Synthesis and thermal stability of ZrO2@SiO2 core-shell submicron particles.

ZrO2@SiO2 core-shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol-gel method and pre-calcined at 400 °C. Silica shells were grown on these particles (average size: ∼270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stöber approach. To study the thermal stability of bare ZrO2 cores and ZrO2@SiO2 core-shell particles they were calcined at 450 to 1200 °C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO2 particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 °C. Increasing the temperature to 600 °C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 °C. In striking contrast, SiO2-encapsulation significantly inhibited grain growth and the t → m transition progressed considerably only after heating to 1000 °C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 °C. Thus, ZrO2@SiO2 core-shell particles are suited for high-temperature applications up to ∼1000 °C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO2@SiO2 core-shell particles.

Chemistry of Shape-Controlled Iron Oxide Nanocrystal Formation.

Herein, we demonstrate that meticulous and in-depth analysis of the reaction mechanisms of nanoparticle formation is rewarded by full control of the size, shape, and crystal structure of superparamagnetic iron oxide nanocrystals during synthesis. Starting from two iron sources, iron(II) and iron(III) carbonate, a strict separation of oleate formation from the generation of reactive pyrolysis products and concomitant nucleation of iron oxide nanoparticles was achieved. This protocol enabled us to analyze each step of nanoparticle formation independently in depth. The progress of the entire reaction was monitored via matrix-assisted laser desorption ionization time-of-flight mass spectrometry and gas chromatography, thus providing insight into the formation of various iron oleate species prior to nucleation. Interestingly, due to the intrinsic strongly reductive pyrolysis conditions of the oleate intermediates and redox process in early stages of the synthesis, pristine iron oxide nuclei were composed exclusively from wüstite irrespective of the oxidation state of the iron source. Controlling the reaction conditions provided a very broad range of size- and shape-defined monodispersed iron oxide nanoparticles. Curiously, after nucleation, star-shaped nanocrystals were obtained that underwent metamorphism toward cubic-shaped particles. Electron energy loss spectroscopy tomography revealed ex post oxidation of the primary wustite nanocrystal, providing a full 3D image of Fe2+ and Fe3+ distribution within. Overall, we developed a highly flexible synthesis, yielding multi-gram amounts of well-defined iron oxide nanocrystals of different sizes and morphologies.

Pore geometry effect on the synthesis of silica supported perovskite oxides.

The formation of perovskite oxide nanoparticles supported on ordered mesoporous silica with different pore geometry is here presented. Systematic study was performed varying both pore shape (gyroidal, cylindrical, spherical) and size (7.5, 12, 17nm) of the hosts. LaFeO3, PrFeO3 and LaCoO3 were chosen as target guest structures. The distribution of the oxide nanoparticles on silica was comprehensively assessed using a multi-technique approach. It could be shown that the pore geometry plays a determining role in the conversion of the infiltrated metal nitrates to metal oxide. In particular, slow degradation kinetic was observed in highly curved pores, which fostered nucleation and crystallization of the guest species. In spherical pore systems the enhancement of pore size caused a remarkable delay of the decomposition of the metal salts, but at the same time improved the homogeneous distribution of the oxide particles in the matrix.

Site-Specific Ligand Interactions Favor the Tetragonal Distortion of PbS Nanocrystal Superlattices.

We analyze the structure and morphology of mesocrystalline, body-centered tetragonal (bct) superlattices of PbS nanocrystals functionalized with oleic acid. On the basis of combined scattering and real space imaging, we derive a three-dimensional (3D) model of the superlattice and show that the bct structure benefits from a balanced combination of {100}PbS-{100}PbS and {111}PbS-{111}PbS interactions between neighboring layers of nanocrystals, which uniquely stabilizes this structure. These interactions are enabled by the coaxial alignment of the atomic lattices of PbS with the superlattice. In addition, we find that this preferential orientation is already weakly present within isolated monolayers. By adding excess oleic acid to the nanocrystal solution, tetragonal distortion is suppressed, and we observe assembly into a bilayered hexagonal lattice reminiscent of a honeycomb with grain sizes of several micrometers.

Organically linked iron oxide nanoparticle supercrystals with exceptional isotropic mechanical properties.

It is commonly accepted that the combination of the anisotropic shape and nanoscale dimensions of the mineral constituents of natural biological composites underlies their superior mechanical properties when compared to those of their rather weak mineral and organic constituents. Here, we show that the self-assembly of nearly spherical iron oxide nanoparticles in supercrystals linked together by a thermally induced crosslinking reaction of oleic acid molecules leads to a nanocomposite with exceptional bending modulus of 114 GPa, hardness of up to 4 GPa and strength of up to 630 MPa. By using a nanomechanical model, we determined that these exceptional mechanical properties are dominated by the covalent backbone of the linked organic molecules. Because oleic acid has been broadly used as nanoparticle ligand, our crosslinking approach should be applicable to a large variety of nanoparticle systems.

Solid-State Chemistry on the Nanoscale: Ion Transport through Interstitial Sites or Vacancies?

How can ion-exchange process occur in nanocrystals without the size and shape changing and why is the ion transport much faster than in classical interdiffusion processes in macrocrystalline solids? We have investigated these processes at the molecular level by means of high-resolution and analytical electron microscopy in temperature-dependent kinetic experiments for several model reactions. The results clearly show a diffusion process that proceeds exclusively through the interstitial lattice positions with a subsequent "kick out" to remove individual ions from lattice sites without the formation of vacancies. This mechanism has not been observed in nanocrystalline systems before.

Quantum-confined emission and fluorescence blinking of individual exciton complexes in CdSe nanowires.

One-dimensional semiconductor nanostructures combine electron mobility in length direction with the possibility of tailoring the physical properties by confinement effects in radial direction. Here we show that thin CdSe quantum nanowires exhibit low-temperature fluorescence spectra with a specific universal structure of several sharp lines. The structure strongly resembles the pattern of bulk spectra but show a diameter-dependent shift due to confinement effects. Also the fluorescence shows a pronounced complex blinking behavior with very different blinking dynamics of different emission lines in one and the same spectrum. Time- and space-resolved optical spectroscopy are combined with high-resolution transmission electron microscopy of the very same quantum nanowires to establish a detailed structure-property relationship. Extensive numerical simulations strongly suggest that excitonic complexes involving donor and acceptor sites are the origin of the feature-rich spectra.

CdSe/CdS-quantum rods: fluorescent probes for in vivo two-photon laser scanning microscopy.

CdSe/CdS-Quantum-dots-quantum-rods (QDQRs) with an aspect ratio of ∼ 6 are prepared via the seeded growth method, encapsulated within a shell of crosslinked poly(isoprene)-block-poly(ethylene glycol) (PI-b-PEG) diblock copolymer, and transferred from the organic phase into aqueous media. Their photoluminescence quantum yield (PLQY) of 78% is not compromised by the phase transfer. Within a period of two months the PLQY of QDQRs in aqueous solution at neutral pH decreases only slightly (to ∼ 65%). The two-photon (TP) action cross sections of QDQRs (∼ 10(5) GM) are two orders of magnitude higher than those of CdSe/CdS/ZnS-core/shell/shell quantum dots (QDs, ∼ 10(3) GM) with comparable diameter (∼ 5 nm). After applying PI-b-PEG encapsulated QDQRs onto the small intestinal mucosa of mice in vivo, their strong red fluorescence can easily be observed by two-photon laser scanning microscopy (TPLSM) and clearly distinguished from autofluorescent background. Our results demonstrate that PI-b-PEG encapsulated CdSe/CdS-QDQRs are excellent probes for studying the uptake and fate of nanoparticles by two-photon imaging techniques in vivo.

Size dependent structural and magnetic properties of FeO-Fe3O4 nanoparticles.

The magnetic properties of monodisperse FeO-Fe3O4 nanoparticles with different mean sizes and volume fractions of FeO synthesized via decomposition of iron oleate were correlated to their crystallographic and phase compositional features by exploiting high resolution transmission electron microscopy, X-ray diffraction, Mössbauer spectroscopy and field and zero field cooled magnetization measurements. A model describing the phase transformation from a pure Fe3O4 phase to a mixture of Fe3O4, FeO and interfacial FeO-Fe3O4 phases as the particle size increases was established. The reduced magnetic moment in FeO-Fe3O4 nanoparticles was attributed to the presence of differently oriented Fe3O4 crystalline domains in the outer layers and paramagnetic FeO phase. The exchange bias energy, dominating magnetization reversal mechanism and superparamagnetic blocking temperature in FeO-Fe3O4 nanoparticles depend strongly on the relative volume fractions of FeO and the interfacial phase.

Highly stable monodisperse PEGylated iron oxide nanoparticle aqueous suspensions: a nontoxic tracer for homogeneous magnetic bioassays.

Uniformly sized and shaped iron oxide nanoparticles with a mean size of 25 nm were synthesized via decomposition of iron-oleate. High resolution transmission electron microscopy and Mössbauer spectroscopy investigations revealed that the particles are spheres primarily composed of Fe3O4 with a small fraction of FeO. From Mössbauer and static magnetization measurements, it was deduced that the particles are superparamagnetic at room temperature. The hydrophobic particles were successfully transferred into water via PEGylation using nitrodopamine as an anchoring group. IR spectroscopy and thermogravimetric analysis showed the success and efficiency of the phase transfer reaction. After PEGylation, the particles retained monodispersity and their magnetic core remained intact as proven by photon cross-correlation spectroscopy, ac susceptibility, and transmission electron microscopy. The particle aqueous suspensions revealed excellent water stability over a month of monitoring and also against temperature up to 40 °C. The particles exhibited a moderate cytotoxic effect on in vitro cultured bone marrow-derived macrophages and no release of inflammatory or anti-inflammatory cytokines. The PEGylated particles were functionalized with Herceptin antibodies via a conjugation chemistry, their response to a rotating magnetic field was studied using a fluxgate-based setup and was compared with the one recorded for hydrophobic and PEGylated particles. The particle phase lag rose after labeling with Herceptin, indicating the successful conjugation of Herceptin antibodies to the particles.

Ultrasmall biocompatible nanocomposites: a new approach using seeded emulsion polymerization for the encapsulation of nanocrystals.

We report a novel approach of seeded emulsion polymerization in which nanocrystals are used as seeds. Ultrasmall biocompatible polymer-coated nanocrystal with sizes between 15 and 110 nm could be prepared in a process that avoids any treatment with high shear forces or ultrasonication. The number of nanocrystals per seed, the size of the seeds, and the shell thickness can be independently adjusted. Single encapsulated nanocrystals in ultrasmall nanobeads as well as clusters of nanocrystals can be obtained. Polysorbat-80 was used as surfactant. It consists of poly(ethylene glycol) (PEG) chains, giving the particles outstanding biofunctional characteristics such as a minimization of unspecific interactions.

Energy transfer from CdSe/CdS nanorods to amorphous carbon.

Semiconductor nanocrystals placed nearby a metal film significantly change their optical properties. In this work, we examine the change in fluorescence intensity, lifetime, and blinking behavior of individual CdSe/CdS nanorods close to a 9 nm thick amorphous carbon film. Energy transfer between the donor and acceptor was investigated in detail yielding a R(-4) distance dependence for the nanorod-carbon system. The Förster critical distance was determined to be R0=24.9 nm, which is nearly identical with the theoretical value of 24.8 nm predicted by the classical approach. Additionally, antibunching measurements were performed in order to prove the presence of single isolated emitters.

Thermoelectric properties of lead chalcogenide core-shell nanostructures.

We present the full thermoelectric characterization of nanostructured bulk PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles followed by spark plasma sintering. An unusually large thermopower is found in both materials, and the possibility of energy filtering as opposed to grain boundary scattering as an explanation is discussed. A decreased Debye temperature and an increased molar specific heat are in accordance with recent predictions for nanostructured materials. On the basis of these results we propose suitable core-shell material combinations for future thermoelectric materials of large electric conductivities in combination with an increased thermopower by energy filtering.

Solution-liquid-solid synthesis of semiconductor nanowires using clusters as single-source precursors.

CdSe clusters react with liquid Bi- nanocatalysts to form one-dimensional CdSe nanowires in solution. The use of cluster precursors as compared to molecular precursors leads to a high local CdSe concentration, which favors the growth of nanowires with thin diameters and a narrow diameter distribution.

Wet-chemical synthesis of nanoscale iron boride, XAFS analysis and crystallisation to α-FeB.

The reaction of lithium tetrahydridoborate and iron bromide in high boiling ether as reaction medium produces an ultrafine, pyrophoric and magnetic precipitate. X-ray and electron diffraction proved the product to be amorphous. According to X-ray absorption fine structure spectroscopy (XAFS) the precipitate has FeB structure up to nearly two coordination spheres around an iron absorber atom. Transmission electron microscopy (TEM) confirms the ultrafine powder to be nanoscale. Subsequent annealing at 450 °C causes the atoms to arrange in a more distinct FeB structure, and further thermal treatment to 1050 °C extends the local structure to the α-modification of FeB. Between 1050 °C and 1500 °C α-FeB is transformed into ß-FeB.

Tuning size and sensing properties in colloidal gold nanostars.

Gold nanostars are multibranched nanoparticles with sharp tips, which display extremely interesting plasmonic properties but require optimization. We present a systematic investigation of the influence of different parameters on the size, morphology, and monodispersity of Au nanostars obtained via seeded growth in concentrated solutions of poly(vinylpyrrolidone) in N,N-dimethylformamide. Controlled prereduction of Au(3+) to Au(+) was found to influence monodispersity (narrower plasmon bands), while the [HAuCl(4)]/[seed] molar ratio significantly affects the morphology and tip plasmon resonance frequency. We also varied the size of the seeds (2-30 nm) and found a clear influence on the final nanostar dimensions as well as on the number of spikes, while synthesis temperature notably affects the morphology of the particles, with more rounded morphologies formed above 60 °C. This rounding effect allowed us to confirm the importance of sharp tips on the optical enhancing behavior of these nanoparticles in surface-enhanced raman scattering (SERS). Additionally, the sensitivity toward changes in the local refractive index was found to increase for larger nanostars, though lower figure of merit (FOM) values were obtained because of the larger polydispersity.

Ultrathin PbS sheets by two-dimensional oriented attachment.

Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, oriented attachment of nanocrystal building blocks, a self-assembly of particles into larger single-crystalline objects, is one of the most promising approaches in nanotechnology. We report here the two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment.

ZT enhancement in solution-grown Sb(2-x)BixTe3 nanoplatelets.

We report a solution-processed, ligand-supported synthesis of 15-20 nm thick Sb(2-x)BixTe3 nanoplatelets. After complete ligand removal by a facile NH3-based etching procedure, the platelets are spark plasma sintered to a p-type nanostructured bulk material with preserved crystal grain sizes. Due to this nanostructure, the total thermal conductivity is reduced by 60% in combination with a reduction in electric conductivity of as low as 20% as compared to the bulk material demonstrating the feasibility of the phonon-glass electron-crystal concept. An enhancement in the dimensionless thermoelectric figure of merit of up to 15% over state-of-the-art bulk materials is achieved, meanwhile, shifting the maximum to significantly higher temperatures.

Reversible attachment of platinum alloy nanoparticles to nonfunctionalized carbon nanotubes.

The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to nonfunctionalized single-walled and multi-walled carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. These ligands not only provide a way to tune the chemical composition, size, and shape of the nanoparticles but also control a strong reversible interaction with carbon nanotubes and permit controlling the nanoparticle coverage. Raman spectroscopy reveals that the sp(2) hybridization of the carbon lattice is not modified by the attachment. In order to better understand the interaction between the directly attached nanoparticles and the nonfunctionalized carbon nanotubes, we employed first-principles calculations on model systems of small Pt clusters and both zigzag and armchair single-walled carbon nanotubes. The detailed comprehension of such systems is of major importance since they find applications in catalysis and energy storage.