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.

Two-dimensional layered materials such as graphene, transition metal dichalcogenides, and black phosphorus have demonstrated outstanding properties due to electron confinement as the thickness is reduced to atomic scale. Among the phosphorus allotropes, black phosphorus, and violet phosphorus possess layer structure with the potential to be scaled down to atomically thin film. For the first time, the plasma-assisted synthesis of atomically layered violet phosphorus has been achieved. Material characterization supports the formation of violet phosphorus/InN over InP substrate where the layer structure of violet phosphorus is clearly observed. The identification of the crystal structure and lattice constant ratifies the formation of violet phosphorus indeed. The critical concept of this synthesis method is the selective reaction induced by different variations of Gibbs free energy (ΔG) of reactions. Besides, the Hall mobility of the violet phosphorus on the InP substrate greatly increases over the theoretical values of InP bulk material without much reduction in the carrier concentration, suggesting that the mobility enhancement results from the violet phosphorus layers. Furthermore, this study demonstrates a low-cost technique with high compatibility to synthesize the high-mobility atomically layered violet phosphorus and open the space for the study of the fundamental properties of this intriguing material as a new member of the fast growing family of 2D crystals.

In this work, polymethylmethacrylate (PMMA) as a superior mediate for the pressure welding of silver nanowires (Ag NWs) networks as transparent electrodes without any thermal treatment is demonstrated. After a pressing of 200 kg cm , not only the sheet resistance but also the surface roughness of the PMMA-mediated Ag NWs networks decreases from 2.6 kΩ sq to 34.3 Ω sq and from 76.1 to 12.6 nm, respectively. On the other hand, high transparency of an average transmittance in the visible wavelengths of 93.5% together with a low haze value of 2.58% can be achieved. In terms of optoelectronic applications, the promising potential of the PMMA-mediated pressure-welded Ag NWs networks used as a transparent electrode in a green organic light-emitting diode (OLED) device is also demonstrated. In comparison with the OLED based on commercial tin-doped indium oxide electrode, the increments of power efficiency and external quantum efficiency (EQE) from 80.1 to 85.9 lm w and 19.2% to 19.9% are demonstrated. In addition, the PMMA-mediated pressure welding succeeds in transferring Ag NWs networks to flexible polyethylene naphthalate and polyimide substrates with the sheet resistance of 42 and 91 Ω sq after 10 000 times of bending, respectively.