Single-molecule nanoelectronic biosensors fabricated via electrical breakdown
Keywords: nanobiosensors
dielectric breakdown
nanopores
single-molecule biosensing

In the past several decades, there has been significant interest in the development of nanoelectronic devices capable of detecting and characterising analytes at the single-molecule level. Applications for such devices are diverse and range from fundamental biological studies to clinical diagnostics. Unfortunately, due to the nanoscale dimensions required, the fabrication of these devices has often relied on time consuming processes that require specialised equipment. This has limited the availability of these devices to the wider research community and in turn slowed their development. This thesis focuses on the fabrication of single-molecule nanoelectronic biosensors via electrical breakdown techniques. These fabrication techniques are rapid, don't require specialised equipment, and can be fully automated. As such, they have the potential to significantly accelerate research into nanoelectronic single-molecule biosensors.
The first part of this thesis is focused on further developing the controlled breakdown (CBD) technique to fabricate nanopores. This technique relies on applying a large electric field across a thin dielectric membrane to induce breakdown and subsequent nanopore formation. Firstly, the mechanism of electrical conduction and nanopore formation during CBD is explored. It is demonstrated that, depending on the membrane stoichiometry, conduction is limited by either oxidation reactions that must occur at the electrolyte-membrane interface (Si-rich SiNx) or charge transport across the dielectric (stoichiometric Si3N4). Next, these results are built upon to introduce a new CBD protocol that enables the independent fabrication of multiple nanopores in a single membrane. This technique relies on applying the breakdown voltage between an on-chip electrode and an electrolyte solution in contact with the other side of the membrane. In this CBD protocol, electrolyte is only present on one side of the membrane which ensures that previously created nanopores are not affected by each new breakdown event. Lastly, this technique is extended to enable the integration of nanopores with on-chip nanostructures. Here, a large current density is passed through an on-chip metal nanoconstriction simultaneous to CBD. This leads to localised Joule heating at the center of the constriction which confines nanopore formation to this region.
In the second part of this thesis, a feedback controlled electroburning technique is utilised to fabricate graphene nanoelectronic devices with a view towards their use in single-molecule biosensing applications. It is demonstrated that these devices display extremely large noise amplitudes. The observed noise can be explained by a charge noise model by incorporating fluctuations in the source-drain and gate voltages. Interestingly, in some cases, this charge noise results in two-level fluctuations in the current level. The implications of the observed noise for single-molecule biosensing applications are discussed.
The results demonstrated in this thesis will hopefully provide a path towards the accessible fabrication of complex nanostructures often required for single-molecule nanoelectronic biosensing applications. The development of such fabrication techniques would significantly accelerate research in this field.