Ultrasmall High‐Entropy Materials: Nanoscale Effects, Synthesis, and Mechanistic Insights

Ultrasmall high-entropy nanomaterials (USHENMs, <10 nm) merge multicomponent chemistry with size-dependent effects, forming a distinct class of materials with unprecedented properties. This review presents a focused overview of USHENMs and their key advantages, including high defect densities, strong atomic interactions, abundant active sites, fast reaction kinetics, and unique binding energetics. To rationalize their formation, we classify recent synthesis strategies according to the dominant driving force, namely entropy, kinetics, or enthalpy. Entropy-driven routes, commonly involving high-temperature melting-fusion-solidification processes, exploit configurational entropy and enable single-phase USHENMs through various established approaches. Kinetically controlled methods, guided by the LaMer nucleation-growth model, offer effective ways to controlling size and provide the most accessible means of reliably achieving sub-10 nm particles under low-temperature conditions. In contrast, enthalpy-driven routes are rarely reported, and producing USHENMs is particularly challenging because growth is dominated by sluggish, diffusion-controlled processes. At such extreme dimensions, surface and interface enthalpies determine phase stability, even when (bulk) thermodynamics predicts segregation. Finally, the review concludes by emphasizing future needs in long-term stabilization, in situ characterization, and machine-learning-assisted discovery to accelerate progress in materials design and expand application possibilities.