Direct Observation of Early-Stage Quantum Dot Growth Mechanisms with High-Temperature Ab Initio Molecular Dynamics

Colloidal
quantum dots (QDs) exhibit highly desirable size- and
shape-dependent properties for applications from electronic devices
to imaging. Indium phosphide QDs have emerged as a primary candidate
to more toxic CdSe QDs, but production of InP QDs with the desired
properties lags behind other QD materials due to a poor understanding
of how to tune the growth process. Using high-temperature ab initio
molecular dynamics (AIMD) simulations, we report the first direct
observation of early-stage intermediates and subsequent formation
of an InP cluster from indium and phosphorus precursors. In our simulations,
indium agglomeration precedes formation of In–P bonds. We observe
a predominantly intercomplex pathway in which In–P bonds form
between one set of precursor copies, and the carboxylate ligand of
a second indium precursor in the agglomerated indium abstracts a ligand
from the phosphorus precursor. This process produces an indium-rich
cluster with structural properties comparable to those in bulk zinc-blende
InP crystals. Minimum energy pathway characterization of the AIMD-sampled
reaction events confirms these observations and identifies that In–carboxylate
dissociation energetics solely determine the barrier along the In–P
bond formation pathway, which is lower for intercomplex (13 kcal/mol)
than intracomplex (21 kcal/mol) mechanisms. The phosphorus precursor
chemistry, on the other hand, controls the thermodynamics of the reaction.
Our observations of the different roles of precursors in controlling
QD formation strongly suggest that the challenges thus far encountered
in InP QD synthesis optimization may be attributed to an overlooked
need for a cooperative tuning strategy that simultaneously addresses
the chemistry of both indium and phosphorus precursors.

胶体量子点（Colloidal Quantum Dots, QDs）具备极具应用价值的尺寸与形状依赖特性，可广泛应用于电子器件至成像技术等诸多领域。磷化铟量子点（Indium Phosphide QDs, InP QDs）已成为替代高毒性硒化镉量子点（Cadmium Selenide QDs, CdSe QDs）的主流候选材料，但由于对生长过程的调控机制认知不足，具备理想性能的InP QDs的制备进度仍落后于其他量子点材料。本研究借助高温从头算分子动力学（Ab Initio Molecular Dynamics, AIMD）模拟，首次直接观测到反应早期中间体的形成过程，以及由铟与磷前驱体生成InP团簇的完整路径。模拟结果显示，铟的团聚过程先于In-P键的形成。我们观测到一条主要的跨复合物反应路径：In-P键在一组前驱体分子之间形成，而团聚铟前驱体的羧酸配体可从磷前驱体处夺取配体。该过程生成了富铟团簇，其结构特性与块体闪锌矿（Zinc-blende）结构的InP晶体相近。对AIMD采样得到的反应事件进行最低能量路径表征，验证了上述观测结果，并明确In-羧酸配体解离的能量学特性是决定In-P键形成路径能垒的唯一因素：跨复合物路径的能垒（13 kcal/mol）低于复合物内路径（21 kcal/mol）。另一方面，磷前驱体的化学特性决定了反应的热力学过程。我们观测到前驱体在量子点形成过程中的不同调控作用，这强烈表明，目前InP QDs合成优化中遇到的难题，可能源于此前被忽视的协同调控策略需求——需同时兼顾铟与磷前驱体的化学特性。