Hung-Chin Wu

Nonvolatile memory based on pentacene organic field-effect transistors with polystyrenepara-substituted oligofluorene pendent moieties as polymer electrets

We report the nonvolatile memory characteristics of pentacene-based organic field-effect transistors (OFET) using polystyrenepara-substituted with π-conjugated oligofluorenes (P(St-Fl)n (n = 1–3)) as chargeable polymer electrets. Effects of fluorene conjugated length on the surface structure and memory characteristics of pentacene OFET were investigated. Among these polymer electrets, the device with the P(St-Fl) exhibited the highest field-effect mobility of 0.47 cm2 V−1 s−1 due to the largest grain size of pentacene growth. The device with P(St-Fl)3 revealed the largest hysteresis window of 76 V due to it having the longest fluorene conjugation length among the studied electrets. The smallest difference of the HOMO energy level between pentacene and P(St-Fl)3 facilitated the charge transfer from pentacene to the polymer electret. The shifts on the transfer curves in both positive and negative directions could be reversibly controlled when applied an external gate bias of ±100 V for a short time (1 μs), indicating the fast trapping-detrapping ability of the polymer electrets. The devices showed excellent nonvolatile behaviors for bistable switching. The ON and OFF states were maintained over 104 s with the Ion/Ioff current ratio of 105–106. The write-read-erase-read (WRER) cycles could be operated over 100 cycles. This study suggested that surface characteristics, charge transport, and memory characteristics of pentacene-based OFET can be manipulated by polymer electrets with different pendent conjugation length.

A poly(fluorene-thiophene) donor with a tethered phenanthro[9,10-d]imidazole acceptor for flexible nonvolatile flash resistive memory devices.

A conjugated poly(fluorene-thiophene) donor and a tethered phenanthro[9,10-d]imidazole acceptor (PFT-PI) was used as the active layer in flexible nonvolatile resistor memory devices with low threshold voltages (±2 V), low switching powers (∼100 μW cm(-2)), large ON/OFF memory windows (10(4)), good retention (>10(4) s) and excellent endurance against electric and mechanical stimulus.

A silole copolymer containing a ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells

We report a combination of a silole containing ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells. This new class of PSiNO polymer possesses a planar, rigid backbone and a low-ordering framework. This unique feature facilitates chain extension, leading to high hole mobility and hence a high PCE of 8.37% without further thermal annealing.

Magnetic hardening mechanism study in FePt thin films

Fe100−xPtx alloy thin films with x=25–67 at. % were prepared by dc magnetron sputtering on naturally oxidized Si substrates. Effects of film composition, annealing temperature (300–650 °C), annealing time (5–120 min), and cooling rate (furnace cooling or ice water quench cooling) on the magnetic properties were investigated. Optimum conditions for saturation magnetization and coercivity of the Fe100−xPtx alloy films were found with x=50 at.%, annealed at 600 °C for 30 min and cooled by ice water quenching. Our experimental data suggests that the magnetic hardening in Fe100−xPtx alloy thin films is mainly due to the fct γ1-FePt phase and the domain wall pinning effect. The domain nucleation mechanism is dominated in samples with furnace cooling; the domain wall pinning mechanism dominates in samples cooled with ice water quenching.

Microstructure and magnetic properties of the (FePt)100−xCrx thin films

The (FePt)100−xCrx alloy thin films with x=0–16 at. % were fabricated on natural-oxidized Si(111) substrate by dc magnetron sputtering. The as-deposited films were annealed between 300 and 750 °C in order to transform the soft magnetic fcc γ-FePt phase to the hard magnetic fct γ1-FePt phase. The addition of Cr in the FePt thin films will reduce its saturation magnetization and coercivity, however, it could inhibit the grain growth during annealing of the samples. The optimum condition for high-density magnetic recording purpose of the (FePt)100−xCrx alloy films was found with x=5 at. %, annealing at 650 °C for 15 min, and ice water quench cooling. According to the transmission electron microscopy study, the average grain size in the annealed (FePt)100−xCrx alloy thin films decrease from 60 to 5 nm with increasing x from 0 to 16.

Biaxially extended quaterthiophene-thiophene and -selenophene conjugated polymers for optoelectronic device applications

New biaxially extended quaterthiophene (4T) conjugated polymers, including poly(5,5′′′-di-(2-ethylhexyl)[2,3′;5′,2′′4′′,2′′′]quaterthiophene) (P4T) and their copolymers with thiophene(P4TT), bithiophene (P4T2T), selenophene(P4TSe) and biselenophene (P4T2Se) were synthesized by Stille coupling reactions under microwave heating. The effects of the ring number of thiophene and selenophene moieties on the physical properties and polymer structures were systematically investigated experimentally and theoretically. With the increased ring number of the unsubstituted thiophene and selenophene moieties, the band gaps and the main-chain torsional angles were reduced. However, the side-chain torsional angles were increased with increasing the ring number, and thus significantly affected the carrier transporting characteristics. Among these studied conjugated polymers, the field-effect transistor (FET) based on P4TSe showed the highest hole mobility of up to 4.28 × 10−2 cm2 V−1 s−1 and an on/off ratio of 1.12 × 104. The photovoltaic device prepared from P4TSe/PC71BM exhibited the highest power conversion efficiency (PCE) of 2.6%, which resulted from more balanced hole/electron mobility and a smaller band gap. The above results revealed that the conformation, charge-transporting and optoelectronic device characteristics of biaxially extended 4T-based conjugated copolymers could be manipulated by incorporating the heteroaromatic ring spacer.

Donor–acceptor conjugated polymers of arylene vinylene with pendent phenanthro[9,10-d]imidazole for high-performance flexible resistor-type memory applications

We report the synthesis and resistive memory device characteristics of new donor–acceptor (D–A) conjugated poly(arylene vinylene) polymers, PVC-PI, PVT-PI, and PVTPA-PI, consisting of carbazole (C), thiophene (T), and triphenylamine (TPA) with pendent phenanthro[9,10-d]imidazole (PI). PVC-PI, PVT-PI, and PVTPA-PI synthesized by Stille coupling reaction have the optical band gaps of 2.86, 1.68, and 2.58 eV, respectively. These polymers possess similar HOMO energy levels (−5.08 to −5.18 eV), but different LUMO energy levels (−2.24, −3.40, and −2.60 eV for PVC-PI, PVT-PI, and PVTPA-PI, respectively). The PVC-PI flexible device with the sandwich configuration of PEN/Al/polymer/Al reveals the volatile static random access memory (SRAM) characteristic while the PVTPA-PI device exhibits the nonvolatile write-once-read-many-times (WORM) switching behavior. The above two devices could operate at low voltages (less than 2.5 V) with high ON/OFF current ratios (over 104) and exhibit excellent durability upon repeated bending tests. However, the PVT-PI device only shows a diode-like electrical behavior. The polymer conformation affects the strength of D–A electrical polarization and charge trapping ability, leading to the variation of the volatility of the memory devices. The present study demonstrates that the prepared vinylene-based donor–acceptor conjugated polymers have excellent memory characteristics with reproducibility and reliability for advanced flexible organic electronics.

Morphology and field-effect transistor characteristics of semicrystalline poly(3-hexylthiophene) and poly(stearyl acrylate) blend nanowires

The morphology and charge transport characteristics of semicrystalline poly(3-hexylthiophene) (P3HT) and semicrystalline poly(stearyl acrylate) (PSA) or amorphous poly(methyl methacrylate) (PMMA) with various blending ratios were systematically investigated using different solvents. The P3HT–PSA films prepared from CH2Cl2 formed well-defined P3HT nanowires with an average diameter of 30 nm, which was larger than that (∼14 nm) of the P3HT–PMMA films, as evidenced by TEM and AFM. The P3HT–PSA nanowire based field effect transistors (FET) could achieve a high hole mobility of 3.2 × 10−3 cm2 V−1 s−1 using only 2 wt% P3HT composition. The maximum FET mobility of 7.86 × 10−3 cm2 V−1 s−1 with the on/off ratio of 105 was obtained in the 10 wt% P3HT–PSA blends, which were higher than those of pristine P3HT and P3HT–PMMA devices. The semicrystalline PSA probably facilitated large P3HT crystallites and led to high FET mobility. Also, the P3HT–PSA FET devices showed lower percolation threshold and better ambient stability than the P3HT–PMMA devices. These results indicated that the crystalline non-conjugated polymers played a critical role in the charge transport and air stability of FETs based on conjugated polymer blends.

Synthesis of monodispersed polystyrene–silver core–shell particles and their application in the fabrication of stretchable large-scale anisotropic conductive films

Monodispersed core–shell conductive particles are designed and produced as efficient electron transporting materials for anisotropic conductive films. Traditionally, particle size control was required usually owing to the demand of anisotropic conductive films. Here, an innovative and facile method is proposed to prepare large-scale anisotropic conductive films by incorporating organic–inorganic core–shell conductive particles. First of all, monodispersed polystyrene–silver (PS–Ag) core–shell particles were prepared by emulsifier-free emulsion polymerization and a modified electroless plating process. A series of variables were used to synthesize the PS–Ag conductive particles to enhance the mobility of electrons in a given medium. The resulting PS–Ag conductive particles had excellent bulk conductivity with Ag nanoshells compactly embedded on the surface of PS colloids. In addition, the PS–Ag conductive particles were further mixed with soft latex particles of poly(styrene-co-butyl acrylate), P(St–BA), and then followed by the film-forming process. After the formation of large-scale anisotropic conductive films by the gravity sedimentation method, a remarkable flexible behavior with good conductivity was obtained. The presented method shows the significance of developments in electronic fields and is expected to be a practical, facile, and general approach for the fabrication of anisotropic conductive films with good flexibility and stretchability.

Transparent deoxyribonucleic acid substrate with high mechanical strength for flexible and biocompatible organic resistive memory devices

Biocompatible deoxyribonucleic acid (DNA), with high mechanical strength, was employed as the substrate for a Ag nanowire (Ag NW) pattern and then used to fabricate flexible resistor-type memory devices. The memory exhibited typical write-once-read-many (WORM)-type memory features with a high ON/OFF ratio (104), long-term retention ability (104 s) and excellent mechanical endurance.