Sculptured Layer‐by‐Layer Films

Abstract

The precise assembly of nanostructured materials into two (2D) and three-dimensional (3D) periodic microscopic arrays is achieved by employing various assembling technologies such as microparticle self-organization, photolithography, holographic lithography, selective chemical etching, ink printing, laser-based polymerization, selective responsive grafting, and inversion of bilayers. Soft lithography, such as microcontact printing has been widely applied to tackle this challenge in a few steps, with a submicrometer resolution, and at low cost. It has been demonstrated that the most popular fabrication of well-defined nanostructured materials with layer-by-layer (LbL) assembly can be combined with photolithography and microprinting to fabricate complex 2D and 3D structures with modulated distribution of different components. Examples of the micropatterned assembly of nanoparticles, microchannels, antireflective coatings, and Raman arrays have already reported. A variety of functional materials have been utilized in the LbL construction to make these structures suitable for prospective applications as antiwetting coatings, sensitive films for solar cells, fuel cells, ultra-strong nanomaterials, microcapsules, and membranes for controlled drug release. On the other hand, microstructural arrays based upon the principles of either diffraction or refraction with in-printed microscopic modulations are extensively used as optical components such as grating, beam splitters, microlenses, displays, and mirrors. A vast majority of LbL structures have been fabricated on solid planar or curved (microparticle) supports and represent essentially 2D planar structures with vertical (along the surface normal) modulations of chemical composition. Micropatterning of LbL films has been recently reviewed by Hammond. Few examples of free-standing LbL structures have been reported either in the form of uniform, micropatterned, or curved shell structures. Therefore, although potential for the fabrication of interesting modulated structures, which provides for optical effects, is inherently present in the LbL technology, few attempts have been made to investigate the feasibility of the fabrication of modulated LbL structures for light control. For example, in a recent study, Rubner et al. have demonstrated that a properly matched LbL coating can serve for constructive–destructive light interferences with efficient antireflective ability. In a related study, Rubner and Cohen have demonstrated that a proper modulation of the reflective index within LBL films might lead to a pronounced structural color effect with selective reflection of visible light controlled by the reflective index modulation rather than the presence of conjugated molecules with proper electronic structures. Although these initial studies showed some potential in LbL technology to generate nanostructured materials with the ability to control visible light diffraction/reflection, they have been limited to one-dimensional modulation of the refractive properties. In this communication, an example of 3D LbL grating structures with the ability to diffract light because of the modulation of local LbL film shape with microscopic periodicity and nanometer-scale vertical modulations is reported. In these freely suspended 3D LbL films, the effective modulation of the refractive properties is caused by the topological variation of the local film shape, thus representing a purely structural color effect. A simple and economical spin-assisted LbL assembly of conjugated polyelectrolytes on a sacrificial microimprinted modulated substrate was employed here to generate a robust, free-standing sculptured LbL structure with an effective thickness of 60 nm and a 160 nm peak-to-peak vertical modulation on a square lattice with 2.5 lm lateral periodicity (Figure 1). These films demonstrate efficient optical grating properties and bright structural colors in a reflective mode controlled by the in-plane spacing and the angle of incidence. The conjugated polyelectrolyte, poly(2,5-methoxypropyloxy sulfonate phenylene vinylene) (MPS-PPV) applied in the LbL assembly synthesized here was critical in providing a mechanically strong structure as has been tested in our previous work to fabricate mechanically robust planar ultrathin LbL films. MPS-PPV is a well known water-soluble conjugated polymer with a highly charged, stiff backbone and strong fluorescence properties. Planar LbL films of MPS-PPV and poly(allylamine hydrochloride) (PAH) demonstrate excellent mechanical properties combined with high fluorescence as has been demonstrated in our previous work. C O M M U N IC A IO N