Charge-state-controlled spin polarization and quantum transport in vanadium-decorated 7-armchair graphene nanoribbons

Transition-metal functionalization and charge-state engineering provide complementary strategies for tailoring the electronic phases of graphene nanoribbons, yet their cooperative influence on spin polarization, optical response, and quantum transport remains poorly understood. Here, we employ spin-polarized density functional theory combined with nonequilibrium Green's function calculations to systematically investigate vanadium-decorated hydrogen-passivated 7-armchair graphene nanoribbons under varying adsorption coverages and charge states. Increasing vanadium coverage progressively strengthens V-3d/C-2p hybridization, enhances adsorption stability through cooperative electronic interactions, and drives the electronic structure from semiconducting to spin-polarized and ultimately half-metallic behavior. Charge-density-difference analysis reveals a transition from localized to delocalized charge redistribution, providing the microscopic origin of the enhanced orbital coupling and magnetic ordering. Charge-state modulation introduces a pronounced electron-hole asymmetry: hole doping reinforces charge transfer, spin polarization, and low-energy optical absorption through stronger V-3d/C-2p hybridization, whereas electron doping produces comparatively weaker electronic reconstruction. The coupled evolution of the electronic structure is reflected in the transport properties, which exhibit a crossover from impurity-assisted conductance enhancement at low vanadium coverage to spin-selective, localization-dominated transport at higher adsorption densities due to the competition between orbital hybridization and carrier scattering. These results establish transition-metal functionalization and charge-state engineering as cooperative approaches for simultaneously tuning the structural, electronic, optical and transport properties of armchair graphene nanoribbons, providing a general framework for the rational design of graphene-based nanoelectronic, optoelectronic, and spintronic devices.