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 are 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 layerbreathing (LB) modes of Mo 0.5 W 0.5 S 2 alloy flakes and their link to the layer number of alloy flakes. 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 C and LB mode frequency 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 2D materials. [6][7][8][9] The 2D alloys are a rich source of 2D materials because they can exhibit tunable properties because the ratio of two end compositions can be controllable. 6,9 The key to form a good quality of 2D monolayer is mixing the end compositions at atomic scale. Individual monolayers in the 2D alloys are kept together by Van der Waals interations. Similar to graphite and graphene, the property of multilayer alloys is a function of their layer number. How to determine the layer number of ultrathin 2D alloys is thus of primary importance for fundamental science and applications.Several optical techniques have been developed to identify the layer number of 2D materials. [10][11][12][13][14][15][16] Nevertheless, most techniques rely on the specific properties of 2D materials. For example, one can identify the layer number, denoted as N, of MoS 2 by the intensity or peak positions of the corresponding photoluminescence (PL) peak and Raman modes. 14,15 The mostly used technique is based upon optical contrast of 2D materials against dielectric substrates, such as a Si substrate covered with a SiO 2 layers. 10,12 To precisely identify N of few-layer 2D materials, the experimental optical contrast must be compared with the theoretical one for different N. However, the theoretical calculations require predetermined parameters, including thickness of a single layer, complex refractive index of the material and information of the substrate, 11,17 which poses great challenge for research on new 2D materials in early stage. Therefore, it is desirable to develop a novel approach to identify the layer number of fewlayer 2D materials, relying on as little prerequisite parameters as possible.The interlayer modes of multilayer 2D materials originates from the relative motions of the rigid monolayer planes themselves, either perpendicular or parallel to their normal, such as the shear (C) modes and the layer breathing (LB) modes. [18][19][20][21][22][23][24] The C and LB modes are the collective vibration modes of all the layers so their frequency (ω C and ω LB ) depe...

Layer‐Number Dependent Optical Properties of 2D Materials and Their Application for Thickness Determination

“…In comparison to NL WS 2 and NL MoS 2 , the E 1 2 g and A 1 g modes in MoWS 2 alloy exhibit broadened profiles due to the disorder effect. However, such disorder effect is generally absent for the interlayer vibrations of the MoWS 2 alloy, as shown in Figure a. The S mode is Raman active in bulk alloy.…”

“…The theoretical (crosses) and experimental (squares) frequencies of e) C and f) LB modes in 2‐11L Mo 0.5 W 0.5 S 2 . Adapted with permission . Copyright 2015, AIP.…”

“…The lower symmetry ( D 2 h , C 2 h , and C i ) leads to the in‐plane anisotropy, which brings one more degree of freedom than isotropic materials and delivers various physical properties . Two or more LMs with similar properties can be alloyed into an additional type of 2DMs, namely, 2D alloy, which can offer tunable band gaps and the corresponding amazing electronic and optical properties . These remarkable properties, including high mobility, high thermal conductivity, unique optical and mechanical properties, and high specific surface area, can be found in 2DMs, which makes them be the ideal materials for promising applications in nanoelectronics and optoelectronics devices …”

Polytypism and unexpected strong interlayer coupling in two-dimensional layered ReS2

“…17,18 The vdW interaction plays an important role in the physical properties of layered materials. 13,[19][20][21] For example, MoS 2 is tuned from direct to indirect bandgap semiconductor from monolayer to bilayer by the interlayer coupling. 13 Moreover, BP is a direct semiconductor with the bang gap varying from 0.3 of the bulk form to 1.5∼2.0 eV of monolayer.…”

Synthesis and applications of multinary layered metal chalcogenide nanomaterials

“…[87] Furthermore, Tan and coworkers reported the measurement of the precise layer number of Mo0.5W0.5S2 nanosheets by the exploration of the shear and layer-breathing modes under Raman spectroscopy because Mo0.5W0.5S2 nanosheet exhibited layer-dependent signals in two different modes. [88] Note that the stoichiometry, complex refractive index and substrate of 2D nanosheet cannot affect the layer-dependent Raman signals in the shear and layer-breathing modes, which makes it one of the most useful skill to identify layer-number of 2D nanosheet. As another example, the Raman spectroscopy study under high pressure on the alloyed Mo0.5W0.5S2…”