Letter pubs.acs.org/NanoLett
Highly Enhanced Fluorescence Signals of Quantum Dot−Polymer Composite Arrays Formed by Hybridization of Ultrathin Plasmonic Au Nanowalls Soo-Yeon Cho,† Hwan-Jin Jeon,‡ Hae-Wook Yoo,§ Kyeong Min Cho,† Woo-Bin Jung,† Jong-Seon Kim,† and Hee-Tae Jung*,† †
Department of Chemical and Biomolecular Engineering (BK-21 Plus), Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ‡ Department of Nano-Structured Materials Research, Korea National Nanofab Center, Daejeon, Republic of Korea § Defence Advanced R&D Center, Agency for Defense Development, Daejeon 305-600, Korea S Supporting Information *
ABSTRACT: Enhancement of the fluorescence intensity of quantum dot (QD)−polymer nanocomposite arrays is an important issue in QD studies because of the significant reduction of fluorescence signals of such arrays due to nonradiative processes in densely packed polymer chains in solid films. In this study, we enhance the fluorescence intensity of such arrays without significantly reducing their optical transparency. Enhanced fluorescence is achieved by hybridizing ultrathin plasmonic Au nanowalls onto the sidewalls of the arrays via single-step patterning and hybridization. The plasmonic Au nanowall induces metal-enhanced fluorescence, resulting in a maximum 7-fold enhancement of the fluorescence signals. We also prepare QD nanostructures of various shapes and sizes by controlling the dry etching time. In the near future, this facile approach can be used for fluorescence enhancement of colloidal QDs with plasmonic hybrid structures. Such structures can be used as optical substrates for imaging applications and for fabrication of QD−LED devices. KEYWORDS: Quantum dots, nanopattern, lithography, metal-enhanced fluorescence, secondary sputtering
Q
printing method using QD composite solutions and a poly(dimethylsiloxane) (PDMS) stamp or a perfluoropolyether elastomeric mold has been performed to prepare QD−polymer composite arrays of various sizes and shapes.11−13 The direct patterning technique using a photosensitive or electron-beamsensitive resin as matrix polymer has been utilized to generate periodic QD polymer composite arrays.14−16 In addition, inkjet printing has been done to minimize the use of QD−polymer composites and to realize multiple deposition of high-resolution patterned layers of QDs.17,18 Despite intensive studies on QD−polymer nanocomposite arrays, these materials still lead to significant reduction of fluorescence signal intensity compared with that obtained with pristine QD array structures. It is well-known that the fluorescence intensity of QDs in polymer composite films is several times lower than that in solution because of the much larger contribution of nonradiative processes in densely packed polymer chains in solid films.19−21 In addition, the amount of QD particles embedded in polymer composites is largely
uantum dot (QD) polymer nanocomposites have attracted considerable attention in recent years because of their high processability, mechanical stability, and compatibility with typical packaging approaches for colloidal QD applications.1−3 Simple spin-coating QD−polymer composites allow precise control of fluorescence location, detection of the parameters of biomolecules, and enhanced energy efficiency for organic solar cells. In this method, QDs are dispersed in polymer matrixes such as polystyrene (PS) and poly(methyl methacrylate), and conjugated polymers are used for spatial control of particles or conformal film formations.4−7 To exploit fully the advantages of using QD polymer nanocomposites, localization of QD polymer composites in predefined two-dimensional (2D) arrays is strongly required. This technique enables localized capture and detection of fluorescent analytes or analysis of single fluorescence intensities of QDs.8,9 With this approach, high-resolution patterning and stacking of QDs, accurate control of registration of pixelated geometries of red−green−blue QDs, and efficient utilization of materials, as well as minimal chemical contamination, may all be realized.10 Several patterning methods, including microcontact printing, direct patterning techniques, and conventional inkjet printing, have been used to localize QD polymer nanocomposites in 2D predefined locations. The microcontact © XXXX American Chemical Society
Received: June 14, 2015 Revised: September 21, 2015
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DOI: 10.1021/acs.nanolett.5b02355 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Schematic illustrations showing the fabrication of a QD/PS pattern hybridized with a Au nanostructure. (a) The QD/PS composite solution is spin-coated onto the glass substrate. (b) A predetermined dot pattern is transferred via microcontact molding of the PDMS mold. (c,d) The residue is removed by dry etching using RIE, and QD/PS patterns of various sizes are fabricated. (e) The Au layer is deposited onto the QD/PS prepatterns and (f) coated onto the side surface of the prepatterns via secondary sputtering. (g) Significantly enhanced fluorescence intensity is confirmed through confocal microscopy. (h) Detailed mechanism of wide-angle sputtering.
reduced and washed away with residue polymer during the patterning and localization processes, resulting in significant reduction of fluorescence intensity.22 These critical limitations result in low efficiency of operation and high power consumption of QD-based technologies, such as QD−LEDs (QLED), biosensors, and QD-based imaging applications. Therefore, novel methods for enhancing the fluorescence intensity should be developed to widen the range of applications of QD−polymer composites. For instances, in biosensing application, we can detect very small amounts of target biomolecules such as protein and DNA while preserving a precise spatial control using the fluorescence enhancing technology. Precise spatial control of target biomolecules on the predefined 2D array is a critical requirement for biosensor or lab-on-chip fabrication. However, when patterning and localization process is applied, only single/few emitters can be on the predefined position. With the fluorescence enhancing technology, easy detection of small amounts of localized target emitters can be realized. In QD based imaging devices field, especially QD-LED, the patterning process of QD emitters should be developed for accurate control of registration of pixelated geometries of red−green−blue QDs. If the fluorescence enhancing technique of QD localized array is developed, increasing emission rate of QD-LED without significant reduction of the optical transparency can be realized,
resulting in low power consumption and high resolution images. To our knowledge, however, there have been few reports on the fluorescence enhancement of patterned QD polymer composites, perhaps because of the difficulties in developing simple processes for QD−polymer nanocomposite arrays and in greatly enhancing the fluorescence signal intensity. In this research, we significantly enhance the fluorescence intensity of QD−polymer composite arrays by hybridizing an ultrathin Au layer (10 nm thick) onto the sidewalls of the composite arrays through single-step patterning and hybridization. The Au nanoparticles (∼5 nm) are etched and coated onto the side surface of the QD composite pattern with a wideangle distribution by low-energy Ar ion bombardment.23−26 Unlike previous approaches, this method performs QD patterning and Au hybridization in a single process, which is difficult to achieve with previous techniques. We show that the strong near-field excitation induced by the plasmonic ultrathin Au layer leads to an approximately 7-fold enhancement of the fluorescence signal of the QD composite array compared with that of a pristine QD composite array without Au nanowalls. This enhancement is attributed to metal-enhanced fluorescence (MEF) resulting from the interactions between excited states of the QDs and induced surface plasmons of thin metal nanostructures.27 We observe that the dimensions of the B
DOI: 10.1021/acs.nanolett.5b02355 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. (a) Line, concentric circle, and dot patterns of the QD/PS nanocomposites. (b) SEM image (left) and AFM profile (right) of the QD/PS composite with a Au nanowall hybrid structure. An ultrathin Au nanowall attached to each QD/PS pattern of the hexagonal array may be clearly observed. (c) Energy-dispersive X-ray spectra show that QDs were perfectly patterned onto the hexagonal array and that the Au nanowall was attached onto the side surface of the QD pattern only.
features (that is, the diameters of the hybridized QD patterns) in the composite array strongly influence MEF enhancement and that the enhancement factor increases (1.4 → 7) as the diameter of the Au nanowall structure decreases (400 → 150 nm), consistent with finite-difference time-domain (FDTD) simulation results. Because of the plasmonic Au nanowall, this technique can provide a confinement structure for mechanical support with good processability and can significantly enhance the energy of electromagnetic radiation incident on the QD− polymer nanocomposite. Results and Discussion. The overall process for singlestep fabrication of the QD−PS composite pattern and hybridization of the ultrathin Au layer is displayed in Figure 1. After solutions of QD (20 mg mL−1 CdSe/ZnS in toluene) and PS (1800 g mol−1 in toluene) were mixed, the resulting mixture was spin-coated onto the target substrate (Figure 1a). Cylindrical QD/PS patterns were then prepared via pattern transfer using a PDMS mold (Figure 1b) in which QD particles embedded in a PS matrix were driven to the voids of the mold pattern by capillary forces. It should be noted that although we used a cylindrical pattern with a 400 nm diameter (d) as the master pattern for the PDMS mold, various types of QD patterns could be fabricated, provided that master patterns with different shapes and feature dimensions are employed. The composite residue was subsequently removed via dry etching using reactive-ion etching (RIE). The remaining composite formed cylindrical QD/PS nanocomposite structures. At this point, we could simultaneously remove the residue film and control the shape and dimensions of the QD nanostructure to within a diameter range of ∼400 nm (Figure 1c) to
