Nguyen, TT; (2016) Transfer and characterisation of graphene for integration with gaseous electron multipliers. Doctoral thesis , UCL (University College London).

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Abstract

Graphene is the latest material to join the large family generated by the high versatility of carbon. Building on the extensive knowledge and large research of its predecessors, graphene has proven to be of a higher potential due to the versatility of its applications. Its most promising properties, a very good electrical conductivity, its two-dimensional nature, combined with its in-plane strength makes it a very good candidate for integration as a membrane in existing electronics. The latter is an important criterion as it allows potential electronic data collection, a technique widely necessary in current technologies. In high-energy physics, the quality of the detection setup is critical to identification of the particles. The gaseous electron multiplier (GEM), a particle detector derived from the multi-wire proportional chamber for which the Nobel Prize was awarded in 1992 is one of the current technologies in use in CERN. Although the GEM currently boosts one of the best quality signal detection, ions produced in the avalanche process have been flowing back into conversion regions of the detector, interfering with the low signals in that section. As a result of this interference, the amplified signals from the particles detected, suffer from induced noise. To remedy to this, this work has overseen the integration of graphene to the device between the amplification and the conversion regions. Due to the thinness of graphene, electrons are expected to flow through it, while ions, of much larger size, are expected to be blocked by the layer. This would make graphene an ideal membrane to prevent ion backflow, while keeping the disturbance of the electron signal to a minimum. In this work, graphene production and integration methods were investigated, and a suitable transfer method was developed for the incorporation of the material into the existing GEMs. The results were the successful transfer of high quality graphene across holes of up to 70 µm, a step up from the 5 µm achieved by previous existing techniques in literature. The integrated layers were then tested within custom built setups in order to insure compatibility and measure graphene transmission properties. This work included a systematic characterisation and optimisation of all parameters to be monitored for the new setups. The properties of graphene were then tested under low electric field configurations in order to avoid potential damage. Results showed the resilience of graphene in low fields, repeatability and the isolation of a field dependence effect which was attributed to field focusing. Secondly, while aiming to improve the electron yield through graphene, the layers were tested in high field conditions. The experiments revealed the possibility of induced permanent damage in the event of high frequency of discharges but a very good resilience of graphene under normal operating conditions. In order to achieve larger coverage, bilayer and trilayer graphene were also tested alongside monolayers. The latter were found to have to low coverage to distinguish field focusing from actual transmission, trilayers were found to be opaque to ions but also to electrons. Finally, bilayers were also found to be opaque to ions as expected, but showed a variable positive electron transmission, a very promising result towards the integration of graphene in GEMs.

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