  Phosphorene, a two-dimensional (2D) monolayer of black phosphorus, has
attracted considerable theoretical interest, although the experimental
realization of monolayer, bilayer, and few-layer flakes has been a significant
challenge. Here we systematically survey conditions for liquid exfoliation to
achieve the first large-scale production of monolayer, bilayer, and few-layer
phosphorus, with exfoliation demonstrated at the 10-gram scale. We describe a
rapid approach for quantifying the thickness of 2D phosphorus and show that
monolayer and few-layer flakes produced by our approach are crystalline and
unoxidized, while air exposure leads to rapid oxidation and the production of
acid. With large quantities of 2D phosphorus now available, we perform the
first quantitative measurements of the material's absorption edge-which is
nearly identical to the material's band gap under our experimental
conditions-as a function of flake thickness. Our interpretation of the
absorbance spectrum relies on an analytical method introduced in this work,
allowing the accurate determination of the absorption edge in polydisperse
samples of quantum-confined semiconductors. Using this method, we found that
the band gap of black phosphorus increased from 0.33 +/- 0.02 eV in bulk to
1.88 +/- 0.24 eV in bilayers, a range that is larger than any other 2D
material. In addition, we quantified a higher-energy optical transition (VB-1
to CB), which changes from 2.0 eV in bulk to 3.23 eV in bilayers. This work
describes several methods for producing and analyzing 2D phosphorus while also
yielding a class of 2D materials with unprecedented optoelectronic properties.
