In colloidal quantum dots (QDs), the geometries of surface ligands may play significant roles in tuning the electronic structure, optical spectra and exciton dynamics. We here propose an effective approach to build a diverse dataset of small QDs, based on which the machine learning force field (MLFF) can be obtained based on the DeePMD framework and the energy of each atom is expressed based on the local atomic structure. Using the obtained QD force field (QDFF), molecular dynamics simulation of large zinc-blende CdSe QDs passivated by carboxylate ligands is successfully carried out, and the complex surface structure is extensively studied. We find that bridging, tilted, chelating and claw geometries are the major geometries of carboxylate ligands in CdSe QDs, and the alkyl chain length of ligands plays a significant role. The Markov state model is utilized to reveal the detailed geometry transformation channels. Due to the high performance of QDFF, the present approach is promising for systematic studies of large QDs with different kinds of ligands that can be synthesized in experiment.

The twist angle between two van der Waals coupled monolayers has emerged as a new and powerful degree of freedom for engineering physical properties of semiconductor homo- and hetero-bilayers. While the interlayer twist has shown prominent effect on electronic and optical properties of transition metal dichalcogenide (TMD) bilayers, it remains unclear how it could be used to manipulate the exciton dynamics, especially exciton-exciton annihilation (EEA) process which is the dominant energy loss channel in TMDs under moderate to high exciton density due to strong Coulomb interaction. Herein, we show that the twist angle in TMD bilayers can act as an effective knob to control the EEA process. Specifically, EEA rate constant increases from 1° twisted WSe₂ bilayers (0.026 cm²/s) by more than twice to 32° twisted bilayers (0.053 cm²/s) and then drops again in 60° twisted bilayers (0.019 cm²/s). This twist-angle dependence can be attributed to the energy difference between indirect and direct excitons arising from the interlayer interaction. Our work opens up the possibility of artificially managing the exciton dynamics in TMD materials for optoelectronic applications via interlayer twist angle.