Thickness‐Dependent Infrared Emissivity of Ultrathin Freestanding MoSiN Nanocomposite Membranes

Metals and dielectrics show opposite trends in thickness‐dependent infrared emissivity: metallic films exhibit a rise in emissivity below a critical thickness, while dielectric films show a corresponding decline. For applications demanding both high emissivity and mechanical strength in membranes with nanoscale thickness, combining these properties in a composite material is a promising approach. In this context, we investigate the infrared mechanism and thickness‐dependent emissivity of MoSiN‐based ultrathin freestanding membranes, a nanocomposite of two intermetallic phases (MoSi 2 and Mo 5 Si 3 ) and a dielectric silicon oxynitride (SiO x N y ) phase. Although MoSiN has a composite nature, its optical response closely resembles that of metals. The membranes exhibit broadband absorption arising from a combination of mechanisms, with free‐carrier contributions present across the entire infrared range (as described by the Drude model), while phonon‐mediated absorption from lattice vibrations becomes more significant at longer wavelengths (far‐IR region). Emissivity shows a pronounced thickness dependence: ultrathin membranes (∼6 nm) display low emissivity (∼0.27), increasing to a maximum (∼0.44) at 16 nm, before declining for thicker films. Optical modeling with Fresnel equations and the generalized matrix method shows that interference and phase shifts drive this nonlinear behavior. These findings provide a fundamental understanding of the infrared mechanism and nanoscale radiative behavior in complex intermetallic membranes, guiding the design of thermally efficient and mechanically robust microheaters for advanced sensor applications. MoSiN nanocomposite membranes are promising materials for microheater applications. Infrared emissivity is crucial for the performance of microheaters as mid‐infrared sources in gas sensors, particularly under cyclic heating, where efficient radiative cooling is key for rapid thermal response, energy efficiency, and operational stability.