Chemoresistive gas sensing properties of two-dimensional materials

Abstract

Semiconducting metal oxides, the most widely used materials in gas sensor applications, still have major problems of high power consumption, thermal safety issues resulting from use of external heaters, poor long-term stability and vulnerability to humidity. To overcome the obstacles, it is of great importance to explore alternative materials and improve their intrinsic properties by diverse strategies such as changing device structures, modification of surface chemistry by noble metal decoration or functionalization and understanding gas sensing mechanisms. Among the alternative materials for gas sensing applications, two-dimensional (2D) materials including graphene, metal oxide nanosheets, and transition metal dichalcogenides (TMDs) are considered as leading candidates for next-generation gas sensors applicable to future electronics because of their unique properties such as transparency, flexibility, high surface-to-volume ratio, numerous active edge sites, and high sensitivity to gas molecules at room temperature. In addition, 2D materials satisfy special requirements for practical uses like low power consumption, low cost, small size, and easy integration into existing technologies. However, 2D materials have also few drawbacks such as poor selectivity, long response time, and irreversible gas sensing behaviors which must be overcome. Therefore, this thesis presents chemoresistive gas sensing properties of self-activated graphene, chemically modified graphene oxide, and liquid-exfoliated MoS2 possessing enhanced sensing characteristics. The sensing mechanisms are identified by first-principles calculations based on density functional theory (DFT). First of all, self-activated gas sensing operation of all graphene gas sensors with high transparency and flexibility has been achieved using simple photolithography and graphene transfer processes on polymer substrates. The all-graphene gas sensors which consist of graphene for both sensor electrodes and active sensing area exhibit highly sensitive, selective, and reversible responses to NO2 without external heating. The theoretical detection limit is calculated to be approximately 6.87 parts per billion (ppb). The sensors show reliable operation under high humidity conditions and bending strain, bending radius of 1 mm. In addition to these remarkable device performances, the significantly facile fabrication process enlarges the potential of the all-graphene gas sensors for use in the Internet of Things (IoT) and wearable electronics. Secondly, we present a facile solution process and the room temperature gas sensing properties of chemically fluorinated graphene oxide (CFGO). The CFGO sensors exhibit improved sensitivity, selectivity, and reversibility upon exposure to NH3 with a significantly low theoretical detection limit of ~6 ppb at room temperature in comparison to NO2 sensing properties. The effect of fluorine doping on the sensing mechanism is examined by first-principles calculations based on density functional theory. The calculations reveal that the fluorine dopant changes the charge distribution on the oxygen containing functional groups in graphene oxide, resulting in the preferred selective adsorption and desorption of NH3 molecules. We believe that the remarkable NH3 sensing properties of CFGO and investigation by first-principles calculations would enlarge the possibility of functionalized 2D materials for practical gas sensing applications. Thirdly, we investigate the oxygen sensing behavior of MoS2 microflakes and nanoparticles prepared by mechanical and liquid exfoliation, respectively. Liquid-exfoliated MoS2 nanoparticles with an increased number of edge sites present high and linear responses to a broad range of oxygen concentrations (1–100%). The energetically favorable oxygen adsorption sites, which are responsible for reversible oxygen sensing, are identified by first-principles calculations based on density functional theory. This study serves as a proof-of-concept for the gas sensing mechanism depending on the surface configuration of 2D materials and broadens the potential of 2D MoS2 in gas sensing applications