Terahertz characterization of graphene conductivity via time-domain reflection spectroscopy on metal-backed dielectric substratesFuscaldo, Walter; De Simone, Sara; Dimitrov, Dimitre; Marinova, Vera; Mussi, Valentina; Beccherelli, Romeo; Zografopoulos, Dimitrios C
doi: 10.1088/1361-6463/ac7759pmid: N/A
A theoretical and experimental framework for the characterization of the terahertz (THz) conductivity of graphene on metal-backed substrates is presented. Analytical equations are derived for the general problem of oblique incidence of the THz beam in a time-domain spectroscopic (TDS) setup working in reflection. The recorded time-domain signals are post-processed in order to retrieve the substrate thickness, its dielectric frequency dispersion, and the complex graphene conductivity frequency dispersion, which is described by a generalized Drude–Smith model. The method is tested on two samples of chemical vapor deposited graphene, transferred on polyethylene terephthalate and cyclo-olefin polymeric substrates of sub-millimetric thickness, and characterized by Raman spectroscopy. By working only with the amplitude spectra, the proposed method circumvents issues stemming from phase uncertainties that typically affect TDS measurements in reflection mode. More important, it allows for a rapid, nondestructive characterization of graphene sheets that can be directly integrated in the production flow of graphene-based passive or active components employing metal-backed resonant cavities, such as THz absorbers, metasurface lenses, or leaky-wave antennas.
Size- and temperature-dependent Young’s modulus of individual ZnS nanobeltsMa, Liang; Jiang, Yijun; Dai, Guozhang; Mead, James L; Yibibulla, Tursunay; Lu, Mingyuan; Huang, Han; Fatikow, Sergej; Wang, Shiliang
doi: 10.1088/1361-6463/ac78a0pmid: N/A
The Young’s modulus of individual ZnS nanobelts with thicknesses ranging from 30 to 110 nm is measured by a mechanical resonance method over a temperature range of 300–650 K. Nanobelts with thicknesses above ∼80 nm exhibit a Young’s modulus very close to their corresponding bulk value of 88 GPa, whilst thinner nanobelts with thicknesses down to 30 nm exhibit a Young’s modulus of ∼70 GPa. Incrementally heating a nanobelt of 110 nm thickness over a temperature range of 300–650 K presents a linearly decreasing Young’s modulus. The nanobelt is thus found to possess a temperature coefficient of Young’s modulus of −125.4 ± 2.8 ppm K−1, which is comparable to their bulk value of −147 ppm K−1. The temperature coefficient of Young’s modulus for nanobelts with thicknesses below 100 nm demonstrates a strong size effect, and is found to dramatically decrease to as low as −201.4 ± 10.2 ppm K−1 for the thickness of 30 nm. A nonlinear temperature dependence of Young’s modulus is experimentally identified for nanobelt with diameters below 100 nm, and the linear-nonlinear transition temperature decreases with the decrease of nanobelt thickness.
Unipolar resistive switching behavior in MoS2–polyvinyl alcohol-based memory deviceLiu, Lifu; Cheng, Shijie; Chen, Wei; Ren, Shuxia; Kang, Xin; Zhao, Xu
doi: 10.1088/1361-6463/ac79dbpmid: N/A
MoS2–polymer-based memory devices have attracted significant interest owing to their mechanical flexibility, convenient solution processability, and affordability. These devices exhibit bipolar resistive switching behavior, and their switching relies on the polarity of the applied bias. This paper presents a memory device in which a MoS2–polyvinyl alcohol (PVA) hybrid film is sandwiched between Ag and Pt electrodes. The developed Ag/MoS2–PVA/Pt device manifests typical unipolar resistive switching (URS) behavior and nonvolatile rewritable memory performance with a low operating voltage, large ON/OFF ratio (105), and multilevel cell storage ability. Notably, 1T-phase MoS2 is crucial for the URS behavior, and this switching behavior can be ascribed to the charge trapping as well as the Joule-heating-induced de-trapping of the S vacancies associated with 1T MoS2. These findings can facilitate the development of new designs for high-performance, high-density data storage.
Defect-control electron transport behavior of gallium nitride/silicon nonplanar-structure heterojunctionZhang, Yan; Jiang, Hang-Hui; Luo, Yuan-Hang; Xiao, Meng-Zhen; Wen, Chao; Xing, Ya-Kun; Li, Xin-Jian
doi: 10.1088/1361-6463/ac78a2pmid: N/A
Compared with a traditional heterojunction, a nonplanar-structure heterojunction can reduce the problems caused by a lattice mismatch through a three-dimensional stress release mechanism, which will be helpful for promoting the performance and stability of related devices. In this paper, we report our study on the electron transport behavior of a gallium nitride (GaN)/silicon (Si) heterojunction with nonplanar-structure interface, which was prepared through growing GaN on a hierarchical structure, Si nanoporous pillar array (Si-NPA). To clarify the electron transport mechanism and promote the device performance, annealing treatment in ammonia atmosphere was carried out to as-prepared GaN/Si-NPA. The formation of the heterojunction was verified by the typical rectification behavior observed in both as-prepared and annealed samples. After annealing treatment, a lower turn-on voltage, a smaller reverse saturation current density, a larger forward current density and a higher reverse breakdown voltage were obtained, which indicate the promotion of the heterojunction performance. By comparatively studying the spectrum evolution of photoluminescence before and after annealing treatment, the underlying mechanism is clarified as the variation of the type and density of point defects such as gallium vacancy (VGa), oxygen substitutional impurity (ON), and their complex defect VGa−ON in GaN. The results illustrate an effective defect-control strategy for optimizing the performance of GaN/Si heterojunction optoelectronic devices.
Chronoamperometric deposition of transparent WO3 film for application as power efficient electrochromic auxiliary electrodeGhosh, Tanushree; Rani, Chanchal; Kandpal, Suchita; Tanwar, Manushree; Bansal, Love; Kumar, Rajesh
doi: 10.1088/1361-6463/ac76f5pmid: N/A
Inorganic oxide electrochromic electrodes can be made more robust by adopting suitable deposition techniques. Electrochemically deposited tungsten oxide (WO3) film through chronoamperometry has been studied here for application as electrochromic auxiliary electrode. Thoroughly characterized film using electron microscopy, x-ray diffraction and Raman spectroscopy has been used for electrochromic measurements. The electrode shows reversible transparent to blue color switching when biased with a very small bias of 1 V. Bias dependent in-situ spectroelectrochemistry measurements have been performed which shows excellent results in terms of reversibility, cyclability, color contrast, appreciable switching time and good current stability at low working potential. The results thus pave the way to an excellent deposition technique designed particularly for electrochromic applications.
Determination of the partial and total pressures of the mixed gases in atomic vapor cells by optical absorptionPeng, Jinpeng; Liu, Zhanchao; Yin, Kaifeng; Zou, Sheng; Yuan, Heng
doi: 10.1088/1361-6463/ac73c0pmid: N/A
We study the partial and total pressures of the mixed gases in a Rb vapor cell from its absorption spectrum under the influence of natural broadening, self-broadening, pressure broadening, the pressure shift, and Doppler broadening. A comprehensive model of the absorption coefficient on the Rb D2 line is developed, which takes into account the influence of multiple gas species. The importance of light intensity selection and frequency calibration to obtain accurate experimental results are discussed. Accurate abundances are deduced from the experimentally measured absorption spectra of vacuum cells and are used to optimize the abundance parameters in our model. We fit the experimentally measured absorption spectrum of a gas-filled cell to the optimized model and obtain a root mean square error better than 0.1%. The extracted partial pressures indicate that the mixture ratio agrees well with the designed value, while the total pressure has an increment of about 17% of its nominal value. We further estimate the amount of helium leakage in a cell to be 89.1 Torr for 6 months. Moreover, using a series of absorption spectra generated by our model as benchmarks, we evaluate the effectiveness of the commonly used single and double Lorentzian fitting models and give some suggestions for using the double Lorentzian model. This study provides a reference for designing the appropriate gas mixture for comagnetometer systems.
Investigation of a vertical 2D/3D semiconductor heterostructure based on GaSe and Ga2O3Sorifi, Sahin; Kaushik, Shuchi; Sheoran, Hardhyan; Singh, Rajendra
doi: 10.1088/1361-6463/ac7987pmid: N/A
Mixed-dimensional heterostructures are emerging to be very promising for future electronic and optoelectronic applications. Here, we report on the fabrication and characterization of a 2D/3D vertical van der Waals p–n heterojunction based on p-type gallium selenide (GaSe) and n-type gallium oxide (Ga2O3). Kelvin Probe Force Microscopic (KPFM) measurements have been conducted to estimate the difference in the surface potential values between GaSe and Ga2O3, which is further used to find out the conduction band offset value at the GaSe/Ga2O3hetero-interface to design the band diagrams. The current–voltage measurements on the device display a diode-like behavior which is attributed to the type-II band alignment, present at the p–n junction interface as per the electron affinities and bandgap values of GaSe and Ga2O3. The device exhibits a high current rectification ratio of ∼2500 extracted at ±5V. The photoresponse properties of the heterostructure are also studied and the figure of merit parameters of the photodetector such as photoresponsivity and specific detectivity have been evaluated for the fabricated device. Since the GaSe/Ga2O3heterojunction holds a great potential in the field of efficient optoelectronic devices, we believe our study could pave the way to designing innovative optoelectronic devices by integrating low-dimensional materials with conventional 3D semiconducting materials.
Effect of dielectric barrier discharge plasma on rice (Oryza sativa L.) seed hydration and hygroscopicityRongsangchaicharean, Thunyapuk; Ruangwong, Khomsan; Onwimol, Damrongvudhi; Tephiruk, Naowarat; Suwannarat, Sawita; Srisonphan, Siwapon
doi: 10.1088/1361-6463/ac791dpmid: N/A
Nonthermal plasma has been explored as a green technology for improving seed wettability and crop productivity. In this investigation, we demonstrate scalable dielectric barrier discharge (DBD) plasma treatment of rice seeds at atmospheric pressure to elucidate the effect of plasma on seed hydration, hygroscopicity, and moisture content (MC). These properties are associated with seedling quality, nutrition, and shelf-life storage. The floating approach was utilized to evaluate seed wettability for a large-scale seed lot because treated seeds become superhydrophilic and sink immediately into water, whereas nontreated seeds float on the surface. We proved that a hydrophilic surface is necessary to improve water absorption, but the extent of physical etching and chemical functionalization had the greatest impact. After 5 h of imbibition, the seeds that were treated with plasma for 10 min absorbed ∼20% more water than the nontreated seeds. After plasma treatment, seed vigor increased dramatically, as evidenced by the radicle emergence times of ∼64 h for treated and ∼69 h for nontreated seeds. Furthermore, the treated seeds yielded seedlings that were ∼10% longer compared to the nontreated seeds on day 14 of germination, even after an artificial aging process. During treatment, the MC of the seed was linearly reduced due to an ionic wind with a velocity of ∼4.7 m s−1, which was generated using a localized nonuniform electric field that was applied around the seed’s surface. DBD plasma can modify seed coatings at the nanoscale level but not the availability of its primary nutrition and hygroscopicity. Although the treated seed absorbed moisture four times faster than the nontreated seed during the first 24 h of storage in a ∼99% relative humidity environment, there was no difference in MC subsequently. Thus, plasma treatment combines the advantages of efficient imbibition and vigor enhancement, and is beneficial for long-term seed preservation.