Gyeong S. Hwang

Long-life Li/polysulphide batteries with high sulphur loading enabled by lightweight three-dimensional nitrogen/sulphur-codoped graphene sponge

Lithium–sulphur batteries with a high theoretical energy density are regarded as promising energy storage devices for electric vehicles and large-scale electricity storage. However, the low active material utilization, low sulphur loading and poor cycling stability restrict their practical applications. Herein, we present an effective strategy to obtain Li/polysulphide batteries with high-energy density and long-cyclic life using three-dimensional nitrogen/sulphur codoped graphene sponge electrodes. The nitrogen/sulphur codoped graphene sponge electrode provides enough space for a high sulphur loading, facilitates fast charge transfer and better immobilization of polysulphide ions. The hetero-doped nitrogen/sulphur sites are demonstrated to show strong binding energy and be capable of anchoring polysulphides based on first-principles calculations. As a result, a high specific capacity of 1,200 mAh g−1 at 0.2C rate, a high-rate capacity of 430 mAh g−1 at 2C rate and excellent cycling stability for 500 cycles with ∼0.078% capacity decay per cycle are achieved.

On the origin of the notching effect during etching in uniform high density plasmas

We present a two-dimensional Monte Carlo simulation of profile evolution during the overetching step of polysilicon-on-insulator structures, which considers explicitly (a) electric field effects during the charging transient, (b) etching reactions of energetic ions impinging on the poly-Si, and (c) forward inelastic scattering effects. Realistic energy and angular distributions for ions and electrons are used in trajectory calculations through local electric fields near and in the microstructure. Transient charging of exposed insulator surfaces is found to profoundly affect local sidewall etching (notching). Ion scattering contributions are small but important in matching experimental notch profiles. The model is validated by capturing quantitatively the notch characteristics and also the effects of the line connectivity and open area width on the notch depth, which have been observed experimentally by Nozawa et al. [Jpn. J. Appl. Phys. 34, 2107 (1995)]. Elucidation of the mechanisms responsible for the effect facilitates the prediction of ways to minimize or eliminate notching.

Epoxide reduction with hydrazine on graphene: A first principles study

Mechanisms for epoxide reduction with hydrazine on a single-layer graphene sheet are examined using quantum mechanical calculations within the framework of gradient-corrected spin-polarized density-functional theory. We find that the reduction reaction is mainly governed by epoxide ring opening which is initiated by H transfer from hydrazine or its derivatives. In addition, our calculations suggest that the epoxide reduction by hydrazine may predominantly follow a direct Eley-Rideal mechanism rather than a Langmuir-Hinshelwood mechanism. We also discuss the generation of various hydrazine derivatives during the reduction of graphene oxide with hydrazine and their potential contribution to lowering the barrier height of epoxide ring opening.

Pattern-Dependent Charging and the Role of Electron Tunneling

We review the prevailing causes of and remedies for profile distortion (notching) resulting from pattern-dependent charging during etching in high density plasmas. A new mechanism for notch reduction, based on electron tunneling through thin gate oxides, is explained through detailed modeling and simulations of charging and profile evolution in polysilicon gate definition. Tunneling currents from the substrate decrease surface charging potentials–responsible for ion deflection–at the bottom of high aspect ratio trenches. The exponential dependence of electron tunneling on the oxide electric field predicts an abrupt transition from severe notching to virtually no notching as the gate oxide thickness is decreased, which has been seen in experiments.

Mechanism of Charging Reduction in Pulsed Plasma Etching.

Numerical simulations of charging and etching in time-modulated high-density plasmas suggest a new mechanism for the reduction of pattern-dependent charging, which is based on low energy positive ions. During the power-off period and before the sheath collapses, the electron temperature and plasma potential decrease rapidly, resulting in low energy ions which can be deflected by smaller local electric fields. The flux of deflected ions to the upper mask sidewalls increases enabling neutralization of the negative charge accumulated there due to the electron shading effect. Current balance at the trench bottom surface is achieved at lower charging potentials, which lead to significantly reduced notching and gate oxide degradation. Pulsing period and duty ratio are examined as parameters to control the performance of pulsed plasmas.

Aspect-ratio-dependent charging in high-density plasmas

The effect of aspect ratio (depth/width) on charge buildup in trenches during plasma etching of polysilicon-on-insulator structures is studied by Monte Carlo simulations. Increased electron shadowing at larger aspect ratios reduces the electron current to the trench bottom. To reach a new charging steady state, the bottom potential must increase, significantly perturbing the local ion dynamics in the trench: the deflected ions bombard the sidewall with larger energies resulting in severe notching. The results capture reported experimental trends and reveal why the increase in aspect ratio that follows the reduction in critical device dimensions will cause more problems unless the geometry is scaled to maintain a constant aspect ratio.

Aspect ratio independent etching of dielectrics

Monte Carlo simulations of pattern-dependent charging during oxide etching predict that the etch rate scaling with aspect ratio breaks down when surface discharge currents are significant. Under conditions of ion-limited etching and no inhibitor deposition, the etch depth depends on the maximum incident ion energy, reaction threshold, and surface discharge threshold, and is the same irrespective of the trench width (<= 0.5 µm).

On the origin of Si nanocrystal formation in a Si suboxide matrix

We examined mechanisms underlying Si nanocrystal formation in Si-rich SiO2 using a combination of quantum mechanical and Monte Carlo (MC) simulations. We find that this process is mainly driven by suboxide penalty arising from incomplete O coordination, with a minor contribution of strain, and it is primarily controlled by O diffusion rather than excess Si diffusion and agglomeration. The overall behavior of Si cluster growth from our MC simulations based on these fundamental findings agrees well with experiments.

Two-dimensional computational model for electrochemical micromachining with ultrashort voltage pulses

We have developed a computational model to simulate electrochemical micromachining of conducting substrates with ultrashort voltage pulses. This theoretical approach integrates (i) a circuit model to describe charging and discharging of electrochemical double layers and electric field variation in electrolytes and (ii) the level set method to simulate feature profile evolution during electrochemical etching. Our simulation results of transient current responses and etch profile evolution are qualitatively in agreement with experimental observations. From our simulations, we find that the resolution of etched features is a strong function of the substrate double layer capacity which may be controlled by electrolyte concentration and pulse duration.

Electrochemical machining with ultrashort voltage pulses: modelling of charging dynamics and feature profile evolution

A two-dimensional computational model is developed to describe electrochemical nanostructuring of conducting materials with ultrashort voltage pulses. The model consists of (1) a transient charging simulation to describe the evolution of the overpotentials at the tool and workpiece surfaces and the resulting dissolution currents and (2) a feature profile evolution tool which uses the level set method to describe either vertical or lateral etching of the workpiece. Results presented include transient currents at different separations between tool and workpiece, evolution of overpotentials and dissolution currents as a function of position along the workpiece, and etch profiles as a function of pulse duration.