Environmental stability is a critical concern for van der Waals heterostructures and 2D material assemblies, as their performance and longevity can be severely compromised by exposure to ambient conditions. Oxidation and humidity are among the most significant degradation mechanisms, leading to changes in electronic properties, mechanical integrity, and optical characteristics. Encapsulation strategies and protective coatings have emerged as essential solutions to mitigate these effects, while accelerated aging tests provide insights into long-term reliability.

Oxidation is a primary degradation pathway for many 2D materials, particularly those with reactive surfaces such as black phosphorus and transition metal dichalcogenides (TMDCs). For instance, monolayer MoS2 undergoes oxidation when exposed to air over extended periods, leading to the formation of MoO3 and SOx species. The oxidation rate depends on environmental factors such as temperature, oxygen concentration, and light exposure. Studies have shown that oxidation proceeds more rapidly at elevated temperatures, with measurable degradation occurring within hours at 200°C in an oxygen-rich environment.

Humidity-induced degradation is another major challenge, especially for hydrophilic materials or those with dangling bonds at defect sites. Water molecules adsorb onto surfaces and intercalate between layers, causing swelling, strain, and eventual delamination. For example, graphene exhibits increased resistivity and reduced carrier mobility when exposed to high humidity due to water adsorption at grain boundaries and defects. In TMDCs, humidity can lead to the formation of sulfuric and molybdic acids, accelerating material breakdown.

Encapsulation strategies are widely employed to shield 2D materials from environmental factors. Hexagonal boron nitride (hBN) is a preferred encapsulation material due to its atomic flatness, chemical inertness, and impermeability to gases. Studies demonstrate that hBN-capped graphene maintains its electronic properties for months in ambient conditions, whereas unprotected graphene shows significant degradation within days. Similarly, TMDCs encapsulated with hBN exhibit prolonged photoluminescence intensity and reduced defect formation.

Polymer-based encapsulation is another approach, with materials such as poly(methyl methacrylate) (PMMA) and polydimethylsiloxane (PDMS) offering flexible protection. However, polymers are less effective than hBN in preventing gas permeation and may introduce strain or doping effects. For instance, PMMA encapsulation reduces but does not eliminate oxidation in black phosphorus, with degradation still observable after several weeks.

Atomic layer deposition (ALD) of oxides like Al2O3 and HfO2 provides a conformal and dense barrier against environmental exposure. Al2O3 coatings as thin as 10 nm have been shown to significantly delay oxidation in MoS2, extending device lifetimes by orders of magnitude. However, ALD processes often require high temperatures or plasma treatments, which may damage sensitive 2D materials. Low-temperature ALD techniques have been developed to address this limitation.

Accelerated aging tests are essential for evaluating long-term stability. These tests subject materials to elevated temperatures, humidity, or UV radiation to simulate years of environmental exposure in a condensed timeframe. For example, a common protocol involves storing devices at 85°C and 85% relative humidity (85/85 test) for hundreds of hours while monitoring performance metrics. Data from such tests reveal that unencapsulated TMDC-based transistors experience a 50% reduction in mobility after 500 hours under 85/85 conditions, whereas hBN-encapsulated devices show negligible degradation.

Protective coatings must also consider mechanical robustness, especially for flexible electronics. Thin metal layers, such as Au or Ni, provide additional shielding but may crack under strain. Hybrid approaches combining hBN with metal or polymer layers offer improved mechanical flexibility while maintaining environmental resistance. For instance, a bilayer of hBN and PDMS has been demonstrated to sustain repeated bending cycles without compromising protection.

Thermal stability is another critical factor, particularly for high-power or high-temperature applications. Some 2D materials, like graphene and hBN, are stable up to 400°C in inert atmospheres but degrade rapidly in air at much lower temperatures. Encapsulation with refractory materials like SiNx can extend operational limits, enabling device functionality at temperatures exceeding 300°C in oxidizing environments.

In addition to external encapsulation, surface passivation techniques can enhance intrinsic stability. Chemical functionalization with hydrophobic molecules, such as fluorinated alkyl chains, reduces water adsorption and oxidation rates. For example, fluorinated MoS2 exhibits a tenfold increase in oxidation resistance compared to untreated samples. However, passivation may alter electronic properties, requiring careful optimization.

Long-term reliability also depends on interfacial quality within heterostructures. Poorly bonded interfaces can trap moisture or oxygen, leading to localized degradation. Advanced transfer techniques that minimize interfacial contaminants are crucial for improving stability. Dry transfer methods and in-situ assembly under vacuum have proven effective in reducing defect densities and enhancing environmental resilience.

Emerging encapsulation materials, such as graphene oxide laminates and metal-organic frameworks (MOFs), show promise for tailored barrier properties. Graphene oxide films provide selective permeability, blocking oxygen and water while allowing other gases to pass. MOFs offer tunable pore sizes for specific environmental exclusion, though their integration with 2D materials remains an active research area.

In conclusion, environmental stability in van der Waals heterostructures demands a multifaceted approach combining encapsulation, passivation, and rigorous testing. While hBN and ALD coatings currently lead in performance, ongoing developments in hybrid barriers and novel materials aim to further extend device lifetimes under harsh conditions. Accelerated aging studies provide critical data for predicting real-world behavior, ensuring that 2D material technologies meet reliability standards for industrial applications.