AN ENHANCED OPTICALLY SENSITIVE MEDIUM USING ORGANIC CHARGE TRANSFER MATERIALS TO PROVIDE REPRODUCIBLE THERMAL/OPTICAL ERASURE

Abstract: An enhanced optically sensitive medium is disclosed which uses an organic charge transfer complex as the switching material. The organic charge transfer complex includes at least one moiety in an oxidized state. An enhancement mechanism is taught which provides a secon­ dary source of natural molecules of said at least one constituent moiety in the neutral or altered oxidation state. For example, with CuTCNQ used as the switching material, the erasing characteristics can be improved by using a covering dispersion having a matrix polymer interspersed with neutral molecules of TCNQ°.

Description: BACKGROUND AND/OR ENVIRONMENT OF THE INVENTION 1. Field of the Invention The present invention pertains generally to the use of organic charge transfer materials to produce an optically sensitive medium; and, more particularly, various enhancement mechanisms for providing improved thermal/optical erasure characteristics. 2. Description of the Contemporary and/or Prior Art. With the advent of the information revolution, recent research activities have focused on developing optical storage medium. Currently, optical memory devices rely on photochemical hole burning (PHB) in which a laser pits the material in an effort to store data. An article entitled "Laser Marking of a Thin Organic Film" by J. J. Wrobel et al, Applied Physics Letter 40, (11), 1 June 1982, describes such a technique using a laser beam to burn holes in a thin organic film. Similarly, optical writing on blue, sputtered iridium oxide films is reported by Mabosch et al in Applied Physics Letter 41, 1 July 1982. This technique uses an optical writing mechanism to thermally induce dehydration at temperatures below the melting point of the optical medium. A patent issued to Ovshinsky (U.S. Patent 3,983,542) discloses a material which undergoes a physical change from an amorphous to a crystalline state when exposed to a light beam. An article entitled "Light-induced Phenomena in Dye-polymer Systems" by V. Novotny et al, The Journal of Applied Physics 50 (3), March 1979, describes an optical marking process based on diffusion in a dye-polymer system. The prior art optical storage systems generally have one overriding disadvantage - prior art optical media is not erasable. As a result, optical storage technology has found little application in computer technology, which requires both read, write and erase functions. The following U.S. patents by R.S. Potember, T.O. Poehler and D.O. Cowan, disclose a class of organic charge transfer salts, such as CuTCNQ, which exhibits stable and reproducible switching between an equilibrium, or first state, and a second state, in the presence of an applied electrical field: (1) "Current Controlled Bistable Electrical Organic Thin Film Switching Device (TCNQ)", issued December 1, 1983, U.S. Patent 4,371,883; and, (2) "Method of Fabricating a Current Control Bistable Electrical Organic Thin Film Switching Device (TCNQ)", filed June 7, 1982,U.S.Patent 4,507,672. The organic charge transfer salts will undergo a reversible electrochemical topotactic redox reaction in the presence of an applied electric field, thereby switching from a first state to a second state. A detectably different impedance occurs between the equilibrium, or first state, and the switched and second state. In specific, an electrical field is applied across a thin film of CuTCNQ, or an equivalent organic charge transfer salt. When the applied electrical field exceeds a threshold value, the impedance across the thin organic film will drop from a relatively high impedance to a relatively low impedance. Two papers written by R. S. Potember et al report that when the organic film is electrically switched, the second state has different optical properties from the equilibrium or first state; (1) "The Vibrational and X-ray Photoelectron Spectra of Semiconducting Copper-TCNQ Films" Chemica Scripta , Vol.17, 219-221 (1981); and, (2) "Electrical Switching and Memory Phenomena in Semiconducting Organic Thin Films" American Chemical Society Symposium Series No. 184 (1982). These articles describe infrared spectroscopic means and reference well-known Raman spectroscopic techniques (S. Matsuzaki et al, "Raman Spectra of Conducting TCNQ Salts" Solid State Communications , Vol. 33, pp. 403-405, 1980) for determining if the CuTCNQ film, switched by an AC or DC electric field is in the first or second state. Follow-up work reported by E.I. Kamitsos et al used Raman spectroscopic techniques to verify the electrochemical charge transfer equation described in the above-referenced articles which cause the CuTCNQ salt to switch from the first to second state: "Raman Study of the Mechanism of Electrical Switching in CuTCNQ films" Solid State Communications , Vol. 42, No. 8, pp. 561-565 (1982). The above articles point out that spectroscopic means can be used to discern whether an area of CuTCNQ switched by an applied electrical field is in the first or second state. Patent 4,574,366 filed 2/7/83 by R.S. Potember, T.O. Poehler and R.C. Benson and entitled "Optical Storage and Switching Devices Using Organic Charge Transfer Salts" teaches that in the presence of optical energy the organic charge transfer salt will undergo a redox reaction and switch from a first to a second state; each state having a detectably different optical spectrum. Spectroscopic means can optically determine if a particular spot on the storage medium is in the first (neutral) or second (switched) state. The application further teaches that thermal energy can reverse the electrochemical reaction and return the material to the first (neutral) state. The above application thus teaches an operable optically sensitive medium which can be switched by an optical beam and erased either in sections or on a bit by bit basis. A paper by R.S. Potember, T.O. Poehler and R.C. Benson entitled "Optical Switching in Semiconductor Organic Thin Films" Applied Physics Letters , Vol. 41 (6), 15 September 1982, and a paper by E.I. Kamitos and W.M. Risen, Jr. entitled "Optically Induced Transformations of Metal TCNQ Materials" Solid State Communications , Vol. 45, No. 2, 1983, discuss the optical switching characteristics of organic charge transfer salts. A U.S. patent application by R. S. Potember and T. O. Poehler entitled "Multistate Optical Switching and Memory Using an Amphoteric Organic Charge Transfer Material" 4,663,270, filed 4/25/84) teaches that organic charge transfer materials can be switched into a plurality of states - making multiple bit storage possible at each spot on an optical storage medium. Although the above references teach the use of organic charge transfer materials in an optically sensitive medium, there has been the need to enhance reproducibility of the erasure process. For an erasable optical storage disc to meet the needs of the current data processing industry, 10²-10⁶ cycles of bit by bit erasure are desirable. SUMMARY OF THE INVENTION Applicants have discovered that the erasure characteristics of the organic charge transfer salt (switching materials) formed from a donor complexed with an acceptor can be improved by a second group of neutral molecules of said donor or said acceptor. Placing such neutral molecules in close proximity to the organic charge transfer complex in the switched state enhances thermal recombination of the charge transfer complex. For example, if an organic charge transfer salt such as CuTCNQ is the switching material, the present invention would provide a secondary source of neutral acceptor TCNQ° molecules. The present invention also teaches the use of a complexing or chelating agent for increasing the compatibility of various constituent moieties in their neutral or altered oxidation states and enhances thermal/optical recombination. For example, the complexing agent helps solubilize the ion-radical salt of CuTCNQ to increase its compatibility with the additional constituent neutral acceptor molecules TCNQ°. In a first embodiment of the invention, a thin film of the organic charge transfer switching material, deposited on a supporting substrate, is covered with a polymer dispersion containing neutral moiety molecules (e.g., neutral acceptor molecules) in an optional complexing agent. The neutral acceptor molecules and complexing agent are interspersed within a polymer matrix forming a solid transparent covering layer. At the interface between the polymer dispersion and the film of organic charge transfer material, enhanced thermal erasure is observed with the application of a beam of light used to generate thermal energy. In a second embodiment of the invented enhancement mechanism, a polymer dispersion is interspersed with both the organic charge transfer switching material (e.g. microcrystals of an organic charge transfer salt, CuTCNQ) and the enhancement material (e.g., neutral constituent acceptor molecules, TCNQ°). Optionally, the polymer dispersion can also be interspersed with a complexing agent to enhance the compatibility between moieties in the switched state. A thin film of the polymer dispersion can be coated onto a supporting base or the polymer dispersion can be molded into the appropriate form (i.e., disc, records, etc.) with the polymer dispersion acting as a self-supporting base material. Various applications of the enhancement mechanism, as taught by the present invention, can improve the erasure characteristics of the switching material. Various organic charge transfer materials, including both bistate and multistate amphoteric charge transfer materials can be used. Molecules of a moiety with a neutral or altered oxidation state can be donor atoms, and/or neutral acceptor molecules, if a bistate switching material is used, or one or more neutral, reduced or oxidized constituent moieties of an amphoteric compound if a multistate switching material is used. Accordingly, it is one object of the present invention to provide an enhancement mechanism which improves the erasure characteristics of organic charge transfer switching materials. The enhancement mechanism provides a secondary source of neutral molecules of at least one moiety of the switching material in a neutral or altered oxidation state thereby enhancing thermal recombination. A second object of the present invention is the use of a complexing or chelating agent to increase the compatibility of the various constituent moieties in the switched state (for example, the complexing agent helps to solubilize the ion-radical salt to increase its compatibility with neutral acceptor moiety molecules). A third objective of the present invention is the use of a polymer matrix as a cover layer to coat the thin film of the organic charge transfer material. The cover layer is a polymer matrix interspersed with the enhancing material (for example, neutral molecules of the acceptor moiety) and interspersed with an optional complexing agent. A fourth object of the present invention is the use of a polymeric dispersion. The polymeric dispersion is a polymer matrix interspersed with microcrystals (or powdered form) of the organic charge transfer switching material and interspersed with molecules or atoms of a constituent moiety in a neutral or other altered oxidation state and with on optional complexing agent (For example, if the organic switching material is CuTCNQ, the enhancement material would be neutral acceptor molecules, TCNQ°). A fifth object of the present invention is the use of polymeric encapsulation to protect the organic charge transfer switching material and to reduce migration or sublimation of constituent moieties (for example, donor and acceptor species) during the switching processes. A sixth object of the present invention is the use of a free standing switching polymeric dispersion which simplifies construction and reduces the cost of fabricating the optical switching medium. The above-mentioned objectives, as well as other objectives of the present invention, will become readily apparent after reading the ensuing description of several non-limiting illustrative embodiments and viewing the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph of the Raman spectral bands for bistate CuTCNQ; Figure 1a shows the spectral bands for neutral TCNQ; Figure 1b shows the spectral bands for CuTCNQ in the unswitched state; Figure 1c shows the spectral bands for CuTCNQ in a second or switched state. Figure 2 is a generalized schematic of the multiple layer enhancement embodiment as taught by the present invention. Figure 3 is a generalized schematic of the single enhanced layer switching dispersion as taught by the present invention. Figure 4 is a generalized schematic of an apparatus used to read/write/erase the enhanced switching material. Figure 5 is a black and white photograph showing sixteen optically switched locations on the enhanced switching material. Figure 6 is a black and white photograph of the same film shown in Figure 5 in which the sixteen switched locations have been optically erased; Figure 6 shows the improved erasure characteristics of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT To understand the enhancement mechanism disclosed herein it is necessary to understand some of the principles of optical switching as described in the co-pending U.S. application entitled "Optical Storage and Switching Devices using Optical Charge Transfer Salts" which is incorporated herein by reference. The redox reaction which occurs when a bistable organic charge transfer salt, such as CuTCNQ, is illuminated with a beam, having sufficient optical field strength, and switched from a first to a second state, is shown below: It is believed that switching occurs because the optical beam (i.e., an electromagnetic field at optical frequencies) causes the bond between the organic electron acceptor (in this case TCNQ) and the donor (represented by M) to break, allowing a charge to transfer from the donor to the organic electron acceptor. The foregoing Equation (1) clearly shows a change in charge distribution as the organic salt switches from a first to a second state. In the first, or base state, the organic electron acceptor moiety is found almost exclusively in the reduced form (TCNQ ). However, in the second, or switched state, the organic electron acceptor moiety is found in both its reduced (TCNQ ) and neutral (TCNQ°) forms. It will be noted from Equation (1) that each state is associated with a unique set of redox species. In the first state the acceptor moiety is found almost exclusively in the reduced form (e.g., only the TCNQ species is present); whereas, in the switched, or second state, the acceptor moiety exists in both the reduced and neutral form (e.g., TCNQ and TCNQ° redox species are present). Visual, spectroscopic, fluorescent and/or other optical means can be used to determine whether the bistate charge transfer salt is in the first or second state by identifying optical characteristics associated with each redox species. Figure 1 shows the Raman spectral bands for the organic charge transfer salt CuTCNQ: Figure 1a shows the spectral bands for neutral TCNQ (e.g., TCNQ°) - note the Raman band (10) at 1451 cm⁻¹; Figure 1b shows the spectral bands for CuTCNQ in the first (unswitched) state where essentially all the organic electron acceptor moiety is in a reduced form (e.g., only the TCNQ redox species is present) - note the Raman band (12) at 1375 cm⁻¹ associated with the TCNQ species; Figure 1c shows the spectral band for CuTCNQ in the second, or switch state, where the organic electron acceptor moiety is in both the reduced and neutral oxidation states (e.g., both the TCNQ and TCNQ° species are present) - note the appearance of Raman bands at 1375 cm⁻¹ (10) and 1451 cm⁻¹ (12). The Raman spectral bands are fundamental absorption modes which are sensitive to the oxidation state of the organic electron acceptor moiety, thereby uniquely identifying the redox species present (i.e., in our example, determining if the TCNQ or TCNQ° redox species are present). When CuTCNQ is used as the organic charge transfer salt one need only analyze the 1451 cm⁻¹ spectral band (10) to determine if the organic salt is in the first or second state. If the CuTCNQ organic charge transfer salt is in the first state, the spectral intensity is low at 1451 cm⁻¹; if the organic charge transfer salt is in the second state, the spectral intensity is high at 1451 cm⁻¹ (10). A number of organic charge transfer salts will switch from the first to second states, as above-described, when illuminated by an optical beam. Various TCNQ derivatives, when complexed with a metal donor, will form an organic charge transfer salt capable of optical switching. Examples of these TCNQ derivatives are shown in the following table: TCNQ(OMe) TCNQ IMe TCNQ(OMe)₂ TCNQI TCNQ(OMe)(OEt) TCNQ(OMe)(OCH₃)₂ TCNQ(OMe)(O-i-Pr) (TCNQ(CN)₂ TCNQ(OMe)(O-i-Bu) (TCNQ(Me) TCNQ(O-i-C₂H₅) TCNQ(Et) TCNQ(OEt)(SMe) TCNQ(i-Pr) TCNQ C1 TCNQ(i-Pr)₂ TCNQ Br TCNQ(i-Pr) TCNQ C1Me TCNQ(i-Pr)₂ TCNQ Br Me Further, if an organic electron acceptor containing at least one cyanomethylene functional group is complexed with a donor moiety to form an organic salt, the organic salt will have memory and switching capabilities. Also, if an organic electron acceptor containing at least one quinoline unit is complexed with a donor moiety to form an organic salt, that organic salt will also have memory and switch capabilities. In specific, if an organic salt is formed from the following organic electron acceptors, the organic salt will be capable of optical memory and/or switching: tetracyanoquinodimethane (TCNQ), tetracyanonapthoquinodimethane (TNAP), tetracyanoethylene (TCNE), and 2, 3 dichloro-5,6 dicyano-1, 4 benzoquinone (DDQ), hexacyanobutalene (HCBD), and 11, 11, 12, 12-tetracyano-1.4 naphthoquinodimethane (benzo TCNQ), and 2,5-bis (dicyanomethylene)-2, 5-dihydrothiophene, and 2,5-bis (dicyanomethylene)-2, 5-selenophene, and thiophene-(T)-TCNQ, and (selenophene-(Se)-TCNQ) and tetracyano-quinoquina-zolinoquinazoline (TCQQ) and hexacyanotrimethylene cyclopropane (HMCTMCP) and 2,4-bis (dicyanomethylene)-1,3-dithietan (BDDT), and any of the TCNQ derivatives shown in the above table. If the following metals are complexed with the above-referenced organic electron acceptor to form an organic salt, the organic salt will switch optically: copper, silver, lead, nickel, lithium, sodium, potassium, barium, chrome, molydenum, tungsten, cobalt, iron, antimony, cesium and magnesium. In addition, the following organic substances can also act as donors, and if complexed with an organic electron acceptor to form an organic salt, the organic salt will be capable of optical memory and/or switching: tetrathioethylenes, dithiodinaminoethylenes, dithiodiselenoethylenes, tetraminoethylenes, azenes, aromatic hydrocyclics. It is to be understood that other organic transfer salts formed with organic electron acceptors having either cyanomethylene functional groups or quinoline units and other organic salts having similar characteristics, may be found with switch from the above-referenced first state to second state in the presence optical radiation. Referring again to Equation (1), it can be seen that the electrochemical reaction is thermally erasable. Applying heat energy to an area of the organic charge transfer salt causes neutral molecules of the donor and electron acceptor found in the switched state to combine chemically and return to the first state. The reversible reaction, which is a thermally activated process, returns the material to its lower energy state. The applied heat can be generated by electrical or optical/thermal means. In a preferred embodiment the thermal energy is generated by a laser. It was found that a CO₂ laser, with an intensity below the switching threshold , can be focused on a location for a period of time sufficient to generate enough thermal energy to transfer that location to the first or neutral state. The present invention is directed to a means for enhancing the erasure characteristics of the optically sensitive organic charge transfer medium. This invention produces an optical sensitive medium that can be repeatedly switched and erased. The enhancement mechanism provides a secondary source of neutral molecules of the electron acceptor and/or donor constituents of the organic charge transfer salt, thereby improving the speed and reproducibility of erasure. Two generalized embodiments of this invention utilize this enhancement mechanism. MULTIPLE LAYER ENHANCEMENT EMBODIMENT The multiple layer embodiment (shown in Figure 2) involves fabricating a thin film 14 of the organic charge transfer salt onto supporting substrate (16) and then covering or coating this film with a polymer dispersion (18) containing additional neutral acceptor molecules in an optional complexing agent. Techniques for complexing the donor and acceptor moiety to form the organic salt, and techniques for forming the film, have been described in the art (see generally, the following Potember et al applications "Optical Storage and Switching Devices using Organic Charge Transfer Salts", and, "Method of Fabricating a Current Controlled Bistable Electrical Organic Thin Film Switching Device" , both incorporated herein and by reference). Various additional techniques exist for sputtering, evaporating, or coating a film on a substrate which are known in the chemical art. The thin covering layer, containing the enhancement mechanism, is produced by first mixing neutral acceptor molecules (the same type of acceptor molecules used to complex the particular organic salt) in a thermally stable polymer matrix to form a dispersion. The dispersion is then coated onto the film of organic salt to form a thin covering layer. The covering layer forms a relatively transparent polymer matrix interspersed with neutral molecules of the acceptor compound. At the interface between the polymer dispersion and the film of organic charge transfer material, enhanced thermal erasure is observed with the application of a beam of light generating thermal energy. This enhanced erasure occurs because excess neutral acceptor molecules, such as TCNQ°, in close proximity to the neutral donor ions can recombined readily to form the original charge transfer complex. The erasure characteristics of the optically sensitive medium can be further enhanced if a complexing or chelating agent is also interspersed in the covering layer. To prepare this version of the covering layer 18, neutral acceptor molecules are mixed with a complexing or chelating agent and with a thermally stable polymer matrix to form a dispersion. This dispersion is then coated on the film 14 of organic charge transfer complex. The effect of the complexing agent is to increase the compatibility of the neutral acceptor molecules interspersed in the polymer matrix with the donor atoms found in switched portions of the organic charge transfer film. Other versions of the covering layer 18 are also envisioned. Instead of interspersing neutral acceptor molecules in the dispersion, neutral donor atoms or molecules can be interspersed in the dispersion. Alternatively, the dispersion can be interspersed or mixed with neutral molecules of both the donor and acceptor moieties. Furthermore, the polymer matrix should be capable of passing optical energy through the covering the covering layer 18 to the charge transfer film 14 where switching occurs. Applicants prefer to use a transparent thermally stable polymer matrix. The process of coating the organic charge transfer film with the covering layer 18, can involve spin casting, brushing, ultraviolet curing spraying or dry coating. Alternatively, the dispersion can be molded (by injection molding or other techniques) into a thin layer. This thin layer is then pressed onto the organic charge transfer film 14 where it bonds to form a secured covering layer. Further, to aid in the coating process, the dispersion also can contain a solvent. After the dispersion is coated onto the organic charge transfer film, the solvent is evaporated to form a solid covering layer. Again it will be noted that the covering layer 18 performs the following functions: (1) excess neutral acceptor molecules interspersed, or mixed, in the polymer matrix provide a secondary source of molecules to combine with the donor atoms or molecules during the erasure step; (2) the complexing agent enhances compatibility between neutral acceptor molecules interspersed into the dispersions and donor atoms or molecules found in the switched portions of the organic charge transfer film; and, (3) the polymer overcoating also acts to protect the organic charge transfer film and to reduce migration or sublimation of donor or acceptor species during the switching process. One example of the multiple layer enhancement embodiment is given by way of explanation and is not meant to limit this invention. Approximately 40 mg of neutral TCNQ (the neutral acceptor molecule) is added to a slurry containing 500 mg of polymethylmethacralete (the polymer matrix) and 250 mg polyethylene oxide (the complexing agent) in 200 ml of dichloroethane (a solvent). The solution is then stirred until a uniformly clear slurry is obtained. The dispersion is then solution cast or deposited onto a substrate containing a thin film of Cu-TCNQ switching material. The polymer matrix dispersed, which is interspersed with excess neutral TCNQ molecules forms a transparent flexible film over the switching material. The solvent (dichloroethane) is evaporated from the polymeric dispersion to form a solid film. A variety of matrix polymers can be used including, but not limited to: Polycarbonates Polymethylmethacrylate (PMMA) Polyacrylonitrile Polyvinylacetal Poly(methyl-2-cyanoacrylate) Poly(4,4'isopropylidine-diphenylcarbonate)(Lexan) Polystyrene Poly(pyromellitimide)(Kaptan) Poly(vinylfluoride) Ethylcellulose Poly(ethylacrylate) Poly(methylacrylate) and mixtures of the above A variety of complexing or chelating agents can be used including, but not limited to: Acetylacetone and its derivatives Polyethylene oxide (Carbowax) Polypropylene glycol Polymeric quanternary ammonium salts Crown ethers Poly(butyleneglycol) Poly(epichlorohydrin) Ethylenediaminetetraacetic acid (EDTA) Nitrilotriacetic acid (NTA) Dimethyglyoxime (DMG) The organic charge transfer salt can be complexed from the non-limiting family of acceptor and donor materials discussed earlier in this specification; and, molecules of the acceptor or donor (of the type complex to form the organic salt) can be interspersed in the matrix polymer. SINGLE ENHANCED LAYER SWITCHING MATERIAL A second embodiment of the invention, shown in Figure 3, involves mixing both the organic charge transfer salt (as a powder or microcrystals) with additional neutral acceptor molecules (the same type of acceptor molecules used to complex the organic salt), and with a matrix polymer to form a single polymer dispersion. The polymer dispersion 20 forms a single layer which is interspersed with both the switching material (the organic charge transfer salt) and the enhancing material 24 (neutral acceptor molecules). A thin film of the polymer dispersion can be coated onto a supporting base or the polymer dispersion can be molded into the appropriate form (i.e., discs, records, etc.) with the polymer dispersion acting as a self-supporting base material. With the polymer dispersion acting to form a self-supporting base material, the need to use complicated vacuum techniques to produce the optical recording medium can be eliminated. Erasure characteristics are enhanced by the availability of a second source of neutral acceptor molecules interspersed within the matrix polymer in close proximity to donor atoms generated locally by the optically induced electrochemical switching reaction. Thermal energy reverses the reaction allowing the additional neutral acceptor molecules or acceptor molecules previously combined in the organic salt to recombine with donor atoms or molecules into a lower energy state. The erasure characteristics can be further enhanced if a complexing or chelating agent 25 is also interspersed within the polymer dispersion. To prepare this version, the switching material (powder or microcrystals of the organic charge transfer salt), the enhancing material (neutral molecules of the organic acceptor) and the complexing agent are mixed with the polymeric matrix to form a single polymer dispersion. The polymer dispersion is then coated on a substrate or molded to form the optical recording disc, record, etc. The complexing agent enhances the homogeneity of the dispersion by reducing the microcrystalization of neutral acceptor molecules. The complexing agent also helps to solubilize the ion-radical salt to increase its compatibility with the neutral acceptor molecules. Both of these factors increase the thermal erasure characteristics of the material. Other versions of the single polymer dispersion embodiment are also envisioned. Instead of interspersing neutral acceptor molecules in the polymer dispersion as the enhancing material, neutral donor atoms or molecules can be interspersed. Alternatively, the polymer dispersion can be interspersed or mixed with neutral molecules of both donor and acceptor moieties. Furthermore, the polymer matrix should be capable of passing optical energy. Applicants prefer to use a transparent thermally stable polymer matrix. The process of coating a base substrate with the polymer dispersion can involve spin casting, brushing, spraying or dry coating. Alternatively, the polymer dispersion can be molded (by injection molding or other techniques) into a thin layer. The thin layer is then pressed onto the substrate. Further, to aid the coating process, the polymer dispersion can also contain a solvent; after the dispersion is coated onto the substrate the solvent is evaporated. If a simply constructed non-erasable disc is desired, one can mix a powder or microcrystals of the organic transfer salt into a polymer matrix to form a switching dispersion. The switching dispersion can be coated onto a base substrate or separately molded with the dispersion forming a self-supporting substrate. One example of the single enhanced layer switching material is given by way of explanation and is not meant to limit this invention. Approximately 500 mg of CuTCNQ powder is mixed into a slurry containing 50 mg of neutral TCNQ, 500 mg polymethylmetacrylate and 250 mg polyethylene oxide in 200 ml of dichloroethane. The solution is then stirred until a uniformly clear slurry is obtained. The composition is then (1) cast or (2) deposited on to smooth substrate material. The polymer matrix containing neutral TCNQ and the CuTCNQ salt forms a transparent flexible film once the solvent has evaporated from the polymeric composition. Generally a wide variety of matrix polymers and complexing agents can be used with the present invention. In specific, the matrix polymers and complexing agents listed with the multiple layer enhancement embodiment can work equally well with this embodiment. Similarly the organic charge transfer salt used with this embodiment can be complexed from the non-limiting family of acceptor and donor materials discussed earlier in this specification; and, molecules of the acceptor or donor (of the type complexed to form the organic salt) can be interspersed in the matrix polymer. Figure 4 is a schematic diagram of the optical storage system using either embodiment of the enhanced optically sensitive material. An optical write beam 26 is focused onto a specific location on the surface of the organic charge transfer film 20. The optical beam can be a high intensity light source or a laser source such as an Argon, CO₂, or solid state laser focused to generate an optical field on the film surface. For memory system, an optical beam with sufficient intensity can be directed by a well known optical means to other locations on the surface of the organic charge transfer salt 20, and can switch such other locations from the first to second state. By turning the beam "ON" or "OFF" at a specific location a "1" or "0" logic state can be stored in the optical storage medium. Alternatively, if a higher optical intensity or longer duration is used, the optical beam 26 can be directed by well known optical means to "draw" a visible on the organic charge transfer salt 20. Once the data has been stored on the enhanced optical storage charge transfer medium, a spectroscopic means can be used to determine if a particular location on the organic film is in its first or second state. The light source 32 can also generate a lower intensity reading optical beam which is directed to illuminate one of the locations 28 with an intensity well below the switching intensity. Alternatively, a separate monochromatic or laser light source could be used. Reflected light 34 from the selected location on the film surface 28 is collected and filtered by optical filter 36 and then passed to a means for measuring the spectral intensity. The spectral measuring means 38 would indicate if the reflected beam 34 pass through filter 36 correspondent to material in the switched or unswitched states. The reading optical beam can be directed by well known optical means to each of the plurality of locations (i.e., 28, 30) on the surface of the enhanced organic charge transfer storage medium to determine if that particular location is in the first or second state, that is to say, if that particular location is storing a "1" or a "0". A thermal erase means which is used to erase the topotactic redox reaction and cause at least one of the plurality of locations on the surface of the enhanced organic charge transfer medium to return to the first or neutral state. Applicants have found that a laser, with an intensity below the switching threshold, can be focused on location 28 for a time period long enough to generate sufficient thermal energy to switch that location back to the first state. Alternatively, the optical heating beam can be generated by the same laser source 32 with an intensity below the optical switching intensity and can be directed by well known optical means to erase other locations on the organic charge transfer salt surface 20. Applicants have found that a variety of laser sources can be used to switch, read, and erase the enhanced optical storage medium. Figure 5 and 6 are two photographs taken in series showing this optical recording and erasure procedure. The first photograph (Figure 5) shows sixteen high contrast spots produced on a blue/black polycrystalline CuTCNQ film overlaid with a transparent polymer matrix interspersed with excess acceptor molecules. The spots were made with an Argon Ion Laser operating at 488 nm. The second photograph (Figure 6) shows the same region of the film after an optically generated heat source has been used to chemically recombine the copper metal with the neutral acceptor molecules. Note that in the second photograph the high contrast spots associated with the formation microscopic amounts of neutral TCNQ have disappeared. Both embodiments of the present invention can work equally well with the bistate switching material, discussed thus far, and the multi-state optical switching material as taught in a U.S. patent application by R.S. Potember and T. O. Poehler entitled "Multistate Optical Switching and Memory Using an Amphoteric Organic Charge Transfer Material" (4,663,270) and incorporated herein by reference. In that patent application the organic switching film can be composed of a plurality of bi-state organic switching salts (which may be linked by a chemical chain) or composed of a single amphoteric organic charge transfer complex. With either multi-state switching film there exists a plurality of acceptor and donor moieties. The enhancement mechanism as taught by the present invention can work with such multi-state switching film by having a secondary source of species of one or more moiety in either a neutral or altered oxidation state interspersed in the polymer dispersion. The acceptor or donor moiety may itself exist as a separate molecule or may be chemically attached to another material. Procedures for fabricating the polymer dispersion would be the same as taught in this specification. For the multiple layer embodiment, the covering layer would have species of at least one moiety in the neutral or altered oxidation state (with the optional complexing agent) interspersed in the matrix polymer. For the single enhanced layer embodiment the polymer dispersion would include a polymer matrix interspersed with microcrystals (or powdered microcrystals) of the multi-state switching material and also interspersed with species of one or more moieties in the neutral or altered oxidation state. For a multistate non-erasable disc, a switching dispersion can be made from at least two organic charge transfer salts interspersed in a polymer matrix. The switching dispersion can be coated onto a base substrate or separately molded with the switching dispersion forming a self-supporting substrate. Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Claims: An optically sensitive medium comprising: at least one organic charge transfer salt formed from a donor complexed with an acceptor, each organic charge transfer salt adapted to switch between at least two different oxidation states in response to the application of optical energy, each oxidation state having discernably different optical characteristics characterized in that it further contains an erasing enhancer containing neutral molecules of said donor or said acceptor in close proximity to the charge transfer salt to enhance the thermal recombination of the charge transfer complex. Medium according to one of the preceeding claims characterized in that the constituent moiety is an oxidized species of said organic electron acceptor moiety of the charge transfer salt. Medium according to claims 1 and 2 characterized in that the constituent moiety is a reduced species of the donor moiety of said charge transfer salt. A medium according to one of the preceeding claims characterized in that the erasing enhancer is in the form of a layer comprising a polymer dispersion interspersed with the neutral molecules, said layer covering at least a portion of the organic charge transfer salt. Medium according to one of the preceeding claims characterized in that the organic charge transfer salt is interspersed in a polymer dispersion. Medium according to one of the preceeding claims characterized in that the polymer dispersion is at least partially transparent to optical energy. Medium according to one of the preceeding claims wherein the dispersion contains a complexing agent. Medium according to claim 7 characterized in that the complexing agent is selected from the group consisting of: Acetylacetone and its derivatives Polyethylene oxide (Carbowax) Polypropylene glycol Polymeric quanternary ammonium salts Crown ethers Poly(butyleneglycol) Poly(epichlorohydrin) Ethylenediaminetetraacetic acid (EDTA) Nitrilotriacetic acid (NTA) Dimethyglyoxime (DMG). Medium according to one of the preceeding claims wherein the polymer is selected from the group consisting of: Polycarbonates Polymethylmethacrylate (PMMA) Polyacrylonitrile Polyvinylacetal Poly(methyl-2-cyanocrylate) Poly(4,4'isopropylidine-diphenylcarbonate) (Lexan) Polystyrene Poly(pyromellitimide) (Kaptan) Poly(vinylfluoride) Ethylcellulose Poly(ethylacrylate) Poly(methylacrylate). Medium according to one of the preceeding claims characterized in that it contains only one type of charge transfer salt and wherein the erasing enhancer comprises a neutral molecules of the donor and/or acceptor of said charge transfer salt. Medium according to one of the claims 1 to 9 characterized in that the charge transfer salt is an amphoteric charge transfer salt having more than one redox state and wherein the erasing enhancer comprises one or more molecules having at least one or more altered oxidation states. Medium according to one of the preceeding claims characterized in that in the form of the wafer wherein the supporting substrate is the base layer, the organic charge transfer complex is contained in a center layer and the polymer dispersion interspersed with the erasing enhancer forms an upper layer. Medium according to one of the preceeding claims characterized in that the organic charge transfer salt is in the form of microcrystals. A process for preparing an optically sensitive medium comprising the steps of depositing an organic charge transfer salt formed from a donor complexed with an acceptor onto a substrate and depositing further onto said organic charge transfer salt an erasing enhancer containing neutral molecules of said donor and/or said acceptor. A process according to claim 14 comprising the steps of mixing the organic charge transfer salt with the erasing enhancer and depositing said mixture onto a substrate. A process according to one of the claims 14 and 15 characterized in that the charge transfer salt and the erasing enhancer deposited in form of a thin film. A process according to one of the claims 14 to 16 wherein the depositing step involves spin casting, brushing, spraying and/or dry coating. A process according to claims 14 to 17 wherein the formation of a thin sheet involves a step of pressing and/or molding. A process according to anyone of the preceeding claims characterized in that it includes a step of adding a complexing agent selected from the group consisting of: Acetylacetone and its derivatives Polyethylene oxide (Carbowax) Polypropylene glycol Polymeric quanternary ammonium salts Crown ethers Poly(butyleneglycol) Poly(epichlorohydrin) Ehtylenediamimetetraacetic acid (EDTA) Nitrilotriacetic acid (NTA) Dimethyglyoxime (DMG). A process according to one of the preceeding claims characterized by the addition of a polymer selected from the group consisting of: Polycarbonates Polymethylmethacrylate (PMMA) Polyacrylonitrile Polyvinylacetal Poly(methyl-2-cyanocrylate) Poly(4,4'isopropylidine-diphenylcarbonate) (Lexan) Polystyrene Poly(pyromellitimide) (Kaptan) Poly(vinylfluoride) Ethylcellulose Poly(ethylacrylate) Poly(methylacrylate).

Inventors: POEHLER, THEODORE OTTO, POTEMBER, RICHARD STANLEY
Applicants: THE JOHNS HOPKINS UNIVERSITY
Application year: 1985
Main classification: G11B7
IPC codes: G03C1, G11B7