On-Surface Reactions In The Growth Of High-Quality CdSe Nanocrystals In Nonpolar Solutions

In solution chemistry synthesis based on precursors,the growth habits of nanocrystals are far from the predictions from classical nucleation theory.Precursor-and/or surface-involved chemical reactions might be a key to interpreting the mechanisms of nanocrystals,while their quatitative researches are still in a preliminary stage.The geometric sizes of nanocrystals fall in the intermediate range between molecules and observable bulk crystals,which is the most critical range in crystallization yet is the most difficult to study.Colloidal semiconductor nanocrystals with their sizes in the quantum confinement regime,named as"quantum dots(QDs)",possess strongly size-dependent optical properties,which provides us unique tools for probing the size of the nanocrystals(clusters)in solution.By choosing certain synthesis schemes and surface ligands,researchers can also get rich surface information from routine measurements.Thus,quantum dots could be used as unique model systems to study crystallization.We designed a cyclic growth strategy to realize high-quality CdSe QDs.Starting with metal carboxylate terminated CdSe seed QDs,we adopted amine-ODE nonpolar solution and precursors and reaction conditions similar to those in standard synthesis.Only counter precursor was allowed in solution exclusively in each reaction step to grow one atomic layer onto QD surfaces,and after reaction completed,QDs were thoroughly purified for the next reaction step.CdSe QDs from this strategy possessed the same crystal structure and crystallinity with those directly obtained from standard synthesis,which meant that the new strategy was suitable for studying on-surface reaction between CdSe QDs and counter precursor solution without being complex with other precursors.Using this strategy,we are able to study reactions between surface atoms of CdSe QDs and counter precursor in bulk solution without being complex with other precursors coexisting in reaction environment.We found that there were three categories of surface half-reactions during CdSe QD growth in cyclic growth strategy,namely,the initial half-reaction,Se-surface half-reaction and Cd-surface half-reaction,while the latter two built up repeating growth cycle.The initial half-reaction and Cd-surface reaction were both reactions between cadmium-terminated CdSe QDs and selemium precursor,whose differences lay on the surface atomic configurations and particle shapes of CdSe QDs;Se-surface half-reaction happened between selenium-terminated CdSe QDs and bulk cadmium carboxylate solution.On-surface reaction mechanisms are studied quantitatively and systematically with fundamental measurements such as steady-state UV-vis absorption and PL spectroscopy,FTIR spectroscopy and TEM.Prior to the repeating growth cycles,pre-synthesized CdSe QD seeds from a conventional scheme went through the initial half-reaction with selemiun precursor,in which there was found to include three elementary steps.The two repeating half-reaction in amine-octadecene solution—Se-surface half-reaction and Cd-surface half-reaction—were both consisted of two elementary steps.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 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.