On the nature and importance of the transition between molecules and nanocrystals: towards a chemistry of "nanoscale perfection"

This paper discusses the importance of the transition between molecular compounds and nanocrystals. The boundary between molecular and nanocrystals/nanoclusters can be defined by the emergence of the bulk phase; atoms in the core of the nanoclusters that are not bound to ligands. This transition in dimensions and structural organization is important because it overlaps with the boundary between atomically defined moieties (molecules can be isolated with increasing purity) and mixtures (nanocrystals have a distribution of sizes, shapes, and defects; they cannot be easily separated into batches of structurally identical species). Passing through this boundary, as the size of a structure increases beyond a few nanometres, the information about the position of each atom gradually disappears. This loss of structural information about a chemical structure fundamentally compromises our ability to use it as a part of a complex chemical system. If we are to engineer complex functions encoded in a chemical language, we will need pure batches of atomically defined (truly monodisperse) nanoscale compounds, and we will need to understand how to make them and preserve them over a broad range of length scales, compositions, and timeframes. In this review we survey most classes of monodisperse nanomaterials (mostly nanoclusters) and highlight the recent breakthroughs in this area which might be spearheading the development of a chemistry of "nanoscale perfection".