Introduction to Nanotechnology - by fjwuxn

VIEWS: 180 PAGES: 79

									          M. Meyyappan
     Center for Nanotechnology
    NASA Ames Research Center
      Moffett Field, CA 94035

email: mmeyyappan@mail.arc.nasa.gov
• Definition

• National Nanotechnology Initiative

• Impact on various economic sectors
     - Electronics and computing
     - Health and medicine
     - Energy
     - Transportation
     -
     -
     -

• Commercial outlook
Nanotechnology is the creation of USEFUL/FUNCTIONAL
materials, devices and systems (of any useful size) through
control/manipulation of matter on the nanometer length scale and
exploitation of novel phenomena and properties which arise because
of the nanometer length scale:
                                                  •   Physical
Nanometer                                         •   Chemical
• One billionth (10-9) of a meter                 •   Electrical
• Hydrogen atom 0.04 nm                           •   Mechanical
• Proteins ~ 1-20 nm                              •   Optical
• Feature size of computer chips 90 nm            •   Magnetic
  (in 2005)                                       •
• Diameter of human hair ~ 10 µm                  •
           What Is Nanotechnology?
  (Definition from the NNI)
 Research and technology development aimed
  to understand and control matter at
  dimensions of approximately 1 - 100
  nanometer – the nanoscale
 Ability to understand, create, and use
    structures, devices and systems that have       Nanoarea Electron Diffraction
    fundamentally new properties and functions       of DW Carbon Nanotube –
    because of their nanoscale structure                     Zuo, et.al


 Ability to image, measure, model, and
    manipulate matter on the nanoscale to exploit
    those properties and functions
 Ability to integrate those properties and
    functions into systems spanning from nano- to
    macro-scopic scales                             Corral of Fe Atoms – D. Eigler
Source: Clayton Teague, NNI
• Examples                                AFM Image of DNA

  - Carbon Nanotubes
  - Proteins, DNA
  - Single electron transistors

• Not just size reduction but phenomena
  intrinsic to nanoscale
  - Size confinement
  - Dominance of interfacial phenomena
  - Quantum mechanics

• New behavior at nanoscale is not
  necessarily predictable from what we
  know at macroscales.
              Unique Properties of Nanoscale Materials

       • Quantum size effects result in unique mechanical,
         electronic, photonic, and magnetic properties of nanoscale
         materials

       • Chemical reactivity of nanoscale materials greatly different
         from more macroscopic form, e.g., gold

       • Vastly increased surface area per unit mass, e.g., upwards
         of 1000 m2 per gram

       • New chemical forms of common chemical elements, e.g.,
         fullerenes, nanotubes of carbon, titanium oxide, zinc oxide,
         other layered compounds
Source: Clayton Teague, NNI
• Atoms and molecules are generally less than a nm and we study
  them in chemistry. Condensed matter physics deals with solids
  with infinite array of bound atoms. Nanoscience deals with the
  in-between meso-world
• Quantum chemistry does not apply (although fundamental laws
  hold) and the systems are not large enough for classical laws of
  physics
• Size-dependent properties
• Surface to volume ratio
         -    A 3 nm iron particle has 50% atoms on the surface
         -    A 10 nm particle         20% on the surface
         -    A 30 nm particle         only 5% on the surface
                        MORE IN SECTION II
• Many existing technologies already depend on nanoscale materials
  and processes
        - photography, catalysts are “old” examples
        - developed empirically decades ago

• In existing technologies using nanomaterials/processes, role of
  nanoscale phenomena not understood until recently; serendipitous
  discoveries
         - with understanding comes opportunities for improvement

• Ability to design more complex systems in the future is ahead
         - designer material that is hard and strong but low weight
         - self-healing materials
                   •    1959 Feynman Lecture “There is Plenty of Room at the
                        Bottom” provided the vision of exciting new discoveries if
                        one could fabricate materials/devices at the atomic/molecular
                        scale.

                   •    Emergence of instruments in the 1980s; STM, AFM
                        providing the “eyes”, “fingers” for nanoscale manipulation,
                        measurement…

                                 •   Recently, there has been an explosion of research
                                     on the nanoscale behavior
                                     - Nanostructures through sub-micron self
                                        assembly creating entities from “bottom-up”
STM
                                        instead of “top-down”
                                     - Characterization and applications
                                     - Highly sophisticated computer simulations to
                                        enhance understanding as well as create
      Image of Highly Oriented          „designer materials‟
         Pyrolitic Graphite
•   For information, www.nano.gov
•   Multiagency Initiative in nanotechnology starting in FY01 “National Nanotechnology
    Initiative (NNI) Leading to the Next Industrial Revolution”
•   FY05 Nano budget is $1.0 Billion

•   Biggest portion of the funding goes to NSF
       - Followed by DoD, DOE, NIH, NASA
       - All these agencies spend most of their nano funding on university programs
•   Very strong activities in Japan, Europe, China, Singapore, fueled by Government Initiatives
•   Nano activities in U.S. companies: IBM, Motorola, HP, Lucent, Hitachi USA, Corning,
    DOW, 3M…
       - In-house R & D
       - Funding of new ventures
•   Nano Centers have been established at Universities all across the world
•   Emerging small companies
      - VC funding on the increase
               NNI Program Component Areas (PCAs)

        • Fundamental Nanoscale Phenomena and
          Processes
        • Nanomaterials
        • Nanoscale Devices and Systems
        • Instrumentation Research, Metrology, and
          Standards for Nanotechnology
        • Nanomanufacturing
        • Major Research Facilities and
          Instrumentation Acquisition
        • Societal Dimensions
Source: Clayton Teague, NNI
•   The U.S. does not dominate
    nanotechnology research. Nearly twice
                                                                         Leadership Position
    as much ongoing research overseas as
    in the U.S.                                  Synthesis & Assembly     U.S.         Europe   Japan

•   Many foreign leaders, companies,             Biological Approaches    U.S./Eur     Japan
    scientists believe that nanotechnology       & Application
    will be the leading technology of the        Dispersions and          U.S./Eur     Japan
    21st century. The fact that there is still   Coatings
    a chance to get on the ground floor          High Surface             U.S.         Europe   Japan
    explains pervasive R & D worldwide.          Area Materials
    Strong nanotechnology programs in
    European Union countries, Japan,             Nanodevices              Japan      Europe     U.S.
    Korea, Switzerland, Singapore,
    Australia, Taiwan, China and Russia.                       Level
                                                                             1           2         3
                                                                             Highest
                                                                                       Source: WTEC Report
•   Academia will play key role in development of nanoscience and technology
    - Promote interdisciplinary work involving multiple departments
    - Develop new educational programs
    - Technology transfer to industry

•   Government Labs will conduct mission oriented nanotechnology research
    - Provide large scale facilities and infrastructure for nanotechnology research
    - Technology transfer to industry

•   Government Funding Agencies will provide research funding to academia, small business, and
    industry through the NNI and other programs (SBIR, STIR, ATP…)

•   Industry will invest only when products are within 3-5 years
    - Maintain in-house research, sponsor precompetitive research
    - Sponsor technology start-ups and spin-offs

•   Venture Capital Community will identify ideas with market potential and help to launch start-ups

•   Professional societies should establish interdisciplinary forum for exchange of information; reach
    out to international community; offer continuing education courses
                                          Sensors,
Organic     Inorganic   Nanoelectronics
                         and Related       NEMS



                                  Structural
          Bio                    Applications




    Nanomaterials               Applications
•   Nanocrystalline materials   •   Molecular electronics
•   Nanoparticles               •   Quantum dots
•   Nanocapsules                •   NEMS, Nanofluidics
•   Nanoporous materials        •   Nanophotonics, Nano-optics
•   Nanofibers                  •   Nanomagnetics
•   Nanowires                   •   Nanofabrication
•   Fullerenes                  •   Nanolithography
•   Nanotubes                   •   Nanomanufacturing
•   Nanosprings                 •   Nanomedicine
•   Nanobelts                   •   Nano-bio
•   Dendrimers                  •
•                               •
            Extraordinary “Space” of Nanomaterials

• Atom clusters
                                                                      SiC Flowers

• Nanotubes, rods, spheres, belts … – carbon and other materials


• Dendrimers    C SWNT
                                 ZnO Belt          ZnO Tube                 GaN Rods


• Macro-molecular structures                                            TiO2
                                                                        Spheres
                           Spherical                     Tubular

• Biomolecular structures
                                            Rotaxane
     Catenane
                                                          STM Image
                                                          of DNA
                                                          Segment


                  STM Image
                  and Model of
                  Porphyrin                                   Source: Clayton Teague, NNI
    As Recommended by the IWGN (Interagency Working Group on Nanotechnology) Panel

                                                                     See www.nano.gov
•    Nanostructure Properties
     - Biological, chemical, electronic, magnetic, optical, structural…
•    Synthesis and Processing
     - Enable atomic and molecular control of material building blocks
     - Bioinspired, multifunctional, adaptive structures
     - Affordability at commercial levels
•    Characterization and manipulation
     - New experimental tools to measure, control
     - New standards of measurement
•    Modeling and simulation
•    Device and System Concepts
     - Stimulate innovative applications to new technologies
•    Application Development
                          (As raised in the IWGN Report)


1. What novel quantum properties will be enabled by nanostructures (at room temp.)?

2. How different from bulk behavior?

3. What are the surface reconstructions and rearrangements of atoms in nanocrystals?

4. Can carbon nanotubes of specified length and helicity be synthesized as pure
   species? Heterojunctions in 1-D?

5. What new insights can we gain about polymer, biological…systems from the
   capability to examine single-molecule properties?

6. How can one use parallel self-assembly techniques to control relative arrangements
   of nanoscale components according to predesigned sequence?

7. Are there processes leading to economic preparation of nanostructures with control
   of size, shape… for applications?

                           This is NOT an exhaustive list
                       • Information Technology
                          - Computing, Memory and Data Storage
                          - Communication
                       • Materials and Manufacturing
                       • Health and Medicine
                       • Energy
                       • Environment
                       • Transportation
                       • National Security
Nanotechnology is an   • Space exploration
enabling technology    •
                       •
•   Ability to synthesize nanoscale building blocks with control on size,
    composition etc.           further assembling into larger structures with
    designed properties will revolutionize materials manufacturing
    - Manufacturing metals, ceramics, polymers, etc. at exact shapes without
       machining
    - Lighter, stronger and programmable materials
    - Lower failure rates and reduced life-cycle costs
    - Bio-inspired materials
    - Multifunctional, adaptive materials
    - Self-healing materials

• Challenges ahead
  - Synthesis, large scale processing
  - Making useful, viable composites
  - Multiscale models with predictive capability
  - Analytical instrumentation
•   Carbon Nanotubes
•   Nanostructured Polymers
•   Optical fiber performs through sol-gel
    processing of nanoparticles
•   Nanoparticles in imaging systems
•   Nanostructured coatings
•   Ceramic Nanoparticles for netshapes




                                             Source: IWGN Report
• Nanostructured metals, ceramics at exact shapes without machining
• Improved color printing through better inks and dyes with
  nanoparticles
• Membranes and filters
• Coatings and paints (nanoparticles)
• Abrasives (using nanoparticles)
• Lubricants
• Composites (high strength, light weight)
• Catalysts
• Insulators
Past
Shared computing        thousands of
people sharing a mainframe computer




                    Present
                    Personal computing

Future
Ubiquitous computing           thousands of computers sharing each
and everyone of us; computers embedded in walls, chairs, clothing,
light switches, cars….; characterized by the connection of things in
the world with computation.
“There is at least as far to go (on a logarithmic scale) from the present as
we have come from ENIAC. The end of CMOS scaling represents both
opportunity and danger.”
                              -Stan Williams, HP

•   A few more CMOS generations left but cost of building fabs going up faster than
    sales. Physics has room for 109x current technology based on 1 Watt
    dissipation, 1018 ops/sec        no clear ways to do it!
          - Molecular nanoelectronics ?
          - Quantum cellular automata ?
          - Chemically synthesized circuits ?

•   Self assembly to reduce manufacturing costs, defect tolerant architectures
    may be critical to future nanoelectronics
• Quantum Computing                                              •   Carbon nanotube transistors by several groups
       - Takes advantage of quantum mechanics
         instead of being limited by it                          •   Molecular electronics: Fabrication of logic gates
       - Digital bit stores info. in the form of „0‟ and             from molecular switches using rotaxane
         „1‟; qubit may be in a superposition state of               molecules
         „0‟ and „1‟ representing both values
         simultaneously until a measurement is made              •   Defect tolerant architecture, TERAMAC computer
       - A sequence of N digital bits can represent                  by HP            architectural solution to the
         one number between 0 and 2N-1; N qubits                     problem of defects in future molecular electronics
         can represent all 2N numbers simultaneously

                 1938                         1998

          Technology engine:           Technology engine:
          Vacuum tube                  CMOS FET

          Proposed improvement:        Proposed improvement:
          Solid state switch           Quantum state switch

          Fundamental research:        Fundamental research:
          Materials purity             Materials size/shape

                                           - Stan Williams, HP
• Processors with declining energy use and cost per gate, thus
  increasing efficiency of computer by 106
• Higher transmission frequencies and more efficient utilization of
  optical spectrum to provide at least 10 times the bandwidth now
• Small mass storage devices: multi-tera bit levels
• Integrated nanosensors: collecting, processing and
  communicating massive amounts of data with minimal size,
  weight, and power consumption
• Quantum computing
• Display technologies
 •   Expanding ability to characterize genetic makeup will
     revolutionize the specificity of diagnostics and
     therapeutics
        - Nanodevices can make gene sequencing more
          efficient

 •   Effective and less expensive health care using remote
     and in-vivo devices

                                       • New formulations and routes for drug
                                         delivery, optimal drug usage

                                       • More durable, rejection-resistant artificial
                                         tissues and organs
Nanotube-based
biosensor for
cancer diagnostics                     • Sensors for early detection and prevention
• DNA microchip arrays using advances for IC industry

• „Gene gun‟ that uses nanoparticles
  to deliver genetic material to
  target cells

• Semiconductor nanocrystals
  as fluorescent biological labels




                                                  Source: IWGN Report
• Energy Production
    - Clean, less expensive sources enabled by novel nanomaterials
      and processes
    - Improved solar cells
• Energy Utilization
    - High efficiency and durable home and
      industrial lighting
    - Solid state lighting can reduce total
      electricity consumption by
      10% and cut carbon emission
      by the equivalent of 28 million tons/year
      (Source: Al Romig, Sandia Lab)
• Materials of construction sensing changing conditions and in
  response, altering their inner structure
• Nanomaterials have a large surface area. For example, single-walled
  carbon nanotubes show ~ 1600 m2/g. This is equivalent to the size of a
  football field for only 4 gms of nanotubes. The large surface area enables:
     - Large adsorption rates of various gases/vapors
     - Separation of pollutants
     - Catalyst support for conversion
       reactions
     - Waste remediation
• Filters and Membranes
      - Removal of contaminants
        from water
      - Desalination
• Reducing auto emissions, NOx conversion
     - Rational design of catalysts
• More efficient catalytic converters
• Thermal barrier and wear resistant coatings

• Battery, fuel cell technology

• Improved displays

• Wear-resistant tires
• High temperature sensors for „under the hood‟; novel
  sensors for “all-electric” vehicles

• High strength, light weight composites for increasing fuel
  efficiency
• Improved collection, transmission, protection of information

• Very high sensitivity, low power sensors for detecting
  chem/bio/nuclear threats

• Light weight military platforms, without sacrificing
  functionality, safety and soldier security
     - Reduce fuel needs and
       logistical requirements

• Reduce carry-on weight of
  soldier gear
     - Increased functionality
       per unit weight
• Advanced miniaturization, a key thrust area to enable new science and
  exploration missions
        - Ultrasmall sensors, power sources, communication, navigation,
            and propulsion systems with very low mass, volume and power
            consumption are needed

• Revolutions in electronics and computing will allow reconfigurable,
  autonomous, “thinking” spacecraft                                       Europa Submarine


• Nanotechnology presents a whole new
  spectrum of opportunities to build
  device components and systems for
  entirely new space architectures
         - Networks of ultrasmall
             probes on planetary surfaces
         - Micro-rovers that drive,
             hop, fly, and burrow
         - Collection of microspacecraft
             making a variety of measurements
•   Short term (< 5 years)
      - Nanoparticles
          * Automotive industry (body moldings, timing belts,
            engine covers…)
          * Packaging industry
          * Cosmetics
          * Inkjet technology
          * Sporting goods

      -   Flat panel displays
      -   Coatings
      -   CNT-based probes in semiconductor metrology
      -   Tools
      -   Catalysts (extension of existing market)
• Medium term (5-10 years)
     -   Memory devices
     -   Fuel cells, batteries
     -   Biosensors (CNT, molecular, qD based)
     -   Biomedical devices
     -   Advances in gene sequencing
     -   Advances in lighting
• Long term (> 15 years)
     -   Nanoelectronics (CNT)
     -   Molecular electronics
     -   Routine use of new composites in Aerospace,
         automotive (risk-averse industries)
     -   Many other things we haven‟t even thought of yet
Timeline for Beginning of Industrial Prototyping and
Nanotechnology Commercialization:Four Generations
             1st Passive Nanostructures
                Ex. Coatings, nanoparticles, nanostructured metals, polymers, ceramics
    ~2000
                       2nd Active Nanostructures
                              Ex. 3D transistors, amplifiers, targeted drugs, actuators,
             ~2005            adaptive structures

                                CMU   3rd Systems of Nanosystems
                                          Ex. Guided assembling; 3D networking and new
                          ~2010           hierarchical architectures, robotics, evolutionary
                                              4th Molecular Nanosystems
                                                  Ex. Molecular devices ‘by design’, atomic
                                                  design, emerging functional systems
                               ~2015-2020




Source: Clayton Teague, NNI                          AIChE Journal 2004, 50, MC Roco
                              Red Herring, May 2002


Commonality: Railroad, auto, computer, nanotech
             all are enabling technologies
NANOSCALE PROPERTIES
• Size-dependent properties
  color, specific heat, melting point, conductivity…..

• I-V of a single nanoparticle

• Adsorption
    - principles
    - some examples

• Nanomaterial reinforcement in composites
    - multifunctionality
    - self-heating
• Cluster
     - A collection of units (atoms or reactive molecules) of up to
       about 50 units
• Colloids
     - A stable liquid phase containing particles in the 1-1000 nm
       range. A colloid particle is one such 1-1000 nm particle.
• Nanoparticle
     - A solid particle in the 1-100 nm range that could be
       noncrystalline, an aggregate of crystallites or a single
       crystallite
• Nanocrystal
     - A solid particle that is a single crystal in the nanometer range
Source: Nanoscale Materials in Chemistry, Ed. K.J. Klabunde, Wiley, 2001
• Spherical iron nanocrystals

• J. Phys. Chem. 1996,
  Vol. 100, p. 12142
              Nanoscale = High Ratio of Surface Area to Vol.
                                     Repeat 24 times




          For example, 5 cubic centimeters – about 1.7 cm per side – of
          material divided 24 times will produce 1 nanometer cubes and
          spread in a single layer could cover a football field
Source: Clayton Teague, NNI
                                               Quantum Size Effect

               Value of physical property                            quantum size
                                                                     effect regime

                                                                                      Regions indicated for
                                                                                      QSE for metals,
                                                                                      semiconductors, and
                                                                                      semimetals are very
                                                     Metals                           rough estimates

                                                               Semiconductors

                                                                         Semimetals

                                            10-100               108
                                            ~ 1 nm            ~ 100 nm

                                              Number of atoms in cluster
Source: Clayton Teague, NNI
• In materials where strong chemical bonding is present, delocalization of valence
  electrons can be extensive. The extent of delocalization can vary with the size
  of the system.

• Structure also changes with size.

• The above two changes can lead to different physical and chemical
  properties, depending on size
     - Optical properties
     - Bandgap
     - Melting point
     - Specific heat
     - Surface reactivity
     -
     -
• Even when such nanoparticles are consolidated into macroscale solids, new
  properties of bulk materials are possible.
     - Example: enhanced plasticity
• For semiconductors such as ZnO, CdS, and Si, the bandgap
  changes with size
       - Bandgap is the energy needed to promote an electron
         from the valence band to the conduction band
       - When the bandgaps lie in the visible spectrum, a change
         in bandgap with size means a change in color

• For magnetic materials such as Fe, Co, Ni, Fe3O4, etc., magnetic
  properties are size dependent
      - The „coercive force‟ (or magnetic memory) needed to
         reverse an internal magnetic field within the particle is
         size dependent
      - The strength of a particle‟s internal magnetic field can be
         size dependent
• In a classical sense, color is caused by the partial absorption of
  light by electrons in matter, resulting in the visibility of the
  complementary part of the light
• On most smooth metal surfaces, light is totally reflected by the
  high density of electrons    no color, just a mirror-like
  appearance.
• Small particles absorb, leading to some color. This is a size
  dependent property.
  Example: Gold, which readily forms nanoparticles but not easily
  oxidized, exhibits different colors depending on particle size.
      - Gold colloids have been used to color glasses since early
        days of glass making. Ruby-glass contains finely dispersed
        gold-colloids.
      - Silver and copper also give attractive colors
• C = ∆Q/m∆T
  Specific heat is the amount of heat ∆Q required to
  raise the temperature by ∆T of a sample of mass m

• Units are J/kg ·K or cal/g ·K

• 1 calorie is the heat needed to raise the temperature
  of 1 g of water by 1 degree.
• Specific heat of polycrystalline materials is given by Dulong-Petit
  law
      - C of solids at room temp. (in J/kg ·k) differs widely from one
         to another; but the molar values (in J/moles ·k) are nearly the
         same, approaching 26 J/mol ·K; Cv = 3 Rg/M
         where M is molecular weight

• Cv of nanocrystalline materials are higher than their bulk
  counterparts. Example:
      - Pd: 48%  from 25 to 37 J/mol.K at 250 K for 6 nm
         crystalline
      - Cu: 8.3%  from 24 to 26 J/mol.K at 250 K for 8 nm
      - Ru: 22%  from 23 to 28 J/mol.K at 250 K for 6 nm
  The melting point of gold particles decreases dramatically as
               the particle size gets below 5 nm




Source: Nanoscale Materials in Chemistry, Wiley, 2001
 • Start from an energy balance; assume the change in internal energy
   (∆U) and change in entropy per unit mass during melting are
   independent of temperature

                     2To / Lr
     ∆   =   Deviation of melting point from the bulk value
     To   =   Bulk melting point
         =   Surface tension coefficient for a liquid-solid interface
         =   Particle density
     r    =   Particle radius


     L    =   Latent heat of fusion
• Lowering of the melting point is proportional to 1/r

•  can be as large as couple of hundred degrees when the
  particle size gets below 10 nm!

• Most of the time,  the surface tension coefficient is unknown;
  by measuring the melting point as a function of radius,  can be
  estimated.

• Note: For nanoparticles embedded in a matrix, melting point may
  be lower or higher, depending on the strength of the interaction
  between the particle and matrix.
• For metals, conductivity is based on their band structure. If the
  conduction band is only partially occupied by electrons, they can
  move in all directions without resistance (provided there is a perfect
  metallic crystal lattice). They are not scattered by the regular
  building blocks, due to the wave character of the electrons.

                    e         v = electron speed
               
                  4 o me v   o = dielectric constant in vacuum

  , mean time between collisions, is /v
• For Cu, v = 1.6 x 106 m/s at room temp.;  = 43 nm,  = 2.7 x 10-14s
• Scattering mechanisms
  (1) By lattice defects (foreign atoms, vacancies, interstitial
       positions, grain boundaries, dislocations, stacking disorders)
  (2) Scattering at thermal vibration of the lattice (phonons)

• Item (1) is more or less independent of temperature while item #2
  is independent of lattice defects, but dependent on temperature.

• Electric current     collective motion of electrons; in a bulk metal,
                          Ohm‟s law: V = RI

• Band structure begins to change when metal particles become
  small. Discrete energy levels begin to dominate, and Ohm‟s law is
  no longer valid.
Source: Nanoscale Materials in Chemistry, Wiley, 2001
• Consider a single nanoparticle between two electrodes, but cushioned by a
  capacitance on each side
    - If an electron is transferred to the particle, its coulomb energy  by
                                   Ec = e2/2c
    - Thermal motion of the atoms in the particle can initiate a charge & Ec,
       leading to further electrons tunneling uncontrollably
    - So, kT << e2/2c
    - Tunneling current I = V/RT
    - Current begins at coulomb voltage Vc = ± e/2c which is called coulomb
       blockade
    - Further electron transfer happens if the coulomb energy of the „quantum
       dot‟ is compensated by an external voltage Vc = ± ne/2c where n is an
       integer
-   Repeated tunneling results in a „staircase‟ with step height in current, e/Rc
-   Possible to charge and discharge a quantum dot in a quantized manner
    principle behind some future computers
                                         Source: Nanoscale Materials in Chemistry, Wiley, 2001

• If a bulk metal is made thinner and thinner, until the electrons can
  move only in two dimensions (instead of 3), then it is “2D quantum
  confinement.”
• Next level is „quantum wire
• Ultimately „quantum dot‟
Adsorption
• Adsorption is like absorption except the adsorbed material is held near the surface
  rather than inside

• In bulk solids, all molecules are surrounded by and bound to neighboring atoms
  and the forces are in balance. Surface atoms are bound only on one side, leaving
  unbalanced atomic and molecular forces on the surface. These forces attract gases
  and molecules  Van der Waals force,  physical adsorption or physisorption

• At high temperatures, unbalanced surface forces may be satisfied by electron
  sharing or valence bonding with gas atoms  chemical adsorption or
  chemisorption
       - Basis for heterogeneous catalysis (key to production of fertilizers,
          pharmaceuticals, synthetic fibers, solvents, surfactants, gasoline, other
          fuels, automobile catalytic converters…)
       - High specific surface area (area per unit mass)
• Physisorption of gases by solids increases with decreasing T and with increasing P
• Weak interaction forces; low heats of adsorption
  < 80 KJ/mole; physisorption does not affect the
  structure or texture of the absorbent
• Desorption takes place as conditions are reversed
• Mostly, testing is done at LN2 temperature
  (77.5 K at 1 atm.). Plot of gas adsorbed as
  volume Va at 0° C and 1 atm (STP) vs. P/Po
  (Po is vapor pressure) is called adsorption
  isotherm.
• Adsorption occurs when the interaction potential energy  is equal
  to the work done to bring a gas molecule to the adsorbed state.
  Assuming that the adsorbed state is at the sat. vap. pressure Po,
                          P
                         V dp  RT ln P / P
                                             0
                          P0


• Assuming that contribution from adsorbate-adsorbate interaction to
    is negligible,  is essentially due to adsorbate-adsorbant
  interaction. Consider surface atom and adsorbate molecule
  separated by r. For physisorption,  is a sum of:
       - Dispersion energy D = -A/r6
       - Close range repulsion R = B/r12
       - Contributions arising from charges on the solid surface
Adsorption in open cylinders
                              D     2
                                     G         
       (mmol / gm)  9.146 f  1         c c
                                D         
  packing factor ~ 0.415
cc  Lennard-Jones parameter for carbon-carbon interaction
• Much stronger interaction than physisorption

• Heat of adsorption up to 800 KJ/mole

• Adsorbing gas or vapor molecule splits into atoms,
  radicals or ions which form a chemical bond with the
  adsorption site.  Sharing of electrons between the gas
  and solid surface; may be regarded as the formation of a
  surface compound.
• Simple reversal is not possible like in physisorption
    - O2 chemisorbed on charcoal  application of heat and
      vacuum will result in CO desorption instead of O2.

• Under favorable T & P, physisorption takes place on all surfaces.
  But chemisorption is localized and occurs on only certain surface
  sites

• Physisorption  with  in T; chemisorption  with  in T

• Same surface can exhibit physiosorption at low T and
  chemisorption at high T. Example: N2 chemisorption on Fe at
  800° C to form iron nitride but physisorption at LN2 temperatures.
• Assumes gases form only one monolayer on a solid
• Gas molecule collision with solid  inelastic, so the gas molecule stays in contact
  with solid, for a time before desorbing
• Writing a balance between the rate at which molecules strike the surface and rate
  at which they leave:
                     Vm bP      Vm= quantity of gas absorbed when the
              Va 
                    1  bP          entire surface is covered with a
                   P      I   P      monolayer
• Rearranging,             
                   Va Vmb Vm
          P
• Plot of Va vs. P should yield a straight line if the equation applies  evaluate
                                           Vm= molar volume of the gas (22414 cm3);
  Vm and b                                 NA= Avogadro number
                           VmN A           = surface area occupied by single adorbed
                        s                     molecule, 16.2 A2 for N2
• Specific surface area     mV0
                                           m = mass of the adsorbing sample
Nanomaterial reinforcement
     in composites
• Processing them into various matrices follow earlier composite
  developments such as
      - Polymer compounding
      - Producing filled polymers
      - Assembly of laminate composites
      - Polymerizing rigid rod polymers
      -
      -

• Purpose
      - Replace existing materials where properties can be superior
      - Applications where traditionally composites were not a
        candidate
• Nanotechnology provides new opportunities for radical changes
  in composite functionality

• Major benefit is to reach percolation threshold at low volumes
  (< 1%) when mixing nanoparticles in a host matrix

• Functionalities can be added when we control the orientation
  of the nanoscale reinforcement.
• This always implies “structure +” since in most cases the major
  function of a structure is to carry load or provide shape.
  Additional functions can be:
• Actuation           controlling position, shape or load
• Electrical          either insulate or conduct
• Thermal             either insulate or conduct
• Health              monitor, control
• Stealth             managing electromagnetic or visible signature
• Self-healing        repair localized damage
• Sensing             physical, chemical variables          NRC Report, 2003
• Building in additional functionalities into load-bearing structures
  is one key example:
       - Sensing function
              * Strain
              * Pressure
              * Temperature
              * Chemical change
              * Contaminant presence
• Miniaturized sensors can be embedded in a distributed fashion to
  add “smartness” or multifunctionality. This approach is „pre-nano‟
  era.
• Nanotechnology, in contrast, is expected to help in assembling
  materials with such functional capabilities
• Possible, in principle, to design any number of composites with multiple levels
  of functionality (3, 4, 5…) by using both multifunctional matrices and
  multifunctional reinforcement additives

       - Add a capsule into the matrix that contains a nanomaterial sensitive to
         thermal, mechanical, electrical stress; when this breaks, would indicate the
         area of damage

       - Another capsule can contain a healant

       - Microcellular structural foam in the matrix may be radar-absorbing,
         conducting or light-emitting

       - Photovoltaic military uniform also containing Kevlar for protection
                  generate power during sunlight for charging the batteries of
         various devices in the soldier-gear
                                                                     NRC Report, 2003
• Carbon nanotubes, nanofibers

• Polymer clay nanocomposites

• Polymer cross-linked aerogels

• Biomimetric hybrids

Expectations:
 - „Designer‟ properties, programmable materials
 - High strength, low weight
 - Low failure rates
 - Reduced life cycle costs
     „Self-healing plastic‟ by Prof. Scott White (U. of Illinois)
                      Nature (Feb. 15, 2001)
• Plastic components break because of mechanical or thermal
  fatigue. Small cracks  large cracks  catastrophic failure.
  „Self-healing‟ is a way of repairing these cracks without human
  intervention.
• Self-healing plastics have small capsules that release a healing
  agent when a crack forms. The agent travels to the crack
  through capillaries similar to blood flow to a wound.
• Polymerization is initiated when the agent comes into contact
  with a catalyst embedded in the plastic. The chemical reaction
  forms a polymer to repair the broken edges of the plastic. New
  bond is complete in an hour at room temperature.
• Surface chemistry is important in catalysis. Nanostructured materials have some
  advantages:
    - Huge surface area, high proportion of atoms on the surface
    - Enhanced intrinsic chemical activity as size gets smaller which is likely
        due to changes in crystal shape
    - Ex: When the shape changes from cubic
        to polyhedral, the number of edges
        and corner sites goes up significantly
    - As crystal size gets smaller, anion/cation
        vacancies can increase, thus affecting
        surface energy; also surface atoms can be
        distorted in their bonding patterns
    - Enhanced solubility, sintering
        at lower T, more adsorptive
        capacity
• Zeolite is an old example which has been around a long time and
  used by petroleum industry as catalysts
• The surface area of a solid increases when it becomes nanoporous;
  this improves catalyst effects, adsorption properties
• Recall „adsorption‟ is like „absorption‟ except the absorbed material
  is held near the surface rather than inside
• How to make nanopores?
     - lithography followed by etching
     - ion beam etching/milling
     - electrochemical techniques
     - sol-gel techniques
• Composite materials formed by transition metal clusters embedded in glass
  matrices exhibit interesting optical properties
        - Candidates for nonlinear integrated optics, photonics  using photons
          instead of electrons to acquire, store, process and transmit information
        - Glass is cheap, ease of processing, high durability, high transparency
            promising glass-based structures

• Dielectric constant is a key property.   , effective dielectric constant:
                 h       m  h                  m  metal, h  host
                        p
                2 h      m  2h                 p = volume fraction

• Preparation techniques
        - ion implantation
        - ion exchange in a molten bath
        - ion irradiation
• Frequently encountered powders:
      - Cement, fertilizer, face powder, table salt, sugar, detergents, coffee
        creamer, baking soda…

• Some products in which powder incorporation is not obvious
     - Paint, tooth paste, lipstick, mascara, chewing gum, magnetic recording
       media, slick magazine covers, floor coverings, automobile tires…

• For most applications, there is an optimum particle size
      - Taste of peanut butter is affected by particle size
      - Extremely fine amorphous silica is added to control the ketchup flow
      - Medical tablets dissolve in our system at a rate controlled by particle size
      - Pigment size controls the saturation and brilliance of paints
      - Effectiveness of odor removers is controlled by the surface area of
        adsorbents.

                              From: Analytical methods in Fine Particle Technology, Webb and Orr
• Adding certain inorganic clays to rubber dramatically improves
  the lifetime and wear-characteristics of tires.

                             Why ?

  The nanoscale clay particles bind to the ends of the polymer
  molecules - which you can think of as molecular strings - and
  prevent them from unraveling.
                                             Price-volume relationship for annual
                                             U.S. consumption of structural materials.
                                             Source: J.H. Westbrook, General Electric
                                             (retired), private communications,
                                             September 27, 2002, NRC Report 2003.




• The relationship between cost and usage in tonnage is inverse

• Value of weight saving should be considered in other aspects as well.
      - Reduction in weight of vehicle (auto, plane)          reduced gasoline
          consumption
      - Spacecraft           cost of launch

								
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