Transition metal dichalcogenides (TMDCs) are a class of two-dimensional materials with unique electronic, optical, and mechanical properties. However, their practical applications are often limited by environmental degradation, particularly oxidation and humidity-induced damage. Understanding these degradation mechanisms and developing effective encapsulation strategies are critical for improving the long-term stability of TMDCs in real-world conditions.

Oxidation is one of the primary degradation pathways for TMDCs, especially in ambient conditions. Molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), for example, undergo oxidation when exposed to oxygen and moisture, leading to the formation of oxides such as MoO3 and WO3. The oxidation process is accelerated at elevated temperatures and under light exposure. Studies have shown that monolayer TMDCs are more susceptible to oxidation than their bulk counterparts due to their higher surface-to-volume ratio. The edges of TMDC flakes are particularly vulnerable, as they contain unsaturated bonds that react readily with oxygen. Over time, oxidation leads to the degradation of electrical and optical properties, reducing carrier mobility and photoluminescence intensity.

Humidity-induced degradation is another major concern for TMDCs. Water molecules adsorb onto the surface of TMDCs, leading to the formation of hydroxyl groups and structural defects. In the case of MoS2, humidity exposure can cause sulfur vacancies, which act as trap states and degrade electronic performance. Prolonged exposure to humid environments can also lead to delamination and mechanical failure in multilayer TMDCs. The presence of water accelerates electrochemical reactions at the material surface, further exacerbating degradation.

To mitigate these degradation mechanisms, encapsulation strategies have been developed to protect TMDCs from environmental factors. Atomic layer deposition (ALD) is a widely used technique for depositing thin, conformal protective layers on TMDCs. Aluminum oxide (Al2O3) and hafnium oxide (HfO2) are common ALD materials that provide excellent moisture and oxygen barrier properties. Studies have demonstrated that ALD encapsulation can significantly extend the lifetime of TMDCs by preventing direct exposure to ambient conditions. However, the ALD process itself must be carefully optimized to avoid damage to the underlying TMDC layers, as some precursors may react with the material surface.

Hexagonal boron nitride (hBN) is another effective encapsulation material for TMDCs. Unlike ALD-deposited oxides, hBN is a van der Waals material that can be mechanically transferred onto TMDCs without introducing interfacial defects. hBN provides superior chemical inertness and impermeability to gases and liquids, making it an ideal protective layer. Encapsulation with hBN has been shown to preserve the electronic properties of TMDCs for extended periods, even under harsh environmental conditions. Additionally, hBN encapsulation minimizes charge scattering and maintains high carrier mobility, which is critical for optoelectronic applications.

Accelerated aging tests are commonly employed to evaluate the effectiveness of encapsulation strategies. These tests involve exposing encapsulated and unencapsulated TMDCs to controlled levels of humidity, temperature, and light to simulate long-term degradation in a shorter time frame. For example, studies have subjected MoS2 samples to 85% relative humidity at 85°C for hundreds of hours to assess stability. Results indicate that ALD-encapsulated TMDCs exhibit significantly slower degradation rates compared to unprotected samples. Similarly, hBN-encapsulated TMDCs show minimal changes in electrical and optical properties even after prolonged exposure.

Comparisons with other 2D materials reveal differences in environmental stability. Graphene, for instance, is relatively resistant to oxidation due to its strong carbon-carbon bonds but can still suffer from humidity-induced doping and mechanical weakening. In contrast, black phosphorus degrades rapidly in ambient conditions due to its high reactivity with oxygen and water, forming phosphorus oxides. TMDCs occupy an intermediate position, with stability that depends on their composition and thickness. Sulfur-based TMDCs like MoS2 are generally more stable than selenium-based ones like WSe2, as selenium is more prone to oxidation.

The choice of encapsulation method depends on the specific application requirements. ALD is suitable for large-scale integration due to its compatibility with semiconductor manufacturing processes, while hBN is preferred for high-performance devices where minimizing interfacial defects is critical. Combining multiple encapsulation layers, such as hBN with an ALD oxide, has also been explored to enhance protection further.

In summary, the environmental stability of TMDCs is a key challenge that must be addressed for their widespread adoption. Oxidation and humidity-induced degradation significantly impact their performance, but encapsulation strategies such as ALD and hBN provide effective solutions. Accelerated aging tests demonstrate that these methods can substantially improve material longevity. Comparisons with other 2D materials highlight the need for tailored protection approaches based on material properties. Continued research into advanced encapsulation techniques will be essential for unlocking the full potential of TMDCs in next-generation technologies.