  Understanding and exploiting the remarkable optical and electronic properties
of phosphorene require mass production methods that avoid chemical degradation.
While solution-based strategies have been developed for scalable exfoliation of
black phosphorus, these techniques have thus far employed anhydrous organic
solvents in an effort to minimize exposure to known oxidants, but at the cost
of limited exfoliation yield and flake size distribution. Here, we present an
alternative phosphorene production method based on surfactant-assisted
exfoliation and post-processing of black phosphorus in deoxygenated water. From
comprehensive microscopic and spectroscopic analysis, this approach is shown to
yield phosphorene dispersions that are stable, highly concentrated, and
comparable to micromechanically exfoliated phosphorene in structure and
chemistry. Due to the high exfoliation efficiency of this process, the
resulting phosphorene flakes are thinner than anhydrous organic solvent
dispersions, thus allowing the observation of layer-dependent photoluminescence
down to the monolayer limit. Furthermore, to demonstrate preservation of
electronic properties following solution processing, the aqueous-exfoliated
phosphorene flakes are employed in field-effect transistors with high drive
currents and current modulation ratios. Overall, this method enables the
isolation and mass production of few-layer phosphorene, which will accelerate
ongoing efforts to realize a diverse range of phosphorene-based applications.
