*Article* **Thin Coatings of Cerium Oxide Nanoparticles with Anti-Reflective Properties**

**Laura J. Romasanta 1, Lauriane D'Alençon 2, Sara Kirchner 3 and Christophe Pradère 3 and Jacques Leng 1,\***


Received: 30 July 2019; Accepted: 5 September 2019; Published: 16 September 2019

**Abstract:** Cerium oxide, in addition to its catalytic properties, is also known for its optical properties such as ultraviolet (UV) radiation filtering and a relatively high refractive index (*n* > 2), which makes it an excellent candidate for multifunctional coatings. Here, we focus on the optical properties of thin deposits (-2 μm) of densely packed CeO2 nanoparticles, which we assemble using two evaporation-based techniques: convective self-assembly (CSA, a type of very slow blade-coating) to fabricate large-scale coatings of controllable thickness—from tens of nanometres to a few micrometers—and microfluidic pervaporation which permits us to add some micro-structure to the coatings. Spectroscopic ellipsometry yields the refractive index of the resulting nano-porous coatings, which behave as lossy dielectrics in the UV-visible regime and loss-less dielectrics in the visible to infra-red (IR) regime; in this regime, the fairly high refractive index (≈1.8) permits us to evidence thickness-tunable anti-reflection on highly refractive substrates, such as silicon, and concomitant enhanced transmissions which we checked in the mid-IR region.

**Keywords:** Cerium oxide nanoparticles; anti-reflection; self-assembly; microfluidics; convective self-assembly
