Albert Cortijos i Aragonès
Published: 2017
Total Pages: 396
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"Inspired by the proposal that single molecules will be functional elements of future nanoelectronic and Spintronics devices, there exists considerable interest in understanding charge transport in individual molecular backbones. To investigate charge transport in single-molecule devices, in the presented thesis is exploit scanning tunneling microscopy-based approaches in the break-junction mode (STM-BJ) designed by Xu and Tao in 2003 under the effects of magnetic and electric force fields, which divide the thesis in two parts. The first block of the first part of the thesis is presented a study performed at room-temperature based on spin-dependent transport in single-molecule devices employing on thermal spin-crossover metal complexes. Here is shown that the interfacial magnetism or Spinterface, resulting from the interaction between a magnetic molecule and a metal surface, becomes the key pillar to engineer nanoscale molecular devices with novel spin-based functionalities, such as conductance switching based on a Spinfilter, because has the capability to spin-polarize the injected current through it. Also in this block are defined the required conditions which have to be gathered by any molecule to behaves as spin-filtering: be paramagnetic and susceptible to an aligned by external magnetic field, interact with the junction metal electrodes enough strongly through the extended electronic states and also present close energy values to the "fermi energy" for one of the electronic spins allowing its transport. The observed results can be summarized as a high magnetoresistive efficiency of two orders of magnitude (10000%) between the two magnetic field orientations. In the second block of the first part is presented a novel way to form highly conductive and tunable molecular wires exploiting supramolecular chemistry schemes. Single metalloporphyrin rings are wired from its metallic center by using strong Lewis bases, resulting in an increase of the conductivity of three orders of magnitude versus previous single-porphyrin wires. This novel platform of wiring individual porphyrins mimics the way nature exploits these systems by orienting the perpendicular porphyrin axis as the easy axis for electron/energy transfer. Employing this new perpendicular molecule's orientation, spin-depending current measurements were performed following the procedure of the first block using Cu and Co metalloporphyrins. results Spinfilter-switch effect. The observed results can be summarized as a medium magnetoresistive efficiency ca. factor 2-4 between the two magnetic field orientations. The third block of the first part is focused on Spin selectivity induced by electron transport through chiral molecules (CISS) replacing the paramagnetic character of the device's central molecules previously studied. A new method to quantify the spin polarization power of chiral molecules is presented using a junction of either a Dextro- or Levo- 22 amino-acid peptide coupled to an Au surface and to a magnetized Ni contact. As a consequence of the molecular property of helicity filtering and the asymmetry in the density of states at the ferromagnetic electrode, the results show how the conductance can be separated in electron helicity channels where the largest contribution is correlated with the molecular filtering effect in the spin-polarized transport through the chiral peptide. In the second part and based on using external electric fields, is demonstrated the use of the STM-BJ approach to study basic mechanisms in chemical catalysis at the nanoscale. Is designed a surface model system to probe electric field catalysis of a Diels-Alder reaction by delivering an oriented electrical field-stimulus across two reactants: a diene, attached to the STM tip electrode and a dienophile attached to the substrate electrode. This method enables studying chemical reactions at the single-molecule level. Was observed how only an external electric field aligned in the specific way respect to the reaction center and pointing from the diene (bearing a negative charge) to the dienophile (bearing a positive charge) can accelerate the Diels-Alder reaction process. Besides using the external electric field strength as tool was possible to tune the reaction processes." -- TDX.