Contents I. Introduction
Self-assembling processes are very powerful approaches that are gaining even more credit in chemistry and materials industry for micro- and nanoscale technology due to their intrinsic advantages of simplicity, versatility and rapidity [1]–[3]. Polymers in liquid phase appear as very suitable candidates to be processed by direct self-assembling methods thus avoiding lithography or long-lasting moulding multi-steps processes [4]– [9]. In fact, the self-assembling strategy could allow the fabrication of components and devices in a single step working directly onto polymer solutions. Nowadays, polymers are multipurpose materials that are ubiquitous in all modern micro and biotechnologies. For instance, polymer three-dimensional (3-D) microstructures have become very promising components in optics, electronic, photonics and biomaterials fields. Microlens arrays, which generally refer to 2-D arrays of small lenses with diameters in the range of ten to hundreds of micrometers, represent important types of miniaturized optical components used in a wide range of applications [10]–[13]. In particular, microlenses from polymer solutions [14], [15] could offer interesting advantages like easy fabrication approach and the use of sustainable/user-friendly materials. High definition displays, photovoltaic devices, semiconductor solar cells, light emitting diodes [16], [17] , sensor [18], biochemical assays [14] and artificial compounds eyes [19] offer just an example of the considerable extension of interest for the micro-technologies industries. Only very recently polymer-based nanocomposite materials have attracted considerable interest because of their excellent properties compared to polymeric materials [20], [21]. In fact, the incorporation of nanofillers into the polymer matrix could modulate the resulting properties of the nanocomposite produced and, at the same time, allows one to use the existing fabrication methods. The growing interest in arrays of polymer microstructures is due to the fact that it is relatively easy for polymers to incorporate colloidal inorganic nanocrystals (NCs) or quantum dots (QDs), thus transforming originally passive micro-optical elements into active photonic components by combining the processability of organic materials with efficient luminescence displayed by the nanofillers [22]. This has been proved to be of great interest for novel applications such as the fabrication of photonic crystals [23] and, notably, of innovative solar cells showing enhanced efficiency [24]–[26]. Here we demonstrate an innovative formation process based on pyro-electrohydrodynamics (pyro-EHD) for direct formation of self-assembled polymer microstructures. Colloidal inorganic NCs embedded into a polymer matrix are dispensed onto a ferroelectric substrate and self-assembled in a single step. Essentially a simultaneous two-fold self-assembling process, involving either EHD instability acting on the hosting liquid polymer in conjunction with dielectrophoretic (DEP) forces operating on NCs, allows the realization of an active array of microstructures just in a single step. Pyro-EHD and pyro-DEP have been discovered and applied separately as advanced processes for the self-assembling of nanofillers [27]–[29], liquids and polymers, including Polydimethylsiloxane (PDMS) [30]– [34] in a multiscale range (i.e., between 25 to 200 μm diameter) with high degree of uniformity. By controlling the polymer instability driven by EHD, different micro-optical structures can be obtained spontaneously, i.e., spherical or toroidal [35]. Here we show how the thermal stimulus applied to a periodically poled lithium niobate crystal (PPLN) is able to drive in a single step the self-assembling and subsequently the cross-linking of the liquid PDMS matrix in form of a 2-D micro-optical polymer array. Meanwhile the liquid polymer is shaped by the EHD instability into a micro-structures array, the NCs are collected through DEP forces at bottom of each optical element of the layer. We show that the formed nanocomposite layer behave indeed as effective active optical elements so that the process does not change the NCs properties. The fabrication procedures of NCs-incorporated and light-converting is illustrated and described. Full optical characterization is also performed and reported. Such self-assembling of nanocomposite polymer could inspire future fabrication techniques for producing layers that could be mounted on top of OLED devices in order to drive the light in a more efficient way, for improving photovoltaic efficiency in energy applications or even for detecting and imaging fluorescent objects in bio-technology.
