Porous silicon has been widely studied for its unique optical, electronic, and biomedical properties. However, its high surface reactivity makes it prone to oxidation and degradation when exposed to ambient or harsh environments. Stabilization methods, including oxidation kinetics control and hydrosilylation, are critical for improving its long-term stability in applications such as photonics, sensing, and drug delivery.

Oxidation of porous silicon follows a logarithmic growth law in the initial stages, transitioning to parabolic kinetics as the oxide layer thickens. The process is influenced by factors such as porosity, pore size distribution, and environmental conditions. At room temperature, oxidation proceeds slowly, forming a native oxide layer of approximately 1-2 nm within days. Elevated temperatures or exposure to oxidizing agents accelerates the reaction, leading to thicker oxide layers that can exceed 10 nm under aggressive conditions. The oxidation mechanism involves the diffusion of oxygen through existing oxide layers, followed by reaction with silicon dangling bonds at the interface. The rate-limiting step shifts from surface reaction control to diffusion control as the oxide grows, consistent with the Deal-Grove model for silicon oxidation.

Hydrosilylation is a widely used chemical passivation technique that stabilizes porous silicon by replacing surface Si-H bonds with more robust organic terminations. The reaction involves the formation of Si-C bonds through thermal, photochemical, or catalytic routes. Thermal hydrosilylation, typically performed at temperatures between 120-200°C, allows alkenes or alkynes to react with hydrogen-terminated porous silicon. The process yields alkyl or alkenyl monolayers that resist oxidation and improve surface hydrophobicity. Photochemical hydrosilylation, using ultraviolet light, offers lower-temperature processing but requires careful control of irradiation conditions to prevent surface damage. Catalytic methods employing platinum or rhodium complexes enable efficient functionalization at near-ambient temperatures. The resulting organic monolayers exhibit improved stability, with some studies reporting resistance to oxidation for months under ambient conditions.

Alternative stabilization approaches include halogenation, where chlorine or bromine termination provides intermediate stability before further functionalization. Electrochemical grafting of organic molecules offers another route for surface modification, enabling the attachment of complex functional groups. Atomic layer deposition (ALD) of alumina or other oxides can encapsulate porous silicon, providing a diffusion barrier against environmental species. However, ALD coatings must be carefully optimized to avoid complete pore filling, which would negate the material's high surface area advantages.

Long-term degradation in ambient environments proceeds through multiple pathways. Even with passivation, residual surface defects or incomplete coverage allow slow oxidation to occur. Water vapor plays a significant role, hydrolyzing Si-C or Si-O bonds over time. The degradation rate depends on relative humidity, with accelerated aging observed above 60% RH. Temperature cycling induces mechanical stress due to mismatched thermal expansion coefficients between silicon and its oxide or organic layers, leading to crack formation. In aqueous or biological environments, dissolution of the silicon skeleton becomes significant, particularly for high-porosity structures with large surface-to-volume ratios.

Harsh environments present additional challenges. Elevated temperatures above 300°C accelerate oxide growth and can degrade organic passivation layers. High humidity combined with thermal cycling promotes crack propagation and delamination. Exposure to UV radiation induces photochemical reactions that break surface bonds, while ionizing radiation generates defects that act as oxidation nucleation sites. In biological fluids, protein adsorption and enzymatic activity may alter surface chemistry unpredictably over time.

Quantitative studies of long-term stability report varied results depending on passivation methods. Thermally hydrosilylated samples show less than 10% change in photoluminescence intensity after six months in dry nitrogen, but degrade within weeks under ambient laboratory conditions. Oxide-coated samples exhibit better stability in humid environments, with some retaining 80% of initial optical properties after one year. Electrochemically grafted layers demonstrate intermediate stability, with degradation rates strongly dependent on molecular structure and packing density.

Comparative studies of stabilization methods reveal trade-offs between processing complexity and long-term performance. Thermal hydrosilylation provides good stability with relatively simple processing but may not withstand prolonged exposure to harsh conditions. ALD coatings offer superior barrier properties but require expensive equipment and precise process control. Multilayer approaches combining chemical passivation with thin oxide coatings have shown promise for demanding applications, though cost and scalability remain concerns.

Accelerated aging tests using elevated temperature and humidity provide useful data but may not perfectly replicate real-world degradation mechanisms. Standardized testing protocols are still lacking, making direct comparison between studies difficult. Long-term stability above five years remains poorly characterized for most passivation methods, highlighting the need for extended-duration studies.

Future developments in stabilization may leverage advanced characterization techniques to better understand failure mechanisms at the atomic scale. Machine learning approaches could optimize passivation protocols by predicting molecular configurations with maximum environmental resistance. Hybrid organic-inorganic coatings combining the flexibility of organic chemistry with the durability of ceramics represent another promising direction.

The choice of stabilization method ultimately depends on application requirements. Optoelectronic devices may prioritize optical stability, while biomedical implants require biocompatibility and corrosion resistance. Cost constraints often dictate practical solutions for commercial applications, favoring simpler chemical passivation over complex multilayer coatings. Continued research into degradation mechanisms and novel passivation strategies remains essential for expanding the practical utility of porous silicon in real-world environments.