-
[show abstract]
[hide abstract]
ABSTRACT: One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor-metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.
Angewandte Chemie International Edition 07/2010; 49(29):4878-97. · 13.45 Impact Factor
-
Advanced Materials 08/2008; 20(22):4312 - 4317. · 13.88 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Linear CdTe|CdSe|CdTe heterostructure nanorods are synthesized by using a colloidal sequential reactant injection technique [Shieh et al., J. Phys. Chem. B 2005, 109, 8538-8542]. The composition profiles of the individual nanorods are verified by using nanobeam elemental mapping by energy dispersive X-ray spectroscopy (EDS) and the photoluminescence emission spectra of the linear CdTe|CdSe|CdTe heterostructure nanorods are measured as a function of the temperature (down to 5 K). Photoluminescence is observed to occur from electron-hole recombination in both the CdSe core and across the heterojunction. Thermally activated trapping is found to influence both luminescence processes, thereby being more significant for the type II recombination across the CdSe|CdTe interface.
ChemPhysChem 07/2008; 9(8):1158-63. · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Visible light photocatalysis is a promising route for harnessing of solar energy to perform useful chemical reactions and to convert light to chemical energy. Nanoscale photocatalytic systems used to date were based mostly on oxide semiconductors aided by metal deposition and were operational only under UV illumination. Additionally, the degree of control over particle size and shape was limited. We report visible light photocatalysis using highly controlled hybrid gold-tipped CdSe nanorods (nanodumbbells). Under visible light irradiation, charge separation takes place between the semiconductor and metal parts of the hybrid particles. The charge-separated state was then utilized for direct photoreduction of a model acceptor molecule, methylene blue, or alternatively, retained for later use to perform the reduction reaction in the dark.
Nano Letters 03/2008; 8(2):637-41. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Colloidal metal/semiconductor hybrid nanoparticles, containing a Group I-VII semiconductor (AgBr) coupled with a noble metal (Ag), are synthesized in an organic solvent using a single-phase, room-temperature method. Methanol is added to a precursor solution containing silver ions complexed with an alkylammonium bromide surfactant. The methanol appears to cause AgBr precipitation while also acting as a reducing agent for silver ions in solution; the reduced silver clusters grow preferentially onto the AgBr, forming dimer particles that are coated with an alkanethiol to passivate the surface. The structure and composition of the hybrid nanocrystals are studied using TEM and XRD. Surprisingly, despite the relatively broad size distribution of the colloids, the ratio between the diameters of the two domains is approximately constant. This suggests that both portions of the dimer nanocrystal nucleate and grow simultaneously, representing a new type of mechanism for the formation of hybrid nanoparticles.
Zeitschrift für anorganische Chemie 10/2007; 633(13‐14):2414 - 2419.
-
Nano Letters. 01/2007; 7(2):541-541.
-
Nano Letters. 01/2007; 7(2):541-541.
-
[show abstract]
[hide abstract]
ABSTRACT: We explore the growth mechanism of gold nanocrystals onto preformed cadmium sulfide nanorods to form hybrid metal nanocrystal/semiconductor nanorod colloids. By manipulating the growth conditions, it is possible to obtain nanostructures exhibiting Au nanocrystal growth at only one nanorod tip, at both tips, or at multiple locations along the nanorod surface. Under anaerobic conditions, Au growth occurs only at one tip of the nanorods, producing asymmetric structures. In contrast, the presence of oxygen and trace amounts of water during the reaction promotes etching of the nanorod surface, providing additional sites for metal deposition. Three growth stages are observed when Au growth is performed under air: (1) Au nanocrystal formation at both nanorod tips, (2) growth onto defect sites on the nanorod surface, and finally (3) a ripening process in which one nanocrystal tip grows at the expense of the other particles present on the nanorod. Analysis of the hybrid nanostructures by high-resolution TEM shows that there is no preferred orientation between the Au nanocrystal and the CdS nanorod, indicating that growth is nonepitaxial. The optical signatures of the nanocrystals and the nanorods (i.e., the surface plasmon and first exciton transition peaks, respectively) are spectrally distinct, allowing the different stages of the growth process to be easily monitored. The initial CdS nanorods exhibit band gap and trap state emission, both of which are quenched during Au growth.
The Journal of Physical Chemistry B 01/2007; 110(50):25421-9. · 3.70 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: The self-assembly of sterically stabilized colloidal copper sulfide nanodisks, 14-20 nm in diameter and 5-7 nm thick, was studied. The nanodisks were observed by electron microscopy and small-angle X-ray scattering to form columnar arrays when evaporated as thin films from concentrated dispersions. These superstructured nanomaterials might give rise to technologically useful properties, such as anisotropic electrical transport and electrorheological and optical properties.
Nano Letters 01/2007; 6(12):2959-63. · 13.20 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Porous polyethylene oxide-b-polyfluorooctylmethacrylate (PEO-b-PFOMA) diblock copolymer films were drop cast onto substrates from Freon (1,1,2-trichlorotrifluoroethane) in a humid atmosphere. The pores in the films exhibit long range hexagonal order in some cases, depending on the PFOMA-to-PEO molecular weight ratio. Films with the best ordered pores were formed with PFOMA-to-PEO ratios of 70 kDa:2 kDa. The pores in the polymer films derive from water droplets that condense as Freon evaporates. The polymer stabilizes the water droplets, or "breath figures," which act as an immiscible template that molds the porous film. Increased polymer hydrophobicity reduces the water wettability of the air/Freon interface, which in turn decreases water droplet nucleation, thus influencing the final pore size and spatial order in the polymer films. We describe how water droplet nucleation influences the final pore size and packing order in the polymer films.
Physical Review E 04/2006; 73(3 Pt 1):031608. · 2.26 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Gold nanocrystal dispersions in toluene−CO2 mixtures were infused into cylindrical pores in mesoporous silica to achieve high loadings over 2 wt % in 24 h. The nanocrystals were highly dispersed according to transmission electron microscopy, and the loadings approached equilibrium. In contrast, the loadings were small for infusion with pure toluene or toluene mixed with an antisolvent, methanol. The differences in loading were correlated with the long-ranged van der Waals forces between gold and silica through the intervening solvent. These van der Waals forces became stronger as CO2 was added to toluene, as a consequence of a reduction in the Hamaker constant of the mixed intervening solvent, resulting in stronger nanocrystal adsorption. The decoupling of the nanocrystal synthesis step and the infusion step leads to exquisite control of the nanocrystal size, morphology, and dispersibility within the pores. The simplicity of the method allows for the facile production of nanocrystal/silica composites for applications such as catalysis and optoelectronics.
11/2005;
-
[show abstract]
[hide abstract]
ABSTRACT: Semiconductor nanocrystal quantum dots (qdots) are now being explored in applications requiring active cellular interfaces, such as biosensing and therapeutics in which information is passed from the qdot to the biological system, or vice versa, to perform a function. These applications may require surface coating chemistry that is different from what is commonly employed for passive interface applications like labeling (i.e., thick polymer coatings such as poly(ethylene glycol) (PEG)), in which the only concern is nonspecific sticking to cells and biocompatibility. The thick insulating coatings that are generally needed for labeling are generally not suitable for active qdot-cell interface applications. There is currently little data regarding the interactions between viable cells and qdots under physiological conditions. Our initial investigations using mercaptoacetic acid-coated CdS and CdTe qdots as a simple model to interface with neuron cell surface receptors under physiological conditions uncovered two significant technological hurdles: nonspecific binding and endocytosis. Nonspecific binding can be extensive and in general there appears to be greater nonspecific binding for larger particle sizes, but this also depends sensitively on the particle surface characteristics and the type of neuron, possibly indicating a detailed relationship between particle-cell affinity and cell membrane chemistry. More importantly, qdot endocytosis occurs rapidly at physiological temperature for the different nerve cell types studied, within the first five minutes of exposure to both CdS and CdTe qdots, regardless of whether the molecular coatings specifically recognize cell surface receptors or not. As a consequence, new strategies for tagging cell surface recognition groups for long-term active interfacing with cells under physiological conditions are needed, which requires more sophisticated ligands than MAA but also the absence of thick insulating coatings.
Talanta 10/2005; 67(3):462-71. · 3.79 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: We report a general synthetic method for the formation of shape-controlled CdS, CdSe and CdTe nanocrystals and mixed-semiconductor heterostructures. The crystal growth kinetics can be manipulated by changing the injection rate of the chalcogen precursor, allowing the particle shape-spherical or rodlike-to be tuned without changing the underlying chemistry. A single injection of precursor leads to isotropic spherical growth, whereas multiple injections promote epitaxial growth along the length of the c-axis. This method was extended to produce linear type I and type II semiconductor nanocrystal heterostructures.
The Journal of Physical Chemistry B 06/2005; 109(18):8538-42. · 3.70 Impact Factor
-
ChemPhysChem 02/2005; 6(1):61-5. · 3.41 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Small-angle X-ray scattering (SAXS) was used to study the size-dependent two-body solvent-mediated interparticle interactions between dodecanethiol-coated gold nanocrystals (2−6 nm in diameter) in dilute toluene dispersions. Using a modified Zimm analysis of concentration-dependent X-ray scattering data, the second virial coefficient B2 was measured as a function of nanocrystal diameter and compared to theoretical predictions. The measured values of B2 are more negative than those expected for hard spheres, indicating that interparticle attractions are significant in this system, even though the particles are dispersed in good solvents for the ligands. The data can be fit using a square well potential to model the pair interactions with nanocrystal size-dependent well depths ranging between 0.1 and 0.4 kT and a range of interaction of 30 Å. The interaction potentials between particles in the larger size range (i.e., >5 nm diameter) are close to those expected from a simple steric stabilization model accounting for the core−core van der Waals attraction modified by an osmotic repulsion between adsorbed chains. Smaller particles, however, exhibited significantly stronger attraction than expected from this simple model, which could possibly be due to decreased ligand surface coverage at the smaller nanocrystal sizes.
10/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Gold nanocrystals, 5 nm in diameter, were synthesized with coatings of thiolated perfluoropolyether (PFPE) ligands. These nanocrystals are dispersed in supercritical carbon dioxide (scCO2). Because of the high CO2 compressibility, the particle dispersibility is a strong function of solvent density, with higher interparticle attraction and increased aggregation at lower density. Using small-angle X-ray scattering (SAXS), the interparticle interactions between PFPE-coated nanocrystals in scCO2 were measured as a function of pressure (from 310 to 120 bar) under isothermal conditions (35 °C). A small peak, which increases in intensity with decreasing pressure, appears at low q in the scattering curve because of interparticle clustering. A model that approximates the nanocrystal interactions with an attractive square-well potential and accounts for the scattering contribution from clustering particles provides excellent agreement with the data. For 5.25-nm-diameter gold nanocrystals dispersed in scCO2 at 35 °C, the square-well potential minimum decreases from −0.6kT to −1.3kT as the pressure is lowered from 310 to 120 bar.
09/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: We describe the single-step self-organization of nanocrystal superlattice films infused with spatially ordered arrays of micrometer-size pores. In a humid atmosphere, water droplets condense on the surface of evaporating thin-film solutions of nanocrystals. Nanocrystals coated with the appropriate ligands stabilize the water droplets, allowing them to grow to uniform size and ultimately pack into very ordered arrays. The droplets provide a temporary template that casts an ordered macroporous nanocrystal film. This process could serve as a reliable bottom-up self-assembly approach for fabricating two-dimensional waveguides with tunable optical properties for single-chip integration of photonic and electronic technologies.
09/2004;
-
[show abstract]
[hide abstract]
ABSTRACT: Thin films of dodecanethiol-passivated Au and Ag nanocrystals drop cast from different solvents were examined by high-resolution scanning electron microscopy (HRSEM). C12-coated Au and Ag nanocrystals, 5-7 nm in diameter, form face-centered cubic (fcc) superlattices oriented with the (111)s planes (subscript s denoting superlattice) parallel to the substrate when deposited from good solvents, such as hexane, chloroform, and toluene. The gross morphology of the films depended on the solvent: hexane produced rough superlattice films whereas chloroform deposited smooth films. The difference in interparticle attraction, which is approximately 20% higher in hexane, appears to give rise to the difference in film morphology. Addition of a poor solvent to the dispersion prior to drop casting led to superlattices with decreased order. Although the superlattices always orient with (111)s as the basal plane on the substrate, superlattices deposited from chloroform grow preferentially in the [110]s direction, whereas hexane deposits superlattices that grow primarily in the [111]s direction.
Langmuir 03/2004; 20(3):978-83. · 4.19 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Cu(2)S nanocrystals with disklike morphologies were synthesized by the solventless thermolysis of a copper alkylthiolate molecular precursor. The nanodisks ranged from circular to hexagonal prisms from 3 to 150 nm in diameter and 3 to 12 nm in thickness depending on the growth conditions. High resolution transmission electron microscopy (HRTEM) revealed the high chalcocite (hexagonal) crystal structure oriented with the c-axis ([001] direction) orthogonal to the favored growth direction. This disk morphology is thermodynamically favored as it allows the extension of the higher energy [100] and [110] surfaces with respect to the [001] planes. The hexagonal prism morphology also appears to relate to increased C-S bond cleavage of adsorbed dodecanethiol along the more energetic [100] facets relative to [001] facets. Monodisperse Cu(2)S nanodisks self-assemble into ribbons of stacked platelets. This solventless approach provides a new technique to synthesize anisotropic metal chalcogenide nanostructures with shapes that depend on both the face-sensitive thermodynamic surface energy and the surface reactivity.
Journal of the American Chemical Society 01/2004; 125(51):16050-7. · 9.91 Impact Factor
-
[show abstract]
[hide abstract]
ABSTRACT: Small-angle X-ray scattering (SAXS) was used to measure the time-dependent evolution of the size and size distribution of tetraoctylammonium-stabilized gold nanocrystals formed by arrested precipitation. The initial growth mechanism was varied by controlling the addition of reducing agent to the ionic gold precursor solution. Slow addition gives high Au0 monomer supersaturation and provides discrete nucleation and growth events and a narrowing of the particle size distribution over short reaction times. Rapid addition of reducing agent leads to broad initial particle size distributions that begin to narrow only after the first few hours of the reaction. After 24 h, the nanocrystals grow to 52 Å in diameter with a very narrow size distribution regardless of the initial growth kinetics. Analysis of the reaction and transport rates reveal that nanocrystal growth is limited by the surface reaction rate of monomer addition to the crystal surface. The final size appears to be thermodynamically metastable, with the system free energy, which includes the gold surface tension and the tetraoctylammonium binding energy, reaching a local minimum. Nanocrystal size is also found to vary inversely with the binding strength of the capping ligand present during growth, allowing the mean particle diameter to be tuned to between 1.7 and 5.2 nm.
12/2003;
