Nontrivial Raman Characteristics in 2D Non-Van der Waals Mo₅N₆.

Abstract

Resonant Raman spectra of a two-dimensional (2D) non-van der Waals (vdW) material, molybdenum nitride (Mo₅N₆), are measured across varying thicknesses, ranging from a few to tens of nanometers. Fifteen distinct Raman peaks are observed experimentally, and their assignments are made using first-principles calculations for the most stable AABB-stacking structure of Mo₅N₆. The assignments are further supported by angular-dependent Raman measurements for all peaks, except the most intense one at 215 cm^-1. Calculations reveal that the 215 cm^-1 peak does not appear for three-dimensional molybdenum nitrides and is not a first-order Raman-active mode. We further investigated the origin of the 215 cm^-1 peak and assigned it as a defect-induced double-resonance peak. Moreover, thickness-dependent Raman measurements reveal that both the 215 and 540 cm^-1 peaks─assigned to out-of-plane and in-plane modes, respectively─blue shift as thickness increases, reaching a plateau around 20 nm. This thickness-dependent Raman shift over a wide thickness range is nontrivial compared to other common vdW 2D materials and is attributed to the much stronger stacking interaction between the constituent layers in non-vdW materials. This finding highlights Raman spectroscopy as a valuable tool for characterizing the thickness of 2D non-vdW materials.