Two kinds of cadmium sulfide (CdS) particulate films have been
generated in situ by exposing stearic acid
(SA) Langmuir monolayer at the air−aqueous CdCl2
interface to hydrogen sulfide (H2S) gas:
particulate
films composed of oriented rodlike nanocrystals (A-type particulate
films), and those of dotlike nanocrystals
which formed a stripelike domain with straight edges aligned with
6-fold symmetry (B-type particulate films).
The SA- coated CdS particulate films were transferred to a solid
substrate and examined by transmission
electron microscopy (TEM) and photoacoustic spectroscopy (PAS).
The dark field image and the transmission
electron diffraction of the TEM were used to study the morphology and
growth mechanisms of the CdS
particulate films in detail. The PAS of the CdS particulate films
composed of the oriented rodlike nanocrystals
shows a new peak at 417.5 nm. This peak implies some new physical
phenomena corresponding to the
ordered alignment of the semiconductor nanocrystals, which exists in
the ordered nanosystems. The novel
synthesis method described here leads to the fabrication of the highly
oriented semiconductor quantum wires
and provides a new method to investigate the structure of the Langmuir
monolayer at the air−water interface.
Cadmium sulfide (CdS) particulate films, composed of highly oriented, rod-like nanocrystals have been generated in situ by the exposure of stearic acid (SA) Langmuir monolayer-coated aqueous CdCl 2 solutions to hydrogen sulfide (H 2 S). The SA-coated CdS particulate films were transferred to a solid substrate and examined by transmission electron microscopy ( TEM) and Auger electron energy spectroscopy. It was found for the first time that the electron diffraction pattern was a composite one with six sets of diffraction patterns which were contributed by the different oriented CdS nanocrystals in these particulate films system. The epitaxial growth of rod-like CdS nanocrystals has been rationalized in terms of matching the d 220 spacing of the cubic CdS crystals and the d 101´0 spacing of the hexagonal closed-packed SA monolayer. The presence of a negatively charged monolayer at the air/water interface was an essential requirement for the oriented growth of CdS nanocrystals. This leads to a novel means of fabrication of highly oriented semiconductor quantum wires.
The formation mechanism of Ge nanocrystals embedded in a SiO2 matrix on Si(100) and quartz substrates was systemically studied with multiple techniques, including x-ray diffraction, transmission electron microscopy, and fluorescence x-ray absorption fine structure. It was found that, on the quartz substrate, the content of GeO2-like species keeps no change during the annealing process. On the contrary, on the Si(100) substrate, the segregated GeO2 from the supersaturated GeO2-SiO2 solid solution can be reduced by Si atoms diffused from the substrate to form Ge nanocrystals. However, the Ge atoms in the stable GeO2-SiO2 solid solution cannot be reduced by the Si substrate under the annealing temperature of 1073 K.
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