Optical trapping and nanoscale sensing using NV centres in diamond

The nitrogen-vacancy (NV) in diamond has proved, in recent years, to be an invaluable tool across physics and biology. This naturally occurring colour centre exhibits an incredible diversity of properties and uses. It can act as bright and photostable fluorescent marker, a solid state qubit, a single photon source, or, as is most relevant to this thesis, a nanoscale probe of many physical quantities. Nanoscale diamond crystals, known as nanodiamonds (NDs), are an ideal host for NV centre nanoscale sensors. The diminutive size ensures that the stand-off distance between the probe NV centre and the probed sample remains small. In order to best take advantage of their nanoscale size, techniques must be used to precisely control the position of the ND. A promising approach for precision manipulation of NDs is optical trapping. Optical tweezers use highly focused laser light to confine micro- and nanoscale particles in an aqueous medium, this makes them a particularly useful tool for biological applications. By trapping NV containing NDs using optical tweezers, the nanoscale probe can be manipulated in all three spatial dimensions, as well as controlling orientation. This thesis presents a suite of research on the combination of nanodiamonds with optical tweezers for highly versatile nanoscale sensing. Integrating these two technologies introduces new challenges, most notably the deleterious effect of the infrared trapping laser on common NV sensing protocols. As such, this work focuses on better understand ing the interaction between optical trapping and NV sensing, as well as the development of experimental methodologies which overcome any adverse effects

Formation and modification of Ag atomic point contacts

There is the hope that Molecular electronics would enable the fabrication of ultra-small sized functional molecular circuits. However, since this is currently not easily possible using the conventional Si technology, this branch of nano-technology requires significant understanding of the various physical processes that take place on the atomic scale which are governed by quantum mechanical principles. Quantum mechanics introduces uncertainties in the behaviour and, therefore, the investigation of atomic and molecular junctions is not very straightforward. Many approaches have been developed to fabricate such junctions in a reliable way, but nevertheless, there still exists a lack of reproducibility amongst measurements. Similarly, along with the increasing miniaturisation demands, not only smaller circuits are desired, but their efficiency and performance also suffers from the increased current densities and local heating. Therefore, while investigating atomic/molecular junctions the approach within this thesis was two-fold: (a) better understanding of the mechanism of electromigration (EM) within nano-structures and (b) fabrication of reproducible atomic point contacts using EM in ultra-thin Ag structures. In this thesis, an unique set-up consisting of a 4-tip SEM/STM UHV chamber was used to perform EM measurements on nano-structures. Multiple Ag nano-structures were fabricated on a Si substrate using a two-step lithography process and presence of the in-situ SEM enabled easy navigation from one nano-structure to the other. The tips were used for contacting the structures and a feedback controlled electromigration (FCE) mechanism was used to control the voltage between them during the EM process. Significant effort was devoted on the development and integration of the EM set-up within the 4-tip SEM/STM UHV chamber in order to establish an in-situ fabrication and characterisation technique of atomic/molecular junctions. Ag bow-tie shaped structures with a centre width between 100 - 200nm were investigated at lN2 temperatures and the in-situ characterisation of the structures was performed before and after EM. Ultra-thin Ag structures deposited on Si exhibited a granular nature with an average grain size of Ag grains between 30 - 40 nm. Therefore, the smallest constriction consisted of more than one grain, which when subjected to EM, led to a complex structure formation. It was observed from the conductance curves that even though these structures depict conductance quantisation while thinning during EM, they could not be re-used for repeatable opening and closing of atomic junctions. This observation led to the conclusion that in order to fabricate reproducible atomic junctions, structure widths below the size of one single grain must be used. To reduce the centre widths below 30 nm, focused ion beam (FIB) patterning was employed, to reliably shape the centre constriction to widths below 20 nm. This extra nano-structuring step allowed precise in-situ local control on the morphology of the structures, which served as a step forward in defining the geometry of the atomic junctions and also improved the reproducibility of the EM technique. EM on these structures produced very well defined conductance plateaus which could be re-opened multiple times, suggesting that atomically precise metallic point contacts were generated. Hence, this dissertation addresses one of the very complex issues in molecular electronics i.e. reproducible fabrication of atomic contacts. Furthermore, CO molecule(s) were adsorbed on these point contacts. Being one of the very simple asymmetric molecules, CO served as a good candidate to understand the role of chemisorption on such junctions. Time-resolved current measurements showed bi-stabilities that were dependent on bias voltages. Conductance could be reproducibly changed between two states just by changing the operating voltage suggesting even the simplest molecular junction possesses the capability to function as switches, or memory devices. In the present case, the exact mechanism behind this behaviour has not been completely comprehended, but few possibilities have been outlined. Hence, this thesis also provides intriguing results on electrical properties of chemisorbed Ag atomic contacts