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Abstract
The objective of this research is to demonstrate high-quality nanocomposites enabling convenient imprinting of nanopatterned optical devices. The nanocomposite developed contains homogeneously dispersed silicon (Si) nanoparticles in a UV-curable prepolymer host medium. Using an optical adhesive NOA73 as host eliminates surface treatment of the silicone mold due to minimal adhesion between the polymer and mold. Moreover, the chosen materials exhibit low shrinkage, enabling faithful replication of the master templates. Tunable refractive index is realized by mixtures of the host polymer with a refractive index of ~1.56 and nanoparticles with a refractive index of ~3.45. For example, using a ~28% Si fraction, an imprintable material with a refractive index of ~2.1 results. Surface functionalization of nanoparticles with polyvinylpyrolidone is shown to reduce agglomeration. Dynamic light scattering (DLS) and Fourier-transform infrared (FTIR) spectroscopy are employed to investigate the stability of nanocomposites and the efficacy of the surface treatment of nanoparticles, respectively. The quality and physical parameters of the fabricated devices are evaluated by the atomic force microscopy (AFM) and scanning electron microscopy (SEM). The thickness of a homogeneous sublayer underneath a periodic layer in the fabricated optical devices is controlled by employing a channel with precisely managed depth. The example resonant filters show acceptable response as measured in the 1600-1800 nm spectral band and reasonable agreement with theoretical predictions.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Organic-inorganic nanocomposites exhibit new properties not available in their separate constituents. Novel and useful properties of nanocomposites originate from the combination of the advantages of their constituent organic and inorganic materials [1]. There are two main types of nanocomposite preparation techniques: The in situ method which contains synthesis and growth of nanoparticles inside an organic material and the ex-situ method that involves uniform dispersion of the premade nanoparticles in the polymer matrix [2, 3]. Numerous studies have been conducted to tune the refractive index of organic materials by incorporating high refractive index inorganic materials such as ZnS [4, 5], ZrO2 [6], TiO2 [7, 8], and PbS [9, 10]. Critical issues associated with nanoparticles are particle size and dispersion quality in the host polymer. Rayleigh scattering and attendant intensity loss of transmitted light from the nanocomposite is managed by choosing the size of the embedded particles to be, for example, below one-tenth of the incident wavelength. Moreover, scattering associated with agglomeration of the nanoparticles takes place especially when the refractive index of nanoparticles is high compared with the host polymer. In addition, as the particle size decreases, the specific surface area, the surface energy, and the mobility of the nanoparticles due to Brownian diffusion increases, which translates to incessant collisions between the particles with proclivity to agglomeration [11, 12]. By using an appropriate dispersion process in the ex-situ method, the agglomeration can be minimized by reducing attractive forces between the nanoparticles. The more reliable method to enhance dispersibility and reduce agglomeration is to keep the nanoparticles separated from each other by attaching organic chains to the surface of the nanoparticles which act as spacers [13]. Through the nanocomposite process, mechanical and optical properties of organic materials can be improved and manipulated.
Whereas the emphasis here is on fabrication methodology, we provide examples of prototype guided-mode resonance filters. The guided-mode resonance effect occurs in thin-film structures containing 1D or 2D periodic layers enabling applications for instance in lasers [14], biosensors [15, 16], and spectral filters [17, 18]. In general, attendant modal resonances stimulate rapid changes in the distribution and localization of electromagnetic nearfields as well as in reflected or transmitted light beams. To obtain leaky mode resonances, an appropriate refractive index contrast modulation is required. Therefore, it is necessary to tune the refractive index of the polymer. The geometric characteristics of resonance devices such as refractive index (n), grating thickness (dg), homogenous layer thickness (dh), grating period (Λ), and fill factor (F) affect the final spectral response. Resonance conditions prevail as an incident light wave couples to leaky, or quasi-guided, lateral Bloch modes supported by the periodic device [19].
Pertinent to the fabrication of the periodic photonic elements exemplified herein, nanoimprint lithography (NIL) exploits flexible patterned stamps fabricated from suitable masters [20]. Patterns on a given stamp can thus be transferred to resist or other imprint materials and consequently either heat or UV light can accomplish the curing process. A peel-off process can be implemented after curing of the imprinted material and the resulting structure, which has dimensions defined by the original master, acts as the final device [21–23]. The strengths of this process compare well with other traditional lithography methods such as holographic interference patterning or e-beam writing and include cost-effective parallel nanofabrication that is fast, simple, repeatable, and mass-production qualified.
Among critical issues in the NIL process is simultaneous control of the uniformity and thickness of the imprint material which in some applications including resonant optical devices play a vital role [24, 25]. A common solution involves applying pressure on the stamp which may not be sufficiently precise for demanding applications. In addition, applying a uniform pressure to all parts of the soft and flexible stamp and imprint material needs accurate control of the applied pressure and special nanoimprint equipment [26–28]. Another solution is placing the imprint stamp in contact with a spin-coated layer of a low viscosity polymer followed with the curing process [29]. Here, we use a convenient method to control the uniformity and thickness of imprint material without applying pressure by fabricating a channel. In this paper, as an example application, we present simulated and fabricated optical devices made of UV curable NOA73 blended with Si nanoparticles based on the NIL method.
2. Experimental methods
Commercially available polycrystalline laser-synthesized Si nanoparticles with diameter < 80 nm and an optical adhesive NOA73 are the nanocomposite constituents applied here. The host polymer with refractive index of 1.56 with low viscosity (130 cps) acts as a matrix for the nanocomposite. The ex-situ method is used to produce the nanocomposite as a resist or imprint material. For ex-situ preparation of the nanocomposite, the Si nanoparticles were homogeneously dispersed in NOA73 at room temperature via a sonication probe for 30 seconds at 40 watts. The dynamic light scattering (DLS) technique was used to measure the particle size distribution of the nanoparticles existing in the polymer matrix. The refractive index of the nanocomposite corresponds to the volume percentage of the incorporated particles in the matrix material. To predict the refractive index (n) of final the nanocomposites the rule of mixture is used as:
which is proportional to the volume fractions Vi and refractive indices ni of the filler (Si) and matrix material (NOA73). Here we select Si as a filler because it has high refractive index with n~3.45 in the spectral band of interest and low density (ρ = 2.33 g/cm3) comparable with other high refractive index materials. Polyvinylpyrolidone (PVP; molecular weight ~55000) is used to functionalize the surface of the Si nanoparticles. To obtain functionalized surfaces, the PVP was attached to the particle surface by addition of 150 mg of Si nanoparticles and 150 mg of PVP to 15 mL of deionized water and then sonicated in a water bath. The mixture was centrifuged to remove the unbound excess of PVP followed by washing and redispersion. Then, unadsorbed PVP was removed by repeated centrifugation/wash/redispersion cycles [30]. Attenuated total reflectance FTIR (ATR-FTIR) spectroscopy enables confirmation of the attachment of the PVP to the surface of the nanoparticles.To fabricate a master template, a thin layer of photoresist was spin coated on a silicon wafer and patterned via the laser interference lithography method. Subsequently, development of the patterned photoresist and a reactive-ion etch (RIE) process were used to transfer patterns from the photoresist to a silicon wafer. A thin layer (~20 nm) of sputtered aluminum followed by an anti-adhesion chemical (dichloromethane) were coated on the patterned silicon wafer to enhance the peel off process. The dichloromethane was applied to a petri dish containing the master template and maintained in a vacuum desiccator for ~2 h. Polydimethylsiloxane (PDMS) was used to make the stamp that contains a negative pattern of the master template. Sylgard 184 silicone elastomer was mixed in a 10:1 ratio of base and curing agent. Then it was cast on a master template and degassed under vacuum until no visible bubbles were observed followed by curing at 75 °C for 4 hours. It was then peeled off from the master to complete the stamp (PDMS thickness ~0.5 cm). In this study, the UV-curable nanocomposite acts as an imprint material, which is patterned with the PDMS stamp. The pattern is transferred via the curing and peel-off process with the PDMS. The NIL process used to fabricate the guided-mode resonance (GMR) device structures is summarized in Fig. 1.
Fig. 1 Nanoimprint device fabrication method. (a) PDMS stamp peeled off from the silicon master. (b) Nanocomposite poured on a glass substrate as spin coating is not applicable. (c) UV curing of the nanocomposite through the PDMS stamp. (d) A final nanoimprinted device with specific physical parameters.
To implement rapid curing of the nanocomposite, we used a high-power UV-cure system with lamp power of 600 watts and irradiance of 175 mW/cm2. Different concentrations of Si nanoparticles were incorporated into the NOA73 matrix with n = 1.56 to tune the refractive index of NOA73 to 1.9, 2, and 2.1 with filler volume percentage of 18%, 23%, and 28%, respectively. These nanocomposites were employed to fabricate nanoimprinted GMR devices with similar geometric characteristics but different refractive indices. Spectral measurements were carried out with a super continuum light source and a near-IR optical spectrum analyzer (OSA).
3. Results and discussion
After mixing the nanoparticles via the sonication probe, dynamic light scattering (DLS) measurements for low filler concentration nanocomposites were carried out to determine the particle size distribution. The DLS results in Fig. 2 show an appropriate dispersion quality immediately after the sonication process and verify the particles size is in the range of the specified nanoparticles. This confirms proper dispersion conditions and uniform distribution. Another measurement after 15 minutes demonstrates formation of agglomeration during this interval of time such that now ~95% of mixture particles have diameters up to 800 nm. On the other hand, the DLS curve for the PVP-coated silicon nanoparticles shows a low level of agglomeration after 15 minutes demonstrating the effectiveness of the surface treatment to reduce agglomeration.
Fig. 2 DLS measurements of as-sonicated, non-treated, and PVP-treated silicon nanoparticles after 15 minutes.
Moreover, ATR-FTIR measurements were done to confirm the attachment of the PVP polymer to the surface of the silicon nanoparticles. Figure 3 depicts the ATR-FTIR spectra related to the pure PVP, the as-received silicon nanoparticles and the PVP-coated silicon nanoparticles. The absorbance peaks at 1660 cm−1, 1425 cm−1, and 1290 cm−1 are related to the characteristic peaks of C = O groups, scissoring bending of CH2 groups, and C-N stretching vibrations of PVP, respectively [31]. These characteristic PVP dips are clearly shown in the transmitted spectrum of the functionalized nanoparticles which indicates existence and formation of the PVP-coated silicon nanoparticles. In contrast, none of these characteristic peaks were detected in as-received nanoparticle spectra.
Fig. 3 ATR-FTIR measurements for pure-PVP, as-received silicon, and PVP-coated silicon nanoparticles.
The SEM images in Figs. 4(a)-(c) demonstrate the importance of the surface modification of the nanoparticles to minimize the formation of agglomeration. Although the surface treatment of the nanoparticles can prevent agglomeration, for highly loaded nanocomposites in which the particles are densely packed, there is a tendency for them to agglomerate during extended curing times. Therefore, in addition to the surface treatment of the silicon nanoparticles to enhance dispersion quality, rapid curing of the nanocomposite will have a beneficial influence to minimize agglomeration. The SEM images illustrate that for the silicon concentration of ~23 vol% combination of rapid curing and surface treatment will give an appropriate particle distribution inside the NOA73 matrix which leads to lower agglomeration and scattering centers.
Fig. 4 Distribution of nanoparticles in a nanocomposite containing ~23 vol% Si. (a) Non-treated nanoparticles under slow curing. (b) Non-treated nanoparticles under rapid curing. (c) Surface treated nanoparticles under rapid curing.
An AFM image of the silicon master after deposition of ~20 nm aluminum is show in Fig. 5(a). Moreover, the fabricated nanoimprinted device is shown in the AFM in Fig. 5(b). It is shown that the profile of the silicon master transfers well to the nanocomposite by means of the PDMS stamp. The quality of the grating lines shown in Fig. 5(b) demonstrates that the peel-off process of PDMS from the cured nanocomposite is done with minimal imperfection.
Fig. 5 AFM images of the (a) Silicon master after aluminum deposition. (b) Fabricated nanoimprinted optical filter device. Insets show 3D views of the grating lines.
Tang et al. mentioned applying an anti-stick monolayer coating to the PDMS before the stamping process to improve the quality of the peel-off process [32]. However, in this study we eliminate all such surface treatment processes. NOA73 is thiolene based and exhibits very poor adhesion to the PDMS [33]. Therefore, employing the thiloene based NOA73 will allow us to accomplish the peel-off process without using any anti-stick chemicals. The AFM images also show that there is no specific shrinkage of the fabricated nanoimprinted devices; thus, the dimensions of the silicon master can be reliably transferred to the final device. This is attributed to the low shrinkage (~1.5%) of NOA73 after curing as compared to high shrinkage at ~20% associated with methods such as organic-inorganic hybrid sol-gel processing. Moreover, in this work we apply UV-curing which is fast and, unlike the sol-gel process, does not need high temperatures and long time to reach the high refractive index desired in some applications.
Figure 6 shows SEM images of the grating ridges and a side view of a device containing 14 vol% Si. The SEM images of the grating lines match well with the AFM images and high-quality grating lines are seen. Figure 6(b) indicates the cross section of the imprinted device. We can see that the obtained homogeneous layer is thick and non-uniform, which is not acceptable for most resonance devices of current interest. One of the challenges of the nanoimprinting technique applied here is controlling the thickness of the homogeneous layer. Previous reports cite applying pressure to the stamp by simple weights [25] or by using complex imprinting equipment [26, 27]. In either case, it is not easy to accurately control the uniformity of the applied pressure to the whole flexible PDMS stamp.
Fig. 6 SEM images of (a) top-view of a nanoimprinted resonance device and (b) cross section of a resonance element with a thick and non-uniform homogeneous layer. The inset in (a) shows a cross-sectional view of the grating.
Good control of thickness and uniformity of the homogeneous layer in our example devices is essential for efficiency and functionality. To bring both parameters under simultaneous control, a convenient channel fabrication method is used. To implement this method, we first spin coated a layer of photoresist (Ultra-i 123) on a glass substrate. Then, by shining UV light through a rectangular mask and developing the photoresist, we realized a narrow channel (~5 mm wide). After that, the PDMS master was placed on the channel in a way that its grating lines were parallel to the long edges of the channel. The nanocomposite was poured at the edge of the PDMS which then diffused and filled the channel. The diffusion time depended on the nanoparticle concentration, taking ~1-3 minutes to fill a ~10-mm-long channel. Therefore, according to the spin speed, we can control the photoresist thickness and consequently, the thickness of the homogeneous layer. The schematic steps of the channel fabrication are shown in Fig. 7(a)-(c). The cross section of the device with controlled homogeneous layer thickness is shown in Fig. 7(d) which is evidence of successful implementation of the channel method.
Fig. 7 (a)-(c) Schematics of the channel fabrication. (d) Cross section of the device fabricated with the channel method to control the thickness of the homogeneous layer.
Spectral transmission measurements of devices with specific constant dimensions but different refractive indices, obtained by different filler concentrations, are shown in Fig. 8. A beam of light from a supercontinuum source illuminated the sample. The beam was collimated and polarized with a spot size of ~1 mm at the sample. The transmitted light was collected by an optical fiber and sent to an OSA. Rigorous coupled-wave analysis (RCWA) was used to simulate the transmission spectrum [34]. Good agreement is found between simulated and experimental results. However, the transmitted efficiencies of the experimental results are lower than the simulated efficiencies. It is seen that the lowest nanoparticle concentration inside the polymer corresponds to the highest transmission efficiency. The reason is that the formation of agglomerations which act as scattering centers is more probable in higher concentrations. It is worth mentioning that without surface functionalization, even at low filler concentrations, the efficiency was less than 10% which confirms the importance of surface treatment to avoid agglomeration. Moreover, no transmission dip is observed for devices with n = 2.0 and n = 2.1 for non-coated nanoparticles even when using the fast curing process. Therefore, to ensure adequate efficiency, the nanoparticle surface treatment is essential.
Fig. 8 Experimental and calculated transmission spectra at normal incidence with TE-polarized light (electric vector of input light lies along the grating grooves). The fabricated devices have different refractive indices but the same physical parameters of Λ = 1.05 μm, dg = 0.34 μm, dh = 0.95 μm, and F = 0.45.
It is clear that rapid curing of nanocomposite under high power UV lamp helps to prevent agglomeration at low filler concentrations. Through the rapid curing process, we can freeze and hold nanoparticles in the polymer matrix, so they will not have time to stick to each other and consequently settle down. However, for high concentrations of incorporated nanoparticles, exceeding 28% in these experiments, the curing time will increase so it is more likely that the particles agglomerate. In summary, taking advantages of both rapid curing and optimal surface treatments enables tuning of the refractive index of the final device-quality polymer.
4. Conclusion
In this work, tunable refractive index was implemented by nanocomposites containing mixtures of a host polymer with refractive index of ~1.56 and Si nanoparticles with refractive index of ~3.45. With a ~28% Si fraction, an imprintable material with refractive index of ~2.1 was made. In principle, by controlling the filling fraction, the tunable range could be from ~1.6 to 2.1. Thereafter, we employed nanoimprint lithography using silicon masters and silicone (PDMS) molds to expeditiously fabricate example resonant optical filters. As the thickness of the homogeneous sublayer underneath the periodic layer is critical in this device class, we showed that we could control its thickness without pressure by employing a channel with precisely managed depth. Thus, by taking advantages of NIL and by controlling the refractive index and homogeneous layer thickness of the imprint material, we demonstrated resonant filters possessing the design parameters in composites with refractive indices of 1.9, 2.0 and 2.1. Here, we avoided use of anti-stick media on the PDMS surface by choosing an appropriate organic material (NOA73) which does not stick to the PDMS stamp. We found negligible shrinkage of the nanocomposite imprint material deriving from low shrinkage (1.5%) of the host medium. This enables fabrication of precise resonance devices which maintain the master’s dimensions. Additionally, by combining the advantages of surface treatment of the nanoparticles and rapid curing we minimized particle agglomeration and consequently fabricated useable resonant filters with acceptable response as measured in the 1600-1800 nm spectral band.
Funding
National Science Foundation (NSF) (IIP-1444922).
Acknowledgments
The authors thank Dr. Kyu Jin Lee, Dr. Amir Koolivand and Nikhil Pandey of UT Arlington for laboratory assistance. We thank Dr. Nelson Claytor and Oscar Lechuga of Fresnel Technologies Inc for technical discussions.
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