Electronic Structure and Nonlinear Optical Properties of Hexagonal Transition-Metal Dichalcogenides Quantum Dots for Nanophotonic Device Applications

With the increasing demand for miniaturized optoelectronic devices, seeking materials with ultrasmall size and high performance are of crucial significance in nanophotonic fields. The electronic structures and nonlinear optical (NLO) response of four transition-metal disulfide (TMD) quantum dots (QDs), labeled MX2 (M = Mo, W; X = S, Se), have been explored by using the quantum chemistry method in the present work. The X atoms play an important role in determining the highest occupied molecular orbital (HOMO) distributions and the energy gap between the HOMO and the lowest unoccupied molecular orbital (LUMO). For the electron transitions corresponding to the maximum absorption, the M atoms along the edge always take the role of “electrons”. The static first hyperpolarizabilities mainly occur in the plane of the hexagonal frameworks, and the X atoms are more decisive for the magnitude of the total values. The dynamic first hyperpolarizabilities under different external fields (λ = 880 nm, 1080 nm, 1280 nm, and 1480 nm) point out that MoSe2 is more desirable for NLO applications, with the largest value 225960.30 × 10–30 esu under 880 nm, revealing a strong resonance at 880 nm. For the other three TMD QDs, the largest value is 2708.96 × 10–30 esu (1280 nm), 1456.60 × 10–30 esu (1280 nm), and 4863.70 × 10–30 esu (880 nm) for MoS2, WS2, and WSe2, respectively. In order to give a deeper insight into the first hyperpolarizabilities, simplified sum-overstates analysis, the unit sphere representation method, and the first hyperpolarizability density analysis have also been performed. Our work is significant not only in searching extra-small TMD-based NLO materials for nonlinear nanophotonic applications but also to provide a theoretical explanation for the experiment.