Self-assembled Systems of Nanomaterials on Langmuir-Blodgett Films

Resumen

[EN]The development and study of the nanomaterials have focused the attention of the scientists to use them as building blocks looking for novel properties inside technological and biological applications. Therefore, it is important the study of their chemical and physical properties for their application in specific technological devices. This thesis deals with this thematic, and presents a research on the film fabrication using the Langmuir-Blodgett (LB) technique and the study of self-assembled systems of nanomaterials at the air-water interface and onto solids. In particular the thesis presents the research on the preparation and investigation of some important nanomaterials, which are styrene-maleic anhydride copolymer monolayers, CdSe semiconductor quantum dots (QDs) and chemically derived graphene. Several techniques have been used in order to characterize the different systems studied, such as Langmuir trough, Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), Ellipsometry, X-Ray photoelectron (XPS) and micro-Raman spectroscopies, Field Emission Scanning Electron Microscopy (FE-SEM) and Fluorescence Lifetime Imaging Microscopy (FLIM). Results about polymer monolayers show that the Langmuir films deposited on saline solutions are more expanded and stable than the monolayer without salt in the subphase. These facts can be attributed to the formation of a complex between the magnesium divalent cation and the carboxylic groups of the polymer molecule. Our results demonstrate that the polymer-Mg2+ complexes increase the viscoelastic behavior of the monolayer. Results also demonstrate surface self-assembly processes for dense polymer monolayers. The aggregates transferred from the air-liquid interface onto silicon wafers present different morphologies depending on the methodology employed to build the LB film, the subphase composition and the polymer nature. The thickness of the LB films obtained by ellipsometry agrees with the film height determined by AFM measurements. In the different approaches proposed to the QDs films preparation, co-spreading and bilayer, different film morphologies were observed. Thus, co-spreading approach leads to two different structures in the LB film depending on the mixed monolayer composition and surface pressure of the Langmuir monolayer precursor. The different film structures, domains and hexagonal networks observed by AFM and TEM, were explained by two dewetting mechanism: spinodal and nucleation and growth of holes, respectively. In contrast, the coating of the substrate with a polymer film promotes the formation of QDs islands or a more dispersed QDs film depending on the ligand nature of the nanoparticles. This behavior could be explained by means of the spreading coefficient. Moreover, we have explored the influence of other approaches as the QDs ligand exchange with a polymer and the application of shearing before the LB deposition to direct the assembly on the film morphology. The photodynamic study of the QD/polymer films allowed us to evaluate the effect of the film morphology on the photoluminescence properties by means of FLIM. The photoluminescence of QD/polymer mixed films was characterized for different conditions of film deposition: bilayer or co-spreading; QD-to-polymer composition; and surface pressure at LB deposition. We propose an approximate model involving energy transfer and trapping to relate the photoluminescence properties with film morphology observed by TEM. Thus, according to the model, the reduction of QDs surface defects and clusters formation in the films leads to higher emission intensity. Finally, in the chemically derived graphene study, we propose a new synthetic route to achieve the functionalization of the reduced graphene oxide (RGO) with a zwitterionic surfactant. This non-covalent functionalization improves the reduction and defect repair degrees of hydrazine and Vitamin C, in accordance with XPS and Raman analysis, and also increases the electric conductivity of the RGO material obtained. In addition, results also show that the Langmuir-Blodgett technique allowed us to deposit a few-layer thick RGO flakes onto silicon wafers observed by AFM and FE-SEM.