Layered two-dimensional transition metal dichalcogenides, due to their semiconducting nature and large surface-to-volume ratio, have created their own niche in the field of gas sensing. Their large recovery time and accompanied incomplete recovery result in inferior sensing properties. Here, we report a composite-based strategy to overcome these issues. In this study, we report a facile double-step synthesis of a MoS 2 /SnO 2 composite and its successful use as a superior room-temperature ammonia sensor. Contrary to the pristine nanosheet-based sensors, the devices made using the composite display superior gas sensing characteristics with faster response. Specifically, at room temperature (30° C), the composite-based sensor exhibited excellent sensitivity (10%) at an ammonia concentration down to 0.4 ppm along with the response and recovery times of 2 and 10 s, respectively. Moreover, the device also exhibited long-term durability, reproducibility, and selectivity toward ammonia against hydrogen sulfide, methanol, ethanol, benzene, acetone, and formaldehyde. Sensor devices made on quartz and alumina substrates with different roughnesses have yielded almost an identical response, except for slight variations in response and recovery transients. Further, to shed light on the underlying adsorption energetics and selectivity, density functional theory simulations were employed. The improved response and enhanced selectivity of the composite were explicitly discussed in terms of adsorption energy. Lowdin charge analysis was performed to understand the charge transfer mechanism between NH 3 , H 2 S, CH 3 OH, HCHO, and the underlying MoS 2 /SnO 2 composite surface. The long-term durability of the sensor was evident from the stable response curves even after 2 months. These results indicate that hydrothermally synthesized MoS 2 /SnO 2 composite-based gas sensors can be used as a promising sensing material for monitoring ammonia gas in real fields.

Half-Heusler compound LiMgBi exhibits pressure driven Topological Insulating (TI) nature with robust spin-locked dissipationless Fermion transport along the surface states.

We employ first-principles calculations to investigate the topological states (TS) and thermoelectric (TE) transport properties of three dimensional (3D) gold iodide (AuI) which belongs to the zincblende family. We explore, semi-metal (SM) to topological conductor (TC) and topological insulator (TI) phase transitions. Under pristine conditions, AuI exhibits Dirac SM nature but, under the influence of mild isotropic compressive pressure the system undergoes electronic quantum phase transition driving it into non-trivial topological state. This state exhibits Dresselhaus like band spin splitting leading to a TC state. In order to realize TI state from the SM state, we break the cubic symmetry of the system by introducing a compressive pressure along (001) crystal direction. The non-trivial TI nature of the system is characterized by the emergence of robust surface states and the Z 2 invariant ν 0 = 1 which indicates a strong TI nature. A novel facet of the phase transition discussed here is, the −s and −p, −d orbital band inversion mechanism which is unconventional as compared to previously explored TI families. This mechanism unravels new path by which TI materials can be predicted. Also, we investigated the lattice and electronic contributions to the TE transport properties. We characterize the TE performance by calculating the figure of merit (zT) and find that, at room temperature (300 K) and for a fixed doping concentration (i.e., n = 1 × 1019 cm−3) the zT is 0.55 and 0.53 for electrons and holes respectively. This is quite remarkable since, higher values of zT are generally predicted at higher temperature scales whereas, zT values as in the present case are desired at room temperatures for various energy applications. The manifestation of non-trivial TS governed by the unconventional band inversion mechanism and the TE properties of AuI make it a unique multi-functional candidate with probable thermoelectric and spintronic applications.

We propose a novel technique of dimensional engineering to realize low dimensional topological insulator from a trivial three dimensional parent. This is achieved by confining the bulk system to one dimension along a particular crystal direction, thus enhancing the quantum confinement effects in the system. We investigate this mechanism in the Half-Heusler compound LiMgAs with face-centered cubic (FCC) structure. At ambient conditions the bulk FCC structure exhibits a semi-conducting nature. But, under the influence of high volume expansive pressure (VEP) the system undergoes a topological phase transition (TPT) from semi-conducting to semi-metallic forming a Dirac cone. At a critical VEP we observe that, spin-orbit coupling (SOC) effects introduce a gap of $$\sim$$ ∼ 1.5 meV in the Dirac cone at high symmetry point $$\Gamma$$ Γ in the Brillouin zone. This phase of bulk LiMgAs exhibits a trivial nature characterized by the $${\mathbb {Z}}_2$$ Z 2 invariants as (0,000). By further performing dimensional engineering, we cleave [111] plane from the bulk FCC structure and confine the system in one dimension. This low-dimensional phase of LiMgAs has structure similar to the two dimensional $${\text {1T-MoS}}_2$$ 1T-MoS 2 system. Under a relatively lower compressive strain, the low-dimensional system undergoes a TPT and exhibits a non-trivial topological nature characterized by the SOC gap of $$\sim$$ ∼ 55 meV and $${\mathbb {Z}}_2$$ Z 2 invariant $$\nu$$ ν = 1. Although both, the low-dimensional and bulk phase exhibit edge and surface states, the low-dimensional phase is far more superior and exceptional as compared to the bulk parent in terms of the velocity of Fermions ($${\text {v}}_f$$ v f ) across the surface states. Such a system has promising applications in nano-electronics.

Topological Insulators (TI) are materials with novel quantum states which exhibit a bulk insulating gap while the edge/surface is conducting. This has been extensively explored in several Half-Heusler (HH) compounds...