Plasmonic metallic nanostructures by direct nanoimprinting of gold nanoparticles

Plasmonic metallic nanostructures by direct

nanoimprinting of gold nanoparticles

Wen-Huei Chang,4 and Chun-Hung Lin1,5,*

1_{Institute of Electro-Optical Science and Engineering, National Cheng Kung University. Tainan 701, Taiwan} 2_{National Nano Device Laboratories, Hsinchu, 300, Taiwan}

3_{Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807,} Taiwan

4_{Department of Chemical Biology, National Pingtung University of Education, Pingtung 900, Taiwan} 5_{Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan 701, Taiwan}

*[email protected]

Abstract: We demonstrated the plasmonic metallic nanostructure fabricated

by direct nanoimprinting of gold nanoparticles (AuNPs). This approach combines the patterning and lift-off processes into a simple one-step process without the need for expensive patterning lithographies and the stringent requirement of the lift-off process for nanostructures. Good imprinting integrity was accomplished with a negligible residual layer. The dynamic optical responses of the imprinted gold pillars from AuNPs to the bulk material during the annealing process were investigated. The localized surface plasmon resonance (LSPR) properties of AuNPs or gold pillar arrays can be controlled and tuned during the annealing process. The sensitivity of the gold pillar array in terms of the wavelength shift per refractive index unit (RIU) reached 259 nm/RIU. The size of the imprinted gold pillars is highly scalable in our process. The corresponding resonance wavelengths can be widely tuned from the visible to infrared region by changing the size of the gold pillars, thus providing a wide range of sensing capability.

©2011 Optical Society of America

OCIS codes: (160.3918) Metamaterials; (240.6680) Surface plasmons; (220.4241) Nanostructure fabrication; (220.3740) Lithography.

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1. Introduction

Metallic metamaterials have been demonstrated to support surface plasmon resonance (SPR) [1]. There are two kinds of SPR. Propagating surface plasmon resonance [2] can be defined as the charge density wave propagating along the continuous metal-dielectric interface. Another kind of SPR is localized surface plasmon resonance (LSPR) [3, 4]. When light with a proper wavelength illuminates the metal structures, free electrons oscillate tremendously in a localized region. The electromagnetic field near metallic surface is highly enhanced. The spectral position of the plasmon resonance is sensitive to the dielectric environment within

#140585 - $15.00 USD Received 11 Jan 2011; revised 9 Feb 2011; accepted 12 Feb 2011; published 25 Feb 2011

near field region. SPR-based sensors have been widely applied for biosensing owing to their benefits of high sensitivity, real-time monitoring, and label-free sample preparation [2, 5].

Traditionally, metallic nanostructures can be patterned by electron beam lithography (EBL) [6] or optical lithography [7] with subsequent metal evaporation and lift-off processes. Direct metallic deposition and patterning with focus ion beam (FIB) lithography is another approach to pattern metallic nanostructures [8]. However, the aforementioned lithographies are expensive, and both EBL and FIB are time-consuming. Moreover, an undercut profile is required to facilitate the lift-off process. The process is difficult to control in a single-layer resist system, especially for lift-off nanostructures. Multilayer schemes were proposed to create undercut features [9, 10], and the whole process became even more complicated. Solution-processible gold nanoparticles (AuNPs) spin coated on the patterned resist were proposed [11, 12] to simplify the fabrication process without the use of a metal evaporation vacuum chamber. Nevertheless, an additional lift-off step was still required to remove the patterned resist. In contrast with the above-mentioned methods, bottom-up approaches, such as nanosphere lithography [13] and hole-mask colloidal lithography [14], provided simple and cost-efficient ways to pattern the metallic nanostructures. Although their self-assembling nature restricted the producible pattern shapes, several pattern shapes, such as nanodisc, triangular, nanoring, crescent, and nanocone, have been fabricated [13–16].

Recently, direct nanoimprinting of AuNPs was investigated to fabricate nano/microscale electronic devices [17, 18]. Nanoimprinting lithography (NIL) has attracted great attention as an alternative nanopatterning technology that allows the fabrication of two-dimensional (2D) or three-dimensional structures with nanoscale resolution [19]. Compared to other lithographies, NIL has the advantages of being high throughput, low cost, and high resolution [20]. With the direct nanoimprinting of AuNPs, this approach combines the patterning and lift-off processes into a simple one-step process. In previous works [17, 18], the electrical properties of nano/microscale electronic devices fabricated by this approach were studied. In contrast, we focused here on the optical behavior of the imprinted metallic nanostructures. 2D photonic crystals of gold pillars were fabricated by the direct nanoimprinting of AuNPs. The fabrication parameters were studied and optimized. The resonance behavior and the sensing property of the fabricated plasmonic nanostructures were demonstrated. The dynamic optical responses of the imprinted gold pillars from AuNPs to bulk material during the annealing process were investigated.

2. Experiment

Figure 1 shows the overall process scheme for direct nanoimprinting of AuNPs. Our home-built imprinting platform with a compressed air press (CAP) is illustrated in Fig. 1(a), and the details are described in Ref [21]. Polydimethylsiloxane (PDMS) based polymers were chosen as the working stamp materials due to their ability to absorb the solvent without deformation [22]. PDMS is porous such that the solvent of the AuNPs can escape from the PDMS, which helps the AuNP solution to solidify, and the AuNP pattern can then be defined. The pattern of the 2D pillar array in a silicon master mold was defined by an electron-beam writer (Leica WEPRINT200) on an oxide layer. After the resist development, the pattern was transferred to the oxide layer by an RIE oxide etcher (TEL TE5000). The height of the pillar structure was controlled by the thickness of the oxide layer. Before transferring the pattern to the PDMS, an anti-sticking treatment was applied by vapor deposition of F13-TCS on the silicon master to

avoid any possible sticking of the PDMS on the silicon [23]. A scheme of the two-layer composite PDMS stamp was employed. The h-PDMS (hard PDMS) was used as a thin, stiff structural layer to increase the mechanical stability [24] to improve the pattern resolution and edge definition of the stamp. A s-PDMS (soft PDMS, Dow Corning Sylgard 184) was employed as a supporting slab to avoid stamp cracking. In fabricating the working stamp, a thin h-PDMS was coated on the silicon master and heated to 60°C for 30 minutes. Then s-PDMS was poured above the h-s-PDMS and was cured at 80°C for 60 minutes (Fig. 1(b)). After the curing process, the two-layer h/s-PDMS stamps could be easily torn off.

#140585 - $15.00 USD Received 11 Jan 2011; revised 9 Feb 2011; accepted 12 Feb 2011; published 25 Feb 2011

(a) Simulation (b) Experiment

Fig. 6. (a) Simulated extinction spectra of the AuNPs with the Mie scattering analysis. The AuNPs were assumed to be ideal gold spheres with a diameter ranging from 20 nm to 200 nm. The dielectric environment was assumed to be air. (b) The measured extinction spectrum of the spin-coated 5% AuNPs on a glass substrate with different annealing times from 0 sec to 45 sec. The annealing temperature was 250°C.

Starting from the annealing time of 15 sec, the LSPR peak of the spin-coated AuNPs appeared at the wavelength of 584 nm as shown in Fig. 6(b). As the annealing time increased, the LSPR peaks of the AuNPs were red-shifted and located at the wavelengths of 596 nm, 636 nm, and 730 nm for the annealing time of 25 sec, 35 sec, and 45 sec, respectively. The extinction strength of the peak also increased with the annealing time. Both phenomena indicate that the AuNPs were fused into larger AuNPs with increasing annealing time, which is consistent with the trend of the simulation shown in Fig. 6(a). This phenomenon was due to the large surface energy of the AuNPs. They tend to aggregate and form larger AuNPs to lower their surface energy, and then the noticeable LSPR peak from the fused AuNPs appeared. For this reason, if a residual layer in the unpatterned region existed, then a LSPR peak should have arisen from the AuNPs in the residues. In our fabrication process, the optimum experimental parameters were tuned to have the least amount residues remaining, as described in previous sections. Therefore, the LSPR arising from the AuNPs was significantly suppressed, as illustrated in Fig. 4.

4. Conclusions

In this study, we synthesized AuNPs and nanoimprinted them into a imprinted plasmonic metallic nanostructure. This approach combines the patterning and lift-off processes into a simple one-step process without the need of expensive patterning lithographies and the stringent requirement of the lift-off process for nanostructures. The process conditions of the imprinting temperature and imprinting pressure were investigated and optimized. Good imprinting integrity was accomplished with a negligible residual layer, which was confirmed from the SEM inspections and the inexistence of the LSPR from the AuNPs. The mechanisms of the proper imprinting conditions were discussed. The dynamic optical responses of the imprinted gold pillars from AuNPs to bulk material during the annealing process were investigated. The LSPR properties from the AuNPs or gold pillar arrays can be controlled and tuned during the annealing process. The sensing performance of the 2D photonic crystals of the gold pillars was investigated with respect to the pillar’s diameter and their dielectric environment. The size of the imprinted gold pillars is highly scalable in our process. The corresponding resonance wavelengths can be widely tuned from the visible to infrared region by changing the size of the gold pillars, providing a wide range of sensing capability.

Acknowledgments

This study was supported by the National Science Council of Taiwan grants NSC 98-2221-E-006-018- and NSC 98-2218-E-009-001-.

#140585 - $15.00 USD Received 11 Jan 2011; revised 9 Feb 2011; accepted 12 Feb 2011; published 25 Feb 2011