Porous silicon: Formation, nanostructuring, development of advanced devices

Porous silicon (PSi) stands as a cornerstone nanomaterial in silicon-based technologies, uniquely combining tunable morphology, CMOS compatibility, and a large specific surface area. This review provides a comprehensive critical analysis of the three principal formation routes—stain etching (SE), metal-assisted chemical etching (MACE), and electrochemical anodic etching—with emphasis on controlled morphological engineering. Experimental results on SE reveal a cyclic growth-and-dissolution behaviour that enables through-wafer etching without external current sources. The defect-deformation (DD) mechanism is presented as a unifying framework that explains spontaneous pore ordering, including quasi-hexagonal arrangements and crystallographically driven transitions between sponge-like and columnar architectures. The application landscape is surveyed across multiple domains: VLSI lateral isolation, one-, two- and three-dimensional photonic crystals, parabolic X-ray lenses, betavoltaic converters, micro fuel cells (μ-FC), and biosensing platforms. Particular attention is paid to composite structures. Platinum-based core-shell catalysts (Pt@Au/PSi) ensure stable μ-FC operation exceeding 600 h. Proton-conducting membranes based on polyvinyl alcohol and phenolsulfonic acid (PVA:PSA) infiltrated into gradient porous silicon (GPSi) matrices achieve record conductivities (0.33 S cm⁻¹ at high humidity), rivalling Nafion®. Carbon nanofibre and graphene-like films deposited inside macropores reduce surface resistance by two to three orders of magnitude and increase specific surface area by a factor up to 3.4. Beyond performance, this review addresses long-term stability and degradation mechanisms—including hydrolysis, structural fatigue, and biofouling—and evaluates passivation strategies (oxidation, hydrosilylation, carbonization, ALD). GPSi-var membranes exhibit anti-reflection coefficients below 0.11%, while nanoporous silicon proves biocompatible, promoting cell proliferation, thereby opening perspectives for biomedical and neuromorphic interfaces. A comparative assessment with emerging materials (graphene, MXenes, MOFs, silicon photonic platforms) highlights PSi’s unique advantages: CMOS integrability, biodegradability, and well-established surface chemistry. Finally, the review critically examines the technological readiness level (TRL) of PSi devices and the remaining challenges for large-scale manufacturing, including substrate inhomogeneities, scale-up of electrochemical etching, and integration with CMOS flows. The results underscore the potential of porous silicon-based composites for next-generation energy, photonic, sensing, and bio-integrated devices, while identifying the key research directions needed to accelerate their commercial deployment.