Classification of self-assembly. Generally, self-assembly of nanoparticles can enable the formation of countless complex structures, all the way from nano to macro scale. However, based on the present state of literature, one can divide self-assembled structures broadly into two classes. The first one encompasses finite clusters and patterns, or superlattices of such clusters. Self-assembled nanoparticle clusters of up to a few hundred nanometres in size are included in this class. Other patterns such as chains of nanoparticles or superlattices of nanoparticle clusters are also included here. In any case, the number of constituting nanoparticles or nanostructures should not be more than ~100. The second class of structures includes primarily nanoparticle films that are extended to "infinity" in two dimensions. In our general terminology, if the area of the nanoparticle array is infinitely larger than the thickness, it can be considered a film. In the specific context of photoactive nanoparticles such as semiconductor photocatalyst nanoparticles and plasmonic nanoparticles, a deviating definition of "infinitely large" area is applicable: If one of the dimensions extends to a length scale that is much larger than the incident light wavelength, then that dimension can be defined as the extension to infinity. For instance, a selfassembled 20 nm Au nanoparticle film of 1x1 cm 2 area irradiated by green light is perceived to extend to "infinity" in two dimensions in the sense that the size of the film in the order of 1 cm is much larger than the visible wavelength range in the order of 500 nm, where the film finds its applications. In the following section, these two classes are discussed in sub-classes defined based on the general synthetic procedures. For a general overview, the classification of self-assembled structures, their synthesis and application are summarized in Figure . Figure . An overview of the classification of self-assembled nanoparticle structures. Broadly, selfassembled structures are divided into two categories: nanoparticle films and, nanoparticle clusters and clusters superlattices. For each category, the well-established synthesis techniques are highlighted. Different applications that are enhanced by self-assembly or self-assembled structures are highlighted at the outer boundary.