Pressure-Tuned Structure and Property of Optically Active Nanocrystals.

Investigations through high-pressure X-ray scattering and spectroscopy in combination with theoretical computations shows that high-pressure compression can systematically tune the optical properties and mechanical stability of the molecular nanocrystals.

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.

Stress-induced nanoparticle crystallization.

We demonstrate for the first time a new mechanical annealing method that can significantly improve the structural quality of self-assembled nanoparticle arrays by eliminating defects at room temperature. Using in situ high-pressure small-angle X-ray scattering, we show that deformation of nanoparticle assembly in the presence of gigapascal level stress rebalances interparticle forces within nanoparticle arrays and transforms the nanoparticle film from an amorphous assembly with defects into a quasi-single crystalline superstructure. Our results show that the existence of the hydrostatic pressure field makes the transformation both thermodynamically and kinetically possible/favorable, thus providing new insight for nanoparticle self-assembly and integration with enhanced mechanical performance.

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.

Self-assembled colloidal superparticles from nanorods.

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.

Porous one-dimensional nanostructures through confined cooperative self-assembly.

We report a simple confined self-assembly process to synthesize nanoporous one-dimensional photoactive nanostructures. Through surfactant-assisted cooperative interactions (e.g., π-π stacking, ligand coordination, and so forth) of the macrocyclic building block, zinc meso-tetra (4-pyridyl) porphyrin (ZnTPyP), self-assembled ZnTPyP nanowires and nanorods with controlled diameters and aspect ratios are prepared. Electron microscopy characterization in combination with X-ray diffraction and gas sorption experiments indicate that these materials exhibit stable single-crystalline and high surface area nanoporous frameworks with well-defined external morphology. Optical characterizations using UV-vis spectroscopy and fluorescence imaging and spectroscopy show enhanced collective optical properties over the individual chromophores (ZnTPyP), favorable for exciton formation and transport.

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.

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.

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.

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.

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.

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.

Monodisperse porous nanodiscs with fluorescent and crystalline wall structure.

We report a facile solution process to synthesize monodisperse porous nanodiscs through confined molecular self-assembly of surfactants and ZnTPyP. The nanodiscs exhibit trimodal pores with fluorescent and crystalline wall structures, and are potentially important for sorption and separation, sensors, catalytic materials, electrode materials, etc.

Hydrogen-bonding-assisted self-assembly: monodisperse hollow nanoparticles made easy.

A facile self-assembly process for synthesizing monodisperse hollow spherical nanoparticles that are less than 50 nm in diameter has been developed. Preferential hydrogen bonding between an amphiphilic block copolymer (polystyrene-b-polyvinylpyridine, PS-PVP) and a hydrogen-bonding agent (HA) enables formation of monodisperse spherical solid polymer nanoparticles with the HA residing in the particle core surrounded by the polymer. Removal of the HA results in monodisperse hollow nanoparticles with tunable hollow cavity size and internal surface reactivity. Formation of ordered hollow nanoparticle films with controlled index of refraction for antireflective coating applications is demonstrated.

Cooperative self-assembly-assisted formation of monodisperse optically active spherical and anisotropic nanoparticles.

We report a new method in which spontaneous self-assembly is employed to synthesize monodisperse polymer nanoparticles with controlled size (<50 nm), shape, tunable functionality, and enhanced solvent and thermal stability. Cooperative noncovalent interactions, such as hydrogen bonding and aromatic pi-pi stacking, assist self-assembly of amphiphilic macromolecules (polystyrene-block-polyvinylpyridine, PS--PVP) and structure directing agents (SDAs) to form both spherical and anisotropic solid polymer nanoparticles with SDAs residing in the particle core surrounded by the polymers. Through detailed investigations by scanning electron microscopy and transmission electron microscopy (TEM), we have rationalized nanoparticle morphology evolution and dependence on factors such as SDA concentration and PVP size. By keeping the PS chain size constant, the particle morphology progresses from continuous films to spherical particles, and on to cylindrical nanowires or rods with increasing the PVP chain size. The final nanoparticles are very stable and can be redispersed in common solvents to form homogenous solutions and thin films of ordered nanoparticle arrays through solvent evaporation processes. These nanoparticles exhibit tunable fluorescent colors (or emissions) depending on the choices of the central SDAs. Our method is simple and general without requiring complicated synthetic chemistry, stabilizing surfactants, or annealing procedures (e.g., temperature or solvent annealing), making scalable synthesis feasible.

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.