Title: Molecular-Scale Engineering: Quantum Transport, Thermoelectricity and Optoelectronics
Author: Pablo Bastante Flores - Instituto de Ciencia de Materiales de Madrid & Departamento de Física de la Materia Condensada, Universidad Autónoma de Madrid
Supervised by: Andrés Castellanos-Gómez (ICMM-CSIC) and Nicolás Agrait de la Puente (UAM)
When: January, 16 - 11AM
Where: Salón de Actos, ICMM-CSIC
Abstract: Engineering materials and electronic components at the atomic and molecular scales is the present limit in terms of miniaturization. The possibility of designing by bottom-up synthesis combined with implementations to integrate these nanostructures into multiple interfaces allows adjusting their properties on demand. This thesis aims to explore different molecular systems and strategies to manipulate and enhance their performance and capabilities. In the first part, electrical transport and thermoelectricity in single-molecule junctions have been studied by the use of a home-built Scanning Tunneling Microscope. Two strategies to improve the transport properties have been proposed: First, quantum interference effects introducing resonances between the HOMO-LUMO gap, by electron-withdrawing and donating side groups, and organic radicals with unpaired electrons. Second, the reduction of the gap where the overlap of the frontier orbitals causes the rise in energy of the middle of the transmission, by metal ions in the molecular structure and compounds with low band gaps by structural design. In the second part, the effect of molecular species on the optoelectronic properties of devices based on single layers of atomically thin semiconductors such as MoS2 has been characterized. First, the sensitivity to ambient adsorbates such as water and oxygen molecules has been monitored over time after thermal annealing and air exposure. Then, the use of photoactive adsorbates to selectively increase the photoresponse is tested by depositing ZnTPP molecules on different semiconductors. Finally, the possibility of extending the operational range to the infrared for broadband photodetection by covalent functionalization with bifunctional molecules and palladium nanostructures is demonstrated. In the third part, an outlook on graphene-based nanotransistors presents the preliminary measurements of electroluminescence and quantum dots in graphene nanoribbons contacted by graphene electrodes, as well as the observed Coulomb blockade features in nanostructures of twisted bilayer graphene near the magic angle. All in all, the technologies explored include the design and assembly of different setups and device architectures by micro and nanofabrication techniques for the characterization of atomic and molecular scale materials, and strategies to tailor and enrich their properties.
