Recently, the MXene-like MOenes provide a novel combination of mechanical, electronic, and optical properties attractive for next-generation low-dimensional quantum and optoelectronic devices. The Janus structure induced by asymmetric surfaces could bring about unique features due to the existence of a built-in electric field. In this study, Janus 2H- and 1T-Ti2OFCl monolayers are systematically scrutinized via the first-principle calculations. 2H- and 1T-Ti2OFCl monolayers are direct semiconductors with a bandgap of 0.75 and 1.12 eV, respectively. Intriguingly, biaxial strains can lead to a three-component 2D pseudospin-1 fermion and a two-component 2D double Weyl fermion during the quantum phase transitions. The effective k·p models further help to characterize these emergent fermions. In addition, the light absorbance calculations including the excitonic effect show that Ti2OFCl monolayers can be potentially applied in infrared detectors and exciton solar cells. This work would further understand the inherently chemical and physical features of the MOene family.

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