Geometric Percolation Threshold Defines Half-Metallic Window in Vacancy-Doped Titanium disulfides
Defect engineering of two-dimensional materials routinely produces local magnetic moments, yet itinerant half-metallic ferromagnetism remains elusive -- experiments frequently yield paramagnetic insulators. We resolve this paradox for vacancy-doped monolayer $1T$-\ptis~by demonstrating that the insulator-to-half-metal transition is governed by universal geometric percolation of the defect network, extending the percolation framework established for three-dimensional diluted magnetic semiconductors into the 2D vacancy-doped regime. Half-metallicity emerges via a two-step mechanism: crystal-field symmetry breaking ($O_h \to C_{4v}$) selectively stabilizes the Ti $3d_{z^2}$ orbital, generating robust local moments ($0.94~μ_B$), but spin-polarized transport requires these moments to form a spanning cluster. At critical vacancy concentration $x_c \approx 12.5\%$, a percolation transition drives the majority-spin impurity band from flat, localized levels ($W < 0.1$~eV) to a dispersive 1.5~eV-wide band with 100\% spin polarization and a minority-spin gap of 1.0~eV. The percolation mechanism is independently corroborated by a striking supercell-size effect: at identical concentration, $2\times2$ cells yield antiferromagnetic order while $4\times4$ cells mandate ferromagnetism, reflecting the presence or absence of a spanning cluster. We estimate a Curie temperature exceeding 300~K from the exchange coupling, and identify a geometric jamming instability at $x > 20\%$ that fragments the network. These results define a narrow functional window ($11\% < x < 15\%$) for half-metallic operation and establish geometric connectivity as a quantitative design principle for defect-engineered 2D spintronics.