Two-dimensional MoS2 2H, 1T, and 1T' crystalline phases with incorporated adatoms: theoretical investigation of electronic and optical properties

Defect dependent electronic properties of two-dimensional transition metal dichalcogenides (2H, 1T, and 1T' phases)

Transition metal dichalcogenides (TMDs) exhibit a wide range of electronic properties due to their structural diversity. Understanding their defect-dependent properties might enable the design of efficient, bright, and long-lifetime quantum emitters. Here, we use density functional theory (DFT) calculations to investigate the 2H, 1T, and 1T' phases of MoS2, WS2, MoSe2, WSe2 and the effect of defect densities on the electronic band structures, focusing on the influence of chalcogen vacancies. The 2H phase, which is thermodynamically stable, is a direct band gap semiconductor, while the 1T phase, despite its higher formation energy, exhibits metallic behavior. 1T phases with spin-orbit coupling show significant band inversions of 0.61, 0.77, 0.24 and 0.78 eV for MoS2, MoSe2, WS2 and WSe2, respectively. We discovered that for all four MX2 systems, the energy difference between 2H, 1T and 1T phases decreases with increasing concentration of vacancies (from 3.13% to 21.88%). Our findings show that the 2H phase also has minimum energy values depending on vacancies. TMDs containing W were found to have a wider bandgap compared to those containing Mo. The band gap of 2H WS2 decreased from 1.81 eV (1.54 eV with SOC included) under GGA calculations to a range of 1.37 eV to 0.79 eV, while the band gap of 2H MoSe2 reduced from 1.43 eV (1.31 eV with SOC) under GGA to a range of 0.98 eV to 0.06 eV, depending on the concentration. Our findings provide guidelines for experimental screening of 2D TMD defects, paving the way for the development of next-generation spintronic, electronic, and optoelectronic devices.

Exfoliation procedure-dependent optical properties of solution deposited MoS2 films

The development of high-precision large-area optical coatings and devices comprising low-dimensional materials hinges on scalable solution-based manufacturability with control over exfoliation procedure-dependent effects. As such, it is critical to understand the influence of technique-induced transition metal dichalcogenide (TMDC) optical properties that impact the design, performance, and integration of advanced optical coatings and devices. Here, we examine the optical properties of semiconducting MoS2 films from the exfoliation formulations of four prominent approaches: solvent-mediated exfoliation, chemical exfoliation with phase reconversion, redox exfoliation, and native redox exfoliation. The resulting MoS2 films exhibit distinct refractive indices (n), extinction coefficients (k), dielectric functions (ε1 and ε2), and absorption coefficients (α). For example, a large index contrast of Δn ≈ 2.3 is observed. These exfoliation procedures and related chemistries produce different exfoliated flake dimensions, chemical impurities, carrier doping, and lattice strain that influence the resulting optical properties. First-principles calculations further confirm the impact of lattice defects and doping characteristics on MoS2 optical properties. Overall, incomplete phase reconfiguration (from 1T to mixed crystalline 2H and amorphous phases), lattice vacancies, intraflake strain, and Mo oxidation largely contribute to the observed differences in the reported MoS2 optical properties. These findings highlight the need for controlled technique-induced effects as well as the opportunity for continued development of, and improvement to, liquid phase exfoliation methodologies. Such chemical and processing-induced effects present compelling routes to engineer exfoliated TMDC optical properties toward the development of next-generation high-performance mirrors, narrow bandpass filters, and wavelength-tailored absorbers.

Floquet engineering of tilted and gapped Dirac bandstructure in 1T[Formula: see text]-MoS[Formula: see text]

We have developed a rigorous theoretical formalism for Floquet engineering, investigating, and subsequently tailoring most crucial electronic properties of 1T[Formula: see text]-MoS[Formula: see text] by applying an external high-frequency dressing field within the off-resonance regime. It was recently demonstrated that monolayer semiconducting 1T[Formula: see text]-MoS[Formula: see text] exhibits tunable and gapped spin- and valley-polarized tilted Dirac bands. The electron-photon dressed states depend strongly on the polarization of the applied irradiation and reflect a full complexity of the low-energy Hamiltonian for non-irradiated material. We have calculated and analyzed the properties of the electron dressed states corresponding to linear and circular polarization of a dressing field by focusing on their symmetry, anisotropy, tilting, direct and indirect band gaps. Circularly polarized dressing field is known for transition into a new electronic state with broken time-reversal symmetry and a non-zero Chern number, and therefore, the combination of these topologically non-trivial phases and transitions between them could reveal some truly unique and previously unknown phenomena and applications. We have also computed and discussed the density of states for various types of 1T[Formula: see text]-MoS[Formula: see text] materials and its modification in the presence of a dressing field.