Abstract
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; therefore, low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal. However, this rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Here, we report that light absorption and emission in MoS2/WS2 monolayer stacks can be tunable from indirect- to direct-gap transitions in both spectral and dynamic characteristics, when the constituent monolayer crystals are coherently stacked without in-plane rotation misfit. Our study suggests that the interlayer rotational attributes determine tunable interlayer excitation as a new set of basis for investigating optical phenomena in a two-dimensional hexagonal monolayer system.
Highlights
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations
We provide evidence that the interlayer excitation is intimately determined by the interlayer rotational degree of freedoms, which are tunable from direct- to indirect-gap transitions
The individual atom positions and the crystallographic ML stacking orders can be thermodynamically stabilized without rotational misfit (Fig. 1b)
Summary
Two-dimensional stacks of dissimilar hexagonal monolayers exhibit unusual electronic, photonic and photovoltaic responses that arise from substantial interlayer excitations. Interband excitation phenomena in individual hexagonal monolayer occur in states at band edges (valleys) in the hexagonal momentum space; low-energy interlayer excitation in the hexagonal monolayer stacks can be directed by the two-dimensional rotational degree of each monolayer crystal This rotation-dependent excitation is largely unknown, due to lack in control over the relative monolayer rotations, thereby leading to momentum-mismatched interlayer excitations. Artificial 2D superlattices, which are vertical stacks of dissimilar h-TMDCs MLs, are even more intriguing because upon light illumination, interlayer excitons can form across the constituent MLs at newly formed 2D electronic superstructures[17,18,19]; these excitons can recombine radiatively, thereby emitting light of different colours and intensities This phenomenon strongly suggests that, upon inter-ML coupling, tunable light absorption and emission, as well as charge separation, can be achieved. We provide evidence that the interlayer excitation is intimately determined by the interlayer rotational degree of freedoms, which are tunable from direct- to indirect-gap transitions
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