posted on 2015-04-28, 00:00authored byBenjamin
T. Diroll, Nicholas J. Greybush, Cherie R. Kagan, Christopher B. Murray
Directing the orientation of anisotropic
nanocrystal assemblies
is important for harnessing the shape-dependent properties of nanocrystal
solids in devices. We control the orientation of smectic B superlattices
of CdSe/CdS dot-in-rod nanocrystals through assembly on different
polar interfaces and quantify the superlattice orientation through
correlated small- and wide-angle grazing-incidence diffraction. Small-angle
scattering is used to determine the phase of the nanorod superlattices
and their preferential growth directions from the subphase. Wide-angle
diffraction is used to quantify the orientations of nanorods within
the superlattices and with respect to the substrate. Not only are
the nanorod long axes aligned within the structures, but truncation
of the short axes also coaligns the crystal axes of the nanorods with
the zone axes in assembled smectic B crystals. Three dimensional orientational
alignment of nanocrystals in superlattices is highly desirable in
device applications. Depending on the subphase used for self-assembly,
the films range from nearly quantitative vertical to horizontal alignment.
Controlling for other variables, we find that the surface tension
of the subphase is strongly correlated with the orientational ordering
of the nanorod superlattices. The microstructure of nanorod superlattices
shows many classic defects of atomic and liquid crystalline systems.
The nature of defect structures supports a mechanism of nuclei formation
on the subphase–solvent interface rather than in solution.
Last, we demonstrate the relationship between optical absorption polarization
and superlattice structure using correlated optical spectroscopy and
electron microscopy.