Interaction of Nucleobases and Aromatic Amino Acids with Graphene Oxide and Graphene Flakes

In this work, we have studied interactions of nucleobases and aromatic amino acids with graphene (G) and graphene oxide (GO) flakes by ab initio density functional theory (DFT). It is evident from the results that GO complexes are stabilized by hydrogen bonding interactions whereas G complexes are stabilized by π–π interactions, leading to enhanced binding energies for GO complexes compared to G complexes. Moreover, time-dependent DFT (TD-DFT) calculations for the optical properties reveal that the GO nanoflakes and GO–nucleobase composite absorb visible light in the range of 400–700 nm, which may be useful for light-emitting devices. The insights obtained from our study will be useful to understand the role of GO flakes as carriers in targeted drug delivery and biosensors

Gap opening and large spin–orbit splitting in (M = Mo,W; X = S,Se,Te) from the interplay between crystal field and hybridisations: insights from <i>ab-initio</i> theory

<p>By means of first-principles density functional calculations, we study the maximally localised Wannier functions for the 2D transition metal dichalcogenides (M = Mo,W; X = S,Se,Te). We have found that part of the energy gap is opened by the crystal field splitting induced by the -like atoms. The inversion of the band character between the and the <i>K</i> points of the Brillouin zone is due to the M–M hybridisation. The consequence of this inversion is the closure of the gap in absence of the M–X hybridisation. The M–X hybridisation is the only one that tends to open the gap at every k-point. It is found that the change in the M–X and M–M hybridisation is the main responsible for the difference in the gap between the different dichalcogenide materials. The inversion of the bands gives rise to different spin–orbit splitting at and <i>K</i> point in the valence band. The different character of the gap at and <i>K</i> point offers the chance to manipulate the semiconducting properties of these compounds. For a bilayer system, the hybridisation between the out-of-plane orbitals and the hybridisation between the in-plane orbitals split the valence band respectively at the and K point. The splitting in the valence band is opened also without spin–orbit coupling and occurs due to the M–M and X–X hybridisation between the two monolayers. The transition from direct to indirect band gap is governed by the hybridisation between out-of-plane orbitals of different layers and in-plane orbitals of different layers.</p

Subsurface Polaron Concentration As a Factor in the Chemistry of Reduced TiO₂ (110) Surfaces

Surface reactivity of rutile TiO₂ (110) surfaces has long been ascribed to bridging oxygen vacancies (V_O), but recently, excess electrons introduced by donor defects are being considered as the main players. However, the spatial distribution of them is not yet clear due to difficulties in interpreting filled state images of scanning tunneling microscopy (STM). In this study, several different images available in the literature are consistently interpreted using density functional theory (DFT). The key factors are polarons in the second layer below Ti_5c row (Ti_5c‑2nd polarons) and a temperature dependence of their concentration. Bright blobs in the experimental images are interpreted as Ti_5c‑2nd polarons. At 78 K, their concentration reaches 33.3% ML, where 1 ML is defined as the density of (1 × 1) unit cells, regardless of V_O coverage. In contrast, at 5 K, it is twice the V_O coverage. This discrepancy is understood by the ionization of donor defects other than V_O, most probably subsurface Ti interstitials, and subsequent diffusion of polarons to Ti_5c‑2nd sites at high temperature. This mechanism explains seemingly contradicting reports on oxygen chemisorption on this surface, which suggests that the so-called oxygen-vacancy model needs to be modified at temperature above at least 78 K

Enhancement of photovoltaic efficiency in CdSexTe1−x (where 0≤x≤1): Insights from density functional theory

Recent advancements in CdTe photovoltaic eciency have come from selenium grading, which reduces the band gap and signicantly improves carrier lifetimes. In this work, density functional theory calculations were performed to understand the structural and electronic eects of Se alloying. Special quasirandom structures were used to simulate a random distribution of Se anions. Lattice parameters decrease lin- early as Se concentration increases in line with Vegard's Law. The simulated band gap bowing shows strong agreement with experimental values. Selenium, by itself does not introduce any defect states in the band gap and no signicant changes to band structure around the Γ point are found. Band oset values suggest a reduction of recombination across the CdSeTe/MgZnO interface at x 0:1875, which corresponds with the Se concentration used experimentally. Band structure analysis of two cases x=0.03125 and x=0.4375, shows a change from dominant Te/Se contributions in the conduction band minimum to Cd/Se contributions as Se concentration is increased, hinting at a change in optical transition characteristics. Further calculations of optical absorption spectra suggest a reduced transition probability particularly at higher energies, which conrms experimental predictions that Se passivates the non-radiative recombination centres

Influence of Electron Correlation on the Electronic Structure and Magnetism of Transition-Metal Phthalocyanines

There exists an extensive literature on the electronic structure of transition-metal phthalocyanines (TMPcs), either as single molecules or adsorbed on surfaces, where explicit intra-atomic Coulomb interactions of the strongly correlated orbitals are included in the form of a Hubbard <i>U</i> term. The choice of <i>U</i> is, to a large extent, based solely on previous values reported in the literature for similar systems. Here, we provide a systematic analysis of the influence of electron correlation on the electronic structure and magnetism of several TMPcs (MnPc, FePc, CoPc, NiPc, and CuPc). By comparing calculated results to valence-band photoelectron spectroscopy measurements, and by determining the Hubbard term from linear response, we show that the choice of <i>U</i> is not as straightforward and can be different for each different TMPc. This, in turn, highlights the importance of individually estimating the value of <i>U</i> for each system before performing any further analysis and shows how this value can influence the final results

Polar Order and Frustrated Antiferromagnetism in Perovskite Pb₂MnWO₆ Single Crystals

Single crystals of the multiferroic double-perovskite Pb₂MnWO₆ have been synthesized and their structural, thermal, magnetic and dielectric properties studied in detail. Pure perovskite-phase formation and stoichiometric chemical composition of the as-grown crystals are confirmed by X-ray single-crystal and powder diffraction techniques as well as energy-dispersive X-ray and inductively coupled plasma mass spectrometry. Detailed structural analyses reveal that the crystals experience a structural phase transition from the cubic space group (s.g.) <i>Fm</i>3̅<i>m</i> to an orthorhombic structure in s.g. <i>Pn</i>2₁<i>a</i> at about 460 K. Dielectric data suggest that a ferrielectric phase transition takes place at that same temperature, in contrast to earlier results on polycrystalline samples, which reported a transition to s.g. <i>Pnma</i> and an antiferroelectric low-temperature phase. Magnetic susceptibility measurements indicate that a frustrated antiferromagnetic phase emerges below 8 K. Density functional theory based calculations confirm that the cationic order between Mn and W is favorable. The lowest total energy was found for an antiferromagnetically ordered state. However, analyses of the calculated exchange parameters revealed strongly competing antiferromagnetic interactions. The large distance between the magnetic atoms, together with magnetic frustration, is shown to be the main reason for the low value of the ordering temperature observed experimentally. We discuss the structure–property relationships in Pb₂MnWO₆ and compare these observations to reported results on related Pb₂BWO₆ perovskites with different B cations