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Electrical characteristic of carbon nanowires

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					ELECTRICAL CHARACTERISTIC OF
CARBON NANOWIRES PRODUCED BY
OXIDATIVE SHRINKING




       Alfredo D. Bobadilla
       Nanotechnology course – Prof Jorge M. Seminario
       Final Project
       Spring 2010
       Texas A&M University
OUTLINE

             MOTIVATION
             INTRODUCTION
             OBJECTIVE
             BASIC CONCEPTS
             REVIEW OF PREVIOUS WORKS
             RESEARCH PROJECT PROPOSAL
             FINAL CONSIDERATIONS
MOTIVATION
 Current 45nm Silicon-based technology has
  reached its physical limit with microprocessors
  speed in the order of ~ 1 GHz (still far of ~ 10
  GHz).
 Molecule-based electronics promise advancing
  towards ~ 1 THz due to novel physical
  phenomena at molecular scale (< 10 nm) and to
  the possibility of an inherent much higher level of
  integration.
INTRODUCTION
 Carbon nanostructured materials own
  outstanding electrical and thermal properties.
 Nonlinear electrical characteristic, approaching
  negative differential resistance behavior, is
  commonly found in molecules, including carbon
  nanomaterials.
 It is possible engineering the structure and
  properties of carbon nanomaterials by using ion
  and electron beams.
OBJECTIVE
 I propose an approach to engineer molecular
  circuits based on carbon nanowires with carbon
  nanotubes serving as interconnects.
 Developing a novel 10nm Carbon-based
  information technology.
BASIC CONCEPTS:
SILICON TECHNOLOGY
 CMOS transistor is
  the basic element for
  making an integrated
  circuit (chip).
 It is a 3-terminal
  device in which a gate
  voltage (Vgs) allows
  controlling the level of
  current (Ids)) through
  the transistor.



                     http://www.cmosvlsi.com/lect3.pdf
BASIC CONCEPTS:
SILICON TECHNOLOGY PHYSICAL LIMITS
   In digital technology the
    transistor is switched
    between two logic states, 0
    and 1, which correspond to
    two different voltage levels,
    i.e. 0V and 5V.
   The 0V and 5V logic levels
    are always mixed with
    thermal noise, as it is shown
    in the bottom picture.
   If the high logic level (5V in
    the example) would change
    to a lower level like 2V,
    thermal noise voltage would
    make it difficult
    distinguishing between 0V
    and 2V.


                AnantAgarwaland Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007.
                     MIT OpenCourseWare(http://ocw.mit.edu/), Massachusetts Institute of Technology.
        BASIC CONCEPTS:
        SILICON TECHNOLOGY PHYSICAL LIMITS
            As the CMOS transistor
             size is decreased, not
             only the number of
             CMOS transistors in a
             chip increase but also
             the power consumption
             and heat generation
             (thermal noise).
            It makes necessary
             lowering the supply
             voltage which, in
             current 45nm Si
             technology, is
             approaching the
             thermal noise voltage.


AnantAgarwaland Jeffrey Lang, course materials for 6.002 Circuits and Electronics, Spring 2007.
     MIT OpenCourseWare(http://ocw.mit.edu/), Massachusetts Institute of Technology.
BASIC CONCEPTS:
OPTICAL LITHOGRAPHY
   It is a micro-patterning
    technique to selectively
    remove parts of a thin
    film photoresist.
   It uses light to transfer
    a geometric pattern
    from a photo mask to a
    (light-sensitive)
    photoresist, then by a
    ‘development’ process
    the exposed area is
    removed.


                                http://cnx.org/content/m1037/latest/5.15.png
BASIC CONCEPTS:
DIFFRACTION LIMIT
   Diffraction can be
    described as the apparent
    bending of waves around
    small obstacles, and its
    effects are generally most
    pronounced for waves
    where the wavelength is
    on the order of the size of
    the diffracting objects.
   Therefore when ligth pass
    through a window size (in
    the mask) comparable to
    light wavelength, the light
    beam will bend. This limit
    the minimum feature size
    able to be patterned in
    optical lithography.


                                  http://cnx.org/content/m1037/latest/5.15.png
BASIC CONCEPTS:
ELECTRON BEAM LITHOGRAPHY
 e-beam lithography is
  a patterning
  technique which allow
  reaching feature sizes
  from 10nm to 100nm.
 The very small
  electron wavelength
  allow reaching a very
  small feature size.




                           http://www.cnf.cornell.edu/image/spiefig1.jpg
BASIC CONCEPTS:
PROXIMITY EFFECT
   An incident electron
    (purple) produces secondary
    electrons (blue). Sometimes,
    the incident electron may
    itself be backscattered, as
    shown in the figure, and
    leave the surface of the
    resist (amber).
   ‘Proximity effect’ refer to
    scattering electrons
    affecting the patterning of
    other nearby zones.
   The minimum feature size
    possible in e-beam
    lithography is not limited by
    the electron wavelength but
    by the ‘proximity effect’.



                                    http://upload.wikimedia.org/wikipedia/en/2/22/Electron_beam_scatteri
BASIC CONCEPTS:
ELECTRON BEAM LITHOGRAPHY

 On it, and high energy
  electron beam is
  scanned on a resist,
  usually PMMA,
  removing selectively
  the exposed area.
 This is followed by a
  development, metal
  deposition and a lift-
  off process, as shown
  in the figure.


                           http://www.aph.kit.edu/wegener/data/image/research/ebl.jpg
BASIC CONCEPTS:
TRANSMISSION ELECTRON MICROSCOPY (TEM)
   TEM is an imaging
    technique which allows
    reaching atomic resolution.
   The transmission electron
    microscope (TEM) operates
    on the same basic principles
    as the light microscope but
    uses electrons instead of
    light.
   What you can see with a
    light microscope is limited
    by the wavelength of light.
    TEMs use electrons as "light
    source" and their much
    lower wavelength makes it
    possible to get a resolution a
    thousand times better than
    with a light microscope.


             http://nobelprize.org/educational_games/physics/microscopes/tem/index.html
BASIC CONCEPTS:
TRANSMISSION ELECTRON MICROSCOPY (TEM)
   On it a highly energetic beam of
    electrons is transmitted through
    an ultra thin specimen.
   This stream is confined and
    focused using metal apertures
    and magnetic lenses into a thin,
    focused, monochromatic beam.
   Interactions occur inside the
    irradiated sample, affecting the
    electron beam.
   Information contained in the
    electron waves exiting from the
    sample is used to form the image.
   The projector lenses allow for the
    correct positioning of this
    electron wave distribution onto
    the viewing system.
        http://www.unl.edu/CMRAcfem/em.htm   http://en.wikipedia.org/wiki/File:Scheme_TEM_en.svg
BASIC CONCEPTS:
ETCHING PROCESSES
   Refer to the process of
    removing thin films previously
    deposited and/or the substrate
    itself. A mask can be used to
    selectively remove a specific
    zone of the film.
   In ‘wet etching’ the material is
    dissolved when immersed in a
    chemical solution.
   In ‘dry etching’ the material is
    sputtered or dissolved using
    reactive ions or a vapor phase
    etchant.
   With dry etching it is possible
    etching almost straight down
    without undercutting, which
    provides much higher
    resolution.




                                       https://www.memsnet.org/mems/processes/etch.html
BASIC CONCEPTS:
FOCUSED ION BEAM (FIB)
    Ions are larger and heavier
     than electrons
    A high energy ion beam is
     able to sputter the surface of
     almost any material and
     cause doping of the surface
     with atoms of the ion beam.
    FIB tools are designed for site
     specific etching or machining
     of surfaces, an ideal FIB
     might machine away one
     atom layer without any
     disruption of the atoms in the
     next layer, or any residual
     disruptions above the surface.


    http://en.wikipedia.org/wiki/Focused_ion_be
BASIC CONCEPTS:
NEGATIVE DIFFERENTIAL RESISTANCE (NDR)

                                                    A NDR device is a 2-terminal
                                                     device, which shows negative
                                                     resistance values, i.e. negative
             From the equation, I0                   values of dI/dV, in the
             and I1 indicate how fast                current-voltage curve.
             ‘V’ changes from ‘0’ state
             to ‘1’ state                           A NDR-based device
                                                     constitute the basic unit to
                                                     create a complete ‘logic
                                                     family’, i.e. it’s possible
                                                     creating all the logic gates
                                                     (AND, OR, etc) required for
                                                     fabricating integrated
                                                     circuits.



        Mathews, R. H. et al. A new RTD-FET logic family. Proc. IEEE 87, 596-605 (1999)
BASIC CONCEPTS:
NEGATIVE DIFFERENTIAL RESISTANCE (NDR)

                                                    When two NDR devices work
                                                     together, as shown in the
                                                     picture, the two logic states,
             From the equation, I0
                                                     i.e. ‘0’ and ‘1’, occur at a low
             and I1 indicate how fast                voltage level which imply a
             ‘V’ changes from ‘0’ state              lower power consumption and
             to ‘1’ state
                                                     lower heat generation.
                                                    In a switching from ‘0’ state to
                                                     ‘1’ state, the currents I0 and I1
                                                     are higher than in typical
                                                     CMOS logic gates, which
                                                     imply the switching velocity is
                                                     higher in NDR-based logic
                                                     gates.


        Mathews, R. H. et al. A new RTD-FET logic family. Proc. IEEE 87, 596-605 (1999)
REVIEW OF PREVIOUS WORKS:
CLONING CARBON NANOTUBES
   The original short SWNT
    seed was a polymer
    wrapped SWNT, end-
    carboxylated, and
    further tethered with Fe
    salts at its ends. The Fe
    salts act as the growth
    catalysts upon
    subsequent reductive
    activation.



      Smalley et al, JACS (2006)
    REVIEW OF PREVIOUS WORKS:
    CLONING CARBON NANOTUBES
   Deposition of the short SWNT-Fe
    tipped species upon an oxide
    surface was followed by heating
    in air to consume the polymer
    wrappers, then reducing the Fe
    salts to Fe(0) under a H2-rich
    atmosphere. During this heating,
    the Fe(0) can etch back into the
    short SWNT. Upon introduction
    of C2H4 as a carbon source the
    short SWNT acts as a template
    for new growth to a long SWNT.
    Analysis indicated that the
    templated VLS-grown long
    SWNT had the same diameter
    and surface orientation as the
    original short SWNT seed.

         Smalley et al, JACS (2006)
REVIEW OF PREVIOUS WORKS:
A BETTER WAY TO MAKE NANOTUBES
 A hoop-shaped chain
  of benzene molecules,
  the shortest segment
  of a carbon nanotube,
  is synthesized.
 This structure enable
  the growing of carbon
  nanotubes in a
  controlled way, with
  each nanotube of
  identical chirality to
  the next.

                           Jasti et al, JACS (2008)
REVIEW OF PREVIOUS WORKS:
SHRINKING A CARBON NANOTUBE
 When carbon
  nanotubes (CNT) are
  exposed to electron
  beam radiation it
  cause the CNT
  shrinking, reducing
  CNT diameter during
  the process.
 At the end of the
  shrinking process,
  carbon nanowires
  are produced.

                  Yuzvinsky et al, Nano Lett (2006)
REVIEW OF PREVIOUS WORKS:
NDR BEHAVIOR IN CARBON NANOWIRE
   CNT current-voltage
    curve is monitored
    during the shrinking
    process, observing
    NDR behavior when a
    carbon nanowire is
    formed.




                      Khoo at al, Nano Lett (2008)
REVIEW OF PREVIOUS WORKS:
JOINING SINGLE-WALLED CARBON NANOTUBES
 Stable junctions of
  various geometries
  are created in situ in a
  transmission electron
  microscope.
 Electron beam
  exposure at high
  temperatures induces
  structural defects
  which promote the
  joining of tubes via
  cross-linking of
  dangling bonds.

                         Terrones et al, PRL (2002)
REVIEW OF PREVIOUS WORKS:
NANOMACHINING CARBON NANOTUBES WITH ION BEAMS

   10 and 30 keV focused
    beams of Ga+ ions are used
    to thin, slice, weld, and alter
    the structure and
    composition of multiwalled
    carbon nanotubes at precise
    locations along the
    nanotube axis.
   Harnessing ion-beam-
    induced defect generation
    and doping could be
    attractive for modulating
    chemical and electrical
    properties along the
    nanotube length, and
    fabricate nanotube                       (a) Prior to irradiation, and (b) after exposure to
    heterostructures and                     1016 ions cm-2 of 10keV Ga+ ions; (d) An example
    networks for device                      of a nanotube network formed by several welds
    applications.                            indicated by arrows; (e) SEM micrograph
                                             showing a CNT welded to the edge of the SiN
                                             membrane (see arrows)
                                  Raghuveer et al, APL (2004)
REVIEW OF PREVIOUS WORKS: DERIVING
CARBON ATOMIC CHAINS FROM GRAPHENE

   Stable and rigid
    carbon atomic chains
    were experimentally
    realized by removing
    carbon atoms row by
    row from graphene
    through the controlled
    energetic electron
    irradiation inside a
    transmission electron
    microscope.


                       Jin et al, PRL (2009)
  REVIEW OF PREVIOUS WORKS: DERIVING
  CARBON ATOMIC CHAINS FROM GRAPHENE
  Consecutive HR-TEM images showing the
dynamics for the formation, breakage of
freestanding carbon atomic chains through
continuous electron beam irradiation. (a) A GNR
with a width of about 1.7 nm was formed
between two holes on the graphene. (b) The GNR
was thinned row-byrow under the continuous
irradiation. (c) A carbon chain consisting of
double strands was formed, and there was a knot
remained on the left chain (marked as the black
arrow). Inset is a representative scheme. The
right chain broke from its bottom end (marked
as white arrow) and detached with the graphene
edge. (d)–(f) The broken chain (on the right)
migrated along the left chain, and finally made a
connection with the edge belonging to the upper
graphene. (g) The carbon was found to be linear
and flexible. (h) The carbon chain made a jump
along the graphene edge with a changing of edge
bonding. The inset is a representative scheme. (i)
The carbon chain broken from its upper head.


                                        Jin et al, PRL (2009)
REVIEW OF PREVIOUS WORKS:
MULTITERMINAL MOLECULAR DEVICES
   DFT and
    nonequilibrium Keldysh
    theory is used to
    analyze electron
    transport through a
    four-terminal device.
   Quantum interference
    between the four
    terminals and the
    central molecule
    originate an NDR
    behavior which is not
    present in a two                      Au, S, C, and H atoms are shown in yellow,
    terminal configuration.               red, cyan, and gray, respectively.
                                          The semi-infinite leads are built out of
                                          Au(111) nanowires.
        Saha et al, Phys. Rev. B (2010)
REVIEW OF PREVIOUS WORKS:
THE NANOCELL
   The Nanocell concept is a
    programmable logic circuit
    based on molecules, showing
    NDR behavior, interconnected
    by metal nanoparticles.
   The structure of the molecular
    circuit is not known but its
    logic is programmable.
   NDR devices change their
    states (ON to OFF or viceversa)
    upon application of voltage
    pulses from the periphery of the
    Nanocell.
   The object in programming or
    training a nanocell is to take a
    random, fixed nanocell and
    turn its switches ON and OFF
    until it functions as a target
    logic device.



          Tour et al, IEEE (2002)      Seminario et al, IEEE (2006).
RESEARCH PROJECT PROPOSAL:
GENERAL DESCRIPTION
   Toward a Carbon-based
    Nanocell. Carbon
    nanowire show a NDR
    behavior. Carbon
    nanotubes serves as
    interconnects between
    nanowires.
   Carbon junctions are
    made possible by
    exposure to electron or
    ion beams.
   The molecular circuit is
    a suspended structure,
    i.e. not in contact with
    the substrate.


                               Figure adapted from Tour et al, IEEE (2002)
  RESEARCH PROJECT PROPOSAL:
  FABRICATION PROCESS
This loop need to be repeated until we find the adequate concentration level
of CNT suspension to get a monolayer of CNTs.
                                                 FE-SEM imaging
          SiO2
                                                          Chromium coating
         Silicon
                                                               SiO2
                                                             Silicon


    CNT in           Spin coating
ethanol solution
  ~ 0.2 ug/ml
                                                       CNT thin film
                                                           SiO2
                                                          Silicon
   RESEARCH PROJECT PROPOSAL
                                                   CNT monolayer
                               After PMMA
                               development               SiO2
                                                        Silicon
         CNT monolayer
             SiO2
                                                      SiO2
           Silicon
                                                     Silicon



PMMA patterned by e-beam lithography
                                       After exposure to Reactive Ion etching
            PMMA
             SiO2                                       PMMA
                                                         SiO2
            Silicon
                                                        Silicon


           SiO2                                       SiO2
          Silicon                                    Silicon
  RESEARCH PROJECT PROPOSAL
                                           Nanocell
           CNT monolayer
                SiO2
               Silicon



              SiO2
             Silicon
                                                      (Inside TEM)
                                                      Electron beam

Au electrodes patterned by e-beam lithography

                 SiO2                                    SiO2

                Silicon                                 Silicon



              SiO2                                     SiO2
             Silicon                                  Silicon
  RESEARCH PROJECT PROPOSAL
               SiO2
              Silicon



             SiO2
            Silicon

                                       After buffered HF to remove all the
                                       SiO2 which is not covered by PMMA,
PMMA patterned by e-beam lithography   including the SiO2 underneath the
              PMMA                     Nanocell.
               SiO2
              Silicon
                                                   SiO2


             SiO2                              Silicon
            Silicon
RESEARCH PROJECT PROPOSAL:
FINAL CONSIDERATIONS
 Carbon nanowires doesn’t own a strong NDR
  behavior but new configurations, like a 4
  terminal device, can be explored by using
  molecular simulation techniques in order to look
  for strong NDR behavior.
 The Nanocell is aimed to be designed suspended
  since It was found the contact to the substrate
  diminish the NDR behavior in CNT.
 Cutting CNTs to get smaller ones (< 10 nm) is
  not a simple procedure. Carbon Nanohoop
  Structures is a novel alternative for growing
  small carbon nanotubes in a more reliable and
  controlled way.
RESEARCH PROJECT PROPOSAL:
FINAL CONSIDERATIONS
 The task for programming the Nanocell should
  take into account the quantum interference
  phenomena it can occur between carbon
  nanowires. In the original ‘Nanocell’ concept this
  aspect is not considered.
 When ion beam is used for welding Carbon
  junctions, it should be taken into account that
  CNT interconnection would result doped with the
  atoms used as ion beam.

				
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