  Monolayer group VI transition metal dichalcogenides have recently emerged as
semiconducting alternatives to graphene in which the true two-dimensionality
(2D) is expected to illuminate new semiconducting physics. Here we investigate
excitons and trions (their singly charged counterparts) which have thus far
been challenging to generate and control in the ultimate 2D limit. Utilizing
high quality monolayer molybdenum diselenide (MoSe2), we report the unambiguous
observation and electrostatic tunability of charging effects in positively
charged (X+), neutral (Xo), and negatively charged (X-) excitons in field
effect transistors via photoluminescence. The trion charging energy is large
(30 meV), enhanced by strong confinement and heavy effective masses, while the
linewidth is narrow (5 meV) at temperatures below 55 K. This is greater
spectral contrast than in any known quasi-2D system. We also find the charging
energies for X+ and X- to be nearly identical implying the same effective mass
for electrons and holes.

Mesoscale and Nanoscale Physics