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^–3 nm/min in the OR stage, whereas thiolactic acid (TLA) was the most labile and produced the highest growth rate of 3 × 10^–3 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 CO₂

Multiphase AgCu thin films were dealloyed using a mixture of hexafluoroacetylacetone (hfacH) and H₂O₂ dissolved in supercritical CO₂. 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^–12 cm²/s for the surface diffusivity in supercritical CO₂

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 NH₃ 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 NH₃ 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 NH₃. 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 × 10¹⁴ atoms/cm² (∼1 N atom per Si surface atom) at 19 mW/cm² and 12 × 10¹⁴ atoms/cm² (∼1.8 N per Si) at 35 mW/cm². 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 NH₃ 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 FeS₂ Nanocubes Synthesized Using Oleylamine as Ligand, Solvent, and Reductant

Pyrite (FeS₂) 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 H₂S and polysulfides of the form S_<i>n</i>^2– are known to form in oleylamine. The slow step is the reaction between FeS and these molecules. Fe²⁺S^2– undergoes nucleophilic attack by H₂S and S_<i>n</i>^2–; S^2– converts to S^– and sulfur is transferred forming pyrite Fe²⁺S₂^2–