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
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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.
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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.
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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.
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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.
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http://www.cmosvlsi.com/lect3.pdf
BASIC CONCEPTS:
SILICON TECHNOLOGY PHYSICAL LIMITS
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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
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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
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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
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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.
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http://www.cnf.cornell.edu/image/spiefig1.jpg
BASIC CONCEPTS:
PROXIMITY EFFECT
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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 liftoff process, as shown
in the figure.
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http://www.aph.kit.edu/wegener/data/image/research/ebl.jpg
BASIC CONCEPTS:
TRANSMISSION ELECTRON MICROSCOPY (TEM)
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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)
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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
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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)
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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)
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From the equation, I0
and I1 indicate how fast
‘V’ changes from ‘0’ state
to ‘1’ state
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A NDR device is a 2-terminal
device, which shows negative
resistance values, i.e. negative
values of dI/dV, in the
current-voltage curve.
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)
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From the equation, I0
and I1 indicate how fast
‘V’ changes from ‘0’ state
to ‘1’ state
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When two NDR devices work
together, as shown in the
picture, the two logic states,
i.e. ‘0’ and ‘1’, occur at a low
voltage level which imply a
lower power consumption and
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
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The original short SWNT
seed was a polymer
wrapped SWNT, endcarboxylated, 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
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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.
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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.
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Yuzvinsky et al, Nano Lett (2006)
REVIEW OF PREVIOUS WORKS:
NDR BEHAVIOR IN CARBON NANOWIRE
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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.
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Terrones et al, PRL (2002)
REVIEW OF PREVIOUS WORKS:
NANOMACHINING CARBON NANOTUBES WITH ION BEAMS
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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-beaminduced defect generation
and doping could be
attractive for modulating
chemical and electrical
properties along the
nanotube length, and
fabricate nanotube
heterostructures and
networks for device
applications.
(a) Prior to irradiation, and (b) after exposure to
1016 ions cm-2 of 10keV Ga+ ions; (d) An example
of a nanotube network formed by several welds
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
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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
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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
terminal configuration.
Saha et al, Phys. Rev. B (2010)
Au, S, C, and H atoms are shown in yellow,
red, cyan, and gray, respectively.
The semi-infinite leads are built out of
Au(111) nanowires.
REVIEW OF PREVIOUS WORKS:
THE NANOCELL
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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
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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
ethanol solution
~ 0.2 ug/ml
Spin coating
CNT thin film
SiO2
Silicon
RESEARCH PROJECT PROPOSAL
After PMMA
development
CNT monolayer
SiO2
Silicon
CNT monolayer
SiO2
Silicon
PMMA patterned by e-beam lithography
PMMA
SiO2
Silicon
SiO2
Silicon
SiO2
Silicon
After exposure to Reactive Ion etching
PMMA
SiO2
Silicon
SiO2
Silicon
RESEARCH PROJECT PROPOSAL
CNT monolayer
Nanocell
SiO2
Silicon
SiO2
Silicon
(Inside TEM)
Electron beam
Au electrodes patterned by e-beam lithography
SiO2
Silicon
SiO2
Silicon
SiO2
Silicon
SiO2
Silicon
RESEARCH PROJECT PROPOSAL
SiO2
Silicon
SiO2
Silicon
PMMA patterned by e-beam lithography
PMMA
SiO2
After buffered HF to remove all the
SiO2 which is not covered by PMMA,
including the SiO2 underneath the
Nanocell.
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.
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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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