  Two-dimensional (2D) atomic crystals, such as graphene and transition-metal
dichalcogenides, have emerged as a new class of materials with remarkable
physical properties. In contrast to graphene, monolayer MoS2 is a
non-centrosymmetric material with a direct energy gap. Strong
photoluminescence, a current on-off ratio exceeding 10^8 in field-effect
transistors, and efficient valley and spin control by optical helicity have
recently been demonstrated in this material. Here we report the spectroscopic
identification in doped monolayer MoS2 of tightly bound negative trions, a
quasi-particle composed of two electrons and a hole. These quasi-particles,
which can be created with valley and spin polarized holes, have no analogue in
other semiconducting materials. They also possess a large binding energy (~ 20
meV), rendering them significant even at room temperature. Our results open up
new avenues both for fundamental studies of many-body interactions and for
opto-electronic and valleytronic applications in 2D atomic crystals.
