Computation of flexoelectric coefficients of a MoS2 monolayer with a model of self-consistently distributed effective charges and dipoles.

Flexoelectricity is an electromechanical coupling phenomenon that can generate noticeable electric polarization in dielectric materials for nanoscale strain gradients. It is gaining increasing attention because of its potential applications and the fact that experimental results were initially an order of magnitude higher than initial theoretical predictions. This stimulated intense experimental and theoretical research to investigate flexoelectric coefficients in dielectric materials such as two-dimensional materials. In this study, we concentrate on the calculation of the flexoelectric coefficients of 2D-MoS2 due to a model using self-consistently determined charges and dipoles on the atoms. More specifically, we study the importance of two contributions that were neglected/omitted in previous papers using this model, namely, the charge term in the total polarization and the conservation of electric charge through a Lagrange multiplier. Our calculations demonstrate that the results for flexoelectric coefficients computed with this improved definition of polarization agree better with experimental measurements, provided that consistent definitions for signs are used. Additionally, we show how two physical contributions with opposite signs compete to give net values of flexoelectric coefficients that can be either positive or negative depending on their relative importance and give net values for the case of MoS2.

Influence of Onion-like Carbonaceous Particles on the Aggregation Process of Hydrocarbons.

Molecular dynamics simulations are performed to characterize the nucleation behavior of organic compounds in the gas phase. Six basic molecular species are considered-ethylene, propylene, toluene, styrene, ethylbenzene, and para-xylene-in interaction with onion-like carbon nanostructures that model soot nanoparticles (NPs) at room temperature. We identify a shell-to-island aggregation process during the physisorption of aromatic molecules on the soot surface: The molecules tend to first cover the NP in a shell, on top of which additional adsorbates form island-shaped aggregates. We present results for the binding energy, suggesting that the NPs lead to the formation of more stable molecular aggregates in comparison with the pure gas phase. Our findings describe a plausible microscopic mechanism for the active role of soot in the formation and growth of organic particulate matter.

An atomistic model for the charge distribution in layered MoS2.

We present an atomistic model for predicting the distribution of doping electric charges in layered molybdenum disulfide (MoS2). This model mimics the charge around each ion as a net Gaussian-spatially distributed charge plus an induced dipole, and is able to predict the distribution of doping charges in layered MoS2 in a self-consistent scheme. The profiles of doping charges in monolayer MoS2 flakes computed by this charge-dipole model are in good agreement with those obtained by density-functional-theory calculations. Using this model, we quantitatively predict the charge enhancement in MoS2 monolayer nanoribbons, with which strong ionic charge-localization effects are shown.

Volume dependence in Handel's model of quartz crystal resonator noise.

Although criticized by many, Handel's quantum model for 1/f noise remains the only model giving a quantitative estimation of the level of intrinsic 1/f noise in quartz crystal resonators that is compatible with the best experimental results. In this paper, we reconsider the volume dependence in this model. We first argue that an acoustic volume, representing the volume in which the vibration energy is trapped, should be used instead of the geometrical volume between the electrodes. Then, we show that because there is an implicit dependence of the quality factor of the resonator with its thickness, the net effect of Handel's formula is not an increase of noise proportionally to the thickness of the resonator, as could be naïvely expected, but a net decrease when thickness increases. Finally, we show that a plot of Q(4)Sy versus the acoustic volume, instead of the usual Sy plot, could be useful to compare the quality of acoustic resonators having very different resonance frequencies.

Optical properties of WO3 thin films modeled by finite-difference time-domain and fabricated by glancing angle deposition.

Optical transmittance spectra between 1.55 eV (800 nm) and 3.10 eV (400 nm) of tungsten oxide (WO3) thin films nanostructured thanks to the Glancing Angle Deposition technique are investigated both experimentally and theoretically, as a function of geometrical parameters. A Finite-Difference Time-Domain code was used to numerically model the films structure and to calculate their optical properties. The corresponding optical index and porosity are considered. It is found that the optical index of columnar structures always follows Cauchy's law as a function of energy and is reduced as the incident angle increases (alpha = 0 to 80 degrees) from n633 = 2.2 to 1.98 for experimental data against 2.1 to 1.75 for those computed with the Finite-Difference Time-Domain code. For zigzag architectures, an increase of the zigzag number from 0.5 to 8, amplifies interference fringes and improves the measured refractive indices. It agrees with modeled optical characteristics since n633 increases from 2.18 to 2.30.