Jia Mei Soon

Electrochemical Double-Layer Capacitance of MoS2 Nanowall Films

Edge-oriented MoS 2 films synthesized by single source precursor chemical vapor deposition exhibit a high density of nanowalls, which can potentially exhibit excellent electrochemical charge storage properties. The electrochemical double-layer capacitance of layered, nanowalled MoS 2 film has been investigated in this work using electrochemical impedance spectroscopy. We show that edge-oriented MoS 2 thin films can behave as a supercapacitor at alternating current frequencies up to 100 Hz. The supercapacitor performance is comparable to that of carbon nanotube array electrodes. In addition to double-layer capacitance, diffusion of the ions into the films at slow scan rates gives rise to faradaic capacitance, which enhances the capacitance significantly. The real and imaginary part of the capacitance of the MoS 2 films was analyzed as a function of frequency, in order to obtain information on the relaxation time constant and frequency dependence of the supercapacitor properties.

Nitrogen-enhanced negative bias temperature instability: An insight by experiment and first-principle calculations

The nitrogen-enhanced negative bias temperature instability (NBTI) effect has been studied experimentally and theoretically. It is observed that both the interface state and positive fixed charge generation increase linearly with interfacial nitrogen concentration. The experimental results can be understood in terms of the reaction energies of the hydrogen trapping reactions at the interface, which are obtained from first-principle calculations. These results improve our understanding of the mechanisms responsible for the nitrogen-enhanced NBTI effect.

Linear relationship between H+-trapping reaction energy and defect generation: Insight into nitrogen-enhanced negative bias temperature instability

In this letter, we report a first-principles calculation which is well correlated to experiment on the role of nitrogen at Si/SiOxNy interface in negative bias temperature instability (NBTI). Our calculation shows that nitrogen’s lone pair electrons can trap dissociated hydrogen species more easily than oxygen. After trapping, a positive charge complex is formed and weakening of bond strength is observed at trapping site. Furthermore, as nitrogen concentration goes beyond 8 at. %, the neighboring effect from nitrogen starts to play a role in further degradation. The interfacial nitrogen dependence of the NBTI-induced defect generation is found to coincide with that of the H+-trapping reaction energy. Eventually, a linear correlation is found between the reaction energy and the defect generation. This provides an insight into nitrogen-enhanced NBTI.

Ab initio studies of borazine and benzene cyclacenes

Abstract Can the borazine cyclacene system provide a conceptual framework for the investigation of the properties of hexagonal BN materials? We perform both Hartree–Fock (HF) and unrestricted density functional theory calculations for the borazine and benzene cyclacenes system to obtain insights into the structural and electronic properties as a function of the number of rings present in cyclacene. The properties of the borazine cyclacene show little dependence on the number of rings ( R ∼4–12) in the peripheral circuits, in contrast to the benzene cyclacene system. The frontier HOMO–LUMO gap in borazine cyclacene increases steadily with R and stabilizes at ∼6.7 eV for R >9, in contrast to the benzene cyclacene system where the gap decreases and approaches that of the semiconducting carbon nanotube system (∼1.13 eV) for R >8. The aromaticity of the rings is investigated by examining the energy separations in the frontier molecular orbitals. Vibrational modes in the borazine cyclacene systems were also calculated using HF method (HF/3-21G) and compared with experimental Raman measurements on hexagonal BN materials.

Highly textured, magnetic Fe(1+x)S nanorods grown on silicon

Highly textured nanorods of iron sulfide Fe(1+x)S (0.1<x<0.2) were deposited on silicon by chemical vapor deposition using the single source precursor iron(III) N,N-diethyldithiocarbamate. The evaporation of this precursor in vacuum, followed by its thermal decomposition on silicon substrate, resulted in the growth of iron-rich iron sulfide semiconductor. Magnetic profiling of the iron-rich Fe(1+x)S rods grown on silicon revealed a Curie temperature of 360°C; the in-plane:out-of-plane magnetic saturation ratio is 4:1, with a saturation magnetization of 19.7emu∕g.

Study of negative-bias temperature-instability-induced defects using first-principle approach

In this letter, we report the study of negative-bias temperature-instability (NBTI)-induced defect using first-principle calculations. From our calculations, we found that the NBTI effect leads to an overall decrease in frontier molecular orbital energy gap at the interface. We studied the interface structure at an atomistic level and proposed an explanation for this phenomenon using molecular orbital theory and band theory. In addition, weakening of bond strength of Si–O at the defect site is observed. It is found that upon formation of a defect, an energy state is created inside the band gap of SiO2. These results improve our understanding of the consequences associated with the NBTI effect, and the self-propagating nature of the NBTI effect.

Transformation from an atomically stepped NiO thin film to a nanotape structure: A kinetic study using x-ray diffraction

Transformation from an atomically stepped epitaxial thin film of NiO to a self-assemble nanotape structure at the step edge was observed in situ using synchrotron x-ray diffraction. The pristine NiO thin film was epitaxially grown on an ultrasmooth sapphire (0001) substrate with a regular step of 0.2nm in height using laser molecular beam epitaxy. Transformation from the thin film to the nanotape structure was facilitated by postannealing in air from room temperature to 620K. From the Arrhenius plot of ln(in-plane domain sizes) versus 1∕T, an atomic-scale transformation energy of ∼0.0135eV/atom was derived.