Atomic layer deposition (ALD) is a highly controlled thin-film growth technique that enables precise coating of nanostructured and porous materials with complex geometries. The self-limiting, sequential surface reactions characteristic of ALD make it uniquely suited for conformal deposition on high-aspect-ratio structures, including aerogels, nanowires, and other intricate frameworks. However, achieving uniform coatings in such materials presents challenges related to precursor diffusion, penetration depth, and reaction site accessibility.

One of the primary advantages of ALD is its ability to deposit films with atomic-level precision, even on highly tortuous surfaces. When coating porous materials like aerogels, the challenge lies in ensuring precursor molecules penetrate deep into the structure without premature reactions or pore clogging. The low-density, high-surface-area nature of aerogels demands careful optimization of deposition parameters to avoid incomplete coverage or excessive precursor consumption. Studies have shown that for silica aerogels with pore sizes below 50 nm, extended precursor exposure times and reduced pressure conditions improve penetration, ensuring uniform film growth throughout the matrix.

Nanowires, with their high aspect ratios and dense packing, present another set of challenges. Conformal coating of individual nanowires within a forest-like array requires precise control over precursor flux and purging cycles. The shadowing effect, where neighboring nanowires block precursor access, can lead to non-uniform deposition. To mitigate this, techniques such as multiple dosing cycles or rotational substrate holders have been employed. Research indicates that for nanowires with aspect ratios exceeding 100:1, increasing the number of ALD cycles while maintaining moderate temperatures enhances sidewall coverage without excessive top-layer thickening.

Penetration depth is a critical factor when depositing on porous or nanostructured materials. In mesoporous structures with pore diameters between 2-50 nm, precursor diffusion limitations often result in thicker coatings near pore entrances and thinner films deeper within. To address this, strategies such as pulsed exposure or alternating precursor sequences have been developed. For example, in the ALD of Al2O3 on porous anodic alumina, extending the precursor pulse duration from 0.1 to 1.0 seconds significantly improves film uniformity at depths beyond 10 micrometers.

Multiple dosing cycles, where precursors are introduced in a split-dose fashion, have proven effective for enhancing conformality in high-aspect-ratio structures. This approach allows for more complete surface reactions before saturation occurs, reducing the risk of incomplete penetration. Experimental data on TiO2 ALD in nanoporous silicon demonstrates that dividing the precursor dose into two shorter pulses increases film thickness uniformity by up to 30% compared to single-pulse dosing.

Temperature plays a crucial role in ALD on nanostructured materials. Lower temperatures slow surface reactions, allowing precursors to diffuse further into porous networks before reacting. However, excessively low temperatures may lead to incomplete ligand exchange or condensation, resulting in defective films. For instance, in ZnO ALD on carbon nanofoams, optimal uniformity is achieved at 150°C, balancing precursor diffusion and reaction kinetics.

The choice of precursors also impacts conformality. Smaller, more volatile precursors such as trimethylaluminum (TMA) for Al2O3 ALD exhibit better penetration into narrow pores compared to bulkier alternatives. Water, as a common oxygen source, diffuses effectively but may condense in nanopores if purging is insufficient. Non-aqueous oxidants like ozone or plasma-assisted processes can mitigate this issue while maintaining high film quality.

Plasma-enhanced ALD (PEALD) offers an alternative route for coating challenging geometries. The use of reactive plasma species enhances precursor dissociation, enabling deposition at lower temperatures and improving step coverage. Studies on PEALD of TiN on silicon nanowires show near-ideal conformality at aspect ratios up to 500:1, attributed to the isotropic nature of plasma-generated radicals.

In conclusion, ALD provides a powerful tool for coating nanostructured and porous materials, but achieving uniform films requires careful optimization of deposition parameters. Techniques such as extended precursor exposure, multiple dosing cycles, and plasma assistance address challenges related to conformality and penetration depth. By tailoring these strategies to specific material architectures, ALD enables precise nanoscale coatings even in the most complex geometries.