# Interface Engineering Strategies for Fabricating Nanocrystal-Based Organic-Inorganic Nanocomposites
## 2 Overview of Ligand Exchange
### Characterization
This strategy offers an advantage compared to NMR characterization in that picomolar-scale detection is feasible due to the high sensitivity of the \({}^{3}\)H nuclei. Specifically, \({}^{3}\)H-labeled oleic acids were first prepared via partial reduction of stearic acid in the presence of tritium gas; then, OA bound to NCs were replaced with isotopically labeled OA ligands, yielding \({}^{3}\)H-labeled OA-capped QDs (Figure 5a). The \({}^{3}\)H-labeled OA-capped QDs were used as starting materials and were subjected to ligand refluxing with various ligands (e.g., trioctylphosphine (TOP), trioctylphosphine oxide (TOPO), OA, and oleylamine) to quantify and understand ligand  
Figure 4: \({}^{1}\)H NMR spectrum of (**a**) free-state OA and (**b**) OA-capped CdSe QDs. Change in alkene resonance upon addition of (**c**) free OA and (**d**) deuterated OA. Reprinted with permission from [59]. Copyright 2012, American Chemical Society. exchange behaviors. Through the detection of a radioactive species (i.e., \({}^{3}\)H-labeled OA), a relative scale of OA displacement strength was established for different ligands (Figure 5b) [60]. For many applications including LEDs, lasers, and bioimaging, the actual concentration of the NCs in solution must be determined. Although thermogravimetric methods are often used to determine the concentration of colloidal nanocrystals, the existence of organic ligands on NC surface hinders accurate measurement. In addition, such techniques are destructive, making an in situ characterization impossible. In this regard, absorption spectroscopy is as an excellent and convenient tool for quantification of semiconducting nanocrystals without damaging the samples. Furthermore, the size of the semiconducting nanocrystals can be determined based on the quantum confinement effect. Several researchers have reported a correlation between the first absorption peak of semiconducting QDs (e.g., CdS, CdSe, PbTe, PbSe, and CdTe) and their size. [PERSON] et al. summarized these values and suggested empirical fitting functions [40; 46; 47]. These equations are only valid in the size range covered by the data shown in Figure 6[47]. The optical properties of the semiconducting quantum dots are summarized in Table 1[40; 47]. Figure 5: (**a**) Synthetic route to form \({}^{3}\)H-labeled OA ligands; (**b**) relative OA detachment strengths of the different ligands. Reprinted with permission from [60]. Copyright 2013, American Chemical Society. Figure 6: (**a**) Size curves vs. the first absorption peak position; (**b**) size-dependent molar absorption coefficients of nanocrystals at the first excitonic absorption. Reprinted with permission from [47]. Copyright 2003, American Chemical Society. The relationship between molar extinction coefficient (\(\varepsilon\)) and NC size has been reported by several groups. It greatly relies on the NC concentration, based on Beer-Lambert's equation [47]. Therefore, the main obstacle for calculating \(\varepsilon\) has been the difficulty in determining the QD concentrations [10; 46; 61]. In the early studies, the extinction coefficient per NC was determined by measuring the NC concentration, based on precursor mass, assuming that all the precursors are converted into NCs. However, this inaccurate assumption resulted in discrepancies in the results and more sophisticated measurement techniques have since been developed, such as inductively coupled plasma mass spectrometry (ICP-MS) and atomic absorption measurements [62]. For example, [PERSON]'s group measured the extinction coefficient of InAs QDs as a function of their size using ICP-MS [62]. The NC concentration was also accurately determined via standard atomic absorption measurements. In detail, a purified NC solution was digested by aqua regia and the cadmium concentration was measured by atomic absorption spectroscopy; this enabled a more accurate calculation of NC concentration by calculating the number of Cd atoms per NC crystal.