Room temperature photosensitive ferromagnetic semiconductor using MoS₂

As CMOS transistors scale below 10 nm, leakage hinders further progress, motivating spintronic devices that exploit electron spin for nonvolatile, ultrafast, low-power computing. Realizing practical spintronics requires materials that are both semiconducting and ferromagnetic at room temperature. While 2D magnetic insulators (e.g., CrI₃, Fe/Cr–Ge₂Te₆, Fe/Mn–PS₃) show magnetism, they suffer from low Curie temperatures, oxidation, or lack of bandgaps/photosensitivity. Hexagonal TMDs possess tunable bandgaps, strong excitonic effects, and attractive optical properties but are intrinsically nonmagnetic; magnetic doping often degrades semiconductivity. Monolayer MoS₂ is a direct-gap (~1.8 eV) semiconductor with exceptional electronic/optical behavior, and theory suggests defect engineering could induce room-temperature ferromagnetism without destroying semiconductivity. The challenge is to experimentally control and probe magnetism in micrometer-scale flakes. Here, we demonstrate that helium ion milling (HIM) can generate controllable nanoholes/vacancies in exfoliated MoS₂, yielding a room-temperature photosensitive ferromagnetic semiconductor and enabling device-level spin–optical switching.

Samples: MoS₂ crystals (SPI Supplies) were micromechanically exfoliated onto Si/SiO₂ (260 nm oxide) substrates. Mono- to multilayer regions (monolayer ~0.7 nm; multilayers ~6–15 nm) were identified via optical microscopy, verified by AFM, and further characterized by Raman/PL mapping (532 nm laser, 8–80 µW, ~0.5 µm spot). Ti/Au (100/30 nm) source/drain electrodes were patterned by e-beam lithography and lift-off to form FETs spanning mono–multilayer junctions.

Helium ion milling (HIM) / irradiation: Exfoliated flakes (~20–30 µm) were exposed to helium ions incident on the basal plane to gently and selectively remove S (and some Mo) atoms, forming nanoholes (AFM/STM-resolved size ~1–4 nm²). Ion energy and dose were varied to control vacancy type/density. Monte Carlo simulations predicted: (i) vacancy number decreases with increasing ion energy; (ii) monolayer vacancy distribution is uniform for fixed parameters (enabling uniform defect densities); (iii) vacancy density increases with depth in multilayers and grows nearly linearly with layer count up to ~10 layers before saturating. For an irradiation dose of 8×10¹³ cm⁻², estimated defects are ~7.5×10²³ cm⁻³, corresponding to ~6×10¹¹ cm⁻² (~1 per 100 unit cells).

Spectroscopy and transport: ARPES mapped valence band dispersions before/after HIM. XPS (Mo 3d, S 2p) tracked binding energy/intensity vs sputter time to assess chemical changes. FET I–V characteristics were measured for monolayer and 5-layer devices pre/post HIM to extract on/off ratios and estimate mobility from linear-regime transconductance using µ = (L/W)(dI/dVg)(1/CV_a), with back-gate capacitance C ≈ εA/t_ox.

Magnetometry and magnetic imaging: Magnetic properties were probed using: (i) field-applied scanning tunneling microscopy (STM) to visualize domain formation; (ii) magnetic force microscopy (MFM, dynamic lift mode, 30 nm lift) to image domains and distinguish treated vs untreated regions and interlayer domain behavior; (iii) magneto-optical Kerr effect (MOKE) magnetometry (focused lateral-mode system within a vector magnet up to 400 Oe) to measure M–T; (iv) m–H loops measured with a VSM 7400 (3.1 T electromagnet). Temperature-dependent measurements employed a thermally stabilized stage with direct thermocouple contact (indium wire mounting) to minimize fluctuations.

Optoelectronic and spin-dependent transport: Photocurrent I–V was recorded in dark and under illumination. Under light, magnetic fields applied parallel or perpendicular to current probed spin-dependent transport and ON/OFF behavior. A spin-filter model at Schottky contacts was invoked to interpret spin-dependent barriers due to exchange-split semiconductor bands and interlayer charge transfer; vacancy engineering can yield ohmic-like behavior for one spin channel, enhancing spin filtering.

First-principles theory: DFT calculations employed PAW potentials and PBE-GGA exchange–correlation (plane-wave cutoff 400 eV). A 6×6×1 MoS₂ supercell with Γ-centered 6×6×1 k-mesh modeled vacancy-induced nanoholes at S- and Mo-edges. Structures were relaxed to forces <0.01 eV/Å and electronic convergence 10⁻⁵ eV. Total and projected DOS (TDOS/PDOS), spin densities, and magnetic moments were computed to assess defect-induced magnetism and spin polarization.

Optical characterization: Low-temperature (78 K) PL measured monolayer and 5-layer samples pre/post HIM, tracking neutral/charged exciton peaks (A ~1.9 eV) and vacancy-bound exciton features (~1.78 eV), their dose dependence, saturation at high excitation, temperature quenching to room temperature, and FWHM/Raman selection-rule relaxation.

• Helium ion milling creates controllable nanoholes (vacancies) in MoS₂ that induce robust room-temperature ferromagnetism while preserving semiconducting behavior.
• ARPES shows the valence band top shifts upward by ~0.5 eV after HIM, indicating an n→p-type shift, consistent with DFT and TDOS asymmetry (p-doping-like behavior).
• XPS (Mo 3d, S 2p) exhibits decreasing binding energies and intensities with sputter time post-treatment; early-stage vacancy generation shows greater Mo loss than S, followed by more pronounced S loss.
• FET transport remains semiconducting with similar on/off ratios before and after HIM: ~10³ (monolayer) and ~10⁴ (multilayer), with only slight mobility reduction attributed to vacancy scattering and band shifts.
• PL at 78 K reveals emergence and strengthening of a vacancy-bound exciton peak near 1.78 eV with increasing dose; main PL peak (~1.90 eV) intensifies (~3×) and blue-shifts in multilayers post-HIM; vacancy-related mid-gap energy ~1.7 eV. Layer- and dose-dependent control of optical transitions is demonstrated.
• Monte Carlo simulations predict uniform monolayer vacancy distributions for fixed ion parameters; vacancy density in multilayers increases with depth and with layer number up to ~10 layers, then saturates. An 8×10¹³ cm⁻² dose yields ~6×10¹¹ cm⁻² defect density (~1 per 100 unit cells).
• DFT: Nanohole edges produce spin-polarized states and ferromagnetic coupling. Calculated magnetic moments: V_S-edge ~3.9 µB, V_Mo-edge ~6.0 µB. Spin polarization derives mainly from S-3p (V_S-edge) and Mo-4d (V_Mo-edge) orbital asymmetry at zigzag edges.
• Magnetic imaging and magnetometry: STM under field shows domain formation post-treatment; MFM images reveal distinct ferromagnetic domains in treated regions (both mono- and multilayer). m–H loops and MOKE T–M confirm room-temperature ferromagnetism with saturation magnetization ~160 emu/cc (~0.6 µB per atom). Magnetization increases with ion dose and with layer number up to an optimum; excessive dose or bulk thickness suppresses magnetism.
• Spin-photocurrent device behavior: Under illumination, magnetic field orientation (parallel vs perpendicular to current) produces ON/OFF switching (on/off ratio >70%). A spin-filtering mechanism at Schottky contacts with spin-dependent barriers is proposed; vacancies can render spin-up channel ohmic, enhancing spin-polarized current.

The study addresses the central challenge of realizing a room-temperature ferromagnetic semiconductor in 2D materials without compromising semiconducting properties. Controlled helium ion milling in MoS₂ generates nanohole/vacancy edges that break local symmetry and create spin-polarized zigzag edge states, yielding ferromagnetic ordering. Spectroscopic (ARPES/XPS) evidence supports a p-type shift and defect-induced states, while optical PL signatures of bound excitons corroborate vacancy formation. Transport measurements show that semiconducting behavior and high on/off ratios are largely preserved, with only modest mobility degradation, indicating the HIM approach is gentle and compatible with device operation. Magnetic domain imaging and MOKE/VSM measurements confirm robust room-temperature magnetism and its tunability with ion dose and layer number. The demonstration of spin-dependent photocurrent switching and a spin-filter effect at Schottky contacts highlights practical spin–optoelectronic functionality, directly connecting the defect-engineered magnetism to device-level control via light and magnetic fields.

Helium ion milling provides a controllable, gentle method to engineer nanoholes in MoS₂, inducing room-temperature ferromagnetism while maintaining semiconducting and photosensitive properties. The work establishes the defect-edge mechanism (zigzag nanohole edges) for magnetism, verifies band structure modulation and excitonic signatures, and demonstrates functional devices exhibiting optical, electrical, and magnetic tunability, including spin-filter-driven ON/OFF switching. Future research should extend HIM-based vacancy engineering to broader 2D material classes, optimize ion parameters and layer thickness for maximal magnetization without degradation, integrate wafer-scale processing, and explore heterostructures exploiting combined spin, valley, and excitonic degrees of freedom for scalable spintronic and opto-spintronic devices.

• Sample size constraints (micrometer-scale flakes) necessitated local probes (STM/MFM/MOKE) rather than bulk magnetometry; signal uniformity across large areas remains to be validated.
• Magnetism depends sensitively on vacancy density, ion dose, and layer number; excessive irradiation degrades crystallinity and suppresses magnetism, indicating a narrow processing window.
• Bulk MoS₂ does not exhibit the induced magnetic properties, limiting thickness scalability; multilayer enhancement saturates beyond ~10 layers.
• Mobility decreases slightly post-HIM due to vacancy scattering; detailed optimization to minimize transport degradation is needed.
• The precise atomic-scale structures of nanohole edges and long-term stability (e.g., environmental effects, oxidation) were not fully explored; wafer-level uniformity and device-to-device variability require further study.