Self-limiting growth mechanisms are critical for achieving precise monolayer control in the synthesis of two-dimensional (2D) materials. These mechanisms rely on thermodynamic or kinetic constraints that naturally halt further deposition after a single atomic layer forms, enabling scalable and reproducible production of uniform ultrathin films. Key strategies include surface passivation, reactant depletion, and steric hindrance, each exploiting distinct physical or chemical principles to restrict growth beyond one layer.

One of the most studied self-limiting processes involves surface passivation, where the reactive sites of a substrate or the growing material itself are chemically saturated, preventing additional layer formation. For example, in the chemical vapor deposition (CVD) of tungsten diselenide (WSe₂), the introduction of selenium (Se) precursors passivates the tungsten (W) adatoms after monolayer coverage. The WSe₂ surface becomes chemically inert due to the formation of stable W-Se bonds, which exhibit lower reactivity compared to the initial nucleation sites on the substrate. This passivation effect is further enhanced by the high binding energy of the monolayer, typically exceeding 5 eV per formula unit, making subsequent layer growth energetically unfavorable under optimized conditions.

Reactant depletion is another self-limiting mechanism observed in 2D material synthesis. In this case, the precursor concentration in the growth environment is carefully controlled to ensure that only enough material is supplied for monolayer formation. For instance, during the CVD growth of molybdenum disulfide (MoS₂), precise dosing of molybdenum trioxide (MoO₃) and sulfur (S) precursors ensures that the Mo species are entirely consumed after forming a single layer. The reaction kinetics are tuned such that the precursor flux diminishes once the monolayer covers the substrate, leaving insufficient reactants for additional layers. This approach requires strict control over gas-phase stoichiometry and residence time to avoid parasitic nucleation.

Steric hindrance plays a role in self-limiting growth when the physical size of precursor molecules or intermediates prevents multilayer formation. In atomic layer deposition (ALD)-like processes, bulky ligands or intermediate species block adsorption sites after the first layer is complete. For example, in the growth of hexagonal boron nitride (hBN), ammonia (NH₃) and borazine (B₃N₃H₆) precursors can form a tightly bonded monolayer that sterically inhibits further precursor adsorption. The van der Waals gap between the monolayer and any potential overlying material creates an energy barrier, effectively suppressing secondary nucleation.

Theoretical limits of thickness control in self-limiting growth are dictated by thermodynamics and kinetics. For transition metal dichalcogenides (TMDCs) like WSe₂, the free energy difference between monolayer and bilayer growth can exceed 50 meV per atom under equilibrium conditions, favoring monolayer stability. Kinetic barriers, such as the diffusion length of adatoms, further constrain multilayer formation. For instance, the mean free path of W adatoms on SiO₂ substrates is typically less than 100 nm at standard CVD temperatures, limiting island coalescence to a single layer before growth terminates.

Experimental studies on WSe₂ demonstrate the effectiveness of self-limiting mechanisms. When grown on sapphire substrates using H₂Se and WO₃ precursors, the WSe₂ monolayer exhibits near-perfect coverage with minimal bilayer defects. Angle-resolved photoemission spectroscopy (ARPES) confirms the absence of electronic states associated with second-layer formation, indicating a self-terminating process. Similarly, in situ X-ray photoelectron spectroscopy (XPS) reveals that the Se/W ratio stabilizes at stoichiometric values after monolayer completion, with no excess Se available for further reactions.

The self-limiting growth of 2D materials is not without challenges. Substrate interactions, precursor purity, and temperature gradients can disrupt the uniformity of the termination process. For example, step edges or defects on the substrate may locally enhance precursor adsorption, leading to sporadic multilayer nucleation. However, advances in substrate engineering, such as the use of epitaxial graphene or hBN buffers, have mitigated these issues by providing atomically smooth surfaces with minimal dangling bonds.

In summary, self-limiting growth mechanisms enable precise monolayer synthesis of 2D materials through surface passivation, reactant depletion, and steric hindrance. These processes are governed by thermodynamic equilibria and kinetic constraints, with materials like WSe₂ serving as exemplary cases. Theoretical models and experimental data confirm that monolayer control is achievable within well-defined processing windows, paving the way for scalable production of ultrathin semiconductors for advanced electronic and optoelectronic applications. Future refinements in precursor chemistry and reactor design will further enhance the reproducibility and scalability of these techniques.