Tupei Chen

Charging phenomena in pentacene-gold nanoparticle memory device

The authors demonstrate a new organic memory system, using pentacene as the active semiconductor layer and citrate-stabilized gold (Au) nanoparticles as charge storage elements. A pronounced clockwise capacitance-voltage (C-V) hysteresis is observed with a memory window of 1.25–2.05V achievable under 5–10V programing range. Similar clockwise C-V hysteresis window and an almost constant full width at half maximum of the conductance peaks in conductance-voltage (G-V) characteristics, obtained in the frequency range of 50kHz–1MHz, indicated that positive charge trapping/detrapping originated mainly from the Au nanoparticles.

Micellar poly(styrene-b-4-vinylpyridine)-nanoparticle hybrid system for non-volatile organic transistor memory

We present organic field-effect transistor (OFET) memories where in-situ synthesized gold (Au) nanoparticles in self-assembled polystyrene-block-poly-4-vinylpyridine (PS-b-P4VP) block copolymer nanodomains successfully functioned as charge storage elements. Both p-type (pentacene) and n-type (perfluorinated copper phthalocyanine) OFET based memories are reported, which have stable large charge capacity and programmable-erasable properties due to charge confinement in the embedded Au nanoparticles. Optical excitation has been utilized to demonstrate photogenerated minority carrier trapping in the Au nanoparticles for efficient erasing operations. The memory devices can hence be written and read electrically and erased optically, resulting in large memory windows (∼9–11 V), high on/off ratio between memory states (103–105) and long retention times (>1000 s). The results clearly indicate the utility of the block copolymer-nanoparticle approach for OFET based memories.

3D hierarchical Co3O4@Co3S4 nanoarrays as cathode materials for asymmetric pseudocapacitors

Three-dimensional (3D) hierarchical Co3O4@Co3S4 nanoarrays (NAs) were synthesized via a stepwise hydrothermal method involving precipitation and in situ sulfurization of Co3O4 nanoneedle arrays (NNAs). By controlling both anion exchange and Ostwald ripening reactions during the sulfurization process, 3D hierarchical Co3O4@Co3S4 NAs with tailored Co3S4 nanostructures have been fabricated as electrode materials for electrochemical capacitor applications. Owing to an interconnected matrix within the 3D architecture, the as-prepared Co3O4@Co3S4 NAs exhibit excellent electrical conductivity, high specific capacity and high cycling stability. It can deliver a high capacitance of 1284.3 F g−1 at 2 mV s−1 and maintain a capacitance retention of 93.1% after 5000 cycles. Moreover, a flexible solid-state asymmetric supercapacitor (ASC) composed of Co3O4@Co3S4 NAs as the positive electrode and activated carbon (AC) as the negative electrode exhibited an energy density of 1.5 mW h cm−3 and a power density of 6.1 W cm−3 at a high operating voltage of 1.6 V. Our results not only present the 3D hierarchical nanostructure of Co3O4@Co3S4 NAs, but they also demonstrate the potential of electrodes for future generation supercapacitors.

Multiferroic properties of sputtered BiFeO3 thin films

A cosputtering method was used to deposit BiFeO3 thin films on Pt∕Ti∕SiO2∕Si substrates. It was confirmed as a polycrystalline film with a tetragonal crystal structure in the annealed state. Both Fe2+ and Fe3+ ions were found to coexist in the film. The leakage current density is as low as 10−3A∕cm2 at 120kV∕cm. This sputtered film shows multiferroic properties exhibiting a saturated ferroelectric loop with a large remnant polarization of 37μC∕cm2 and a saturated ferromagnetic loop with saturation magnetization of 21emu∕cm3 at room temperature.

Memory characteristics of MOSFETs with densely stacked silicon nanocrystal Layers in the gate oxide synthesized by low-energy ion beam

Densely stacked silicon nanocrystal layers embedded in the gate oxide of MOSFETs are synthesized with Si ion implantation into an SiO/sub 2/ layer at an implantation energy of 2 keV. In this letter, the memory characteristics of MOSFETs with 7-nm tunnel oxide and 20-nm control oxide at various temperatures have been investigated. A threshold voltage window of /spl sim/ 0.5 V is achieved under write/erase (W/E) voltages of +12 V/-12 V for 1 ms. The devices exhibit good endurance up to 10/sup 5/ W/E cycles even at a high operation temperature of 150/spl deg/C. They also have good retention characteristics with an extrapolated ten-year memory window of /spl sim/ 0.3 V at 100/spl deg/C.

Emulating the paired-pulse facilitation of a biological synapse with a NiOx-based memristor

We study the paired-pulse-induced response of a NiOx-based memristor. The behavior of the memristor is surprisingly similar to the paired-pulse facilitation of a biological synapse. When the memristor is stimulated with a pair of electrical pulses, the current of the memristor induced by the second pulse is larger than that by the first pulse. In addition, the magnitude of the facilitation decreases with the pulse interval, while it increases with the pulse magnitude or pulse width.

Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions

Strong visible electroluminescence (EL) has been observed from a 30 nm silicon nitride thin film multiply implanted with Si ions and annealed at 1100u2009°C. The EL intensity shows a linear relationship with the current transport in the thin film at lower voltages, but a departure from the linear relationship with a quenching in the EL intensity is observed at higher voltages. The EL spectra show two primary EL bands including the predominant violet band at ∼3.0u2002eV (415 nm) and the strong green-yellow band at ∼2.2u2002eV (560 nm). Two weak bands including the ultraviolet band at ∼3.8u2002eV and the near infrared band at ∼1.45u2002eV emerge at high voltages. The evolution of each EL band with the voltage has been examined. The phenomena observed are explained, and the EL mechanisms are discussed.

Impact of programming mechanisms on the performance and reliability of nonvolatile memory devices based on Si nanocrystals

A nonvolatile memory based on silicon nanocrystals (nc-Si) synthesized with very-low-energy Si/sup +/ implantation is fabricated, and the memory performance under the programming/erasing of either Fowler-Nordheim (FN)/FN or channel hot electron (CHE)/FN at both room temperature and 85/spl deg/C is investigated. The CHE programming has a larger memory window, a better endurance, and a longer retention time as compared to FN programming. In addition, the CHE programming yields less stress-induced leakage current than FN programming, suggesting that it produces less damage to the gate oxide and the oxide/Si interface. Detailed discussions on the impact of the programming mechanisms are presented.

Charging effect on current conduction in aluminum nitride thin films containing Al nanocrystals

The presence of Al nanocrystals (nc-Al) in AlN thin films is found to enhance the current conduction of the thin film system greatly due to the formation of tunneling paths of nc-Al arrays, and the nc-Al∕AlN system shows a quasi-two-dimensional transport following a power law. However, charge trapping in nc-Al reduces the current conduction because of the increase in the tunneling resistance and/or the breaking of some tunneling paths due to Coulomb blockade effect. The current conduction also evolves with a trend towards one-dimensional transport due to the breaking of some transverse tunneling paths as a result of the charge trapping.

Influence of charge trapping on electroluminescence from Si-nanocrystal light emitting structure

We report a study on the influence of charge trapping on electroluminescence (EL) from Si nanocrystal (nc-Si) distributed throughout a 30nm SiO2 thin film synthesized by Si+ implantation into an oxide film thermally grown on a p-type Si substrate. The electron and hole trapping in the nc-Si located near the indium tin oxide gate and the Si substrate, respectively, cause a reduction in the EL intensity. The reduced EL intensity can be recovered after the trapped charges are released. A partial recovery can be easily achieved by the application of a positive gate voltage or thermal annealing at hot temperatures (e.g., 120°C) for a short duration. The present study highlights the impact of charging in the nc-Si on the light emission efficiency and its stability of nc-Si light-emitting devices.