Field-effects on single molecular circuitry. Electronic transport from synthetic to biological models

Abstract

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.

En el primer bloc de la primera part de la tesi es presenta un estudi realitzat a temperatura ambient basat en el transport spin-dependent a través d'en complexos metàl·lics tipus spin-crossover. Aquí es mostra que la Spinterface, esdevé el pilar clau per dissenyar dispositius unimoleculars Spintrònics perquè té la capacitat de spin-polaritzar el corrent injectat a través d'ella. També defineixen les condicions per a les molècules per comportar-se com spin-filtres: paramagnètic i alineació a un camp magnètic, i estats electrònics alineats i interactuant significativament amb els elèctrodes metàl·lics. Els resultats observats es poden resumir com una alta eficiència magnetoresistiva de dos ordres de magnitud (10.000%) entre les dues orientacions del camp magnètic. En el segon bloc de la primera part, fent servir una orientació perpendicular, es van realitzar mesuraments de corrent dependents del spin seguint el procediment descrit i usant metalloporfirinas de Cu i Co Els resultats observats es poden resumir com una eficiència magnetoresistiva mitjana entre un factor 2 i 4. El tercer bloc de la primera part se centrarà en la selectivitat de spin induïda pel transport d'electrons a través de molècules quirals (CISS) que substitueix el caràcter paramagnètic de les molècules centrals. Es va mesurar un pèptid Dextro o Levo de 22 aminoàcids acoblat a elèctrodes d'Au i Ni. A causa de l'spin-filtre fruit de la helicitat i de la asimetria en la densitat d'estats en l'elèctrode ferromagnètic, els resultats mostren com la conductància pot separar-se en canals de helicidad d'electrons on la major contribució es correlaciona amb l'efecte de filtrat molecular en el spin-polaritzat mitjançant el pèptid quiral. A la segona part i basat en l'ús de camps elèctrics s'estudien els mecanismes de catàlisi a la nanoescala emprant un disseny unit a l'elèctrode de punta STM i un dienófil unit a l'elèctrode de substrat. Es va observar que només un camp elèctric alineat en la forma específica respecte al centre de reacció i apuntant des del die al dienófil pot accelerar el procés de reacció de Diels-Alder utilitzant utilitzar la intensitat del camp elèctric com a regulador