Well-Defined Block Copolymers with Triphenylamine and Isocyanate Moieties Synthesized via Living Anionic Polymerization for Polymer-Based Resistive Memory Applications: Effect of Morphological Structures on Nonvolatile Memory Performances

The anionic block copolymerization of 4,4′-vinylphenyl-<i>N</i>,<i>N</i>-bis­(4-<i>tert</i>-butylphenyl)­benzenamine (<b>A</b>) with <i>n</i>-hexyl isocyanate (<b>B</b>) was performed using potassium naphthalenide (K-Naph) in THF at −78 and −98 °C in the presence of sodium tetraphenylborate (NaBPh₄) to afford the well-defined block copolymers for investigating the effect of morphological structures on electrical memory performances. The well-defined functional block copolymers (P<b>BAB</b>) with different block ratios had predictable molecular weights (<i>M</i>_n = 17 700–79 100 g/mol) and narrow molecular weight distributions (<i>M</i>_w/<i>M</i>_n = 1.14–1.19). It was observed from transmission electron microscopy (TEM) that the block copolymers showed different morphological structures depending on block ratios. Although all memory devices fabricated from the resulting block copolymers with different block compositions equally exhibited nonvolatile resistive switching characteristics, which are governed by the trap-controlled space-charge-limited current (SCLC) conduction mechanism and filament formation, it was found that electrical memory performances of each device varied depending on morphological structures of the block copolymer films

1/<i>f</i> Noise Scaling Analysis in Unipolar-Type Organic Nanocomposite Resistive Memory

We studied noise characteristics of a nanocomposite of polyimide (PI) and phenyl-C61-butyric acid methyl ester (PCBM) (denoted as PI:PCBM), a composite for the organic nonvolatile resistive memory material. The current fluctuations were investigated over a bias range that covers various intermediate resistive states and negative differential resistance (NDR) in organic nanocomposite unipolar resistive memory devices. From the analysis of the 1/<i>f</i>^γ type noises, scaling behavior between the relative noise power spectral density <i>S̃</i> and resistance <i>R</i> was observed, indicating a percolating behavior. Considering a linear rate equation of the charge trapping–detrapping at traps, the percolation behavior and NDR could be understood by the modulation of the conductive phase fraction φ with an external bias. This study can enhance the understanding of the NDR phenomena in organic nanocomposite unipolar resistive memory devices in terms of the current path formation and the memory switching

Electric Stress-Induced Threshold Voltage Instability of Multilayer MoS₂ Field Effect Transistors

We investigated the gate bias stress effects of multilayered MoS₂ field effect transistors (FETs) with a back-gated configuration. The electrical stability of the MoS₂ FETs can be significantly influenced by the electrical stress type, relative sweep rate, and stress time in an ambient environment. Specifically, when a positive gate bias stress was applied to the MoS₂ FET, the current of the device decreased and its threshold shifted in the positive gate bias direction. In contrast, with a negative gate bias stress, the current of the device increased and the threshold shifted in the negative gate bias direction. The gate bias stress effects were enhanced when a gate bias was applied for a longer time or when a slower sweep rate was used. These phenomena can be explained by the charge trapping due to the adsorption or desorption of oxygen and/or water on the MoS₂ surface with a positive or negative gate bias, respectively, under an ambient environment. This study will be helpful in understanding the electrical-stress-induced instability of the MoS₂-based electronic devices and will also give insight into the design of desirable devices for electronics applications

Graphene/Pentacene Barristor with Ion-Gel Gate Dielectric: Flexible Ambipolar Transistor with High Mobility and On/Off Ratio

High-quality channel layer is required for next-generation flexible electronic devices. Graphene is a good candidate due to its high carrier mobility and unique ambipolar transport characteristics but typically shows a low on/off ratio caused by gapless band structure. Popularly investigated organic semiconductors, such as pentacene, suffer from poor carrier mobility. Here, we propose a graphene/pentacene channel layer with high-k ion-gel gate dielectric. The graphene/pentacene device shows both high on/off ratio and carrier mobility as well as excellent mechanical flexibility. Most importantly, it reveals ambipolar behaviors and related negative differential resistance, which are controlled by external bias. Therefore, our graphene/pentacene barristor with ion-gel gate dielectric can offer various flexible device applications with high performances

Electrical and Optical Characterization of MoS₂ with Sulfur Vacancy Passivation by Treatment with Alkanethiol Molecules

We investigated the physical properties of molybdenum disulfide (MoS₂) atomic crystals with a sulfur vacancy passivation after treatment with alkanethiol molecules including their electrical, Raman, and photoluminescence (PL) characteristics. MoS₂, one of the transition metal dichalcogenide materials, is a promising two-dimensional semiconductor material with good physical properties. It is known that sulfur vacancies exist in MoS₂, resulting in the n-type behavior of MoS₂. The sulfur vacancies on the MoS₂ surface tend to form covalent bonds with sulfur-containing groups. In this study, we deposited alkanethiol molecules on MoS₂ field effect transistors (FETs) and then characterized the electrical properties of the devices before and after the alkanethiol treatment. We observed that the electrical characteristics of MoS₂ FETs dramatically changed after the alkanethiol treatment. We also observed that the Raman and PL spectra of MoS₂ films changed after the alkanethiol treatment. These effects are attributed to the thiol (−SH) end groups in alkanethiols bonding at sulfur vacancy sites, thus altering the physical properties of the MoS₂. This study will help us better understand the electrical and optical properties of MoS₂ and suggest a way of tailoring the properties of MoS₂ by passivating a sulfur vacancy with thiol molecules