Shape-Directed Binary Assembly of Anisotropic Nanoplates: A Nanocrystal Puzzle with Shape-Complementary Building Blocks

We present the binary self-assembly of two anisotropic nanoplate building blocks mediated by shape complementarity. We use rhombic GdF₃ and tripodal Gd₂O₃ nanoplates as building blocks in which the size and shape are designed to be optimal for complementary organization. A liquid interfacial assembly technique allows the formation of self-assembled binary superlattices from two anisotropic nanoplates over a micrometer length scale. Shape-directed self-assembly guides the position of each anisotropic nanoplate in the binary superlattices, allowing for long-range orientational and positional order of each building block. The design of shape complementary anisotropic building blocks offers the possibility to self-assemble binary superlattices with predictable and designable structures

Binary and Ternary Superlattices Self-Assembled from Colloidal Nanodisks and Nanorods

Self-assembly of multicomponent anisotropic nanocrystals with controlled orientation and spatial distribution allows the design of novel metamaterials with unique shape- and orientation-dependent collective properties. Although many phases of binary structures are theoretically proposed, the examples of multicomponent assemblies, which are experimentally realized with colloidal anisotropic nanocrystals, are still limited. In this report, we demonstrate the formation of binary and ternary superlattices from colloidal two-dimensional LaF₃ nanodisks and one-dimensional CdSe/CdS nanorods via liquid interfacial assembly. The colloidal nanodisks and nanorods are coassembled into AB-, AB₂-, and AB₆-type binary arrays determined by their relative size ratio and concentration to maximize their packing density. The position and orientation of anisotropic nanocrystal building blocks are tightly controlled in the self-assembled binary and ternary lattices. The macroscopic orientation of the superlattices is further tuned by changing the liquid subphase used for self-assembly, resulting in the formation of lamellar-type binary liquid crystalline superlattices. In addition, we demonstrate a novel ternary superlattice self-assembled from two different sizes of nanodisks and a nanorod, which offers the unique opportunity to design multifunctional metamaterials

Expanding the Spectral Tunability of Plasmonic Resonances in Doped Metal-Oxide Nanocrystals through Cooperative Cation–Anion Codoping

We present a generalized cation–anion codoping methodology for the synthesis of monodisperse, doped metal-oxide nanocrystals (NCs) that exhibit near-infrared localized surface plasmon resonance (LSPR) with the highest reported quality factors. We demonstrate that, in addition to the use of common cation dopants, the incorporation of fluorine into the lattice as an anion dopant can further increase the free-carrier concentration within individual NCs; this supports the cooperative effects of mixed cation–anion doping in shifting the LSPR to higher energies. As a result, this method allows the LSPR of doped metal-oxide NCs to become tunable across a significantly broader wavelength range (1.5–3.3 μm), circumventing the prior limitations on the highest possible LSPR energies associated with single-element doping for a given oxide host. The strategy of cation–anion codoping can offer new possibilities for the chemical design of doped semiconductor and metal-oxide NCs with tailored LSPR characteristics

Designing Tripodal and Triangular Gadolinium Oxide Nanoplates and Self-Assembled Nanofibrils as Potential Multimodal Bioimaging Probes

Here, we report the shape-controlled synthesis of tripodal and triangular gadolinium oxide (Gd₂O₃) nanoplates. In the presence of lithium ions, the shape of the nanocrystals is readily controlled by tailoring reaction parameters such as temperature and time. We observe that the morphology transforms from an initial tripodal shape to a triangular shape with increasing reaction time or elevated temperatures. Highly uniform Gd₂O₃ nanoplates are self-assembled into nanofibril-like liquid-crystalline superlattices with long-range orientational and positional order. In addition, shape-directed self-assemblies are investigated by tailoring the aspect ratio of the arms of the Gd₂O₃ nanoplates. Due to a strong paramagnetic response, Gd₂O₃ nanocrystals are excellent candidates for MRI contrast agents and also can be doped with rare-earth ions to form nanophosphors, pointing to their potential in multimodal imaging. In this work, we investigate the MR relaxometry at high magnetic fields (9.4 and 14.1 T) and the optical properties including near-IR to visible upconversion luminescence and X-ray excited optical luminescence of doped Gd₂O₃ nanoplates. The complex shape of Gd₂O₃ nanoplates, coupled with their magnetic properties and their ability to phosphoresce under NIR or X-ray excitation which penetrate deep into tissue, makes these nanoplates a promising platform for multimodal imaging in biomedical applications

Enhanced Charge Transfer Kinetics of CdSe Quantum Dot-Sensitized Solar Cell by Inorganic Ligand Exchange Treatments

Enhancement of the charge transfer rate in CdSe quantum dot (QD) sensitized solar cells is one of the most important criteria determining cell efficiency. We report a novel strategy for enhancing charge transfer by exchanging the native, long organic chain to an atomic ligand, S^2–, with a simple solid exchange process. S^2–-ligand exchange is easily executed by dipping the CdSe QDs sensitized photoanode into a formamide solution of K₂S. The results show that this exchange process leads to an enhancement of the electronic coupling between CdSe QD and TiO₂ by removing the insulating organic barrier to charge transfer, while maintaining its quantum confined band structure. This treatment significantly increases the charge transfer rate at the interfacial region between CdSe QDs and TiO₂ as well as between the CdSe QDs and Red/Ox coupling electrolyte, as verified by time-resolved photoluminescence and electrochemical impedance spectroscopy measurements. Finally, the S^2–-treated photoanode exhibits a much higher photovoltaic performance than the conventional MPA or TGA-capped CdSe QDs sensitized solar cell. The findings reported herein propose an innovative route toward harvesting energy from solar light by enhancing the carrier charge transfer rate

Nanocrystal Size-Dependent Efficiency of Quantum Dot Sensitized Solar Cells in the Strongly Coupled CdSe Nanocrystals/TiO₂ System

Light absorption and electron injection are important criteria determining solar energy conversion efficiency. In this research, monodisperse CdSe quantum dots (QDs) are synthesized with five different diameters, and the size-dependent solar energy conversion efficiency of CdSe quantum dot sensitized solar cell (QDSSCs) is investigated by employing the atomic inorganic ligand, S^2–. Absorbance measurements and transmission electron microscopy show that the diameters of the uniform CdSe QDs are 2.5, 3.2, 4.2, 6.4, and 7.8 nm. Larger CdSe QDs generate a larger amount of charge under the irradiation of long wavelength photons, as verified by the absorbance results and the measurements of the external quantum efficiencies. However, the smaller QDs exhibit faster electron injection kinetics from CdSe QDs to TiO₂ because of the high energy level of CB_CdSe, as verified by time-resolved photoluminescence and internal quantum efficiency results. Importantly, the S^2– ligand significantly enhances the electronic coupling between the CdSe QDs and TiO₂, yielding an enhancement of the charge transfer rate at the interfacial region. As a result, the S^2– ligand helps improve the new size-dependent solar energy conversion efficiency, showing best performance with 4.2-nm CdSe QDs, whereas conventional ligand, mercaptopropionic acid, does not show any differences in efficiency according to the size of the CdSe QDs. The findings reported herein suggest that the atomic inorganic ligand reinforces the influence of quantum confinement on the solar energy conversion efficiency of QDSSCs

Characterization of Shape and Monodispersity of Anisotropic Nanocrystals through Atomistic X‑ray Scattering Simulation

Nanocrystals with anisotropic shape and high uniformity are now commonly produced as a result of significant advances in synthetic control. In most cases, the morphology of such materials is characterized only by electron microscopy, which makes the extraction of statistical information laborious and subject to bias. In this work, we describe how X-ray scattering patterns in conjunction with Debye formula simulations can be used to provide accurate atomisitic models for ensembles of anisotropic nanocrystals to complement and extend microscopic studies. Methods of sample preparation and measurement conditions are also discussed to provide appropriate experimental data. The scripts written to implement the Debye function are provided as a tool to allow researchers to obtain atomisitic models of nanocrystals

Nonaqueous Synthesis of TiO₂ Nanocrystals Using TiF₄ to Engineer Morphology, Oxygen Vacancy Concentration, and Photocatalytic Activity

Control over faceting in nanocrystals (NCs) is pivotal for many applications, but most notably when investigating catalytic reactions which occur on the surfaces of nanostructures. Anatase titanium dioxide (TiO₂) is one of the most studied photocatalysts, but the shape dependence of its activity has not yet been satisfactorily investigated and many questions still remain unanswered. We report the nonaqueous surfactant-assisted synthesis of highly uniform anatase TiO₂ NCs with tailorable morphology in the 10–100 nm size regime, prepared through a seeded growth technique. Introduction of titanium­(IV) fluoride (TiF₄) preferentially exposes the {001} facet of anatase through <i>in situ</i> release of hydrofluoric acid (HF), allowing for the formation of uniform anatase NCs based on the truncated tetragonal bipyramidal geometry. A method is described to engineer the percentage of {001} and {101} facets through the choice of cosurfactant and titanium precursor. X-ray diffraction studies are performed in conjunction with simulation to determine an average NC dimension which correlates with results obtained using electron microscopy. In addition to altering the particle shape, the introduction of TiF₄ into the synthesis results in TiO₂ NCs that are blue in color and display a broad visible/NIR absorbance which peaks in the infrared (λ_max ≈ 3400 nm). The blue color results from oxygen vacancies formed in the presence of fluorine, as indicated by electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) studies. The surfactants on the surface of the NCs are removed through a simple ligand exchange procedure, allowing the shape dependence of photocatalytic hydrogen evolution to be studied using monodisperse TiO₂ NCs. Preliminary experiments on the photoreforming of methanol, employed as a model sacrificial agent, on platinized samples resulted in high volumes of evolved hydrogen (up to 2.1 mmol h^–1 g^–1) under simulated solar illumination. Remarkably, the data suggest that, under our experimental conditions, the {101} facets of anatase are more active than the {001}

Seeded Growth of Metal-Doped Plasmonic Oxide Heterodimer Nanocrystals and Their Chemical Transformation

We have developed a generalized seeded-growth methodology for the synthesis of monodisperse metal-doped plasmonic oxide heterodimer nanocrystals (NCs) with a near-unity morphological yield. Using indium-doped cadmium oxide (ICO) as an example, we show that a wide variety of preformed metal NCs (Au, Pt, Pd, FePt, etc.) can serve as the seeds for the tailored synthesis of metal-ICO heterodimers with exquisite size, shape, and composition control, facilitated by the delayed nucleation mechanism of the CdO phase. The metal-ICO heterodimers exhibit broadly tunable near-infrared localized surface plasmon resonances, and dual plasmonic bands are observed for Au-ICO heterodimers. We further demonstrate that the oxide domain of the Au-ICO heterodimers can be selectively and controllably transformed into a series of partially and completely hollow cadmium chalcogenide nanoarchitectures with unprecedented structural complexity, leaving the metal domain intact. Our work not only represents an exciting addition to the rapidly expanding library of chemical reactions that produce colloidal hybrid NCs, but it also provides a general route for the bottom-up chemical design of multicomponent metal-oxide-semiconductor NCs in a rational and sequential manner

Shape-Dependent Plasmonic Response and Directed Self-Assembly in a New Semiconductor Building Block, Indium-Doped Cadmium Oxide (ICO)

The influence of particle shape on plasmonic response and local electric field strength is well-documented in metallic nanoparticles. Morphologies such as rods, plates, and octahedra are readily synthesized and exhibit drastically different extinction spectra than spherical particles. Despite this fact, the influence of composition and shape on the optical properties of plasmonic semiconductor nanocrystals, in which free electrons result from heavy doping, has not been well-studied. Here, we report the first observation of plasmonic resonance in indium-doped cadmium oxide (ICO) nanocrystals, which exhibit the highest quality factors reported for semiconductor nanocrystals. Furthermore, we are able to independently control the shape and free electron concentration in ICO nanocrystals, allowing for the influence of shape on the optical response of a plasmonic semiconductor to be conclusively demonstrated. The highly uniform particles may be self-assembled into ordered single component and binary nanocrystal superlattices, and in thin films, exhibit negative permittivity in the near infrared (NIR) region, validating their use as a new class of tunable low-loss plasmonic building blocks for 3-D optical metamaterials