Form Birefringence in Resonant Transducers for the Selective Monitoring of VOCs under Ambient Conditions

Abstract

In this work, we have developed a new kind of nanocolumnar birefringent Bragg microcavity (BBM) that, tailored by oblique angle deposition, behaves as a selective transducer of volatile organic compounds (VOCs). Unlike the atomic lattice origin of birefringence in anisotropic single crystals, in the BBM, it stems from an anisotropic self-organization at the nanoscale of the voids and structural elements of the layers. The optical adsorption isotherms recorded upon exposure of these nanostructured systems to water vapor and VOCs have revealed a rich yet unexplored phenomenology linked to their optical activity that provides both capacity for vapor identification and partial pressure determination. This photonic response has been reproduced with a theoretical model accounting for the evolution of the form birefringence of the individual layers upon vapor condensation in nanopores and internanocolumnar voids. BBMs that repel water vapor but are accessible to VOCs have been also developed through grafting of their internal surfaces with perfluorooctyltriethoxysilane molecules. These nanostructured photonic systems are proposed for the development of transducers that, operating under environmental conditions, may respond specifically to VOCs without any influence by the degree of humidity of the medium.

Titania Enhanced Photocatalysis and Dye Giant Absorption in Nanoporous 1D Bragg Microcavities

“… 42 , 43 We assume that this microstructure enables a straightforward diffusion of dissolved molecules or vapors, as proved in previous studies dealing with the photonic analysis of fluids and volatile organic compounds. 26 , 46 , 47 …”

“…42,43 We assume that this microstructure enables a straightforward diffusion of dissolved molecules or vapors, as proved in previous studies dealing with the photonic analysis of fluids and volatile organic compounds. 26,46,47 It is also apparent in Figure 2 that the thickness of the a-TiO 2 photoactive layer in the three samples is about the same (∼400 nm) and the thickness of the external SiO 2 or SiO 2 / Ta 2 O 5 capping stack in S/pA/S or BM/pA/BM sample ranges between 500 and 600 nm.…”

“…Another characteristic feature is that pores arrange along open channels of a rather large size (larger than 10 nm) and that quite small nanopores with a size less than or equal to 2 nm are present within the nanocolumns. , We assume that this microstructure enables a straightforward diffusion of dissolved molecules or vapors, as proved in previous studies dealing with the photonic analysis of fluids and volatile organic compounds. ,, …”