ZnS Quantum Dots Induced by

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Energy Conversion and Storage; Plasmonics and Optoelectronics

Emission Enhancement from CdSe/ZnS Quantum Dots Induced by Strong Localized Surface Plasmonic Resonances without Damping Yingcheng Wang, Yuanhao Jin, Tianfu Zhang, Zhongzheng Huang, Haitao Yang, Jiaping Wang, Kaili Jiang, Shoushan Fan, and Qunqing Li J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b00818 • Publication Date (Web): 16 Apr 2019 Downloaded from http://pubs.acs.org on April 16, 2019

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The Journal of Physical Chemistry Letters

Emission

Enhancement

from

CdSe/ZnS

Quantum Dots Induced by Strong Localized Surface Plasmonic Resonances without Damping Yingcheng Wang1,2, Yuanhao Jin1,2, Tianfu Zhang1,2, Zhongzheng Huang1,2, Haitao Yang1,2, Jiaping Wang1,2, Kaili Jiang1,2, Shoushan Fan1,2, and Qunqing Li1,2* 1State

Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics &

Tsinghua-Foxconn Nanotechnology Research Center, Tsinghua University, Beijing 100084, China 2Collaborative

Innovation Center of Quantum Matter, Beijing, China

*Corresponding author. Tel.: +86 10 62796019; Fax: +86 10 62792457. E-mail address: [email protected]

KEYWORDS: LSPR, quantum dots, nanostructures, coupling, photoluminescence enhancement factor

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ABSTRACT A high-performance exciton-localized surface plasmon (LSP) coupling system consisting of well-designed plasmonic nanostructures and CdSe/ZnS quantum dots was fabricated by first introducing a Ta2O5 layer as both an adhesive coating and coupling medium. It is shown that a larger emission enhancement factor of 6 from CdSe/ZnS quantum dots (QDs) can be obtained from the strong coupling effect between QDs and tri-prism Au nanoarrays, and the high scattering efficiency of LSPs without damping. This can be attributed to the matching conditions and a low extinction coefficient with little damping absorption of the Ta2O5 layer in the system. The radiative scattering rate of ΓLSPs can make a contribution to the spontaneous emission rate Γ and, thus, improve the internal quantum yield of the QDs. This strategy could be promising for practical application of metal-modified fluorescence enhancement.

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Noble metallic nanostructures of subwavelength size are capable of supporting localized surface plasmon (LSP). LSP can be considered to be the collective oscillations of free electron gas at the interface between a conductive nanostructure and a dielectric, which can be excited by an incident electromagnetic field.1–4 The strong coupling between light and LSP has a remarkable effect on the characteristic optical near field in the metallic nanostructures, whose energy and distribution are highly dependent on the geometric shape and size of the metal nanostructures, interparticle gap size, and the surrounding medium environment.4–6 The LSPs can be further optimized to provide for a resonance with a specific wavelength of light via the adjustment of nanostructural parameters, termed localized surface plasmon resonance (LSPR). A markedly large enhancement of the local electromagnetic field in the metallic nanostructures can be obtained, and the light-matter interaction at the nanoscale can also be modulated during the occurrence of LSPR.6–9 Based on their interesting optical properties, LSP resonances have been demonstrated in a large number of applications, including biochemical sensing, surface-enhanced Raman scattering (SERS), and fluorescence enhancement.6,10–14 Recent advancements in the syntheses of colloidal quantum dots (QDs) have contributed to rapid growth in research of nanophotonics and the basic aspect of light-matter interaction.15–18 Colloidal QDs are a class of semiconductor nanocrystals that can be fabricated and processed based on solution techniques.15,16 They exhibit excellent and unique optoelectronic properties relative to other luminescent materials on account of their inherent performance derived from the quantum effect, such as narrow spectral emission linewidth, low manufacturing cost, high photostability, and compatibility with solution systems. Above all, the full-range color can be easily and precisely tuned by ACS Paragon Plus Environment


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variation of the size, composition, and surface state of the QDs.19–21 These attributes have made them one of the most promising materials for fluorescent bioassays, solar cells, and light emitters in optoelectronic devices.22–25 Considerable efforts have hence been made toward techniques that can more efficiently excite the light source of QDs and significantly modify the emission properties, which could lead to improvements in the sensitivity of biological assays and increase the quantum yield in optoelectronic applications.26–30 Interactions between QDs and LSPs represent a topic of significant interest.17,29,30 To date, experiments and theoretical studies have demonstrated that the radiative and nonradiative decay rates of chromophores near the metallic nanostructures can be altered, thereby affecting their emission.31–33 The photoluminescence (PL) intensity of QDs can be enhanced or quenched by the electromagnetic interactions with LSPs, depending on the detailed configurations of the device consisting of a plasmonic system and QD film.33,34 The role that LSPs play in plasmon-enhanced fluorescence is currently under intensive investigation, wherein the focus has concentrated on promoting the coupling between the surface plasmon polaritons and QDs, and improving the PL enhancement factor.35,36 In general, two main factors are primarily responsible for surface plasmon-mediated fluorescence enhancement: excitation enhancement and emission enhancement. The former is based on the optical excitation of localized surface plasmon, increasing the excitation rate of QDs by the generation of “hot spots”. The latter involves the coupling of emitter exciton and surface plasmon, which can alter radiative and nonradiative decay rate of QDs and improve the emission efficiency.36 Related to the latter, much attention has been paid to spectrally tuning the LSP or Surface Plasmon Polariton (SPP) resonances by patterning periodic metallic nanostructures and balancing enhancement and quenching of ACS Paragon Plus Environment

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QD PL by inserting a spacer between the QDs and plasmonic metal,33,36,37 but little work has been undertaken to restrain the damping of LSP in the coupling system, i.e., an enhanced radiative rate with the highest coupling efficiency and the smallest absorption, which affects the plasmon-enhanced emission from QDs. We herein report on an exciton-LSP coupling system that consists of engineered plasmonic nanostructures and CdSe/ZnS quantum dots (see Figure 1). The plasmonic nanostructures are designed through finite difference time domain (FDTD) simulations and an electron beam lithography (EBL) technique, in which Ta2O5 is introduced as both an adhesive coating and coupling medium. CdSe/ZnS QDs are assembled onto the LSP device with the well-defined placement of polymethyl methacrylate (PMMA) spacer between the metallic nanostructures and QDs to compose the exciton-LSP coupling system. The effect of the thickness of PMMA spacer on the sensitivity of enhanced PL was also examined, and this has been demonstrated by earlier works.34,38,39 It is found that the introduction of Ta2O5 on the plasmonic nanostructures plays a critical role in the intense PL enhancement of CdSe/ZnS QDs. Ta2O5 has a low extinction coefficient with a small absorption factor, and the introduction of it into the metallic nanostructures can efficiently couple light into LSP with strong resonance because of its matching dielectric constant. This contributes to the enhanced radiative rate and, therefore, the emission enhancement. Both the experiments and simulated results show that this strategy can dramatically improve the total fluorescence efficiency of QDs in an exciton-LSP coupling system compared to that in a system using conventional adhesive coating, e.g. Cr and Ti. The fluorescent enhancement in the as-fabricated exciton-LSP coupling system was found to reach a maximum at a spacer thickness of 40 nm. An overall maximum PL enhancement ACS Paragon Plus Environment


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by a factor of 6 was observed in the system with Ta2O5 adhesive coating, which is two times larger than that with Cr adhesive coating. It is believed that the strategy around this LSPR-based fluorescence enhancement may aid the development of a wealth of biomedical applications and improvement in optoelectronic device performances. Specifically, periodic arrays of Au tri-prism nanostructures with a sharp metal tip were employed. The first aim is to tune the LSP resonance of the plasmonic nanostructures to a wavelength around 590 nm by controlling their size, the arrangement on the substrate, and the annealing process, which can be well matched with the emission wavelength of CdSe/ZnS QDs. The FDTD method is considered an effective means of calculating the optical resonance of noble metallic nanostructures. Furthermore, a standard transmission setup can be used to evaluate the LSP resonances. We, therefore, strove for the far-field transmission spectra of localized surface plasmon devices with different Au thicknesses to determine the resonance. The Au tri-prism nanostructures are sketched in Figure 1, in which a double array with staggered pattern was created where the samples have a period of 500 nm and an edge length of 100 nm. Figure 2(a) shows the simulated results for transmission spectra of the plasmonic tri-prism nanoarrays with a series of Au thicknesses from 30 nm to 120 nm. From this we can see that every sample exhibits a transmission valley at visible frequencies, revealing strong resonances of the plasmonic nanostructures at the corresponding wavelength. As the Au layer thickness is increased the plasmon resonance wavelength shifts towards the blue and exhibits multiple modes from a single mode.


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Figure 1. (a) A three-dimensional and (b) cross-sectional schematic illustration of the exciton-LSP coupling system consisting of plasmonic nanoarrays and QD film. In the experiments, plasmonic Au tri-prism nanoarrays were designed through an EBL technique, followed by the e-beam deposition of Au thin film. Considering that the annealing process can yield a better crystallinity and clear corner of noble metallic nanostructures, and subsequently has a good effect on their thermal stability and optical properties, strong coupling between the generated LSPs and QDs can be obtained with a slight blue-shift of the resonance.40,41 We fabricated the localized surface plasmon devices according to the FDTD optimization of the nanostructures and annealing processes to get a LSP resonance of 590 nm. Ultimately, a thickness of 40 nm Au was selected to construct the LSP device with a period of 500 nm and edge length of 100 nm. For the following synthesis of exciton-LSP coupling system, a 3-nm thick adhesive coating is necessarily introduced between the Au layer and the substrate. A thin film of Ta2O5 was deposited to be used as both an adhesive coating and coupling medium for the first time. For comparison, the LSP devices with a conventional adhesive coating of Cr layer were also fabricated. UV-vis transmission spectra were obtained using a microspectrometer (CRAIC 308 PV) in the range 400–800 nm. Figure 2(b) shows the transmission spectra of the tri-prism Au nanoarrays with Ta2O5 and Cr adhesive coating, from which we can see that ACS Paragon Plus Environment


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the transmission of the LSP device with Ta2O5 adhesive coating has a smaller full width at half maximum (FWHM) and lower transmissivity compared with that of the system with Cr adhesive coating, revealing the stronger resonant intensity of the LSP device that used Ta2O5 film as an adhesive coating. Furthermore, the LSP devices with either Ta2O5 or Cr possess the designed resonance wavelength of about 630 nm before annealing treatment, which is consistent with the calculated results. The as-fabricated devices were annealed at 180 °C for 2 h under hydrogen gas ambient conditions to form a clear corner of nanostructures with better crystallinity and high thermal stability. This is favorable for coupling the generated LSPs with QDs and enhances the PL of QDs. The changes in optical properties are shown in Figure 2(c), where a blue-shift from 630 nm to about 590 nm can be observed in the annealed samples. Meanwhile, a stronger LSP resonance of the device with Ta2O5 adhesive coating can be typically seen, resulting from the decrease in plasmon transmissivity in the corresponding resonant wavelength after annealing treatment, while there is some increase in transmissivity and, therefore, a decrease in the resonant intensity of the LSP device with Cr adhesive coating, as shown by the arrows in Figure 2(c). Additionally, the same conclusion is reached that the LSP device with Ta2O5 adhesive coating has a much stronger plasmon resonance compared with the device with Cr adhesive coating after annealing, resulting from the lower extinction coefficient with a small absorption of Ta2O5 film and its matching dielectric constant with the Au layer in the excitation of the localized surface plasmon. Figure 2(d) shows representative SEM images of the tri-prism Au nanoarrays before and after annealing. A clear and smooth corner of nanostructure can be seen from the right image (with annealing process). Therefore, a reliable and ACS Paragon Plus Environment

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high-performance LSP device with a well-defined plasmon resonance of about 590 nm was achieved by the introduction of Ta2O5 adhesive film, wherein the resonant wavelength would be matched well with the emission wavelength of CdSe/ZnS QDs. Further, the fluorescence enhancement in the case of QD emission and the matching LSP resonance are striking.

Figure 2. (a) Simulated transmission spectra of the plasmonic tri-prism nanoarrays for different Au thicknesses and with an edge length l = 100 nm and period p = 500 nm. (b) Experimental and calculated transmission spectra of the tri-prism Au nanoarrays with Ta2O5 and Cr adhesive coating. (c) Transmission spectra of the plasmonic tri-prism Au nanoarrays with Ta2O5 and Cr adhesive coating before (blue trace) and after (red trace) annealing treatment. (d) SEM images of the tri-prism Au nanoarrays with Ta2O5 adhesive coating before (left) and after (right) annealing treatment. Structural parameters for (b)–(d): height h = 40 nm, edge length l = 100 nm and period p = 500 nm.



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To explicitly construct the high-performance exciton-LSP coupling system, a PMMA layer was used as a spacer between the metallic nanostructures and QDs. The plasmon-based fluorescence enhancement factor (EF) exhibits nonmonotonic character with the spacer distance, whose correlation has been demonstrated earlier using different fluorescent material systems. When the fluorescent material is in close contact with metallic nanostructures, PL quenching of semiconductor nanocrystals would happen due to nonradiative energy transfer from the fluorophore to the metal. The expected and maximal enhancement in PL of a fluorescent material can be obtained at an optimal spacer distance. Further increase in the spacer distance results in the decrease of PL intensity because of the short-range effect of LSP.34 We also examined the effect of the thickness of PMMA spacer on the sensitivity of the enhanced PL, searching for a maximum boost in the PL intensity of the CdSe/ZnS nanocrystals. The PMMA layer was first spin-coated onto the LSP devices, see the AFM image shown in Figure 3(a). The corresponding cross-sectional plots in Figure 3(d) illustrate the in-plane configuration and out-of-plane height of the LSP devices, in which the plasmonic tri-prism Au nanoarrays exhibit the designed period and edge length with about a 3-nm thick Ta2O5 adhesive coating and an Au layer thickness of 40 nm. The spacer thickness was controlled by the processing parameters in the spin-coating and thinning treatment with reactive ion etching (RIE), as a representative AFM scan in Figure 3(b) shows. In addition, a flat surface with low roughness of PMMA layer can be seen from Figure 3(e), which is also used as a planarization layer on the LSP device, facilitating assembly of the CdSe/ZnS QDs.


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Figure 3. AFM images of the plasmonic tri-prism Au nanoarrays before (a) and after (b) coating with a PMMA layer. (c) CdSe/ZnS quantum dots on the surface of a plasmonic device coated with a spacer layer of PMMA. (d)–(f) Corresponding cross-sectional plots (yellow dashed line) illustrate the in-plane configuration and out-of-plane height of the exciton-LSP coupling system. The CdSe/ZnS QDs have a fluorescence emission centered at ~590 nm with a FWHM of 30 nm and a crystal diameter of around 4.3 nm. These were first diluted by toluene from a bulk concentration of 12 μM to the target concentration of 1 μM, and completely dispersed under thorough ultrasonication for approximately 30 min. Then, the same amount of CdSe/ZnS QDs solution was spread on the surface of each plasmonic device coated with different spacer thicknesses of PMMA to form a thin film by spin coating, followed by being cured at 150 °C for 10 min under vacuum. During the process, avoiding aggregation of QDs and their assembly on the device surface with high uniformity are crucial to the subsequent characterization and detection of PL from the exciton-LSP ACS Paragon Plus Environment


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coupling system. Therefore, the spin-coating process carefully began with a low speed at 500 rpm for 10 s, followed by a high speed at 1500 rpm for 1 min. A quasi-monolayer of QD nanocrystals could be clearly observed after evaporation of the solutions, as, respectively, the AFM image and its corresponding cross-sectional plots in Figure 3(c) and 3(f) show. A relatively uniform QDs film can be formed over the whole sample surface without the large-proportion aggregation, which shows the feasibility of this technology in our exciton-LSP coupling system and its benefit for the following characterization. Room-temperature PL spectra of the exciton-LSP coupling systems were acquired using a 514 nm excitation source from an air-cooled Ar laser with a spot size of ~2 μm and power 0.2 mW. The PL signals (integrated from λ = 450–700 nm) were recorded at normal angle and analyzed using a Peltier-cooled charge coupled device (CCD) in conjunction with a BH-2 microscope with 50× objective lens (NA 0.5). To study the enhancement effect quantitatively, we define the PL enhancement factor as the ratio of the total photon intensity collected from QD nanocrystals near the plasmon device to that from the QD IPL

background without the tri-prism Au nanoarrays, which can be described as EF = I0 . PL

Figures 4(a) and (b) show the PL enhancement factor of CdSe/ZnS QDs based on the coupling of exciton and LSP for different thicknesses of PMMA spacer from 20 nm to 210 nm, corresponding to the systems with Cr and Ta2O5 adhesive coating, respectively. The enhanced PL wavelength from two types of the exciton-LSP coupling systems are all located in a region at ~590 nm, which is consistent with the emission of pure QD nanocrystals [a photograph of pink PL is shown in the inset of Figure 4(a)] and the maximum of plasmon resonance in the transmission spectra of tri-prism Au nanoarrays. Dramatic enhancement in the PL intensity from the exciton-LSP systems is distinctly ACS Paragon Plus Environment

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observed after optimization of the PMMA spacer thickness between the metallic nanostructures and QDs, which is ascribed to the intensive coupling of QDs with LSP resulting from the tri-prism Au nanoarrays. However, the exciton-LSP coupling systems with either Ta2O5 or Cr adhesive coating inevitably result in PL quenching if the QDs are assembled close to the tri-prism Au nanoarrays, as can be seen from a representative PL spectrum from the system with a PMMA spacer thickness of 20 nm. This occurs primarily because nonradiative energy transfer or electron transfer from QDs to Au nanostructures are dominant relative to the electric field coupling with plasmon, which can be understood by the damping of the dipole oscillations induced by the nearby Au nanostructures. With increasing spacer thickness, it is found that the electric field induced PL enhancement increases and reaches a maximum at the spacer thickness of 40 nm. Eventually, the PL intensity falls back to the level of that for pure QDs film without Au nanoarrays, i.e., a PL enhancement factor of approximately 1. The trend in the effect of PMMA spacer thickness on the sensitivity of the PL enhancement factor discussed above is in good agreement with previous results for the coupling between QDs and plasmonic metallic nanostructures. Most importantly, the maximal PL enhancement factor from the exciton-LSP coupling systems is approximately twice that from conventional systems, in which a thin film of Ta2O5 is first introduced to be used as both an adhesive coating and coupling medium to replace the Cr layer. To directly compare this effect of the plasmonic device with Ta2O5 and Cr adhesive coating on the PL enhancement of QDs, the PL intensity enhancement factor as a function of spacer thickness for these two adhesive coatings are extracted from Figures 4(a) and (b), as shown in Figure 4(c). The emission from the exciton-LSP coupling system with Ta2O5 adhesive coating shows larger enhancement with respect to that with ACS Paragon Plus Environment


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Cr adhesive coating, despite the variation in the PMMA spacer thickness. In particular, the strongest enhancement factor of about 6 is obtained from this proposed and well-designed coupling system with the optimal spacer thickness resulting from the introduction of the versatile Ta2O5 layer. We can primarily attribute metal-enhanced fluorescence to the strong coupling of the emitter exciton from QDs and LSP stimulated in the Au nanostructures. That is, a nearby metal can increase the intrinsic radiative decay rate of fluorescent QDs. It is informative to consider the mechanism of a larger enhancement factor from the coupling system and the modified form when the Ta2O5 layer is introduced into the plasmonic Au nanostructures.

Figure 4. Photoluminescence enhancement factor of CdSe/ZnS QDs based on the coupling of exciton and LSP with (a) Cr and (b) Ta2O5 adhesive coating at different thicknesses of PMMA spacer, the normalized PL intensity enhancement for pure QDs film without Au nanoarrays are shown as references. (c) PL intensity enhancement factor as a function of spacer thickness for different adhesive coatings. (d) Diagram to illustrate the ACS Paragon Plus Environment

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mechanism of emission enhancement from QDs mediated by localized surface plasmon. The underlying enhancement in PL of QDs can be evaluated by the internal quantum yield (IQE), 𝜂int, which reflects the competition between the radiative decay and nonradiative processes.42,43 The IQE of pure QDs nanocrystals without modification of Au nanoarrays depends on the radiative and nonradiative combination rates, and can be expressed as 𝜂𝑖𝑛𝑡 = Γrad/(Γrad + Γnr). While in the case of a metal-enhanced fluorescence system, another new rate, ΓLSPs, would be added to the spontaneous emission rate due to the coupling of QDs and LSPs from near tri-prism Au nanoarrays, and the IQE 𝜂,𝑖𝑛𝑡 of the fluorescent system is given by: Γ𝑟𝑎𝑑 + Γ𝐿𝑆𝑃𝑠

𝐹Γ𝑟𝑎𝑑

𝜂,𝑖𝑛𝑡 = Γ𝑟𝑎𝑑 + Γ𝐿𝑆𝑃𝑠 + Γ𝑛𝑟 = 𝐹Γ𝑟𝑎𝑑 + Γ𝑛𝑟,

(1)

where Γrad and Γnr are the radiative and nonradiative decay rate, respectively, and ΓLSPs is the metal-modified combination rate due to emission of LSPs. The Purcell enhancement factor, F = (Γrad + ΓLSPs)/Γrad, quantifies the average PL enhancement of QDs in the presence of plasmonic nanostructures. A PL enhancement from QDs means an increase in IQE because ΓLSPs can also contribute to the η𝑖𝑛𝑡, just as is the same for Γ𝑟𝑎𝑑. When the plasmonic nanostructures are carefully designed for the high coupling efficiency with low absorption and a matching dielectric material, and the QDs are placed at suitable distances from the Au nanoarrays, the fluorescent QDs can undergo strong modifications to their radiative decay rate and, therefore, their far-field scattering properties. To well understand the inherent mechanism, a diagram illustrating the LSP-mediated fluorescent enhancement of QDs is shown in Figure 4(d). Once the system is irradiated by the incident 514 nm laser source, two processes are considered simultaneously, including direct excitation of localized surface plasmon in the tri-prism Au nanoarrays (labeled as ACS Paragon Plus Environment


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route 2), and the stimulation of QD emitters with a fluorescent emission of 590 nm (labeled as route 1). For route 2, although the Au nanostructures can increase the excitation rate of QDs by concentrating the electromagnetic field and the subsequent emission, the effect of excitation enhancement provides a relatively small proportion to the overall PL enhancement because of the minimal spectral overlap between the plasmonic resonant wavelength of the tri-prism Au nanoarrays and the excitation laser wavelength. As a result, the emission enhancement for route 1 plays a dominant role in the proposed exciton-LSP coupling system, in which there are several loss channels responsible for the energy dissipation during the excitation of the QD emitters. In addition to the usual radiative and nonradiative decay rates of Γrad and Γnr, the energy would be resonantly coupled to the plasmonic metallic nanostructures, i.e., a strong LSPR. This occurs because the emitted wavelength of QD nanocrystals centered at ~590 nm matches the LSP resonance of the well-designed nanostructures with perfect spectral overlap, and the LSPR occurs with the associated light absorption peak (also the corresponding valley in the transmission spectra). Supposing that ε(ω) = 𝜀𝑟(𝜔) +𝑖𝜀𝑖(𝜔) is the frequency-dependent dielectric permittivity of bulk Au with, respectively, 𝜀𝑟(ω) and 𝜀𝑖(ω) as the real and imaginary components of ε(ω), and 𝜀𝑚 is the dielectric constant of the surrounding medium, the strongest resonance would occur when the formula 𝜀𝑟(ω) = ―2𝜀𝑚 is satisfied for the specific electromagnetic frequency ω.44 In the proposed system, 𝜀𝑟 ≈ ― 9.1 for the bulk Au at wavelength of ~590 nm was acquired from experimental results of previous works, 45 and 𝜀𝑚 = 4.51 of Ta2O5 layer was measured using a spectroscopic ellipsometer (Horiba Jobin-Yvon UVISEL). A stronger LSPR with high coupling efficiency can thus be pursued because of the matching conditions between the Au nanostructures and Ta2O5 layer. After ACS Paragon Plus Environment

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that, the energy of the LSPR generally dissipates into absorption with nonradiative damping ΓLabs and the radiative far-field scattering ΓLSPR rad , where the latter increases the overall PL of the system. This greatly depends on the scattering efficiency, i.e., the extent of absorption. There exists almost zero absorption with an extinction coefficient of 0.013 for the Ta2O5 layer, which is far less than that of the Cr layer (an extinction coefficient of about 4.37 at wavelength 590 nm). Therefore, the strong LSPR induced by the efficient coupling between QDs and Au tri-prism nanostructures can more efficiently scatter into far-field without damping after the introduction of the versatile Ta2O5 layer. An improvement in the internal quantum yield, ηint, was obtained by the addition of ΓLSPs (the sum of dominant ΓLSPR and negligible ΓLSP rad rad ) to the spontaneous emission rate Γ, thereby increasing the PL with a higher Purcell enhancement factor, F.

Figure 5. (a) High-resolution SEM image of the tri-prism Au nanoarray. (b)–(d) Simulated intensity distribution of the local electromagnetic field near the tri-prism Au arrays with Cr adhesive coating (b), Ta2O5 adhesive coating (c), and without adhesive coating (d). The ACS Paragon Plus Environment


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insets show the corresponding intensity distribution in the XZ profile. Finally, to further verify the high performance of the as-fabricated plasmonic device and investigate the interactions of an electromagnetic field with the tri-prism Au nanoarrays, the local near-field distribution of the nanostructures was calculated by the FDTD method. A unit model with staggered Au pattern was constructed, where the structural parameters, including the period of the nanoarrays, morphology characteristics, and size distribution of each layer, are based on the well-designed results and the local details of high-resolution SEM images, as shown in Figure 5(a). Three models of plasmonic devices with Cr and Ta2O5 adhesive coatings, and without adhesive coating, were selected and built with periodic boundary conditions in the x and y directions. The calculations were performed under the perpendicular irradiation of a laser with wavelength of 590 nm, and using a maximal mesh size of 2 nm. The dielectric permittivity for bulk Au and Cr was acquired from Palik’s work43 and the optical constant of the Ta2O5 layer is 𝜀𝑚 = 4.51. Figures 5(b)– (d) show simulated intensity distributions of the normalized local electromagnetic field near the tri-prism Au arrays with Cr and Ta2O5 adhesive coatings and without adhesive coating, respectively. The intensity distribution in the XZ profile is also shown in the inset of the corresponding Figure. Dramatic enhancement in the electromagnetic field is indicated by 𝐸 2

the factor of (𝐸0) that is very clearly observed. In addition, the maximal electromagnetic field enhancement factor in the XY profile (10 nm away from the structural surface) for the plasmonic device with a Ta2O5 layer can reach 72 by rigorous simulation and analyzation, which is much larger than that for devices with a Cr layer or those without adhesive coating. The calculated results confirm the aforementioned conclusion that a stronger LSPR can be induced under the incident light and far-field scattering more efficiently by ACS Paragon Plus Environment

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the introduction of a Ta2O5 layer. It is also demonstrated that the proposed exciton-LSP coupling system can greatly enhance the PL emission from CdSe/ZnS QDs. To summarize, we report the LSPR-induced emission enhancement from CdSe/ZnS QDs near well-engineered tri-prism Au nanoarrays through a high-performance exciton-LSP coupling system with a versatile Ta2O5 layer. By combining the matching conditions and a low extinction coefficient with minimal damping absorption of the Ta2O5 layer in the system, stronger LSPR can be induced by the efficient coupling between QDs and Au tri-prism nanostructures and efficiently scatter into the far-field without damping. The plasmon-enhanced emission from QDs with high PL enhancement factor was obtained by the coupling of excitons and LSPs. Additionally, the emission enhancement plays a dominant role in the improvement of the internal quantum yield by increasing the spontaneous emission rate, Γ. An overall maximum PL enhancement factor of 6 was observed from this well-engineered coupling system with the optimal spacer thickness of 40 nm, which is larger than that of the system with conventional adhesive coating. It is believed that this design concept and method with greatly enhanced light-matter interactions will open up avenues towards actual applications of LSPR-based fluorescence enhancement in biomedicine and optoelectronic devices.




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AUTHOR INFORMATION Corresponding Author *[email protected] Notes The authors declare no competing financial interest.



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ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (11574171, 51532008), and the National Key Research and Development Program of China (2017YFA0205800).


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