Showing papers by "Christopher B. Murray published in 2007"

Synergism in binary nanocrystal superlattices leads to enhanced p-type conductivity in self-assembled PbTe/Ag2Te thin films

Abstract: The ordered cocrystallization of nanoparticles into binary superlattices enables close contact of nanocrystals with distinct physical properties, providing a route to 'metamaterials' design. Here we present the first electronic measurements of multicomponent nanocrystal solids composed of PbTe and Ag(2)Te, demonstrating synergistic effects leading to enhanced p-type conductivity. First, syntheses of size-tuneable PbTe and Ag(2)Te nanocrystals are presented, along with deposition as thin-film nanocrystal solids, whose electronic transport properties are characterized. Next, assembly of PbTe and Ag(2)Te nanocrystals into AB binary nanocrystal superlattices is demonstrated. Furthermore, binary composites of varying PbTe-Ag(2)Te stoichiometry (1:1 and 5:1) are prepared and electronically characterized. These composites show strongly enhanced (conductance approximately 100-fold increased in 1:1 composites over the sum of individual conductances of single-component PbTe and Ag(2)Te films) p-type electronic conductivity. This observation, consistent with the role of Ag(2)Te as a p-type dopant in bulk PbTe, demonstrates that nanocrystals can behave as dopants in nanostructured assemblies.

Dipole-dipole interactions in nanoparticle superlattices.

Abstract: Nanoparticles often self-assemble into hexagonal-close-packed (hcp) structures although it is predicted to be less stable than face-centered-cubic (fcc) packing in hard-sphere models In addition to close-packed fcc and hcp superlattices, we observe formation of nonclose-packed simple-hexagonal (sh) superlattices of nearly spherical PbS, PbSe, and γ-Fe2O3 nanocrystals This surprisingly rich phase diagram of monodisperse semiconducting nanoparticles is explained by considering the interactions between nonlocal dipoles of individual nanoparticles By calculating the total electrostatic and dispersive energies, we explain stability of the hcp and sh nanoparticle superlattices, introduce the superlattice phase diagram, and predict antiferroelectric ordering in dipolar nanoparticle superlattices

Synthesis of Colloidal PbSe/PbS Core−Shell Nanowires and PbS/Au Nanowire−Nanocrystal Heterostructures

Abstract: The oriented attachment of PbSe nanocrystals results in single-crystalline colloidal PbSe nanowires. The addition of different surfactants allows tailoring nanowire morphology, choosing between straight, zigzag, helical, and branched nanowires. In this work, we studied the formation of coaxial PbSe/PbS core−shell heterostructures and observed Stranski−Krastanov growth regime of PbS shell on PbSe nanowires at low reaction temperature (150 °C) that switched to layer-by-layer epitaxial growth above 180 °C. The protection of PbSe nanowires with a PbS shell substantially improves the nanowire stability against oxidation. We also developed a technique for controllable decoration of colloidal PbSe nanowires with Au nanoparticles and found that the morphology of nanowire template had a strong effect on nucleation and growth of gold nanoparticles.

Alignment, Electronic Properties, Doping, and On-Chip Growth of Colloidal PbSe Nanowires

Abstract: Single-crystalline straight, zigzag, helical, and branched nanowires can be synthesized by oriented attachment of PbSe nanocrystals followed by their fusion in the presence of different surfactants. These colloidal nanowires can be aligned in external electrical fields, facilitating their integration into electronic circuits. We show how the nanowire morphology affects the interaction of nanowires with the electric field. Conducting PbSe nanowires can be assembled from nanocrystal building blocks directly on a chip, growing along the electric field. As-formed PbSe nanowires show a p-type conduction that can be switched to an n-type by charge-transfer doping of the nanowires with hydrazine. In this work, we demonstrate strategies for device integration of solution-phase synthesized semiconductor nanowires and explore their electronic properties.

Resonant energy transfer within a colloidal nanocrystal polymer host system

Abstract: The authors report on resonant energy transfer of nonequilibrium excitons in an amorphous polyfluorene donor CdSe∕ZnS core-shell nanocrystal acceptor system. By time-resolved photoluminescence (PL) spectroscopy, they have investigated the PL decay behavior of the primarily excited polyfluorene in dependence of the concentration of the nanocrystals. The authors are able to demonstrate that the finite rise time of the PL transient monitored at the spectral position of the acceptor PL directly reflects the PL decay of the donor.

Hybrid magnetic nanoparticles derived from wüstite disproportionation reactions at the nanoscale

Abstract: We present Mossbauer studies of hybrid iron-oxide nanoparticles obtained by the disproportionation of oleic acid stabilized wustite (FexO) nanoparticles produced by selective oxidation of iron pentacarbonyl in high boiling temperature organic solvents. The results support X-ray diffraction and Transmission Electron Microscope studies of the presence of mixed FexO and Fe3O4 phases within the nanoparticles whose relative content can be altered through heat treatment in nitrogen atmosphere. Furthermore, the Mossbauer study gives evidence of the presence of an amorphous, spin-glass like phase due to spin frustration at the FexO/Fe3O4 interface.

Method for producing nano-size particles from transition metals

Abstract: A method of forming nanoparticles includes steps of forming a metal precursor solution from a transition metal, injecting the metal precursor solution to the surfactant solution, adding a flocculent to cause nanoparticles to precipitate out of solution without permanent agglomeration, and adding a hydrocarbon solvent to redisperse or repeptize the nanoparticles.