Transition metal dichalcogenides (TMDs) have received immense research interest in particular for their outstanding electrochemical and optoelectrical properties. Lately, chemical gas sensor applications of TMDs have been recognized as well owing to the low operating temperatures of devices, which is a great advantage over conventional metal oxide based sensors. In this work, we elaborate on the gas sensing properties of WS2 and MoS2 thin films made by simple and straightforward thermal sulfurization of sputter deposited metal films on silicon chips. The sensor response to H2, H2S, CO and NH3 analytes in air at 30 °C has been assessed and both MoS2 and WS2 were found to have an excellent selectivity to NH3 with a particularly high sensitivity of 0.10 ± 0.02 ppm−1 at sub-ppm concentrations in the case of WS2. The sensing behavior is explained on the bases of gas adsorption energies as well as carrier (hole) localization induced by the surface adsorbed moieties having reductive nature.
Owing to their higher intrinsic electrical conductivity and chemical stability with respect to their oxide counterparts, nanostructured metal sulfides are expected to revive materials for resistive chemical sensor applications. Herein, we explore the gas sensing behavior of WS 2 nanowire-nanoflake hybrid materials and demonstrate their excellent sensitivity (0.043 ppm-1) as well as high selectivity towards H 2 S relative to CO, NH 3 , H 2 , and NO (with corresponding sensitivities of 0.002, 0.0074, 0.0002, and 0.0046 ppm-1 , respectively). Gas response measurements, complemented with the results of X-ray photoelectron spectroscopy analysis and first-principles calculations based on density functional theory, suggest that the intrinsic electronic properties of pristine WS 2 alone are not sufficient to explain the observed high sensitivity towards H 2 S. A major role in this behavior is also played by O doping in the S sites of the WS 2 lattice. The results of the present study open up new avenues for the use of transition metal disulfide nanomaterials as effective alternatives to metal oxides in future applications for industrial process control, security, and health and environmental safety.
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 m 2 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.
On-chip energy storage and management will have transformative impacts in developing advanced electronic platforms with built-in energy needs for operation of integrated circuits driving a microprocessor. Though success in growing stand-alone energy storage elements such as electrochemical capacitors (super and pseusocapacitors) on a variety of substrates is a promising step towards this direction. In this work, on-chip energy storage is demonstrated using architectures of highly aligned vertical carbon nanotubes (CNTs) acting as supercapacitors, capable of providing large device capacitances. The efficiency of these structures is further increased by incorporating electrochemically active nanoparticles such as MnOx to form pseudocapacitive architectures thus enhancing device capacitance areal specific capacitance of 37 mF/cm2. The demonstrated on-chip integration is up and down-scalable, compatible with standard CMOS processes, and offers lightweight energy storage what is vital for portable and autonomous device operation with numerous advantages as compared to electronics built from discrete components.
Laser speckle contrast imaging (LSCI) is a well-known and versatile approach for the noninvasive visualization of flows and microcirculation localized in turbid scattering media, including biological tissues. In most conventional implementations of LSCI the ergodic regime is typically assumed valid. However, most composite turbid scattering media, especially biological tissues, are non-ergodic, containing a mixture of dynamic and static centers of light scattering. In the current study, we examined the speckle contrast in different dynamic conditions with the aim of assessing limitations in the quantitative interpretation of speckle contrast images. Based on a simple phenomenological approach, we introduced a coefficient of speckle dynamics to quantitatively assess the ratio of the dynamic part of a scattering medium to the static one. The introduced coefficient allows one to distinguish real changes in motion from the mere appearance of static components in the field of view. As examples of systems with static/dynamic transitions, thawing and heating of Intralipid samples were studied by the LSCI approach.
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