Exciton spectroscopy and unidirectional transport in MoSe2-WSe2 lateral heterostructures encapsulated in hexagonal boron nitride

Abstract

Chemical vapor deposition (CVD) allows lateral edge epitaxy of transition metal dichalcogenide heterostructures. Critical for carrier and exciton transport is the material quality and the nature of the lateral heterojunction. Important details of the optical properties were inaccessible in as-grown heterostructure samples due to large inhomogeneous broadening of the optical transitions. Here we perform optical spectroscopy of CVD grown MoSe2-WSe2 lateral heterostructures, encapsulated in hBN. Photoluminescence (PL), reflectance contrast and Raman spectroscopy reveal optical transition linewidths similar to high quality exfoliated monolayers, while PL imaging experiments uncover the effective excitonic diffusion length of both materials. The typical extent of the covalently bonded MoSe2-WSe2 heterojunctions is 3 nm measured by scanning transmission electron microscopy (STEM). Tip-enhanced, sub-wavelength optical spectroscopy mapping shows the high quality of the heterojunction which acts as an excitonic diode resulting in unidirectional exciton transfer from WSe2 to MoSe2.

Interface engineering of charge-transfer excitons in 2D lateral heterostructures

“…In particular, we address the competition between Coulomb-induced spatial confinement of excitons (Bohr radius) and interface widths. Considering the exemplary case of hBN-encapsulated MoSe 2 –WSe 2 lateral heterostructures 14 , 20 , we predict for small junction widths and low temperatures the appearance of an additional low-energy resonance in PL spectra that we assign to a bound CT exciton. To test this, we perform cryogenic PL measurements in hBN-encapsulated MoSe 2 –WSe 2 lateral heterostructures with a high-quality, very narrow junction width of ~2–3 nm 14 .…”

“…Considering the exemplary case of hBN-encapsulated MoSe 2 –WSe 2 lateral heterostructures 14 , 20 , we predict for small junction widths and low temperatures the appearance of an additional low-energy resonance in PL spectra that we assign to a bound CT exciton. To test this, we perform cryogenic PL measurements in hBN-encapsulated MoSe 2 –WSe 2 lateral heterostructures with a high-quality, very narrow junction width of ~2–3 nm 14 . We find PL emission peaks at the heterojunction in the high-quality samples that are below the MoSe 2 and WSe 2 intralayer excitons and that present a strong indication for the bound CT excitons predicted by our microscopic theory.…”

“…These are characterized by spatially separated interlayer excitons forming an out-of-plane dipole and thus allowing, e.g., a gate-controllable exciton transport 4 , 5 . In comparison, much less is known about lateral TMD heterostructures 6 – 14 , where two different TMD monolayer materials are grown sequentially and covalently bond in the plane 6 – 12 (Fig. 1 a).…”

“…We also perform cryogenic photoluminescence (PL) measurements to directly check the theoretical predictions. Motivated by the recent progress in the growth of lateral heterostructures with atomically sharp interfaces 9 , 14 , 19 – 21 , we theoretically investigate optimal conditions to find CT excitons (i) via interface engineering (interface widths, band offsets) and (ii) dielectric engineering (surrounding substrates). In particular, we address the competition between Coulomb-induced spatial confinement of excitons (Bohr radius) and interface widths.…”

“…Furthermore, we predict CT exciton binding energies of just a few tens of meV as well as extraordinarily large dipole moments for hBN-encapsulated materials. This indicates that lateral heterostructures with ultrathin junctions and weakly bound CT excitons to have also technological relevance for optoelectronic devices due to the expected high exciton mobility 13 , efficient exciton dissociation, and diode-like exciton transport across the interface 14 .…”