2D PIEZOTRONICS AND SPIN-ORBITRONICS IN SELECTED MATERIALS: A DFT PERSPECTIVE

Abstract

This thesis encompasses a range of advanced research domains, spanning nanoelectromechanical energy conversion (piezoelectricity), spintronics, valleytronics, and charge-spin interconversion, all to enable futuristic self-powered multifunctional miniaturized device applications. Employing a theoretical microscope, namely, first-principles-based density functional theory (DFT) calculations, this thesis delves into the intricate mechanisms underlying these physical phenomena. The first research endeavor explores nanoscale piezoelectricity, and charge carrier mobility in Janus MXY monolayers (M = Ge, Sn and X/Y = S/Se, X ≠ Y), derived from the parent 1T phase of Group-IV(A) di-chalcogenide pristine MX2 monolayers. Breaking inversion symmetry, Janus MXY monolayers induce notable in-plane (𝑑22) and shear (𝑑15) piezoelectric coefficients in GeSSe (SnSSe) monolayer. High flexibility (Young's modulus 60-70 N/m) allows large-scale strain engineering, with a 7% uniaxial tensile strain increasing 𝑑22 and 𝑑15 dramatically. Stacking Janus GeSSe monolayers into a van der Waals homo-bilayer further enhances piezoelectric properties. Moreover, the Janus GeSSe monolayer exhibits higher hole mobility (µℎ) than electron mobility (µ𝑒), enhancing charge carrier lifetime. These properties make Janus GeSSe monolayers and bilayers promising for energy harvesting, nanopiezotronic devices, sensors, actuators, transducers, etc. [Nanoscale 13, 5460 (2021)] [1] The second research work in the thesis examines the origin of negative in-plane and positive out of-plane piezoelectric coefficients in novel SnNBr systems, providing atomic-scale insights often overlooked in previous studies. It reveals the all-round stability, direct bandgap nature, high carrier mobility, and durable mechanical stability of non-toxic 2D SnNBr. The research explains a positive clamped-ion contribution to piezoelectricity, differing from typical bulk materials. The study also highlights the tunability of these commendable physical properties with varying layer numbers, enhancing their potential for device applications. This versatility underscores the importance of experimentally synthesizing 2D SnNBr for advanced piezotronic devices. [J. Phys. Chem. C 127, 23551 (2023)] [2] The third research endeavor investigates charge-spin conversion and spin density dynamics in non-magnetic SnNBr monolayers under time-reversal invariance. The absence of an out-of-plane mirror and inversion symmetry and the presence of spin-orbit coupling (SOC) induce a notable effect. Theoretical calculations reveal significant Rashba spin splitting and intrinsic spin Hall conductivity, with modulation through in-plane biaxial strains. The Rashba parameter varies with changes in the built-in out-of-plane electric field. These findings highlight the potential of the non-centrosymmetric SnNBr monolayers for advanced spintronics, spin-orbitronics, and piezo spintronic applications, encouraging further experimental research. [J. Appl. Phys. 135, 234302 (2024)] [3] The fourth research work explores spin-orbit coupling and nanoscale piezoelectricity in Janus MXSiN2 (M = Cr/Mo/W; X = S/Se/Te) monolayers, hybrids of transition metal dichalcogenides and MSi2N4 monolayers. These Janus monolayers exhibit concurrent vertical and horizontal electric polarization, leading to in-plane and out-of-plane polarized spins. Breaking out-of-plane mirror and inversion symmetry, coupled with spin-orbit coupling, results in significant Rashba spin splitting (0.01 - 0.62 eV Å), Zeeman-type valley spin splitting (4 - 374 meV), and valley-contrasting Berry curvature (6.22 - 13.30 Ų). The biaxial strain affects spin polarizations through macroscopic charge distribution and atomic orbitals. Further, the study identifies suitable Rashba descriptors. Moreover, Janus MXSiN2 monolayers exhibit moderate in-plane and out-of-plane piezoelectricity, thereby highlighting the potential for spintronics, valleytronics, piezotronics, and self-powered piezo-spintronic devices. [Manuscript under revision] [4] The thesis emphasizes the vital role of sustainable energy harvesting and the advancement of next-generation nanoelectronic devices by harnessing the various degrees of freedom, including charge, spin, and valley, and through the tailored engineering of 2D materials. This approach provides useful pointers to experimentalists and technologists, and aims to pave the way for innovative solutions in energy and nanoelectronics.