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Fluorescence Enhancement and Spectral Shaping of Silicon Quantum Dot Mono-Layer by Plasmonic Gap Resonances Shiho Yashima, Hiroshi Sugimoto, Hiroyuki Takashina, and Minoru Fujii J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09124 • Publication Date (Web): 29 Nov 2016 Downloaded from http://pubs.acs.org on December 2, 2016

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The Journal of Physical Chemistry

Fluorescence Enhancement and Spectral Shaping of Silicon Quantum Dot Mono-Layer by Plasmonic Gap Resonances Shiho Yashima, Hiroshi Sugimoto, Hiroyuki Takashina, and Minoru Fujii* Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Rokkodai, Nada, Kobe 657-8501, Japan

ABSTRACT. A monolayer of silicon quantum dots (Si-QDs) 2.8 and 3.9 nm in diameters is placed in a gap between a gold (Au) thin film and a Au nanoparticle and the photoluminescence (PL) properties are studied. By the metal nanoparticle over mirror (MNPoM) structure, the PL spectra of Si-QDs are strongly modified; the full width at half maximum is reduced to ~170 meV, which is less than half of that of Si-QDs on a silica substrate. The spectral shape coincides almost perfectly with that of the scattering spectrum of the MNPoM structure, indicating efficient coupling of the luminescence of Si-QDs with the gap surface plasmon modes. The luminescence intensity of Si-QDs in the gap is estimated to be enhanced about 700-fold compared to those on a Au film.

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Introduction Colloidal semiconductor quantum dots (QDs) have superior properties as phosphors such as wide tunability of the photoluminescence (PL) wavelength, high photo-stability, etc., and have been considered to be an alternative to traditional phosphors such as organic dyes and rare-earth or transition-metal doped inorganic phosphors. However, toxic heavy metal elements in commercially available II-IV and IV-VI semiconductor QDs are always a concern for the biomedical applications, and the development of heavymetal-free QDs is an urgent research subject. Silicon (Si) QDs are one of the candidates of the next generation QD phosphors due to the non-toxicity as an element and the high bio-compatibility.1 The technology for the fabrication of colloidal Si-QDs has been improved very rapidly and the quality has been improved significantly.

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Recently, in

order to further enhance the optical response of Si-QDs, the coupling with surface plasmon resonances of metal nanostructures have been explored. The proof-of-concept research on the coupling of excitons in Si-QDs with surface plasmons has been performed for Si-QDs embedded in solid matrices and the enhancement of the radiative recombination rate was demonstrated.12–15 Recently, various types of nanocomposites composed of metal nanoparticles, i.e., nanospheres and nanorods, and Si-QDs have been developed by using colloidal suspension of Si-QDs.16–19 In this work, we employ a metal nanoparticle over mirror (MNPoM) structure20 for the enhancement and shaping of luminescence from Si-QDs. In the MNPoM structure, a metal nanoparticle (NP) is placed on a metal thin film with a gap of a few nm. Due to the coupling of surface plasmon polaritons (SPP) of a metal film with localized surface plasmons (LSP) of a metal NP, very strong electric fields are induced in the gap (gap plasmon modes).21 As a result, luminescence22,23 and Raman scattering24–28 of materials in the gap are strongly enhanced.29 An advantage of the structure is that the surface


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plasmon resonance wavelength can be controlled in a wide range by the gap length and the metal NP size .21,30,31 By utilizing these advantages of a MNPoM structure, we can expect strong enhancement of luminescence from Si-QDs if a monolayer of precisely size-controlled Si-QDs is successfully placed in the gap and also if the luminescence and the gap plasmon wavelengths are overlapped. In this paper, we first demonstrate formation of a MNPoM structure with a monolayer of Si-QDs 2.8 and 3.9 nm in diameter in the gap. We study the light scattering properties of individual MNPoM structures and show that the gap plasmon resonance wavelength can be controlled by the size of Si-QDs. We then show that the photoluminescence (PL) spectra of Si-QDs in the gap are strongly modified from that on a silica substrate. The width of the PL band is significantly reduced and the PL intensity is strongly enhanced. Analyses of the experimental data reveal that the PL enhancement factor reaches 700-fold at the hot spot.

Preparation method of a MNPoM structure with a monolayer of Si-QDs in the gap First, we briefly explain the property of Si-QDs used in this work2–4,32,33The Si-QD used in this work has a heavily B and P codoped Si shell, which induces negative potential on the surface and makes QDs dispersible in polar solvents almost perfectly without organic ligands (see supporting information for brief description of the preparation procedure).2 Figure 1a shows a photograph of a methanol solution of the codoped Si-QDs. The solution is very clear and light scattering by agglomerates cannot be seen. Because of the perfect dispersion of the QDs in solution, i.e., no agglomerates of QDs in solution, a two dimensional array of QDs can be produced on a substrate by spincoating or drop-casting (Figure 1a). The average diameter (dave) of the QDs can be controlled by the growth temperature.4 In this work, we use Si-QDs grown at 1050°C


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(dave = 2.8 nm, standard deviation σ = 0.6 nm) and 1100°C (dave = 3.9 nm, σ = 0.8 nm). Detailed TEM analyses of the QDs are found in Ref. [4]. The codoped Si-QDs exhibit very stable and size-controllable PL in the visible to near infrared (NIR) range. The PL properties are insensitive to the environment; almost identical PL spectral shape and quantum yields are obtained in methanol, in water and in air.3 Figure 1b shows the PL spectra of colloidal Si-QDs with two different average sizes. The excitation source is monochromatized 405 nm light from a Xe lamp (450 W) (Fluorolog-3, Horiba Jovin Yvon). The PL of the sample with dave =2.8 nm has a peak at 1.65 eV with the full width at half maximum (FWHM) of 0.4 eV, while that with dave =3.9 nm has a peak at 1.45 eV with the FWHM of 0.47 eV. It should be noted that the PL energy of the codoped Si-QDs is several hundred meV smaller than those reported in undoped Si-QDs, 4 due to the involvement of the impurity states to the optical transition. 2 Hereafter, we denote Si-QDs with dave=2.8 nm and 3.9 nm as Si-QD(2.8) and Si-QD(3.9), respectively. The MNPoM structure developed in this work is schematically shown in Figure 1c. A Au/Ti film (200/10 nm) was deposited by thermal evaporation on a Si substrate. A 6Amino-1-hexanethiol, hydrochloride (6-AHT) self-assembled monolayer was formed on a Au film by immersing a Au-deposited substrate in a 1mM 6-AHT solution for 24h. A onto a substrate and dried in air at room temperature. The QD concentration was controlled so that a monolayer film is formed. Finally, diluted solution of citratestabilized Au NPs (1.10×10 /ml) were dropped on the Si-QD monolayer and dried. The concentration of Au NPs was about 7.3× 106 cm-2, which is sparse enough to make optical measurements of a single MNPoM structure possible and to prevent inter-particle electromagnetic coupling.


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Figure 1. (a) Photograph of a methanol solution, TEM image and High-resolution TEM image of Si-QDs. The average diameter estimated from the TEM image is 3.9 nm. (b) Normalized PL spectra of Si-QDs with the average diameters of 2.8 nm (red) and 3.9 nm (black). (c) Schematic illustration of a MNPoM structure.

Before the formation of the structure, we estimate the resonance wavelength of the gap plasmon modes and the degree of the field enhancement at the gap by numerical simulations using the MNPBEM code,34,35 which is based on the rigorous boundary element method. The model structure for the calculation is shown in Figure 2a. A Au NP 80 nm in diameter is positioned above a Au film, separated by a spacer. The structure is irradiated by plane wave with the incident angle (θ) of 65° and the scattering crosssections at different wavelengths are calculated. Figure 2b shows calculated scattering spectra for the gap length of 4 and 5 nm. The gap length corresponds to the sum of the diameter of a Si-QD (2.8 or 3.9 nm) and 6-AHT (~1.2 nm). The refractive index of the gap is assumed to be 1.7, which corresponds to that of a Si-QD layer estimated by spectroscopic ellipsometry.36 The spectra in Figure 2b are obtained by averaging those calculated for s- and p-polarized light. The spectra are asymmetric and consist of two peaks. The main peak around 1.8-2.0 eV is due to coupling of the dipolar mode of a Au


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NP with SPP of a Au film. The smaller peak around 2.2-2.4 eV originates from the quadrupolar plasmon mode.37,38 The quadrupolar resonance acquires part of the “bright” character of the dipolar mode, becoming visible on the scattering spectrum due to hybridization of the dipolar and quadrupolar plasmons of Au NPs caused by the symmetry-breaking by the presence of the Au film. The scattering main peak shifts from 1.8 to 2.0 eV with increasing the gap length from 4 to 5 nm. By comparing Figure 1b and Figure 2b, we can see better overlap of the Si-QD PL band and the LSP resonance of the MNPoM structure in Si-QD(2.8), i.e., when the gap length is 4 nm. In that case, the field enhancement (|E/E0|) at the middle of the gap reaches 20-fold at 1.9 eV (Figure 2c), suggesting strong enhancement of the luminescence from Si-QDs in the gap. Figure 2d shows the result of the same calculation at the PL excitation energy discussed later (3.06 eV (405 nm)). |E/E0| is about 1 at the peak. Strong enhancement of the PL excitation rate is thus not expected.

Figure 2. (a) Schematic illustration of the MNPoM structure for the calculation of scattering cross-sections and field patterns. (b) Calculated scattering spectra (Au NP


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diameter: 80 nm, gap length: 4 nm and 5 nm, gap refractive index: 1.7). Electric field enhancement (|E/E0|) at the middle of the gap (c) at the scattering peak energy (1.9 eV) and (d) at the excitation energy for PL measurements (3.06 eV) when the gap length is 4 nm.

Result and Discussion Figure 3a shows the scattering spectra of single MNPoM structures with different materials in the gap, i.e., SAM (black), Si-QD(2.8)/SAM (red) and Si-QD(3.9)/SAM (blue). The data of a Au NP on a silica substrate (Au NP/Silica) (green) is also shown as a reference. Au NP/Silica has a scattering peak around 2.2 eV and the scattering image is green. In the MNPoM structures, the peak shifts to the lower-energy and the image turns to red. It is around 1.76 eV when the spacer is SAM (~1.2 nm), while it is 1.80 eV and 1.90 eV in Si-QD(2.8)/SAM and Si-QD(3.9)/SAM, respectively. The spectral shape of the MNPoM structure with Si-QD(2.8)/SAM (red) in the gap is asymmetric with a small bump at the high energy side. The shape is very similar to that of calculated spectra in Figure 2b. The average peak energies obtained for more than 10 individual MNPoM structures are plotted in Figure 3b. The error bars represent the standard deviation (σ) of the peak energies. The scattering peak energy exhibits clear blue shift with increasing the gap length. This is a typical behavior of a MNPoM structure and is due to decreased coupling of the two plasmon modes with increasing the gap length.

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The strength of the

coupling appears also in the scattering intensity.41 Figure 3c plots the scattering peak intensity as a function of the peak energy. Although the data are scattered, a clear correlation can be seen.42 The scattering of the MNPoM structures is stronger than that of


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the references (Au NP/Silica),28,43 and the smaller peak energy, i.e., smaller gap length, results in larger scattering intensity. 23,30,38,41 In Figure 3b, scattering peaks obtained by numerical calculations are also shown. As described above, the refractive index of a Si-QD layer is about 1.7. The average refractive index of the gap is considered to be close to the value but may be slightly different due to the existence of citrate acid on Au NP surface and SAM on Au film surface. Therefore, in the calculation, the refractive index in the gap is changed from 1.5 to 1.9. The experimental

scattering peaks agree fairly well with those of numerical calculations. The fairly good agreement between experiments and calculations is also seen in the relation between the scattering intensity and the scattering peak energy in Figure 3c.


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Figure 3. (a) Scattering spectra of single MNPoM structures with different materials in the gap, SAM (black), Si-QD(2.8)/SAM (red) and Si-QD(3.9)/SAM (blue). Spectrum of a single Au NP on a silica substrate (green) is also shown. The scale bars of the dark-field scattering images are 1 µm. (b) The relation between the scattering peak energy and the gap length. The error bars represent the standard deviation of the measured values. Gray symbols and lines are calculated results for the refractive indices designated in the figure. (c) The relation between the scattering intensity and the scattering peak energy. Calculated data are shown by gray symbols and lines.


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Figure 4a shows a PL spectrum of Si-QD(2.8)/SAM coupled to a MNPoM structure (red symbols). As references, the spectra of Si-QD(2.8)/SAM on a Au film without a Au NP (blue symbols) and Si-QD(2.8) on a silica substrate (gray) are shown. The PL spectrum of a Si-QD(2.8)/SAM on a Au film is obtained in the region very close to that of a MNPoM structure. All the measurements were performed with the same setup as the scattering spectra. The excitation wavelength is 3.06 eV (405 nm). The PL spectral shape of Si-QD(2.8) on a silica substrate is almost identical with that of Si-QD(2.8) in methanol (Figure 1b). As described above, PL properties of B and P codoped Si-QDs are very insensitive to the environment and we obtain bright and stable PL from a monolayer of Si-QDs in air.3,4 On the other hand, the PL is strongly quenched on a Au film. The strong quenching is due to coupling of the Si-QDs with lossy surface wave and SPP of a Au film.44 We will discuss the degree of the PL quenching on a Au film later (supporting information, “Radiative and nonradiative rates and QY on a flat Au film”). When a Au NP is placed on a Si-QD(2.8)/SAM/Au structure, i.e., Si-QD(2.8)/SAM coupled to the MNPoM structure, the PL is drastically recovered. In Figure 4a, the PL intensity is enhanced 10-fold. More importantly, the PL spectral shape is strongly modified from that of Si-QD (2.8) on a silica substrate. The PL peak shifts from 1.65 eV to 1.77 eV and the spectrum becomes much narrow; the FWHM reduces from 0.40 to 0.25 eV. This strong modification of the spectral shape indicates that light emission of SiQDs is coupled with the gap plasmon modes of the MNPoM structure. In Figure 4a, the scattering spectrum of the same MNPoM structure is also shown (black broken curve). The spectral shape of the scattering and the PL is almost identical, supporting the coupling between the emission in Si-QDs and the gap plasmon modes of the MNPoM structure. It should be stressed here that the observed PL enhancement of ~10-fold in the


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MNPoM structure compared to Si-QDs on a Au film in Figure 4a is the average enhancement factor over the PL detection area, which is much larger than the actual area of hot spot in the gap. Therefore, PL enhancement in the hot spot is expected to be much larger than the observed value. We will estimate the enhancement factor at the hot spot later. As described above, very precise tuning of the structural parameters is necessary to achieve high PL enhancement. In Figure 4b, we replace Si-QD(2.8) with Si-QD(3.9). This shifts the PL peak energy from 1.67 to 1.45 eV, and the scattering peak from 1.8 eV to 1.9 eV. Therefore, the overlap between the PL of Si-QDs and the gap plasmon resonance becomes small and thus strong PL enhancement is not expected. In fact, in Figure 4b, PL of Si-QD(3.9)/SAM in a MNPoM structure (red) is only slightly enhanced compared with that of Si-QD(3.9)/SAM/Au (blue), and the spectral shape does not coincide with the scattering spectrum.34,45 We perform the same measurements as in Figure 4a for more than 20 single MNPoM structures to study the correlation between PL and scattering more in detail. In Figure 4c, the relation between the PL and scattering peak energies is plotted. The PL peak energy of Si-QD(2.8) on a silica substrate is shown by a horizontal dashed line (1.65 eV). We can clearly see that in the MNPoM structure, the PL peak always shifts to higher energy. Furthermore, we can see clear correlation between the PL peak energy and the scattering peak energy as shown by the green shade. In Figure 4d, the FWHMs of the PL and scattering spectra are shown. The FWHM of Si-QD(2.8) on a silica substrate is shown by a horizontal broad line (0.4-0.45 eV). The PL FWHM of all the single MNPoM structures is much narrower than that of Si-QD(2.8) on a silica substrate. The smallest FWHM reaches 170 mV. The PL FWHM can be as narrow as the scattering FWHM. All these


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results are clear evidences of the coupling of the emission from Si-QDs with the gap plasmon mode and very strong enhancement of the PL intensity at the gap. In Figure 4e, PL intensity of 23 single MNPoM structures are plotted (red). In the same column, PL intensity of Si-QD(2.8)/SAM on a Au film without a Au NP obtained in the region very close to each MNPoM structure is shown (blue). In almost all samples, the PL intensity of Si-QDs in the gap of a MNPoM structure is much larger than that on a Au film. The largest enhancement factor is 13. Since there are several MNPoM structures with very small enhancement factors, the average enhancement factor for the 23 MNPoM structures is 7.0. The large variation of the PL intensities may arise from non-uniformity of a Si-QDs monolayer, roughness of a Au film, size distribution of a Au NP, etc.


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Figure 4. (a) PL spectrum of a MNPoM structure with Si-QD(2.8)/SAM in the gap (red symbols). The scattering spectrum of the same single MNPoM structure is shown by a black broken curve. Si-QD(2.8) on a silica substrate (gray) and on a flat Au film (blue symbols) are also shown. (b) PL spectrum of a MNPoM structure with Si-QD(3.9)/SAM in the gap (red symbols). The scattering spectrum of the same single MNPoM structure is shown by a black broken curve. Si-QD(3.9) on a silica substrate (gray) and on a flat Au


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film (blue symbols) are also shown. (c) Relation between PL energy and scattering energy for single MNPoM structures with Si-QD(2.8)/SAM in the gap. (d) Relation between PL FWHM and scattering FWHM for single MNPoM structures with Si-QD(2.8)/SAM in the gap. (Broken) horizontal lines in (c) and (d) correspond to the data of Si-QD(2.8)/SAM on a silica substrate.

(e) PL intensities of single MNPoM structures with Si-

QD(2.8)/SAM in the gap (red). The abscissa corresponds to the sample number. The data obtained in the region very close to each MNPoM structure, i.e., Si-QD(2.8)/SAM on a flat Au film without a Au NP, are shown in the same column (blue).

In a MNPoM structure, the largest PL enhancement is expected only in the region just beneath a Au NP. We adopt the size of a Au NP as the hot spot size as have been assumed in previous studied on similar structures.22,46,47 This assumption determines the lower limit of the PL enhancement factor in the hot spot. The radius of the hot spot is ~40 nm ( = 5.0×103 nm2), which is much smaller than the PL detection area ( = ~0.56

µm2). By taking into account these numbers, the observed enhancement factor (7.0 in average) corresponds to the enhancement factor in the hot spot of 700-fold. We also estimate the enhancement factor of the PL integrated intensity at the hot spot. It is 214fold, which is smaller than that of the peak intensity due to the spectral narrowing in a MNPoM structure. In this work, the PL is measured in the linear region with respect to the excitation power. The experimentally obtained PL enhancement factor ( . .) is thus expressed as, 46–48

. . ∝

Γ , (1) Γ

where is the emission collection efficiency and Γ is the excitation rate and is the quantum yield. Each of these values of MNPoM structures is attached “MNPoM” and


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that of on the flat Au film is attached “Au”. The radiation pattern of a MNPoM structure is not strongly modified from that of a dipole placed on a flat Au film.49 Therefore, the collection efficiency by an objective (NA=0.9) is expected to be similar (~50 %). On the other hand, the excitation rate, which is proportional to the square of the incident electric field, is slightly enhanced (~2.2-fold; see supporting information for the estimation). Therefore, the largest contribution to the observed giant PL enhancement factor comes from the very large value of / , which is estimated to be ~97 from the observed enhancement factor of the integrated intensity (214-fold). The quantum yield ( ) of Si-QDs coupled to metal nanostructures is expressed as, 48,50,51

=

, (2) + ! + (1 − )/

where is the enhancement factor of the radiative rate by the metal nanostructure (Purcell factor), ! is the normalized nonradiative rate induced by the metal nanostructure, and is the quantum efficiency of Si-QDs in vacuum. When Si-QDs are placed on a flat Au film, ! (~270) is two orders of magnitude larger than (~0.47) due to efficient excitation of lossy-surface wave and SPPs of a Au film. From Eq. (2), this decreases the quantum yield of Si-QDs from the intrinsic value ( =~10 %) to ~0.17%

( ) (supporting information, “Radiative and nonradiative rates and QY on a flat Au film”). Since the ratio of the QYs between a MNPoM structure and on a flat Au film ( / ) is 97, is estimated to be 16.5%. Therefore, in a MNPoM structure, the QY of Si-QDs is enhanced about 1.65 fold, indicating that the enhancement of the radiative decay rate can overcome non-radiative processes induced by the formation of the MNPoM structure. 52,53


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Conclusion We have succeeded in producing a MNPoM structure with a monolayer of NIR luminescent Si-QDs in the gap. We demonstrated that the PL spectra of Si-QDs, especially the FWHM and the peak energy, were strongly modified by the MNPoM structure. The narrowest FWHM is 170 meV, which is less than half of that of Si-QDs on a silica substrate. The PL peak intensity of Si-QDs in the gap is estimated to be ~700-fold enhanced compared to that on a flat Au film. The strong contrast of the PL intensity between Si-QDs in the MNPoM structure and on a flat Au film arises from large enhancement of the quantum yield in the gap in the MNPoM structure and also from strong deterioration of that on a flat Au film. The very large contrast of the luminescence intensity between Si-QDs on a Au film and those in the MNPoM structure may be very useful to achieve large bind to free ratio in Si-based fluorescent biosensing devices.

Method Measurements of scattering and PL spectra of single MNPoM structures Scattering and PL spectra of single MNPoM structures were measured using a custom built optical microscope. For the measurements of the dark-field scattering images and the spectra, the samples were illuminated by a halogen lamp via a dark-field objective lens and the scattered light was collected by the same objective (100×, NA = 0.9). The scattered light was transferred onto the entrance slit of a monochromator (SpectraPro300i, Acton Research Corp.) and detected by a liquid-N2 cooled charge coupled device (CCD) (Princeton Instruments). For the measurements of PL images and spectra, a MNPoM structure was excited via the objective by a 405 nm semiconductor laser with the power density of 2.26 W/cm2.


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ASSOCIATED CONTENT Supporting Information. Preparation procedure of B and P codoped all-inorganic colloidal Si-QDs, calculation of electric fields in the gap of a MNPoM structure and on a flat Au film, and calculation of radiative and nonradiative rates of a dipole placed on a flat Au film. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * [email protected]

ACKNOWLEDGMENT This work was partly supported by the 2015 JST Visegrad Group (V4)-Japan Joint Research Project on Advanced Materials and JSPS KAKENHI Grant Number 16H03828. HS acknowledges support from Grant-in-Aid for JSPS Research Fellow (26-3120).

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Fluorescence Enhancement and Spectral Shaping ... - ACS Publications

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