Figure 3. Optical absorption spectra.

(a,b) Optical absorption spectra of few-layer BP for light incident in the c (z) direction and polarized along the a (x) and b (y) directions, respectively. Black dashed lines show an approximate linear fit used to estimate the band edges for the first absorption peak, which are highly anisotropic between x and y. The band edge drops rapidly with sample thickness for x (from 1.55 eV for the monolayer (red) to 0.60 eV for five layers (orange)) but only slightly for y (from 3.14 eV to 2.90 eV). (c) Schematic illustration of a proposed experimental geometry to determine the orientation of few-layer BP structures using optical absorption spectroscopy, and thus to utilize the anisotropic electronic properties of BP. The light is linearly polarized in a chosen orientation and near-normally incident on the sample. The sample should be rotated to identify the a or b direction by monitoring the absorption signal, after which the source and drain electrodes, denoted as ‘S’ and ‘D’ respectively, may be deposited to fabricate an FET device.