Acetone improves the topographical homogeneity of liquid phase exfoliated few-layer black phosphorus flakes

Liquid phase exfoliation of 2D materials has issues related to the sorption of the solvent, the oxidation of the sample during storage, and the topographical inhomogeneity of the exfoliated material. N-methyl-2-pyrrolidone (NMP), a common solvent for black phosphorus (BP) exfoliation, has additional drawbacks like the formation of by-products during sonication and poor solvent volatility. Here we demonstrate an improvement in the topographical homogeneity (i.e. thickness and lateral dimensions) of NMP-exfoliated BP flakes after resuspension in acetone. The typical size of monolayers and bilayers stabilised in acetone was 99.8 ± 27.4 nm and 159.1 ± 57 nm, respectively. These standard deviations represent a threefold improvement over those of the NMP-exfoliated originals. Phosphorene can also be exfoliated directly in acetone by very long ultrasonication. The product suspension enjoys the same dimensional homogeneity benefits, which confirms that this effect is an intrinsic property of the acetone-BP system. The quality and stability of the exfoliated flakes was checked by XRD, TEM, electron diffraction and Raman spectroscopy. Thermal expansion coefficients of the [Formula: see text] B_2g and [Formula: see text] Raman modes were calculated for drop-casted samples as -0.018 28 cm^-1 K^-1, -0.030 56 cm^-1 K^-1 and -0.032 19 cm^-1 K^-1, respectively. The flakes withstand 20 min in O₂ flow at 373 K without lattice distortion.