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Showing papers by "Christopher B. Murray published in 2010"


Journal ArticleDOI
22 Jul 2010-Nature
TL;DR: This work reports a general method of growing centimetre-scale, uniform membranes of BNSLs that can readily be transferred to arbitrary substrates, and demonstrates the construction of magnetoresistive devices by incorporating large-area B NSL membranes that clearly show that device magnetoresistance is dependent on the structure (stoichiometry) of the BNSS.
Abstract: The spontaneous organization of multicomponent micrometre-sized colloids or nanocrystals into superlattices is of scientific importance for understanding the assembly process on the nanometre scale and is of great interest for bottom-up fabrication of functional devices. In particular, co-assembly of two types of nanocrystal into binary nanocrystal superlattices (BNSLs) has recently attracted significant attention, as this provides a low-cost, programmable way to design metamaterials with precisely controlled properties that arise from the organization and interactions of the constituent nanocrystal components. Although challenging, the ability to grow and manipulate large-scale BNSLs is critical for extensive exploration of this new class of material. Here we report a general method of growing centimetre-scale, uniform membranes of BNSLs that can readily be transferred to arbitrary substrates. Our method is based on the liquid-air interfacial assembly of multicomponent nanocrystals and circumvents the limitations associated with the current assembly strategies, allowing integration of BNSLs on any substrate for the fabrication of nanocrystal-based devices. We demonstrate the construction of magnetoresistive devices by incorporating large-area (1.5 mm x 2.5 mm) BNSL membranes; their magnetotransport measurements clearly show that device magnetoresistance is dependent on the structure (stoichiometry) of the BNSLs. The ability to transfer BNSLs also allows the construction of free-standing membranes and other complex architectures that have not been accessible previously.

757 citations


Journal ArticleDOI
TL;DR: A one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation and an interfacial assembly strategy to organize these nanocrystals into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly are reported.
Abstract: We report a one-pot chemical approach for the synthesis of highly monodisperse colloidal nanophosphors displaying bright upconversion luminescence under 980 nm excitation. This general method optimizes the synthesis with initial heating rates up to 100 °C/minute generating a rich family of nanoscale building blocks with distinct morphologies (spheres, rods, hexagonal prisms, and plates) and upconversion emission tunable through the choice of rare earth dopants. Furthermore, we employ an interfacial assembly strategy to organize these nanocrystals (NCs) into superlattices over multiple length scales facilitating the NC characterization and enabling systematic studies of shape-directed assembly. The global and local ordering of these superstructures is programmed by the precise engineering of individual NC’s size and shape. This dramatically improved nanophosphor synthesis together with insights from shape-directed assembly will advance the investigation of an array of emerging biological and energy-related nanophosphor applications.

408 citations


Journal ArticleDOI
TL;DR: The Mn-Pt nanocubes, which are enclosed by (100) surfaces, showed better activities than their spherical counterparts and are promising new candidates as cathode and anode catalysts in fuel cells.
Abstract: Mn-Pt nanocubes were synthesized from platinum acetylacetonate and manganese carbonyl in the presence of oleic acid and oleylamine. The Mn-Pt nanocubes were converted into an ordered MnPt(3) intermetallic phase upon annealing. The electrocatalytic properties of the Mn-Pt nanocubes and spheres (including oxygen reduction, formic acid oxidation, and methanol oxidation) were tested and found to be shape-dependent. The Mn-Pt nanocubes, which are enclosed by (100) surfaces, showed better activities than their spherical counterparts. These materials are promising new candidates as cathode and anode catalysts in fuel cells.

322 citations


Journal ArticleDOI
TL;DR: Monodisperse, high-quality, single-crystal PbSe nanorods were synthesized in a catalyst-free, one-pot reaction using a new phosphine selenide precursor to provide liquid-crystalline alignment or vertical alignment under controlled evaporation conditions, indicating that oriented attachment could be involved to provide anisotropic Pb Se nanostructures.
Abstract: Monodisperse, high-quality, single-crystal PbSe nanorods were synthesized in a catalyst-free, one-pot reaction using a new phosphine selenide precursor. PbSe nanorods were assembled to provide liquid-crystalline alignment or vertical alignment under controlled evaporation conditions. The growth of nanorods was monitored by TEM and absorption spectroscopy, indicating that oriented attachment could be involved to provide anisotropic PbSe nanostructures. In-plane XRD showed an enhanced (200) peak for PbSe nanorods, indicating the preferred alignment of nanorods on the substrates and their growth along the ⟨100⟩ direction. Absorption and emission spectra, along with lifetime measurements, show the differences between nanoscale PbSe spheres and rods.

198 citations


Journal ArticleDOI
TL;DR: The size- and shape-selective formation of metal nanostructures including Pt nanocubes, Pd spherical NCs, and Au nanowires (NWs) using carbon monoxide (CO, generated at point-of-use) as a reducing agent is reported.
Abstract: ties unobtainable simply by tuning the size of the spheres. The synthesis of metal NCs typically employs the reduction or decomposition of metal precursors in the presence of ligands, which prevent aggregation and improve the colloidal stability of the NCs. Among the wide spectrum of reducing agents that have been used, gases such as hydrogen under pressure have proven effective in delicately manipulating the growth kinetics and thus tailoring the size and morphology of the metal NCs. [15, 16] Despite these efforts, a one-pot synthesis of highly monodisperse metal NCs at ambient pressure using gaseous reducing agents generated at point-of-use is still an important advance. Herein we report the size- and shape-selective formation of metal nanostructures including Pt nanocubes, Pd spherical NCs, and Au nanowires (NWs) using carbon monoxide (CO, generated at point-of-use) as a reducing agent. We also discuss the implications of our observation on several recent reports of the preparation of Pt NCs utilizing metal carbonyls. In catalysis, it is well-known that particle shape (the facets exposed) can be as important as the particle surface area in activity and selectivity. For example, Pt(100) exhibits higher electrocatalytic activity than Pt(111) for the oxygen reduction reaction in H2SO4 electrolyte. [17, 18] Pt(100) also shows different selectivity from Pt(111) towards hydrogenation reactions. [19] Thus Pt nanocubes with well-defined {100} facets provide a model system for understanding microscopic surface phenomena in many catalytic processes. We report the synthesis of Pt nanocubes employing CO (generated by dehydration of formic acid; Supporting Information, Fig

188 citations


Journal ArticleDOI
TL;DR: The growth and magnetic characterization of large-area BNSL membranes self-assembled from distinct magnetic NCs at the liquid-air interface are reported, which exhibit a single-phase-like magnetization alignment process, which is not observed in the phase-separated NC mixtures having the same stoichiometry.
Abstract: Co-assembly of two types of nanocrystals (NCs) into binary NC superlattices (BNSLs) provides a solution-based, inexpensive way to create novel metamaterials with rationally designed properties. The fundamental challenge is to probe and understand the nature and extent of complex interparticle interactions present in BNSLs, which can lead to collective properties that differ from their dispersed constituents or phase-separated counterparts. Here, we report the growth and magnetic characterization of large-area (∼1 cm2) BNSL membranes self-assembled from distinct magnetic NCs at the liquid−air interface. The resulting BNSL membranes exhibit a single-phase-like magnetization alignment process, which is not observed in the phase-separated NC mixtures having the same stoichiometry. This single-phase-like magnetic behavior is attributed to the collective interparticle dipolar interactions between two NC components in BNSLs, corroborated by calculation of the random dipolar fields as well as Monte Carlo simulati...

142 citations


Journal ArticleDOI
TL;DR: In this article, a theory of the electronic-structure and excitonic absorption spectra of PbS and PbSe nanowires and nanorods in the framework of a four-band effective mass model is presented.
Abstract: A theory of the electronic-structure and excitonic absorption spectra of PbS and PbSe nanowires and nanorods in the framework of a four-band effective-mass model is presented. Calculations conducted for PbSe show that dielectric contrast dramatically strengthens the exciton binding in narrow nanowires and nanorods. However, the self-interaction energies of the electron and hole nearly cancel the Coulomb binding, and as a result the optical absorption spectra are practically unaffected by the strong dielectric contrast between PbSe and the surrounding medium. Measurements of the size-dependent absorption spectra of colloidal PbSe nanorods are also presented. Using room-temperature energy-band parameters extracted from the optical spectra of spherical PbSe nanocrystals, the theory provides good quantitative agreement with the measured spectra.

132 citations


Journal ArticleDOI
19 Mar 2010-ACS Nano
TL;DR: The formation of ico-AB(13) and AlB(2) B NSLs from electrostatically neutral Fe3O4 nanoparticles is consistent with the space-filling principles and further supports entropy as the dominant factor during the growth of these BNSLs.
Abstract: Multicomponent nanocrystal assemblies have received great attention due to their fundamental role in the study of self-assembly and novel physical properties arising from particle interactions. Here, we report the formation of the first binary nanocrystal superlattices (BNSLs) consisting of different-sized Fe3O4 nanocrystals. We establish a framework to systematically study the structure of BNSLs using a dual-axis tomography TEM holder. The tilt series obtained not only allows us to map the three-dimensional (3D) structure of icosahdedral AB13 (ico-AB13) and AlB2-type BNSLs but also uncovers the structural differences among the projections of ico-AB13, cuboctahedral AB13 (cub-AB13), and AlB2. This structural characterization method is general and is important for further exploration of structural diversity in BNSLs and in the development of rigorous structure−property relationships in BNSLs. The formation of ico-AB13 and AlB2 BNSLs from electrostatically neutral Fe3O4 nanoparticles is consistent with the ...

56 citations


Journal ArticleDOI
TL;DR: The first temperature-dependent thermopower measurements in doped and undoped NC solids are reported, which serve as a unique electronic spectroscopy tool to quantitatively reveal the energetics of carriers and electronic states inNC solids.
Abstract: Single component and multicomponent nanocrystal (NC) solids represent an exciting new form of condensed matter, as they can potentially capture not only the quantum features of the individual building blocks but also novel collective properties through coupling of NC components. Unlike bulk semiconductors, however, there is no current theory for how introduction of dopants will impact the electronic structure and transport properties of NC solids. Empirically, it is known that in semiconductor NC systems, mixing two different materials of NCs electronically dopes the film. However, it has been challenging to connect the macroscopic measurements of doping effects on transport behavior to a microscopic understanding of how the identity, placement, and abundance of dopants impact these measurements. In this Letter, we report the first temperature-dependent thermopower measurements in doped and undoped NC solids. In combination with temperature-dependent electrical conductivity measurements, how the doping af...

43 citations


Proceedings ArticleDOI
16 May 2010
TL;DR: In this article, micro-photoluminescence measurements of PbS nanocrystals coupled to air-slot mode-gap photonic crystal nanocavities with Q∼15,000 and V eff ∼0.02 (λ/n air )3.
Abstract: We demonstrate micro-photoluminescence measurements of PbS nanocrystals coupled to air-slot mode-gap photonic crystal nanocavities with Q∼15,000 and V eff ∼0.02 (λ/n air )3. The ultrahigh Q/V ratios are critical for applications in cavity QED, nonlinear optics and sensing.

3 citations