# Recent Advances in Research from Nanoparticle to Nano-Assembly: A Review
## 2 Oriented Assembly of Nanomaterials
For instance, uniform dodecylamine (DDA)-capped CdSe/CdS NRs were fabricated and linked via their ends, as illustrated in the representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image in Figure 1a [43]. These structures are solution-processable and possess high colloidal stability (see Inset Figure 1a). NRs are activated by cation exchange reactions with Ag\({}^{+}\), resulting in small regions of Ag\({}_{2}\)S only at the rod tips. Next, an acid-base reaction is carried out using octadecyl phosphonic acid (ODPA), resulting in an insoluble salt with DDA. This process involves removing ligands from Ag\({}_{2}\)S surfaces and linking NRs together into long chains. Domains of Ag\({}_{2}\)S are the only ones involved in linking. Due to the stoichiometric and site-selective nature of the linking process, particle ordering can be controlled. The linkage sites are determined by the positions of the Ag\({}_{2}\)S domains. The final assembly can be controlled by modifying the driving force for their nucleation. Due to the curvature of NR ends, reducing the curvature greatly lowers the barrier to heterogeneous nucleation [44; 45]. This is because the critical nucleus size is proportional to the curvature of the NR ends. To illustrate this effect, more complex particles were utilized. Matchstick-shaped NRs, for example, have bulged heads at one end and thin tails with different curvatures at the other. Cation exchange with Ag\({}^{+}\) results in the nucleation of Ag\({}_{2}\)S only at the tail end of Ag\({}_{2}\)S. The asymmetric NRs, however, produced dimers only when exchanged with Ag\({}^{+}\), as shown in Figure 1b. Figure 1: (**a**) Low-resolution HAADF-STEM images of CdSe/CdS nanorods (-50 nm in length and -4.5 nm diameter) linked by Ag\({}_{2}\)S (-2 nm diameter). Inset shows photograph of well-dispersed, linked structures. nano heterostructures in organic solution without any noticeable aggregation over prolonged times. **(b)** Representative TEM image of typical tail\(-\)tail dimers when \(-\)70 nm long asymmetric nanorods were linked by cation exchange with Ag\({}^{+}\). **(c)** Representative TEM image of head\(-\)head dimers linked through cation exchange with Cu\({}^{+}\). **(d)** TEM image of a hierarchical linked structure comprising three tail\(-\)tail dimers joined through their head ends, fabricated by sequential cation exchange with Ag\({}^{+}\) and Cu\({}^{+}\). TEM images of hybrid structures formed by using Ag\({}^{+}\) to link. These images were cited and reproduced from the reference [43]. **(e)** Schematic showing hetero-linking of Ag\({}_{2}\)Se-tipped CdSe NRs (green) to Ag\({}_{2}\)Te-tipped CdTe TPs (yellow) via their Ag chalcogenide ends. Due to competitive cation exchange, not all CdSe NRs undergo exchange with Ag\({}^{+}\) and remain as pristine NRs. **(f)** HAADF-STEM image of CdSe NR-linked CdTe TPs. The dotted circles denote the linkage regions. **(g)** HAADF-STEM image of CdSe NR-linked CdTe TP nanostructures after Cd\({}^{2+}\) back-exchange and size-selective precipitation. (Inset) Typical Cd\({}^{2+}\) back-exchanged CdTe\(-\)CdSe (TP\(-\)NR) HtNS. The scale bar is 20 nm. **(h)** Pump-fluence-dependent PL spectra of linked nanostructures dispersed in toluene. (Inset) Zoomed-in image of PL spectra, showing the evolution of the type II emission at different pump intensities. These images were cited and reproduced from the reference [46]. **(i)** Schematic illustrating the formation of branched heterostructures with a controlled number of branches as well as orientation. **(j-l)** Mechanism of formation of bipods and tripods. These images were cited and reproduced from the reference [47]. **(m)** Schematic illustrating the fabrication of 2D Rafts from NRs. **(n-q)** Representative TEM images showing the various stages of 2D raft formation **(n)**NRs, **(o)** chains of NRs, **(p)** Longer chains of NRs, and **(q)** 2D Rafts [48]. Nucleation can also be affected by the chemical composition of the linkages.