Determining layer number of two dimensional flakes of transition-metal dichalcogenides by the Raman intensity from substrate
arXiv:1602.05378 · doi:10.1088/0957-4484/27/14/145704
Abstract
Transition-metal dichalcogenide (TMD) semiconductors have been widely studied due to their distinctive electronic and optical properties. The property of TMD flakes is a function of its thickness, or layer number (N). How to determine N of ultrathin TMDs materials is of primary importance for fundamental study and practical applications. Raman mode intensity from substrates has been used to identify N of intrinsic and defective multilayer graphenes up to N=100. However, such analysis is not applicable for ultrathin TMD flakes due to the lack of a unified complex refractive index () from monolayer to bulk TMDs. Here, we discuss the N identification of TMD flakes on the SiO/Si substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm) SiO/Si substrates underneath TMD flakes and that from bare SiO/Si substrates. We assume the real part of of TMD flakes as that of monolayer TMD and treat the imaginary part of as a fitting parameter to fit the experimental intensity ratio. An empirical , namely, , of ultrathin MoS, WS and WSe flakes from monolayer to multilayer is obtained for typical laser excitations (2.54 eV, 2.34 eV, or 2.09 eV). The fitted of MoS has been used to identify N of MoS flakes deposited on 302-nm SiO/Si substrate, which agrees well with that determined from their shear and layer-breathing modes. This technique by measuring Raman intensity from the substrate can be extended to identify N of ultrathin 2D flakes with N-dependent . For the application purpose, the intensity ratio excited by specific laser excitations has been provided for MoS, WS and WSe flakes and multilayer graphene flakes deposited on Si substrates covered by 80-110 nm or 280-310 nm SiO layer.
arXiv comment10 pages, 4 figures. Accepted by Nanotechnology
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