Electron transport in molecular nanostructures
Keywords: quantum transport
1-D physics
graphene
nanoribbons
quantum computing

The past decades have seen huge efforts in trying to integrate carbon nanomaterials into electronic devices. Although microelectronic processors running on carbon nanotubes have demonstrated excellent performance and low power consumption, there are still limitations inherent to these materials, which restrict their applicability and integrability into devices. The lack of a bandgap in graphene, the mixture of metallic and semiconducting behaviours in carbon nanotubes, and tendency of nanotube to aggregate, are some examples of why it is necessary to devise alternative carbon nanostructures.
In this thesis, I investigate the electronic properties of nanodevices where molecularly defined graphene nanoribbons have been integrated as functional elements. These systems offer several advantages: they are intrinsically semiconducting, with atomically precise topology that guarantees, in principle, low scattering and ultraclean behaviour, as well as access to topological states. On the other hand, their integration into devices is just in its infancy. After detailing the background of these systems, and of the most advanced devices that can be created out of them, I will describe the instrumentation and device architectures used to measure the electronic-transport properties. I then demonstrate a molecular quantum singleelectron transistor that can work up to room temperature, and detail the transport regimes that appear in the different temperature regions. These findings are not only the first observation of quantum operation at room temperature in a molecular structure, but also clarify the physics of how such devices work.
I then address the second issue that has plagued devices based on carbon nanotubes: the presence of aggregates that can compromise, by intermolecular forces, device performance. I demonstrate how chemical functionalisation of the edges by the appropriate side groups can overcome these issues, allowing us to produce some the cleanest electronic nanodevices and features of single-electron transport, ever obtained. The clarity of the transport features is comparable of what has been seen for carbon nanotubes after more than 10 years of efforts, and allow us to address fundamental interactions that can regulate the transport, such as the electron–vibration coupling. I observe very strong vibronic coupling, with clear features of a Franck–Condon blockade.
Eventually I examine how to control the chemical potential of molecular nanoribbons, and dope them at will. This is first addressed by investigating dopant groups attached to the ribbon edges. The energy shift induced in the nanoribbons by the dopant is, however, weak and impractical at the device level. I then present measurements in the first field-effect transistor of a unique class of materials, where dopants and metal atoms are inserted directly into the honeycomb lattice, and I propose possible improvements.
These results offer a first step towards the realisation of practical and extremely clean electronic devices based on molecular nanoribbons. They thus open the path to the use of electronics for the investigation of spin states, topological effects, and the physics of quantum systems at high temperatures. They offer a picture of the transport mechanisms at play, and, from the practical level, offer the first recipes to overcome longstanding issues that have hindered the development of carbon-based electronic devices, and may thus be instrumental in the development of the first nanoribbon-based microprocessors.