Defect engineering, a key technology in materials science, involves manipulating a material's properties by controlling its defects. While homogeneous doping is common, creating various defect distributions offers potential for inducing desired physical properties, particularly those associated with symmetry breaking. Spintronics, leveraging both spin and charge of electrons, benefits from defect engineering. A critical goal is breaking inversion symmetry to activate the Rashba interaction, coupling spin and momentum of electrons, creating an alternative channel for spin sources without ferromagnets. Theoretical studies suggest organized defect distributions can achieve this, but experimental realization has been challenging. Two-dimensional transition metal dichalcogenides (TMDs) are suitable platforms for defect engineering due to their crystallinity and varied elemental compositions, allowing selective reactions, particularly via plasma treatment. Mild plasma treatment can selectively create vacancies, while controlling the plasma's energy and the TMD's composition can enable layer-by-layer defect gradients. This study focuses on generating a defect gradient in PtSe₂ to activate the Rashba interaction, offering a novel approach for spin-orbitronic applications.

Extensive research explores defect engineering in materials science, demonstrating its impact in semiconductor technology through homogeneous doping. However, the potential of non-uniform defect distributions to manipulate material properties, especially through symmetry breaking, has only recently begun to be explored. In spintronics, defect engineering aims to break time-reversal symmetry by inducing magnetic moments or breaking inversion symmetry to generate the Rashba interaction. While theoretical studies propose methods to achieve this in 2D materials, experimental realization of organized defect distributions has been difficult. Previous work shows defect-induced magnetism in graphene and TMDs, but generating a controlled inversion symmetry breaking for the Rashba effect remains a challenge. Two-dimensional TMDs, particularly those amenable to selective plasma etching, are promising platforms for this kind of defect engineering.

Van der Waals layered PtSe₂ was chosen for its controlled reaction to plasma and air stability. Mechanically exfoliated PtSe₂ films were treated with plasma etching using a mixture of Ar and SF₆ gases. Ar etches Se atoms, while SF₆ reacts chemically with Pt atoms, leading to vaporization. The plasma's energy disperses, affecting subsequent layers and creating a layer-by-layer defect gradient. Scanning electron microscopy imaged the sample for magneto-transport measurements. Atomic-resolution scanning transmission electron microscopy (STEM), using energy-dispersive X-ray spectroscopy (EDS) and electron energy-loss spectroscopy (EELS), analyzed the structure and elemental composition at the atomic level. X-ray photoelectron spectroscopy (XPS) investigated the oxidation states of Pt before and after plasma treatment. Magneto-transport measurements, including residual resistance ratio (RRR) and magnetoresistance (MR), were performed using a physical property measurement system (PPMS). Nonreciprocal transport characteristics were studied using both DC and AC currents to probe the Rashba effect. Density functional theory (DFT) calculations, employing the virtual crystal approximation, simulated the defect-gradient and its effect on the electronic band structure, specifically focusing on the Rashba spin-splitting.

STEM analysis revealed a defect gradient in the plasma-treated PtSe₂ films. Se vacancies were dominant, extending to a depth of 7 nm from the surface. The Se/Pt ratio showed a clear linear gradient along the layers. EDS and EELS data consistently confirmed the Se deficiency near the surface. XPS analysis showed a change in Pt oxidation states after plasma treatment. Magneto-transport measurements showed a significant reduction in the RRR and MR, consistent with the introduction of defects. Crucially, nonreciprocal charge transport was observed, indicating the Rashba effect. The nonreciprocal magnetoresistance (MR) was linear with the applied magnetic field and exhibited a clear dependence on current direction. Angle-dependent MR measurements allowed estimation of the effective Rashba field, which showed a linear relationship with applied current. The nonreciprocal effect persisted up to room temperature. DFT calculations corroborated the experimental results, showing large Rashba-type spin-splitting at the Fermi level due to the asymmetric Se vacancy distribution. Calculations showed that both spin-orbit coupling and the inversion-breaking field from the defect gradient were necessary for the Rashba splitting.

The findings directly address the hypothesis that a controlled defect gradient can induce the Rashba effect. The observation of nonreciprocal transport, consistent with theoretical predictions and DFT calculations, firmly establishes the presence of the Rashba interaction stemming from the broken inversion symmetry caused by the Se vacancy gradient. The room-temperature persistence of the effect significantly enhances its potential for practical applications. This work contributes a novel approach to creating a controllable Rashba effect in a 2D material, offering a new pathway for developing functional spintronic devices.

This study successfully demonstrated the generation of a defect gradient and the resulting Rashba effect in PtSe₂ using selective plasma etching. The combination of experimental techniques and DFT calculations provided compelling evidence for the mechanism. The room-temperature operation of the Rashba effect highlights the potential of this approach for applications in spintronics and other electronic devices. Future research could explore other TMD materials and different defect engineering techniques to further broaden the applicability of this method.

The current study focused on a specific range of PtSe₂ thicknesses and plasma treatment conditions. Further investigation is needed to explore the effects of varying these parameters on the magnitude and stability of the Rashba effect. The spatial resolution of the STEM analysis might limit the precise determination of the defect distribution. A more detailed analysis, using advanced characterization techniques, could provide further insight into the precise arrangement and distribution of defects.