Write-once-read-many-times Research Articles

Development of indigo-based nonvolatile write-once-read-many-times memory device

The development of a nonvolatile organic write-once-read-many-times (WORM) memory device, consisting of a 100nm layer of indigo sandwiched between indium tin oxide cathode and Al anode, is reported. This device is found to be at its ON state and can be switched to the OFF state by applying a positive bias with the ON/OFF current ratio being up to 106. The device exhibits storage stability of over 108h in air without encapsulation. This device offers the possibility of a low-cost biodegradable data storage as Indigo is inexpensive, bio-degradable and non-toxic. The operational mechanism of the device is discussed in terms of dipoles induced at the Al-indigo interface.

Achieving tunable memory performance from nonvolatile to volatile by altering the trap depth of charge trapping sites in functional imides containing carbazole moieties

In this work, two functional imides were designed and synthesized to elucidate the influence of charge trap depth on the memory behavior. Two dianhydrides, 3,3′,4,4'-diphenylsulfonetetracarboxylic dianhydride and 4,4'-(hexafluoroisopropylidene)diphthalic anhydride were selected as the electron acceptor considering their functional moieties of different charge-trapping depth and electron-withdrawing ability. Electrical characterization indicated that the memory devices based on the two imides exhibited nonvolatile write once read many times (WORM) memory behavior and volatile static random access memory (SRAM) behavior. Mechanisms associated with the distinct memory effect were demonstrated based on the molecular simulation. Analysis results indicated that charge trapping process is responsible for the electrical bistability and the different depth of charge-trapping sites in the two imides accounts for the distinct memory behavior of corresponding memory devices. Meanwhile, the two memory devices both show excellent long term operation stability. This paper not only reports two novel memory materials but also provided some guiding principle to the design of organic based memory devices.

Triggering WORM/SRAM Memory Conversion in a Porphyrinated Polyimide via Zn Complexation as the Internal Electrode

We design two novel solution processable polyimides (PIs), NH-Por-6FDA and Zn-Por-6FDA, with 5,15-bis(4,-aminophenyl)-10,20-diphenylporphyrin (trans-DATPP) (electron donor) and 4,4′-(hexafluoroisoprpoylidine)diphthalic anhydride (6FDA) (electron acceptor) as the building blocks for polymer memory applications. The chemical structures of the two polymers are mostly identical with the only difference lying in the zinc ion (Zn2+) insertion into the porphyrin core in the Zn-Por-6FDA. Electrical characterization indicates that the NH-Por-6FDA possesses bidirectional nonvolatile write once read many times (WORM) memory behavior, while the Zn-Por-6FDA shows vastly different volatile static random access memory (SRAM) behavior. Both polymer memory devices show high ON/OFF current ratio up to 106 and exhibit excellent long-term operation stability in 108 read cycles and retention time of 4000 s with no current degradation. The charge transfer (CT) and function of the donor/acceptor moiety in the polymers related w...

Tunable Switching Characteristics of Low Operating Voltage Organic Bistable Memory Devices Based on Gold Nanoparticles and Copper Phthalocyanine Thin Films

The influence of the top contact electrode on the switching characteristics of a low operating voltage organic bistable memory device, using copper phthalocyanine and gold nanoparticle thin films, was investigated using Au, Al, and Hg electrodes. While the ON/OFF ratio higher than 105 was achieved for all the devices, the nature of the memory behavior was found to be dependent on the top electrodes. Thermally evaporated Au and Al electrodes resulted in write-once–read-many times (WORM) behavior, whereas Hg drop soft contact led to write–read–erase–read (rewritable) characteristics, with the device retaining the ON state in the former and returning to the OFF state in the latter. The switching voltage was found to be influenced by the top electrode with the devices switching to the ON state at around 2 V for Hg and close to 1 V for Au and Al electrodes. Additionally, though the ON state conduction mechanism was dominated by Fowler–Nordheim (FN) tunneling through AuNP trap states in all the devices, the dyn...

Main chain copolysiloxanes with terthiophene and perylenediimide units: synthesis, characterization and electrical memory

A main chain donor–acceptor copolysiloxane PBIClSi-alt-PTSi for resistor type memory has been designed and synthesized. The PBIClSi-alt-PTSi possesses high thermal stability and shows nonvolatile write-once-read many times (WORM) memory characteristics.

A reversible bipolar WORM device based on AlOxNy thin film with Al nano phase embedded

An Al-rich AlOxNy thin film based reversible Write-Once-Read-Many-Times (WORM) memory device with MIS structure could transit from high resistance state (HRS, ∼1011Ω) to low resistance state (LRS, ∼105Ω) by sweeping voltage up to ∼20V. The first switching could be recorded as writing process for WORM device which may relate to conductive path are formed through the thin film. The conductive path should be formed by both Al nano phase and oxygen vacancies. Among of them, Al nano phases are not easy to move, but oxygen vacancies could migrate under high E-field or at high temperature environment. Such conductive path is not sensitive to charging effect after it formed, but it could be broken by heating effect, which may relate to the migration of excess Al ions and oxygen vacancies at high temperature. After baking LRS (ON state) WORM device at 200°C for 2min, the conductivity will decrease to HRS which indicates conductive path is broken and device back to HRS (OFF state) again. This phenomenon could be recorded as recovery process. Both writing and recovery process related to migration of oxygen vacancies and could be repeated over 10 times in this study. It also indicates that there is no permanent breakdown occurred in MIS structured WORM device operation. We suggest that this conductive path only can be dissolved by a temperature sensitive electro-chemical action. This WORM device could maintain at LRS over 105s with on-off ratio over 4 orders.

Electrical bistable properties of nonvolatile memory device based on hybrid ZCIS NCs:PMMA film

A write-once–read–many-times (WORM) electrical bistable device was fabricated utilizing hybrid quaternary Zn-Cu-In-S nanocrystals (ZCIS NCs) and poly(methyl methacrylate) (PMMA) as active layer. A low turn-on voltage of 1.0V and the maximum ON/OFF current ratio of 4×103 were found from current-voltage (I-V) characteristics. The current stability in the ON and OFF state has a fluctuation loss less than 5% in the measurement duration of 104s. The electron transport and memory mechanism are described based on the fitted I-V curves and the energy band diagrams. The conduction models in the OFF state from 0V to 1.1V are described as thermionic emission (TE), space-charge-limited current (SCLC) and Fowler–Nordheim (FN) tunneling, respectively. And the Ohmic conduction dominates when the device is transited to the ON state at the voltage over 1.1V. The high-density defects in quaternary ZCIS NCs and the high barrier between ZCIS NCs and the PMMA are considered to contribute to the electron retention behavior. For device performance optimization, an additional PMMA layer is pre-deposited on the ITO substrate surface. The 50% reduction at turn-on voltage and over 50 times enhancement in ON/OFF ratio are reasonably attributed to the better film quality and decreased series resistance from the decrease of root-mean-square average surface roughness (RMS) of the ITO surface from 0.877 to 0.216nm. The better retention capacity is considered to relate to thickened tunnel barrier by PMMA inserted layer.

High-performance stretchable resistive memories using donor–acceptor block copolymers with fluorene rods and pendent isoindigo coils

Diblock copolymers consisting of electron-donating poly[2,7-(9,9-dihexylfluorene)] (PF) rods and electron-withdrawing poly(pendent isoindigo) (Piso) coils were designed and synthesized through a click reaction. The electronic properties and interchain organization of the copolymers could be tuned by varying the PF/Piso ratio (PF14-b-Pison (n=10, 20, 60 and 100)). The highest occupied molecular orbital and lowest unoccupied molecular orbital energy levels of the studied polymers were progressively reduced as the length of Piso increased, affecting the charge trapping and intramolecular charge transfer environment between PF and Piso domains. Thermally treated PF14-b-Pison thin films exhibited a clear nanofibrillar structure, and the d-spacing was enhanced systematically as the Piso chain length increased. Resistive memory characteristics were explored with a sandwich indium tin oxide/PF14-b-Pisons/Al device configuration. The enhanced conjugated PF conducting channels led to stable resistance switching behavior, exhibiting volatile SRAM (static random access memory) (PF14-b-Piso10) and nonvolatile WORM (write-once-read-many-times) (PF14-b-Piso20, PF14-b-Piso60, PF14-b-Piso100) characteristics with a large ON/OFF ratio (106) and a stable retention time (104 s). A more appealing feature is that such memory cells were integrated on a soft poly(dimethylsiloxane) substrate, allowing for the development of a stretchable data storage device. Reliable and reproducible electrical characteristics, including SRAM- and WORM-type memories, could be explored as the device was stretched under an applied tensile strain ranging from 0 to 50%. The studied donor–acceptor copolymers indeed showed great potential for stretchable electronic applications with controllable digital information storage characteristics. A copolymer whose electrical properties can be tuned by varying the ratio of its two blocks is promising for wearable data storage devices. Polymer–based memory devices have excited much interest because of their flexibility, low cost and printability, but so far no stretchable memory devices based on copolymers made up of electron–donating and –accepting blocks have been made. Now, Wen–Chang Chen of National Taiwan University and co–workers have fabricated a series of copolymers made of electron-donating rods and electron–accepting coils. The rods impart the copolymer with superior electrical properties, while the coils give it flexibility. Varying the ratio of the rods to coils allows the copolymer's properties to be varied. The researchers demonstrated that reproducible electrical characteristics can be obtained even when tensile strains of up to 50% are applied. Stretchable resistive memories were created using donor–acceptor rod-coil diblock copolymers, poly(polyfluorene)-b-poly(pendent isoindigo), with different block ratios. Reliable and reproducible electrical characteristics of volatile SRAM- and nonvolatile WORM-type memories were exhibited under applied tensile strains ranging from 0 to 50 %, demonstrating the potential to serve as promising candidates for high-performance wearable devices.

Quadrupolar (A-π-D-π-A) Tetra-aryl 1,4-Dihydropyrrolo[3,2-b]pyrroles as Single Molecular Resistive Memory Devices: Substituent Triggered Amphoteric Redox Performance and Electrical Bistability

A series of quadrupolar (A-π-D-π-A) tetra-aryl 1,4-dihydropyrrolo[3,2-b]pyrrole (DHPP) derivatives synthesized are showcased as potential organic resistive memory (ORM) devices for the first time. The experimental observations coupled with density functional theory (DFT) calculations probe in detail the role of terminal substituent groups (p-NH2, p-Cl, p-CN, p-NO2, m-NO2) on the optical and electrical properties. Electrochemical studies reveal that the 3- and 4-dinitro derivatives form an unusual class of tetra-aryl DHPPs that exhibit intrinsic amphoteric redox behavior contrary to the literature reports. The bipolar nature within a single molecule was harnessed to design operational ORMs. Interestingly, the memory devices fabricated using the structural isomers exhibited dissimilar memory characteristics. While the p-NO2 derivative displays permanent Write Once Read Many times (WORM) memory, its meta-counterpart represents a behavior akin to rewriteable flash memory. The noticeably higher ON/OFF ratio (∼...

Memory behavior of multi-bit resistive switching based on multiwalled carbon nanotubes

Multiwalled carbon nanotube (MWCNT) displays peculiar electrical behavior, with its nano-configuration consisting of exceptional graphene flakes. As for MWCNTs blended into polymethyl methacrylate (PMMA), sandwiched Ni/MWCNTs(2 wt%):PMMA/ITO was manufactured to investigate into the multi-bit resistive switching regarding the turn-on compliance current as well as the thickness of the active layer. It bears ternary write-once read-many-times (WORM) memory, whose current proportionality between ON-state and OFF-state can exceed 107. Moreover, the memory performance, covering the long-term retention (>106 s), better endurance (>1012 cycles) and device-to-device profiles, confirms the excellent ternary memory of MWCNTs:PMMA. Concentration on multi-bit resistive switching in respect of MWCNTs underlies performance enhancement, higher integration and advanced architecture.

Synthesis and memory characteristics of novel soluble polyimides based on asymmetrical diamines containing carbazole

Two novel asymmetrical carbazole-based diamines 9-(2-(1,1′-binaphthyl-4-yl)benzyl)-9H-carbazole-3,6-diamine (BNBCD) and 9-((4′-(9H-carbazol-9-yl)biphenyl-2-yl)methyl-9H-carbazole-3,6-diamine (CBMCD) were synthesized. A series of novel soluble aromatic polyimides were prepared from these diamines by poly-condensation with Pyromelitic dianhydride (PMDA) and 2,2′,3,3′-biphenyl tetracarboxylic dianhydride (BPDA) via a two-step procedure. The resulting polymers were fully characterized, they exhibited excellent organosolubility and high thermal stability with the temperature of 5% weight loss under nitrogen atmosphere over 400 °C. Resistive switching devices with the configuration of Al/polymer/ITO were constructed from these polyimides by using conventional solution coating process. The memory devices based on PI-a, PI-b and PI-c exhibited a flash type memory capability, whereas the PI-d presented a write once read many times (WORM) memory capability.

Structural Phase Transition Effect on Resistive Switching Behavior of MoS2 -Polyvinylpyrrolidone Nanocomposites Films for Flexible Memory Devices.

The 2H phase and 1T phase coexisting in the same molybdenum disulfide (MoS2 ) nanosheets can influence the electronic properties of the materials. The 1T phase of MoS2 is introduced into the 2H-MoS2 nanosheets by two-step hydrothermal synthetic methods. Two types of nonvolatile memory effects, namely write-once read-many times memory and rewritable memory effect, are observed in the flexible memory devices with the configuration of Al/1T@2H-MoS2 -polyvinylpyrrolidone (PVP)/indium tin oxide (ITO)/polyethylene terephthalate (PET) and Al/2H-MoS2 -PVP/ITO/PET, respectively. It is observed that structural phase transition in MoS2 nanosheets plays an important role on the resistive switching behaviors of the MoS2 -based device. It is hoped that our results can offer a general route for the preparation of various promising nanocomposites based on 2D nanosheets of layered transition metal dichalcogenides for fabricating the high performance and flexible nonvolatile memory devices through regulating the phase structure in the 2D nanosheets.

Write-once memory effects observed in Ga-doped ZnO/organic semiconductor/MoO3/Au structures

Electrical switching phenomena in a device configuration of inverted organic light-emitting diodes have been observed. The device structure was Ga-doped ZnO (GZO)/N,N′-dicarbazolyl-3,5-benzene (m-CP)/MoO3 (10 nm)/Au (50 nm) fabricated by solution coating. The devices are write-once-read-many-times (WORM) memory devices with low switching voltage (<3 V) and long retention time (>700 h). Equivalent circuits in OFF and ON states are determined from the Cole–Cole plots measured by impedance spectroscopy. The switching phenomena and nonvolatile memory behavior are attributable to the formation conductive Au filaments in the m-CP layer of the ON state.

Bi-stable switching behaviors of ITO/EVA:ZnO NPs/ITO transparent memory devices fabricated using a thermal roll lamination technique

This research reported the transparent non-volatile write-once-read-many-times (WORM) memory behaviors of memory devices based on zinc oxide nanoparticles (ZnO NPs) embedded in an insulating poly(ethylene-co-vinyl acetate) (EVA), sandwiched between two ITO-coated flexible polyethylene naphthalate (PEN) substrates. The memory devices were fabricated employing a thermal roll lamination technique with the laminated-structure of PEN/ITO/EVA:ZnO NPs/ITO/PEN. The average transmittance of the laminated memory device was over 70% for an optical visible range of 400–800 nm. The maximum ON/OFF current ratio for the memory device was about 103. In addition, a reliability study for continuous read operations in a long time memory device is presented. The conductance switching mechanisms of the laminated memory device were analyzed using theoretical models on the basis of the experimental data.

The influence of thickness on memory characteristic based on nonvolatile tuning behavior in poly(N-vinylcarbazole) films

The memory characteristic based on nonvolatile tuning behavior in indium tin oxide/poly(N-vinylcarbazole)/aluminum (ITO/PVK/Al) was investigated, the different memory behaviors were first observed in PVK film as the film thickness changing. By control of PVK film thickness with different spinning speeds, the nonvolatile behavior of ITO/PVK/Al sandwich structure can be tuned in a controlled manner. Obviously different nonvolatile behaviors, such as (i) flash memory behavior and (ii) write-once-read-many times (WORM) memory behavior are from the current–voltage (I–V) characteristics of the PVK films. The results suggest that the film thickness plays a key part in determining the memory type of the PVK.

Improving Memory Performances by Adjusting the Symmetry and Polarity of O-Fluoroazobenzene-Based Molecules.

Three O-fluoroazobenzene-based molecules were chosen as memory-active molecules: FAZO-1 with a D-A2-D symmetric structure, FAZO-2 with an A1-A2-A1 symmetric structure, and FAZO-3 with a D-A2-A1 asymmetric structure. Both FAZO-1 and FAZO-2 had a lower molecular polarity, whereas FAZO-3 had a higher polarity. The fabricated indium-tin oxide (ITO)/FAZO-1/Al (Au) and ITO/FAZO-2/Al (Au) memory devices both exhibited volatile static random access memory (SRAM) behavior, whereas the ITO/FAZO-3/Al (Au) device showed nonvolatile ternary write-once-read-many-times (WORM) behavior. It should be noted that the reproducibility of these devices was considerably high, which is significant for practical application in memory devices. In addition, the different memory performances of the three active materials were determined to be attributable to the stability of electric-field-induced charge-transfer complexes. Therefore, the switching memory behavior could be tuned by adjusting the molecular polarity.

Fully transparent nonvolatile resistive polymer memory

ABSTRACTWe present a fully transparent nonvolatile resistive polymer memory device based on an anthracene‐containing partially aliphatic polyimide along with indium tin oxide (ITO) top and bottom electrodes. High transmittance of over 90% in the wavelength range of 400 to 800 nm is accomplished with an ITO/polyimide/ITO/glass device. The device shows unipolar write‐once‐read‐many times (WORM) memory behavior with an ON/OFF current ratio of ∼2 × 103, and the ratio remained without any significant degradation for over 104 s. The memory behavior of the device is considered to be governed by trap‐controlled space‐charge limited conduction (SCLC) and local filament formation. Based on molecular simulation of the polyimide, the location of energy states is different from that in the conventional charge transfer (CT) mechanism. Despite the relatively low ON/OFF current ratio, our results can give insight into the development of fully transparent memory device. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016, 54, 918–925

The effect of changing electrode metal on solution-processed flexible titanium dioxide memristors

Flexible solution-processed memristors show different behaviour dependent on the choice of electrode material. Use of gold for both electrodes leads to switchable WORM (Write Once Read Many times) resistive devices. Use of aluminium for both electrodes allows both curved (wholly non-linear) and triangular (linear ohmic low resistance state) memristive switching resistance memories. A comparison device with an aluminium bottom electrode and gold top electrode only exhibited significant memristive resistance switching when the aluminium electrode was the anode, suggesting that the electrode is acting as a source/sink of oxygen anions. Using the gold electrode as the anode causes oxygen evolution and electrode deformation. We conclude aluminium is helpful for stabilising and promoting memristive behaviour in sol–gel TiO2 devices. On and Off resistance states were found to correlate to device size, and the relative proportions of curved to triangular switching devices could be affected by vacuum curing of the gel layer and compliance current. We postulate that: A. the curved memristor switching is a bulk action compliant with Chua's description of a memristor; B. the triangular switching involves a filament conduction for the ohmic low resistance state.

Changing the stability of polymer-based memory devices in high conductivity state via tuning the red-ox property of Hemin

Two polymer-Hemin composites, AB type block copolymer (PStCH-b-P4VP): Hemin and homopolymer of poly(4-vinylpyridine) (P4VP): Hemin, can be fabricated into sandwich memory devices. The memory device characteristics of the PStCH-b-P4VP: Hemin blends can show Flash memory behavior, while the P4VP polymer with Hemin only shows write-once-read-many-times (WORM) performance. Charge transfer and nanocomposite redox are probably responsible for the conductance-switching behavior. The results of cyclic voltammogram (CV) measurement indicate that the reversible FeIII/FeII redox reaction is preserved well in the composite PStCH-b-P4VP: Hemin, while only polymer oxidation is observed in the P4VP: Hemin system. Due to the hindrance of StCH moiety, the coordination of pyridine and FeII can keep the FeII active. The P4VP has a more flexible chain that saturates the FeII center and results in stable coordination; FeIII/FeII redox reaction is therefore an obstacle. By tuning the coordination surroundings of the metal ion, different memory types can be obtained.

Functionalization of graphene with self-doped conducting polypyrrole by click coupling

The synthesis of self-doped conducting polypyrrole-grafted graphene sheets (GS-PPy) for non-volatile memory applications is reported. First, the alkyne-modified graphene sheets (GS-alkyne) were covalently functionalized with a water-soluble polymer containing numerous anionic SO3− dopants by a copper-catalyzed click reaction. Then, polypyrrole was covalently grafted onto the functionalized graphene sheets by chemical oxidative polymerization to produce GS-PPy hybrids. The GS-PPy hybrids showed a uniform coating of PPy on the GS sheets, good dispersion in aqueous solutions, high electrical conductivity, and red-shifted absorption peak in the UV/Visible spectra. The non-volatile memory device composed of a Al/(GS-PPy/poly(vinyl alcohol))/Al structure, produced by spin coating of the aqueous GS-PPy/poly(vinyl alcohol) solution, showed a good write-once read-many times memory behavior, which was due to good electrical and optical absorption properties of the GS-PPy hybrids. The findings of this study provide a potential solution for the fabrication of water-soluble graphene-based hybrids for non-volatile resistive-memory-based applications.