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

This first-principles study reveals that while pristine chalcogen vacancies in MoS$_2$ and MoSe$_2$ do not induce magnetism, the adsorption of azanide (NH$_2$) and ammonia (NH$_3$) on these defective monolayers generates localized magnetic moments, with MoSe$_2$ exhibiting a notable 2.0 $\mu_B$ moment upon NH$_3$ dissociation, thereby demonstrating a viable strategy for tuning magnetism in 2D materials for spintronic applications.

The Gist

Imagine a sheet of material so thin it's only one atom thick, like a microscopic sheet of paper made of molybdenum and sulfur (or selenium). Scientists call these "2D materials." Usually, these sheets are like quiet, calm lakes—they don't have any magnetic properties. They are non-magnetic, meaning they wouldn't stick to a fridge magnet.

However, this paper explores what happens when you poke a tiny hole in that sheet (a "defect") and then drop a tiny chemical "visitor" onto that hole. The visitors in this story are Ammonia (the stuff in some cleaning products) and Azanide (a piece of ammonia missing a hydrogen atom).

Here is the story of their discovery, broken down into simple concepts:

1. The Empty Hole vs. The Visited Hole

The researchers first tried just poking a hole in the sheet.

• The Result: Nothing happened. The sheet remained calm and non-magnetic. It was like poking a hole in a piece of paper; the paper didn't suddenly start singing or glowing.
• The Twist: When they brought in the ammonia visitors and let them sit in or near those holes, the sheet suddenly woke up. It started generating a tiny magnetic field. It was as if the hole was a silent stage, and the ammonia visitor was the actor that made the stage come alive with "spin" (a quantum property that creates magnetism).

2. The "Magic" of Molybdenum vs. The "Silence" of Tungsten

The team tested two types of sheets: one made with Molybdenum (Mo) and one made with Tungsten (W).

• Molybdenum Sheets: When ammonia visited the holes in these sheets, they became magnetic. In one specific case with Molybdenum and Selenium, the ammonia broke apart (like a Lego set snapping into two pieces) right on the surface. This created a surprisingly strong magnetic pulse, about 2.0 units of magnetism.
• Tungsten Sheets: The researchers tried the exact same experiment on Tungsten sheets. They poked holes, added the same ammonia visitors, and waited. Nothing happened. The Tungsten sheets remained completely non-magnetic.
• The Lesson: It's not just about having a hole or a visitor; it's about who is hosting the party. The Molybdenum atoms are like a sensitive microphone that picks up the visitor's presence and amplifies it into magnetism. The Tungsten atoms are like a soundproof wall; they ignore the visitor completely.

3. The "Same Side" vs. "Opposite Side" Game

The researchers played a game of positioning. They asked: "What if we put two ammonia molecules on the same side of the sheet? What if we put one on the top and one on the bottom?"

• For Molybdenum Sulfide (MoS2): It didn't matter much. Whether the visitors were on the same side or opposite sides, the sheet still got magnetic, though the strength varied slightly.
• For Molybdenum Selenide (MoSe2): The position mattered a lot!
  • If the ammonia broke apart and both pieces stayed on the same side, the sheet became strongly magnetic (the 2.0 units mentioned earlier).
  • If the pieces were on opposite sides (one on top, one on bottom), the magnetism vanished. The sheet went back to being quiet.
  • Analogy: Think of it like two people pushing a swing. If they push from the same side at the same time, the swing goes high (strong magnetism). If one pushes from the front and one from the back, they cancel each other out, and the swing stops (no magnetism).

4. The "Smaller Visitor" (Azanide)

They also tested a smaller visitor, Azanide (NH2), which is just ammonia without one hydrogen atom.

• This smaller visitor also made the Molybdenum sheets magnetic.
• However, unlike the full ammonia molecule, making more holes (two holes instead of one) didn't make the magnetism get much stronger. It seemed like the Azanide visitor only cared about the immediate neighborhood of the hole it was sitting in, rather than the whole sheet.

The Bottom Line

This paper is a report on a specific experiment: If you take a Molybdenum-based sheet, poke a hole in it, and let ammonia (or its fragments) sit there, you can turn that non-magnetic sheet into a magnetic one.

• Key Finding 1: Holes alone don't create magnetism; you need the ammonia visitor.
• Key Finding 2: Molybdenum sheets react; Tungsten sheets do not.
• Key Finding 3: The arrangement of the ammonia molecules (especially if they break apart) changes how strong the magnetism is.

The authors suggest this is a way to "tune" or control magnetism in these tiny materials, but they stop there. They describe the "how" and the "what" of the experiment, showing that specific combinations of defects and molecules can switch magnetism on and off in Molybdenum sheets.

Technical Summary

Technical Summary: Magnetism Induced by Azanide and Ammonia Adsorption in Defective Molybdenum Disulfide and Diselenide

Problem Statement
Two-dimensional transition metal dichalcogenides (TMDs) possess tunable electronic and structural properties, yet their intrinsic spin-related behaviors are often limited. While point defects (such as mono- and divacancies) and molecular adsorption are known to modify TMD properties, the specific mechanisms by which small adsorbates like azanide ($NH_2$) and ammonia ($NH_3$) interact with defective chalcogen vacancies to induce magnetism remain insufficiently explored. Previous literature indicates that pristine chalcogen vacancies in Mo-based dichalcogenides typically do not generate spin density variations, whereas metal vacancies can. Furthermore, while the dissociation of molecules like $H_2O$ at vacancy sites has been linked to spin modifications, the combined effects of specific vacancy configurations (mono- vs. divacancies) and the adsorption of nitrogen-based species on the local magnetic environment of MoS$_2$ and MoSe$_2$ require systematic investigation.

Methodology
The study employs first-principles spin-polarized density functional theory (DFT) simulations using the SIESTA code. The exchange-correlation effects are modeled using the PBE functional with a DZP (double-$\zeta$ polarization) basis set. The computational setup includes:

• Structural Models: Pristine, mono-vacancy ($V_X$), and di-vacancy ($2V_X$) configurations for MoS$_2$ and MoSe$_2$ monolayers, constructed within $3 \times 3 \times 1$ supercells. Di-vacancies are modeled in two arrangements: both vacancies on the same side of the monolayer and alternating sides.
• Adsorption Scenarios: The study investigates the adsorption of single and double molecules of $NH_3$ and $NH_2$, as well as the dissociation of $NH_3$ into $NH_2$ and H fragments on the same or opposite sides of the surface.
• Comparative Analysis: Equivalent tests are performed on W-based dichalcogenides ($WS_2$ and $WSe_2$) to isolate the role of the transition metal.
• Parameters: Calculations utilize a real-space mesh cutoff of 400 Ry, a vacuum region of 25 Å, and Γ-centered Monkhorst-Pack grids appropriate for the unit cell and supercell sizes. Structural relaxation is performed until residual forces are below 0.05 eV/Å.

Key Results

1. Vacancy-Induced Magnetism: Consistent with prior reports, the creation of S or Se vacancies alone in MoS$_2$ and MoSe$_2$ does not generate a net magnetic moment.
2. Adsorption-Induced Magnetism in Mo-Based TMDs:
   • $NH_3$ Adsorption: The adsorption of $NH_3$ (and $NH_2$) onto vacancy sites induces localized magnetic moments on Mo atoms near the defect.
   • Divacancy Effects: In MoS$_2$, a divacancy configuration on the same side of the monolayer leads to a pronounced enhancement of the local magnetic moment (approx. 21.3% increase compared to single vacancy cases). In MoSe$_2$, this configuration yields a significantly larger increase (approx. 200%).
   • $NH_2$ Adsorption: Similar to $NH_3$, $NH_2$ adsorption induces magnetic moments. However, unlike the $NH_3$ case, the magnetic response in MoS$_2$ does not scale significantly with divacancy density, suggesting a more localized interaction.
   • Dissociation of $NH_3$: A notable finding occurs in MoSe$_2$ where $NH_3$ dissociates into $NH_2$ and H fragments on the same side of the surface, producing a net magnetic moment of 2.0 $\mu_B$. In contrast, an alternating arrangement (fragments on opposite sides) results in no net magnetization. For MoS$_2$, dissociation induces a smaller net moment (0.444 $\mu_B$) regardless of spatial arrangement.
3. W-Based TMDs: Under equivalent conditions (vacancies and $NH_3$ adsorption), $WS_2$ and $WSe_2$ exhibit no magnetic response. Non-zero spin density in W-based systems was only observed when W atoms were removed (metal vacancies), not chalcogen vacancies.

Significance and Claims
The authors claim that molecular adsorption, when combined with defect engineering, serves as a practical approach to tune magnetism in 2D Mo-based dichalcogenides. The study demonstrates that while chalcogen vacancies alone are non-magnetic, the introduction of specific adsorbates ($NH_2$, $NH_3$) and their dissociation products can create localized magnetic moments. The magnitude of this effect is highly sensitive to:

• The type of transition metal (Mo vs. W).
• The specific chalcogen atom (S vs. Se).
• The configuration of the vacancy (mono- vs. divacancy, same-side vs. alternating).
• The spatial arrangement of adsorbed or dissociated species.

The paper concludes that these findings provide insights into adsorbate-driven spin distortions in Mo-based dichalcogenides, contributing to the understanding of defect-engineered 2D materials with tunable spin properties, which holds potential relevance for spintronic and sensing applications. The work explicitly notes that W-based analogs do not exhibit this behavior under the same conditions, highlighting the critical role of the transition metal identity in determining magnetic outcomes.