Single-molecule spin transistors: exploiting the use of graphene-based electrodes for the next generation of molecular spintronic devices

The massive advancements in performance from the first computing machines to the current electronic devices are mainly due to the extreme miniaturization of their active components. At present, further downscaling represents an enormous technological challenge, as the size of the devices is reaching the ultimate limit of the molecular and atomic scale. The evolution of a novel discipline, molecular spintronics, is contributing to develop new concepts and tools to control matter at the single-molecule scale and to manipulate spins and charges in electronic devices containing one or more molecules. In this framework, single-molecule magnets are expected to be suitable candidates as building blocks for molecular spintronic devices. One of the most interesting applications is the realization of electronic circuits addressing individual molecules in the three-terminal configuration, the so-called single-molecule transistors. Groundbreaking results have been achieved, including the electrical read-out and manipulation of an individual nuclear spin. In this context, the use of graphene has recently been proposed as a valid solution for the fabrication of molecular-scale electrodes. Indeed, its covalent-bond structure assures a high mechanical stability even above room temperature and its two-dimensional fabric limits the size mismatch between electrode and molecule, allowing also for the anchoring of a wide spectrum of different molecules. In the first part of the thesis, a feedback-controlled electroburning (EB) procedure was employed to open nanometer-sized gaps in graphene junctions suitable to contact single molecules. The EB process was systematically characterized on different types of (substrate-supported or suspended) graphene devices both under air and vacuum conditions. The EB exploits the chemical reaction of carbon atoms with oxygen at the high temperatures induced by Joule heating at large current densities. The presence of a fast feedback loop is vital to avoid the abrupt breaking of graphene and allows for a more accurate control on the final structure of the nm-sized gap. With that, it was possible to increase the yield of the process up to about 90%. By means of this EB procedure, three-terminal molecular devices were prepared in which a Tb-based single-molecule magnet (TbPc2 or Tb2Pc3) was embedded between two nm-sized graphene electrodes. At low temperature, these devices worked as molecular spin transistors allowing to detect the Tb electronic spin flip during the sweeping of an external magnetic field. The magnetic exchange coupling between the current passing through the molecular system in the Coulomb blockade regime and the Tb electronic spin was also characterized. In the second part of the thesis, microwave (MW) pulses were used to induce the spin reversal in an individual Tb ion enclosed in a “double decker” single-molecule magnet (TbPc2) and embedded between two nano-gapped gold electrodes obtained via electromigration. The Tb electronic spin being exchange-coupled to the read-out quantum dot formed by the Pc ligands, the conductance through the read-out dot is spin-dependent, allowing for an electrical and non-destructive read-out of the Tb electronic spin state. It was shown that a reversal of the Tb electronic spin can be actually induced by applying sequences of MW pulses. Data analysis revealed that the spin flip process is non-resonant and non-coherent with the MW pulses and it does not depend on the applied magnetic field. To explain these experimental findings, a simple model was derived taking into account the exchange of energy and momentum between the spin and the surrounding environment.