Graphene-Based Sensors for Soil Analysis: A Spectroscopic and Theoretical Study

This PhD research investigates the electronic and photophysical properties of graphene and its derivatives, focusing on the relationship between these properties and structures of functionalised graphene materials for their potential application in phosphate sensors for agricultural soil analysis as part of the Signals in the Soils (SitS) project. Graphene-based materials were synthesised by our partners within the SitS project, and this study aimed to explore their structural and electronic characteristics which can influence their performance in sensors. Gaining an understanding of the relationship between functionalised graphene’s structure and properties helps address a critical gap in the field of nanomaterials to enable their application in sensor technologies.
A combination of ultrafast spectroscopy and computational modelling techniques was employed to investigate functionalised graphenes’ structure-property relationship. Experimental methods, including ultraviolet-visible spectroscopy, infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS), were used to investigate the structural and bonding characteristics of graphene-based materials. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) provided key insights into the morphology, particle shape, and particle size distribution of the materials. Transient absorption spectroscopy (TAS) enabled the study of electron dynamics on ultrafast timescales, revealing the impact of oxygen-induced trap states. Density Functional Theory (DFT) simulations using the SIESTA software were conducted to model the electronic and optical properties of these materials and the influence of multiple factors on these properties.
This research showed that electron dynamics in functionalised graphene is a complex process involving multiple decay pathways on a range of timescales. Moreover, this research demonstrated presence of oxygen groups and morphological changes in functionalised graphene. Charge dynamics was found to be directly dependent on the functionalisation of graphene, in particular, on the amount of oxygen present in the samples. The study also showed that oxygen content affects the optical absorption and prolongs carrier lifetimes. Theoretical calculations revealed that oxygen functionalisation, particularly the introduction of hydroxyl and epoxide groups, has a significant impact on the electronic properties of graphene by opening bandgaps. This tunability enhances graphene’s optoelectronic properties, making it a promising candidate for sensor applications.
Overall, this research advances the understanding of how oxygen functional groups alter the morphology, electronic structure, and optical properties of graphene, offering new avenues for tailoring its properties for next-generation electronic and sensing devices. The combination of experimental techniques and theoretical modelling provides valuable insights into the design and optimisation of graphene-based materials for technological applications.