CONSPECTUS: Nanostructures based on molybdenum disulfide (MoS2) are by far the most common and well-studied systems among two-dimensional (2D) semiconducting materials. Although still being characterized as a "promising material", catalytic activity of MoS2 nanostructures has been found, and applications in lubrication processes are pursued. Because exfoliation techniques have improved over the past years, monolayer MoS2 is easily at hand; thus, experimental studies on its electronic properties and applicability are in scientific focus, and some MoS2-based electronic devices have been reported already. Additionally, the improvement of atomic force microscopy led to nanoindentation experiments, in which the exceptional mechanical properties of MoS2 could be confirmed. In this Account, we review recent results from density-functional based calculations on several MoS2-based nanostructures; we have chosen to follow several experimental routes focusing on different nanostructures and their specific properties. MoS2-based triangular nanoflakes are systems that are experimentally well described and studied with a special focus on their optical absorption. The interpretation of our calculations fits well to the experimental picture: the absorption peaks in the visible light range show a quantum-confinement effect; they originate from excitations into the edge states. Additionally, delocalized metallic-like states are present close to the Fermi level, which do not contribute to photoabsorption in the visible range. Additionally, nanoindentation experiments have been simulated to obtain mechanical properties of the MoS2 material and to study the influence of deformation on the system's electronics. In these molecular-dynamics simulations, a tip penetrates a MoS2 monolayer, and the obtained Young's modulus and breaking stress agree very well with experimentally obtained values. Whereas the structural properties, such as bond lengths and layer contraction, vary locally differently upon indentation, the electronic structure in terms of the density of states, the gap between occupied and unoccupied states, or the quantum transport change only slightly. The robustness of the material with respect to electronic and mechanical properties makes monolayer MoS2 special. However, it is important to note that this robustness refers to a local disturbance through deformation and still seems to be dependent on the defect concentration. Finally, we present a comparison of the thermodynamic stabilities of different MoS2-based nanostructures with a focus on nanoflakes, fullerene-like nanooctahedra, and smaller Chevrel-type and non-Chevrel-type clusters (nanowires). All studied systems are stable in comparison to MoS2, Mo bulk, and the S8 crown, but only the studied nanoflakes and nanowires show specific stoichiometries, either sulfur-rich or sulfur-poor, whereas the nanooctahedra may adopt both. From the thermodynamic stabilities, it should be possible to deliberately choose specific nanostructures by thoughtful choices of the synthesis c...

The results from calculations of optical and electronic properties of triangular MoS2 nanoflakes with edge lengths ranging from 1.6 to 10.4 nm are presented. The optical spectra were calculated using the time-dependent extension of the density-functional tight-binding method (TD-DFTB). The size effect in the optical absorption spectra is clearly visible. With decreasing length of the nanoflakes edges, the long-wavelength absorption in the range of visible light is shifted toward short-wavelength absorption, confirming a quantum-confinement-like behavior of these flakes. In contrast, the edges of the nanoflakes exhibit a distinct metallic-like behavior. The relation of the absorption properties to the observed photoluminescence of MoS2 nanoflakes is discussed in a qualitative manner.

Defects usually play an important role in tuning and modifying various properties of semiconducting or insulating materials. Therefore we study the impact of point and line defects on the electronic structure and optical properties of MoS 2 monolayers using density-functional methods.The different types of defects form electronic states that are spatially localized on the defect. The strongly localized nature is reflected in weak electronic interactions between individual point or line defect and a weak dependence of the defect formation energy on the defect concentration or line defect separation. In the electronic energy spectrum the defect states occur as deep levels in the band gap, as shallow levels very close to the band edges, as well as levels in-between the bulk states. Due to their strongly localized nature, all states of point defects are sharply peaked in energy. Periodic line defects form nearly dispersionless one-dimensional band structures and the related spectral features are also strongly peaked. The electronic structure of the monolayer system is quite robust and it is well preserved for point defect concentrations of up to 6%. The impact of point defects on the optical absorption for concentrations of 1% and below is found to be very small. For higher defect concentrations molybdenum vacancies were found to quench the overall absorption and sulfur defects lead to sharp absorption peaks below the absorption edge of the ideal monolayer. For line defects, we did not find a considerable impact on the absorption spectrum. These results support recent experiments on defective transition metal chalcogenides.