Abnormal Out-of-Plane Vibrational Raman Mode in Electrochemically Intercalated Multilayer MoS₂

Raman spectroscopy is a powerful technique to probe structural and doping behaviors of two-dimensional (2D) materials. In MoS₂, the always coexisting in-plane (E_2g¹) and out-of-plane (A_1g) vibrational modes are used as reliable fingerprints to distinguish the number of layers, strains, and doping levels. In this work, however, we report an abnormal Raman behavior, i.e., the absence of the A_1g mode in cetyltrimethylammonium bromide (CTAB)-intercalated MoS₂ superlattice. This unusual behavior is quite different from the softening of the A_1g mode induced by surface engineering or electric-field gating. Interestingly, under a strong laser illumination, heating, or mechanical indentation, an A_1g peak gradually appears, accompanied by the migration of intercalated CTA⁺ cations. The abnormal Raman behavior is mainly attributed to the constraint of the out-of-plane vibration due to intercalations and resulting severe electron doping. Our work renews the understanding of Raman spectra of 2D semiconducting materials and sheds light on developing next-generation devices with tunable structures.