Resumen de Optoelectronic Properties of Excitonic and Biexcitonic Complexes in Metal Chalcogenide Semiconductor Nanoplatelets
David Francisco Macias Pinilla
Among the big family of colloidal nanocrystals (NCs), quasi-two-dimensional NCs -usually known as nanoplatelets (NPLs)- are of particular interest for their unique properties. NPLs have a precisely controlled thickness of a few atomic monolayers (MLs), and lateral dimensions ranging from hundreds of nm to few nms, where quantum confinement is already felt. Additionally, NPLs are usually surrounded by organic ligands with weak polarizability, thus exerting a strong dielectric confinement that, together with the quantum confinement, serves to modulate the optical properties. These characteristics make NPLs present short fluorescence lifetimes (5-11 ns), large exciton binding energy (up to 300 meV), extremely narrow emission (∼ 40 meV) and lower rate of Auger recombination. All in all, NPLs have potential optoelectronic applications as bright LEDs with high color purity, low-threshold of amplified spontaneous emission and lasing.
Even though NPLs are studied for many types of materials and architectures, core-only CdSe and PbS NPLs have proven to be stable, uniform and with a relatively high photoluminescence quantum yield, making them interesting systems for direct applications. Consequently, the main objective of this PhD work is to study at a theoretical level the optoelectronic properties of excitonic and biexcitonic complexes in semiconductor core-only PbS and CdSe NPLs. To this end, we shall analyze the electronic structure of exciton complexes confined inside, and the resulting optical spectrum. The aforementioned dielectric confinement, together with the quasi-two-dimensional geometry of NPLs, turns this system into one of strong electronic correlations, which poses a methodological challenge for the study of the electronic structure. We address through density functional theory (DFT) and k·p theory, coupled to variational and variational quantum Monte Carlo (VQMC) methods, some of the open questions that currently exist about the electronic nature of these NPLs. We give a description of these methods together with the codes developed, which are freely accessible to the scientific community.
This work is divided into the following specific sections:
1. We analyze the orthorhombic modification of the rock-salt structure evidenced in PbS NPLs. Through DFT calculations and k·p Hamiltonians coupled to variational methods to account for Coulomb interactions, we obtain the electronic bands and the effects of dielectric and quantum confinement on the exciton ground state. We associate the low luminescence reported for these NPLs to the indirect gap character calculated by DFT. From k·p calculations, we infer that confinement directions other than the shortest orthorhombic crystallographic axis could enhance the luminescence. We obtain strong excitonic effects with high binding energies, similar to previous calculations in NPLs with rock-salt structure.
2. We study the ground state electronic properties of trions in terms of emission energy, binding energy and oscillator strength in CdSe and PbS NPLs. With the image charge method and the VQMC included in effective mass Hamiltonians, we study the dependency of the trion optical properties on lateral and dielectric confinement. We derive the trion oscillator strength behaviour and calculate the positive and negative trion emission to compare to the neutral exciton. Finally, we contrast our results with previous theoretical and experimental work.
3. We use k·p theory coupled to VQMC model to be the first work to study biexcitonic properties in CdSe NPLs. We obtain an stable biexciton with tetrahedral geometry and show the effects of quantum confinement, dielectric confinement, relative electron-hole masses, and Coulomb correlations on the biexcion optical properties. We calculate the biexciton/exciton binding energy ratio (EXXb/EXb) and compare it with values expected from earlier worksdeveloped for epitaxial quantum wells (QWs). Finally, we estimate biexciton radiative lifetimes.
By drawing connections between structural conditions of NPLs and the electronic structure of exciton complexes, this PhD determines a number of influential factors determining the optical properties of NPLs, and provides guidelines for future theoretical and experimental studies regarding strong electronic correlations and multiexcitonic behavior in these nanostructures.
