Colloidal quantum dots (CQDs) offer promise in flexible electronics, light sensing and energy conversion. These applications rely on rectifying junctions that require the creation of high-quality CQD solids that are controllably n-type (electron-rich) or p-type (hole-rich). Unfortunately, n-type semiconductors made using soft matter are notoriously prone to oxidation within minutes of air exposure. Here we report high-performance, air-stable n-type CQD solids. Using density functional theory we identify inorganic passivants that bind strongly to the CQD surface and repel oxidative attack. A materials processing strategy that wards off strong protic attack by polar solvents enabled the synthesis of an air-stable n-type PbS CQD solid. This material was used to build an air-processed inverted quantum junction device, which shows the highest current density from any CQD solar cell and a solar power conversion efficiency as high as 8%. We also feature the n-type CQD solid in the rapid, sensitive, and specific detection of atmospheric NO2. This work paves the way for new families of electronic devices that leverage air-stable quantum-tuned materials.

Perovskite solar cells (PSCs) have attracted extensive research interest in the last decade due to their high power conversion efficiency (PCE) and simple solution-based fabrication process. [1,2] Evolved from dye-sensitized solar cells (DSSCs), [3] typical PSCs usually employ a mesoporous TiO 2 as the electron-transport layer (ETL), which also functions as the scaffold for depositing the perovskite absorbing layer. [4,5] Although it is criticized that the high-temperature (>450 °C) sintering process for the mesoporous TiO 2 layer makes the device manufacturing complex and energy consumptive, which also hinders the integration of PSCs with flexible substrates and electronics, such mesoscopic PSCs have been dominating the efficiency breakthroughs of PSCs from certified 14.1% in 2013 to 23.7% in 2019. [5-8] The latest 25.2% is highly possible also obtained by mesoscopic PSCs. [9] The ambipolar charge transport characteristics and long charge carrier diffusion length of lead halide perovskites offers the possibility of replacing the mesoporous ETL by a planar one, and constructing planar-structured PSCs with low-temperature (≤150 °C) processes. [10,11] For inverted (p-in) planar PSCs, there are plenty of options available for ETLs and hole-transport layers (HTLs), such as poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and phenyl-C61-butyric acid methyl ester (PC 61 BM), attributing to years of research on organic solar cells. [12,13] For regular (n-i-p) planar PSCs, compact TiO 2 layer was first used as the ETL, which soon aroused the attention on the anomalous hysteresis phenomenon for PSCs. [14,15] It was claimed that the low electron mobility of compact TiO 2 resulted in charge accumulations at the TiO 2 /perovskite interface and thus caused significant hysteresis. [16,17] Then, it was further found out that the electronic contact between the TiO 2 ETL and the perovskite layer played an essential role in the hysteresis behaviors of PSCs. [18] This is in agreement with the fact that the mesoporous TiO 2-based PSCs usually show much reduced hysteresis, [6,7] since the mesoscopically structured ETL can provide much larger surface area for contacting the perovskite absorber with stabilized properties. Along with the defects at the interfaces, ion migration and trap states in the perovskite layer have also been considered as the origin of the hysteresis Perovskite solar cells (PSCs) have rapidly developed and achieved power conversion efficiencies of over 20% with diverse technical routes. Particularly, planar-structured PSCs can be fabricated with low-temperature (≤150 °C) solution-based processes, which is energy efficient and compatible with flexible substrates. Here, the efficiency and stability of planar PSCs are enhanced by improving the interface contact between the SnO 2 electron-transport layer (ETL) and the perovskite layer. A biological polymer (heparin potassium, HP) is introduced to regulate the arrangement of SnO 2 nanocrystals, and induce vertically aligned crystal growth of perovski...

Metal oxide/graphene nanocomposites are emerging as one of the promising candidate materials for developing high-performance gas sensors. Here, we demonstrate sensitive room-temperature H 2 S gas sensors based on SnO 2 quantum wires that are anchored on reduced graphene oxide (rGO) nanosheets. Using a one-step colloidal synthesis strategy, the morphology-related quantum confinement of SnO 2 can be well-controlled by tuning the reaction time, because of the steric hindrance effect of rGO. The assynthesized SnO 2 quantum wire/rGO nanocomposites are spin-coated onto ceramics substrates without further sintering to construct chemiresistive gas sensors. The optimal sensor response toward 50 ppm of H 2 S is 33 in 2 s, and it is fully reversible upon H 2 S release at 22 °C. In addition to the excellent gas adsorption of ultrathin SnO 2 quantum wires, the superior sensing performance of SnO 2 quantum wire/rGO nanocomposites can be attributed to the enhanced electron transport resulting from the favorable charge transfer of SnO 2 /rGO interfaces and the superb transport capability of rGO. The easy fabrication and roomtemperature operation make our sensors highly attractive for ultrasensitive H 2 S gas detection with less power consumption.

The surface-initiated polymerization with different initiator densities and different polymerization rates is investigated using molecular dynamics simulation method. We find that the initiator density, together with the polymerization rate, greatly determines the polymer brush structure, the initiation efficiency, and the graft density, especially when the initiator density is high. The excluded volume effect also plays a crucial role in the system when the chains are densely grafted. By tuning the initiator density and modifying the polymerization rate, we can obtain the polymer brushes with different degrees of polydispersity. This study partially emphasizes the importance of considering the effects of polymerization rate in further investigations.

Pt/H-ZSM5 and Pt/Al2O3 with similar surface Pt particle sizes and chemical states were prepared by incipient wetness impregnation as catalysts for soot oxidation. Pt/H-ZSM5 exhibits obviously higher activities in both O2 and NO + O2 than Pt/Al2O3. On the basis of the results obtained with in situ DRIFT and other structural and surface property characterizations, the high catalytic activity of Pt/H-ZSM5 is currently attributed to two main factors. First, the acidic H-ZSM5 support inhibits NO2 adsorption on the catalyst, leading to a preferential adsorption of NO2 on the surface of soot and providing more chances for NO2–soot reactions. Second, the strong acid sites on the surface of Pt/H-ZSM5 can participate in the catalytic formation and decomposition of surface oxygenated complexes. Consequently, a high catalytic efficiency for soot oxidation is achieved on the Pt/H-ZSM5 catalyst.

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