This experimental and theoretical work shows how external impurities organize within confined systems, especially when the system sizes are so small that a single impurity can interact with large portion of the assembly. We have used a defocused laser beam to form two-dimensional colloidal crystallites containing foreign dopants, which reveal surprising position dependence in the fate of the dopant. This phenomenon occurs due to the interplay between the internal energy and entropy of the system which act in opposite directions creating a free energy barrier which 
decides whether the system self purifies or the dopant gets permanently internalized. The insight gained here can be used to understand self purification of nano crystals and designing new type of soft nano materials.
Website: https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.117.258002 chosen as Editor’s suggestion
