Quasicrystal Nanosheet/α-Fe2O3 Heterostructure-Based Low Power NO2 Sensors: Experimental and DFT Studies.

Industrial emissions, environmental monitoring, and medical fields have put forward huge demands for high-performance and low power consumption sensors. Two-dimensional quasicrystal (2D QC) nanosheets of metallic multicomponent Al70Co10Fe5Ni10Cu5 have emerged as a promising material for gas sensors due to their excellent catalytic and electronic properties. Herein, we demonstrate highly sensitive and selective NO2 sensors developed by low-cost and scalable fabrication techniques using 2D QC nanosheets and α-Fe2O3 nanoparticles. The sensitivity (ΔR/R%) of the optimal amount of 2D QC nanosheet-loaded α-Fe2O3 sensor was 32%, which is significantly larger about 3.5 times than bare α-Fe2O3 sensors for 1 ppm of NO2 at 150 °C operating temperature. The sensors exhibited p-type conduction, and resistance was reduced when exposed to NO2, an oxidizing gas. The enhanced sensing characteristics are a result of the formation of nanoheterojunctions between 2D QC and α-Fe2O3, which improved the charge transport and provided a large sensing signal. In addition, the heterojunction sensor demonstrated excellent NO2 selectivity over other oxidizing and reducing gases. Furthermore, density functional theory calculation examines the adsorption energy and charge transfer between NO2 molecules on the α-Fe2O3(110) and QC/α-Fe2O3(110) heterostructure surfaces, which coincides well with the experimental results.

Janus HfSSe monolayer: a promising candidate for SO2and COCl2gas sensing.

Janus monolayers based on transition metal dichalcogenides have garnered significant interest as potential materials for nano electronic device applications due to their exceptional physical and electronic properties. In this study, we investigate the stability of the Janus HfSSe monolayer usingab initiomolecular dynamics simulations and analyze the electronic properties in its pristine state. We then examine the impact of adsorbing toxic gas molecules (AsH3, COCl2, NH3, NO2, and SO2) on the monolayer's structure and electronic properties, testing their adsorption on different active sites on top of hafnium, selenium, and sulfur. The sensitivity of the gas molecules is quantified in terms of their adsorption energy, with the highest and lowest energies being observed for SO2(-0.278 eV) and NO2(-0.095 eV), respectively. Additionally, we calculate other properties such as recovery time, adsorption height, Bader charge, and charge difference density to determine the sensitivity and selectivity of the toxic gas molecules. Our findings suggest that the Janus HfSSe monolayer has the potential to function as SO2and COCl2gas sensor due to its high sensitivity for these two gases.

Unveiling transient current response in bilayer oxide-based physical reservoirs for time-series data analysis.

Physical reservoirs employed to map time-series data and analyze extracted features have attracted interest owing to their low training cost and mitigated interconnection complexity. This study reports a physical reservoir based on a bilayer oxide-based dynamic memristor. The proposed device exhibits a nonlinear current response and short-term memory (STM), satisfying the requirements of reservoir computing (RC). These characteristics are validated using a compact model to account for resistive switching (RS) via the dynamic evolution of the internal state variable and the relocation of oxygen vacancies. Mathematically, the transient current response can be quantitatively described according to a simple set of equations to correlate the theoretical framework with experimental results. Furthermore, the device shows significant reliability and ability to distinguish 4-bit inputs and four diverse neural firing patterns. Therefore, this work shows the feasibility of implementing physical reservoirs in hardware and advances the understanding of the dynamic response.

Enhancement of H2S sensing performance of rGO decorated CuO thin films: experimental and DFT studies.

We demonstrate a highly selective and sensitive Cupric oxide (CuO) thin film-based low concentration Hydrogen sulfide (H2S) sensor. The sensitivity was improved around three times by decorating with reduced graphene oxide (rGO) nanosheets. CuO thin films were deposited by Chemical Vapor Deposition followed by inter-digital electrode fabrication by a thermal evaporations system. The crystal structure of CuO was confirmed by x-ray diffraction. The sensing response of pristine CuO was found around 54% at 100 °C to 100 ppm of H2S. In contrast, the sensing response was enhanced to 167% by decorating with rGO of 1.5 mg ml-1concentration solution. The sensing was improved due to the formation of heterojunctions between the rGO and CuO. The developed sensor was examined under various gas environments and found to be highly selective towards H2S gas. The improvement in sensing response has been attributed to increased hole concentration in CuO in the presence of rGO due to the Fermi level alignment and increased absorption of H2S molecules at the rGO/CuO heterojunction. Further, electronic structure calculations show the physisorption behavior of H2S molecules on the different adsorption sites. Detailed insight into the gas sensing mechanism is discussed based on experimental results and electronic structure calculations.

Low-Temperature Highly Robust Hydrogen Sensor Using Pristine ZnO Nanorods with Enhanced Response and Selectivity.

We report the hydrogen-sensing response on low-cost-solution-derived ZnO nanorods (NRs) on a glass substrate, integrated with aluminum as interdigitated electrodes (IDEs). The hydrothermally grown ZnO NRs on ZnO seed-layer-glass substrates are vertically aligned and highly textured along the c-axis (002 plane) with texture coefficient ∼2.3. An optimal hydrogen-sensing response of about 21.46% is observed for 150 ppm at 150 °C, which is higher than the responses at 100 and 50 °C, which are ∼12.98 and ∼10.36%, respectively. This can be attributed to the large surface area of ∼14.51 m2/g and pore volume of ∼0.013 cm3/g, associated with NRs and related defects, especially oxygen vacancies in pristine ZnO nanorods. The selective nature is investigated with different oxidizing and reducing gases like NO2, CO, H2S, and NH3, showing relatively much lower ∼4.28, 3.42, 6.43, and 3.51% responses, respectively, at 50 °C for 50 ppm gas concentration. The impedance measurements also substantiate the same as the observed surface resistance is initially more than bulk, which reduces after introducing the hydrogen gas during sensing measurements. The humidity does not show any significant change in the hydrogen response, which is ∼20.5 ± 1.5% for a large humidity range (from 10 to 65%). More interestingly, the devices are robust against sensing response, showing no significant change after 10 months or even more.

First-principles simulation of neutral and charged oxygen vacancies in m-ZrO2: an origin of filamentary type resistive switching.

Metal oxide ZrO2has been widely explored for resistive switching application due to excellent properties like high ON/OFF ratio, superior data retention, and low operating voltage. However, the conduction mechanism at the atomistic level is still under debate. Therefore, we have performed comprehensive insights into the role of neutral and charged oxygen vacancies in conduction filament (CF) formation and rupture, which are demonstrated using the atomistic simulation based on density functional theory (DFT). Formation energy demonstrated that the fourfold coordinated oxygen vacancy is more stable. In addition, the electronic properties of the defect included supercell confirm the improvement in electrical conductivity due to the presence of additional energy states near Fermi energy. The CF formation and rupture using threefold and fourfold oxygen vacancies are demonstrated through cohesive energy, electron localization function, and band structure. Cohesive energy analysis confirms the cohesive nature of neutral oxygen vacancies while the isolated behavior for +2 charged oxygen vacancies in the CF. In addition, nudged elastic band calculation is also performed to analyze the oxygen vacancy diffusion energy under different paths. Moreover, we have computed the diffusion coefficient and drift velocity of oxygen vacancies to understand the CF. This DFT study described detailed insight into filamentary type resistive switching observed in the experimentally fabricated device. Therefore, this fundamental study provides the platform to explore the switching mechanism of other oxide materials used for memristor device application.

Synaptic Emulation via Ferroelectric P(VDF-TrFE) Reinforced Charge Trapping/Detrapping in Zinc-Tin Oxide Transistor.

Brain inspired artificial synapses are highly desirable for neuromorphic computing and are an alternative to a conventional computing system. Here, we report a simple and cost-effective ferroelectric capacitively coupled zinc-tin oxide (ZTO) thin-film transistor (TFT) topped with ferroelectric copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) for artificial synaptic devices. Ferroelectric dipoles enhance the charge trapping/detrapping effect in ZTO TFT, as confirmed by the transfer curve (ID-VG) analysis. This substantiates superior artificial synapse responses in ferroelectric-coupled ZTO TFT because the current potentiation and depression are individually improved. The ferroelectric-coupled ZTO TFT successfully emulates the essential features of the artificial synapse, including pair-pulsed facilitation (PPF) and potentiation/depression (P/D) characteristics. In addition, the device also mimics the memory consolidation behavior through intensified stimulation. This work demonstrates that the ferroelectric-coupled ZTO synaptic transistor possesses great potential as a hardware candidate for neuromorphic computing.

Strain tailored thermodynamic stability, electronic transitions, and optoelectronic properties of III (In, Ga and Al)-nitride monolayers.

The thermodynamic stability of III-nitride monolayers is calculated using the phonon band structure. Electronic properties are computed using the generalized gradient approximation-Perdew-Burke-Ernzerhof exchange-correlation potentials, which show the semiconducting behavior with bandgap 0.59 eV, 2.034 eV, and 2.906 eV for InN, GaN, and AlN monolayers, respectively. The biaxial tensile and compressive strains are used as external stimuli to understand their impact on the optoelectronic properties of these monolayers. The thermodynamic stability of strained monolayers is investigated to explore the maximum possible strains, i.e. flexibility limit, these monolayers can sustain. These monolayers are more sensitive to compressive strains, showing thermodynamic instability even at 1% compressive strain for all the considered monolayers. Further, the III-nitride monolayers are more robust with the tensile strain. InN, GaN, and AlN monolayers can sustain up to 4%, 16%, and 18% tensile strain, respectively. More interestingly, the electronic transitions, such as direct to indirect and semiconducting to metallic, are noticed with strain in the considered monolayers. The optical properties also exhibit strong strain dependency at the different transition points.

Ultrahigh sensitivity with excellent recovery time for NH3 and NO2 in pristine and defect mediated Janus WSSe monolayers.

We demonstrated ultrahigh sensitivity with excellent recovery time for H2S, NH3, NO2, and NO molecules on the sulfur and selenium surfaces of Janus WSSe monolayers using density functional theory. The selenium surface of the WSSe monolayer showed strong adsorption in comparison to the sulfur surface. The respective adsorption energies for H2S, NH3, NO2 and NO molecules are -0.193 eV, -0.220 eV, -0.276 eV, and -0.189 eV. These values are higher than the experimentally reported values for ultrahigh sensitivity gas sensors based on MoS2, MoSe2, WS2, and WSe2 monolayers. The computed adsorption energy and recovery time suggest that the desorption of gas molecules can be achieved easily in the WSSe monolayer. Further, the probable vacancy defects SV, SeV, and (S/Se)V and antisite defects SSe, and SeS are considered to understand their impact on the adsorption properties with respect to the pristine WSSe monolayer. We observed that the defect-including WSSe monolayers showed enhanced adsorption energy with fast recovery, which makes the Janus WSSe monolayer an excellent material for nanoscale gas sensors with ultrahigh sensitivity and excellent recovery time.

Multisite-Occupancy-Driven Intense Narrow-Band Blue Emission from Sr5SiO4Cl6:Eu2+ Phosphor with Excellent Stability and Color Performance.

The development of an efficient blue phosphor with remarkable thermal stability required for high-quality white-light-emitting diodes (WLEDs) remains an exigent task and mainly concerned BaMgAl10O17:Eu2+ (BAM:Eu). Despite the outstanding performance of BAM:Eu, the reduction in luminescence efficiency under long-term operation results in numerous researches on new hosts having lattice rigidity with symmetrical coordination environment. Therefore, we have synthesized a competent blue-emitting Eu2+-activated Sr5SiO4Cl6 (SSC) phosphors. The admirable rigidity of these phosphors with three Sr polyhedra Sr(I)O9, Sr(II)O7, and Sr(III)O8 assessed from Rietveld refinement indicate the dense connectivity in the crystal structure, and the ab initio calculations further support the firm electronic band structure. The broad excitation from 250 to 450 nm suitably matches the absorption band of a near-UV (n-UV) LED chip. The phosphor exhibited bright blue emission with internal quantum yield and color purity > 90% which contribute to the slender fwhm of 33 nm. The first-principle calculation indicates the most stable site for Eu2+ substitution as Sr(III)O8, and the experimental results agreed with this fact as well. The synthesized phosphor displayed an excellent thermal stability which is superior to that of the commercial BAM:Eu phosphor. The excellent thermal stability may be owed to the highly symmetric coordination environment of Eu2+ in the SSC host that are revealed from the distortion and charge density distribution calculation by density functional theory. The blue phosphor was further utilized for WLEDs and displayed white light with a high color-rendering index and suitable correlated color temperature, which is ideal for practical applications in warm WLEDs.

Complex magnetic structure and magnetocapacitance response in a non-oxide NiF2 system.

We report here on the complex magnetic structure and magnetocapacitance in NiF2, a non-oxide multifunctional system. It undergoes an anti-ferromagnetic transition near 68.5 K, superimposed with canted Ni spin driven weak ferromagnetic ordering, followed by a metastable ferromagnetic phase at or below 10 K. Our density functional calculations account for the complex magnetic structure of NiF2 deduced from the temperature and the field dependent measurements. Near room temperature, NiF2 exhibits a relatively large dielectric response reaching >103 with a low dielectric loss of <0.5 at frequencies >20 Hz. This is attributed to the intrinsic grain contribution in contrast to the grain boundary contribution in most of the known dielectric materials. The response time is 10 µs or more at 280 K. The activation energy for such temperature dependent relaxation is ~500 meV and is the main source for grain contribution. Further, a large negative magneto capacitance >90% is noticed in 1 T magnetic field. We propose that our findings provide a new non-oxide multifunctional NiF2, useful for dielectric applications.