Photopatternable Quantum Dots Forming Quasi-Ordered Arrays

Jun 28, 2010 - Samsung Advanced Institute of Technology, Mt. 14-1, Yongin-si, Gyeonggi-do ... Corresponding authors: e-mail [email protected], phone +82 42...
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Photopatternable Quantum Dots Forming Quasi-Ordered Arrays Jong-Jin Park,†,| Prem Prabhakaran,‡,| Kyung Kook Jang,‡ YoungGu Lee,† Junho Lee,† KwangHee Lee,† Jaehyun Hur,† Jong-Min Kim,*,† Namchul Cho,‡ Yong Son,§ Dong-Yol Yang,§ and Kwang-Sup Lee*,‡ †

Samsung Advanced Institute of Technology, Mt. 14-1, Yongin-si, Gyeonggi-do 449-712, South Korea, ‡ Department of Advanced Materials, Hannam University, Daejeon 305-811, South Korea, and § Department of Mechanical Engineering, Korea Advanced Institute of Science and Technology, Daejeon 305-701, South Korea ABSTRACT We have functionalized core-shell CdSe/ZnS quantum dots (QDs) with a photosensitive monolayer, rendering them solution processable and photopatternable. Upon exposure to ultraviolet radiation, films composed of this material were found to polymerize, forming interconnected arrays of QDs. The photoluminescence properties of the nanocrystal films increased with photocuring. The material was found to be suitable for spin casting and was used as the active layer in a green electroluminescent device. The electroluminescence efficiency of devices containing a photocured active layer was found to be largely enhanced when compared to devices containing nonphotocured active layers. The material also showed excellent adhesion to both organic and inorganic substrates because of the unique combination of a siloxane and a photopatternable layer as ligands. The pristine functionalized nanocrystals could easily be used for two-dimensional patterning on organic and inorganic substrates. The photopatternable quantum dots were uniformly dispersed into a photopolymerizable resin to fabricate QD embedded three-dimensional microstructures. KEYWORDS Photopatternable quantum dots, photoordering, light-emitting diodes, hybrid functionalization

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corona constituting methacrylate and an inner siloxane layer, with a view making them photoresponsive and increasing their stability. Silanes are capable of forming dense films on nanoparticles. These dense films could act as a passivating layer and also play a role in reducing the toxicity of the QDs. This passivation activity enables the application of core-shell QDs as photoluminescent markers in the study of biological systems.18,19 To ascertain the role played by the siloxane layer and the methacrylate corona, we have synthesized two control samples. The first one is stabilized by a long alkyl chains with no siloxane or photopolymerizable group, and the second with siloxane layer but without the methacrylate groups. Quantum dots synthesized by conventional methods are unsuited for lithographic applications because of their undesirable aggregation through interdigitation of stabilizing ligand alkyl terminals when mixed into a patternable matrix. This is a common nanoparticles related phenomenon well reported in literature.20-23 We inferred that by proper functionalization QDs could be coustomized for photopatterning and also could be well integrated into photopatternable resins. For this study regarding patternable QDs, we have synthesized a series of three different types of QDs given in Figure 1a. By varying the ligand corona around the QDs, we looked at the difference in behavior of the nanocrystals based on their surface functionalization. Oleic acidstabilized CdSe/ZnS QDs were synthesized following a procedure reported earlier in the literature.24 The stabilizing ligand was then replaced by undecanethiol to give QDs with

he controlled fabrication of two- or three-dimensional micro- and nanoscale structures containing nanoparticles is of great scientific importance for the development of efficient optoelectronic devices.1,2 Quantum dots (QDs) are inorganic semiconductor nanocrystals that have tunable electrical and optical properties originating from their size-confined structure. QDs are used in various applications, including solar cells, photodetectors, and bioimaging.3-7 In recent years there have been many attempts to incorporate QDs into two-dimensional (2D) and threedimensional (3D) patterns through lithographic techniques.8,9 The widespread applicability of QDs depends on their stability and ease of solution processing.10-13 The stabilization of QDs against external factors, which could adversely affect their properties, can be achieved in two ways, namely, the introduction of a core-shell structure with an epitaxial shell of a higher-band gap material grown on it or by passivation with organic ligands. The former method of passivation employs ZnS, ZnSe, or CdS for the epitaxial layer and the latter introduction of a variety of organic ligands containing different functional groups.14-17 We have designed and synthesized green functionalized quantum dots composed of a photopolymerizable outer * Corresponding authors: e-mail [email protected], phone +82 42 629 8857, fax +82 42 629 8854; e-mail [email protected], phone +82 31 280 9311, fax +82280-9349. |

These authors contributed equally to this work. Received for review: 09/4/2009 Published on Web: 06/28/2010 © 2010 American Chemical Society

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FIGURE 1. (a) Shows the three types of quantum dots synthesized for this study, the alkyl-terminated 1, siloxane inner layer containing alkyl-terminated 2, and the photpatternable QD with siloxane inner layer and the photopolymerizable methacrylate corona 3. (b, c) Working of the photopolymerizable corona. The methacrylate groups in the outer periphery of 3 are photopolymerized in (b). A magnified view of the changes on the nanoparticle surface is given in (c).

an alkyl corona, 1, as given in Figure 1a. The siloxane containing 2 with alkyl-terminated ligands was obtained through a two-step process. The oleic acid on the QDs was first replaced with mercapto undecanol, the resulting QDs were then reacted with trimethoxy(octyl)silane to obtain 2. The methacrylate-terminated 3 was obtained by treating the mercapto undecanol capped QDs from the above procedure with 3-(trimethoxysilyl)propyl methacrylate. The functioning of the photopolymerizable methacrylate corona is summarized in parts b and c of Figure 1. (See Supporting Information for a detailed synthetic procedure.) The optical properties of the QDs were investigated before and after photofunctionalization with ultraviolet (UV) spectroscopy and photoluminescence (PL) studies. UV-vis characterization was performed using a Perkin-Elmer Lambda 14 spectrophotometer. PL spectra were obtained with a Fluoro Max-2 fluorescence spectrophotometer and fluorescence spectrometer (F-7000, HITACHI Co.). The UV-PL properties of the QDs were seen not to vary significantly with changing surface functionalization in the given series (see Figure S2 in the Supporting Information). To investigate the effect of photopolymerization on the emission properties of QDs, we carried out PL measure© 2010 American Chemical Society

ments on spin-cast QD films with and without UV exposure. Roughly 30 mg/mL solution of 1 (in chloroform), 2 (ethanol), or 3 (ethanol) was spin coated on a cleaned glass substrate. The spin-cast QD film was baked at 90 °C for 1 min to remove the solvent, and PL measurements were done for all cases on the film before and after photocuring for 24 h. The results from the PL studies are summarized in Figure 2. A general trend of increase in PL emission after UV irradiation was observed in all samples. For a given film of 2 or 3, a very high increase in PL intensity was observed when comparing photocured film with the nonphotocured film. In both cases the PL intensity increased over an order of magnitude after photocuring, with the latter showing a greater extent of enhancement. To further investigate the cause of this fluorescence enhancement, we looked at the morphology of the QDs before and after UV exposure by transmission electron microscopy (TEM) measurements. The TEM images of unexposed samples of 1, 2, and 3 are given in parts a, b, and c of Figure 3, respectively. The postexposure TEM images of 1, 2, and 3 are given in parts d, e, and f of Figure 3, respectively. The alkyl-terminated QDs do not show much change between the irradiated and nonirradiated samples 2311

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that UV exposure led to the cross-linking of QDs into an interconnected nanomatrix comprising periodically spaced particles. Closely packed arrays of QDs are generated through radical polymerization. The selective UV irradiation of such films can be implemented to rearrange the QDs through the creation of close-packed arrays composed of nanoparticles covalently cross-linked to each other by strong rigid bonds. There have been several reports of UV-induced PL enhancement in the past. Many different mechanisms have been proposed to explain photoinduced enhancement of PL in quantum dots; stabilization of surface defects on the quantum dots through irradiation, photobleaching of nanocrystals, solvent-induced enhancement of PL intensity, PL enhancement due to radiation-induced heat induction, PL enhancement due to influence of substrate surface on QD films during illumination, etc., are some of them.21-27 Photobleaching is usually characterized by a blue shift in UV absorption due to the decreasing active size of the QDs. There have also been reports of photodegradation in siloxane layer containing QDs because of oxygen trapped in the porous siloxane layer.27,32 It is difficult to pin down the mechanism of enhancement seen here to any one particular mechanism reported previously because of the novelty of the materials. Besides the mechanism of illumination induced enhancement is known to vary depending on the nature of the quantum dots and their ambient conditions.32 Some of the above mechanisms like heat induction due to illumination and substrate-assisted enhancement of PL could contribute to the observed enhancement of PL luminescence intensity in multilayered QD films.29,30 We could observe a clear increase in the extent of enhancement when going from siloxane-coated QDs (2) to the methacrylate-terminated QDs (3) for the same period of irradiation. Acrylate monomers undergo shrinkage when they are polymerized; this effect in the methacrylate-functionalized QDs can increase the optical density in photocured films. The shrinkage associated with polymerization of acrylates is a welldocumented phenomenon.33-36 Domains of ordered quantum dots extending over several tens of nanometers could be observed for the photopatternable quantum dots. However the dimensionality of ordering is not clear from our studies and would require further study. The length scales of the arrays and their properties would depend on several factors including variations in film thickness during spincoating, drying temperature, drying time, nanoparticle concentration, and UV exposure time (see Figure S3 of the Supporting Information for more TEM images). We believe that in the case of UV-irradiated photopolymerized core-shell QDs both the outer polymerized corona and the siloxane layer forming the inner corona around the QDs play pivotal roles in enhancing photoluminescence. It is difficult to verify the composition of the ligand monolayers on the QDs. To obtain the conclusive evidence of the increasing particle density in photocured films, we carried out energy-filtered TEM (EF-TEM) imaging employing the

FIGURE 2. Effect of irradiation on PL spectra. (a) The PL spectra of 1 before and after UV irradiation for one day. (b) The photoluminescence of 2 before and after UV irradiation under same conditions. (c) The large enhancement in PL of the photopatternable QD film after UV exposure compared with that before irradiation.

as seen in TEM images. Siloxane containing alkyl-terminated 2, shows largely disordered films after irradiation. This can be attributed to the aggregation of siloxane shell containing QDs due to their partial solubility in ethanol. In the case of photopolymerizable methacrylate-terminated 3, we found © 2010 American Chemical Society

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FIGURE 3. (a), (b), and (c) Respective TEM images of alkyl-terminated QDs, 1, siloxane-coated alkyl-terminated QDs, 2, and the photopatternable QDs, 3 before UV irradiation; the scale bar in (a) and (b) is 20 nm while that in (c) is 100 nm. (d) The TEM image of 1 after UV irradiation (scale 20 nm). (e) TEM image of 2 after irradiation (scale 20 nm). (f) 3 after irradiation (scale 100 nm). The inset in (f) shows a closer looks at the cross-linked QDs (scale 10 nm). (g, h) TEM and EF-TEM images of the same region of a TEM grid containing photocured domains of 3. The yellow regions represent the siloxane coating and the blue-green regions depict the core-shell CdSe@ZnS QDs (scale 20 nm). (i) A pictoral demonstration of the concept of photoinduced array formation in the QDs. Radical polymerization of the photopolymerizable corona leads to the formation of ordered arrays of nanocrystals (on a scale of tens of nanometers).

silicon postedge measurement at 99 eV (L23).37-39 TEM and EF-TEM (FEI Co. model Titan and Gatan Co. model 865 GIF Tridiem) images of the same area of a photopolymerized nanocrystal film are shown in parts g and h of Figure 3. With the help of this high-contrast technique, we could observe a yellow-colored corona of siloxane around the blue-green QDs. For nonpolymerized films, the silicon signals were not strong enough to be detected whereas these could be easily detected after photocuring. A graphic introducing the concept of photoordering is given in Figure 3i. © 2010 American Chemical Society

To further support the idea of the formation of a photoinduced network array of nanoparticles, we investigated the absorbance of dispersions, 3, during extended UV irradiation. The UV absorbance was observed over time to study several spectral features, such as the position, width, and height of the first exciton absorption of the QDs. The long UV exposure brought about a visual color change followed by a red shift in the excitonic peak and the formation of an amber-colored deposit after 17 h. It has been reported that at similar wavelengths, the absorption cross section of QDs 2313

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is proportional to the number of nanocrystals per unit volume.40 Thus, the increased UV absorbance of 3 can be explained by an increase in the nanonetwork array volume resulting from photopolymerization in solution. As previously reported, the peak position and intensity of the first exciton absorption band of the QD nanocrystals are good indicators of the CdSe core size and of the concentration of suspended QDs.41 The different changes in the peak position, width, and height observed during extended irradiation of 3 for 17 h are summarized in Figure 4a. The peak position of the first exciton absorption peak progressively moved to higher wavelength (496 nm f 500 nm, red-shifted 7 nm during the first 17 h of UV exposure) as a consequence of photopolymerization, see inset of Figure 4a. The increase in peak width can be attributed to the presence of polymerized fragments of different sizes in the QD dispersion. The red shift of the spectral response can be attributed to the formation of photopolymerized nanonetworks within the solution. The inherent properties of photopolymerizable QDs, such as their stability, PL, and ease of solution processability, make them suitable materials for the active layers of electroluminescent devices. The first example of a QD-based light-emitting diode (QD LED) was a device based on an indium-tin-oxide (ITO) supported hole-transporting poly(phenylene vinylene) (PPV) film interfaced to a capped CdSenanoparticle film with Mg top electrode.2 To evaluate the potential of this material, we fabricated electroluminescent devices in which the active layer was a spin-cast film of photofunctionalized QDs. To evaluate the role of photordering in the device performance, we carried out studies on devices with and without exposure of their active layer to UV light. The structure and characteristics of these devices are summarized in parts b and c of Figure 4. The EL properties of the QD-LED devices were examined using a five-layer LED-device configuration, as shown in the inset of Figure 4b, where the conducting polymer PEDOT was used as the hole-injecting layer. The thickness of the ITO anode was 150 nm, and the sheet resistance was 10 Ω. The hole-injection layers were spin-coated to give 55 nm thick films on top of the ITO glass after surface treatment (for 1 min) with oxygen plasma. Subsequently, the surfaces were baked in air at 110 °C for 2 h. An interlayer was spin-coated (the spin-coating step was 500 rpm for 5 s, 2000 rpm for 50 s, 500 rpm for 5 s) on the HILs (hole-injection layers), using TFB to obtain a 20 nm thick film, and then baked at 180 °C for 30 min and subsequently at 90 °C for 1 min. Then, a 30 mg/mL nanocrystal solution was spin-coated (the spin-coating step was 500 rpm for 5 s, 3000 rpm for 50 s, 500 rpm for 5 s) on top of the interlayer and baked at 90 °C for 1 min. Subsequently, a 30 nm thick tris(8-hydroxyquinoline)aluminum(III) (Alq3) film was thermally deposited onto the QD layers, thereby serving as electron transport layer (ETL). LiF (1 nm) and Al (100 nm) layers were sequentially deposited (under vacuum; P < 10-6 Torr) onto the ETL. © 2010 American Chemical Society

FIGURE 4. (a) Results obtained from the prolonged UV exposure of 3. The graph compares the UV absorption of a solution of 3 before continuous UV irradiation to absorptions obtained for prolonged irradiation times ranging from 400 s to 17 h. The red shift of the excitonic peak is indicative of polymerization. The inset highlights the change. (b) Structure and performance of electroluminescent devices fabricated with and without photocured green-quantum-dot layer as the active component. The graph shows the EL efficiency vs current density characteristic of the devices before and after UV exposure, and the inset shows the structure of the device. The cured device shows maximum external quantum efficiency (EQE) of 0.62% at 588 cd/m2 whereas the uncured device shows an EQE of 0.53% at 703 cd/m2. (c) Comparison of the green emissions from the quantum dot sand AlQ3. Because of the distinctive signature of both compounds, they are easily recognizable in the CIE coordinate system given in the inset. A second inset shows an image of the working device. 2314

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Finally, the QD-LED unit devices were encapsulated with a cover glass for characterization in the glovebox using a UVcurable epoxy resin and a CaO getter. The current-voltageluminescence (I-V-L) characteristics of the devices were obtained using a Keithley 238 unit and a Photo Research PR650 spectrophotometer. The image of a working device is given in one of the insets in Figure 4c. In green electroluminescent devices involving Alq3, it is usually difficult to distinguish between the electroluminescence of the green active component and that of Alq3. In the case of the present device, we could readily distinguish the electroluminescence associated with green QDs because of the distinctive fingerprints of the QD and Alq3 emissions, as given in CIE coordinates in the inset of Figure 4c. From the electroluminescence (EL) spectrum, it could be clearly inferred that light emission was generated from the QDs and not from Alq3 (see Figure 4c). The device whose active layer was exposed to UV light showed a strong green emission with a quite low turn-on voltage (i.e., the voltage needed to achieve a brightness of 1 cd/m2) at 4.8 V. The luminance reached a value of 4384 cd/m2 at a drive voltage of 13 V and a current density of 182 mA/cm2, corresponding to an efficiency of 2.40 cd/A and a luminosity of 0.58 lm/W. The maximum external quantum efficiency was 0.62% at 588 cd/m2 (with a bias of 9.5 V). It is well-known that conjugated polymers absorbing visible light degrade upon intense irradiation. The photoinduced degradation of conjugated polymers is accelerated in the presence of oxygen. Photooxidation severely affects the molecular structures of poly(3, 4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) and poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sbutylphenyl))diphenylamine) (TFB) upon UV exposure after coating photosensitive QDs as the emitting layer. In the case of the device whose active layer was not exposed to UV radiation, the luminance reached a value of 2996 cd/m2 at a drive voltage of 13 V and a current density of 191 mA/ cm2; this corresponds to an efficiency of 1.57 cd/A and a luminosity of 0.38 lm/W. The maximum external quantum efficiency is 0.53% at 703 cd/m2 and a bias of 10.0 V. Although the photooxidation of the conducting polymer affects the device performance, the photoinduced nanomatrix array composed of QDs enhances the EL efficiency by up to 52% when compared to the efficiency of the devices without UV exposure. Photopatternable acrylated QDs exhibit hybrid organicinorganic properties which result from the different components attached to them. The formation of inorganic siloxane on their surface improves their adhesion to inorganic substrates while the photopolymerizable organic layer interacts with organic substrates, leading to flexible adhesion properties. An ITO glass was cleaned several times in neutral detergent and isopropyl alcohol (alternatively) and then treated in an ozone generator for 15 min. Then, a photofunctionalized QD solution (148.5 mg/mL) was spin-coated (the spin-coating step was characterized by 500 rpm for 5 s, © 2010 American Chemical Society

2000 rpm for 20 s, 500 rpm for 5 s) on top of the ITO glass and baked at 90 °C for 0.5 min and then at 90 °C for 1 min. The coated ITO glass was then exposed (for 5 min) to UV light through a photomask. A broad range of UV light was used (i.e., 300-400 nm), and the source power was 33.2 mW/cm2 at 360 nm. After the mask was removed, the unexposed part of the nanocrystal film was rinsed with ethanol (for 1 min) and dried at 90 °C for a minute. Following a process similar to the ITO coating, we fabricated 2D patterns on flexible polyethylene terephthalate (PET) films, except that the drying conditions were 60 °C for 0.5 min. Parts a and b of Figure 5 show fluorescing nanocrystal patterns on glass and PET substrates, respectively (excited at 365 nm). The prominent fluorescence of the PET film in this region of the spectrum causes the green fluorescent QD patterns to appear blue. Different lithographic techniques have been employed in the past to achieve quantum dot embedded three-dimensional microstructures since they are of enormous importance in photonic devices.9,20,42 A pre-existing problem in all the previous cases where lithography was attempted with photopatternable materials mixed with QDs is the detrimental aggregation effects of the nanocrystals.20,42 These effects are governed by the interactions between the surface ligands of the QDs, as well as their incompatibility with the photopatternable material in which they are dispersed. We reasoned that the acrylate-terminated QDs could be integrated well into acrylate and urethane acrylate resins due to their photopatternable corona. A urethane acrylate resin SCR 500 (1 g) was mixed with 3 in ethanol (2 mL, 1.125 mg/mL in ethanol). The excess solvent was removed by slow vacuum evaporation. The resultant resin did not show large aggregates of QDs as has been the case with previous studies.42 This resin was used to fabricate two- and three-dimensional quantum dot dispersed microstructures by two-photon nanostereolithography.43-45 A highly efficient flourene-based twophoton sensitizer TP-MOSF-TP (1 mg) was mixed into the QD dispersed resin before the lithographic process. The fabricated structures were visualized by both scanning electron microscopy (SEM) (JSM-6400 JEOL) and confocal microscopy, and the images are summarized in panels c-f in Figure 5. Confocal microscopy was employed to image various planes of the microfabricated 3D structure to discern the successful and uniform incorporation of nanocrystals within the microstructure. The two-photon dye used as the photosensitizer was chosen such that the peak fluorescence of the dye does not coincide with that of the QD fluorescence when imaging the QD-impregnated structures. A rotated image of a three-dimensional woodpile structure generated by stacking the confocal images of different layers of the 3D woodpile is given in Figure 5f. In conclusion we have synthesized new photopatternable quantum dot with an inner siloxane layer and a photopatternable methacrylate corona. We have been able to demonstrate the influence of photopatternable functionalization 2315

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FIGURE 5. (a, b) Hybrid inorganic-organic properties of the photofunctionalized quantum dots by which they could be readily spin-cast and patterned by irradiation on both glass (scale 22.5 mm) and the flexible organic substrate PET (scale 35 mm). The inset in (a) shows a magnified image of the pattern on the glass substrate. (c-f) TEM and confocal fluorescence microscopy images of a two-dimensional map structure (map of Korea and Japan) and a three-dimensional wood-pile-like array structure fabricated by incorporating the acrylate-functionalized quantum dots into a urethane acrylate resin. (g) Series of rotated 3D stacks of confocal images of various planes of the 3D structure, demonstrating the uniform cross-linking of quantum dots throughout the structure.

on the optical properties of QD films after photocuring. Photopatternable QD films were found to form dense quasiordered arrays postexposure. Photopolymerization driven shrinkage of patternable QD films resulted in an increase of particle density in the photocured films, as well as a large enhancement in photoluminescence. The phenomenon of photodriven ordering in functionalized QDs is previously unreported to the best of our knowledge and provides a new method to control the microstructure of the quantum dot films. An exposed active layer in an electroluminescent device was found to show greater electroluminescence efficiency when compared to devices with unexposed QD active layers. The hybrid nature of the photopatternable QD makes it readily suitable for solution processing on both inorganic and organic substrates and subsequent photopatterning. Finally the chemical compatibility of photopattern© 2010 American Chemical Society

able QD as an active emitting material in polymeric microstructures has been successfully demonstrated by the fabrication of three-dimensional shining structures from QD dispersed urethane acrylate resins. Acknowledgment. This work was supported by the Samsung Research Grant. One of authors (K.-S. Lee) would like to acknowledge support of the National Research Foundation of Korea (Project No. 2010-000499 and 20100001696) and the Asian Office of Aerospace Research and Development (AOARD-09-4035), AFOSR. D.-Y. Yang also thanks for the support by the Nano R&D program of NRF 20090082831. Supporting Information Available. Details of materials used for synthesis and synthetic procedure for various QDs used in this study (Figure S1), the UV-PL spectra of surface 2316

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modified quantum dots at different stages of synthesis of the photopatternable QD (Figure S2), procedure for TEM imaging, TEM images showing the photoordering of quantum dots (Figure S3), the I-V characteristics and luminance characteristics of the QD-OLED devices with and without photocured active layer (Figure S4), details of the laser setup used for two-photon nanostereolithography, details of the confocal microscopy used to image the structures fabricated by two-photon lithography, and the structure of the twophoton sensitizer dye as well as comparison of its emission to that of photopatternable QDs (Figure S5). This material is available free of charge via the Internet at http://pubs.acs. org.

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DOI: 10.1021/nl101609s | Nano Lett. 2010, 10, 2310-–2317