Guofeng Tian

Dynamic random access memory effect and memory device derived from a functional polyimide containing electron donor-acceptor pairs in the main chain.

A functional polyimide, hexafluoroisopropyl bis(phthalic dianhydride)/3,6-diaminocarbazole (6FDA/DAC), in which DAC serves as electron donor and 6FDA as electron acceptor, has been synthesized in our present work. Electrical characterization results on the sandwiched polyimide memory device (ITO/Thin polyimide Layer/Au) indicate that the polyimide possesses electrical bistability and the device exhibits two accessible conductivity states, which can be reversibly switched from the low-conductivity (OFF) state to the high-conductivity (ON) state with an ON/OFF current ratio of about 10(4). Different from the widely reported write-once-read-many-times (WORM) effects, the device with the 6FDA/DAC polyimide as the active layer shows dynamic random access memory (DRAM) behavior. The ON state of the device was lost immediately after removal of the applied voltage, while by applying a constant bias (e.g., 3 V) the ON state can be electrically sustained. The roles of donor and acceptor components in the polyimide main chain were elucidated through molecular simulation.

Nonvolatile memory effect of a functional polyimide containing ferrocene as the electroactive moiety

A functional polyimide, hexafluoroisopropyl bis(phthalic dianhydride)/4-(bis(4-aminophenyl) methyl)phenol grafted with ferrocene, was synthesized. Electrical characterization results indicate that the sandwiched devices using our synthesized polyimide as the active layer possess electrical bistability and exhibit nonvolatile memory behavior with an ON/OFF current ratio of about 103. Molecular orbitals and electronic properties are investigated by molecular simulation and cyclic voltammetry characterization. The charge transfer mechanisms in the OFF and ON states of the memory device were reasonably interpreted by using the thermionic emission and space-charge-limited-current model, respectively.

Nonvolatile write-once read-many-times memory device based on an aromatic hyperbranched polyimide bearing triphenylamine moieties

An aromatic hyperbranched polyimide, poly(N,N,N′,N′-tetrakis(4-aminophenyl)benzidine-N,N- 4,4′-hexafluoroisopropylidene-diphthalimide) (6F-TEAPBD PI), was synthesized. Semiconductor parameter analysis on the sandwich devices using the synthesized polyimide as the active layer indicates that the polymer possesses distinct electrical bi-stable states with an ON/OFF current ratio of about 300 and a switching voltage at around 2.0 V, which could be applied as nonvolatile write-once read-many-times (WORM) memory. Mechanisms associated with the electrical switching effect are discussed on the basis of the experimental and quantum simulation results. It is suggested that the electric-field-induced charge transport from triphenylamine moieties to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are responsible for the observed electrical memory effect.

Tuning the electrical memory characteristics from WORM to flash by α- and β-substitution of the electron-donating naphthylamine moieties in functional polyimides

Two novel functional aromatic polyimides (PIs), 6F-αNA PI and 6F-βNA PI, in which the hexafluoroisopropylidene-diphthalic anhydride (6FDA) serves as the electron-accepting unit and the diphenylnaphthylamine (DPNA) functions as the electron-donating species, were synthesized for memory device applications. The 6F-αNA PI shows distinct electrical bistable states with an ON/OFF current ratio up to 106, and can be switched on bi-directionally with no polarity, which could be applied as the nonvolatile write-once read-many times (WORM) memory. Whereas, the 6F-βNA PI-based memory device exhibits flash type memory characteristics with a switching-on voltage at ca. 1.1 V and an ON/OFF ratio of 104. Both polyimides exhibit good long-term operation stability, survive up to 108 reading cycles with no current degradation, and show ultrafast switching with a response time less than 20 ns. Mechanisms associated with the electrical switching behaviors are discussed on the basis of the experimental and quantum simulation results. The electric-field-induced electronic transition from diphenylnaphthylamine units to hexafluoropropylidene phthalimide units and the subsequent formation of charge-transfer complexes are supposed to be responsible for the observed electrical memory effects. Molecular simulation suggests that α-tethering of the naphthyl group results in more non-coplanar conformation of the DPNA species in the 6F-αNA PI, as compared to that of the β-tethering in the 6F-βNA PI, therefore producing a higher energy barrier that prevents the back charge transfer processes, consequently leading to the WORM vs. flash memory behaviors. The 6F-αNA PI differs from 6F-βNA PI only in the substitution position of the naphthyl group, i.e., α-tethering vs. β-tethering. The distinct memory effects observed here suggest the significance of the electron-donating structures on the memory effects, and the tailorability of the memory characteristics through fine structure adjustment.

Solution-processed high-performance flexible 9, 10-bis(phenylethynyl)anthracene organic single-crystal transistor and ring oscillator

Organic semiconductor of 9, 10-bis(phenylethynyl)anthracene (BPEA) single crystal ribbon with ultra-long length has been prepared by solution drop casting method, where the growth direction was controlled with the seed crystal. The BPEA single crystal ribbon based field-effect transistors show high hole mobility up to 3.2 cm2/V·s, and the inverters exhibited the highest gain of 92. The complex device such as 5-stage ring oscillator consisting of 10 transistors was also constructed on a single crystal ribbon. This straightforward methodology was applied to fabricate plastic transistors on the flexible substrate, showing high performance even after repeatedly bending of 300 times.

Nonvolatile electrical switching behavior observed in a functional polyimide thin film embedded with silver nanoparticles

In this paper, we report our works on the synthesis of a silver-nanoparticle-embedded polyimide (PI) thin film and its electrical bistability. A soluble PI, (4,4′-(hexafluoroisopropylidene) diphthalic anhydride/4,4′-oxydianiline (6FDA/ODA), where the 6FDA part serves as an effective electron-accepting moiety, was synthesized in our current work as the polymer matrix. Silver nanoparticles (Ag NPs) with diameters less than 7 nm were subsequently generated in situ in the parent PI film via ultraviolet (UV) reduction of the (1,1,1-trifluoro-2,4-pentadionato) silver(I) complex (AgTFA) previously incorporated in the matrix. Electrical characterization results on the sandwiched device (ITO|PI (6FDA/ODA)/silver nanohybrid film|Au) indicate that the nanohybrid material possesses electrical bistability and the device exhibits two accessible conductivity states, which can be reversibly switched from the low-conductivity state to the high-conductivity state with an ON/OFF current ratio of about 102. The device with the PI (6FDA/ODA)/silver nanohybrid film as the active layer shows nonvolatile memory behavior. The high-conductivity state and the low-conductivity state of the device can be sustained after the removal of the applied voltage. Mechanisms regarding the charge transfer in the nanohybrid material were discussed.

Enhanced surface free energy of polyimide fibers by alkali treatment and its interfacial adhesion behavior to epoxy resins

Abstract In this article, polyimide (PI) fibers were modified by alkali treatment, and PI fiber-reinforced epoxy composites were fabricated. The effects of different alkali treatment times on the surface properties of the PI fibers and the adhesion behaviors of PI fiber/epoxy composites were studied. The surface morphologies, chemical compositions, mechanical properties, and surface free energy of the PI fibers were characterized by atomic force microscopy, X-ray photoelectron spectroscopy, single-fiber tensile strength analysis, and dynamic contact angle analysis, respectively. The results show that alkali treatment plays an important role in the improvement of the surface free energy and the wettability of PI fibers. We also found that, after the 3 min, 30 °C, 20 wt% NaOH solution treatment, the fibers possessed good mechanical properties and surface activities, and the interlaminar shear strength of the composites increased to 64.52 MPa, indicating good interfacial adhesion properties.

Interfacial growth of controllable morphology of silver patterns on plastic substrates.

Controllable growth of newly born silver nanoparticles to fractal, cauliflower-like, microscale disks and continuous silver layers with high conductivity and reflectivity on plastic substrates has been developed via solid-liquid interfacial reduction and growing of ion-doped polymeric films. Such approaches involve polyimide (PI) films as substrates, its corresponding silver-ion-doped precursors as solid oxidants, and facile immersion of ion-doped polymeric films in aqueous reducing solution. The solution reducing process belongs to liquid-solid interfacial reduction processes, during which silver ions doped in polymeric matrix transformed to newly born silver nanoparticles which further aggregated and migrated along the liquid-solid interface to form dendrite, cauliflower-like and lamella disk-like architecture and/or severely compact continuous silver nanolayers with highly reflective and conductive properties. Time-dependent morphology evolutions of silver particles were traced by scanning electron microscopy (SEM), atomic force microscopy (AFM), and transmission electron microscopy (TEM). This strategy can also extend to synthesis of many other metals on polymeric films while maintaining outstanding metal-polymer adhesion based on incorporation of various metal ions, and may offer an opportunity to fabricate large scale, high-output, cost-effective processes for metal patterns on flexible polymeric substrates.

Dielectric behavior of a flexible three-phase polyimide/BaTiO3/multi-walled carbon nanotube composite film

A three-phase composite film was produced by inserting multi-walled carbon nanotubes (MWCNTs) and BaTiO3 nanoparticles into polyimide (PI). The combination of in-situ polymerization and water-based preparation involved in the experiment ensured fillers’ homogeneous dispersion in the matrix, which led to flexible shape of the composite films. The dielectric properties of composite films as a function of the frequency and the volume fraction of MWCNTs were studied. Such composite film displayed a high dielectric constant (314.07), low dielectric loss and excellent flexibility at 100Hz in the neighborhood of percolation threshold (9.02 vol%) owing to the special microcapacitor structure. The experimental results were highly consistent with the power law of percolation theory.

Enhanced Dielectric Permittivity and Thermal Stability of Graphene-Polyimide Nanohybrid Films

In this paper, we report our works on the fabrication of reduced graphene oxide (RGO) and graphene oxide (GO) embedded polyimide (PI) nanohybrid films and their dielectric and thermal properties. The RGO-PI films exhibit higher dielectric permittivity than the GO-PI films and the RGO-PI films show a lower percolation threshold of 0.0172 in volume fraction. The TGA and DMA analysis indicate that all the films have excellent thermal stability and thermo-mechanical properties, and the RGO-PI films exhibit a higher thermal decomposition temperature than GO-PI and pure PI films.