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Multifunctional surfaces structured with electroactive and magnetic molecules for electronic and spintronic devices

Final Report Summary - ELECTROMAGIC (Multifunctional surfaces structured with electroactive and magnetic molecules for electronic and spintronic devices)

ELECTROMAGIC is a project embedded in the field of molecular electronics and molecular spintronics. Generally, molecular electronics aims at studying the use of molecular building blocks for the fabrication of electronic components. The intrinsic limitations of the silicon technology for miniaturizing the size of the electronic devices and the prospect of size reduction offered by molecular-level control properties, have led to an increased effort to progress in molecular electronic devices. Further, molecular spintronics aims at fabricating novel devices exploiting both the electron spin and the electron charge of the molecule for information processing.
Taking all this into account, the first main idea of the project was to transfer the well studied properties of promising molecular switches from solution to solid state. In our case, we proposed to transfer these intrinsic characteristics to a solid support through the preparation of self-assembled monolayers (SAMs) for memory, sensing, surface energy modulation and for spintronics applications. In ELECTROMAGIC the functional units forming the SAMs are redox-active molecules. To achieve the main objectives of the project we have chosen five different families of compounds: ferrocene (Fc), tetrathiafulvalene (TTF), anthraquinone (AQ), endohedral metallofullerene (EMF) and polychlorotriphenylmethyl (PTM) radicals. We have employed mainly two different types of substrates: gold and indium tin oxide (ITO).
ELECTROMAGIC is a multidisciplinary project that brings together different disciplines like organic synthesis, physical chemistry and engineering. For this we have put a lot of effort, both on the design and synthesis of novel molecules with the desired electronic properties and chemical structure as well as on the optimization of different methodologies for preparing the hybrid systems (molecule/substrate) and on the characterization of the resulting functional surfaces. We have implemented in our group the use of the electrochemical impedance spectroscopy (EIS) to characterize the molecular switches and we have designed different electrochemical cells suitable to characterize our systems.
Along the duration of the project we have achieved most of the proposed intermediate and final objectives. The targets initially proposed were: 1) preparation of electrochemical switches; 2) preparation of mixed self-assembled monolayers to fabricate 3-state switches and 3) study and characterization of radical based SAMs for their application in molecular spintronics. In general terms, the different molecular switches that have been prepared are based on the tuning of the redox properties of the grafted molecules. The basic working principle of the prepared surface-confined molecular switches is based on the application of a specific bias (voltage) to the modified substrate. With this, we can tune the redox state of the molecules. This has allowed us to study the resulting functional substrates as charge storage molecular devices, to exploit the intrinsic properties of each redox state (e.g. magnetic properties) as a read-out of the switch or to modulate surface characteristics, like wetability. The principal scientific results achieved during ELECTROMAGIC are:
1) A novel EMF, synthesized by the group of Prof. Akasaka in Japan, was synthesized with a functional group able to react with gold and hence form SAMs on this substrate. After carrying out the functionalization, the resulting surface displayed the properties of the EMF in solution. In particular, a gold surface was chemically modified with a novel La@C82 derivative. Thanks to the electrochemical properties, the magnetic behavior was electrically triggered and switched between two different redox states.
2) The wetability of a gold surface was possible to be modified in a control manner by functionalizing the surface with two different TTF molecules, differing between them by the terminal groups. The function of these smart surfaces relies on the modulation of the surface energy by changing the oxidation state and, thus the charge of the TTF molecules. The net charge on the surface has a huge impact on the hydrophobicity of the surface. Also, the surface energy was tuned by exchanging the counter-ions used to stabilize the charge of the oxidized SAMs.
3) Moreover, as mentioned above, the different redox states can act as memory bits, then, a higher number of states would be reflected as a higher storage capacity of the fabricated molecular switch. For this, mixed self-assembled monolayers based on two electro-active molecules, Fc and AQ have been prepared. The e-donor character of the Fc and the e-acceptor of the AQ have allowed accessing to the different redox states in a narrower voltage window. We have achieved a four-state molecular switch.
4) Based on the good results obtained with the mixed SAM, we have designed a more complex system that, instead of having the two redox-active components mixed on the same surface, we have locally confined the molecules on certain areas of the surface. The idea is to have a multifunctional surface in which the redox properties can be tuned in a localized area. In this case these SAMs are not going to be only applied for memory functions but also for recognition of bio-molecules. As a proof of concept we have used nanoparticles bearing recognition units that can bind with the molecules on the surface in collaboration with Prof. Ravoo in Münster. The interaction between the analyte and the grafted molecules can be tuned by the redox state of the molecule. We have already achieved a good control on the modification of the patterned substrate as well as on the local binding of the analyte. We expect that this system can be then implemented in microfluidic chips to guide the recognition in the channels, as sensors for charged biomolecules, for a controlled growing of charged polyelectrolytes, etc.
5) During the duration of the project, we have put a lot of effort to use the impedance spectroscopy to characterize the redox-active monolayers. This is a technique that allows having detailed information on the interface between the modified electrode and the electrolyte. We have successfully characterized a ferrocene based monolayer, being able to write and read the switch (the different redox states) by extracting the capacitance of the double layer, i.e. to fully electrically trigger the switch. In addition, we have been able to prepare a fully solid molecular switch by making use of a solid electrolyte. The combination of the electrical input/electrical output and the solid electrolytic medium represents a step forward for their integration in more real electronic devices. Up to now we were always limited to use a liquid environment which hindered the real exploitation of these types of systems. We believe that this work will have a noticeable impact in the molecular electronic device society.
6) Finally, and more in line with the molecular spintronics target of ELECTROMAGIC, we have designed and synthesized a family of PTM radicals with varying chain length. Our goal was to determine the charge transport mechanism through the molecular junctions electrode/radical SAM/electrode, and to elucidate the role of the unpaired electron. In view of integrating organic free radicals in electronic devices, the understanding of the transport mechanism is crucial. For this, we have put a lot of effort on the synthesis and on the electronic and structural characterization of the radical based SAMs. This information has been essential to understand the electrical measurements. Thanks to the obtained results we can experimentally support the theory that was predicting a key role of the radical on the charge transport across the molecular junctions. These results open new perspective to exploit organic radicals in molecular spintronics.