Transition Metal Dichalcogenide MoS${}_2$: oxygen and fluorine functionalization for selective plasma processing

TL;DR

This work tackles selective chalcogen removal in $MoS_2$ during low-temperature plasma processing by functionalizing the surface with oxygen or fluorine to lower the sputtering threshold energy $E_{sputt}$. AIMD simulations show $E_{sputt}$ dropping from about $3\times 10^{1}$ eV in pristine MoS$_2$ to approximately $14$ eV for MoS$_2$O and $9.5$ eV for MoS$_2$F, with MoS$_2$O retaining lattice order and MoS$_2$F exhibiting substantial disorder and reduced angular sensitivity. A two-step chemically enhanced sputtering mechanism is proposed, involving formation and desorption of SO$_2$ or SF$_4$ intermediates, which broadens the Ar energy window for selective etching; this is complemented by a simple, parameter-free theory linking $E_{sputt}$ to temperature $T$ and incidence angle, validated by MD. The study integrates a 2-body collision model, detailed DFT considerations, spin treatment rationales, and free-energy simulations to provide practical guidelines for damage-free, spatially selective TMD processing, including mask-based patterning and potential cryogenic operating benefits. Overall, the results offer actionable insights for designing plasma processes that achieve selective chalcogen removal while preserving the metal lattice in 2D TMDs.

Abstract

Low-temperature plasma processing is a promising technique for tailoring the properties of transition metal dichalcogenides (TMDs) because it allows for precise control of radical and ion energies and fluxes. For chalcogen substitution, a key challenge is to identify the ion energy window that enables selective chalcogen removal while preserving the metal lattice. Using ab-initio molecular dynamics (AIMD), we demonstrate that oxygen and fluorine functionalization through thermal chemisorption significantly lowers the sputtering energy threshold ($E_{sputt}$) of MoS${}_2$ from $\sim 35$ eV to $\sim 10$ eV. In addition, we find that a non-orthogonal impact angle $\sim 30{}^{\circ}$ reduces the sputtering energy threshold, while cryogenic-range TMD temperatures may increase. To explain the observed trends, a multi-step sputtering mechanism is proposed. Our results show that oxygen/fluorine functionalization, impact angle, and material temperature are key control parameters for selective, damage-free chalcogen removal in TMD processing.

Figures (18)

• Figure 1: Simulation setup schematic. (A): Side view of the full MoS2O equilibrated supercell (4x4) initial state. Ar atom's trajectory is shown by a rainbow line with color representing time from blue to red. (B,C): Representative parts of the MoS2F and MoS2 initial equilibrated states (side view), respectively. Irregularity of fluorine atoms' positions is discussed in the main text. (D): Top view of MoS2O. The purple shade triangle shows the minimal set of hit-points that represents all other points by symmetry. The in-plane far-field approach angle, $\varphi$, shows its origin $\varphi = 0^{\circ}$ and its positive direction $\varphi = 60^{\circ}$. The hit angle remains very close to the initial far-approach angle in cases considered in this study.
• Figure 2: Probability to eject sulfur from pristine and functionalized MoS2 back into the semi-space the impact came from. Functionalization with O and F substantially lowers the sputtering threshold energy, indicating enhanced chalcogen removal efficiency at reduced Ar energies. For each of 3 TMDs, the impact point was optimized to minimize $E_{sputt}$ (details on Figure \ref{['fig:F2S5']}. Materials were equilibrated at 116K.
• Figure 3: (Top): A typical orthogonal Ar collision with MoS2O. Opaque atoms are those that ultimately play a major role in the collision. Gray numbers are timestamps in [fs]. The sputtering mechanism of MoS2O that realizes at the lowest required projectile energy involves an O atom being pushed in the direction of the nearest S atom that already has another O bound to it, allowing them to combine and form SO2, which then escapes. Small red numbers innumerate O atoms to help following them during collisions. A collision at an angle is shown in video V1. (Middle and Bottom): A typical orthogonal Ar collision with MoS2F. Side and top views are both provided to illustrate more complex atom movements during the collisions. Small red numbers innumerate F atoms to help following them during collisions. Both materials were equilibrated at 116 K, both impacts were with 15 eV.
• Figure 4: Angular dependence of the sputtering threshold energy $E_{sputt}$ for MoS2 (dark yellow), MoS2F (light green) and MoS2O (red) for head-on impacts. Thermal fluctuations of target S/F/O atoms in the system are suppressed by formally setting $T = 1$K. Each curve is obtained after optimizing the impact point and the in-plane angle $\varphi$ to minimize $E_{sputt}$, as shown on Figure \ref{['fig:F4S2']} for MoS2O. Simulations at $\theta = 45^{\circ}$ were done for MoS2 and MoS2O and they showed $E_{MoS_2}(45^{\circ}) > 35$ eV and $E_{MoS_2O}(45^{\circ}) > 14$ eV. Error bars reflect the step of the energy grid used to pin down the threshold.
• Figure 5: (A): A schematic showing an Ar impacting an O atom. The Ar velocity is directed exactly at the O equilibrium position, which was shown to be the most damage-susceptible point of MoS2O in Figure \ref{['fig:F2S5']}. However, the impact is not head-on due to a thermal fluctuation of magnitude $\sigma(T)$ of the O atom. (B): Red, blue and yellow: Our predictions for $E_{\perp, sputt}(T)$ of MoS2O for inherent $\theta$-spreads of the incoming Ar of $0^{\circ}$, $7^{\circ}$, and $10^{\circ}$ respectively. Green: the $T$-independent $E_{sputt}$ of MoS2F. No fitting parameters were used in the theory, except for data interpolation of $E_{Ar-O}(\theta)$ on Figure \ref{['fig:F4']}. Curves for no inherent spread ($\theta_{inherent} = 0$) show a good match to MD simulation results (Figure \ref{['fig:F5S0']}).