ZnS Nanoparticle Monolayer

We present spontaneous emission control of a core–shell CdSe/ZnS nanoparticle array assembled with polymer ultrathin films consisting of polymer nan...
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Spontaneous Emission Control of CdSe/ZnS Nanoparticle Monolayer in Polymer Nanosheet Waveguide Assembled on a One-Dimensional Silver Grating Surface Masaya Mitsuishi,*,† Shimpei Morita,† Keiko Tawa,‡ Junji Nishii,§ and Tokuji Miyashita† †

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Health Research Institute, AIST, Ikeda, Osaka 563-8577, Japan § Research Institute for Electronic Science, Hokkaido University, Kita 21 Nishi 10, Kita-ku, Sapporo 001-0021, Japan ‡

S Supporting Information *

ABSTRACT: We present spontaneous emission control of a core−shell CdSe/ZnS nanoparticle array assembled with polymer ultrathin films consisting of polymer nanosheets on a silver grating substrate, which served as a unique and simple photonic cavity. The grating-coupled waveguide modes enabled 103 order luminescence enhancement and one-fourth spectral narrowing. The light emission from a CdSe/ZnS nanoparticle array can be controlled by tuning the film thickness of hybrid polymer nanoassemblies, which provides multiple emission performance with good tuning ability from red to green at low-power continuous wave laser excitation (∼μW).

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grating-coupled waveguide mode at visible light wavelength with high coupling efficiency if the grating has a waveguide medium on its surface. Controlling the waveguide medium film thickness at a nanometer length-scale allows for the manipulation of the emission properties of CdSe/ZnS Nps. Thereby, spontaneous emission is effectively guided into the single mode. Our strategy is to direct light emissions from singly excited CdSe/ZnS Nps in an ordered array using lowpower laser excitation. In this study, we demonstrate spectral narrowing of CdSe/ZnS Nps occurring, even at low-power excitation, in a well-defined nanostructure.18,19 Bottom-up approaches enable us to prepare a CdSe/ZnS Np monolayer with the CdSe/ZnS Nps distributed uniformly and twodimensionally. The monolayer was assembled with polymer Langmuir−Blodgett (LB) films (polymer nanosheets), which serve as a waveguide. The multilayer was combined with a silver grating surface to excite and couple a waveguide mode (single mode) more effectively at excitation and emission wavelengths. Surprisingly, narrow band emission (full-width at halfmaximum (fwhm) ca. 6 nm) was observed at precisely controlled nanostructures. The nanostructure also provides strong, narrow, and directional (vertical) emission. Figure 1 depicts a schematic illustration of a polymer nanosheet waveguide coupled with a one-dimensional silver grating surface. Polymer nanosheets consisting of poly(Ndodecylacrylamide) (pDDA, Supporting Information Figure

emiconductor CdSe/ZnS nanoparticles (CdSe/ZnS Nps) have attracted much attention because of their high quantum yield, photostability, and quantum size effect, i.e., size-dependent band gap,1−3 which lead to their high potential for applications as biological/chemical sensors,4 lasers,5 OLEDs,6 and solar cells.7 However, their omnidirectionality and inherent Auger recombination processes are severe drawbacks for light emission control.8 Although laser operation has been achieved by exciting CdSe/ZnS Nps with ultrashort (ca. 100 fs) pump pulses using distributed feedback,9−11 whisper gallery mode,12,13 and random lasing configurations, it is generally extremely difficult to observe spectral narrowing below the lasing threshold.4,14 Realizing a solid-state photonics application is more challenging because nonradiative processes such as Auger recombination processes occur in CdSe/ZnS Nps under multiple excitation and depopulate the excitons.15,16 Moreover, the femtosecond laser pumping system is expensive and power-consuming. Applications of CdSe/ZnS Nps may be extended if their emission could be narrowed to laserlike levels. The key issue is therefore how to control the CdSe/ZnS Np light emission at low-power excitation, i.e., spontaneous emission. For example, a Yagi−Uda nanoantenna provides unidirectional light emission of CdSe/ZnS Nps.8 Optical waveguide and wavelength-scale microcavities produce a narrowed emission spectrum.14,17 These local microenvironments will alter the spatial, temporal, and spectral properties of the light emission, which motivates us to design well-defined nanostructures with CdSe/ZnS Nps. We specifically examined nanoscale metal gratings because of their superior optical properties. They are highly reflective and enable generation of © 2012 American Chemical Society

Received: October 27, 2011 Revised: January 20, 2012 Published: January 20, 2012 2313

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Figure 1. (a) Schematic of hybrid polymer nanoassemblies on a grating substrate. The CdSe/ZnS Np monolayer was embedded in a polymer nanosheet waveguide supported by a one-dimensional silver grating substrate. (b) Emission spectrum of CdSe/ZnS monolayer assembled on a flat quartz surface. (c) Emission spectrum of the CdSe/ZnS Np monolayer.

polymer nanosheet waveguide on a silver grating surface (Figure 1c). The CdSe/ZnS Np monolayer assembled on a flat quartz surface (without pDDA nanosheets) was used as a reference. The reference exhibits strong fluorescence with fwhm = 27 nm (Figure 1b). The fwhm value decreases to 6 nm as seen in Figure 1c, which is one-fourth less than that of the CdSe/ZnS Np monolayer on the flat surface, and reached the resolution limit. Furthermore, the emission intensity increased to more than the 103-fold level compared to that of the CdSe/ ZnS Np array on the flat quartz surface (Figure S6). It should be mentioned that the luminescence intensity of CdSe/ZnS Np array depends on the substrate. For example, in the case of CdSe/ZnS Np arrays assembled on a flat silver surface and a silver grating surface without pDDA nanosheets, the luminescence was quenched. The luminescence intensity decreased to one-fourth that of the CdSe/ZnS Np array on the flat quartz surface. We needed to separate the CdSe Np array 70 nm far from the silver surface to avoid the luminescence quenching. The emission intensity increased almost linearly along with the excitation power (Figure 2a). The phenomenon has no threshold behavior because the emission is due to linearly excitation dependent cavity-coupled spontaneous emission. Consequently, the narrowed band was obtained even at an excitation level of 5 μW (Figure 2b). The emission holds a linear-polarization character: transverse electric (TE) polarization. This constitutes indirect evidence of waveguide mode

S1) enable control of the assembly thickness as well as producing a CdSe/ZnS Np monolayer. The monolayer thickness of the polymer nanosheets was determined to be 1.76 nm (pDDA) and 2.0 nm (p(DDA/DONH), Figure S1) using surface plasmon spectroscopy and atomic force microscopy (Figure S2). The grating surface was prepared using a nanoimprinting method. The surface has a period of the grating of Λ = 480 nm, depth of 30 nm, and duty ratio of 56:44 (Figure 1).20 The polymer nanoassemblies maintain the grating surface shape even after multilayer deposition (Figure S3). We made a sandwich structure in which carboxyl-terminated CdSe/ ZnS Nps form a monolayer in the middle. We immobilized carboxyl-terminated CdSe/ZnS Nps onto cationic p(DDA/ DONH) nanosheets through electrostatic interaction. The monolayer formation of CdSe/ZnS Nps was confirmed by AFM and UV absorption measurement (Figure S4); the surface number density of the CdSe/ZnS Np monolayer was determined to be ∼1.8 × 103 μm−2. We used three different CdSe/ZnS Nps (emission peak wavelength λmax = 525, 605, and 650 nm). The sample was set on a computer-controlled rotation stage (Figure S5). The direction of the incident light (Ar+ ion laser, excitation wavelength = 488 nm) was set parallel to the grating vector of the silver grating. Light emission from the nanoassemblies was monitored by varying the incident angle (θex) and detection angles (θD) independently. A narrowed emission spectrum was observed from the CdSe/ZnS Np monolayer (λmax = 650 nm) embedded in a 2314

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the electromagnetic field distribution at excitation and emission wavelengths under grating-coupled waveguide modes (18.5° for 488 nm and 0.0° for 605 nm) using FDTD calculations. Comparison of the experimental emission intensities (filled circles in Figure 3d) with the electromagnetic field distribution at 488 nm (green line in Figure 3d) and 605 nm (red line in Figure 3d) reveals that the bandwidth is much narrower than those of the calculated electromagnetic field distribution. It shows good agreement with the electromagnetic field distribution, resulting in the production of the electromagnetic field distribution at both 488 and 605 nm (black line in Figure 3d). In other words, both grating-coupled waveguide modes affect the enhanced spontaneous emission. No such emission enhancement was observed for dtotal < 100 nm, also implying a cutoff for waveguide mode, which is indispensable for emission narrowing and enhancement. It should be described that no spectral narrowing was observed through surface-plasmoncoupled emission so far.23 In our case, grating-coupled surface plasmon excitation was possible for the CdSe/ZnS Np monolayer deposited directly onto the silver grating surface without pDDA nanosheets (dtotal = 25 nm). However, the fwhm value was 10 nm, which is less effective than that for the waveguide mode (Figure S7). Because of the low-power laser excitation, the enhanced emission observed is not related with lasing. As mentioned above, the emission intensity and the wavelength observed are angle-dependent. We never observed an intense spot; the beam angle was broadened as the distance become far from the sample. The linear relationship between the light intensity and the input laser power (Figure 2) implies that only enhanced spontaneous emission was observed and that the enhancement level was not reached amplified stimulated emission. The reason why the enhancement occurred in hybrid polymer nanoassemblies on the silver grating surface is confirmed by time-resolved lifetime measurements (Figure 4). The lifetime of CdSe/ZnS Np monolayer embedded in the hybrid polymer waveguide on the silver grating surface displayed twocomponent decay (red symbols in Figure 4, a1 = 0.95, τ1 = 0.5 (ns), a2 = 0.24, τ2 = 9.8 (ns)). The fast component is predominant and much shorter than that of the CdSe/ZnS Np monolayer on the quartz substrate (blue symbols in Figure 4, a1 = 0.44, τ1 = 1.5 (ns), a2 = 0.53, τ2 = 12.9 (ns)). The lifetime of the latter is similar to that found in the literature.24 The decrease in the lifetime of the hybrid polymer waveguide resulted from the Purcell effect;25 i.e., the enhanced density of states resulted in the faster emission rate. This finding indicates that the polymer waveguide serves as a photonic cavity that can couple the emission to a grating-coupled waveguide and enhance the emission rate. As an advantage of bottom-up nanoassembly, multiple light emission control was conducted by preparing the multilayered hybrid polymer nanoassemblies (Figure 5). Three different CdSe/ZnS Np arrays were embedded in a hybrid polymer nanoassembly with separation distance of 20 nm. The distance is larger than the Förster critical distance (7.3 nm) observed in similar material systems.26 The CdSe/ZnS Np monolayer was placed in the polymer nanosheet so that the position matches the maximum electromagnetic field of the waveguide modes at each emission wavelength. Narrowed emission (fwhm of ca. 6 nm) was observed at 12.9°, 0.7°, and 6.0°, for 525, 605, and 650 nm light emission wavelength. No significant decrease in the luminescence intensity was observed comparing with the single monolayer systems, indicating that the reabsorption effects

Figure 2. (a) Plot of the luminescence intensity vs input laser power. (b) Emission spectra from hybrid polymer nanoassemblies as a function of excitation light intensity. CdSe/ZnS Nps (λmax = 650 nm) were embedded in the assemblies. (Inset) emission spectra excited with 5 μW CW Ar+ ion laser beam (488 nm).

excitation because surface plasmon excitation involves only transverse magnetic (TM) polarization, as discussed later. It is noteworthy that the narrowed and enhanced emission was observed at specific angles. For example, the hybrid polymer nanoassembly (total thickness dtotal = 238.0 nm) generated narrowed emission at angles of 7.7° and −7.7° from the surface normal. The extent of the emission narrowing is independent of the incident angle of the excitation light. However, the maximum luminescence intensity was achieved by setting the incident light direction at a specific angle which satisfies the grating-coupled waveguide mode condition. Evidence of the grating-coupled waveguide mode operation was confirmed by varying the film thickness and the CdSe/ZnS Np monolayer position in the polymer nanosheet waveguide. It is particularly interesting that the emission direction is controllable by adjusting the film thickness (Figure 3a). This result also indicates that the phenomenon is based on a wellknown effect of distributed feedback cavities.21,22 Figure 3b shows emission spectra of hybrid polymer nanoassembly as a function of the film thickness. The emission peak wavelength shifted toward longer wavelength as the film thickness increased (Figure 3b). A linear relation is apparent between the emission peak wavelength observed at θD = 0° and the film thickness (Figure 3c). For example, the vertical emission from the CdSe/ZnS Np (λmax = 605 nm) monolayer was observed at dtotal = 268.0 nm (dashed line in Figure 3c). We also measured the light emission intensity at 605 nm as a function of the Y position (the position on the Y axis (Figure 3d)) of the CdSe/ ZnS monolayer while dtotal was kept constant at dtotal = 268.0 nm (Figure 3d). The emission intensity depends strongly on the Y position of the CdSe/ZnS Np monolayer. We calculated 2315

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Figure 3. (a) (red symbols) Emission direction (θD) of hybrid polymer nanoassemblies of CdSe/ZnS Np monolayer (λmax = 605 nm) as a function of the film thickness. (b) Emission spectra of CdSe/ZnS Np monolayer embedded in polymer nanosheets with different film thicknesses: (from left) 247.0, 254.0, 261.0, 268.0, and 275.0 nm. (c) Emission peak wavelength detected in the vertical direction vs film thickness. The emission peak shifted to a longer wavelength linearly as the film thickness increased. (d) The emission intensity from CdSe/ZnS Np monolayers at λem = 605 nm as a function of the Y position. Three lines correspond to the electromagnetic distribution at 488 nm (green) and 605 nm (red). For black line, see the text.

observed at 77 k for the smaller CdSe/ZnS Nps.15 It requires cooling because of the contribution of the Auger recombination process. Auger recombination is a process which deactivates multi electron−hole pair states in nanocrystals. The process requires nonlinear optical effects, leading to ASE and/or lasing. The consequence of ASE and lasing is spectral narrowing. In conclusion, we demonstrated spontaneous emission control of CdSe/ZnS Np monolayers using nanostructured hybrid polymer nanoassemblies as a novel photonic cavity. For laser operation, the lasing wavelength of CdSe/ZnS Nps appeared at longer wavelength than the emission peak because of the gain. No such restriction was necessary for our system; the narrowed light emission wavelength can be tuned precisely by controlling the film thickness and the detection angle. The narrowed light emission is based on linear cavity-coupled spontaneous emission. Polymer nanosheets are good candidates for producing integrated optically driven photonic devices, with good transparency and fine thickness tuning. The periodical nanoscale grating allows spectral narrowing as well as a coupled waveguide mode at both incident and emission light wavelengths. The coupling efficiency (80% at excitation wavelength, Figure S8) will be improved by optimizing the grating structure: pitch, depth, grating film thickness, and so forth. The system will be applicable to develop low-power photonic devices.

Figure 4. Time-resolved decay curves for CdSe/ZnS Np monolayer embedded in polymer waveguide on the silver grating surface (red, dtotal = 268.0 nm) and CdSe/ZnS Np monolayer assembled on a flat quartz surface (blue, dtotal = 25 nm). The data was taken at θex= 6.0, and θD = 0° for the silver grating and θex = 45°, and θ0 = 0° for the flat quartz. The experimental geometry in the waveguide matched the grating-coupled waveguide mode condition. A linearly polarized Nd:YAG laser beam (TE polarization, 532 nm, 300 ps pulse width, 60 μJ) was used as an excitation source.

might be negligible. It is particularly interesting that the spontaneous emission control allows spectral narrowing at room temperature for the 525 nm-CdSe/ZnS Np monolayer; the amplified stimulated emission (ASE) performance was 2316

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ACKNOWLEDGMENTS We thank Mr. Chikara Yasui, Kansai University, for grating preparation. This work was supported by a Grant-in-aid for Priority Area (No. 470, No. 21020005) and the Cooperative Research Program of “Network Joint Research Center for Materials and Devices”, from the Ministry of Education, Culture, Sports, Science and Technology of Japan. M. M. thanks Tokuyama Science Foundation for their financial support. K. T. thanks Toyo Gosei Co. Ltd. for providing UVcurable resin PAK-02-A.

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Figure 5. (top) Schematic of hybrid polymer nanossemblies with different CdSe/ZnS Np monolayers. (bottom) Emission spectra of (solid curves) hybrid polymer nanossemblies with different CdSe/ZnS Np monolayers. Regarding the detection conditions, see the text. (dashed curves) Emission spectra of CdSe/ZnS Nps in solution.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details of synthesis and preparation, film thickness of polymer nanosheets, AFM images of hybrid polymer nanoassembly, characterization of CdSe/ZnS Np monolayer, optical setup, luminescence image of hybrid polymer nanoassembly, emission spectrum for surface-plasmon excitation, angle-dependent reflectivity, and FDTD calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 2317

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