Polymer based memristors
10538618 ยท 2020-01-21
Assignee
Inventors
- Joe B. Gilroy (London, CA)
- Giovanni Fanchini (London, CA)
- Joseph A. Paquette (Blind River, CA)
- Sabastine C. Ezugwu (London, CA)
Cpc classification
C08G61/08
CHEMISTRY; METALLURGY
C08G61/02
CHEMISTRY; METALLURGY
H10N70/826
ELECTRICITY
H10B63/80
ELECTRICITY
C08G2261/1426
CHEMISTRY; METALLURGY
C08G2261/418
CHEMISTRY; METALLURGY
G11C2213/52
PHYSICS
G11C2213/77
PHYSICS
International classification
C08G61/02
CHEMISTRY; METALLURGY
Abstract
Disclosed herein are redox-active 6-oxoverdazyl polymers having structures (S1) and (S2) synthesized via ring-opening metathesis polymerization (ROMP) and their solution, bulk, and thin-film properties investigated. Detailed studies of the ROMP method employed confirmed that stable radical polymers with controlled molecular weights and narrow molecular weight distributions (<1.2) were produced. Thermal gravimetric analysis of a representative example of the title polymers demonstrated stability up to 190 C., while differential scanning calorimetry studies revealed a glass transition temperature of 152 C. An ultrathin memristor device was produced using these polymers, namely a 10 nm homogeneous thin film of poly-[1,5-diisopropyl-3-(cis-5-norbornene-exo-2,3-dicarboxiimide)-6-oxoverdazyl] (P6OV), a poly-radical with three tunable charge states per each radical monomer: positive, neutral and negative.
Claims
1. A memristor, comprising: a support substrate, a first array of electrodes or integrated circuit located on the support substrate; a layer of a verdazyl polymer located on said first electrode array, said layer having a thickness in a range from about 1 nm to about 1 m, and said verdazyl polymer having a structure (S1): ##STR00007## in which B is a bridging group which is any one of CO, CS, CH.sub.2, and CHAr where Ar is a phenyl or substituted phenyl; R.sup.1 and R.sup.5 are N-substituents which include any one of methyl, isopropyl, tertbutyl, phenyl, substituted phenyls, thiophene, furan, pyrrole, imidazole, pyridine, dimethylaminopyridine, pyrimidine, and indole; PG is the polymerized form of a group, the group including any one of cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, vinyl, alkynyl, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, and ethylene oxide; and n is an integer equal to or greater than 1; a second array of electrodes or integrated circuit located on a top surface of the layer of a verdazyl polymer; and one of said first and second array of electrodes or integrated circuits being a cathode, said cathode having a work function in range of about 3 to about 6 eV, and the other array of electrodes being an anode, said anode having a work function in range of about 2 to about 5 eV.
2. The memristor according to claim 1 wherein said cathode electrode has a work function in range of about 3.0 to about 4.8 eV.
3. The memristor according to claim 2 wherein said anode electrode has a work function in a range of about 2.5 to about 3.5 eV.
4. The memristor according to claim 1, wherein said layer of verdazyl polymer has a thickness in a range from about 1 nm to about 25 nm.
5. The memristor according claim 1, wherein R.sup.1 and R.sup.5 are isopropyl, and wherein B is CO, and wherein PG is cis-5-norbornene-exo-2,3-dicarboximide, and wherein n is between 1 and about 100.
6. A memristor, comprising: a support substrate, a first array of electrodes or integrated circuit located on the support substrate; a layer of a verdazyl polymer located on said first electrode array, said layer having a thickness in a range from about 1 nm to about 1 m, and said verdazyl polymer having a structure (S2): ##STR00008## in which B is a bridging group which is any one of CO, CS, CH.sub.2 and CHAr where Ar is a phenyl or substituted phenyl; R.sup.1 and R.sup.5 are N-substituents which include any one of methyl, isopropyl, tertbutyl, phenyl, substituted phenyls, thiophene, furan, pyrrole, imidazole, pyridine, dimethylaminopyridine, pyrimidine, and indole; PG is the polymerized form of a group, the group including any one of cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, vinyl, alkynyl, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, and ethylene oxide; and n is an integer equal to or greater than 1; a second array of electrodes or integrated circuit located on a top surface of the layer of a verdazyl polymer; and one of said first and second array of electrodes or integrated circuits being a cathode, said cathode having a work function in range of about 3 to about 6 eV, and the other array of electrodes being an anode, said anode having a work function in range of about 2 to about 5 eV.
7. The memristor according to claim 6 wherein said cathode electrode has a work function in range of about 3.0 to about 4.8 eV.
8. The memristor according to claim 7 wherein said anode electrode has a work function in range of about 2.5 to about 3.5 eV.
9. The memristor according to claim 6, wherein said layer of verdazyl polymer has a thickness in a range from about 1 nm to about 25 nm.
10. The memristor according to claim 1, wherein said bridging group B is CO, said R.sup.1 and R.sup.5 are isopropyl, phenyl, or substituted phenyl and wherein PG is the polymerized form of group cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, or ethylene oxide.
11. The memristor according to claim 1, wherein R.sup.1 and R.sup.5 are identical.
12. The memristor according to claim 1, wherein R.sup.1 and R.sup.5 are different.
13. The memristor according to claim 1, wherein n is in a range from 1 to about 25,000.
14. The memristor according to claim 1, wherein the substituted phenyls include phenyl rings bearing one or more methyl, methoxy, alcohol, amine, nitro, cyano, dimethylamino, fluoro, chloro, bromo, iodo, sulfoxy, carboxy, alkynyl, and alkenyl substituents.
15. The memristor according to claim 6, wherein said bridging group B is CO, said R.sup.1 and R.sup.5 are isopropyl, phenyl, or substituted phenyl and wherein PG is the polymerized form of group cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, or ethylene oxide.
16. The memristor according to claim 6, wherein R.sup.1 and R.sup.5 are identical.
17. The memristor according to claim 6, wherein R.sup.1 and R.sup.5 are different.
18. The memristor according to claim 1, wherein n is in a range from 1 to about 25,000.
19. The memristor according to claim 6, wherein the substituted phenyls include phenyl rings bearing one or more methyl, methoxy, alcohol, amine, nitro, cyano, dimethylamino, fluoro, chloro, bromo, iodo, sulfoxy, carboxy, alkynyl, and alkenyl substituents.
20. The memristor according to claim 6, wherein n is in a range from 1 to about 25,000.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) Embodiments disclosed herein will be more fully understood from the following detailed description thereof taken in connection with the accompanying drawings, which form a part of this application, and in which:
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DETAILED DESCRIPTION
(43) Various embodiments and aspects of the disclosure will be described with reference to details discussed below. The following description and drawings are illustrative of the disclosure and are not to be construed as limiting the disclosure. The drawings are not to scale. Numerous specific details are described to provide a thorough understanding of various embodiments of the present disclosure. However, in certain instances, well-known or conventional details are not described in order to provide a concise discussion of embodiments of the present disclosure.
(44) As used herein, the terms comprises and comprising are to be construed as being inclusive and open ended, and not exclusive. Specifically, when used in the specification and claims, the terms comprises and comprising and variations thereof mean the specified features, steps or components are included. These terms are not to be interpreted to exclude the presence of other features, steps or components.
(45) As used herein, the term exemplary means serving as an example, instance, or illustration, and should not be construed as preferred or advantageous over other configurations disclosed herein.
(46) As used herein, the terms about and approximately are meant to cover variations that may exist in the upper and lower limits of the ranges of values, such as variations in properties, parameters, and dimensions.
(47) As used herein, the phrase memristor refers to a device capable of memorizing digital information that carries different levels of resistance depending on the amount of current, or voltage, that was previously applied to it.
(48) As used herein, the phrase integrated circuit means a set of electronic circuits made up of various transistors on a single piece of semiconducting material (i.e., a chip). The integration of large numbers of transistors into a small chip will result in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed for discrete electronic components (i.e., arrays of electrodes). In the organic electronics community this is a common term. Essentially the array of electrodes use in the characterization of the present memristors is for prototype purposes only. For integrated circuits, the integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed for discrete electronic components. Therefore, any commercial application of the present memristors will require the use of integrated circuits.
(49) As will be discussed below, memristors made using 6-oxoverdazyl polymers will be discussed. The memristors include a substrate, a first array of electrodes, or integrated circuit, laid down on the substrate, a thin 6-oxoverdazyl polymer layer formed on the first array of electrodes and a second array of electrodes, or integrated circuit, applied on the top surface of the 6-oxoverdazyl polymer film.
(50) In an embodiment the present disclosure provides 6-oxoverdazyl polymers, having a structure (S1).
(51) ##STR00003##
in which B is a bridging group which is any one of CO, CS, CH.sub.2, and CHAr, where Ar is a phenyl or substituted phenyl. R.sup.1 and R.sup.5 are N-substituents and these N-substituents may include any one of methyl, isopropyl, tertbutyl, phenyl, substituted phenyls, thiophene, furan, pyrrole, imidazole, pyridine, dimethylaminopyridine, pyrimidine, and indole.
(52) Non-limiting examples of substituted phenyls in the bridging group B and the N-substituents R.sup.1 and R.sup.5 may include phenyl rings bearing one or more methyl, methoxy, alcohol, amine, nitro, cyano, dimethylamino, fluoro, chloro, bromo, iodo, sulfoxy, carboxy, alkynyl, and alkenyl substituents. Non-limiting specific examples of substituted phenyls that may be used include 2-methylphenyl, 3-methylphenyl, 4-methylphenyl, 2-methoxyphenyl, 3-methoxyphenyl, 4-methoxyphenyl, 2-nitrophenyl, 3-nitrophenyl, 4-nitrophenyl, 2-cyanophenyl, 3-cyanophenyl, 4-cyanophenyl, 2-(dimethylamino)phenyl, 3-(dimethylamino)phenyl, 4-(dimethylamino)phenyl, 2-fluorophenyl, 3-fluorophenyl, 4-fluorophenyl, 2-chlorophenyl, 3-chlorophenyl, 4-chlorophenyl, 2-bromophenyl, 3-bromophenyl, 4-bromophenyl, 2-iodophenyl, 3-iodophenyl, 4-iodophenyl, 2-sulfoxyphenyl, 3-sulfoxyphenyl, 4-sulfoxyphenyl, 2-carboxyphenyl, 3-carboxyphenyl and 4-carboxyphenyl.
(53) PG is the polymerized form of a group, and the group may include any one of cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, vinyl, alkynyl, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, and ethylene oxide, n is an integer equal to or greater than 1. In some embodiments R.sup.1 and R.sup.5 may be identical in the polymer, however it will be understood that in some embodiments R.sup.1 and R.sup.5 may be different from each other.
(54) In particular embodiments of the polymer the substituted phenyls include any one of methylphenyl, methoxyphenyl and nitrophenyl while the polymerizable group PG may include any one of cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, and ethylene oxide. Similarly, in particular embodiments the R.sup.1 and R.sup.5 N-substituents may include methyl, isopropyl, phenyl, methylphenyl, methoxyphenyl and nitrophenyl. The integer n may be in a range from 1 to about 25,000, but for many applications it is contemplated to be a range from 1 to 1000.
(55) In an exemplary, non-limiting example, S1 may have R.sup.1 and R.sup.5 being isopropyl, and B is CO, and wherein PG is cis-5-norbornene-exo-2,3-dicarboximide, and wherein n is between 1 and about 100, and the synthesis of this polymer 13 is illustrated hereinafter.
(56) In another embodiment the 6-oxoverdazyl polymer has a structure (S2).
(57) ##STR00004##
The bridging group B, R.sup.1 and R.sup.5 N-substituents, the polymerized form of a group PG and the integer n may be the same as in structure (S1) with the only difference being structure (S2) is lacking the aromatic carbon ring structure between the PG group and the nitrogen ring structure. The inventors contemplate that polymers represented by structure (S1) are contemplated to have greater thermal and chemical stability than those represented by structure (S2). However, polymers represented by structure (S2) are expected to have a greater number of unpaired electrons per unit mass, potentially enhancing their resistive/conductive properties and charge storage capacity.
(58) Non-limiting examples of 6-oxoverdazyl polymer structures represented by structure (S1), their methods of synthesis and their characterization will now be given.
EXAMPLE
(59) Experimental
(60) General
(61) All reactions and manipulations were carried out under a nitrogen atmosphere using standard Schlenk techniques unless otherwise stated. Solvents were obtained from Caledon Laboratories, dried using an Innovative Technologies Inc. solvent purification system, collected under vacuum, and stored under a nitrogen atmosphere over 4 molecular sieves. All reagents were purchased from Sigma-Aldrich, Alfa Aesar, or Oakwood Chemicals and used as received unless otherwise stated. 2,4-Di-isopropylcarbonohydrazide bis-hydrochloride 8.sup.27 and N-(3-hydroxylpropyl)-cis-5-norbornene-exo-2,3-dicarboximide 11.sup.28 were prepared according to published procedures. NMR spectra were recorded on a 400 MHz (.sup.1H: 400.1 MHz, .sup.13C: 100.4 MHz) Varian INOVA instrument. .sup.1H NMR spectra were referenced to residual CD.sub.3SOCD.sub.2H (2.50 ppm) and .sup.13C{.sup.1H} NMR spectra were referenced to CD.sub.3SOCD.sub.3 (39.5 ppm). Mass spectrometry data were recorded in positive-ion mode using a high resolution Finnigan MAT 8200 spectrometer using electron impact ionization. UV-vis absorption spectra were recorded in CH.sub.2Cl.sub.2 solutions using a Cary 300 Scan instrument. Four separate concentrations were run for each sample, and molar extinction coefficients were determined from the slope of a plot of absorbance against concentration. FT-IR spectra were recorded on a PerkinElmer Spectrum Two FT-IR as KBr pellets. Elemental analysis (C, H, N) was carried out by Laboratoire dAnalyse lmentaire, Universit de Montral, Montral, QC, Canada.
(62) Gel Permeation Chromatography (GPC)
(63) GPC experiments were conducted in chromatography grade THF at concentrations of 5 mg mL.sup.1 using a Viscotek GPCmax VE 2001 GPC instrument equipped with an Agilent PolyPore guard column (PL1113-1500) and two sequential Agilent PolyPore GPC columns packed with porous poly(styrene-co-divinylbenzene) particles (MW range 200-2,000,000 g mol.sup.1; PL1113-6500) regulated at a temperature of 30 C. Signal response was measured using a Viscotek VE 3580 RI detector, and molecular weights were determined by comparison of the maximum RI response with a calibration curve (10 points, 1,500-786,000 g mol.sup.1) established using monodisperse polystyrene purchased from Viscotek.
(64) Thermal Analysis
(65) Thermal degradation studies were performed using a TA Instruments 0600 SDT TGA and processed using TA Universal Analysis software. Samples were placed in an alumina cup and heated at a rate of 10 C. min.sup.1 tram 25 to 800 C. under a flow of nitrogen (100 mL min.sup.1). Glass transition temperatures were determined using differential scanning calorimetry (DSC) on a TA Instruments DSC 020. The polymer samples were placed in an aluminum Tzero pan and heated from room temperature to 180 C. at a scan rate of 10 C. min.sup.1 under a flow of nitrogen (50 mL min.sup.1) and cooled down to 0 C. at a scan rate 10 C. min.sup.1 before they underwent two more heating/cooling cycles. The glass transition temperature (T.sub.g) was determined from the second heating/cooling cycle.
(66) Cyclic Voltammetry (CV)
(67) CV experiments were performed with a Bioanalytical Systems Inc. (BASi) Epsilon potentiostat and analyzed using BASi Epsilon software. Typical electrochemical cells consisted of a three-electrode setup including a glassy carbon working electrode, platinum wire counter electrode, and silver wire pseudo-reference electrode. Experiments were run at 100 mV s.sup.1 in degassed MeCN/CH.sub.2Cl.sub.2 (1:1) solutions of the analyte (1 mM) and electrolyte (0.1 M nBu.sub.4NPF.sub.6). Voltammograms were referenced internally against the ferrocene/ferrocenium redox couple (1 mM internal standard) and corrected for internal cell resistance using the BASi Epsilon software.
(68) Electron Paramagnetic Resonance (EPR) Spectroscopy
(69) EPR measurements were made on ca. 10.sup.5 M CH.sub.2Cl.sub.2 solutions of 6-oxoverdazyl monomer 12 and polymer 13 that had been subjected to three freeze-pump-thaw cycles in 0.4 mm quartz tubes using a JEOL JES-FA200 EPR spectrometer. All measurements were made at 20 C. and g-factors were referenced relative to a built-in manganese oxide marker within the resonant cavity of the instrument. Quantification of the number of unpaired electrons present in polymer 13 was done by comparing an EPR spectrum collected for a TEMPO solution of known concentration in CH.sub.2Cl.sub.2 that was compared to the manganese oxide marker signal as outlined above. The integration of the TEMPO signal with respect to the manganese oxide marker was compared to that of the radical polymer. By assuming one molecule of TEMPO contributes one unpaired electron, the number of unpaired electrons present in the radical polymer sample was determined.
(70) X-ray Crystallography
(71) Crystals of monomer 12 suitable for X-ray diffraction were grown by vapor diffusion of hexanes into a saturated CH.sub.2Cl.sub.2 solution at 30 C. The sample was mounted on a MiTeGen polyimide micromount with a small amount of Paratone N oil. All X-ray measurements were made on a Nonius KappaCCD Apex2 diffractometer at a temperature of 110 K. The frame integration was performed using SAINT.sup.29. The resulting raw data was scaled and absorption corrected using a multi-scan averaging of symmetry equivalent data using SADABS..sup.30 The structure was solved by using a dual space methodology using the SHELXT program.sup.31. All non-hydrogen atoms were obtained from the initial solution. The hydrogen atoms were introduced at idealized positions and the positional parameters but not the displacement parameters were allowed to refine. The structural model was fit to the data using full matrix least-squares based on F.sup.2. The calculated structure factors included corrections for anomalous dispersion from the usual tabulation. The structure was refined using the SHELXL-2014 program from the SHELX suite of crystallographic software.sup.32. Graphic plots were produced using the Mercury program suite. See Table 1 and CCDC 1428231 for X-ray diffraction data collection and refinement details.
(72) TABLE-US-00001 TABLE 1 Selected X-ray diffraction data collection and refinement details for monomer 12. Compound 12 Chemical Formula C.sub.27H.sub.32N.sub.5O.sub.5 Formula Weight (g mol.sup.1) 506.57 Crystal Dimensions (mm) 0.349 0.325 0.115 Crystal Color and Habit Red prism Crystal System Triclinic Space Group P 1 Temperature (K) 110 a () 10.063(2) b () 11.316(2) c () 12.516(2) () 100.839(4) () 112.235(5) () 98.624(5) V (.sup.3) 1257.0(4) Z 2 (g cm.sup.1) 1.338 () 1.54178 (cm.sup.1) 0.769 Diffractometer Type Nonius KappaCCD Apex2 R.sub.merge 0.0245 R.sub.1.sup.a 0.0352 R.sub.2.sup.b 0.0900 R.sub.1 (all data) 0.0391 R.sub.2 (all data) 0.0940 GOF.sup.c 1.028 .sup.aR.sub.1 = (|F.sub.o| |F.sub.c|)/ F.sub.o .sup.bR.sub.2 = [((F.sub.o.sup.2 F.sub.c.sup.2).sup.2)/(F.sub.o.sup.4)].sup.1/2 .sup.cGOF = [((F.sub.o.sup.2 F.sub.c.sup.2).sup.2)/(No. of reflns. No. of params.)].sup.1/2
Thin-Film Preparation and Electrical Conductivity Measurements
(73) Thin films of various thickness were prepared from polymer 13 and their electrical properties were measured. Film preparation and electrical measurements were both carried out in a glove box loaded with N2 (Nexus II, Vacuum Atmospheres Co.) attached to an ultra-high vacuum (UHV) chamber for sample metallization and contacting. Samples can be transferred to/from this chamber from/to the glove box without any direct exposure to air. O.sub.2 and H.sub.2O contents in the glove box were below 3 ppm during the entire fabrication and measurement process. To prepare the thin films, the polymer was dissolved at 12.5 mg mL.sup.1 in anhydrous chlorobenzene. The solution was stirred overnight at 50 C., filtered through 0.8 m pore size syringe filters and spun on glass substrates with pre-deposited indium tin-oxide (ITO) contacts (15 /square sheet resistance, Sigma-Aldrich) using a KW-4A spin coater (Chemat Technologies Inc.) located in the glove box. ITO substrate patterning on glass was previously obtained by coating the ITO area to be retained with Kapton tape and etching the remaining area in a 2:1:1 H.sub.2O:HCl:HNO.sub.3 mixture at 55 C.
(74) Different spinning speeds, from 500 to 3000 rpm, were used to obtain a set of thin films at thicknesses from 504 nm to 104 nm, respectively. These thicknesses were measured by atomic force microscopy (AFM) from samples identical to those used for electrical measurements. In order to perform the thickness measurements, part of the substrate was masked prior to spin coating the polymer solution. The mask was then removed and samples were extracted from the glove box and analyzed in contact-mode using a Witec Alpha300S AFM microscope, from which topography profiles of the step in the correspondence of the masked area were obtained. Additional AFM profiles were recorded in the correspondence of scratches made on the polymer film using soft probes that were known not to affect the substrate. Referring to
(75) Electrical measurements were performed in a sandwich configuration in the glove box. To complete the sandwich structure, samples were transferred in the aforementioned UHV chamber directly accessible from the glove box and 100 nm thick aluminum contacts were thermally evaporated on top of the polymer films, with contact thickness measured in situ using a Sycom STM-2 thickness monitor. The temperature was kept below 50 C. during the entire thermal evaporation process. A first set of current voltage (I-V) characteristics of the thin films were recorded at 1 V using a computer automated Keithley 2400 source meter with 10 mV scan step. After this set of measurements a significantly higher voltage, V.sub.o=5 V, was applied to the samples, and electrical measurements were repeated. The breakdown voltage was determined to be 8-12 V for the thinnest sample and is therefore significantly higher than any voltages used during our experiments. Consistent electrical measurements were successfully reproduced on different sets of identical samples.
1,5-di-isopropyl-3-(4-carboxyphenyl)-6-oxotetrazane (9)
(76) To a refluxing solution of 2,4-di-isopropylcarbazide (2.00 g, 8.09 mmol) and sodium acetate (1.33 g, 16.2 mmol) in MeOH (50 mL) was added dropwise over a 3 h period, a solution of 4-carboxybenzaldehyde (1.21 g, 8.09 mmol) and sodium acetate (0.66 g, 8.1 mmol) in MeOH (50 mL). The solution was stirred at reflux overnight, removed from the heat and allowed to cool to room temperature. The reaction mixture was then acidified to pH3 using 1 M HCl, followed by the removal of MeOH in vacuo. The white precipitate that crashed out of the resulting aqueous solution was filtered and washed with two portions of deionized H.sub.2O (50 mL) to give tetrazane 9 as a white microcrystalline powder. Yield=2.34 g, 94%. .sup.1H NMR (400.1 MHz, d.sub.6-DMSO): 13.00 (s, 1H, COOH), 7.97 (d, 2H, .sup.3J.sub.HH=6 Hz, aryl CH), 7.68 (d, 2H, .sup.3J.sub.HH=6 Hz, aryl CH), 5.05 (d, 2H, .sup.3J.sub.HH=11 Hz, NH), 4.50-4.43 (m, 3H, NCHN and CHMe.sub.2), 1.06-1.04 (m, 12H, CH.sub.3). .sup.13C {.sup.1H}NMR (100.6 MHz, d.sub.6-DMSO): 167.0, 153.4, 141.1, 130.6, 129.3, 126.9, 72.2, 46.8, 19.6, 18.4. FT-IR (ranked intensity, assignment), KBr pellet: 3249 (13, NH), 2981 (7), 2935 (11), 2872 (12), 1694 (3, CO), 1586 (1, CO), 1423 (2), 1227 (5), 1125 (6), - +ve mode): exact mass calculated for C.sub.15H.sub.22N.sub.4O.sub.3: 306.1692; found: 306.1688; difference: 1.3 ppm.
1,5-di-isopropyl-3-(4-carboxyphenyl)-6-oxoverdazyl (10)
(77) To a deionized H.sub.2O/THF (2:1, 45 mL) solution of 1,5-di-isopropyl-3-(4-carboxyphenyl)-6-oxotetrazane 9 (2.34 g, 7.64 mmol) and sodium hydroxide (0.31 g, 7.6 mmol) open to air was added sodium periodate (2.45 g, 11.5 mmol) in deionized H.sub.2O (30 mL) dropwise over 30 min. The reaction was stirred at room temperature for 18 h and slowly turned a dark red colour. The mixture was then acidified to pH3 using 1 M HCl, followed by removal of THF in vacuo. The resultant dark-red precipitate was filtered and washed with two portions of deionized H.sub.2O (50 mL) to give verdazyl 10 as a red microcrystalline powder. Yield=2.15 g, 93%. FT-IR (ranked intensity, assignment), KBr pellet: 3434 (4, br, OH), 3198 (8), 2985 (6), 2937 (9), 1721 (3, CO), 1680 (1, CO), 1656 (2), 1612 (7), 1432 (12), 1386 (10), 1290 (11), 1219 (5) cm.sup.1. UV-vis (CH.sub.2Cl.sub.2): .sub.max 419 nm (=1,875 M.sup.1 cm.sup.1), 405 nm (=1,600 M.sup.1 cm.sup.1), 270 nm (=29,750 M.sup.1 cm.sup.1). Mass Spec. (EI, +ve mode): exact mass calculated for C.sub.15H.sub.19N.sub.4O.sub.3: 303.1457; found: 303.1459; difference: +0.7 ppm.
1,5-diisopropyl-3-(cis-5-norbornene-exo-2,3,-dicarboxiimide)-6-oxoverdazyl (12)
(78) To a solution of N,N-dicyclohexylcarbodiimide (DCC) (0.75 g, 3.6 mmol) and 4-dimethylaminopyridine (DMAP) (0.48 g, 4.0 mmol) in dry CH.sub.2Cl.sub.2 (20 mL) was added 1,5-di-isopropyl-3-(4-carboxyphenyl)-6-oxoverdazyl 10 (1.00 g, 3.30 mmol) before the mixture was stirred for 10 min at room temperature. To this solution was added N-(3-hydroxylpropyl)-cis-5-norbornene-exo-2,3-dicarboximide 11 (0.73 g, 3.3 mmol) and a further 10 mL of dry CH.sub.2Cl.sub.2 (for rinsing). The reaction mixture was stirred for 4 h at room temperature. The solution was filtered to remove salt, which was rinsed with dry CH.sub.2Cl.sub.2 before the organic phases were combined and taken to dryness in vacuo. The resulting orange oil was purified by column chromatography [75 mL neutral alumina, hexanes/EtOAc (35:65), R.sub.t=0.45], and recrystallized from a hot saturated solution of hexanes to give 12 as dark-red needles. Yield=1.18 g, 71%. FT-IR (ranked intensity, assignment), KBr pellet: 2978 (11), 2935 (12), 2877 (14), 1770 (13, CO), 1697 (1, CO), 1679 (2, CO), 1611 (10), 1386 (9), 1367 (8), 1268 (2), 1230 (5), 1174 (6), 776 (7), 705 (4) cm.sup.1. UV-vis (CH.sub.2Cl.sub.2): .sub.max 418 nm (=1,900 M.sup.1 cm.sup.1), 403 nm (=1,400 M.sup.1 cm.sup.1), 268 nm (=31,025 M.sup.1 cm.sup.1). Mass Spec. (EI, +ve mode): exact mass calculated for C.sub.27H.sub.32N.sub.5O.sub.5: 506.2403; found: 506.2399; difference: 0.8 ppm. Anal. Calcd. (%) for C.sub.27H.sub.32N.sub.5O.sub.5: C, 64.02; H, 6.37; N, 13.82. Found: C, 63.88; H, 6.50; N, 13.65.
Representative Synthesis of poly[1,5-diisopropyl-3-(cis-5-norbornene-exo-2,3,-dicarboxiimide)-6-oxoverdazyl] (13)
(79) A grease-free Schlenk flask was charged with monomer 12 (0.50 g, 0.99 mmol) and degassed CH.sub.2Cl.sub.2 (12 mL, 3 freeze-pump-thaw cycles). The monomer solution was cooled to 0 C. in an ice bath for 10 min before a 1 mg mL.sup.1CH.sub.2Cl.sub.2 solution of Grubbs' 3rd generation catalyst (8.73 mL, 9.8710.sup.3 mmol) was rapidly added in one portion. The polymerization proceeded for 1 h before it was terminated with ethyl vinyl ether (2.37 mL, 24.7 mmol), and stirred for an additional 30 min while warming to room temperature. The crude mixture was filtered through a short neutral alumina column (4 cm2.5 cm, CH.sub.2Cl.sub.2) before the solvent was removed in vacuo. The resultant polymer, an orange oil was dissolved in THF (10 mL) and precipitated thrice into cold hexanes (90 mL) to afford 13 as an orange powder. Yield=0.46 g, 92%. FT-IR (ranked intensity, assignment), KBr pellet: 2975 (13), 2939 (14), 2871 (16), CO ester 1775 (15), 1698 (1, CO), 1682 (2, CO), 1611 (12), 1387 (10), 1368 (8), 1270 (3), 1228 (9), 1173 (6), 1104 (7), 1101 (5), 776 (11), 705 (4) cm.sup.1. UV-vis (CH.sub.2Cl.sub.2): .sub.max 419 nm (e=2,050 M.sup.1 cm.sup.1), 402 nm (=1,475 M.sup.1 cm.sup.1), 270 nm (e=29,900 M.sup.1 cm.sup.1). GPC (THF, conventional calibration relative to polystyrene standards): M.sub.n=46,100 g mol.sup.1, M.sub.w=49,000 g mol.sup.1, =1.07).
(80) Kinetic Studies of the ROMP of Monomer 12
(81) Catalyst Loading:
(82) Using 0.05 g of monomer 12 each, a series of five reactions were carried out according to the procedure described above. The catalyst molar feed stock ratios (monomer:catalyst) were: 20, 40, 60, 80, and 100. The polymerization times were held constant at 30 min. The degree of polymerization was measured by GPC analysis using conventional calibration relative to polystyrene standards.
(83) Timed Aliquots:
(84) A 1 mg mL.sup.1 CH.sub.2Cl.sub.2 solution of Grubbs' 3.sup.rd generation catalyst (3.5 mL, 4.010.sup.3 mmol) was rapidly added in one portion to a 42 mg mL.sup.1 CH.sub.2Cl.sub.2 solution of monomer 12 (4.8 mL, 0.40 mmol) and the mixture was stirred at 0 C. Six samples were taken at 150 s intervals and added into separate reaction flasks containing ethyl vinyl ether (0.94 mL, 9.9 mmol) to terminate polymerization. The number average molecular weights (M.sub.n) were measured by GPC analysis using conventional calibration relative to polystyrene standards.
(85) Results and Discussion
(86) Synthesis
(87) The synthesis of monomer 12 (Scheme 1) began with the condensation reaction between bis-hydrazide2HCl salt 8 and 4-formylbenzoic acid to afford tetrazane 9 as a white powder in 94% yield (see
(88) ##STR00005##
(89) X-ray diffraction studies of single crystals of monomer 12 afforded a solid-state structure (
(90) With monomer 12 in hand, we began to explore its ROMP behavior. In a representative reaction, polymerization was initiated by the rapid introduction of a solution of Grubbs' 3rd generation catalyst (G3) to a stirring solution of monomer 12 in dry and degassed CH.sub.2Cl.sub.2 at 0 C. with a feed molar ratio of 100 (monomer:catalyst=100:1). Reaction progress was monitored by gel permeation chromatography (GPC), confirming the reaction was near completion after approximately 15 min. Nonetheless, the solution was stirred for an additional 45 min to ensure complete monomer conversion before a large excess of ethyl vinyl ether (EVE) was added to terminate the polymerization. The reaction mixture was passed through a plug of neutral alumina to remove residual catalyst. Subsequent precipitations from THF into cold hexanes, followed by centrifugation afforded polymer 13 as an orange powder in 92% yield.
(91) It is worth noting that during our investigations of the ROMP of monomer 12, approximately 20% of our reactions yielded polymer samples that contained a high molecular weight shoulder in their GPC chromatograms (e.g.,
(92) The inventors performed two separate experiments designed to further probe the ROMP of monomer 12. The scope of the present studies were limited due to the paramagnetic nature of polymer 13, which precluded the use of integration data obtained from .sup.1H NMR spectroscopy for the determination of the number average degree of polymerization (DP.sub.n) and monomer consumption as a function of time. The first study involved ROMP of monomer 12 at five different molar feedstock ratios (
(93) Polymer Characterization
(94) Thermal gravimetric analysis (TGA) revealed that polymer 13 was thermally stable up to a temperature of 190 C., where rapid degradation occurred in three steps (see
(95) To confirm the presence of 6-oxoverdazyl radicals in the polymer, careful comparison of the spectroscopic and electrochemical properties of monomer 12 and polymer 13 were made.
(96) In order to further support these findings, electron paramagnetic resonance (EPR) spectroscopy was performed to quantitatively determine the number of unpaired 6-oxoverdazyl units present in polymer 13. This experiment showed that ca. 94% of the repeating units in the polymer contain an unpaired electron, supporting our IR and UV-vis absorption spectroscopy-based claims discussed above. An EPR spectrum of monomer 12 was also obtained and compared to the spectrum of polymer 13 (
(97)
(98) The electrochemical properties of monomer 12 and polymer 13 were studied using cyclic voltammetry (CV) in a CH.sub.2Cl.sub.2/MeCN (1:1) solvent mixture (
(99) TABLE-US-00002 TABLE 2 Electrochemical data for 6-oxoverdazyl monomer 12 and polymer 13. Compound E.sub.1/2,ox (V vs. Fc/Fc.sup.+) E.sub.1/2,red (V vs. Fc/Fc.sup.+) 12 0.24 1.36 13 0.25 1.35
Electrical Properties of Thin Films of Polymer 13
(100) Based on the spectroscopic and electrochemical studies described above, we concluded that polymer 13 may have interesting and potentially unique thin-film properties. To this end, we investigated the electrical properties of thin solid films of this polymer, from approximately 10 nm to 50 nm in thickness, by using a sandwich architecture. Thin polymeric films may have very peculiar morphological and structural properties as a consequence of the preferential alignment of the polymeric chains with respect to the substrate. Specifically, the alignment may be strongly dependent on the thickness, which may sometimes lead to non-ohmic conductivity mechanisms and thickness-dependent resistivity in ultrathin polymeric films.
(101)
(102)
where A is the area of the region in which the top and bottom contacts overlap, d is the film thickness and R=V/I is the thin film resistance, obtained from the slope of the I-V characteristics in the proximity of the origin. The I-V curves recorded from our films of polymer 13 are shown in
(103) It is noteworthy that the I-V characteristics before high-voltage treatment of our films are typically non-ohmic, as can be inferred from
(104)
where .sub.o corresponds to the polymer resistivity in the absence of traps, q is the electron charge, is the voltage barrier electrons must cross at low voltage to hop from one charged trap to another, .sub.m is the dielectric permittivity of the polymer relative to vacuum, and k.sub.BT=0.025 eV at room temperature.
(105) While the high-conductivity state observed in
(106) The switchable conductor-insulator transitions in thin films of polymer 13 and their relationship with the redox processes in this material requires further investigations. However, the observation of Poole-Frenkel type transport in the low conductivity state leads us to tentatively suggest that such state corresponds to a situation in which only a few repeating units in a polymer chain are charged, while most of them are in a neutral state. At sufficiently high voltage (i.e., VV.sub.o5 V) charges may directly tunnel from the electrodes into some of the neutral repeating units, charging them either positively or negatively. When a sufficient concentration of charged repeating units is reached in a thin film, a percolating pathway may be established between such repeating units, leading to switching to the high conductivity state, dominated by transport between extended electronic states. This conductor-insulator transition makes polymer 13 uniquely positioned for several applications in bistable electronics.
(107) With respect to the class of compounds S2, SCHEME 2 below outlines a representative synthesis of these S2 polymers, with polymer 14 being a non-limiting example.
(108) ##STR00006##
Summary
(109) As a result of this work, we have demonstrated that ROMP using the 3-bromopyridine derivative of Grubbs' 3rd generation catalyst can be used to produce 6-oxoverdazyl polymers with up to ca. 100 repeating units, narrow molecular weight distributions (<1.2), and high radical content (ca. 94%). A representative sample of the 6-oxoverdazyl polymers produced was thermally stable up to a temperature of 190 C. and had a glass transition temperature of 152 C. Comparison of the same polymer, which is stable towards air and moisture, to the monomer employed using several spectroscopic techniques, including IR, UV-vis absorption, and EPR spectroscopy and cyclic voltammetry confirmed the identity and properties of the pendant 6-oxoverdazyl groups were maintained after ROMP. The redox properties of the polymers described led us to explore their thin-film electrical transport properties, revealing a significant decrease in sheet resistance from 240 to 26 M upon application of a potential, V.sub.o=5 V.
(110) Memristor Device
(111) The phrase P6OV refers to polymer 13 and is the 6-oxoverdazyl polymer structures (S1). The 6-oxoverdazyl polymer structures (S1) and (S2) disclosed above may be used to produce memristor devices. These memristors include a support substrate, a first array of electrodes or integrated circuit located on the support substrate, a thin layer of the 6-oxoverdazyl polymer (structures (S1) or (S2)) with the polymer layer having a thickness in a range from about 1 nm to about 1 m. A second array of electrodes or integrated circuit located is then applied on the top surface of the layer of the 6-oxoverdazyl polymer. One of the electrode arrays or integrated circuit acts as a cathode with the other acts as an anode. The cathode array has a work function in range of about 3 to about 6 eV, and the anode array has a work function in range of about 2 to about 5 eV.
(112) In some embodiments the 6-oxoverdazyl polymer may have CO as its bridging group B, R.sup.1 and R.sup.5 may be any one or combination of isopropyl, phenyl, and substituted phenyl, and PG may be the polymerized form of any of the groups cis-5-norbornene-exo-2,3-dicarboximide, norbornene, substituted norbornenes, styrene, acrylate, methacrylate, substituted silacyclobutanes, ethylene glycol, and ethylene oxide. In these embodiments the cathode electrode is selected to have a work function in range of about 3.0 to about 4.8 eV and the anode electrode is selected to have a work function in range of about 2.5 to about 3.5 eV.
(113) The thickness of the layer of 6-oxoverdazyl polymer may have a thickness in a range from about 1 nm to about 25 nm.
(114) Non-limiting examples of memristors produced using 6-oxoverdazyl polymer (polymer 13) structures represented by structure (S1) will now be given.
EXAMPLE
(115) The chemical structure of P6OV (as synthesized above), the polyradical used for the present memristor design and optimization is shown in
(116) Sixteen memristors were simultaneously fabricated as shown in
(117) 2.1. Electronic Energy Levels in P6OV
(118) The typical current-voltage (J-V) characteristics of one of our devices is shown in
(119) In order to optimize the performance of P6OV as an active memory material, it is imperative to construct an energy-level matched electronic structure for the devices. P6OV in its neutral state is expected to possess a singly-occupied molecular orbital (SOMO) at about mid-gap between the doubly-occupied highest occupied molecular orbital (HOMO, -bonding), and the lowest unoccupied molecular orbital (LUMO, *-antibonding). Considering that P6OV is a non-conjugated polymer, the HOMO-LUMO energy band gap E.sub.*, corresponding to .fwdarw.* interband optical transitions is expected to sit in the near-UV photon energy range, comparable to or slightly lower than 5.6 eV in benzene. From our UV-visible (UV-vis) absorption measurements, seen in
(120) From
(121) Notation Used to Indicate the Electron Energy Levels in P6OV-based Memristors
(122) A significant advantage of the present P6OV polyradicals is the fact that it exhibits three tunable charge states: positive, neutral and negative. These charge states are respectively represented with the symbols (+), (0), and (). As is generally the case, the electronic state due to -bonding and -antibonding orbitals of the radical P6OV polymer is represented with the symbol and * respectively. In order to distinguish between the levels, the symbol , with the appropriate subscript, were used to indicate the energy levels of P6OV relative to vacuum as shown in
(123) TABLE-US-00003 TABLE 3 Transition type Initial Final E HOMO .fwdarw. Initial state: Final state: E.sub.* LUMO at 6.2 eV * at 1.6 eV 4.6 eV .fwdarw. * SOMO .fwdarw. Initial state: Final states: E.sub.(0)(+,*) LUMO (0) SOMO at 3.9 eV (+) hole at 4.7 eV 3.1 eV (0) .fwdarw. (+,*) and * at 1.6 eV HOMO .fwdarw. Initial states: Final state: E.sub.(0,)() SOMO at 6.2 eV and () electron at 3.1 eV 3.1 eV (0,) .fwdarw. () (0) SOMO at 3.9 eV
(124) Although the energies E.sub.(0),(+,*) and E.sub.(0,),() we assign to (0).fwdarw.(+, *) (i.e., SOMO.fwdarw.LUMO) and (0, ).fwdarw.() (i.e., HOMO.fwdarw.SOMO) transitions in
(125) In few-electron systems, the actual position of an electron energy level is dramatically affected by their occupation number. The subsequent change in energy levels, known as the Hubbard interaction energy U, corresponds to the energy lost or gained by the system due to the addition or removal of one electron as a consequence of the increase or decrease in electron-electron Coulombic repulsion. When positively charged, a neutral (0) SOMO state sitting at .sub.(0) below the vacuum level is expected to generate an unoccupied energy level situated at .sub.(+)=.sub.(0)+U. Conversely, when the same SOMO state is negatively charged due to the addition of one extra electron, negative electron-electron Coulombic repulsion energy is more significant and the corresponding doubly occupied energy level will be situated at .sub.()=.sub.(0)U. Consequently, under the assumption that .sub.(0) sits at midgap, nonzero Hubbard interaction energy results in (0).fwdarw.(+,*) and (0,).fwdarw.() optical transitions occurring at
E.sub.(0)(+,*)=.sub.*.sub.(+)E.sub.*U; E.sub.(0,*)()=.sub.().sub.E.sub.*U (3)
respectively, which indicates U0.8 eV, consistent with similar values that were previously found in small carbon clusters.
(126) In order to corroborate our UV-vis analysis and confirm the positions of .sub., .sub.(), .sub.(0), .sub.(+) and .sub.* energy levels, we used Kelvin-probe force microscopy (KPFM) images to directly measure the surface potential of P6OV. Surface potentials determined by KPFM may either correspond to the position of the HOMO level, the SOMO level, or a combination of the two. In the specific case of P6OV, a polyradical exhibiting memory effects, the voltage used for KPFM imaging may influence the occupation number of mid-gap states, which may transition from neutral to positively or negatively charged, depending on the value of the nullifying bias voltage applied to the atomic force microscopy (AFM) tip used for the measurements, and affect the actual value of the surface potential. In order to solve this issue, our KPFM images were performed after a first scan at which a high constant voltage +7 V, higher than the erase voltage of the device, was applied to the tip. Under those conditions, the device is erased, and the value of the surface potential is expected to reflect the position of the HOMO and SOMO levels.
(127)
(128) From
(129) Memristor Design Criteria
(130)
.sub.H(ITO)>.sub.H(FLG)>.sub.H(Al)>.sub.H(Ca/Al)(4)
and, therefore, is higher for ITO-based devices, of which the cathode work function aligns with the surface potential of positively charged P6OV, .sub.ITO=.sub.(+)=4.7 eV. In the other three types of devices, with FLG, Al and Ca/Al cathodes and .sub.(+), .sub.H decreases at decreasing work function of cathode, proportionally to exp(|.sub.(+)|/k.sub.BT). .sub.H is lowest in devices with symmetric anode and cathode, both made out of Ca-coated aluminum. Even though their electrode work function matches the surface potential of negatively charged P6OV at .sub.Ca/Al=.sub.()=3.1 eV, the (+) energy level is much higher, at 4.7 eV, which leads to very significant contact resistance. The nearly ohmic characteristics of the J-V curves, demonstrated in
(131) Further insight into the bistability and transport properties of P6OV memristors can be gained by observing their behavior in low-conductivity regime. In such regime, their conductivity can be inferred from the J-V curves in
.sub.L(Al)>.sub.L(FLG)>.sub.L(ITO)>.sub.L(Ca/Al).(5)
Consequently, .sub.L is higher for cathodes of pure aluminum, with work function .sub.Al=.sub.(0)=4.0 eV, aligning to the SOMO of neutral P6OV. In the other three types of devices, with FLG, ITO and Ca/Al cathodes at .sub.(0), c decreases as exp(|.sub.(0)|/k.sub.BT). This indicates that the conductivity of P6OV memristors in the low-conductivity regime is determined by the contact resistance between the SOMO and the cathode. Such an observation agrees with previous findings that the transport properties in such regime obey a Poole-Frenkel transport mechanism, demonstrated in
(132)
(133) Application of sufficiently high electric fields leads to charge polarization of thin film dielectric materials. This is consistent with the fact that the 3.5 V threshold for erase voltages in our devices correspond to an energy e V.sub.E4 U, which is the sum of the formation energies of a doubly-occupied, negatively charged monomer and an unoccupied and positively charged monomer. This configuration, shown in
(134) It is worthwhile noting that, in this regime, optimal hole injection occurs for charges transferred from ITO to the .sub.(+) level of P6OV, both at 4.7 eV, while optimal electron injection of electrons occurs for charges transferred from Ca/Al to the .sub.() level of P6OV, both at 3.1 eV. For practical memristor applications, the on current in the high-conductivity state needs to be maximized, while the off current in the low-conductivity state needs to be kept to a minimum. Therefore, Al/Ca-P6OV-ITO architectures are optimal for the fabrication of polyradical thin film memristors based on P6OV, and our design criteria can be extended to ultrathin memory devices from a large variety of polyradicals, clearly indicating the generality of our study.
(135) It is also important to bear in mind that the transport mechanism proposed in
(136) Memristor Performance
(137) To demonstrate the important significance of the electrode work functions in relation to the energy levels of the polyradical and the performance of the memory fabricated devices, the present inventors carried out multiple consecutive cycles of electrical switching between low and high-conductivity regimes, as illustrated in
(138)
(139) Effect of Switching Speed on the Performance of P6OV-based Memristors
(140) In order to explore the role of switching speed on the current-voltage (I-V) characteristics of P6OV-based thin film memristors, devices with Al/Ca-P6OV-ITO architecture were tested by recording I-V scans at different scan speeds, from 2.5 V/ms to 2.5 10.sup.3 mV/ms, as shown in
(141) As these results demonstrate, no significant changes in the write and erase voltages were observed, even at sweep rates as fast as 2.5 V/ms. This indicates that the present devices can be operated at least up to frequencies in the kHz regime. Once the devices switched to either state, they produced output currents commensurate with the conductivity in such regime, accordingly to equations (2) and (3). Dramatically different behaviors and on/off ratios are therefore expected with different cathode materials.
(142) Current changes during multiple sequences of write-read-erase-read cycles are demonstrated by the top curves in
(143) These results point to the strong significance of the work function of the cathode materials on the performance of these memristors and with the fact that ITO cathodes are optimal for the performance of our devices, consistently with the band diagrams drawn in
(144)
(145) In view of the above mentioned considerations, the major obstacle to polyradical memristor performance is in carefully engineering the contact resistance between the polymer thin films and the electrodes.
(146) The capability of optimized memristors to retain information for a long period of time was further investigated. Once the on state of a device was activated, it was left in the glove box under nitrogen atmosphere without applied bias voltage. After a prolonged period of time, lasting up to several days, the device was still observed to remain in the same state and switched states only with appropriate voltage bias.
CONCLUSION
(147) The present disclosure provides a design criterion for ultrathin memristors based on a homogeneous active layer of polyradical material, with P6OV (structure (A)) as a case study. Devices presented in this work utilized a sandwich configuration with aluminum-calcium bilayers as anode material, and four cathode materials at different work functions, ranging from =4.7 eV for ITO electrodes to =3.1 eV for calcium-aluminum bilayers. The energy levels of the positively, neutral and negatively charge states of P6OV have been determined from a combination of UV-Vis and KPFM measurements. The behavior of the SOMO energy level of P6OV at different degrees of charging could be understood in the framework of the Hubbard model by assuming a correlation energy U=0.8 eV. Knowledge of the positive, neutral and negative energy levels were vital to configure our memristor architectures for maximum performance.
(148) Although switching effects have been demonstrated here in a sandwich configuration, conducting AFM will be essential as a future tool for probing memory switching effects in P6OV at the nanoscale. Conducting AFM measurements are particularly challenging in this system because of the thinness of the soft polyradical samples, which are easily damaged by the AFM tip when working in contact-mode as required by conducting scanning probe measurements. Furthermore, native oxide at the AFM tip surface may affect spin polarization. This may be a significant issue in radical polymers in which transport is highly spin polarized, as shown in
(149) Current-voltage characteristics show that a high-conductivity regime can be created, and subsequently eliminated by applying an erase voltage to the devices of the order of V.sub.E4 U/e. Resistivity in the high-conductivity regime was found to be lower for ITO-based devices, in which the work function of the cathode matches the surface potential of positively charged P6OV radicals: .sub.ITO=.sub.(+)=4.7 eV. In the other three types of devices, with FLG, Al and Ca/Al cathodes and <.sub.(+), the on current decreased at decreasing work function of the cathode. In the low-conductivity regime, the resistivity of P6OV memristors was determined by the contact resistance between the SOMO energy of neutral P6OV and the cathode. Consequently, the electrical conductivity was higher for devices with cathodes of pure aluminum, with work function .sub.Al=.sub.(0)=4.0 eV, but decreases in the other types of devices with FLG, ITO and Ca/Al cathodes, as the mismatch between the SOMO level .sub.(0) and the cathode work function increases. We demonstrated that the electrical bistability of our devices is due to two distinct transport regimes in P6OV, extended states and Poole-Frenkel.
(150) Optimized flash memory effects were demonstrated in Al/Ca-P6OV-ITO, in which the energy levels of the positively and negatively charged states of the polyradical align, respectively, to the cathode and anode work functions. More than 10.sup.3 seconds of write-read-erase-read cycles were performed without significant current degradation. Conversely, Al/Ca-P6OV-Al devices, in which the cathode work function aligned with the neutral energy level of P6OV, exhibited WORM effects. The present results demonstrate the importance of energy-level matched electronic structures as the basis on which ultrathin, single-layer memristors can be successfully implemented.
(151) The foregoing description of the preferred embodiments of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims and their equivalents.
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