Graphene is often used as an acceptor in highly efficient energy transfer processes between its electrons and neighbouring optical emitters such as quantum dots, fluorescent molecules and color centres in crystals. Here we demonstrate that graphene can act not only as an acceptor in energy transfer processes, but also an acceptor of charge donated by photoexcited quantum emitters. Specifically, we use heterostructures comprised of graphene and hexagonal boron nitride (hBN) to demonstrate a reversible charge transfer process from quantum emitters in hBN to graphene. The process acts as a controllable, energy-resolved filter that quenches quantum emitters with ground states located above the Fermi level of graphene. Our findings shed light on the positions of hBN defect states within the bandgap of hBN, and are important for the design of devices based on 2D heterostructures, opening new avenues to technologies based on electrical excitation, manipulation, and readout of the quantum states of optical emitters.
We
demonstrate a general protocol that uses a metastable phase
as a template, followed by chalcogen substitution and phase transformation
to obtain superlattices, or single crystals, of layered transition
metal dichalcogenides (TMDs). In particular, the single-crystalline
2H-MoTe2 thin film, with the available wafer-scale synthesis,
is selected as the template to study the chalcogen substitution mechanism.
Analogous to the initiation polymerization process, a Te vacancy-initiated
and S diffusion-mediated mechanism is proposed to describe the sequential
substitution: the complete sulfurization of the top and bottom MoTe2 layers at the first stage, followed by the alloying process
in the middle layers and finally the full conversion of the flake
into MoS2. The substitution in each layer starts from the
vacancy and expands to the nearby region catalyzed by the strain field,
whereas the sulfurization sequence in the multilayer system is mediated
by the cross-layer S diffusion process. This is confirmed by the cross-sectional
observation of the intermediate state by scanning transmission electron
microscopy and density functional theory studies. This unique mechanism
enables us to fabricate the sub-centimeter scale, composition-tunable,
and symmetric MoS2/MoTe2(1–x)S2x
/MoS2 superlattices.
Our work presents a new tool for the large-scale synthesis of TMD-based
heterostructures toward industrial applications.
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