Piezoresistive carbon foams in sensing applications

Abstract Mechanical strain sensing is ubiquitous, found in applications such as heart rate monitoring, analysis of body part motion, vibration of machines, dilatation in buildings and large infrastructure, and so forth. Piezoresistive materials and sensors based on those offer versatile and robust solutions to measure strains and displacements and can be implemented even in acceleration and pressure analyses. In this paper, we overview the most prominent piezoresistive materials, and present a case study on carbon foams as well as on their hierarchical hybrid structures with carbon nanotubes/nanofibers. Our results show highly non-linear electrical resistance and mechanical stress dependence on uniaxial strain in both types of materials up to 50% compression. The Young’s moduli increase with compressive strain between 1–65 and 0.1–92 kPa for the foam and hierarchical structure, respectively. The foams have giant gauge factors (up to −1000) with large differential gauge factors (on the scale of −10) and differential pressure sensitivity of 0.016 Pa⁻¹ (at 10% strain) making them suitable for detecting both small and large displacements with excellent accuracy of electrical readout as we demonstrate in detecting building wall displacement as well as in monitoring heart rate and flexing of fingers

Recent advances in synthesis of water-stable metal halide perovskites and photocatalytic applications

Abstract Solar-driven photocatalytic reactions have attracted wide interest as a viable method to generate green energy and alleviate environmental challenges posed by fossil fuels. Although, various classes of photocatalysts have been explored during the past decades, the pursuit towards even more efficient ones is still ongoing. Metal halide perovskites (MHPs) have been recently proposed as novel photocatalysts owing to their wide light absorption range and excellent optoelectronic properties. However, the instability of MHPs in water is the main obstacle that impedes their applications in practice and prompts stabilization strategies to be developed. This review focuses on the recent approaches for stabilizing MHPs in water, including surface engineering, common-ion effect, and intrinsic water stability. The photocatalytic applications of water-stable MHPs are summarized and an outlook with perspectives over the current challenges are provided

Native oxide formation on pentagonal copper nanowires:a TEM study

Abstract Hydrothermally synthesized copper nanowires were allowed to oxidize in air at room temperature and 30% constant humidity for the period of 22 days. The growth of native oxide layer was followed up by high-resolution transmission electron microscopy and diffraction to reveal and understand the kinetics of the oxidation process. Copper oxides appear in the form of differently oriented crystalline phases around the metallic core as a shell-like layer (Cu2O) and as nanoscopic islands (CuO) on the top of that. Time dependent oxide thickness data suggests that oxidation follows the field-assisted growth model at the beginning of the process, as practically immediately an oxide layer of ∼2.8 nm thickness develops on the surface. However, after this initial rapid growth, the local field attenuates and the classical parabolic diffusion limited growth plays the main role in the oxidation. Because of the single crystal facets on the side surface of penta-twinned Cu nanowires, the oxidation rate in the diffusion limited regime is lower than in polycrystalline films

Visible range photoresponse of vertically oriented on-chip MoS2 and WS2 thin films

Abstract The excellent electrical properties of transition metal dichalcogenide (TMD) 2D materials promise a competitive alternative to traditional semiconductor materials for applications in optoelectronics, chemical sensing, as well as in energy harvesting and conversion. As the typical synthesis methods of TMDs produce nanoparticles, such as single or multi-layered nanoflakes, subsequent strenuous integration steps are necessary to obtain devices. Direct synthesis of the material on substrates would simplify the process and provide the means for large-scale integration and production of practical devices. In our approach, we synthesize MoS2 and WS2 thin films with a simple sulfurization of the respective metal films deposited by sputtering on Si/SiO2 chips, and study their optoelectrical properties at wavelengths of 661 nm, 552 nm, and 401 nm using pulsed lasers. Both TMD thin films are found to show photoresponsivities of up to ∼5 × 10−6 A W−1 with corresponding quantum efficiencies of ∼10−5, which are unexpectedly moderate, and can be attributed to their columnar microstructure, in which the basal planes of the hexagonal lattices are perpendicular to the substrate, thus, limiting the electron transport in the films parallel to the plane of the substrate

Porous low-loss silica–PMMA dielectric nanocomposite for high-frequency bullet lens applications

Abstract Several devices of the future generation wireless telecommunication technologies that use bands in THz frequencies for data transmission need low-loss and low-permittivity materials to enable ideal conditions for the propagation of electromagnetic waves. Herein, a lightweight dielectric bullet-shaped lens operating in the frequency range of 110–170 GHz is demonstrated to collimate electromagnetic waves, thus increasing the intensity of the electric field. The material of the lens is based on a composite of silica nanoshells and poly(methylmethacrylate) made by the impregnation of the nanoshells with the polymer followed by hot pressing in a mold. As the polymer acts only as an adhesive between the hollow nanospheres without filling the inner cavity of the shells and their interparticle spaces, the composite is highly porous (67%) and has low dielectric permittivity and loss tangent (1.5 and 4 × 10⁻³, respectively, below 200 GHz). The size of the collimated beam and the increase of the corresponding field strength are measured to vary from 2.2 to 1.2 mm and from 17.2 to 8.98 dB depending on the frequency of the waves (110–170 GHz)

Biodegradable multiphase poly(lactic acid)/biochar/graphite composites for electromagnetic interference shielding

Abstract High performance electromagnetic interference (EMI) shielding materials that are of lightweight, thin, and easy-to-design are extremely important for future portable and wearable electronic and telecommunication devices. Achieving such functionalities using environmentally friendly and biodegradable materials is becoming increasingly important and adds on even more value. Herein, we demonstrate two- and three-phase composites of poly(lactic acid), graphite and biochar that can be produced in large quantities and practically in any shape or form by applying facile twin-screw extrusion and subsequent hot-pressing methods. The developed conducting polymer composites with a wide range of filler loading level enable excellent shielding with an effectiveness beyond 30 dB at K-band frequency range (18–26.5 GHz) using very thin films (0.25 mm). The corresponding specific shielding effectiveness 890 dB cm2 g−1 is particularly high among biodegradable shielding materials. The outstanding performance is due to scattering and thus increased propagation distance of electromagnetic waves in the multiphase composite medium. The presented novel environmentally friendly and biodegradable materials are not only promising for wearables and portable devices but also for other applications, where size and weight matters such as in aeronautics, astronautics and robotics

Green carbon nanofiber networks for advanced energy storage

Abstract Energy storage devices such as supercapacitors of high performance are in great need due to the continuous expansion of digitalization and related devices for mobile electronics, autonomous sensors, and vehicles of different kinds. However, the nonrenewable resources and often complex preparation processes associated with electrode materials and structures pose limited scale-up in production and difficulties in versatile utilization of the devices. Here, free-standing and flexible carbon nanofiber networks derived from renewable and abundant bioresources are demonstrated. By a simple optimization of carbonization, the carbon nanofiber networks reach a large surface area of 1670 m² g–1 and excellent specific gravimetric capacitance of ∼240 F g–1, outperforming many other nanostructured carbon, activated carbon, and even those decorated with metal oxides. The remarkable electrochemical performance and flexibility of the green carbon networks enable an all-solid-state supercapacitor device, which displays a device capacitance of 60.4 F g–1 with a corresponding gravimetric energy density of 8.4 Wh kg–1 while maintaining good mechanical properties

Ultrafast photoresponse of vertically oriented TMD films probed in a vertical electrode configuration on Si chips

Abstract Integrated photodetectors based on transition metal dichalcogenides (TMDs) face the challenge of growing their high-quality crystals directly on chips or transferring them to the desired locations of components by applying multi-step processes. Herein, we show that vertically oriented polycrystalline thin films of MoS2 and WS2 grown by sulfurization of Mo and W sputtered on highly doped Si are robust solutions to achieve on-chip photodetectors with a sensitivity of up to 1 mA W−1 and an ultrafast response time in the sub-μs regime by simply probing the device in a vertical arrangement, i.e., parallel to the basal planes of TMDs. These results are two orders of magnitude better than those measured earlier in lateral probing setups having both electrodes on top of vertically aligned polycrystalline TMD films. Accordingly, our study suggests that easy-to-grow vertically oriented polycrystalline thin film structures may be viable components in fast photodetectors as well as in imaging, sensing and telecommunication devices