Abstract
Phase transitions of electron–hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. To investigate interfacial dynamical transitions of charged quasiparticles, however, remains a grand challenge. By employing ultrafast mid-infrared microspectroscopic probes to detect excitonic internal quantum transitions and two-dimensional atomic device fabrications, we are able to directly monitor the interplay between free carriers and insulating interlayer excitons between two atomic layers. Our observations reveal unexpected ultrafast formation of tightly bound interlayer excitons between conducting graphene and semiconducting MoSe2. The result suggests carriers in the doped graphene are no longer massless, and an effective mass as small as one percent of free electron mass is sufficient to confine carriers within a 2D hetero space with energy 10 times larger than the room-temperature thermal energy. The interlayer excitons arise within 1 ps. Their formation effectively blocks charge recombination and improves charge separation efficiency for more than one order of magnitude.
Highlights
Phase transitions of electron–hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics
The distinctly different opto-electro properties of graphene and MoSe2 and other direct bandgap 2D transition metal dichalcogenide (TMDC) materials[2,18] promise ideal combinations for atomically thin optoelectronic devices in which 2D TMDC functions as photon absorbers and graphene layers serve as transparent electrodes[16,17]
The results indicate that around 25% (120 fs/490 fs) of the free carriers’ photogenerated in graphene transform into interlayer excitons within hundreds of fs, and by the time the majority of other free carriers have recombined and released heat in graphene
Summary
Phase transitions of electron–hole pairs on semiconductor/conductor interfaces determine fundamental properties of optoelectronics. At this detection frequency, the heterostructure and graphene follow essentially the same dynamics, and both are slower than the intralayer free-carrier dynamics[4] in monolayer MoSe2 excited with 3.1 eV photons. The detection frequency dependent signals and dynamics (Fig. 3b–f) resemble those free carrier/ exciton transitions of GaAs quantum wells observed in the THz
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