Abstract
Van der Waals heterostuctures, made from stacks of two-dimensional materials, exhibit unique light-matter interactions and are promising for novel optoelectronic devices. The performance of such devices is governed by near-field coupling through, e.g., interlayer charge and/or energy transfer. New concepts and experimental methodologies are needed to properly describe two-dimensional heterointerfaces. Here, we report on interlayer charge and energy transfer in atomically thin metal (graphene)/semiconductor (transition metal dichalcogenide (TMD, here MoSe$_2$)) heterostructures using a combination of photoluminescence and Raman scattering spectroscopies. The photoluminescence intensity in graphene/MoSe$_2$ is quenched by more than two orders of magnitude and rises linearly with the photon flux, demonstrating a drastically shortened ($\sim 1~\tr{ps}$) room temperature MoSe$_2$ exciton lifetime. Key complementary insights are provided from analysis of the graphene and MoSe$_2$ Raman modes, which reveals net photoinduced electron transfer from MoSe$_2$ to graphene and hole accumulation in MoSe$_2$. Remarkably, the steady state Fermi energy of graphene saturates at $290\pm 15~\tr{meV}$ above the Dirac point. This behavior is observed both in ambient air and in vacuum and is discussed in terms of band offsets and environmental effects. In this saturation regime, balanced photoinduced flows of electrons and holes may transfer to graphene, a mechanism that effectively leads to energy transfer. Using a broad range of photon fluxes and diverse environmental conditions, we find that the presence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene/MoSe$_2$. This absence of correlation strongly suggests that energy transfer to graphene is the dominant interlayer coupling mechanism between atomically-thin TMDs and graphene.
Highlights
Charge and energy transfer (CT, ET) play a prominent role in atomic, molecular, and nanoscale systems
Using a broad range of incident photon fluxes and diverse environmental conditions, we find that the existence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene=MoSe2
The distinct charge transfer dynamics observed under ambient conditions and in vacuum shed light on the role of molecular adsorbates at the surface of the van der Waals heterostructures (vdWHs)
Summary
Charge and energy transfer (CT, ET) play a prominent role in atomic, molecular, and nanoscale systems. Using a broad range of incident photon fluxes and diverse environmental conditions, we find that the existence of net photoinduced charge transfer has no measurable impact on the near-unity photoluminescence quenching efficiency in graphene=MoSe2 This absence of correlation strongly suggests that energy transfer to graphene (either in the form of Dexter or Förster processes) is the dominant interlayer coupling mechanism between atomically thin TMDs and graphene. B PL is very similar in the three cases and scales linearly with Φph [see Fig. 2(d)] These observations suggest (i) that interlayer coupling does not significantly affect exciton formation and exciton decay until a population of A excitons is formed, and (ii) that the A-exciton lifetime in Gr=MoSe2 is not appreciably longer than the B → A decay time. Using the estimated decay time of bare MoSe2 and a typical quenching factor of 300 (i.e., a quenching efficiency of 99.7%) in the low fluence limit, we can assume a conservative upper bound of a few ps for the exciton lifetime in coupled Gr=MoSe2
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