Carbon nanotubes (CNTs) are cylinders of one or more layers of graphene
(lattice). Diameters of single-walled carbon nanotubes (SWNTs) and
multi-walled carbon nanotubes (MWNTs) are typically 0.8 to 2 nm and 5 to
20 nm, respectively, although MWNT diameters can exceed 100 nm. CNT
lengths range from less than 100 nm to 0.5 m.
Individual CNT walls can be metallic or semiconducting depending on the orientation of the lattice with respect to the tube axis, which is called chirality. MWNT's cross-sectional area offers an elastic modulus approaching 1 TPa and a tensile strength of 100 GPa, over 10-fold higher than any industrial fiber. MWNTs are typically metallic and can carry currents of up to 109 A cm−2. SWNTs can display thermal conductivity of 3500 W m−1 K−1, exceeding that of diamond.
As of 2013, carbon nanotube production exceeded several thousand tons per year, used for applications in energy storage, device modelling, automotive parts, boat hulls, sporting goods, water filters, thin-film electronics, coatings, actuators and electromagnetic shields. CNT-related publications more than tripled in the prior decade, while rates of patent issuance also increased. Most output was of unorganized architecture. Organized CNT architectures such as "forests", yarns and regular sheets were produced in much smaller volumes. CNTs have even been proposed as the tether for a purported space elevator.
Individual CNT walls can be metallic or semiconducting depending on the orientation of the lattice with respect to the tube axis, which is called chirality. MWNT's cross-sectional area offers an elastic modulus approaching 1 TPa and a tensile strength of 100 GPa, over 10-fold higher than any industrial fiber. MWNTs are typically metallic and can carry currents of up to 109 A cm−2. SWNTs can display thermal conductivity of 3500 W m−1 K−1, exceeding that of diamond.
As of 2013, carbon nanotube production exceeded several thousand tons per year, used for applications in energy storage, device modelling, automotive parts, boat hulls, sporting goods, water filters, thin-film electronics, coatings, actuators and electromagnetic shields. CNT-related publications more than tripled in the prior decade, while rates of patent issuance also increased. Most output was of unorganized architecture. Organized CNT architectures such as "forests", yarns and regular sheets were produced in much smaller volumes. CNTs have even been proposed as the tether for a purported space elevator.
3D carbon nanotube scaffolds
Recently, several studies have highlighted the prospect of using
carbon nanotubes as building blocks to fabricate three-dimensional
macroscopic (>1mm in all three dimensions) all-carbon devices.
Lalwani et al. have reported a novel radical initiated thermal
crosslinking method to fabricated macroscopic, free-standing, porous,
all-carbon scaffolds using single- and multi-walled carbon nanotubes as
building blocks.
These scaffolds possess macro-, micro-, and nano- structured pores and
the porosity can be tailored for specific applications. These 3D
all-carbon scaffolds/architectures may be used for the fabrication of
the next generation of energy storage, supercapacitors, field emission
transistors, high-performance catalysis, photovoltaics, and biomedical
devices and implants.
Biological and biomedical research
Researchers
from Rice University and State University of New York – Stony Brook
have shown that the addition of low weight % of carbon nanotubes can
lead to significant improvements in the mechanical properties of
biodegradable polymeric nanocomposites for applications in tissue
engineering including bone, cartilage, muscle and nerve tissue.
Dispersion of low weight % of graphene (~0.02 wt.%) results in
significant increases in compressive and flexural mechanical properties
of polymeric nanocomposites. Researchers at Rice University, Stony Brook
University, Radboud University Nijmegen Medical Centre and University
of California, Riverside have shown that carbon nanotubes and their
polymer nanocomposites are suitable scaffold materials for bone tissue
engineering and bone formation.
CNTs exhibit dimensional and chemical compatibility with biomolecules, such as DNA and proteins. CNTs enable fluorescent and photoacoustic imaging, as well as localized heating using near-infrared radiation.
SWNT biosensors exhibit large changes in electrical impedance and
optical properties, which is typically modulated by adsorption of a
target on the CNT surface. Low detection limits and high selectivity
require engineering the CNT surface and field effects, capacitance,
Raman spectral shifts and photoluminescence for sensor design. Products
under development include printed test strips for estrogen and progesterone detection, microarrays for DNA and protein detection and sensors for NO
2 and cardiac troponin. Similar CNT sensors support food industry, military and environmental applications.
2 and cardiac troponin. Similar CNT sensors support food industry, military and environmental applications.
CNTs can be internalized by cells, first by binding their tips to cell membrane receptors. This enables transfection of molecular cargo attached to the CNT walls or encapsulated by CNTs. For example, the cancer drug doxorubicin was loaded at up to 60 wt % on CNTs compared with a maximum of 8 to 10 wt % on liposomes. Cargo release can be triggered by near-infrared radiation. However, limiting the retention of CNTs within the body is critical to prevent undesirable accumulation.
CNT toxicity remains a concern, although CNT biocompatibility may
be engineerable. The degree of lung inflammation caused by injection of
well-dispersed SWNTs was insignificant compared with asbestos
and with particulate matter in air. Medical acceptance of CNTs requires
understanding of immune response and appropriate exposure standards for
inhalation, injection, ingestion and skin contact. CNT forests
immobilized in a polymer did not show elevated inflammatory response in
rats relative to controls. CNTs are under consideration as low-impedance
neural interface electrodes and for coating of catheters to reduce thrombosis.
CNT enabled x-ray sources for medical imaging are also in
development. Relying on the unique properties of the CNTs, researchers
have developed field emission cathodes that allow precise x-ray control
and close placement of multiple sources. CNT enabled x-ray sources have
been demonstrated for pre-clinical, small animal imaging applications,
and are currently in clinical trials.
In November 2012 researchers at the American National Institute of Standards and Technology (NIST) proved that single-wall carbon nanotubes may help protect DNA molecules from damage by oxidation.
A highly effective method of delivering carbon nanotubes into cells is Cell squeezing, a high-throughput vector-free microfluidic platform for intracellular delivery developed at the Massachusetts Institute of Technology in the labs of Robert S. Langer.
Carbon nanotubes have furthermore been grown inside microfluidic
channels for chemical analysis, based on electrochromatography. Here,
the high surface-area-to-volume ratio and high hydrophobicity of CNTs
are used in order to greatly decrease the analysis time of small neutral
molecules that typically require large bulky equipment for analysis.
Composite materials
Because
of the carbon nanotube's superior mechanical properties, many
structures have been proposed ranging from everyday items like clothes
and sports gear to combat jackets and space elevators.
However, the space elevator will require further efforts in refining
carbon nanotube technology, as the practical tensile strength of carbon
nanotubes must be greatly improved.
For perspective, outstanding breakthroughs have already been
made. Pioneering work led by Ray H. Baughman at the NanoTech Institute
has shown that single and multi-walled nanotubes can produce materials
with toughness unmatched in the man-made and natural worlds.
Carbon nanotubes being spun to form a yarn, CSIRO
Carbon nanotubes are also a promising material as building blocks in
hierarchical composite materials given their exceptional mechanical
properties (~1 TPa in modulus, and ~100 GPa in strength). Initial
attempts to incorporate CNTs into hierarchical structures (such as
yarns, fibres or films)
has led to mechanical properties that were significantly lower than
these potential limits. The hierarchical integration of multi-walled
carbon nanotubes and metal/metal oxides within a single nanostructure
can leverage the potentiality of carbon nanotubes composite for water
splitting and electrocatalysis. Windle et al. have used an in situ chemical vapor deposition (CVD) spinning method to produce continuous CNT yarns from CVD-grown CNT aerogels.
CNT yarns can also be manufactured by drawing out CNT bundles from a
CNT forest and subsequently twisting to form the fibre (draw-twist
method, see picture on right). The Windle group have fabricated CNT
yarns with strengths as high as ~9 GPa at small gage lengths of ~1 mm,
however, strengths of only about ~1 GPa were reported at the longer gage
length of 20 mm.
The reason why fibre strengths have been low compared to the strength
of individual CNTs is due to a failure to effectively transfer load to
the constituent (discontinuous) CNTs within the fibre. One potential
route for alleviating this problem is via irradiation (or deposition)
induced covalent inter-bundle and inter-CNT cross-linking to effectively
'join up' the CNTs, with higher dosage levels leading to the
possibility of amorphous carbon/carbon nanotube composite fibres. Espinosa et al.
developed high performance DWNT-polymer composite yarns by twisting and
stretching ribbons of randomly oriented bundles of DWNTs thinly coated
with polymeric organic compounds. These DWNT-polymer yarns exhibited an
unusually high energy to failure of ~100 J·g−1 (comparable to one of the toughest natural materials – spider silk), and strength as high as ~1.4 GPa. Effort is ongoing to produce CNT composites that incorporate tougher matrix materials, such as Kevlar, to further improve on the mechanical properties toward those of individual CNTs.
Because of the high mechanical strength of carbon nanotubes,
research is being made into weaving them into clothes to create
stab-proof and bulletproof clothing. The nanotubes would effectively
stop the bullet from penetrating the body, although the bullet's kinetic
energy would likely cause broken bones and internal bleeding.
Mixtures
MWNTs
were first used as electrically conductive fillers in metals, at
concentrations as high as 83.78 percent by weight (wt%). MWNT-polymer
composites reach conductivities as high as 10,000 S m−1 at 10
wt % loading. In the automotive industry, CNT plastics are used in
electrostatic-assisted painting of mirror housings, as well as fuel
lines and filters that dissipate electrostatic charge. Other products include electromagnetic interference (EMI)–shielding packages and silicon wafer carriers.
For load-bearing applications, CNT powders are mixed with
polymers or precursor resins to increase stiffness, strength and
toughness. These enhancements depend on CNT diameter, aspect ratio,
alignment, dispersion and interfacial interaction. Premixed resins and
master batches employ CNT loadings from 0.1 to 20 wt%. Nanoscale
stick-slip among CNTs and CNT-polymer contacts can increase material
damping, enhancing sporting goods, including tennis racquets, baseball
bats and bicycle frames.
CNT resins enhance fiber composites, including wind turbine
blades and hulls for maritime security boats that are made by enhancing carbon fiber
composites with CNT-enhanced resin. CNTs are deployed as additives in
the organic precursors of stronger 1-μm diameter carbon fibers. CNTs
influence the arrangement of carbon in pyrolyzed fiber.
Toward the challenge of organizing CNTs at larger scales,
hierarchical fiber composites are created by growing aligned forests
onto glass, silicon carbide (SiC), alumina and carbon fibers, creating so-called "fuzzy" fibers. Fuzzy epoxy
CNT-SiC and CNT-alumina fabric showed 69% improved crack-opening (mode
I) and/or in-plane shear interlaminar (mode II) toughness. Applications
under investigation include lightning-strike protection, deicing, and
structural health monitoring for aircraft.
MWNTs can be used as a flame-retardant additive to plastics due to changes in rheology by nanotube loading. Such additives can replace halogenated flame retardants, which face environmental restrictions.
CNT/Concrete blends offer increased tensile strength and reduced crack propagation.
Buckypaper (nanotube aggregate) can significantly improve fire resistance due to efficient heat reflection.
Textiles
The
previous studies on the use of CNTs for textile functionalization were
focused on fiber spinning for improving physical and mechanical
properties.
Recently a great deal of attention has been focused on coating CNTs on
textile fabrics. Various methods have been employed for modifying
fabrics using CNTs. produced intelligent e-textiles for Human
Biomonitoring using a polyelectrolyte-based coating with CNTs.
Additionally, Panhuis et al. dyed textile material by immersion in
either a poly (2-methoxy aniline-5-sulfonic acid) PMAS polymer solution
or PMAS-SWNT dispersion with enhanced conductivity and capacitance with a
durable behavior.
In another study, Hu and coworkers coated single-walled carbon
nanotubes with a simple “dipping and drying” process for wearable
electronics and energy storage applications.
In the recent study, Li and coworkers using elastomeric separator and
almost achieved a fully stretchable supercapacitor based on buckled
single-walled carbon nanotube macrofilms. The electrospun polyurethane
was used and provided sound mechanical stretchability and the whole cell
achieve excellent charge-discharge cycling stability.
CNTs have an aligned nanotube structure and a negative surface charge.
Therefore, they have similar structures to direct dyes, so the
exhaustion method is applied for coating and absorbing CNTs on the fiber
surface for preparing multifunctional fabric including antibacterial,
electric conductive, flame retardant and electromagnetic absorbance
properties.
Later, CNT yarns and laminated sheets made by direct chemical vapor deposition
(CVD) or forest spinning or drawing methods may compete with carbon
fiber for high-end uses, especially in weight-sensitive applications
requiring combined electrical and mechanical functionality. Research
yarns made from few-walled CNTs have reached a stiffness of 357 GPa and a
strength of 8.8 GPa for a gauge length comparable to the
millimeter-long CNTs within the yarn. Centimeter-scale gauge lengths
offer only 2-GPa gravimetric strengths, matching that of Kevlar.
Because the probability of a critical flaw increases with volume,
yarns may never achieve the strength of individual CNTs. However, CNT's
high surface area may provide interfacial coupling that mitigates these
deficiencies. CNT yarns can be knotted without loss of strength.
Coating forest-drawn CNT sheets with functional powder before inserting
twist yields weavable, braidable and sewable yarns containing up to 95
wt % powder. Uses include superconducting wires, battery and fuel cell
electrodes and self-cleaning textiles.
As yet impractical fibers of aligned SWNTs can be made by
coagulation-based spinning of CNT suspensions. Cheaper SWNTs or spun
MWNTs are necessary for commercialization. Carbon nanotubes can be dissolved in superacids such as fluorosulfuric acid and drawn into fibers in dry jet-wet spinning.
DWNT-polymer composite yarns have been made by twisting and
stretching ribbons of randomly oriented bundles of DWNTs thinly coated
with polymeric organic compounds.
Body armor—combat jackets Cambridge University developed the fibres and licensed a company to make them. In comparison, the bullet-resistant fiber Kevlar fails at 27–33 J/g.
Synthetic muscles offer high contraction/extension ratio given an electric current.
SWNT are in use as an experimental material for removable, structural bridge panels.
In 2015 researchers incorporated CNTs and graphene into spider silk, increasing its strength and toughness to a new record. They sprayed 15 Pholcidae spiders with water containing the nanotubes or flakes. The resulting silk had a fracture strength up to 5.4 GPa, a Young’s modulus up to 47.8 GPa and a toughness modulus up to 2.1 GPa, surpassing both synthetic polymeric high performance fibres (e.g. Kevlar49) and knotted fibers.
Carbon nanotube springs
"Forests" of stretched, aligned MWNT springs can achieve an energy density
10 times greater than that of steel springs, offering cycling
durability, temperature insensitivity, no spontaneous discharge and
arbitrary discharge rate. SWNT forests are expected to be able to store
far more than MWNTs.
Alloys
Adding
small amounts of CNTs to metals increases tensile strength and modulus
with potential in aerospace and automotive structures. Commercial
aluminum-MWNT composites have strengths comparable to stainless steel (0.7 to 1 GPa) at one-third the density (2.6 g cm−3), comparable to more expensive aluminium-lithium alloys.
Coatings and films
CNTs can serve as a multifunctional coating material. For example, paint/MWNT mixtures can reduce biofouling of ship hulls by discouraging attachment of algae and barnacles. They are a possible alternative to environmentally hazardous biocide-containing paints.
Mixing CNTs into anticorrosion coatings for metals can enhance coating
stiffness and strength and provide a path for cathodic protection.
CNTs provide a less expensive alternative to ITO for a range of
consumer devices. Besides cost, CNT's flexible, transparent conductors
offer an advantage over brittle ITO coatings for flexible displays. CNT
conductors can be deposited from solution and patterned by methods such
as screen printing. SWNT films offer 90% transparency and a sheet
resistivity of 100 ohm per square. Such films are under development for
thin-film heaters, such as for defrosting windows or sidewalks.
Carbon nanotubes forests and foams can also be coated with a
variety of different materials to change their functionality and
performance. Examples include silicon coated CNTs to create flexible
energy-dense batteries, graphene coatings to create highly elastic aerogels and silicon carbide coatings to create a strong structural material for robust high-aspect-ratio 3D-micro architectures.
There is a wide range of methods how CNTs can be formed into coatings and films.
Optical power detectors
A
spray-on mixture of carbon nanotubes and ceramic demonstrates
unprecedented ability to resist damage while absorbing laser light. Such
coatings that absorb as the energy of high-powered lasers without
breaking down are essential for optical power detectors that measure the
output of such lasers. These are used, for example, in military
equipment for defusing unexploded mines. The composite consists of
multiwall carbon nanotubes and a ceramic made of silicon, carbon and
nitrogen. Including boron boosts the breakdown temperature. The
nanotubes and graphene-like carbon transmit heat well, while the
oxidation-resistant ceramic boosts damage resistance. Creating the
coating involves dispersing the nanotubes in toluene,
to which a clear liquid polymer containing boron was added. The mixture
was heated to 1,100 °C (2,010 °F). The result is crushed into a fine
powder, dispersed again in toluene and sprayed in a thin coat on a
copper surface. The coating absorbed 97.5 percent of the light from a
far-infrared laser and tolerated 15 kilowatts per square centimeter for
10 seconds. Damage tolerance is about 50 percent higher than for similar
coatings, e.g., nanotubes alone and carbon paint.
Radar absorption
Radars
work in the microwave frequency range, which can be absorbed by MWNTs.
Applying the MWNTs to the aircraft would cause the radar to be absorbed
and therefore seem to have a smaller radar cross-section. One such application could be to paint the nanotubes onto the plane. Recently there has been some work done at the University of Michigan regarding carbon nanotubes usefulness as stealth technology
on aircraft. It has been found that in addition to the radar absorbing
properties, the nanotubes neither reflect nor scatter visible light,
making it essentially invisible at night, much like painting current stealth aircraft
black except much more effective. Current limitations in manufacturing,
however, mean that current production of nanotube-coated aircraft is
not possible. One theory to overcome these current limitations is to
cover small particles with the nanotubes and suspend the
nanotube-covered particles in a medium such as paint, which can then be
applied to a surface, like a stealth aircraft.
In 2010, Lockheed Martin Corporation
applied for a patent for just such a CNT based radar absorbent
material, which was reassigned and granted to Applied NanoStructure
Solutions, LLC in 2012. Some believe that this material is incorporated in the F-35 Lightning II.
Microelectronics
Nanotube-based transistors, also known as carbon nanotube field-effect transistors (CNTFETs), have been made that operate at room temperature and that are capable of digital switching using a single electron.
However, one major obstacle to realization of nanotubes has been the
lack of technology for mass production. In 2001 IBM researchers
demonstrated how metallic nanotubes can be destroyed, leaving
semiconducting ones behind for use as transistors. Their process is
called "constructive destruction," which includes the automatic
destruction of defective nanotubes on the wafer. This process, however, only gives control over the electrical properties on a statistical scale.
SWNTs are attractive for transistors because of their low electron
scattering and their bandgap. SWNTs are compatible with field-effect
transistor (FET) architectures and high-k dielectrics. Despite progress
following the CNT transistor's appearance in 1998, including a tunneling
FET with a subthreshold swing of less than 60 mV per decade (2004), a radio
(2007) and an FET with sub-10-nm channel length and a normalized current
density of 2.41 mA μm−1 at 0.5 V, greater than those obtained for silicon devices.
However, control of diameter, chirality, density and placement
remains insufficient for commercial production. Less demanding devices
of tens to thousands of SWNTs are more immediately practical. The use of
CNT arrays/transistor increases output current and compensates for
defects and chirality differences, improving device uniformity and
reproducibility. For example, transistors using horizontally aligned CNT
arrays achieved mobilities of 80 cm2 V−1 s−1, subthreshold slopes of 140 mV per decade and on/off ratios as high as 105. CNT film deposition methods enable conventional semiconductor fabrication of more than 10,000 CNT devices per chip.
Printed CNT thin-film transistors (TFTs) are attractive for driving organic light-emitting diode displays, showing higher mobility than amorphous silicon (~1 cm2 V−1 s−1) and can be deposited by low-temperature, nonvacuum methods. Flexible CNT TFTs with a mobility of 35 cm2 V−1 s−1 and an on/off ratio of 6×106
were demonstrated. A vertical CNT FET showed sufficient current output
to drive OLEDs at low voltage, enabling red-green-blue emission through a
transparent CNT network. CNTs are under consideration for radio-frequency identification tags. Selective retention of semiconducting SWNTs during spin-coating and reduced sensitivity to adsorbates were demonstrated.
The International Technology Roadmap for Semiconductors suggests that CNTs could replace Cu
in microelectronic interconnects, owing to their low scattering, high
current-carrying capacity, and resistance to electromigration. For this,
vias comprising tightly packed (greater than 1013 cm−2)
metallic CNTs with low defect density and low contact resistance are
needed. Recently, complementary metal oxide semiconductor
(CMOS)–compatible 150-nm-diameter interconnects with a single
CNT–contact hole resistance of 2.8 kOhm were demonstrated on full
200-mm-diameter wafers. Also, as a replacement for solder bumps, CNTs
can function both as electrical leads and heat dissipaters for use in
high-power amplifiers.
Last, a concept for a nonvolatile memory based on individual CNT
crossbar electromechanical switches has been adapted for
commercialization by patterning tangled CNT thin films as the functional
elements. This required development of ultrapure CNT suspensions that
can be spin-coated and processed in industrial clean room environments
and are therefore compatible with CMOS processing standards.
Transistors
Carbon nanotube field-effect transistors (CNTFETs) can operate at room temperature and are capable of digital switching using a single electron. In 2013, a CNT logic circuit was demonstrated that could perform useful work. Major obstacles to nanotube-based microelectronics include the absence of technology for mass production, circuit density, positioning of individual electrical contacts, sample purity, control over length, chirality and desired alignment, thermal budget and contact resistance.
One of the main challenges was regulating conductivity. Depending on subtle surface features, a nanotube may act as a conductor or as a semiconductor.
Another way to make carbon nanotube transistors has been to use random networks of them. By doing so one averages all of their electrical differences and one can produce devices in large scale at the wafer level. This approach was first patented by Nanomix Inc. (date of original application June 2002). It was first published in the academic literature by the United States Naval Research Laboratory
in 2003 through independent research work. This approach also enabled
Nanomix to make the first transistor on a flexible and transparent
substrate.
Since the electron mean free path in SWCNTs can exceed 1 micrometer, long channel CNTFETs exhibit near-ballistic transport
characteristics, resulting in high speeds. CNT devices are projected to
operate in the frequency range of hundreds of gigahertz.
Nanotubes can be grown on nanoparticles of magnetic metal (Fe, Co) that facilitates production of electronic (spintronic)
devices. In particular control of current through a field-effect
transistor by magnetic field has been demonstrated in such a single-tube
nanostructure.
History
In 2001
IBM researchers demonstrated how metallic nanotubes can be destroyed,
leaving semiconducting nanotubes for use as components. Using
"constructive destruction", they destroyed defective nanotubes on the wafer. This process, however, only gives control over the electrical properties on a statistical scale. In 2003 room-temperature ballistic transistors with ohmic metal contacts and high-k gate dielectric were reported, showing 20–30x more current than state-of-the-art siliconMOSFETs. Palladium is a high-work function metal that was shown to exhibit Schottky barrier-free contacts to semiconducting nanotubes with diameters >1.7 nm.
The potential of carbon nanotubes was demonstrated in 2003 when
room-temperature ballistic transistors with ohmic metal contacts and high-k gate dielectric were reported, showing 20–30x higher ON current than state-of-the-art Si MOSFETs.
This presented an important advance in the field as CNT was shown to
potentially outperform Si. At the time, a major challenge was ohmic
metal contact formation. In this regard, palladium, which is a high-work function metal was shown to exhibit Schottky barrier-free contacts to semiconducting nanotubes with diameters >1.7 nm.
The first nanotube integrated memory circuit was made in 2004.
One of the main challenges has been regulating the conductivity of
nanotubes. Depending on subtle surface features a nanotube may act as a
plain conductor or as a semiconductor. A fully automated method has however been developed to remove non-semiconductor tubes.
In 2013, researchers demonstrated a Turing-complete prototype micrometer-scale computer. Carbon nanotube transistors as logic-gate circuits with densities comparable to modern CMOS technology has not yet been demonstrated.
In 2014 networks of purified semiconducting carbon nanotubes were used as the active material in p-type thin film transistors. They were created using 3-D printers using inkjet or gravure methods on flexible substrates, including polyimide and polyethylene (PET) and transparent substrates such as glass. These transistors reliably exhibit high-mobilities (greater than 10 cm2 V−1 s−1)
and ON/OFF ratios (greater than 1000) as well as threshold voltages below 5 V.
They offer current density and low power consumption as well as
environmental stability and mechanical flexibility. Hysterisis in the current-voltage curses as well as variability in the threshold voltage remain to be solved.
In 2015 researchers announced a new way to connect wires to SWNTs
that make it possible to continue shrinking the width of the wires
without increasing electrical resistance. The advance was expected to
shrink the contact point between the two materials to just 40 atoms in
width and later less. They tubes align in regularly spaced rows on
silicon wafers. Simulations indicated that designs could be optimized
either for high performance or for low power consumption. Commercial
devices were not expected until the 2020s.
Thermal management
Large
structures of carbon nanotubes can be used for thermal management of
electronic circuits. An approximately 1 mm–thick carbon nanotube layer
was used as a special material to fabricate coolers, this material has
very low density, ~20 times lower weight than a similar copper
structure, while the cooling properties are similar for the two
materials.
Buckypaper has characteristics appropriate for use as a heat sink for chipboards, a backlight for LCD screens or as a faraday cage.
Solar cells
One of the promising applications of single-walled carbon nanotubes
(SWNTs) is their use in solar panels, due to their strong UV/Vis-NIR
absorption characteristics. Research has shown that they can provide a
sizable increase in efficiency, even at their current unoptimized state.
Solar cells developed at the New Jersey Institute of Technology use a carbon nanotube complex, formed by a mixture of carbon nanotubes and carbon buckyballs (known as fullerenes) to form snake-like structures. Buckyballs trap electrons, but they can't make electrons flow.
Add sunlight to excite the polymers, and the buckyballs will grab the
electrons. Nanotubes, behaving like copper wires, will then be able to
make the electrons or current flow.
Additional research has been conducted on creating SWNT hybrid
solar panels to increase the efficiency further. These hybrids are
created by combining SWNT's with photo-excitable electron donors to
increase the number of electrons generated. It has been found that the
interaction between the photo-excited porphyrin
and SWNT generates electro-hole pairs at the SWNT surfaces. This
phenomenon has been observed experimentally, and contributes practically
to an increase in efficiency up to 8.5%.
Nanotubes can potentially replace indium tin oxide
in solar cells as a transparent conductive film in solar cells to allow
light to pass to the active layers and generate photocurrent.
CNTs in organic solar cells help reduce energy loss (carrier
recombination) and enhance resistance to photooxidation. Photovoltaic
technologies may someday incorporate CNT-Silicon heterojunctions to
leverage efficient multiple-exciton generation at p-n junctions formed
within individual CNTs. In the nearer term, commercial photovoltaics may
incorporate transparent SWNT electrodes.
Hydrogen storage
In
addition to being able to store electrical energy, there has been some
research in using carbon nanotubes to store hydrogen to be used as a
fuel source. By taking advantage of the capillary effects of the small
carbon nanotubes, it is possible to condense gases in high density
inside single-walled nanotubes. This allows for gases, most notably
hydrogen (H2), to be stored at high densities without being
condensed into a liquid. Potentially, this storage method could be used
on vehicles in place of gas fuel tanks for a hydrogen-powered car. A
current issue regarding hydrogen-powered vehicles is the on-board
storage of the fuel. Current storage methods involve cooling and
condensing the H2 gas to a liquid state for storage which
causes a loss of potential energy (25–45%) when compared to the energy
associated with the gaseous state. Storage using SWNTs would allow one
to keep the H2 in its gaseous state, thereby increasing the storage
efficiency. This method allows for a volume to energy ratio slightly
smaller to that of current gas powered vehicles, allowing for a slightly
lower but comparable range.
An area of controversy and frequent experimentation regarding the
storage of hydrogen by adsorption in carbon nanotubes is the efficiency
by which this process occurs. The effectiveness of hydrogen storage is
integral to its use as a primary fuel source since hydrogen only
contains about one fourth the energy per unit volume as gasoline.
Studies however show that what is the most important is the surface area
of the materials used. Hence activated carbon with surface area of 2600
m2/g can store up to 5,8% w/w. In all these carbonaceous materials,
hydrogen is stored by physisorption at 70-90K.
Experimental capacity
One experiment sought to determine the amount of hydrogen stored in CNTs by utilizing elastic recoil detection analysis (ERDA). CNTs (primarily SWNTs) were synthesized via chemical vapor disposition
(CVD) and subjected to a two-stage purification process including air
oxidation and acid treatment, then formed into flat, uniform discs and
exposed to pure, pressurized hydrogen at various temperatures. When the
data was analyzed, it was found that the ability of CNTs to store
hydrogen decreased as temperature increased. Moreover, the highest
hydrogen concentration measured was ~0.18%; significantly lower than
commercially viable hydrogen storage needs to be. A separate
experimental work performed by using a gravimetric method also revealed
the maximum hydrogen uptake capacity of CNTs to be as low as 0.2%.
In another experiment, CNTs were synthesized via CVD and their structure was characterized using Raman spectroscopy. Utilizing microwave digestion,
the samples were exposed to different acid concentrations and different
temperatures for various amounts of time in an attempt to find the
optimum purification method for SWNTs of the diameter determined
earlier. The purified samples were then exposed to hydrogen gas at
various high pressures, and their adsorption
by weight percent was plotted. The data showed that hydrogen adsorption
levels of up to 3.7% are possible with a very pure sample and under the
proper conditions. It is thought that microwave digestion helps improve
the hydrogen adsorption capacity of the CNTs by opening up the ends,
allowing access to the inner cavities of the nanotubes.
Limitations on efficient hydrogen adsorption
The
biggest obstacle to efficient hydrogen storage using CNTs is the purity
of the nanotubes. To achieve maximum hydrogen adsorption, there must be
minimum graphene,
amorphous carbon, and metallic deposits in the nanotube sample. Current
methods of CNT synthesis require a purification step. However, even
with pure nanotubes, the adsorption capacity is only maximized under
high pressures, which are undesirable in commercial fuel tanks.
Electronic components
Various companies are developing transparent, electrically conductive CNT films and nanobuds to replace indium tin oxide
(ITO) in LCDs, touch screens and photovoltaic devices. Nanotube films
show promise for use in displays for computers, cell phones, Personal digital assistants, and automated teller machines. CNT diodes display a photovoltaic effect.
Multi-walled nanotubes (MWNT coated with magnetite) can generate strong magnetic fields. Recent advances show that MWNT decorated with maghemite nanoparticles can be oriented in a magnetic field and enhance the electrical properties of the composite material in the direction of the field for use in electric motor brushes.
A layer of 29% iron enriched single-walled nanotubes (SWNT) placed on top of a layer of explosive material such as PETN can be ignited with a regular camera flash.
CNTs can be used as electron guns in miniature cathode ray tubes (CRT) in high-brightness, low-energy, low-weight displays. A display would consist of a group of tiny CRTs, each providing the electrons to illuminate the phosphor of one pixel, instead of having one CRT whose electrons are aimed using electric and magnetic fields. These displays are known as field emission displays (FEDs).
CNTs can act as antennas for radios and other electromagnetic devices.
Conductive CNTs are used in brushes for commercial electric motors. They replace traditional carbon black.
The nanotubes improve electrical and thermal conductivity because they
stretch through the plastic matrix of the brush. This permits the carbon
filler to be reduced from 30% down to 3.6%, so that more matrix is
present in the brush. Nanotube composite motor brushes are
better-lubricated (from the matrix), cooler-running (both from better
lubrication and superior thermal conductivity), less brittle (more
matrix, and fiber reinforcement), stronger and more accurately moldable
(more matrix). Since brushes are a critical failure point in electric
motors, and also don't need much material, they became economical before
almost any other application.
Wires for carrying electric current may be fabricated from
nanotubes and nanotube-polymer composites. Small wires have been
fabricated with specific conductivity exceeding copper and aluminum; the highest conductivity non-metallic cables.
CNT are under investigation as an alternative to tungsten filaments in incandescent light bulbs.
Interconnects
Metallic carbon nanotubes have aroused research interest for their applicability
as very-large-scale integration (VLSI) interconnects because of their high thermal stability, high thermal conductivity and large current carrying capacity.
An isolated CNT can carry current
densities in excess of 1000 MA/sq-cm without damage even at an elevated
temperature of 250 °C (482 °F), eliminating electromigration reliability
concerns that plague Cu interconnects. Recent modeling work comparing the two has shown that CNT bundle interconnects can potentially offer advantages over copper. Recent experiments demonstrated resistances as low as 20 Ohms using different architectures,
detailed conductance measurements over a wide temperature range were
shown to agree with theory for a strongly disordered
quasi-one-dimensional conductor.
Hybrid interconnects that employ CNT vias in tandem with copper
interconnects may offer advantages from a reliability/thermal-management
perspective.
In 2016, the European Union has funded a four million euro project over
three years to evaluate manufacturability and performance of composite
interconnects employing both CNT and copper interconnects. The project
named CONNECT (CarbON Nanotube compositE InterconneCTs)
involves the joint efforts of seven European research and industry
partners on fabrication techniques and processes to enable reliable
Carbon NanoTubes for on-chip interconnects in ULSI microchip production.
Electrical cables and wires
Wires
for carrying electric current may be fabricated from pure nanotubes and
nanotube-polymer composites. It has already been demonstrated that
carbon nanotube wires can successfully be used for power or data
transmission. Recently small wires have been fabricated with specific conductivity exceeding copper and aluminum;
these cables are the highest conductivity carbon nanotube and also
highest conductivity non-metal cables. Recently, composite of carbon
nanotube and copper have been shown to exhibit nearly one hundred times
higher current-carrying-capacity than pure copper or gold. Significantly, the electrical conductivity of such a composite is
similar to pure Cu. Thus, this Carbon nanotube-copper (CNT-Cu) composite
possesses the highest observed current-carrying capacity among
electrical conductors. Thus for a given cross-section of electrical
conductor, the CNT-Cu composite can withstand and transport one hundred
times higher current compared to metals such as copper and gold.
Energy storage
The use of CNTs as a catalyst support in fuel cells can potentially reduce platinum usage by 60% compared with carbon black. Doped CNTs may enable the complete elimination of Pt.
Supercapacitor
MIT Research Laboratory of Electronics uses nanotubes to improve supercapacitors.
The activated charcoal used in conventional ultracapacitors has many
small hollow spaces of various size, which create together a large
surface to store electric charge. But as charge is quantized into
elementary charges, i.e. electrons, and each such elementary charge
needs a minimum space, a significant fraction of the electrode surface
is not available for storage because the hollow spaces are not
compatible with the charge's requirements. With a nanotube electrode the
spaces may be tailored to size—few too large or too small—and
consequently the capacity should be increased considerably.
A 40-F supercapacitor with a maximum voltage of 3.5 V that
employed forest-grown SWNTs that are binder- and additive-free achieved
an energy density of 15.6 Wh kg−1 and a power density of 37 kW kg−1. CNTs can be bound to the charge plates of capacitors to dramatically increase the surface area and therefore energy density.
Batteries
Carbon
nanotubes' (CNTs) exciting electronic properties have shown promise in
the field of batteries, where typically they are being experimented as a
new electrode material, particularly the anode for lithium ion batteries. This is due to the fact that the anode requires a relatively high reversible capacity
at a potential close to metallic lithium, and a moderate irreversible
capacity, observed thus far only by graphite-based composites, such as
CNTs. They have shown to greatly improve capacity and cyclability of lithium-ion batteries,
as well as the capability to be very effective buffering components,
alleviating the degradation of the batteries that is typically due to
repeated charging and discharging. Further, electronic transport in the
anode can be greatly improved using highly metallic CNTs.
More specifically, CNTs have shown reversible capacities from 300 to 600 mAhg−1, with some treatments to them showing these numbers rise to up to 1000 mAhg−1. Meanwhile, graphite, which is most widely used as an anode material for these lithium batteries, has shown capacities of only 320 mAhg−1.
By creating composites out of the CNTs, scientists see much potential
in taking advantage of these exceptional capacities, as well as their
excellent mechanical strength, conductivities, and low densities.
MWNTs are used in lithium ion batteries cathodes.
In these batteries, small amounts of MWNT powder are blended with
active materials and a polymer binder, such as 1 wt % CNT loading in LiCoO
2 cathodes and graphite anodes. CNTs provide increased electrical connectivity and mechanical integrity, which enhances rate capability and cycle life.
2 cathodes and graphite anodes. CNTs provide increased electrical connectivity and mechanical integrity, which enhances rate capability and cycle life.
Paper batteries
A paper battery is a battery engineered to use a paper-thin sheet of cellulose (which is the major constituent of regular paper, among other things) infused with aligned carbon nanotubes. The potential for these devices is great, as they may be manufactured via a roll-to-roll process,
which would make it very low-cost, and they would be lightweight,
flexible, and thin. In order to productively use paper electronics (or
any thin electronic devices), the power source must be equally thin,
thus indicating the need for paper batteries. Recently, it has been
shown that surfaces coated with CNTs can be used to replace heavy metals
in batteries.
More recently, functional paper batteries have been demonstrated, where
a lithium-ion battery is integrated on a single sheet of paper through a
lamination process as a composite with Li4Ti5O12 (LTO) or LiCoO2 (LCO).
The paper substrate would function well as the separator for the
battery, where the CNT films function as the current collectors for both
the anode and the cathode. These rechargeable energy devices show potential in RFID tags, functional packaging, or new disposable electronic applications.
Improvements have also been shown in lead-acid batteries, based
on research performed by Bar-Ilan University using high quality SWCNT
manufactured by OCSiAl.
The study demonstrated an increase in the lifetime of lead acid
batteries by 4.5 times and a capacity increase of 30% on average and up
to 200% at high discharge rates.
Chemical
CNT can be used for desalination.
Water molecules can be separated from salt by forcing them through
electrochemically robust nanotube networks with controlled nanoscale
porosity. This process requires far lower pressures than conventional reverse osmosis methods. Compared to a plain membrane, it operates at a 20 °C lower temperature, and at a 6x greater flow rate.
Membranes using aligned, encapsulated CNTs with open ends permit flow
through the CNTs' interiors. Very-small-diameter SWNTs are needed to
reject salt at seawater concentrations. Portable filters containing CNT
meshes can purify contaminated drinking water. Such networks can
electrochemically oxidize organic contaminants, bacteria and viruses.
CNT membranes can filter carbon dioxide from power plant emissions.
CNT can be filled with biological molecules, aiding biotechnology.
CNT have the potential to store between 4.2 and 65% hydrogen
by weight. If they can be mass-produced economically, 13.2 litres
(2.9 imp gal; 3.5 US gal) of CNT could contain the same amount of energy
as a 50 litres (11 imp gal; 13 US gal) gasoline tank.
CNTs can be used to produce nanowires of other elements/molecules, such as gold or zinc oxide. Nanowires in turn can be used to cast nanotubes of other materials, such as gallium nitride. These can have very different properties from CNTs—for example, gallium nitride nanotubes are hydrophilic, while CNTs are hydrophobic, giving them possible uses in organic chemistry.
Mechanical
Oscillators based on CNT have achieved speeds of > 50 GHz.
CNT electrical and mechanical properties suggest them as alternatives to traditional electrical actuators.
Actuators
The
exceptional electrical and mechanical properties of carbon nanotubes
have made them alternatives to the traditional electrical actuators for
both microscopic and macroscopic applications. Carbon nanotubes are very
good conductors of both electricity and heat, and they are also very
strong and elastic molecules in certain directions.
Loudspeaker
Carbon
nanotubes have also been applied in the acoustics (such as loudspeaker
and earphone). In 2008 it was shown that a sheet of nanotubes can
operate as a loudspeaker if an alternating current is applied. The sound
is not produced through vibration but thermoacoustically.
In 2013, a carbon nanotube (CNT) thin yarn thermoacoustic earphone
together with CNT thin yarn thermoacoustic chip was demonstrated by a
research group of Tsinghua-Foxconn Nanotechnology Research Center in
Tsinghua University, using a Si-based semi-conducting technology compatible fabrication process.
Near-term commercial uses include replacing piezoelectric speakers in greeting cards.
Optical
Carbon nanotube photoluminescence (fluorescence) can be used to
observe semiconducting single-walled carbon nanotube species.
Photoluminescence maps, made by acquiring the emission and scanning the
excitation energy, can facilitate sample characterization.
Nanotube fluorescence is under investigation for biomedical imaging and sensors.
The reflectivity of buckypaper produced with "super-growth" chemical vapor deposition is 0.03 or less, potentially enabling performance gains for pyroelectricinfrared detectors.
Nanotube fluorescence is under investigation for biomedical imaging and sensors.
The reflectivity of buckypaper produced with "super-growth" chemical vapor deposition is 0.03 or less, potentially enabling performance gains for pyroelectricinfrared detectors.
Environmental
Environmental remediation
A
CNT nano-structured sponge (nanosponge) containing sulfur and iron is
more effective at soaking up water contaminants such as oil,
fertilizers, pesticides and pharmaceuticals. Their magnetic properties
make them easier to retrieve once the clean-up job is done. The sulfur
and iron increases sponge size to around 2 centimetres (0.79 in). It
also increases porosity due to beneficial defects, creating buoyancy and
reusability. Iron, in the form of ferrocene
makes the structure easier to control and enables recovery using
magnets. Such nanosponges increase the absorption of the toxic organic solvent dichlorobenzene from water by 3.5 times. The sponges can absorb vegetable oil up to 150 times their initial weight and can absorb engine oil as well.
Earlier, a magnetic boron-doped MWNT nanosponge that could absorb
oil from water. The sponge was grown as a forest on a substrate via
chemical vapor disposition. Boron puts kinks and elbows into the tubes
as they grow and promotes the formation of covalent bonds. The nanosponges retain their elastic property after 10,000 compressions in the lab. The sponges are both superhydrophobic, forcing them to remain at the water's surface and oleophilic, drawing oil to them.
Water treatment
It
has been shown that carbon nanotubes exhibit strong adsorption
affinities to a wide range of aromatic and aliphatic contaminants in
water,
due to their large and hydrophobic surface areas. They also showed
similar adsorption capacities as activated carbons in the presence of
natural organic matter.
As a result, they have been suggested as promising adsorbents for
removal of contaminant in water and wastewater treatment systems.
Moreover, membranes made out of carbon nanotube arrays have been
suggested as switchable molecular sieves, with sieving and permeation
features that can be dynamically activated/deactivated by either pore
size distribution (passive control) or external electrostatic fields
(active control).
Other applications
Carbon nanotubes have been implemented in nanoelectromechanical systems, including mechanical memory elements (NRAM being developed by Nantero Inc.) and nanoscale electric motors.
Carboxyl-modified single-walled carbon nanotubes (so called
zig-zag, armchair type) can act as sensors of atoms and ions of alkali
metals Na, Li, K.
In May 2005, Nanomix Inc. placed on the market a hydrogen sensor that
integrated carbon nanotubes on a silicon platform. Since then, Nanomix
has been patenting many such sensor applications, such as in the field
of carbon dioxide, nitrous oxide, glucose, DNA detection, etc. End of
2014, Tulane University researchers have tested Nanomix's fast and fully
automated point of care diagnostic system in Sierra Leone to help for
rapid testing for Ebola. Nanomix announced that a product could be launched within three to six months.
Eikos Inc of Franklin, Massachusetts and Unidym Inc. of Silicon Valley, California are developing transparent, electrically conductive films of carbon nanotubes to replace indium tin oxide (ITO). Carbon nanotube films are substantially more mechanically robust than ITO films, making them ideal for high-reliability touchscreens
and flexible displays. Printable water-based inks of carbon nanotubes
are desired to enable the production of these films to replace ITO. Nanotube films show promise for use in displays for computers, cell phones, PDAs, and ATMs.
A nanoradio, a radio receiver consisting of a single nanotube, was demonstrated in 2007.
The use in tensile stress or toxic gas sensors was proposed by Tsagarakis.
A flywheel
made of carbon nanotubes could be spun at extremely high velocity on a
floating magnetic axis in a vacuum, and potentially store energy at a density
approaching that of conventional fossil fuels. Since energy can be
added to and removed from flywheels very efficiently in the form of
electricity, this might offer a way of storing electricity,
making the electrical grid more efficient and variable power suppliers
(like wind turbines) more useful in meeting energy needs. The
practicality of this depends heavily upon the cost of making massive,
unbroken nanotube structures, and their failure rate under stress.
Carbon nanotube springs
have the potential to indefinitely store elastic potential energy at
ten times the density of lithium-ion batteries with flexible charge and
discharge rates and extremely high cycling durability.
Ultra-short SWNTs (US-tubes) have been used as nanoscaled capsules for delivering MRI contrast agents in vivo.
Carbon nanotubes provide a certain potential for metal-free catalysis
of inorganic and organic reactions. For instance, oxygen groups
attached to the surface of carbon nanotubes have the potential to
catalyze oxidative dehydrogenations or selective oxidations. Nitrogen-doped carbon nanotubes may replace platinum catalysts used to reduce oxygen in fuel cells.
A forest of vertically aligned nanotubes can reduce oxygen in alkaline
solution more effectively than platinum, which has been used in such
applications since the 1960s. Here, the nanotubes have the added benefit
of not being subject to carbon monoxide poisoning.
Wake Forest University engineers are using multiwalled carbon nanotubes to enhance the brightness of field-induced polymer electroluminescent
technology, potentially offering a step forward in the search for safe,
pleasing, high-efficiency lighting. In this technology, moldable
polymer matrix emits light when exposed to an electric current. It could
eventually yield high-efficiency lights without the mercury vapor of compact fluorescent lamps or the bluish tint of some fluorescents and LEDs, which has been linked with circadian rhythm disruption.
Candida albicans
has been used in combination with carbon nanotubes (CNT) to produce
stable electrically conductive bio-nano-composite tissue materials that
have been used as temperature sensing elements.
The SWNT production company OCSiAl
developed a series of masterbatches for industrial use of single-wall
CNTs in multiple types of rubber blends and tires, with initial trials
showing increases in hardness, viscosity, tensile strain resistance and
resistance to abrasion while reducing elongation and compression
In tires the three primary characteristics of durability, fuel
efficiency and traction were improved using SWNTs. The development of
rubber masterbatches built on earlier work by the Japanese National
Institute of Advanced Industrial Science & Technology showing rubber
to be a viable candidate for improvement with SWNTs.
Introducing MWNTs to polymers can improve flame retardancy and retard thermal degradation of polymer.
The results confirmed that combination of MWNTs and ammonium
polyphosphates show a synergistic effect for improving flame retardancy.
