Conformational Dependence and Capacitance in the Study of Helicenes for Molecular Electronics

• Autores: Tamara de Ara García
• Directores de la Tesis: Carlos Untiedt Lecuona (dir. tes.), Carlos Sabater Piqueres (dir. tes.)
• Lectura: En la Universitat d'Alacant / Universidad de Alicante ( España ) en 2024
• Idioma: inglés
• Tribunal Calificador de la Tesis: Nicolás Agraït de la Puente (presid.), Linda Ángela Zotti (secret.), Mads Brandbyge (voc.)
• Programa de doctorado: Programa de Doctorado en Nanociencia y Nanotecnología por la Universidad de Alicante; la Universidad de Castilla-La Mancha; la Universidad de La Laguna; la Universidad Jaume I de Castellón y la Universitat de València (Estudi General)
• Materias:
  • Física
    • Física del estado sólido
      • Propiedades de transporte de electrones
• Enlaces
  • Tesis en acceso abierto en: RUA
• Resumen

  • In this study, we explore the electronic transport properties of a series of helical molecules between metallic electrodes, their deposition, and the role of organic solvents. Additionally, the capacitance between electrodes is investigated. To tackle these challenges, the electronic characteristics of the molecular junctions are typically studied using techniques such as scanning tunneling microscopy or break-junction techniques, which enable the investigation of individual molecules. This dissertation presents experimental work on electronic transport within molecular electronics, specifically on helicoidal molecules. The study explores the conformational dependence of single molecules on charge transport and delves into the molecular transport mechanism of a series of helicenes. A STM study provides insights into molecular deposition and orientation on gold electrodes. Mechanically controlled break junction (MCBJ) experiments are conducted to probe electronic conduction mechanisms, supported by ab initio calculations, contributing to a comprehensive understanding of the electronic transport mechanism in the molecules. A study on monocyclic molecules, derived from the primary objective, sheds light on the impact of binding conformation in tuning charge transport using STM. We observe physisorption through topography and explore molecular junctions via the break junction technique. Density functional theory (DFT) calculations and molecular dynamics (MD) simulations enhance our understanding of the intricate relationship between molecular geometry and quantum transport. While considering the contact as a resistance is a common approach, the systems are inherently more complex, and electrostatic effects may come into play. For instance, capacitative contributions become apparent as tunneling occurs and the distance increases, before the classic picture becomes relevant. Therefore, in this study, we have approached the resistance and the capacitance effect between electrodes at short distances. This dissertation addresses the capacitance contribution between electrodes, aiming to study the variation in the density of states measured due to the presence of molecules. We use a STM at low temperatures to measure capacitance as a function of the distance. Measurements show three distinct regimes: leakage of capacitance, quantum capacitance limit influenced by the Pauli exclusion principle, and the classical capacitive effect. The capacitance is modeled, revealing a correction that limits the classical behavior at short distances, related to the finite density of states. This research provides insights into the interplay between quantum effects and capacitance in charge transport.