Magnetism Induced by Azanide and Ammonia Adsorption in Defective Molybdenum Disulfide and Diselenide: A First-Principles Study

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted considerable attention due to their tunable structural, electronic, and spin-related properties, particularly in the presence of point defects and molecular adsorbates. Motivated by these aspects, we have investigated using first-principles methods the magnetic properties induced by azanide (NH$_2$) and ammonia (NH$_3$) adsorption on defective monolayers of Molybdenum Disulfide (MoS$_2$) and Diselenide(MoSe$_2$). Spin-polarized density functional theory (DFT) was employed to investigate the impact of mono- and di-vacancies on the local spin environment and the role of molecular adsorption in modifying magnetic behavior. The results show that pristine chalcogen vacancies do not generate magnetism, whereas the adsorption of NH$_2$ and NH$_3$ creates localized magnetic moments in Mo-based dichalcogenides. A notable case occurs for MoSe$_2$, where NH$_3$ dissociation into NH$_2$ and H fragments on the same side of the surface produces a net magnetic moment of 2.0 $μ_B$. Tests performed on W-based dichalcogenides under equivalent conditions showed no magnetic response, and are reported here only for comparison. These findings demonstrate that molecular adsorption combined with defect engineering can be a practical approach to tune magnetism in 2D materials, with potential relevance for spintronic and sensing applications.

💡 Research Summary

This paper investigates how point defects and the adsorption of small nitrogen‑containing molecules can induce magnetism in two‑dimensional transition‑metal dichalcogenides (TMDs), focusing on molybdenum disulfide (MoS₂) and molybdenum diselenide (MoSe₂). Using spin‑polarized density‑functional theory (DFT) as implemented in the SIESTA code, the authors model monolayers with single chalcogen vacancies (V_S, V_Se) and double vacancies (2V_S, 2V_Se) arranged either on the same side of the sheet or on opposite sides. Structural relaxations employ the PBE‑GGA functional, a double‑ζ polarized (DZP) basis set, a 400 Ry real‑space mesh, and k‑point samplings of 8 × 8 × 1 for the primitive cell and 4 × 4 × 1 for the 3 × 3 supercell. Convergence criteria are set to 10⁻⁴ for the density matrix and forces below 0.05 eV/Å.

Key findings are as follows:

1. Defect‑only systems are non‑magnetic. Both MoS₂ and MoSe₂ with isolated chalcogen vacancies show negligible spin density, confirming earlier reports that only metal‑site vacancies generate localized moments in Mo‑based TMDs.

2. NH₃ adsorption creates localized magnetic moments. A single NH₃ molecule binds near a vacancy at ~2.38 Å (Mo–S) or ~2.36 Å (Mo–Se) from the nearest Mo atom. In MoS₂ this adsorption increases the total magnetic moment by roughly 21 % (≈0.21 μ_B), whereas in MoSe₂ the increase is about 200 % (≈0.44 μ_B). When two vacancies are placed on the same side, the enhancement is more pronounced; opposite‑side double vacancies behave similarly to the single‑vacancy case.

3. NH₂ adsorption induces stronger local moments in MoS₂. The azanide radical, being a stronger electron donor, produces a sizable local moment of up to 0.91 e⁻ (≈0.91 μ_B) on a specific Mo atom in MoS₂. The effect does not scale with vacancy density, and in MoSe₂ the induced moments remain below 0.5 e⁻, indicating a weaker interaction.

4. Dissociation of NH₃ (NH₂ + H) is highly geometry‑dependent. For MoS₂, both configurations—NH₂ and H on the same side or on opposite sides—yield a modest net moment of ~0.44 μ_B. In contrast, MoSe₂ exhibits a dramatic 2.0 μ_B only when NH₂ and H reside on the same side of the sheet; the opposite‑side arrangement produces no net magnetization. This suggests that the simultaneous presence of both fragments on one surface side amplifies spin polarization, likely through enhanced charge transfer and spin‑orbit coupling associated with the heavier Se atoms.

5. W‑based dichalcogenides (WS₂, WSe₂) remain non‑magnetic. Under identical defect and adsorption conditions, no spin polarization is observed, highlighting the crucial role of the transition‑metal d‑orbital character. Tungsten’s broader d‑band and stronger metallic character suppress the localization needed for magnetic moment formation.

The authors interpret these results as evidence that (i) chalcogen vacancies alone are insufficient to generate magnetism, (ii) adsorption of electron‑donating molecules can polarize the local electronic structure of Mo‑based TMDs, and (iii) the spatial arrangement of dissociated fragments critically controls the magnitude of the induced moment, especially in the heavier MoSe₂ system. The pronounced response of MoSe₂ compared with MoS₂ is attributed to stronger spin‑orbit coupling and a more polarizable electronic environment around selenium.

From an application perspective, the study demonstrates a viable route to engineer switchable magnetic states in 2D materials via defect creation and controlled molecular adsorption. The sensitivity of MoSe₂ to NH₃ dissociation suggests potential for high‑sensitivity gas sensing, where magnetic signals could be read out electrically. Moreover, external stimuli (electric fields, light) could be used to trigger NH₃ dissociation on demand, enabling reconfigurable spintronic devices such as magnetic switches or memory elements based on a single atomic layer.

Overall, the paper provides a comprehensive first‑principles analysis of how point defects and small adsorbates synergistically induce magnetism in Mo‑based TMDs, while also clarifying why analogous W‑based systems do not exhibit the same behavior. The insights gained pave the way for defect‑engineered spintronic and sensing technologies that exploit the tunable magnetic properties of two‑dimensional semiconductors.