A 3D percolation model for multicomponent nanocarbon composites: the critical role of nematic transition.

A three-dimensional (3D) continuum percolation model has been developed on the basis of Monte Carlo simulation to investigate the percolation behavior of an electrically insulating matrix reinforced with multiple conductive fillers of different dimensionalities. Impenetrable fillers of large aspect ratio are found to preferentially align with each other to maximize the packing entropy rather than forming randomly oriented clusters. This entropy-driven transition from isotropic to nematic phase is shown to critically affect the percolation threshold. It suggests that an isotropic phase with a smaller nematic order parameter leads to a reduction in percolation threshold. In addition, a combination of two fillers with different dimensionalities can achieve a working concentration below the percolation threshold of single component system, which is further validated by the experiments of electrical conductivity in multicomponent multidimensional nanocarbon composites.

Monte Carlo simulations of electrical percolation in multicomponent thin films with nanofillers.

We developed a 2D disk-stick percolation model to investigate the electrical percolation behavior of an insulating thin film reinforced with 1D and 2D conductive nanofillers via Monte Carlo simulation. Numerical predictions of the percolation threshold in single component thin films showed good agreement with the previous published work, validating our model for investigating the characteristics of the percolation phenomena. Parametric studies of size effect, i.e., length of 1D nanofiller and diameter of 2D nanofiller, were carried out to predict the electrical percolation threshold for hybrid systems. The relationships between the nanofillers in two hybrid systems was established, which showed differences from previous linear assumption. The effective electrical conductance was evaluated through Kirchhoff's current law by transforming it into a resistor network. The equivalent resistance was obtained from the distribution of nodal voltages by solving a system of linear equations with a Gaussian elimination method. We examined the effects of stick length, relative concentration, and contact patterns of 1D/2D inclusions on electrical performance. One novel aspect of our study is its ability to investigate the effective conductance of nanocomposites as a function of relative concentrations, which shows there is a synergistic effect when nanofillers with different dimensionalities combine properly. Our work provides an important theoretical basis for designing the conductive networks and predicting the percolation properties of multicomponent nanocomposites.

Prediction of a two-dimensional organic topological insulator.

Topological insulators (TI) are a class of materials exhibiting unique quantum transport properties with potential applications in spintronics and quantum computing. To date, all of the experimentally confirmed TIs are inorganic materials. Recent theories predicted the possible existence of organic TIs (OTI) in two-dimensional (2D) organometallic frameworks. However, those theoretically proposed structures do not naturally exist and remain to be made in experiments. Here, we identify a recently experimentally made 2D organometallic framework, consisting of π-conjugated nickel-bis-dithiolene with a chemical formula Ni3C12S12, which exhibits nontrivial topological states in both a Dirac band and a flat band, therefore confirming the existence of OTI.

Quantum manifestations of graphene edge stress and edge instability: a first-principles study.

We have performed first-principles calculations of graphene edge stresses, which display two interesting quantum manifestations absent from the classical interpretation: the armchair edge stress oscillates with a nanoribbon width, and the zigzag edge stress is noticeably reduced by spin polarization. Such quantum stress effects in turn manifest in mechanical edge twisting and warping instability, showing features not captured by empirical potentials or continuum theory. Edge adsorption of H and Stone-Wales reconstruction are shown to provide alternative mechanisms in relieving the edge compression and hence to stabilize the planar edge structure.