This study was on the optoelectronic properties of multilayered two-dimensional MoS₂ and WS₂ materials on a silicon substrate using sputtering physical vapor deposition (PVD) and chemical vapor deposition (CVD) techniques. For the first time, we report ultraviolet (UV) photoresponses under air, CO₂, and O₂ environments at different flow rates. The electrical Hall effect measurement showed the existence of MoS₂ (n-type)/Si (p-type) and WS₂ (P-type)/Si (p-type) heterojunctions with a higher sheet carrier concentration of 5.50 × 10⁵ cm⁻² for WS₂ thin film. The IV electrical results revealed that WS₂ is more reactive than MoS₂ film under different gas stimuli. WS₂ film showed high stability under different bias voltages, even at zero bias voltage, due to the noticeably good carrier mobility of 29.8 × 10² cm²/V. WS₂ film indicated a fast rise/decay time of 0.23/0.21 s under air while a faster response of 0.190/0.10 s under a CO₂ environment was observed. Additionally, the external quantum efficiency of WS₂ revealed a remarkable enhancement in the CO₂ environment of 1.62 × 10⁸ compared to MoS₂ film with 6.74 × 10⁶. According to our findings, the presence of CO₂ on the surface of WS₂ improves such optoelectronic properties as photocurrent gain, photoresponsivity, external quantum efficiency, and detectivity. These results indicate potential applications of WS₂ as a photodetector under gas stimuli for future optoelectronic applications.

Vanadium oxide (VO₂) is considered a Peierls–Mott insulator with a metal–insulator transition (MIT) at T_c = 68^◦ C. The tuning of MIT parameters is a crucial point to use VO₂ within thermoelectric, electrochromic, or thermochromic applications. In this study, the effect of oxygen deficiencies, strain engineering, and metal tungsten doping are combined to tune the MIT with a low phase transition of 20^◦C in the air without capsulation. Narrow hysteresis phase transition devices based on multilayer VO₂, WO₃, Mo_0.2W_0.8O_3, and/or MoO₃ oxide thin films deposited through a high vacuum sputtering are investigated. The deposited films are structurally, chemically, electrically, and optically characterized. Different conductivity behaviour was observed, with the highest value towards VO_1.75/WO_2.94 and the lowest VO_1.75 on FTO glass. VO_1.75/WO_2.94 showed a narrow hysteresis curve with a single-phase transition. Thanks to the role of oxygen vacancies, the MIT temperature decreased to 35^◦C, while the lowest value (T_c = 20^◦C) was reached with Mo_0.2W_0.8O₃/VO₂/MoO₃ structure. In this former sample, Mo_0.2W_0.8O₃ was used for the first time as an anti-reflective and anti-oxidative layer. The results showed that the MoO₃ bottom layer is more suitable than WO₃ to enhance the electrical properties of VO₂ thin films. This work is applied to fast phase transition devices.

The distinctive properties of strongly correlated oxides provide a variety of possibilities for modulating the properties of 2D transition metal dichalcogenides semiconductors; which represent a new class of superior optical and optoelectronic interfacing semiconductors. We report a novel approach to scaling-up molybdenum disulfide (MoS₂) by combining the techniques of chemical and physical vapor deposition (CVD and PVD) and interfacing with a thin layer of monoclinic VO₂. MoWO₃/VO₂/MoS₂ photodetectors were manufactured at different sputtering times by depositing molybdenum oxide layers using a PVD technique on p-type silicon substrates followed by a sulphurization process in the CVD chamber. The high quality and the excellent structural and absorption properties of MoWO₃/VO₂/MoS₂/Si with MoS₂ deposited for 60 s enables its use as an efficient UV photodetector. The electronically coupled monoclinic VO₂ layer on MoS₂/Si causes a redshift and intensive MoS₂ Raman peaks. Interestingly, the incorporation of VO₂ dramatically changes the ratio between A-exciton (ground state exciton) and trion photoluminescence intensities of VO₂/(30 s)MoS₂/Si from < 1 to > 1. By increasing the deposition time of MoS₂ from 60 to 180 s, the relative intensity of the B-exciton/A-exciton increases, whereas the lowest ratio at deposition time of 60 s refers to the high quality and low defect densities of the VO₂/(60 s)MoS₂/Si structure. Both the VO₂/(60 s)MoS₂/Si trion and A-exciton peaks have higher intensities compared with (60 s) MoS₂/Si structure. The MoWO₃/VO₂/(60 s)MoS₂/Si photodetector displays the highest photocurrent gain of 1.6, 4.32 × 10⁸ Jones detectivity, and ~ 1.0 × 10¹⁰ quantum efficiency at 365 nm. Moreover, the surface roughness and grains mapping are studied and a low semiconducting-metallic phase transition is observed at ~ 40 °C.

The greenhouse effect is involving global heating and climate change within the world. Carbon dioxide (CO₂)is one of the major gas at the origin of this effect, but also the byproduct of human activity. Therefore, monitoring the indoor/outdoor CO₂ emission by gas sensors is one of the priorities for environmental preservation. In this paper, the sensing performance of CO₂ towards two different O-rich films have been studied; graphene oxide (GO)and vanadium dioxide (VO₂). The preparation of GO film has been carried out by spray pyrolysis on fluorine tin oxide (FTO)prepared by the modified Hummers method. While the VO2 film has been sol-gel spin-coated on a glass substrate. Both films have been characterized using XRD, SEM and electrical properties. The CO₂ gas sensing mechanism and the role of oxygen vacancies in VO₂ are addressing. The oxygen functional groups in GO play a main role in the CO₂ gas the sensitivity level and response time. Their gas sensing performances have been investigated based on measuring the response vs recovery time, dynamic response curve analysis and sensitivity. In order to better understand the sensing mechanism, characterization has been done with different gas concentrations. Both GO and VO₂ based CO₂-sensors are acted as an n-type sensor. Sensing behavior of GO at RT has explained to be mainly mediated by the oxygen functional groups and a wide range of active sites. In the other hand, VO₂ contains oxygen vacancies and more defect sites which play a main role in the RT sensing activity and low recovery time.

The study of the plasmonic coupling between metallic nanorods (NRs) is essential for many modern applications. In this paper, we present theoretical studies of scattering cross sections for Au-Au, Au-Ag, and Ag-Ag dimers based on the finite element method (FEM) through COMSOL Multiphysics software. Hybridization model has proposed to explain the plasmon coupling in the different dimers configurations. The single Au nanorod (Au NR) exhibits only a single plasmon resonance, which is red-shifted with increasing the NR length because intra-rod restoring forces are reduced. For asymmetric Au-Au and Au-Ag with end-to-end configuration, two peaks are appearing in the spectrum corresponding to bright (low energy) and dark (high energy) modes. The bright mode interacts strongly with the incident field compared to weak interaction of the dark mode. The bright mode is progressively blue shifted while the dark mode is slightly red-shifted as the gap between NRs increases. In the case of Ag-Ag asymmetric configuration, the two NRs are placed perpendicular to each other; the transverse and longitudinal surface plasmon modes are observed. Also, many dark plasmon hybridization coupling modes are illustrated in the range from 100 to 400 nm and ascribed to higher order plasmonic modes due to the dipole-dipole interaction. The coupling interactions between two metallic NRs can lead to significant plasmon shifts and enormous electric field enhancements. These are very useful for environmental detections, nanoantenna, Raman spectroscopy and solar cell. Also, the results of asymmetric Ag-Ag plasmonic produce new plasmon modes in the UV region that open up new opportunities for designing plasmonic devices.

In this report, the structures, morphologies, optical, electrical and gas sensing properties of ZnO and ZnO: Na spin-coated films are studied. X-ray diffraction (XRD) results reveal that the films are of a single phase wurtzite ZnO with a preferential orientation along (002) direction parallel to c-axis. Na doping reduces the crystalline quality of the films. The plane surface of ZnO film turned to be wrinkle net-work structure after doping. The reflectance and the optical band gap of the ZnO film decreased after Na doping. The wrinkle net-work nanostructured Na-doped film shows an unusually sensitivity, 81.9% @ 50 sccm, for CO₂ gas at room temperature compared to 1.0% for the pure ZnO film. The signals to noise ratio (SNR) and detection limit of Na-doped ZnO sensor are 0.24 and 0.42 sccm, respectively. These enhanced sensing properties are ascribed to high surface-to-volume ratio, hoping effect, and the increase of O-vacancies density according to Kroger VinK effect. The response time increased from 179 to 240 s by the incorporation of Na atoms @50 sccm. This response time increased as the CO₂ concentration increased. The recovery time is increased from 122 to 472 s by the incorporation of Na atoms @50 sccm.