Optical seminar. Kershaw, Ghazaryan

1. Steve Kershaw (Research fellow, City University of Hong Kong)

"Chemistry and Engineering of IR QDs"

Abstract: My talk will present some of our work on NIR to MIR emitting quantum dots, mostly based on HgTe QDs. It will describe some of what we have learned about the dependence of non-radiative and radiative recombination processes upon the QD’s environment and how we can exert some influence on each of these. I will also present some results from our more recent synthetic method where the QDs are synthesized in aprotic rather than aqueous or organic solvents and describe how we follow the progress of the reactions in order to gain some insight into how better to control the range of processes that together lead to QD growth, i.e. how to control QD sizes and size distributions when enlarging dots over a large size range. This is a particular concern when working with low bandgap IR emitting QDs – syntheses often span a far larger range of sizes than usually encountered with visible emitting materials and consequently tight control can be harder to maintain for the whole duration of the growth. In the second part of my talk I will present some of the work on IR QD-based devices that our collaborators have carried out using our QDs. These include IR photoconductors, photodiodes and phototransistors that are intended for applications such as gas sensing and automated plastic sorting, etc. This includes early stage work on the commercial development of a multielement HgTe phototransistor array detector operated at room temperature, by a HK university spin-out company. I will also describe work on an integrated IR photodetector by one of our collaborators, and work on enhancing the properties of our HgTe QDs by combining them with gold nanoplasmonic arrays.

2. Davit Ghazaryan

"Localised electronic states of hexagonal boron nitride; tunnelling spectroscopy"

Abstract: Localised electronic states, pinpointed to the bandgap of an insulating layer, are of special interest for the further establishment of electronic properties of nanoscale devices based on two-dimensional (2D) materials. In particular, the emergence of individual electronic states amid the bandgap of monolayer hexagonal boron nitride resulted in the observation of quantum emission, relevant to applications in nanophotonics and quantum information processing; and the origination of keen conducting channels when investigating tunnel effect across the few-layer thick hexagonal boron nitride barrier, relevant to applications of tunnel current amplification. Notably, the process of resonant tunnelling through the localised electronic states within the insulating barrier should likely coexist with the other mechanisms, as its emergence is mainly attributed to the presence of lattice imperfections (defects, vacancies, impurities) within the layers completing the tunnel barrier. In the case of hexagonal boron nitride, such defects can be acquired unexpectedly during the imperfect fabrication procedure even for the purest crystals, but also, can be deliberately introduced utilizing functionalisation. Usually, in general experiments on tunnel effect, the extent of the localised electronic states within the barrier is serving as a marker of its pristinity. However, these states offer a great platform of investigations due to their potential of revealing other resonant tunnelling processes (that require coupling with the defects to manifest), and direct probing of the electronic densities of states of emitter and collector contacts. In this direction, I will report on the pattern analysis of resonant tunnelling processes and conductive channels through such localised states within the hexagonal boron nitride integrated with the variety of graphene electrodes, mainly in the dual-gated configuration, into different van der Waals heterostructures.