer enhances the angular emission patterns in the horizontal directions. This observation serves to contribute to the geometric tuning of the SPR and enhancement of the emission of perovskite-polymer composite film using sputtered AuNPs. The optimization of the AuNPs/polymer layer stack with this strategy can result in high-quality LCD backlight emission, SSCs, and photodetectors.
Appendix A: SEM and extinction analysis of sputtered AuNPs for 40 s
There is thickness limitation of Au film to get similar plasmonic resonance peak. With increase in the sputtering time, thickness of Au film is increased and size of AuNPs is not controlled by annealing. On comparing Fig. 2(a-c) with Fig. 9(a), we can clearly see that, size of AuNPs for Nd-D is bigger but for Nd-A – Nd-C AuNPs are almost of the same size. The average AuNPs size in films A-C are 19 nm, 21 nm, and 26 nm, respectively. It is noted that the average AuNPs size in film D remarkably increased to 41 nm. Due to this size increment the extinction for Nd-D gets big red-shifted as shown in Fig. 9(b), which is not beneficial condition for LSPR enhanced emission of our emissive composite film.
Fig. 9. (a) SEM for 40 s (Nd-D-film D) of AuNPs (b) Red-shift in the extinction for 40 s sputtering time.
Appendix B: Density (Nd) of AuNPs analysis with AFM
Atomic force microscopic (AFM) images of sputtered AuNPs are shown in Fig. 10, where Nd-A, Nd-B, Nd-C of AuNPs are shown in Fig. 10(a), (b) and (c) respectively. From Fig. 10(c), it is clear that particle boundaries are expanding and inter-particle spacing is decreased. AFM spectral calculations showed an increase in root mean square roughness along with an increased number density of AuNPs. Which showed that more nanoparticles undergone with less space among them, which can cause scattering.
Figure 10(d) showing a plot of particle boundary variation with increased sputtering time. On analyzing AFM images and comparing surface morphology of AFM images we find spacing among particles is reducing with a slight increase in the size of AuNPs and a fast increase in the number of grown-up AuNPs with the increased sputtering time.
Fig. 10. (a-c) AFM of AuNPs sputtered for 10s (Nd-A), 20s (Nd-B), 30s (Nd-C) respectively. (d) Variation of AuNPs boundary with respect to the sputtering time.
Appendix C: SEM cross-section analysis of the device
Figure 11 is showing SEM cross section image with AuNPs. It is clearly seen from the SEM image that PQDs-PAN composite film is stacked uniformly over PMMA layer, it shows that PMMA/Au interface is following the particle structure of AuNPs up till their immersion into PMMA and PMA/PQDs-PAN is smooth, hence top side of device is very smooth. It can provide very uniform base for the coating of next layer in the fabrication of solar cells.
Appendix D: PL emission and transmittance analysis of the devices for Nd-A –Nd-D of sputtered AuNPs substrates
The further increasing the sputtering time to 40 s, the AuNPs size is bigger and results in a red-shift of the plasmonic maximum (Fig. 9(b)) and then it is not beneficial for the further enhancement of the PL.
Without PMMA incorporation, such remarkable LSPR red-shift for Nd-D results in the decrease of the PL enhancement comparing to Nd-C as shown in Fig. 12(a). With PMMA and Nd-D, the enhancement of the PL is still less than for Nd-C as shown in Fig. 12(b). The PMMA layer thickness large increment may reduce the plasmonic effect of AuNPs on the PL, as the electromagnetic field near the AuNPs becomes weaker away from its surface. So, PMMA of specific thickness reduces PL quenching and improves the back reflectance of the emitted photons.
Fig. 12. (a) PL emission for Nd-A to Nd-D of sputtered AuNPs without PMMA (b) PL emission for Nd-A to Nd-D of sputtered AuNPs with PMMA.
The transmission of the device depends upon the thickness of the layers as well as the nature of interfaces involved in it.
With increasing sputtering time transmittance of the devices also decreases. In our device without PMMA transmittance is reduced from 64% without-AuNPs to 37% with AuNPs with Nd-D (without PMMA) Fig. 13(a). With PMMA as back reflectance is increased so transmittance is reduced from 53% to 20% and is lesser than that without PMMA Fig. 13(b). A detailed study of transmittance is not part of the present work, would be studied next.
Appendix E: Theoretical investigation of QE enhancement
Quantum efficiency is enhanced by enhancing the density of the number of particles [42,43]. The simulated QE remains high for a wide range of gap thicknesses greater than 6 nm, so we found QE enhanced from 12.22 to 13.55 by adding PMMA to the AuNPs layer as shown in Fig. 14.
Fig. 14. (a)-(b) Simulation of QE for devices over glass and Au and for devices over PMMA and over PMMA containing Au at its bottom, respectively.
Furthermore, the QE at the position of highest emission rate is also high, pointing to the possibility of integrating single emitters. Notably, due to the large emission rate enhancements, large enhancements in the radiative rate are possible.
Funding
National Natural Science Foundation of China (21603012, 2171101421, 61722502).
Acknowledgments
We want to thank Dr. Linghai Meng for helping in sample fabrication and we acknowledge Belarussian Foundation for Fundamental Research.
Disclosures
The authors present no conflict of interest.
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