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Register a new userEnergy, phonon, and dynamic stability criteria of 2d materials
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First-principles calculations have become a powerful tool to exclude the Edisonian approach in search of novel 2d materials. However, no universal first-principles criteria to examine the realizability of hypothetical 2d materials have been established in the literature yet. Because of this, and since the calculations are always performed in an artificial simulation environment, one can unintentionally study compounds that do not exist in the experiments. Although investigations of physics and chemistry of unrealizable materials can provide some fundamental knowledge, the discussion of their applications can mislead experimentalists for years and increase the gap between experimental and theoretical research. By analyzing energy convex hull, phonon spectra, and structure evolution during ab initio molecular dynamics simulations for a range of synthesized and recently proposed 2d materials, we construct energy, phonon, and dynamic stability filters which need to be satisfied before proposing novel 2d compounds. We demonstrate the power of the suggested filters for several selected 2d systems, revealing that some of them cannot be ever realized experimentally.
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A method was developed to calculate the free energy of 2D materials on substrates and was demonstrated by the system of graphene and {gamma}-graphyne on copper substrate. The method works at least 3 orders faster than state-of-the-art algorithms, and the accuracy was tested by molecular dynamics simulations, showing that the precision for calculations of the internal energy achieves up to 0.03% in a temperature range from 100 to 1300K. As expected, the calculated the free energy of a graphene sheet on Cu (111) or Ni (111) surface in a temperature range up to 3000K is always smaller than the one of a {gamma}-graphyne sheet with the same number of C atoms, which is consistent with the fact that growth of graphene on the substrates is much easier than {gamma}-graphyne.
Raman spectroscopy is one of the most extended experimental techniques to investigate thin-layered 2D materials. For a complete understanding and modeling of the Raman spectrum of a novel 2D material, it is often necessary to combine the experimental investigation to density-functional-theory calculations. We provide the experimental proof of the fundamentally different behavior of polar 2D vs 3D systems regarding the effect of the dipole-dipole interactions, which in 2D systems ultimately lead to the absence of optical phonons splitting, otherwise present in 3D materials. We demonstrate that non-analytical corrections (NACs) should not be applied to properly model the Raman spectra of few-layered 2D materials, such as WSe$_{2}$ and h-BN, corroborating recent theoretical predictions [Nano Lett. 2017, 17 (6), 3758-3763]. Our findings are supported by measurements performed on tilted samples that allow increasing the component of photon momenta in the plane of the flake, thus unambiguously setting the direction of an eventual NAC. We also investigate the influence of the parity of the number of layers and of the type of layer-by-layer stacking on the effect of NACs on the Raman spectra.
We study theoretically the role of carrier multiplication due to impact ionization after an ultrafast optical excitation in a model system of a quasi-two dimensional material with a small band gap. As a mechanism for the photo-induced band gap narrowing we use coherent phonons, which mimics the quenching of an insulator phase. We discuss the importance of impact ionization in the ultrafast response, and investigate the interplay between carrier and band dynamics. Our model allows us to compare with recent experiments and identify signatures of carrier multiplication in typical electronic distribution curves as measured by time-resolved photoemission spectroscopy. In particular we investigate the influence of the shape of the bands on the carrier multiplication and the respective contributions of band and carrier dynamics to electronic distribution curves.
Recently, there have been increasing interests in phonon thermal transport in low dimensional materials, due to the crucial importance for dissipating and managing heat in micro and nano electronic devices. Significant progresses have been achieved for one-dimensional (1D) systems both theoretically and experimentally. However, the study of heat conduction in two-dimensional (2D) systems is still in its infancy due to the limited availability of 2D materials and the technical challenges in fabricating suspended samples suitable for thermal measurements. In this review, we outline different experimental techniques and theoretical approaches for phonon thermal transport in 2D materials, discuss the problems and challenges in phonon thermal transport measurements and provide comparison between existing experimental data. Special focus will be given to the effects of the size, dimensionality, anisotropy and mode contributions in the novel 2D systems including graphene, boron nitride, MoS2, black phosphorous, silicene etc.
Recently a new group of two dimensional (2D) materials, originating from the group V elements (pnictogens), has gained global attention owing to their outstanding properties.
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