*2.1. Methods for the Self-Assembly of NCs*

Highly ordered NC superlattices can be prepared by the self-assembly of colloidal NCs. Mostly, colloidal inorganic NCs are synthesized by the high-temperature solvothermal decomposition process, which yields highly monodispersed NCs coated with alkyl chain ligands [6–8]. A few approaches have been adopted to build highly ordered NC superlattices with the prepared NC building blocks. One of the methods is simply drop-casting colloidal NCs in non-polar solvents such as hexane, toluene, or chloroform onto a solid substrate and allowing them to dry for a couple of minutes [9]. When the solvent evaporates, the NCs are densely solidified through various kinds of interactions including NC–NC interactions (i.e., van der Waals force and electrostatic interaction) and ligand–ligand interactions (i.e., hydrogen bonding). In addition to the simple drop-casting method, the recrystallization method has been used for the preparation of three-dimensional (3D) ordered NC superlattices by using a polar solvent to destabilize NC dispersion in a non-polar solvent, as shown in Figure 1a–c [10,11]. As the non-polar solvent slowly evaporates from the QD dispersion in the presence of the polar solvent, QDs start crystallizing because of their decreased solubility in the solution. Consequently, 3D micro-sized NC superlattices are formed because of van der Waals interactions between the NCs and the change in free energy during the crystallization process.

**Figure 1.** (**a**) Dark field optical micrograph of colloidal crystals formed by 2 nm CdSe nanocrystals (NCs) (Reproduced with permission from [10], Copyright American Association for the Advancement of Science, 1995). Scanning electron microscope (SEM) images of self-assembled supercrystals of (**b**) octahedral and (**c**) cubic Pt NCs (Reproduced with permission from [11]. Copyright American Chemical Society, 2013). Schematic illustration of the self-assembly of NCs by (**d**) the slow evaporation of NC solution under vacuum (Reproduced with permission from [12], Copyright American Chemical Society, 2006). and (**e**) liquid–air interface assembly (Reproduced with permission from [13]. Copyright Springer Nature, 2010). Scale bars in Figure 1b,c represent 500 nm and 200 nm, respectively.

Another method for the self-assembly of NCs involves the slow evaporation of the solvent from the NC solution, inducing the crystallization of the NCs [12]. As shown in Figure 1d, a substrate (e.g., transmission electron microscope (TEM) grid or silicon wafer) is placed in a container with the NC solution. Then, the container is placed in a chamber and tilted by 60–70◦. Next, the solvent is slowly evaporated under a low-pressure vacuum at 45 ◦C. As the concentration of NCs in the solution increases, it reaches the solubility limit of NCs in the solution, leading to the crystallization of NCs. Consequently, a well-ordered NC superlattice is formed on the substrate. A similar approach has been adopted to obtain NC superlattices at the liquid–air interface [13,14]. When NCs in non-polar solvents such as hexane and toluene are drop-casted on top of an immiscible polar solvent (e.g., ethylene glycol and diethylene glycol) in a well, followed by covering the top with a slide glass, the non-polar solvent on top of the liquid substrate slowly evaporates, and as the concentration increases, NCs are crystallized. Finally, a thin, long-range ordered NC film forms on top of the polar solvent, which is then transferred to a solid substrate for characterization. An advantage of the liquid–air interface self-assembly technique is that it yields uniform NC superlattice thin-films over a large area within a short time.
