Meso-Crystallographic Study of a Three-Dimensional Self-Assembled Bimodal Nanocrystal Superlattice

Nanocrystal superlattices are attracting significant interest due to novel and peculiar collective properties arising from the interactions of the nanocrystals forming the superlattice. A large variety of superlattice structures can be obtained, involving one or more types of nanocrystals, with different sizes and concentrations. Engineering of the superlattice properties relies on accurate structural and morphological characterization, able to provide not only a fundamental feedback for synthesis procedures, but also relevant insight into their structural properties for possible applications. Electron microscopy and X-ray based techniques are complementary approaches for nanoscale structural imaging, which however become challenging in the presence of building blocks only a few nanometers in size. Here, a structure solution for a three-dimensional (3D) self-assembly of PbS nanocrystals with bimodal size distribution is obtained, by exploiting small-angle X-ray diffraction, transmission electron microscopy, crystallographic procedures, and geometric constraints. In particular, analysis of small-angle X-ray diffraction data, based on the Patterson function and on the single crystal model, is shown to provide relevant information on the 3D superlattice structure as well as on particle size, the scattering signal being sensitive to particles as small as 1.5 nm. The combined approach here proposed is thus demonstrated to effectively overcome important resolution limitations in the imaging of superlattices including small nanocrystals

Near Infrared Emission from Monomodal and Bimodal PbS Nanocrystal Superlattices

PbS colloidal nanocrystal (NC) assemblies with monomodal and bimodal size distribution have been fabricated by slow evaporation of solvent on silicon substrates. The interparticle distances of the assembled structures have been carefully defined, both in the plane and in the <i>z</i> direction, perpendicular to the substrate, thanks to the combination of small and wide-angle X-ray diffraction and TEM measurements. The spectroscopic characteristics of PbS NCs, both in solution and organized in a superlattice, have been investigated by steady-state and time-resolved photoluminescence measurements. The optical results reveal the occurrence of a Förster resonant energy transfer (FRET) mechanism between closed-packed neighboring PbS NCs. The occurrence of FRET is dependent on NC assembly geometry, and thus on their interparticle distance, suggesting that only when NCs are close enough, as in the close-packed arrangement of the monomodal assembly, the energy transfer can be promoted. In bimodal assemblies, the energy transfer between large and small NCs is negligible, due to the low spectral overlap between the emission and absorption bands of the different sized nanoparticles and to the large interparticle distance. Moreover, recombination lifetimes on the microsecond time scale have been observed and explained in terms of dielectric screening effect, in agreement with previous findings on lead chalcogenide NC optical properties

Photoactive Hybrid Material Based on Pyrene Functionalized PbS Nanocrystals Decorating CVD Monolayer Graphene

A simple and facile solution-based procedure is implemented for decorating a large area, monolayer graphene film, grown by chemical vapor deposition, with size-tunable light absorbing colloidal PbS nanocrystals (NCs). The hybrid is obtained by exposing a large area graphene film to a solution of 1-pyrene butyric acid surface coated PbS NCs, obtained by a capping exchange procedure onto presynthesized organic-capped NCs. The results demonstrate that at the interface, multiple and cooperative π–π stacking interactions promoted by the pyrene ligand coordinating the NC surface lead to a successful anchoring of the nano-objects on the graphene platform which concomitantly preserves its aromatic structure. Interligand interactions provide organization of the nano-objects in highly interconnected nanostructured multilayer coatings, where the NCs retain geometry and composition. The resulting hybrid exhibits a sheet resistance lower than that of bare graphene, which is explained in terms of electronic communication in the hybrid, due to the interconnection of the NC film and to a hole transfer from photoexcited PbS NCs to graphene, channelled at the interface by pyrene. Such a direct electron coupling makes the manufactured hybrid material an interesting component for optoelectronics, sensors and for optical communication and information technology

Polyelectrolyte Multilayers As a Platform for Luminescent Nanocrystal Patterned Assemblies

The fabrication of uniform and patterned nanocrystal (NC) assemblies has been investigated by exploiting the possibility of carefully tailoring colloidal NC surface chemistry and the ability of polyelectrolyte (PE) to functionalize substrates through an electrostatic layer-by-layer (LbL) strategy. Appropriate deposition conditions, substrate functionalization, and post-preparative treatments were selected to tailor the substrate surface chemistry to effectively direct the homogeneous electrostatic-induced assembly of NCs. Water-dispersible luminescent NCs, namely, (CdSe)­ZnS and CdS, were differently functionalized by (1) ligand-exchange reaction, (2) growth of a hydrophilic silica shell, and (3) formation of a hydrophilic inclusion complex, thus providing functional NCs stable in a defined pH range. The electrostatically charged functional NCs represent a comprehensive selection of examples of surface-functionalized NCs, which enables the systematic investigation of experimental parameters in NC assembly processes carried out by combining LbL procedures with microcontact printing and also exploiting NC emission, relevant for potential applications, as a prompt and effective probe for evaluating assembly quality. Thus, an ample showcase of combinations has been investigated, and the spectroscopic and morphological features of the resulting NC-based structures have been discussed

Fabrication of photoactive heterostructures based on quantum dots decorated with Au nanoparticles

<p>Silica based multifunctional heterostructures, exhibiting near infrared (NIR) absorption (650–1200 nm) and luminescence in the visible region, represent innovative nanosystems useful for diagnostic or theranostic applications. Herein, colloidal synthetic procedures are applied to design a photoactive multifunctional nanosystem. Luminescent silica (SiO₂) coated quantum dots (QDs) have been used as versatile nanoplatforms to assemble on their surface gold (Au) seeds, further grown into Au spackled structures. The synthesized nanostructures combine the QD emission in the visible region, and, concomitantly, the distinctive NIR absorption of Au nanodomains. The possibility of having multiple QDs in a single heterostructure, the SiO₂ shell thickness, and the extent of Au deposition onto SiO₂ surface have been carefully controlled. The work shows that a single QD entrapped in 16 nm thick SiO₂ shell, coated with Au speckles, represents the most suitable geometry to preserve the QD emission in the visible region and to generate NIR absorption from metal NPs. The resulting architectures present a biomedical potential as an effective optical multimodal probes and as promising therapeutic agents due to the Au NP mediated photothermal effect.</p

Three-Dimensional Self-Assembly of Networked Branched TiO₂ Nanocrystal Scaffolds for Efficient Room-Temperature Processed Depleted Bulk Heterojunction Solar Cells

In this work, we report on ∼4% power conversion efficiency (PCE) depleted bulk heterojunction (DBH) solar cells based on a high-quality electrode with a three-dimensional nanoscale architecture purposely designed so as to maximize light absorption and charge collection. The newly conceived architecture comprises a mesoporous electron-collecting film made of networked anisotropic metal-oxide nanostructures, which accommodates visible-to-infrared light harvesting quantum dots within the recessed regions of its volume. The three-dimensional electrodes were self-assembled by spin-coating a solution of colloidal branched anatase TiO₂ NCs (BNC), followed by photocatalytic removal of the native organic capping from their surface by a mild UV-light treatment and filling with small PbS NCs via infiltration. The PCE ∼ 4% of our TiO₂ BNC/PbS QD DBH solar cell features an enhancement of 84% over the performance obtained for a planar device fabricated under the same conditions. Overall, the DBH device fabrication procedure is entirely carried out under mild processing conditions at room temperature, thus holding promise for low-cost and large-scale manufacturing

Two-Dimensional Plasmonic Superlattice Based on Au Nanoparticles Self-Assembling onto a Functionalized Substrate

Au nanoparticles (NPs) self-assembled by means of a simple solvent evaporation strategy in a two-dimensional (2D) superlattice with a highly controlled geometry and extending over micrometers squared when drop cast onto a suitably functionalized silicon substrate. The assembly procedure was defined by carefully monitoring experimental parameters, namely, dispersing solvent, deposition temperature, Au NP concentration, and chemistry of supporting substrate. The investigated parameters were demonstrated to play a significant role on the delicate energetic balance of the mutual NPs as well as NP–substrate interactions, ultimately directing the NP assembly. Remarkably, substrate surface chemistry revealed to be decisive to control the extent of the organization. Scanning electron microscopy demonstrated that the 2D superlattice extends uniformly over hundreds of square micrometers. Grazing-incidence small-angle X-ray scattering investigation validated the Au NP organization in crystalline domains and confirmed the role played by the surface chemistry of the substrate onto the 2D lattice assembly. Finally, preliminary spectroscopic ellipsometry investigation allowed extraction of optical constants of NP assemblies. The localized surface plasmon resonance modes of the NP assemblies were studied through a combined analysis of reflection, transmission, and ellipsometric data that demonstrated that the plasmonic properties of the Au NP assemblies strongly depend on the substrate, which was found to influence NP ordering and near-field interactions between NPs