Realization of write-once-read-many-times memory devices based on poly(N-vinylcarbazole) by thermally annealing

A nonvolatile write-once-read-many-time (WORM-time) memory device based on poly(N-vinylcarbazole) (PVK) films was realized by thermally annealing. The device can be fabricated using a simple spin coat method. It was found that the control of PVK film surface morphology by thermally annealing plays an important role in achieving the WORM memory properties. The memory device showed an ON/OFF current ratio as high as 10(4) and the retention time was over 2000 s without degradation.

The morphology control of pentacene for write-once-read-many-times memory devices

We realized write-once-read-many-times (WORM) memory devices based on pentacene and demonstrated that the morphology control of the vacuum deposited pentacene thin film is greatly important for achieving the unique nonvolatile memory properties. The resulted memory devices show a high ON/OFF current ratio (10(4)), long retention time (over 12 h), and good storage stability (over 240 h). The reduction of the barrier height caused by a large interface dipole and the damage of the interface dipole under a critical bias voltage have been used to explain the transition processes.

Origin of negative differential resistance and memory characteristics in organic devices based on tris(8-hydroxyquinoline) aluminum

Negative differential resistance (NDR) and memory phenomenon have been realized in current-voltage (I-V) characteristics of indium tin oxide/tris(8-hydroxyquinoline) aluminum/aluminum devices. The I-V curves have been divided into three operational regions that are associated with different working regimes of the devices: (i) bistable region, (ii) NDR region, and (iii) monotonic region. The bistable region disappeared after a couple of voltage sweeps from zero to a positive voltage. The bistable nature can be reinstated by applying a suitable negative voltage.

Organic Thin-Film Transistor Memory With Nanoparticle Floating Gate

Organic thin-film transistor memory devices were realized by inserting a layer of nanoparticles (such as Ag or CaF2) between two Nylon 6 gate dielectrics as the floating gate. The transistor memories were fabricated on glass substrates by full thermal deposition. The transistors exhibit significant hysteresis behavior in current-voltage characteristics, due to the separated Ag or CaF2 nanoparticle islands that act as charge trap centers. The mechanism of the transistor memory operation was discussed.

Write-Once Read-Many-Times Memory Based on a Single Layer of Pentacene

We realized an organic electrical memory device with a simple structure based on single-layer pentacene film embedded between Al and ITO electrodes. The optimization of the thickness and deposition rate of pentacene resulted in a reliable device with an on/off current ratio as high as nearly 10(6), which was two orders of magnitude higher than previous results, and the storage time was more than 576 h. The current transition process is attributed to the formation and damage of the Interface dipole at different electric fields, in which the current conduction showed a transition from ohmic conductive current to Fowler-Nordheim tunneling current. After the transition from ON- to OFF-state, the device tended to remain in the OFF-State even when the applied voltage was removed, which indicated that the device was very promising for write-once read-many-times memory.

Nonvolatile memory devices based on gold nanoparticle and poly (N-Vinylcarbazole) composite

A rewritable polymer memory device based on gold nanoparticle doped poly (N-vinylcarbazole) (PVK), which can be easily fabricated by simple spin coating, has been described. An electrical bistable phenomenon is observed in the current-voltage characteristics of this device, and it is found that the electrical bistability is repeatable by proper writing voltage and erasing voltage. The unique behavior of the devices provides an interesting approach such that doping nanoparticles in polymer can be used to realize high performance nanovolatile polymer memory devices.

Shape memory effect of poly(L-lactide)-based polyurethanes with different hard segments

A series of biodegradable polylactide-based polyurethanes (PLAUs) were synthesized using PLA diol (M-n = 3200) as soft segment, 4,4 '-diphenylmethane diisocyanate (MDI), 2,4-toluene diisocyanate (TDI), and isophorone diisocyanate (IPDI) as hard segment, and 1,4-butanediol as chain extender. The structures and properties of these PLAUs were studied using infrared spectroscopy, differential scanning calorimetry, tensile testing, and thermomechanical analysis. Among them, the MDI-based PLAU has the highest T-g, maximum tensile strength, and restoration force, the TDI-based PLAU has the lowest T-g, and the IPDI-based PLAU has the highest tensile modulus and elongation at break. They are all amorphous. The shape recovery of the three PLAUs is almost complete in a tensile elongation of 150% or a twofold compression. They can keep their temporary shape easily at room temperature (20 degrees C). More importantly, they can deform and recover at a temperature below their T-g values. Therefore, by selecting the appropriate hard segment and adjusting the ratio of hard to soft segments, they can meet different practical demands for shape memory medical devices.

Biodegradable polyurethane based on random copolymer of L-lactide and epsilon-caprolactone and its shape-memory property

A series of biodegradable polyurethanes (PUs) are synthesized from the copolymer diols prepared from L-lactide and epsilon-caprolactone (CL), 2,4-toluene diisocyanate, and 1,4-butanediol. Their thermal and mechanical properties are characterized via FTIR, DSC, and tensile tests. Their T(g)s are in the range of 28-53 degrees C. They have high modulus, tensile strength, and elongation ratio at break. With increasing CL content, the PU changes from semicrystalline to completely amorphous. Thermal mechanical analysis is used to determine their shape-memory property. When they are deformed and fixed at proper temperatures, their shape-recovery is almost complete for a tensile elongation of 150% or a compression of 2-folds. By changing the content of CL and the hard-to-soft ratio, their T(g)s and their shape-recovery temperature can be adjusted. Therefore, they may find wide applications.

Shape-memory and biocompatibility properties of segmented polyurethanes based on poly(L-lactide)

A series of segmented poly (L-lactide)-polyurethanes (PLA-PU) were synthesized by a two-step method, with oligo-poly(L-lactide) (PLA) as the soft segments and the reaction product of 2,4-toluene diisocyanate(TDI) and ethylene glycol(EG) as the hard segments. The shape memory properties of PLA-PUs were examined. The processed PLA-PUs could recover almost 100% to their original shape within 10 degrees C from the lowest recovery temperature. In the recovery process, the PLA-PUs showed a maximum contracting stress of shape change in the range of 1.5-4 MPa depending on the PLA segmental length and the hard-segmental content and higher than that of poly (e-caprolactone polyurethane) (PCL-PU). Besides, the influence of deforming and fixing temperatures on shape memory properties of PLA-PU was studied in detail. They could affect not only the recovery temperature but also the maximum contracting stress. The experiments of cell incubation were used to evaluate the biocompatibility of PLA-PU. The results show that the biocompatibility of PLA-PU is comparable to that of the pure PLA. This kind of polyurethane can be used as implanted medical devices with a shape memory property.

The influence of hard-segments on two-phase structure and shape memory properties of PCL-based segmented polyurethanes

Poly(epsilon-caprolactone)-based segmented polyurethanes (PCLUs) were prepared from poly(epsilon-caprolactone) diol, diisocyanates (DI), and 1,4-butanediol. The DIs used were 4,4'-diphenylmethane diisocyanate (MDI), 2,4-toluenediisocyanate (TDI), iso-phorone diisocyanate (IPDI), and hexamethylene diisocyanate (HDI). Differential scanning calorimetry, small-angle X-ray scattering, and dynamic mechanical analysis were employed to characterize the two-phase structures of all PCLUs. It was found that HDI- and MDI-based PCLUs had higher degree of microphase separation than did IPDI- and TDI-based PCLUs, which was primarily due to the crystallization of HDI- and MDI-based hard-segments. As a result, the HDI-based PCLU exhibited the highest recovery force up to 6 MPa and slowest stress relaxation with increasing temperature. Besides, it was found that the partial damage in hard-segment domains during the sample deformation was responsible for the incomplete shape-recovery of PCLUs after the first deformation, but the damage did not develop during the subsequent deformation.

Negative differential resistance and memory effect in diodes based on 1,4-dibenzyl C60 and zinc phthalocyanine doped polystyrene hybrid material

Negative differential resistance (NDR) and memory effect were observed in diodes based on 1,4-dibenzyl C60 (DBC) and zinc phthalocyanine doped polystyrene hybrid material. Certain negative starting sweeping voltages led to a reproducible NDR, making the hybrid material a promising candidate in memory devices. It was found that the introduction of DBC enhanced the ON/OFF current ratio and significantly improved the memory stability. The ON/OFF current ratio was up to 2 orders of magnitude. The write-read-erase-reread cycles were more than 10(6), and the retention time reached 10 000 s without current degradation.

Memory devices based on lanthanide (SM3+, EU3+, Gd3+) complexes

Memory effects in single-layer organic light-emitting devices based on Sm3+, Gd3+, and Eu3+ rare earth complexes were realized. The device structure was indium-tin-oxide (ITO)/3,4-poly(ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT)/Poly(N-vinyl carbazole) (PVK): rare earth complex/LiF/Ca/Ag. It was found experimentally that all the devices exhibited two distinctive bistable conductivity states in current-voltage characteristics by applying negative starting voltage, and more than 10(6) write-read-erase-reread cycles were achieved without degradation. Our results indicate that the rare earth organic complexes are promising materials for high-density, low-cost memory application besides the potential application as organic light-emitting materials in display devices.

Performance improvement by charge trapping of doping fluorescent dyes in organic memory devices

We studied the memory effect in the devices consisting of dye-doped N, N'-di(naphthalene-1-yl)-N, N'-diphenyl-benzidine sandwiched between indium-tin oxide and Ag electrodes. It was found that the on/off current ratio was greatly improved by the doped fluorescent dyes compared with nondoping devices. A mechanism of charge trapping was demonstrated to explain the improvement of the memory effect. For the off state, the conduction process is dominated by the trapping current, which is a characteristic of the space-charge limited current, whereas the on state is dominated by the detrapping current, and interpreted by Poole-Frenkel emission.