Applications of Kelvin Probe Force Microscopy in the characterization of 2D materials and their composites

Kelvin Probe Force Microscopy (KPFM), since its relatively recent introduction in 1991, has become a widely used technique to assess surface charge distribution and work function of metal/semiconductor interfaces in electronic devices at the nanoscale. Today, this characterization technique is employed in many nanotechnology-related applications, with 2D layered material being a notable example due to the necessity of characterizing various electrical properties with high spatial resolution to better understand how these parameters scale or change relatively with synthesis, deposition and environment conditions. In this thesis I will discuss the main applications of KPFM characterization that I have encountered during my three years of doctorate, showing its usefulness in various fields, its main limitations and some practical considerations about the best practices I developed to optimize sample preparation. Regarding the study of fundamental electronic properties I will present my results on the correlation of work function of 2D materials like MoS2 and their thickness (Chapter 3), and how laser induced 3D structures can locally tune the work function of the graphene basal plane for possible future localized functionalization (Chapter 4). As an example of the use of KPFM characterization of 2D composites, I will present the analyses of a metal/graphene interface, more specifically the silver nanowire (AgNW)/graphene interface, selected as it represents a very promising type of hybrid for devices requiring highly conductive transparent electrodes (Chapter 5). Finally, I will show how KPFM can be employed even in a macroscopic conductive 2D material/polymer composite to elucidate the structure or to verify the presence of the conductive elements inside the insulating polymeric matrix (Chapter 6). At the start of this work, I will present a theory background on the origin and principles of the technique, its evolution and main limitations (Chapter 1) followed by a brief introduction of the main other characterization techniques employed and my personal consideration on the best deposition techniques (Chapter 2).