Intentional defect introduction during the synthesis of two-dimensional (2D) materials is a critical strategy for tailoring their electronic, optical, and catalytic properties. Defects such as vacancies, dopants, and grain boundaries can be deliberately incorporated into 2D materials like graphene, transition metal dichalcogenides (TMDCs), and hexagonal boron nitride (hBN) to enhance their functionality for specific applications. These defects are introduced during the growth phase using techniques such as plasma treatment, precursor doping, or controlled synthesis conditions, rather than through post-growth modifications.

One common method for introducing defects is plasma treatment during chemical vapor deposition (CVD), a widely used technique for synthesizing 2D materials. Plasma exposure can create vacancies or functionalize the material by breaking bonds and introducing heteroatoms. For example, nitrogen or oxygen plasma treatment of graphene introduces nitrogen or oxygen dopants, altering its electronic structure and enhancing its catalytic activity. Similarly, sulfur vacancies in MoS2 can be generated by controlling the sulfur precursor flow or using argon plasma during growth. These vacancies serve as active sites for catalytic reactions such as hydrogen evolution.

Precursor doping is another intentional defect incorporation strategy. By introducing dopant atoms into the precursor mixture during synthesis, foreign atoms can be substitutionally or interstitially integrated into the 2D lattice. For instance, doping graphene with boron or nitrogen during CVD growth modifies its charge carrier concentration and work function. In TMDCs like WS2, transition metal dopants such as iron or cobalt can replace tungsten atoms, introducing magnetic properties or improving electrocatalytic performance. The concentration of dopants can be precisely controlled by adjusting the precursor ratios, enabling fine-tuning of material properties.

Controlled synthesis conditions, such as temperature, pressure, and gas flow rates, also play a crucial role in defect engineering. Lower growth temperatures or reduced precursor availability can lead to higher defect densities, including chalcogen vacancies in TMDCs. For example, selenium-deficient MoSe2 exhibits enhanced catalytic activity due to the presence of selenium vacancies that serve as reaction centers. Similarly, carbon vacancies in graphene can be introduced by optimizing the methane-to-hydrogen ratio during CVD growth, influencing its mechanical and electronic characteristics.

Defective 2D materials find significant applications in catalysis, where vacancies and dopants act as active sites to lower reaction barriers. Nitrogen-doped graphene exhibits improved oxygen reduction reaction (ORR) activity compared to pristine graphene, making it suitable for fuel cells. Sulfur vacancies in MoS2 enhance its performance as a catalyst for hydrogen evolution reactions (HER) by facilitating proton adsorption and hydrogen desorption. Similarly, defect-rich hBN has been explored for oxidative dehydrogenation reactions due to the presence of boron vacancies that activate reactant molecules.

Beyond catalysis, defect engineering during synthesis also impacts electronic and optoelectronic applications. Dopants in TMDCs can induce n-type or p-type conductivity, enabling the fabrication of complementary electronic devices. Vacancies in graphene quantum dots influence their photoluminescence properties, making them useful for sensing and light-emitting applications. The intentional introduction of defects thus provides a versatile approach to customize 2D materials for diverse technological needs.

In summary, intentional defect introduction during 2D material synthesis is a powerful tool for property modulation. Techniques such as plasma treatment, precursor doping, and controlled growth conditions enable precise defect incorporation, enhancing material performance in catalysis, electronics, and beyond. By understanding and optimizing these methods, researchers can unlock new functionalities in 2D materials for next-generation applications.