New optical features of substitutional defects in 2D materials*

Defect engineering of 2D materials offers enormous opportunities to tune material properties. Compared to the most common vacancies in 2D materials, substitutional defects require unique and controllable approaches to generate. We have studied two types of substitutional defects in 2D materials, self-limited along the out-of-plane and in-plane directions, respectively. The first type is atomic substitution: a nitrogen atom substituting a chalcogen atom in 2D transition metal dichalcogenides (TMDs), which yields new distinct photoluminescence features well separated from the free excitons of 2D TMDs [1]. The second type is layer substitution: an entire layer of chalcogen atoms in 2D TMD substituted by another type of chalcogen atoms, namely, Janus TMDs. Due to the intrinsic vertical dipole, Janus TMDs form unconventional interaction with adjacent materials including other 2D material layers. These unconventional interactions were probed by optical signature changes such as ultra-low frequency Raman modes and photoluminescence yield change [2,3]. The engineering of such defects in 2D materials presents unique opportunities for optoelectronic devices and quantum information platforms.