Strain Control And Boundary Magnetism Of Photocurrent In Phosphorene Devices

Phosphorene is a rising two-dimensional material,which is a puckered honeycomb structure formed by covalently-bonded phosphorus atoms.Phosphorene possesses a tunable direct band gap and highly-anisotropic band structure.These properties make phosphorene a promising material for optical and optoelectronic applications.Based on first-principle calculations,we investigate the strain manipulation of photocurrent in phosphorene-based two-terminal device and edge magnetism in phosphorene nanoribbons.(1)The effect of geometry relaxation on edge magnetism and band structure has been studied for phosphorene nanoribbons.For the nanoribbons tailored directly from phosphorene crystal,our numerical calculations indicate that the lowest enengy appears when ferromagnetically ordered spins align at each edge and the spin directions at two edges are opposite(FM-A state).An armchairedged nanoribbon behaves as a metal,while a zigzag-edged nanoribbon behave as an indirect-gap semiconductor.As the width increases,the edge magnetism vanishes gradually.For phosphorene nanoribbons with full geometry relaxation,our calculations indicate that armchair-edged nanoribbons have no edge magnetism and behave as indirect-gap semiconductors.A zigzag-edged relaxed nanoribbon shows FM-A state only under a given width.In comparison,for graphene nanoribbons geometry relaxation has a minor effect on the edge magnetism and band structure.(2)Strain manipulation of photocurrent has been investigated for a phosphorenebased two-terminal device irradiated by a linear-polarized light.An out-of-plane gate voltage is applied within the irradiated region to break the inversion symmetry of pristine phosphorene so that photocurrent is obtained under zero sourcedrain bias.Such a photogalvanic current is calculated by the nonequilibrium Green's function formalism combined with density-functional theory.The photocurrent in the strain-free system is highly anisotropic and depends on the applied region of the gate voltage.Under a small tensile strain with strength ? 2%,the photoresponse for lights in the whole visible range is suppressed greatly.Thisindicates that the considered phosphorene device acts as a mechanical switch of photocurrent.The photocurrent along the zigzag direction decays quickly with the strain strength.In contrast,the photocurrent along the armchair direction can be enhanced by a moderate strain strength ? 8% in some region of photon energy.Such a mechanical tunability of photogalvanic current could offer additional functionality of phosphorene-based photodetectors.