Fabrication of microlens arrays by gel trapping of self-assembled particle monolayers at the decane–water interface

Abstract

A new technique for producing ordered microlens arrays has been developed which is based entirely on self-assembly of charged latex particles spread at an oil–water interface. It includes using a gel trapping technique and replication of the ordered particle monolayers by casting with PDMS. Microlens arrays were fabricated by taking an inverse replica of the PDMS template with a photopolymer.

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Over the past few years microlens arrays have attracted significant interest^1–5 for optoelectronics applications, including image capture and processing systems, photosensors, illumination devices as laser diode arrays and displays, and confocal microscopy.⁶ Various techniques for preparation of microlens arrays have been suggested which are usually based on directed assembly¹ or deposition^2–4 of the lens material (e.g. particles or polymers) on prefabricated photoresist templates followed by subsequent melting. Recently, an alternative technique of UV proximity printing has also been suggested.⁵

Here we report a new generic technique for preparation of hexagonally ordered microlens arrays which is entirely based on self-assembly of repulsive latex particles adsorbed at an oil–water interface followed by templating the exposed part of the ordered particle monolayer. We used the “gel trapping technique” (GTT), recently developed for determination of the contact angle of microparticles adsorbed at air–water and oil–water surfaces,⁷ to prepare and replicate monolayers of monodisperse polystyrene microparticles in a controlled way. The GTT is based on spreading of colloid particles on an air–water or an oil–water surface and a subsequent gelling of the water sub-phase with suitable non-adsorbing hydrocolloid (Fig. 1). The exposed surface area of the particles is controlled by their reduced weight and contact angle (wettability) when adsorbed at the oil–water interface. The particle monolayer trapped on the gel surface is then replicated and lifted by casting with polydimethylsiloxane (PDMS) elastomer. Very recently, we utilised the GTT technique to produce PDMS surfaces of controlled surface microporosity⁸ by removing ordered particle monolayers, partially embedded on the surface of the PDMS matrix. In the present communication, we produce elastomer surfaces of controlled microporosity defined by not only by the size and the contact angle of the templated particles but also by the interparticle distance within the monolayer. We template the microporous PDMS surface with a photopolymer which produces microlens arrays of defined properties (see Fig. 1). In our method the PDMS templates for fabrication of the microlens arrays are produced by a purely self-assembling mechanism which is due to the long-range repulsion between charged particles adsorbed at the oil–water interface.^8–10 Here we describe the procedure in more detail.

Fig. 1 Scheme of the gel trapping technique (GTT) for fabrication of microlens arrays: (A) spreading of charged latex particles at the interface between an oil and a hot aqueous solution of agarose. After the gel sets the particle monolayer is trapped at the interface (B) and the oil phase is replaced with curable PDMS (C). By peeling the cured PDMS (D) and dissolving the latex particles in toluene, the PDMS template (E) is cast with a photopolymer (F) which produces a microlens array on a glass support (G).

n-Decane (from Sigma) was used as an oil phase after multiple passes through chromatographic alumina to remove any surface active components. For the series of experiments reported here we used 3.9 µm sulfate polystyrene (PS) latex particles (IDC, USA), delivered in the form of an aqueous suspension, 8.5% wt solid, 2.14 nm² surface area per sulfate group. Isopropanol was used as a spreading agent for the particles at the decane–water interface. In a typical procedure for the preparation of microlens arrays, hot agarose solution was prepared by hydration of 2% low melting point agarose (NBS Biologicals, UK) in water at 75 °C for 10 min and topped with the pre-warmed decane phase in a Petri dish. A sample of 50–150 µL of latex particle suspension–isopropanol solution (50 : 50) was injected at the decane–water interface at 60 °C. After setting the agarose gel at 25 °C, the decane phase was replaced with PDMS (Sylgard 184 from Dow Corning) and cured for 48 hours at room temperature. Then the solidified PDMS replica was peeled off the gel and kept in hot water (95 °C) for 10 min to remove agarose residues from its surface. To fabricate the template for microlens arrays, the PDMS–particle monolayer composite was treated with toluene to dissolve the latex particles. After washing with pure toluene, the PDMS template was dried under vacuum and coated with a layer of Norland optical adhesive 61 (Lot 225, Norland products, Inc., USA) covered with a microscope glass slide. After UV curing of the optical adhesive, the PDMS template was peeled off to produce a microlens array on a glass support (see also Figs. 1G and F).

Figs. 2a and b present SEM images of the monolayer of monodisperse 3.9 µm sulfate PS latex particles captured on PDMS with the GTT at the decane–water interface. Note that although the monolayer is quite diluted, the particles are ordered in a nearly hexagonal lattice. This order results from the super-long range electrostatic repulsion between charged particles adsorbed at the oil–water interface as was recently discovered by Aveyard et al.^9–11 We found that the particles can also be removed from the surface by local stretching of the PDMS⁸ which produces a nearly regular array of circular microcavities as a result of the self-assembly of the repulsive particles on the templated oil–water interface. In a separate study we have been able to produce similar arrays of smaller microcavities after templating monolayers of smaller particles (2.5 µm COOH PS latex) with PDMS.⁸ Such materials may find applications as antireflective coatings, surfaces/filters of controlled microporosity, etc.⁸ Similar microporous PDMS surfaces were also obtained in this study by dissolving the PS particles in toluene as described above. By controlling the amount of particles spread on the original oil–water surface (or the area of the oil–water interface) we can control the distance between the repulsive particles within the monolayer and hence the density of microcavities on the surface of the PDMS replica.

Fig. 2 SEM images of ordered particle monolayers partially embedded into a PDMS elastomer. (A) A monolayer of 3.9 µm sulfate latex particle monolayer on PDMS. Particles are equally spaced due to the repulsive interaction at the original decane–water interface (Fig. 1). (B) The PDMS template after partial removal (mechanically) of the particles. Similar result is obtained by dissolving the particles in toluene.

Figs. 3A and B show typical SEM images of ordered hexagonal microlens arrays fabricated from the PDMS templates by moulding with a photopolymer. The inter-lens distance is controlled by the spacing between the particles in the original ordered particle monolayer at the oil–water interface. The area of the ordered particle monolayer at the oil–water interface that we have obtained can be as large as several square centimetres (although not perfectly hexagonal) and was limited only by the size of the Petri dish containing the monolayer. The typical domain size of a perfectly hexagonal lattice includes about 100 × 100 particles. This order can be further improved by compressing the particle monolayer under controlled conditions at the oil–water interface in a Langmuir trough before gelling the aqueous sub-phase. Fig. 3C present the optical microscope image of a single object (black triangle) projected through a microlens array similar to that shown in Fig. 3A. Note that the lens diameters here are of the order of 3 µm, i.e. less that the diameter of the sulfate latex particles (3.9 µm) used to produce the microlens array. One sees that the object features are well resolved by the microlens array. Here we also remark upon the possibility for control over the lens focal length by manipulating the particle contact angle at the original oil–water interface.

Fig. 3 SEM images of the microlens arrays of Norland optical adhesive 61 produced by taking a replica from the PDMS templates, like those in Fig. 2. The latter are fabricated by moulding ordered monolayers of 3.9 µm sulfate latex particles self-assembled at the decane–water interface with the GTT. The amounts of particle suspension spread at the oil–water interface were: (A) 50 µL, (B) 150 µL. (C) Optical microscope images obtained with a transmission optical microscope by projecting a single black triangle on a transparent background through a 2D array of microlenses (from Fig. 3A).

In summary, we have developed a new technique for producing ordered microlens arrays which is based entirely on self-assembly of charged latex particles spread at the oil–water interface. The method includes using of a gel trapping technique and replication of the ordered particle monolayers by casting with curable PDMS. Microlens arrays have been fabricated by taking an inverse replica of the PDMS template with a photopolymer. Our method allows immediate control of the microlens array lattice constant by using different amounts of particles spread at the original oil–water interface. An interesting extension of this technique would be to control the microlens shape and size by controlling the particle three-phase contact angle at the oil–water interface or using particles of different diameter. Such work is in progress and will be reported in a follow-up publication.

Notes and references

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