Showing papers in "NANO in 2007"

Clay nanotubes for encapsulation and sustained release of drugs

Abstract: 50-nm diameter halloysite clay nanotubules with 15 nm lumen were used for loading poorly soluble drugs and sustaining their release. Loading was optimized by varying pH and alcohol/water ratio in the solvent with a maximum drug loading of 12 volume%. Near linear release of Dexamethasone, Furosemide, and Nifedipine was demonstrated for 5–10 hours. Its capacity for the time release of drugs, along with its simple loading procedure and the biocompatibility of halloysite nanotubules makes this method a prospective drug delivery system.

Dynamic response of carbon nanotubes dispersed in nematic liquid crystal

Abstract: The alignment and dynamic response of carbon nanotubes (CNTs) in nematic liquid crystal (NLC) medium induced by strong electric field have been observed through polarizing optical microscope. Density-functional calculations suggest that LC molecule anchors helically to the CNT wall to enhance π-stacking with a binding energy of nearly -2.0 eV due to a considerable amount of charge transfer from LC molecule to CNT, resulting in the formation of excess charges and permanent dipole moment in CNTs. Under strong electric field, the motion of CNTs distorted the director of adjacent LC molecules. Our detailed analysis of dynamics revealed that the four-lobe textures in vertical cell and two vertical stripes in in-plane switching cell were strongly correlated, i.e., the side view of textures by the vertical motion of CNTs in vertical cell was similar to the textures in in-plane switching cell. Interestingly, the magnitude of textures in microscope was strongly dependent on the size of CNTs and the applied field strength. The statistical size distribution of textures similar to that of CNTs provided information for the degree of dispersion of CNTs.

Dependence of material quality on performance of flexible transparent conducting films with single-walled carbon nanotubes

Abstract: Several single-walled carbon nanotubes (SWCNTs) prepared by different methods have been used to investigate the material dependence on the optimal film performance of flexible transparent conducting films. The nanotubes were dispersed in water with sodium dodecyl sulfate by sonication. These SWCNT solutions were then sprayed onto the Poly(ethylene terephthalate) substrate by a spray coater to form the film. Several factors such as purity, diameter, defects, metallicity, and degree of dispersion were evaluated individually to examine how they affect the film performance. We found that the metallicity of SWCNTs and the degree of dispersion are the most crucial factors in determining the film performance. We also proposed a material quality factor to estimate the material quality of SWCNTs as a figure of merit for the film performance.

Field emission and application of carbon nanotubes

Abstract: The mechanism of field emission from a metal surface was well explained based on the quantum mechanics in early 20th century. Since then, various materials have been studied for field emitters. However, so far, we have been using only limited materials as a field emitter and an application in some area requires further scientific understandings and technological advancements. In this paper, we review the current status of researches in field emission and emission phenomena of carbon nanotubes (CNTs). This may include current saturation induced by gas adsorbates, screening effects, high current emission, degradation of emitter, and field enhancement factor. We also introduce the present status in the development of various CNT-based field emission devices and discuss their performances. In this part, various potential applications such as field emission display, ionization gauge, X-ray gun, and lamp will be presented.

Experimental nanomechanics of one-dimensional nanomaterials by in situ microscopy

Abstract: This paper provides a comprehensive review on the methodological development and technical applications of in situ microscopy, including transmission electron microscopy (TEM), scanning electron microscopy (SEM) and atomic force microscopy (AFM), developed in the last decade for investigating the structure-mechanical-property relationship of a single one-dimensional nanomaterial, such as nanotube, nanowire and nanobelt. The paper covers both the fundamental methods and detailed applications, including AFM-based static elastic and plastic measurements of a carbon nanotube, external field-induced resonance dynamic measurement of elastic modulus of a nanotube/nanowire, nano-indentation, and in situ plastic deformation process of a nanowire. Details are presented on the elastic property measurements and direct imaging of plastic to superplastic behavior of semiconductor nanowires at atomic resolution, providing quantitative information on the mechanical behavior of nanomaterials. The studies on the Si and SiC nanowires clearly demonstrated their distinct, "unexpected" and superior plastic mechanical properties. Finally, a perspective is given on the future of nanomechanics.

Ripening of silver nanoparticles on carbon nanotubes

Abstract: An electrostatic-force-directed-assembly technique was used to coat multiwalled carbon nano-tubes (MWCNTs) with aerosol Ag nanoparticles produced from a mini-arc plasma source. The deposition of Ag nanoparticles onto CNTs was confirmed by transmission electron microscopy (TEM), high-resolution TEM, scanning electron microscopy, and X-ray photoelectron spectroscopy. Ripening of Ag nanoparticles on CNTs was observed via successive TEM imaging after heating the nanoparticle–nanotube hybrid structures in air to three different temperatures ranging from 100°C to 300°C. With temperatures at and above 200°C, the areal density of Ag nanoparticles decreased and the average particle size increased. In particular, migration and coalescence of Ag nanoparticles have been observed at this relatively low temperature, which suggests a van der Waals nanoparticle–nanotube interaction.

Continuum modeling of interfaces in polymer matrix composites reinforced by carbon nanotubes

Abstract: The interface behavior may significantly influence the mechanical properties of carbon nanotube (CNT)-reinforced composites due to the large interface area per unit volume at the composite. The modeling of CNT/polymer interfaces has been a challenge in the continuum modeling of CNT-reinforced composites. This paper presents a review of recent progress to model the CNT/matrix interfaces via a cohesive law established from the van der Waals force. A simple, analytical cohesive law is obtained from the inter-atomic potential, and is used to study the effect of CNT/matrix interfaces on the macroscopic properties of CNT-reinforced composites.

New amine functional ionic liquid as building block in nanotechnology

Abstract: In this communication, we report the synthesis of an amino-functionalized ionic liquid (AFIL) and its application as building block in the construction of new functional nanomaterials. This AFIL has been used as surfactant in the synthesis of gold nanoparticles (Au NPs) and in the development of functional CNTs/IL hybrid nanomaterials by mixing the AFIL with multi-walled carbon nanotubes (MWCNTs). The presence of an amine group in the ionic liquid provides to the resultant nanomaterial with new opportunities in several applications in the nanobio field.

Adhesion between carbon nanotubes and substrate: mimicking the gecko foot-hair

Abstract: A continuum model is developed based on the van der Waals interactions to study the adhesion between multi-wall carbon nanotubes (MWCNT) and substrates. Simple, analytical expressions for the binding energy and pulling force are obtained in terms of the MWCNT radius, number of walls, and parameters in the van der Waals potential. For a 1 cm by 1 cm template with densely packed MWCNTs (50 nm in spacing), the total pulling force can reach 26.6 N and 21.9 N for the graphite and polyethylene substrates, respectively.

Electronic structure of finite-length carbon nanotubes: crossover from fullerenes to nanotubes

Abstract: Finite-size effects in armchair (n,n) carbon nanotubes are studied as a function of the tube length by using the generalized tight-binding calculations. End structures of the tubes considered here are caps formed from hemispherical pieces of Ih fullerenes and hydrogen-free open ends. It has been clarified that the dimensionality in electronic structures of the finite-length nanotubes only depends on an aspect ratio of the tube diameter to the length of the cylindrical region. The aspect ratio where the finite-length tubes exhibit one-dimensional properties is found to be about four. The results corroborate that the nanotubes with their length of 10–100 nm experimentally observed could be regarded as one-dimensional electron systems.

Theory–experiment relationship of the "shrinking hot giant" road of dynamic fullerene self-assembly in hot carbon vapor

Abstract: Though subject to intensive studies, the formation mechanism of buckminsterfullerene C60 and related higher fullerenes has long evaded discovery. To elucidate their atomistic self-assembly mechanism, we have performed high-temperature quantum chemical molecular dynamics simulations on carbon vapor model systems initially consisting of C2 molecules. Our simulations reveal a coherent mechanism how highly ordered fullerene cages naturally self-assemble under nonequilibrium conditions, following a series of irreversible processes from the polymerization of C2 molecules to vibrationally excited giant fullerenes, which then shrink by C2 evaporation down to the smallest spherical, isolated pentagon rule obeying species C70 and C60 as the smallest and kinetically most stable species of the shrinking process. We show that the potential energy surface associated with giant fullerene cage growth, measured by an average cluster curvature, is downhill all the way, and in agreement with high-level energetics from density functional theory. This fullerene formation mechanism is a good example of dynamic self-assembly leading to dissipative structures far from thermodynamic equilibrium, and the "shrinking hot giant" road provides a natural explanation for the observed cage size distributions in a random optimization process consistent with several important experimental observations.

Growth of carbon nanotubes in etched ion tracks in silicon oxide on silicon

Abstract: Carbon nanotubes (CNTs) were selectively grown in etched ion tracks in SiO2 layers on Si. For this sake, Ni-catalyst nanocrystals were initially deposited within the ion tracks by galvanic deposition. The characteristics of plasma-enhanced chemical vapor deposition (PECVD)- and thermal chemical vapor deposition (TCVD)-grown CNTs, such as structural details and length distribution, were investigated. In addition, field emission properties were studied. The analysis revealed that the emerging PECVD-grown CNTs were of cylindrical and/or conical shape and usually had diameters as large as the etched tracks. The exponential length distribution of these CNTs can be well understood by applying a simple defect-growth model. For contrast, many narrow and curled CNTs were found to cluster in spots well separated from each other, after applying TCVD instead of PECVD. The Raman investigations of PECVD-grown CNTs showed that Si–O–C and Si–C phases had formed during the growth of the CNTs. These ion-track-correlated PECVD-grown CNTs open the way for the production of novel 3D nanoelectronic devices based on the TEMPOS concept. These structures are also excellent candidates for experiments on channeling in CNTs. Application as field emitting devices, however, appears unfavorable due to poor mean-field enhancement factors and insufficient stability.

Microbe-mediated nanotransformation: cadmium

Abstract: An eco-friendly microbe (lactobacillus sp.)-mediated synthesis of cadmium nanoparticles is reported. The synthesis is performed at room temperature. X-ray and transmission electron microscopy analyses are performed to ascertain the formation of Cd nanoparticles. Individual nanoparticles as well as a number of aggregates almost spherical in shape having a size of 40–60 nm are found.

＂surface-programmed assembly＂ of nanotube/nanowire-based integrated devices

Abstract: We present a review on recently-developed "surface-programmed assembly" strategy for massive production of nanotube/nanowire-based devices. In this process, surface molecular patterns guide the assembly and alignment of nanotubes/nanowires onto specific locations on solid substrates without relying on external forces. The assembled structures were further utilized to fabricate functional devices such as field effect transistors and sensors. Control experiments provided us rich scientific insights including "sliding kinetics" and "lens effect" during the assembly process. Since this method does not require high-temperature processing steps or unconventional fabrication facilities, it is readily available to conventional device industry and may open up new nanodevice industry based on carbon nanotubes and nanowires.

Simple and catalyst-free synthesis of silicon oxide nanowires and nanocoils

Abstract: In this paper, a simple method for synthesizing SiOx nanowires and nanocoils is presented. Si substrates with an oxide layer were placed in a tube furnace exposed to temperatures ranging from 900°C to 1200°C for a few hours under a mixture of flowing Ar and H2 gas maintained at ambient pressure. Nanowires were grown from the surface when the furnace temperature was above 1000°C and a high yield could be achieved at 1100°C. SiOx nanocoils have also been observed and the sample treated at 1000°C had the highest concentration of them. TEM images show that the nanowires and the nanocoils have an amorphous structure and analysis of EDX spectra (obtained in the TEM) shows that x varies from 1.2 to 2.0. The mechanism of growth is discussed.

Tyrosinase biosensor based on zinc oxide nanorods

Abstract: A phenol biosensor based on the skillful immobilization of tyrosinase on zinc oxide (ZnO) nanorods was proposed. The ZnO nanorods fabricated by a simple vapor-phase transport method possess a high aspect ratio, good electron communication, chemical purity, smooth and positive charged surface and are ready for immobilization of biochemicals with low isoelectric point (IEP). Electrochemical measurement and Scanning Electron Microscopic (SEM) analysis showed that the enzyme of tyrosinase with IEP 4.5 can be adsorbed on the surface of ZnO nanorods and kept its bioactivity of the oxidation of phenol to a large extent. This led us to develop phenol biosensor with good stability and reproducibility. The proposed method creates a new way to develop biosensors using nanostructured materials with high IEP.

Synthesis and properties of one-dimensional aluminum nitride nanostructures

Abstract: This article presents a brief review of the recent research progresses achieved in the field of one-dimensional (1D) aluminum nitride (AlN) nanostructures. It mainly covers three aspects: The first one is to introduce the synthetic strategies for several classic 1D AlN nanostructures (such as nano fibers, nanobelts, nanorods, nanowires, nanotips, etc.) including template-confined reaction, arc discharge, catalyst-assisted growth, and vapor transport and related growth methods. The second is to elaborate some special physical properties, such as field emission and photoluminescence, which associate with the uniqueness of 1D AlN nanostructures. It is revealed that aligned AlN 1D nanostructures have low turn-on and threshold voltages, high emission current and small current fluctuation, and that the photoluminescence of AlN nanobelts are different from those of conventional AlN material. The third is to briefly illustrate the potential application of these 1D AlN nanostructures in composite materials. It is found that AlN nanowire is a good reinforcement for improving the mechanical and thermal properties of metal matrix composites, which can be expected to be utilized as packaging material with high strength and low thermal expansion. Finally, we summarize the major challenges in this field. Among them, a thorough understanding of the growth mechanism of 1D AlN nanostructures is the most important issue, and more precisely controlled growth is required to obtain tailored AlN nanostructures according to device applications.

Enhancement of nano-RC switching delay due to the resistance blow-up in ingaas

Abstract: RC and transit-time delays in a nanocircuit, where the resistor is a few nanometers in length, are evaluated taking into account the velocity and current saturation and applied to RC switching delay in InGaAs heterojunction field effect transistor (HFET). Transit time delay is the dominant factor in the ohmic regime where the applied voltage V is less than the critical voltage Vc for the onset of nonlinear nonohmic behavior. However, RC time constant is predominant in the nonohmic regime and increases linearly with the applied step voltage. The power in the nanocircuit is smaller and rises linearly in the nonohmic regime as compared to the quadratic behavior in the ohmic regime.

Synthesis and spectroscopic characterization of salmon dna-wrapped single-wall carbon nanotubes

Abstract: Single-wall carbon nanotubes (SWNTs) can be well-dispersed in water by wrapping with short segments of natural DNA from salmon sperm. We report here the isolated DNA-wrapped SWNT hybrids. Measurements were carried out using UV-vis-NIR, near-infrared photoluminescence (PL) spectrum and atomic force microscopy (AFM). A possible charge transport between SWNTs and salmon-DNA is discussed in terms of observed spectral shifts in the photoluminescence spectra.

Pressure-assisted spinning: a unique and versatile approach for directly fabricating membranes with micro- and nanofibers

Abstract: Fibers are structurally interesting components most useful in a range of applications spanning the physical and life science areas of research. These membrane (scaffold) forming fibers have been explored in applications ranging from microfilteration to advanced biological investigations in tissue engineering to controlled and targeted drug delivery. One such robust fiber generation approach investigated for over a century, which has recently been exploited, is the well-established threading process referred to as electrospinning. In this technique, single- or multi-phase media are charged within a conducting needle and later exposed to an electric field which promotes the formation of a continuous micro- to nanosized fiber(s) which over a period of collection time has been reported for forming scaffolds and membranes. This process has been explored for a wide range of polymer composite-based materials and the technique has now reached the point where it has moved into industrial production. We report here as a first example a comparable fiber to membrane fabrication approach completely driven by the coupling of a coaxial needle system with a pressure. We refer to this novel methodology as pressure-assisted spinning (PAS) where the hazardous element of high voltage (as in the case of electrospinning) is nonexistent. Hence, our discovery introduces both a directly competing fiber, scaffold to membrane fabrication approach, which is versatile and has no associated hazards as those in electrospinning. Furthermore as our technique is nonelectric field driven, the media spun into fibers could have a high electrical conductivity, which in this case has no effect on the stability in processing near-uniform fibers/scaffolds to membranes. The fabricated fibers and membranes generated by means of this approach could directly be used for a plethora of applications spanning the engineering and biological areas of research.

DIFFUSION IN SIMULATED SiO2 NANOPARTICLES

Abstract: Diffusion in liquid SiO2 nanoparticle with the diameter of 4 nm has been studied in a spherical model containing 2214 atoms via Molecular Dynamics simulation (MD). Diffusion constant of atomic species has been calculated over temperatures ranged from 2100 K to 7000 K and compared with those observed for the bulk. We found that temperature dependence of diffusion constant of atomic species in nanoparticle shows an Arrhenius law at temperatures above 2100 K and at T > 4200 K, it deviates from an Arrhenius law. However, unlike those observed in the bulk at T > 4200 K, it does not show a power law, D ~ (T - Tc)γ. Moreover, at relatively low temperatures, diffusion constant in nanoparticle is higher than that in the bulk indicating the surface dynamics effects.

Facile synthesis of copper nanoparticle chains

Abstract: The copper nanoparticle chains have been synthesized through a self-assembly process with sodium polymethacylic acid as the template. The resulted nanoparticle chains were 0.8–1.5 μm long, and the composed nanoparticles were about 30 nm in diameter. The self-assembly formation mechanism has been proposed.

Silicon-based half-metal: metal-encapsulated silicon nanotube

Abstract: We performed first-principles calculation to show that a host–guest silicon nanostructure can exhibit half-metallic properties, wherein the host is a single-walled hexagonal silicon nanotube while the guest is a hybrid atomic chain of Mn and Co (encapsulated in the host nanotube). The calculated electronic band structures indicate that the Fermi level intersects only in the spin-up band, whereas the spin-down band exhibits semiconducting characteristics.

Extraction of metallic nanotubes of zeolite-supported single-walled carbon nanotubes synthesized from alcohol

Abstract: Single-walled carbon nanotubes (SWNTs) synthesized by catalytic decomposition of an alcohol were purified by extraction. The purified SWNTs were characterized on the basis of visible-near infrared (vis-NIR) absorption, photoluminescence and Raman spectroscopic analyses, scanning electron microscopy (SEM) observation, and thermal analysis. Selective extraction of metallic nanotubes was also achieved by the extraction condition.

EVOLUTION OF STRUCTURE OF LIQUID AND AMORPHOUS Al2O3·2SiO2 NANOPARTICLES UPON COOLING FROM THE MELTS

Abstract: Structural properties of liquid and amorphous Al2O3·2SiO2 nanoparticles have been investigated by molecular dynamics (MD) method. The simulations were done in a spherical model with different sizes of 2 nm, 3 nm and 4 nm under nonperiodic boundary conditions. We use the Born–Mayer type interatomic potentials. We determined the changes in mean interatomic distances, coordination number distributions, partial radial distribution functions (PRDFs) and bond-angle distributions of Al2O3·2SiO2 nanoparticles upon cooling towards glassy state. Calculations show that liquid and amorphous Al2O3·2SiO2 nanoparticles have a slightly distorted tetrahedral network structure with the mean coordination number ZAl - O ≈ 4.0 and ZSi - O ≈ 4.0, which is contrary to that of the liquid and amorphous bulk. Temperature dependence of structural defects in nanoparticles was found and discussed. In addition, we found size effects on glass transition temperature of Al2O3·2SiO2 nanoparticles in that it decreases with decreasing size.

Synthesis of calcium oxalate assembly structure and conversion

Abstract: The conversion from one assembly structure to other composite assembly structures is valuable to both theoretical research and actual application in the nano/micromaterials science. In this paper, firstly, the flower-like calcium oxalate assembly structure was synthesized using a supramolecule template; then, through a facile process, the calcium oxalate was converted to a sphere-cluster-like core/shell CaC2O4/CaWO4 nanocomposite assembly structure. The converted product remained the basic structure of original product, and possessed some new optical properties such as fluorescence, etc.

ZnO-BASED NANOWIRES

Abstract: ZnO has properties well-suited to UV/visible light emitters, transparent thin film electronics and a variety of gas and chemical sensor applications. There has been extensive interest in recent times in synthesis of ZnO nanowires by a number of methods using both catalyst and catalyst-free approaches. In this paper we review our recent results on developing functional nanowires in the ZnMgO systems and show some applications in hydrogen gas sensing, pH sensing, transparent transistors and UV detectors. In terms of sensors, the main selling points are large surface-to-volume ratio for improved detection sensitivity and also low power requirements.

Oil/water interface synthesis and optical property of strontium tungstate nanorods

Abstract: Strontium tungstate nanorods with uniform morphology were controllably synthesized for the first time via a facile liquid–solid–solution (LSS) process through oil/water interface. These nanorods have diameters of about 50 nm and length of about 500 nm. XRD curves indicated that as-obtained nanorods were body-centered tetragonal phases with lattice parameters of a = 5.416 A and c = 11.95 A, which is consistent with the values reported in JCPDS card No. 08-0490. Study on the optical properties of as-obtained nanorods showed that SrWO4 nanorods possess different optical properties from bulk counterpart. A mechanism of the formation process was also discussed.

Thermal conductivity in nanoporous gold films during electron-phonon nonequilibrium.

Abstract: The reduction of nanodevices has given recent attention to nanoporous materials due to their structure and geometry. However, the thermophysical properties of these materials are relatively unknown. In this article, an expression for thermal conductivity of nanoporous structures is derived based on the assumption that the finite size of the ligaments leads to electron-ligament wall scattering. This expression is then used to analyze the thermal conductivity of nanoporous structures in the event of electron-phonon nonequilibrium.

Tunable electric conductivities of Au-doped boron nitride nanotubes

Abstract: Tunable electronic properties of nanotubes are an essential requirement for building nanotube circuits and devices. We have produced BN nanotube films with controlled electric conductivities by Au doping. The Au-doped BN nanotube films have been found to exhibit from semiconducting to metallic behavior under different Au contents. Both experimental and computing simulation studies reveal that the conductivities of the doped film are improved by the incorporation of Au atoms into nanotube walls. The doped BN nanotubes and films are expected to have potential applications in catalysts, sensors and nanoelectronics.