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Colloidal semiconductor nanocrystals (NCs) are promising building blocks for various applications. This is mainly due to the ability to modify their physical and chemical properties by controlling the particles size and shape in the nanometer scale. The inorganic NCs surface is usually covered by an organic ligands shell, which has a crucial role in controlling the size and shape of the NCs during the colloidal synthesis. The properties of the ligand shell also determine the NCs dispersibility in various solvents and matrices and their physical and chemical properties. Although the importance of the ligand shell its exact properties and specifically the effect of the NC size and shape are still not well understood. This is mainly due to the lack of experimental tools that will enable to study the ligand shell in situ. In our research we have uniquely studied the physical properties of the ligand shell on the surface of spherical quantum-dots (QDs), of various sizes. We have utilized dye molecules that are embedded within the organic ligand layer and adopt its properties to optically study the effective viscosity of the ligand shell. Tracing the reorientation times of the dye molecules we were able to calculate the effective viscosity of the shell. We have found that as the size of the QD decreases (and hence the curvature increases), the effective viscosity of the shell is decreasing. Modifying the physical properties of the ligand shell by changing the shape of the surface is a unique property of NCs. Further investigation of the ligand shell will allow rational design of the surface to achieve desired properties, providing an additional important knob for tuning their functionality.
