No Arabic abstract
One-dimensional inorganic nanotubes hold promise for technological applications due to their distinct physical/chemical properties, but so far advancements have been hampered by difficulties in producing single-wall nanotubes with a well-defined radius. In this work we investigate, based on Density Functional Theory (DFT), the formation mechanism of 135 different inorganic nanotubes formed by the intrinsic self-rolling driving force found in asymmetric 2D Janus sheets. We show that for isovalent Janus sheets, the lattice mismatch between inner and outer atomic layers is the driving force behind the nanotube formation, while in the non-isovalent case it is governed by the difference in chemical bond strength of the inner and outer layer leading to steric effects. From our pool of candidate structures we have identified more than 100 tubes with a preferred radius below 35 {AA}, which we hypothesize can display unique properties compared to their parent 2D monolayers. Simple descriptors have been identified to accelerate the discovery of small-radius tubes and a Bayesian regression approach has been implemented to assess the uncertainty in our predictions on the radius.
The structural stability of thermoelectric materials is a subject of growing importance for their energy harvesting applications. Here, we study the microscopic mechanisms governing the structural stability change of zinc antimony at its working temperature, using molecular dynamics combined with experimental measurements of the electrical and thermal conductivity. Our results show that the temperature-dependence of the thermal and electrical transport coefficients is strongly correlated with a structural transition. This is found to be associated with a relaxation process, in which a group of Zn atoms migrates between interstitial sites. This atom migration gradually leads to a stabilizing structural transition of the entire crystal framework, and then results in a more stable crystal structure of $beta-$ Zn4Sb3 at high temperature.
The rotation of octahedra (octahedral tilting) is common in ABO3 perovskites and relevant to many physical phenomena, ranging from electronic and magnetic properties, metal-insulator transitions to improper ferroelectricity. Hydrostatic pressure is an efficient way to tune and control octahedral tiltings. However, the pressure behavior of such tiltings can dramatically differ from one material to another, with the origins of such differences remaining controversial. In this work, we discover several new mechanisms and formulate a set of simple rules that allow to understand how pressure affects oxygen octahedral tiltings, via the use and analysis of first-principles results for a variety of compounds. Besides the known A-O interactions, we reveal that the interactions between specific B-ions and oxygen ions contribute to the tilting instability. We explain the previously reported trend that the derivative of the oxygen octahedral tilting with respect to pressure (dR/dP) usually decreases with both the tolerance factor and the ionization state of the A-ion, by illustrating the key role of A-O interactions and their change under pressure. Furthermore, three new mechanisms/rules are discovered. We further predict that the polarization associated with the so-called hybrid improper ferroelectricity could be manipulated by hydrostatic pressure, by indirectly controlling the amplitude of octahedral rotations.
We report the stability and electronic structures of the boron nitride nanotubes (BNNTs) with diameters below 4 A by semi-empirical quantum mechanical molecular dynamics simulations and ab initio calculations. Among them (3,0), (3,1), (2,2), (4,0), (4,1) and (3,2) BNNTs can be stable well over room temperature. These small BNNTs become globally stable when encapsulated in a larger BNNT. It is found that the energy gaps and work functions of these small BNNTs are strongly dependent on their chirality and diameters. The small zigzag BNNTs become desirable semiconductors and have peculiar distribution of nearly free electron states due to strong hybridization effect. When such a small BNNT is inserted in a larger one, the energy gap of the formed double-walled BNNT can even be much reduced due to the coupled effect of wall buckling difference and NFE-pi hybridization.
We have applied the quantum Monte Carlo method and tight-binding modelling to calculate the binding energy of biexcitons in semiconductor carbon nanotubes for a wide range of diameters and chiralities. For typical nanotube diameters we find that biexciton binding energies are much larger than previously predicted from variational methods, which easily brings the biexciton binding energy above the room temperature threshold.
Much of our understanding of complex structures is based on simplification: for example, metal-organic frameworks are often discussed in the context of nodes and linkers, allowing for a qualitative comparison with simpler inorganic structures. Here we show how such an understanding can be obtained in a systematic and quantitative framework, by combining atom-density based similarity (kernel) functions and unsupervised machine learning with the long-standing idea of coarse-graining atomic structure. We demonstrate how the latter enables a comparison of vastly different chemical systems, and use it to create a unified, two-dimensional structure map of experimentally known tetrahedral AB2 networks - including clathrate hydrates, zeolitic imidazolate frameworks (ZIFs), and diverse inorganic phases. The structural relationships that emerge can then be linked to microscopic properties of interest, which we exemplify for structural heterogeneity and tetrahedral density.