Transition metal dichalcogenide (TMDC) monolayers (MLs) have emerged as a promising 2D crystal family with several characteristic features. Their direct band gap and the strong spin-orbit coupling combined with the lack of inversion symmetry leads to a unique coupling of the spin and valley degrees of freedom. Due to this distinct feature, elastic scattering of charge carriers is possible only by simultaneously flipping the spins of two carriers, and this severe restriction leads to small intervalley scattering rates and therefore long spin and valley lifetimes. These properties make the TMDCs promising candidates for nanoelectronics and optoelectronics applications. It is important to understand the conventional method to obtain stable magnetism in semiconductors TMDC ML. Hence, In the first part of this thesis we have studied a high-Curie-temperature ferromagnetic metal (a cobalt monolayer) on a single WS2 monolayer. . Density functional computations and Monte Carlo simulations are performed to investigate structural, electronic, magnetic, and thermodynamic properties of Co/WS2 nanolayer, a semiconductor WS2 monolayer covered by a ferromagnetic cobalt monolayer. In addition to a conventional semilocal exchange-correlation functional, three nonlocal functional, including the novel ACBN0 scheme, are applied to obtain reliable electronic and magnetic properties. It is argued that the ACBN0 scheme is very efficient for first principles description of the Co/WS2 nanolayer.The obtained electronic structures evidence a trustworthy half-metallic gap in the majority spin channel of the lowest energy configuration of the nanolayer, promising for spintronic applications. The obtained magnetic thermodynamics properties from Monte Carlo simulations predict a Curie temperature of about 110 K, which is far small for device applications of this nanolayer. The electronic and magnetic properties of the system are calculated under various compressive and tensile strains and it is shown that a tensile strain of about 4% may effectively improve thermal stability of half-metallic ferromagnetism in the Co/WS2 nanolayer. The second subject is to understand the nature of the electronic interface between metals and a TMDC monolayer. Hence, in this project, we employ first-principles computations to study the properties of the interface WS2/metal. Slabs of the hexagonally close-packed (hcp) metals Zr and Co are used to model the (0001) surfaces of Zr bulk or Co bulk, which serve as templates to place the hexagonal layer of WS2. WS2 has attracted appreciable interest among TMDC MLs due to the presence of a high valence band splitting and a high mobility. The atomic and electronic structure of single-layer WS2 attached to Zr and Co contacts are determined. We considered Zr and Co as suitable candidates for contact properties. Zr emerges as an ideal candidate with only 1.5% mismatch to WS2. Co appears less suitable at first, due to a large lattice mismatch, but spin-injection from such a high- Curie-temperature ferromagnetic metal to a single TMDC layer is very important since it would give access to TMDC based spintronic devices. We observed that both metals form stable interfaces that are promising as contacts for injection of n-type carriers into the conduction band of WS2 with Schottky barriers of 0.45 eV and 0.62 eV for Zr and Co, respectively. With the help of quantum traort calculations, we address the conductive properties of a freestanding WS2 sheet suspended between two Zr contacts. It is found that such a device behaves like a diode with steep I–V characteristics. Spin-polarized traort is calculated for such a device with a floating-gate Co electrode added. Depending on the geometrical shape of the Co gate and the energy of the carriers in WS2, the transmission of spin majority and minority electrons may differ by up to an order of magnitude. Thus the steep I–V characteristics of the nanoscale device makes it possible to realize a spin filter.