First-principles Study Of Exciton Properties Regulation In Quantum Dot Materials

Charge transfer excitons,which have an intrinsic dipole moment and a radiative decay lifetime of up to microseconds,are an important platform for exploring quantum many-body phenomena such as BoseEinstein condensation and superfluidity.However,there is an inherent contradiction between the spatial separation of electrons and holes and the stability of excitons,and one of the current challenges in research on charge transfer excitons is how to maximize the spatial separation of electrons and holes while maintaining good exciton thermal stability.Quantum dots have excellent optical properties,such as broad absorption and narrow emission,and tunable spectral continuity,making them promising in many fields such as lightemitting diodes,solar cells,and bio-detection.The three-dimensional confinement of quantum dots and weaker screening effects compared to bulk materials make them an important platform for realizing charge transfer excitons.In existing research,quantum dot materials mainly achieve charge transfer excitons through core-shell engineering and ligand engineering.However,the strong dependence of the confinement effect on the thickness of the shell quantum dot in core-shell engineering will reduce the carrier transport rate and device efficiency as the shell thickness increases.The diversity of ligands makes it a promising way to induce charge transfer excitons in a medium,but the interaction mechanism between ligands and quantum dots and the selection of ligands need further exploration.Based on the time-dependent density functional theory,this thesis studies the regulation of exciton properties in quantum dot materials and focuses on several possible ways to achieve charge transfer excitons in this material system.The main results are as follows:(1)External fusion quantum dots are proposed as a potential platform for achieving charge transfer excitons.It is first proved that the criterion of type II band alignment for achieving charge transfer excitons is invalid in heterostructure quantum dot molecules with strong quantum confinement effects,resulting in localized Frenkel excitons.Due to the important role of excitonic effects in the strong confinement system of quantum dots,different properties of excitonic states cause energy reversal.Secondly,twist is introduced into the external fusion quantum dots,and it is demonstrated that charge transfer excitons can be achieved in heterostructure quantum dot molecules with type II and quasi-type II band alignments,and even in homogenous quantum dot molecules,through the relative orientation of quantum dots.Twist plays a dual role in changing the orbital coupling between quantum dots and modulating the ground state orbital wave function.This work innovatively extends twist electronics to zero-dimensional nanosystems,providing a new means of manipulating the optical properties of quantum dots and related molecular systems.(2)A strategy is proposed to construct an organic-inorganic interface and achieve charge transfer excitons through the regulation of quantum dot surface ligands.The II-type staggered energy levels at the nanoscale quantum ligand-quantum dot interface may not necessarily form charge transfer excitons due to the strong excitonic effect.The introduction of ligand molecules can contribute to the highest occupied molecular orbital,changing the overall energy distribution of the quantum ligand-quantum dot and forming a typical electron-hole-separated charge transfer exciton.Furthermore,the characteristics of the charge transfer exciton can still be maintained as the number of ligand molecules increases.The structure of the ligand molecule is diverse,and the introduction of electron-withdrawing and donating groups through ligand exchange can change the structure of the ligand molecule,thereby changing the energy level distribution and promoting the formation of charge transfer excitons.Through the study of the regulation of exciton properties in quantum dots,this thesis verifies the pathways for realizing charge transfer excitons in pure inorganic systems and ligand-quantum dot systems in quantum dots,respectively,and provides favorable guidance for the experimental preparation of charge transfer excitons with favorable energy.