We investigate the geometrical and electronic structures of various configurations of 2Si-doped two-dimensional (2D) bilayers of black phosphorene (alpha P)(2), in which two P atoms are substituted by Si atoms. Our first-principles calculations suggest that doping is cooperative, which is clearly manifested in the formation of Si-Si bonds in the two most stable configurations. As a result, both configurations become indirect-gap semiconductors, which differ from that of the pristine 2D bilayer. On the one hand, 2Si-doped armchair phosphorene nanoribbon (APNR) bilayers possess pseudodirect band gaps in the most stable configuration, which are one-dimensional materials cut with armchair edges saturated with hydrogen atoms. Comparisons of the deformation energy and the activation barrier suggest that Stone-Wales (SW) deformation can occur substantially more easily in the doped APNR than in carbon nanotubes, and molecular dynamics simulations show that the SW defect will be kinetically stable. This is because the deformation brings about shortening and strengthening of weak Si-Si bonds. As a result, the APNRs turn into real direct-gap materials.
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