5 research outputs found
Solvent-Triggered Self-Assembly of CdTe Quantum Dots into Flat Ribbons
Diluting an aqueous colloid containing
purified CdTe quantum dots
(QD) by injecting into common organic solvents triggered self-assembly
into a variety of structures. Nanoribbons formed in methanol with
aspect ratios near 1000 containing discrete dots lacking a packing
order. The flat ribbons were 30–90 nm (8–22 QDs) wide
based on AFM and TEM, about 8–18 nm (2–5 QDs) high based
on AFM, and 0.5–10 μm long based on SEM. Passivation
of defect sites, likely by S, enhanced the photoluminescence of the
ribbons relative to the raw QDs. Multibranched clusters containing
fused dots formed in IPA as well as ribbons with pendent nodules.
The photoluminescence of the assortment was attenuated compared to
the raw QDs. Injecting into acetone not only yielded ribbons and clusters
but also dissolved the dots over a period of 20 days, forming flower-like
assemblies whose petals consisted of bundles of CdS wires. Diluting
in solvents with lower dielectric constants than water initially aggregated
the dots by reducing the electrostatic screening between the negatively
charged thioglycolic acid (TGA) ligand layers. The solubility of TGA
in the solvents determined the superstructure that formed. Extracting
the smallest portion of this layer in methanol promoted vectorial
growth into ribbons consistent with dipole–dipole attractive
and charge–charge repulsive interactions. Removing more of
the TGA layer in IPA caused the dots to fuse into webs containing
clustered ribbons and branches, and the directional nature of the
superstructure was lost. Completely deprotecting the surface in acetone
promoted photoetching and dissolved the dots. Control of the ligand
surface density by means of the solubility adds another method to
direct spontaneous self-organization of QDs
Ligand-Controlled Growth of ZnSe Quantum Dots in Water during Ostwald Ripening
A strong ligand effect was observed for the aqueous-phase
growth
of ZnSe quantum dots (QDs) in the Ostwald ripening (OR) stage. The
QDs were made by injecting Se monomer at room temperature followed
by a ramp to 100 °C. The ramp produced a second, more gradual
increase in the concentrations of both Zn and Se monomers fed by the
dissolution of QDs below the critical size. The dissolution process
was followed using measurements of the mass of Zn in QDs and in the
supernatant by inductively coupled plasma optical emission spectroscopy
(ICP-OES). Despite the flux of monomers, there was little growth in
the QDs of average size based on UV–vis absorption spectra,
until the temperature reached 100 °C, when there was a period
of rapid growth followed by a period of linear growth. The linear
growth stage is the result of OR as the total mass of Zn in QDs and
in the solvent remained constant. The growth data were fit to a continuum
model for the limiting case of surface reaction control. The rate
is proportional to the equilibrium coefficient for ligand detachment
from the QD surface. The ligand 3-mercaptopropionic acid (MPA) was
the most tightly bound to the surface and produced the lowest growth
rate of (1.5–2) × 10<sup>–3</sup> nm/min in the
OR stage, whereas thiolactic acid (TLA) was the most labile and produced
the highest growth rate of 3 × 10<sup>–3</sup> nm/min.
Methyl
thioglycolate (MTG) and thioglycolic acid (TGA) produced rates in
between these values. Ligands containing electron-withdrawing groups
closer to the S atom and branching promote growth, whereas longer,
possibly bidendate, ligands retard it. Mixed ligand experiments confirmed
that growth is determined by ligand bonding strength to the QD. Photoluminescence
spectroscopy showed that the more labile the ligand, the more facile
the repair of surface defects during the exposure of the QDs to room
light
Dealloying Multiphase AgCu Thin Films in Supercritical CO<sub>2</sub>
Multiphase AgCu thin films were dealloyed
using a mixture of hexafluoroacetylacetone
(hfacH) and H<sub>2</sub>O<sub>2</sub> dissolved in supercritical
CO<sub>2</sub>. AgCu alloys exhibit eutectic phase behavior, allowing
the composition of the two phases to be fixed while varying the average
size of the phase domains from 250 to 1000 nm by increasing the annealing
temperature. Selective removal of Cu from both phases was observed,
and higher concentrations of the etching solution increased the etching
rate between 45 and 75 °C, where the reaction exhibited an apparent
activation energy of ∼38 kJ/mol. The morphology after dealloying
was quantified using the fast Fourier transform power spectrum obtained
from electron microscopy images. For phase domains smaller than ∼500
nm, Ag atoms released in the open regions that had been occupied by
the Cu-rich phase diffused separately or as clusters to the Ag-rich
phase, forming a nanostructured morphology that mimicked the starting
microstructure. For larger domains, stable Ag clusters (50 nm diameter)
formed in the open regions, because the diffusion limit was reached,
yielding an estimate of 4 × 10<sup>–12</sup> cm<sup>2</sup>/s for the surface diffusivity in supercritical CO<sub>2</sub>
Ammonia Photodissociation Promoted by Si(100)
Using
in situ X-ray photoelectron spectroscopy measurements after
reaction, we show that hydrogen-terminated Si(100) perturbs the bonding
of physisorbed NH<sub>3</sub> enabling a photochemical decomposition
pathway at wavelengths different from those characteristic of either
the molecule in the gas phase or the semiconductor bandgap. UV illumination
only of gas phase NH<sub>3</sub> at partial pressures from 0.1 to
100 Torr produced a maximum at 10 Torr in the N surface coverage.
This is in good agreement with a model of the radical production rate
showing that at this pressure the gas density balances the flux of
photons at the surface with energies sufficient to dissociate NH<sub>3</sub>. UV illumination of both the gas phase and the surface produced
a monotonic increase in the N coverage with pressure as well as coverages
that were 3–10 times higher than when only the gas phase was
illuminated. The amine saturation coverage scaled with the UV fluence
at 10 Torr and 75 °C, reaching 6.9 × 10<sup>14</sup> atoms/cm<sup>2</sup> (∼1 N atom per Si surface atom) at 19 mW/cm<sup>2</sup> and 12 × 10<sup>14</sup> atoms/cm<sup>2</sup> (∼1.8
N per Si) at 35 mW/cm<sup>2</sup>. Monochromatic illumination showed
that the wavelengths driving deposition were not correlated with the
Si bandgap, but instead were roughly the same as gas phase photodissociation
(λ < 220 nm). The primary driving force to replace the hydrogen
termination with amine groups was direct photodissociation of NH<sub>3</sub> molecules whose electronic structure was perturbed by interaction
with the surface. Amine groups enhanced the surface reaction of water
present as a contaminant in the source gas. These results show that
molecules in weakly bound surface states can have a dramatic impact
on the photochemistry
Phase Pure Pyrite FeS<sub>2</sub> Nanocubes Synthesized Using Oleylamine as Ligand, Solvent, and Reductant
Pyrite
(FeS<sub>2</sub>) nanocrystals with a narrow size distribution
and optical absorption from 1600 to 380 nm were synthesized in a single
stage by reacting ferrous chloride with elemental sulfur in oleylamine
(OAm) without an additional ligand. X-ray diffraction, Raman, and
scanning electron microscopy showed that pyrite cubes with dimensions
of 88 ± 14 nm formed at 200 °C, an S to Fe ratio of 6, and
1 h reaction time in OAm. The time required to form phase pure pyrite
depended on the S to Fe ratio. Phase purity was obtained in less than
1 h at a ratio of 6, but times as long as 24 h were necessary at a
ratio of 2.75. The rate of pyrite formation increased with higher
sulfur concentrations, which shows that molecules containing sulfur
are involved in the rate-determining step. Both H<sub>2</sub>S and
polysulfides of the form S<sub><i>n</i></sub><sup>2–</sup> are known to form in oleylamine. The slow step is the reaction between
FeS and these molecules. Fe<sup>2+</sup>S<sup>2–</sup> undergoes
nucleophilic attack by H<sub>2</sub>S and S<sub><i>n</i></sub><sup>2–</sup>; S<sup>2–</sup> converts to S<sup>–</sup> and sulfur is transferred forming pyrite Fe<sup>2+</sup>S<sub>2</sub><sup>2–</sup>