Flexible molecular-scale electronic devices.

Flexible materials and devices could be exploited in light-emitting diodes, electronic circuits, memory devices, sensors, displays, solar cells and bioelectronic devices. Nanoscale elements such as thin films, nanowires, nanotubes and nanoparticles can also be incorporated into the active films of mechanically flexible devices. Large-area devices containing extremely thin films of molecular materials represent the ultimate scaling of flexible devices based on organic materials, but the influence of bending and twisting on the electrical and mechanical stability of such devices has never been examined. Here, we report the fabrication and characterization of two-terminal electronic devices based on self-assembled monolayers of alkyl or aromatic thiol molecules on flexible substrates. We find that the charge transport characteristics of the devices remain stable under severe bending conditions (radius ≤ 1 mm) and a large number of repetitive bending cycles (≥1,000). The devices also remain reliable in various bending configurations, including twisted and helical structures.

Organic nonvolatile memory devices with charge trapping multilayer graphene film.

We fabricated an array-type organic nonvolatile memory device with multilayer graphene (MLG) film embedded in polyimide (PI) layers. The memory devices showed a high ON/OFF ratio (over 10(6)) and a long retention time (over 10(4) s). The switching of the Al/PI/MLG/PI/Al memory devices was due to the presence of the MLG film inserted into the PI layers. The double-log current-voltage characteristics could be explained by the space-charge-limited current conduction based on a charge-trap model. A conductive atomic force microscopy found that the conduction paths in the low-resistance ON state were distributed in a highly localized area, which was associated with a carbon-rich filamentary switching mechanism.

Structural and electrical characterization of a block copolymer-based unipolar nonvolatile memory device.

Electronic devices based on a series of synthesized block copolymers are demonstrated. In particular, a block copolymer system with a lamellar structure exhibits unipolar switching behavior. This study provides a simple strategy based on the adjustment of the block ratio in block copolymers to control the polymer morphology and thus the electrical and switching properties of polymer-based memory devices.

Nonvolatile write-once-read-many times memory devices based on the composites of poly(4-vinylphenol)/Vulcan XC-72.

We fabricated write-once-read-many times (WORM) type organic memory devices in 8 x 8 cross-bar structure. The active material for organic based WORM memory devices is mixture of both poly(4-vinyphenol) (PVP) and Vulcan XC-72s. From the electrical characteristics of the WORM memory devices, we observed two different resistance states, low resistance state and high resistance state, with six orders of ON/OFF ratio (I(ON)/I(OFF) - 10(6)). In addition, the WORM memory devices were maintained for longer than 50000 seconds without any serious degradation.

Flexible organic memory devices with multilayer graphene electrodes.

We fabricated 8 × 8 cross-bar array-type flexible organic resistive memory devices with transparent multilayer graphene (MLG) electrodes on a poly(ethylene terephthalate) substrate. The active layer of the memory devices is a composite of polyimide and 6-phenyl-C61 butyric acid methyl ester. The sheet resistance of the MLG film on memory device was found to be ∼270 Ω/◻, and the transmittance of separated MLG film from memory device was ∼92%. The memory devices showed typical write-once-read-many (WORM) characteristics and an ON/OFF ratio of over ∼10(6). The memory devices also exhibited outstanding cell-to-cell uniformity with flexibility. There was no substantial variation observed in the current levels of the WORM memory devices upon bending and bending cycling up to 10 000 times. A retention time of over 10(4) s was observed without fluctuation under bending.

Unipolar bistable switching of organic non-volatile memory devices with poly(styrene-co-styrenesulfonic acid Na).

We demonstrated unipolar organic bistable memory devices with 8 x 8 cross-bar array type structure. The active material for the organic non-volatile memory devices is poly(styrene-co-styrenesulfonic acid Na) (PSSANa). From the electrical measurements of the PSSANa organic memory devices, we observed rewritable unipolar switching behaviors with a stable endurance and narrow cumulative probability. Also the PSSANa memory devices exhibited a uniform cell-to-cell switching with a high ON/OFF ratio of approximately 10(5) and good retention time of approximately 10(4) seconds without significant degradation.

Tunable electronic transport characteristics of surface-architecture-controlled ZnO nanowire field effect transistors.

Surface-architecture-controlled ZnO nanowires were grown using a vapor transport method on various ZnO buffer film coated c-plane sapphire substrates with or without Au catalysts. The ZnO nanowires that were grown showed two different types of geometric properties: corrugated ZnO nanowires having a relatively smaller diameter and a strong deep-level emission photoluminescence (PL) peak and smooth ZnO nanowires having a relatively larger diameter and a weak deep-level emission PL peak. The surface morphology and size-dependent tunable electronic transport properties of the ZnO nanowires were characterized using a nanowire field effect transistor (FET) device structure. The FETs made from smooth ZnO nanowires with a larger diameter exhibited negative threshold voltages, indicating n-channel depletion-mode behavior, whereas those made from corrugated ZnO nanowires with a smaller diameter had positive threshold voltages, indicating n-channel enhancement-mode behavior.

Comparison of Si doping effect on GaN nanowires and films synthesized by metal-organic chemical vapor deposition.

We investigated Si doping effect on GaN nanowires and GaN films grown by metal-organic chemical vapor deposition (MOCVD). Si as n-type dopant is incorporated to GaN nanowires and GaN films controlled by SiH4 flow rate (0, 1, 5, 8, and 10 sccm). The charge concentration and mobility of GaN films increased and decreased, respectively, as increasing the SiH4 flow rate, whereas those for GaN nanowires were not influenced by the SiH4 flow rate. Significant vacancies and impurities resulted in the intense yellow band in GaN nanowires as compared with GaN films, which leads to the large device-to-device variation and negligible dependence of Si doping and the SiH4 flux rate on the electrical properties of GaN nanowires.