Interstitial TM–P pairing in P ₃ -coordinated wide-gap quantum dots: spin-selective insulating states and enhanced hyperpolarizability

The variations in electronic, magnetic, and nonlinear optoelectronic properties due to interstitial doping on group-12 based single-atom thick ternary metal-phosphorus-chalcogenide quantum dots (MPC QDs) have been studied with density functional theory computations. This novel doping strategy intended to examine the impacts of P₃-TM hybridizations in the surface-bound region and how it systematically regulates the multifunctional behavior of these nanoflakes. It is found that the placement of a transition metal (TM) atom at a hollow site, in proximity to the substituted phosphorus, leads to localized magnetic moments in these honeycomb-shaped nanoflakes. Moreover, a few configurations retain their nonmagnetic character despite the interstitial coordination (spin compensation), while nonlocal chalcogen coordination within the host framework modulates the overall magneto-electronic response. The spin-polarization can be tuned to achieve specific magnetic ordering with S = 1, 3/2, 2, and 3, confirming the constrained spatial extent of the stable spin density around the dopant. Pristine MPC QDs have energy gaps of 2.7-7.37 eV, which increase for Zn/Cd and decrease for Hg with chalcogens, while Co-, Ni-, Mn-, or V-doped cases have energy gaps ranging from 4.53-9.10 eV (E↑g) and from 3.89-7.13 eV (E↓g). Moreover, linear polarizability increases with chalcogens (S to Te) for the pristine and ternary cases, while interstitial cases show enhanced static first-hyperpolarizability due to co-doping-induced charge asymmetry. Overall, understanding doping-induced local hybridization in wide-gap nanoflakes, which gives rise to proximal magnetic moments with controllable HOMO-LUMO distributions and enhanced hyper(polarizability), enables the effort to engineer spin-filtering devices, spin-based quantum computation, second-harmonic generation (SHG), and electro-optic modulation.