Flexible carbon nanotube (CNT) chemoresistors offer a scalable, low‐cost platform for wearable gas detection. However, a clear understanding of their sensing mechanism remains essential to optimize sensitivity, selectivity, and overall reliability. In this work, we complement electrical response measurements in both dry air and ultra‐high vacuum (UHV) with synchrotron‐based in situ X‐ray photoelectron spectroscopy (XPS) to directly probe ammonia (NH ₃ )–CNT interactions. In both environments, the response can be described by Langmuir‐type adsorption–desorption kinetics. In dry air (3–50 ppm), the devices exhibit a reproducible increase in resistance with a sensitivity of ∼0.4% ppm ⁻¹ . Under UHV, a nominal NH ₃ concentration of ∼8 ppm produces a smaller relative response of ∼0.5%, comparable to that obtained at 3 ppm in dry air. In situ micro‐focused XPS reveals reversible (≈1 eV) shifts in the C 1s core‐level binding energy during NH ₃ exposure, confirming that NH ₃ acts as an electron donor. This spectroscopic evidence correlates quantitatively with the chemoresistive response, establishing intra‐CNT charge transfer as the dominant transduction mechanism. These findings underscore the effectiveness of coupling XPS with electrical analysis to unravel gas‐sensor transduction in nanomaterials and pave the way for the rational design of high‐performance CNT‐based sensors.

Transition metal dichalcogenides, especially molybdenum disulfide (MoS₂), exhibit exceptional properties that make them suitable for a wide range of applications. However, the interaction between MoS₂ and technologically relevant substrates, such as platinum (Pt) electrodes, can significantly influence its properties. This study investigates the growth and properties of MoS₂ thin films on Pt substrates using ionized jet deposition, a versatile, low-cost vacuum deposition technique. We explore the effects of the roughness of Pt substrates and self-heating during deposition on the chemical composition, structure, and strain of MoS₂ films. By optimizing the deposition system to achieve crystalline MoS₂ at room temperature, we compare as-deposited and annealed films. The results reveal that as-deposited MoS₂ films are initially amorphous and conform to the Pt substrate roughness, but crystalline growth is reached when the sample holder is sufficiently heated by the plasma. Further post-annealing at 270 °C enhances crystallinity and reduces sulfur-related defects. We also identify a change in the MoS₂–Pt interface properties, with a reduction in Pt–S interactions after annealing. Our findings contribute to the understanding of MoS₂ growth on Pt and provide insights for optimizing MoS₂-based devices in catalysis and electronics.