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Elena V Shevchenko

Publications and source records attributed to Elena V Shevchenko.

10 recordsLinked to original sources
PubMed2006

Structural characterization of self-assembled multifunctional binary nanoparticle superlattices.

Nanocrystals of different size and functionality (e.g., noble metals, semiconductors, oxides, magnetic alloys) can be induced to self-assemble into ordered binary superlattices (also known as opals or colloidal crystals), retaining the size tunable properties of their constituents. We have built a variety of binary superlattices from monodisperse PbS, PbSe, CoPt3, Fe2O3, Au, Ag, and Pd nanocrystals, mixing and matching these nanoscale building blocks to yield multifunctional nanocomposites (metamaterials). Superlattices with AB, AB2, AB3, AB4, AB5, AB6, and AB13 stoichiometry with cubic, hexagonal, tetragonal, and orthorhombic symmetries have been identified. Assemblies with the same stoichiometry can be produced in several polymorphous forms by tailoring the particle size and deposition conditions. We have identified arrays isostructural with NaCl, CuAu, AlB2, MgZn2, MgNi2, Cu3Au, Fe4C, CaCu5, CaB6, NaZn13, and cub-AB13 compounds emphasizing the parallels between nanoparticle assembly and atomic scale crystal growth and providing confidence that many more structures will follow. Recently, we have demonstrated that electrical charges on sterically stabilized nanoparticles in addition to such parameters as particle size ratio and their concentrations can provide the formation of a much broader pallet of binary nanoparticle superlattices as compared with the limited number of possible superlattices formed by hard noninteracting spheres. In this contribution, we demonstrate a large variety of different binary superlattices, provide their detailed structural characterization, and discuss the role of energetic and kinetic factors in the cocrystallization process. We found that Coulomb, van der Waals, charge-dipole, dipole-dipole, and other interactions can contribute equally to cocrystallization, allowing superlattice formation to be dependent on a number of tunable parameters. We present binary superlattices as a new class of materials with a potentially unlimited library of constituents over a wide range of tunable structures.

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PubMed2006

Self-assembly of PbTe quantum dots into nanocrystal superlattices and glassy films.

Monodisperse lead telluride (PbTe) nanocrystals ranging from approximately 4 to 10 nm in diameter are synthesized to provide quantum dot building blocks for the design of novel materials for electronic applications. Two complementary synthetic approaches are developed that enable either (1) isolation of small quantities of nanocrystals of many different sizes or (2) the production of up to 10 g of a single nanocrystal size. PbTe nanocrystals are characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), and optical absorption. Assembly of PbTe nanocrystals is directed to prepare nanocrystal solids that display either short-range (glassy solids) or long-range (superlattices) packing order by varying deposition conditions. Film order and average interparticle spacing are analyzed with grazing-incidence small-angle X-ray scattering (GISAXS) and high-resolution scanning electron microscopy (HRSEM). We perform the first optical and electronic studies of PbTe solids and demonstrate that chemical activation of these films enhances conductivity by approximately 9-10 orders of magnitude while preserving their quantum dot nature.

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PubMed2006

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.

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PubMed2005

Polymorphism in AB(13) nanoparticle superlattices: an example of semiconductor-metal metamaterials.

Colloidal crystallization of nanoparticles with different functionalities into multicomponent assemblies provides a route to new classes of ordered nanocomposites with novel properties tunable by the choice of the constituent building blocks. While theories based on hard sphere approximation predict crystallization of only a few stable binary phases (NaCl-, AlB(2)- and NaZn(13)-type), we find that additional polymorphs of lower packing density are possible. We demonstrate that PbSe and Pd nanoparticles can be reproducibly crystallized into two polymorphs with AB(13) stoichiometry. One polymorph is isostructural with the intermetallic compound NaZn(13) and is consistent with dense packing of hard spheres driven by entropy. The second unanticipated polymorph is of lower packing density. This observation underscores the shortcomings of applying simple space-filling principles to the crystallization of organically passivated nanocrystals and further motivates the development of models that incorporate combinations of hard-sphere, van der Waals, dipolar, and hydrophobic forces. This work demonstrates that ordered periodic structures with lower packing density are achievable and provides the first example of a binary semiconductor-metal superlattice using a combination of PbSe-Pd nanocrystals.

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PubMed2005

Determination of nanocrystal sizes: a comparison of TEM, SAXS, and XRD studies of highly monodisperse CoPt3 particles.

One of the most fundamental tasks in nanoscience is the accurate determination of particle sizes. Various methods have been developed to elucidate the mean particle diameter and the standard deviation for an ensemble of nanocrystals. However, good agreement between the results from different methods is not always encountered in the literature. In this study, we investigate colloidally prepared, highly monodisperse CoPt3 nanoparticles by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), and powder X-ray diffraction (XRD). The results are compared in order to examine to which extent agreement is obtained by the different techniques when applied to small nanocrystals in the size range below 10 nm. In particular, the applicability of the simple Scherrer formula for size determination from the broadening of XRD reflections is checked. When the different techniques are correctly applied, the results from all methods are in good agreement.

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PubMed2004

CdSe and CdSe/CdS nanorod solids.

We demonstrate the self-organization of CdSe nanorods into nematic, smectic, and crystalline solids. Layered colloidal crystals of CdSe nanorods grow by slow destabilization of a nanocrystal solution upon allowing the diffusion of a nonsolvent into the colloidal solution of nanocrystals. The colloidal crystals of nanorods show characteristic birefringence, which we assign to specific spherulite-like texture of each nanorod assembly. To demonstrate the general character of nanorod self-assembly technique, CdSe/CdS heterostructure nanorods were organized into highly luminescent superlattices.

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PubMed2003

Study of nucleation and growth in the organometallic synthesis of magnetic alloy nanocrystals: the role of nucleation rate in size control of CoPt3 nanocrystals.

High quality CoPt(3) nanocrystals were synthesized via simultaneous reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid and hexadecylamine as stabilizing agents. The high flexibility and reproducibility of the synthesis allows us to consider CoPt(3) nanocrystals as a model system for the hot organometallic synthesis of metal nanoparticles. Different experimental conditions (reaction temperature, concentration of stabilizing agents, ratio between cobalt and platinum precursors, etc.) have been investigated to reveal the processes governing the formation of the metal alloy nanocrystals. It was found that CoPt(3) nanocrystals nucleate and grow up to their final size at an early stage of the synthesis with no Ostwald ripening observed upon further heating. In this case, the nanocrystal size can be controlled only via proper balance between the rates for nucleation and for growth from the molecular precursors. Thus, the size of CoPt(3) nanocrystals can be precisely tuned from approximately 3 nm up to approximately 18 nm in a predictable and reproducible way. The mechanism of homogeneous nucleation, evolution of the nanocrystal ensemble in the absence of Ostwald ripening, nanocrystal faceting, and size-dependent magnetic properties are investigated and discussed on the example of CoPt(3) magnetic alloy nanocrystals. The developed approach was found to be applicable to other systems, e.g., FePt and CoPd(2) magnetic alloy nanocrystals.

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PubMed2002

Colloidal synthesis and self-assembly of CoPt(3) nanocrystals.

Reduction of platinum acetylacetonate and thermodecomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic acid were employed in different coordinating mixtures to produce monodisperse, highly crystalline CoPt(3) nanoparticles. The mean particle size can be varied from 1.5 to 7.2 nm by controlling the reaction conditions and the type of coordinating mixture. As-synthesized CoPt(3) particles represent single crystal domains and have chemically disordered face-centered cubic (fcc) structure. Nearly spherical CoPt(3) nanocrystals were found to assemble into two- (2D) and three-dimensional (3D) structures. An AB(5) type superlattice is observed by TEM after mixing two nanoparticle samples with different mean sizes. Slow precipitation led to the formation of facetted colloidal crystals with sizes up to 20 microm.

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PubMed2002

Dynamic distribution of growth rates within the ensembles of colloidal II-VI and III-V semiconductor nanocrystals as a factor governing their photoluminescence efficiency.

The distribution of properties within ensembles of colloidally grown II-VI and III-V semiconductor nanocrystals was studied. A drastic difference in the photoluminescence efficiencies of size-selected fractions was observed for both organometallically prepared CdSe and InAs colloids and for CdTe nanocrystals synthesized in aqueous medium, indicating a general character of the phenomenon observed. The difference in the photoluminescence efficiencies is attributed to different averaged surface disorder of the nanocrystals originating from the Ostwald ripening growth mechanism when larger particles in the ensemble grow at the expense of dissolving smaller particles. At any stage of growth, only a fraction of particles within the ensemble of growing colloidal nanocrystals has the most perfect surface and, thus, shows the most efficient photoluminescence. This is explained by a theoretical model describing the evolution of an ensemble of nanocrystals in a colloidal solution. In an ensemble of growing nanocrystals, the fraction of particles with the highest photoluminescence corresponds to the particle size having nearly zero average growth rate. The small average growth rate leads to the lowest possible degree of surface disorder at any given reaction conditions.

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