Chirality is a fascinating property manifesting on all size scales in the universe. At the nanoscale, interactions between circularly polarized light and chiral matter can result in chiroptical activity. Combined with the unique optical properties of ultrathin semiconducting nanocrystals, this creates a rich playground for the creation of novel chiral materials. However, many fundamental questions remain regarding the factors influencing chirality in inorganic nanoscale materials. This thesis aims to understand how chirality can be induced in ultrathin systems. This is achieved through electronic ligand-nanocrystal coupling, and by deforming ultrathin sheets into chiral shapes. The first chapter provides an introduction to relevant scientific concepts drawn from the literature. The second chapter demonstrates large magnitudes of circular dichroism and circularly polarized luminescence in methylammonium lead bromide perovskite nanoplatelets through ligand-induced chirality. Samples are prepared using a precise mixture of chiral and nonchiral ligands to optimize chiroptical signals. The competitive ligand binding is described using an equilibrium model, elucidating relationships between the surface ligands and chiroptical properties. The third chapter demonstrates structural chirality by controlling the conformation of helical CdSe nanoplatelets through temperature-dependent surface ligand stresses. By altering the ligand functional group and alkyl chain, the radius of curvature is changed in both magnitude and sign, resulting in nanoplatelet ``shapeshifting''. These changes are attributed to multiple factors including ligand binding configuration, ligand desorption and decomposition, and nanocrystal surface reconstruction. The fourth chapter searches for new ultrathin shapeshifting systems by understanding the interplay between surface chemistry, crystal structure, and conformation. First, patch-wise phosphonic acid ligand exchange is shown to flatten CdSe nanohelices. Lead sulfide and lanthanide oxide nanoplatelets are prepared, the phase transformation of tungsten disulfide nanomonolayers is characterized, and lastly, the unrolling of indium sulfide nanocoils is demonstrated. This work advances the understanding of mechanics at the nanoscale, helping to elucidate the relationship between nanoplatelet deformation and surface-ligand stress. By uncovering the mechanisms behind ligand-induced chirality and nanoplatelet deformation, this work paves the way towards the rational design of ultrathin chiral nanocrystals.
Agenda de l'ENS de Lyon