Single Molecules Studied by Low-Temperature Microwave Scanning Tunneling Microscopy

Dielectric spectroscopy, a versatile analytical technique, investigates the dielectric properties of materials by measuring their response to an external periodically time-varying electric field across a broad frequency range. It has applications in understanding material properties such as permittivity, conductivity, and molecular dynamics, with significant relevance in fields like polymer science, biology, and molecular electronics. Using this powerful tool, this thesis investigates the dielectric and conductive properties of single organic molecules, specifically Bis-(acetylsulfanyl) 7 helicene (BA7H), adsorbed on the single-crystal Ag(111) surface. Utilizing low-temperature microwave scanning tunneling microscopy, reflection coefficient S11(z, f) measurements at GHz frequencies were performed to quantify molecular conductance G(z, f) and capacitance C(z, f) at the nanoscale. Experimental results were derived using retraction curves obtained over BA7H and pristine Ag(111). The analysis involved repeating the measurement at least five times at each different frequency value to ensure reproducibility and accuracy in determining the complex-valued dielectric function ε∗(ω). The findings highlighted the capabilities of a microwave-adapted scanning tunneling
microscope in unraveling the dielectric absorption and polarization dynamics at the molecular level. Through careful optimization and parameter tuning, this study addresses technical challenges associated with conductance measurements at GHz frequency. These advancements offer valuable insights into the intrinsic properties of functional organic molecules and their potential applications in nanoscale devices. Future work will focus on expanding this methodology to study larger assemblies and more complicated molecular systems to further enhance the understanding of molecular-scale electrical phenomena.