This work presents a more realistic study on the potential of titanium carbide MXene (Ti3C2T x ) for gas sensing, by employing first principle calculations. The effects of different ratios of different functional groups on the adsorption of NH3, NO, NO2, N2O, CO, CO2, CH4, and H2S gas molecules on Ti3C2T x were analyzed. The results indicated that Ti3C2T x is considerably more sensitive to NH3, among the studied gas molecules, with a charge transfer of −0.098 e and an adsorption energy of −0.36 eV. By analyzing the electrostatic surface potential (ESP) and the projected density of states (PDOS), important physical and mechanical properties that determine the strength and nature of gas-substrate interactions were investigated, and also, the significant role of electrostatic effects on the charge transfer mechanism was revealed. Further, the Bader charge analysis for the closest oxygen and fluorine atoms to NH3 molecule showed that oxygen atoms have 60% to 180% larger charge transfer than fluorine atoms, supporting that Ti3C2T x substrate with a relatively lower ratio of fluorine surface terminations has a stronger interaction with NH3 gas molecules. The calculations show that in the presence of water molecules, approximately 90% smaller charge transfer between NH3 molecule and the Ti3C2T x occurs. Ab initio molecular dynamics simulations (AIMD) were also carried out to evaluate the thermal stabilities of Mxenes. The comprehensive study presented in this work provides insights and paves the way for realizing sensitive NH3 sensors based on Ti3C2T x that can be tuned by the ratio of surface termination groups.

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The temperature coefficient of resistance (TCR) of thin metal lines is often used for applications in thermometry, bolometers, or thermal accelerometers. However, metal TCR is much degraded in nanometer-thin films due to strong surface scattering, preventing their use as fast thermal sensors, which simultaneously require low thermal mass and large TCR. In contrast, here we show that the TCR of doped two-dimensional (2D) semiconductors is large (∼0.3% K−1 at 300 K in MoS2 and MoTe2) even at sub-nanometer thickness. This is larger than that of any metals with thicknesses up to ∼35 nm and larger than that of ∼95 nm thick Cu lines (0.25% K−1) at 300 K. At 100 K, the TCR of these 2D materials is doubled, ∼0.6% K−1. Comparison with detailed 2D transport models suggests that the TCR could be further enhanced (up to 0.45% K−1 at 300 K and ∼2.5% K−1 at 100 K) by reducing the density of Coulomb impurities and scattering centers. Such high TCR in atomically thin 2D semiconductors could lead to the design of fast thermal sensors.

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