Beatriz H. Juárez

Ultrathin PbS Sheets by Two-Dimensional Oriented Attachment

Manufacturing Nanomaterials The exploration of many materials at the nanoscale has revealed properties that only emerge when working at these small dimensions. For device manufacture, materials need to be deposited or assembled in specific patterns. Schliehe et al. (p. 550; see the cover) show the oriented attachment of lead sulfide nanocrystals into two-dimensional sheets. The packing is driven by the choice of solvents that influence the interactions between the nanocrystals. The nanocrystals have excellent photoconductive properties and were incorporated into a photodetector without any additional chemical processing. Self-assembled two-dimensional nanocrystals of lead sulfide have excellent photoconductive properties. Controlling anisotropy is a key concept in the generation of complex functionality in advanced materials. For this concept, oriented attachment of nanocrystal building blocks, a self-assembly of particles into larger single-crystalline objects, is one of the most promising approaches in nanotechnology. We report here the two-dimensional oriented attachment of lead sulfide (PbS) nanocrystals into ultrathin single-crystal sheets with dimensions on the micrometer scale. We found that this process is initiated by cosolvents, which alter nucleation and growth rates during the primary nanocrystal formation, and is finally driven by dense packing of oleic acid ligands on {100} facets of PbS. The obtained nanosheets can be readily integrated in a photodetector device without further treatment.

Quantum Dot Attachment and Morphology Control by Carbon Nanotubes

Novel applications in nanotechnology rely on the design of tailored nanoarchitectures. For this purpose, carbon nanotubes and nanoparticles are intensively investigated. In this work, we study the influence of nonfunctionalized carbon nanotubes on the synthesis of CdSe nanoparticles by means of organometallic colloidal routes. This new synthesis methodology not only provides an effective path to attach nanoparticles noncovalently to carbon nanotubes but represents also a new way to control the shape of nanoparticles.

Tunable Plasmon Coupling in Distance-Controlled Gold Nanoparticles

Plasmons are resonant excitations in metallic films and nanoparticles. For small enough static distances of metal nanoparticles, additional plasmon-coupled modes appear as a collective excitation between the nanoparticles. Here we show, by combining poly(N-isopropylacrylamide) micro- and nanospheres and Au nanoparticles, how to design a system that allows controllably and reversibly switching on and off, and tuning the plasmon-coupled mode.

Self-assembly approach to optical metamaterials

Photonic crystals can be viewed just as a subclass of a larger family of material systems called metamaterials in which the properties largely derive from the structure rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings for further templating other materials with photonic applications for instance. The tortuous route from materials to devices might perhaps find reward in the ease and low cost of fabrication of these materials. In this paper we present a review of recent work and work under way in our laboratory tending towards synthesis based on self-assembly to realize metamaterials in the optical range. This comprises the formation of the templates (opals) and subsequent synthesis of guest materials such as semiconductors, metals and insulators. The possibility of further processing allows additional two-dimensional and quasi-two-dimensional patterning for the design of new structures. In this paper we show how the raw matter can be checked for quality and learn how to use its optical properties to evaluate application potential. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined. We show some examples where opals are used to pattern the growth of other materials with photonic applications (such as metals and semiconductors) and developments leading to both vertical and lateral engineering are shown.

Optical and morphological study of disorder in opals

An optical and morphological study has been carried out to understand the role of intrinsic defects in the optical properties of opal-based photonic crystals. By doping poly(methylmethacrylate) (PMMA) thin-film opals with larger polystyrene (PS) spheres, structural disorder has being generated perturbing the PMMA matrix periodicity. It is shown that this disorder dramatically affects the optical response of the system worsening its photonic properties. It has been found that the effect of doping is highly dependent not only on the concentration but also on the relative size of the dopant with reference to the matrix. Through a detailed scanning electron microscopy inspection, the sort of structural defects involved, derived from the different particle size used, has been characterized. A direct relationship between the observed optical response with the different perturbations generated in the lattice has been found. In addition, from this study it can be concluded that it is possible to grow high quality...

3-D characterization of CdSe nanoparticles attached to carbon nanotubes

The crystallographic structure of CdSe nanoparticles attached to carbon nanotubes has been elucidated by means of high resolution transmission electron microscopy and high angle annular dark field scanning transmission electron microscopy tomography. CdSe rod-like nanoparticles, grown in solution together with carbon nanotubes, undergo a morphological transformation and become attached to the carbon surface. Electron tomography reveals that the nanoparticles are hexagonal-based with the (001) planes epitaxially matched to the outer graphene layer.

Carbon supported CdSe nanocrystals.

Insights to the mechanism of CdSe nanoparticle attachment to carbon nanotubes following the hot injection method are discussed. It was observed that the presence of water improves the nanotube coverage while Cl containing media are responsible for the shape transformation of the nanoparticles and further attachment to the carbon lattice. The experiments also show that the mechanism taking place involves the right balance of several factors, namely, low passivated nanoparticle surface, particles with well-defined crystallographic facets, and interaction with an organics-free sp2 carbon lattice. Furthermore, this procedure can be extended to cover graphene by quantum dots.

CdSe/CdS nanoparticles immobilized on pNIPAm-based microspheres

In this work, CdSe/CdS semiconductor ligand-exchanged nanoparticles have been immobilized on poly-(N-isopropylacrylamide) (pNIPAM)-based microspheres. The size and the shrinkage capacity of the spheres can be tuned by the ratio of NIPAM/styrene (pNIPAM-PS spheres) or NIPAM/BIS (N-N′-methylene-bis-acrylamide) and MA (maleic acid) (pNIPAM-BIS-MA spheres). A ligand-exchange procedure for the transfer of initially organic compatible nanoparticles into aqueous solution using amine-modified or thiol-modified poly(ethylene oxide)s (PEOs) has been carried out prior to their immobilization. We observed that the interaction of the nanoparticles with the pNIPAM-based system depends on the nature of the ligands and the chemical composition of the microspheres. Nanoparticles capped with amine- or mercapto- poly(ethylene oxide)s ligands interact with pNIPAM–PS beads while only amine-capped ones show a clear tendency to interact with pNIPAM containing acid groups which leads to a high nanoparticle coverage. Dynamic light scattering measurements, atomic force microscopy and optical spectroscopy hint that nanoparticles are placed on the surface of pNIPAM-BIS-MA beads while being partially incorporated into pNIPAM-PS network. Cell culture studies demonstrate that the fluorescent composites show non-specific binding to fibroblasts. These features may be very valuable to develop materials for drug delivery and specific targeting of cells combined with the outstanding optical properties of semiconductor nanoparticles as fluorescent labelers.

Reversible attachment of platinum alloy nanoparticles to nonfunctionalized carbon nanotubes.

The formation of monodisperse, tunable sized, alloyed nanoparticles of Ni, Co, or Fe with Pt and pure Pt nanoparticles attached to carbon nanotubes has been investigated. Following homogeneous nucleation, nanoparticles attach directly to nonfunctionalized single-walled and multi-walled carbon nanotubes during nanoparticle synthesis as a function of ligand nature and the nanoparticle work function. These ligands not only provide a way to tune the chemical composition, size, and shape of the nanoparticles but also control a strong reversible interaction with carbon nanotubes and permit controlling the nanoparticle coverage. Raman spectroscopy reveals that the sp(2) hybridization of the carbon lattice is not modified by the attachment. In order to better understand the interaction between the directly attached nanoparticles and the nonfunctionalized carbon nanotubes, we employed first-principles calculations on model systems of small Pt clusters and both zigzag and armchair single-walled carbon nanotubes. The detailed comprehension of such systems is of major importance since they find applications in catalysis and energy storage.

Interfacing Quantum Dots and Graphitic Surfaces with Chlorine Atomic Ligands

The performance of devices based on semiconductor nanocrystals (NCs) improves both with stronger interface interactions among NCs and between NCs and solid electrode surfaces. The combination of X-ray photoelectron spectroscopy (XPS) and solid (31)P CP/MAS NMR (cross-polarization/magic angle spinning nuclear magnetic resonance) shows that the selective substitution of long organic chains by chlorine atomic ligands during the colloidal synthesis by the hot injection method promotes the adsorption of CdSe NCs to carbon sp(2) surfaces, leading to the formation of well-ordered NC monolayers on graphitic materials.