In the development of next-generation electronic devices and photovoltaic (PV) solar cells to replace the conventional silicon technology, it is necessary that these devices are lightweight, economical and highly efficient. Novel nano-scale materials play a vital role in this technological transition. Particularly, low-dimensional semiconductors are of intense research interests owing to their unique electronic and optical properties. The reduced dimensionality compared with bulk materials results in lower dielectric screening and strong electron-electron interactions. Importantly, given the devices typically operate at room temperature, the scattering of electrons by phonons and phonon-assisted optical transitions are significant factors to be considered and controlled. In this dissertation, we apply first-principles simulations to understand and quantify exciton-phonon interactions in low-dimensional materials. For organic one-dimensional (1D) and inorganic monolayer two-dimensional (2D) materials, we utilize density functional theory (DFT) and many-body perturbation theory (MBPT) to describe the electronic properties, and a combination of MBPT and recently developed special displacement method (SDM) to describe optical properties under electron-phonon interactions. For both classes of materials, electron-phonon interactions are expected to be significant but are not well-understood.
For a 1D π-stacked array of perylene diimide molecules, we demonstrate that phonon-assisted transitions lower the optical gap by 0.5 eV but do not result in significant modification of the exciton wavefunction, indicating that intermolecular coupling survives the presence of phonons. Hence, we expect that electron mobility and exciton delocalization will be maintained at room temperature. For 2D germanium selenide (GeSe), we explore the phonon-induced renormalization of the exciton wavefunctions, excitation energies, and oscillator strengths. We determine that the onset of optical absorption is red-shifted by 0.1 eV due to phonon-induced renormalization and that the 2D Wannier exciton is distorted but not localized. Both optical and acoustic phonons were found to couple to the excited-state, with the strongest coupling with optical phonons at ~ 100 cm−1 and indication of phonon-assisted inter-valley scattering of electrons. We also determined that the exciton-phonon coupling is similar between the bulk and the monolayer. Based on the lessons learned in this study, we have initiated a high-throughput framework to study electron-phonon couplings for a series of 2D materials extracted from a recently-developed database.
In summary, by applying state-of-the-art first-principles theory, we extract fundamental physical properties related to electron-phonnon interactions. Such an understanding will allow for the design of materials with tailored properties for new nanoelectronic and optical devices.
| Identifer | oai:union.ndltd.org:bu.edu/oai:open.bu.edu:2144/45058 |
| Date | 26 August 2022 |
| Creators | Huang, Tianlun |
| Contributors | Sharifzadeh, Sahar |
| Source Sets | Boston University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
| Rights | Attribution 4.0 International, http://creativecommons.org/licenses/by/4.0/ |
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