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.
It has been generally accepted that
iron-group metals (iron, cobalt,
nickel) consistently show the highest catalytic activity for the growth
of carbon nanomaterials, including carbon nanotubes (CNTs) and graphene.
However, it still remains a challenge for them to obtain uniform graphene,
because of their high carbon solubility, which can be attributed to
an uncontrollable precipitation in cooling process. The quality and
uniformity of the graphene grown on low-cost iron-group metals determine
whether graphene can be put into the mass productions or not. Here,
we develop a novel strategy to form an antiperovskite layer using
ambient-pressure chemical vapor deposition (APCVD), which, so far,
is the only known way for iron-group metals to prepare uniform monolayer
graphene with 100% surface coverage. Our strategy utilizes liquid
metal (e.g., gallium) to assist iron-group metals to form an antiperovskite
layer that is chemically stable throughout the high-temperature growth
process and then to seal the passageway of carbon segregation from
the metal bulk during cooling. With the advantage of forming antiperovskite
structure, the uniform monolayer graphene can always be obtained under
the variations of experimental conditions. Our strategy solves the
problem about how to get uniform graphene film on high-solubility
carbon substrate, to utilize the high catalytic activity of low-cost
iron-group metals and to realize low-temperature growth by chemical
vapor deposition.
The low photoluminescence (PL) quantum yields of transition metal dichalcogenide monolayers have been a limiting factor for their optoelectronic applications. Various and even inconsistent mechanisms have been proposed to modulate...
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