Silver nanoparticles and quantum dots (QDs) are both high-refractive-index materials. In addition, silver possesses plasmonic properties, and QDs are highly efficient fluorescers. These properties can be employed in various optical devices. However, aggregation of the nanoparticles presents challenges. They are prone to agglomerate to reduce a very high surface energy. The size, shape, and extent of aggregation affects the optical properties of noble metal nanoparticles. Consequently, it is necessary to control the agglomeration to control the optical properties of microstructures containing them. This can be achieved either by chemical functionalization of the nanoparticle surface or by controlling the formation dynamics of nanomaterials inside hybrid structures.
We have successfully used two-photon lithography (TPL) to fabricate microstructures containing silver or QD nanoparticles. TPL is a fast-prototyping method that photoinduces chemical reactions in a patternable medium, allowing direct writing of microstructures. Femtosecond lasers with high repetition rates induce very specific chemical changes at the focal spot of the laser within a photoactive medium. Two-photon sensitivity is key to initiating chemical changes during microfabrication. For this reason, the patternable medium should contain a two-photon-absorbing (TPA) material acting as a sensitizer or an initiator. The spatial selectivity of the chemical process arises from the inverse dependence of two-photon absorption on the intensity of the laser beam. Polymeric as well as inorganic-organic hybrid materials have previously been used to fabricate the microstructures.1–4
We used a combination of photocuring and thermal curing to generate silver nanoparticles inside cured epoxy microstructures.5We used SU8 (an epoxy resin widely employed in microelectromechanical systems), a highly active spirofluorene-based TPA dye as a sensitizer, gamma-butyrolactone (GBL) as a solvent, silver trifluoroacetate (AgTFA) as the silver source, and triphenylsulfonium hexafluoroantimonate as a photo acid generator. The photo acid generator serves to initiate the ring-opening cationic polymerization in the SU8 epoxy resin (see Figure 1). Photoreduction of a 3wt% solution of AgTFA in GBL formed a large number of nucleates. The average size of the nanoparticles generated was 2.5±1.0nm. We compared a solution photocured for 120 seconds with one that was cured at 240 seconds and found an increase in intensity in the 325nm region, indicating an increase in the number of nucleates rather than an increase in the aspect ratio of the particles. Thermal curing another solution of the same concentration showed larger nanoparticles with fewer nucleates and larger nanoparticles (red-shift in UV absorption). This could easily be explained by the fusion of smaller nanoparticles into bigger ones. The average particle size was found to be 3.1±1.2nm.
Figure 1. (a) Functionalized SU8 epoxy resin, (b) gamma-butyrolactone, (c) triphenylsulfonium hexafluoroantimonate photo acid generator, (d) silver trifluoroacetate, (e) spirofluorene-based two-photon-absorbing dye. N: Nitrogen.
Having studied the effect of photo as well as thermal curing, we combined the two to cure epoxy resin (SU8) films, to which we had added 8wt% AgTFA. We photocured the films for two minutes before heating them at 120°C for another two minutes. The films' UV absorption showed that the photocuring generates nucleates, with typical surface plasmon absorption at 325nm for small ones. On heating the films, the UV absorption shifted to a longer wavelength region, indicating an increase in the aspect ratio of the nanoparticle. We confirmed this by transmission electron microscopy (TEM), which showed nanoparticles with an average particle size of 4.5±2.0nm. We also recorded films' UV spectra and took TEM images: see Figure 2(a) and (b).
Figure 2. UV spectrum of silver nanoparticles generated inside a cured SU8 film by a combination of photo and thermal curing (a). Transmission electron microscope images of silver nonoparticles generated inside a cured SU8 film with a combination of photo and thermal curing (b). Scanning electron microscope (SEM) image of 3D woodpile structures fabricated using two-photon lithography of SU8 resin (c). SEM image of silver nanoparticles embedded in microstructures (d). (Reproduced with permission from Wiley.)
Silver nanoparticles show high optical nonlinearities and plasmonic effects. During two-photon lithography, the interaction of the laser with previously synthesized nanoparticles in photopatternable resins can cause undesirable effects. We already knew that photocuring is efficient in generating large numbers of nucleates and that thermal curing can fuse smaller nanoparticles into larger ones. Combining the two techniques gave us 3D epoxy-cured microstructures containing embedded silver nanoparticles made in situ. We used an SU8 resin containing 0.2wt% AgTFA to fabricate microstructures. The nanoparticles nucleated during two-photon lithography and grew during subsequent baking at 90°C for 10 minutes. The embedded nanoparticles can be clearly seen in scanning electron microscope images of a woodpile microstructure fabricated by this approach: see Figure 2(d).
Unlike silver, nanoparticle QDs do not exhibit intense plasmonic interaction with a laser during fabrication. Though these materials show optical nonlinearities, they are not as strong as those of silver. Effective surface functionalization is a prerequisite for successful application of QDs.6 This helps to reduce the aggregation of QDs and to increase their dispersability in photopatternable media. We have rendered the QDs photopatternable by stabilizing them with ligands containing appropriate groups. We surface-functionalized cadmium selenide/zinc sulfide (CdSe@ZnS) QDs to have a spacer ligand followed by an inner siloxane layer and a photopatternable methacrylate outer layer: see Figure 3(a).7 We achieved this through an initial ligand exchange reaction followed by a series of condensation reactions.
Figure 3. Photopatternable cadmium selenide/zinc sulfide quantum dots (QDs) (a). One-photon patterning on glass (b). One-photon patterning on flexible poly(ethylene terephthalate) substrate (c). SEM image of QD-doped polymeric structure fabricated by two-photon lithography (d). Confocal microscopy image of a QD-doped polymeric microstructure (e). (One-photon patterning refers to conventional lithography carried out with low-intensity light sources. The ease with which the QDs can be patterned using this technique demonstrates the versatility in application of functionalized QDs.)
The siloxane groups and the methyl methacrylate groups on the QDs give them a hybrid nature that allows them to be used on inorganic as well as organic substrates. We could easily spin-coat the material onto substrates of glass or plastic. The inner siloxane layer promotes surface interaction and is also thermally crosslinkable. The outer methacrylate groups can crosslink on UV irradiation. For planar one-photon patterning this material is spin-coated and thermally cured at 90°C to remove the solvent. On photocuring, the QDs were found to increase in density because of crosslinking, which enhanced the photoluminescence properties by almost two orders of magnitude. We also tested this material in an electroluminescence device, which showed a similar enhancement of electroluminescence properties when the active QD layer was photocured. These QDs could be doped into photopatternable urethane acrylate resins and used to make complex 3D structures with two-photon lithography. We used the TPA dye in Figure 1(e) as a sensitizer for the microfabrication. The results from patterning experiments involving QDs are summarized in Figure 3(b–e). Photocuring presents a new pathway of controlling the microstructure and properties of nanoparticle-containing films.
In summary, incorporating nanoparticles into microstructures through two-photon lithography provides a fast alternative to using the combination of lithography and vacuum deposition techniques for creating high dielectric prototypes for photonics. We have demonstrated the possibility of using chemistry to solve issues in such fabrication techniques by developing methods for in situ synthesis of nanoparticles in the case of silver and photopatternable functionalization of QDs. These methods are effective in uniformly incorporating nanoparticles and minimizing their aggregation. It is tedious to optically characterize these nanoparticle-incorporated microstructures because of problems with accurately aligning optical components on the microscale. We are currently investigating how to characterize these systems' optical properties. We can obtain better results by varying the size and shape of the microstructures as well as the extent of nanoparticle incorporation.
This work is supported by the National Research Foundation of Korea (projects R11-2007-050-0000-0 and 2010-000499) and the Asian Office of Aerospace and Development, Air Force Office of Scientific Research.
Kwang-Sup Lee, Prem Prabhakaran, Kyung Kook Jang
Department of Advanced Materials
Daejeon, South Korea
Kwang-Sup Lee is a professor whose current research focuses on the development of organic optoelectronic materials, including two-photon, solar cell, and organic semiconducting materials.
Samsung Advanced Institute of Technology
Yongin, South Korea
Dong-Yol Yang, Yong Son
Department of Mechanical Engineering
Korea Advanced Institute of Science and Technology
Daejeon, South Korea
1. S. H. Park, T. W. Lim, S. H. Lee, D.-Y. Yang, H. J. Kong, K.-S. Lee, Fabrication of microstructures using double contour scanning (DCS) method by two-photon polymerization, Polymer (Korea) 29, pp. 146–150, 2005.
2. T. A. Pham, D. P. Kim, T. W. Lim, S. H. Park, D.-Y. Yang, K.-S. Lee, Three-dimensional SiCN ceramic microstructures via nano-stereolithography of inorganic polymer photoresists, Adv. Funct. Mater. 16, pp. 1235–1241, 2006.
3. K.-S. Lee, R. H. Kim, D.-Y. Yang, S. H. Park, Advances in 3D nano/microfabrication using two-photon initiated polymerization, Prog. Polym. Sci. 33, pp. 631–681, 2008.
4. S.-H. Park, D.-Y. Yang, K.-S. Lee, Two-photon stereolithography for realizing ultraprecise three-dimensional nano/microdevices, Laser Photon. Rev. 3, pp. 1–11, 2009.
5. J.-J. Park, X. Bulliard, J. M. Lee, J. Hur, K. Im, J.-M. Kim, P. Prabhakaran, N. Cho, K.-S. Lee, S.-Y. Min, T.-W. Lee, S. Yong, D.-Y. Yang, Pattern formation of silver nanoparticles in 1-, 2-, and 3D microstructures fabricated by a photo- and thermal reduction method, Adv. Funct. Mater. 20, pp. 2296–2302, 2010.
6. W. J. Kim, S. J. Kim, K.-S. Lee, M. Samoc, A. N. Cartwright, P. N. Prasad, Solution-processed pentacene quantum-dot polymeric nanocomposite for infrared photodetection, Nano Lett. 8, pp. 3262–3265, 2008.
7. J.-J. Park, P. Prabhakaran, K. K. Jang, Y. Lee, J. Lee, K. Lee, J. Hur, J.-M. Kim, N. Cho, Y. Son, D.-Y. Yang, K.-S. Lee, Photopatternable quantum dots forming quasi-ordered arrays, Nano Lett. 10, pp. 2310–2317, 2010.