Showing papers in "Small in 2005"

Chemically functionalized carbon nanotubes.

Abstract: Since their discovery, carbon nanotubes have attracted the attention of many a scientist around the world. This extraordinary interest stems from their outstanding structural, mechanical, and electronic properties. In fact, apart from being the best and most easily available one-dimensional (1D) model system, carbon nanotubes show strong application potential in electronics, scanning probe microscopy, chemical and biological sensing, reinforced composite materials, and in many more areas. While some of the proposed applications remain still a far-off dream, others are close to technical realization. Recent advances in the development of reliable methods for the chemical functionalization of the nanotubes provide an additional impetus towards extending the scope of their application spectrum. In particular, covalent modification schemes allow persistent alteration of the electronic properties of the tubes, as well as to chemically tailor their surface properties, whereby new functions can be implemented that cannot otherwise be acquired by pristine nanotubes.

Synthesis, Properties, and Applications of Iron Nanoparticles

Abstract: Iron, the most ubiquitous of the transition metals and the fourth most plentiful element in the Earth's crust, is the structural backbone of our modern infrastructure. It is therefore ironic that as a nanoparticle, iron has been somewhat neglected in favor of its own oxides, as well as other metals such as cobalt, nickel, gold, and platinum. This is unfortunate, but understandable. Iron's reactivity is important in macroscopic applications (particularly rusting), but is a dominant concern at the nanoscale. Finely divided iron has long been known to be pyrophoric, which is a major reason that iron nanoparticles have not been more fully studied to date. This extreme reactivity has traditionally made iron nanoparticles difficult to study and inconvenient for practical applications. Iron however has a great deal to offer at the nanoscale, including very potent magnetic and catalytic properties. Recent work has begun to take advantage of iron's potential, and work in this field appears to be blossoming.

Nanoscience, nanotechnology, and chemistry.

Abstract: “Nanoscience” is the emerging science of objects that are intermediate in size between the largest molecules and the smallest structures that can be fabricated by current photolithography; that is, the science of objects with smallest dimensions ranging from a few nanometers to less than 100 nanometers. In chemistry, this range of sizes has historically been associated with colloids, micelles, polymer molecules, phase-separated regions in block copolymers, and similar structures—typically, very large molecules, or aggregates of many molecules. More recently, structures such as buckytubes, silicon nanorods, and compound semiconductor quantum dots have emerged as particularly interesting classes of nanostructures. In physics and electrical engineering, nanoscience is most often associated with quantum behavior, and the behavior of electrons and photons in nanoscale structures. Biology and biochemistry also have a deep interest in nanostructures as components of the cell; many of the most interesting structures in biology—from DNA and viruses to subcellular organelles and gap junctions—can be considered as nanostructures. These very small structures are intensely interesting for many reasons. First, many of their properties mystify us. How does the flagellar motor of E. coli run? How do electrons move through organometallic nanowires? Second, they are challenging to make. Molecules are easily synthesized in large quantities, and can be characterized thoroughly. Colloids and micelles and crystal nuclei have always been more difficult to prepare (in fact, most can only be made as mixtures—a characteristic that contributes to the difficulty of colloid science) and to characterize; developing a “synthetic chemistry” of colloids that is as precise as that used to make molecules is a wonderful challenge for chemistry. Synthesizing or fabricating ordered arrays and patterns of colloids poses a different and equally fascinating set of problems. Third, because many nanoscale structures have been inaccessible and/or off the beaten scientific track, studying these structures leads to new phenomena. Very small particles, or large, ordered, aggregates of molecules or atoms, are simply not structures that science has been able to explore carefully. Fourth, nanostructures are in a range of sizes in which quantum phenomena—especially quantum entanglement and other reflections of the wave character of matter—would be expected to be important (and important at room temperature!). Quantum phenomena are, of course, the ultimate basis of the properties of atoms and molecules, but are largely hidden behind classical behavior in macroscopic matter and structures. Quantum dots and nanowires have already been prepared and demonstrated to show remarkable electronic properties; there will, I am certain, be other nanoscale materials, and other properties, to study and exploit. Fifth, the nanometer-sized, functional structures that carry out many of the most sophisticated tasks of the cell are one frontier of biology. The ribosome (Figure 1), histones and chromatin, the Golgi apparatus, the interior structure of the mitochondrion, the flagellar micromotor, the photosynthetic reaction center, and the fabulous ATPases that power the cell are all nanostructures we have only just begun to understand. Sixth, nanostructures will be the basis of nanoelectronics and -photonics. The single most important fabrication technology of our time is, arguably, microlithography: its progeny—the microprocessors and memories that it generates—are the basis for the information technology that has so transformed society in the last half-century (Figure 2). Microelectronic technology has relentlessly followed a single law—Moore s law—for almost 50 years; the popular expression of this law is “smaller is cheaper and faster”. 21] Enthusiasm for “smaller” as the guiding ideology in circuit design has recently cooled, and other features—heat dissipation, power distribution, clock synchronization, intrachip communication—have become increasingly important. Still, technical evolution in the semiconductor industry has brought the components of [*] Prof. G. M. Whitesides Department of Chemistry and Chemical Biology Harvard University Cambridge MA 02138 (USA) Fax: (+1) 617-495-9857 E-mail: gwhitesides@gmwgroup.harvard.edu

Symmetric and Asymmetric Ostwald Ripening in the Fabrication of Homogeneous Core–Shell Semiconductors

Abstract: Two methodic concepts, symmetric and asymmetric Ostwald ripening, are elucidated by solution-route syntheses of oxide and sulfide semiconductors. While the original shape of a crystallite aggregate forms the exterior appearance, the preorganization of the crystallites determines the ultimate interior space structure of the aggregate upon Ostwald ripening. Further investigations on the design of crystallite preorganization and control of the solution process will allow the construction of complex architectures, including nonspherical configurations.

Physicochemical evaluation of the hot-injection method, a synthesis route for monodisperse nanocrystals.

Abstract: The quintessence of the hot-injection method, a synthesis route for monodisperse, highly luminescent semiconductor nanocrystals, is reviewed. The separate stages of nucleation and growth of the nanocrystals are discussed in the framework of classical nucleation theory and an equilibrium model proposed by Debye. We also review the numerous adaptations of the original synthesis that currently provide colloidal nanocrystals with well-defined, size-dependent optical, electrical, and magnetic properties. The availability of these remarkable materials is one of the most promising developments in nanoscience and nanotechnology.

Extracellular biosynthesis of bimetallic Au-Ag alloy nanoparticles.

Abstract: The exposure of a mixture of 1 mM HAuCl4 and 1 mM AgNO3 solutions to different amounts of fungal biomass (Fusarium oxysporum) results in the formation of highly stable Au-Ag alloy nanoparticles with dimensions of 8-14 nm depending on metal molar fraction. The amount of cofactor NADH released by the F. oxysporum fungus plays an important role in controlling the composition of the alloy nanoparticles.

Guided growth of large-scale, horizontally aligned arrays of single-walled carbon nanotubes and their use in thin-film transistors.

Abstract: A convenient process for generating large-scale, horizontally aligned arrays of pristine, single-walled carbon nanotubes (SWNTs) is described. The approach uses guided growth, by chemical vapor deposition (CVD), of SWNTs on miscut single-crystal quartz substrates. Studies of the growth reveal important relationships between the density and alignment of the tubes, the CVD conditions, and the morphology of the quartz. Electrodes and dielectrics patterned on top of these arrays yield thin-film transistors that use the SWNTs as effective thin-film semiconductors. The ability to build high-performance devices of this type suggests significant promise for large-scale aligned arrays of SWNTs in electronics, sensors, and other applications.

Nanomaterial-based amplified transduction of biomolecular interactions.

Abstract: This article reviews progress in the development of nanomaterials for amplified biosensing and discusses different nanomaterial-based bioamplification strategies. Signal amplification has attracted considerable attention for ultrasensitive detection of disease markers and biothreat agents. The emergence of nanotechnology is opening new horizons for highly sensitive bioaffinity and biocatalytic assays and for novel biosensor protocols that employ electronic, optical, or microgravimetric signal transduction. Nucleic acids and antibodies functionalized with metal or semiconductor nanoparticles have been employed as amplifying tags for the detection of DNA and proteins. The coupling of different nanomaterial-based amplification platforms and amplification processes dramatically enhances the intensity of the analytical signal and leads to ultrasensitive bioassays. The successful realization of the new nanoparticle-based signal amplification strategies requires proper attention to nonspecific adsorption issues. The implications of such nanoscale materials on amplified biodetection protocols and on the development of modern biosensors are discussed.

Gram-scale synthesis of soluble, near-monodisperse gold nanorods and other anisotropic nanoparticles.

Abstract: An aqueous surfactant-based colloidal chemical method is reported for the synthesis of anisotropic noble-metal nanoparticles on the milligram to multigram scale. Fine control of the nucleation-growth kinetics and rodlike micelle-induced breaking of symmetry at the early stage of particle growth are responsible for high-quality anisotropic nanoparticles. Near-monodisperse gold and silver nanorods, spheroids, nanowires, platelets, or cubes of 4-50 nm dimension and controllable aspect ratio can be prepared. The method may also be extended to semiconductor systems.

Recent Developments in Nanofabrication Using Focused Ion Beams

Abstract: Focused ion beam (FIB) technology has become increasingly popular in the fabrication of nanoscale structures. In this paper, the recent developments of the FIB technology are examined with emphasis on its ability to fabricate a wide variety of nanostructures. FIB-based nanofabrication involves four major approaches: milling, implantation, ion-induced deposition, and ion-assisted etching of materials; all these approaches are reviewed separately. Following an introduction of the uniqueness and strength of the technology, the ion source and systems used for FIB are presented. The principle and specific techniques underlying each of the four approaches are subsequently studied with emphasis on their abilities of writing structures with nanoscale accuracy. The differences and uniqueness among these techniques are also discussed. Finally, concluding remarks are provided where the strength and weakness of the techniques studied are summarized and the scopes for technological improvement and future research are recommended.

Fabrication of nanoscale rings, dots, and rods by combining shadow nanosphere lithography and annealed polystyrene nanosphere masks

Abstract: The application of shadow nanosphere lithography for the preparation of large-area, two-dimensional, metallic nanostructures of different shape is described. Through changing the mask morphology by temperature processing and varying the evaporation conditions, particles with morphologies such as rings, rods, and dots have been produced. This process allows outstanding control of the size and morphology of the particles. The efficient technique is shown to scale down the size of metallic nanoparticles from 200 to 30 nm, while preserving the original nanosphere spacing and order. The 150-nm-diameter Fe rings produced by this method show ferromagnetic behavior, which was predicted by theoretical simulation. All the experimental results were confirmed by computer simulations, which also showed the possibility of creating periodic arrays of any other geometrical shape.

Water‐Soluble Sacrificial Layers for Surface Micromachining

Abstract: This manuscript describes the use of water-soluble polymers for use as sacrificial layers in surface micromachining. Water-soluble polymers have two attractive characteristics for this application: 1) They can be deposited conveniently by spin-coating, and the solvent removed at a low temperature (95-150 degrees C), and 2) the resulting layer can be dissolved in water; no corrosive reagents or organic solvents are required. This technique is therefore compatible with a number of fragile materials, such as organic polymers, metal oxides and metals-materials that might be damaged during typical surface micromachining processes. The carboxylic acid groups of one polymer-poly(acrylic acid) (PAA)-can be transformed by reversible ion-exchange from water-soluble (Na+ counterion) to water-insoluble (Ca2+ counterion) forms. The use of PAA and dextran polymers as sacrificial materials is a useful technique for the fabrication of microstructures: Examples include metallic structures formed by the electrodeposition of nickel, and freestanding, polymeric structures formed by photolithography.

Nanoengineered polymer capsules: tools for detection, controlled delivery, and site-specific manipulation.

Abstract: We present the concept of multifunctional nanoengineered polymer capsules and outline their applications as new drug delivery systems or supramolecular toolboxes containing, for example, enzymes capable of converting nontoxic prodrugs into toxic drugs at a designated location. Such functionalized nanocontainers offer a wide range of applications including enzymatic catalysis, controlled release, and directed drug delivery in medicine due to their multifunctionality. The unique advantage of capsules in comparison to other systems is that they can be functionalized or loaded simultaneously with the above-mentioned components, thus permitting multifunctional processes in single cells.

Electromechanical transducers at the nanoscale: actuation and sensing of motion in nanoelectromechanical systems (NEMS).

Abstract: Electromechanical devices are rapidly being miniaturized, following the trend in commercial transistor electronics. Miniature electromechanical devices--now with dimensions in the deep sub-micrometer range--are envisioned for a variety of applications as well as for accessing interesting regimes in fundamental physics. Among the most important technological challenges in the operation of these nanoelectromechanical systems (NEMS) are the actuation and detection of their sub-nanometer displacements at high frequencies. In this Review, we shall focus on this most central concern in NEMS technology: realization of electromechanical transducers at the nanoscale. The currently available techniques to actuate and detect NEMS motion are introduced, and the accuracy, bandwidth, and robustness of these techniques are discussed.

Preparation and Characterization of Aligned Carbon Nanotube–Ruthenium Oxide Nanocomposites for Supercapacitors

Abstract: A novel type of ruthenium oxide (RuO(2))-modified multi-walled carbon nanotube (MWNT) nanocomposite electrode (RuO(2)/MWNT) for supercapacitors has been prepared. The nanocomposites were formed by depositing Ru by magnetic-sputtering in an Ar/O(2) atmosphere onto MWNTs, which were synthesized on Ta plates by chemical vapor deposition. Cyclic voltammetry, chronopotentiometry, and electrochemical impedance measurements were applied to investigate the performance of the RuO(2)/MWNT nanocomposite electrodes. The capacitance of the MWNT electrodes in 1.0 M H(2)SO(4) is significantly increased from 0.35 to 16.94 mF cm(-2) by modification with RuO(2). The RuO(2) film on the surface of the nanotubes is composed of small crystal grains with tilted bundle-like microstructures, as observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results demonstrate a promising route to prepare RuO(2)/MWNT-based double-layer supercapacitors.

Flow Focusing: A Versatile Technology to Produce Size- Controlled and Specific-Morphology Microparticles**

Abstract: The Flow Focusing platform is especially advantageous for micro- and nanoparticle production. This versatile technique is amenable to designing the size, surface treatment and internal topology of the particles; mechanical stresses are minimal—an optimal feature for the manipulation of delicate substances. Multiplexing and high-rate production are readily implemented. Adaptive operational design can lead, in one single step, to finely tuned microcapsules encasing different products within a targeted morphology. This achievement is of great significance for most microcapsule applications in the biosciences (for example, drug delivery, cell encapsulation, and the production of bead arrays).

Simple Synthesis of Hierarchically Ordered Mesocellular Mesoporous Silica Materials Hosting Crosslinked Enzyme Aggregates

Abstract: Hierarchically ordered mesocellular mesoporous silica materials (MMS) were synthesized using a single structure directing agent under neutral conditions for the first time. The mesocellular pores are synthesized without adding any pore expander, and the walls of cellular pores in MMS are composed of SBA-15 type mesopores. The small-angle X-ray scattering (SAXS) pattern of MMS revealed the presence of ordered pore structures with two different length scales. The current MMS possesses four different pore systems; complementary micro/mesopores, main 13 nm mesopores, 40 nm mesocellular spherical pores, and textural inter-particle macropores. Nanometer-scale enzyme reactors (NER) were developed in mesocellular mesoporous silica (MMS) via a ship-in-a-bottle approach, which employs adsorption of enzymes followed by cross-linking using glutaraldehyde (GA) treatment. The resulting NER show an impressive stability and activity with an extremely high loading of enzymes. For example, NER containing α-chymotrypsin (NER-CT) could hold 0.5 g CT in 1 g of silica, but the specific activity of NER-CT was 10.4 times higher than that of the adsorbed CT with a lower loading (0.07 g CT per 1 g of silica), which was further decreased by a continuous leaching of adsorbed CT. NER-CT showed excellent stability without any leaching, i.e. no activity decrease atmore » all in a rigorously-shaking condition for two weeks (a half-life with 3.8 years), while the conventional adsorption method resulted in a half-life of 3.6 days in the same condition.« less

Direct Observation of the Growth Process of MgO Nanoflowers by a Simple Chemical Route

Abstract: Novel flowerlike nanostructures consisting of MgO nanofibers were successfully synthesized by a simple chemical route with H(2)O at 950 degrees C in an Ar atmosphere. Various durations of heating gave different growth stages that led to varied product morphologies. The synthesized products were systematically studied by X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and energy-dispersive X-ray analysis. The results show that the nucleation and growth process of the nanoflowers seems to be a vapor-solid mechanism, and that the total heating time during the reaction process is a critical factor for the development of MgO nanoflowers. Initially, Mg particles formed on the Si substrate, followed by the formation of MgO clusters as nucleation centers on the magnesium melt surface and the nucleation of short MgO nanofibers, then growth of the MgO nanofibers occurred, and finally MgO nanoflowers were formed. Besides nanoflowers, novel hierarchical MgO nanostructures were also observed. These nanostructures may be used as three-dimensional composite materials and as supports for other materials.

Nanometer-scale modification and welding of silicon and metallic nanowires with a high-intensity electron beam.

Abstract: We demonstrate that a high-intensity electron beam can be applied to create holes, gaps, and other patterns of atomic and nanometer dimensions on a single nanowire, to weld individual nanowires to form metal-metal or metal-semiconductor junctions, and to remove the oxide shell from a crystalline nanowire. In single-crystalline Si nanowires, the beam induces instant local vaporization and local amorphization. In metallic Au, Ag, Cu, and Sn nanowires, the beam induces rapid local surface melting and enhanced surface diffusion, in addition to local vaporization. These studies open up a novel approach for patterning and connecting nanomaterials in devices and circuits at the nanometer scale.

Top‐Down Meets Bottom‐Up: Dip‐Pen Nanolithography and DNA‐Directed Assembly of Nanoscale Electrical Circuits

Abstract: Interfacing bottom-up chemical and biological assembly schemes with top-down lithography to fabricate complex devices is presently a major goal in nanoscience and technology. Bridging the gap between self-assembly techniques and modern top-down lithography offers a way to incorporate additional functionality (for example, in the form of chemical or biological recognition and sensing capabilities) into conventional electronic and optical devices, and provides a rapid means to test the potential viability of multiple chemically synthesized device components and self-assembly strategies. In this paper we present a method that allows multiple, independent chemical recognition events to be incorporated in close proximity in a single device. We focus on the problem of assembling specific metal nanoparticles into electrical junctions using DNA-directed assembly because we believe such an approach ultimately offers the opportunity to combine self-assembly and biosensor development with basic studies of electrical transport through biofunctionalized nanostructures. Many studies have focused on understanding the electrical properties of chemically synthesized structures including