The term “hierarchy”, which was originated from a Greek word (hierarkhia [íεραρχíα]: leader of sacred rites) has been widely used in a variety of fields including social science, education, zoology, science of religion, computer science, math, etc. The definition and meaning of “hierarchy” or “hierarchical” vary depending on the referenced field. Words such as rank, class, food chain, pyramid, caste system, many branches may be associated with the term “hierarchy”.

In general, the “hierarchical” structure in nanoscience and engineering can be defined as an integrated structure that is composed of many, low-dimensional nano-building blocks.¹  In other words, a hierarchical nanostructure has a configuration of higher dimension assembled from lower dimensional nanomaterials including nanoparticles (0-D), nanowires/rods/tubes (1-D) and nanosheets (2-D). Therefore a “hierarchical structure” invokes a 3-D image resembling trees or flowers having very dense branches with extremely high surface areas. If both backbones and branches are wire-type structures, tree-like hierarchical structures^2,3  are formed while spherical particles comprise urchin-like or chestnut-like hierarchical structures⁴  when combined with wire-type branches. In this chapter, the words “hierarchical structure” rather signify structures with a distinct backbone (also called a trunk or core) to which a plethora of branches are attached as seen in Figure 8.1. Therefore, tetrapod,^5,6  cauliflower⁷  and chrysanthemum⁸  structures are not examined in this chapter, though they are also often referred to as hierarchical structures in a broader sense.