44 research outputs found

    Ion-Gel-Gated Graphene Optical Modulator with Hysteretic Behavior

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    We propose a graphene-based optical modulator and comprehensively investigate its photonic characteristics by electrically controlling the device with an ion-gel top-gate dielectric. The density of the electrically driven charge carriers in the ion-gel gate dielectric plays a key role in tuning the optical output power of the device. The charge density at the ion-gel–graphene interface is tuned electrically, and the chemical potential of graphene is then changed to control its light absorption strength. The optical behavior of the ion-gel gate dielectric exhibits a large hysteresis which originates from the inherent nature of the ionic gel and the graphene–ion-gel interface and a slow polarization response time of ions. The photonic device is applicable to both TE- and TM-polarized light waves, covering two entire optical communication bands, the O-band (1.26–1.36 μm) and the C-band (1.52–1.565 μm). The experimental results are in good agreement with theoretically simulated predictions. The temporal behavior of the ion-gel–graphene-integrated optical modulator reveals a long-term modulation state because of the relatively low mobility of the ions in the ion-gel solution and formation of the electric double layer in the graphene–ion-gel interface. Fast dynamic recovery is observed by applying an opposite voltage gate pulse. This study paves the way to the understanding of the operational principles and future applications of ion-gel-gated graphene optical devices in photonics

    One-Transistor–One-Transistor (1T1T) Optoelectronic Nonvolatile MoS<sub>2</sub> Memory Cell with Nondestructive Read-Out

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    Taking advantage of the superlative optoelectronic properties of single-layer MoS<sub>2</sub>, we developed a one-transistor–one-transistor (1T1T)-type MoS<sub>2</sub> optoelectronic nonvolatile memory cell. The 1T1T memory cell consisted of a control transistor (CT) and a memory transistor (MT), in which the drain electrode of the MT was connected electrically to the gate electrode of the CT, whereas the source electrode of the CT was connected electrically to the gate electrode of the MT. Single-layer MoS<sub>2</sub> films were utilized as the channel materials in both transistors, and gold nanoparticles acted as the floating gates in the MT. This 1T1T device architecture allowed for a nondestructive read-out operation in the memory because the writing (programming or erasing) and read-out processes were operated separately. The switching of the CT could be controlled by light illumination as well as the applied gate voltage due to the strong light absorption induced by the direct band gap of single-layer MoS<sub>2</sub> (∼1.8 eV). The resulting MoS<sub>2</sub> 1T1T memory cell exhibited excellent memory performance, including a large programming/erasing current ratio (over 10<sup>6</sup>), multilevel data storage (over 6 levels), cyclic endurance (200 cycles), and stable retention (10<sup>3</sup> s)

    Polyelectrolyte Interlayer for Ultra-Sensitive Organic Transistor Humidity Sensors

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    We demonstrate low-voltage, flexible, transparent pentacene humidity sensors with ultrahigh sensitivity, good reliability, and fast response/recovery behavior. The excellent performances of these devices are derived from an inserted polyelectrolyte (poly­[2-(methacryloyloxy)­ethyltrimethylammonium chloride-<i>co</i>-3-(trimethoxysilyl)­propyl methacrylate] (poly­(METAC-<i>co</i>-TSPM)) interlayer, which releases free Cl<sup>–</sup> ions in the electrolyte dielectric layer under humid conditions and boosts the electrical current in the transistor channel. This has led to extreme device sensitivity, such that electrical signal variations exceeding 7 orders of magnitude have been achieved in response to a 15% change in the relative humidity level. The new sensors exhibit a fast responsivity and a stable performance toward changes in humidity levels. Furthermore, the humidity sensors, mounted on flexible substrates, provided low voltage (<5 V) operation while preserving the unique ultrasensitivity and fast responsivity of these devices. We believe that the strategy of utilizing the enhanced ion motion in an inserted polyelectrolyte layer of an OFET structure can potentially improve sensor technologies beyond humidity-responsive systems

    Multifunctional Graphene Optoelectronic Devices Capable of Detecting and Storing Photonic Signals

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    The advantages of graphene photodetectors were utilized to design a new multifunctional graphene optoelectronic device. Organic semiconductors, gold nanoparticles (AuNPs), and graphene were combined to fabricate a photodetecting device with a nonvolatile memory function for storing photonic signals. A pentacene organic semiconductor acted as a light absorption layer in the device and provided a high hole photocurrent to the graphene channel. The AuNPs, positioned between the tunneling and blocking dielectric layers, acted as both a charge trap layer and a plasmonic light scatterer, which enable storing of the information about the incident light. The proposed pentacene-graphene-AuNP hybrid photodetector not only performed well as a photodetector in the visible light range, it also was able to store the photonic signal in the form of persistent current. The good photodetection performance resulted from the plasmonics-enabled enhancement of the optical absorption and from the photogating mechanisms in the pentacene. The device provided a photoresponse that depended on the wavelength of incident light; therefore, the signal information (both the wavelength and intensity) of the incident light was effectively committed to memory. The simple process of applying a negative pulse gate voltage could then erase the programmed information. The proposed photodetector with the capacity to store a photonic signal in memory represents a significant step toward the use of graphene in optoelectronic devices

    Halide Welding for Silver Nanowire Network Electrode

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    We developed a method of chemically welding silver nanowires (AgNWs) using an aqueous solution containing sodium halide salts (NaF, NaCl, NaBr, or NaI). The halide welding was performed simply by immersing the as-coated AgNW film into the sodium halide solution, and the resulting material was compared with those obtained using two typical thermal and plasmonic welding techniques. The halide welding dramatically reduced the sheet resistance of the AgNW electrode because of the strong fusion among nanowires at each junction while preserving the optical transmittance. The dramatic decrease in the sheet resistance was attributed to the autocatalytic addition of dissolved silver ions to the nanowire junction. Unlike thermal and plasmonic welding methods, the halide welding could be applied to AgNW films with a variety of deposition densities because the halide ions uniformly contacted the surface or junction regions. The optimized AgNW electrodes exhibited a sheet resistance of 9.3 Ω/sq at an optical transmittance of 92%. The halide welding significantly enhanced the mechanical flexibility of the electrode compared with the as-coated AgNWs. The halide-welded AgNWs were successfully used as source–drain electrodes in a transparent and flexible organic field-effect transistor (OFET). This simple, low-cost, and low-power consumption halide welding technique provides an innovative approach to preparing transparent electrodes for use in next-generation flexible optoelectronic devices

    Water-Gel for Gating Graphene Transistors

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    Water, the primary electrolyte in biology, attracts significant interest as an electrolyte-type dielectric material for transistors compatible with biological systems. Unfortunately, the fluidic nature and low ionic conductivity of water prevents its practical usage in such applications. Here, we describe the development of a solid state, megahertz-operating, water-based gate dielectric system for operating graphene transistors. The new electrolyte systems were prepared by dissolving metal-substituted DNA polyelectrolytes into water. The addition of these biocompatible polyelectrolytes induced hydrogelation to provide solid-state integrity to the system. They also enhanced the ionic conductivities of the electrolytes, which in turn led to the quick formation of an electric double layer at the graphene/electrolyte interface that is beneficial for modulating currents in graphene transistors at high frequencies. At the optimized conditions, the Na-DNA water-gel-gated flexible transistors and inverters were operated at frequencies above 1 MHz and 100 kHz, respectively

    Micropatterned Single-Walled Carbon Nanotube Electrodes for Use in High-Performance Transistors and Inverters

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    We demonstrated the solution-processed single-walled carbon nanotube (SWNT) source–drain electrodes patterned using a plasma-enhanced detachment patterning method for high-performance organic transistors and inverters. The high-resolution SWNT electrode patterning began with the formation of highly uniform SWNT thin films on a hydrophobic silanized substrate. The SWNT source–drain patterns were then formed by modulating the interfacial energies of the prepatterned elastomeric mold and the SWNT thin film using oxygen plasma. The SWNT films were subsequently selectively delaminated using a rubber mold. The patterned SWNTs could be used as the source–drain electrodes for both n-type PTCDI-C8 and p-type pentacene field-effect transistors (FETs). The n- and p-type devices exhibited good and exactly matched electrical performances, with a field-effect mobility of around 0.15 cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup> and an ON/OFF current ratio exceeding 10<sup>6</sup>. The single electrode material was used for both the n and p channels, permitting the successful fabrication of a high-performance complementary inverter by connecting a p-type pentacene FET to an n-type PTCDI-C8 FET. This patterning technique was simple, inexpensive, and easily scaled for the preparation of large-area electrode micropatterns for flexible microelectronic device fabrication

    Highly Sensitive and Reusable Membraneless Field-Effect Transistor (FET)-Type Tungsten Diselenide (WSe<sub>2</sub>) Biosensors

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    In recent years when the demand for high-performance biosensors has been aroused, a field-effect transistor (FET)-type biosensor (BioFET) has attracted great interest because of its high sensitivity, label-free detection, fast detection speed, and miniaturization. However, the insulating membrane in the conventional BioFET, which is essential in preventing the surface dangling bonds of typical semiconductors from nonspecific bindings, has limited the sensitivity of biosensors. Here, we present a highly sensitive and reusable membraneless BioFET based on a defect-free van der Waals material, tungsten diselenide (WSe<sub>2</sub>). We intentionally generated a few surface defects that serve as extra binding sites for the bioreceptor immobilization through weak oxygen plasma treatment, consequently magnifying the sensitivity values to 2.87 × 10<sup>5</sup> A/A for 10 mM glucose. The WSe<sub>2</sub> BioFET also maintained its high sensitivity even after several cycles of rinsing and glucose application were repeated

    Flexible and Mechanically Robust Organic Light-Emitting Diodes Based on Photopatternable Silver Nanowire Electrodes

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    We developed a simple methodology for fabricating silver nanowire (AgNW) micropatterns on a plastic substrate using a photocurable polymer. The patterning method began with the lamination of a UV-curable prepolymer film onto the AgNW-coated rigid glass substrate. Selective UV exposure of the UV-curable prepolymer film through a photomask solidified the exposed regions, and the unexposed regions were simply removed by the solvent. AgNW micropatterns of various sizes and shapes could be readily formed across the entire plastic substrate. Importantly, this photopatterning process enabled the embedding of the AgNW structures into the polymer matrix, which dramatically reduced the surface roughness and enhanced the mechanical stability of the AgNW film. The AgNW structures served as transparent anode electrodes in organic light-emitting diodes (OLEDs) that performed well compared to OLEDs fabricated using conventional indium tin oxide (ITO) or conducting polymer electrodes. This simple, inexpensive, and scalable AgNW patterning technique provides a novel approach to realizing next-generation flexible electronics

    Observation of the Inverse Giant Piezoresistance Effect in Silicon Nanomembranes Probed by Ultrafast Terahertz Spectroscopy

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    The anomalous piezoresistance (a-PZR) effects, including giant PZR (GPZR) with large magnitude and inverse PZR of opposite, have exciting technological potentials for their integration into novel nanoelectromechanical systems. However, the nature of a-PZR effect and the associated kinetics have not been clearly determined yet. Even further, there are intense research debates whether the a-PZR effect actually exists or not; although numerous investigations have been conducted, the origin of the effect has not been clearly understood. This paper shows the existence of a-PZR and provides direct experimental evidence through the performance of well-established electrical measurements and terahertz spectroscopy on silicon nanomembranes (Si NMs). The clear inverse PZR behavior was observed in the Si NMs when the thickness was less than 40 nm and the magnitude of the PZR response linearly increased with the decreasing thickness. Observations combined with electrical and optical measurements strongly corroborate that the a-PZR effect originates from the carrier concentration changes via charge carrier trapping into strain-induced defect states
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