PHYchip Corporation. SCU Nanotechnology Course presentation. Dhaval Brahmbhatt President & CEO. Friday, April 29 th, 2005

Transcription

1 SCU Nanotechnology Course presentation Dhaval Brahmbhatt President & CEO Friday, April 29 th, 2005, San Jose, CA

2 Course Books (1) Primary Book: Introduction to Nanoscale Science and Technology Edited by Massimiliano Di Ventra, Stephane Evoy and James R. Heflin, Jr. Kulver Academic Publishers (2) Other Books: Nanosystems. Molecular Machinery, Manufacturing and Computation Author: K. Eric Drexler Wiley Interscience Publication (3) NANOTECHNOLOGY & HOMELAND SECURITY. New weapons for new wars. Authors: Daniel Ratner & Mark A. Ratner. Forwarded by James Murday, Office of Naval Research ADDISON-WESLEY PROFESSIONAL PRENTICE HALL PTR 2

3 NANOCOMPOSITES DEFINED AS MULTIPHASE METERIALS WHERE ONE OR MORE OF THE PHASES HAVE AT LEAST ONE DIMENSION OF THE ORDER OF 100 NM OR LESS Most nanocomposites that have been developed and are of significance are TWO PHASE MATERIALS, they are of three principal types: (1) Composites with alternate layers of nanoscale dimensions (2) Nanofilamentary composites composed of a matrix with embeded and generally aligned nanoscale diameter filaments (3) Nanoparticulate composites composed of matrix with embedded nanoscale particles Nanocomposites display improvements over those of component phases individually, with nanoscale sometimes unusual and enhanced properties can be realized Have large ratio of interface area to volume, this results in novel and enhanced properties that can be exploited technically 3

4 Λ (a) d (b) d (c) Figure 8.1. Schematic representations of nanocomposite materials with characteristic length scale: (a) nanolayered composites with nanoscale bilayer repeat length Λ; (b) nanofilamentary (nanowire) composites composed of a matrix with embedded filaments of nanoscale diameter d; (c) nanoparticulate composites composed of a matrix with embedded particles of nanoscale diameter d. 4

5 NANOCOMPOSITES functional Nanocomposites are either functional materials (based on their electrical, magnetic, and/or optical behavior) or as structural material (based on mechanical properties) Example of functional material would be alternate layers of single crystal GaAs and GaAlAs. When the layer thicknesses are reduced below the electronic mean free path of the bulk three dimensional material, novel electronic and photonic properties can be realized owing to quantum confinement effects Fig. 8.2 shows a schematic energy band diagram for GaAs/GaAlAs superlattice, an electron in the GaAs layer is partially confined in a quantum well of barrier height determined by the difference in energies of the bottom of their conduction bands. By changing the width of the well, the electron energies can be tuned for certain electronic and photonic applications 5

6 E c ψ ΔE E gap for Al x Ga 1 x As E gap for GaAs E v thickness of GaAs layer Figure 8.2. Schematic energy band diagram of GaAs/GaAl x As 1-x quantum well. An electron (represented by its wavefunction ψ) can be considered as partially confined in the quantum well of width equal to the GaAs thickness. The barrier height ΔE is equal to the difference in the energies of the bottom of the conduction band E c for the two layer materials. E v is the energy of the top of the valence band and E gap is the band gap energy. 6

7 NANOCOMPOSITES structural An example would be ductile metal matrix embedded with reinforcing second phase composed of hard ceramic nanoparticles Yield strength governed by stress necessary for mobile dislocations to overcome obstacles to their motion If the particles are very close (distance much smaller than particle size) then yield strength determined by stress needed for dislocations to overcome the particles by bowing around them Orowan Bowing Mechanism It is possible to achieve significant strengthening with a relatively small particle volume fraction Some applications for polymer multilayers of alternate brittle and ductile materials are in heavy duty wrapping and packaging materials 7

8 hard precipitate particles dislocation λ Figure 8.3. Precipitate particles of spacing λ acting as obstacles to dislocation motion. 8

9 NANOLAYERED COMPOSITES Advanced thin film deposition techniques allow fabrication of high quality multilayered materials Artificially multilayered materials composed of layers of different phase are generally known as heterostructures Multilayers composed of many single crystals layers that posses the same crystal structure and where there is perfect lattice matching at the surface (interphase interfaces) are called superlattices, an important example is GaAs/GaAlAs Physical Vapor Deposition (PVD) methods, such as evaporation and sputtering have been widely used to produce metallic, ceramic, and semiconductor artificially layered thin films Some interesting applications for ferromagnetic materials are in data storage, semiconductor materials such as Mo/Si have been used in Bragg reflectors for X-ray optics elements 9

10 Figure 8.4. High resolution transmission electron micrograph showing a cross-sectional view of an InAs-GaSb (100) superlattice (Reproduced with kind permission of M. Twigg.) 10

11 Nanofilamentary & Nanowire Composites A matrix embedded with aligned second phase filaments gives rise to two kinds of nanocomposites; Nanofilamentary composites, associated with mechanical processing methods to produce materials with enhanced mechanical strength and some other properties Embedded nanowire array, generally produced by electrodeposition in a compliant matrix and gives rise to interesting functional properties of nanowires 11

12 Nanofilamentary Composites These are wires that are composed of a metal matrix with aligned second phase metal filaments Two phases are metal that show little to no solid solubility, most common system is Cu/Nb where Nb volume is few tens of % To produce nanocomposite, the two-phase ingot is subjected to large scale deformation such as drawing and swaging In case of Cu/Nb, the Nb wire diameters can be reduced to tens of nanometers, with a large aspect ratio of 10 3 to 10 6 Nanofilamentary composites show enhanced tensile strength (approaching theoretical limits) and high electrical conductivity that are maintained at very low temperatures where Nb is superconducting, so these nanowires have application in as windings for high field pulsed magnets Swaging is a metal forming technique in which the metal is plastically deformed (non-reversible change of shape in response to an applied force) to its final shape. Swaging differs from forging in that the metal is Cold worked. The most common use of swaging is to attach fittings to pipes or cables; the parts loosely fit together, and a mechanical or hydraulic tool compresses and deforms the fitting, creating a permanent joint. Pipe flaring machines are another example. 12

13 Nanowire Composites Functional materials whose behavior is generally associated with quantum confinement effects Electrodeposition is an extremely important synthesis method, method uses nanoporous membrane such as polycarbonate. On one side of the membrane, gold electrode is deposited by sputtering. During the plating, pores are filled with deposited material, resulting in an array of nanowires within the membrane. The next page picture shows SEM of FeCo nanowires produced by this method after the membrane has been dissolved With this approach, nanowire composites with arrays of metals & alloys, conducting polymers, and semiconductors can be produced Potential applications include field emitter areas for flat panel displays and magnetic data storage 13

14 Figure 8.5. Scanning electron micrograph of electrodeposited FeCo nanowires (the polycarbonate matrix in which the wires were embedded has been completely dissolved). 14

15 Nanoparticulate Composites These materials are composed of nanoscale metal particles embedded in an immiscible metal, ceramic, or semiconductor matrix. Granular metals have been generally produced by simultaneous thin film deposition of two immiscible phases by evaporation or sputtering. The figure next page shows Ni/SiO 2 granular metal films. The ceramic phase is amorphous and the metal is in the form of nanoscale single crystal particles that are spherical in shape In these films, metal phase displays a percolation threshold at a composition between 50 and 60 volume percent (above, metal is an interconnected network, below it is metal is in distinct particles) Structure nanocomposites produced using layered clay materials such as montmorilonite and hectorite are the most significant Another type of commercially important nanocomposites are thermoplastics reinforced with Carbon nanotubes, the high tensile strength of carbon nanotubes make them very attractive filler materials for nanocomposite polymers 15

16 Figure 8.6. Bright field transmission electron micrographs of Ni/SiO 2 granular metal films. (From Ref. 29 by permission of Elsevier Science B.V.) 16

17 Figure 8.7. Transmission electron micrographs of binary nanoparticle assemblies. (a) Fe 3 O 4 (4nm)-Fe 58 Pt 42 (4nm) assembly; (b) Fe 3 O 4 (8nm)- Fe 58 Pt 42 (4nm) assembly; Fe 3 O 4 (12 nm)- Fe 58 Pt 42 (4 nm) assembly. (From Ref. 46 by permission PHYchip of Macmillan Corporation Magazines Ltd.) 17

18 Nanoparticulate Composites electrical & magnetic properties Below the percolation threshold, the nanocomposite displays the conductivity of a dielectric material, above is an electrical conductor. Thus associated with the percolation transition is a several orders of magnitude change in conductivity If the metal is a ferromagnet, the nanocomposite will behave like a ferromagnet above the percolation threshold. Below the percolation threshold and when the metal particles are smaller can lead to novel magnetic behavior such as giant magnetoresistance (GMR) The Giant Magnetoresistive Effect (GMR) is a quantum mechanical effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers. The effect manifests itself as a significant increase in resistance of the structure when two ferromagnetic layers contain electrons with opposite spin, compared to a lower level of resistance when the two layers contain electrons with parallel spin. The effect was first discovered in pure crystal layers in 1988 by Peter Grünberg of the Jülich Research Centre and Albert Fert of the University of Paris-Sud, working independently. The possibilities of using the effect in a magnetic field sensor, and hence as a new type of reading head in a computer hard drive, were quickly recognised by an IBM research team led by Stuart Parkin, who replicated the effect in polycrystalline layers. IBM produced the first commercial device based on this effect in December Currently, research has focused on employing multilayered nanowires (which offer greater sensitivity than the thin films now used in hard drives), which also exhibit giant magnetoresistance. 18

19 Structural Materials Reinforcing second phase particles in a nanoparticulate composite can result in significant mechanical property enhancements using a relatively small volume fraction of the second phase e.g. Ni/Al 2 O 3 and Si 3 N 4 /TiN films This behavior makes these materials attractive as coatings Ceramic nanocomposites such as Al 2 O 3 /SiC have shown promise for enhanced wear and creep resistance Polymer nanocomposites using layered clay and carbon nanotube reinforcing particles are already used for automobile material applications such as timing chain covers and mirror housings 19