Colloidal Monolayer TMDs: synthesis, phase transitions and theoretical insights.

Context:

Group VI transition metal dichalcogenide (TMD) monolayers have garnered significant attention due to their exceptional optoelectronic properties, arising from their stable hexagonal (1H phase) crystal structure, characterized by trigonal prismatic coordination of the transition metal atom. However, alternative crystal phases, such as the metastable 1T (1T’) phases, where metal atoms exhibit octahedral coordination, are emerging as a promising research area. These unconventional phases display intriguing properties like metallicity, semi-metallicity, superconductivity, and catalytic potential.[1] Despite their promise, current synthetic methods for these monolayers are limited, especially in term of achieving high quality and for large-scale production.

Project:

This PhD project will use the synthesis of colloidally stable 1T’-WS₂ monolayers we previously developed as a model system to understand the origin of metastable phase stabilization and colloidal stability in two-dimensional TMDs. Although WS₂ can already be synthesized in the 1T’ phase using colloidal methods, the factors governing its stabilization remain unclear. In particular, the role of surface ligands, both on the basal planes and at the edges of the monolayers, is still poorly understood.

The project will address these questions by combining colloidal synthesis, advanced structural and spectroscopic characterization, and density functional theory calculations. The main objective will be to identify the structural and chemical factors responsible for stabilizing the 1T’ phase (both colloidally and structurally), and to clarify the respective contributions of lattice distortions, surface chemistry, edge structure, and colloidal interactions.

This understanding will then be used to rationalize and guide the synthesis of more complex TMD monolayers. In a second stage, the project will exploit this knowledge to develop new morphologies, such as 1H triangles and 1T’ ribbons, and to extend the approach to other compositions, including MoS₂ and WSe₂. Alloyed systems will also be explored as a way to continuously tune structural parameters and investigate their influence on phase stability.

By combining the tunability of colloidal synthesis with theoretical modelling, this project aims to establish design rules for the stabilization of unconventional TMD phases. The expected outcomes include a deeper understanding of the structural, electronic, and colloidal properties of metastable TMD monolayers, ultimately paving the way toward new functional two-dimensional materials.

Workplan:

During the project, the PhD student will pursue in parallel two objectives: the synthetic development of colloidal monolayers in their metastable phase and the study of their stabilizations (colloidal and structural) through DFT calculations.

In the synthesis part, the student will explore monolayers of increasing complexity: beginning with well-known protocols producing size-controlled monolayers [2], he/she will then produce alloyed monolayers [3] and finally other compositions.

In parallel he/she will systematically characterize the monolayers (see figure) from routine phase identification using UV-Vis spectroscopy and X-ray diffraction to HRSTEM using the St Etienne Jeol NeoARM microscope. Phase transition behavior will be monitored in-situ using Raman spectroscopy and XPS in collaboration with Debora Pierrucci at INSP.

Bibliography:

[1] Sokolikova, M. S. & Mattevi, C. Direct synthesis of metastable phases of 2D transition metal dichalcogenides. Chem. Soc. Rev. 49, 3952–3980 (2020).

[2] Shahmanesh, A. et al. Monodisperse size-controlled 1T’-WS2 nano-monolayers with high colloidal stability. ChemRxiv (2023) doi:10.26434/chemrxiv-2023-tnggx.

[3] Shahmanesh, A. et al. 2D Monolayer of the 1T’ Phase of Alloyed WSSe from Colloidal Synthesis. J. Phys. Chem. C 125, 11058–11065 (2021).

Environment:

The PhD student will be jointly supervised by Benoît Mahler and Stephan Steinmann, at Institut Lumière Matière (iLM) Villeurbanne and Laboratoire de Chimie (LCH) Lyon respectively.

The Luminescence Team at Institut Lumière Matière research institute has a strong expertise in light–matter interactions, nanomaterials, spectroscopy, and advanced characterization. The PhD student will benefit from an interdisciplinary environment combining colloidal chemistry, nanomaterials synthesis, optical spectroscopy, and electron microscopy.

The group of Theoretical Chemistry and Molecular Thermodynamics of the Laboratoire de chimie (LCH) in Lyon, France, is recognized at the international level in the atomistic modeling of reactions and nanostructures. Located on the campus of the ENS de Lyon, the applicant will beneficiate form a strong international context, with a dynamic research environment in chemistry.

Students from all-over the world have been or are part of the teams, where English is the main language for scientific discussions. Lyon is listed by UNESCO as a World Heritage Site, recognizing the history of the city. It is also recognized for its gastronomy and, last but not least, Lyon is a vivid city with thousands of students.

Master’s degree in Chemistry or a related field.

Student with a taste for interdisciplinarity, highly motivated, curious, and capable of working independently as well as in a collaborative environment.