Nontrivial Raman\nCharacteristics in 2D Non-Van der\nWaals Mo<sub>5</sub>N<sub>6</sub>

Resonant Raman spectra of a two-dimensional (2D) non-van\nder Waals\n(vdW) material, molybdenum nitride (Mo<sub>5</sub>N<sub>6</sub>),\nare measured across varying thicknesses, ranging from a few to tens\nof nanometers. Fifteen distinct Raman peaks are observed experimentally,\nand their assignments are made using first-principles calculations\nfor the most stable AABB-stacking structure of Mo<sub>5</sub>N<sub>6</sub>. The assignments are further supported by angular-dependent\nRaman measurements for all peaks, except the most intense one at 215\ncm<sup>–1</sup>. Calculations reveal that the 215 cm<sup>–1</sup> peak does not appear for three-dimensional molybdenum nitrides and\nis not a first-order Raman-active mode. We further investigated the\norigin of the 215 cm<sup>–1</sup> peak and assigned it as a\ndefect-induced double-resonance peak. Moreover, thickness-dependent\nRaman measurements reveal that both the 215 and 540 cm<sup>–1</sup> peaksassigned to out-of-plane and in-plane modes, respectivelyblue\nshift as thickness increases, reaching a plateau around 20 nm. This\nthickness-dependent Raman shift over a wide thickness range is nontrivial\ncompared to other common vdW 2D materials and is attributed to the\nmuch stronger stacking interaction between the constituent layers\nin non-vdW materials. This finding highlights Raman spectroscopy as\na valuable tool for characterizing the thickness of 2D non-vdW materials.