**1. Introduction**

Cerium oxide nanoparticles have been applied in diverse fields including catalysis, luminescence, and nanomedicine, etc. [1–3]. Here, we investigate their optical behaviour when assembled as coatings onto a substrate. As cerium oxide (CeO2) has a fairly high optical refractive index, we expect that dense coatings made out of CeO2nanoparticles will also exhibit a high refractive index.

We follow a low-tech approach to engineer simple coatings out of dispersions of CeO2 nanoparticles. The coatings are realized using evaporation-based techniques out of water-based dispersions, making the processes fairly appealing in terms of energy consumption and toxicity: Their fabrication requires virtually no external pressure, no toxic or hazardous gases, no elevated temperatures, no etching, no vacuum, and no toxic solvents. The coatings we obtain have simple ye<sup>t</sup> non trivial structures and they perform very well from an optical point of view, in particular for anti-reflection on highly refractive materials, which we demonstrate on silicon as a case study.

First, we re-demonstrate that the evaporation-based blade-coating method is efficient for producing coatings of controllable thickness, in this case, out of CeO2 dispersions. These results conform to the pioneering work of O. Velev and co-workers [4,5] who coated SiO2 dispersions on glass and silicon. The very slow regime of blade-coating, were evaporation competes favourably with the

casting velocity enhances the formation of a well structured, thin to thick deposit. We also add some complexity via micro-patterned structures using an evaporation-based micro moulding technique. We measure the refractive index of these coatings which behave as lossy dielectrics in the ultraviolet (UV)-visible regime and loss-less dielectrics in the visible to infra-red (IR) regime; in the latter regime, the fairly high refractive index (≈1.8) permits us to demonstrate thickness-tunable anti-reflection properties on substrates with a high refractive index such as silicon substrates, and concomitant enhanced transmission which we tested in the mid-IR region, thereby covering a large spectral range.

#### **2. Thin and Structured Coatings of Densely Packed** CeO2 **Nanoparticles**

We use two different techniques that permit us to assemble an initially dispersed state of nanoparticles into a solid made of the same, densely packed nanoparticles: convective self-assembly (CSA) and microfluidic pervaporation (μ-pervaporation). A detailed description of these methods will be given in Section 4.2.

In brief, CSA is a modified blade-coating technique where the withdrawing velocity of the substrate on which the coating will be deposited is so small that evaporation competes with the film casting rate, so that the dispersion is pre-concentrated at the level of the deposition meniscus; on top of this, it enhances the structure of the deposit [6–9].

μ-pervaporation takes advantage of the extremely well-defined poly(dimethyl siloxane) (PDMS) geometries crafted by soft-lithography [10,11], as well as the ability of some solvents to pervaporate across the elastomer PDMS matrix [12]. Pervaporation induces a concentration mechanism of the solute, which was initially solubilized/dispersed in the solvent, thus leading to the formation of a solid that grows in a neat geometry [13]. It is an 'augmented' version of the moulding of solids into micro-capillaries, a seminal and inspiring piece of work described in [14].
