CVD Grown Hybrid MoSe$_2$-WSe$_2$ Lateral/Vertical Heterostructures with Strong Interlayer Exciton Emission

Lateral heterostructures of 2D transition metal dichalcogenide offer a powerful platform to investigate photonic and electronic phenomena at atomically sharp interfaces. However, their controlled engineering, including tuning lateral domain size and integration into vertical van der Waals heterostructures with other 2D materials, remains challenging. Here, we present a facile route for the synthesis of two types of heterostructures consisting of monolayers of MoSe$_2$ and WSe$_2$ - purely lateral (HS I) and hybrid lateral/vertical (HS II) - using liquid precursors of transition metal salts and chemical vapor deposition (CVD). Depending on the growth parameters, the heterostructure type and their lateral dimensions can be adjusted. We characterized properties of the HS I and HS II by complementary spectroscopic and microscopic techniques including Raman and photoluminescence spectroscopy, and optical and atomic force microscopy, and scanning electron and transmission electron microscopy. The photoluminescence measurements reveal strong interlayer exciton emission in the MoSe$_2$/WSe$_2$ region of HS II, which dominates the spectrum at 4 K and persisting up to room temperature. These results demonstrate high optical quality of the grown heterostructures which in combination with scalability of the developed approach paves the way for fundamental studies and device applications based on these unique 2D quantum materials.

💡 Research Summary

In this work the authors present a scalable, single‑step chemical vapor deposition (CVD) strategy for the growth of high‑quality MoSe₂–WSe₂ heterostructures using water‑soluble sodium molybdate (Na₂MoO₄) and sodium tungstate (Na₂WO₄) as liquid precursors. By spin‑coating aqueous solutions of these salts onto 300 nm SiO₂/Si substrates and subsequently exposing the coated substrates to selenium vapor in a quartz‑Knudsen‑cell‑equipped furnace, they achieve two distinct heterostructure configurations: (i) a purely lateral heterostructure (HS I) in which a MoSe₂ core is laterally stitched by WSe₂, and (ii) a hybrid lateral/vertical heterostructure (HS II) in which a monolayer MoSe₂ base is overgrown by a lateral MoSe₂–WSe₂ alloy that forms a second layer, creating a vertical MoSe₂/WSe₂ interface.

The key to selecting between HS I and HS II is the molar ratio of the Mo and W precursors. A low Mo‑to‑W ratio (1 : 3) favors the nucleation of monolayer MoSe₂ followed by epitaxial lateral growth of WSe₂, yielding HS I. Conversely, a high Mo‑to‑W ratio (3 : 1) leads to the early formation of a bilayer MoSe₂ seed; the additional MoSe₂ layer then serves as a template for the lateral growth of WSe₂, producing the vertical component of HS II. The heating ramp of 15 °C min⁻¹ is critical; faster ramps generate mixed‑phase domains, while slower ramps preserve phase purity. Growth temperatures of 770 °C, 820 °C, and 900 °C do not change the heterostructure type but affect lateral domain sizes, with higher temperatures expanding HS I domains up to ~200 µm and HS II top‑layer domains up to ~60 µm.

Raman spectroscopy confirms the presence of both materials: MoSe₂ exhibits A′₁ (≈240 cm⁻¹) and E′ (≈286 cm⁻¹) modes, while WSe₂ shows E′/A′₁ (≈249 cm⁻¹) and 2LA (≈260 cm⁻¹) modes. Narrow full‑width‑at‑half‑maximum values (1.7 cm⁻¹ for MoSe₂, 3.7 cm⁻¹ for WSe₂) indicate excellent crystallinity comparable to solid‑precursor growth. Raman intensity maps reveal uniform distribution of each component across the respective regions. AFM measurements give a step height of ~0.7 nm, confirming monolayer thickness for both the lateral domains and the underlying MoSe₂ layer in HS II.

High‑resolution HAADF‑STEM combined with energy‑dispersive X‑ray spectroscopy (EDX) provides nanoscale compositional insight. A ~120 nm wide intermixing zone containing both Mo and W is observed at the MoSe₂/WSe₂ boundary, indicating a narrow alloyed transition region. Within this alloyed region the interface width is only ~5 nm, as evidenced by atomic‑number contrast and selected‑area electron diffraction (SAED) patterns that display a single six‑fold symmetry with identical {10‑10} spacings across MoSe₂, alloy, and WSe₂ domains, confirming a single‑crystal in‑plane heterojunction.

Photoluminescence (PL) at room temperature shows neutral exciton peaks at 1.50 eV (MoSe₂) and 1.57 eV (WSe₂) for HS I, both red‑shifted relative to strain‑free values, which the authors attribute to tensile strain induced during cooling due to mismatched thermal expansion coefficients. The PL intensity of the WSe₂ region is ~3.5 times higher than that of MoSe₂, consistent with previous reports on lateral MoSe₂–WSe₂ heterostructures.

The most striking optical result concerns HS II. Temperature‑dependent PL from 4 K to 300 K reveals a dominant interlayer exciton (IE) emission centered around 1.35 eV that overwhelms the intralayer exciton peaks at low temperature and remains clearly visible at room temperature. This strong, temperature‑robust IE signal demonstrates efficient charge transfer across the vertically stacked MoSe₂/WSe₂ interface and indicates that the liquid‑precursor CVD method can produce heterostructures with interlayer excitonic properties comparable to, or even surpassing, those obtained by mechanical stacking or solid‑precursor CVD.

In summary, the authors have established a versatile, bottom‑up CVD platform that uses easily tunable liquid metal‑salt precursors to fabricate both purely lateral and hybrid lateral/vertical MoSe₂–WSe₂ heterostructures with atomically sharp interfaces, high crystallinity, and pronounced interlayer exciton emission persisting to room temperature. This approach offers a pathway toward large‑area integration of 2D heterostructures for next‑generation optoelectronic, valleytronic, and quantum photonic devices.