Patent Application
20090146134
The present invention relates to a composition comprising carbon nanotubes in a semiconductive matrix. These compositions are useful in printing semiconducting portions of thin film transistors.
Blanchet et al. in U.S. patent application Ser. No. 10/374,875 describes formulations of carbon nanotubes in a conducting polyaniline matrix.
It was found that there is a need for a formulation of carbon nanotubes in a polymer or oligomer resulting in a semiconducting matrix.
The present invention is a composition comprising a host matrix and 0.01 to 10% of volume of carbon nanotubes, preferably 0.01 to 1% which have been separated from the large ropes of nanotubes which are formed during their production. The large ropes are separated by being dispersed into an aqueous solution and then redispersed in an organic solvent. The nanotubes are subsequently linked by semiconducting materials.
The present invention is also a composition comprising a semiconducting host and 0.01 to 10% of volume of carbon nanotubes, preferably 0.01 to 1% that have been separated from the large ropes of nanotubes which are formed during their production into an aqueous solution. The nanotubes are subsequently dispersed in a semiconducting matrix.
A further embodiment of the present invention is a process comprising coating the above-cited composition on a donor element, contacting the donor element with a receiver element such that the coating lies between the donor element and the receiver element, and irradiating the coating through the donor element with a laser to transfer the coating on the donor element to the receiver element.
A yet further embodiment of the present invention is a process comprising inking the protruded regions of a stamp or flexographic plate with a solution of the above-cited composition, contacting the stamp or plate onto a receiver element such that the inking solution is transferred onto the receiver element with the pattern of the stamp protrusions.
Another embodiment of the present invention is a process comprising delivering a solution of the above-cited compositions onto a receiver element via an ink jet nozzle.
A further embodiment of the present invention is a transistor with a semiconductor comprising the above-cited compositions.
a, b, c, d show AFM images of SWNT's spun from two different solution concentrations at (a) 5 mg/L and (c) 20 mg/L and (b) and (d) are AFM images of the corresponding bi-layers with 200-Å-thick pentacene evaporated at 0.2 Å/s on top of SWNT's. The letters S and D indicate the Au source and drain electrodes.
This invention discloses a composition comprising carbon nanotubes dispersed in a semiconducting or insulating matrix. It demonstrates an alternative path for achieving high transconductance organic transistors in spite of relatively large source to drain distances. The improvement of the electronic characteristic of such a scheme is equivalent to a 60-fold increase in mobility of the underlying organic semiconductor. The method is based on networks, which are created from a dispersion of individual single wall carbon nanotubes (SWNT) and narrow ropes within an organic semiconducting host.
The carbon nanotubes described herein have been separated from the ropes of nanotubes, which form during their production process. They are dispersed at a concentration of 0.01 to 10%, preferably 0.01 % to 1%, in a matrix. This leads to the formation of networks of carbon nanotubes, which may be connected via semiconducting polymers, semiconducting oligomers, or barely conducting polymers coated on the nanotubes. At nanotube concentrations below the formation of a percolation network, the majority of current paths between source and drain follow the metallic nanotubes, but require a short, switchable semiconducting link to complete the circuit. Such electrically semiconductive organic composite can be patterned such that the film retains adequate electron mobility.
The carbon nanotubes can be dispersed into small amounts of semiconducting material prior to their dispersion into an insulating matrix. This leads to the formation of semiconducting carbon nanotube networks in an organic matrix for applications in which the resulting semiconducting layer functions as the transport layer in an organic electronic device. With these nanotube/semiconducting composites, a 60×reduction is effectively achieved in source to drain distance, which is equivalent to 60-fold increase of the mobility of the starting semiconducting material with a minor decrease of the on/off current ratio. These field-induced networks allow for the fabrication of high-transconductance transistors having a relatively large source to drain distances that can be manufactured by commercially available printing techniques.
It has been found that approximately two thirds of individual carbon nanotubes are semiconducting and the remaining one third is conducting. Although, the semiconducting to metallic ratio is also dependent on the nanotube synthesis method and specific catalyst used in their preparation. When the nanotubes are separated or dispersed from the large ropes in which they agglomerate during their production, networks of small ropes and individual tubes are obtained and can be dispersed in a matrix. At nanotube concentrations in the range of 0.01 to 10 percent by volume, the connectivity of the carbon nanotube varies considerably. While the onset of a measurable off current points to the formation of a few conducting pathways between source and drain at concentrations as low as 0.0025%, many others, remain interrupted by stretches of semiconducting material. However, interrupted links that fall close to the interface to the dielectric are switchable and can be turned on and off via the gate, which creates a thin electron channel within the semiconductor. It is this switchable network that becomes the active component between source and drain rather than it being any homogeneous material. Carriers move largely within the highly conducting metallic nanotubes from source to drain. Only occasionally and for distances short compared to the s-d length do they travel through the activated semiconducting channel. This represents an effective shortening of the s-d distance, giving raise to an equivalent increase in the transconductance. This notion represents the central part of our invention. A nearly percolating network of nanotubes is a network where complete 3 dimensional pathway of contact between the desired points (such as a source and a drain electrode) is almost, but not completely established. Gaps remain in the conductive pathway of touching nanotubes. The existence of these gaps is manifested in an on/off ratio of a composite transistor of 10,000 or greater. While the onset of a measurable Ioff, points to the formation of a few conducting pathways between source and drain, many others, remain interrupted by stretches of semiconductor. However, interrupted links that fall close to the interface to the dielectric are switchable and can be turned on and off via the gate, which creates a thin electron channel within the semiconductor. It is this switchable network that becomes the active component between source and drain rather than it being any homogeneous material. Carriers move largely within the highly conducting metallic nanotubes from source to drain. Only occasionally and for distances short compared to the s-d length do they travel through the activated PHT channel. This represents an effective shortening of the s-d distance, giving raise to an equivalent increase in the transconductance.
As the CNT concentration increases, the number of switchable current paths increases and the transconductance, gm, rises. Since mobility enters the transconductance linearly, μapp tracks gm. However, according to our model it would be more appropriate to think of the enhancement in gm as an effective reduction of the channel length, |eff ∝1/μapp, while the mobility of the semiconducting links remains constant.
As the nanotube concentration increases, the on/off ratio of these semiconducting composites when used as the transport layer of thin film transistors decreases due to the increased content of conducting tubes. At considerable higher concentrations, a 3-dimensional percolating network of conducting nanotubes are formed and the composite of nanotubes in the matrix is also conducting. Thus, composites containing semiconducting and metallic carbon nanotubes can be formed to be semiconductors for use in thin film transistors. The presence of the metallic tubes shortens the channel length increasing the effective mobility.
If the host matrix in which the network is embedded is a polymer, the composite may also be deposited as the active semiconducting layer in a transistor by various printing processes. The semiconducting layer can be printed via a thermal transfer process, printed using a photo-imageable printing plate (e.g., offset and flexo), an elastomeric molded plate such as a micro-contact printing plate or ink jetted. Improved electron mobility may also be achieved through the addition of semiconductive media such as semiconducting nanorods with high aspect ratios and semiconducting like mobilities. Since the nanotube concentration is considerably lower than that required of fillers, the processability of the host polymer is maintained while the mobility is increased.
Organic semiconductors such as pentacene and polythiophene, who have a π-electron system in their backbone consist of a sequence of aromatic rings. In particular, the mobility of these materials is quite low relative to their inorganic counterparts. Over the last 10 years, there has been considerable interest in developing organic semiconductors with high mobility that could be used in active electronic devices.
Tailoring the transport properties of organics has been achieved utilizing three different strategies:
Solution-processable polymers can be potentially used in a reel-to-reel production process of thin film transistors, thus reducing manufacture cost further compared with vacuum deposited organic films. In principle, organic materials have greater flexibility and easier tunability relative to the silicon-based counterparts. However, solution-based organic materials have low field-effect mobilities (10−3-10−6 cm2/Vs). Thus, considerable activity has been focused on the development of semiconductor materials with high mobilities for applications in TFT's due to vast variety of organic materials available. Similarly, semiconducting oligomers, which could be deposited via thermal evaporation, also show moderate mobilities relative to inorganic counterparts. Poly(alkylthiophenes), oligothiophenes, pentacene, phthalocyanines are just a few examples of such semiconductors. In addition, commercialization of organic electronic devices requires the ability to pattern the semiconducting layer. Imaging processes such as laser thermal transfer, ink jet or micro-contact printing have been described for such applications and are appropriate methods to deposit patterns of the compositions of the present invention in the production of TFTs. Throughout imaging processes, the resolution of the images as well as device performance is controlled. In particular, the mobility of the organic semiconducting film must be preserved throughout the imaging process. The mobility of organic semiconducting oligomers requires a considerable degree of crystalline order with large grain size and limited number of grain boundaries. Semiconducting polymers require instead a high degree of regio-regularity to achieve high mobility. In both these systems imaging via a laser process disrupts crystallinity, order and thus mobility. This invention presents paths towards increasing mobility by designing single wall carbon nanotube (SWCNT) composite materials. TFTs using the composite as transport channel have been fabricated. In contrast, the semiconducting networks of this invention can be imaged via a laser transfer technique, micro-contact, photo-imageable plates and ink jet. In addition, since the bulk of the material is not actively contributing to the overall film mobility, it can be selected for its processability, nanotube affinity and compatibility with a specific printing method. Using the present invention, one can assemble organic semiconductors with potentially much higher mobility than today's choice (pentacene) and considerably higher processability. Unlike pentacene, these networks can be potentially imaged with high resolution using thermal transfer methods, micro-contact printing and ink jet. The materials disclosed here are appropriate for applications as transport layer in plastic TFT transistors in microelectronics.
The compositions of the present invention require that the nanotubes be dispersed from the agglomerate ropes, which are formed during the production of the nanotubes into narrow ropes and individual tubes. As outlined in the examples, this can be done by dispersing the nanotubes into an aqueous solution and then re-dispersing them in an organic solvent.
Additionally, the carbon nanotubes may be coated prior to dispersion in the host matrix to increase the electron mobility above that of composites where the nanotube merely touch. The coating may be a semiconductor or insulator or a barely conductive polymer. By “barely conductive,” it is meant that the electrical conductivity is less than 10−6 S/cm.
By carbon nanotubes herein is meant carbon atoms bonded together in a hexagonal pattern to form long cylinders. Nanotubes can also be formed of multiple layers of walls. Carbon nanotubes were discovered about 1991. The nanotubes used herein were obtained from Rice University, Houston, Tex., U.S.A.
Preferred solvents herein are selected from the group consisting of ortho di-chloro benzene, water, xylenes, toluene, cyclohexane, chloroform, or mixture thereof with polar solvent such as isopropanol, 2-butoxyethanol, where the content of the polar solvent is preferably less than 25% by weight, toluene, cyclohexane, chloroform, isopropanol, 2-butoxyethanol and mixtures thereof.
These examples illustrate the effect of single wall carbon nanotubes (SWNT) dispersed in polythiophene. Single wall nanotubes obtained from Rice University, Houston, Tex., were dispersed in ortho di-chloro benzene to a concentration of 0.01 mg/ml. SWNT resulting from this dispersion are a few nanometer in diameter or single tubes.
To prepare the polythiophene matrix, 1 gram of Aldrich polythiophene (PTH) was purified in house following standard purification procedures. A 0.5% by weight solution of PTH in anhydride chloroform was prepared in a dry box. The solution was stirred with a stir bar at room temperature for about 48 hours until no solids remained. The thin film of the control sample was prepared by spin coating a thin film at 2000 RPM for 30 seconds onto a clean Si/SiO2 wafer with Au pattern sets of source and drains. The spun semiconducting layer was then baked at 80° C. for 30 minutes. This provided the transport layer of a thin film transistor in a bottom gate configuration (Example 1). The doped Si wafer was used as the gate electrode. A 250 nm thermally grown SiO2 film on the Si wafer was used as the dielectric onto which 40 sets of source and drains of various widths (W) and channel lengths (L) were patterned by photolithography. The patterned wafers were cleaned following the following procedure: 1) acetone rinse 3 times, 2) methanol rinse 3 times, 3) de-ionized water rinse, 4) blow dry and 5) O2 plasma for 5 minutes.
The thiophene solutions with carbon nanotubes are illustrated in Examples 2 and 3. A dispersion of single wall carbon nanotubes in ortho di-chloro benzene (ODCB) at 0.15 mg/ml concentration was tip sonicated for 5 minutes. The solution was then placed in the dry box and mixed into the polythiophene solution to make composites at 0.01, 0.02, 0.05 0.1 and 0.2% by weight CNT's. The I,V characteristics of the transistors. were then measured using a standard Hewlett-Packard 4155 probe station. I,V measurements were performed in a dry box in the dark to avoid the degradation known to be caused on PTP by oxygen and light.
This example demonstrates an alternative path for achieving high transconductance organic transistors by assembling bi-layers of pentacene onto random arrays of single-walled carbon nanotubes (SWNT). As in Example 3, for non-percolating SWNT arrays, the majority of current paths between source and drain follow the highly conducting nanotubes with short, switchable pentacene links completing the circuit. We show here that by varying the connectivity of the underlying nanotube network, the channel length of a thin film transistor can be reduced by nearly two orders of magnitude. Thus, enabling the increase in device transconductance without reduction in the on/off ratio.
Hipco SWNT's ropes, fabricated by CNI, Houston, Tex., were separated into individual tubes with the aid of surfactants. The resulting aqueous dispersion containing metallic and semiconducting tubes was filtered and the surfactant fully removed. The tubes (and small diameter ropes) were dried and re-dispersed in ortho-dichloro benzene (ODCB) at 5 mg/L, 10 mg/L, 20 mg/L, 35 mg/L and 50 mg/L concentrations. The various dispersions were spun at 1000 RPM onto a clean Si wafer with a 2500 Å thermal oxide and pre-patterned Au source/drain electrodes of various channel widths (W) and lengths (L). A 200 Å pentacene overlay evaporated at a base pressure of ˜7×10−8 torr. and at 0.2 Å/s, completed the device. Electrical performance was characterized using an Agilent 4155° C.
AFM images of SWNT arrays spun onto Si/SiO2 wafers from 5 and 20 mg/L SWNT dispersions and the corresponding SWNT/pentacene bi-layers are shown in
The effective field effect mobilities and transconductance of pentacene-SWNT TFT's bi-layers as a function of SWNT concentrations are shown in
In TFT devices comprising non-percolating nanotube networks, the presence of conducting SWNT rods merely reduces the distance between source and drain. In contrast, conducting pathways above percolation lead to a rapid increase in off current, thus lowering of the on/off ratio. In this example the semiconducting overlay is pentacene.
As in the single layer composite work, the majority of the current paths between source and drain follow the highly conducting nanotubes with short, switchable pentacene links completing the circuit. In principle, one would expect the increase in transconductance to scale inversely with channel length reduction. Thus, nearly 2 orders of magnitude increase in transconductance reflecting a 100×reduction in channel length.
The effective channel length in
A short channel length transistor was created through the exploitation of non-percolating SWNT arrays that are connected via semiconducting links that have a similar morphology to the underlying nanotube network. This method can raise the transconductance of our device by nearly two orders of magnitude to 1 cm2/V sec, mobility of a-Si. The factor of 40 observed here for the amorphous bi-layers reflects a significant improvement relative to Example 4 in which the pentacene crystallinity was reduced by the presence of their underlying nanotube network. The example illustrates that the potential of the bi-layers can be achieved with more amorphous semiconductor that grow conforming to the underlying network of tubes. Since the origin of this mobility improvement relies on the reduction of the effective source drain distance via the formation of a non-percolating arrays of SWNT's, on/off ratio of 105 can be maintained. The material alkyl antracene is described in U.S. provisional patent application 672177 by Hong Meng. The AFM of a 400 Å semiconducting films evaporated onto a Si wafer with a SiO2 layer is shown in
These examples illustrate a semiconducting carbon nanotube network in a polyaniline (PANI) matrix. The polyaniline was slightly doped to a conductivity of 10−5ohm-cm. The single wall carbon nanotubes (SWNT) were well dispersed in water to a concentration of 0.015 mg/ml with 1% surfactant (SDS) provided by Strano. The SWNT resulting from this dispersion were mostly single tubes. The dispersion was used in the composites without further sonication. A 3% by weight solution of polyaniline in distilled water as prepared at room temperature. The polyaniline solution was then mixed with the SWNT in water as previously described to SWNT concentrations of 0, 1%, 2%, 5% and 10%. Zonyl FSN (by DuPont, Wilmington, Del.) to 6 to 10% by weight concentration was added as a coating aid. The amounts of the composites used in Examples 6-10 are indicated below:
Example 6, control 250 mg PANI, no nanotubes
Example 7, 1% CNT in PANI: 5 ml SWNT solution, 250 mg PANI
Example 8, 2% CNT in PANI: 5 ml SWNT solution, 120 mg PANI
Example 9, 5% CNT in PANI: 5 ml SWNT solution, 68 mg PANI
Example 10, 10% CNT in PANI: 10 ml SWNT solution, 68 mg PANI
The solutions were spun onto the SiO2/Si wafers described in Examples 1-3 at 2000 rpm and baked in an oven at 60° C. for 5 minutes. The TFT characteristics were measured as previously described. The Igate sweepcurves are shown in
These examples illustrate the formation of a semiconducting carbon nanotube network in an insulating matrix. The host matrix is an insulating terpolymer of methyl methacrylate/butyl methacrylate/methacrylic acid/glycydil methacrylate in a ratio of 70/25/3/2. This has a glass transition (Tg) of 70° C. The latex was 33% by weight in water. The single wall carbon nanotubes were well dispersed in water to a concentration of 0.015 mg/ml with 1% surfactant (SDS) which were provided by Michael Strano from University of Illinois. The SWNT resulting from this dispersion were mostly single tubes and were used in the composites without further sonication. The SWNT dispersion was then mixed with the latex. Zonyl FSN was added to 1 part in 106 by weight of total solution to aid with coating. In Example 11 (control sample), the latex was spun onto the patterned clean wafers previously described. I,V curves are shown in
The formulations were then spun onto the clean pattern Si wafers previously described at 2000 rpm. The spun samples were then baked in an oven at 60° C. for 5 minutes. The I,V curves were measured and mobility calculated from the linear regime.
These examples illustrate the formation of a semiconducting carbon nanotube network coated with a conducting polyaniline (1:4 ratio) in an insulating matrix. As in Example 4, the polyaniline is soluble in water to a concentration of 3% by weight. As in Example 9, the host matrix is an insulating terpolymer of methyl methacrylate/butyl methacrylate/methacrylic acid/glycydil methacrylate. This has a glass transition (Tg) of 70° C. The latex was 33% by weight in water. The single wall carbon nanotubes were well dispersed in water to a concentration of 0.015 mg/ml with 1% surfactant (SDS) were provided by Strano. The SWNT resulting from this dispersion were mostly single tubes and were used in the composites without further sonication. Zonyl FSN was added to 1 part in 106 by weight of total solution to aid with coating. The SWNT dispersion was mixed with the PANI solution in a ratio of 1:4. That is 18 ml containing approximately 0.27 mg of SWNTs was mixed with 38 ml of 3% PANI solution containing 1.14 mg of PANI. This was finally mixed with 228 ml of 33% latex solution containing about 76 mg of latex.
The formulation was then spun onto the clean patterned Si wafers previously described at 2000 rpm. The spun samples were then baked in an oven at 60° C. for 5 min. The I,V curves were measured and mobility calculated from the linear regime.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US05/30632 | 8/25/2005 | WO | 00 | 3/5/2008 |
| Number | Date | Country | |
|---|---|---|---|
| 60605343 | Aug 2004 | US |
