Sungwon Moon

Passive electrical properties of multi-walled carbon nanotubes up to 0.1 THz

Metallic carbon nanotubes (CNTs) are promising as transmission lines or interconnects in radio-frequency nanoelectric circuits. This paper presents passive network properties of individual multi-walled carbon nanotubes (MWNTs) up to 110 GHz measured at room temperature. From the S-parameter data, frequency-dependent electric properties of the MWNT were extracted using an equivalent R-L-C circuit model. The ac impedance of the MWNT decreases significantly with increasing frequency, as predicted by earlier theoretical work. In particular, the equivalent resistance decreases a few hundred times. Our findings show that MWNTs can carry high-frequency currents much better than dc.

Intrinsic high-frequency characteristics of graphene layers

The experimental results for the high-frequency transport character- istics (from 0.5 to 110GHz) of graphene sheets are presented. Samples with graphene sheets of a different number of layers, as well as samples with- out graphene, were fabricated and compared by their room-temperature radio- frequency (RF) transmission properties. From RF two-port network experiments, the circuit parameters of resistance, inductance and capacitance were extracted using a lumped circuit model that consists of graphene, metal electrodes and con- tacts. Self-inductance was suppressed with decreasing number of layers, possibly due to minimized interlayer conduction or scattering. The graphene-electrode contact property shows dependence on the number of graphene layers. Our investigation may promote an understanding of the intrinsic graphene character- istics and graphene-electrode contact configuration in passive graphene devices for RF applications.

Electro-actuation characteristics of Cl2 and SF6 plasma-treated IPMC actuators

This paper describes plasma treatments that improve the actuation properties by modifying the surface morphology of ionic polymer metal composites (IPMC). The proposed Cl2 and SF6 plasmas change the surface appearance of the electroactive polymer, and scanning electron microscopy (SEM) of the plasma-treated surfaces reveals the development of round and cone-shaped microstructures. After electroless chemical metal plating, these microstructures significantly alter the characteristics of the IPMC electrode. In plasma-treated IPMCs, the densely packed platinum nanoparticles have produced a relatively thick electrode layer. This configuration has led to the improvement in the electrical properties of the IPMC: surface resistance is noticeably decreased, whereas electrical capacitance is increased. These changes in the electrical properties have considerably enhanced the actuation parameters: displacement, force and operational life are increased by more than three times relative to the conventional IPMC. Our experimental data suggest a relationship between the IPMC actuators electrical properties and actuation parameters: actuators with lower surface resistance generate large deflection and actuators with higher capacitance generate large actuation force. The actuation tests including coin lifting suggests the potential of the modified IPMC for artificial muscle applications.

Nonlinear characteristics in radio frequency nanoelectromechanical resonators

The nonlinear oscillations of nanoelectromechanical resonators have previously been studied both experimentally and analytically. Nanoresonators have achieved superior sensitivity and high quality factors in many applications. However, the linear operating range of nanoresonators is significantly limited because of the small dimensions and thus the linear regime of nanoresonators may be required to expand performance in various conditions. In order to increase the linear operating range, we proposed that proper adjustments of simultaneous application of drive and electrothermal power can be used to optimize the resonance performance, providing a wider linear range as well as to tune the resonance frequency. For a nanoresonator operated by simultaneous drive and electrothermal power, experimental data are theoretically supported using nonlinear damping and spring terms. In the transition between linearity and nonlinearity by proper combinations of ac drive and dc electrothermal power, the experimental data can be better fitted, by theoretical study, with newly derived nonlinear damping terms. We believe that better understanding of these effects with different ac/dc combinations on radio frequency oscillation is crucial for utilizing nanoresonators for various applications such as sensors, oscillators and filters.

Cerebral glucose metabolism in oculopalatal tremor

No study adopted the statistical parametric mapping (SPM) analyses of 18F‐fluorodeoxy glucose (FDG) PET in a large number of patients with oculopalatal tremor (OPT). To determine regional cerebral glucose metabolism in patients with OPT, nine patients with OPT underwent FDG‐PET of the brain. Their glucose metabolism was compared with that of 50 normal controls (NC) by using SPM analyses. Three patients had bilateral and six showed unilateral pseudohypertrophic degeneration of the inferior olivary nucleus (ION) on MRI. Compared with NC, OPT patients did not show any metabolic derangement in the anterolateral medulla where the pseudohypertrophic ION locates. Instead, six patients with unilateral ION changes had hypometabolism in ipsilesional pontine tegmentum and hypermetabolism in contralesional thalamus. Their metabolic changes did not depend on the lateralization of ION changes. Our study failed to present any metabolic evidence for the role of ION in the generation of OPT. In part, the failure might originate from the different pathomechanism between OPT and pure palatal tremor or sensitivity/specificity issues of PET and SPM analyses. But, our results suggest that impaired cell groups of the paramedian tract and thalamic tremor cells may contribute to the generation of OPT.

Microfabricated coupled-cavity backward-wave oscillator for terahertz imaging

A design study of a 0.1-THz coupled-cavity backward-wave oscillator (CCBWO) is presented using a finite element method and a particle-in-cell simulation. In experiments, the Si-based technology of 2-step deep reactive ion etching (DRIE) and eutectic bonding shows a good potential for the microfabrication of the interaction circuit. A miniaturized field emission cathode using carbon nanotubes grown by chemical vapor deposition (CVD) is also a good candidate for the electron beam generation. An integration of the cathode and the interaction circuit will be discussed. In addition, a preliminary design of a continuous-wave (CW) THz imaging system is demonstrated for the THz propagation and detection.

Optimization of Decentralized Task Assignment for Heterogeneous UAVs

Abstract In this paper, the optimization of a decentralized task assignment strategy for heterogeneous UAVs in a probabilistic engagement scenario is investigated. In the engagement scenario, each UAV selects its targets by employing the consensus-based bundle algorithm (CBBA). This paper uses a scoring matrix to reflect heterogeneity among the UAVs and targets with different capabilities. Therefore, a performance improvement of CBBA is closely connected with the scoring matrix and it should be optimally selected. The values of scoring matrix can be obtained by employing an episodic parameter optimization (EPO). The EPO algorithm is performed during the numerous repeated simulation runs of the engagement and the reward of each episode is updated using reinforcement learning. The candidate scoring matrices are selected by using particle swarm optimization. The optimization results show that the team survivability of the UAVs is increased after performing the EPO algorithm and the values of the optimized score matrix are also optimally selected.

Cooperation with Ground and Arieal Vehicles for Multiple Tasks: Decentralized Task Assignment and Graph Connectivity Control

Maintenance and improvement of the graph connectivity is very important for decentralized multi-agent systems. Although the CBBA (Consensus-Based Bundle Algorithm) guarantees suboptimal performance and bounded convergence time, it is only valid for connected graphs. In this study, we apply a decentralized estimation procedure that allows each agent to track the algebraic connectivity of a time-varying graph. Based on this estimation, we design a decentralized gradient controller to maintain the graph connectivity while agents are traveling to perform assigned tasks. Simulation result for fully-actuated first-order agents that move in a 2-D plane are presented.