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Edge‐Passivated Monolayer WSe2 Nanoribbon Transistors

AbstractThe ongoing reduction in transistor sizes drives advancements in information technology. However, as transistors shrink to the nanometer scale, surface and edge states begin to constrain their performance. 2D semiconductors like transition metal dichalcogenides (TMDs) have dangling‐bond‐free surfaces, hence achieving minimal surface states. Nonetheless, edge state disorder still limits the performance of width‐scaled 2D transistors. This work demonstrates a facile edge passivation method to enhance the electrical properties of monolayer WSe2 nanoribbons, by combining scanning transmission electron microscopy, optical spectroscopy, and field‐effect transistor (FET) transport measurements. Monolayer WSe2 nanoribbons are passivated with amorphous WOxSey at the edges, which is achieved using nanolithography and a controlled remote O2 plasma process. The same nanoribbons, with and without edge passivation are sequentially fabricated and measured. The passivated‐edge nanoribbon FETs exhibit 10 ± 6 times higher field‐effect mobility than the open‐edge nanoribbon FETs, which are characterized with dangling bonds at the edges. WOxSey edge passivation minimizes edge disorder and enhances the material quality of WSe2 nanoribbons. Owing to its simplicity and effectiveness, oxidation‐based edge passivation could become a turnkey manufacturing solution for TMD nanoribbons in beyond‐silicon electronics and optoelectronics.
Publications

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi2Se3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. By applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (∼6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.
Publications

Near‐Field Nano‐Optical Imaging of van der Waals Materials

Abstract2D van der Waals (vdW) materials are emerging as the next generation platform for optical and electronic devices with their wide coverage of the energy bandgaps. The strong light–matter interactions in 2D vdW layers allow for exploring novel optical and electronic phenomena such as 2D polaritons exhibiting ultrahigh field confinement, defects‐induced new quantum states, and strain‐modulated quantum confinement of 2D excitons. Far‐field optical imaging techniques are extensively used to characterize the 2D vdW materials so far, however, subdiffraction spatial resolution is required for comprehensive investigations of 2D vdW materials of which physical properties are greatly influenced by local defects and strain. This article aims to cover historical advances, fundamental principles, and distinct features of emerging near‐field optical imaging techniques: scattering‐type scanning near‐field optical microscopy, tip‐enhanced Raman spectroscopy, tip‐enhanced photoluminescence techniques, and photo‐induced force microscopy. The recent developments toward spectroscopic analysis of near‐field imaging and applications for unveiling unique properties of 2D polaritons, nanoscale defects, and mechanical strains in 2D vdW materials, are also discussed. This review article provides an understanding of emerging near‐field imaging techniques and suggests prospective applications for exploring 2D vdW materials.
Publications

Nonlinear photocarrier dynamics in multilayer W S e 2 induced by intense terahertz pulses

Non-equilibrium photocarriers in multilayer WSe2 injected by femtosecond laser pulses exhibit extraordinary nonlinear dynamics in the presence of intense THz fields. The THz absorption in optically excited WSe2 rises rapidly in the low THz field regime and gradually ramps up at high intensities. The strong THz pulses drive the photocarriers into sidebands of higher mobility and release trapped charge carriers, which consequently enhance the transient conductivity of WSe2. The spectrally analyzed conductivity reveals distinctive features, indicating that the photocarriers undergo resonant interactions such as carrier-photon scattering.
Publications

Order parameter dynamics in Mn3Sn driven by DC and pulsed spin–orbit torques

We numerically investigate and develop analytic models for both the DC and pulsed spin–orbit-torque (SOT)-driven response of order parameter in single-domain Mn3Sn, which is a metallic antiferromagnet with an anti-chiral 120° spin structure. We show that DC currents above a critical threshold can excite oscillatory dynamics of the order parameter in the gigahertz to terahertz frequency spectrum. Detailed models of the oscillation frequency vs input current are developed and found to be in excellent agreement with the numerical simulations of the dynamics. In the case of pulsed excitation, the magnetization can be switched from one stable state to any of the other five stable states in the Kagome plane by tuning the duration or the amplitude of the current pulse. Precise functional forms of the final switched state vs the input current are derived, offering crucial insights into the switching dynamics of Mn3Sn. The readout of the magnetic state can be carried out via either the anomalous Hall effect or the recently demonstrated tunneling magnetoresistance in an all-Mn3Sn junction. We also discuss possible disturbance of the magnetic order due to heating that may occur if the sample is subject to large currents. Operating the device in a pulsed mode or using low DC currents reduces the peak temperature rise in the sample due to Joule heating. Our predictive modeling and simulation results can be used by both theorists and experimentalists to explore the interplay of SOT and the order dynamics in Mn3Sn and to further benchmark the device performance.

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