The structure of A-b B diblock copolymer (BCP) thin films is often exploited for 'tailoring the self-assembly of nanoparticles into various geometries, characterized by long-range order Mechanistically, the nanoparticles are sequestered within the A or B domains of the copolymer, and the domains act as scaffolds to direct the assembly of the nanoparticles We show that in BCP thin films, which order at suboptimal thicknesses dislocations play a dominant role in determining the spatial organization of sufficiently large nanoparticles The nanoparticles preferentially segregate to the core of edge dislocations, which are ubiquitous in these materials To this end, the two-dimensional planar shape of the islands and holes determine the self assembled geometry of the nanoparticles We also show that the so-called "healing length", lambda increases with film thickness, h, in a manner consistent with lambda proportional to h(1/2), in accordance with theory In films of thickness between L < h < 3L, where L is the domain spacing, lambda largely determines the average size of the region within which the nanoparticles are sequestered