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Exciton Dynamics in van der Waals Heterostructures
[Thesis]. Manchester, UK: The University of Manchester; 2020.
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Abstract
This work details the fabrication and measurement of various two-dimensional (2D) material light emitting diodes, constructed as heterostructures of graphene, hexagonal boron nitride and transition metal dichalcogenide (TMD) monolayers. Devices are created for two different projects in order to study two interest topics currently at the forefront of 2D materials research: interlayer excitons (IXs) and single photon emitters (SPEs). For the IX studies, devices are fabricated in order to electrically inject electrons and holes into a type-II band alignment heterojunction formed from WSe2 and MoS2 monolayers. Electroluminescence measurements of the devices demonstrated the formation and emission of the expected interlayer exciton, between the two K/K' points of the materials. The observed emission is confirmed to be the interlayer exciton by its bias dependent emission energy, which undergoes a blueshift to higher energies as the bias is increased. This behaviour is explainable by considering how the relative band alignment of each material shifts with increasing bias. In addition, an upconversion process arising from IX-IX interactions is also observed in three of the devices and a similar upconversion-like process in one more. In this case, IX-IX interactions allow the population of intralayer exciton states in each TMD at biases where direct electrical injection into these states is not possible. A mechanism for this is proposed by which a large population of charge carriers may be achieved due to the different positions of the K/K' points in the two TMDs, making radiative recombination unfavourable. For the SPE project, a method of fabrication is developed, building upon previous reports, in order to purposely induce SPEs in specific locations of WSe2 and WS2 which could then be transferred to other locations while retaining the SPEs. The second half of Chapter 6 discusses photoluminescence and electroluminescence phenomena observed in an LED fabricated using the developed method. Narrow peaks indicative of SPEs are observed with a wide range of different bias dependent and magnetic field dependent behaviours. These different behaviours are quantified in that section.