The detection and monitoring of gases in exhaled human breath up to date has been limited by the lack of appropriate materials and technologies which could rapidly and selectively identify the presence and monitor the concentration of trace levels of specific analytes-biomarkers. We present a metal oxide-based nanosensor that is highly specific to ammonia gas in breath-simulating environments at low part-per-billion concentrations. The design of a handheld breath analyzer for gas detection in exhaled human breath is described. Semiconducting ceramics are presented as suitable sensor materials for easy and affordable non-invasive diagnostics.

We have constructed heterojunction iodide ligand PbS quantum dot (QD) and ZnO nanowire (NW) solar cells. In these interdigitated structures, PbS QDs are well-embedded within ZnO NWs grown on a dense ZnO layer. A Au back contact is directly formed on the iodide ligand PbS QD surface layer. In the widely studied colloidal QD-based heterojunction solar cells, the PbS QD active layer is sandwiched between the hole-blocking layer (or electron-accepting layer) and the electron-blocking layer (EBL) (or hole-transport layer). In contrast, our solar cells contain no EBL except an additional thin PbS QD capping layer, which was deposited on the interdigitated layer to ensure the complete infiltration of the PbS QDs, and prevent shortening between the ZnO NWs and Au back contact. The interdigitated layer was constructed using a layer-by-layer dip-coating method to achieve sufficient infiltration of PbS QDs in the ZnO NWs. Owing to highly efficient charge separation in the interdigitated structure, our cell achieved a high external quantum efficiency in the visible and infrared regions, despite using only iodide ligand PbS QDs. Using cross-sectional Kelvin probe force microscopy, we confirmed that the PbS QD capping layer formed between the ZnO NW and Au back contact intrinsically functioned as an EBL. Consequently, the solar cells with ZnO NWs and iodide ligand PbS QDs maintained their performance after 500 days of storage in air without encapsulation. This stability performance surpasses most of the previously reported solar cells with PbS QDs. An encapsulated 1.0 cm 2 solar cell maintained its power output for over 75 h under continuous one-sun illumination with no significant degradation. The proposed solar cell architecture with an automatically embedded EBL can realize highly efficient and stable colloidal QD-based solar cells. Moreover, the interdigitated architecture concept can be extended to various organic and inorganic hybrid solar cells.

AgBiS 2 nanocrystals (NCs) are nontoxic, lead-free, and near-infrared absorbing materials. Eco-friendly solar cells were constructed using interdigitated layers of ZnO nanowires (NWs) and AgBiS 2 NCs, with the aim of elongating the otherwise short carrier diffusion length of the AgBiS 2 NC assembly. AgBiS 2 NCs were uniformly infiltrated into the ZnO NW layers using a low-cost and easily scalable dip coating method. The resulting ZnO NW/AgBiS 2 NC interdigitated structures provided efficient carrier pathways in constructed nanowire solar cells (NWSCs), composed of a transparent electrode/ZnO NW/AgBiS 2 NC interdigitated layer/P3HT hole transport layer/Au. The photocurrent external quantum efficiency (EQE) in the visible to nearinfrared regions was enhanced compared to those of the control solar cells made with ZnO/AgBiS 2 tandem layered structures. The maximum EQE for the NWSCs reached 82% in the visible region, which is higher than the EQE values previously reported for solar cells fabricated with ZnO/AgBiS 2 NCs. Air stability tests on unsealed NWSCs demonstrated that 90% or more of the initial power conversion efficiency was maintained even after 6 months.

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