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.

中文翻译：

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二维非范德华 Mo5N6 中的非平凡拉曼特性


二维 （2D） 非范德华 （vdW） 材料氮化钼 （Mo₅N₆） 的共振拉曼光谱可在从几纳米到几十纳米的不同厚度上进行测量。通过实验观察到 15 个不同的拉曼峰，并使用第一性原理计算对 Mo₅N₆ 最稳定的 AABB 堆叠结构进行分配。所有峰的角度相关拉曼测量进一步支持这些分配，但 215 cm^–1 处强度最高的峰除外。计算表明，215 cm^–1 峰不会出现在三维氮化钼中，也不是一阶拉曼活性模式。我们进一步研究了 215 ^cm-1 峰的来源，并将其归类为缺陷诱导的双共振峰。此外，与厚度相关的拉曼测量表明，分别分配给面外和面内模式的 215 和 540 cm^–1 峰都随着厚度的增加而蓝移，达到 20 nm 左右的平台期。与其他常见的 vdW 2D 材料相比，这种在较宽厚度范围内的厚度依赖性拉曼偏移非同寻常，这归因于非 vdW 材料中组成层之间的堆叠相互作用要强得多。这一发现突出了拉曼光谱是表征 2D 非 vdW 材料厚度的宝贵工具。