Ultrathin films of layered coordination polymerscharge transport and spin crossover at the nanoscale

Resum

This thesis tackles the challenge of integrating coordination polymers (CPs) and metal-organic frameworks (MOFs) as active interfaces in electronic and magnetic devices. This dissertation highlights the importance of processing them as high-quality ultrathin films through advanced bottom-up strategies and of the careful examination of their electronic and magnetic properties at the nanoscale. Chapter 1 provides an overview of the different approaches to render these materials electrically conductive, as well as the strategies to nanostructure and process them as thin films to act as active interfaces in electronic devices. Special attention is given to metal-organic graphene analogues (MOGs), thus far the most promising candidates for the above goal of MOF integration in electronic devices. Chapter 2 shows the fabrication of ultrathin films of a 2D porphyrin-based CP (NAFS-1) and subsequent study of its electronic properties through a soft contacting method. Nanosheets of NAFS-1 are synthesised and compressed into a continuous floating film using the Langmuir-Blodgett (LB) technique. Surface functionalization with self-assembled monolayers (SAMs) allowed to transfer NAFS-1 single layers to non- conventional substrates, such as ferromagnetic permalloy. The LB strategy to prepare and transfer ultrathin films onto SAM-functionalized substrates was used again in Chapter 3 with a conductive MOG. Homogeneous crystalline ultrathin films of Cu-CAT-1 were produced with preferential crystalline orientation and thickness control. Moreover, this approach allowed us to fabricate and study the electrical conductivity of MOF-based devices of the thinnest semiconductive MOF film reported thus far (10 nm thick). These MOF-based devices were then used to unveil the origin of the chemiresistive response of Cu-CAT-1. Our study combines theoretical and experimental studies that point to guest interactions with CuII affecting the electronic and structural properties of Cu-CAT-1. These changes are ultimately responsible for the different electrical response of the films under different atmospheres. Chapter 4 reports the fabrication of crystalline ultrathin films of a FeII Hofmann- type coordination polymer with variable thicknesses for close analysis of their structure and spin crossover (SCO) properties. Atomic force microscopy (AFM), synchrotron grazing incidence X-ray diffraction and X-ray absorption spectroscopy data revealed serious limitations to the ultimate thickness that might be representative of the behaviour of the bulk upon nanostructuration of SCO materials as ultrathin films. This originates from the formation of non-uniform layers that result from rapid nucleation at the early stages of film growth. Reducing the thickness below a critical value can have acutely detrimental effects over the magnetic properties of the film leading to a drastic reduction of the completeness of the high spin/low spin transition and to the complete disappearance of magnetic cooperativity.