Abstract
Electronic charge rearrangement between components of a heterostructure is the fundamental principle to reach the electronic ground state. It is acknowledged that the density of state distribution of the components governs the amount of charge transfer, but a notable dependence on temperature is not yet considered, particularly for weakly interacting systems. Here, it is experimentally observed that the amount of ground-state charge transfer in a van der Waals heterostructure formed by monolayer MoS2 sandwiched between graphite and a molecular electron acceptor layer increases by a factor of 3 when going from 7 K to room temperature. State-of-the-art electronic structure calculations of the full heterostructure that accounts for nuclear thermal fluctuations reveal intracomponent electron-phonon coupling and intercomponent electronic coupling as the key factors determining the amount of charge transfer. This conclusion is rationalized by a model applicable to multicomponent van der Waals heterostructures.
Highlights
The discovery that atomically thin layers can exist at room temperature in air marked the launch of research on two-dimensional (2D) materials.[1,2] A single layer of a 2D material may be insulating, semiconducting (e.g., MoS2), or conducting, and all electrical material properties required for the construction of an electronic or optoelectronic device are available in the monolayer limit.[3,4,5,6] Stacks of such monolayers are typically bound by weak van der Waals interlayer interactions, and vdW heterostructures have emerged as prime candidates for realizing electronic and optoelectronic functions in the smallest possible volume
For a vdW heterostructure consisting of ca. 0.5 ML F6TCNNQ on ML-MoS2/highly oriented pyrolytic graphite (HOPG) the corresponding energy distribution curves (EDCs) in the near-EF region from angle-resolved photoelectron spectroscopy (ARPES) measurements are shown in Figure 2a, for different temperatures
At room temperature (300 K), this energy region is dominated by two features centered at 0.2 eV and 0.8 eV binding energy, as the valence band onset of MoS2 is at a binding energy higher than 1.5 eV
Summary
The discovery that atomically thin layers can exist at room temperature in air marked the launch of research on two-dimensional (2D) materials.[1,2] A single layer of a 2D material may be insulating (e.g., hexagonal boron nitride), semiconducting (e.g., MoS2), or conducting (e.g., graphene), and all electrical material properties required for the construction of an electronic or optoelectronic device are available in the monolayer limit.[3,4,5,6] Stacks of such monolayers are typically bound by weak van der Waals (vdW) interlayer interactions, and vdW heterostructures have emerged as prime candidates for realizing electronic and optoelectronic functions in the smallest possible volume. Charge rearrangement and charge transfer (CT) phenomena that define the electronic ground state of vdW heterostructures must be thoroughly understood for knowledge-guided device design. Beyond layered inorganic 2D materials, organic molecular semiconductors are attractive as a component in vdW heterostructures, because they extend the range of available energy gap values and feature strong light-matter interaction.[8,9] it has been recognized that strong molecular electron acceptors and donors can be employed as dopants for numerous 2D semiconductors, for transition metal dichalcogenides (TMDCs).[10,11,12,13,14] This doping, on the one hand, allows controlling the Fermi level (EF) position and mobile carrier density in the semiconductor, and, on the other hand, enables manipulation of optical quasi-particles, such as charged excitons (positive or negative trions) in TMDC monolayers.[15,16,17,18,19] Such vdW heterostructures can be ideal model systems for exploring intriguing physical phenomena,[20,21] and they can pave the way for a broader class of device applications
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