Helicity-selective Raman scattering from in-plane anisotropic {\alpha}-MoO3_3

Hyperbolic crystals like {\alpha}-MoO$_3$ can support large wavevectors and photon density as compared to the commonly used dielectric crystals, which makes them a highly desirable platform for compact photonic devices. The extreme anisotropy of the dielectric constant in these crystals is intricately linked with the anisotropic character of the phonons, which along with photon confinement leads to the rich physics of phonon polaritons. However, the chiral nature of phonons in these hyperbolic crystals have not been studied in detail. In this study, we report our observations of helicity selective Raman scattering from flakes of {\alpha}-MoO$_3$. Both helicity-preserving and helicity-reversing Raman scattering are observed. We observe that helical selectivity is largely governed by the underlying crystal symmetry. This study shed light on the chiral character of the high symmetry phonons in these hyperbolic crystals. It paves the way for exploiting proposed schemes of coupling chiral phonon modes into propagating surface plasmon polaritons and for compact photonic circuits based on helical polarized light

Dual Selective Gas Sensing Characteristics of 2D α‑MoO_3–<i>x</i> via a Facile Transfer Process

Metal oxide-based gas sensor technology is promising due to their practical applications in toxic and hazardous gas detection. Orthorhombic α-MoO3 is a planar metal oxide with a unique layered structure, which can be obtained in a two-dimensional (2D) form. In the 2D form, the larger surface area-to-volume ratio of the material facilitates significantly higher interaction with gas molecules while exhibiting exceptional transport properties. The presence of oxygen vacancies results in nonstoichiometric MoO3 (MoO3–x), which further enhances the charge carrier mobility. Here, we study dual gas sensing characteristics and mechanism of 2D α-MoO3–x. Herein, conductometric dual gas sensors based on chemical vapor deposited 2D α-MoO3–x are developed and demonstrated. A facile transfer process is established to integrate the material into any arbitrary substrate. The sensors show high selectivity toward NO2 and H2S gases with response and recovery rates of 295.0 and 276.0 kΩ/s toward NO2 and 28.5 and 48.0 kΩ/s toward H2S, respectively. These gas sensors also show excellent cyclic endurance with a variation in ΔR ∼ 112 ± 1.64 and 19.5 ± 1.13 MΩ for NO2 and H2S, respectively. As such, this work presents the viability of planar 2D α-MoO3–x as a dual selective gas sensor

Visible-Active Artificial Synapses Based on Ultrathin Indium Oxide

One of the key requirements to emulate synaptic features in optoelectronic devices is the presence of persistent photoconductivity (PPC). While there are several visible-active materials, transparent semiconducting oxides (TSOs) have commercially established production processes and applications. Despite the inherently exceptional optoelectronic properties in many atomically thin TSOs along with PPC, their wide band gap renders them feasible only for ultraviolet (UV)-active synaptic applications. Hence, approaches need to be developed that allow one to tailor such semiconductors for visible-active optoelectronic synapses that are a strong emerging area of research. Over the past few years, liquid metal (LM) printing techniques have enabled the realization of many nonstratified oxides in an atomically thin form, resulting in oxide systems with enhanced optoelectronic performances, which can be further engineered using postsynthesis processing techniques. Here, we utilize a nonlayered ultrathin oxide, indium oxide (In2O3), engineered to demonstrate a photoelectrical response in the visible spectrum with a peak responsivity of 6.67 × 103 A/W at 455 nm. The 2.2 nm thin sheets operating under a driving voltage of 200 mV are successfully able to detect short pulses under 500 ms while showcasing PPC characteristics without additional bias. Key synaptic and multisynaptic functionalities are replicated using blue and green light sources, demonstrating a viable pathway to integrate atomically thin oxide semiconductors for visible light-active optoelectronic synaptic applications

Gas sensors based on the oxide skin of liquid indium

Various non-stratified two-dimensional (2D) materials can be obtained from liquid metal surfaces that are not naturally accessible

Electrically Activated UV‑A Filters Based on Electrochromic MoO_3–<i>x</i>

Chromism-based optical filters is a niche field of research, due to there being only a handful of electrochromic materials. Typically, electrochromic transition metal oxides such as MoO3 and WO3 are utilized in applications such as smart windows and electrochromic devices (ECD). Herein, we report MoO3–x-based electrically activated ultraviolet (UV) filters. The MoO3–x grown on indium tin oxide (ITO) substrate is mechanically assembled onto an electrically activated proton exchange membrane. Reversible H+ injection/extraction in MoO3–x is employed to switch the optical transmittance, enabling an electrically activated optical filter. The devices exhibit broadband transmission modulation (325–800 nm), with a peak of ∼60% in the UV-A range (350–392 nm). Comparable switching times of 8 s and a coloration efficiency of up to 116 cm2 C–1 are achieved