Phosphorene: Synthesis, Scale-up, and Quantitative Optical Spectroscopy

TL;DR

This paper presents a scalable liquid exfoliation method for producing large quantities of monolayer and few-layer phosphorene, along with a new optical spectroscopy technique to accurately measure its band gap and optical properties.

Contribution

It introduces a rapid, scalable exfoliation process and a novel analytical method for precise optical characterization of 2D phosphorus materials.

Findings

Achieved large-scale production of phosphorene at the 10-gram scale.

Determined the band gap increases from 0.33 eV in bulk to 1.88 eV in bilayers.

Developed an analytical method for accurate absorption edge measurement in polydisperse samples.

Abstract

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.