Qi-Dan Ling

An Effective Friedel−Crafts Postfunctionalization of Poly(N-vinylcarbazole) to Tune Carrier Transportation of Supramolecular Organic Semiconductors Based on π-Stacked Polymers for Nonvolatile Flash Memory Cell

Poly(N-vinylcarbazole) (PVK) and its derivatives are π-stacked polymers of the most important supramolecular organic semiconductors (SOSs), in which semiconducting features are originated from intra-supramolecular interactions. An effective Friedel−Crafts method has been developed to postfunctionalize PVK to a PVK−PF SOS and to tune the fundamental electronic structures and transporting properties of the resulting SOS. Stable nonvolatile flash memory effect from the SOS has been demonstrated in an ITO/PVK−PF/metal sandwich device. The device exhibited an ON/OFF current ratio up to 104, and writing/erasing voltages around +2.2/−2.0 V, respectively. The unique electrical bistability can be attributed to the ordering alignment effect induced by electric field and the hindrance effect arisen from bulky moieties. Thus, PVK-containing complicated 9,9-diarylfluorenes (CDAFs) are promising materials for information storage applications.

Electrical Conductance Tuning and Bistable Switching in Poly(N-vinylcarbazole)−Carbon Nanotube Composite Films

By varying the carbon nanotube (CNT) content in poly(N-vinylcarbazole) (PVK) composite thin films, the electrical conductance behavior of an indium-tin oxide/PVK-CNT/aluminum (ITO/PVK-CNT/Al) sandwich structure can be tuned in a controlled manner. Distinctly different electrical conductance behaviors, such as (i) insulator behavior, (ii) bistable electrical conductance switching effects (write-once read-many-times (WORM) memory effect and rewritable memory effect), and (iii) conductor behavior, are discernible from the current density-voltage characteristics of the composite films. The turn-on voltage of the two bistable conductance switching devices decreases and the ON/OFF state current ratio of the WORM device increases with the increase in CNT content of the composite film. Both the WORM and rewritable devices are stable under a constant voltage stress or a continuous pulse voltage stress, with an ON/OFF state current ratio in excess of 10(3). The conductance switching effects of the composite films have been attributed to electron trapping in the CNTs of the electron-donating/hole-transporting PVK matrix.

Bistable electrical switching and write-once read-many-times memory effect in a donor-acceptor containing polyfluorene derivative and its carbon nanotube composites

A conjugated copolymer of 9,9-didodecylfluorene and 4-triphenylamino-2,6-bis(phenyl)pyridine (F12TPN), containing both electron donor and acceptor moieties, was synthesized via Suzuki coupling polymerization. Polymer memory devices, based on thin films of F12TPN and its carbon nanotube composites, were fabricated. The current density-voltage characteristics of the indium tin oxide (ITO)/F12TPN/Al sandwich structure could be switched from a low conductivity (off) state to a high conductivity (on) state, when operated under negative bias with ITO as the anode. The switch-on voltage of the as-fabricated device was around −2.3V. The on/off state current ratio was about 10. The on/off state current ratio could be enhanced to 105 and the switch-on voltage reduced to about −1.7V by doping the F12TPN layer with about 1wt% of carbon nanotubes. Once the memory devices had been switched on, they could not be erased (switched off) with a reverse bias (positive bias). Thus the memory effect was characteristics of that...

Volatile electrical switching in a functional polyimide containing electron-donor and -acceptor moieties

A solution-processable functional polyimide (PYTPA-PI), containing triphenylamine-substituted diphenylpyridine moieties (PYTPA, electron donors) and phthalimide moieties (PI, electron acceptors), was synthesized. The copolymer exhibits a high glass transition temperature of 342 °C. A switching device, based on a solution-cast thin film of PYTPA-PI sandwiched between an indium-tin oxide (ITO) bottom electrode and an Al top electrode, exhibits two accessible conductivity states and can be switched from the low-conductivity (OFF) state to the high-conductivity (ON) state, with an ON/OFF current ratio of more than 103, at the threshold voltage of about 2.7 V. The ON state is volatile and relaxes readily to the OFF state. However, it can be electrically sustained by a refreshing voltage pulse of 2 V. The ON state can also be reset to the initial OFF state by a reverse bias of −0.9 V. The ability to write, erase, read, and refresh the electrical states fulfills the functionality of a dynamic random access memor...

Electrically Bistable Thin-Film Device Based on PVK and GNPs Polymer Material

We present an electrical-bistability device based on MIM-sandwiched structure. Poly(N-vinylcarbazole) (PVK) mixed with gold nanoparticles (GNPs) serve as the active layer between two metal electrodes. After applying a voltage, the as-fabricated device can transit from low conductivity state to high conductivity state. By simply using a reverse bias, the high conductivity state can return to the low conductivity state. An on/off current ratio as high as 105 at room temperature has been achieved. The memory effect is attributed to electric-field-induced charge transfer complex formed between the PVK and the GNPs. The device shows a good stability under stress test for both states and exhibits a high potential on Flash-type memory applications

Theoretical Studies of the Structural, Electronic, and Optical Properties of Phosphafluorenes

Phosphafluorenes have drawn increasing attention recently in the applications of organic electronic devices due to their particular optoelectronic properties. To reveal their molecular structures, optoelectronic properties, and structure-property relationships of the newly emerged functional materials, an in-depth theoretical investigation was elaborated via quantum chemical calculations. The optimized geometric and electronic structures in both ground and exited states, the mobility of the hole and electron, the absorption and emission spectra, and the singlet exciton generation fraction of these novel phosphors-containing materials have been studied by density functional theory (DFT), single excitation configuration interaction (CIS), time-dependent density functional theory (TDDFT) methods, and the polarizable continuum model (PCM). The results show that the highest occupied molecular orbitals (HOMOs), the lowest unoccupied molecular orbitals (LUMOs), triplet energies ((3)E(g)), energy gaps (E(g)), as well as some other electronic properties including ionization potentials (IPs), electron affinities (EAs), reorganization energies (lambda), the singlet exciton generation fraction, radiative lifetime, and absorption and emission spectra can be easily tuned by chemical modifications of the phosphorus atom via methyl, phenyl, oxygen, sulfur, or selenium substitution, indicating that the phosphafluorenes are interesting optoelectronic functional materials, which have great potential in the applications of OLEDs, organic solar cells, organic storage, and sensors.

Monochromatic light-emitting copolymers of N-vinylcarbazole and Eu-complexed 4-vinylbenzoate and their single layer high luminance PLEDs

A series of bifunctional copolymers containing hole transporting units and light-emitting units in the main chain had been synthesized via radical copolymerization of N-vinylcarbazole and Eu-complexed 4-vinylbenzoate. The chemical structure and composition of the copolymers were characterized by FT-IR and UV-visible absorption spectroscopy, 1H and 13C NMR spectroscopy, GPC, ICP, mass spectroscopy, and elemental analysis. The copolymers exhibited good solubility, as well as good thermal stability and high glass transition temperatures. The photoluminescence (PL) properties of the copolymers in solution and in film form were investigated. Intramolecular energy transfer from the carbazole groups to the europium complex occurred even in diluted solutions. The efficiency of this process depended also on the composition of the copolymers. The PL spectra of the copolymers consisted of two emission bands, one in the 340–550 nm region and another at around 612 nm, corresponding to the π* → π transitions of the carbazole moieties and the f–f transitions of the europium ions, respectively. In the solid state, emission from the carbazole groups was suppressed and the absorbed excitation energy was transferred effectively to the europium complexes in the copolymer. Nearly monochromatic red emission with a turn-on voltage of 8 V was detected from the single layer copolymer light-emitting diode (PLED). This single layer PLED exhibited a maximum luminance of 126 cd m−2 at 22 V and 328 mA cm−2, and a luminance efficiency of 0.56 cd A−1.

Thermally stable polymer memory devices based on a π-conjugated triad

Polymer memory devices based on a π-conjugated poly [9,9-bis(4-diphenylaminophenyl)-2,7-fluorene] covalently bridged, Disperse Red 1 (a dye) triad were fabricated. The devices exhibit electrical bistability in the I-V characteristics and can be used to perform read-write-erase memory functions. The memory devices exhibit good performance with an on/off current ratio up to 103 and stable on and off states under a constant voltage stress and read pulses. Furthermore, memory retention tests show that it is possible to preserve both states at 150°C under ambient atmosphere for about 1h when using Cu as the top electrode.

Memory performance of a thin-film device based on a conjugated copolymer containing fluorene and chelated europium complex

A memory device based on the sandwiched structure of a conjugated copolymer (PF8Eu), containing fluorene and chelated europium complex, has been fabricated. An electrical bistability phenomenon was observed on this device: low conductivity state for the as-fabricated device and high conductivity state after device transition by applying a voltage of /spl sim/3 V. At the low conductivity state, the device showed a charge injection controlled current and at the high conductivity state, the device showed a space charge limited current. At the same applied voltage, the device exhibited two distinguishable conductivities with an ON/OFF current ratio as high as 10/sup 6/ at room temperature. After transition to the high conductivity state, the device tended to remain in the high conductivity state even when the applied voltage was removed. Thus, the device is a potential write-once-read-many-times memory device.

WORM-Type Memory Device Based on a Conjugated Copolymer Containing Europium Complex in the Main Chain

A conjugated copolymer of 9,9-dihexylfluorene and Eu-complexed benzoate (PF6Eu) for write-once read-many-times (WORM) memory application was demonstrated in a sandwich structure of Al/PF6Eu/ITO. The device exhibited: a high ON/OFF current ratio up to 10 7 , stable ON and OFF states with read cycles up to 10 8 at a read voltage of 1 V, and projected stability up to 10 years at a constant stress of I V.