Raman Enhancement Effect on Two-Dimensional Layered Materials: Graphene, h-BN and MoS2

Abstract

Realizing Raman enhancement on a flat surface has become increasingly attractive after the discovery of graphene-enhanced Raman scattering (GERS). Two-dimensional (2D) layered materials, exhibiting a flat surface without dangling bonds, were thought to be strong candidates for both fundamental studies of this Raman enhancement effect and its extension to meet practical applications requirements. Here, we study the Raman enhancement effect on graphene, hexagonal boron nitride (h-BN), and molybdenum disulfide (MoS₂), by using the copper phthalocyanine (CuPc) molecule as a probe. This molecule can sit on these layered materials in a face-on configuration. However, it is found that the Raman enhancement effect, which is observable on graphene, hBN, and MoS₂, has different enhancement factors for the different vibrational modes of CuPc, depending strongly on the surfaces. Higher-frequency phonon modes of CuPc (such as those at 1342, 1452, 1531 cm^–1) are enhanced more strongly on graphene than that on h-BN, while the lower frequency phonon modes of CuPc (such as those at 682, 749, 1142, 1185 cm^–1) are enhanced more strongly on h-BN than that on graphene. MoS₂ demonstrated the weakest Raman enhancement effect as a substrate among these three 2D materials. These differences are attributed to the different enhancement mechanisms related to the different electronic properties and chemical bonds exhibited by the three substrates: (1) graphene is zero-gap semiconductor and has a nonpolar C–C bond, which induces charge transfer (2) h-BN is insulating and has a strong B–N bond, while (3) MoS₂ is semiconducting with the sulfur atoms on the surface and has a polar covalent bond (Mo–S) with the polarity in the vertical direction to the surface. Therefore, the different Raman enhancement mechanisms differ for each material: (1) charge transfer may occur for graphene; (2) strong dipole–dipole coupling may occur for h-BN, and (3) both charge transfer and dipole–dipole coupling may occur, although weaker in magnitude, for MoS₂. Consequently, this work studied the origin of the Raman enhancement (specifically, chemical enhancement) and identifies h-BN and MoS₂ as two different types of 2D materials with potential for use as Raman enhancement substrates.