Perfectly absorbing ultra thin interference coatings for hydrogen sensing

Abstract

Here we numerically demonstrate a straightforward method for optical detection of hydrogen gas by means of absorption reduction and colorimetric indication. A perfectly absorbing metal-insulator-metal (MIM) thin film interference structure is constructed using a silver metal back reflector, silicon dioxide insulator, and palladium as the upper metal layer and hydrogen catalyst. The thickness of silicon dioxide allows the maximizing of the electric field intensity at the Air∕SiO 2 interface at the quarter wavelengths and enabling perfect absorption with the help of highly absorptive palladium thin film ∼7 nm. While the exposure of the MIM structure to H 2 moderately increases reflection, the relative intensity contrast due to formation of metal hydride is extensive. By modifying the insulator film thickness and hence the spectral absorption, the color is tuned and eyevisible results are obtained. Hydrogen H 2 sensing requires development of highly sensitive, reliable, and short response-time sensors due to its use in critical applications such as fuel-cells, pharmaceuticals, and petroleum and chemical production. Palladium (Pd) can absorb H 2 upon exposure and transform into palladium hydride PdH x , and the optical and electrical properties of Pd metal are changed by such transformation [1]. Recent advancements in plasmonic research and nanofabrication techniques led the development of surface and localized surface plasmon-based (LSPR) hydrogen sensors using Pd [2][3][4]. Nanoparticle or LSPR sensors show good potential for faster response times [5,6]. However, the broad spectral response of LSPR-based hydrogen sensors arises as an issue, which, later on, led the researchers to reduce the linewidth with the help of a whispering gallery mode cavity [7], and to develop highly sensitive hybrid sensors such as using Pd with a good plasmonic metal [8,9]. For example, Alivisatos and co-workers have demonstrated highly sensitive hydrogen sensors by placing Pd nanoparticles close to the sharp corners of nanoantennas [10].Such sensing mechanisms are based on probing of the scattering properties of nanoantenna-Pd nanoparticle structure, rather than the direct probing of scattering properties of Pd nanoparticles that shows small wavelength shifts upon hydrogen exposure. Other important plasmon-based hydrogen sensing mechanism is based on plasmonic-perfect absorbers [11][12][13], and single crystal Pd and its alloy-coated nanowires [14,15]. The motivation behind the absorbance-based sensors is to minimize the requirement for the bulky optical setups, expensive light sources, and spectrometers by only probing the scattered light intensity at perfect-absorption wavelength. Moreover, some of the recent studies report the visual detection of hydrogen by using thin film optical coatings to circumvent the problems of hydrogen sensors based on electrical readout [16,17]. Despite the demonstration of visual detection of H 2 , the thin film structure did not take full advantage of the lossy nature of Pd films. In this study, we demo...

Dynamic Optical Properties of Metal Hydrides

“…Palladium and palladium alloys have been widely used for such sensors, structured as both thin films and nanoparticles. [16][17][18][19][20][21][22][23] Yttrium and lanthanum have been investigated for their use as switchable mirrors due to their metal to dielectric transition upon hydrogenation. 7,8 Magnesium has seen recent interest for use in reversible color filters due to its optically dramatic shift from metal to dielectric upon hydrogenation.…”

“…These metal hydrides are of great interest for switchable photonic devices, particularly for applications involving optical hydrogen sensors and switchable mirrors. Palladium and palladium alloys have been widely used for such sensors, structured as both thin films and nanoparticles. − Yttrium and lanthanum have been investigated for their use as switchable mirrors due to their metal to dielectric transition upon hydrogenation. , Magnesium has seen recent interest for use in reversible color filters due to its optically dramatic shift from metal to dielectric upon hydrogenation . Hafnium has been introduced as an optical hydrogen sensor that can span 6 orders of magnitude .…”

Thin Film and Nanostructured Pd-Based Materials for Optical H2 Sensors: A Review

“…Another family of CN materials are those that combine layers of thin films and arrays of isolated nanodiscs. These materials, referred to as plasmonic perfect-absorberbased H 2 optical sensors [92][93][94][95], exploit changes in the reflectance during hydrogenation/dehydrogenation [92]. An example structure of such a material comprises of an array of Pd nanodiscs that is placed on top of a dielectric spacer (MgF 2 ) while the bottom layer is a metallic film (e.g., Au), which plays the role of a mirror (Figure 11a).…”

Strong Interference in Planar, Multilayer Perfect Absorbers: Achieving High-Operational Performances in Visible and Near-Infrared Regimes

“…The proposed MIM resonator has a reflection dip in the Vis range. As PdHx is formed, the effective permittivity of the top layer changes, imposing a red shift in its dipspectral position [118]. This colorimetric diagnostic can be achieved by using SPRbased structures.…”