Quantum Confinement and Phase Transition in PbS nanowire

We report first principles density functional calculations of electronic structures and energy bandgaps ($\Delta E_g$) in PbS nanowires (NW). The $\Delta E_g$ is tuned by varying the diameter of the NW - {\it revealing the role of quantum confinement}. The compressive radial strain (CS) on the NW is shown to be responsible for semiconducting to metallic phase transition. The conduction band (CB) of the NW, which has significant contribution from the excited 3{\it d}-orbital of S, is found to be more sensitive to the CS with the CB minimum shifting towards and eventually crossing the Fermi energy with increasing CS. The origin of the observed phase transition in a recent experiment is attributed to the CS on the PbS nanowire.