Heterostructures between a tin-based intermetallic compound and a layered semiconductor for gas sensing

UV-light-assisted gas sensor based on PdSe2/InSe heterojunction for ppb-level NO2 sensing at room temperature

The fabrication of van der Waals (vdWs) heterostructures mainly extends to two-dimensional (2D) materials. Nevertheless, the current processes for obtaining high-quality 2D films are mainly exfoliated from their bulk counterparts or by high-temperature chemical vapor deposition (CVD), which limits industrial production and is often accompanied by defects. Herein, we first fabricated the type-II p-PdSe₂/n-InSe vdWs heterostructure using the ultra-high vacuum laser molecular beam epitaxy (LMBE) technique combined with the vertical 2D stacking strategy, which is reproducible and suitable for high-volume manufacturing. This work found that the introduction of 365 nm UV light illumination can significantly improve the electrical transport properties and NO₂ sensing performance of the PdSe₂/InSe heterojunction-based device at room temperature (RT). The detailed studies confirm that the sensor based on the PdSe₂/InSe heterojunction delivers the comparable sensitivity (R_a/R_g = ∼2.6 at 10 ppm), a low limit of detection of 52 ppb, and excellent selectivity for NO₂ gas under UV light illumination, indicating great potential for NO₂ detection. Notably, the sensor possesses fast response and full recovery properties (275/1078 s) compared to the results in the dark. Furthermore, the mechanism of enhanced gas sensitivity was proposed based on the energy band alignment of the PdSe₂/InSe heterojunction with the assistance of investigating the surface potential variations. This work may pave the way for the development of high-performance, room-temperature gas sensors based on 2D vdWs heterostructures through the LMBE technique.

Boosting room-temperature NO2 detection via in-situ interfacial engineering on Ag2S/SnS2 heterostructures

The effective detection of hazardous gases has become extremely necessary for the ecological environment and public health. Interfacial engineering plays an indispensable role in the development of innovative materials with exceptional properties, thus triggering a new revolution in the realization of high-performance gas sensing. Herein, the rational designed Ag₂S/SnS₂ heterostructures were synthesized via a facile in-situ cation-exchange method. The coshared S atoms derived from in-situ interfacial engineering enable intimate atomic-level contact and strong electron coupling between SnS₂ and Ag₂S, which efficiently assist interfacial charge redistribution and transport as confirmed theoretically and experimentally. Benefiting from the high-quality interface of the heterostructures, the resultant Ag₂S/SnS₂ sensor delivered an ultrahigh response (286%) together with short response/recovery time (17 s/38 s) to 1 ppm NO₂. The sensor also demonstrated superior sensing selectivity and reliable repeatability at room-temperature. Such excellent sensing performance could be synergistically ascribed to the junction effect and interfacial engineering of Ag₂S/SnS₂ heterostructures, which not only modulates the electronic properties of SnS₂ but also provides abundant adsorption sites for gas sensing. This study offers guidance for engineering heterostructures with high-quality interface, which might stimulate the exploitation of other novel materials and widen their potential applications.

Ligand-assisted deposition of ultra-small Au nanodots on Fe2O3/reduced graphene oxide for flexible gas sensors

The development of flexible room-temperature gas sensors is important in environmental monitoring and protection. In this contribution, by using 1-octadecanethiol (ODT) as a surface ligand, Au nanodots (NDs) with ultra-small size of ∼1.7 nm were deposited on the surface of α-Fe₂O₃/reduced graphene oxide (rGO). The Au ND-ODT/α-Fe₂O₃/rGO composite was fabricated into flexible gas sensors, which could detect NO₂ gas down to 200 ppb at room temperature. Compared with α-Fe₂O₃/rGO, Au ND-ODT/α-Fe₂O₃/rGO showed enhanced sensing performance because of the beneficial effects of Au NDs, including facilitating the adsorption of NO₂ molecules and forming ohmic-like contact with rGO and α-Fe₂O₃. In addition, the sensing performance of the composite was also influenced by the surface ligands of the Au NDs. Ligands with less polar terminal groups were found to be beneficial to charge transfer in the sensing film. Moreover, Au ND-ODT/α-Fe₂O₃/rGO-based flexible sensors showed negligible performance deterioration under moderately bent conditions, suggesting their potential to be used in portable and wearable devices.

Realization of Oriented and Nanoporous Bismuth Chalcogenide Layers via Topochemical Heteroepitaxy for Flexible Gas Sensors

Most van der Waals two-dimensional (2D) materials without surface dangling bonds show limited surface activities except for their edge sites. Ultrathin Bi₂Se₃, a topological insulator that behaves metal-like under ambient conditions, has been overlooked on its surface activities. Herein, through a topochemical conversion process, ultrathin nanoporous Bi₂Se₃ layers were epitaxially deposited on BiOCl nanosheets with strong electronic coupling, leading to hybrid electronic states with further bandgap narrowing. Such oriented nanoporous Bi₂Se₃ layers possessed largely exposed active edge sites, along with improved surface roughness and film forming ability even on inkjet-printed flexible electrodes. Superior room-temperature NO₂ sensing performance was achieved compared to other 2D materials under bent conditions. Our work demonstrates that creating nanoscale features in 2D materials through topochemical heteroepitaxy is promising to achieve both favorable electronic properties and surface activity toward practical applications.