Jason S Ross, Sanfeng Wu, Hongyi Yu, Nirmal J Ghimire, Aaron M Jones, Grant Aivazian, Jiaqiang Yan, David G. Mandrus, Di Xiao, Wang Yao, Xiaodong Xu
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 play key roles in photonic, optoelectronic, and optically driven quantum devices in solids but 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 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, which provides the first confirmation of their recent description as massive Dirac fermions. Our work demonstrates that monolayer MoSe2 is an ultimate 2D semiconductor opening the door for the investigation of phenomena such as exciton condensation and the Fermi-edge singularity, as well as for a new generation of optoelectronic devices such as light emitting diodes (LEDs) and excitonic circuits.
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http://arxiv.org/abs/1211.0072
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