First-principles Based Investigation of the Adsorption and Optoelectronic Properties of Polymorphic Borophene

Abstract

The discovery of graphene has prompted interest in other two-dimensional materials, especially borophene, which is a single layer of boron atoms that forms a variety of crystal shapes. Borophene, the lightest member of the 2D material family, stands out due to its exceptional polymorphism, which has a wide range of applications, including energy storage and transistors. In this thesis, we adopt a first-principles atomistic modeling technique to examine surface adsorption and optoelectronic properties in various borophene polymorphs. We begin by investigating the adsorption of alkali ions on borophene battery electrode. The hereditary polymorphism of borophene has obscured the alkali-ion adsorption mechanisms. Previous computational investigations have relied on the uniform-adsorption model, which fails to reflect the adsorption-induced phase transition in borophene, resulting in unrealistic specific capacity estimates. Here we use global-minima-search strategies to get atomistic insight into the polymorphism-driven sodium- and magnesium-ion binding process. Our well-designed computational strategy employs two distinct search techniques and exposes several nonidealities (e.g., bond cleavage, electroplating, phase transition, etc.). In contrast with previous studies, our work indicates borophene to be an exceptional contender for Mg-ion batteries, but not so promising for Na-ion storage due to comparatively high formation energies. Next, we focus on 8-Pmmn borophene, which is notable due to its tilted-Dirac fermions. The properties of interfaces between 8-Pmmn and metal substrates have yet to be investigated, despite the vital importance of their application in electronic devices. We demonstrate that when 8-Pmmn borophene is interfaced with common electrode materials such as Au, Ag, or Ti, the distinctive tilted-Dirac feature is totally lost. This is due to the high chemical reactivity of borophene, as determined by crystal orbital Hamilton population and electron localization function study. In order to recover the Dirac property, we insert a graphene/hexagonal-boron-nitride (hBN) layer between 8-Pmmn and metal, a strategy used in previous experiments with other 2D materials. We demonstrate that whereas the insertion of graphene successfully recovers the Dirac nature for all three metals, hBN fails to do so when interface with Ti. It is, remarkable to note that all the synthesized polymorphs of borophene are found to be metallic although bulk boron is semiconducting in nature. In the third work, we study the optoelectronic properties of clustered-P1, a semiconducting phase of borophene, recently discovered by a genetic-algorithm based structure-search technique. Our results obtained through extensive ab-initio calculations of many-body interactions indicate that this phase can exhibit an optical gap of 0.74 eV and has a substantial binding energy of 1.5 eV at room temperature It also exhibits an impressive zero-point renormalization of the quasi-particle direct and indirect gaps, resulting in large shifts of 73 and 91 meV, respectively. The crucial phonon modes that are responsible for the lowest bound exciton are found to be in the spectral region of 350-950 cm-1 with high immunity to temperature fluctuations. The lowest bound exciton's room temperature nonradiative lifetime (~40 fs) is found to be much shorter than its intrinsic radiative lifetime (~0.6 ns). The results obtained from our work on global-minima-search for Na- and Mg- adsorbed polymorphic borophene are reliable as an experimental guideline for the design of Mg-ion based batteries as they are more accurate compared previous works using uniform adsorption model. The effect of interface with metal on the dirac nature has been reported for graphene but not for borophene. The quantum chemical insights presented in our work aid in-to access the Dirac properties of 8-Pmmn borophene in experiments The study of temperature dependent optical processes of clustered-P1 borophene includes the impact of electron-phonon couplings at non-zero temperatures which was explored in very few previous works. It gives us a better estimate of the radiative and non-radiative excitonic lifetimes and a better understanding of the feasibility of this phase of borophene for optoelectronic applications. These findings could provide useful aids for future experimental efforts on different phases of borophene.