Thèses

Vendredi 28 Octobre 2022 à 14h00.

Colloidal synthesis and characterization of tungsten dichalcogenides: from nanomonolayers to heterostructures

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Directeur de thèse / thesis director :
Dr. Yannick Guyot and Dr. Benoit Mahler

Membres du jury / jury members :

Reviewer: Pr. Kevin SIVULA
Reviewer: Pr. Iwan MOREELS
Examiner: Pr. Catherine JOURNET
Examiner: Pr. Benjamin ABECASSIS
Examiner: Dr. Jannika LAUTH
Invited: Dr. Hanako OKUNO

Résumé / Abstract :

Tungsten dichalcogenides are members of the transition metal dichalcogenides family, which is a family of van der Waals solids, with a strong covalent in-plane bonding and weak van der Waals inter-layer one with different properties and applications. This group of materials can possess semiconducting properties in its stable crystal structure, or semi-metallic properties in the metastable crystal structure. Reducing the size of semiconductors to single layers induces indirect to direct bandgap transition. These promising properties show great potential for applications such as electronics, optoelectronics, catalysis, and electrode materials for ionic batteries. Additionally, due to bandgap size and bandgap position, these materials can be used for water splitting and hydrogen evolution reaction.

I developed a colloidal protocol to synthesize well-dispersed and size-controlled WS2 nano-monolayers (NMLs). This aim was reached by introducing 1-octadecanethiol as the tungsten complexing agent. Then, 1T’ to 2H phase transformation was studied. The aim was to investigate the fundamentals of 1T’ to 2H phase change. I obtained 2H-WS2 nanosheets by annealing them in oleylamine at 340°C for 19 hours. UV-Vis spectroscopy, powder X-ray diffraction analysis, and X-ray photoelectron spectrometry have been used to characterize the phase change. Moreover, due to the different crystal structures, scanning transmission electron microscopy illustrated the phase transformation very well. Extension of the protocol developed for 1T’-WS2 to the other TMDCs resulted in new protocols by which we can produce single layer nanosheets of TiS2, TiSe2, WSe2, and WSSe alloys. Finally, I used these NMLs as seeds to create more complex structures through a reinjection protocol. Preliminary results demonstrate that it is now possible to further extend the NMLs size, control their shape and create heterostructures using such an approach. I studied heterostructures of WS2-WSSe, WS2-WSe2, and 1T’-WS2/2H-WS2 by reinjecting the second phase. UV-Vis spectroscopy and electron microscopy methods have been utilized to investigate the formation of the heterostructures as the absorbance and size of nanosheets change by formation of heterostructures.

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