Showing papers by "Christopher B. Murray published in 2012"

Nonaqueous Synthesis of TiO2 Nanocrystals Using TiF4 to Engineer Morphology, Oxygen Vacancy Concentration, and Photocatalytic Activity

Abstract: Control over faceting in nanocrystals (NCs) is pivotal for many applications, but most notably when investigating catalytic reactions which occur on the surfaces of nanostructures. Anatase titanium dioxide (TiO2) is one of the most studied photocatalysts, but the shape dependence of its activity has not yet been satisfactorily investigated and many questions still remain unanswered. We report the nonaqueous surfactant-assisted synthesis of highly uniform anatase TiO2 NCs with tailorable morphology in the 10–100 nm size regime, prepared through a seeded growth technique. Introduction of titanium(IV) fluoride (TiF4) preferentially exposes the {001} facet of anatase through in situ release of hydrofluoric acid (HF), allowing for the formation of uniform anatase NCs based on the truncated tetragonal bipyramidal geometry. A method is described to engineer the percentage of {001} and {101} facets through the choice of cosurfactant and titanium precursor. X-ray diffraction studies are performed in conjunction wi...

Improved Size-Tunable Synthesis of Monodisperse Gold Nanorods through the Use of Aromatic Additives

Abstract: We report an improved synthesis of colloidal gold nanorods (NRs) by using aromatic additives that reduce the concentration of hexadecyltrimethylammonium bromide surfactant to ∼0.05 M as opposed to 0.1 M in well-established protocols. The method optimizes the synthesis for each of the 11 additives studied, allowing a rich array of monodisperse gold NRs with longitudinal surface plasmon resonance tunable from 627 to 1246 nm to be generated. The gold NRs form large-area ordered assemblies upon slow evaporation of NR solution, exhibiting liquid crystalline ordering and several distinct local packing motifs that are dependent upon the NR’s aspect ratio. Tailored synthesis of gold NRs with simultaneous improvements in monodispersity and dimensional tunability through rational introduction of additives will not only help to better understand the mechanism of seed-mediated growth of gold NRs but also advance the research on plasmonic metamaterials incorporating anisotropic metal nanostructures.

Bandlike transport in strongly coupled and doped quantum dot solids: a route to high-performance thin-film electronics.

Abstract: We report bandlike transport in solution-deposited, CdSe QD thin-films with room temperature field-effect mobilities for electrons of 27 cm2/(V s). A concomitant shift and broadening in the QD solid optical absorption compared to that of dispersed samples is consistent with electron delocalization and measured electron mobilities. Annealing indium contacts allows for thermal diffusion and doping of the QD thin-films, shifting the Fermi energy, filling traps, and providing access to the bands. Temperature-dependent measurements show bandlike transport to 220 K on a SiO2 gate insulator that is extended to 140 K by reducing the interface trap density using an Al2O3/SiO2 gate insulator. The use of compact ligands and doping provides a pathway to high performance, solution-deposited QD electronics and optoelectronics.

Metal-Enhanced Upconversion Luminescence Tunable through Metal Nanoparticle–Nanophosphor Separation

Abstract: We have demonstrated amplification of luminescence in upconversion nanophosphors (UCNPs) of hexagonal phase NaYF4 (β-NaYF4) doped with the lanthanide dopants Yb3+, Er3+ or Yb3+, Tm3+ by close proximity to metal nanoparticles (NPs). We present a configuration in which close-packed monolayers of UCNPs are separated from a dense multilayer of metal NPs (Au or Ag) by a nanometer-scale oxide grown by atomic layer deposition. Luminescence enhancements were found to be dependent on the thickness of the oxide spacer layer and the type of metal NP with enhancements of up to 5.2-fold proximal to Au NPs and of up to 45-fold proximal to Ag NPs. Concomitant shortening of the UCNP luminescence decay time and rise time is indicative of the enhancement of the UCNP luminescence induced by resonant plasmonic coupling and nonresonant near-field enhancement from the metal NP layer, respectively.

Synthesis, Shape Control, and Methanol Electro-oxidation Properties of Pt–Zn Alloy and Pt3Zn Intermetallic Nanocrystals

Abstract: We report the first synthesis of highly monodisperse Pt(3)Zn nanocrystals (NCs). Shape-controlled synthesis generates cubic and spherical Pt-Zn NCs. Reaction temperature is the key to incorporate Zn into Pt, even in the absence of a strong reducing agent. The Pt-Zn NCs are active toward methanol oxidation, with the spherical NCs exhibiting higher activity than the cubic NCs. The Pt-Zn alloy phase can be transformed into the Pt(3)Zn intermetallic phase, upon annealing. The intermetallic Pt(3)Zn shows better performance than the alloy phase Pt-Zn. Besides the activity toward methanol oxidation, Pt-Zn NCs show excellent poisoning tolerance. With activities comparable to the commercial Pt catalyst, enhanced poisoning tolerance and lower cost, Pt-Zn and Pt(3)Zn NCs are a promising new family of catalysts for direct methanol fuel cells.

The state of nanoparticle-based nanoscience and biotechnology: Progress, promises, and challenges

Abstract: Colloidal nanoparticles (NPs) have become versatile building blocks in a wide variety of fields. Here, we discuss the state-of-the-art, current hot topics, and future directions based on the following aspects: narrow size-distribution NPs can exhibit protein-like properties; monodispersity of NPs is not always required; assembled NPs can exhibit collective behavior; NPs can be assembled one by one; there is more to be connected with NPs; NPs can be designed to be smart; surface-modified NPs can directly reach the cytosols of living cells.

Flexible and low-voltage integrated circuits constructed from high-performance nanocrystal transistors

Abstract: Colloidal semiconductor nanocrystals are emerging as a new class of solution-processable materials for low-cost, flexible, thin-film electronics. Although these colloidal inks have been shown to form single, thin-film field-effect transistors with impressive characteristics, the use of multiple high-performance nanocrystal field-effect transistors in large-area integrated circuits has not been shown. This is needed to understand and demonstrate the applicability of these discrete nanocrystal field-effect transistors for advanced electronic technologies. Here we report solution-deposited nanocrystal integrated circuits, showing nanocrystal integrated circuit inverters, amplifiers and ring oscillators, constructed from high-performance, low-voltage, low-hysteresis CdSe nanocrystal field-effect transistors with electron mobilities of up to 22 cm(2) V(-1) s(-1), current modulation >10(6) and subthreshold swing of 0.28 V dec(-1). We fabricated the nanocrystal field-effect transistors and nanocrystal integrated circuits from colloidal inks on flexible plastic substrates and scaled the devices to operate at low voltages. We demonstrate that colloidal nanocrystal field-effect transistors can be used as building blocks to construct complex integrated circuits, promising a viable material for low-cost, flexible, large-area electronics.

Highly active Pt3Pb and core-shell Pt3Pb-Pt electrocatalysts for formic acid oxidation.

Abstract: Formic acid is a promising chemical fuel for fuel cell applications. However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of direct formic acid fuel cells has been impeded by the low activity of existing electrocatalysts at desirable operating voltage. We report the first synthesis of Pt3Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electrocatalysts. The activity can be further improved by manipulating the Pt3Pb–Pt core–shell structure. Combined experimental and theoretical studies suggest that the high activity from Pt3Pb and the Pt–Pb core–shell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps. Therefore, the Pt3Pb and Pt–Pb core–shell nanocrystals can improve the performance of direct formic acid fuel cells at desired operating voltage to enable their practical application.

Dendritic upconverting nanoparticles enable in vivo multiphoton microscopy with low-power continuous wave sources

Abstract: We report a group of optical imaging probes, comprising upconverting lanthanide nanoparticles (UCNPs) and polyanionic dendrimers. Dendrimers with rigid cores and multiple carboxylate groups at the periphery are able to tightly bind to surfaces of UCNPs pretreated with NOBF4, yielding stable, water-soluble, biocompatible nanomaterials. Unlike conventional linear polymers, dendrimers adhere to UCNPs by donating only a fraction of their peripheral groups to the UCNP–surface interactions. The remaining termini make up an interface between the nanoparticle and the aqueous phase, enhancing solubility and offering multiple possibilities for subsequent modification. Using optical probes as dendrimer cores makes it possible to couple the UCNPs signal to analyte-sensitive detection via UCNP-to-chromophore excitation energy transfer (EET). As an example, we demonstrate that UCNPs modified with porphyrin–dendrimers can operate as upconverting ratiometric pH nanosensors. Dendritic UCNPs possess excellent photostability, solubility, and biocompatibility, which make them directly suitable for in vivo imaging. Polyglutamic dendritic UCNPs injected in the blood of a mouse allowed mapping of the cortical vasculature down to 400 μm under the tissue surface, thus demonstrating feasibility of in vivo high-resolution two-photon microscopy with continuous wave (CW) excitation sources. Dendrimerization as a method of solubilization of UCNPs opens up numerous possibilities for use of these unique agents in biological imaging and sensing.

Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots.

Abstract: We examine the population dynamics of multiple excitons in PbS quantum dots using spectrally resolved ultrafast supercontinuum transient absorption (SC-TA) measurements. We simultaneously probe the first three excitonic transitions. The transient spectra show the presence of bleaching of absorption for the 1Sh–1Se transition, as well as transients associated with the 1Ph–1Pe transition. We examine signatures of carrier multiplication (multiple excitons arising from a single absorbed photon) from analysis of the bleaching features in the limit of low absorbed photon numbers (⟨Nabs⟩ ∼ 10–2) for pump photon energies from two to four times that of the band gap. The efficiency of multiple-exciton generation is discussed both in terms of the ratio between early- to long-time transient absorption signals and of a broadband global fit to the data. Analysis of the population dynamics shows that bleaching associated with biexciton population is red shifted with respect to the single exciton feature, which is in acc...

Magnetic anisotropy considerations in magnetic force microscopy studies of single superparamagnetic nanoparticles.

Abstract: In recent years, superparamagnetic nanoparticles (SPNs) have become increasingly important in applications ranging from solid state memory devices to biomedical diagnostic and therapeutic tools. However, detection and characterization of the small and unstable magnetic moment of an SPN at the single particle level remains a challenge. Further, depending on their physical shape, crystalline structure or orientation, SPNs may also possess magnetic anisotropy, which can govern the extent to which their magnetic moments can align with an externally applied magnetic field. Here, we demonstrate how we can exploit the magnetic anisotropy of SPNs to enable uniform, highly-sensitive detection of single SPNs using magnetic force microscopy (MFM) in ambient air. Superconducting quantum interference device magnetometry and analytical transmission electron microscopy techniques are utilized to characterize the collective magnetic behavior, morphology and composition of the SPNs. Our results show how the consideration of magnetic anisotropy can enhance the ability of MFM to detect single SPNs at ambient room temperature with high force sensitivity and spatial resolution.

Functionalizing molecular wires: a tunable class of α,ω-diphenyl-μ,ν-dicyano-oligoenes

Abstract: We describe the synthesis and characterization of a new class of cyano-functionalized oligoenes and their derivatives. We have made the vinylogous series of α,ω-diphenyl-μ,ν-dicyano-oligoenes (DPDCn) comprised of each odd-numbered member from 3 to 13 linear conjugated olefins. Installing cyano groups onto the oligoene backbone lowers HOMO and LUMO energies by up to ∼0.7 eV, thereby stabilizing the molecule with respect to oxidative decomposition; this exemplifies a new approach to the stabilization of conjugated oligoenes. UV-vis absorption spectra and redox potentials across the DPDCn series reveal that the molecular band gap ranges from 2.80 to 1.75 eV. This gap can be further tuned by the facile installation of a variety of aryl end-groups. The choice of end-groups also greatly affects the physical properties such as solubility and the solid-state packing. We also present the longest oligoene crystal structure reported to date. Moreover, we find that the prototypical linear structure makes oligoenes suitable as molecular wires and connectors in the bottom-up construction of nanoscale architectures. As a proof of concept, carboxylic acid terminated oligoenes were used to position 10-nm Fe3O4 nanoparticles on a GaAs (100) substrate.

Coating Evaluation and Purification of Monodisperse, Water-Soluble, Magnetic Nanoparticles Using Sucrose Density Gradient Ultracentrifugation.

Abstract: Magnetic nanoparticles have numerous applications in biomedicine and biomedical research.1 The bioavailability of these nanomedicines is a complex function of several physicochemical and material properties, and, as such, requires tight control of various chemical and physical nanoparticle properties that include size, shape, flexibility, surface charge, and surface chemistry.2 The most widely used magnetic nanoparticles contain iron oxide nanocrystals because the toxicity, metabolism, and pharmacokinetics of intravenously injected iron oxide based nanoparticles have been extensively documented.3 However, increasing in vivo sensitivity and efficiency requires nanocrystals with specific compositions. Thus, recent research has focused on modifying the magnetic properties of iron oxide nanocrystals by doping with transition metal ions like cobalt, manganese, nickel, and zinc.4, 5

Highly Active Pt 3 Pb and CoreShell Pt 3 PbPt Electrocatalysts for Formic Acid Oxidation

Abstract: Formic acid is a promising chemical fuel for fuel cell applications However, due to the dominance of the indirect reaction pathway and strong poisoning effects, the development of directformicacidfuelcellshasbeenimpededbythelowactivityofexistingelectrocatalysts at desirable operating voltage We report the first synthesis of Pt3Pb nanocrystals through solution phase synthesis and show they are highly efficient formic acid oxidation electro- catalysts The activity can be further improved by manipulating the Pt3PbPt coreshell structure Combined experimental and theoretical studies suggest that the high activity from Pt3Pb and the PtPb coreshell nanocrystals results from the elimination of CO poisoning and decreased barriers for the dehydrogenation steps Therefore, the Pt3Pb and PtPb coreshellnanocrystalscanimprovetheperformanceofdirectformicacidfuelcellsatdesired operating voltage to enable their practical application

Ultrafast supercontinuum spectroscopy of multiple exciton states in lead chalcogenide nanorods and nanocrystals

Abstract: We examine the multiple exciton population dynamics in lead chalcogenide nanostructures by ultrafast supercontinuum transient absorption. Carrier multiplication is revealed in the limit of low absorbed photon number, along with biexciton dynamics.

Liquid Cell in-situ Electron Microscopy: Interfacial Phenomena and Electrochemical Deposition

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.

TEM Analysis of Fe3O4/GaAs Hybrid Ferromagnet/Semiconductor Nanostructures

Abstract: Extended abstract of a paper presented at Microscopy and Microanalysis 2012 in Phoenix, Arizona, USA, July 29 – August 2, 2012.