Surface ligands of semiconductor nanocrystals (NCs) play key roles in determining their colloidal stability and physicochemical properties and are thus enablers also for the NCs flexible manipulation toward numerous applications. Attention is usually paid to the ligand binding group, while the impact of the ligand chain backbone structure is less discussed. Using isothermal titration calorimetry (ITC), we studied the effect of structural changes in the ligand chain on the thermodynamics of the exchange reaction for oleate coated CdSe NCs, comparing linear and branched alkylthiols. The investigated alkylthiol ligands differed in their backbone length, branching position, and branching group length. Compared to linear ligands, lower exothermicity and entropy loss were observed for an exchange with branched ligands, due to steric hindrance in ligand packing, thereby justifying their previous classification as "entropic ligands". Mean-field calculations for ligand binding demonstrate the contribution to the overall entropy originating from ligand conformational entropy, which is diminished upon binding mainly by packing of NC-bound ligands. Model calculations and the experimental ITC data both point to an interplay between the branching position and the backbone length in determining the entropic nature of the branched ligand. Our findings suggest that the most entropic ligand should be a short, branched ligand with short branching group located toward the middle of the ligand chain. The insights provided by this work also contribute to a future smarter NC surface design, which is an essential tool for their implementation in diverse applications.
Bibliographical noteFunding Information:
Israel Science Foundation-National Science Foundation China (ISF-NSFC) joint research program (grant No. 2495/17).
This research was supported by the Israel Science Foundation-National Science Foundation China (ISF-NSFC) joint research program (grant No. 2495/17). O.E and O.A. acknowledge support from the Hebrew University Center for Nanoscience and Nanotechnology. U.B. thanks the Alfred & Erica Larisch memorial chair. The authors would like to thank Dr. Kobi Oded for insightful discussions regarding the synthesis and analysis of the branched alkylthiol ligands.
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- CdSe nanocrystals
- branched ligands
- conformational entropy
- isothermal titration calorimetry
- ligand exchange