The gas sensor market is growing fast, driven by many socioeconomic and industrial factors. Mid-infrared (MIR) gas sensors offer excellent performance for an increasing number of sensing applications in healthcare, smart homes, and the automotive sector. Having access to low-cost, miniaturized, energy efficient light sources is of critical importance for the monolithic integration of MIR sensors. Here, we present an on-chip broadband thermal MIR source fabricated by combining a complementary metal oxide semiconductor (CMOS) micro-hotplate with a dielectric-encapsulated carbon nanotube (CNT) blackbody layer. The micro-hotplate was used during fabrication as a micro-reactor to facilitate high temperature (>700 ∘C) growth of the CNT layer and also for post-growth thermal annealing. We demonstrate, for the first time, stable extended operation in air of devices with a dielectric-encapsulated CNT layer at heater temperatures above 600 ∘C. The demonstrated devices exhibit almost unitary emissivity across the entire MIR spectrum, offering an ideal solution for low-cost, highly-integrated MIR spectroscopy for the Internet of Things.

Introduction
Gas sensors are at the center of increasing research and development efforts, driven by many scientific, industrial and commercial applications1. These include the monitoring of environmental pollutants from deforestation2, vehicles and industry3 and also air quality within buildings4. There is an increased awareness of the impact of air pollution on human health3, leading to a rise in demand for low cost, accessible, compact, and readily deployable air quality monitoring5. To sustain the emerging global demand, gas sensors must meet a suitable and challenging balance between performance and cost1. In addition to being economically viable, an increasing number of sensors have stringent power and volume constraints1, e.g. those deployed within the Internet of Things (IoT)6 and in mobile platforms7. These requirements motivate researchers to explore novel materials, designs and technologies to achieve miniaturization, monolithic integration of components, low cost, reduced power consumption and manufacturability1.

Amongst the various different sensing technologies, optical gas sensors offer several advantages in terms of selectivity and long-term operational stability1. Notably, nondispersive infrared (NDIR) sensors currently dominate the carbon dioxide (CO2) gas sensor market, and also serve many other applications8. However, despite their inherent advantages (e.g., for spectroscopic sensing), NDIR gas sensors are currently mostly employed for the detection of single analytes, or a few species at the same time. A limit to wider adoption has been availability of miniaturised broadband MIR light sources which are low cost and optically efficient (arguably the core of an optical gas sensor)1. Bulb based thermal sources have traditionally been used but are fragile, bulky and have limited optical efficiency at wavelengths above 5 μm. Light-emitting diodes (LED) offer improved integration and reliability but are costlier to fabricate due to the use of specialist III–V semiconductor technologies9.

Read more: A highly stable, nanotube-enhanced, CMOS-MEMS thermal emitter for mid-IR gas sensing