Switchable assembly of ultra narrow CdS nanowires and nanorods

Colloidal semiconductor nanocrystals: from bottom-up nanoarchitectonics to energy harvesting applications

Colloidal semiconductor nanocrystals (NCs) have been extensively investigated owing to their unique properties induced by the quantum confinement effect. The advent of colloidal synthesis routes led to the design of stable colloidal NCs with uniform size, shape, and composition. Metal oxides, phosphides, and chalcogenides (ZnE, CdE, PbE, where E = S, Se, or Te) are few of the most important monocomponent semiconductor NCs, which show excellent optoelectronic properties. The ability to build quantum confined heterostructures comprising two or more semiconductor NCs offer greater customization and tunability of properties compared to their monocomponent counterparts. More recently, the halide perovskite NCs showed exceptional optoelectronic properties for energy generation and harvesting applications. Numerous applications including photovoltaic, photodetectors, light emitting devices, catalysis, photochemical devices, and solar driven fuel cells have demonstrated using these NCs in the recent past. Overall, semiconductor NCs prepared via the colloidal synthesis route offer immense potential to become an alternative to the presently available device applications. This feature article will explore the progress of NCs syntheses with outstanding potential to control the shape and spatial dimensionality required for photovoltaic, light emitting diode, and photocatalytic applications. We also attempt to address the challenges associated with achieving high efficiency devices with the NCs and possible solutions including interface engineering, packing control, encapsulation chemistry, and device architecture engineering.

Electronic Structure and Excited State Dynamics of Cadmium Chalcogenide Nanorods

The cylindrical quasi-one-dimensional shape of colloidal semiconductor nanorods (NRs) gives them unique electronic structure and optical properties. In addition to the band gap tunability common to nanocrystals, NRs have polarized light absorption and emission and high molar absorptivities. NR-shaped heterostructures feature control of electron and hole locations as well as light emission energy and efficiency. We comprehensively review the electronic structure and optical properties of Cd-chalcogenide NRs and NR heterostructures (e.g., CdSe/CdS dot-in-rods, CdSe/ZnS rod-in-rods), which have been widely investigated over the last two decades due in part to promising optoelectronic applications. We start by describing methods for synthesizing these colloidal NRs. We then detail the electronic structure of single-component and heterostructure NRs and follow with a discussion of light absorption and emission in these materials. Next, we describe the excited state dynamics of these NRs, including carrier cooling, carrier and exciton migration, radiative and nonradiative recombination, multiexciton generation and dynamics, and processes that involve trapped carriers. Finally, we describe charge transfer from photoexcited NRs and connect the dynamics of these processes with light-driven chemistry. We end with an outlook that highlights some of the outstanding questions about the excited state properties of Cd-chalcogenide NRs.

Dispersion and Tunable Alignment of Colloidal Silver Nanowires in a Nematic Liquid Crystal for Applications in Electric-Optic Devices

The dispersion and tunable alignment of colloidal nanomaterials is desirable for practical applications in electric-optic (E-O) devices; however, it remains challenging for large one-dimensional nanomaterials with a large aspect ratio. Here, we demonstrate a large-scale, simple, multi-microdomain, and noncontact photoalignment technology to align colloidal silver nanowires (AgNWs, length ∼4.5 μm, diameter ∼70.6 nm) in a liquid crystal (LC) with a high two-dimensional order parameter (about 0.9). The AgNWs are precisely self-assembled via photomasks with twisted nematic and planar alignment models in microdomain regions. The AgNW orientation is tuned with an electric field, through the rotation of an LC director n, which allows three-dimensional (3D) tunable orientation combined with photoalignment. The colloidal dispersions of AgNWs in the LC cell influenced the ion transfer, elastic constant, dielectric anisotropy, and near LC alignment, changing the E-O properties of the LC devices. The 3D tunable orientation of an AgNW by photoalignment and an electric field could provide a new way to assemble large colloidal nanomaterials and fabricate functional E-O devices.

One-pot preparation of cationic charged Pt nanoparticles by the autocatalytic hydrolysis of acetylthiocholine

Autocatalytic synthesis of cationic charged platinum nanoparticles.

Hierarchical superstructure of alkylamine-coated ZnS nanoparticle assemblies

We describe methodology for producing highly uniform, ordered and reproducible superstructures of surfactant-coated ZnS nanorod and nanowire assemblies, and propose a predictive multiscale "packing model" for superstructure formation based on electron microscopy and powder X-ray diffraction data on the superstructure, as well as on individual components of the nanostructured system. The studied nanoparticles showed a hierarchical structure starting from the individual faceted ZnS inorganic cores, onto which the crystalline surfactant molecules are adsorbed, to the superstructure of the nanoparticle arrays. Our results point out the critical role of the surfactant headgroup and polarity in nanoparticle assembly, and demonstrate the relationship between the molecular structure of the surfactant and the resulting superstructure of the nanoparticle assemblies.

Three synthetic routes to single-crystalline PbS nanowires with controlled growth direction and their electrical transport properties

Single-crystalline rock-salt PbS nanowires (NWs) were synthesized using three different routes; the solvothermal, chemical vapor transport, and gas-phase substitution reaction of pregrown CdS NWs. They were uniformly grown with the [100] or [110], [112] direction in a controlled manner. In the solvothermal growth, the oriented attachment of the octylamine (OA) ligands enables the NWs to be produced with a controlled morphology and growth direction. As the concentration of OA increases, the growth direction evolves from the [100] to the higher surface-energy [110] and [112] directions under the more thermodynamically controlled growth conditions. In the synthesis involving chemical vapor transport and the substitution reaction, the use of a lower growth temperature causes the higher surface-energy growth direction to change from [100] to [110]. The high-resolution X-ray diffraction pattern and X-ray photoelectron spectroscopy results revealed that a thinner oxide-layer was produced on the surface of the PbS NWs by the substitution reaction. We fabricated field effect transistors using single PbS NW, which showed intrinsic p-type semiconductor characteristics for all three routes. For the PbS NW with a thinner oxide layer, the carrier mobility was measured to be as high as 10 cm(2) V(-1) s(-1).

Scalable alignment and transfer of nanowires based on oriented polymer nanofibers

We develop a simple and scalable method based on oriented polymer nanofiber films for the parallel assembly and transfer of nanowires at high density. Nanowires dispersed in solution are aligned and selectively deposited at the central space of parallel nanochannels formed by the well-oriented nanofibers as a result of evaporation-induced flow and capillarity. A general contact printing method is used to realize the transfer of the nanowires from the donor nanofiber film to a receiver substrate. The mechanism, which involves ordered alignment of nanowires on oriented polymer nanofiber films, is also explored with an evaporation model of cylindrical droplets. The simplicity of the assembly and transfer, and the facile fabrication of large-area well-oriented nanofiber films, make the present method promising for the application of nanowires, especially for the disordered nanowires synthesized by solution chemistry.

One-dimensional CdS nanostructures: synthesis, properties, and applications

One-dimensional (1D) semiconductor nanostructures are of prime interest due to their potential in investigating the size and dimensionality dependence of the materials' physical properties and constructing nanoscale electronic and optoelectronic devices. Cadmium sulfide (CdS) is an important semiconductor compound of the II-VI group, and its synthesis and properties have been of growing interest owing to prominent applications in several fields. This article provides a comprehensive review of the state-of-the-art research activities that focus on the rational synthesis, novel properties and unique applications of 1D CdS nanostructures in nanotechnology. It begins with the rational design and synthesis of 1D CdS nanostructures, and then highlights a range of unique properties and applications (e.g. photoluminescence, cathodoluminescence, electrochemiluminescence, photocatalysis, lasers, waveguides, modulators, solar cells, field-effect transistors, photodetectors, field-emitters, and nanogenerators) associated with them. Finally, the review is concluded with the author outlook of the perspectives with respect to future research on 1D CdS nanostructures.

Properties and emerging applications of self-assembled structures made from inorganic nanoparticles

Just as nanoparticles display properties that differ from those of bulk samples of the same material, ensembles of nanoparticles can have collective properties that are different to those displayed by individual nanoparticles and bulk samples. Self-assembly has emerged as a powerful technique for controlling the structure and properties of ensembles of inorganic nanoparticles. Here we review different strategies for nanoparticle self-assembly, the properties of self-assembled structures of nanoparticles, and potential applications of such structures. Many of these properties and possible applications rely on our ability to control the interactions between the electronic, magnetic and optical properties of the individual nanoparticles.

Reaction of alkylamine surfactants with carbon dioxide: relevance to nanocrystal synthesis

Exposure of tetradecylamine, hexadecylamine, and octadecylamine to CO(2) results in their transformation to alkylammonium alkylcarbamate (AAAC) pairs, which we find is a major source of irreproducibility in nanoparticle synthesis. Controlled exposure to CO(2) allows for highly uniform, ultranarrow ZnS nanorods coated with tetradecylamine to be reproducibly obtained in a single step. The crystal structures of the alkylamines and their AAAC analogs were investigated by powder X-ray diffraction and their isostructural three-dimensional unit cells are reported.

Synthesis of InP nanoneedles and their use as Schottky devices

Indium phosphide (InP) nanostructures have been synthesized by means of colloidal chemistry. Under appropriate conditions needle-shaped nanostructures composed of an In head and an InP tail with lengths up to several micrometers could be generated in a one-pot synthesis. The growth is interpreted in terms of simultaneous decomposition of the In precursor and in situ generation of In and InP nanostructures. Owing to their specific design such In/InP nanoneedles suit the use as ready-made Schottky transistors. Their transfer and output characteristics are presented.

A study of simultaneous patterning and alignment of semiconductor nanorods via polymerization-induced phase separation

We report simultaneous patterning and alignment of semiconductor nanorods (NRs) in nanorod-polymer films by using photolithographic polymerization-induced phase separation (PIPS). Exposure of the nanoparticle-monomer mixture to UV irradiation through a mask resulted in the site-specific photoinitiated polymerization of the monomer, which was followed with flow of the NRs from the areas rich in polymer to the areas rich in monomer. The orientation of NRs in the direction of flow was trapped in the polymerized films and characterized in polarization absorption experiments.

Single Crystalline Cadmium Sulfide Nanowires with Branched Structure

In this article, we report the synthesis of branched single crystal CdS nanowires. This branched CdS nanostructure is prepared by a simple surfactant-directing method, which is of particular interest as it uses readily available reagents and provides a convenient route to high-yield single crystal nanowires but with branched shape. These branched nanowires have an average diameter of about 40 nm and length up to several micrometers. A possible mechanism has been proposed and the addition of surfactant dodecylthiol into the two mixed-solvents would play an importance effect on the structure of the product. Based on the mechanism, by controlling the synthesis conditions, such as the ratios between the surfactant, inorganic solvent, and organic solvent, other kinds of nanostructures based on CdS nanowires were also prepared. Photoluminescence (PL) measurement reveals that the branched CdS nanowires have a strong emission at about 700 nm which might be due to its special structure.

Multifunctional Nanoparticles as Biocompatible Targeted Probes for Human Cancer Diagnosis and Therapy

The use of nanoparticles in biological application has been rapidly advancing toward practical applications in human cancer diagnosis and therapy. Upon linking the nanoparticles with biomolecules, they can be used to locate cancerous area as well as for traceable drug delivery with high affinity and specificity. In this review, we discuss the engineering of multifunctional nanoparticle probes and their use in bioimaging and nanomedicine.

Synthesis of high aspect ratio quantum-size CdS nanorods and their surface-dependent photoluminescence

Colloidal CdS nanorods with diameters near 4 nm and narrow size distributions ( approximately +/-10%) were synthesized up to 300 nm long by a sequential reactant injection technique that utilizes phosophonic acids as capping ligands. The phosphonic acid strongly passivates the nonpolar CdS surfaces and sequential reactant injection provides controlled CdS formation kinetics to enable heterogeneous and facet-selective CdS deposition on the more reactive {002} surfaces. With this process, the nanorod length can be systematically increased by increasing reactant addition to extend nanorod growth. The phosphonic acid concentration, however, is quite important, as "low" concentrations allow radial deposition and branching to occur. These high aspect ratio (>100) CdS nanorods luminesce with relatively high efficiencies of 10.8% quantum yield at room temperature. The luminescence, however, mostly arises from trap-related recombination, and the emission is significantly red-shifted from the absorption edge. Various surface passivation treatments were explored to eliminate trap emission and increase the luminescence quantum yield. Thiol and amine passivation both significantly reduced trap emission and enhanced band-edge emission, but the total luminescence quantum yields dropped significantly, with a maximum measured value of 1.5% for the amine-passivated CdS nanorods.

The role of interparticle and external forces in nanoparticle assembly

The past 20 years have witnessed simultaneous multidisciplinary explosions in experimental techniques for synthesizing new materials, measuring and manipulating nanoscale structures, understanding biological processes at the nanoscale, and carrying out large-scale computations of many-atom and complex macromolecular systems. These advances have led to the new disciplines of nanoscience and nanoengineering. For reasons that are discussed here, most nanoparticles do not 'self-assemble' into their thermodynamically lowest energy state, and require an input of energy or external forces to 'direct' them into particular structures or assemblies. We discuss why and how a combination of self- and directed-assembly processes, involving interparticle and externally applied forces, can be applied to produce desired nanostructured materials.

Sonochemistry-assisted microwave synthesis and optical study of single-crystalline CdS nanoflowers

Well-defined flower-like CdS nanostructures have been synthesized by applying ultrasound and microwave simultaneously, which consist of hexagonal nanopyramids and/or nanoplates depending on different sulfur sources. It is shown that the synergistic effect of microwave and sonochemistry is the main mechanism for the formation of the nanoflowers. Optical characterization of the nanoflowers shows a large blue-shift up to 100 nm in comparing with simple low-dimensional CdS nanostructures. This structure induced shift in optical properties may have potential applications in optoelectronics devices, catalysis, and solar cells.

2D self-bundled CdS nanorods with micrometer dimension in the absence of an external directing process

In the absence of an external direction-controlling process, exclusive self-bundled arrays of CdS nanorods are formed using a facile solution-based method involving trioctylphosphine (TOP) and tetradecylphosphonic acids (TDPA) as cosurfactants. CdS self-bundled arrays with an area of as large as 2.0 microm(2) could be obtained. A detailed mechanistic investigation leads us to conclude that the matching in nanorod concentration, intrinsic properties of CdS, and the hydrocarbon chains of the surfactants between adjacent CdS rods play key roles in the self-assembly. In sharp contrast to the defect dominant emission in solutions, the self-bundled CdS nanorods exhibit optical emission nearly free from the defect-states, demonstrating their potential for applications in luminescence and photovoltaic devices.

Challenges and breakthroughs in recent research on self-assembly

The controlled fabrication of nanometer-scale objects is without doubt one of the central issues in current science and technology. However, existing fabrication techniques suffer from several disadvantages including size-restrictions and a general paucity of applicable materials. Because of this, the development of alternative approaches based on supramolecular self-assembly processes is anticipated as a breakthrough methodology. This review article aims to comprehensively summarize the salient aspects of self-assembly through the introduction of the recent challenges and breakthroughs in three categories: (i) types of self-assembly in bulk media; (ii) types of components for self-assembly in bulk media; and (iii) self-assembly at interfaces.

Synthesis, two-dimensional assembly, and surface pressure-induced coalescence of ultranarrow PbS nanowires

Ultranarrow (1.8 nm) PbS nanowires are synthesized in a single step, under benchtop conditions at relatively low temperature (90 degrees C). The nanowires exhibit a nearly perfect crystal lattice, high width uniformity, and tight side-by-side registry. Two-dimensional (2D) assembly over large areas (>15 microm2) is achieved using the Langmuir Blodgett method. The wire width can be readily controlled in the range 1.8-10 nm by a surface pressure-induced coalescence reaction, as monitored by transmission electron microscopy and Raman spectroscopy. The fluorescence of the 2D assembly shows strong polarization dependence along the long axis of the wires, making the system potentially suitable for orientation-sensitive devices.

Forces between surfaces across nanoparticle solutions: role of size, shape, and concentration

Using a surface force apparatus, we have measured the normal forces between mica surfaces across various types of nanoparticles consisting of ZnS cores coated with a monolayer of physisorbed surfactant, dispersed in organic solvents. We focused on the effects of nanoparticle size, shape, and concentration on the force-distance profiles. Forces were exponentially repulsive when the surfactant layers were strongly bound to the nanoparticles and were roughly linear when there was adhesion between the nanoparticle cores, i.e., when the surfactant layers detached from the nanoparticles. In both cases, the range and magnitude of the forces were dependent upon the particle size, shape, and solution concentration. Fine details in the otherwise smooth force-distance profiles indicate specific effects due to particle chemistry and geometry and the existence of first-order disorder-order phase transitions upon confinement. Small amounts of water in the (hydrophobic) organic solvents had dramatic effects on the measured forces. Understanding and controlling the effects of particle shape, size, and concentration and the presence of water (or other surface-active solutes) on particle-particle and particle-surface interactions are important for the processing of nanoparticles into ordered superstructured materials.