  Semiconductor nanocrystals are a promising class of materials for a variety
of novel optoelectronic devices, since many of their properties, such as the
electronic gap and conductivity, can be controlled. Much of this control is
achieved via the organic ligand shell, through control of the size of the
nanocrystal and the distance to other objects. We here simulate ligand-coated
CdSe nanocrystals using atomistic molecular dynamics, allowing for the
resolution of novel structural details about the ligand shell. We show that the
ligands on the surface can lie flat to form a highly anisotropic 'wet hair'
layer as opposed to the 'spiky ball' appearance typically considered. We
discuss how this can give rise to a dot-to-dot packing distance of one ligand
length since the thickness of the ligand shell is reduced to approximately
one-half of the ligand length for the system sizes considered here; these
distances imply that energy and charge transfer rates between dots and nearby
objects will be enhanced due to the thinner than expected ligand shell. Our
model predicts a non-linear scaling of ligand shell thickness as the ligands
transition from 'spiky' to 'wet hair'. We verify this scaling using TEM on a
PbS nanoarray, confirming that this theory gives a qualitatively correct
picture of the ligand shell thickness of colloidal quantum dots.
