Surface reactions with oxygen are a fundamental cause of the degradation of phosphorene. Using first-principles calculations, we show that for each oxygen atom adsorbed onto phosphorene there is an energy release of about 2 eV. Although the most stable oxygen adsorbed forms are electrically inactive and lead only to minor distortions of the lattice, there are low energy metastable forms which introduce deep donor and/or acceptor levels in the gap. We also propose a mechanism for phosphorene oxidation and we suggest that dangling oxygen atoms increase the hydrophilicity of phosphorene.PACS numbers: 73.20. At,73.20.Hb Phosphorene, a single layer of black phosphorus [1,2], has revealed extraordinary functional properties which make it a promising material not only for exploring novel physical phenomena but also for practical applications. In contrast to graphene, which is a semi-metal, phosphorene is a semiconductor with a quasiparticle band gap of 2 eV. The optical band gap is reduced to 1.2 eV, because of the large exciton binding energy (800 meV) [3,4]. Phosphorene's peculiar structure of parallel zig-zag rows leads to very anisotropic electron and hole masses, optical absorption and mobility [5][6][7]. Both its gap and the effective masses can be tuned by stressing phosphorene's naturally pliable waved structure. Strain along the zigzag direction can switch the gap between direct and indirect [8] and compression along the direction perpendicular to the layers can in principle even transform the material into a metal or semimetal [1]. Phosphorene has also scored well as a functional material for two-dimensional electronic and optoelectronic devices. Multi-layer phosphorene field effect transistors have already been demonstrated to exhibit on-off current ratios exceeding 10 5 , field-effect mobilities of 1000 cm 2 /Vs[9], and fast and broadband photodetection [10].An invariable issue encountered in the manipulation of phosphorene is the control of the oxidation. The presence of exposed lone pairs at the surface makes phosphorus very reactive to air. Surface oxidation is made apparent by the roughening, which grows exponentially during the first hour after exfoliation [11] and contributes to increasing contact resistance, lower carrier mobility and possibly to the mechanical degradation and breakdown. Thus, identifying the mechanisms of phospherene oxidationincluding the electrically active forms of oxygen and how they are introduced -is essential to understanding the real material and its applications.In this Letter, we show that oxygen chemisorption onto phosphorene is exoenergetic and leads to the formation of neutral defects, as well as to metastable electrically active defect forms. We also discuss the conditions necessary for extensive oxidation and propose strategies to control it.Oxygen defects were modeled using first-principles calculations based on density functional theory (DFT), as implemented in the Quantum ESPRESSO package [12]. We used three different approximations for the exchangecorrelation e...

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The stability of the surface of in situ cleaved black phosphorus crystals upon exposure to atmosphere is investigated with synchrotron-based photoelectron spectroscopy. After 2 days atmosphere exposure a stable subnanometer layer of primarily P2O5 forms at the surface. The work function increases by 0.1 eV from 3.9 eV for as-cleaved black phosphorus to 4.0 eV after formation of the 0.4 nm thick oxide, with phosphorus core levels shifting by <0.1 eV. The results indicate minimal charge transfer, suggesting that the oxide layer is suitable for passivation or as an interface layer for further dielectric deposition.