Synaptic Characteristics of Atomic Layer-Deposited Ferroelectric Lanthanum-Doped HfO (La:HfO) and TaN-Based Artificial Synapses.

Analog synaptic devices have made significant advances based on various electronic materials that can realize the biological synapse properties of neuromorphic computing. Ferroelectric (FE) HfO-based materials with nonvolatile and low power consumption characteristics are being studied as promising materials for application to analog synaptic devices. The gradual reversal of FE multilevel polarization results in precise changes in the channel conductance and allows analogue synaptic weight updates.

Ultrastrong Hybrid Fibers with Tunable Macromolecular Interfaces of Graphene Oxide and Carbon Nanotube for Multifunctional Applications.

Individual carbon nanotubes (CNT) and graphene have unique mechanical and electrical properties; however, the properties of their macroscopic assemblies have not met expectations because of limited physical dimensions, the limited degree of dispersion of the components, and various structural defects. Here, a state-of-the-art assembly for a novel type of hybrid fiber possessing the properties required for a wide variety of multifunctional applications is presented. A simple and effective multidimensional nanostructure of CNT and graphene oxide (GO) assembled by solution processing improves the interfacial utilization of the components.

Effects of etching process and annealing temperature on the ferroelectric properties of atomic layer deposited Al-doped HfOthin film.

An investigation was conducted with regard to the effect of etching process on the ferroelectric (FE) characteristics of different device structures with Al-doped HfOthin films; further, the effect of the rapid thermal annealing temperature on the FE properties was elucidated using metal-ferroelectric-metal (MFM) capacitors using TiN electrodes with varying thickness and 4 at.% Al-doped HfOFE layer. The capacitors were annealed at different temperatures after lithography and etching process; this was aimed at incorporating the FE-orthorhombic phase.

Ultrahigh strength, modulus, and conductivity of graphitic fibers by macromolecular coalescence.

Theoretical considerations suggest that the strength of carbon nanotube (CNT) fibers be exceptional; however, their mechanical performance values are much lower than the theoretical values. To achieve macroscopic fibers with ultrahigh performance, we developed a method to form multidimensional nanostructures by coalescence of individual nanotubes. The highly aligned wet-spun fibers of single- or double-walled nanotube bundles were graphitized to induce nanotube collapse and multi-inner walled structures.

Surface Modification of Sulfur-Assisted Reduced Graphene Oxide with Poly(phenylene sulfide) for Multifunctional Nanocomposites.

The sulfur on the sulfur-assisted reduced graphene oxide (SrGO) surface provides the origin of poly(phenylene sulfide) PPS-grafting via SAr mechanism. In-situ polymerization from sulfur on SrGO afforded surface modification of SrGO, resulting in enhanced dispersibility in PPS. The tensile strength, electrical and thermal conductivities, and flame retardancy of PPS-coated SrGO were efficiently enhanced using highly concentrated SrGO and masterbatch (MB) for industrial purposes.

Analog Synaptic Transistor with Al-Doped HfO Ferroelectric Thin Film.

Neuromorphic computing has garnered significant attention because it can overcome the limitations of the current von-Neumann computing system. Analog synaptic devices are essential for realizing hardware-based artificial neuromorphic devices; however, only a few systematic studies in terms of both synaptic materials and device structures have been conducted so far, and thus, further research is required in this direction. In this study, we demonstrate the synaptic characteristics of a ferroelectric material-based thin-film transistor (FeTFT) that uses partial switching of ferroelectric polarization to implement analog conductance modulation.

Analysis of the effect of organic solvent-sheet interfacial interaction on the exfoliation of sulfur-doped reduced graphene oxide sheets in a solvent system using molecular dynamics simulations.

In this study, the effect of interfacial interaction between solvent and sheets on the exfoliation of sulfur-doped reduced graphene oxide (SrGO) sheets was studied, using molecular dynamics simulations. Four organic solvents of toluene, tetrahydrofuran, N-methyl-2-pyrrolidone, and sulfolane, were used in this simulation. An insertion simulation considering the size effect of insertion molecules was used to determine the insertion efficiency of the solvent molecules.

The coexistence of threshold and memory switching characteristics of ALD HfO memristor synaptic arrays for energy-efficient neuromorphic computing.

The development of bioinspired electronic devices that can mimic the biological synapses is an essential step towards the development of efficient neuromorphic systems to simulate the functions of the human brain. Among various materials that can be utilized to attain electronic synapses, the existing semiconductor industry-compatible conventional materials are more favorable due to their low cost, easy fabrication and reliable switching properties. In this work, atomic layer deposited HfO2-based memristor synaptic arrays are fabricated.

Carbon nanotube fibers with enhanced longitudinal carrier mobility for high-performance all-carbon thermoelectric generators.

With the increase in practical interest in flexible thermoelectric (TE) generators, the demand for high-performance alternatives to brittle TE materials is growing. Herein, we have demonstrated wet-spun CNT fibers with high TE performance by systematically controlling the longitudinal carrier mobility without a significant change in the carrier concentration. The carrier mobility optimized by CNT alignment increases the electrical conductivity without decreasing the thermopower, thus improving the power factor.

Study of in Situ Silver Migration in Amorphous Boron Nitride CBRAM Device.

We report the dependence of the thickness of amorphous boron nitride (a-BN) on the characteristics of conductive bridge random access memory (CBRAM) structured with the Ag/a-BN/Pt stacking sequence. The a-BN thin film layers of three different thicknesses of 5.5, 11, and 21.

Molecular Design and Property Prediction of Sterically Confined Polyimides for Thermally Stable and Transparent Materials.

To meet the demand for next-generation flexible optoelectronic devices, it is crucial to accurately establish the chemical structure-property relationships of new optical polymer films from a theoretical point of view, prior to production. In the current study, computer-aided simulations of newly designed poly(ester imide)s (PEsIs) with various side groups (⁻H, ⁻CH₃, and ⁻CF₃) and substituted positions were employed to study substituent-derived steric effects on their optical and thermal properties. From calculations of the dihedral angle distribution of the model compounds, it was found that the torsion angle of the C⁻N imide bonds was effectively constrained by the judicious introduction of di-, tetra-, and hexa-substituted aromatic diamines with ⁻CF₃ groups.

Influences of carboxyl functionalization of intercalators on exfoliation of graphite oxide: a molecular dynamics simulation.

In this study, the influences of the carboxyl functionalization of intercalators on exfoliation of graphite oxide were analyzed using molecular dynamics (MD) simulations. Molecular models of four-layered graphene oxide (GO) sheets, four different solvents (ethanol, dimethylformamide, tetrahydrofuran, and N-methyl-2-pyrrolidone), and four different intercalators (anthracene, 2-anthracenecarboxylic acid, 2,3-anthracenedicarboxylic acid, and 2,6-anthracenedicarboxylic acid) were used in the MD simulations. A separation simulation of GO sheets was performed to determine the point at which the GO sheets begin to exfoliate.

Synthesis of biologically-active reduced graphene oxide by using fucoidan as a multifunctional agent for combination cancer therapy.

A therapeutic reduced graphene oxide (RGO) is synthesized by using fucoidan (Fu) as the reducing and surface functionalizing agent. The synthesized Fu-RGO exhibits promising characteristics for therapeutic applications such as high dispersity in aqueous media, biocompatibility, selective cytotoxicity to cancer cells, high loading capacity of the anticancer drug, and photothermal conversion effect. Therefore, Fu-GO is successfully harnessed as a combinatorial cancer treatment platform through bio-functional (Fu), chemo (doxorubicin (Dox)) and photothermal (RGO with near-infrared irradiation) modalities.

Engineering synaptic characteristics of TaO/HfO bi-layered resistive switching device.

We performed various pulse measurements on an atomic layer deposited (ALD) HfO-based resistive switching random access memory (RRAM) device and investigated its electronic synaptic characteristics. Unlike requirements for RRAM device application, to achieve the multi-state conductance changes required for the synaptic device, we employed additional sputtered TaO thin film formation on the ALD HfO switching medium, which leads to engineering the concentration of oxygen vacancies and modulating the conductive filaments. With this TaO/HfO bi-layered device, we attained gradual resistive switching, linear and symmetric conductance change, improved endurance and reproducibility characteristics compared to a single HfO device.

Compliance-Free, Digital SET and Analog RESET Synaptic Characteristics of Sub-Tantalum Oxide Based Neuromorphic Device.

A two terminal semiconducting device like a memristor is indispensable to emulate the function of synapse in the working memory. The analog switching characteristics of memristor play a vital role in the emulation of biological synapses. The application of consecutive voltage sweeps or pulses (action potentials) changes the conductivity of the memristor which is considered as the fundamental cause of the synaptic plasticity.

Facile Supramolecular Processing of Carbon Nanotubes and Polymers for Electromechanical Sensors.

We herein report a facile, cost-competitive, and scalable method for producing viscoelastic conductors via one-pot melt-blending using polymers and supramolecular gels composed of carbon nanotubes (CNTs), diphenylamine (DP), and benzophenone (BP). When mixed, a non-volatile eutectic liquid (EL) produced by simply blending DP with BP (1:1 molar ratio) enabled not only the gelation of CNTs (EL-CNTs) but also the dissolution of a number of commodity polymers. To make use of these advantages, viscoelastic conductors were produced via one-pot melt-blending the EL and CNTs with a model thermoplastic elastomer, poly(styrene-b-butadiene-b-styrene) (SBS, styrene 30 wt %).

Signal-Induced Release of Guests from a Photolatent Metal-Phenolic Supramolecular Cage and Its Hybrid Assemblies.

The coordination chemistry of plant polyphenols and metal ions can be used for coating various substrates and for creating modular superstructures. We herein explored this chemistry for the controlled release of guests from mesoporous silica nanoparticles (MSNs). The selective adsorption of tannic acids (TAs) on MSN silica walls opens the MSN mesoporous channels without disturbing mass transport.

Direct observation of morphological evolution of a catalyst during carbon nanotube forest growth: new insights into growth and growth termination.

In this study, we develop a new methodology for transmission electron microscopy (TEM) analysis that enables us to directly investigate the interface between carbon nanotube (CNT) arrays and the catalyst and support layers for CNT forest growth without any damage induced by a post-growth TEM sample preparation. Using this methodology, we perform in situ and ex situ TEM investigations on the evolution of the morphology of the catalyst particles and observe the catalyst particles to climb up through CNT arrays during CNT forest growth. We speculate that the lifted catalysts significantly affect the growth and growth termination of CNT forests along with Ostwald ripening and sub-surface diffusion.

Atomically-thin molecular layers for electrode modification of organic transistors.

Atomically-thin molecular layers of aryl-functionalized graphene oxides (GOs) were used to modify the surface characteristics of source-drain electrodes to improve the performances of organic field-effect transistor (OFET) devices. The GOs were functionalized with various aryl diazonium salts, including 4-nitroaniline, 4-fluoroaniline, or 4-methoxyaniline, to produce several types of GOs with different surface functional groups (NO2-Ph-GO, F-Ph-GO, or CH3O-Ph-GO, respectively). The deposition of aryl-functionalized GOs or their reduced derivatives onto metal electrode surfaces dramatically enhanced the electrical performances of both p-type and n-type OFETs relative to the performances of OFETs prepared without the GO modification layer.

Synthesis and mechanistic study of in situ halogen/nitrogen dual-doping in graphene tailored by stepwise pyrolysis of ionic liquids.

New halogen/nitrogen dual-doped graphenes (X/N-G) with thermally tunable doping levels are synthesized via the thermal reduction of graphite oxide (GO) with stepwise-pyrolyzed ionic liquids. The doping process of halogen and nitrogen into the graphene lattice proceeds via substitutional or covalent bonding through the physisorption or chemisorption of in situ pyrolyzed dopant precursors. The doping process is performed by heating to 300-400 °C of ionic liquid, and the chemically assisted reduction of GO is facilitated by ionic iodine, resulting in I/N-G materials possessing about three and two orders of magnitude higher conductivity (∼22,200 S m(-1)) and charge carrier density (∼10(21) cm(-3)), compared to those of thermally reduced GO.

Effects of nitrogen doping from pyrolyzed ionic liquid in carbon nanotube fibers: enhanced mechanical and electrical properties.

Nitrogen doping in carbon nanotube (CNT) fibers using pyrolyzed ionic liquid induced interfacial hydrogen bonding between individual CNTs, enhancing mechanical properties and electrical conductivity simultaneously. In particular, the nitrogen doped CNT fiber using the ionic liquid BMI-I exhibited about 104%, 714%, and 38% increased tensile strength (0.65 N/tex), elastic modulus (83 N/tex), and electrical conductivity (1350 S cm(-1)), respectively, compared to pristine CNT fiber.

Catalyst and doping methods for arc graphene.

Nitrogen-doped graphene synthesis with ∼g scale has been accomplished using the arc discharge method. The defects formed in the synthesis process were reduced by adding various metal catalysts, among which Bi2O3 was found to be the most effective. Adding dopants to the starting materials increased the electrical conductivity of the graphene product, and the doping concentration in graphene was tuned by adjusting the amount of nitrogen dopants.

Carbon nanotube core graphitic shell hybrid fibers.

A carbon nanotube yarn core graphitic shell hybrid fiber was fabricated via facile heat treatment of epoxy-based negative photoresist (SU-8) on carbon nanotube yarn. The effective encapsulation of carbon nanotube yarn in carbon fiber and a glassy carbon outer shell determines their physical properties. The higher electrical conductivity (than carbon fiber) of the carbon nanotube yarn overcomes the drawbacks of carbon fiber/glassy carbon, and the better properties (than carbon nanotubes) of the carbon fiber/glassy carbon make up for the lower thermal and mechanical properties of the carbon nanotube yarn via synergistic hybridization without any chemical doping and additional processes.

Chemical method for improving both the electrical conductivity and mechanical properties of carbon nanotube yarn via intramolecular cross-dehydrogenative coupling.

Chemical post-treatment of the carbon nanotube fiber (CNTF) was carried out via intramolecular cross-dehydrogenative coupling (ICDC) with FeCl3 at room temperature. The Raman intensity ratio of the G band to the D band (IG/ID ratio) of CNT fiber increased from 2.3 to 4.

Defect healing of reduced graphene oxide via intramolecular cross-dehydrogenative coupling.

A chemical defect healing of reduced graphene oxide (RGO) was carried out via intramolecular cross-dehydrogenative coupling (ICDC) with FeCl3 at room temperature. The Raman intensity ratio of the G-band to the D-band, the IG/ID ratio, of the RGO was increased from 0.77 to 1.