Substrate-free layer-number identification of two-dimensional materials: A case of Mo0.5W0.5S2 alloy

Abstract

Any of two or more two-dimensional (2D) materials with similar properties can be alloyed into a new layered material, namely, 2D alloy. Individual monolayer in 2D alloys is kept together by van der Waals interactions. The property of multilayer alloys is a function of their layer number. Here, we studied the shear (C) and layer-breathing (LB) modes of Mo0.5W0.5S2 alloy flakes and their link to the layer number. The study reveals that the disorder effect is absent in the C and LB modes of 2D alloys, and the monatomic chain model can be used to estimate the frequencies of the C and LB modes. We demonstrated how to use the frequencies of C and LB modes to identify the layer number of alloy flakes deposited on different substrates. This technique is independent of the substrate, stoichiometry, monolayer thickness, and complex refractive index of 2D materials, offering a robust and substrate-free approach for layer-number identification of ultrathin flakes of 2D materials, such as 2D crystals and 2D alloys.