SELECTED MULTIFUNCTIONAL 2D MATERIALS FOR ENERGY CONVERSION AND DEVICES: PLANE WAVE DFT-BASED APPROACHES

Abstract

This thesis delves into a diverse array of advanced research fields, ranging from energy conversion to futuristic electronic devices, by harnessing the novel properties and various electron degrees of freedom in atomically thin two-dimensional (2D) semiconductors and van der Waals heterostructures. Utilizing first-principles density functional theory (DFT), the thesis provides atomistic insights into the electronic, mechanical, piezoelectric, carrier mobility, spintronic, valleytronic, ferroelectric, and ferromagnetic properties of selected 2D materials. The initial part of the thesis explores the relatively unexplored realm of nanoscale negative piezoelectricity in the dialkali metal monochalcogenide family, 𝑀₂𝑋 (𝑀 = Na, K, Rb, or Cs; 𝑋 = O, S, Se, or Te) monolayers and their van der Waals (vdW) heterostructures 1 . The vdW heterostructures derived from these semiconducting monolayers exhibit an anomalous negative out-of-plane piezoelectricity, with Na₂Te/Cs₂S heterostructure demonstrating the highest negative piezoelectric coefficient (𝑑₃₃ = −39 pmV⁻¹). Additionally, the heterostructure is highly responsive to external stimuli, such as electric field. A vertical electric field causes the band gap to gradually narrow and close, leading to a semiconducting-to-metallic transition at low electric fields of 0.31 and −0.33 V/Å. Consequently, low-power data storage devices could be efficiently realized through the application of small gate voltages. And, the pronounced negative piezoelectric properties suggest strong potential for applications in piezotronic energy harvesting devices and advanced electronic technologies. The subsequent part emphasizes the importance of accurately determining carrier mobility to evaluate the performance of electronic devices, focuses on the transport properties of highly anisotropic, semiconducting 2D transition metal trichalcogenide (TMTC) monolayers, 𝑀𝑋 3 (𝑀 = Ti, Zr or Hf; 𝑋 = S or Se) 2 . The integration of the Boltzmann Transport Equation (BTE) with DFT markedly enhances the precision of electron carrier mobility predictions for the TiS₃ monolayer (10 3 cm²/V·s), in stark contrast to the mobility (10 4 cm²/V·s) determined by the effective mass approximation in previous studies. This comprehensive study underscores the necessity of employing the DFT-BTE methodology to accurately assess carrier mobility in monolayers and advance the understanding of TMTC monolayers. vBeyond charge degrees of freedom, the thesis delves into the intricate manipulation of spin and valley degrees of freedom for information storage and processing within the domains of spintronics and valleytronics. Utilizing high-throughput DFT calculations, the work reveals a pronounced coupling between Rashba and valley spin splitting across a broad array of 2D monolayers. The Rashba spin splitting is primarily influenced by key physical parameters, including the anti-crossing of Rashba-split bands and the Born effective charge (Z*). Furthermore, the coupling between the Rashba and valley spin splitting is facilitated by both out-of-plane and in-plane orbital contributions at critical K points within the band spectra. This in-depth study significantly aids in the identification of monolayers with substantial spin splitting and spin polarization, thereby contributing to the design and optimization of high- performance 2D materials 3 . The last part presents the electrical manipulation of magnetic spin ordering, enabling low- power electrical writing and non-destructive optical reading for a spin-constrained photoelectric memory device. By employing ferroelectric switching in the GeS monolayer, the study induces transitions in the ground state configuration, shifting from ferromagnetic to antiferromagnetic orderings within the NiCl₂ magnetic layer 4 . This transition results in light- induced charge transfer that generates either spin-polarized (―1‖) or unpolarized current (―0‖). The research highlights NiCl₂/GeS heterostructure as a promising candidate for spin- dependent photoelectric memory, enabling the integration of memory and processing in a single device using layered multiferroic heterostructures. This thesis advances the field of next-generation electronics by harnessing the unique properties of 2D materials and van der Waals heterostructures. It provides critical insights into their diverse attributes and potential applications, from energy conversion to spintronics and memory devices. Ultimately, it paves the way for innovative technologies by integrating multifunctional capabilities into a single device.