**1. Introduction**

In the past decades, colloidal inorganic nanocrystals (NCs) have received considerable attention in several research fields because of their interesting size-dependent properties, such as their quantum confinement effect and localized surface plasmonic effect, which are not be observed in the bulk [1,2]. With extensive research efforts, there has been significant progress in the development of synthetic methods for inorganic NCs, enabling the precise tuning of their size, chemical composition, crystallinity, and shape, which is very important in controlling their properties. In addition to rendering unique material properties to individual NCs, the self-assembly of NCs provides a "bottom-up" approach for the fabrication of micro- or macroscale NC-based films with highly complicated nanostructures, which are difficult to achieve through conventional lithography-based fabrication processes. Moreover, NC building blocks enable solution-based, cheap, and scalable processes, which are highly beneficial for industrial applications.

One of the most interesting applications of NC building blocks is the colloidal self-assembly of NCs into ordered crystalline structures; that is, NC building blocks form various crystal structures, including face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal close packed (hcp) structures, similar to how atoms or ions form crystalline structures [3,4]. More importantly, the use of two different types of NCs can yield highly ordered binary NC superlattices (BNSLs) with various packing structures, such as NaCl, MgZn2, and NaZn13, depending on the size and concentration ratio [5]. BNSLs exhibit not only structural diversity for tuning NC–NC interactions through the choice of NC constituents or packing symmetry, but also collective properties that are distinct from the sum

of the individual characteristics. It is highly important to understand the self-assembly behavior of NCs for the development of novel materials because of their superior controllability in material design. In this review, we will broadly describe the self-assembly of colloidal NCs, including the fabrication method, formation mechanism of self-assembly, structural diversity of BNSLs, and mesoscale structure of self-assembled NC superlattices.
