Probing Compositional Variation within Hybrid Nanostructures

We present a detailed analysis of the structural and magnetic properties of solution-grown PtCo-CdS hybrid structures in comparison to similar free-standing PtCo alloy nanoparticles. X-ray absorption spectroscopy is utilized as a sensitive probe for identifying subtle differences in the structure of the hybrid materials. We found that the growth of bimetallic tips on a CdS nanorod substrate leads to a more complex nanoparticle structure composed of a PtCo alloy core and thin CoO shell. The core-shell architecture is an unexpected consequence of the different nanoparticle growth mechanism on the nanorod tip, as compared to free growth in solution. Magnetic measurements indicate that the PtCo-CdS hybrid structures are superparamagnetic despite the presence of a CoO shell. The use of X-ray spectroscopic techniques to detect minute differences in atomic structure and bonding in complex nanosystems makes it possible to better understand and predict catalytic or magnetic properties for nanoscale bimetallic hybrid materials

Enhanced Semiconductor Nanocrystal Conductance via Solution Grown Contacts

We report a 100,000-fold increase in the conductance of individual CdSe nanorods when they are electrically contacted via direct solution phase growth of Au tips on the nanorod ends. Ensemble UV-Vis and X-Ray photoelectron spectroscopy indicate this enhancement does not result from alloying of the nanorod. Rather, low temperature tunneling and high temperature (250-400 K) thermionic emission across the junction at the Au contact reveal a 75percent lower interface barrier to conduction compared to a control sample. We correlate this barrier lowering with the electronic structure at the Au-CdSe interface. Our results emphasize the importance of nanocrystal surface structure for robust device performance and the advantage of this contact method

Size dependent tunneling and optical spectroscopy of CdSe quantum rods

Photoluminescence excitation spectroscopy and scanning tunneling spectroscopy are used to study the electronic states in CdSe quantum rods that manifest a transition from a zero dimensional to a one dimensional quantum confined structure. Both optical and tunneling spectra show that the level structure depends primarily on the rod diameter and not on length. With increasing diameter, the band-gap and the excited state level spacings shift to the red. The level structure was assigned using a multi-band effective-mass model, showing a similar dependence on rod dimensions.Comment: Accepted to PRL (nearly final version). 4 pages in revtex, 4 figure

Shape, Size, and Assembly Control of PbTe Nanocrystals

The small band gap and the large Bohr radius 1 of the Pbchalcogenide enable unique optical, electrical, and chemical properties, presenting this family of materials as a good candidate for potential application in solar cells, 2-4 thermoelectric (TE) 5 devices, telecommunication, 6 field effect transistors (FET), Tremendous efforts have been put toward controlling the properties of the nanocrystals by controlling their size due to quantum confinement phenomena. Several groups have reported controlled synthesis of PbTe nanocrystals, 18 all obtaining cube-shape structures with sizes ranging from 15 to 20 nm. Herein, we demonstrate the synthesis of PbTe nanocrystals with controllable shape and size. We establish a new method for controlling the shape of Pb-chalcogenide materials by controlling the reaction kinetic, using proper surfactant, the right temperature, and changing the molar ratio between Pb and Te. The reaction was carried out using trioctylphosphine (TOP) or diphenyl ether (DPE) as the growth solvent and phosphonic acid or amines (hexadecylamine, HDA, and dodecylamine, DDA) as the stabilizer surfactant. Two separate precursor solutions were prepared for Pb and Te. The Pb solution contained Pb-acetate trihydrate and oleic acid in TOP or DPE, and the Te solution was prepared by dissolving Te powder in TOP (see Supporting Information). The precursor was first injected at 250°C followed by a constant growth temperature of 170-180°C for 3-4 min. The PbTe nanoparticle structures were studied by high-resolution transmission electron microscopy (HRTEM) and are shown to be single-crystalline as presented in Achieving shape control using surfactant becomes one of the most studied effects in the nanoparticle shape control field. To study the effect of the surfactant, we have also tested tetradecylphosphonic acid (TDPA) or one of its derivatives (octadecyl-and hexylphosphonic acids), instead of the amines. The phosphonic acid leads to a slow growth rate; it takes ∼10 s for the nucleation to take place after the precursors' injection (change in the color of growth solution from colorless to black) compared to an immediate nucleation when the amine is used. This is due to the strong binding of the phosphonic acid to the nanoparticles' surface. The ratio between the precursors (Pb:Te) was also examined for this surfactant, and it was changed from 5:1 to 1:5. Cube particles were obtained when the ratio between the Pb:Te precursors is 1:5. However, the cuboctahedra were produced when the ratio for Pb:Te is 5:1 to 1:3. Octahedral particles were not achieved by using the phosphonic acid surfactant, and again this could be related to the fact that growing octahedra nanocrystals required a fast growth rate of the (100) direction compared to the (111) facet, and this is not achievable using phosphonic acid surfactant since the growth rate is slower using phosphonic acid. Size uniformity of the cubic, cuboctahedral, and octahedral nanoparticles prepared using TOP and phosphonic acid can be controlled by the proper choice of surfactants and the reaction time (cuboctahedra shown in The cubic, octahedral, and cuboctahedral shape PbTe particles could have a special interest for thermoelectric and photovoltaic application once they are processed into thin film form. We have so far already established an approach for assembling the PbTe nanocrystals and succeeded in preparing a large area close-packed film using the Langmuir-Blodgett technique, as shown i

Length and Diameter Control of Ultrathin Nanowires of Substoichiometric Tungsten Oxide with Insights into the Growth Mechanism

Tungsten oxide ultrathin nanowires have potential applications in electrochromic devices, dye sensitized solar cells, gas sensors, and as photocatalysts. Herein, we report a synthesis for solution phase ultrathin nanowires with independent control over the length and diameter. W­(CO)₆ is used as the tungsten source, and the solvents octadecanol and octadecene are used. Morphological control is obtained by varying the ratios between these three components as well as reaction conditions such as time and temperature. Such precise synthetic tuning will enable future investigations on the role of the aspect ratio and diameter for the above-mentioned applications. We can infer from experimental data a plausible nucleation pathway that involves the formation of tungsten alkoxide clusters. Raman spectroscopy, X-ray photoelectron spectroscopy, electron microscopy, and X-ray and electron diffraction are used to characterize the nanowires, and the results indicate the phase to be a crystallographic sheer structure, such as W₂₀O₅₈

Role of the Counteranions on the Formation of Different Crystal Structures of Iron Oxyhydroxides via Redox Reaction

Redox reactions have been employed for the conversion of various metal oxide nanocrystals, where several parameters were examined to understand the reaction mechanism. However, the role of the counteranions still has not yet been studied. Herein, we present the influence of the counteranions (sulfate, chloride, and acetate) on the formation of iron oxyhydroxide nanofiber structure (100 nm × few microns; diameter × length) via redox reaction using iron salt and manganese oxide as a template. We found that different structures of iron oxide/oxyhydroxide (goethite, hydrohematite, and maghemite) were obtained when different anions (sulfate, chloride, and acetate, respectively) were used. Moreover, we studied the effect of various parameters such as the concentration of iron precursor, the time, and the temperature of the reaction. Finally, we converted the iron oxyhydroxide film to hematite and examined its photoelectrochemical properties

Formation of Copper Oxide Nanotextures on Porous Calcium Carbonate Templates for Water Treatment

The necessity of providing clean water sources increases the demand to develop catalytic systems for water treatment. Good pollutants adsorbers are a key ingredient, and CuO is one of the candidate materials for this task. Among the different approaches for CuO synthesis, precipitation out of aqueous solutions is a leading candidate due to the facile synthesis, high yield, sustainability, and the reported shape control by adjustment of the counter anions. We harness this effect to investigate the formation of copper oxide-based 3D structures. Specifically, the counter anion (chloride, nitrate, and acetate) affects the formation of copper-based hydroxides and the final structure following their conversion into copper oxide nanostructures over porous templates. The formation of a 3D structure is obtained when copper chloride or nitrate reacts with a Sorites scaffold (marine-based calcium carbonate template) without external hydroxide addition. The transformation into copper oxides occurs after calcination or reduction of the obtained Cu2(OH)3X (X = Cl− or NO3−) while preserving the porous morphology. Finally, the formed Sorites@CuO structure is examined for water treatment to remove heavy metal cations and degrade organic contaminant molecules