paper

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 ( $n ~$ ) from monolayer to bulk TMDs. Here, we discuss the N identification of TMD flakes on the SiO $_{2}$ /Si substrate by the intensity ratio between the Si peak from 100-nm (or 89-nm) SiO $_{2}$ /Si substrates underneath TMD flakes and that from bare SiO $_{2}$ /Si substrates. We assume the real part of $n ~$ of TMD flakes as that of monolayer TMD and treat the imaginary part of $n ~$ as a fitting parameter to fit the experimental intensity ratio. An empirical $n ~$ , namely, $n ~_{e f f}$ , of ultrathin MoS $_{2}$ , WS $_{2}$ and WSe $_{2}$ flakes from monolayer to multilayer is obtained for typical laser excitations (2.54 eV, 2.34 eV, or 2.09 eV). The fitted $n ~_{e f f}$ of MoS $_{2}$ has been used to identify N of MoS $_{2}$ flakes deposited on 302-nm SiO $_{2}$ /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 $n ~$ . For the application purpose, the intensity ratio excited by specific laser excitations has been provided for MoS $_{2}$ , WS $_{2}$ and WSe $_{2}$ flakes and multilayer graphene flakes deposited on Si substrates covered by 80-110 nm or 280-310 nm SiO $_{2}$ layer.

10 pages, 4 figures. Accepted by Nanotechnology