REGULATION OF EXCITONIC PROPERTIES IN TWO DIMENSIONAL MATERIALS FOR LIGHT HARVESTING: AN AB-INITIO STUDY

Abstract

In the pursuit of a balanced approach to environmental sustainability and development, the exploration of solar-based energy sources has become essential. Photocatalytic water splitting and solar cells, in particular, have shown significant potential in meeting sustainable development goals. The advent of two-dimensional (2D) materials has further accelerated efforts to harness solar energy, owing to their increased surface-to-volume ratio and enhanced light-matter interactions. However, the reduced dielectric screening in 2D materials leads to a pronounced excitonic effect, unlike in bulk materials where it is minimal. The properties of excitons, especially their binding energy, are crucial in determining a material's suitability as a photocatalyst or in solar cell applications, as the efficiency of these technologies depends on the ease with which excitons can dissociate. In this thesis work, we strategically aim to manipulate the exciton properties, especially their spatial localization and hence the exciton binding energy (EBE). Since excitons in 2D semiconductors necessarily reside near a surface, their fundamental properties (size, binding energy) are expected to be strongly influenced by any additional screening from the dielectric environment surrounding the monolayer. Additionally, the external magnetic field can also potentially influence the excitonic properties. Therefore, we study the effect of a dielectric environment, and magnetic field, and the combined effect on exciton properties for the application of PWS and solar cells. The initial phase of this thesis investigates non-oxide-based 2D monolayers, specifically metal-telluro-halides (XTeI, where X = Ga or In) and metal phosphorous triselenides (MPSe3, where M = Zn or Cd). These materials have valence band maxima that are higher than the O_2p orbitals found in conventional metal oxides like TiO2 and SnO2, making them well suited for visible-light-driven photocatalysis. Their capability to facilitate both water oxidation and hydrogen reduction half-reactions simultaneously stems from the sufficient driving forces provided by the photogenerated electrons and holes in these monolayers. In this study, we demonstrate the tuning of the ground state exciton binding energy through dielectric screening engineering, achieving an order of magnitude reduction in EBE by adjusting the surrounding environment's dielectric constant from ~1 to ~3. v Next, we investigate the excitonic and photocatalytic properties of the metal-telluro-halide GaTeCl/InTeBr heterostructure. In this study, we explore the excited states of excitons and analyze how the surrounding dielectric environment influences these excited states. Inefficient exciton dissociation can also limit the efficiency of photovoltaics. Moving to the next work, we demonstrate the role of the dielectric environment similar to prior works but additionally examine the effect of asymmetric halogenation and magnetic field in 2D Ti2O MOene as an efficient strategy for regulating exciton binding energy towards spontaneous exciton dissociation. We determine the quantitative impact of varying dielectric screening and magnetic field strength on exciton binding energy for different excited states (1s, 2s, 3s, 4s, and so on). We extend the combined effect of the surrounding dielectric screening and magnetic field on interlayer exciton in the metal-telluro-hallide heterostructure.