Abstract

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization. Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-WSe₂) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi–van der Waals epitaxy, effectively suppressing strong interfacial bonding and promoting the growth of quasi free-standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveals an indirect-gap electronic structure with the valence-band maximum at the Γ point, as well as a pronounced spin–orbit-driven splitting of 520 ± 20 meV at the K point. Analysis of the measured dispersions yields hole effective masses of 0.46 ± 0.04 mₑ and 0.75 ± 0.06 mₑ for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi–free-standing 3R-WSe₂ and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion-symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.

Abstract

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization. Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-WSe₂) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi–van der Waals epitaxy, effectively suppressing strong interfacial bonding and promoting the growth of quasi free-standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveals an indirect-gap electronic structure with the valence-band maximum at the Γ point, as well as a pronounced spin–orbit-driven splitting of 520 ± 20 meV at the K point. Analysis of the measured dispersions yields hole effective masses of 0.46 ± 0.04 mₑ and 0.75 ± 0.06 mₑ for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi–free-standing 3R-WSe₂ and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion-symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.

Abstract

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization. Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-WSe₂) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi–van der Waals epitaxy, effectively suppressing strong interfacial bonding and promoting the growth of quasi free-standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveals an indirect-gap electronic structure with the valence-band maximum at the Γ point, as well as a pronounced spin–orbit-driven splitting of 520 ± 20 meV at the K point. Analysis of the measured dispersions yields hole effective masses of 0.46 ± 0.04 mₑ and 0.75 ± 0.06 mₑ for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi–free-standing 3R-WSe₂ and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion-symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.

Abstract

Rhombohedral-stacked transition metal dichalcogenides (TMDs) break inversion symmetry between adjacent layers, giving rise to an intrinsic out-of-plane ferroelectric polarization. Controlling the formation of this stacking polytype is therefore essential for harnessing ferroelectric effects in two-dimensional materials. In this work, we demonstrate the epitaxial growth of rhombohedral bilayer tungsten diselenide (3R-WSe₂) on a cubic W(110) single crystal by molecular beam epitaxy. We show that selenium passivation of the substrate is key to enable a quasi–van der Waals epitaxy, effectively suppressing strong interfacial bonding and promoting the growth of quasi free-standing bilayer films. The 3R stacking order is confirmed through a combination of Raman spectroscopy and high-resolution angle-resolved photoemission spectroscopy (ARPES), supported by density functional theory (DFT) calculations. ARPES and DFT reveals an indirect-gap electronic structure with the valence-band maximum at the Γ point, as well as a pronounced spin–orbit-driven splitting of 520 ± 20 meV at the K point. Analysis of the measured dispersions yields hole effective masses of 0.46 ± 0.04 mₑ and 0.75 ± 0.06 mₑ for the upper and lower valence bands at K point, respectively. These results establish a robust route for synthesizing quasi–free-standing 3R-WSe₂ and provide a platform for exploring the electronic, optical, and ferroelectric functionalities that emerge from inversion-symmetry breaking in layered TMDs. Our findings further highlight the potential of cubic substrates for deterministic fabrication of rhombohedral TMD heterostructures and ferroelectric devices at the nanoscale.