Gate-Tunable Thermal Metal-Insulator Transition in VO2 Monolithically Integrated into a WSe2 Field-Effect Transistor

Plasmonic switches based on VO2as the phase change material

In this paper, a comprehensive review of the recent advancements in the design and development of plasmonic switches based on vanadium dioxide (VO2) is presented. Plasmonic switches are employed in applications such as integrated photonics, plasmonic logic circuits and computing networks for light routing and switching, and are based on the switching of the plasmonic properties under the effect of an external stimulus. In the last few decades, plasmonic switches have seen a significant growth because of their ultra-fast switching speed, wide spectral tunability, ultra-compact size, and low losses. In this review, first, the mechanism of the semiconductor to metal phase transition in VO2is discussed and the reasons for employing VO2over other phase change materials for plasmonic switching are described. Subsequently, an exhaustive review and comparison of the current state-of-the-art plasmonic switches based on VO2proposed in the last decade is carried out. As the phase transition in VO2can be activated by application of temperature, voltage or optical light pulses, this review paper has been categorized into thermally-activated, electrically-activated, and optically-activated plasmonic switches based on VO2operating in the visible, near-infrared, infrared and terahertz frequency regions.

Topochemical Transformation of Two-Dimensional VSe2 into Metallic Nonlayered VO2 for Water Splitting Reactions in Acidic and Alkaline Media

The engineering of the structural and morphological properties of nanomaterials is a fundamental aspect to attain desired performance in energy storage/conversion systems and multifunctional composites. We report the synthesis of room temperature-stable metallic rutile VO₂ (VO₂ (R)) nanosheets by topochemically transforming liquid-phase exfoliated VSe₂ in a reductive Ar-H₂ atmosphere. The as-produced VO₂ (R) represents an example of two-dimensional (2D) nonlayered materials, whose bulk counterparts do not have a layered structure composed by layers held together by van der Waals force or electrostatic forces between charged layers and counterbalancing ions amid them. By pretreating the VSe₂ nanosheets by O₂ plasma, the resulting 2D VO₂ (R) nanosheets exhibit a porous morphology that increases the material specific surface area while introducing defective sites. The as-synthesized porous (holey)-VO₂ (R) nanosheets are investigated as metallic catalysts for the water splitting reactions in both acidic and alkaline media, reaching a maximum mass activity of 972.3 A g^-1 at -0.300 V vs RHE for the hydrogen evolution reaction (HER) in 0.5 M H₂SO₄ (faradaic efficiency = 100%, overpotential for the HER at 10 mA cm^-2 = 0.184 V) and a mass activity (calculated for a non 100% faradaic efficiency) of 745.9 A g^-1 at +1.580 V vs RHE for the oxygen evolution reaction (OER) in 1 M KOH (overpotential for the OER at 10 mA cm^-2 = 0.209 V). By demonstrating proof-of-concept electrolyzers, our results show the possibility to synthesize special material phases through topochemical conversion of 2D materials for advanced energy-related applications.

Role of Surface Termination in the Metal-Insulator Transition of V2O3(0001) Ultrathin Films

Surface termination is known to play an important role in determining the physical properties of materials. It is crucial to know how surface termination affects the metal-insulator transition (MIT) of V₂O₃ films for both fundamental understanding and its applications. By changing growth parameters, we achieved a variety of surface terminations in V₂O₃ films that are characterized by low-energy electron diffraction (LEED) and photoemission spectroscopy techniques. Depending upon the terminations, our results show that MIT can be partially or fully suppressed near the surface region due to the different fillings of the electrons at the surface and subsurface layers and the change of screening length compared to the bulk. Across MIT, a strong redistribution of spectral weight and its transfer from a high-to-low-binding energy regime is observed in a wide energy scale. Our results show that the total spectral weight in the low-energy regime is not conserved across MIT, indicating a breakdown of the "sum rules of spectral weight", signature of a strongly correlated system. Such a change in spectral weight is possibly linked to the change in hybridization, lattice volume (i.e., effective carrier density), and the spin degree of freedom in the system that occurs across MIT. We find that MIT in this system is strongly correlation-driven, where the electron-electron interactions play a pivotal role. Moreover, our results provide better insight into the understanding of the electronic structure of strongly correlated systems and highlight the importance of accounting for surface effects during interpretation of the physical property data mainly using surface-sensitive probes, such as surface resistivity.

Experimental and Computational Investigation of Layer-Dependent Thermal Conductivities and Interfacial Thermal Conductance of One- to Three-Layer WSe2

Two-dimensional materials such as graphene and transition metal dichalcogenides (TMDCs) have received extensive research interest and investigations in the past decade. In this research, we used a refined opto-thermal Raman technique to explore the thermal transport properties of one popular TMDC material WSe₂, in the single-layer (1L), bilayer (2L), and trilayer (3L) forms. This measurement technique is direct without additional processing to the material, and the absorption coefficient of WSe₂ is discovered during the measurement process to further increase this technique's precision. By comparing the sample's Raman spectroscopy spectra through two different laser spot sizes, we are able to obtain two parameters-lateral thermal conductivities of 1L-3L WSe₂ and the interfacial thermal conductance between 1L-3L WSe₂ and the substrate. We also implemented full-atom nonequilibrium molecular dynamics simulations (NEMD) to computationally investigate the thermal conductivities of 1L-3L WSe₂ to provide comprehensive evidence and confirm the experimental results. The trend of the layer-dependent lateral thermal conductivities and interfacial thermal conductance of 1L-3L WSe₂ is discovered. The room-temperature thermal conductivities for 1L-3L WSe₂ are 37 ± 12, 24 ± 12, and 20 ± 6 W/(m·K), respectively. The suspended 1L WSe₂ possesses a thermal conductivity of 49 ± 14 W/(m·K). Crucially, the interfacial thermal conductance values between 1L-3L WSe₂ and the substrate are found to be 2.95 ± 0.46, 3.45 ± 0.50, and 3.46 ± 0.45 MW/(m²·K), respectively, with a flattened trend starting the 2L, a finding that provides the key information for thermal management and thermoelectric designs.

Phase Transition-Induced Temperature-Dependent Phonon Shifts in Molybdenum Disulfide Monolayers Interfaced with a Vanadium Dioxide Film

We report the optical phonon shifts induced by phase transition effects of vanadium dioxide (VO₂) in monolayer molybdenum disulfide (MoS₂) when interfacing with a VO₂ film showing a metal-insulator transition coupled with structural phase transition (SPT). To this end, the monolayer MoS₂ directly synthesized on a SiO₂/Si substrate by chemical vapor deposition was first transferred onto a VO₂/c-Al₂O₃ substrate in which the VO₂ film was prepared by a sputtering method. We compared the MoS₂ interfaced with the VO₂ film with the as-synthesized MoS₂ by using Raman spectroscopy. The temperature-dependent Raman scattering characteristics exhibited the distinct phonon behaviors of the E_2g¹ and A_1g modes in the monolayer MoS₂. Specifically, for the as-synthesized MoS₂, there were no Raman shifts for each mode, but the enhancement in the Raman intensities of E_2g¹ and A_1g modes was clearly observed with increasing temperature, which could be interpreted by the significant contribution of the interface optical interference effect. In contrast, the red-shifts of both the E_2g¹ and A_1g modes for the MoS₂ transferred onto VO₂ were clearly observed across the phase transition of VO₂, which could be explained in terms of the in-plane tensile strain effect induced by the SPT and the enhancement of electron-phonon interactions due to an increased electron density at the MoS₂/VO₂ interface through the electronic phase transition. This study provides further insights into the influence of interfacial hybridization for the heterogeneous integration of 2D transition-metal dichalcogenides and strongly correlated materials.

Barrier Formation at the Contacts of Vanadium Dioxide and Transition-Metal Dichalcogenides

Phase-transition field-effect transistors (FETs) are a class of steep-slope devices that show abrupt on/off switching owing to the metal-insulator transition (MIT) induced in the contacting materials. An important avenue to develop phase-transition FETs is to understand the charge injection mechanism at the junction of the contacting MIT materials and semiconductor channels. Here, toward the realization of high-performance phase-transition FETs, we investigate the contact properties of heterojunctions between semiconducting transition-metal dichalcogenides (TMDCs) and vanadium dioxide (VO₂) that undergoes a MIT at a critical temperature (T_c) of approximately 340 K. We fabricated transistors based on molybdenum disulfide (MoS₂) and tungsten diselenide (WSe₂) in contact with the VO₂ source/drain electrodes. The VO₂-contacted MoS₂ transistor exhibited n-type transport both below and above T_c. Across the MIT, the on-current was observed to increase only by a factor of 5, in contrast to the order-of-magnitude change in the resistance of the VO₂ electrodes, suggesting the existence of high contact resistance. The Arrhenius analyses of the gate-dependent drain current confirmed the formation of the interfacial barrier at the VO₂/MoS₂ contacts, irrespective of the phase state of VO₂. The VO₂-contacted WSe₂ transistor showed ambipolar transport, indicating that the Fermi level lies near the mid gap of WSe₂. These observations provide insights into the contact properties of phase-transition FETs based on VO₂ and TMDCs and suggest the need for contact engineering for high-performance operations.