Colloidal Cu 2-x(S ySe 1-y) alloy nanocrystals with controllable crystal phase: Synthesis, plasmonic properties, cation exchange and electrochemical lithiation

We report synthetic routes to both cubic and hexagonal phase Cu 2-x(S ySe 1-y) alloy nanocrystals exhibiting a well-defined near-infrared valence band plasmon resonance, the spectral position of which is dependent mainly on x, i.e. on Cu stoichiometry, and to a lesser extent on the crystal phase of the NCs. For cubic Cu 2-x(S ySe 1-y) nanocrystals y could be varied in the 0.4-0.6 range, while for hexagonal nanocrystals y could be varied in the 0.3-0.7 range. Furthermore, the Cu 2-x(S ySe 1-y) nanocrystals could be transformed into the corresponding Cd-based alloy nanocrystals with comparable S ySe 1-y stoichiometry, by cation exchange. The crystal phase of the resulting Cd(S ySe 1-y) nanocrystals was either cubic or hexagonal, depending on the phase of the starting nanocrystals. One sample of cubic Cu 2-x(S ySe 1-y) nanocrystals, with S 0.5Se 0.5 chalcogenide stoichiometry, was then evaluated as the anode material in Li-ion batteries. The nanocrystals were capable of undergoing lithiation/delithiation via a displacement/conversion reaction (Cu to Li and vice versa) in a partially reversible manner. © 2012 The Royal Society of Chemistry

Про конституційно-правові засади національного суверенітету в Україні

Розглядаються характеристики народу та нації як суб’єктів конституційно-правових відносин, зв’язок етнічного і політичного чинників у змісті національного суверенітету.Рассматриваются характеристики народа и нации как субъектов конституцион­но-правовых отношений, связь этнического и политического в содержании националь­ного суверенитета.Describe the nations and the peoples as subjects of constitutionals law relatios, the con­nection between ethnos and political in content of national sovereignty

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

Nanocrystal Film Patterning by Inhibiting Cation Exchange via Electron-Beam or X‑ray Lithography

In this Letter we report patterning of colloidal nanocrystal films that combines direct e-beam (electron beam) writing with cation exchange. The e-beam irradiation causes cross-linking of the ligand molecules present at the nanocrystal surface, and the cross-linked molecules act as a mask for further processing. Consequently, in the following step of cation exchange, which is performed by directly dipping the substrate in a solution containing the new cations, the regions that have not been exposed to the electron beam are chemically transformed, while the exposed ones remain unchanged. This selective protection allows the design of patterns that are formed by chemically different nanocrystals, yet in a homogeneous nanocrystal film. Spatially resolved compositional analysis by energy-dispersive X-ray spectroscopy (EDS) corroborates that the selective exchange occurs only in the nonirradiated regions. We demonstrate the utility of this lithography approach by fabricating conductive wires and luminescent patterns in CdSe/CdS nanocrystal films by converting nonirradiated regions to Cu_2–<i>x</i>Se/Cu_2–<i>x</i>S. Furthermore, we show that X-ray irradiation too can lead to protection from cation exchange

Ultrafast Optical Mapping of Nonlinear Plasmon Dynamics in Cu2–xSe Nanoparticles

We report on the experimental investigation and theoretical modeling of the ultrafast nonlinear optical response exhibited by Cu2−xSe nanoparticles in a broad range of wavelengths, from the red to the near-infrared, under excitation with intense femtosecond-laser pulses. The ultrafast dynamics of the localized plasmon resonance of the system is mapped by pump−probe differential transmission measurements. Deviations from theoretical predictions based on standard two-temperature model (TTM) are revealed in the subpicosecond time-scale where the nonlinear phenomenon is more pronounced. The key-role played by nonthermalized carriers is investigated in detail on the basis of an extended TT

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

Selective Cation Exchange in the Core Region of Cu_2–<i>x</i>Se/Cu_2–<i>x</i>S Core/Shell Nanocrystals

We studied cation exchange (CE) in core/shell Cu_2–<i>x</i>Se/Cu_2–<i>x</i>S nanorods with two cations, Ag⁺ and Hg²⁺, which are known to induce rapid exchange within metal chalcogenide nanocrystals (NCs) at room temperature. At the initial stage of the reaction, the guest ions diffused through the Cu_2–<i>x</i>S shell and reached the Cu_2–<i>x</i>Se core, replacing first Cu⁺ ions within the latter region. These experiments prove that CE in copper chalcogenide NCs is facilitated by the high diffusivity of guest cations in the lattice, such that they can probe the whole host structure and identify the preferred regions where to initiate the exchange. For both guest ions, CE is thermodynamically driven as it aims for the formation of the chalcogen phase characterized by the lower solubility under the specific reaction conditions

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