Structural Relaxation Governs the Stoichiometry Dependence of Core-level Shifts in MoS x

X-ray photoelectron spectroscopy (XPS) is widely used to probe composition and chemical environment in transition-metal chalcogenides, but composition-dependent core-level shifts are often interpreted without explicit consideration of structural relaxation. Here, combining first-principles calculations and experiment, we show that the Mo 3 d –S 2 p binding-energy difference in MoS x remains approximately linear with sulfur stoichiometry, while its slope is strongly controlled by the structural boundary conditions. Fully relaxed bulk, bilayer, and monolayer MoS x yield a slope close to 1.2 eV –1, whereas constrained structures approach 3.0 eV –1 . Experimentally, bulk sputtered MoS x exhibits a slope close to 1.2 eV –1, while a transferred monolayer MoS 2 sample shows a comparable trend within uncertainty, consistent with the fully relaxed limit. By contrast, monolayer and 6-layer MoS 2 films grown directly by ALD on SiO 2 exhibit significantly steeper slopes, in the range 2.6–2.9 eV –1, close to the constrained regime. These results show that the slope of the Mo 3 d –S 2 p binding-energy difference is not determined by stoichiometry alone, but also provides a simple XPS descriptor of structural accommodation and interfacial constraint in low-dimensional materials.