ACTA PHYSICA POLONICA A No. 4 Vol. 140 (2021) Proceedings of the Mustansiriyah International Conference on Applied Physics (MICAP-2021) Preparation of High-Performance Room Temperature ZnO Nanostructures Gas Sensor Haitham M. Mikhlif a,* , Mohammed O. Dawood a , Oday M. Abdulmunem a and Mohanad K. Mejbel b a Department of Physics, College of Science, Mustansiriyah University, Palestine St., 10052, Baghdad, Iraq b Materials Techniques Engineering Department, Engineering Technical College-Baghdad, Middle Technical University (MTU), Baghdad, Iraq Doi: 10.12693/APhysPolA.140.320 ∗ e-mail: haitham.mikhlif@uomustansiriyah.edu.iq Zinc oxide (ZnO) nanostructures were deposited on glass substrates by physical vapor deposition tech- nique. To improve the crystallinity of ZnO, oxidation treatment was conducted at 400 ◦ for 1 h in an atmospheric environment. The films characteristics of the films were examined by X-ray diffrac- tion, ultraviolet–visible spectroscopy, atomic force microscopy, and scanning electron microscopy. The X-ray diffraction results illustrated that the deposited films have a polycrystalline hexagonal structure. The ultraviolet-visible spectrum showed that the transmittance of the ZnO film has an energy gap of about 3.225 eV. The atomic force microscopy images indicated that the films have good homogeneity, and the scanning electron microscopy images reveal that they consist of spherical nanosized grains with a granular surface. The ZnO films revealed good sensing performance to acetone and ethanol gases at an operating temperature of 25 ◦ C with suitable recovery and response times. The sensitivity measured by homemade gas sensor system was approximately 21.39, 29.63, 23.8% for acetone gas, and 15.96, 21.28, 20.97% for ethanol gas, at 125, 250, 500 ppm concentrations, respectively. topics: polycrystalline crystal, ZnO nanostructure, physical vapor deposition, gas sensor 1. Introduction In the last decade, ZnO nanostructures have be- come worthwhile because of their significant physi- cal and chemical properties as well as various tech- nological applications. ZnO is an n-type II–VI group compound with a hexagonal crystalline (wurtzite) structure with a direct energy gap of 3.37 eV at room temperature and a high exciton voltage bond- ing of 60 meV [1–3]. ZnO nanocrystalline is es- sential for many applications in electrochemical, electromechanical, electronic and optoelectronic de- vices [4–8] such as solar cells [9–12], ultraviolet (UV) lasers [13, 14], field emission devices [15–17], light- emitting diodes [18], nanosensors [19–21] and nano- piezotronics [22–24]. Zinc oxide thin films are grown by chemical and physical techniques. Physical methods such as evaporation by electron beam epitaxy tech- nique [25], pulsed-laser evaporation [26], reactive magnetron sputtering [27], physical vapor depo- sition (PVD) [28] etc. allow the production of high quality polycrystalline–crystalline nanostruc- tures with homogeneous surface, high efficiency and safe. There are a variety of chemical methods used to deposit ZnO films, for example, chemical bath deposition (CBD) [29], electrode position [30], thermal evaporation [31], spray pyrolysis [32] and multi precipitation method [33]. The zinc oxide thin film has a high electrical resistance when annealing is done in atmosphere air because of oxygen adsorp- tion/desorption process of surface, thus, electrical resistance of the sensing material changes [34]. Acetone and ethanol gases are usually classified as volatile organic compound (VOC) [35]. Acetone evaporates when exposed to the atmosphere and ex- hibits slight toxicity in normal use. There is also some evidence of chronic health effects if no precau- tions are followed. Although ethanol is a common material used, it is also a toxic chemical and highly flammable. In alcoholic beverages, for example, the consumption of ethanol alone can cause coma and death. Therefore, it must be treated with great care at home or in the work. According to the above, gas sensors should be small in size, quick reacting, have a long lifetime and good sensitivity to detect acetone and ethanol gases in low concentrations. The particle size and surface morphology are the main variables relevant for the gas sensing properties for the metal oxide 320