Kelvin Probe Force Microscopy on Graphene Thin Films for Solar Cell and Biosensing Applications
Doctor of Philosophy
Dr. Giovanni Fanchini
Graphene, a one atom thick planer sheet of carbon atoms, has attracted much attention in scientific and technological communities due to its remarkable electronic and physical properties. Graphene has been widely studied as an alternative to transparent conducting Indium Tin Oxide (ITO) electrodes in organic photovoltaics fabrication. Graphene platforms have also attracted interest in biological applications. However, large area graphene films are not yet widely commercialized because the fabrication techniques needed to prepare high quality graphene are expensive and non-scalable. More importantly, most of the low cost fabrication techniques require using toxic materials, which are not biocompatible for using graphene in bio-applications.
In this work, we have proposed ribonucleic acid (RNA), a nonionic surfactant, for exfoliation of graphite in single and few layered graphene flakes in water solution and the subsequent preparation of transparent and conducting graphene-RNA thin films. A number of pre- and post-deposition treatments were performed to improve the electrical and optical performance of graphene-RNA thin films. We further assembled organic photovoltaics on such graphene electrodes using poly(3-hexyl-thiophene):phenyl-C61-butyric acid methyl ester blended photoactive layers. Such photovoltaic devices exhibited high open circuit voltages compared with the reference ITO-based devices. The origin of open circuit voltage was investigated using Kelvin Probe Force Microscopy (KPFM) in the dark and under light irradiation.
In our study, we have also demonstrated the fabrication of photovoltaic devices from cost-effective, water-soluble, and bio-sensitive acridine orange molecules. We investigated the morphology and work function of acridine orange using Atomic Force Microscopy and KPFM. Acridine orange’s ability to generate sufficient amounts of photo carriers was also demonstrated using KPFM under laser irradiation.
To this end, the adhesion of eight proteinogenic amino acids, including Arginine, Tryptophan, Histidine, Lysine, Phenylalanine, Alanine, Asparagine, and Aspartic acid, on the graphene samples was studied using KPFM. The adhesion energy was increased in the order of Ala< Asp< Asn< Lys< Phe< His< Trp< Arg. The results were in good agreement with the previous theoretical calculations of the van der Waals contributions for the adsorption energies of different amino acids with the graphene surface.
Sharifi, Faranak, "Kelvin Probe Force Microscopy on Graphene Thin Films for Solar Cell and Biosensing Applications" (2014). Electronic Thesis and Dissertation Repository. 2597.