Edge Magnetism in Colloidal MoS2 Triangular Nanoflakes

TL;DR

This work shows that triangular MoS2 nanoflakes with sulfur-terminated, hydrogen-passivated edges exhibit intrinsic, localized edge magnetism that depends on the edge length, emerging above a critical size of about $1.5~\mathrm{nm}$. Using spin-polarized density functional theory, the authors map a landscape of competing magnetic states in flakes $r_4$ through $r_7$, with local moments residing on specific Mo-edge atoms arranged in pseudo-triangular motifs; these states are nearly degenerate within ~$25$ meV, suggesting high tunability by external fields. Electronic structure analyses reveal a transition from semiconducting behavior in the smaller, nonmagnetic flakes to metallic behavior in the larger magnetic ones, with Mo $d$-states crossing the Fermi level and edge-localized magnetism underpinning the spin-active islands. The findings establish S-terminated, H-passivated triangular MoS2 nanoflakes as a stable and experimentally accessible platform for nanoscale spintronic applications, including spin filters and spin-qubit schemes via electrostatic or magnetic-field control, while acknowledging limitations of the exchange-correlation treatment used.

Abstract

The control of localized magnetic domains at the nanoscale holds great promise for next-generation spintronic applications. Colloidal transition metal dichalcogenides nanostructures are experimentally accessible and chemically tunable platforms for spintronics, deserving dedicated research to assess their potential. Here, we investigate from first principles free-standing triangular MoS2 nanoflakes with sulfur-terminated, hydrogen-passivated edges, to probe intrinsic spin behavior at varying side lengths. We find a critical edge length of approximately 1.5 nm separating nonmagnetic nanoflakes from larger ones with a magnetic ground state emerging from several, energetically competing spin configurations. In these systems, the magnetic activity is not uniformly distributed along the edges but localized on specific "magnetic islands" around molybdenum edge atoms. The localization of magnetic moments is robust even in non-equilateral nanoflake geometries, highlighting their intrinsic stability regardless of the (high) symmetry of the hosting structure. These findings establish that the S-terminated, H-passivated triangular MoS2 nanoflakes are a stable and experimentally accessible platform via colloidal synthesis for low-dimensional, next-generation spintronic devices.

Figures (5)

• Figure 1: Hydrogen-passivated triangular MoS$_2$ nanoflakes, with an increasing number of rings along the edge length (L) and labeled according to them. The atoms are color-coded according to the magnitude of their displacement from the initial positions after structural optimization.
• Figure 2: Total energy difference ($E_{diff}$) relative to the magnetic ground state (red square) for various fixed total magnetic moments ($\mu$) of the considered MoS$_2$ nanoflake. Less stable configurations discussed in the text are squared in black. Insets: Zoom-in to nearly degenerate magnetic orderings.
• Figure 3: Spatial distribution of the local magnetic moments in the r$_4$ nanoflake at varying total magnetization (in $\mu_B)$ indicated in each panel. The color scale for the atoms represents the magnitude and orientation of the local spins.
• Figure 4: Spatial distribution of the local magnetic moments in selected (a) r$_5$, (b) r$_6$, and (c) r$_7$ magnetic configurations. The color scale for the atoms represents the magnitude and orientation of the local spins.
• Figure 5: Spin-polarized atom-projected density of states (PDOS) for the magnetic ground state of each MoS$_2$ nanoflake. The Fermi energy (E$_F$) is set to 0 eV and marked by a vertical dashed bar.