# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 2 Overview of Ligand Exchange
### Organic-Inorganic Hybrid Nanocomposites  
Ligand exchange through refluxing has been demonstrated to be a simple and effective method to bind CP ligands to a NC surface. For example, oligomer ligands (e.g., pentathiophene phosphonic acid and terthiophene phosphonic acid) that have functionalities for anchoring to NC surfaces were tethered onto the surface of CdSe NCs via ligand refluxing [24; 28]. The resulting oligohexylthiophene-capped CdSe NCs showed excellent solubility in most organic solvents and good miscibility with CPs. [PERSON] et al. grafted carbodithioate-oligothiophenes onto a CdSe QD surface via ligand exchange for solar cells [65]. Since ligand exchange relies on the kinetics of the adsorption-desorption process, the grafting density of long-chain CPs on NC surfaces is usually low. To address this entropic penalty, short-chain bifunctional ligands are introduced on NC surfaces through ligand refluxing, followed by grafting with the desired CPs [66]. Rationally designed bifunctional ligands (e.g., arylbromide-functionalized phosphine oxides or thiols) were tethered to the CdSe NC surfaces and then grafted with end-functionalized P3 HT, yielding P3 HT-CdSe NC nanocomposites (Figure 7) [24]. The grafting density was 250 and 400 P3 HT chains per CdSe nanorod (NR) for phosphine oxide- and thiol-functionalities, respectively. This result is in good agreement with the fact that thiol ligands co \begin{table}
\begin{tabular}{c c c} \hline \hline
**Materials** & **Diameter (nm)** & **Molar Absorption Coefficient** \\ \hline \multirow{2}{*}{**CdS**} & \(D=-\big{(}6.86521\times 10^{-8}\big{)}\lambda^{3}+\big{(}1.79557\times 10^{-4} \big{)}\lambda^{2}\) & \multirow{2}{*}{\(\varepsilon=21536\left(D\right)^{2.3}\)} \\  & \(-\big{(}9.2352\times 10^{-2}\big{)}\lambda+(13.29\big{)}\) & \\ \hline \multirow{2}{*}{**CdSe**} & \(D=\big{(}1.6122\times 10^{-9}\big{)}\lambda^{4}-\big{(}2.6575\times 10^{-6} \big{)}\lambda^{3}\) & \multirow{2}{*}{\(\varepsilon=5857\left(D\right)^{2.65}\)} \\  & \(+\big{(}1.6242\times 10^{-3}\big{)}\lambda^{2}-(0.4277)\lambda+(41.57)\) & \\ \hline \multirow{2}{*}{**CdTe**} & \(D=\big{(}9.8127\times 10^{-7}\big{)}\lambda^{3}-\big{(}1.7147\times 10^{-3} \big{)}\lambda^{2}\) & \multirow{2}{*}{\(\varepsilon=10043\left(D\right)^{2.12}\)} \\  & \(+(1.0064)\lambda-(194.84)\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Optical properties and diameters of CdS, CdSe, and CdTe QDs. than phosphine oxide ligands. Despite the high grafting density, this process involves a two-step procedure and is thus time-consuming. To further simplify the process of forming hybrid nanocomposites, the bifunctional ligand-grafted NCs can be synthesized directly. This dispenses with the need for tedious ligand exchange procedures. Specifically, a bifunctional ligand, 4-bromobenzyl phosphonic acid (BBPA), which coordinates strongly with the \((01\overline{1}0)\) and \((11\overline{2}0)\) facets of CdSe NCs was used during the synthesis of CdSe NCs, yielding BBPA-capped CdSe NCs [25]. It should be noted that BBPA possesses a bromide moiety that can be used as a chemical coupling site. Subsequently, vinyl- and ethynyl-terminated P3 HT were grafted with the bromide group of BBPA-capped CdSe NRs via Heck coupling and click chemistry, respectively. These chemically tethered P3 HT-CdSe NC nanocomposites not only showed excellent dispersion in various organic solvents, but also demonstrated improved electronic interactions between the electron-accepting inorganic NCs (e.g., CdTe tetrapods and CdSe QDs) and the electron-donating CPs (e.g., P3 HT). Photoluminescence measurements show the facilitated charge transport between these components [26]. Since the use of small fractions of aliphatic ligands is unavoidable, techniques involving direct growth of NCs in proximity to CPs have been studied. It avoids the use of further treatments like ligand refluxing as NCs are directly synthesized in the presence of a CP matrix. [PERSON] et al.