The invention concerns the field of semiconductors. More specifically, the invention concerns new conjugated organic semiconductors, composites and methods for their use.
Since the original reports on the conductivity of polyacetylene (Chiang et al., 1977), there have been considerable efforts to develop semiconducting polymers for use in electronic devices (Angelopoulos, 2001). The primary focus has been on chemical modification in order to tune the band gap (Faied et al., 1995) as well as to control the carrier type (p and n-type) (MacInnes et al., 1981) and the carrier concentrations (Bredas & Street, 1985). Emphasis has been placed on developing materials that function in a manner that is analogous to inorganic semiconductors and on creating devices with traditional architectures such as transistors (Burroughes et al., 1988), p-n junctions (Cheng et al., 2004) and organic light emitting diodes (OLEDs) (Burroughs et al., 1990). At the same time, considerable focus has been placed on understanding and controlling the movement of counter ions into and out of semiconducting polymers in contact with electrolytes, and a wide range of devices have been developed based on redox switching such as batteries, supercapacitors and electrochromic devices (Gurunathan et al., 1999). Control over the identity of the ionic charge carrier can be imposed by immobilizing anions within the polymer either covalently (Patil et al., 1987) or by physical entrapment (Bidan et al., 1988), thereby forcing charge to be carried by smaller and more mobile cations.
In typical conjugated polymer systems, current conduction can be either electrode or bulk limited. For most conducting polymers, such as polypyrrole (PPy), with Au electrodes, the current is expected to be bulk limited (Blom et al., 1997). Several mechanisms of bulk conduction of charge can be dominant depending on geometry, field strength and carrier mobility among other things (Blom et al., 1997; Pai, 1970). At lower potentials between contact electrodes, ohmic behavior is expected. As the potential between the electrodes increases space charge limited currents (SCLC) often become dominate (Blom et al., 1997). With space charge limited current the injected charge carriers create a potential that limits the flow of charge (Blom et al., 1997). This results in a current which is nonlinear and increases as V2 and 1/L3 in conventional semiconductors, in the absence of traps. However unlike conventional semiconductors, in conjugated polymers the mobility of the charge carriers increases with the applied field (Blom et al., 1997; Pai, 1970). This results in the current increasing more rapidly with voltage than would be predicted in the absence of field enhanced mobility (Blom et al., 1997; Pai, 1970). The modulation of field generated carriers by introducing ion pairs into a neutral conjugated polymer is described here.
In certain aspects, the present invention concerns organic semiconductors. In some aspects, organic compounds for use according to the invention may comprise conductive polymers or mixtures of conductive polymers such as polypyrrole (PPy), polyacetylene (PA), polythiophene (PT), polyaniline, polyphenylene (PPP), poly(phenylene vinylene) or derivatives thereof. Furthermore, in certain cases, organic polymers comprise doping with positively charged molecules (cation) and negatively charged molecules (anions). Preferably, doped polymers of the invention comprise a large immobile ion of one charge (i.e., a cation or anion) and a small mobile ion of a reverse charge. For example, a doped polymer may comprise at least a first small, mobile cation and at least a first large, less mobile anion. Thus, in certain aspects, an organic polymer may comprise doping with at least a first polyelectrolyte. Thus, ions comprised in a doped polymer of the invention may be mobilized upon the application of an electric potential. For example, a polymer of the invention may exhibit a change in current (upon potential application) equal to an effective change in effective length of between about 5 and 20%.
As discussed supra, in certain aspects, an organic polymer of the invention comprises doping with a large anion or a polyelectrolyte comprising a large anion or a mixture of large anions. In further aspects, an anion for use in the invention may have a molecular weight (MW) of between about 100 and 1,000,000. Furthermore, in some instances, an anion may be further defined as surfactant. In some cases, an anion for use in the invention may be monovalent, however it is also contemplated that divalent, trivalent or multivalent anions may be used. For example, in some very specific cases, an anion may be a polyacrylamidoglycolic acid (PAGA), poly(diallyldimethylammonium chloride) (PDMA), poly(sodium styrenesulfonate) (PSS), polystyrene sulfonate (SPS), poly(acrylic acid) (PAA), poly(vinyl phosphate) (PVP), poly(2-acrylamido-2-methyl-1-propanesulfonicacid) (PAMPS), poly(2-acrylamidoglycolic acid), poly(2-hydroxy-4-N-methacrylamidobenzoic acid) (PHMA), poly(sodium thiophene-3-carboxylate) (PSTC), poly(sodium phenylenecarboxylate) (PSPC), a sulfonated poly(benzobisthiazole) (PBT), sulfated poly((3-hydroxyether), sulfated poly(butadiene), sulfated poly(imide), sulfated poly(methacrylate), bis(2-ethylhexyl)sulfosuccinate, dodecylbenzenesulfonic acid (DBSA), dodecylbenzenesulfonate (DBS), dodecylsulfate, tetradecyltrimethylammonium bromide (TTAB), tetraethylammonium p-toluensulfonate, toluenesulfonate, pyrenesulfonate, pyrene-1,3,6,8-tetrasulfonate, dodecylbenzenesulfonate, 1,2-bis(decyloxycarbonyl)ethanesulfonate, octachloro-dirhenate (Re2Cl8) or tetraphenylborate anion or a combination thereof For instance, in some cases, an organic polymer may comprise doping with a dodecylbenzenesulfonate− (DBS) anion or a polyelectrolyte such as sodium dodecylbenzenesulfonate. Furthermore, in some instances, a doped organic polymer of the invention may be defined by the concentration of an anion comprised in the polymer. Thus, in some cases, a doped polymer may comprise a ratio of polymer to anion of about 6:1, about 5:1 or about 4:1. In even more specific aspects the concentration of an anion in a polymer may be defined. For example, a doped polymer of the invention may comprise about 1×1021 anion molecules per cm3 as exemplified herein.
In still further aspects, it will be understood that doped polymers of the invention may comprise a small, mobile cation or a mixture of small mobile cations. For example, in certain cases, polymers may comprise doping with a second polyelectrolyte comprising a mobile cation. Furthermore, in some aspects, this doping process may proceed under reducing conditions. In preferred aspects, cations for use in the invention are small mobile cations having molecular weight of less than about 100, such as single atom ions. For instance, cations may be an alkali metal such as lithium. Thus, in certain cases, a polymer of the invention may be doped with a polyelectrolyte such as lithium perchlorate.
In yet further embodiments, polymers of the invention may comprise additional polymer layers or other ion containing layers. In some cases, such additional layers may be used to provide a barrier to lock in field produced I-V asymmetry. In still other aspects, polymer layers may be altered to modify ion mobility. For instance, polymers maybe driven through the glass transition temperature, or plasticizers may be added or removed to alter the glass transition temperature.
In still further embodiments, the invention provides a method comprising applying a first potential across an organic polymer of the invention, and applying a second potential across said polymer to generate a current that is dependent on the first potential that was applied across the organic polymer. In certain aspects, the second potential may be a reverse polarity with respect to the first potential. Preferably, the magnitude of the first and second potentials will be different. For example, in some cases, the difference in the magnitude of the first potential and the second potential is between about 3 and about 4.5 V. In still further aspects, the first potential may be greater than the magnitude of the second potential or visa versa. In some cases, the current generated by the second potential may be assessed, which may comprise measuring the current generated by second potential. Thus, in some aspects, assessing the current generated by said second potential may comprise determining whether the current generated by the second potential increases or decreases over time. In certain further aspects the distance of polymer over which a potential is applied may be defined. For example, a potential may be applied over a distance of polymer of between about 0.1 and about 100 μm, or between about 1 and about 20 μm, or between about 100 nm and about 500 nm, or about 200 nm.
Thus, in certain aspects, there is provided a method for determining whether a first potential has been applied across a doped organic polymer of the invention comprising (i) applying a potential across the polymer and (ii) applying (e.g., assessing) the current resulting from said potential thereby determining whether a potential has been previously applied across the polymer. Such a method therefore may used in the storage of binary data (e.g., 0 is no previous potential has been applied, 1 if a previous potential has been applied).
As exemplified herein, doped organic polymers of the invention may in certain cases, be used in electronic circuits. For example, in some aspects, the invention provides a circuit comprising an organic polymer doped with at least a first polyelectrolyte wherein said organic polymer is in electronic communication with at least a first and second conductor. Hence, organic polymers of the invention may fill a gap between two conductor materials. In certain cases, a circuit of the invention may be defined by the length of the gap filled by a doped polymer. For example, a doped polymer of the invention may fill a gap of between about 0.001 and 100 μm, or between about 0.1 and 100 μm, or between about 1 and 20 μm. In certain aspects the first or second conductor may be further defined as an injecting or blocking electrode. Furthermore, in certain cases the first or second conductor (or both) may comprise Au, Pt, Cu, Ag or an alloy or mixture thereof. In some preferred aspects, a circuit of the invention may be comprised in a non reactive environment such as a vacuum or an inert gas such as nitrogen or a noble gas. In some specific aspects a circuit of the invention maybe further defined as a memory circuit as exemplified herein.
Furthermore, in certain aspects, doped polymers of the invention may comprise additional polymer layer. For example, and additional polymer layer may be used to block the motion of ions that can be triggered by electric fields, magnetic fields, heat or light. Thus, such additional layer may reduce interference from undesired external sources.
In yet further aspects, the invention concerns an array comprising two or more circuits. Such an array for example may be comprised within an electronic device. For example, the invention provided in some aspects, a computer comprising a circuit or an array of circuits of the invention. In other embodiments, the array may be a sensor array.
Some embodiments of the present disclosure involve crossbar devices having a first conductor adjacent to a crossbar junction region, a second conductor adjacent to the crossbar junction region, and a doped organic polymer (e.g., as described above) that is within the crossbar junction region and is in electronic communication with the first conductor and the second conductor. In these embodiments, the first conductor and the second conductor may each have widths of less than about 100 μm and may be separated from each other by about 1 μm or less. In some embodiments, the first conductor and the second conductor may be separated from each other by as little as about 1 nm or less. Other dimensions can be used, as would be understood by those having ordinary skill in the art, with the benefit of this disclosure.
These crossbar devices can be configured such that a first potential can be applied across the organic polymer using the first conductor and the second conductor, and a second potential can be applied across the organic polymer using the first conductor and the second conductor to generate a current that passes through a portion of the first conductor, a portion of the organic polymer, and a portion of the second conductor. In these embodiments, the current that passes through the organic polymer (and therefore the crossbar junction region) when the second potential is applied is dependent on the first potential.
In some of these embodiments, the first conductor and the second conductor may be separated by between about 1 nm and about 500 nm. In some of these embodiments, this separation distance may be about 200 nm. In other embodiments, other dimension can be used, as would be understood by those having ordinary skill in the art, with the benefit of this disclosure.
In some embodiments of the present disclosure, the width of the first conductor and the width of the second conductor may be each about 20 μm or less. In some embodiments, the first conductor or the second conductor (or both) may be Au, Pt, Cu, Ag, tungsten oxide, or other metal oxides. Other materials can be used, as would be understood by those having ordinary skill in the art, with the benefit of this disclosure. In some embodiments, the materials contained in the first conductor and the materials contained in the second conductor may be identical. In some embodiments, the materials contained in the first conductor and the materials contained in the second conductor may be different.
Some embodiments of the present the disclosure involve memory devices. For example, two or more crossbar devices may form an array. In some embodiments, the array is a sensor array. In other embodiments, a computer contains the crossbar devices or arrays containing the crossbar devices. One of ordinary skill in the art, with the benefit of this disclosure, would understand that embodiments of the present disclosure may involve memory devices such as, for example, RAM, SRAM, DRAM, etc., that may be used in applications such as, for example, computers, cell phones, PDA devices, cameras, mobile electronics, etc.
Embodiments discussed in the context of a methods and/or composition of the invention may be employed with respect to any other method or composition described herein. Thus, an embodiment pertaining to one method or composition may be applied to other methods and compositions of the invention as well.
As used herein the specification, “a” or “an” may mean one or more. As used herein in the claim(s), when used in conjunction with the word “comprising”, the words “a” or “an” may mean one or more than one.
The use of the term “or” in the claims is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive, although the disclosure supports a definition that refers to only alternatives and “and/or.” As used herein “another” may mean at least a second or more.
Throughout this application, the term “about” is used to indicate that a value includes the inherent variation of error for the device, the method being employed to determine the value, or the variation that exists among the study subjects.
The terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include” (and any form of include, such as “includes” and “including”) and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. Thus, a method comprising certain steps is a method that includes at least the recited steps, but is not limited to only possessing the recited steps.
Other objects, features and advantages of the present invention will become apparent from the following detailed description. It should be understood, however, that the detailed description and the specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
The following drawing is part of the present specification and is included to further demonstrate certain aspects of the present invention. The invention may be better understood by reference to the drawing in combination with the detailed description of specific embodiments presented herein.
Herein there is provided examples demonstrating conjugated semiconducting polymer composite comprising immobile anions that act as a counter ion for the oxidized (more conductive) form of the polymer and highly mobile cations that can balance the charge of the immobile anions in the presence of the reduced (less conductive) form of the polymer. This material exhibits a field-dependent resistance in the solid state with a time dependence that is a function of the mobility of the cation. Using this approach, a functioning dynamic memory device has been demonstrated. These findings should open-up new directions for the development of organic-based electronics that utilize field- and time-dependent behavior of organic semiconducting composites.
The material system described in this work has significant advantages for application in nanometer-scale electronics, since the junctions are electrochemically grown and they can be fabricated after all conductor layers have been deposited and patterned. For example, in a cross-bar memory architecture, these junctions can be grown after the formation of the crossbars rather than between the metal layers (Green et al., 2007). The bulk dominated conductance of this system should result in better scaling with device size. If the dimensions of the device were all scaled by a factor of 1/s then the space charge limited current would scale as s3 with the device length and 1/s2 with device area resulting in an overall scaling of s3/s2=s. In interface-dominated devices, such as diodes, the current would scale as s−2 resulting in currents for nanometer-scale devices being very small (Cerofolini & Mascolo, 2006).
In summary, the invention provides a new approach for the design of conjugated conducting polymer composites that exhibit dynamic asymmetric electronic behavior based on the movement of charge in response to the application of a field. This work opens up new avenues for device design and fabrication. Devices utilizing this material offer several potential advantages including ease of fabrication, simple structures and more favorable scalability factors.
The following examples are included to further illustrate various aspects of the invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent techniques and/or compositions discovered by the inventor to function well in the practice of the invention, and thus can be considered to constitute preferred modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments which are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention.
In order to produce IDA/polymer structures, a PPy composite material containing immobilized dodecylbenzenesulfonate (DBS) and lithium (Li+) in the form of a thin film spanning two metal electrodes was created (
Preparation Methods
The gold interdigitated array electrodes (IDAs) were obtained from
Biomedical Microsensors Laboratory at North Carolina State University. Each of these arrays contains 2.8 mm×0.075 mm gold electrodes with a gap width of 20 μm having a total exposed area of 6.09 mm2. The polypyrrole (PPy) films were grown across the IDA electrodes using an aqueous solution of freshly distilled pyrrole monomer (100 mM) and an electrolyte (100 mM, NaDBS or LiClO4) at a constant potential of +0.65 V vs. Ag/AgCl. The thickness of the polypyrrole films were controlled by passing specific amount of charge (200 mC/cm2 for 1 μm thick film) (Smela, 1999) during the electrodeposition. In each case, 1.23 C/cm2 of charge was passed to achieve complete bridging of the IDA electrodes and resulted in a film thickness close to 6 μm as seen in
Samples were prepared by mechanical cutting and shaving of the PPy-IDA cross-sectional interface using a razor blade. The finely cut samples were sputter-coated (Edward) with a thin layer of gold and the images were acquired with a Cambridge Stereoscan 120 Scanning Electron Microscope.
The X-ray photoelectron spectra of polypyrrole NaDBS composite on gold was determined (
The current-voltage (I-V) properties of the PPy-IDA devices were characterized under a nitrogen atmosphere with CHI660 electrochemical workstation (CH Instruments) or a Hewlett Packard 4145A semiconductor parameter analyzer. In the configuration described above (PPy0Li+DBS−,
The bulk-limited current behavior of the PPy devices can be observed in current versus voltage curves (
The time-dependant behavior of the current through these devices offers further evidence of this internal reconfiguration leading to a structure as shown in
This redistribution of cations with voltage and time dependence offers new opportunities to produce electronic devices, such as dynamic memory cells and sensor arrays. For example, the redistribution of cations, that results in a junction with a reduced effective length, remains for a period of time after the field is removed since the cations do not instantaneously return to their equilibrium position. Indeed, one would expect that upon reversing the applied potential, the internal configuration of charge and carriers remains largely unchanged. A reverse potential can be used to determine the magnitude of the previously applied potential. If the magnitude of the reversed potential is greater than the previous forward potential, the magnitude of the current will increase with time as the cations drift and reduce the effective length of the junction. If the magnitude of the reversed potential is smaller than the previous forward potential, the current will decrease with time as the cations drift to increase the effective junction width. This memory effect is analogous to an “echo” related to the potential applied prior to the potential reversal. This echo effect increases with increasing initial potentials as is observed in
Since the distribution of cations that results in the echo is detectable for periods over 60 s (
Using this principle, a simple memory circuit was constructed (
Testing Methods
The voltage applied to the PPy device in series with a resistor (R=50 Ω) was supplied with a DS345 Synthesized Function Generator, and its synchronous signal (CLK1) was used to trigger pulses (CLK2, CLK3 and CLK4) generated by the HP 8016A word generator. The current flowing through the device was read out from voltage across the resistor, which was then amplified 10 times by a differential amplifier (Op AMP AD621). Furthermore, two sample and hold amplifiers (LF398A) were used to record initial and stable states of the current, and their values were compared using comparator (LM311). At the final stage, a dual D-type flip/flop (SN74LS74A) was used to capture the memory data.
A smaller polypyrrole composite device was fabricated having a ˜1 μm gap as opposed to the larger gap in the device shown in
Current-voltage behavior for the device in
Current versus time behavior of polypyrrole composite device in
Referring to
In the embodiment depicted in
Organic polymer 1040 may be within crossbar junction region 1030 and in contact with both first conductor 1010 and second conductor 1020. Although
An exemplary embodiment of the present crossbar junction was fabricated. In this embodiment, width 1011 of first conductor 1010 and width 1021 of second conductor 1020 were each 20 μm. Separation distance 1050 was 200 nm. To form organic polymer 1040, polypyrrole (PPy) thin films were electrochemically grown across crossbar junction region 1030 from the bottom of second conductor 1020 to the top of first conductor 1010 using an aqueous solution of freshly distilled pyrrole monomer (100 mM) and an electrolyte (100 mM, NaDBS) at a constant potential of +0.65 V vs. Ag/AgCl. A polymer in the form of PPy+DBS− was synthesized in an oxidized state (e.g., a P-doped conducting state). The thickness of the thin films was controlled by passing specific amount of charge (200 mC/cm2 for 1 μm thick film) during the electrochemical deposition. The oxidized film was then reduced in an electrolyte LiClO4 through incorporating small Li cations into the polymer for balancing the charges of DBS anions, which changes the conductivity of the polymer into a semiconducting/insulting state. Thus, the polymer composite in the form of PPy0(DBS-Li+) in a charge neutral state was created. For PPy composite junctions in this embodiment, all electrochemical experiments were performed using a CHI660 electrochemical workstation (CH Instruments), and their electrical transport were characterized under a nitrogen atmosphere with CHI660 electrochemical workstation (CH Instruments) or a Hewlett Packard 4145A semiconductor parameter analyzer. Other embodiments may employ different materials in different dimensions.
The current-voltage characteristics of crossbar device 1000 were investigated. Referring to
Further increasing first potential 1060 drove the conductance state into a regime (FGCC region 1130) where the field generated charge carriers (FGCC) began to make a contribution to total current. The FGCC was produced because the drift of mobile Li+ ions (under the external field and the space charge induced field) left behind the immobile anions (DBS) stabilized region in a high conducting state. As a result, the configuration of internal ions species was changed and caused reduction of the effective conductance path L that the space charge limited current flowed through, thus giving rise to the FGCC current.
The FGCC current was evidenced by measuring the time dependence of current 1070 flowing through crossbar junction region 1030. Referring to
The time dependent conductance related to the field driven ions redistribution was also observed with further measurements as shown in
For cases where a very short field-free time was implemented before reversing the applied field, the cations could not return to their original equilibrium positions after the field was removed. Therefore, the internal configuration of charges and carriers that was established under the forward field did not change significantly. In contrast, for cases where the field-free time was more than 1 second, the ion distribution that was established under the forward field returned to its initial equilibrium state, and the current under the reversed field behaved the same as when the forward field was applied at the first time.
As experimentally observed above, the conductance state of crossbar junction region 1030 can be determined by the internal configuration of charges and carriers that are pre-established under the field. This can produce useful transient current behaviors that can be controlled by reversing the field relative to the previously applied field. Referring to
It will be recognized by one of skill in the art that a sensor array may also be implemented using the crossbar devices and the methodology described above, where first potential 1060 represents an input to a crossbar device sensor element, and second potential 1061 is applied to “read” the state of the sensor element that results from the input.
Referring to
Embodiments of the present disclosure may also find application in reconfigurable electronic devices of a more general purpose nature. For example tuning the characteristics of transistors for analog electronic applications where device characteristics must be matched in order to achieve certain levels of performance.
All of the compositions and methods disclosed and claimed herein can be made and executed without undue experimentation in light of the present disclosure. While the compositions and methods of this invention have been described in terms of preferred embodiments, it will be apparent to those of skill in the art that variations may be applied to the compositions and methods and in the steps or in the sequence of steps of the method described herein without departing from the concept, spirit and scope of the invention. More specifically, it will be apparent that certain agents which are both chemically and physiologically related may be substituted for the agents described herein while the same or similar results would be achieved. All such similar substitutes and modifications apparent to those skilled in the art are deemed to be within the spirit, scope and concept of the invention as defined by the appended claims.
The appended claims are not to be interpreted as including means-plus-function limitations, unless such a limitation is explicitly recited in a given claim using the phrase(s) “means for” and/or “step for,” respectively.
The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference.
This application claims priority to U.S. Provisional Patent Application Ser. No. 60/940,478, filed May 29, 2007, which is incorporated by reference.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2008/003470 | 5/28/2008 | WO | 00 | 5/14/2010 |
Number | Date | Country | |
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60940478 | May 2007 | US |