High-Temperature Atomic Layer Deposition of GaN on 1D Nanostructures.

Silica nanosprings (NS) were coated with gallium nitride (GaN) by high-temperature atomic layer deposition. The deposition temperature was 800 °C using trimethylgallium (TMG) as the Ga source and ammonia (NH3) as the reactive nitrogen source. The growth of GaN on silica nanosprings was compared with deposition of GaN thin films to elucidate the growth properties. The effects of buffer layers of aluminum nitride (AlN) and aluminum oxide (Al2O3) on the stoichiometry, chemical bonding, and morphology of GaN thin films were determined with X-ray photoelectron spectroscopy (XPS), high-resolution x-ray diffraction (HRXRD), and atomic force microscopy (AFM). Scanning and transmission electron microscopy of coated silica nanosprings were compared with corresponding data for the GaN thin films. As grown, GaN on NS is conformal and amorphous. Upon introducing buffer layers of Al2O3 or AlN or combinations thereof, GaN is nanocrystalline with an average crystallite size of 11.5 ± 0.5 nm. The electrical properties of the GaN coated NS depends on whether or not a buffer layer is present and the choice of the buffer layer. In addition, the IV curves of GaN coated NS and the thin films (TF) with corresponding buffer layers, or lack thereof, show similar characteristic features, which supports the conclusion that atomic layer deposition (ALD) of GaN thin films with and without buffer layers translates to 1D nanostructures.

Emergent Electrical Properties of Ensembles of 1D Nanostructures and Their Impact on Room Temperature Electrical Sensing of Ammonium Nitrate Vapor.

Ammonium nitrate is an explosive agent that has a very low vapor pressure, which makes airborne detection very challenging. Detection of ammonium nitrate vapor has been achieved by using silica nanospring mats coated with a thin semiconducting layer of zinc oxide. The sensor was operated at room temperature and under ambient conditions in air. Lock-in amplification was employed to measure the change in electrical resistance of the sensor upon exposure to the said target gas analyte. The sensor showed fast detection, only taking ∼15 s to reach its peak response, and exhibited a moderate recovery time of approximately 0.5 min/20 ppm for <40 ppm exposures. A comparison between the ZnO coated nanospring sensor and ZnO thin film sensor demonstrated that the nanospring sensor has superior sensitivity and responsiveness over the thin film sensor. A percolation-based model is proposed to explain the greater sensitivity at low analyte concentrations of the ZnO-nanospring sensor, as compared to a ZnO thin film sensor.