Monolayer transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2), tungsten diselenide (WSe2) and tungsten disulfide (WS2), have shown in the last years remarkable physical properties. These direct badgap three-atoms thick monolayers, with a broken inversion symmetry, present a unique coupling between the spin and valley degrees of freedom originated from the relativistic spin¿orbit interaction. Together with their mechanical flexibility, result in materials promising for flexible, transparent and low power electronic, optoelectronic and spin/valleytronic applications. In this thesis, we investigate optoelectronic and spin/valleytronic devices based on monolayer MoS2, the most studied monolayer from the TMD familly, with 1.9 eV direct band gap and 6.5 Å thick. We show a monolayer MoS2 photodetector with 100,000-fold improved photoresponsivity from previous monolayer MoS2 phototransistors, first time reported entire junction light emitting diodes (LED) and solar cells based on monolayer MoS2/p-type silicon heterojunctions, which can work as an avalanche photodiodes too, and first spin-valley tunable light emitting diode made with monolayer MoS2/monolayer WSe2. Utilizing high-quality monolayer MoS2, we achieved a broad spectral range phototransistor with photoresponsivity of 880 A/W and low noise equivalent power (NEP) of 1.8 x 10^-15 W/Hz^1/2. Afterwards, we used the two-dimensional (2D) n-type monolayer MoS2 combined with three-dimensional (3D) p-type silicon to build vertical p-n junctions. The entire junction area of our 2D/3D heterostructures emitted light with a low emission threshold power density of 3.2 W/cm^2 and spectrum related to the direct band gap of monolayer MoS2. The heterojunction diode could operate as a solar cell with an external quantum efficiency (EQE) of 4.4% and a broad spectral response. With the use of large area chemical vapor deposition (CVD) grown monolayer MoS2, we scaled up the manufacturing process and showed the capability of these heterostructures to work as avalanche photodiodes (APD) with a multiplication exceeding 1000 for -10 V. Finally, we combined monolayer MoS2 with WSe2 or WS2 and made 2D/2D heterojunctions able to emit light with different characteristic spectrums related to the type of heterojunction. Monolayer WS2/monolayer MoS2 showed one of the emission peaks in the green region of the visible spectrum while all heterojunctions showed peaks in the red region. Spin-polarized charge carriers were injected across the Schottky barrier between a ferromagnetic electrode and the monolayer WSe2 of a monolayer WSe2/monolayer MoS2 heterostructure, resulting in valley polarization due to spin-valley locking. The degree of spin/valley polarization was controlled by a magnetic field between a polarization of ± 20%. A slope of 0.47 ± 0.08 meV/T was also seen related to the valley Zeeman effect.