INVESTIGATING ELECTRONIC, OPTICAL AND STRUCTURAL PROPERTIES OF MATERIALS/STRUCTURES FOR OPTOELECTRONIC AND PHOTOCATALYTIC APPLICATIONS

Abstract

Perovskite and two-dimensional materials have emerged as a revolutionary class of material in optoelectronics, characterized by their remarkable light-harvesting capacity, tuneable bandgap, and high charge-carrier mobility. These materials are extensively utilized in the fabrication of solar cells, due to high power conversion efficiencies with cost effective and scalable fabrication. Their utilization in photovoltaics and optoelectronic circuits make them a potential candidate for next-generation electronic and photonic applications. The present work researches into modulating the structural, electronic, and optical properties of perovskites (LiNbO₃, SrTiO₃) and two-dimensional materials (SnS, SnSe, ZnS, ZnSe) for optoelectronic applications. This research focuses on three key techniques: doping, strain, and layering for bandgap modulation of the materials and subsequently improving the visible region absorption. The aim is to improve the optical properties by employing techniques doping, strain, and layering and making them suitable for advanced optoelectronic, photovoltaic, and solar applications. This work investigates the electronic and optical properties of the oxide perovskites SrTiO₃ and LiNbO₃ under the influence of doping and strain by utilizing density functional theory (DFT). The pristine LiNbO₃ is a wide bandgap material with bandgap value of 3.56 eV, having optical absorption primarily in the UV region of optical spectrum. The hexagonal unit cell of pristine LiNbO₃ is doped with various metal dopants. Due to the metal doping, the bandgap of LiNbO₃ is reduced in comparison to the pristine cell. Specifically, the doping of plasmonic metal dopants such as gold, silver, aluminum, and copper leads to a significant reduction in the bandgap. The decrease in the bandgap is highest for silver-doped and gold-doped LiNbO₃ with values 2.38 eV and 2.45 eV, respectively. This shift extends optical absorption into the visible spectrum, making it more suitable for optoelectronic applications. Additionally, to investigate the impact of doping on the optical properties, the refractive index and dielectric constant are also calculated for pristine and doped structures. The dielectric constant and refractive index increase upon doping, with silver-doped LiNbO₃ exhibiting the highest enhancement. These findings indicate that metal-doped LiNbO₃ can be a promising material for applications in photovoltaics, photonic, and optoelectronics. Likewise, pristine SrTiO₃, characterized by a wide bandgap of 3.20 eV, predominantly absorbs ultraviolet (UV) light within the 300–400 nm range, which limits its applicability in visible- vii light-driven technologies such as photovoltaics and photocatalysis. To overcome this limitation, metal doping with plasmonic metal dopants such as Ag, Al, Au, and Cu has been employed to modify the bandgap and optical properties of SrTiO₃. Among these dopants, Cu has been found the most effective dopant in reducing the bandgap, lowering it to 2.0 eV, thereby extending its optical absorption into the visible spectrum (380–800 nm). Additionally, Au and Ag doping enhance visible-light absorption through surface plasmon resonance effects, improving the efficiency of light energy conversion into electronic transitions. These doping- induced modifications noticeably improve the optoelectronic performance of SrTiO₃, making it a highly promising candidate for applications in photocatalysis, solar cells, and other energy- harvesting technologies. Further, Strain engineering plays a crucial role in modifying the electronic and optical properties of oxide perovskite like SrTiO₃ and LiNbO₃, enabling their application in optoelectronics and photovoltaics. The application of biaxial tensile and compressive strains on pristine SrTiO₃ significantly enhances its optical absorption by shifting the absorption spectrum into the visible range. Under 20% compressive strain, SrTiO₃ exhibited absorption peaks in the visible spectrum (380–800 nm), with a significant redshift and increased absorption intensity. Similarly, pristine LiNbO₃, with an intrinsic bandgap of 3.56 eV and UV- only absorption, showed remarkable improvements under strain. Application of 20% tensile strain shift its absorption into the visible range (400–800 nm), with a peak absorption coefficient of ~500 k at 600 nm. These structural modifications also resulted in enhancement in dielectric constants and refractive indices, improving light propagation and optical performance. Comparatively, unstrained SrTiO₃ and LiNbO₃ exhibited negligible absorption in the visible spectrum, underscoring the transformative impact of strain on their optical properties. These enhancements demonstrate that strain engineering not only narrows the bandgap but also expands the utility of these materials to better utilize solar energy, paving the way for advanced applications in energy and photonics. Furthermore, the coexistence of strain and doping demonstrated an even greater effect on the optical performance of these materials. In SrTiO₃, doping with metals such as copper (Cu), silver (Ag), aluminum (Al), and gold (Au) introduced impurity bands that significantly reduced the bandgap. For instance, Cu-doped SrTiO₃ under 20% compressive strain exhibited absorption peaks throughout the visible spectrum, with an absorption coefficient of ~650 k. Similarly, for LiNbO₃, metal doping combined with strain yielded extraordinary improvements. viii Au-doped LiNbO₃, with an initial bandgap of 1.36 eV (near the Shockley-Queisser limit), exhibited a further redshift when subjected to 20% tensile strain, enhancing visible light absorption to ~550 k. Ag-doped LiNbO₃ under 20% compressive strain also showed peak absorption values of ~650 k in the visible range, demonstrating comparable optical performance to Cu-doped structures. Additionally, Cu-doped LiNbO₃, under both compressive and tensile strains, exhibited a peak absorption intensity of ~500 k, making it another strong candidate for optoelectronic applications. These enhancements, attributed to the synergistic effects of strain and doping, position SrTiO₃ and LiNbO₃ as leading materials for photovoltaic and optoelectronic technologies. Additionally, the two-dimensional materials, pristine monolayers of SnS, SnSe, ZnS, and ZnSe exhibit bandgaps of 1.70 eV, 1.46 eV, 2.35 eV, and 1.46 eV, respectively. The SnS and SnSe, with their relatively low bandgaps, absorb photons in the 350–600 nm range (part of the visible spectrum), while ZnS and ZnSe, with their wide bandgaps, absorb primarily in the UV range (100–300 nm) with zero visible-light absorption. The application of strain modifies their bandgaps and optical properties, enabling significant redshifts in absorption. The tensile strain of 10% reduces the bandgap of ZnSe from 1.46 eV to 0.58 eV, shifting absorption entirely into the visible region (400–700 nm). Similarly, ZnS exhibits a bandgap reduction from 2.35 eV to 1.35 eV under 10% tensile strain, with absorption extending into the visible spectrum. Strain also improves optical parameters such as the refractive index and dielectric constant, enabling efficient photon interaction and energy absorption in the visible range. These results highlight strain engineering as a versatile tool for tailoring the optoelectronic properties of 2D materials. In this work, the formation of van der Waals heterostructures, such as SnS/SnSe, SnS/ZnS, and SnS/ZnSe, provides an additional strategy for optimizing optical absorption. For instance, combining SnS (bandgap: 1.70 eV) with SnSe (bandgap: 1.46 eV) results in a heterostructure with a reduced bandgap of 1.04 eV, shifting absorption peaks into the visible region. Similarly, SnS/ZnS and SnS/ZnSe heterostructures exhibit bandgaps of 0.6 eV and 0.9 eV, respectively, enabling complete absorption within the visible spectrum (350–650 nm). The heterostructures also demonstrate enhanced and broad optical absorption compared to individual monolayers, owing to synergistic effects from combining the properties of the constituent layers. The peaks of energy-dependent absorption coefficients for heterostructures are concentrated in the 1.0– 4.0 eV range, indicating superior absorption performance in the visible region. Furthermore, ix these heterostructures exhibit improvements in refractive index and dielectric constant, essential for optoelectronic device performance. In summary, this research work demonstrates the transformative potential of metal doping, strain engineering, and heterostructure formation in tailoring the electronic and optical properties of perovskites and 2D materials. The oxide perovskite is doped with the metal dopants and further strain is applied to tune the optical properties for optoelectronic applications. In monolayers of 2D materials the layering and strain is utilized to modulate the electronic and optical properties. The reduction in bandgap and redshifts in optical absorption induced by these modifications enable efficient visible-light absorption and energy conversion, making these materials suitable for advanced applications in photovoltaics, photodetectors, and optoelectronic devices. These findings pave the way for next-generation materials with precisely tuned properties to meet diverse technological needs.