Stephen O'Brien

STRUCTURAL DIVERSITY IN BINARY NANOPARTICLE SUPERLATTICES

Assembly of small building blocks such as atoms, molecules and nanoparticles into macroscopic structures—that is, ‘bottom up’ assembly—is a theme that runs through chemistry, biology and material science. Bacteria, macromolecules and nanoparticles can self-assemble, generating ordered structures with a precision that challenges current lithographic techniques. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice (BNSL) can provide a general and inexpensive path to a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. Maximization of the nanoparticle packing density has been proposed as the driving force for BNSL formation, and only a few BNSL structures have been predicted to be thermodynamically stable. Recently, colloidal crystals with micrometre-scale lattice spacings have been grown from oppositely charged polymethyl methacrylate spheres. Here we demonstrate formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic and magnetic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nanoparticles determine BNSL stoichiometry; additional contributions from entropic, van der Waals, steric and dipolar forces stabilize the variety of BNSL structures.

Three-dimensional binary superlattices of magnetic nanocrystals and semiconductor quantum dots.

Recent advances in strategies for synthesizing nanoparticles—such as semiconductor quantum dots, magnets and noble-metal clusters—have enabled the precise control of composition, size, shape, crystal structure, and surface chemistry. The distinct properties of the resulting nanometre-scale building blocks can be harnessed in assemblies with new collective properties, which can be further engineered by controlling interparticle spacing and by material processing. Our study is motivated by the emerging concept of metamaterials—materials with properties arising from the controlled interaction of the different nanocrystals in an assembly. Previous multi-component nanocrystal assemblies have usually resulted in amorphous or short-range-ordered materials because of non-directional forces or insufficient mobility during assembly. Here we report the self-assembly of PbSe semiconductor quantum dots and Fe2O3 magnetic nanocrystals into precisely ordered three-dimensional superlattices. The use of specific size ratios directs the assembly of the magnetic and semiconducting nanoparticles into AB13 or AB2 superlattices with potentially tunable optical and magnetic properties. This synthesis concept could ultimately enable the fine-tuning of material responses to magnetic, electrical, optical and mechanical stimuli.

Metal Acetylacetonates as General Precursors for the Synthesis of Early Transition Metal Oxide Nanomaterials

A versatile, convenient, and nontoxic solvothermal method for the synthesis of nanocrystalline iron, chromium, and manganese oxides is described. This method employs the reactions of metal acetylacetonate precursors and oxygen-containing solvents in a reaction to prepare metal oxide nanoparticles. Characterization of these nanocrystalline materials was carried out employing transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), and elemental analysis.

Orientated assembly of single-walled carbon nanotubes and applications

Single-walled carbon nanotubes (SWNTs) have unique structural, mechanical, thermal and electrical properties, which make them attractive and important building blocks for nanotechnology. Orientated assembly of SWNTs presents a crucial prerequisite for both fundamental research at the individual SWNT level and nanotube-based device fabrication and applications. In this feature article, we review recent progress in the field with a focus on the orientated assembly of SWNTs (horizontally aligned and vertically aligned) from solution deposition and direct chemical vapor deposition, and on possible growth mechanisms. We also discuss our own research efforts in orientation control by chemical vapor deposition, and how this enables the fundamental characterization of individual SWNTs, and contributes to nanotube-based device fabrication and applications.

Flexible BaTiO3/PVDF gradated multilayer nanocomposite film with enhanced dielectric strength and high energy density

Organic–inorganic 0–3 nanocomposites, which combine the potentially high dielectric strength of the organic matrix and the high dielectric permittivity of the inorganic filler, are extensively studied as energy-storage dielectrics in high-performance capacitors. In this study, a gradated multilayer BaTiO3/poly(vinylidene fluoride) thin film structure is presented as a means to achieve both a higher breakdown strength and a superior energy-storage capability. The central layer of this film, designed to provide high electric displacement, is composed of a high volume fraction of 6–10 nm BTO nanocrystals produced by a TEG-sol method. The small particle size contributes to a high dispersibility of the nanocrystals in polymer media, as well as a high interfacial area to mitigate the local electric field concentration. The outer layers of the structure are predominantly PVDF, with a significantly low volume fraction of BTO, taking advantage of the high dielectric strength of pure PVDF at the electrode–nanocomposite interface. The film is mechanically flexible, and can be removed from the substrate, with total thicknesses in the range of 1.2–1.5 μm. Parallel plate capacitance devices exhibit highly improved dielectric performances with low-frequency permittivity values of 20–25, a maximal discharge energy density of 19.37 J cm−3 and dielectric (breakdown) strengths of up to 495 kV mm−1.

Influence of capping groups on the synthesis of γ-Fe2O3 nanocrystals

Monodisperse and uniform γ-Fe 2 O 3 (maghemite) nanocrystals of variable size were prepared by thermal decomposition of iron pentacarbonyl [Fe(CO) 5 ] in the presence of surfactants, following controlled oxidation with trimethylamine N -oxide as a mild oxidant. The influence of carboxylic acids with variable alkyl carbon chain lengths on the synthesis of γ-Fe 2 O 3 nanocrystals was investigated. The effect of the molar ratios of surfactant to iron precursor was also studied. The nanocrystals were characterized by x-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD showed the particles were highly crystalline at the nanometer scale. The results showed that the size and shape of the nanocrystal is strongly influenced by the decomposition temperature of iron pentacarbonyl and closely related to the length of carbon chain of the capping groups and the molar ratio of surfactant to iron precursor. Following controlled evaporation from nonpolar solvents, self-assembly into two-dimensional arrays could be observed by TEM. It was also found that the distance between the nanocrystals in self-assembled structures matched the length of the capping molecules very well.

Intrinsic dielectric frequency dependent spectrum of a single domain tetragonal BaTiO3

The intrinsic dielectric frequency dependent spectrum of single domain barium titanate (BaTiO3) at room temperature is investigated by considering the vibration of phonons and the conductivity of the tetragonal system in a wide frequency range up to THz. The proposed model combines Debye type of dissipation, soft mode theory, and the influence of conductivity on the dielectric loss to obtain a more precise dielectric frequency spectrum. The calculated results were compared with experimental data on single domain nanocrystals of BaTiO3, both free standing and suspended in a low dielectric medium. The comparisons provide insight into the mechanism for the dielectric behavior, which can be extended to apply to a range of composites that comprise single domain dielectrics embedded in continuous media. At the lower frequency range, conductivity plays a dominant role in the contribution to the dielectric loss along both a- and c-axes, while the phonon vibration controls the dielectric behavior of the system at ...

Diameter Control and Photoluminescence of ZnO Nanorods from Trialkylamines

A novel solution method to control the diameter of ZnO nanorods is reported. Small diameter (2-3 nm) nanorods were synthesized from trihexylamine, and large diameter (50–80 nm) nanorods were synthesized by increasing the alkyl chain length to tridodecylamine. The defect (green) emission of the photoluminescence (PL) spectra of the nanorods varies with diameter, and can thus be controlled by the diameter control. The small ZnO nanorods have strong green emission, while the large diameter nanorods exhibit a remarkably suppressed green band. We show that this observation supports surface oxygen vacancies as the defect that gives rise to the green emission.

Formation of silica–surfactant mesophases studied by real-time in situ X-ray powder diffraction

Real-time in situ energy dispersive synchrotron X-ray powder diffraction data provides evidence for the formation of an intermediate lamellar silica–surfactant intercalate during the synthesis of the hexagonal mesophase derived from the layered polysilicate kanemite, whereas no intermediate phases are observed during the formation of the silica–surfactant mesophase that leads to the mesoporous material MCM-41.

Synchrotron x-ray scattering of ZnO nanorods: Periodic ordering and lattice size

We demonstrate that synchrotron x-ray powder diffraction (XRD) is a powerful technique for studying the structure and self-organization of zinc-oxide nanostructures. Zinc-oxide nanorods were prepared by a solution-growth method that resulted in uniform nanorods with 2-nm diameter and lengths in the range 10-50 nm. These nanorods were structurally characterized by a combination of small-angle and wide-angle synchrotron XRD and transmission electron microscopy (TEM). Small-angle XRD and TEM were used to investigate nanorod self-assembly and the influence of surfactant/precursor ratio on self-assembly. Wide-angle XRD was used to study the evolution of nanorod growth as a function of synthesis time and surfactant/precursor ratio.