Showing papers in "International Journal of Nanotechnology in 2005"

Exchange bias in ferromagnetic nanoparticles embedded in an antiferromagnetic matrix

Abstract: The fabrication process and magnetic properties of three types of system consisting of ferromagnetic (FM) particles embedded in an antiferromagnetic (AFM) matrix are discussed. The preparation techniques are ball milling, H2 partial reduction of oxides and nanoparticle gas condensation. The magnetic properties of the FM/AFM composites are shown to depend strongly on the morphology of the system (e.g., nanoparticle size), the AFM anisotropy and the AFM-FM coupling. For example, all the studied systems exhibit coercivity enhancement below the Neel temperature of the AFM. However, while Co nanoparticles embedded in CoO exhibit loop shifts of thousands of Oe, Fe nanoparticles in Cr2O3 only show a few Oe shifts. An interesting effect evidenced in all systems is the increase of remanence (MR) which, in the case of Co-CoO, ultimately leads to an improvement of the superparamagnetic blocking temperature of the nanoparticles.

Field emission measurement techniques for the optimisation of carbon nanotube cathodes

Abstract: We have developed advanced measurement techniques for the investigation of the field emission (FE) properties of high-technology materials. Flat samples up to 30 mm in diameter can be optimised either for strong or weak electron emission, i.e., cold FE cathodes operating at field levels below 10 V/μm or high-voltage vacuum devices preventing FE up to 200 V/μm. The integral measurement system with luminescent screen (IMLS) provides a quick overview of the microscopic distribution and long- and short-term stability of the emission current, and facilitates a controlled conditioning of cold cathode samples at various pressures. The field emission scanning microscope (FESM) is incorporated within a UHV surface analysis system and enables high-resolution emitter distributions and local FE investigations down to the nanoscale. Exemplary results on carbon nanotube samples will be given to demonstrate that the combination of both instruments yields a very effective means for an improved understanding and tailoring of materials for cold cathode applications.

Application of the standard porosimetry method for nanomaterials

Abstract: A new method of Standard Porosimetry (MSP) for investigation of any type of porous materials, including soft, frail, amalgamated materials, films and powders has been developed. The method is relatively simple and non-destructive and can be used for measurements in a wide range of pore sizes from 0.3 nm to 3 × 10 6 nm. The method is currently being used to study porous materials such as carbon nanotubes, membranes, thin films, ceramics, electrodes for batteries, paper etc., and can be used in other applications, including nanotechnology. The MSP employs numerous manual operations. In order to avoid them, the Automated Standard Porosimeter (ASP) has been developed based on the MSP by POROTECH, Ltd. Reference to this paper should be made as follows: Volfkovich, Y.M., Sakars, A.V. and Volinsky, A.A. (2005) 'Application of the standard porosimetry method for nanomaterials', Int. J. Nanotechnology, Vol. 2, No. 3, pp.292-302. Biographical notes: Yury M. Volfkovich is the Chief Scientist at C&T Lab Inc., Toronto, ON, Canada and simultaneously he is a Professor, Head of the laboratory at Frumkin Institute of Electrochemistry of Russian Academy of Sciences (FIE), Moscow, Russia. He obtained his PhD Degree in 1969 from FIE, thesis title: Micro- and Macrokinetics of Electro-oxidation of Methanol on Smooth and Porous Electrodes. He obtained his Dr.Sc. Degree in 1983, thesis title: Macrokinetics of Electrochemical Cells with Capillary Membranes and Methods of Standard Porosimetry. His research interests are Methods and devices development for investigation of porous structure. He authored over 300 scientific publications and holds more than 40 patents.

On the thermodynamic stability of metal oxide nanoparticles in aqueous solutions

Abstract: Divided and ultra-divided systems such as colloidal and nanoparticle dispersions are generally unstable with regard to the size and number of their constituents because the solid-solution interfacial tension, acting as a driving force, leads to a reduction of the surface area to minimise the dispersion free enthalpy. Such phenomenon known as surface energy minimisation induces an increase in average particle size as a result of the decrease of the surface area at constant volume. For such a reason such dispersions are usually considered thermodynamically unstable. However, they can be thermodynamically stabilised if, by adsorption, the interfacial tension of the system becomes very low. This phenomenon, well known for microemulsions, is for the first time quantitatively modelled and demonstrated for transition metal oxide nanoparticles. When the pH of precipitation is sufficiently far from the point of zero charge and the ionic strength sufficiently high, the ripening of nanoparticles is avoided and their size can be monitored over one order of magnitude by tailoring solution pH and ionic strength. A model based on Gibbs adsorption equation leads to an analytical expression of the water-oxide interfacial tension as a function of the pH and the ionic strength of the dispersion/precipitation medium. The stability condition, defined by a 'zero' interfacial tension, corresponds to the chemical and electrostatic saturation of the water-oxide interface. In such a condition, the density of charged surface groups reaches its maximum, the interfacial tension its minimum and further adsorption forces the surface area to expand and consequently, the size of nanoparticles to decrease. An excellent agreement was found between the model prediction and the experimental results obtained from the aqueous precipitation of magnetite (Fe3O4) nanoparticles in basic medium. A general control of the metal oxide nanoparticle size when precipitated far from their point-of-zero-charge is thus expected.

Mesoscale modelling: recent developments and applications to nanocomposites, drug delivery and precipitation membranes

Abstract: Mesoscale simulations have traditionally been used to investigate structural morphology of polymers in solution, melts and blends. Recently, such modelling methods are being pushed to important areas of Nanotechnology and Drug delivery that are well out of reach of classical molecular dynamics. This paper describes our current efforts in three important emerging areas: polymer-nanotube composites; drug diffusivity through cell membranes; solvent-exchange in nanoporous membranes. Enhancements to existing codes, and future directions of research are indicated as well.

From magnetoelectronic to biomedical applications based on the nanoscale properties of advanced magnetic materials

Abstract: In this paper, we review some results obtained in our group in the framework of research projects concerning magnetoelectronic and biomedical applications with advanced magnetic materials. First, we focus on the type of materials used, and then we describe specific magnetoelectronic devices based on spin-dependent tunnelling and magnetic nanoparticles for biomedical applications. Special attention is drawn to the phenomena occurring at the nanometric scale, which in most cases completely determine the observed macroscopic properties.

Carbon nanotubes: from production to functional composites

Abstract: In this paper we present our work on carbon nanotubes, including our most recent results, in two important fields: production of carbon nanotubes itself and fabrication of carbon nanotube based composites with enhanced functionality. Firstly, we focus on the three main carbon nanotube production techniques, namely electric arc, laser evaporation and CVD. Based on our results, for each method the importance of the key parameters is highlighted. A comparison of production possibilities towards commercialisation concludes this section. Secondly, we report on the synthesis of a soluble and highly functional polyaniline/carbon nanotube composite and emphasise the general importance of conformational changes in order to achieve proper carbon nanotube-matrix interactions.

Nanotubes and nanowires : the effect of impurities and defects on their electronic properties

Abstract: In recent years, our laboratory has conducted extensive research studies on nanoscale systems. We give here a brief overview of these activities, and also present some previously unpublished results on the influence of impurities and defects on the electronic properties of carbon nanotubes and metallic nanowires. In the case of carbon nanotubes, we show that the presence of structural defects such as strong bends can lead to the presence of electronic states in the gap (for semiconducting nanotubes), which we correlate with recent scanning tunnel microscopy and spectroscopy measurements. In the case of metallic nanowires, we discuss the effect of the presence of carbon impurities, which have been suggested to be present in atomic gold nanowires, on their electronic transport properties. In particular, we show that the conductance in the presence of impurities deviates considerably from that of the pure gold atomic wires.

Dielectrophoresis as a tool for nanoscale DNA manipulation

Abstract: The use of the dielectrophoresis as a tool for DNA manipulation is demonstrated experimentally, using both unmodified 48,500 base pairs long bacteriophage lambda dsDNA (λ-DNA), ∼16 μm in length and 414 base pairs long thiol modified natural dsDNA (avDNA), ∼140 nm in length. We show that both the dsDNA types used, are effectively directed between the planar gold electrodes by the positive dielectrophoresis while applying an AC voltage at frequencies between 500 kHz and 1 MHz. With high concentrations of dsDNA in buffer the attached dsDNA molecules are shown to form bundles or clumps (both λ-DNA and avDNA). Furthermore, we demonstrate the attaching of a single avDNA molecule to an electrode via gold-thiol bonding. Also the clear orientation and straightening along the electric field is seen in this case. In addition, the electrical conductivity of dsDNA is studied by measuring the full I-V characteristics of the samples.

Nanomechanics of lung epithelial cells

Abstract: Nanobiotechnology provides powerful tools for manipulating cells with nanometric resolution and with simultaneous measurement of force with pN resolution. We review the application of atomic force microscopy (AFM) and magnetic tweezers for probing cell nanomechanics. AFM measures the mechanical properties of the cell by indenting its surface by means of a flexible cantilever with a sharp tip at its end. Magnetic tweezers probe cell mechanics by twisting or pulling a magnetic microbead bound to the cell surface. The stress-strain relationship allows us to compute the complex shear modulus of the cell. Lung epithelial cells probed with AFM and magnetic tweezers exhibit a power-law dynamics with a weak exponent. This dynamics conforms to the rheology of soft glassy materials, suggesting that cytoskeleton internal disorder and matrix agitation could govern the mechanical behaviour of the cell.

Liposome encapsulated gold nanoshells for NanoPhototherapy induced hyperthermia

Abstract: Gold nanoshells have been extensively studied since their invention and their role in both in vitro and in vivo photothermal therapy has been explored and demonstrated. In this work, we investigate liposomal delivery of gold nanoshells and evaluate their effects for in vitro NanoPhototherapy induced hyperthermia in human mammary carcinoma cells. In addition, we compare the application of liposome encapsulated gold nanoshells and free standing gold nanoshells in NanoPhototherapy. NanoPhotoTherapy induced hyperthermia was performed using a 785 nm near-infrared light from a diode laser and the in vitro effects were evaluated using nucleic acid molecular probes by fluorescence microscopy. Additionally, we monitored apoptosis by detecting capase-9 activity.

The challenge of structural control on the nanoscale: bottom-up self-assembly of nucleic acids in 3D

Abstract: Control of the structure of matter has been a major challenge to humankind throughout its entire history. The finer the features that we are able to engineer, the greater is the level of control that we have. Here, we summarise progress made in the bottom-up control of structure that is based on the self assembly of nucleic acids. Nucleic acids are unique among molecular systems in that their intermolecular interactions can be programmed, from the perspectives of both affinity and of structure. Structural DNA nanotechnology has been based on directing the cohesion of branched DNA motifs by the same cohesive interactions used by genetic engineers. As a result, multiply-connected objects, periodic and aperiodic arrays and nanomechanical devices have been produced by these systems. Current experiments are directed at using nucleic acid systems to scaffold the spatial assembly of other species.

Nucleation phenomenon in nanoparticle self-assemblies

Abstract: Dispersed nanoparticles self-assemble into complex structures when segregated from the solvent either by evaporation or precipitation. Thus, different micro- and macroscopic structures (opals, fractals, liquid crystals) formed by nanoparticles are observed as a result of the balance between electrostatic forces, surface tension, volume exclusion, substrate topography and affinity, and size, shape and concentration of the particles. The formation process is close to the hard-sphere case with two special features: nanoparticles are coated with surfactant molecules of roughly 1 nm in length, and nanoparticles are in the nanometric scale resulting in an increased mobility. Moreover, self-assembly does not take place in solution; hence, a minimum concentration is needed in order to start self-assembly. This process occurs in contact with the substrate, when the solvent cannot re-disperse the particles anymore.

Engineering hybrid nano-devices powered by the F1-ATPase biomolecular motor

Abstract: Hybrid nano-devices that encompass organic and inorganic components offer truly unique and flexible functionalities. With integrated biomolecules and active proteins, it is plausible to envision seamless interfacing with nastive biological environments. The ability to incorporate inorganic features not only enables the synthesis of complex systems, it provides a myriad of options in custom tailoring devices to pertinent biomedical applications as well. This is especially true with the advent of more advanced nano-fabrication technologies in recent years. F1-ATPase is a motor protein whose rotary motion has been well characterised [1,2]. The biomolecular motor hydrolyses ATP to generate forces compatible with currently fabricated nano-mechanical structures. We have fabricated and operated a hybrid organic-inorganic nano-device powered by the F1-ATPase biomolecular motor [3–5]. The scope of the hybrid device developments was manifold including the local movement of surrounding suspended particles by functional organic-inorganic devices and in other efforts, the capability of implementing control mechanisms [6]. In addition, a renewable biological fuel (ATP) source has recently been developed through sol–gel encapsulation of proteoliposomes in an effort towards creating a self-sustaining device. The following paper is a review of the knowledge that has transpired upon developing hybrid organic-inorganic nano-devices to hopefully elicit future advancements of bio-nano-systems.

Nanoscopic surface architecture based on molecular self-assembly and scanning probe lithography

Abstract: Scanning probe lithography and molecular self-assembly have been integrated for constructing surface nanostructures. In the first part of this paper, organosilane monolayers grown through molecular self-assembly, i.e., self-assembled monolayers (SAMs) are applied as patterning media in scanning probe lithography based on scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) with an electrical conductive probe. The SAMs are successfully applied to resist films patternable in scanning probe lithography and etching masks for nanostructuring Si. Some of advanced techniques, namely, current-regulated AFM lithography, multilayer resist system and integration with photolithography, are presented as well. In the latter part, chemical methods to fabricate surface nanostructures are shown. Using SAM samples, on which nanopatterns formed by scanning probe lithography, as templates, minute objects including, organic molecules, nanoparticles and biomolecules are spatially arranged.

Electrical properties in granular Co-ZrO2 thin films

Abstract: Granular films composed of well-defined nanometric Co particles embedded in an insulating ZrZrO2 matrix were prepared by laser ablation in a wide range of Co concentration. The temperature-dependent resistivity depends very critically on the Co-volume fraction with percolation threshold in the range 0.45 < x < 0.52. In the dielectric regime, AC transport arises from the competition between the inter-particle thermally assisted tunnelling (R) and capacitive (C) channels. Consequentially, a complex low-frequency absorption phenomenon is observed that mimics the universal response of disordered dielectric materials. The experimental observation can be successfully modelled by a random R-C network.

Atomic scale design of carbon nanotubes: the way to produce bolt and nut pairs

Abstract: The structures of double wall carbon nanotubes (DWNTs) with atomic scale defects that can operate as bolt and nut pairs are analysed. The barriers and threshold forces for the relative motion of walls along the thread line and for twistoff are calculated for various types of defects. It is found that a type of defect does not influence the qualitative characteristics of a thread. The possibility of producing of the DWNT (which can operate as bolt and nut pair) by selforganisation is proposed.

Quantum confinement in carbon-nanotube systems

Abstract: Carbon nanotubes are graphene cylinders of nanometric diameter which can be either semiconducting or metallic depending on their geometry. Joining different kinds of nanotubes by means of topological defects, one can design all-carbon quantum dots and, in principle, achieve electronic confinement in quasi-zero dimensional systems. In this work, we review different ways of quantum electron confinement in carbon nanotubes: by matching a finite metallic nanotube portion to two semi-infinite semiconducting nanotubes, quantum dot states appear in the system due to confinement by energy barriers; all-metallic carbon nanotube structures are shown to have completely localised states because of the symmetry gap between the nanotube components; quasi-localised states, showing up as sharp resonances, are demonstrated to arise as a result of wave vector mismatch in all-metallic systems made of nanotubes without any common symmetry. The different conductance behaviour of the various structures is also studied.

Towards a universal embedded-atom method: empirically adjusted and consistent set of atomic densities for all elements of the periodic table

Abstract: The Hartree-Fock-Slater model of atom has been modified by using individual values of the exchange parameter αex for each atom. Each value of αex was adjusted to reproduce the empirical value of the first ionisation energy of the atom considered. The expectation values of energies and radial functions for all elements of the periodic table have been evaluated on the basis of the Hartree-Fock-Slater model and individual exchange parameters. Qualitatively, the expectation values compare well with Mann's numerical Hartree-Fock values but contain some influence of correlation and relativistic phenomena. A consistent set of universal atomic electron density tables for all elements of the periodic table, suitable for embedded-atom method (EAM) type calculations, is presented. We considered the electron density distribution as the key variable linking the total energy and inter-particle separation in the EAM molecular dynamics study. The universal set of atomic densities have been tested using the XMD molecular dynamics package.

Electrical properties of long molecules: single-walled carbon nanotubes and DNA

Abstract: A fundamental requirement for a molecule to be considered a molecular wire (MW) is the ability to transport electrical charge with a reasonably low resistance. From this point of view we performed experiments of electrical transport on carbon nanotubes and individual DNA molecules. By using a conductive atomic force microscopy (AFM) tip as a mobile electrode, we can experimentally determine the electron mean free path of carbon nanotubes with one end connected to a macroscopic gold electrode. We also investigated the radial electromechanical properties of carbon nanotubes by using AFM, showing that when a high strain is applied to the nanotube a gap in the electronic band structure of the tube is opened. Finally we show how the electrostatic force between the carbon nanotubes and the AFM tip can be used to investigate the conductance properties of carbon nanotubes. When similar experiments are performed on single DNA molecules, we do not find any traces of conductance.

STREL: A versatile computational environment for the study and design of nanostructures

Abstract: Fabrication techniques have advanced greatly and many nanostructures have been produced, studied and have found technological application. The study of the nanostructures, both experimentally and theoretically is very demanding, as many degrees of freedom are important in determining their properties. In order to meet these complex tasks, we have prepared a computational environment to facilitate the study of the structural, electronic and optical properties of nanostructures. In this environment, it is possible to tailor the properties of specific nanostructures.

Label-free detection of biomaterials using terahertz technology

Abstract: We present terahertz (THz) absorption spectra of various biomolecules (tripeptide gly-gly-gly and oligo-nucleotides poly[A] and poly[C]). While the tripeptide shows unique resonances in the THz range due to intramolecular vibrations, the different oligo-nucleotides can be distinguished due to different absorption properties. In space-resolved THz free-space measurements, we were able to repeatedly distinguish between dried drops of poly[A] and poly[C]. In addition, we present the design for a new class of filters for on-chip analysis of biomaterials. As compared to filters described earlier in the literature, the new structures have an increased sensitivity and require much less biomaterial.

Creating functional nanostructured materials at the crossroad of physics, chemistry and materials science

Abstract: Controlled assembly and manipulation of three-dimensional (3D) nanostructures with well-defined shapes, profiles and functionalities present a significant challenge to nanotechnology. In this paper we summarise our recent efforts in an attempt to solve this problem by developing a highly selective surface-templated layer-by-layer assembly, where top-down approaches such as soft lithography, focused-ion beam lithography and voltage-controlled nanopipet delivery are combined with bottom-up techniques like self-assembly, molecular recognition and layer-by-layer assembly to controllably deposit and grow 2D to 3D micro/nanostructures with functional materials. This opens up wide applications in highly sensitive biosensors, miniaturised assays, and functional 3D nano-assemblies and devices.

The Spanish presence in the molecular sieve technology scenario

Abstract: Molecular sieve research is a mature, but growing, area of interest within the Spanish scientific community. Indeed, Spanish researchers produce about 10% of the total number of publications in the zeolite field, covering new aspects of synthesis, characterisation and catalytic applications of those materials. More recently, there has been an emergent area focusing more on nanotechnological applications. New sensors and photodevices based on molecular sieve materials have also been reported. Furthermore, the void volumes of these micro- or meso-porous materials have been used as casts for synthesising nanoparticles inside the molecular sieves.

Many-body effects and quantum coherence in electron transport through quantum dots

Abstract: In this paper, I briefly review some recent aspects of electron transport through lateral quantum dots in the few-electron regime. These systems are very useful in studying a wide range of fascinating phenomena. Many of these phenomena originate from the intricate interplay between electron interactions and quantum coherence. In particular, I describe recent developments concerning Kondo physics in quantum dots and the realisation of charge qubits in double quantum dots. These two examples illustrate how recent experimental advances in transport studies of quantum dots provide us with an excellent testing ground to study fundamental problems of condensed-matter physics (for instance, the physics of correlated electrons and quantum dissipation) with unprecedented control. Some of the effects appearing in nanostructures have no counterpart in conventional systems. The nanotechnology for fabricating and measuring sophisticated devices now exists and may soon help us to gain new insights into a number of fundamental physical concepts related to correlations and quantum coherence in low dimensions.

Directed self-organisation of soluble proteins on surfaces

Abstract: Incorporation of proteins on surfaces is a first step in the lengthy process of using active biological molecules in man-made nanodevices. There is therefore a strong motivation to develop strategies for organising active proteins on solid substrates and it is very desirable to take advantage of the principles of self-assembly. In the present work, I summarise two strategies to orient and selectively adsorb proteins on gold surfaces previously modified with specially designed self-assembled monolayers. I also report on the on-going research describing the aggregating properties of a bacterial cytoskeletal protein on a surface to illustrate nature's ability to direct the self-assembly of dynamic structures.

A compact model for electrostatic discharge protection nanoelectronics simulation

Abstract: In nanoelectronics, snapback phenomena play an important role in electrostatic discharge (ESD) protection devices, in particular for gigascale, very large scale integration (VLSI) circuit design. In this paper we present a new ESD equivalent circuit model for deep submicrion and nanoscale semiconductor device simulation. By considering the geometry effect in the formulation of snapback characteristics, our model can be directly incorporated into electronic circuit simulation for the whole chip ESD protection circuit design. With the developed ESD model, we can investigate robust enhancement problems and perform a SPICE based whole chip ESD protection circuit design in nanoelectronics.