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The authors compute the surface energies, equilibrium Wulff morphology, and facet-dependent band-edge alignments across four exposed crystallographic orientations of the quasi-one-dimensional van der Waals semiconductor ZrSe$_3$. Using HSE06 hybrid-functional calculations with spin-orbit coupling, they reveal that ionization potential and electron affinity shift by up to 0.45 eV and 0.80 eV across facets, driving a thickness-dependent gap reduction in the catalytically active (210) facet and metallicity in the (010) surface. These electronic variations are dictated by whether surface termination preserves or distorts covalent Se–Se dimers, establishing an intuitive design rule for the broader transition-metal trichalcogenide family.
Breaking the Se–Se dimer controls whether a ZrSe$_3$ facet remains semiconducting or turns metallic, establishing an intuitive structural descriptor that governs facet-dependent band offsets across the entire transition-metal trichalcogenide family.
Transition-metal trichalcogenides MX$_3$ are quasi-one-dimensional van der Waals materials whose layers form through lateral chain binding, giving them an interlayer cleavage plane and in-plane electronic anisotropy. Among them, ZrSe$_3$ is stable under ambient conditions and semiconducting. Exfoliated crystals expose four crystallographic planes, and electrocatalytic measurements on an individual crystal assigned the hydrogen evolution activity to the high-energy (210) facet rather than to the basal plane. Those non-basal surfaces remain uncharacterized, since the available calculations treat either the bulk crystal or the free-standing basal monolayer. Here, we compute the surface energies and the equilibrium morphology of these four surfaces with a dispersion-corrected semilocal functional, and their band-edge alignment with the HSE06 hybrid functional including spin-orbit coupling. The surface energies span nearly an order of magnitude, from \SI{0.097}{\joule\per\meter\squared} for (001) to \SI{0.869}{\joule\per\meter\squared} for (010), and the Wulff shape exposes the four observed planes with area fractions of \SI{17.1}{\percent}, \SI{4.4}{\percent}, \SI{71.4}{\percent}, and \SI{7.0}{\percent}. The ionization potential varies by \SI{0.45}{\electronvolt} between facets and the electron affinity by \SI{0.80}{\electronvolt}, the gap of (210) falls from 1.43 to \SI{0.94}{\electronvolt} between 10 and \SI{23}{\angstrom}, and (010) remains metallic. The facets separate according to whether the cut preserves or distorts the Se-Se dimer, and the distorted dimers concentrate the frontier states. Because the same dimer governs the bulk electronic structure of the MX$_3$ family, the criterion transfers to the related trichalcogenides.