Brian M. Leonard

The monoclinic 2M phase of tungsten disulfide (2M-WS ₂ ) has emerged as a promising platform for topological superconductivity, combining topological surface states, anisotropic thermoelectric response, and superconductivity with a critical temperature of ∼8.8K. A detailed understanding of its electron–phonon coupling and phonon anharmonicity is, however, still lacking. Here, we investigate the phonon properties of few-layer 2M-WS ₂ as functions of laser power, temperature, polarization, and magnetic field using polarized Raman spectroscopy down to 12.5 K. We uncover an unusual, non-linear temperature dependence of the Raman shifts in selected modes, consistent with strong, mode-dependent electron–phonon interactions that dominate over phonon–phonon scattering at low temperatures. Angle-resolved polarized Raman measurements further reveal a pronounced change in the polarization behavior of the A g (4) mode at ∼316cm ⁻¹ upon cooling, indicating a modification of the underlying lattice and electronic anisotropy. In contrast, the applied magnetic field has a negligible influence on the Raman response over the measured temperature range. Together, these results advance the microscopic understanding of vibrational dynamics and electron–phonon coupling in few-layer 2M-WS ₂ and provide important insight into the lattice-electronic interplay relevant for its unconventional superconductivity and potential quantum and thermoelectric device applications.

Chiral phonons (CPs)—vibrational modes carrying angular momentum—offer a new degree of freedom for phononic quantum information and topological energy transport. However, their microscopic identification and control within van der Waals materials remain limited. Here, we report the direct observation of domain-dependent CP junctions in layered ferroelastic WO₂I₂, whose intrinsic micro-strain twinning provides a natural platform for CP localization. Using helicity-resolved and angle-resolved Raman spectroscopy, we reveal distinct circular polarization responses in specific domains, where A _g modes at 115 cm ⁻¹ and 753 cm ⁻¹ exhibit strong helicity contrast, while the B _g mode remains helicity-independent. These chiral responses are confined to ferroelastic domains with higher local strain. Density functional theory (DFT + U ) calculations accurately reproduce the experimental Raman features and show that strain induces degeneracy lifting in the A _g modes, consistent with domain-resolved observations. Our results demonstrate that ferroelastic strain engineering can be used to generate and manipulate microscopic CP junctions, establishing WO₂I₂ as a promising candidate for strain-tunable quantum phononic devices.

A novel and facile route for intercalating alkali-metal ions and ammonium ions into the layered mixed-ion compound tungsten oxychloride (WO₂Cl₂) has been developed using the iodide–triiodide redox couple as a mild redox-active reagent. Unlike traditional intercalation techniques employing highly reducing and air-sensitive reagents such as n-butyllithium, alkali triethylborohydride, and naphthalenide, the I⁻/I₃⁻ redox system operates at a moderate potential (0.536 V vs. SHE), enabling safer handling under ambient conditions without stringent inert-atmosphere requirements. This redox pair promotes the reduction of W⁶⁺ to W⁵⁺, thereby facilitating cation insertion into the van der Waal (vdW) gaps of WO₂Cl₂. This method uniquely enables ammonium ion intercalation into WO₂Cl₂, a first for this system. Intercalation was confirmed by X-ray diffraction, scanning electron microscopy (SEM/EDS), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), with measured lattice expansion correlating well with Shannon ionic radii and coordinating environments. Electrical transport measurements reveal a transition from insulating WO₂Cl₂ to a semiconducting phase for K_0.5WO₂Cl₂, exhibiting a resistance drop of over four orders of magnitude. This work demonstrates the I⁻/I₃⁻ couple as a general, safe, and versatile method for layered mixed-anion materials, broadening the chemical toolkit for low-temperature, solution-based tuning of structures and properties.

Anisotropic materials with low symmetries hold significant promise for next-generation electronic and quantum devices. 2M-WS₂, which is a candidate for topological superconductivity, has garnered considerable interest. However, a comprehensive understanding of how its anisotropic features contribute to unconventional superconductivity, along with a simple, reliable method to identify its crystal orientation, remains elusive. Here, we combine theoretical and experimental approaches to investigate angle- and polarization-dependent anisotropic Raman modes of 2M-WS₂. Through first-principles calculations, we predict and analyze the phonon dispersion and lattice vibrations of all Raman modes in 2M-WS₂. We establish a direct correlation between their anisotropic Raman spectra and high-resolution transmission electron microscopy images. Finally, we demonstrate that anisotropic Raman spectroscopy can accurately determine the crystal orientation and twist angle between two stacked 2M-WS₂ layers. Our findings provide insights into the electron-phonon coupling and anisotropic properties of 2M-WS₂, paving the way for the use of anisotropic materials in advanced electronic and quantum devices.

Leveraging the reciprocal-space proximity effect between superconducting bulk and topological surface states (TSSs) offers a promising way to topological superconductivity. However, elucidating the mutual influence of bulk and TSSs on topological superconductivity remains a challenge. Here, we report pioneering transport evidence of a thickness-dependent transition from conventional to unconventional superconductivity in 2M-phase WS₂ (2M-WS₂). As the sample thickness reduces, we see clear changes in key superconducting metrics, including critical temperature, critical current, and carrier density. Notably, while thick 2M-WS₂ samples show conventional superconductivity, with an in-plane (IP) upper critical field constrained by the Pauli limit, samples under 20 nm exhibit a pronounced IP critical field enhancement, inversely correlated with 2D carrier density. This marks a distinct crossover to unconventional superconductivity with strong spin-orbit-parity coupling. Our findings underscore the crucial role of sample thickness in accessing topological states in 2D topological superconductors, offering pivotal insights into future studies of topological superconductivity.

The investigation of exotic properties in two-dimensional (2D) topological superconductors has garnered increasing attention in condensed matter physics, particularly for applications in topological qubits. Despite this interest, a reliable way of fabricating topological Josephson junctions (JJs) utilizing topological superconductors has yet to be demonstrated. Controllable structural phase transition presents a unique approach to achieving topological JJs in atomically thin 2D topological superconductors. In this work, we report the pioneering demonstration of a structural phase transition from the superconducting to the semiconducting phase in the 2D topological superconductor 2M-WS₂. We reveal that the metastable 2M phase of WS₂ remains stable in ambient conditions but transitions to the 2H phase when subjected to temperatures above 150 °C. We further locally induced the 2H phase within 2M-WS₂ nanolayers using laser irradiation. Notably, the 2H phase region exhibits a hexagonal shape, and scanning tunneling microscopy uncovers an atomically sharp crystal structural transition between the 2H and 2M phase regions. Moreover, the 2M to 2H phase transition can be induced at the nanometer scale by a 200 kV electron beam. The electrical transport measurements further confirmed the superconductivity of the pristine 2M-WS₂ and the semiconducting behavior of the laser-irradiated 2M-WS₂. Our results establish a novel approach for controllable topological phase change in 2D topological superconductors, significantly impacting the development of atomically scaled planar topological JJs.

A series of hexagonal (Cr_xFe_1-x)₇C₃ solid solution compounds were synthesized for the first time as nanomaterials using a unique amine-metal oxide composite (AMOC) method. These materials are related to stainless steel where chromium provides protection from oxidation and could be potential low-cost catalyst materials for a variety of important reactions. Thus, a simple synthesis method coupled with a detailed understanding of their surface chemistry and composition is extremely important. X-ray photoelectron spectroscopy (XPS) studies revealed that both Fe and Cr in the bulk of (Cr_xFe_1-x)₇C₃ solid solutions are mostly metallic. Further, the surface of these nanomaterials maintained exposed zero valent metals (∼10 at%) after long term air exposure demonstrating the corrosion and oxidation resistant nature of this Cr-Fe-C ternary system. This suggests that these compounds have stable surface chemistry which makes them potentially good candidates for catalysts. These bimetallic carbides were subsequently tested as electrochemical catalysts for both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) applications in acidic (0.1 M HClO₄) and alkaline (0.1 M KOH) electrolytes. All (Cr_xFe_1-x)₇C₃ solid solutions exhibit better ORR and OER activities than Cr₇C₃, Fe₃C, and Co₃O₄ and both (Cr_0.4Fe_0.6)₇C₃ and (Cr_0.5Fe_0.4)₇C₃ are good OER catalysts in alkaline media demonstrating their potential for future catalysis applications.

We studied CdS QDs capped with chiral epimers, structural analogs, and positional isomers and determined that match/mismatch stereo-effects together with position and type of functional group affect the optical and chiroptical properties of CdS.

Research and catalytic testing of platinum group transition metal carbides have been extremely limited due to a lack of reliable, simple synthetic approaches. Powder samples have been reported to phase separately above 1%, and only thin-film samples have been reported to have appreciable amounts of precious metal doping. Herein, we demonstrated, through the simple co-precipitation of Pd and W or Mo precursors and their subsequent annealing, the possibility to readily form ternary carbide powders. During the investigation of the Pd-W ternary system, we discovered a new hexagonal phase, (PdW)₂C, which represents the first non-cubic Pd ternary carbide. Additionally, the solubility of Pd in the Pd-W-C and Pd-Mo-C systems was increased to 24 and 32%, respectively. As a potential application, these new materials show an enhanced activity for the methanol oxidation reaction (MOR) compared to industrial Pd/C.