Metal halide perovskite materials are emerging as highly promising materials for high performance optoelectronic devices thus triggered broad attention. Nanostructured perovskite materials have wide applications in nanoelectronics and nano-optoelectronics. Due to incompatibility of metal halide perovskite materials with conventional lithography techniques, it is preferable to achieve nano-perovskite material growth and assembly at the same time for further device applications. In our work, we have developed a chemical vapor deposition (CVD) process to grow ordered three-dimensional (3-D) metal halide nanowire (NW) arrays in nanoengineering templates. This unique CVD process utilizes metal nanoclusters at the bottom of vertical nanochannels to initiate high quality NW growth. As the nanochannels have largely controllable geometrical factors, namely, periodicity, diameter and depth, NW geometry can also be precisely nanoengineered. As the result, the ordered 3-D NW arrays can achieve ultra-high NW density in the range of ~109/cm2. The 3-D NW arrays are conspicuously promising for 3-D integrated nano-electronics/optoelectronics. To further demonstrate the technological potency of the perovskite NW arrays, they have been fabricated into photodetectors and proof-of-concept image sensors. Each image sensor consists of 1,024 photodiode pixels made of vertical perovskite NWs, and the imaging functionality has been verified by recognizing various optical patterns projected on the sensor. In addition, we have also discovered that the chemically and mechanically robust template can effectively protect perovskite NWs from water and oxygen invasion thus the material stability is significantly better than planar perovskite films confirmed by photoluminescence and photoelectric measurements.
Zinc oxide nanowires are configured as <i>n</i>-channel field effect transistors. These transistors are implemented as highly sensitive chemical sensors for detection of various gases such as O<sub>2</sub>, NO<sub>2</sub>, NH<sub>3</sub>, and CO at room temperature. They show oxidizing sensing property to oxygen and nitrogen dioxide. Nanowires' ammonia sensing behavior is observed to switch from oxidizing to reducing when temperature increased from 300 to 500 K. This effect is attributed to the temperature dependent chemical potential shift. Carbon monoxide is found to increase the nanowire conductance in the presence of oxygen. Due to a Debye screening length comparable to the nanowire diameter, the electric field applied over the back gate significantly affects the sensitivity as it modulates the carrier concentration. A strong negative field is utilized to refresh the sensors by an electro-desorption mechanism. In addition, different chemisorbed species could be distinguished from the "refresh" threshold voltage and the temporal response of the conductance. These results demonstrate a refreshable field effect sensor with a potential gas identification function.
Low dimensional systems such as nanotubes and nanowires have fascinating, and technologically useful, optical and electrical properties. Studies on these systems advance our knowledge on the science at the nanoscale, while simultaneously provides the possibility for developing miniaturized electronics and optoelectronics. The material system attracting increasing attention is zinc oxide (ZnO),<sup>1-6</sup>which is a <i>II-VI</i>compound semiconductor with a wide and direct banc gap of 3.37 eV at room temperature. ZnO has demonstrated unique properties and potential applications in manifold fields, such as transparent electronics, ultraviolet (UV) light emitter, surface acoustic wave devices, gas sensors and magnetoelectronics. It is shown to have wurtzite structure with lattice constant a = 3.249 Α, c = 5.207 Α. Its large exciton binding energy (60 meV), which is much greater than the thermal energy at room temperature, makes it a promising candidate for applications in blue-UV light emission and room temperature UV lasing<sup>7</sup>. Furthermore, its high piezoelectric constant (d<sub>33</sub>=246) makes it a highly valuable material for fabricating mechanical devices, such as acoustic transducers, sensors and actuators.