Gas sensors based on two dimensional (2D) nanostructured materials have attracted much attention due to their unique chemical and physical properties. During the last decade, although the development of 2D materials has led to considerable advances in the field of high-performance gas sensors, many fundamental relationships between design parameters and device performance have yet been explored.

The thesis presents a variety of methods based on size and interface modulation in 2D materials for the enhancement of gas sensing properties. The thesis begins with an introduction to 2D nanomaterials synthesis, which focuses on the strategies in size control and sensing mechanism. In chapter 2, a strategy for optimizing room temperature gas sensing via the control of nanosheet aspect ratio has been presented, i.e., tuning both the lateral dimension and thickness of nanosheets simultaneously. Through elucidating the tradeoff between the quantity of active gas sensing sites and interface resistances as nanosheet dimensions change, systematic design of gas sensors is thus possible. As a demonstration, a room temperature formaldehyde gas sensor has been fabricated using aspect ratio controlled TaS₂ few-layer nanosheets, synthesized by a centrifugation-assisted liquid-phase exfoliation method. Sensor based on the TaS₂ nanosheets with optimal aspect ratio enabled an ultra-large room temperature response of 78% toward 10 ppm formaldehyde. This work illuminates the potential of nanosheet aspect ratio as a primary design parameter towards systematic optimization of nanosheet gas sensing properties.

In chapter 3, a functional group engineering strategy based on in-situ plasma exposure for optimizing gas sensing performance of Ti₃C₂T_x MXene has been presented. For performance assessment and sensing mechanism elucidation, the few-layered Ti₃C₂T_x MXene with grafted functional groups via in-situ plasma treatment has been synthesized. Both experimental and theoretical analyses including X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and density function theory (DFT) indicate consistently that the amount of -O functional group contributes to the enhanced gas sensing performance of the MXene based sensor. DFT calculations reveal that -O functional groups are associated with increased NO₂ adsorption energy, thereby enhancing charge transport. The -O functionalized Ti₃C₂T_x sensor shows a response of 13.8% toward 10 ppm NO₂, good selectivity, and long-term stability at room temperature. The proposed technique is also capable of improving selectivity, a well-known challenge in chemoresistive gas sensing. This work paves the way to the possibility of using plasma grafting for precise functionalization of MXene surfaces towards practical realization of electronic devices.

In chapter 4, a d-band center modulation strategy for MXene realized by functionalizing Fe onto the surface of Ti₃C₂T_x nanosheets has been presented, which demonstrates enhanced gas sensing response and selectivity. The optimized gas sensor, through the incorporation of Fe, shows a response of 50% towards 10 ppm NO₂ at room temperature, which is over six-fold improvement from its pristine counterpart. XPS, valence band, and DFT analyses consistently relate the underlying enhancement mechanism to the tuning of the d-band center energy towards the Fermi level. This work provides a new design strategy based on the optimization of the d-band center energy, adds a much needed systematic and quantitative method to the design of two-dimensional materials based semiconducting gas sensors.

Chapter 5 concludes the thesis with several perspectives on possible future directions of investigation. Findings from this thesis provide important insights to the rational design for 2D material-based gas sensor and would also be an instrumental reference for the functionalization of high-performance TMD and MXene based 2D materials.