Reversing the Properties of 2D Black Phosphorus through Water Rinsing

Abstract

Two-dimensional (2D) materials have gained a significant attention due to their unique properties. Among various 2D materials, black phosphorus is known for its excellent electrical and optical properties. Not only does its mobility can reach as high as 1000 cm2/V·s, the on/off current ratio has the maximum value of 105 . BP is also remarkable in the fact that it has direct bandgap regardless of number of layers and the bandgap can be controlled from 0.3 eV in bulk to 2.0 eV in monolayer BP. Despite all these advantageous properties, the instability of BP under air condition is critical in studying BP and applying to various fields. It has been already revealed by previous researches that BP readily degrades through oxidation process of phosphorus atoms reacting with oxygen chemisorbed on the surface. Once oxygen molecules chemisorb, BP surface becomes hydrophilic and water molecules physisorb, forming large bubble on the surface. These bubbles on thin BP flake are initially small and great in number, however, they coalesce as the exposure time in air condition increases and eventually become one large bubble, which can be even thicker than 500 nm, covering the whole surface. This bubble is the result of reaction between phosphorus and water molecules and it was found out that phosphoric acids are the product components of the reaction. This fact has been confirmed once more when the bubble was successfully rinsed with deionized (DI) water; phosphoric acid is known to be very soluble in water. Thus the bubble is very likely to be consisted of phosphoric acid components dissolved in water. We have investigated the effect of DI rinsing and employed this method plus further treatments to recover the pristine surface of BP. After intentionally causing degradation on the surface using UV lamp, the samples were immersed in DI water and annealed under vacuum condition. Field effect transistors were also fabricated using BP as the channel material in order to analyze the change in electrical properties using semiconductor parameter analyzer. In addition, the morphology and optical properties were observed using atomic force microscopy and Raman spectroscopy, respectively. In conclusion, the pristine properties could be obtained even after degradation that is rather inevitable during characterizations and fabrication processes, meaning BP-based devices with higher performance can still be fabricated without strictly controlled vacuum system. Further results and discussion will be presented in detail.