Please cite this article in press as: I. Sandu, et al., Hanging colloidal drop: A new photonic crystal synthesis route, Photon Nanostruct: Fundam Appl (2018), https://doi.org/10.1016/j.photonics.2018.02.001 ARTICLE IN PRESS G Model PNFA-635; No. of Pages 7 Photonics and Nanostructures – Fundamentals and Applications xxx (2018) xxx–xxx Contents lists available at ScienceDirect Photonics and Nanostructures – Fundamentals and Applications jou rn al hom epage: www.elsevier.com/locate/photonics Hanging colloidal drop: A new photonic crystal synthesis route Ion Sandu, Marius Dumitru ∗ , Claudiu Teodor Fleaca, Florian Dumitrache National Institute for Lasers, Plasma and Radiation Physics, Lasers Dept., Bucharest, Magurele, 409, Atomistilor Street, 077125, Romania a r t i c l e i n f o Article history: Received 19 September 2017 Received in revised form 9 February 2018 Accepted 9 February 2018 Available online xxx Keywords: Photonic crystals Self-assembly Drop drying Liquid/gas interface a b s t r a c t High-quality photonic crystals (hundreds of micrometres in thickness) were grown by the free evapo- ration of a colloidal drop consisting of silica and polystyrene nanospheres with dimensions of 300 nm, 500 nm, and 1000 nm. The essence of experimental findings is that the drop has to hang on a pillar. This leads to the inhibition of the droplet spreading, the minimisation of the convective force, and the zero- ing of the static frictional force between nanospheres and the liquid/air interface, where the first layer is formed. The theoretical essence is the continuous adjustment of nanospheres positions during the growth of photonic crystal, a key condition of the self-assembling phenomenon. © 2018 Elsevier B.V. All rights reserved. 1. Introduction The study of colloidal particle self-assembly [1] is of great inter- est to many scientists because, in addition to theoretical gain, it allows the growth of some photonic crystals [2–4]. If, by conven- tion, a crystal is a periodic arrangement of atoms and molecules, in the case of photonic crystals, the “atoms or molecules” are replaced by macroscopic materials such as nano- or microspheres. In pho- tonic crystals the motion of photons is affected in much the same way that ionic lattices affect electrons in solids; thus, they can be used in photonic band-gap devices. Current modern technology, light and electron lithography [5], can produce photonic crystals of one or several layers, but at very high prices. Quite inexpensive but still of good quality monolayers and bilayers can be produced by natural lithography [6], which is also based on the self-assembly phenomenon. Thick photonic crystals known as opals [7], inverse- opals [8], and ordered macroporous materials [9] can be produced by self-assembly of spherical particles of polystyrene, silica (SiO 2 ), polymethylmethacrylate (PMMA), or other polymethacrylates. The ability of colloidal particles to self-assemble into a variety of crys- talline phases lies at the heart of many materials-science studies, especially in the fields of photonics, catalysis, sensors, tissue engi- neering, and lithography [2–4]. In these applications the colloids can either serve directly as the functional building blocks or form a template for making opals and inverse-opal structures [2–4]. ∗ Corresponding author. E-mail address: marius.dumitru@inflpr.ro (M. Dumitru). There is a variety of assembly methods available in the litera- ture for the fabrication of colloidal crystals which have attracted considerable interest as potential alternatives to conventional top- down processes for their scalable and low-cost synthesis processes. The most common methods are drop casting [10], sedimentation [11], centrifugation [12], spin-coating [13], dip-coating [14], ver- tical deposition [15], shear ordering [16], Langmuir-Blodgett [17], direct assembly on water surface [18], and electrophoretic [19] and magnetic self-assembly [20]. However, none of these methods can grow a perfect and thick photonic crystal. Generally, it is difficult to obtain structural uniformity over length scales beyond ∼50 m. Even more, each synthesis method has its own drawback; a large amount of time necessary for the opal to grow in the case of the sedimentation method, a relatively high technological complexity for the vertical deposition, a multi-step processing as in Langmuir- Blodgett, and others. As a common weakness we can mention the relatively small thickness (tens of layers) of the synthesised opals and inverted opals. High thickness can be important in applications such as catalysis [21], filtering [22], Li-ion batteries [23], and con- ductometric sensors [24], which necessitates high porous volumes. Extended order is necessary in photonic band-gap devices such as optical sensors [25] and biosensors [26]. From the theoretical point of view, the self-assembly phe- nomenon defined as “the autonomous organisation of components into patterns or structures without human intervention” [1] requires the existence of few fundamental conditions such as the identity of components and their ability to move and equilibrate [1]. A self-assembling system is usually of extreme complex- ity. Few or more forces such as electrostatic, van der Waals, https://doi.org/10.1016/j.photonics.2018.02.001 1569-4410/© 2018 Elsevier B.V. All rights reserved.