Lai Runchen, Pu Chaodan, Peng Xiaogang
Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry , Zhejiang University , Hangzhou 310027 , China.
J Am Chem Soc. 2018 Jul 25;140(29):9174-9183. doi: 10.1021/jacs.8b04743. Epub 2018 Jul 12.
On-surface reaction mechanisms during the growth of high-quality CdSe nanocrystals are studied quantitatively and systematically by introducing a cyclic growth scheme. Prior to the repeating growth cycles, presynthesized CdSe QD seeds from a conventional scheme are reacted with an activated Se precursor, which is found to include three elementary steps and generate Se-terminated CdSe QDs. The cyclic growth in amine-octadecene solution includes two repeating half-reactions. The first half-reaction is between cadmium carboxylates in the bulk solution and the Se-terminated QDs, and the other is between the Se precursor in the bulk solution and the Cd-terminated QDs generated by the first half-reaction. While two elementary steps in the Se-surface half-reaction can be quantitatively treated as parallel kinetics, two elementary steps for the Cd-surface half-reaction must be treated as consecutive steps. These elementary steps are found to possess substantially different reaction rates as well as activation energies. Results indicate that, in the growth of compound semiconductor nanocrystals with metal carboxylates as cationic precursor (or ligands), the elementary step between activated anionic precursors in the bulk solution and the cationic sites on the surface of nanocrystals would be the rate-limiting step. This rate-limiting step should be the one that causes nucleation (or formation of small clusters by solution reactions) to be substantially faster than the corresponding growth through on-surface reactions.
通过引入循环生长方案，对高质量CdSe纳米晶体生长过程中的表面反应机制进行了定量和系统的研究。在重复生长循环之前，将传统方案预先合成的CdSe量子点种子与活化的硒前驱体反应，发现该反应包括三个基本步骤，并生成硒端接的CdSe量子点。在胺-十八烯溶液中的循环生长包括两个重复的半反应。第一个半反应是本体溶液中的羧酸镉与硒端接的量子点之间的反应，另一个是本体溶液中的硒前驱体与第一个半反应生成的镉端接的量子点之间的反应。虽然硒表面半反应中的两个基本步骤可以定量地视为平行动力学，但镉表面半反应的两个基本步骤必须视为连续步骤。发现这些基本步骤具有显著不同的反应速率以及活化能。结果表明，在用金属羧酸盐作为阳离子前驱体（或配体）生长化合物半导体纳米晶体时，本体溶液中活化的阴离子前驱体与纳米晶体表面阳离子位点之间的基本步骤将是速率限制步骤。这个速率限制步骤应该是使成核（或通过溶液反应形成小簇）比通过表面反应的相应生长快得多的步骤。
