Huimeng Wu

Self-assembled colloidal superparticles from nanorods

Beyond Quantum Dots Semiconducting colloidal nanoparticles—quantum dots—are of interest for their unusual properties. One current challenge is the controlled assembly of colloidal particles into larger structures, such as two-dimensional lattices on a substrate, or three-dimensional superparticles. Wang et al. (p. 358) present a two-step self-assembly of CdSe/CdS semiconductor nanorods to form mesoscopic colloidal superparticles. The particles show well-defined super-crystalline domains with dimensions ranging from hundreds of nanometers to several microns, and with the particle morphology controlled by the number of constituent rods. Films of the needle-shaped superparticles were able to act as polarizing light-emitting diodes. Colloidal rods self-assemble into semiconducting superparticles with a shape controlled by the number of rods. Colloidal superparticles are nanoparticle assemblies in the form of colloidal particles. The assembly of nanoscopic objects into mesoscopic or macroscopic complex architectures allows bottom-up fabrication of functional materials. We report that the self-assembly of cadmium selenide–cadmium sulfide (CdSe-CdS) core-shell semiconductor nanorods, mediated by shape and structural anisotropy, produces mesoscopic colloidal superparticles having multiple well-defined supercrystalline domains. Moreover, functionality-based anisotropic interactions between these CdSe-CdS nanorods can be kinetically introduced during the self-assembly and, in turn, yield single-domain, needle-like superparticles with parallel alignment of constituent nanorods. Unidirectional patterning of these mesoscopic needle-like superparticles gives rise to the lateral alignment of CdSe-CdS nanorods into macroscopic, uniform, freestanding polymer films that exhibit strong photoluminescence with a striking anisotropy, enabling their use as downconversion phosphors to create polarized light-emitting diodes.

Templated Photocatalytic Synthesis of Well-Defined Platinum Hollow Nanostructures with Enhanced Catalytic Performance for Methanol Oxidation

Hollow metallic nanostructures exhibit important applications in catalysis, sensing, and phototherapy due to their increased surface areas, reduced densities, and unique optical and electronic features. Here we report a facile photocatalytic process to synthesize and tune hollow platinum (Pt) nanostructures. Through hierarchically structured templates, well-defined hollow Pt nanostructures are achieved. These nanostructures possess interconnected nanoporous framework as shell with high surface area for enhanced catalytic performance/mass transport for methanol oxidation.

Surface-functionalization-dependent optical properties of II-VI semiconductor nanocrystals.

We report a study of the surface-functionalization-dependent optical properties of II-VI zinc-blende semiconductor nanocrystals on the basis of ligand-exchange chemistry, isomaterial core/shell growth, optical spectroscopy, transmission electron microscopy, and X-ray powder diffraction. Our results show that the transition energy and extinction coefficient of the 2S(h3/2)1S(e) excitonic band of these nanocrystals can be strongly modified by their surface ligands as well as ligand associated surface atomic arrangement. The oleylamine exchange of oleate-capped zinc-blende II-VI nanocrystals narrows the energy gap between their first and second excitonic absorption bands, and this narrowing effect is size-dependent. The oleylamine exchange results in the quenching, subsequent recovery, and even enhancing of the photoluminescence emission of these II-VI semiconductor nanocrystals. In addition, the results from our X-ray powder diffraction measurements and simulations completely rule out the possibility that oleate-capped zinc-blende CdSe nanocrystals can undergo zinc-blende-to-wurtzite crystal transformation upon ligand exchange with oleylamine. Moreover, our theoretical modeling results suggest that the surface-functionalization-dependent optical properties of these semiconductor nanocrystals can be caused by a thin type II isomaterial shell that is created by the negatively charged ligands (e.g., oleate and octadecyl phosphonate). Taking all these results together, we provide the unambiguous identification that II-VI semiconductor nanocrystals exhibit surface-functionalization-dependent excitonic absorption features.

Deviatoric Stress Driven Formation of Large Single-Crystal PbS Nanosheet from Nanoparticles and in Situ Monitoring of Oriented Attachment

Two-dimensional single-crystal PbS nanosheets were synthesized by deviatoric stress-driven orientation and attachment of nanoparticles (NPs). In situ small- and wide-angle synchrotron X-ray scattering measurements on the same spot of the sample under pressure coupled with transmission electron microscopy enable reconstruction of the nucleation route showing how enhanced deviatoric stress causes ordering NPs into single-crystal nanosheets with a lamellar mesostructure. At the same time that deviatoric stress drives SC(110) orientation in a face-centered-cubic supercrystal (SC), rocksalt (RS) NPs rotate and align their RS(200) and RS(220) planes within the SC(110) plane. When NPs approach each other along the compression axis, enhanced deviatoric stress drives soft ligands passivated at RS(200) and RS(220) surfaces to reorient from a group of SC(110) in-planes to the interspace of SC[110]-normal planes. While the internal NP structure starts a rocksalt-to-orthorhombic transition at 7.1 GPa, NPs become aligned on RS(220) and RS(200) and thus become attached at those faces. The transition-catalyzed surface atoms accelerate the inter-NP coalescing process and the formation of low-energy structure nanosheet. Above 11.6 GPa, the nucleated single-crystal nanosheets stack into a lamellar mesostructure that has a domain size comparable to the starting supercrystal.

Cylindrical Superparticles from Semiconductor Nanorods

In this communication, we report a synthesis of anisotropic colloidal superparticles (SPs) from CdSe/CdS semiconductor nanorods. These anisotropic SPs are cylindrical disks or stacked-disk arrays. We attribute the major driving forces controlling the SP shape to interparticle interactions between nanorods and solvophobic interactions between a superparticle and its surrounding solvent. According to their sizes (or volumes), the SPs adopt either single- or multilayered structures. In addition, these SPs exhibit linearly polarized emissions, demonstrating their potential role as useful components in devices such as polarized light-emitting diodes and electrooptical modulators.

Interfacial Self-Assembly Driven Formation of Hierarchically Structured Nanocrystals with Photocatalytic Activity

We report the synthesis of hierarchical structured nanocrystals through an interfacial self-assembly driven microemulsion (μ-emulsion) process. An optically active macrocyclic building block Sn (IV) meso-tetraphenylporphine dichloride (tin porphyrin) is used to initiate noncovalent self-assembly confined within μ-emulsion droplets. In-situ studies of dynamic light scattering, UV-vis spectroscopy, and electron microscopy, as well as optical imaging of reaction processes suggest an evaporation-induced nucleation and growth self-assembly mechanism. The resulted nanocrystals exhibit uniform shapes and sizes from ten to a hundred nanometers. Because of the spatial ordering of tin porphyrin, the hierarchical nanocrystals exhibit collective optical properties resulting from the coupling of molecular tin porphyrin and photocatalytic activities in the reduction of platinum nanoparticles and networks and in photodegradation of methyl orange (MO) pollutants.

Gas-Bubble Effects on the Formation of Colloidal Iron Oxide Nanocrystals

This paper reports that gas bubbles can be used to tailor the kinetics of the nucleation and growth of inorganic-nanocrystals in a colloidal synthesis. We conducted a mechanistic study of the synthesis of colloidal iron oxide nanocrystals using gas bubbles generated by boiling solvents or artificial Ar bubbling. We identified that bubbling effects take place through absorbing local latent heat released from the exothermic reactions involved in the nucleation and growth of iron oxide nanocrystals. Our results show that gas bubbles display a stronger effect on the nucleation of iron oxide nanocrystals than on their growth. These results indicate that the nucleation and growth of iron oxide nanocrystals may rely on different types of chemical reactions between the iron-oleate decomposition products: the nucleation relies on the strongly exothermic, multiple-bond formation reactions, whereas the growth of iron oxide nanocrystals may primarily depend upon single-bond formation reactions. The identification of exothermic reactions is further consistent with our results in the synthesis of iron oxide nanocrystals with boiling solvents at reaction temperatures ranging from 290 to 365 °C, by which we determined the reaction enthalpy in the nucleation of iron oxide nanocrystals to be -142 ± 12 kJ/mol. Moreover, our results suggest that a prerequisite for effectively suppressing secondary nucleation in a colloidal synthesis is that the primary nucleation must produce a critical amount of nuclei, and this finding is important for a priori design of colloidal synthesis of monodispersed nanocrystals in general.

Formation of Heterodimer Nanocrystals: UO2/In2O3 and FePt/In2O3

This Article reports a mechanistic study on the formation of colloidal UO(2)/In(2)O(3) and FePt/In(2)O(3) heterodimer nanocrystals. These dimer nanocrystals were synthesized via the growth of In(2)O(3) as the epitaxial material onto the seed nanocrystals of UO(2) or FePt. The resulting dimer nanocrystals were characterized using X-ray powder diffraction (XRD), energy dispersion spectroscopy, transmission electron microscopy (TEM), scanning transmission electron microscopy, and high-resolution TEM (HRTEM). The results from XRD and HRTEM clearly show that lattice strains exist in both of these dimer nanocrystals. Interestingly, the lattice of In(2)O(3) expands in UO(2)/In(2)O(3) dimers, whereas FePt/In(2)O(3) dimers exhibit compressed In(2)O(3) lattices. Using HRTEM and nanocrystal structure simulations, we have identified the crystallographic orientation of the attachment of the two segments in these two types of dimers. An unconventional Miller index was introduced to describe the crystallographic orientation of these heterodimer nanocrystals. On the basis of the results herein as well as those from other researchers, we propose an empirical law for the determination of the crystallographic attachment orientation in heterodimers: instead of growth on the facet of the seed nanocrystals where lattice mismatch is minimized, the growth of an epitaxial material often chooses the crystal facets where the first atomic monolayer of this material has the strongest affinity for the seed nanocrystals.

Nanostructured Gold Architectures Formed through High Pressure-Driven Sintering of Spherical Nanoparticle Arrays

We have demonstrated pressure-directed assembly for preparation of a new class of chemically and mechanically stable gold nanostructures through high pressure-driven sintering of nanoparticle assemblies at room temperature. We show that under a hydrostatic pressure field, the unit cell dimension of a 3D ordered nanoparticle array can be reversibly manipulated allowing fine-tuning of the interparticle separation distance. In addition, 3D nanostructured gold architecture can be formed through high pressure-induced nanoparticle sintering. This work opens a new pathway for engineering and fabrication of different metal nanostructured architectures.

Morphology-Controlled Self-Assembly and Synthesis of Photocatalytic Nanocrystals

Abilities to control the size and shape of nanocrystals in order to tune functional properties are an important grand challenge. Here we report a surfactant self-assembly induced micelle encapsulation method to fabricate porphyrin nanocrystals using the optically active precursor zinc porphyrin (ZnTPP). Through confined noncovalent interactions of ZnTPP within surfactant micelles, nanocrystals with a series of morphologies including nanodisk, tetragonal rod, and hexagonal rod, as well as amorphous spherical particle are synthesized with controlled size and dimension. A phase diagram that describes morphology control is achieved via kinetically controlled nucleation and growth. Because of the spatial ordering of ZnTPP, the hierarchical nanocrystals exhibit both collective optical properties resulted from coupling of molecular ZnTPP and shape dependent photocatalytic activities in photo degradation of methyl orange pollutants. This simple ability to exert rational control over dimension and morphology provides new opportunities for practical applications in photocatalysis, sensing, and nanoelectronics.