Are ceramic nanofilms a soft matter?

Conventional ceramic films (metal oxides, metal carbides and metal nitrides) are commonly known as hard materials. However, much evidence appears to show that the softness and hardness of ceramic films are dependent on the bonding interaction on the atomic and molecular scale as well as on the structures on the microscopic scale. When ceramic films become extremely thin, i.e., ceramic nanofilms, they are in fact a soft matter. In this paper, the authors discuss the conceivable factors that affect the softness and hardness of a material on different length scales, and review both their recent work and others' that present the evidence of the softness of metal oxide nanofilms and inorganic layered materials.