The intercalated water into nanopores exhibits anomalous properties such as an ultralow dielectric constant. Multiscale modeling and simulations are used to investigate the dielectric properties of various crystalline two-dimensional ices and bulk ices. Although the structural properties of two-dimensional (2D) ices have been extensively studied, much less is known about their electronic and optical properties. First, by using density functional theory and density functional perturbation theory (DFPT), we calculate the key electronic, optical, and dielectric properties of 2D ices. Performing DFPT calculations, both the ionic and electronic contributions of the dielectric constant are computed. The in-plane electronic dielectric constant is found to be larger than the out-of-plane dielectric constant for all the studied 2D ices. The in-plane dielectric constant of the electronic response (ε el ) is found to be isotropic for all the studied ices. Second, we determined the dipolar dielectric constant of 2D ices using molecular dynamics simulations at finite temperature. The total out-of-plane dielectric constant is found to be larger than 2 for all the studied 2D ices. Within the framework of the random-phase approximation, the absorption energy ranges for 2D ices are found to be in the ultraviolet spectra. For comparison purposes, we also elucidate the electronic, dielectric, and optical properties of four crystalline ices (ice VIII, ice XI, ice Ic, and ice Ih) and bulk water.
The self rotation of a graphene flake over graphite is controlled by the size, initial misalignment and temperature. Using both ab initio calculations and molecular dynamics simulations, we investigate annealing effects on the self rotation of a graphene flake on a graphene substrate. The energy barriers for rotation and drift of a graphene flake over graphene is found to be smaller than 25 meV/ atom which is comparable to thermal energy. We found that small flakes (of about ∼4 nm) are more sensitive to temperature and initial misorientation angles than larger one (beyond 10 nm). The initial stacking configuration of the flake is found to be important for its dynamics and time evolution of misalignment. Large flakes, which are initially in the AA-or AB-stacking state with small misorientation angle, rotate and end up in the AB-stacking configuration. However small flakes can they stay in an incommensurate state specially when the initial misorientation angle is larger than 2 . Our results are in agreement with recent experiments.
Using density functional theory, we study the electronic properties of a graphene-silicene bilayer (GSB). A single layer of silicene binds to the graphene layer with adhesion energy of about 25 meV/atom. This adhesion energy between the two layers follows accurately the well-known −1/z 2 dispersion energy as found between two infinite parallel plates. In small flakes of GSB with hydrogenated edges, negative charge is transferred from the graphene layer to the silicene layer, producing a permanent and a switchable polar bilayer, while in an infinite GSB, the negative charge is transferred from the silicene layer to the graphene layer. The graphene-silicene bilayer is a good candidate for a nanocapacitor with piezoelectric capabilities. We found that the permanent dipole of the bilayer can be tuned by an external perpendicular electric field.
Because of their unusual fundamental behavior that arises at the molecular scale, the electrical conductivity of stacked graphene oxide (GO) sheets in the presence of external parameters is not adequately understood. Previous studies concentrated on the DC response of thin GO membranes giving their resistive switching properties. Here, we observe anomalous low-frequency (<1 Hz) oscillations in the electrical conductivity of micrometer size GO, which is repeated in a process over and over in an ongoing feedback loop. Such vibrations and their unique trajectories are not only fundamentally important but also have characteristic frequencies that can be directly linked to the formation and destruction of regions with sp2 hybridization. Also, the reported switching time (of the order of seconds) makes our resistive switching system different than all the previously reported systems and introduces a new class of switching phenomena. The observed phenomena improve our understanding of the electrical conductivity of GO membranes and the corresponding microscopic details that pave the way for the promising application of these new observed low-frequency oscillations.
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