A Pd/HNb3O8 flexible humidity sensor is developed with long-term stability at a high humidity level (∼90% RH). Using the sensor, wireless monitoring of human respirations and finger moisture variation under exercise is successfully demonstrated.

In this work, we, for the first time, observed the remarkable thermoelectric properties of a few high-quality PtSe2 layered films fabricated by a post selenization of Pt thin films. An excellent power factor of ≳200 μW/mK2 with a Seebeck coefficient of >100 μV/K in the PtSe2 layered film of 10 layers can be experimentally demonstrated over a wide temperature range, which is much better than those of most of the two-dimensional materials reported in the literature. Optical absorption spectra and DFT (density functional theory) calculations indicate a semiconductor–metal transition at a critical thickness once the thickness increases from 7.7 (15 layers) to 14.3 nm (30 Layers). The results are consistent with the experimental results of the dramatic reduction in the power factor, the magnitude of the Seebeck coefficient, and the resistivity when the thickness increases from 7.7 (15 layers) to 14.3 nm (30 Layers). Nevertheless, the semiconductor–metal transition would occur when the thickness increases from 1.5 nm (3 layers) to 2 nm (4 layers). To figure out this unusual performance, a detailed material examination has been conducted. After the transmission electron microscopy examination, ∼7% biaxial compressive strain built in the polycrystalline PtSe2 thin film can be observed. The strain, as revealed by our DFT calculations, plays an important role in opening the electronic energy gap and hence significantly improves the thermoelectric performance. Boltzmann transport calculation results suggested that both the strain and the hole concentration in the p-type specimens are well optimized. We further propose that an even better power factor can be achieved with n-type-doped PtSe2.

Transition metal dichalcogenides (TMDCs) have recently attracted a tremendous amount of attention owing to their superior optical and electrical properties as well as the interesting and various nanostructures that are created by different synthesis processes. However, the atomic thickness of TMDCs limits the light absorption and results in the weak performance of optoelectronic devices, such as photodetectors. Here, we demonstrate the approach to increase the surface area of TMDCs by a one-step synthesis process of TMDC nanowalls from WO x into three-dimensional (3D) WS 2 nanowalls. By utilizing a rapid heating and rapid cooling process, the formation of 3D nanowalls with a height of approximately 150 nm standing perpendicularly on top of the substrate can be achieved. The combination of core−shell colloidal quantum dots (QDs) with three different emission wavelengths and 3D WS 2 nanowalls further improves the performance of WS 2 -based photodetector devices, including a photocurrent enhancement of 320−470% and shorter response time. The significant results of the core−shell QD−WS 2 hybrid devices can be contributed by the high nonradiative energy transfer efficiency between core−shell QDs and the nanostructured material, which is caused by the spectral overlap between the emission of core−shell QDs and the absorption of WS 2 . Besides, outstanding NO 2 gas-sensing performance of core−shell QDs/WS 2 devices can be achieved with an extremely low detection limit of 50 ppb and a fast response time of 26.8 s because of local p−n junctions generated by p-type 3D WS 2 nanowalls and n-type core−shell CdSe-ZnS QDs. Our work successfully reveals the energy transfer phenomenon in core−shell QD−WS 2 hybrid devices and shows great potential in commercial multifunctional sensing applications.