Direct nanoscale mapping of band alignment in single-layer semiconducting lateral heterojunctions

TL;DR

This work demonstrates direct nanoscale probing of band alignment in monolayer MoSe$_2$-WSe$_2$ lateral heterostructures using nanoARPES, revealing how interface stoichiometry and defects shape valence-band offsets. By comparing atomically sharp and compositionally diffusive interfaces, the authors show that VBM offsets and excitonic energies depend on local composition and the presence of interstitials or vacancies, with support from μPL and DFT analyses. The combination of spatially resolved electronic structure and optical spectroscopy provides a nanoscale picture of band-edge evolution, corroborated by defect-physics insights that explain reduced VB offsets at sharp interfaces. These findings establish stoichiometric engineering as a practical route to tailor band offsets and carrier dynamics in 1D TMDC heterostructures for advanced electronic, optoelectronic, and quantum devices.

Abstract

Atomic-scale control over band alignment in single-layer lateral heterostructures (LHSs) of dissimilar transition metal dichalcogenides (TMDCs) is critical for nextgeneration electronic, optoelectronic, and quantum technologies. However, direct experimental access to interfacial electronic states with nanometer precision remains a significant challenge. Here, we employ angle-resolved photoemission spectroscopy with nanoscale spatial resolution (nanoARPES) to directly map the epitaxial alignment and valence band evolution across MoSe2-WSe2 LHSs. By combining nanoARPES with spatially resolved photoluminescence, we correlate the evolution of the valence band maximum and exciton features across both atomically sharp and compositionally graded diffusive interfaces. We identified type-II band alignments governed by both material composition and interstitial-induced modifications of band offsets, in close agreement with density functional theory calculations. These results reveal fundamental mechanisms of electronic structure modulation at 1D TMDC heterointerfaces and provide a robust platform for tailored band engineering in van der Waals materials.

Figures (11)

• Figure 1: (a) Schematic of the nanoARPES experiment on a monolayer MoSe$_{2}$-WSe$_{2}$ LHS on hBN atop a gold-coated SiO$_2$/Si substrate. (b) Optical micrographs of the two samples that have the stated interface properties; green and red dotted boundaries are MoSe$_2$ and WSe$_2$ domains respectively. Scale bars are 5 µm unless stated otherwise. (c) Atomic-resolution HAADF-STEM images of both interface types. (d) Momentum-integrated core-level photoemission spectra from WSe$_2$ and MoSe$_2$ domains, the inset shows a magnified view of the Se 3d peaks for MoSe$_2$ and WSe$_2$. (e) Energy- and momentum-resolved ARPES intensity along the $\Gamma$–K direction. Spectra from MoSe$_2$ and WSe$_2$ were obtained from the areas demarcated by green and red dots in (b). (f) Energy distribution curves (EDCs) at $\Gamma$ and K for WSe$_2$ and MoSe$_2$ represented by red and green curves, respectively.
• Figure 2: (a) NanoARPES intensity maps of VB near the $\Gamma$-point across diffusive and sharp interface LHS, highlighting the MoSe$_{2}$ and WSe$_{2}$ domains in yellow and orange colors respectively. (b) ARPES (E,k) spectra across the interface, moving from left (MoSe$_{2}$) to right (WSe$_{2}$) side with a step of 0.5 $\mu$m. (c) Line scans (extracted along white line in (a)) at the $\Gamma$-point of VB across both diffusive and sharp interfaces. The red dots represent the extracted peak energies along the scan direction, perpendicular to the interface. The black curves are fitted profiles using sigmoid functions. The widths of both diffusive and sharp interfaces were extracted by differentiating the fit function, as shown as blue dotted curves with their full width at the half maximum (FWHM).
• Figure 3: (a) Contour colour plots of the PL intensity, showing measurement position versus PL energy across the interface for both sharp and diffusive interfaces. (b) PL spectra recorded while scanning laterally from MoSe$_{2}$ to WSe$_{2}$ across the interface, highlighting spectral shifts near the junction. (c) Extracted peak energy (red dots) of the spectra in (b), the black curves are fitted profiles using sigmoid functions.
• Figure 4: (a) Calculated band structure of pristine MoSe$_2$ and WSe$_2$. (b) VBM and conduction band minimum (CBM) energies of W$_{1-x}$Mo$_x$Se$_2$ alloys ($0 < x < 1$) with varying composition. Inset: Top-view of relaxed structures for W-rich ($x = 0.25$) and Mo-rich ($x = 0.75$) alloys; gray, yellow, and green denote W, Mo, and Se atoms, respectively. (c–d) Energy level diagrams showing defect-induced band edges in (c) WSe$_2$ and (d) MoSe$_2$. Red dashed lines represent VBM/CBM of pristine materials, green solid/dashed lines show occupied/unoccupied defect states. (e) Formation energies of different defects: Mo interstitial (Mo$_i$), W interstitial (W$_i$), Mo adatom (Mo$_{ad}$), W adatom (W$_{ad}$), Se adatom (Se$_{ad}$), and Se vacancy (V$_{Se}$). (f) Energy level diagram for the MoSe$_2$/WSe$_2$ sharp interface. Red line: WSe$_2$; green lines: MoSe$_2$ solid lines: VB; dashed lines: CB. Black dashed double-sided arrow marks the VBM offset. Inset: Side-view of Mo interstitial in WSe$_2$ and W interstitial in MoSe$_2$, used as models for the interfacial regions.
• Figure 5: Schematic of composition dependent band alignment across the diffusive and sharp interface, where black and red lines represent the VBM and exciton states respectively. (a) Interplay between band offset, band alignment, and excitonic states across diffusive interface, and (b) interplay between band offset, band alignment, and excitonic states across sharp interface.