Interlayer orientation-dependent light absorption and emission in monolayer semiconductor stacks

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.

Visualization of Local Conductance in MoS2/WSe2 Heterostructure Transistors

“…A flake of MoS2 was first picked up by the PDMS/PPC stamp at 40 C and then stacked onto another WSe2 flake on the target SiO2 (300 nm)/Si substrate at 110 C. The high temperature enabled delamination and release of the PPC film onto the target substrate, and was critical to reduce the trapped moisture 8 and gases in the heterostructure interface region. After the PPC was released, the target substrate was soaked in chloroform, acetone and isopropanol consecutively to remove the PPC residue.…”

Interlayer‐State‐Coupling Dependent Ultrafast Charge Transfer in MoS2/WS2 Bilayers

“…The MX 2 materials share similar crystalline structures and symmetries, but possess distinct electronic properties in bandgaps, photoabsorption, and spin–orbit coupling strength 7, 8, 9. The heterostructures vertically reassembled from different 2D materials form even richer material systems, and thus provide a new platform for investigating new physics12, 13, 14, 15, 16 and exploring new applications 17, 18, 19, 20, 21, 22, 23, 24. The heterostructures of two MX 2 are of particular interests because many of them form type II heterojunctions,25, 26, 27 which facilitate the efficient separation of photoexcited electrons and holes28, 29 and therefore exhibit great potentials in the applications of photodetectors,30, 31 photovoltaic cells,32, 33 and light emitters 34…”

Epitaxially Stabilized Oxide Composed of Twisted Triangular-Lattice Layers

“…The above result indicates that the new compound is composed of the triangular-lattice Rh In general, two triangular lattices coupled at a twist angle form a quasiperiodic structure that has no unit cell. [18][19][20][21] As evidenced by the in-plane x-ray and electron diffractions (Figures 2e and 3j), however, the new layered compound shows periodically modulated hexagonal patterns with the in-plane cell parameter of 8.05(±0.05) Å. The selected-area electron diffraction detects the single pattern from only one type of the two crystallographic domains, while their both patterns are observed in the entire-area in-plane XRD.…”

“…Actually, in the research field of atomic layer materials such as graphene or transition metal dichalcogenides, the twist angle has been recognized as a new important parameter for tuning electrical and optical properties of stacked layers. [18][19][20][21] In the following, we present a new compound with a twisted stack of oxide layers stabilized by epitaxy technique. For this purpose, we adopt a simple Bi-Rh-O system, because Bi with a relatively large ionic radius is expected to form a large triangular-lattice layer mismatched to the RhO 2 layer.…”