Study on Temporal Growth and Luminescence Properties of ZnO Nanostructures

Abstract

Zinc Oxide (ZnO) has attracted significant attention due to its unique characteristics and promising potential in various fields such as photo-detectors, sensors, LEDs and solar cells etc. Optically, ZnO NWs are highly desirable due to their wide energy band gap (approximately 3.27 eV) and large exciton binding energy (approximately 60 meV) at room temperature. These properties, along with their high surface-to-volume ratio, make ZnO NWs ideal for use in optoelectronics. Additionally, its non-toxicity, bio-compatibility and variety of synthesis methods make it more attractive. The objective of this study is to examine the growth and aging of ZnO nanoparticles, as well as the influence of the chemical environment on these phenomena. Additionally, the study explores the correlation between growth duration and temperature and their impact on the optical characteristics of nanowires (NWs). The scientific community has extensively documented the study of the temporal growth and aging of nanoparticles, with numerous experimental and theoretical models available. However, these studies often necessitate the use of intricate equipment, highlighting the importance of proper handling. In this study, we employed a simple chemical approach to synthesize colloidal nanoparticles (NPs) and examined their temporal growth and aging in the growth solution using experimental and theoretical models. Our experimental procedures involved UV-vis and TEM spectroscopy techniques to investigate the in situ growth of NPs. The results from both methods demonstrate that nucleation occurs within 2 minutes in the growth solution, followed by the growth of particles. Additionally, we also studied atomically balanced and unbalanced precursors and discovered that atomically balanced reactions lead to the growth and aging of NPs, while atomically unbalanced reactions result in the decoupling of growth due to the presence of excess Zn environment around ZnO nuclei. Subsequently, we understood and modelled the growth of NPs using a phase field model (PFM) for different sets of parameters at various time steps. In addition, we investigated the impact of temperature variations on the morphological and optical properties of ZnO nanowires. Our results indicate that even small changes in growth temperature can result in significant modifications in the morphology of ZnO nanostructures. For instance, a growth temperature of 70°C produced non-uniform structures, while broken and hollow tips of NWs were achieved at a temperature of 120°C, and the formation of hexagonal NWs was observed at 90°C. We also discovered that changes in morphology or growth temperature can alter the optical band gap, while still leading to the same number of defect states at the same energy position within the band gap. Despite the same defect energy states at the same energy position, they show a shift in their NBE emission peak.