# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 3 Inorganic Ligand Exchange
This approach, however, is not applicable to metal oxides and dielectrics; thus, more generalized techniques for surface functionalization are necessary. The devices fabricated with MCC-capped NCs exhibited improved performance when used in QD-sensitized solar cells (Figure 11) [76]. In this regard, [PERSON] et al. introduced a generalized ligand exchange strategy that can be applied to metal oxides (e.g., Fe\({}_{3}\)O\({}_{4}\) and TiO\({}_{2}\)), metal alloys (FePt and CoPt\({}_{3}\)), and dielectrics (NaYF\({}_{4}\)) [77]. The strategy involves the use of nitrosonium tetrafluoroborate or diazonium tetrafluoroborate compounds to replace original ligands and introduce BF\({}_{4}\)\({}^{-}\) anions on NC surfaces, thus stabilizing the NCs in various polar solvents such as \(N\),\(N\)-dimethylformamide (DMF), dimethylsulfoxide (DMSO), and acetonitrile. The reactive nature of NO\({}^{+}\) with solvated water resulted in the release of a considerable number of protons, thus facilitating the removal of organic ligands by protonation. It is worth noting that the failure of detachment of ligands when NaBF\({}_{4}\) was used instead of NOBF\({}_{4}\) or HBF\({}_{4}\) strongly suggests the importance of the H\({}^{+}\) ions. More significantly, the weak binding affinity of BF\({}_{4}\)\({}^{-}\) anions to the NC surface allows for fully reversible phase transfer of the NC between hydrophobic and hydrophilic media, enabling a secondary ligand exchange. The immediate phase transfer of NCs from a polar solvent (e.g., DMF) to a non-polar solvent (e.g., hexane) was observed upon the addition of  
Figure 11: Scheme of a CdSe QD-sensitized solar cell (QDSC) and J-V curves for oleylamine, S\({}_{2}\)\({}^{-}\), and S\({}_{2}\)O\({}_{3}\)\({}^{2-}\)-capped CdSe QDs-based QDSCs (FTO is fluorine-doped tin oxide glass). Reprinted with permission from [76]. Copyright 2017, American Chemical Society. organic ligands such as oleic acid, oleylamine, and tetradecyl phosphonic acid, demonstrating the complete recovery of surface functionalization, as shown in Figure 12[77]. MCC ligands bound to the NC surface via bridging of chalcogenides contain foreign metals (e.g., Sn, As). The presence of such foreign metals in proximity to the NCs often affects both the chemical and physical properties. For example, Sn\({}_{2}\)S\({}_{6}\)\({}^{4-}\) ligands can participate in redox processes by switching the oxidation state from Sn\({}^{\rm IV}\) to Sn\({}^{\rm II}\). In this context, metal-free inorganic ligands such as chalcogenides and hydrochalcogenides (S\({}^{2-}\), HS\({}^{-}\), Se\({}^{2-}\), HSe\({}^{-}\), Te\({}^{2-}\), and HTe\({}^{-}\)), mixed chalcogenides (TeS\({}_{3}\)\({}^{2-}\)), as well as OH\({}^{-}\) and NH\({}_{2}\)\({}^{-}\), are used in order to minimize the possible influence of foreign metals on the NC properties [78]. It should be highlighted that the use of metal-containing ligands often introduces recombination centers. The electron mobility (\(\mu\)) of FET devices fabricated using CdSe QD-treated metal-free ligands (i.e., (NH\({}_{4}\))\({}_{2}\)S stabilized CdSe QDs) was \(\mu\) = 0.4 cm\({}^{2}\) V\({}^{-1}\)s\({}^{-1}\) and the on-off threshold (I\({}_{\rm on}\)/I\({}_{\rm off}\)) was \(\sim\)10\({}^{3}\). The electronic coupling among NCs can be enhanced by employing ammonium thiocyanate (NH\({}_{4}\)SCN) [79]. The use of ammonium thiocyanate in cadmium chalcogenides (CdS, CdSe, and CdTe) and lead analogs (PbS, PbSe, and PbTe) yielded NCs-terminated NCs and removed more than 90% of the organic ligands. It is worth noting that NH\({}_{4}\)SCN is environmentally benign, available in kiloton-scale production, and air-stable. Based on the FTIR vibrational characteristics of thiocyanate (SCN) bound to NC surfaces (PbS, PbSe, CdSe, and CdS), it was found that the identity of the  
Figure 12: Schematic illustration of the secondary ligand exchange procedure and the corresponding FTIR spectra. Reprinted with permission from [77]. Copyright 2009, American Chemical Society. metal determines the vibrational frequency of thiocyanate.