Fig. 3.

Band gap of thin film and monolayer black P. (A) Polarization-resolved infrared relative extinction spectra of a black P thin film when light is polarized along the six directions, as shown in Inset (an optical micrograph of a black P flake with a thickness of around 30 nm). (Scale bar, 20 μm.) (B) Two representative tunneling spectra plotted on a log scale and measured on a black P surface, showing a wide band gap with an estimated size of 2.05 eV. Inset shows high-resolution STM image (V_bias = +1.2 V, I_set = 150 pA) with a scan size of 2.4 $\times$ 3.6 nm. A and B are reproduced with permission from refs. 2 and 81, respectively.