9 research outputs found
Template-Based Electrochemically Controlled Growth of Segmented Multimetal Nanorods
Multisegmented one-dimensional nanostructures composed of gold, copper, and nickel have been fabricated by depositing metals electrochemically in the pores of anodic aluminum oxide (AAO) templates. The electrodeposition process has been carried out using a direct current in a two-electrode electrochemical cell, where a silver-evaporated AAO membrane and a platinum plate have served as a working electrode and a counter electrode, respectively. The striped multimetal rods with an average diameter of about 300ânm have tunable lengths ranging from a few hundred nanometers to a few micrometers. The lengths and the sequence of metal segments in a striped rod can be tailored readily by controlling the durations of electrodeposition and the order of electroplating solutions, respectively
Reversible Tunability of the Near-Infrared Valence Band Plasmon Resonance in Cu(2-x)Se Nanocrystals
We demonstrate that colloidal Cu(2-x)Se nanocrystals exhibit a well-defined infrared absorption band due to the excitation of positive charge carrier oscillations (i.e., a valence band plasmon mode), which can be tuned reversibly in width and position by varying the copper stoichiometry. The value of x could be incrementally varied from 0 (no plasmon absorption, then a broad peak at 1700 nm) to 0.4 (narrow plasmon band at 1100 nm) by oxidizing Cu(2)Se nanocrystals (upon exposure either to oxygen or to a Ce(IV) complex), and it could be incrementally restored back to zero by the addition of a Cu(I) complex. The experimentally observed plasmonic behavior is in good agreement with calculations based on the electrostatic approximation
Influence of Chloride Ions on the Synthesis of Colloidal Branched CdSe/CdS Nanocrystals by Seeded Growth
We studied the influence of chloride ions (Cl<sup>â</sup>), introduced as CdCl<sub>2</sub>, on the seeded growth synthesis of colloidal branched CdSe(core)/CdS(pods) nanocrystals. This is carried out by growing wurtzite CdS pods on top of preformed octahedral sphalerite CdSe seeds. When no CdCl<sub>2</sub> is added, the synthesis of multipods has a low reproducibility, and the side nucleation of CdS nanorods is often observed. At a suitable concentration of CdCl<sub>2</sub>, octapods are formed and they are stable in solution during the synthesis. Our experiments indicate that Cl<sup>â</sup> ions introduced in the reaction reduce the availability of Cd<sup>2+</sup> ions in solution, most likely <i>via</i> formation of strong complexes with both Cd and the various surfactants. This prevents homogeneous nucleation of CdS nanocrystals, so that the heterogeneous nucleation of CdS pods on top of the CdSe seeds is the preferred process. Once such optimal concentration of CdCl<sub>2</sub> is set for a stable growth of octapods, the pod lengths can be tuned by varying the relative ratios of the various alkyl phosphonic acids used. Furthermore, at higher concentrations of CdCl<sub>2</sub> added, octapods are initially formed, but many of them evolve into tetrapods over time. This transformation points to an additional role of Cl species in regulating the growth rate and stability of various crystal facets of the CdS pods
Hollow and Concave Nanoparticles via Preferential Oxidation of the Core in Colloidal Core/Shell Nanocrystals
Hollow and concave nanocrystals find
applications in many fields,
and their fabrication can follow different possible mechanisms. We
report a new route to these nanostructures that exploits the oxidation
of Cu<sub>2â<i>x</i></sub>Se/Cu<sub>2â<i>x</i></sub>S core/shell nanocrystals with various etchants.
Even though the Cu<sub>2â<i>x</i></sub>Se core is
encased in a thick Cu<sub>2â<i>x</i></sub>S shell,
the initial effect of oxidation is the creation of a void in the core.
This is rationalized in terms of diffusion of Cu<sup>+</sup> ions
and electrons from the core to the shell (and from there to the solution).
Differently from the classical Kirkendall effect, which entails an
imbalance between in-diffusion and out-diffusion of two different
species across an interface, the present mechanism can be considered
as a limiting case of such effect and is triggered by the stronger
tendency of Cu<sub>2â<i>x</i></sub>Se over Cu<sub>2â<i>x</i></sub>S toward oxidation and by fast Cu<sup>+</sup> diffusion in copper chalcogenides. As the oxidation progresses,
expansion of the inner void erodes the entire Cu<sub>2â<i>x</i></sub>Se core, accompanied by etching and partial collapse
of the shell, yielding Cu<sub>2â<i>x</i></sub>S<sub><i>y</i></sub>Se<sub>1â<i>y</i></sub> concave
particles
Enhanced Long-term and Thermal Stability of Polymer Solar Cells in Air at High Humidity with the Formation of Unusual Quantum Dot Networks
Due
to the practical applications of polymer solar cells (PSCs),
their stability recently has received increasing attention. Herein,
a new strategy was developed to largely enhance the long-term and
thermal stability of PSCs in air with a relatively high humidity of
50â60% without any encapsulation. In this strategy, semiconductor
PbS/CdS core/shell quantum dots (QDs) were incorporated into the photoactive
blend of polyÂ(3-hexylthiophene) (P3HT) and phenyl-C<sub>61</sub>-butyric
acid methyl ester (PCBM). By replacing the initial ligands of oleic
acid with halide ligands on the surface of PbS/CdS QDs via solution-phase
ligand exchange, we were able to form unusual, continuous QD networks
in the film of P3HT:PCBM, which effectively stabilized the photoactive
layer. Air-processed PSCs based on the stabilized P3HT:PCBM film showed
excellent long-term stability under high humidity, providing over
3% of power conversion efficiency (PCE) simultaneously. Around 91%
of pristine PCE was retained after 30 days storage in high-humidity
air without encapsulation. This constitutes a remarkable improvement
compared to âŒ53% retained PCE for the QD-free devices, which
can be ascribed to the efficient suppression of both PCBM aggregation
and oxidation of the thiophene ring in P3HT, thanks to the formation
of robust QD networks. Furthermore, the presence of QD networks was
able to enhance the stability of the P3HT:PCBM film against thermal
stress/oxidation under high-humidity environment (50â60%) as
well. The device kept 60% of pristine PCE after thermal treatment
for 12 h at 85 °C in air, which is more than twice higher than
that for the QD-free device. To the best of our knowledge, the work
represents the first unambiguous demonstration of the formation of
QD networks in the photoactive layer and of their important contribution
to the stability of PSCs. This strategy is highly promising for other
fullerene-based PSCs and opens a new avenue toward achieving PSCs
with high PCE and excellent stability