Architectural control of magnetic semiconductor nanocrystals

Facile fabrication of NH4CoPO4·H2O nano/microstructures and their primarily application as electrochemical supercapacitor

Various NH(4)CoPO(4)·H(2)O nano/microstructures (oblong plate, microplate, microflower, hierarchical architectures) have been synthesized through a facile chemical precipitation method without surfactants and templates. More importantly, the supercapacitive performances of NH(4)CoPO(4)·H(2)O nano/microstructures were firstly studied using cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy methods in 3.0 M KOH solution. These results indicated that NH(4)CoPO(4)·H(2)O hierarchical architectures electrodes exhibit effective supercapacitive characteristics in aqueous KOH electrolyte. The specific capacitance of NH(4)CoPO(4)·H(2)O electrode is up to 369.4 F g(-1) at a current density of 0.625 A g(-1) and the material has a long cycle life which can maintain 99.7% of initial specific capacitance after 400 cycles.

Synthesis and applications of CdSe nano-tetrapods in hybrid photovoltaic devices

Tetrapod-shaped nanocrystals have attracted increasing interest for optoelectronic applications in recent years due to their rich morphologies. With unique properties such as a direct band-gap and excellent photoelectrical characteristics, CdSe nano-tetrapods are promising nanostructures for applications in such fields as photodetectors, field emitters, and photovoltaic devices. This review mainly describes the remarkable progress made in synthesis and hybrid photovoltaic applications of CdSe nano-tetrapods over the last few years. In particular, the “blinking” effect observed from these nano-tetrapods in chloroform solution is highlighted. This overview covers the current state of the art as well as an outlook on possibilities and limitations.

Effect of metal ions on photoluminescence, charge transport, magnetic and catalytic properties of all-inorganic colloidal nanocrystals and nanocrystal solids

Colloidal semiconductor nanocrystals (NCs) provide convenient "building blocks" for solution-processed solar cells, light-emitting devices, photocatalytic systems, etc. The use of inorganic ligands for colloidal NCs dramatically improved inter-NC charge transport, enabling fast progress in NC-based devices. Typical inorganic ligands (e.g., Sn(2)S(6)(4-), S(2-)) are represented by negatively charged ions that bind covalently to electrophilic metal surface sites. The binding of inorganic charged species to the NC surface provides electrostatic stabilization of NC colloids in polar solvents without introducing insulating barriers between NCs. In this work we show that cationic species needed for electrostatic balance of NC surface charges can also be employed for engineering almost every property of all-inorganic NCs and NC solids, including photoluminescence efficiency, electron mobility, doping, magnetic susceptibility, and electrocatalytic performance. We used a suite of experimental techniques to elucidate the impact of various metal ions on the characteristics of all-inorganic NCs and developed strategies for engineering and optimizing NC-based materials.

Heteroepitaxial growth of GaSb nanotrees with an ultra-low reflectivity in a broad spectral range

We report on the growth of GaSb nanotrees on InAs { ̅1 ̅1 ̅1}(B) substrates by chemical beam epitaxy. GaSb nanotrees form by the nucleation of Ga droplets on the surface of < ̅1 ̅1 ̅1>(B) oriented GaSb nanowires followed by the epitaxial growth of branches catalyzed by these Ga droplets. In the tip region, the trunks of the GaSb nanotrees are periodically twinned, which is attributed to a change of the effective V/III ratio at the later stage of growth as a consequence of the change in surface structure. The reflectivity of a forest of nanotrees was measured for a broad spectral range and compared to the reflectivity of a GaSb ( ̅1 ̅1 ̅1)(B) wafer and of GaSb nanowires. At wavelengths from 500 to 1700 nm, the presence of GaSb nanotrees decreased the reflection by three orders of magnitude compared to a blank GaSb substrate.

Epitaxial II-VI tripod nanocrystals: a generalization of van der Waals epitaxy for nonplanar polytypic nanoarchitectures

We report for the first time the synthesis of nonplanar epitaxial tripod nanocrystals of II-VI compounds (ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, and CdTe) on muscovite mica substrate. With CdS as a case study, we conclude via Raman spectroscopy and electron microscopy studies that the tripods, which are found to be polytypic, followed a seeded growth mechanism. The epitaxy, manifested by the in-plane alignment of the legs of the tripods within a substrate, is attributed to the van der Waals interaction between the tripod bases and the mica surface, instead of to the covalent chemical bond which would require lattice matching between the epilayer and the substrate. The results demonstrated herein could have widespread immediate implications, including the potential of van der Waals epitaxy to be applicable in producing ordered arrays of more complex nanoarchitectures from various classes of compounds toward a broad range of technological applications.

In situ reversible tuning of photoluminescence of Mn2+-doped ZnS quantum dots by redox chemistry

Herein we report the development of a new method for in situ reversible tuning of photoluminescence properties of quantum dots (Qdots) by partial oxidation of population of the emitting species and subsequent chemical reduction of the oxidized form. The concept has been demonstrated using Mn(2+)-doped ZnS Qdots stabilized by acetyl acetonate. Treatment of an aqueous solution of the Qdots (with Mn(OAc)(2) being the source of Mn used for the synthesis of the Qdots) by potassium peroxodisulfate (KPS) led to reduction of intensity of emission due to Mn(2+) ((4)T(1)-(6)A(1)). Subsequent treatment of the solution containing KPS-treated Qdots with NaBH(4) led to regaining of intensity, thus providing reversibility to the tuning, which was possible for more than one cycle. Electron spin resonance (ESR) spectroscopic investigations revealed reduction of the population of Mn(2+) upon treatment with KPS, whereas it went back up upon further treatment with NaBH(4). Interestingly, a mixed population of oxidation states of Mn was indicated to be present in the Qdots prepared using KMnO(4) as the source of Mn. The fluorescence intensity of the Qdots so prepared increased upon treatment with NaBH(4) following synthesis, which was not possible when the source of Mn was Mn(OAc)(2). Transmission electron microscopic and X-ray diffraction studies indicated that oxidation and reduction did not change the sizes of Qdots significantly.

Sonocatalytic degradation and catalytic activities for MB solution of Fe treated fullerene/TiO2 composite with different ultrasonic intensity

Fe-fullerene/TiO(2) composite catalysts were prepared with titanium (IV) n-butoxide (TNB) by a sol-gel method. The samples were characterized by scanning electron microscopy (SEM), transmission electron microscope (TEM), specific surface area (BET), X-ray diffraction analysis (XRD) and energy dispersive X-ray spectroscopy (EDX). The catalytic activities were evaluated by the catalytic oxidation of methylene blue (MB) solution. XRD patterns of the composites showed that the Fe-fullerene/TiO(2) composite contained a typical single and clear anatase phase. The surface properties shown by SEM present a characterization of the texture on Fe-fullerene/TiO(2) composites and showed a homogenous composition in the particles for the titanium sources used. The EDX spectra for the elemental identification showed the presence of C and Ti with strong Fe peaks for the Fe-fullerene/TiO(2) composite. The degradation of MB solution by ultrasonic irradiation in the presence of Fe-fullerene/TiO(2) compounds was investigated in complete darkness. With the increase in the amount of Fe, the degradation rate of methylene blue solution also increased.

Control of luminescence emitted by Cd 1-x Mn x S nanocrystals in a glass matrix: x concentration and thermal annealing

Cd(1-x)Mn(x)S nanocrystals (NCs) were successfully grown in a glass matrix and investigated by photoluminescence (PL), electron paramagnetic resonance (EPR) and magnetic force microscopy (MFM). We verified that the luminescent properties of these NCs can be controlled both by changing the x concentration and by thermal annealing of the samples. The EPR and PL data showed that the characteristic emission of Mn(2+) ions ((4)T(1)-(6)A(1)) is only observed when this magnetic impurity is substitutionally incorporated in the Cd(1-x)Mn(x)S NC core (site S(I)). Besides, it was observed that the emission ((4)T(1)-(6)A(1)) suppression, caused by the Mn(2+) ion presence near the surface (site S(II)) of the Cd(1-x)Mn(x)S NCs, is independent of the host material. The MFM images also confirmed the high quality of the Cd(1 - x)Mn(x)S NC samples, showing a uniform distribution of total magnetic moments in the nanoparticles.

Size-controlled aggregation of cube-shaped EuS nanocrystals with magneto-optic properties in solution phase

Size-controlled formation of colloidal aggregates composed of cube-shaped EuS nanocrystals, EuS NCs, in solution phase are reported and their optical and magneto-optical properties are studied. The average size of the colloidal particles of the EuS NCs-aggregates formed in 1-pentanol, 1-hexanol, and 1-octanol were ca. 800, 500, and 100 nm, respectively. Self-organized cubic-type superlattice structure was evaluated in the colloidal aggregates by means of the small-angle X-ray diffraction measurements, which is similar to those in the macroscopic 3D superlattice structures. The distances between NCs in the EuS NCs-aggregates are dependent on alkyl chain length of the solvent alcohol. Magneto-optic properties of EuS NCs-aggregates and the monomeric EuS nanocrystals in liquid media are characterized with magnetic circular dichroism spectra. The active wavelength of EuS NCs-aggregates is considerably longer than that of the monomeric EuS nanocrystals.

Colloidal hybrid nanostructures: a new type of functional materials

One key goal of nanocrystal research is the development of experimental methods to selectively control the composition and shape of nanocrystals over a wide range of material combinations. The ability to selectively arrange nanosized domains of metallic, semiconducting, and magnetic materials into a single hybrid nanoparticle offers an intriguing route to engineer nanomaterials with multiple functionalities or the enhanced properties of one domain. In this Review, we focus on recent strategies used to create semiconductor-metal hybrid nanoparticles, present the emergent properties of these multicomponent materials, and discuss their potential applicability in different technologies.

Complex ZnO nanotree arrays with tunable top, stem and branch structures

Hierarchical tree-, mushroom- and cockscomb-like ZnO arrays with increasing branching order and complexities have been grown in situ on cheap zinc plates by a simple hydrothermal oxidation approach. Their morphology, crystal structure and orientation relationship are characterized by powder X-ray diffraction, scanning electron microscopy (SEM) and cross-sectional high-resolution transmission electron microscopy (HRTEM). The wurtzite ZnO arrays, growing mainly in the [0001] direction, show a special orientation relationship between the stem and the branch as well as a novel stem-branch boundary which might be attributed to the least mismatch between [symbol: see text] and (0002) lattice planes. The co-solvent ethylenediamine (en) was used to control the morphology and complexing of these complex ZnO nanostructures. Correspondingly, the physical properties of ZnO nanostructure assembly arrays were tuned and a stronger UV emission was observed with negligible emissions in the visible range, indicating the highly crystalline features of the complex ZnO micro-/nanostructured materials.

Ascorbate-assisted growth of hierarchical ZnO nanostructures: sphere, spindle, and flower and their catalytic properties

A simple solution-based method to prepare mainly flowerlike zinc oxide (ZnO) nanostructures using the ascorbate ion as a shape-directing/capping agent at relatively low temperature (ca. 30 and 60 degrees C) was described. However, we observed that different shapes of hierarchical ZnO nanostructures such as flowerlike, spindlelike, and spherical could be obtained with an increase in the synthesis temperature from 60 to 90 degrees C. The effects of other organic capping agents on the shape of hierarchical ZnO nanostructures were also studied. FTIR, FESEM, and XRD characterization were performed on the formed ZnO nanostructures to understand the role of ascorbate in the growth of flowerlike morphology. The nucleation and growth process can regulate by changing the metal precursor and ascorbate ion concentrations. We were able to identify intermediate nanostructures such as spherical/quasi-spherical and spindle that are very much on the pathway of formation of large, flowerlike ZnO nanostructures. Electron microscopy results indicated that these spherical/quasi-spherical ZnO nanoparticles might aggregate through oriented attachment to produce spindlelike and flowerlike nanostructures. On the basis of these results, a possible growth mechanism for the formation of flowerlike ZnO nanostructures was described. The optical properties of these differently shaped ZnO nanostructures were also described. The catalytic activities of the as-synthesized spherical and flowerlike ZnO nanostructures were tested in the Friedel-Crafts acylation reaction of anthracene with benzoyl chloride. The catalysis results indicated that the catalytic activity of flowerlike ZnO nanostructures is slightly higher than the spherical counterpart.

Mn-doped ZnO nanonails and their magnetic properties

Mn-doped ZnO nanonails composed of hexagonal caps and shafts with large doping concentrations from 11.7 to 35.1 at.% were synthesized via a simple and straightforward non-aqueous solution-based route. The chemical composition, morphology and structure of the obtained nanocrystals were characterized by energy-dispersive x-ray analysis, powder x-ray diffraction and transmission electron microscopy. The results show that the nanonails possess a single-crystal wurtzite structure. Electron paramagnetic resonance results indicate that Mn(2+) ions substitute for the zinc sites in the ZnO matrix, which is also confirmed by high-resolution electron microscopy. Thermal tolerance experiments show that the Mn-doped nanonails can maintain their crystalline structure up to 700 degrees C. The influence of reaction conditions on the size and shape of the ZnO-based samples has been investigated in detail, and the possible growth mechanism of the nanonails has also been discussed.

Shape-controlled synthesis of highly crystalline titania nanocrystals

A versatile synthetic method based on solvothermal technique has been developed for the fabrication of TiO(2) nanocrystals with different shapes such as rhombic, truncated rhombic, spherical, dog-bone, truncated and elongated rhombic, and bar. The central features of our approach are the use of water vapor as hydrolysis agent to accelerate the reaction and the use of both oleic acid and oleylamine as two distinct capping surfactants which have different binding strengths to control the growth of the TiO(2) nanoparticles. We also show that the presence of an appropriate amount of water vapor along with the desired oleic acid/oleylamine molar ratio plays a crucial role in controlling size and shape of TiO(2) nanocrystals.

Unpredicted nucleation of extended zinc blende phases in wurtzite ZnO nanotetrapod arms

Tailoring the structural and electronic properties of 3D nanostructures via bottom-up techniques would pave the way for novel low-cost applications. One of such possibilities is offered by ZnO branched nanostructures like tetrapods, that have recently attracted attention for nanodevice applications from nanoelectronics to drug delivery. The conventional picture is that ZnO arms are thermodynamically stable only in the wurtzite phase. Here, we provide the first experimental evidence of unpredicted extended zinc blend phases (50-60 nm long) embedded in the arms of ZnO wurtzite tetrapods. In particular, decisive evidence is obtained from the one-to-one correlation between high lateral resolution cathodoluminescence spectroscopy, monochromatic contrast maps, and atomic resolution transmission electron microscopy images of ZnO single TPs. This observation is not specific to ZnO and can have a general validity for the understanding of the nucleation mechanisms in semiconducting 3D nanostructures for device applications.

Design and synthesis of (E)-1-((3-ethyl-2,4,4-trimethylcyclohex-2-enylidene)methyl-4-substituted benzenes from 1-(2,6,6-trimethylcyclohex-1-enyl)ethanol

A novel strategy for synthesizing (E)-1-((3-ethyl-2,4,4-trimethylcyclohex-2-enylidene)methyl-4-substituted benzenes from 1-(2,6,6-trimethylcyclohex-1-enyl)ethanol has been developed; the allylic alcohol was treated with PPh(3).HBr in methanol followed by aldehydes in the presence of a base and furnished the 1,3-dienes in moderate to good yields (79-86%).

Sonocatalytic degradation of some dyestuffs and comparison of catalytic activities of nano-sized TiO2, nano-sized ZnO and composite TiO2/ZnO powders under ultrasonic irradiation

Here, a novel sonocatalyst, composite TiO2/ZnO powder, was prepared through the combination of nano-sized TiO2 and ZnO powders. Because of the appropriate adsorbability to organic pollutants and special crystal interphase between TiO2 and ZnO particles, the composite TiO2/ZnO powder exhibits a high sonocatalytic activity under ultrasonic irradiation during the degradation of acid red B. Especially, the sonocatalytic activity of composite TiO2/ZnO powder with 4:1 molar proportion treated at 500 degrees C for 50 min showed obvious improvement compared with pure nano-sized TiO2 and ZnO powders. When the experimental conditions such as 10mg/L acid red B concentration, 1.0 g/L catalyst addition amount, pH=7.0, 20 degrees C system temperature, 100 min ultrasonic time and 50 mL total volume were adopted, the satisfactory degradation ratio and rate were obtained. All experiments indicate that the sonocatalytic method using composite TiO2/ZnO powder may be a more advisable choice for the treatments of non- or low-transparent organic wastewaters in future.

Mn-doped near-infrared quantum dots as multimodal targeted probes for pancreatic cancer imaging

This work presents a novel approach to producing manganese (Mn)-doped quantum dots (Mnd-QDs) emitting in the near-infrared (NIR). Surface functionalization of Mnd-QDs with lysine makes them stably disperse in aqueous media and able to conjugate with targeting molecules. The nanoparticles were structurally and compositionally characterized and maintained a high photoluminescence quantum yield and displayed paramagnetism in water. The receptor-mediated delivery of bioconjugated Mnd-QDs into pancreatic cancer cells was demonstrated using the confocal microscopy technique. Cytotoxicity of Mnd-QDs on live cells has been evaluated. The NIR-emitting characteristic of the QDs has been exploited to acquire whole animal body imaging with high contrast signals. In addition, histological and blood analysis of mice have revealed that no long-term toxic effects arise from MnD-QDs. These studies suggest multimodal Mnd-QDs have the potentials as probes for early pancreatic cancer imaging and detection.

The influence of capping thioalkyl acid on the growth and photoluminescence efficiency of CdTe and CdSe quantum dots

The influence of thioalkyl acid ligand was evaluated during aqueous synthesis at 100 °C and under hydrothermal conditions (150 °C) of CdTe and CdSe quantum dots (QDs). Experiments performed with 3-mercaptopropionic acid (MPA), 6-mercaptohexanoic acid (MHA) and 11-mercaptoundecanoic acid (MUA) demonstrated that the use of MHA and MUA allowed for the preparation of very small nanoparticles (0.6-2.5 nm) in carrying out the reaction under atmospheric pressure or in an autoclave and that the photophysical properties of QDs were dependent on the ligand and on the synthesis conditions. The influence of various experimental conditions, including the Te-to-Cd ratio, temperature, and precursor concentration, on the growth rate of CdTe or CdSe QDs has been systematically investigated. The fluorescence intensities of CdTe QDs capped with MPA, MHA, or MUA versus pH were also found to be related to the surface coverage of the nanoparticles.

Coalescence and interface diffusion in linear CdTe/CdSe/CdTe heterojunction nanorods

Colloidal nanorods with linear CdTe/CdSe/CdTe heterojunctions were synthesized by sequential reactant injection. After CdTe deposition at the ends of initially formed CdSe nanorods, continued heating in solution leads to Se-Te interdiffusion across the heterojunctions and coalescence to decreased aspect ratio. The Se-Te interdiffusion rates were measured by mapping the composition profile using nanobeam energy dispersive X-ray spectroscopy (EDS) along the lengths of individual nanorods aged in hot solvent for different amounts of time. The rate of nanorod coalescence was also measured and compared to model predictions using a continuum viscous flow model, which appears to provide a reasonable estimate of the coalescence rate.

Effects of a 10 T external magnetic field on the thermal decomposition of Fe, Ni, and Co acetyl acetonates

The current investigation is centered on the thermal decomposition (700 degrees C) of acetyl acetonates of Ni, Co, and Fe in a closed reactor that was conducted by employing an external magnetic field (MF) of 10T. Interestingly, reactions of Co and Ni acetyl acetonates under a 10T MF produce Co and Ni nanoparticles (NPs) coated with carbon, while Fe acetyl acetonate produces Fe3O4 uncoated with carbon. Additionally, it is observed that all the as-formed magnetic particles tend to align in one dimension along applied MF; thus, this process can be used to fabricate large arrays of magnetic nanoparticles. The effect of an applied MF to synthesize morphologically and compositionally different products from corresponding precursors with their mesoscopic organization is the key theme of the present paper, explained with a plausible mechanism.

Fabrication and characterization of superhydrophobic Sb(2)O(3) films

Superhydrophobic Sb(2)O(3) films with micro-nanoscale hierarchical structures have been successfully synthesized for the first time. The resultant materials were characterized in detail by x-ray diffraction, scanning electron microscopy, energy-dispersive x-ray spectroscopy, transmission electron microscopy and water contact angle measurements. The water static contact angle of the obtained Sb(2)O(3) film is about 159° ± 2° and the sliding angle is less than 5°. Reasonable mechanisms for the formation of micro-nanoscale hierarchical structures and for the superhydrophobic properties with a small sliding angle of the obtained Sb(2)O(3) film are also presented in this work.

Self-organization of Te nanorods into V-shaped assemblies: a Brownian dynamics study and experimental insights

Computer modeling of nanoscale processes provides critical quantitative insights into nanoscale self-organization, which is hard to achieve by other means. Starting from a suspension of Te nanorods, it was recently found that short nanorods (50 nm) self-organized into checkmark-like V-shaped assemblies over a period of a few days, whereas long nanorods (2200 nm) did not. This experimental fact was difficult to explain, and so here we use Brownian dynamics simulations of a dilute suspension of hard spherocylinders to better understand the process of self-organization. With the assumption that close encounters between nanorod tips result in their merger into V-particles, it was found that the ratio of the initial rate of nanorod formation for the short and long rods was 3760. By systematically varying the length and the concentration, we found that the concentration of the nanorods, rather their length, was primarily instrumental in setting the initial rate of checkmark formation. Using a simple kinetic model in conjunction with experimental data, we find that approximately 30,000 close encounters are required on average for a single successful merger. This study gives an important reference point for understanding the mechanism of the formation of complex nanostructured system by oriented attachment; it also can be extended to and provides conceptual leads for other self-organized systems.