1. Field of the Invention
The present invention relates to a nanomaterials and, more particularly, to flexible devices containing nanomaterials.
2. Description of Related Art
Electronic devices are responsible for the majority of advancements in technology in the last century, and the complexity of these devices grows in proportion to the exponential advancements in the field. As electronic devices continue to expand into every facet of everyday life, the demand for more durable, robust, and flexible electronic devices becomes increasingly significant. Recent discoveries with respect to nanomaterials have opened new doors with respect to enabling durable, robust, and flexible electronic devices, but these new discoveries come with equally challenging obstacles.
One of the significant challenges encountered with use of nanomaterials in electronic devices relates to a dichotomy between conductive and flexible materials. More specifically, a general dichotomy exists between materials with superior electrical, optoelectronic, semiconductor and/or structural characteristics and materials that are highly compatible with flexible, malleable, and durable composites. Typically materials that have excellent transport properties, regarding conductivity and carrier mobility, are difficult or impossible to directly process on or with materials that are flexible, malleable, and durable. Conversely, those materials that have poor electrical properties are more likely to be compatible with flexible, malleable, and durable materials. In accordance with this dichotomy, nanomaterial structures that have excellent transport or structural properties, such as carbon nanotubes or semiconductor nanowires, are difficult to process and fabricate with flexible, malleable, and durable materials such as polymer substrates. Therefore, it is highly desired to develop methods and systems which permit the incorporation of nanomaterial structures into flexible, malleable, and durable materials, such as polymer substrates.
As nanomaterial structures, such as carbon nanotubes and semiconductor nanowires, are the product of recent discoveries, their applications and uses are widely undeveloped and undiscovered. The discovery of carbon nanotubes by Sumio Iijima, a Japanese physicist, in 1991 may ultimately prove to be one of the most significant discoveries of the twentieth century as it spurned the growth of research in the 1-D nanomaterials revolution. Prior to Dr. Iijima's discovery, solid state carbon was know to appear only in four basic structures: “diamond structures,” “graphite structures,” “non-crystalline structures,” and “fullerene molecules.” In 1991, Dr. Iijima discovered a tube-shaped material made up of carbon in a continuous hexagonal mesh and with a diameter measuring on the order of a nanometer, one-billionth of a meter. The far-reaching benefits of carbon nanotubes (“CNTs”) stem from their unique and novel electronic, thermal, and structural properties. A CNT is cylindrical in structure and may be around one nanometer in diameter and up to several micrometers in length. In other words, the length of a CNT may potentially be millions of times greater than their molecularized diameter. The small diameter of the CNT is due to the fact that its tubular body is typically only a few atoms in circumference. The CNTs are hollow and have a linear fullerene structure. Due to the carbon to carbon covalent bonding and the seamless hexagonal network, CNTs are quite possibly the strongest known molecular structure. For example, the strength to weight ratio is 500 times greater than that of aluminum. CNTs have a tensile strength of 63 gigapascals (“GPa”), compared to high carbon steel at 1.2 GPa. CNTs are light, flexible, stable and generally inert.
CNTs can be formulated to exhibit varying degrees of conductivity, depending upon their chirality. The chirality, or “twist” of the nanotube structure can alter the density of the hexagonal lattice structure and thus effect the conductivity of the nanotube. Therefore, CNTs can be formulated to be either metallic or semiconductive. Metallic CNTs, exhibit electrical conductivity on the order of six times greater than that of copper. In addition to good conductance, CNTs exhibit a very high current carrying capacity. Significantly, with lengths of several microns and diameters of a few nanometers, CNTs form microtips with high aspect ratios, which are excellent field emitters.
CNTs are generally good thermal conducts in the axial direction, along their tube axis, and insulative in the radial direction, along an axis lateral to the tube axis. Furthermore, CNTs are incredibly efficient conductors of heat, with a potential thermal conductivity of 3000 W/mK.
CNTs can be of two main types of structures, Multiwalled Nanotbues (MWNTs) and Single Walled Nanotubes (SWNTs). A SWNT is simply one cylindrical hexagonal carbon structure that can be very long in length. MWNTs have multiple layers of encapsulated cylindrical hexagonal carbon structures. MWNTs can have multiple SWNTs concentric cyldinders inside a large SWNT or one SWNT inside a larger SWNT.
Carbon nanotubes are generally produced by three main techniques, arc discharge, laser ablation and chemical vapor deposition. In arc discharge, a vapor is created by an arc discharge between two carbon electrodes with or without catalyst. Nanotubes self-assemble from the resulting carbon vapor. In the laser ablation technique, a high-power laser beam impinges on a volume of carbon-containing feedstock gas (methane or carbon monoxide). Laser ablation typically produces a small amount of clean nanotubes, whereas arc discharge methods generally produce large quantities of impure material.
Chemical vapor deposition (CVD) synthesis is achieved by putting a carbon source in the gas phase and using an energy source, such as a plasma or a resistively heated coil, to transfer energy to a gaseous carbon molecule. Commonly used gaseous carbon sources include methane, carbon monoxide and acetylene. The energy source is used to “crack” the molecule into reactive atomic carbon. Then, the carbon diffuses towards the substrate, which is heated and coated with a catalyst (usually a first row transition metal such as Ni, Fe, Mo or Co) where it will bind. Carbon nanotubes will be formed if the proper parameters are maintained through the vapor liquid solid growth mechanism. Excellent alignment, as well as positional control on nanometer scale, can be achieved by using CVD. Control over the diameter, as well as the growth rate of the nanotubes can also be maintained. The appropriate metal catalyst can preferentially grow single rather than multi-walled nanotubes.
The current applications and potential applications for CNTs are amazingly varied and wide in range. The CNTs superior and unique properties afford an almost unending number of novel implementations and improvements to a variety of fields. CNTs are currently used or contemplated for use in nanoelectronics, biosensors, chemical sensors, optical sensors, solar cells, magnets, slick surfaces, combat jackets, transistors, oscillators, high strength composites, and superconductors.
The unique properties of CNTs present novel possibilities with respect to nanoelectronics, biosensors, chemical sensors, optical sensors, and similar devices. For example, nanomaterials are ideal for chemical sensors because they have very large surface areas. This large surface area translates into large adsorption rates of gases and vapors. Similarly, every atom in CNT is on the surface, thus it is incredibly sensitive to the environment, and small changes in charge environment can drastically change the electrical properties of the CNT. Indeed, SWNT field effect transistors have been fabricated wherein a single SWNT or a film of multiple SWNTs forms the conducting channel. Therefore, the conductance of the channel will change upon exposure of SWNT to certain chemical gases. Chemical sensing can thus be executed by monitoring the conductance of the channel.
A significant potential application for carbon nanotubes that has been the subject of much research and development is their potential use in flexible electronic devices. Conventionally, flexible electronic devices have relied upon advancements in semiconductor fabrication. In particular, the plasma-enhanced deposition of amorphous silicon onto polymer substrates has been utilized to create flexible semiconductor devices. Despite its advantages, the use of amorphous silicon in flexible electronic devices has significant drawbacks. For instance, in general, the processing temperature requirements for amorphous silicon limit its compatibility to a small number of polymers. Furthermore, the low transport mobility of amorphous silicon, on the order of 1 cm2/Vs, limits the applications in which the resulting flexible electronic device can be utilized.
The use of carbon nanotubes has been contemplated to overcome the inherent problems of amorphous silicon in flexible electronic device applications. Carbon nanotubes have transport mobilities, which are several orders of magnitude greater than amorphous silicon. The performance of random CNT network devices has demonstrated electron mobility as high as 270 cm2/Vs and transistor on-off ratios as high as 10,000. Therefore, a significant desire exists to be able to incorporate CNTs into flexible electronic devices. Unfortunately, many problems exist in the compatibility of CNTs with flexible polymer substrates. More specifically, the synthesis of CNTs directly onto polymer substrates is not feasible due to the high temperatures or harsh chemical environments under which they are synthesized.
Conventional methods have attempted to address the challenge of incorporating CNTs into flexible polymer substrates with solution-based transfers. Solution-based carbon nanotube transfer processes, such as spin casting, flow-directed alignment, electrophoretic trapping, chemical functionalization, or microcontact printing, involves suspending the CNTs in an “ink” solution. The solution may act as the vehicle to transport the CNTs to the surface of the polymer substrate.
For example, U.S. Pat. No. 6,436,221 to Chang, et al., filed Feb. 7, 2001, describes a solution-based method of transfer of carbon nanotubes to a substrate. In the disclosed method, a conductive pattern is coated on a substrate by screen-printing a conductive slurry containing silver through a patterned screen. Next, a CNT paste, consisting of an organic bonding agent, resin, silver powder, and CNTs, is screen-printed through a mesh pattern screen onto the substrate. Subsequently, the substrate is baked at a predetermined temperature to remove the solvent, then sintered to solidify the CNT to the conductive pattern. Finally an adhesive film, such as tape is closely attached on the cathode substrate and then is then removed so as to discard the badly bonded CNT portions and to vertically pull up a portion of the CNT which laid down during the sintering.
U.S. Patent Application No. 2003/0092207 to Yaniv, et al., published May 15, 2003, describes a solution-based CNT transfer process. Yaniv, et al. discloses a process involving obtaining carbon nanotube powder, grinding the powder into shorter length CNTs, and mixing the powder in a solution in an ultrasonic process to disperse the CNTs. After the mixture solution has been created it is spayed onto the substrate with an atomizer, then tape is used to remove a portion of the CNTs from the surface, thereby leaving a layer of CNTs on the surface of the substrate.
U.S. Patent Application No. 2005/0165155 to Blanchet-Fincher, published Jul. 28, 2005, describes a method of creating a composition comprising carbon nanotubes and conductive polyaniline. The method disclosed involves creating a mixture of carbon nanotubes and conductive polyaniline by dispersing carbon nanotubes in xylenes and then adding a solution of doped polyaniline to the dispersion. Subsequently, a solution of insulating polymer is added to the dispersion, the dispersion is deposited on a substrate, and the solvent is allowed to evaporate. The result is a polymer composition containing carbon nanotubes.
Many problems exist with the solution-based CNT transfer methods, such as the methods disclosed in Chang, et al., Yaniv, et al. and Blanchet-Fincher, due to, among other things, the fact that the carbon nanotubes are fragile, vulnerable to separation, and randomly oriented in the solution. The random orientation in the solution results in a random orientation of the carbon nanotubes on the substrate. This random orientation of the CNTs can be detrimental to many applications, including creating non-optimized electrical field distribution and resulting in shielding effects between adjacently positioned CNTs. Additionally, the inks and solvents used to transfer the carbon nanotubes must be compatible with the polymer substrate. This significantly limits the choice of ink and solvents and the choice of polymers. Furthermore, the flexible electronic devices created by these solution-based transfer methods are very vulnerable, due to the fact that the CNTs simply reside on the surface of the substrate to which they applied. The development of solution-based CNT transfer inks require extensive fabrication time cycles. The create of the ink requires a significant amount of time to disperse the CNTs throughout the solution. Furthermore, the solution-based method involves many steps. Even after the solution has been applied to the substrate, the solution must be printed and the “solvent” evaporated (requiring more processing time). Additional steps must be taken if control over the arrangement and orientation is desired in solution based deposition techniques.
Similar problems are encountered in the manufacture of flexible electronic devices that incorporate nanomaterial structures other than CNTs. For example, known methods for the incorporation of nanowires into flexible polymer substrates suffer from the same problems as the described conventional methods for CNT transfer. In transferring the nanowires to a polymer substrate, the integrity of the nanowire structure is often lost or degraded and the nanowires are only tenably attached to the polymer substrate.
Accordingly, there is a need in the art for an efficient method by which to create an effective flexible electronic device incorporating nanomaterials.
Additionally, there is a need in the art for an efficient method to mass produce flexible electronic devices that incorporate nanomaterials with minimal processing steps.
Additionally, there is a need in the art for an efficient method to mass produce flexible electronic devices that incorporate carbon nanotubes with minimal processing steps.
Additionally, there is a need in the art for method to transfer carbon nanotubes to a flexible electronic device.
Additionally, there is a need in the art for method to transfer nanowires to a flexible electronic device.
Additionally, there is a need in the art for a method to transfer nanomaterials to a variety of polymer types.
Additionally, there is a need in the art for a method to transfer carbon nanotubes to a variety of polymer types.
Additionally, there is a need in the art for a flexible electronic device comprised of systematically arranged nanomaterials.
Additionally, there is a need in the art for a flexible electronic device comprised of systematically arranged carbon nanotubes.
Additionally, there is a need in the art for a flexible electronic device comprised of systematically arranged nanowires.
Additionally, there is a need in the art for method to create a flexible electronic device comprised of systematically arranged carbon nanotubes.
Additionally, there is a need in the art for method by which to integrate carbon nanotubes into a polymer substrate in a manner that enable control over the general orientation of a portion of the carbon nanotubes.