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

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

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

Copper Sulfide Nanocrystals with Tunable Composition by Reduction of Covellite Nanocrystals with Cu⁺ Ions

Platelet-shaped copper sulfide nanocrystals (NCs) with tunable Cu stoichiometry were prepared from Cu-rich covellite (Cu_1.1S) nanoplates through their reaction with a Cu­(I) complex ([Cu­(CH₃CN)₄]­PF₆) at room temperature. Starting from a common sample, by this approach it is possible to access a range of compositions in these NCs, varying from Cu_1.1S up to Cu₂S, each characterized by a different optical response: from the metallic covellite, with a high density of free carriers and strong localized surface plasmon resonance (LSPR), up to Cu₂S NCs with no LSPR. In all these NCs the valency of Cu in the lattice stays always close to +1, while the average −1 valency of S in covellite gradually evolves to −2 with increasing Cu content; i.e., sulfur is progressively reduced. The addition of copper to the starting covellite NCs is similar to the intercalation of metal species in layered transition metal dichalcogenides (TMDCs); i.e., the chalcogen–chalcogen bonds holding the layers are progressively broken to make room for the intercalated metals, while their overall anion sublattice does not change much. However, differently from the TMDCs, the intercalation in covellite NCs is sustained by a change in the redox state of the anion framework. Furthermore, the amount of Cu incorporated in the NCs upon reaction is associated with the formation of an equimolar amount of Cu­(II) species in solution. Therefore, the reaction scheme can be written as: Cu_1.1S + 2γCu­(I) → Cu_1.1+γS + γCu­(II)