Effect of the Dopant Configuration on the Electronic Transport Properties of Nitrogen-Doped Carbon Nanotubes

Nitrogen-doped carbon nanotubes (N-CNTs) show promise in several applications related to catalysis and electrochemistry. In particular, N-CNTs with a single nitrogen dopant in the unit cell have been extensively studied computationally, but the structure-property correlations between the relative positions of several nitrogen dopants and the electronic transport properties of N-CNTs have not been systematically investigated with accurate hybrid density functional methods. We use hybrid density functional theory and semiclassical Boltzmann transport theory to systematically investigate the effect of different substitutional nitrogen doping configurations on the electrical conductivity of N-CNTs. Our results indicate significant variation in the electrical conductivity and the relative energies of the different dopant configurations. The findings can be utilized in the optimization of electrical transport properties of N-CNTs.

Conditions for electronic hybridization between transition-metal dichalcogenide monolayers and physisorbed carbon-conjugated molecules

Hybridization effects play a crucial role in determining the electronic properties of hybrid inorganic/organic interfaces. To gain insight into these important interactions, we perform a first-principles study based on hybrid density-functional theory including spin-orbit coupling, focusing on eight representative systems formed by two carbon-conjugated molecules-pyrene and perylene-physisorbed on the transition-metal dichalcogenide monolayers (TMDCs) MoS2, MoSe2 WS2, and WSe2. By means of band unfolding techniques, we analyze the band structures of the considered materials, identifying the contributions of the individual constituents as well as the signatures of their hybridization. Based on symmetry and energetic arguments, we derive general conditions for electronic hybridization between conjugated molecules and underlying TMDCs even when the former do not lie planar on the latter, thus providing the key to predict how their mutual arrangement affect their electronic interactions.

Overcoming Asymmetric Contact Resistances in Al-Contacted Mg2(Si,Sn) Thermoelectric Legs

Thermoelectric generators are a reliable and environmentally friendly source of electrical energy. A crucial step for their development is the maximization of their efficiency. The efficiency of a TEG is inversely related to its electrical contact resistance, which it is therefore essential to minimize. In this paper, we investigate the contacting of an Al electrode on Mg2(Si,Sn) thermoelectric material and find that samples can show highly asymmetric electrical contact resistivities on both sides of a leg (e.g., 10 µΩ·cm2 and 200 µΩ·cm2). Differential contacting experiments allow one to identify the oxide layer on the Al foil as well as the dicing of the pellets into legs are identified as the main origins of this behavior. In order to avoid any oxidation of the foil, a thin layer of Zn is sputtered after etching the Al surface; this method proves itself effective in keeping the contact resistivities of both interfaces equally low (<10 µΩ·cm2) after dicing. A slight gradient is observed in the n-type leg’s Seebeck coefficient after the contacting with the Zn-coated electrode and the role of Zn in this change is confirmed by comparing the experimental results to hybrid-density functional calculations of Zn point defects.

Electromechanical Characteristics by a Vertical Flip of C70 Fullerene Prolate Spheroid in a Single-Electron Transistor: Hybrid Density Functional Methods

In this study, the B3LYP hybrid density functional theory was used to investigate the electromechanical characteristics of C70 fullerene with and without point charges to model the effect of the surface of the gate electrode in a C70 single-electron transistor (SET). To understand electron tunneling through C70 fullerene species in a single-C70 transistor, descriptors of geometrical atomic structures and frontier molecular orbitals were analyzed. The findings regarding the node planes of the lowest unoccupied molecular orbitals (LUMOs) of C70 and both the highest occupied molecular orbitals (HOMOs) and the LUMO of the C70 anion suggest that electron tunneling of pristine C70 prolate spheroidal fullerene could be better in the major axis orientation when facing the gate electrode than in the major (longer) axis orientation when facing the Au source and drain electrodes. In addition, we explored the effect on the geometrical atomic structure of C70 by a single-electron addition, in which the maximum change for the distance between two carbon sites of C70 is 0.02 Å.

Iron in Hydroxyapatite: Interstitial or Substitution Sites?

Iron-doped hydroxyapatite (Fe-HAp) is regarded as a promising magnetic material with innate biocompatibility. Despite the many studies reported in the literature, a detailed theoretical description of Fe inclusions is still missing. There is even no consensual view on what kind of Fe defects take place in Fe-HAp—iron interstitial or calcium substitutions? In order to address these questions, we employ modern first-principles methodologies, including hybrid density functional theory, to find the geometry, electronic, magnetic and thermodynamic properties of iron impurities in Fe-HAp. We consider a total of 26 defect configurations, including substitutional (phosphorus and calcium sites) and interstitial defects. Formation energies are estimated considering the boundaries of chemical potentials in stable hydroxyapatite. We show that the most probable defect configurations are: Fe3+ and Fe2+ substitutions of Ca(I) and Ca(II) sites under Ca-poor conditions. Conversely, Fe interstitials near the edge of the hydroxyl channel are favored in Ca-rich material. Substitutional Fe on the P site is also a probable defect, and unlike the other forms of Fe, it adopts a low-spin state. The analysis of Fe K-XANES spectra available in the literature shows that Fe-HAp usually contains iron in different configurations.

Benchmarking computational methods to calculate the octanol/water partition coefficients of a diverse set of organic molecules

In the discovery process of new drugs and the development of novel therapies in medicine, computational modeling is a complementary tool for the design of new molecules by predicting for example their solubility in different solvents. Here, we benchmarked several computational methods to calculate the partition coefficients of a diverse set of 161 organic molecules with experimental logP values obtained from the literature. In general, density functional theory methods yielded the best correlations and lower average deviations. Although results are obtained faster with semiempirical and molecular mechanics methodologies, these methods yielded higher average deviations and lower correlation coefficients than hybrid density functional theory methods. We recommend the use of an empirical formula to correct the calculated values with each methodology tested.

Optimization of a Range-Separated UW12 Hybrid Functional

In a previous paper we presented a new hybrid functional B-LYP-osUW12-D3(BJ) containing the Unsöld-w12 (UW12) hybrid correlation model. In this paper we present a new 15-parameter range-separated hybrid density functional using a power series expansion together with UW12 correlation. This functional is optimised using the survival of the fittest strategy developed for the ωB97X-V functional, fitted to data from the Main Group Chemistry Database (MGCDB84). In addition we optimize a standard hybrid and double hybrid using the same method. We show that our fully self-consistent UW12 hybrid functional WM21-D3(BJ) outperforms both of these functionals and other range-separated hybrid functionals.