Surface chemistry of colloidal semiconductor quantum dots on graphitic substrates

Resumen

In the search of promising synergetic effects for future technological applications such as the generation of photocurrent in photovoltaics, the combination of semiconductor nanoparticles (or colloidal quantum dots) and graphitic surfaces such as those of carbon nanotubes has attracted considerable attention. The experimental results shown in this thesis are the outcome of combined work that includes both synthesis of nanomaterials (CdSe semiconductor nanoparticles, NPs) and surface chemical characterization when they are in contact to graphitic surfaces. The work has been carried out in chemical laboratories belonging to IMDEA Nanoscience and Universidad Autónoma of Madrid and in the Surface Science Laboratory at the Universidad Autónoma of Madrid (LASUAM). From the synthetic point of view it has been confirmed that little variations in the amount of a chlorinated co-solvent added during the synthesis of CdSe NPs lead to chemical and morphological changes on the NPs, modifying their shape from rod-like to pyramidal. Furthermore, if a graphitic surface is added to the synthetic mixture (such as highly oriented pyrolytic graphite (HOPG) or carbon nanotubes, once the NPs acquire the pyramidal shape they are prone to decorate graphitic surfaces in contrast to rod-like NPs, which do not show this tendency. The main part of this work has focused on the elucidation of the CdSe NPs surface in contact to graphitic surfaces, in particular to HOPG. CdSe NPs composed of an inorganic core covered by organic molecules (the so-called ligand shell) have been analyzed mainly by X-ray photoelectron spectroscopy (XPS). XPS analysis confirms the presence of two different phosphorous chemical environments on the NPs surface, which have been assigned to two different organic ligands (molecules) capping the NPs surface. It has been elucidated that the final morphology of the NPs depends not only on the concentration of these molecules but also on the ratio between them and the presence of chlorine on the surface. Correlated Scanning Electron Microscopy and XPS studies allow extracting information about the role of the chlorinated co-solvent during the synthesis of the NPs and their effect in the interaction to graphitic surfaces. In particular, if the synthetic process takes place in the absence chlorinated co-solvents, both P-containing species co-exists on the surface leading to the production of rod-like NPs. If the process takes place in the presence of the chlorinated co-solvent partial and selective displacement of one of the P-environments occurs, boosting the shape transformation and further interaction to the graphitic surfaces. This knowledge allows obtaining control over the coverage and monodispersity of CdSe NPs monolayer arrangements on graphitic surfaces. Other methods to produce monolayer arrangements of NPs on surfaces have been studied and compared with the above mentioned approach. Among them, NP depositions by means Langmuir Blodgett, convective assembling and electrospray in high-vacuum conditions have been carried out. Finally, morphological changes of pyramidal CdSe NPs on HOPG upon annealing treatments have been characterized by means of Scanning Tunneling Microscopy (STM) along with correlated high resolution XPS analysis. These studies point to several mechanisms taking place either in individual NPs (intra-particle ripening) or among NPs (inter-particle ripening) depending on the temperature. The former mechanism taking place at relatively low temperature yields a shape relaxation of the NPs while mass transfer occurs during the second one taking place at temperatures above 150ºC. A correlation between the desorpted species from the NPs surface and the subsequent ripening process has been established. ¿