Showing papers in "Small in 2006"

Optical properties of ZnO nanostructures.

Abstract: We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Metal and Metal Oxide Nanoparticles in Chemiresistors: Does the Nanoscale Matter?

Abstract: Sensor technology is one of the most important key technologies of the future with a constantly increasing number of applications, both in the industrial and in the private sectors. More and more gas sensors are used for the control of technical processes, in environment monitoring, healthcare, and automobiles. Consequently, the development of fast and sensitive gas sensors with small cross sensitivity is the subject of intense research, propelled by strategies based on nanoscience and -technology. Established systems can be improved and novel sensor concepts based on bottom-up approaches show that the sensor properties can be controlled by molecular design. This Review highlights the recent developments and reflects the impact of nanoscience on sensor technology.

Metal nanoparticles and related materials supported on carbon nanotubes: methods and applications.

Abstract: A review. C nanotubes are one of the most intensively explored nanostructured materials. In particular, C nanotubes are unique and ideal templates onto which to immobilize nanoparticles allowing the construction of designed nanoarchitectures that are extremely attractive as supports for heterogeneous catalysts, for use in fuel cells, and in related technologies that exploit the inherent smallness and hollow characteristics of the nanoparticles. Here we overview the recent developments in this area by exploring the various techniques in which nanotubes can be functionalized with metals and other nanoparticles and explore the diverse applications of the resulting materials. [on SciFinder(R)]

Semiconductor nanowires: from self-organization to patterned growth

Abstract: The synthesis of semiconductor nanowires has been studied intensively worldwide for a wide spectrum of materials. Such low-dimensional nanostructures are not only interesting for fundamental research due to their unique structural and physical properties relative to their bulk counterparts, but also offer fascinating potential for future technological applications. Deeper understanding and sufficient control of the growth of nanowires are central to the current research interest. This Review discusses the various growth processes, with a focus on the vapor-liquid-solid process, which offers an opportunity for the control of spatial positioning of nanowires. Strategies for position-controlled and nanopatterned growth of nanowire arrays are reviewed and demonstrated by selected examples as well as discussed in terms of larger-scale realization and future prospects. Issues on building up nanowire-based electronic and photonic devices are addressed at the end of the Review, accompanied by a brief survey of recent progress demonstrated so far on the laboratory level.

Nanomachining by colloidal lithography

Abstract: Colloidal lithography is a recently emerging field; the evolution of this simple technique is still in progress. Recent advances in this area have developed a variety of practical routes of colloidal lithography, which have great potential to replace, at least partially, complex and high-cost advanced lithographic techniques. This Review presents the state of the art of colloidal lithography and consists of three main parts, beginning with synthetic routes to monodisperse colloids and their self-assembly with low defect concentrations, which are used as lithographic masks. Then, we will introduce the modification of the colloidal masks using reactive ion etching (RIE), which produces a variety of nanoscopic features and multifaceted particles. Finally, a few prospective applications of colloidal lithography will be discussed.

Inkjet Printing of Electrically Conductive Patterns of Carbon Nanotubes

Abstract: The advantageous physical properties of carbon nanotubes (CNTs), such as excellent thermal conductivity, good mechanical strength, optional semiconducting/metallic nature, and advanced field-emission behavior, have been utilized in a number of different devices for several years. The area-selective synthesis of well-organized CNTs on prepatterned growth templates using either catalytic or plasma-enhanced chemical vapor deposition methods (CCVD and PECVD, respectively) opens up further novel fields for advanced future applications. However, these promising techniques require complex lithography processes and sophisticated deposition facilities (PECVD) or are limited to thermally durable growth substrates (CCVD). Recent advances in nanotube chemistry enable both the dissolution and dispersion of CNTs in various solvents. These results suggest new alternatives for fabricating CNT patterns by simply dispensing/printing the dissolved/dispersed particles on substrates. Alternatively, controlled flocculation of CNT suspensions in flow channels or on prepatterned stamps can be accomplished to produce patterns of nanotubes on various surfaces. Herein, a cost-effective and scaleable deposition method for generating conductive multi-walled carbon nanotube (MWCNT) patterns on paper and polymer surfaces is presented. MWCNTs grown by CCVD were chemically modified to make the nanotubes dispersible in water, and in turn the aqueous dispersion was dispensed on various substrates using a commercial desktop inkjet printer. The electrical behavior of the printed patterns is investigated and the limitations of the process are discussed. For functionalization (Figure 1a), the MWCNTs were first refluxed in nitric acid to produce carboxyl, hydroxyl, and carbonyl groups at the defect sites of the outer graphene layer of the nanotubes. In a subsequent step, these hydroxyl and carbonyl groups were oxidized further with potassium permanganate solution (in perchloric acid) to achieve additional carboxyl groups on the surfaces of the nanotubes. Modifications of the as-grown CNT structure may be identified by comparison of the Raman spectra ACHTUNGTRENNUNGof the as-produced nanotubes (Figure 1b) and the fully ACHTUNGTRENNUNGfunctionalized nanotubes (Figure 1c) in the vicinity of the

Realization of a silicon nanowire vertical surround-gate field-effect transistor.

Abstract: Semiconducting nanowires have recently attracted considerable attention. With their unique electrical and optical properties, they offer interesting perspectives for basic research as well as for technology. A variety of technical applications, such as nanowires as parts of sensors, and electronic and photonic devices have already been demonstrated. In particular, electronic applications come more and more into focus, as the ongoing miniaturization in microelectronics demands new innovative solutions. Semiconducting nanowires, in particular epitaxially grown silicon (Si) nanowires, are considered as promising candidates for post-CMOS (CMOS: complementary metal–oxide semiconductor) logic elements owing to their potential compatibility with existing CMOS technology. One major advantage of vapor–liquid– solid(VLS-) grown nanowires compared to top-down fabricated devices is that they have well-defined surfaces. This reduces surface scattering, an issue which becomes important for devices on the nanoscale. Moreover, epitaxially grown nanowires circumvent the problem of handling and positioning nanometer-sized objects that arises in the conventional pick-and-place approach, where devices are fabricated by manipulating horizontally lying VLS-grown nanowires. The first step towards a technical realization of a nanowire logic element is the design and manufacturing of a nanowire transistor. The epitaxial growth of vertical nanowires offers advantages over other approaches: For example, the transistor gate can be wrapped around the vertically oriented nanowire. Such a wrapped-around gate allows better electrostatic gate control of the conducting channel and offers the potential to drive more current per device area than is possible in a conventional planar architecture. In this Communication, a generic process for fabricating a vertical surround-gate field-effect transistor (VS-FET) based on epitaxially grown nanowires is described. Exemplarily, we used Si nanowires and present a first electrical characterization proving the feasibility of the process developed and the basic functionality of this device. Figure 1a shows a schematic cross section through a conventional p-type MOSFET. In such a device, an inversion channel can be created close to the gate by applying a negative gate voltage. This forms a conducting channel that connects the p-doped regions between the source and drain contacts electrically. Using this concept, a silicon nanowire VS-FET would ideally require a nanowire that is n-doped in the region of the gate and p-doped elsewhere. Unfortunately, such a p-n-p structure with abrupt transitions appears difficult to realize if the nanowires are grown by means of the vapor–liquid–solid mechanism using gold as a catalyst. The difficulty here is that the dopant atoms, which are dissolved in the catalyst droplet, might act as a reservoir, thus creating a graded transition when switching to another dopant. Therefore, we used a structure consisting of an n-doped silicon nanowire grown on a p-type substrate (see Figure 1b). If the gate–drain and gate–source distances are not too long, it is electrostatically still possible to create an inversion channel along the length of the entire wire. In the proposed configuration, the p–n junction at the source contact (Figure 1a) is replaced by a Au/n-Si Schottky contact at the nanowire tip. In order to investigate the influence of the Au/n-Si Schottky contact on the nanowire (current–voltage) I–V characteristics, an array of n-doped nanowires vertically grown on an n-type (111)-oriented substrate was imbedded in a spin-coated SiO2 matrix. After removing the thin SiO2 coverage from the Au tips by a short reactive ion etching, contacts 0.6 mm in size were defined by evaporating aluminum onto the sample, such that approximately 10 nanowires were contacted in parallel. The temperature-dependent measurements (shown in Figure 2) were performed by applying a voltage to the Si substrate, while the Al top contact was held at a constant potential. The measurements reveal a strong rectifying behavior with a thermally activated current possessing an activation energy of 0.6 eV. This can be explained by the Au/n-Si Schottky contact dominating the I–V behavior. The fact that the Schottky contact is forward-biased for negative voltages furthermore proves that, as expected, electrons act as majority charge carries. Figure 1. Schematics of a) a conventional p-channel MOSFET and b) a silicon nanowire vertical surround-gate field-effect transistor.

Adverse effects of citrate/gold nanoparticles on human dermal fibroblasts.

Abstract: Nanoscale engineering is one of the most dynamically growing areas at the interface between electronics, physics, biology, and medicine. As there are no safety regulations yet, concerns about future health problems are rising. We investigated the effects of citrate/gold nanoparticles at different concentrations and exposure times on human dermal fibroblasts. We found that, as a result of intracellular nanoparticle presence, actin stress fibers disappeared, thereby inducing major adverse effects on cell viability. Thus, properties such as cell spreading and adhesion, cell growth, and protein synthesis to form the extracellular matrix were altered dramatically. These results suggest that the internal cell activities have been damaged.

Methotrexate-immobilized poly(ethylene glycol) magnetic nanoparticles for MR imaging and drug delivery.

Abstract: We report the development of a biostable methotrexate-immobilized iron oxide nanoparticle drug carrier that may potentially be used for real-time monitoring of drug delivery through magnetic resonance imaging. Methotrexate (MTX) was immobilized on the nanoparticle surface via a poly(ethylene glycol) self-assembled monolayer (PEG SAM). The cytotoxicity of the nanoparticle-drug conjugate (NP-PEG-MTX) to target cells was studied with 9L glioma cells. Cellular uptake experiments showed that the uptake of NP-PEG-MTX conjugates by glioma cells was considerably higher than that of control nanoparticles. Magnetic resonance imaging in 9L cells cultured with NP-PEG-MTX of various concentrations showed significant contrast enhancement. NP-PEG-MTX demonstrated higher cytotoxicity in 9L cells to free MTX in vitro. Leucovorin, an MTX antidote, was used to rescue the cells that had been exposed to NP-PEG-MTX or free MTX, and the experiment verified the biocompatibility of NP-PEG-MTX conjugates and the MTX on NP-PEG-MTX conjugates to be the true source of the cytotoxicity to the target cells. TEM results showed that NP-PEG-MTX conjugates were internalized into the 9L cellular cytoplasm and retained its crystal structure therein for up to 144 h, as identified by electron diffraction. This prolonged particle retention may allow physicians to image tumor cells exposed to the NP-PEG-MTX conjugate over an extended therapeutic time course.

Extracellular biosynthesis of magnetite using fungi.

Abstract: The development of synthetic processes for oxide nanomaterials is an issue of considerable topical interest. While a number of chemical methods are available and are extensively used, the collaborations are often energy intensive and employ toxic chemicals. On the other hand, the synthesis of inorganic materials by biological systems is characterized by processes that occur at close to ambient temperatures and pressures, and at neutral pH (examples include magnetotactic bacteria, diatoms, and S-layer bacteria). Here we show that nanoparticulate magnetite may be produced at room temperature extracellularly by challenging the fungi, Fusarium oxysporum and Verticillium sp., with mixtures of ferric and ferrous salts. Extracellular hydrolysis of the anionic iron complexes by cationic proteins secreted by the fungi results in the room-temperature synthesis of crystalline magnetite particles that exhibit a signature of a ferrimagnetic transition with a negligible amount of spontaneous magnetization at low temperature.

Shape Control of II–VI Semiconductor Nanomaterials

Abstract: Anisotropic II-VI semiconductor nanocrystals and nanoparticles have become important building blocks for (potential) nanotechnological applications. Even though a wide variety of differently shaped nanoparticles of this class can be prepared, the underlying mechanisms are mostly not fully understood. This Review article provides a brief overview of the currently studied shape-evolution mechanisms and the most prominent synthesis methods for such particles, with an aim to provide a fundamental understanding on how different morphologies evolve, and to function as a tool to aid in the preparation of specific nanocrystals.

Controlled Arrangement of Nanoparticle Arrays in Block-Copolymer Domains

Abstract: This Review describes recent results on the precise spatial distribution control of metal and semiconductor nanoparticles into domains of microphase-separated block copolymers. Specific focus is directed towards selective incorporation into a specific microphase of a block copolymer. Details on theoretical aspects concerning nanoparticle incorporation as well as practical examples are given. Furthermore, examples on applications and technological aspects of the resulting nanoparticle/polymer nanocomposites are provided.

Specific Targeting, Cell Sorting, and Bioimaging with Smart Magnetic Silica Core–Shell Nanomaterials

Abstract: Magnetic nanoparticles (MNPs) have been used in various areas, such as in the manufacture of bearings, seals, lubricants, heat carriers, and in printing, recording, and polishing media. [1] One of the rapidlydeveloping research subjects involving MNPs is their application in biological systems, including their application in magnetic resonance imaging (MRI), targeted drug delivery, rapid biological separation, biosensors, and magnetic hyperthermia therapy. [2] The exploration of the interaction between nanostructured materials and living systems is of fundamental and practical interest, and it opens new doors to novel interdisciplinaryresearch field, “nanobioscience”. MNPs exhibited great potential for in vitro and in vivo biomedical application, [3] and the biodistribution of MNPs is stronglyinfluenced bytheir size, charge, and surface chemistry. [4] Recentlypublished reports indicate that magnetic nanoparticles (or microparticles), Fe3O4, conjugated with various targeting molecules or antibodies, can be used to target specific cells in vitro. [5] However, the noncovalent surface modification of nanoparticles

Beaklike SnO2 nanorods with strong photoluminescent and field-emission properties.

Abstract: Beaklike SnO2 nanorods were synthesized by a vapor-liquid-solid approach using Au as a catalyst. The nanorods grow along the [10 1] direction and the beak is formed by switching the growth direction to [1 12] through controlling the growth conditions at the end of the synthesis. The photoluminescence (PL) spectrum of the nanorods exhibits visible light emission with a peak at 602 nm. The field-emission (FE) properties of the nanorods have been measured to exhibit a turn-on field of 5.8 V microm(-1). A comparative study of FE measurements between SnO2 nanorods with uniform diameters and these beaklike nanorods suggests that the shape and curved tips are important factors in determining the FE properties.

Reversible and controllable switching of a single-molecule junction.

Abstract: This phenomen-on is caused by statisticalfluctuations in the moleculeitself or the molecule–metalcontact. The lack of controlover this type of switchingmakesitessentiallyundesira-ble for any technological ap-plication. Recent experi-ments aiming to identify thefundamental mechanisms re-sponsibleforvoltage-inducedswitching in sandwich struc-tures

Cicada wings: a stamp from nature for nanoimprint lithography.

Abstract: Nanoimprint lithography (NIL) has been proposed as an alternative approach for the fabrication of nanostructures with critical dimensions on the wafer scale (sub 100 nm). The smallest structures that have been successfully fabricated by NIL so far are 6-nm half-pitch lines. Also, imprinting processes have already been demonstrated on 8-inch wafers with lines, dots and rings. Compared with other lithography techniques, such as scanning-probe lithography (SPL), electron-beam lithography (EBL), and photolithography (PL), NIL has advantages of high throughput, high resolution, and low cost, and thus has proved to be a promising candidate for next-generation lithography. However, a lot of challenges in terms of overprinting, imprinting with microand nanocombined structures, and imprinting on irregular surfaces remain. Many efforts have been made to develop the alignment system to resolve the problem of microand nanostructure imprinting, and to perform the nanoimprint lithography on nonflat surfaces. Preparation of stamps with high resolution over a large area is a key procedure in NIL. Besides the traditional methods mentioned above, there are many other developed methods of preparing the stamps. For example, a stamp with a silicon-tip array has been fabricated by Ar ions bombarding Ti dots on a silicon wafer. Large-area pillar arrays have been prepared using polymer-colloid masks, and biomolecules can also act as a stamp in imprinting. However, these are usually time-consuming and complicated and in some cases, expensive to carry out. Many nanostructures exist in nature. A very good example is the microand nanostructures that exist on the Morpho sulkowskyi butterfly scale, which, through lightscattering and diffraction, result in the butterfly8s charming blue color. 18] Combined microand nanostructures found on the surface of the leaves of some plants exhibit superhydrophobicity, which is described as the “lotus effect”. In addition, arrays of tapered pillars have also been found on cicada wings and moth eyes, which can greatly minimize the reflectivity on their surfaces over broad angles or frequency ranges. In fact, many efforts have been made to replicate or directly utilize these bio ACHTUNGTRENNUNGnanostructures for special applications. 25] In this work, the direct use of these nanostructures as stamps for NIL is reported. With these natural stamps, nanowell arrays (negative structures of cicada wings) have been fabricated on a polymer support. Further more, the nanowell arrays can be transferred to the silicon substrate by reactive ion etching (RIE), and thus exhibit an antireflective property. With patterned poly(methyl methacrylate) (PMMA) as a mold, hexagonal gold-pillar arrays similar to the surface of cicada wings can also be obtained by thermodeposition. The cicadas (Cryptympana atrata Fabricius) are either captured locally or bought from specimen factories. As shown in Figure 1, the microscopic structures of the cicada

Conversion of a Bi nanowire array to an array of Bi-Bi2O3 core-shell nanowires and Bi2O3 nanotubes.

Abstract: A template-based heat-treatment method has been developed to convert metal nanowire arrays into arrays of metal–metal oxide core–shell nanowires and single-crystalline metal oxide nanotubes. This process is demonstrated by kinetically controlling the conversion of single-crystalline Bi nanowires to Bi–Bi2O3 core–shell nanowires via a multistep, slow oxidation method, and then controlling their further conversion to a single-crystalline Bi2O3 nanotube array via fast oxidation. This process can conveniently be extended to fabricate a free-standing, easily oxidized metal–metal oxide nanowire and metal oxide nanotube array, which may have future applications in nanoscale optics, electronics, and magnetics.

Dramatic effect of dispersed carbon nanotubes on the mechanical and electroconductive properties of polymers derived from ionic liquids.

Abstract: Free-radical polymerization of an imidazolium ion-based ionic liquid bearing a methacrylate group, gelling with single-walled carbon nanotubes (SWNTs), allows fabrication of a mechanically reinforced, electroconductive soft material (bucky plastic). A film sample of this material displays an excellent conductivity of 1 S cm(-1) and a 120-fold enhancement of the Young's modulus at a 7 wt % content of SWNTs. The conductivity is temperature-dependent in the range 5-300 K, suggesting that the conductive process involves carrier hopping. Scanning electron and atomic force micrographs of a bucky plastic film display the presence of crosslinked networks consisting of finely dispersed SWNTs. Such nanotube networks, developed in the polymer matrix, likely suppress slipping of entrapped polymer molecules via a strong interfacial interaction and also facilitate intertubular carrier transport. Although a bucky plastic derived from a vinylimidazolium ion-based ionic liquid monomer shows a comparable conductivity to that of the methacrylate version, the film is brittle irrespective of the presence or absence of SWNTs.

Template-assisted large-scale ordered arrays of ZnO pillars for optical and piezoelectric applications.

Abstract: Spatially separated ZnO pillars, typically 300 nm in diameter and 2 microm in height, are fabricated via a template-directed approach that leads to long-range hexagonal order. The templates of Au nanodisk arrays are obtained by using metal membranes as a lithography mask. The growth of ZnO pillars is performed in a double-tube system through vapor diffusion-deposition. The growth mechanism of the pillars is studied in detail and is proposed to be a combination of vapor-liquid-solid and vapor-solid models. The piezoelectric and optical properties of single pillars are characterized using piezoresponse force microscopy and micro-photoluminescence spectroscopy, respectively. The pillars show strong excitonic emissions up to room temperature, which indicate a relatively low defect density and good crystalline quality. The obtained piezoelectric coefficient d(33) is (7.5+/-0.6) pm V(-1), which is to our knowledge the first reported value for a single nanopillar.

Rapid, room-temperature synthesis of antibacterial bionanocomposites of lysozyme with amorphous silica or titania.

Abstract: : Lysozyme directs the formation of silica or titania nanoparticles under ambient conditions and is simultaneously entrapped while in the active bactericidal form. The ability to encapsulate an active antimicrobial protein within inorganic nanoparticles provides an opportunity to create bionanocomposite materials that resist bacterial activity, for use as broad spectrum antifouling materials.

pH-sensitive dispersion and debundling of single-walled carbon nanotubes: lysozyme as a tool.

Abstract: Highly dispersed and debundled carbon nanotubes were prepared in an aqueous solution of lysozyme using a combination of ultrasonication and ultracentrifugation. The product is a pH-sensitive dispersion, which remains in a highly dispersed state at pH 11, but in an aggregated state at pH 8-11. Photoluminescence measurements show that by changing the pH value, a reversible conversion of the highly dispersed state to the aggregated state (or vice versa) could be observed. Circular dichromism analysis confirmed that the secondary structure, as well as the majority of the tertiary structure, remains intact. Some lysozyme molecules were irreversibly bound to the nanotubes, which is possibly due to pi-pi or hydrophobic interactions. However, these interactions alone are not enough to produce fine dispersions of the nanotubes. Protonated amine interactions on the defect sites of the nanotubes play a vital role in the stabilization of the nanotubes below the isoelectric point and amine adsorption on the sidewalls of nanotubes occurs in cases where the pH value is higher than the isoelectric point.

The empirical correlation between size and two-photon absorption cross section of CdSe and CdTe quantum dots.

Abstract: The tuning of CdSe quantum dot (QDs) sizes, and consequently their corresponding two-photon absorption (TPA) cross section, has been systematically investigated. As the size (diameter) of the quantum dots increases, the TPA cross section is found to be empirically related via a power-law proportionality of 3.5±0.5 and 5.6±0.7 to the diameters of CdSe and CdTe QDs, respectively. The results are tentatively rationalized via a theoretical model of two-photon excitation properties in a system incorporating excitons and defects.