3 research outputs found
Twinning and Phase Control in Template-Directed ZnS and (Cd,Zn)ÂS Nanocrystals
We report on the nucleation and growth
of ZnS and (Cd<sub><i>x</i></sub>ÂZn<sub>1–<i>x</i></sub>)ÂS
nanocrystals on polydiacetylene Langmuir films. It was found that
the 3–5 nm nanocrystals form ordered linear arrays aligned
at a constant 27° angle with respect to the conjugated direction
of the polydiacetylene film, as derived from the optimal alignment
between the two phases. ZnS nanocrystals were found to nucleate specifically
from the zinc blende (001) face. Because of closely matched interfacial
relations, twinning defects were induced on the {111} planes. These
nanometer-sized twin crystals exhibit extra {111} electron diffraction
reflections due to elongation of reciprocal space spots, despite their
off axis orientation. The composition of solid solution (Cd,Zn)ÂS
nanocrystals depends on the Zn<sup>2+</sup>\Cd<sup>2+</sup> ratio
in the aqueous subphase. Their structure is affected by the template
mismatch both by twinning, as is the case for ZnS, for which continuous
compositional shift is observed, and by phase shift to hexagonal wurtzite,
with a pure CdS composition. The nanocrystals exhibited a continuous
energy-gap shift, reflecting the Zn/Cd ratio in the solid solution.
We demonstrate control over the nanocrystals’ crystal structure,
defect structure, orientation, and composition, providing a potentially
effective tool for band-gap engineering in organic–inorganic
hybrid assemblies
New Nanocrystalline Materials: A Previously Unknown Simple Cubic Phase in the SnS Binary System
We
report a new phase in the binary SnS system, obtained as highly symmetric
nanotetrahedra. Due to the nanoscale size and minute amounts of these
particles in the synthesis yield, the structure was exclusively solved
using electron diffraction methods. The atomic model of the new phase
(<i>a</i> = 11.7 Ă…, <i>P</i>2<sub>1</sub>3<i>)</i> was deduced and found to be associated with the
rocksalt-type structure. Kramers–Kronig analysis predicted
different optical and electronic properties for the new phase, as
compared to α-SnS
Oriented Attachment: A Path to Columnar Morphology in Chemical Bath Deposited PbSe Thin Films
We have studied columnar PbSe thin
films obtained using chemical
bath deposition. The columnar microstructure resulted from an oriented
attachment growth mechanism, in which nuclei precipitating from solution
attached along preferred crystallographic facets to form highly oriented,
size-quantized columnar grains. This is shown to be an intermediate
growth mechanism between the ion-by-ion and cluster growth mechanisms.
A structural zone model depicting the active growth mechanisms is
presented for the first time for semiconductor thin films deposited
from solution. The columnar films showed well-defined twinning relations
between neighboring columns, which exhibited 2D quantum confinement,
as established by photoluminescence spectroscopy. In addition, anisotropic
nanoscale electrical properties were investigated using current sensing
AFM, which indicated vertical conductivity, while maintaining quantum
confinement