Properties of nano-objects — Critical lengths, size and shape effects

The reduction in size of an object to nanometric dimensions modifies its properties quantitatively and qualitatively. This is why, whereas the properties of a macroscopic material depend solely on its structure and composition, the properties of a nano-object also result from its size and shape. So, while at the usual scales, engineers have to search for the ideal chemical composition and crystallographic structure to obtain specific properties, at the nanometric scale, the same engineer has the additional variables of the object's size and shape. The space of accessible parameters thus expanded is the source of many potential applications. However, there is no universal rule for predicting below what critical size the properties of a nano-object differ from those of the same bulk material from which it was formed. In the case of heterogeneous materials, composed of domains or grains, the critical size is that of a domain. For homogeneous materials, any property being characterized by one (or more) characteristic lengths, a nano-object must have a critical size of the order of magnitude of or smaller than this characteristic length for its properties to differ from those of the bulk material.

Without claiming to be exhaustive, the aim of this article is to analyze the mechanical, electronic, optical, magnetic, transport and thermodynamic properties of nano-objects, paying particular attention to the characteristic lengths of the properties studied, and thus to highlight the critical sizes below which nano-objects can exhibit specific properties linked to their size but also to their shape. Particular attention is paid to nanoparticles, ultrathin films and two-dimensional materials, as well as nanowires or nanotubes, which are nano-objects with nanometric dimensions in three, two or one direction respectively. Nanostructured or nanocomposite materials, which, although macroscopic in size, can exhibit novel properties induced by their nanoscale structuring, are also discussed.

To facilitate reading, each section dedicated to a property is preceded by a brief reminder (in an insert) followed by a definition of the associated characteristic lengths. The main effects of size and shape on the properties of nano-objects below these critical sizes are then described, and a few examples of applications presented.