Strain engineering of bright defects in quasi-one-dimensional TiS₃ and NbS₃ nanowires

Defects in atomically thin van der Waals (vdW) materials have recently been investigated as sources of spin-photon entanglement with sensitivity to strain tuning. Unlike many two-dimensional materials, quasi-one-dimensional (1D) materials such as transition metal trichalcogenides (TMTCs) exhibit in-plane anisotropy leading to axis-dependent material responses to compressive and tensile strains. Herein, we present our theoretical work to characterize the tunability of the spin and optical properties of intrinsic vacancy defects in titanium trisulfide (TiS₃) and niobium trisulfide (NbS₃) nanowires. Within our ab initio approach, we show that sulfide and disulfide vacancies (V_S and V_D, respectively) in TiS₃ and NbS₃ adopt strain-dependent defect geometries between tensile strains of -3 and 3%. The calculated electronic structures indicate that both V_S and V_D originate in-gap defect states with optically bright electronic transitions whose positions relative to the valence band maximum vary with in-plane strain. Further, our calculations predict that V_S in TiS₃ and V_D in NbS₃ exhibit transitions in their ground state spins; specifically, a compressive strain of 0.3% in the y-direction causes a shift from a ground triplet state to a ground singlet state for the V_S defect in TiS₃, whereas a y-direction tensile strain of 2.9% of the V_D defect in NbS₃ induces a triplet ground state. Our work shows that TiS₃ and NbS₃ nanowires offer exceptional tunability of optically active, high-spin point defects that can be used in quantum information applications.