Showing papers by "Christopher B. Murray published in 2006"

Structural diversity in binary nanoparticle superlattices

Abstract: The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice is a promising way of synthesizing a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. In theory only a few stable binary superlattice structures can assemble from hard spheres, potentially limiting this approach. But all is not lost because at the nanometre scale there are additional forces (electrostatic, van der Waals and dipolar) that can stabilize binary nanoparticulate structures. Shevchenko et al. now report the synthesis of a dozen novel structures from various combinations of metal, semiconductor, magnetic and dielectric nanoparticles. This demonstrates the potential of self-assembly in designing families of novel materials and metamaterials with programmable physical and chemical properties. 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. Bacteria1, macromolecules2 and nanoparticles3 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)3,4,5,6,7 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 formation3,8,9, 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 spheres10,11. 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.

Structural Characterization of Self-Assembled Multifunctional Binary Nanoparticle Superlattices

Abstract: 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 cr...

Magnetotransport of magnetite nanoparticle arrays

Abstract: We combine a material self-assembly with conventional lithographic processes in order to fabricate magnetoelectronic devices composed of ordered three-dimensional arrays of magnetite $({\mathrm{Fe}}_{3}{\mathrm{O}}_{4})$ nanoparticles. The device magnetoresistance reaches 35% at 60 K, corresponding to an electron spin polarization of 73%. Magnetoresistance of 12% remains at room temperature. Magnetoresistance decreases with both increasing temperature and bias voltage, however, the magnetoresistance of nanoparticle-based structures is only weakly dependent on the voltage---a favorable attribute for application to electronics.

Nanomaterials with tetrazole-based removable stabilizing agents

Abstract: Nanoparticles coated with tetrazole and methods for preparing them are provided. The nanoparticles can be coated onto a substrate and used in fabricating devices such as field effect transistors, switches, sensors, solar cells and spring exchange magnets.

Novel inorganic materials for solution-processed electronics

Abstract: While conventional crystalline inorganic semiconductors offer superior charge carrier mobilities, they are generally difficult to form by low cost processes. Crystallization of inorganic semiconductors requires high-temperature treatments that force trade-offs between device performance, cost and compatibility with plastic substrates. The development of applications ranging from displays, photovoltaic cells and light-emitting devices to "smart cards", radio frequency tags and sensors could be accelerated by introducing lower cost alternatives to conventional silicon technology. Solution-based processes such as spin coating, dip coating or inkjet printing offer substantial cost reductions for fabrication of electronic and optoelectronic devices. We provide an overview of several new approaches to solution-processed inorganic semiconductors. Colloidal semiconductor nanocrystals enable room temperature solution-based fabrication of field-effect devices [1]. We fabricated thin-film transistor channels formed by self-assembly of 9 nm PbSe nanocrystals (Figure 1). Cross-linking of the nanocrystals with hydrazine increased exchange coupling, raising film conductance by about 10 orders of magnitude, yielding n-type device with charge-carrier mobility of 1 cm2/Vs. Reversible switching between nand p-type transport in nanocrystal arrays is possible upon adsorption of nor p-type doping molecules on nanocrystal surface (Figure 1d). Annealing of doped nanocrystal arrays at 200°C increased carrier mobility by about an order of magnitude. Electron mobility of 11 cm2/Vs was observed for arrays of 8 nm PbTe nanocrystals [2]. Self-assembly of multifunctional nanoparticle building blocks provides a powerful modular approach to the design of composite materials that combine properties of semiconducting, metallic and magnetic constituents (Figure 2) [3]. We demonstrate that these materials can be employed for solution-processed electronic and optoelectronic devices. Liquid-phase colloidal synthesis allows engineering size, shape and composition of nanomaterials. Various semiconductors can be prepared in form of nanoscale spheres, rods, discs, tetrapods, nanowires and nanorings. Some of these structures are interesting for ultra-small electronic devices. For example, Figure 3 shows a single electron transistor based on a CdTe tetrapod with arms 8 nm in diameter and 150 nm in length [5]. Another promising approach to solution-processed semiconductors is based on using molecular precursors that transform into crystalline inorganic semiconductors upon heating at elevated temperatures. A novel class of inexpensive soluble precursors for high-mobility inorganic chalcogenides has been developed [4]. For example, spin-coated films of In2Se3 exhibited electron mobilities as high as 16 cm2/Vs (Figure 4) [6]. Soluble hydrazine-based precursors can be synthesized for a range of materials with promising electronic, thermoelectric, and photovoltaic properties.