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This study presents a new S-band-receiving phased-array antenna with a phase-deviation-minimized calibration method for the ground station of a low Earth orbit (LEO) satellite. The proposed antenna consists of 16 subarrays, 16 beamforming receiving RF modules (BF-RFMs), a power/control board, and a 16-way feed network. The subarray was achieved by joining two 8 × 1 arrays with a two-way power combiner. The 16-element antenna subarrays showed a gain of 16.1 dBi and a reflection coefficient of less than −10 dB from 2.12 GHz to 2.45 GHz. The BF-RFM, which consists of three low-noise amplifiers (LNAs), a power combiner, a phase shifter, and a digital attenuator, was designed and fabricated. The BF-RFMs were provided by the power/control board and showed a gain of 30.8 ± 0.8 dB, an amplitude root-mean-square (RMS) error from 0.25 dB to 0.28 dB, and a phase RMS error from 1.8° to 2.5° over the Rx frequency range. The arrangement procedures of the 16 BF-RFMs are presented to increase beam pointing accuracy at the desired angle. A commercial 16-way feed network was employed to combine all the output ports of the 16 BF-RFMs. The assembled antenna, which has dimensions of 1.58 m × 1.58 m × 0.2 m, was measured by partial and full scans in the near-field scanning system. The back-projected algorithm was employed to calibrate the antenna’s gain patterns in the partial scan. The implemented phased-array antenna had a gain greater than 28.14 dBi, sidelobe levels less than −17.1 dB, and beam pointing errors less than 0.07° over the beam pointing angle of −20~+20°. Based on the implemented antenna system, we conducted a field test using KOMPSAT-5, which is actually operating in South Korea, in order to verify the performance of the low Earth orbit (LEO) satellite ground station system.

Recently, transparent conducting oxides (TCOs) have attracted increasing attention in the development of electronic devices and energy harvesting applications. Standard materials used as TCOs are Sn-doped indium oxide (ITO) and F-doped tin oxide (FTO), which are ternary systems and require a doping process. Here, we introduce the synergistic effect of using microwave (MW) irradiation and Au nanoparticles (NPs) together on tin dioxide (SnO2) thin films grown by atomic layer deposition (ALD), which have a binary system and do not need any additional doping process. For comparison to as-deposited SnO2 thin films, we prepared MW-irradiated SnO2 thin films with and without Au NP decoration. SnO2 thin films with Au NPs added during MW irradiation have carrier concentration, Hall mobility, and resistivity of 1.45 × 1022 cm−3, 5.4 cm2/Vs, and 7.93 × 10−5 Ω cm, respectively. Compared with as-deposited SnO2 thin films, these electrical properties are significantly enhanced, and plasmonic effects also occurred due to the presence of Au NPs. The enhanced properties are mainly attributed to generation of oxygen vacancies in SnO2 thin films. Our results demonstrate the potential of using SnO2 thin films as binary system TCOs, and point to the synergistic effects of using MW and Au NPs together.

Metal oxide semiconductors with branched structures, such as branched nanowires (b-NWs), have promising properties for being used in gas sensors. In this work, we synthesized Pt-decorated Bi2O3-branched SnO2 nanowires (NWs). NO2 sensing studies revealed the superior capacity of a Pt-decorated Bi2O3-branched SnO2 NWs gas sensor relative to pristine and branched SnO2 gas sensors, and it worked at near room temperature (50 °C). The increased sensing capacity was related to the synergistic effects of Pt decoration and Bi2O3 branching, particularly the morphology of the gas sensor with branched structures, the promising effects of Pt as a noble metal with good catalytic activity, and the generation of homo- and heterojunctions in the Pt-decorated Bi2O3-branched SnO2 NWs gas sensor. The results obtained in this work are useful for design and development of NO2 gas sensors using a simple strategy, which can be easily extended to various metal oxides.

In this study, reduced graphene oxide (RGO) nanosheets were synthesized and then functionalized with Pt nanoparticles (NPs) for gas sensing studies at room temperature. Pt-functionalized RGOs (Pt-RGOs) were irradiated by a 2 MeV electron beam with different doses of 100 and 500 kGy (Pt-RGO-100 and Pt-RGO-500) to study the role of electron beam irradiation (EBI) on properties and NO2 gas sensing characteristics. Different characterization techniques verified the generation of structural defects after EBI. Additionally, NO2 sensing tests at room temperature revealed that the Pt-RGO-100 sensor had the highest response to NO2 gas as well as the shortest response time. We surmise that the improved sensing signal with respect to NO2 gas for Pt-RGO-100 sensor was related to the role of oxygen functional groups. However, decreases in the response for Pt-RGO-500 sensor were related to the creation of reactive oxygen gases that could be adsorbed on the active defects of the RGO by the interaction of electron beams with oxygen species in the ambient air, leading to a decrease of the effective defect sites and decreases of the sensing response. This work suggests progress towards an improved performance of RGO-based gas sensors that can work at low temperature, by using EBI as a clean tool applied at room temperature with Pt-functionalization.

Graphene-based fibers (GFs) have aroused enormous interest in portable, wearable electronics because of their excellent mechanical flexibility, electrical conductivity, and weavability, which make them advantageous for wearable electronic devices. Herein, we report the development of metal binder-free Ti3C2Tx MXene/graphene hybrid fibers by a scalable wet-spinning process. These hybrid fibers exhibit excellent mechanical and electrical properties for applications in flexible wearable gas sensors. The synergistic effects of electronic properties and gas-adsorption capabilities of MXene/graphene allow the created fibers to show high NH3 gas sensitivity at room temperature. The hybrid fibers exhibited significantly improved NH3 sensing response (ΔR/R0 = 6.77%) compared with individual MXene and graphene. The hybrid fibers also showed excellent mechanical flexibility with a minimal fluctuation of resistance of ±0.2% and low noise resistance even after bending over 2000 cycles, enabling gas sensing during deformation. Furthermore, flexible MXene/graphene hybrid fibers were woven into a lab coat, demonstrating their high potential for wearable devices. We envisage that these exciting features of 2D hybrid materials will provide a novel pathway for designing next-generation portable wearable gas sensors.

In this work, a low-temperature (100 °C) gas sensor based on heterojunctions of porous Si and SnO2 nanowires (NWs) is presented. Porous Si was obtained from p-Si wafers by electrochemical etching, and SnO2 NWs were fabricated by a vapor-liquid-solid route. Different characterization techniques were used to verify the formation of porous Si/SnO2 NW heterojunctions. H2S gas sensing results showed enhanced gas sensing performance of the porous Si/SnO2 NW sensor in comparison with that of a porous Si sensor. The reasons for such enhancement are discussed in detail. This study demonstrates the promising effects of SnO2 NWs in combination with porous Si to realize low-temperature H2S gas sensors that are highly compatible with existing Si processing technology.

To attain high life standards, it is important to develop high-performance non-toxic gas sensors for public safety, environmental pollutant control, industrial processes, etc. Because reports on single element semiconductor-coated semiconducting metal oxides for sensing applications are rare, we synthesized SnO2 nanowires and coated them with a 5 nm-thick or 10 nm-thick Si layer for H2S gas sensing studies. SnO2 nanowires were successfully synthesized using a highly pure metallic Sn powder at high temperature in a tube furnace by the vapor-liquid-solid method and Si was deposited on the nanowires by the sputtering technique. The desired morphology and composition of the synthesized nanowires were confirmed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Moreover, the gas sensing characteristics of pristine and Si-coated SnO2 nanowires toward H2S, CO, H2, C6H6, C2H5OH and C6H7 gases were investigated. The sensing results revealed a good response to H2S at the optimum operational temperature of 100 °C. Notably, Si-coated SnO2 nanowire sensors showed a better response to H2S than pristine SnO2 nanowires. The mechanism of H2S sensing is discussed in detail here. This study shows that the Si coating on the SnO2 nanowire enhances its sensing performance and decreases the sensing temperature required for H2S gas detection. (Received May 3, 2019; Accepted September 7, 2019)

A new gas sensor working in room temperature, which is compatible with silicon fabrication technology is presented. Porous silicon nanowires (NWs) were synthesized by metal-assisted chemical etching method and then TeO₂ NWs branches were attached to their stem by thermal evaporation of Te powders in the presence of air. Afterwards TeO₂ branched porous Si NWs were functionalized by Pt via sputtering followed by low temperature thermal annealing. Scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy collectively confirmed successful formation of TeO₂ branched porous Si NWs functionalized by Pt nanoparticles. Their gas sensing properties in the presence of CO, C6H6 and C7H8 were tested at room temperature, for Si wafer, pristine porous Si NWs, pristine TeO₂ branched porous Si NWs, and Pt functionalized TeO₂ branched porous Si NWs sensors. Pt functionalized TeO₂ branched porous Si NWs have higher responses to all tested gases than the other sensors. The origin of high response is discussed in detail. This new room temperature gas sensor can open a new aperture for development of gas sensors with minimum energy consumption which are compatible with silicon fabrication technology.

Branched nanowires (NWs) are a novel class of composite materials with increased surface area relative to their pristine one-dimensional counterparts. Accordingly, they are good choice for gas sensing studies. In this study, p-n, TeO2-branched SnO2 NWs were produced by a two-step catalyst-assisted vapor-liquid-solid (VLS) growth technique for gas sensing studies. First, SnO2 NWs were synthesized from highly pure Sn powders, and TeO2 branches were subsequently added. The fabricated samples were well characterized in terms of morphology, crystallinity, and chemical composition. Gas sensing results exhibited the enhanced NO2 sensing capability of TeO2 branched SnO2 NW sensors relative to pristine SnO2 NWs. In particular, the maximum responses (Rg/Ra) of pristine and TeO2 branched SnO2 sensors to 10 ppm NO2 were 6.34 and 10.25, respectively. Furthermore, dynamics of TeO2 branched sensor at the optimal temperature was faster. Superior sensing properties of TeO2 branched SnO2 NWs were related to the high surface area of the branched sensors and creation of p-n heterojunctions on the surfaces of this sensor. We believe that branching is a good way to realize gas sensors for practical usages.

A simple yet powerful flame chemical vapor deposition technique is proposed that allows free control of the surface morphology, microstructure, and composition of existing materials with regard to various functionalities within a short process time (in seconds) at room temperature and atmospheric pressure as per the requirement. Since the heat energy is directly transferred to the material surface, the redox periodically converges to the energy dynamic equilibrium depending on the energy injection time; therefore, bidirectional transition between the semiconductor/metal is optionally available. To demonstrate this, a variety of Sn-based particles were created on preformed SnO2 nanowires, and this has been interpreted as a new mechanism for the response and response times of gas-sensing, which are representative indicators of the most surface-sensitive applications and show one-to-one correspondence between theoretical and experimental results. The detailed technologies derived herein are clearly influential in both research and industry.

In view of the various qualifications required of materials nowadays, efforts to change the characteristics of inherent materials have continued. However, most material conversion techniques that have been used in the past, such as alloy design and doping effect, cannot overcome the limitation that properties are only added to the original characteristics of pristine materials. Therefore, herein, we introduced a new material design technique, a so-called “Molecular Group System”, which is completely different from existing methods. Since whole-set-systems are considered one-energetic-unit-system, either only the merits of the constituent elements can be emphasized or new materials completely different from the raw ones can be synthesized. In this study, block-stacking bottom-up approach was employed to form a one group system from SnO2, SnOx, Sn, and graphene powders, and a binder, using high-energy irradiation. Then, we discussed theoretical verifications such as SnO2-reduction and Sn-channeling.

A novel gas sensor fabricated from ZnO-shelled Si nanowires (SiNWs) is presented. After coating a thin layer of Au on the surfaces of Si NWs, ZnO layers were formed on the surfaces of p-SiNWs by thermal evaporation of Zn powders and a subsequent oxidation process. Microscopic analysis confirmed the successful formation of ZnO-Si core-shell NWs with Au nanoparticles present on the shell surface. The gas sensing performance of the gas sensor fabricated using the p-Si/n-ZnO core-shell NWs was evaluated for various gases. The sensor exhibited outstanding response and selectivity to H2S gas. The gas sensing mechanism was evaluated in detail and attributed to various factors, including the formation of ZnO/Si and Au/ZnO heterojunctions and the chemical attraction between ZnO and Au. The results demonstrate a new sensing material for H2S detection in various fields that can be easily incorporated into Si-based devices.

Branched nanowires (b-NWs) have been used for advanced applications because of their nanoscale dimensions, high surface area, and scalable synthetic methods. Herein, for the first time, we report the fabrication of Au-functionalized ZnO-branched SnO2 NWs for enhanced gas sensing application. As-fabricated NWs were structurally and morphologically investigated by X-ray diffraction, scanning electron microscopy as well as transmission electron microscopy to reveals the crystallinity and morphology of NWs. By comparing the gas sensing properties of Au-functionalized b-NWs, b-NWs, and pristine SnO2 NWs gas sensors, in terms of their sensitivity and cross-sensitivity towards NO2, we demonstrate the superior sensing properties of Au-functionalized b-NWs sensors. In particular the Au-functionalized b-NWs sensor showed a response (Rg/Ra) of 13.07 to 10 ppm NO2 gas which was higher than those of b-NWs, and pristine SnO2 NWs gas sensors. In addition to the special architecture of sensors (i.e. ZnO b-NWs on SnO2 NWs), which produces a large number of hetero- and homojunctions (a branching effect), Au acts as a catalyst to further improve the sensing characteristics of this particular sensor (a functionalization effect). According to the obtained results, we believe that metal oxide gas sensors based on noble metal-functionalized b-NWs, which use a facile synthesis procedure and produce enhanced sensing performance, will play a vital role in the future of the sensor industry.

We report the electrical and optical characteristics of SnO2 thin films irradiated by microwaves (MWs) and grown using atomic layer deposition in a commercial MW oven operating at a frequency of 2.45 GHz. The properties of the MW-irradiated SnO2 thin films were compared with those of the as-deposited SnO2 thin films. After MW irradiation, the conductivity and transparency of the thin films were enhanced. In addition, the samples irradiated for 5 min showed optimal carrier concentration, Hall mobility, resistivity, and transmittance values of 1.5 × 1020 cm−3, 4.6 cm2/V s, 8 × 10−3 Ω cm, and 95.77%, respectively. The improved properties of the MW-irradiated samples were attributed mainly to the formation of an oxygen vacancy in the SnO2 lattice during MW irradiation. Our results can be applied for the fabrication of pure SnO2-based transparent conductive oxides; these oxides are generally doped with other elements.

We report the gas-sensing properties of pristine and SnS2 functionalized TeO2 nanowires (NWs). TeO2 NWs were synthesized by a vapor–liquid–solid growth method, and SnS2 functionalization was performed using an atomic layer deposition technique followed by thermal treatment. Structural and morphological analyses verified the formation of pristine and SnS2 functionalized TeO2 NWs with desired composition, phase, and morphology. Interestingly, sensing results showed that the pristine TeO2 NW gas sensor had better sensing properties relative to the SnS2 functionalized TeO2 NW gas sensor. An underlying sensing mechanism is explained in detail, and reasons for the decrease of sensing performance with the SnS2 functionalized TeO2 NW sensor was attributed to the coverage of TeO2 surface by the SnS2 nanoparticles.

We present a highly sensitive and selective NO2 sensor based on Bi2O3 branched SnO2 nanowires (NWs). SnO2 NWs were first synthesized by a vapor-liquid-solid method, were coated with an Au layer, and Bi2O3 branches were grown on their stems by the same procedure used for pure Bi powders. The fabricated sensor showed a high response (Rg/Ra) of 56.92 to 2 ppm of NO2 gas at an optimal temperature. Furthermore, its response to other interfering gases such as ethanol, acetone, toluene, and benzene, was less than 1.55, which demonstrated excellent selectivity of the sensor towards NO2 gas. For comparison and to better understand the sensing mechanism, a pristine SnO2 NWs sensor was also tested. The superior sensing properties of the branched NW sensor relative to the pristine sensor were mainly attributed to the high surface area of the sensor resulting from Bi2O3 branching, as well as the formation of homo-and heterojunctions (Bi2O3-SnO2). In addition, several factors including the presence of Au contributed to the excellent selectivity to NO2 gas. Based on the results obtained in this work, we believe that the present sensor with an easy fabrication method, along with its high sensitivity and selectivity towards NO2, can be used for the detection of NO2 gas in real applications.

Cross-interference with humidity is a major limiting factor for the accurate detection of target gases in semiconductor metal-oxide gas sensors. Under humid conditions, the surface-active sites of metal oxides for gas adsorption are easily deactivated by atmospheric water molecules. Thus, development of a new approach that can simultaneously improve the two inversely related features for realizing practical gas sensors is necessary. This paper presents a facile method to engineer surface-point defects based on proton-beam irradiation. The sensor irradiated with a proton beam shows not only an improved NO

With the recent rapid development of portable smart electronic devices, there is a great demand for gas sensors having high performance, high flexibility, and low energy consumption. We explored the effects of SnO

Breakthrough process technologies have been introduced that can increase the chemical sensitivity of an interface at which reactions occur without significantly altering the physico-chemical properties of the material. Such an interfacial treatment method is based on amorphous-carbon as a base so that fluids can be deposited, and the desired thickness and quality of the deposition can be ensured irrespective of the interface state of the material. In addition, side effects such as diffusion and decreasing strength at the interface can be avoided. This is simpler than existing vacuum-based deposition technology and it has an unmatched industrial advantage in terms of economics, speed, accuracy, reliability, accessibility, and convenience. In particular, this amorphous-carbon interface treatment technology has been demonstrated to improve gas-sensing characteristics of NO2 at room temperature.

Theoretical advances in science are inherently time-consuming to realise in engineering, since their practical application is hindered by the inability to follow the theoretical essence. Herein, we propose a new method to freely control the time, cost, and process variables in the fabrication of a hybrid featuring Au nanoparticles on a pre-formed SnO2 nanostructure. The above advantages, which were divided into six categories, are proven to be superior to those achieved elsewhere, and the obtained results are found to be applicable to the synthesis and functionalisation of other nanostructures. Furthermore, the reduction of the time-gap between science and engineering is expected to promote the practical applications of numerous scientific theories.

We present a dual sensitization gas sensor for selective detection of either H2S or C2H5OH gases, based on ZnO/CuO nanoparticle (NP)-decorated MWCNTs. First, Cu-Zn layers of three different thickness (3, 6, 9 nm) were successfully deposited on the surfaces of CNTs by a sputtering process and subsequently converted to their corresponding oxides by thermal annealing. The gas sensing characteristics of metal oxide decorated-MWCNTs were studied in the presence of three gases, namely NO2, H2S, and C2H5OH. The best sensing properties were obtained when the Cu-Zn deposition layer was 6 nm thick. The optimized sensor showed extraordinary responses to both H2S and C2H5OH gases at working temperatures of 100 °C and 200 °C, respectively. Therefore, selectivity was tuned by selection of the working temperature. The sensing mechanisms of the metal oxide-decorated CNTs sensors are discussed in detail. The approach described, namely co-decoration of the surfaces of MWCNTs with different metal oxides, will be of great utility to researchers who wish to fabricate dual sensitive gas sensors.

In this study, a peculiar configuration for a gas sensor consisting of vertically aligned silicon nanowires (VA-Si NWs) synthesized by metal-assisted chemical etching (MACE) is reported. Si NWs were prepared via a facile MACE method and subsequent thermal annealing. Etching was performed by generation of silver nanoparticles (Ag NPs) and subsequent etching in HF/H2O2 aqueous solution; the growth conditions were optimized by changing the process parameters. Highly vertically oriented arrays of Si NWs with a straight-line morphology were obtained, and a top–top electrode configuration was applied. The VA-Si NW gas sensor showed good sensing performance, and the VA-Si NWs exhibited a remarkable response (Rg/Ra = 11.5 ∼ 17.1) to H2 gas (10–50 ppm) at 100 °C which was the optimal working temperature. The formation mechanism and gas sensing mechanism of VA-Si NWs are described. The obtained results can suggest new approaches to making inexpensive, versatile, and portable sensors based on Si NWs having a novel top–top electrode structure that are fully compatible with well-developed Si technologies.

In this study, multi-walled carbon nanotubes (MWCNTs) were decorated by the Cu2O/CuO nanoparticles for gas sensing. For this purpose, Cu layer with different thicknesses of 3, 6, and 9 nm was coated on multi-walled carbon nanotubes using sputtering technique for different times of 1, 2, and 3 min at 25°C, respectively, followed by annealing at 500°C to produce isolated Cu2O/CuO islands. The synthesized products were fully characterized and their expected morphology, chemical composition and phases were confirmed. The gas sensors were fabricated and the optimal sensing temperatures for H2S sensing was found to be relatively low (150 °C). With control of the size of the Cu2O/CuO nanoparticles, very high sensor response ((Rg – Ra)/Ra x 100) of about 1244 % to 1 ppm H2S gas was obtained, with response time and recovery time of 219 and 77 s, respectively. Since sensor response, response time, and recovery time became significantly higher, shorter, and shorter, respectively, by the Cu2O/CuO functionalization, the associated mechanisms were explained in regard to the selective sensitivity to H2S gas. In fact, Cu2O/CuO decorations not only increased the effective surface area for sensing studies, but also they acted as effective catalyst for H2S gas. By considering the energy bands, among the multi-walled carbon nanotubes-comprising interfaces, the multi-walled carbon nanotubes/CuS heterointerfaces were effective in the enhancing the sensing behavior.

Cross-interference with humidity is a major limiting factor for the accurate detection of target gases in semiconductor metal-oxide gas sensors. Under humid conditions, the surface-active sites of metal oxides for gas adsorption are easily deactivated by atmospheric water molecules. Thus, development of a new approach that can simultaneously improve the two inversely related features for realizing practical gas sensors is necessary. This paper presents a facile method to engineer surface-point defects based on proton-beam irradiation. The sensor irradiated with a proton beam shows not only an improved NO2 response but also considerable tolerance toward humidity. Based on surface analyses and DFT calculations, it is found that proton beams induce three types of point defects, which make NO2 molecules preferentially adsorb on the ZnO surfaces compared to H2O molecules, eventually enabling improved NO2 detection with less humidity interference.

With the recent rapid development of portable smart electronic devices, there is a great demand for gas sensors having high performance, high flexibility, and low energy consumption. We explored the effects of SnO2 shell thickness and operating voltage on the sensing behavior of WS2 nanosheets (NSs) deposited over a flexible substrate in self-heating mode. Commercial WS2 nanowires (NWs) were used as the core and SnO2 shells with various thicknesses were deposited on the core by an advanced physical technique, namely atomic layer deposition (ALD). With regard to CO sensing, a shell thickness of 15 nm operating at 3.4 V, was optimal. Alternatively, for NO2 sensing, the optimal shell thickness was 30 nm. Therefore, using engineering design principles to determine the shell material and shell thickness, it is possible to selectively detect reducing gases such as CO, while the response to oxidizing gases is weak. We have also discussed the details of this sensing mechanism. We believe that our results can lead to further study of C–S NSs for sensing studies from different points of views.

Introduction NO2 gas, with its highly toxic nature, is mainly produced from automobile exhaust or the burning of fossil fuels [1]. This gas could lead to damage in the respiratory system when inhaled [2]. Moreover, when exposed to high concentration, could lead to cancer or related disease. One unique type of nanowires is core-shell (C-S) nanowires, where nanowires are covered by a thin shell. In this study, SnO2 nanowires covered by ZrO2 shell has been explored. The oxygen ions in ZrO2 are actively transported after it reaches 600°C limiting the application field. SnO2 has often been applied for gas sensing due to its physiochemical stability and high mobility of charge carriers. SnO2-ZrO2 C-S NWs are expected to show a prominent result in respect of sensor temperature and sensor response. This study has been published on Sensors and Actuators B: Chemical 319 (2020) 128309. Method SnO2 NWs were fabricated using the vapor-liquid-solid (VLS) method. Metallic Sn powders in alumina tube were placed into a furnace and were thermally evaporated at 900°C for 15 min. Evaporated Sn reacts with oxygen from the air, finally, SnO2 NWs are synthesized. ZrO2 was coated on SnO2 NWs by a thermal ALD system (Lucida D100, NCD Technology, Korea). The precursor and reactant for the ALD process were tris(dimethylamino) cyclopentadienyl zirconium (Cp-Zr) and ozone (O3), respectively. Cp-Zr was heated to 90°C in a canister, and Ar gas was used as a bubbler. The pressure and temperature of the ALD process were kept at 0.5 Torr and 300°C, respectively [3,4]. A bi-layer electrode (200-nm thick Au and 50-nm thick Ti) was sputter-deposited on the samples for making of the gas sensors. The sensors were placed inside a gas chamber in a tubular furnace. The gas concentrations were controlled by mass flow controllers with air as a background gas. The total flow rate was fixed to 500 sccm during all measurements. Result and Conclusions Fig. 1(A,B), (C,D), (E,F) and (G,H) show TEM and high resolution TEM (HR-TEM) images of SnO2-ZrO2 C-S NWs with 50, 100, 150, 200 ALD cycles respectively. Owing to the very thin nature of the shell, the lattice fringes from the underlying SnO2 core were able to be observed. Fig. 2A and B show EDAX data of SnO2-ZrO2 C-S NWs after 50 cycles for core and shell parts, respectively. For evaluation of the presence of Zr in the core and shell regions, the ratio of Zr/(Zr+Sn) was used. The ratio of Zr/(Zr+Sn) in the core region was 0.08 and that of the shell region was 0.98. This demonstrates the successful deposition of Zr in the shell region. The same trend can be seen for other C-S NWs. Therefore, it can be concluded that ZrO2 is mainly concentrated in the outer regions, whereas the SnO2 is concentrated in the core region, regardless of ALD cycle time. The value of Zr/(Zr+Sn) in the core region becomes higher at higher ALD cycles, as the ZrO2 shell gets thicker, whereas the SnO2 core is constant. Fig. 3 depicts the response to 10 ppm NO2 gas versus temperature for pristine and C-S gas sensors. At low temperatures, the response is low and after reaching a peak at a particular temperature, the response decreases. The highest response for pristine SnO2 gas sensor occurred at 250°C, which was 20.9. For C-S NW gas sensors with a ZrO2 shell deposited at 50, 100, 150, and 200 cycles, the maximum responses were 14.9, 16.2, 24.7, and 14.2, respectively. The sensor with ZrO2 shell being deposited with 150 ALD cycles exhibited the highest gas response. All C-S gas sensors except that deposited with 100 ALD cycles showed their optimal sensing temperature at 150°C. However, for the sensor with ZrO2 shell deposited with 100 ALD cycles, the optimal sensing temperature was 100°C. The selectivity of SnO2-ZrO2 (150 cycles) C-S NW to 10 ppm of various gases at 150°C is presented in Fig. 4. The response to NO2 (24.7) was much higher than that of the other gases. To conclude, the optimal sensing temperature was varied from 100°C for the sensor with a ZrO2 shell deposited with 100 cycles to 250°C for pristine SnO2 gas sensor. Also, NO2 gas sensing results demonstrated a dependence of response to the thickness of the ZrO2 shell. The optimal sensor with a ZrO2 shell thickness of 24.1 nm (150 ALD cycles) revealed a high response to NO2 gas and also a good selectivity to NO2 gas. References [1] S. Jain, A. Paliwal, V. Gupta, M. Tomar, Long Range Surface Plasmons assisted highly sensitive and room temperature operated NO2 gas sensor, Sens. Actuators B Chem. 311 (2020) 127897. doi: https://doi.org/10.1016/j.snb.2020.127897 [2] P.G. Su, J.H. Yu, Enhanced NO2 gas-sensing properties of Au-Ag bimetal decorated MWCNTs/WO3 composite sensor under UV-LED irradiation, Sens. Actuators A Phys. 303 (2020) 111718. doi: https://doi.org/10.1016/j.sna.2019.111718 [3] K. Lee, W. Jang, H. Kim, H. Lim, B. Kim, H. Seo, H. Jeon, Leakage current suppression in spatially controlled Si-doped ZrO2 for capacitors using atomic layer deposition, Thin Solid Films 657 (2018) 1–7. doi: https://doi.org/10.1016/j.tsf.2018.04.033 [4] H. Song, H. Jeon, C. Shin, S. Shin, W. Jang, J. Park, J. Chang, J.H. Choi, Y. Kim, H. Lim, H. Seo, H. Jeon, The effect of ozone concentration during atomic layer deposition on the properties of ZrO2 films for capacitor applications, Thin Solid Films 619 (2016) 317–322. doi: https://doi.org/10.1016/j.tsf.2016.10.044 Figure 1

We explored NO2 sensing performance of bare and SnS nanoparticle (NP)-functionalized SnO2 nanowires (NWs). The SnO2 NWs were synthesized and functionalization with SnS NPs was done by atomic layer deposition with subsequent annealing. The NO2 gas sensing results revealed that at temperatures up to 100 °C, the performance of the SnS-coated SnO2 NWs was better than bare SnO2 NWs. Moreover, the response time of SnS-coated SnO2 NWs to 2 ppm NO2 was 69 s at 100 °C, namely 9.1 times faster than that of bare SnO2 NWs. The sensing mechanisms related to the SnS-functionalized SnO2 gas sensor were proposed. The obtained results could present possibilities for the realization of low-temperature SnO2 NW-based gas sensors.

Complex metal oxides such as functionalized branched nanowires (NWs) are a new category of nanocomposites with unique properties for gas sensing applications. In the present work, we studied the gas sensing properties of Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs, inspired by their high surface area and numerous resistive points. These NWs were studied and compared with pristine SnO 2 NWs and ZnO-branched SnO 2 NWs. To prepare the functionalized NWs, ZnO-branched SnO 2 NWs were sputter-coated with a Cr layer and then annealed at 500 °C to produce isolated Cr 2 O 3 NPs on the ZnO branches. Results of X-ray diffraction, scanning electron microscopy, and lattice-resolved transmission electron microscopy collectively showed that Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs were successfully formed. Cr 2 O 3 functionalization was found to greatly improve the sensors’ response to NO 2 gas. Furthermore, the sensors’ good selectivity was demonstrated by testing them in the presence of various interfering gases. The underlying gas sensing mechanisms are discussed herein in detail. We believe that the Cr 2 O 3 -functionalized ZnO-branched SnO 2 NWs reported herein are promising sensors for the highly sensitive and selective detection of NO 2 gas.

Microwave (MW) irradiation has obtained extensive importance in the field of synthesis and treatment of nanoparticles, because of its faster, cleaner and cost effectiveness than the other conventional and wet chemical methods. In this study, ZnO/graphene nanocomposites were prepared and subsequently post-treated by MW irradiation. Responses of the MW irradiated ZnO/graphene nanocomposites sensors were tested towards various gases including NO2, ethanol, acetone, toluene and CO and the results were compared with those of pristine ZnO and ZnO/graphene sensors without MW irradiation. It was demonstrated that the MW irradiated sensor had much higher response particularly to NO2 gas along with superior selectivity and shorter response/recovery times in comparison to unirradiated ZnO/graphene and pristine ZnO sensors. The possible underlying mechanism of this behavior is discussed in detail, mainly in terms of the MW-induced surface defects and the generation of finer ZnO nanoparticles. The results obtained demonstrated the beneficial effect of MW irradiation for enhancing the NO2-gas sensing behavior of ZnO/graphene nanocomposites, opening a new door not only to a novel synthesis of semiconductors/graphene nanocomposites, but also to a cost-effective way of improving their sensing capabilities.

We obtained extremely high and selective sensitivity to NO2 gas by fabricating graphene–SnO2 nanocomposites using a commercial microwave oven. Structural characterization revealed that the products corresponded to agglomerated structures of graphene and SnO2 particles, with small secondary SnOx (x ≤ 2) nanoparticles deposited on the surfaces. The overall oxygen atomic ratio was decreased with the appearance of an SnOx (x < 2) phase. By the microwave treatment of graphene–SnO2 nanocomposites, with the graphene promoting efficient transport of the microwave energy, evaporation and redeposition of SnOx nanoparticles were facilitated. The graphene–SnO2 nanocomposites exhibited a high sensor response of 24.7 for 1 ppm of NO2 gas, at an optimized temperature of 150 °C. The graphene–SnO2 nanocomposites were selectively sensitive to NO2 gas, in comparison with SO2, NH3, and ethanol gases. We suggest that the generation of SnOx nanoparticles and the SnOx phase in the matrix results in the formation of SnO2/SnO2 ho...

In this work, ZnO-decorated multi-walled carbon nanotube (MWCNT) nanocomposites prepared using an in-situ method involving the thermal evaporation of Zn powders in the presence of MWCNTs. The gas sensing characteristics of the MWCNT/ZnO nanocomposites are studied, and results for the material characterizations for the synthesized nanocomposites confirm the formation of well-distributed ZnO nanoparticles onto MWCNTs, creating MWCNT/ZnO nanocomposites. The gas sensing properties of the MWCNT/ZnO nanocomposite gas sensor, such as response, cross-sensitivity, and response-recovery time, are investigated and compared with a bare MWCNT sensor. The decoration of ZnO nanoparticles greatly improves the gas sensing properties of bare MWCNTs. We discussed the possible mechanisms for the enhancement of sensing capabilities. The results suggest that decoration of n-type semiconducting oxide materials, such as ZnO in the form of nanoparticles, is a promising strategy for improvement of gas sensing properties of p-MWCNTs.

We report a novel method to significantly improve the C 6 H 6 -sensing performance of Si nanowires through the combination of TeO 2 branches and Pd sensitization. The morphological investigation revealed that TeO 2 branches were densely formed on the stem nanowires (NWs). The sensor responses of the Pd-functionalized branched NWs exhibited superior sensor responses of 55.19 to C 6 H 6 gas. In particular, Pd nanoparticles enhanced the sensor response to C 6 H 6 gas most efficiently, increasing the sensor response by 173.5%. Possible mechanisms for the sensing of the Pd-decorated branched nanowires will be associated with resistance modulation along the branch TeO 2 nanowires (including catalytic Pd effects), at the networked homojunctions between the branch TeO 2 nanowires, at the boundaries of the TeO 2 nanograins, and at the Pd/TeO 2 heterojunctions.

We prepared nanocomposites of n-SnO2/p-Co3O4 for application in chemical sensors. In order to fabricate p-Co3O4-decorated n-SnO2 nanowires, we sputtered a Co layer and subsequently annealed the material in air ambient. Characterization revealed that crystalline cubic Co3O4 with a tubular-like structure was attached to the surface of SnO2 core nanowires. We carried out sensing tests at 573 K in at NO2 gas concentrations ranging between 2 and 10 ppm. The sensor response was increased both by adding the Co3O4 layer and also by decreasing the thickness of the Co3O4 layer from 19.2 to 6.4 nm. We proposed possible mechanisms to explain the enhanced sensor properties obtained by Co3O4-functionalization. Co3O4-functionalized SnO2 nanowires exhibited a higher sensor response than pristine nanowires, not only due to the heterostructure-induced depletion of n-SnO2 region but also due to the surface effects of Co3O4. The generation of hole-accumulated Co3O4 layer in case of thicker-layered nanowires will decrease the sensor response. We demonstrated that Co3O4-functionalized SnO2 nanowire sensors can be used as gas sensors at very low concentrations.

This letter presents a two-element conformal antenna for receiving the global positioning system L1, L2, and GLONASS L1 signals for precision artillery applications. The antenna consists of two stub-loaded shorted conformal patches fed by coaxial cables and a stripline power combiner. The patch is realized on a cylindrical polymer substrate wrapped around an artillery fuze body. Two patches are placed 180° apart and combined to realize an omnidirectional pattern. The fabricated antenna shows good Global Navigation Satellite System signal receiving performance.

Herein, we present the effect of electron-beam irradiation (EBI) on the gas-sensing properties of Pd-functionalized reduced graphene oxide (RGO). Scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the synthesized products. The samples were irradiated using electron beams at doses of 0 (Pd-RGO-0), 100 (Pd-RGO-100), and 500 kGy (Pd-RGO-500), and the NO2 gas-sensing properties were investigated. It was found that irradiation by electron beams has a critical effect on the gas-sensing properties of the samples, and the Pd-RGO-500 sensor showed the highest response to NO2 gas. In particular, the response of the unirradiated sensor and the sensor irradiated at doses of 100 and 500 kGy to 10 ppm NO2 were 1.027, 1.045, and 1.047, respectively. The response times of Pd-RGO-0, Pd-RGO-100, and Pd-RGO-500 to 10 ppm NO2 gas were 389, 335, and 345 s, respectively. The corresponding recovery times for these sensors were 808, 766, and 816 s, respectively. The increased numbers of oxygen functional groups and high-energy defects were the main reasons for the increased gas response. This study will eventually lead to increased performance levels of RGO-based sensors that use EBI, as this technology can introduce changes into materials in a non-contact, clean, and powerful manner.

The use of conventional doping methods requires consideration of not only the energy connection with the base material but also the limits of the type and doping range of the dopant. The scope of the physico-chemical change must be determined from the properties of the base material, and when this limit is exceeded, a large energy barrier must be formed between the base material and the dopant as in a heterojunction. Thus, starting from a different viewpoint, we introduce a so-called metallization of surface reduction method, which easily overcomes the disadvantages of existing methods while having the effect of doping the base material. Such new synthetic techniques enable sequential energy arrangements–gradients from the surface to the centre of the material–so that free energy transfer effects can be obtained as per the energies in the semiconducting band, eliminating the energy discontinuity of the heterojunction.

A new gas sensor working in room temperature, which is compatible with silicon fabrication technology is presented. Porous silicon nanowires (NWs) were synthesized by metal-assisted chemical etching method and then TeO₂ NWs branches were attached to their stem by thermal evaporation of Te powders in the presence of air. Afterwards TeO₂ branched porous Si NWs were functionalized by Pt via sputtering followed by low temperature thermal annealing. Scanning electron microscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy collectively confirmed successful formation of TeO₂ branched porous Si NWs functionalized by Pt nanoparticles. Their gas sensing properties in the presence of CO, C

Theoretical advances in science are inherently time-consuming to realise in engineering, since their practical application is hindered by the inability to follow the theoretical essence. Herein, we propose a new method to freely control the time, cost, and process variables in the fabrication of a hybrid featuring Au nanoparticles on a pre-formed SnO2 nanostructure. The above advantages, which were divided into six categories, are proven to be superior to those achieved elsewhere, and the obtained results are found to be applicable to the synthesis and functionalisation of other nanostructures. Furthermore, the reduction of the time-gap between science and engineering is expected to promote the practical applications of numerous scientific theories.

This paper presents a new high-performance five-element monopulse feed with a simple comparator network for Cassegrain reflector applications. The radiating element is a tapered dielectric rod fed by a circular waveguide which is in turn excited by a printed dipole. The center element is used for the sum pattern, while top and bottom elements are used for the elevation difference pattern, and left and right elements for the azimuth difference pattern. Out-of-phase excitation for the difference pattern is obtained by antisymmetrically placing two dipoles and combining their outputs with a power combiner. The optimally designed feed has been fabricated and measured. Measurement shows that the feed has the maximum sum pattern gain of 16.8 dBi, the difference pattern null depth of –21 dBi, and the reflection coefficient of less than –10 dB over 13.9–16.7 GHz.