Showing papers by "Yang Yang published in 2004"

Programmable polymer thin film and non-volatile memory device.

Abstract: Building on the success of organic electronic devices, such as light-emitting diodes and field-effect transistors, procedures for fabricating non-volatile organic memory devices are now being explored. Here, we demonstrate a novel organic memory device fabricated by solution processing. Programmable electrical bistability was observed in a device made from a polystyrene film containing gold nanoparticles and 8-hydroxyquinoline sandwiched between two metal electrodes. The as-prepared device, which is in a low-conductivity state, displays an abrupt transition to a high-conductivity state under an external bias of 2.8 V. These two states differ in conductivity by about four orders of magnitude. Applying a negative bias of 1.8 V causes the device to return to the low-conductivity state. The electronic transition is attributed to the electric-field-induced charge transfer between the gold nanoparticles and 8-hydroxyquinoline. The transition from the low- to the high-conductivity state takes place in nanoseconds, and is non-volatile, indicating that the device may be used for low-cost, high-density memory storage.

Rigidity, computation, and randomization in network localization

Abstract: We provide a theoretical foundation for the problem of network localization in which some nodes know their locations and other nodes determine their locations by measuring the distances to their neighbors. We construct grounded graphs to model network localization and apply graph rigidity theory to test the conditions for unique localizability and to construct uniquely localizable networks. We further study the computational complexity of network localization and investigate a subclass of grounded graphs where localization can be computed efficiently. We conclude with a discussion of localization in sensor networks where the sensors are placed randomly.

Effects of Nitrogen Deficiency on Gas Exchange, Chlorophyll Fluorescence, and Antioxidant Enzymes in Leaves of Rice Plants

Abstract: Gas exchange, chlorophyll (Chl) fluorescence, and contents of photosynthetic pigments, soluble proteins (ribulose-1,5-bisphosphate carboxylase/oxygenase, RuBPCO), and antioxidant enzymes were characterized in the fully expanded 6th leaves in rice seedlings grown on either complete (CK) or on nitrogen-deficient nutrient (N-deficiency) solutions during a 20-chase period. Compared with the control plants, the lower photosynthetic capacity at saturation irradiance (Pmax) was accompanied by an increase in intercellular CO2 concentration (Ci), indicating that in N-deficient plants the decline in Pmax was not due to stomatal limitation but due to the reduced carboxylation efficiency. The fluorescence parameters ΦPS2, Fv′/Fm′, electron transport rate (ETR), and qP showed the same tendency as Pmax in N-deficient plants. Correspondingly, a higher qN paralleled the rise of the ratio of carotenoid (Car) to Chl contents. However, Fv/Fm was still diminished, suggesting that photoinhibition did occur in the photosystem 2 (PS2) reaction centres. In addition, the activities of antioxidant enzymes on a fresh mass basis were gradually lowered, leading to the aggravation of membrane lipid peroxidation with the proceeding N-deficiency. The accumulation of malonyldialdehyde resulted in the lessening of Chl and soluble protein content. Analyses of regression showed PS2 excitation pressure (1 - qP) was linearly correlated with the content of Chl and inversely with soluble protein (particularly RuBPCO) content. There was a lag phase in the increase of PS2 excitation pressure compared to the decrease of RuBPCO content. Therefore, the increased excitation pressure under N-deficiency is probably the result of saturation of the electron transport chain due to the limitation of the use of reductants by the Calvin cycle. Rice plants responded to N-deficiency and high irradiance by decreasing light-harvesting capacity and by increasing thermal dissipation of absorbed energy.

Organic thin-film transistors with nanocomposite dielectric gate insulator

Abstract: High-performance organic thin-film transistors (OTFTs) with a nanoparticle composite dielectric layer have been demonstrated. The dielectric layer consists of cross-linked poly-4-vinylphenol (PVP) and high-dielectric titanium dioxide (TiO2) nanoparticles. Because of the nanosize of TiO2, it disperses well in the organic solvent, which makes it possible to use solution-processable methods to prepare the dielectric layer. OTFTs with pentacene as the semiconducting layers have been demonstrated; it was found that the OTFTs with the nanocomposite dielectric layer have higher field-induced current than that of conventional devices because the dielectric constant of the gate insulator is increased. This finding opens an interesting direction for the preparation of high-performance OTFTs without complicated sputtering of high-κ dielectric materials.

Unique architecture and concept for high-performance organic transistors

Abstract: We report an organic transistor with a vertically stack structure, which consists of a layer-by-layer active cell (drain/organics/source) on top of a capacitor cell (source/dielectrics/gate); the middle source electrode is shared by the capacitor cell and active cell. Three unique characteristics of this transistor, (a) its very thin and rough middle source electrode; (b) its capacitor cell with high charge-storage capability, allow the active cell to be influenced when the gate is biased; and (c) the large cross-section area and small distance between the source and the drain allow current flowing between the source and drain electrodes. Devices have been fabricated by thermal evaporation with the source-drain current well modulated by the gate potential. We have achieved organic transistors with low working voltage (less than 5V) and high current output (up to 10mA or 4A∕cm2) and an ON/OFF ratio of 4×106. A model is proposed for the device operation mechanism. The demonstrated device with its enhanced operating characteristics may open directions for organic transistors and their applications.

Organic nonvolatile memory by controlling the dynamic copper-ion concentration within organic layer

Abstract: Copper (Cu) migration into semiconductor materials like silicon is a well-known and troublesome phenomenon often causing adverse effect on devices. Generally a diffusion barrier layer is added to prevent Cu metallization. We demonstrate an organic nonvolatile memory device by controlling the Cu-ion (Cu+) concentration within the organic layer. When the Cu+ concentration is high enough, the device exhibits a high conductive state due to the metallization effect. When the Cu+ concentration is low, the device displays a low conductance state. These two states differ in their electrical conductivity by more than seven orders of magnitude and can be precisely switched by controlling the Cu+ concentration through the application of external biases. The retention time of both states can be more than several months, and the device is promising for flash memory application. Discussions about the device operation mechanism are provided.

Efficient single-layer “twistacene”-doped polymer white light-emitting diodes

Abstract: Bright, efficient, and stable white polymer light-emitting diodes based on blue polyfluorene doped by a “twistacene,” 6, 8, 15, 17-tetraphenyl-118, 45, 910, 1314-tetrabenzoheptacene (3) (TBH), are demonstrated In “twistacene” the terminal pyrene moieties serve two functions: (i) to stabilize the inherently unstable heptacene and (ii) to enable the oligoacene to be a strongly fluorescent molecule As a result, efficient and very bright white polymer light-emitting diodes are obtained The maximum luminance of the devices exceeds 20000cd∕m2 The maximum luminous efficiency is 355cd∕A at 4228cd∕m2 while the maximum power efficiency is 16lm∕W at 310cd∕m2 The device obtains a stable white balance by a combination of energy transfer from the blue polyfluorene to TBH by 1% TBH doping plus the host emission The device emission color is not a function of bias current, which is ideal for various applications, from lighting to the backlight for liquid crystal displays

Memory devices based on electric field programmable films

Abstract: A composition for the formation of an electric field programmable film, the composition comprising a matrix precursor composition or a dielectric matrix material, wherein the dielectric matrix material comprises an organic polymer and/or a inorganic oxide; and an electron donor and an electron acceptor of a type and in an amount effective to provide electric field programming. The films are of utility in data storage devices.

Polymer Optoelectronic Devices with High-Conductivity Poly(3,4-Ethylenedioxythiophene) Anodes

Abstract: The conductivity of poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) film can be enhanced by more than two orders in magnitude by adding a compound with two or more polar groups, such as ethylene glycol (EG), meso‐erythritol (IUPAC name: 1,2,3,4‐tetrahydroxybutane), or 2‐nitroethanol, into the PEDOT:PSS aqueous solution. The mechanism of the increase in conductivity for PEDOT:PSS has been studied using Raman spectroscopy and atomic force microscope (AFM). Here we propose that the change in conductivity is due to the conformational change of PEDOT chains in the film. In untreated PEDOT:PSS films, coil, linear, or expanded‐coil conformations of the PEDOT chains may be present. In treated PEDOT:PSS films, the linear or expanded‐coil conformations may becomes the dominant form for PEDOT chains. This conformational change results in the enhancement of charge‐carrier mobility in the film and leads to enhanced conductivity. The high‐conductivity PEDOT:PSS film is ideal as the electrode ...

Organic Thin-Film Memory

Abstract: Recently, organic nonvolatile memory devices have attracted considerable attention due to their low cost and high performance. This article reviews recent developments in organic nonvolatile memory and describes in detail an organic electrical bistable device (OBD) that has potential for applications. The OBD consists of a tri-layer of organics/metal nanoclusters/organics sandwiched between top and bottom electrodes. A sufficiently high applied bias causes the metal nanoparticle layer to become polarized, resulting in charge storage near the two metal/organic interfaces. This stored charge lowers the resistance of the device and leads to an electrical switching behavior. The ON and OFF states of an OBD differ in their conductivity by several orders of magnitude and show remarkable bistability—once either state is reached, the device tends to remain in that state for a prolonged period of time. More important, the conductivity states of an OBD can be precisely controlled by the application of a positive voltage pulse (to write) or a negative voltage pulse (to erase). Device performance tests show that the OBD is a promising candidate for high-density, low-cost electrically addressable data storage applications.

High-speed and high-current density C60 diodes

Abstract: A high-performance organic diode is demonstrated by using C60 sandwiched between a cathode and an anode using metals with different diffusivity and donor ability. In this letter, copper (Cu) and aluminum (Al) are selected as the cathode and anode, respectively. C60 is used as the organic electron-acceptor for its high stability and high carrier mobility. The as-prepared diode shows poor performance. However, after heat treatment, the Cu/C60 interface becomes an Ohmic contact through Cu diffusion and charge-transfer processes, allowing highly efficient electron injection from the Cu electrode. On the other hand, a rectified C60/Al contact is formed, prohibiting efficient electron injection from the Al electrode into C60. Hence, a high-performance organic diode is formed through a heat treatment process, not by the selection of metals with different work functions. Due to the high mobility of C60, the device shows megahertz frequency response, and it can also handle rather high current density (363 A/cm2 at...

Single-band Hubbard model for the transport properties in bistable organic/metal nanoparticle/organic devices

Abstract: The transport properties of bistable organic/metal nanoparticle/organic devices are investigated within the single-band Hubbard model. The effect of two electrodes on the molecules with the nanoparticles is taken into account by using the Newns' chemisorption theory. The Coulomb interactions between the electrons in the Hubbard model are treated by the spectral density approach. The transmission probabilities of the system are calculated as a function of the energy, the organic layers' thickness, and the hopping term for the organic layers. At small bias, the transmission probability is small near the Fermi level if no charges are trapped in the system, which corresponds to the low-conductance state of the device. Above a threshold bias, the electrons within the nanoparticles will tunnel resonantly from one side to the other side, and the resulting positive-negative charges are trapped at both sides of the nanoparticle layer, in which case the transmission probability increases tremendously near the Fermi level, resulting in the high-conductance state. The layer-dependent densities of states are used to investigate the phenomena in detail. The transmission probability decreases exponentially as the thickness of the organic layers increases.

Maximally flexible assignment of orthogonal variable spreading factor codes for multirate traffic

Abstract: In universal terrestrial radio access (UTRA) systems, orthogonal variable spreading factor (OVSF) codes are used to support different transmission rates for different users. In this paper, we first define the flexibility index to measure the capability of an assignable code set in supporting multirate traffic classes. Based on this index, two single-code assignment schemes, nonrearrangeable and rearrangeable compact assignments, are proposed. Both schemes can offer maximal flexibility for the resulting code tree after each code assignment. We then present an analytical model and derive the call blocking probability, system throughput and fairness index. Analytical and simulation results show that the proposed schemes are efficient, stable and fair.

Enhanced efficiency of plastic photovoltaic devices by blending with ionic solid electrolytes

Abstract: One of the major technology bottlenecks of polymer photovoltaic cells is the low photoinduced current, due to the low carrier mobility and short exciton migration distance. In this letter we demonstrated that the electric current for polymer PV cells can be significantly enhanced by adding a small amount of ionic solid electrolyte. Heterojunction polymer photovoltaic devices, consisting of poly[2-methoxy-5-(2′-ethyl-hexyoxy)-1,4-phenylene vinylene] (MEH-PPV) C60 and/or methanofullerene([6,6]-phenyl C61-butyric acid methyl ester) (PCBM) as the active materials, were fabricated. It has been found that the power efficiency of the organic was enhanced by blending ionic solid electrolyte, such as polyethylene oxide into the active layer. It is believed that the optimized polymer morphology, the improved electrical conductivity, and the in situ photodoping of MEH-PPV contribute to this enhancement of photovoltaic efficiency.

Analysis of power ramping schemes for UTRA-FDD random access channel

Abstract: The random access channel (RACH) in a universal terrestrial radio access frequency division duplex (UTRA-FDD) system is a contention-based channel mainly used to carry control information from mobile stations to base stations. The transmission of a random access request contains two steps, preamble transmission and message transmission. In preamble transmission, a power ramping technique is used to favor the delayed preambles by stepping up the transmission power after each unsuccessful access. In doing so, the success of transmitting a long-delayed preamble is increased due to the power capture effect. We analyze the blocking and throughput performance of preamble transmission under three power ramping schemes with fixed, linear and geometric step sizes. Also, we compare the interference caused by different power ramping schemes.

Transport, ultrasound, and structural properties for the charge-ordered Pr 1-x Ca x MnO 3 ( 0.5⩽x⩽0.875 ) manganites

Abstract: The effects of the cooperative Jahn-Teller effect on the crystal structure and the stability of the charge-ordered ($\text{CO}$) state were studied by measurements of powder x-ray diffraction, resistivity, and ultrasound for ${\text{Pr}}_{1\ensuremath{-}x}{\text{Ca}}_{x}{\text{MnO}}_{3}$ $(0.5\ensuremath{\leqslant}x\ensuremath{\leqslant}0.875)$. Powder x-ray diffraction revealed a change of the crystal structure from tetragonally compressed to tetragonally elongated orthorhombic between $x=0.75$ and $x=0.8$ in the $\text{CO}$ state, resulting from the crossover of the cooperative Jahn-Teller vibration mode from ${Q}_{2}$ to ${Q}_{3}$. The relative stiffening of the ultrasound $(\ensuremath{\Delta}V∕V)$ reflecting the magnitude of the cooperative Jahn-Teller lattice distortion in the $\text{CO}$ state increases with increasing $x$ from 0.5 to 0.625, reaching its maximum and being almost $x$ independent for $0.625\ensuremath{\leqslant}x\ensuremath{\leqslant}0.8$, and drops steeply with further increase of $x$. Coincident with the variation of $\ensuremath{\Delta}V∕V$ with $x$, the stability of the $\text{CO}$ state reflected by the magnetoresistance effect increases with increasing $x$ from 0.5 to 0.625, reaching the most stable for $0.625\ensuremath{\leqslant}x\ensuremath{\leqslant}0.825$, and becomes unstable with further increase of $x$. These features demonstrate that the cooperative Jahn-Teller lattice distortion is one of the key ingredients in understanding the essential physics of the $\text{CO}$ state in manganites.

Rewritable nano-surface organic electrical bistable devices

Abstract: A bistable electrical device that is convertible between a low resistance state and a high resistance state. The device includes at least one layer of organic low conductivity material that is sandwiched between two electrodes. A buffer layer is located between the organic layer and at least one of the electrodes. The buffer layer includes particles in the form of flakes or dots of a low conducting material or insulating material that are present in a sufficient amount to only partially cover the electrode surface. The presence of the buffer layer controls metal migration into the organic layer when voltage pulses are applied between the electrodes to convert the device back and forth between the low and high resistance states.

Three-terminal electrical bistable devices

Abstract: A three terminal electrical bistable device that includes a tri-layer composed of an electrically conductive mixed layer sandwiched between two layers of low conductivity organic material that is interposed between a top electrode and a bottom electrode. The conducting mixed layer serves as the middle electrode. The device includes two memory cells composed of electrode/organic layer/mixed layer, where the interfaces between the electrically conductive mixed layer and the low conductivity organic layer exhibit bistable behavior.

Analysis of random access channel in UTRA‐TDD on AWGN channel

Abstract: The random access channel (RACH) in UTRA-TDD systems is an uplink contention-based transport channel that is mainly used to carry control information from mobile stations to base stations. In this paper, we study the performance of RACH on an additive white Gaussian noise (AWGN) channel whereby successful transmission of a burst requires the spreading code chosen to be collision-free and the burst is error-free after convolutional decoding. Based on this model, the code-collision probability, the data bit error probability and the RACH channel capacity are derived. The random retransmission delay mechanism is not specified in UTRA-TDD. We therefore choose an access mechanism with binary exponential backoff delay procedure similar to that in IEEE 802.11. Based on that mechanism, the blocking probability and the first two moments of the delay are also derived. Compared with the mean, the standard deviation is found to be very high. Copyright © 2004 John Wiley & Sons, Ltd.

EGG and acoustic test for the diagnosis of laryngeal diseases

Abstract: OBJECTIVE: To explore the relationship between electroglottography/acoustic test parameter and disease diagnosis. METHOD: 3.0 software by Dr. Speech was used to test synchronically the electroglottography and acoustic test parameter of 503 cases so as to make a statistical analysis and comparison. RESULT: The shimmer was similar with the jitter on the inspection of polyp of vocal cord and vocal nodules, but the shimmer was more susceptive than the jitter. Compared with the healthy group, mean F0, jitter and shimmer of polyp of vocal cord increased, while the Harmonic-to-Noise decreased, so did the vocal nodules and carcinoma of larynx. Compared with the normal, the contact index of carcinoma of larynx increased, but the contact quotient remained on the same level. CONCLUSION: EGG, acoustic test number comparison and wave change provide an objective quantized index for the diagnosis of laryngeal diseases.

Multicode Multirate Compact Assignment of OVSF Codes for QoS Differentiated Terminals

Abstract: Orthogonal Variable Spreading Factor (OVSF) codes are used in both UTRA-FDD and UTRA-TDD of the third-generation (3G) mobile communication systems. They can support multirate transmissions for mobile terminals with multicode transmission capabilities. In this paper, a new OVSF code assignment scheme, namely “Multicode Multirate Compact Assignment” (MMCA), is proposed and analyzed. The design of MMCA is based on the concept of “compact index” and takes into consideration mobile terminals with different multicode transmission capabilities and Quality of Service (QoS) requirements. Specifically, priority differentiation between multirate realtime traffic and best-effort data traffic is supported in MMCA. Analytical and simulation results show that MMCA is efficient and fair.

Polymer electrical bistable device and memory cells

Abstract: Electrical bistable states with the conductivity different by more than four orders in magnitude were observed in a polymer film sandwiched between two metal electrodes. This polymer film was composed of gold nanoparticles, 8-hydroxyquinoline and polystyrene, and was formed by a solution process. The film can be programmed between the two electrical states by an electric field. The as-prepared device, which was in a low conductivity state, exhibited an abrupt increase of current when the device was scanned up to 2.8 volt (V). The high conductivity state can be returned to the low conductivity state at a voltage of –1.8 V in the reverse direction. The device has a good stability in both the states. The transitions are nonvolatile, and the transition from the low to the high conductivity state takes place in nanoseconds, so that the device can be used as a low-cost, high-density, high-speed, and nonvolatile memory. The switching mechanism was studied by investigating the current-voltage characteristics, the temperature dependence of the current, the surface potential atomic force microscopy and the energy levels of the materials. The electronic transition is attributed to the electric-field induced charge transfer between the gold nanoparticles and 8-hydroxyquinoline molecules.

Polymer Morphology and Device Performance in Polymer Electronics

Abstract: Since the first successful fabrication of the polymer light-emitting diode (PLED) using poly(p-phenylenevinylene) (PPV) as the active material,1 there has been widespread research interest in conjugated polymers. The lightweight, flexibility, ease of processing, and the unique electrical and photonic semiconducting properties are among the most attractive characteristics of this class of materials. Because some of these materials can be obtained in solution form, spin-coating has become the most commonly used technique for obtaining uniform thin films of these materials. It is often noted that results published by different groups are inconsistent. For example, as shown in Fig. 6.1, emission spectra of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene (MEH-PPV) thin films can be dramatically different. In addition, fluorescence decay dynamics of MEH-PPV films with a photoluminescence (PL) spectrum similar to curve a in Fig. 6.1 reported by Jakubiak Rothberg2 are significantly different from that with a PL spectrum similar to curve b in Fig. 6.1 reported by Samuel et al.3 The dispute of the charge carriers’ mobility is another example. Scott et al.,4,5 found that the electron is highly mobile in MEH-PPV films. Blom et al.6 suggested that the charge recombination zone is next to the cathode, which suggests that the holes move faster than the electrons. It was also noted that hole transport is dispersive and, therefore, an interpretation in terms of charge carrier mobility is not meaningful.7

Double Sense Multiple Access for Hidden Terminal Avoidance in Wireless Ad Hoc Networks

Abstract: Double Sense Multiple Access In wireless ad hoc networks, it is a challenging problem to design an efficient media access control (MAC) protocol that can completely avoid the interference from the hidden terminals, which are defined as the terminals beyond the communication range of the transmitter but within that of the receiver. In [1], Haas and Deng have proposed the dual busy tone multiple access (DBTMA) protocol where the transmitter and the receiver can clear other packet transmissions in their communication ranges by broadcasting two different busy tone (BT) signals. To avoid the still possible packet collisions due to the propagation delay, a mandatory waiting time is inserted between the successful reception of a ready-to-send (RTS) packet and the transmission of a data packet. In addition, all terminals are required to keep sensing the BT signals, even while they are transmitting packets. In this paper, we propose and analyse a novel dual channel random access protocol, called “Double Sense Multiple Access” (DSMA), for solving the hidden terminal problem. Two time-slotted channels are used for transmitting control (e.g. RTS) and data packets separately. Under DSMA, a transmitter will sense the BT signals twice before sending the data packet to the receiver. In doing so, the transmitter can identify if it is the intended transmitter for sending a data packet and, therefore, the possible data packet collisions due to propagation delay can be completely avoided. Compared with DBTMA, our DSMA protocol has the following advantages: (1) higher channel efficiency by completely avoiding the collisions between control and data packets; (2) higher power efficiency by setting lower data rate, i.e. lower transmission power, in the control channel; (3) shorter transmission delay by removing the “mandatory waiting time” in DBTMA; and (4) less complexity and cost by reducing the “keep sensing” requirement in DBTMA to “sense twice” in DSMA.

Synthesis and functionalization of mesoporous zirconium (IV) phosphate-phenylphosphonate with small mesopores

Abstract: The synthesis and functionalization of mesoporous zirconium (IV) phosphate-phenylphosphonate have been systematically described as a new non-siliceous mesoporous material The mesostructure formation and functionalization could be proved through XRD, BET, 31 PMASNMR, IR, TEM and TGA characterization methods The samples prepared have worm-like 3D-connected tubular mesopores, higher surface areas and better thermal stabilities in contrast to the layered analogs The amount of phenyl groups inside the samples could be adjusted by controlling the molar ratio of the phosphoric and phosphonic acids during the synthesis procedure Furthermore, the phenyl groups inside the pore channels of samples can be easily functionalized through phenyl substitution reactions to incorporate sulphonate, bromic, nitro groups After functionalization, the sulphonated and brominated samples still kept the mesostructure but the nitrified sample did not The results of the NH 3 adsorption microcalorimetric and TGA measurements demonstrated that the sulphonated sample had more strong acid sites and higher thermal stability, and consequently possess higher acid catalytic activity for the liquid phase esterification reaction between benzoic acid and methanol