Thèses

Vendredi 22 Septembre 2023 à 14h00.

Low-dimensional systems under extreme pressure conditions: environment and stability

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Directeur de thèse / thesis director : Alfonso San Miguel

Membres du jury / jury members :

ALVAREZ Laurent
BENDIAB Nedjma
DUNSTAN David
GOMES DE SOUZA FILHO Antonio
JOURNET GAUTIER Catherine

Résumé / Abstract :

The extraordinary properties of low dimensional systems opened up opportunities for the development of novel technologies due to their excellent mechanical, electrical and optical properties. Moreover, their low-dimensional nature intrinsically connects them to the surrounding environment which can be used to tune their properties.

In the present work we investigated the role of pressure as a thermodynamic tuning variable for enhancing sample-environment interaction. We focussed principally on graphene and its stacking, and carbon nanotubes. Several instrumental developments have been necessary for those studies which resulted in the improvement of a transfer system for 2D materials, in the development of a spectroscopic mapping system and an upgraded system for high pressure application using sapphire anvils.

Using those technologies we studied, in an initial part of this work, new pathways to detect and characterize the collapse of carbon nanotubes at high pressure. We used the Raman D-band of carbon nanotubes as a novel spectroscopic signature of their collapse to a flattened shape, allowing us to detect their geometrical changes when surrounded by different environments. Furthermore, we focussed on the study of individualized tubes and follow the evolution of an individualized 1.68 nm diameter tube, characterizing the signatures of its collapse by its optical and vibrational properties.

In a second part of the work we focussed on the study of graphene and few-layer graphene by investigating the role of the environment surrounding two-dimensional systems in high pressure experiments. The interaction with the underlying substrate was thoroughly explored by fabricating suspending structures for two-dimensional systems adapted for high pressure. We found excellent mechanical strain and charge carrier transmission between the supported and suspended regions of the sample in bilayer graphene. Using thin graphites our results show that the pressure response of the sample is affected by the geometry of the suspension and an improved chemical interaction with the environment in the suspended region. Additionally, we studied the effects of water on the high pressure transition of few-layer graphene to diamondene by creating a heterostructure graphene/hBN as well as by interchanging different substrates. We found that, together with the chemical interaction with water, the biaxial strain induced by the substrate on the sample is an essential tuning parameter for the transition, opening up opportunities for novel ways of fabricating diamondene at high pressure.

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