Nanosilver Colloids-Filled Photonic Crystal Arrays for Photoluminescence Enhancement
© The Author(s) 2010
Received: 29 April 2010
Accepted: 30 June 2010
Published: 14 July 2010
For the improved surface plasmon-coupled photoluminescence emission, a more accessible fabrication method of a controlled nanosilver pattern array was developed by effectively filling the predefined hole array with nanosilver colloid in a UV-curable resin via direct nanoimprinting. When applied to a glass substrate for light emittance with an oxide spacer layer on top of the nanosilver pattern, hybrid emission enhancements were produced from both the localized surface plasmon resonance-coupled emission enhancement and the guided light extraction from the photonic crystal array. When CdSe/ZnS nanocrystal quantum dots were deposited as an active emitter, a total photoluminescence intensity improvement of 84% was observed. This was attributed to contributions from both the silver nanoparticle filling and the nanoimprinted photonic crystal array.
Silver nanoparticles, which are among the most accessible and optoelectronically functional nanomaterials reported to date, can be applied directly to fabricate nanosilver dot arrays to produce localized surface plasmon resonance (LSPR)-coupled fluorescence enhancement. When a surface plasmon is formed on two-dimensional periodic arrays of nanosilver dots, the SPR energy is confined to each isolated dot, which is known as localized SPR (LSPR). The localized photoelectron energy in neighboring silver dots can be subject to electromagnetic field interactions. These lead to enhanced SPR effects with minimized in-plane propagation losses and provide improved sensitivity and coupled emission efficiency [1–3]. LSPR has therefore attracted considerable recent interest for applications in the fields of sensors and photo- and electroluminescence devices [4, 5].
One of the most active areas of research is the development of a method for fabricating silver nanopattern periodic arrays in a cost-effective, large area processible manner. Various top–down fabrication approaches have been proposed, including nanoimprinting followed by deposition  or lift-off [6, 7], and holographic lithography followed by reactive ion etching (RIE) . Alternatively, self-organization methods have attracted increasing research interest due to their large area processing capability and more competitive production costs than the top–down process. It has been reported that randomly distributed silver nanoclusters can be self-transformed from the sputter-coated silver film by the dewetting phenomenon resulting from the increased surface energy at elevated temperatures. This can be achieved on either horizontally leveled substrates [9, 10] or angled substrates with ion beam sputtered surface morphology . These randomly distributed array of nanosilver aggregates have been applied to introduce LSPR coupling effects in light emitting devices [12–15], antireflection [16, 17] and photovoltaics . However, this mechanism involves elevated temperatures to induce restructuring, which imposes thermal constraints on the processing, and has issues with controllability of the nanosilver array and size.
As an alternative solution for improving the structural control and process repeatability of silver nanodots, nanosilver colloid can be used to fill the predefined hole array via various self-guiding assembly strategies, such as electrochemical deposition , surface chemistry modulation , and PMMA layer lift-off . By applying different template pattern designs, various silver pattern array configurations can be reproduced successfully. Hence, with its enhanced patterning reliability, it should be an effective method for circumventing the technical limitations of the continuous metal thin-film self-transformation method described above. The predefined pattern can be generated by nanoimprint technology [19–21] with the template produced via either top–down fabrication or bottom–up self-organization methods. There have been a number of recent reports of self-organized template patterning, such as block copolymers, where an ionized nanosilver solution was introduced into the removed trench  and anodized porous alumina . However, this is a less preferred means of controlling pattern configuration than the top–down fabrication method.
For the improvement in LSPR-coupled photoluminescence efficiency, the present study present a much simpler process strategy to achieve a controlled array of nanosilver dots by directly filling nanosilver colloids into the nanoimprinted hole array. No processing step was included for the removal of imprint residue. Multiple spin coatings were applied to increase the nanosilver colloid filling rate, followed by optimized thermal annealing and removal of the colloidal residue. In comparison with previous methods, we have achieved the photoluminescence enhancement efficiency of greater than 80% over the reference sample, which should be due to the silver nanoarray-induced localized SPR with a two-dimensional photonic crystal structural effect.
Results and Discussion
where λ is the wavelength of the pump light, and ε 1 and ε 2 are the real parts of the dielectric constant of SiO2 and the silver nanoparticles, respectively. By trying several thicknesses of SiO2 (ranging from 20 to 80 nm), we found that the maximum efficiency was 60 nm, in good agreement with Eq. 1 based on the theoretical value of 52.9 nm found in previous reports [26, 27].
For the emission enhancement in organic light emitting devices, previous papers have achieved around 50% improvement [28, 29] and 56%  by the photonic crystal effect only. In these studies, the high-refractive index dielectric oxide filled the photonic crystal structure to increase the out-of-domain light directionality, and for planarization to reduce the current leakage during the electroluminescence operation. As a result, they produced a larger photonic crystal effect than in the present study, 33%, where the oxide fill-deposition of dielectric oxide for planarization was not applied. Therefore, direct comparison of the photonic crystal effect between this and previous studies is meaningless. Rather, it should be noted that the present approach of filling nanoimprinted hole arrays with nanosilver colloids creates LSPR coupling as well as simultaneously providing the planarization effect that otherwise ultimately gives rise to current leakage and efficiency degradation. Consequently, the 84% increase in photoluminescence over the control is considerably more than that achieved in previous studies using only the photonic crystal effect [28–30].
As a result of inserting nanosilver-filled photonic crystal structure array, the accumulated enhancement in the PL intensity from a layer of QDs, 84%, was achieved due to hybrid effect of silver nanoarray-induced localized SPR and outcoupling of wave-guided light in two-dimensional nanopattern array. Even in comparison with many previous studies that have focused on developing the process of metal pattern array only for LSPR coupling, the present approach of utilizing colloids provide a unique and competitive method of realizing metal nanopattern array on predefined patterns. Such competitive advantages should be derived in view of the greater process accessibility and repeatability even over conventional nanoimprinting because it is based on highly efficient direct nanoimprint, further without requiring residual layer removal and separate planarization steps.
There are several other opportunities to further increase the enhancement factors, probably by a factor close to or above 2 by improving several of the processing steps, including the silver colloid filling rate, residual colloid removal, and optimization of the nanopillar configuration, all of which are currently under investigation.
This research was supported by a grant (08K1401-00210) from the Center for Nanoscale Mechatronics & Manufacturing, one of the 21st Century Frontier Research Programs, and the Nano R&D program (Grant 2008-02773) supported by the Ministry of Education, Science and Technology of Korea.
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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