# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 3 Inorganic Ligand Exchange  
Despite repeated purification and ligand refluxing, various researchers have reported that detachment of covalently bonded ligands (X-type) does not really occur; rather, they remain on the NC surfaces unless proton transfer is possible [56]. These remaining aliphatic ligands detrimentally affect the optoelectronic properties when used in various applications including LEDs, lasers, and solar cells [9]. Thereby, robust techniques are required for complete removal of insulating aliphatic ligands capped on NCs. To address this issue, an alternative surface coating method was developed by employing a new class of surface ligands: hydrazine-based metal chalcogenide complexes (MCC) e.g., N\({}_{2}\)H\({}_{2}\)-MCCs, (N\({}_{2}\)H\({}_{5}\))\({}_{4}\)Sn\({}_{2}\)S\({}_{6}\), (N\({}_{2}\)H\({}_{4}\))ZnTe, and N\({}_{4}\)H\({}_{6}\)Cu\({}_{7}\)S\({}_{4}\)[72; 73]. The complexes are prepared by dissolving metal chalcogenides in N\({}_{2}\)H\({}_{4}\), which possesses a unique solvating and reducing capability. Molecular metal chalcogenide anions (e.g., SnS\({}_{4}\)\({}^{-}\), SnS\({}_{2}\)S\({}_{6}\)\({}^{-}\), SnTe\({}_{4}\)\({}^{-}\), AsS\({}_{3}\)\({}^{-}\), and MoS\({}_{4}\)\({}^{2-}\)) solvated in a polar solvent attach to NC surfaces, replacing the original bulky organic ligands and providing colloidal stability. MCC-treated NCs were prepared via a simple phase transfer of NCs from the non-polar to the polar solvent. Specifically, aliphatic-ligand-capped NCs dispersed in a non-polar solvent (e.g., hexane) were transferred to a polar solvent containing hydrazine-chalcogenide anions, indicating successful displacement of the insulating ligands. It is worth noting that MCCs enable stable and facile electronic interactions between NCs and provides colloidal stabilization by adhering to the NCs. The zeta potential measurements of the NC surface show excellent dispersion of NCs in a polar solvent despite the lower steric hindrance of MCCs than the bulky aliphatic ligands. TGA characterization showed that the 2-10 wt % of MCCs is sufficient to stabilize colloidal NCs in variety of solvents [72]. Figure 10: Schematic illustration of the formation of hybrid nanocomposites using a template. Reprinted with permission from [71]. Copyright 2013, Springer Nature. Intriguingly, the utilization of N\({}_{2}\)H\({}_{4}\) results in thermal decomposition of N\({}_{2}\)H\({}_{4}\)-MCCs into metal chalcogenides, which act as an electronic glue. The FTIR peaks attributed to C-H, S-H, and N-H bonding completely disappeared, demonstrating successful ligand detachment. Indeed, the film fabricated with MCC-treated NCs (e.g., (N\({}_{2}\)H\({}_{5}\))\({}_{2}\)Sn\({}_{2}\)S\({}_{6}\)-capped Au, and (N\({}_{2}\)H\({}_{5}\))\({}_{2}\)Sn\({}_{2}\)S\({}_{6}\)-capped CdSe NCs) showed improved conductivity when used in field-effect transistors [72]. However, hydrazine is not only highly toxic and unstable, but it also strongly alters the properties of NCs owing to its strong chemical reactivity. Thus, environmentally benign and air-stable synthetic routes to form MCC-capped NCs have been developed. These techniques use metal sulfide MCCs such as Na\({}_{4}\)SnS\({}_{4}\), Na\({}_{4}\)Sn\({}_{2}\)S\({}_{6}\), Na\({}_{3}\)As\({}_{3}\), (NH\({}_{4}\))\({}_{4}\)Sn\({}_{2}\)S\({}_{6}\), and (NH\({}_{4}\))\({}_{3}\)AsS\({}_{3}\) instead of hydrazine-MCCs [74, 75]. Treatment with these metal sulfide MCCs has been applied to a wide range of NCs, including CdS, CdSe, CdSe/ZnS, CdTe, PbS, PbTe, Au, and FePt. It should be highlighted that CdSe and PbS QDs retain their luminescent properties even after repeated ligand exchange, which is in contrast to the observation after a N\({}_{2}\)H\({}_{2}\)-MCCs treatment. FTIR studies demonstrated the successful displacement of aliphatic ligands with metal sulfide MCCs. The surface coverage of SnS\({}^{4-}\) ligands on NCs was \(\sim\)20.8%, based on ICP-OES measurements.