  We study perfect valley polarization in a molybdenum disulfide MoS2
nanoribbon monolayer using two bands Hamiltonian model and non-equilibrium
Green's function method. The device consists of a one-dimensional quantum wire
of MoS2 monolayer sandwiched between two zigzag MoS2 nanoribbons such that the
sites A and B of the honeycomb lattice are constructed by the molecular orbital
of Mo atoms, only. Spin-valley coupling is seen in energy dispersion curve due
to the inversion asymmetry and time-reversal symmetry. Although, the time
reversal symmetry is broken by applying an external magnetic field, the valley
polarization is very small. A valley polarization equal to 46% can be achieved
using an exchange field of 0.13 eV. It is shown that a particular spin-valley
combination with perfect valley polarization can be selected based on a given
set of exchange field and gate voltage as input parameters. Therefore, the
valley polarization can be detected by detecting the spin degree of freedom.
