Electrostatic Gating of Phosphorene Polymorphs.

The ability to directly monitor the states of electrons in modern field-effect transistors (FETs) could transform our understanding of the physics and improve the function of related devices. In particular, phosphorene allotropes present a fertile landscape for the development of high-performance FETs. Using density functional theory-based methods, we have systematically investigated the influence of electrostatic gating on the structures, stabilities, and fundamental electronic properties of pristine and carbon-doped monolayer (bilayer) phosphorene allotropes. The remarkable flexibility of phosphorene allotropes, arising from intra- and interlayer van der Waals interactions, causes a good resilience up to equivalent gate potential of two electrons per unit cell. The resilience depends on the stacking details in such a way that rotated bilayers show considerably higher thermodynamical stability than the unrotated ones, even at a high gate potential. In addition, a semiconductor to metal phase transition is observed in some of the rotated and carbon-doped structures with increased electronic transport relative to graphene in the context of real space Green's function formalism.

Artificial Neural Network-Derived Unified Six-Dimensional Potential Energy Surface for Tetra Atomic Isomers of the Biogenic [H, C, N, O] System.

Recognition of different structural patterns in different potential energy surface regions, such as in isomerizing quasilinear tetra atomic molecules, is important for understanding the details of underlying physics and chemistry. In this respect, using three variants of artificial neural networks (ANNs), we investigated the six-dimensional (6-D) singlet potential energy surfaces (PES) of tetra atomic isomers of the biogenic [H, C, N, O] system. At first, we constructed a separate ANN potential for each of the studied isomers. In the next step, a comparative assessment of the separate ANN models led to the setting up of a unified 6-D singlet PES equally and accurately describing all studied isomers. The constructed unified model yields relative energies comparable to those obtained either from the gold standard CCSD(T) method or from separate ANNs for each of the studied isomers. The accuracy of the unified singlet PES is on the order of 10-4 Hartrees (0.1 kcal/mol). The developed PES in this work captures the main features of nonlinear and quasilinear tetra atomic isomers of this biogenic system.

Electronic and structural properties of Lin @Be2 B8 (n = 1-14) and Lin @Be2 B36 (n = 1-21) nanoflakes shed light on possible anode materials for Li-based batteries.

Systematic addition of Li atoms to the Be2 B8 and Be2 B36 backbones has been studied by density functional theory-based calculations with the aim to investigate properties of interest on possible anode materials for Li-based batteries. For the Be2 B8 Lin (n = 1-8) and the Be2 B36 Lin (n = 1-20) systems, lithium salts are dominant whereas a clear electride feature shows up for Be2 B8 Lin (n = 9-14) and Be2 B36 Li21 . Addition of hydrogen radicals to these systems shows that the Be2 B8 Li14 electride becomes a Be2 B8 Li14 H2 hydride electride whereas Be2 B36 Li21 leads to a Be2 B36 Li21 H salt. Moreover, for the addition of Li atoms to Be2 B8 and the Be2 B36 backbones, large values of the interaction and of the adsorption energy per Li atom, high specific capacity of Be2 B8 Li14 and of Be2 B36 Li21 (1860 and 1017 mAh g-1 , respectively) and low and flat voltage associated with lithiation have been found. Likewise, the considerable thermodynamic driving force (ΔG° = -29.66 kcal/mol) and the small energy barrier ( ΔG# = 0.26 kcal/mol) associated with electron transfer in Be2 B36 Li21 and Be2 B36 species confirm that boron rich species have potential abilities to be used in the Li-based battery. © 2018 Wiley Periodicals, Inc.

Prediction of many-electron wavefunctions using atomic potentials.

For a given many-electron molecule, it is possible to define a corresponding one-electron Schrödinger equation, using potentials derived from simple atomic densities, whose solution predicts fairly accurate molecular orbitals for single- and multi-determinant wavefunctions for the molecule. The energy is not predicted and must be evaluated by calculating Coulomb and exchange interactions over the predicted orbitals. Potentials are found by minimizing the energy of predicted wavefunctions. There exist slightly less accurate average potentials for first-row atoms that can be used without modification in different molecules. For a test set of molecules representing different bonding environments, these average potentials give wavefunctions with energies that deviate from exact self-consistent field or configuration interaction energies by less than 0.08 eV and 0.03 eV per bond or valence electron pair, respectively.

Influence of NO and (NO)2 adsorption on the properties of Fe-N4 porphyrin-like graphene sheets.

Detection of NO in biological systems and removing or reducing NO for environment protection is paramount. Herein, we investigate the influence of NO and (NO)2 adsorption on the properties of Fe-N4 porphyrin-like graphene (G-Fe-N4) sheets using periodic DFT calculations with the dispersion correction. The results show that NO can be converted into N2O through adsorbed (NO)2 with a total energy barrier of 0.92 eV. The adsorption of N2O and of two NO on O/G-Fe-N4 sheets can proceed through (N2)gas + (O2)ads and (N2O)gas + (O2)ads, respectively. Both paths have a rate-determining step with a high (∼1.80 eV) energy barrier. Nevertheless, the formation of (O2)ads on the G-Fe-N4 can be regarded as an oxygen reduction reaction (ORR) precursor. Detailed analyses of the electronic properties of the various systems involved in this reaction reveal the increased spin filter characteristics for some structures. Hence, the obtained spin filter parameters of the NO@G-Fe-N4 and (NO)2@G-Fe-N4 structures are 72.53% and 47.96%, respectively. Also, it is found that the adsorption of NO gas molecules induces different energy antiresonant dips not found in G-Fe-N4, which are induced by quasi-bound states related to the adsorbate and Fe-N4 defect.

Theoretical study of electronic and tribological properties of h-BNC2/graphene, h-BNC2/h-BN and h-BNC2/h-BNC2 bilayers.

Density functional theory based methods are used to investigate the interlayer sliding energy landscape (ISEL), binding energy and interlayer spacing between h-BNC2/graphene (I), h-BNC2/h-BN (II) and h-BNC2/h-BNC2 (III) bilayer structures for three, six and fourteen different stacking patterns, respectively. Our results show that, in the studied cases, increasing the atomic variety of the ingredient monolayers leads to an ISEL corrugation increase as well. For the studied bilayers the ISEL is obtained by means of the registry index. For sufficiently large flakes of h-BNC2 on graphene sheets with the largest incommensurability and the least monolayer anisotropy, a robust superlubricity occurs regardless of the relative interlayer orientation. On the other hand, for the h-BNC2/h-BNC2 bilayer exhibiting the least incommensurability and the most monolayer anisotropy, the occurrence of robust superlubricity depends on the relative interlayer orientation.

Reactivity of the free and (5,5)-carbon nanotube-supported AuPt bimetallic clusters towards O2 activation: a theoretical study.

Density functional theory (DFT)-based calculations were carried out to predict the geometry, energy and electronic structures of the small bimetallic AumPtn (2 ≤m + n≤ 4) clusters deposited on a single-wall (5,5)-carbon nanotube (CNT). The chemical reactivity of these supported bimetallic clusters towards O2 reduction reaction was also considered. The calculations indicate that Au atoms tend to avoid the CNT atoms, whereas the opposite occurs for Pt atoms, a behavior which can be rationalized through analyses of the density of states plots. Compared to isolated clusters, the supported counterparts are found to have significant superiority in catalytic activity towards O2 reduction. The adsorption configuration and identity of the metal (Au or Pt) exposed to the O2 molecule adsorption are the dominant factors in determining the catalytic activity of the supported particles. Most notably, high catalytic activity of the supported clusters is associated with a drastic decrease in adsorption energy of the O2 molecule.

Line defects and induced doping effects in graphene, hexagonal boron nitride and hybrid BNC.

Effects on the atomic structure and electronic properties of two-dimensional graphene (G) and h-BN sheets related to the coexistence of dopants and defects are investigated by using density functional theory based methods. Two types of extended line defects are considered for pristine G and h-BN sheets. In these sheets, the presence of individual doping increases the charge transport character. The coexistence of dopants and defects tunes the band gap towards lower values and causes the direct-indirect band gap change. The relative stability and the electronic properties of various BxNyCz systems are analyzed in detail. We find that the structural properties of these types of systems strongly depend on the orientation of grain boundaries and whether these are parallel or perpendicular to the extended line defects. The electronic structure analysis of the different systems evidences the shift of absorption to the visible region.

Cold column trapping-cloud point extraction coupled to high performance liquid chromatography for preconcentration and determination of curcumin in human urine.

A cold column trapping-cloud point extraction (CCT-CPE) method coupled to high performance liquid chromatography (HPLC) was developed for preconcentration and determination of curcumin in human urine. A nonionic surfactant, Triton X-100, was used as the extraction medium. In the proposed method, a low surfactant concentration of 0.4% v/v and a short heating time of only 2min at 70°C were sufficient for quantitative extraction of the analyte. For the separation of the extraction phase, the resulted cloudy solution was passed through a packed trapping column that was cooled to 0 °C. The temperature of the CCT column was then increased to 25°C and the surfactant rich phase was desorbed with 400µL ethanol to be directly injected into HPLC for the analysis. The effects of different variables such as pH, surfactant concentration, cloud point temperature and time were investigated and optimum conditions were established by a central composite design (response surface) method. A limit of detection of 0.066mgL(-1) curcumin and a linear range of 0.22-100mgL(-1) with a determination coefficient of 0.9998 were obtained for the method. The average recovery and relative standard deviation for six replicated analysis were 101.0% and 2.77%, respectively. The CCT-CPE technique was faster than a conventional CPE method requiring a lower concentration of the surfactant and lower temperatures with no need for the centrifugation. The proposed method was successfully applied to the analysis of curcumin in human urine samples.

On the adsorption and formation of Pt dimers on the CeO2(111) surface.

The direct adsorption of Pt(2) dimers on CeO(2)(111) and their formation from isolated adsorbed Pt atoms have been studied using periodic slab model calculations based on density functional theory and including the so-called on-site Hubbard parameter (GGA + U). In the most stable configuration Pt(2) is found to be almost parallel to the surface; the electronic ground state is closed shell and there is no evidence of charge transfer towards or from the surface. The formation of Pt(2) from two single adsorbed Pt atoms involves a rather small energy barrier of ~0.10 eV only. On the contrary, dissociation of adsorbed Pt(2) requires to overcome a considerable barrier of ~1.43 eV. This indicates that once Pt(2) is formed it will remain on the surface, thus likely triggering the growth of larger supported Pt particles.

Theoretical insights into the trends in molecular properties of HCY, HSiY and HGeY molecules where Y = N, P, As.

To obtain insights into the factors that govern the analogy between HCN and its isostructures, HXY where X = C, Si, Ge and Y = N, P, As, the electronic and structural properties of these species in ground, cationic and anionic states at the QCISD, MP2 and B3LYP levels with 6-311++G** basis set and the first exited state with TD-B3LYP method have been presented. The results suggest that there are some correlations between structural and thermodynamic properties of the smallest member of this group (HCN) and heavier congers. The results of computation at these levels also predict the stability of HCAs in the ground state and HCN, HSiN and HGeN in the cationic state from the energetic point of view. Molecular electrostatic potential map inspection shows that in HXN species nucleophilic region positions on N atom but in HXP and HXAs molecules by increasing the size of central atom nucleophilic region shifts from region near X atom toward terminal atom. Finally, the nature of bonds of HXY molecules are systematically studied through atoms in molecules (AIM) and natural bond orbital analyses (NBO).