Dicalcogenetos de metais de transição em poucas camadas: o papel das espécies de calcogênio e suas propriedades não convencionais
Resumen
Two-dimensional (2D) Transition Metal Dichalcogenides (TMDs) constitute the most
studied class of 2D materials after graphene, as they have a variety of applications already
demonstrated in the laboratory, including sensors, nanoelectronics and catalysis. In this
context, simulations based on Density Functional Theory (DFT) allow exploring different
compositions and crystalline phases, searching for the most promising ones for specific
applications. This doctoral thesis presents a systematic investigation of few-layer 2D TMDs
with compositions MQ2, where M belongs to the groups 8 and 10 of the periodic table and
Q=S, Se or Te. We investigate systems that vary between one and six in the number of
layers. Our study focuses on elucidating the structural, energetic and electronic properties
of these materials through DFT-based numerical simulations. The structures are optimized
with the GGA-PBE semi-local exchange-correlation functional, where we included the
D3 van de Waals correction on the forces and total energy. Furthermore, we incorporate
corrections for the self-interaction error inherent in PBE through the HSE06 hybrid
functional and spin-orbit coupling. Our investigation reveals significant variations in lattice
parameters and exfoliation energies with changes in the number of layers, mostly influenced
by chalcogen species. Compositions with transition metals within the same column of
the periodic table exhibit similar lattice parameters for the same choice of chalcogens,
making them suitable for constructing commensurable heterostructures. Furthermore, the
decreasing electronegativity trend from S to Te results in stronger exfoliation energies
due to lower surface charges, thus governing the structural and electronic characteristics
of these materials. We delved deeper into the electronic properties, and found unusual
features in other materials, such as (i) increases in band gap generated by spin-orbit
coupling for certain compositions, (ii) emergence of polarization electric fields due to the
breaking of point inversion symmetry, and (iii) semiconductor-to-metal transitions with
the addition of just one or two sheets to the monolayer. The presence of sulfur on the
surface results in greater variations in the work function (in relation to the Se and Te
terminations) when the number of layers varies, allowing precise adjustment of the work
function and electronic affinity with the number of layers and with the choice of transition
metal species. These findings highlight the unique and versatile nature of TMDs from
groups 8 and 10, motivating diverse applications in nanoelectronics and catalysis, where
tuning properties with the number of layers can be leveraged for technological advances.
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