Study of the microstructure and emissive properties of ultrathin freestanding MoSiN nanocomposite membranes

The growing demand for compact, energy-efficient, and robust mid-infrared (mid-IR) light sources is driving the development of advanced thermal emitters for next-generation high-temperature gas sensing technologies. Mid-IR gas sensors, in particular, benefit from this spectral range due to the strong and selective molecular absorption features of most gases. However, their development remains constrained by the lack of efficient, low-power, and thermally stable emitter materials capable of reaching the high temperature (600-1000°C) needed for strong emission across the mid-IR broadband range. Conventional icroheater-based emitters suffer from high power consumption, slow thermal response, and material degradation during repeated heating cycles. Addressing these limitations requires materials that combine high emissivity, thermal stability, and mechanical resilience at nanoscale thicknesses. Reducing the membrane thickness lowers thermal mass, minimizing power consumption and enabling faster heating and cooling for enhanced thermal response. For nanoscale membranes where both high emissivity and mechanical strength are critical, integrating these attributes within a composite material offers an effective and promising solution. In this context, this study develops and investigates ultrathin freestanding MoSiN nanocomposite membranes, examining their structural, optical, and thermal properties to assess their suitability as efficient, low-power, and thermally stable mid-IR emitters for compact and reliable gas sensing applications. Freestanding MoSiN membranes, 6-16 nm thick with lateral dimensions of 10 mm × 10 mm supported on silicon frames, were fabricated and characterized. The microstructure and composition of the ultrathin MoSiN membranes were thoroughly studied to elucidate the distribution of intermetallic and dielectric phases. The microstructural analysis reveals that MoSiN possess a complex nanocomposite structure, comprising intermetallic phases (MoSi2 and Mo5Si3) and silicon oxynitride (SiOxNy) dielectric phase. The intermetallic phases facilitate free-carrier absorption in the mid-IR region, while the dielectric phase enhances mechanical integrity and provides strong resistance to oxidation. Membrane exhibits metal-like emissivity (~0.41) and retains thermal and mechanical stability at high temperature (900°C). Repeated laser pulsed-heating experiments confirm rapid thermal response, excellent repeatability, and efficient radiative cooling, validating its potential as a durable microheater emitter material. FTIR spectroscopy shows broadband absorption in the 2-8 µm range, dominated by free-carrier processes, with negligible phonon absorption beyond 8 µm. Emissivity shows a strong thickness dependence due to interference effects, as confirmed by optical modeling using Fresnel equations and the generalized matrix method. Electrical characterization shows resistivity (~10-4 Ω·cm) with a positive temperature coefficient, indicating metallic conduction. Overall, MoSiN behaves as a semimetal combining metallic absorption and dielectric-induced stability. These results highlight ultrathin freestanding MoSiN membranes as a compelling platform for mid-infrared emitters, offering a synergistic combination of low power, thermal efficiency, mechanical durability, and CMOS compatibility for gas sensing applications in harsh environments.