Tuning the Optical and Electrical Properties of Few-Layer Black Phosphorus via Physisorption of Small Solvent Molecules.

Black phosphorus (BP) is recently becoming more and more popular among semiconducting 2D materials for (opto)electronic applications. The controlled physisorption of molecules on the BP surface is a viable approach to modulate its optical and electronic properties. Solvents consisting of small molecules are often used for washing 2D materials or as liquid media for their chemical functionalization with larger molecules, disregarding their ability to change the opto-electronic properties of BP. Herein, it is shown that the opto-electronic properties of mechanically exfoliated few-layer BP are altered when physically interacting with common solvents. Significantly, charge transport analysis in field-effect transistors reveals that physisorbed solvent molecules induce a modulation of the charge carrier density which can be as high as 1012 cm-2 in BP, i.e., comparable to common dopants such as F4 -TCNQ and MoO3 . By combining experimental evidences with density functional theory calculations, it is confirmed that BP doping by solvent molecules not only depends on charge transfer, but is also influenced by molecular dipole. The results clearly demonstrate how an exquisite tuning of the opto-electronic properties of few-layer BP can be achieved through physisorption of small solvent molecules. Such findings are of interest both for fundamental studies and more technological applications in opto-electronics.