Novel polymeric biomaterial poly(butyl-2-cyanoacrylate) nanowires: synthesis, characterization and formation mechanism

Poly(butyl-2-cyanoacrylate) (PBCA) nanoparticles have been widely elaborated for nearly half a century. However, PBCA nanowires (PNWs) were seldom investigated. Here, new polymeric biomaterial PNWs were prepared via emulsion polymerization based on the sodium dodecyl sulfate (SDS)-assisted emulsion process. Results indicated that SDS micelles and PBCA polymer can develop surfactant-polymer complexes by self-assembly at room temperature. SDS concentration was confirmed to be the critical parameter for the association of the surfactant and the polymer. With the addition of SDS (0-40 mM), the interaction between SDS and PBCA led to a series of transitions from nanoparticles to nanowires. These morphology transitions were triggered by changing the electrostatic repulsion in the SDS-PBCA system, confirmed by the variety of zeta potential with increasing molar contents of SDS. To overcome the electrostatic repulsion, the complexes underwent transitions from spherical, worm-like (short-cylindrical), to elongated-cylindrical form. Finally, associated with the results from scanning / transmission electron microscopy (SEM / TEM), the elongated-cylindrical PNWs acquired at 20 mM of SDS were chosen to execute cell viability assay, which showed that they had no toxicity but with good-biocompatibility at the doses ≤ 50 μg/ml. These results indicate that the PNWs prepared by this facile-green and low-toxic strategy can potentially work as promising biomaterials in the biomedicine field.

3D and 2D structural characterization of 1D Al/Al2 O3 biphasic nanostructures

1D Al/Al2 O3 nanostructures have been synthesized by chemical vapour deposition (CVD) of the molecular precursor [(t) BuOAlH2 ]2 . The deposited nanostructures grow chaotically on the substrate forming a layer with a high porosity (80%). Depending on the deposition time, diverse nanostructured surfaces with different distribution densities were achieved. A three-dimensional (3D) reconstruction has been evaluated for every nanostructure density using the Focus Ion Beam (FIB) tomography technique and reconstruction software tools. Several structural parameters such as porosity, Euler number, geometrical tortuosity and aspect ratio have been quantified through the analysis with specified software of the reconstructions. Additionally roughness of the prepared surfaces has been characterized at micro- and nanoscale using profilometry and AFM techniques, respectively. While high aspects ratio around 20-30 indicates a strong anisotropy in the structure, high porosity values (around 80%) is observed as a consequence of highly tangled geometry of such 1D nanostructures.

Recent developments and future directions in the growth of nanostructures by van der Waals epitaxy

Here we review the characteristics of "van der Waals epitaxy" (vdWE) as an alternative epitaxy mechanism that has been demonstrated as a viable method for circumventing the lattice matching requirements for epitaxial growth. Particular focus is given on the application of vdWE for nonplanar nanostructures. We highlight our works on the vdWE growth of nanowire arrays, tripods, and tetrapods from various semiconductors (ZnO, ZnTe, CdS, CdSe, CdSxSe1-x, CdTe, and PbS) on muscovite mica substrates, irrespective of the ensuing lattice mismatch. We then address the controllability of the synthesis and the growth mechanism of ZnO nanowires from catalyst-free vdWE in vapor transport growth. As exemplified herein with optical characterizations of ZnO and CdSe nanowires, we show that samples from vdWE may possess properties that are as excellent as those from conventional epitaxy. With our works, we aim to advocate vdWE as a prospective universal growth strategy for nonplanar epitaxial nanostructures.

Synthesis and optical properties of II-VI 1D nanostructures

1D nanostructures from II-VI semiconductors have been demonstrated to exhibit outstanding optical properties with strong promise for novel optoelectronic devices with augmented performance and functionalities. Herein, we present a comprehensive review discussing important topics pertinent to the fundamental properties and applications of II-VI 1D nanostructures. With practical applications in mind, the considerations, principles and experimental techniques on the sample preparation of high quality 1D nanostructures are highlighted. Fundamentals on the optical properties of II-VI materials, along with relevant investigation techniques and recent progress in the field, are also extensively discussed. With the steady development of their synthesis, characterization and device fabrication, it is strongly expected that II-VI 1D nanostructures will assume a unique position in future technology.

Composition and bandgap-graded semiconductor alloy nanowires

Semiconductor alloy nanowires with spatially graded compositions (and bandgaps) provide a new material platform for many new multifunctional optoelectronic devices, such as broadly tunable lasers, multispectral photodetectors, broad-band light emitting diodes (LEDs) and high-efficiency solar cells. In this review, we will summarize the recent progress on composition graded semiconductor alloy nanowires with bandgaps graded in a wide range. Depending on different growth methods and material systems, two typical nanowire composition grading approaches will be presented in detail, including composition graded alloy nanowires along a single substrate and those along single nanowires. Furthermore, selected examples of applications of these composition graded semiconductor nanowires will be presented and discussed, including tunable nanolasers, multi-terminal on-nanowire photodetectors, full-spectrum solar cells, and white-light LEDs. Finally, we will make some concluding remarks with future perspectives including opportunities and challenges in this research area.

Quantum maximum-entropy principle for closed quantum hydrodynamic transport within a Wigner function formalism

By introducing a quantum entropy functional of the reduced density matrix, the principle of quantum maximum entropy is asserted as fundamental principle of quantum statistical mechanics. Accordingly, we develop a comprehensive theoretical formalism to construct rigorously a closed quantum hydrodynamic transport within a Wigner function approach. The theoretical formalism is formulated in both thermodynamic equilibrium and nonequilibrium conditions, and the quantum contributions are obtained by only assuming that the Lagrange multipliers can be expanded in powers of h(2). In particular, by using an arbitrary number of moments, we prove that (1) on a macroscopic scale all nonlocal effects, compatible with the uncertainty principle, are imputable to high-order spatial derivatives, both of the numerical density n and of the effective temperature T; (2) the results available from the literature in the framework of both a quantum Boltzmann gas and a degenerate quantum Fermi gas are recovered as a particular case; (3) the statistics for the quantum Fermi and Bose gases at different levels of degeneracy are explicitly incorporated; (4) a set of relevant applications admitting exact analytical equations are explicitly given and discussed; (5) the quantum maximum entropy principle keeps full validity in the classical limit, when h → 0.

CHEMICAL ROUTES TO NANOCRYSTALS, NANOWIRES AND NANOTUBES

Soft chemical routes are employed effectively for the synthesis of nanocrystals of semiconductor materials. Several methods have been developed for the synthesis of multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs) and specially noteworthy are the precursor route and nebulized spray pyrolysis for the synthesis of MWNTs and junction nanotubes. Nanotubes of inorganic layered materials are obtained by ingenious chemical methods. Nanowires of inorganic materials can be synthesized not only by high-temperature methods such as the carbon-assisted route but also by soft chemical routes.

Growth of GaN nanotubes by halide vapor phase epitaxy

We have investigated low temperature growth of GaN nanostructures using halide vapor phase epitaxy on c-oriented Al(2)O(3) and Au coated Al(2)O(3) substrates. Depending on the III/V ratio and the growth temperature, the shape and density of the structures could be controlled. By increasing the GaCl partial pressure, the structure changed from dot-like to nanotubes. The nanotubes, which could be open or closed, were about 1 µm long with a diameter of typically 200 nm. In addition, it was observed that the nanostructures were spontaneously nucleated at droplets of Ga or, when using Au coated Al(2)O(3), on droplets of Au/Ga alloy. By varying the growth temperature, the inner diameter of the nanotubes could be controlled. The experimental results suggest that this approach with pre-patterned Au coated Al(2)O(3)substrates has the potential for fabrication of well-organized nanotubes with a high density.

Equilibrium Analysis of Lattice-Mismatched Nanowire Heterostructures

The quality of lattice-mismatched semiconductor heterojunctions is often limited by the presence of misfit dislocations. Nanowire geometries offer the promise of creating highly mismatched, yet dislocation free heterojunctions. A simple model, based upon the critical thickness model of Matthews and Blakeslee for misfit dislocation formation in planar heterostructures, illustrates that there exists a critical nanowire radius for which a coherent heterostructured nanowire system is unstable with respect to the formation of misfit dislocations. The model indicates that within the nanowire geometry, it should be possible to create perfect heterojunctions with large lattice-mismatch.

ZnS nanostructure arrays: a developing material star

Semiconductor nanostructure arrays are of great scientific and technical interest because of the strong non-linear and electro-optic effects that occur due to carrier confinement in three dimensions. The use of such nanostructure arrays with tailored geometry, array density, and length-diameter-ratio as building blocks are expected to play a crucial role in future nanoscale devices. With the unique properties of a direct wide-bandgap semiconductor, such as the presence of polar surfaces, excellent transport properties, good thermal stability, and high electronic mobility, ZnS nanostructure arrays has been a developing material star. The research on ZnS nanostructure arrays has seen remarkable progress over the last five years due to the unique properties and important potential applications of nanostructure arrays, which are summarized here. Firstly, a survey of various methods to the synthesis of ZnS nanostructure arrays will be introduced. Next recent efforts on exploiting the unique properties and applications of ZnS nanostructure arrays are discussed. Potential future directions of this research field are also highlighted.

Stages in molecular beam epitaxy growth of GaAs nanowires studied by x-ray diffraction

GaAs nanowires were grown by molecular beam epitaxy and studied by glancing-angle x-ray diffraction during five different stages of the growth process. An entire forest of randomly positioned epitaxial nanowires was sampled simultaneously and a large variation in the Au-Ga catalyst was found. Au, AuGa, AuGa(2) and the hexagonal beta phase were all identified in several orientations and in similar amounts. The nanowires are shown to consist of regular zinc blende crystal, its twin and the hexagonal wurtzite. The evolution of the various Au-Ga catalysts and the development in the twin to the wurtzite abundance ratio indicate that the Au catalyst is saturated upon initiation of growth leading to an increased amount of wurtzite structure in the wires. A specular x-ray scan identifies the various Au-Ga alloys, three Au lattice constants and a rough interface between nanowires and catalyst. Reciprocal space maps were obtained around Au Bragg points and show the development of the Au catalyst from a distribution largely oriented with respect to the lattice to a non-uniform distribution with several well-defined lattice constants.

Synthesis methods, microscopy characterization and device integration of nanoscale metal oxide semiconductors for gas sensing

A comparison is made between SnO₂, ZnO, and TiO₂ single-crystal nanowires and SnO₂ polycrystalline nanofibers for gas sensing. Both nanostructures possess a one-dimensional morphology. Different synthesis methods are used to produce these materials: thermal evaporation-condensation (TEC), controlled oxidation, and electrospinning. Advantages and limitations of each technique are listed. Practical issues associated with harvesting, purification, and integration of these materials into sensing devices are detailed. For comparison to the nascent form, these sensing materials are surface coated with Pd and Pt nanoparticles. Gas sensing tests, with respect to H₂, are conducted at ambient and elevated temperatures. Comparative normalized responses and time constants for the catalyst and noncatalyst systems provide a basis for identification of the superior metal-oxide nanostructure and catalyst combination. With temperature-dependent data, Arrhenius analyses are made to determine activation energies for the catalyst-assisted systems.

Specific heat and thermal conductivity measurements for anisotropic and random macroscopic composites of cobalt nanowires

We report simultaneous specific heat (c(p)) and thermal conductivity (κ) measurements for anisotropic and random macroscopic composites of cobalt nanowires (Co NWs), from 300 to 400 K. Anisotropic composites of Co NW consist of nanowires grown within the highly ordered, densely packed array of parallel nanochannels in anodized aluminum oxide. Random composites are formed by drop-casting a thin film of randomly oriented Co NWs, removed from the anodized aluminum oxide host, within a calorimetric cell. The specific heat measured with the heat flow parallel to the Co NW alignment ([Formula: see text]) and that for the random sample (c(p)(R)) deviate strongly in temperature dependence from that measured for bulk, amorphous, powder cobalt under identical experimental conditions. The thermal conductivity for random composites (κ(R)) follows a bulk-like behavior though it is greatly reduced in magnitude, exhibiting a broad maximum near 365 K indicating the onset of boundary-phonon scattering. The thermal conductivity in the anisotropic sample ([Formula: see text]) is equally reduced in magnitude but increases smoothly with increasing temperature and appears to be dominated by phonon-phonon scattering.

Integratable nanowire transistors

We report a structure to control nanowire location and growth direction and demonstrate top-gated, metal-oxide-semiconductor, field-effect transistors (MOSFETs) using this structure. The nanowires wereengineered to grow against an oxide surface of a (001), silicon-on-insulator substrate, enabling straightforward fabrication of MOSFETs exhibiting an Io/Ioff ratio approximately 104 and a subthreshold slope of approximately 155 mV/decade. Though nanowires were engineered to grow in (110) directions, the nanowires still grew by the addition of {111) planes.

Electrical properties of self-assembled branched InAs nanowire junctions

We investigate electrical properties of self-assembled branched InAs nanowires. The branched nanowires are catalytically grown using chemical beam epitaxy, and three-terminal nanoelectronic devices are fabricated from the branched nanowires using electron-beam lithography. We demonstrate that, in difference from conventional macroscopic junctions, the fabricated self-assembled nanowire junction devices exhibit tunable nonlinear electrical characteristics and a signature of ballistic electron transport. As an example of applications, we demonstrate that the self-assembled three-terminal nanowire junctions can be used to implement the functions of frequency mixing, multiplication, and phase-difference detection of input electrical signals at room temperature. Our results suggest a wide range of potential applications of branched semiconductor nanostructures in nanoelectronics.

Gold/boron core-shell nanocables synthesized from gold-boron eutectic droplets

Metal/semiconductor core-shell coaxial nanocables are promising building blocks for nanoelectronic devices while in situ growth of these nanocables remains challenging due to the distinctly different synthesis temperature ranges required for metals and semiconductors. To overcome this difficulty, we have developed a vapor-liquid-solid and oxide-assisted bimodal competition growth strategy for in situ metal/semiconductor core-shell nanocable growth. Using this process, gold/boron core-shell nanocables were obtained. A core-shell Au-B/BO(x) eutectic droplet formed via hydrogen gas-assisted rapid cooling was found critical for initiation of the nanocable growth. In addition, the large difference in the boron nanowire growth rates in the vapor-liquid-solid and oxide-assisted mechanisms facilitates the layered growth in the nanocables. The compatibility of this method with the vapor-liquid-solid process applied widely for semiconductor nanowire growth allows in situ connection of metal/semiconductor nanocables with semiconductor nanowires.

Dispersibility, stabilization, and chemical stability of ultrathin tellurium nanowires in acetone: morphology change, crystallization, and transformation into TeO2 in different solvents

The dispersibility and stabilization of freshly synthesized ultrathin tellurium nanowires with diameters of 4-9 nm using poly(vinyl pyrrolidone) (PVP) as a capping agent can be well controlled through an easy acetone-addition process. Ultrathin Te nanowires synthesized by a hydrothermal method using PVP as a capping agent will aggregate in a water/acetone system, and their aggregation state strongly relies on the volume of water and acetone in this mixed solution. This phenomenon is due to the different solubility of PVP in water and acetone, which has significant influence on the dispersibility and stabilization of the nanowires. The results also demonstrate that the freshly prepared Te nanowires are not stable after being stored for a prolonged time in contact with air, ethanol, and water. Ultrathin Te nanowires can be oxidized easily with various final morphologies, which are core-shell structures in contact with air, amorphous nanoparticles and nanoplatelets in ethanol, and large square flakes in water. The entire conversion process from crystalline Te nanowires to amorphous TeO2 nanoparticles or single-crystal paratellurite (TeO2) at room temperature was carefully studied, implying that tellurium nanowires synthesized by other chemical methods and other nanomaterials after synthesis could also not be stable, and their storage methods require special attention.

Few electron double quantum dots in InAs/InP nanowire heterostructures

We report on fabrication of double quantum dots in catalytically grown InAs/InP nanowire heterostructures. In the few-electron regime, starting with both dots empty, our low-temperature transport measurements reveal a clear shell structure for sequential charging of the larger of the two dots with up to 12 electrons. The resonant current through the double dot is found to depend on the orbital coupling between states of different radial symmetry. The charging energies are well described by a capacitance model if next-neighbor capacitances are taken into account.

A novel ultrasonic-assisted solution-phase approach for the fabrication of tellurium bundles of nanowhiskers

An ultrasonic-assisted solution-phase approach to tellurium bundles of nanowhiskers has been successfully established. It is a convenient and efficient process for the formation of Te nanowhiskers and their simultaneous assembly into tellurium bundles without using any templates. It was found that the transformation from Te powder to Te nanowhiskers involved the reversible disproportionation of tellurium. Sonication played a critical role for the formation of the Tellurium bundles of nanowhiskers. The bandgap of the Te nanowhiskers was calculated to be about 3.8 eV based on the UV-visible spectrum. The simplicity and rapidity of the procedure, and the newly discovered uniform morphology of the products made this synthesis promising and potential for related future applications.

High-quality luminescent tellurium nanowires of several nanometers in diameter and high aspect ratio synthesized by a poly (vinyl pyrrolidone)-assisted hydrothermal process

Large-scale selective synthesis of uniform single crystalline tellurium nanowires with a diameter of 4-9 nm, and microbelts with a width of 250-800 nm and tens of micrometers in length, can be realized by a poly (vinyl pyrrolidone) (PVP)-assisted hydrothermal process. The formation of tellurium nanowires and nanobelts in the presence of PVP is strongly dependent on the reaction conditions such as temperature, the amount of PVP, and reaction time. The results demonstrated that the keys for selective synthesis of Te nanobelts and nanowires are to modulate the growth rates of (100), (101), and (110) planes in the presence of PVP and to precisely control the reaction kinetics. High-quality luminescent ultrathin t-Te nanowires with a diameter of 4-9 nm display strong luminescent emission in the blue-violet region. This approach provides a facile route for the production of high-quality tellurium nanostructures with an interesting optical property. Furthermore, the synthesized ultrathin nanowires with deep blue color and nanobelts in gray color by this approach can be well dispersed in water or ethanol, making it possible for further engineering of their surfaces to prepare other hybrid core-shell nanostructures.

A general route for the rapid synthesis of one-dimensional nanostructured single-crystal Te, Se and Se-Te alloys directly from Te or/and Se powders

A general and template-free 'disproportionation and reversal' route was developed to synthesize one-dimensional (1D) nanostructures of Te, Se and Se-Te alloys directly from Te or/and Se powders. The products were characterized by x-ray diffraction (XRD), transmission electron microscopy (TEM), selected area electron diffraction (SAED), and scanning electron microscopy (SEM). Te nanorods and nanowires with a width varying from about 40 nm to about 300 nm, Se nanowires with a width of 60-100 nm and a length of 4-6 µm, and SexTe100-x alloy nanorods with x in a wide range, and with a width of 30-70 nm and an aspect ratio of three to five, were prepared. The mechanism of formation of the nanorods and nanowires and the effects of the experimental conditions, such as solution concentration, cooling rate, solvent nature and heating process, on the morphology and size of the products have been discussed. We believe that this general route and some other proper reversible processes between solid state and solution state can be extended to the transformations from various bulk materials into nanosized materials with various morphologies.

Self-assembled boron nanowire Y-junctions

In this work, we demonstrate that boron nanowire Y-junctions can be synthesized in a self-assembled manner by fusing two individual boron nanowires grown inclined toward each other. We show that the presence of a second liquid, in addition to the liquid Au catalyst, is critical to the inclination of the boron nanowire. The structure of the BNYJ arrays that we report here may allow construction of three- or multiple-terminal nanowire devices directly on Si-based readout circuits through controlled nanowire growth.

Buckling instabilities in GaN nanotubes under uniaxial compression

We report experimental observations of shell buckling instabilities in free-standing, vertically aligned GaN nanotubes subjected to uniaxial compression. Highly uniform arrays of the GaN nanotubes standing on a GaN template were fabricated and subjected to uniaxial compression using a nanoindenter. The buckling load was found to be of the order of 150 microN for the GaN nanotubes with an outer radius of 40 nm, an inner radius of 20 nm, and heights of 500 and 300 nm. Good agreement was found between the experimental observations, the stress-strain relation equation study findings and the predictions from the cylindrical shell buckling theory.

Thermal stability of benzorods: molecular-dynamics simulations

Thermal stability of benzorods 2C6-20C6, which are obtained by stacking n (n=2-20) dehydrogenated benzene, have been investigated by molecular-dynamics simulations. It has been found that these structures assume a geometrical form depending on the number of dehydrogenated benzene layers, and they are stable under heat treatment up to elevated temperatures with a dependence on length.

Sonochemical Fabrication and Characterization of Stibnite Nanorods

Regular stibnite (Sb(2)S(3)) nanorods with diameters of 20-40 nm and lengths of 220-350 nm have been successfully synthesized by a sonochemical method under ambient air from an ethanolic solution containing antimony trichloride and thioacetamide. The as-prepared Sb(2)S(3) nanorods are characterized by employing techniques including X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, and optical diffuse reflection spectroscopy. Microstructural analysis reveals that the Sb(2)S(3) nanorods crystallize in an orthorhombic structure and predominantly grow along the (001) crystalline plane. High-intensity ultrasound irradiation plays an important role in the formation of these Sb(2)S(3) nanorods. The experimental results show that the sonochemical formation of stibnite nanorods can be divided into four steps in sequence: (1) ultrasound-induced decomposition of the precursor, which leads to the formation of amorphous Sb(2)S(3) nanospheres; (2) ultrasound-induced crystallization of these amorphous nanospheres and generation of nanocrystalline irregular short rods; (3) a crystal growth process, giving rise to the formation of regular needle-shaped nanowhiskers; (4) surface corrosion and fragmentation of the nanowhiskers by ultrasound irradiation, resulting in the formation of regular nanorods. The optical properties of the Sb(2)S(3) amorphous nanospheres, irregular short nanorods, needle-shaped nanowhiskers, and regular nanorods are investigated by diffuse reflection spectroscopic measurements, and the band gaps are measured to be 2.45, 1.99, 1.85, and 1.94 eV, respectively.