Nanomaterials

The importance of carbon


In the periodic table, carbon is located in column IV of period 2 and has an atomic number of 6.   The critical location of carbon indicates that a single atom contains six protons and six electrons, four of which are valence electrons in the second energy level.   The complete electron configuration of carbon is given by,

  1s2 2s2 2p2  

The atomic configuration gives carbon three characteristics crucial to nanotechnology:

•  Carbon bonds well with an assortment of different atoms through covalent bonding.  
  A property called electronegativity allows carbon to share electrons with other atoms (covalent bonding) instead of giving up electrons (ionic bonding).   As a general rule, if the differenece in electronegativity between two atoms is no more than 2, the atom can make covalent bonds.   Carbon's electronegativity (2.55) is in the middle between the highest (fluorine at 3.98) and lowest (francium at 0.7) electronegativity values of the periodic table.   Atoms with higher electronegativity values tend to form ionic bonds with atoms of the lowest electronegativity (i.e group VII elements with group I elements). Carbon is less than 2 Pauling units from francium and fluorine, so carbon nearly always forms covalent bonds.
  
  
•  Carbon can bond with up to four other atoms.  
  Carbon contains four electrons in its outermost energy level (the valence).   To achieve stability, carbon needs a total of eight electrons in its valence.   The four empty energy states enable carbon to bond with four other atoms.   The four other atoms do not have to be the same.   Bonding with different atoms creates larger chains of molecules with unique properties.
  
  
• Carbon atoms can bond with other carbon atoms stronger and in more arrangements than other elements.  
  Shorter molecular chains containing multiple carbon atoms may cause a gas such as propane ( C3H8 ) to form.   Longer, larger chains of carbon and other atoms result in various solids from softer plastics to 2D and 3D lattices such as graphite and carbon nanotubes.

Basic structures
Two basic carbon-based molecules provide a valuable insight into the properties of more complex molecules important to nanotechnology.  

Benzene  C6H6  
A single carbon atom uses one electron for each of the two covelent bonds to adjacent carbon atoms.   A third electron is used to bond with a hydrogen atom, leaving the fourth valence electron unbonded.   The total of six electrons are termed delocalized and can travel freely between the other atoms in the molecule.   Each carbon atom's p-orbital containing the extra electron overlaps with the adjacent atom's p-orbital to create a ring-like orbital in which electrons can travel.   The animation is an exaggeration of the free movement of the moving electrons but demonstrates how carbon, a nonmetal, can still conduct current through delocalized electrons.  

Graphite  
Much like benzene, graphite posses delocalized electrons and has the ability to conduct electricity.   Each carbon atom in graphite bonds to three adjacent carbon atoms (except on the edges where the carbon bonds to hydrogen as in benzene) using three of its four valence electrons.   The fourth electron is then delocalized and available to move for electrical conduction.   Graphite is composed of 2-dimensional layers of carbon rings.   Even though each individual layer is only about 1 nm thick, the sheets of carbon have tremendous strength.   When the layers are stacked, van der Waals' force is the only attraction holding the sheets together.   As a result, each sheet can be sheered off without much effort.  

Two of the most widely researched nanostructures, buckminsterfullerenes and carbon nanotubes, can be thought of as a single sheet of graphite rolled into spheres and different types of tubes.

•  Nanotubes
  
  A carbon nanotube is a lattice of covalently bonded carbon atoms formed into the shape of a cylinder.   Each carbon atom is bonded with three other atoms, allowing the extra electron to become delocalized for conductions.   Two other structures aid in the visualization of the carbon nanotube: graphite and the buckyball.   Some nanotubes have open ends while others have closed ends.   An open-ended nanotube is much like a single 2D sheet of graphite (1 carbon atom thick) which is rolled up in different directions.   Based on the hexagonal geometry formed from the carbon bonding, different orientations of nanotubes with different properties form.   A closed nanotube is in essence an elongated buckyball.   Cutting the buckyball into halves and inserting the cylindrical carbon lattice will result in a nanotube with closed ends.  
  
  Most of the current nanotube research is focused on single-walled nanotubes (SWNT), which have walls only one atom thick and a total diameter of about 1 nm.   However, multi-walled nanotubes (MWNT) have been discovered.   A MWNT has multiple single-walled nanotubes inside of one another, much like the concentric annual rings of a tree.  
  
  Nanotubes can have three different orientations when they are first formed: armchair, zigzag and chiral.   An actual naming convention is used to describe the types of nanotubes with pairs of indices.   The orientation of the hexagonal carbon bonding defines the nanotubes electrical properties.   Armchair nanotubes have hexagonal carbon rings repeating parallel to the center axis of the nanotube.   Zigzag nanotubes, on the other hand, have the carbon geometry repeating perpendicular to the center axis.   The carbon rings in chiral nanotubes repeat at an angle to the center axis in a spiral fashion around the cylinder of the nanotube.  
  
  All armchair nanotubes have the electrical properties of a metal, but only a third of zigzag and chiral orientations exhibit metallic properties.   Instead, most of the zigzag and chiral nanotubes have semiconductor properties.   In metallic nanotubes, the collection of delocalized electrons can be viewed of as a "sea of electrons" or a electron cloud surrounding the length of the carbon nanotube.   Due to the small size of the nanotubes, electron transportation is different than normal movement in conventional metal materials such as copper.   Electrons travel along the length of the nanotube in a type of transportation called ballistic transport, which is governed by quantum mechanics.   Thus, the electrons have almost no impedance to movement.   The electrons in semiconductor nanotubes do not readily conduct electricity.   Without any external stimuli, semiconductor nanotubes act as good insulators.   With the addition of energy to some delocalized electrons, thought, electricity can be conducted. More details about electrical properties as well as heat, hardness, and other properties are given below.
  
  
•  C60 Buckminsterfullerene
  
  The fullerene family of molecules consists of structures which are composed entirely of carbon.   The term fullerene comes from American architect, Buckminster Fuller, who popularized the geodesic dome.   The spherical fullerenes exhibit geometry similar to Fuller's geodesic dome.   The most abundant, naturally occuring form from the fullerene family is the C60 buckyball.   Unlike a sheet of graphite or a shaft of a carbon nanotube, the 60 carbon atoms of the buckyball are not all arranged in hexagonal rings.   For a sphere of carbon atoms to form, some of the carbon atoms must form pentagon rings.   More specifically, research has found that the C60 buckyball is a stable structure when the carbon atoms from 12 pentagons and 20 hexagons.   The buckyball comes in other sizes containing more carbon atoms, such as the C70 and C80 buckyballs as well as the large C540 buckyball.   The C60 buckyball is the smallest of the fullerenes, measuring only about 1 nm in diameter.  
  
  
•  Quantum Dots
  
  Quantum dots are crystalline semiconductor nanostructures which contain unique semiconductor properties due to there small size.   The unique properties depend on the semiconductor material as well as the size of the quantum dot.   For example, quantum dots can used as tunable light source by varying the size of the dot.   Typical semiconductor crystals are made from elements in periodic table group combinations such III-V and IV-VI.   Other materials, which are more frequently used for quantum dots, originate from the group combination IIB-VI.   Examples are CdTe, CdSe, and ZnS.   Quantum dots originate from bulk semiconductors (material with dimensions much larger than 10 nm) and posses properties of the bulk semiconductors.   However, some aspects of the bulk semiconductor change when the size of the crystalline becomes small enough to be a quantum dot.   To grasp the usefullness of the quantum dot, some semiconductor basics should be examined.  
  
  
  • Semiconductor Concepts
    Consider for a moment a single hydrogen atom.   From the research of Planck and Bohr, the single electron of hydrogen was found to have discrete quantized energy level.   Furthermore, Shcrödinger's wave equation was instrumental in determining where an electron can or cannot be found.   The result of the wave equation shows that the probability of finding hydrogen's electron in its lowest energy state is radially distributed with respect to the nucleus.   The wave equation does not just apply to hydrogen.   The equation can also be used to show the behaviour of semiconductor elements in the form crystal lattices.   When two atoms are brought into close proximity to one another as they are in a crystal, the energy levels of the atoms interact.   Hydrogen is not a semiconductor element and does not form crystals, but it can be used to show the interactive behaviour.   If two hydrogen atoms are close together, the two electrons' wave functions (i.e. the probability distribution function) overlap.   The Pauli exclusion principle sill must hold true in the case of electron interaction.   If no more energy states are available within a given energy level, then by the Pauli exclusion principle, the energy level overlap is not allowed.   Interaction between energy levels causes the energy levels to split into discrete quantized energy levels.   Instead of just two atoms interacting, consider the result of many atoms periodically interacting in a crystal.   As the atoms are brought closer together, the energy levels still split into band of permittable discrete energy levels.   However, if the material is a bulk semiconductor (containing 1018 electrons, for example), then by the Pauli exclusion principle, all of the electrons must maintain a unique energy (quantum) state.   If the band of allowed energy levels spans 2 eV, for example, and the discrete energy levels are equally spaced, then the energy difference between each of the quantized energy levels is a mere 5•10-17 eV.   Therefore, the energy bands of a bulk semiconductor are considered to be continuous even though in actuality they are discrete.   When energy levels of an atom with more than one electron interact with other energy levels of adjacent atoms, bands of both allowable and forbidden energies form.   Consider silicon's outermost energy level (n = 3) which has a similar electron configuration as carbon with just a higher energy level.   As the 3s and 3p energy levels overlap with decreasing interatomic distance, the band of allowed energies split into upper and lower bands containing an equal number of available energy states.   The lower band is called the valence band, while the upper band is termed the conduction band.   The region between the two bands is known as the energy bandgap or the forbidden energy band.   The width of the forbidden energy band is unique to each material.   The energy band theory is a result of the Kronig-Penney model, which utilized Shcrödinger's wave equation and quantum mechanical analysis. The Kronig-Penney model is extremely useful because it describes where Shcrödinger's wave equation is valid or where solutions do not exist.   Thus, the model shows which range of energies an electron can possiblty occupy.   Electrons in the valence band will always have lower energies than electrons present in the conduction band.   At absolute zero, no electrons exist in the conduction and all the energy states in the valence band are completely full.   In order for an electron to cross the forbidden energy gap, energy must stimulate the electron to break free from its covelent bond.   The energy can come in various forms such as heat, ultraviolet rays, a static electric field, and X-rays.   However, the electron reaches the conduction band, the excited electron will always leave behind a hole (considered to be positively charged) in the valence band.   The corresponding electron-hole pair is known as an exciton. A certain distance called the exciton Bohr radius exists between the electron in the conduction band and the hole in the valence band.   The electron posses an attractive force to the oppositely charged hole similar to that of the single electron in hydrogen to the proton in the nucleus.   The distant present between the two charges is dependent on the type of material.   Unlike the hydrogen model in which the electron is already at its lowest energy level, the electron of the exciton does not stay in the conduction band indefinitely.   Eventually, the electron will naturally recombine with the hole in the valence band to be achieve its initial energy state.   Energy must be released in order for the exciton electron to cross the bandgap and recombine with the hole in the valence band.   Many times the energy is released in the form of visible electromagnetic radiation.   When an electron recombines with a hole, electrons usually congregate towards the bottom of the conduction band and holes towards the top of the valence band.   The amount of energy released then just depends on the width of the forbidden energy band.   Therefore, the wavelength of the radiation emitted from the recombination is unique to each material due to different widths of the forbidden energy gap, which is constant providing doping densities and temperature do not change.  
    
    
  The core difference between the bulk semiconductor and the quantum dot lies in the size of the prospective material in relation to the exciton Bohr radius.   In the bulk semiconductor, the size of the material is many orders of magnitude larger than the exciton Bohr radius.   Thus, the exciton may travel throughout the semiconductor at will while maintaining its average Bohr radius.   When the size of the material is decreased to the same order of magnitude of the exciton Bohr radius or even smaller, three things happen:
  
  
  •  Energy levels expand:  Consider the forces between two oppositely charge magnets.   When the two magnets are far apart, there is a negligible attraction.   However, bring the two close together, and the force becomes too large to keep them from attaching.   The same sort of thing happens when the size of the material becomes smaller than the exciton Bohr radius.   The normal energy between an electron-hole pair in the quantum dot must be greater than that of the exciton at its natural Bohr radius in a bulk material.
    
    
  •  Energy levels become discrete:  In the bulk semiconductor, energy levels were continuous due to the enormous amount of electrons (energy states) covering a finite range of available energies.   With a much smaller amount of electrons spanning a larger range of energies, the energy levels are not continuous because each available energy level is too far apart.   The formation of discrete levels is known as quantum confinement because the exciton no longer possesses the same freedom to travel as it did in the bulk.
    
    
  •  The width of the forbidden energy band increases:  As the size decreases, the width of the bandgap increases, allowing any electron-hole recombinations to produce higher energy radiation with a shorter wavelength.   In terms of color, the radiation of the quantum dot is more blue in color.   Compared to emitted radiation of bulk semiconductors, the radiation from the quantum dot is considered to be blue-shifted.
  
  
  The effects of the all-important exciton Bohr radius on the behaviour of nanomaterials such as quantum dots are profound.   The wavelength of light emitted by a quantum dot can be tuned through changing either the size or the geometry of the quantum dot.   Changing the locations of a few atoms or adding and subtracting some atoms to make the quantum dot larger or smaller could change the hue of light given off by the quantum dot.   As stated earlier, larger quantum dots emit lower energy radiation in the red spectrum, and smaller quantum dots emit higher energy radiation in the blue spectrum.   Potential applications of quantum dots in the medical field may prove to be invaluable.   Examples of such applications are illuminating cancer cells for more effective surgical removal or illuminating different organs for high contrast medical imaging.
  
  

Novel Properties of Nanostructures

Nanotubes

• Electrical Properties
  
  • Metallic
    Because of the size of nanotubes, the electrical properties differ from bulk materials just as the quantum dot exhibits some unique properties that larger scale materials cannot posses.   Consider the size of a single-walled carbon nanotube.   The lengthwise dimension of the nanotube is much longer (as much as 4 orders of magnitude) than the dimension perpendicular to the nanotube axis.   Therefore, the carbon nanotube is considered as one of the most ideal 1-dimensional structures yet discovered.   Almost all other 1-dimensional materials are unstable for conduction at the nanoscale.   The instability is known as Peierls instability and results in a widened energy bandgap due to the changes in the atoms' movements at the nanoscale.   In a metal, the energy bandgap is negligible or nonexistant, so a material with Peierls instability must be considered an insulator.   Carbon nanotubes do not exhibit this instability because the carbon-carbon bonds form the strong lattice.   One area where the difference between bulk materials and the nanotube can be seen is that of resistivity.   In metallic circuit elements, resistance is observed in part due to an electron's mean free path in relation to the dimensions of the metal.   For a bulk material, the mean free path is much smaller than the dimensions of the element.   The electron almost always collides with another electron before reaching the physical boundaries of the metal.   Resistance, of course, increases with temperature because the velocity and kinetic energy of the electrons increase, making collisions more frequent.   In nanotubes, however, an electron's mean free path is larger than the sidewall boundaries, so an electron's movement is not impeded by collisions with other electrons.   Instead an electron has the ability to travel uninterrupted down the lengthwise axis of the nanotube.   This type of electron transport is known as ballistic transport and is governed by the rule of quantum mechanics.   Ballistic transport does not imply that metallic carbon nanotubes exhibit no resistance.   Resistivity depends on different factors such as temperature, the number of defects in the nanotube, and bending angles of the nanotube.   Instead the benefit of ballistic transport due to quantum mechanics is that an electron will pass from one end of the nanotube to the other with certainty, regardless of the length.   Conductivity of a metallic carbon nanotube does not change if the length of the nanotube is increased.   Measurements of the conductance of single-walled and multi-walled nanotubes have been observed.   Particularly intriguing is an experiment with a MWNT performed by Stefan Frank at Georgia Tech University's nanotube lab.   Frank contacted one end of the MWNT to a scanning probe microscope (SPM) while the other end was placed in mercury.   Mercury conducts electricity, so by running a current through the nanotube to the mercury, conductance can be measured.   As the nanotube was gradually lowered into the mercury, the outer nanotube wall was the first which made electrical contact.   Continuing to lower the nanotube allowed each inner nanotube wall to make contact with the mercury.   The result of the experiment showed that the MWNT's conductance had quantized values which depend on the number of walls conducting current.   An equation to represent the conductance is G = GoM = (2e2/h)M, where Go is (12.9kΩ)-1, e = -1.6×10-19, h is Planck's constant, and M is an integer.   Experimentally, M may not be an integer as nanotubes can exhibit impurities and defects as well distortions and other factors due to the nanotube-contact interface which may alter the overall electrical properties of the nanotube.   If the nanotube was pure and undistorted, M would theoretically be an integer dependant on the number of nanotube walls performing the electrical conductance.   Frank's experiment shows the quantum effects of a carbon nanotube as a quantum wire.  
    
    A metallic nanotube can carry a current as much as 25 µA.   If a single nanotube is 1.3 nm in diameter, then there are nearly 7.7 million nanotubes in a centimeter.   The corresponding current density for a cross-sectional area of 1 cm2 would be approximately 1.5×109 A/cm2, a very high current density by anyone's standards.
    
    
  • Semiconducting
    Semiconducting nanotubes (zigzag and chiral nanotubes particularly) are especially important for the future development of nanoscale electronics.   Specifically, much research is focused on the use of carbon nanotubes in MOSFETs, the integral component in today's electronics.   Details on MOSFETs and nanoelectronics are given on the following page.   Like metallic nanotubes, conductance in semiconducting nanotubes can depend on several things such as the diameter of the nanotube, chemical doping, and temperature.   The principal difference between the semiconducting and metallic nanotubes is the presence of the energy bandgap.   The width of the bandgap (in eV) is inversely proportional to the diameter of the tube, namely Eg ≈ 0.84/d, where d is the diameter of the nanotube in nm.   The energy bandgap is an approximate since there are other depending factors.   Generally, the bandgap of a 1 nm semiconducting nanotube is between 0.7 eV and 0.9 eV.   Two important concepts in semiconductor theory are important to understand the conduction of electricity in a semiconducting nanotube: the Fermi energy level and the electron and hole density of states.   These two concepts dictate the number of electrons and holes present in the conduction and valence band, respectively, and the concentration of charge carriers determines the ability to conduct.   The Fermi energy is defined as the energy at which the probability of an electron occupying an energy state in the conduction band is 0.5.   The Fermi energy level is a result of the Fermi-dirac distribution.   Even though the Fermi function is written for electrons, it can also be used to find the probability of holes in the valence band.   Since the hole is the direct opposite of the electron, it follows that the probability of hole occupying an energy state in the valence is opposite that of the electron in the conduction band.   Therefore, the probability of a hole in the valence band is 1 - F(E).   The Fermi energy level is nearly always located somewhere in the energy bandgap.   Factors such as temperature and doping (impurity) levels can cause the Fermi energy to creep closer to the bottom of the conduction band (n-type semiconductor) or the bottom of the valence band (p-type semiconductor).   If the Fermi energy happens to be located inside the conduction band or valence band, then the semiconductor is considered to be degenerative, and the material no longer functions like a normal semiconductor.   Going hand in hand with the Fermi function is the concept of density of states.   The density of states indicates how closely energy levels are to each other in a semiconductor.   A high density of states reveals that a material is capable of having more charge carriers in its conduction and valence bands.   As discussed in the quantum dot section, the available energy states are so close together in bulk semiconductors that the conductance and valence bands have continuous energy level.   At the nanoscale, though, energy levels become more spread out.   In the quantum dot, those energy levels are quantized.   Observations of nanotube density of states with Raman spectroscopy and a scanning tunneling miscroscope (STM) shows that energy levels are not quantized like the quantum dot, though they are close to be quantized.   Peaks or kinks in the density of states function known as van Hove singularities occur in nanotubes.   The equation for the density of states of electrons and holes gives the number of energy levels per unit volume per unit energy.   By multiplying the density of states by the probability that an electron occupies an energy state (i.e. the Fermi function), the number of charge carriers can be found.   The transport of the charge carriers (drift and diffusion in bulk materials and ballistic in nanomaterials) is the current through the material.   Since the carbon nanotube can be considered as a 1-dimensional structure, a 1D space model must be used to derive the density of states function.  
    
    INSERT 1D DOS model and compare theoretical/observed results
    
    
  
  
  
• Electromechanical Properties
  Ideally, a semiconducting nanotube in devices such as MOSFETs or a metallic nanotube in a quantum wire would be straight to ensure that the inherent properties of the nanotubes will remain unchanged.   When mechanical strain is placed on bulk semiconductors such as silicon and germanium, the band structure is altered from changes in the atomic lattice (i.e. the distances between atoms).   Slight changes in bending as well as twisting angles, for example, can lengthen or shorten bond lengths between carbon atoms.   In turn, the energy bandgap or even the density of states may change along the nanotube's axis.   Phaedon Avouris and his research team at IBM have shown variation in electrical properties due to modifications to the nanotubes.   A metallic nanotube will induce a bandgap when twisted, indicating that a metallic nanotube can act like a semiconducting nanotube.   Changes in the density of states can also occur along the length of the nanotube when bent at various angles.   The density of states at different locations on the nanotube is called the local density of states.   Modifying nanotubes can be a disadvantage when their expected electrical properties are desired for nanoelectronic devices.   However, slight changes in band structure and the density of states can be exploited for the development of sensors as changes in band structure affect the conductivity or resistivity.   The phenomenon of a material changing its resistivity with mechanical strain is known as the piezoresistive effect.   In contrast, the piezoelectric effect occurs when a voltage is induced in the material when it subjected to stress.   The piezoelectric effect has not yet been observed in nanotubes, but the piezoresistive effect has been documented.   Now that research has found that electrical properties of nanotubes also change with stress, there is great potential for developing tiny, durable, and lightweight sensors.  
  
  
• Tensile Strength
  Comparing the mechanical properties of nanotubes to other more common materials, the nanotubes display excellent potential.   Tensile strength is the force required to break a sample of material.   The force which is measured by tensile strength is applied parallel to the nanotube axis.   If the nanotube were a rope and a force gauge was placed on one end while pulling the other end until the rope broke, the tensile strength can be found.   In bundles of SWNTs, the nanotubes are only held together by van der Waals' force, so shearing forces are present.   The high tensile strength of nanotubes is attributed to the sp2 carbon bonds, one of the strongest bonds in nature.   Carbon bonds to itself stronger than any other element in the periodic table can bond to itself.   The nanotube also doesn't have the seams in the lattice which steel exhibits in its grain boundaries.   The fact that the nanotube is one large molecule is an advantage over other materials.  
  
  
• Elasticity
  Young's modulus is the constant which is used to define the elasticity or stiffness of material.   Not only can the nanotube withstand high axial force as measured by tensile strength, but the nanotube returns to its original position after large forces are applied.   Young's modulus is dependant on the diameter of the nanotube.   Smaller diameter SWNTs have higher elasticity.   In MWNTs, the innermost nanotubes contribute to the majority of Young's modulus, since they have the smallest diameter.   The outer SWNTs only add to the elasticity of the entire MWNT.   The size dependence is also present in bundles of SWNT.   Experimental observations show that elasticity increases 100 GPa to 1 TPa (1000 GPa) when the diameter of the SWNT bundle decreases from 20 nm to 3 nm.  
  
  
• Thermal Conductivity
  Thermal conductivity, denoted by κ, is a measure of how well a material conducts heat.   The highest known thermal conductivity was a pure diamond measured at about 41000 W/m·K.   Theoretical calculations of thermal conductivity have shown that the carbon nanotube can have a range of values from 2800 W/m·K to more than 37000 W/m·K.   Much like electrical conductivity, the thermal conductivity changes with temperature.   As temperature rises from absolute zero, thermal conductivity exhibits a linear behaviour, which is characteristic of ballistic transport governed by quantum mechanics.   At a certain temperature, though, the thermal conductivity saturates and then starts decreasing again.   Thermal measurements of isolated SWNTs have not yet been accomplished.   However, experiments have allowed some to observe how well a MWNT can conduct heat.   A value of over 3000 W/m·K has been measured at 320 K (more than 115° F) in a MWNT with a diameter of 14 nm.   A tangled bundle of SWNTs has been measured at about 25 W/m·K, while an aligned bundle of SWNTs yielded 200 W/m·K.   The result indicates that thermal transport, like electrical transport, is effected by orientation or bundling of nanotubes.   In bulk metal materials, heat is conducted mainly through the transport of electrons.   In nanotubes, though, heat is transported through the vibrations of the atoms around their position in the lattice.   On the nanoscale, the vibrations of atoms in a lattice are called phonons and are quantum mechanical in nature.   The thermal properties of nanomaterials are playing an increasing role in research since future applications, especially in electronic circuit cooling, are on the horizon.  
  
  

Nanostructure-polymer Composites
A composite is composed of two or more materials.   A few examples of composites are plywood, plastics, carbon fiber, and epoxy.   A composite can be thought of as a fiber and a matrix.   The fiber is the backbone of the material, while the matrix is the glue which surrounds the fiber to hold the intertwined parts together.   Various companies and research groups are focusing on utilizing the mechanical and electrical properties of carbon nanotubes as fibers in a composite.   By using carbon nanotubes in a composite, one can develop a multifunctional material which can conduct electricity as well as provide incredible strength and resistance to breakdown.   The goal of using nanotubes may be to safely prevent electrostatic discharge damage to electrical circuits by making conductive polymers or making an even stronger and more durable carbon fiber and ceramics.   Ideally, the backbone or fibers of the composite should be as evenly dispersed in the matrix as much as possible to provide consistent properties across the entire material and to maximize the binding between each fiber and the matrix.   However, due to Van der Waals' force, nanotubes have the tendency to stick together into a thick bundle.   The tangling or bundling of nanotubes presents a problem when forming composites.   In order to be dispersed at all, the nanotubes must be soluble in the matrix, which is usually a polymer of some kind.   If they are not soluble, the matrix would not bind to the fibers, and the two would just pull apart.   Researchers have found that wrapping other polymers around the nanotube will increase solubility.   One way of obtaining good dispersion is to functionalize nanotubes with certain molecules called poly(aryleneethynylene)s or PPEs.   The PPE-SWNT composite, first introduced by a company called Zyvex, wraps half way around the nanotube and allows the nanotubes to be soluble in another polymer.   The advantage of using a PPE polymer over other materials is that the nanotube becomes noncovalently functionalized, allowing the nanotube to retain its desirable properties (in particular, its electrical properties).   Much of composite research involves the use of SWNTs rather than MWNTs or bundles of nanotubes.   MWNTs generally have more defects than SWNTs, and the bundles of SWNTs have poor mechanical properties due to shearing or slipping of nanotubes inside the bundle.   A benefit of dispersed SWNTs in composites is a lower percolation threshold.   MWNTs and bundles of SWNTs negatively affect the percolation threshold.   Because of their larger diameters, MWNTs and bundles of nanotubes have lower aspect ratios (i.e. length to diameter), and lower aspect ratios cause higher percolation thresholds.   Higher percolation thresholds cause the cost of the material to increase, and there is a risk of negatively effecting the physical properties of the hosting matrix.   A collaboration of researchers at Zyvex and Michigan Tech University have experimented with the PPE-SWNTs in both polystyrene and polycarbonate polymer matrices.   By themselves, the polystyrene and polycarbonate have conduction values of approximately 10-14 S/m and 10-13 S/m, respectively.   However, the addition of the PPE-SWNT reinforcement to the composite resulted in conductions of 6.89 S/m for polystyrene and 4.81×102 for the polycarbonate (both values were measured with a 7 wt % SWNT loading) — an increase of about 1015 S/m in both cases.   The percolation thresholds for both the polystyrene and polycarbonate are incredibly low at 0.045 wt % and 0.11 wt %;   The small amount of SWNTs needed to achieve electrical conduction through the composite indicates excellent 3-dimesnional dispersement of the nanotubes.   Images of the PPE-SWNT/polystyrene composite show the 3-dimesional dispersement of the noncovalently functionalized nanotubes.  

Product Examples of Nanomaterials

The Difficulties:  Efficiency in Making and Sorting Nano Building Blocks
There are few things impeding the production of nano-based products on the market.   One obvious thing is the research.   The novel properties of nanomaterials are still being tested, and though much progress has been made regarding actual applications, consumer products are still in the future.   However, as of now, the cost of nanomaterials is high because of the price of production.   A highly effeciently way of producing nanotubes, for example, would greatly impact both research and the speed at which applications will hit the market.  

Many processes have been used to develop or synthesize carbon nanotubes — arc discharge, laser ablation, high pressure carbon monoxide deposition (HiPCO), and chemical vapor deposition (CVD).   The arc discharge process was the first method used to produce nanotubes, though it was orginally intended to make fullerenes.   In the arc discharge method, currents as high as 100 A are discharged through a pair rod-shaped graphite electrodes in a pressure controlled enviroment filled with helium gas, argon gas, or a mixture of the two gases.   The high currents heat the electrodes, causing carbon in cathode to change phase from a solid to gas vapor (sublimation).   The carbon then self-assembles into nanotubes back onto the surface of the anode.   By varying the ratio of helium and argon gas as wells as the dopant in the graphite electrodes, SWNTs of different diameters can be synthesized.   MWNTs can be made from using pure graphite electrodes.   Arc discharge produces more impure or defective nanotubes than pure ones, produces many bi-products, and has a smaller yield of about 30%.  

Laser ablation was first used by Richard Smalley with other researchers at Rice University.   The process, compared to arc discharge, still yields smaller quantities of nanotubes, but the quality is much better.   In the laser ablation method, a graphite sample is placed in a 1200° C oven, which is filled with helium or argon gas.   A high-intensity laser (either pulsed or continuous) is fired at the graphite.   The carbon vaporizes, and the flow of inert gas in the chamber transfers the vapor to a cooled piece of copper where the carbon nanotubes are deposited as a solid.   Similar to arc discharge, SWNTs are formed from a doped piece of graphite, while MWNTs are formed from a pure sample.   The major drawback of laser ablation is the cost — it is more expensive than arc discharge or CVD.  

Due to cost as well as the yield from both arc discharge and laser ablation, companies are turning towards HiPCO depostion and chemical vapor deposition to produce more nanotubes at less cost.   HiPCO deposition uses carbon monoxide gas as a feedstock to a hot chamber where iron atoms are floating.   Contact of the carbon monoxide with the iron separates the carbon monoxide into its carbon and oxygen atoms.   The oxygen atom form carbon dioxide as a bi-product, while the resulting carbon atom bonds with other separated carbon atoms to begin forming of the nanotube.   HiPCO has a higher rate of production (about 450mg/hr) than the former two methods, and the nanotube purity is excellent at about 97%.   HiPCO also contains a good carbon nanotube yield of 70%.  

The other method used for mass production is chemical vapor deposition.   CVD also involves a heated chamber and a gas.   Gases used in CVD, though, are hydrocarbons such as methane, ethylene, and acetylene.   A stationary catalyst-covered substrate is located in the chamber (the types of catalyst can vary, but as with laser ablation, iron particles can be used as well as nickel and cobalt).   As the temperature in the chamber transfers energy to the incoming hydrocarbon gas, the gas is forced to breakdown into its different elements.   As the separated carbon atoms come into contact with the catalyst on the substrate, the carbon becomes attached to the substrate and form bonds with surrounding carbon atoms to complete the carbon lattice of the nanotube.   Etching the catalyst on the substrate gives control over where the nanotubes grow.   The addition of a strong electric field and a plasma in plasma-enhanced chemical vapor deposition (PECVD) causes nanotubes to grow vertically or perpendicular to the substrate.   Without plasma, the Van der Waals' force, once again, causes the nanotubes to tangle.   Of the various ways to produce nanotubes, CVD currently is the most effecient, maintaining the lowest cost per nanotube ratio.  

One thing that is limiting research and affecting production cost in the area of nanotubes is the difficulty in sorting through a tangled mess of nanotubes to get specific types, sizes, and orientations.   Van der Waals' force is the culprit in the sticky, tangled bundles of various nanotubes.   In the case of the arc discharge method, extra soot and extra undesired material is generated, making it cumbersome to sort the products from the bi-products of the process.   To promote more specific research and create specific devices, it is ideal to separate metallic (armchair) and semiconducting nanotubes (zigzag and chiral orientations) or to create just metallic or semiconducting nanotubes.   Phaedon Avouris and his group at IBM have found a way to weed out the semiconducting nanotubes from a diverse sample of nanotubes.   IBM calls the process constructive destruction because the metallic nanotubes are destroyed in the process, while the semiconducting nanotubes remain untouched.   An entire sample is placed on a silicon dioxide (SiO2) substrate.   Using photolithography, two electrodes are placed on either side over the nanotube sample.   The electrodes are used to control whether the semiconducting tubes are conducting are insulating.   When the semiconductor nanotubes are insulated, a voltage is applied across the wafer to give an electric shock to the metallic nanotubes.   The shock destroys the metallic nanotubes, but because the semiconducting nanotubes are insulated, they cannot conduct current and do not breakdown.   Besides creating semiconducting nanotubes, the process is also useful for tuning the properties of multiwalled nanotubes.   Using the same shock, individual layers of MWNTs can be taken off to manipulate the electrical properties.