Engineering Metastability in Atomic Layer Deposition: Polymorph and Valence Control.

Metastable phases characterized by their higher-energy states offer promising functionalities for electronic, catalytic, and energy applications. However, their synthesis is often hindered by high formation energy barriers and thermodynamic constraints. Atomic layer deposition (ALD) has attracted significant interest for a wide range of applications, including semiconductors, advanced electronics, and energy-related applications, owing to its exceptional features, including low-temperature processing, precise atomic-scale control, and excellent conformality. Despite these advantages, the inherently low thermal budget of ALD poses significant challenges for the synthesis of metastable phases. This review presents a comprehensive overview of the recent advances in the engineering of metastability via ALD. This review categorizes the manifestations of metastability in ALD into two main directions: polymorphic transformations and valence state control. For polymorphs, strategies, such as temperature modulation, substrate-induced lattice matching, grain-size refinement, doping, and solid-solution formation, enable selective phase stabilization. Approaches for valence control include temperature modulation, the design and selection of the precursor/reactant, and post-deposition treatments. By linking reaction mechanisms with material phases, this review offers insights into the stabilization of metastable phases and practical design principles for achieving them. These insights will pave the way for new functional materials that surpass conventional thermodynamic limitations and advance next-generation devices and technologies.