We fabricated highly sensitive and selective ammonia gas sensors based on quartz crystal microbalance (QCM) platforms that were functionalized with electrospun polyvinyl acetate (PVAc) nanofibers and doped with various organic acids (i.e., oxalic, tartaric, and citric acids). The structural and chemical surface conditions of the nanofiber-based active layers on top of the QCMs were confirmed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and Fourier-transform infrared (FTIR) spectroscopy. The sensitivity of the PVAc nanofiber-based QCM sensor doped with citric acid was found to be the highest (2.95 Hz/ppm) among others with a limit of detection (LOD) of down to the subppm level (550 ppb). It also exhibited good selectivity, rapid response, short recovery time, and decent repeatability. This simple yet low-cost alternative solution based on chemical modification of nanofibers could improve the performance of QCM-based ammonia gas sensors in many areas including for smart electronic nose applications.

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Production of high-aspect-ratio silicon (Si) nanowire-based anode for lithium ion batteries is challenging particularly in terms of controlling wire property and geometry to improve the battery performance. This report demonstrates tunable optimization of inductively coupled plasma reactive ion etching (ICP-RIE) at cryogenic temperature to fabricate vertically-aligned silicon nanowire array anodes with high verticality, controllable morphology, and good homogeneity. Three different materials [i.e., photoresist, chromium (Cr), and silicon dioxide (SiO2)] were employed as masks during the subsequent photolithography and cryogenic ICP-RIE processes to investigate their effects on the resulting nanowire structures. Silicon nanowire arrays with a high aspect ratio of up to 22 can be achieved by tuning several etching parameters [i.e., temperature, oxygen/sulfur hexafluoride (O2/SF6) gas mixture ratio, chamber pressure, plasma density, and ion energy]. Higher compressive stress was revealed for longer Si wires by means of Raman spectroscopy. Moreover, an anisotropy of lattice stress was found at the top and sidewall of Si nanowire, indicating compressive and tensile stresses, respectively. From electrochemical characterization, half-cell battery integrating ICP-RIE-based silicon nanowire anode exhibits a capacity of 0.25 mAh cm−2 with 16.67% capacity fading until 20 cycles, which has to be improved for application in future energy storage devices.