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Crystal Splitting in the Growth of Bi2S3

Description: Novel Bi{sub 2}S{sub 3} nanostructures with a sheaf-like morphology are obtained via reaction of bismuth acetate-oleic acid complex with elemental sulfur in 1-octadecence. We propose these structures form by the splitting crystal growth mechanism, which is known to account for the morphology some mineral crystals assume in nature. By controlling the synthetic parameters, different forms of splitting, analogous to observed in minerals, are obtained in our case of Bi{sub 2}S{sub 3}. These new and complex Bi{sub 2}S{sub 3} nanostructures are characterized by TEM, SEM, XRD and ED.
Date: June 15, 2006
Creator: Tang, Jing & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Preparation of Asymmetric Nanostructures through Site SelectiveModification of Tetrapods

Description: CdTe tetrapods have been deposited on a substrate and partially coated with a protective polymer layer, exposing just one arm. The exposed arm was then decorated with Au nanoparticles in a site selective fashion. The modified arms were readily broken off from the remainder of the tetrapods and released from the substrate, yielding CdTe nanorods asymmetrically modified with Au nanoparticles. These nanostructures with reduced symmetry may show interesting optoelectronic properties.
Date: September 9, 2004
Creator: Liu, Haitao & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Nanocrystal Diffusion in a Liquid Thin Film Observed by in situ Transmission Electron Microscopy

Description: We have directly observed motion of inorganic nanoparticles during fluid evaporation using a Transmission Electron Microscope. Tracking real-time diffusion of both spherical (5-15 nm) and rod-shaped (5x10 nm) gold nanocrystals in a thin-film of water-15percentglycerol reveals complex movements, such as rolling motions coupled to large-step movements and macroscopic violations of the Stokes-Einstein relation for diffusion. As drying patches form during the final stages of evaporation, particle motion is dominated by the nearby retracting liquid front.
Date: April 17, 2009
Creator: Zheng, Haimei; Claridge, Shelley A.; Minor, Andrew M.; Alivisatos, A. Paul & Dahmen, Ulrich
Partner: UNT Libraries Government Documents Department

Strain-dependent photoluminescence behavior in three geometries of CdSe/CdS nanocrystals

Description: In recent years, a new generation of quantum confined colloidal semiconductor structures has emerged, with more complex shapes than simple quantum dots1, 2. These include nanorods3 and tetrapods4. Beyond shape, it is also now possible to spatially vary the electron and hole potentials within these nanoparticles by varying the composition. Examples of these new structures include seeded dots, rods, and tetrapods, which contain a CdSe core embedded within a CdS shell5, 6. These structures may have many uses beyond those envisioned for simple quantum dots, which are frequently employed in luminescent applications7. This paper is concerned with changes in the optoelectronic properties of tetrapods when the arms are bent. We demonstrate that seeded tetrapods can serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications ranging from sensitive optomechanical devices to biological force investigations.
Date: May 26, 2009
Creator: Choi, Charina L; Koski, Kristie J; Sivasankar, Sanjeevi & Alivisatos, A Paul
Partner: UNT Libraries Government Documents Department

Nanoheterostructure Cation Exchange: Anionic Framework Conservation

Description: In ionic nanocrystals the cationic sub-lattice can be replaced with a different metal ion via a fast, simple, and reversible place-exchange, allowing post-synthetic modification of the composition of the nanocrystal, while preserving its size and shape. Here, we demonstrate for the first time that during such an exchange, the anionic framework of the crystal is preserved. When applied to nanoheterostructures, this phenomenon ensures that compositional interfaces within the heterostructure are conserved throughout the transformation. For instance, a morphology composed of a CdSe nanocrystal embedded in a CdS rod (CdSe/CdS) was exchanged to a PbSe/PbS nanorod via a Cu2Se/Cu2S structure. During every exchange cycle, the seed size and position within the nanorod were preserved, as evident by excitonic features, Z-contrast imaging, and elemental line-scans. Anionic framework conservation extends the domain of cation exchange to the design of more complex and unique nanostructures.
Date: May 11, 2010
Creator: Jain, Prashant K.; Amirav, Lilac; Aloni, Shaul & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Correlation Analysis of TEM Images of Nanocrystal Molecules

Description: Quantitative characterizataion of images of naocrystals and nanostructures is a challenging but important task. The development and optimization of methods for the construction of complex nanostructures rely on imaging techniques. Computer programs were developed to quantify TEM images of nanocrystal/DNA nanostructures, and results are presented for heterodimers annd trimers of gold nanocrystals. The programs presented here have also been used to analyze more complex trimers and tetramers linked by branched DNA, as well as for structures made from attaching gold nanocrystals to CdSe/ZnS core-shell quantum dots. This work has the additional goal of enabling others to quickly and easily adapt the methods for their own use.
Date: May 21, 2008
Creator: Micheel, Christine; Zanchet, Daniele & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

From Artificial Atoms to Nanocrystal Molecules: Preparation and Properties of More Complex Nanostructures

Description: Quantum dots, which have found widespread use in fields such as biomedicine, photovoltaics, and electronics, are often called artificial atoms due to their size-dependent physical properties. Here this analogy is extended to consider artificial nanocrystal molecules, formed from well-defined groupings of plasmonically or electronically coupled single nanocrystals. Just as a hydrogen molecule has properties distinct from two uncoupled hydrogen atoms, a key feature of nanocrystal molecules is that they exhibit properties altered from those of the component nanoparticles due to coupling. The nature of the coupling between nanocrystal atoms and its response to vibrations and deformations of the nanocrystal molecule bonds are of particular interest. We discuss synthetic approaches, predicted and observed physical properties, and prospects and challenges toward this new class of materials.
Date: October 20, 2009
Creator: Choi, Charina L & Alivisatos, A Paul
Partner: UNT Libraries Government Documents Department

Pyramidal and Chiral Groupings of Gold Nanocrystals Assembled Using DNA Scaffolds

Description: Nanostructures constructed from metal and semiconductor nanocrystals conjugated to, and organized by DNA are an emerging class of material with collective optical properties. We created discrete pyramids of DNA with gold nanocrystals at the tips. By taking small angle X-ray scattering (SAXS) measurments from solutions of these pyramids we confirmed that this pyramidal geometry creates structures which are more rigid in solution than linear DNA. We then took advantage of the tetrahedral symmetry to demonstrate construction of chiral nanostructures.
Date: March 30, 2009
Creator: Mastroianni, Alexander; Claridge, Shelley & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Colloidal nanocrystal synthesis and the organic-inorganicinterface

Description: Colloidal nanocrystals are nanometer-sized, solution-grown inorganic particles stabilized by a layer of surfactants attached to their surface. The inorganic cores exhibit useful properties controlled by composition as well as size and shape, while the surfactant coating ensures that these structures are easy to fabricate and process. It is this combination of features that makes colloidal nanocrystals attractive and promising building blocks for advanced materials and devices. But their full potential can only be exploited if we achieve exquisite control over their composition, size, shape, crystal structure and surface properties. Here we review what is known about nanocrystal growth and outline strategies for controlling it.
Date: May 12, 2005
Creator: Yin, Yadong & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

High-Temperature Microfluidic Synthesis of CdSe Nanocrystals inNanoliter Droplets

Description: The high-temperature synthesis of CdSe nanocrystals innanoliter-volume droplets flowing in a perfluorinated carrier fluidthrough a microfabricated reactor is presented. A flow-focusing nanojetstructure with a step increase in channel height reproducibly generatedoctadecene droplets in Fomblin Y 06/6 perfluorinated polyether atcapillary numbers up to 0.81 and with a droplet:carrier fluid viscosityratio of 0.035. Cadmium and selenium precursors flowing in octadecenedroplets through a high-temperature (240-300 degrees C) glassmicroreactor produced high quality CdSe nanocrystals, as verified byoptical spectroscopy and transmission electron microscopy. Isolating thereaction solution in droplets prevented particle deposition andhydrodynamic dispersion, allowing the reproducible synthesis ofnanocrystals at three different temperatures and four different residencetimes in the span of four hours. Our synthesis of a wide range ofnanocrystals at high temperatures, high capillary numbers, and lowviscosity ratio illustrates the general utility of droplet-basedmicrofluidic reactors to encapsulate nanoliter volumes of organic oraqueous solutions and to precisely control chemical or biochemicalreactions.
Date: June 9, 2005
Creator: Chan, Emory M.; Alivisatos, A. Paul & Mathies, Richard A.
Partner: UNT Libraries Government Documents Department

Controlled Assembly of Hybrid Bulk-Heterojunction Solar Cells bySequential Deposition

Description: This work presents a technique to create ordered and easily characterized hybrid nanocrystal-polymer composites by sequential deposition of tetrapod-shaped cadmium telluride (CdTe) nanocrystals and poly(3-hexlythiophene). With controlled fabrication and composite morphology, these devices offer several advantages over traditional codeposited hybrid cells, and provide a model system for detailed investigation into the operation of bulk-heterojunction cells.
Date: August 13, 2006
Creator: Gur, Ilan; Fromer, Neil A. & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Faceting of Nanocrystals during Chemical Transformation: FromSolid Silver Spheres to Hollow Gold Octahedra

Description: Sustained progress in nanocrystal synthesis has enabled recent use of these materials as inorganic, macromolecular precursors that can be chemically transformed into new nanostructures. The literature now contains several cases with chemical transformations being accompanied by varying degrees of modification of properties, including crystal structure and particle shape. As a recent example, we demonstrated that as-synthesized metallic nanocrystals yield, upon oxidation, nanostructures with modified morphologies such as hollow particles. This morphological change derives from directional material flows due to differing diffusivities for the reacting atomic species, in a nanoscale version of the well-known Kirkendall Effect. This general methodology has since been extended by other groups to produce nanostructures with various compositions and shapes. We demonstrate that performing a replacement reaction on single crystalline Ag nanospheres of {approx}10 nm in diameter in an organic solvent produces hollow Au nanocrystals with an octahedral shape. Different from those Au shells made by starting with Ag particles about one order of magnitude larger, which largely reproduce that of the sacrificial Ag counterparts, the hollow nanocrystals obtained in this work show significant changes in the external morphology from the spherical Ag precursors. This evolution of a faceted external morphology during chemical transformation is made possible by the enhanced role of surface effects in our smaller nanocrystals. The competition between the Au atom deposition and Ag atom dissolution on various nanocrystal surfaces is believed to determine the final octahedral shape of the hollow Au nanocrystals. Simultaneous achievement of surface-mediated shape control and a hollow morphology in a one-pot, single-step synthetic procedure in this study promises an avenue to finer tuning of particle morphology, and thus physical properties such as surface plasmon resonance.
Date: June 23, 2006
Creator: Yin, Yadong; Erdonmez, Can & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Optical properties of ZnO/ZnS and ZnO/ZnTe heterostructures forphotovoltaic applications

Description: Although ZnO and ZnS are abundant, stable, environmentallybenign, their band gap energies (3.44 eV, 3.72 eV) are too large foroptimal photovoltaic efficiency. By using band-corrected pseudopotentialdensity-functional theory calculations, we study how the band gap,opticalabsorption, and carrier localization canbe controlled by formingquantum-well like and nanowire-based heterostructures ofZnO/ZnS andZnO/ZnTe. In the case of ZnO/ZnS core/shell nanowires, which can besynthesized using existing methods, we obtain a band gap of 2.07 eV,which corresponds to a Shockley-Quiesser efficiency limitof 23 percent.Based on these nanowire results, we propose that ZnO/ZnScore/shellnanowires can be used as photovoltaic devices with organic polymersemiconductors as p-channel contacts.
Date: May 1, 2007
Creator: Schrier, Joshua; Demchenko, Denis O.; Wang, Lin-Wang & Alivisatos,A. Paul
Partner: UNT Libraries Government Documents Department

Electrical Contacts to Individual Colloidal Semiconductor Nanorods

Description: We report the results of charge transport studies on single CdTe nanocrystals contacted via evaporated Pd electrodes. Device charging energy, E{sub c}, monitored as a function of electrode separation drops suddenly at separations below {approx}55 nm. This drop can be explained by chemical changes induced by the metal electrodes. This explanation is corroborated by ensemble X-Ray photoelectron spectroscopy (XPS) studies of CdTe films as well as single particle measurements by transmission electron microscopy (TEM) and energy dispersive X-Rays (EDX). Similar to robust optical behavior obtained when Nanocrystals are coated with a protective shell, we find that a protective SiO2 layer deposited between the nanocrystal and the electrode prevents interface reactions and an associated drop in E{sub c,max}. This observation of interface reactivity and its effect on electrical properties has important implications for the integration of nanocrystals into conventional fabrication techniques and may enable novel nano-materials.
Date: April 1, 2008
Creator: Trudeau, Paul-Emile; Sheldon, Matt; Altoe, Virginia & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Synthesis and photovoltaic application of coper (I) sulfide nanocrystals

Description: We present the rational synthesis of colloidal copper(I) sulfide nanocrystals and demonstrate their application as an active light absorbing component in combination with CdS nanorods to make a solution-processed solar cell with 1.6percent power conversion efficiency on both conventional glass substrates and flexible plastic substrates with stability over a 4 month testing period.
Date: June 24, 2008
Creator: Wu, Yue; Wadia, Cyrus; Ma, Wanli; Sadtler, Bryce & Alivisatos, A.Paul
Partner: UNT Libraries Government Documents Department

Strain-Dependent Photoluminescence Behavior of CdSe/CdS Nanocrystals with Spherical, Linear, and Branched Topologies

Description: The photoluminescence of CdSe/CdS core/shell quantum dots, nanorods, and tetrapods is investigated as a function of applied hydrostatic and non-hydrostatic pressure. The optoelectronic properties of all three nanocrystal morphologies are affected by strain. Furthermore, it is demonstrated that the unique morphology of a tetrapod is highly sensitive to non-isotropic stress environments. Seeded tetrapods can thereby serve as an optical strain gauge, capable of measuring forces on the order of nanonewtons. We anticipate that a nanocrystal strain gauge with optical readout will be useful for applications ranging from sensitive optomechanical devices to investigations of biomechanical processes.
Date: August 13, 2009
Creator: Choi, Charina L.; Koski, Kristie J.; Sivasankar, Sanjeevi & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Independent control of the shape and composition of ionic nanocrystals through sequential cation exchange reactions

Description: Size- and shape-controlled nanocrystal growth is intensely researched for applications including electro-optic, catalytic, and medical devices. Chemical transformations such as cation exchange overcome the limitation of traditional colloidal synthesis, where the nanocrystal shape often reflects the inherent symmetry of the underlying lattice. Here we show that nanocrystals, with established synthetic protocols for high monodispersity, can be templates for independent composition control. Specifically, controlled interconversion between wurtzite CdS, chalcocite Cu2S, and rock salt PbS occurs while preserving the anisotropic dimensions unique to the as-synthesized materials. Sequential exchange reactions between the three sulfide compositions are driven by the disparate solubilites of the metal ion exchange pair in specific coordinating molecules. Starting with CdS, highly anisotropic PbS nanorods are created, which serve as an important material for studying strong 2-dimensional quantum confinement, as well as for optoelectronic applications. Furthermore, interesting nanoheterostructures of CdS|PbS are obtained by precise control over ion insertion and removal.
Date: July 6, 2009
Creator: Luther, Joseph Matthew; Zheng, Haimei; Sadtler, Bryce & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department

Hetero-Epitaxial Anion Exchange Yields Single-Crystalline Hollow Nanoparticles

Description: Anion exchange with S was performed on ZnO colloidal nanoparticles. The resulting hollow ZnS nanoparticles are crystal whose shape is dictated by the initial ZnO. Crystallographic and elemental analyses provide insight into the mechanism of the anion exchange.
Date: August 26, 2009
Creator: Park, Jungwon; Zheng, Haimei; Jun, Young-wook & Alivisatos, A. Paul
Partner: UNT Libraries Government Documents Department