Department of Chemical Engineering and Materials Science, University of Minnesota , Minneapolis, Minnesota 55455, United States.
Langmuir. 2017 Aug 22;33(33):8239-8245. doi: 10.1021/acs.langmuir.7b01924. Epub 2017 Aug 10.
Semiconductor quantum dots synthesized using rapid mixing of precursors by injection into a hot solution of solvents and surfactants have surface ligands that sterically stabilize the dispersions in nonpolar solvents. Often, these ligands are exchanged to disperse the quantum dots in polar solvents, but quantitative studies of quantum dot surfaces before and after ligand exchange are scarce. We studied exchanging trioctylphosphine (TOP) and trioctylphosphine oxide (TOPO) ligands on as-synthesized CdSe quantum dots dispersed in hexane with a 2000 g/mol thiolated poly(ethylene glycol) (PEG) polymer. Using infrared spectroscopy we quantify the absolute surface concentration of TOP/TOPO and PEG ligands per unit area before and after ligand exchange. While 50-85% of the TOP/TOPO ligands are removed upon ligand exchange, only a few are replaced with PEG. Surprisingly, the remaining TOP/TOPO ligands outnumber the PEG ligands, but these few PEG ligands are sufficient to disperse the quantum dots in polar solvents such as chloroform, tetrahydrofuran, and water. Moreover, as-synthesized quantum dots once easily dispersed in hexane are no longer dispersible in nonpolar solvents after ligand exchange. A subtle coverage-dependent balance between attractive PEG-solvent interactions and repulsive TOP/TOPO-solvent interactions determines the dispersion stability.
使用注射法将前体快速混合到热溶剂和表面活性剂溶液中合成的半导体量子点具有空间位阻稳定分散在非极性溶剂中的表面配体。通常，这些配体被交换以在极性溶剂中分散量子点，但在配体交换前后对量子点表面的定量研究很少。我们研究了在己烷中分散的合成的 CdSe 量子点上用 2000g/mol 的巯基化聚（乙二醇）（PEG）聚合物交换三辛基膦（TOP）和三辛基氧化膦（TOPO）配体。使用红外光谱，我们定量地确定了配体交换前后单位面积上 TOP/TOPO 和 PEG 配体的绝对表面浓度。虽然配体交换后有 50-85%的 TOP/TOPO 配体被去除，但只有少数被 PEG 取代。令人惊讶的是，剩余的 TOP/TOPO 配体数量超过了 PEG 配体，但这些少数的 PEG 配体足以在极性溶剂如氯仿、四氢呋喃和水中分散量子点。此外，在己烷中很容易分散的合成量子点在配体交换后不再能在非极性溶剂中分散。在吸引力 PEG-溶剂相互作用和排斥性 TOP/TOPO-溶剂相互作用之间存在微妙的覆盖依赖性平衡，决定了分散体的稳定性。
