Exciton Geometry and Dynamics in Low-dimensional Silicon Structures

Description

When the dimension of a silicon (Si) structure shrinks to the exciton Bohr radius(4.9nm) of bulk Si, the quantum confinement effect becomes significant, facilitatingmany interesting and manipulatable physical properties and paving the way for multipleelectronic and optoelectronic applications of the material. These low-dimensional Sistructures, including quantum dots (0D SiQDs), nanowires (1D SiNWs), and nanosheets(2D SiNSs), share some common properties and functionalities but present different andinteresting dimensionality effects. Our recent studies show that the exciton in smallSiQDs and SiNSs can be localized at an individual bond, but can also be delocalized inSiNWs. A closer study of these details of all the above mentioned nanostructures,particularly in terms of their exciton geometry and dynamics, is strategically importantfor their emerging applications.In this project, we will systematically investigate the geometry and dynamics of variouspossible excitons of these Si nanostructures with different dimensions, sizes, boundariesand symmetries. Using time-dependent density functional theory and its tight bindingapproximation, we will perform excited-state molecular dynamics (MD) simulations forthe Si nanostructures mentioned above. We will explore the formation, trapping, andmigration of the exciton in order to understand these ultrafast and dynamic processes.We will also identify the atoms and bonds affected by the excitations and observe thetime evolution of the charge density, such as the migration of the excited electron fromits initial position to neighboring bonds as well as its localization. The simulation will beconducted at different temperatures including 50K, 100K, and 300K in the MDsimulations, in order to examine the phonon effect. These exciton processes may takeplace at a sub picosecond scale, suitable for the quantum mechanical level simulations.We expect our simulations to reveal the interplay between the material geometricproperties (relating to size, boundary, symmetry, defect, passivant and adsorbent) andexciton geometry and dynamics, including the localization and trapping of the exciton aswell as the possible transition between the Frenkel and Wannier-Mott types in thesehighly confined environments. We aim to provide important insights into photoinduceddynamic phenomena and their crucial implications for modulating the exciton transportefficiency in strongly confined low-dimensional systems. The study can advance ourunderstanding of the structure-dependent optoelectronic properties of Si nanostructuresand guide their applications in nanoscale optoelectronic devices.