Thermal Stabilization of Self-Assembled Monolayers by ALD-Grown Metal Oxide Overlayers

The integration of functional organic materials, such as self-assembled monolayers (SAMs), into robust electronic and optical devices is often hampered by their poor thermal stability in ambient environments. Here, we demonstrate a nanolevel metal oxide sealing strategy to enhance the thermal durability of SAMs by covering them with tens of nanometers of inorganic nanolayers via atomic layer deposition (ALD). Using a model system of phosphonic acid SAMs on ZnO nanowires, we systematically investigated and compared the protective efficacy of three different metal oxides: Al2O3, TiO2, and HfO2. Spectroscopic analysis reveals that a 10 nm thick Al2O3 layer, deposited at a low temperature of 150 °C, provides exceptional protection, preserving over 90% of the SAM’s molecular structure even after harsh annealing at 300 °C for 2 h in air. In contrast, TiO2 and HfO2 coatings offer significantly less protection under the same conditions. Through advanced 4D-scanning transmission electron microscopy (4D-STEM), we correlate this superior performance to the nanostructure of the protective layer. The Al2O3 film is found to be dense and largely amorphous, effectively blocking oxygen diffusion to the underlying SAM. Conversely, the TiO2 and HfO2 films exhibit nanocrystalline domains with more permeable grain boundaries, which act as pathways for oxidative degradation. This study not only provides a practical method for stabilizing organic monolayers but also elucidates that the amorphous nature of the protective nanolayer is a key design principle for achieving a high thermal durability in organic–inorganic hybrid materials.