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 300 nm 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^–), introduced as CdCl₂, 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 CdCl₂ 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 CdCl₂, octapods are formed and they are stable in solution during the synthesis. Our experiments indicate that Cl^– ions introduced in the reaction reduce the availability of Cd²⁺ 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 CdCl₂ 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 CdCl₂ 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_2–<i>x</i>Se/Cu_2–<i>x</i>S core/shell nanocrystals with various etchants. Even though the Cu_2–<i>x</i>Se core is encased in a thick Cu_2–<i>x</i>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⁺ 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_2–<i>x</i>Se over Cu_2–<i>x</i>S toward oxidation and by fast Cu⁺ diffusion in copper chalcogenides. As the oxidation progresses, expansion of the inner void erodes the entire Cu_2–<i>x</i>Se core, accompanied by etching and partial collapse of the shell, yielding Cu_2–<i>x</i>S_<i>y</i>Se_1–<i>y</i> 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 50–60% without any encapsulation. In this strategy, semiconductor PbS/CdS core/shell quantum dots (QDs) were incorporated into the photoactive blend of poly­(3-hexylthiophene) (P3HT) and phenyl-C₆₁-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 ∼53% 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 (50–60%) 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