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Bandgaps, band edges and structural properties of domain walls (DWs) in multiferroic BiFeO3 (BFO) are studied using density functional theory. The ferroelectric DWs (namely, the 71°, 109° and 180° DWs) in rhombohedral BFO are considered, with a view of critically examining the relationships between local bandgap, polar ordering and octahedral distortion reported for this material. Our study reveals that the domain-wall electronic properties of BFO sensitively depend on the polarization behavior and structural distortion at the wall center. The value of the bandgap at the wall decreases with the weakening of ferroelectricity at the wall center. Compared to all the wall types, DWs with the paraelectric-like phase at the wall center exhibit a significantly reduced bandgap. The bandgap reduction is also found to be closely related to two key parameters of the octahedral distortion at the wall: Fe-O bonds and O-Fe-O angles. The reduction in the Fe-O bond deviation and the straightening in the O-Fe-O angles at the wall appear with a marked bandgap reduction in the BFO DWs. Such a structural change at BFO DWs is reminiscent of a cubic structure, which is consistent with the analysis of paraelectric-like behavior at DWs. However, our analysis reveals that the straightening of Fe-O-Fe does not have any obvious correlation with bandgap reduction as suggested in previous works. The influence of the octahedral distortion at DWs is further clarified by investigating the bandgap and structural properties of various BFO bulk phases including Pnma, Pna21, tetragonal (P4mm), and Amm2. Our result establishes important correlations between electronic properties, polar ordering and structural distortion in BFO DWs. This study also suggests that photo nanocatalysts employing BFO DWs may be a promising strategy to enhance photocatalytic activity. [ABSTRACT FROM AUTHOR]

The morphological development of the hydrogen bond network in the protonated hydrogen fluoride clusters, H+(HF)n (n=2–9), is investigated in detail by ab initio methods. We find a dominance of the linear morphology, which is energetically well separated from the other minimum energy morphologies of the clusters. The geometry for these clusters shows a pattern due to the cooperativity effect prevalent in the hydrogen bonds, as a result of the difference in electronegativities of hydrogen and fluorine atom in the HF molecule. The variations in the covalent HF and hydrogen bond distances in the clusters are in turn reflected in the vibrational spectra. Distinct HF stretching modes for the linear and ring with tail structures were identified. We have discussed the signature peaks for the two possible ion-core morphologies present in the clusters. The highly corrosive nature of HF makes it difficult to study using experiments. We, thus, believe that our structure and vibrational spectra calculations would be useful in understanding the key features in these systems. [ABSTRACT FROM AUTHOR]

Structural isomer population of a hydrogen-bonded cluster generally depends on temperature. Therefore, determination of an isomer population profile in a wide temperature range is important to understand the nature of hydrogen bond networks of the cluster. To explore an isomer population profile, stepwise changes of internal vibrational energy of a protonated hydrogen-bonded cluster are performed by inert gas tagging. We observe infrared spectra of the protonated methanol pentamer with various tag species. The bare protonated methanol pentamer practically has only two possible isomer types. With the tagging, the relative population of the two isomer types changes according to the binding energy with the tag species. The observed relative population follows its theoretically predicted temperature dependence. [ABSTRACT FROM AUTHOR]

A surprising switch of the protonated site from methanol to water in protonated methanol−water mixed clusters, H+(MeOH)m(H2O)1(m= 1−9), was investigated by a joint theoretical and vibrational spectroscopic study. Extensive density functional calculations on all possible structural isomers revealed that the switch of the ion core is correlated with the size dependence and structural development of the hydrogen-bond network: (1) the CH3OH2+ion core is preferred for the small-sized clusters of m= 1 and 2, (2) coexistence of the H3O+and CH3OH2+ion cores is highly plausible for 3 ≤ m≤ 7 clusters, and (3) obvious preference of the H3O+ion core appears from m≥ 8 with the appearance of the characteristic “tricyclic” structure of the hydrogen-bond network. The ion core switch at m≈ 8 is experimentally supported by the infrared photodissociation spectra of the size-selected clusters and the size dependence of the fragmentation channel following vibrational excitation. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations of protonated methanol−water mixed clusters, H +(MeOH) 1(H 2O) n( n= 1−8), were extensively carried out to analyze the hydrogen bond structures of the clusters. Various structural isomers were energy optimized, and their relative energies with zero point energy corrections and temperature dependence of the free energies were examined. Coexistence of different morphological isomers was suggested. Infrared spectra were simulated on the basis of the optimized structures. The infrared spectra were also experimentally measured for n= 3−9 in the OH stretching vibrational region. The observed broad bands in the hydrogen-bonded OH stretch region were assigned in comparison with the simulations. From the DFT calculations, the preferential proton location was also investigated. Clear correlations between the excess proton location and the cluster morphology were found. [ABSTRACT FROM AUTHOR]

Extensive density functional theory (DFT) calculations are carried out on various structural isomers of protonated methanol clusters, H(MeOH)n(n 2−9), to analyze the morphological development of the hydrogen bond network in the clusters with an increase of the cluster size. Coexistence of multiple structural isomers is demonstrated by the nearly degenerated energies. Moreover, significant temperature dependence of the preferential isomer structure is shown by the calculated Gibbs free energies. The previously reported infrared spectra of H(MeOH)n(J. Phys. Chem. A2005, 109, 138) are revisited on the basis of the spectral simulations of the isomers by DFT calculations. [ABSTRACT FROM AUTHOR]

We have studied the structure of ice-VI by examining all ice-rule-allowed structures in its primary unit cell of 10 water molecules with first principles methods. A significant amount of static distortions in the oxygen positions away from their crystallographic positions are found, which is in good agreements with significant higher-order terms in the atomic displacement parameters obtained from X-ray and neutron diffraction data. Structural anomalies (such as exceptionally short OH bonds and small H−O−H angles) noted in conventional crystal structure refinements were not seen in our ab initio calculations, and it is evident that these structural anomalies arose from oversimplified models in which static distortions are not properly accounted for. Our results also show that the molecular geometry of water in ice-VI is similar to but richer than those in ice-Ih and ice-VII. Larger distortions in bond lengths/angles and correlation between the molecular geometry and the neighboring environments were found. Different proton-ordering schemes proposed in the literature were examined, and our calculations provide evidence in favor of a ferroelectric phase of the proton-ordered counterpart of ice-VI at about 80 K. [ABSTRACT FROM AUTHOR]

The hydrogen bond network, oxygen K edge x-ray absorption spectra (XAS), and electronic structure of ice XIII under compression have been extensively studied by density functional theory (DFT). We showed that DFT methods yield a ground state consistent with previous neutron scattering experiment and a few low-enthalpy metastable states are likely to coexist from the total enthalpy calculations. Oxygen K edge XAS of four low-enthalpy configurations was studied with the aim to shed light on the local structure in these configurations. We demonstrated that pre-edge of oxygen K edge XAS is a common feature appearing in all these four structures while major spectral differences exist in the main peak area. Therefore, we arrived at the conclusion that the main peak is more sensitive to the local hydrogen bond environment and could be used as an effective tool to distinguish these four configurations. We also found that the pre-edge has main contribution from O 1s-4a1 transitions and its intensity was suppressed by pressure while the main peak is mostly coming from O 1s-2b2 transitions. [ABSTRACT FROM AUTHOR]

Single-sheet graphene/boron nitride superlattices (SLs) are studied by a first-principles density functional theory approach to form a basic understanding for engineering a graphene band gap. The formation energy and electronic properties of SLs with different orientations (armchair or zigzag boundary) and stripe widths (2.16-14.80 Å) are studied. The trends of the formation energy with respect to the stripe widths and orientations are explained by the differences in bond energies and simple coulombic forces between ions. Regarding the electronic properties, it is found that not all SLs have a nonzero band gap. The band gap size and band edge locations can be explained by the Brillouin zone folding, the symmetry of SLs, and the effects from the graphene/boron nitride edges. The factors that cause the band gap opening and those that lilt it are analyzed and discussed. The detailed electronic states near the band edges of each SL group are analyzed by directly plotting the charge densities, as well as utilizing a simple tight-binding model to resolve their characteristics. [ABSTRACT FROM AUTHOR]

Understanding the structures of medium-sized protonated water clusters [H + (H 2 O) n ] has made a significant advancement recently thanks to the development of new experimental techniques and high-level computational methods. A combination of vibrational predissociation spectroscopy and ab initio calculations was shown to be effective in elucidating the structures of the clusters as a function of their temperature and size. The combined study revealed several intriguing features (such as symmetric hydrogen bonds) that could not be found for neutral water clusters. However, similar to its neutral counterpart, the number of stable isomers increases exponentially with cluster size ( n ), making direct structural identification of medium-sized clusters difficult. Despite the difficulties, both experimental and computational results indicated a smooth change in hydrogen-bond topology from tree-like, single-ring, multiple-ring to polyhedron-like structures (and their mixtures) as n increases from 5 to 28. The excess proton can be symmetrically hydrated at n   =  6–8. Five-membered ring isomers can form at n   =  7 and 8 as the low-lying minima. Only a single feature (∼⃒3695  cm -1 ) in the free OH stretching region was observed for H + (H 2 O) 21 and H + (H 2 O) 28 , suggesting that all surface water molecules are linked in a similar 3-coordinated (double-acceptor-single-donor, (AAD)) configuration in both “magic number” clusters. The clathrate-like structures open up at higher temperatures, as evidenced by the increased intensity of the free-OH stretching absorption band (∼⃒3715  cm -1 ) of 2-coordinated (single-acceptor-single-donor, (AD)) water molecules. Further understanding of the structures and thermal properties of these clusters is gained through the studies with Monte Carlo (MC) and molecular dynamics simulations, ion reactivity and thermal dissociation measurements, as well as Ar tagging experiments. [ABSTRACT FROM AUTHOR]

Infrared spectra of large-sized protonated methanol-water mixed clusters, H+(MeOH)m(H2O)n (m=1–4, n=4–22), were measured in the OH stretch region. The free OH stretch bands of the water moiety converged to a single peak due to the three-coordinated sites at the sizes of m+n=21, which is the magic number of the protonated water cluster. This is a spectroscopic signature for the formation of the three-dimensional cage structure in the mixed cluster, and it demonstrates the compatibility of a small number of methanol molecules with water in the hydrogen-bonded cage formation. Density functional theory calculations were carried out to examine the relative stability and structures of selected isomers of the mixed clusters. The calculation results supported the microscopic compatibility of methanol and water in the hydrogen-bonded cage development. The authors also found that in the magic number clusters, the surface protonated sites are energetically favored over their internal counterparts and the excess proton prefers to take the form of H3O+ despite the fact that the proton affinity of methanol is greater than that of water. [ABSTRACT FROM AUTHOR]

Hydrogen-bonded network structures and preferential ion core in the protonated methanol–water mixed clusters, H+(methanol)n–(water)1 (n = 6–10), were explored by a combination of infrared spectroscopy and theoretical calculations. Infrared spectra of the OH stretch region of the clusters were measured at the two different temperature ranges by using Ar-tagging. Stable isomer structures of the clusters were searched by the multiscale modeling approach and temperature dependent infrared spectra were simulated based on the statistical populations of the isomers. The combined experimental and theoretical studies revealed that the characteristic multiring structures begin to form at n = 7 under the low temperature condition and they are preferential at the wide temperature range in n ≥ 8. It was also demonstrated that the preferential ion core type changes from methanol (MeOH2+) to water (H3O+) with increasing cluster size. In n ≤ 8, the observed infrared spectral features partly depend on the monitoring vibrational predissociation channel, and weak correlations between the hydrogen-bonded network structure and preferential dissociation channels were suggested. However, the ion core type does not necessarily correlate to the preferential dissociation channel. This implies that large rearrangement of the hydrogen-bonded network structure occurs prior to the dissociation. [ABSTRACT FROM AUTHOR]

Hydrogen bonding exists in all hybrid organic-inorganic lead halide perovskites MAPbX3 (X = Cl, Br, or I). It has a strong influence on the structure, stability, and electronic and optical properties of this perovskite family. The hydrogen-bonding state between the H atoms of the methylammonium (MA) cation and the halide ions is resolved by combining ab initio calculations with temperature-dependent Raman scattering and powder X-ray diffraction measurements on MAPbBr3 hybrid perovskites. When the compounds are cooled, the H-bonding in the NH3 end of the MA group shows sequential changes while the H atoms in the CH3 end form H bonds with only the Br ions in the orthorhombic phase, leading to a decrease in the degree of rotational freedom of MA and a narrowing for MA Raman modes. Hydrogen bonding drives the evolution of temperature-dependent rotations of the MA cation and the concomitant tilting of PbX6 octahedra with the consequent dynamical change in the electronic band structures, from indirect bandgap to direct bandgap along with ~60-fold PL emission enhancement upon cooling. We experimentally and theoretically reveal the evolution of hydrogen bonding during polymorphic transformations and how the different types of hydrogen bonding lead to specific optoelectronic properties and device applications of hybrid perovskites. [ABSTRACT FROM AUTHOR]

The stacking configuration in few-layer two-dimensional (2D) materials results in different structural symmetries and layer-to-layer interactions, and hence it provides a very useful parameter for tuning their electronic properties. For example, ABA-stacking trilayer graphene remains semimetallic similar to that of monolayer, while ABC-stacking is predicted to be a tunable band gap semiconductor under an external electric field. Such stacking dependence resulting from many-body interactions has recently been the focus of intense research activities. Here we demonstrate that few-layer MoS2 samples grown by chemical vapor deposition with different stacking configurations (AA, AB for bilayer; AAB, ABB, ABA, AAA for trilayer) exhibit distinct coupling phenomena in both photoluminescence and Raman spectra. By means of ultralow-frequency (ULF) Raman spectroscopy, we demonstrate that the evolution of interlayer interaction with various stacking configurations correlates strongly with layer-breathing mode (LBM) vibrations. Our ab initio calculations reveal that the layer-dependent properties arise from both the spin-orbit coupling (SOC) and interlayer coupling in different structural symmetries. Such detailed understanding provides useful guidance for future spintronics fabrication using various stacked few-layer MoS2 blocks. [ABSTRACT FROM AUTHOR]

Understandingthe spectral features for solvated hydronium hasbeen hindered due to the strong and complex vibrational couplingsthat lead to broad bands in the aqueous phase. In this work, utilizing ab initiovibrational calculations, we determine how thevibrational couplings induced by the Ar microsolvation in H3O+Armm=0–3 affect the observed spectra. With theoretical peak intensitiesand peak positions, we assign the experimental spectra. We also showthat an increase in the number of Ar atoms results in an anticooperativeblue shifting in the Ar-tagged OH stretching bands. This change inpeak position of the OH stretching fundamental modulates the Fermiresonance with the bending overtone. This is observed as a distinctdoublet feature at 3200 cm–1with varying intensitiesfor H3O+Ar2and H3O+Ar3. The coupling between the in-plane rotationof the hydronium and the bending modes of H3O+leads to the existence of a strong association bands around 1900cm–1. [ABSTRACT FROM AUTHOR]

Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulse width and an ångström resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces. [ABSTRACT FROM AUTHOR]

Although new spintronic devices based on the giant spin-orbit splitting of single-layer MoS2 have been proposed, such splitting has not been studied effectively in experiments. This Letter reports the valence band spin-orbit splitting in single-layer MoS2 for the first time, probed by the triply resonant Raman scattering process. We found that upon 325 nm laser irradiation, the second order overtone and combination Raman modes of single-layer MoS2 are dramatically enhanced. Such resonant Raman enhancement arises from the electron-two-phonon triple resonance via the deformation potential and Fröhlich interaction. As a sensitive and precise probe for the spin-orbit splitting, the triply resonant Raman scattering will provide a new and independent route to study the spin characteristics of MoS2. [ABSTRACT FROM AUTHOR]

A theoretical study on the thermodynamic stability and electronic properties of wurtzite (WZ) and zincblende (ZB) ZnOxS1-x as been carried out with first-principles methods. Special quasi-random structures (SQSs) are employed to model the random alloys and from the calculated phase diagram, it is clear that the solubility limits in the mother lattices (sulfur-doped WZ-ZnO and oxygen-doped ZB-ZnS) are very small for both WZ and ZB phases. Detailed electronic and structural properties of quasi-ordered structures were investigated by examining alloy configurations in different supercells. In particular, we found that the concentration dependence of the band gap is highly nonlinear and the large bowing coefficients are also sensitive to the composition. The intriguing difference in band gaps and bowing coefficient between WZ- and ZB-ZnO1-xSx is evident. This result from first-principles calculations predicts that the band gap bowing with the WZ structure can be quite different from that with the ZB structure in some semiconductor alloys and deserves further experimental investigation for verification. [ABSTRACT FROM AUTHOR]

We propose a multiscale method to explore the energy landscape of water clusters. An asynchronous genetic algorithm is employed to explore the potential energy surface (PES) of OSS2 and TTM2.1-F models. Local minimum structures are collected on the fly, and the ultrafast shape recognition algorithm was used to remove duplicate structures. These structures are then refined at the B3LYP/6-31+G* level. The number of distinct local minima we found (21, 76, 369, 1443, and 3563 isomers for n= 4−8, respectively) reflects the complexity of the PES of water clusters. [ABSTRACT FROM AUTHOR]

Ground-state structures, vibrational frequencies, HOMO−LUMO energy gap, electron affinities, and cluster mixing energy of binary semiconductor clusters SimGenin the range smn ≤ 7 have been investigated using the B3LYP-DFT and CCSD(T) methods with the basis of 6-311G(d). SimGenclusters are found to have similar structural patterns and the same spin multiplicities as those of corresponding elemental clusters of Sisand Gesbut with more isomeric structures and lower symmetries. Notable structural changes induced by the additional electron were observed, except for s4. The mixing energies of binary clusters are negative, which suggests that the mixed clusters are more stable than pure Si and Ge clusters. Both the HOMO−LUMO gaps and the calculated mixing energy show that binary clusters SiGe2, Si2Ge2, Si2Ge3, Si2Ge4, and Si2Ge5are species with high stability and more likely to be produced experimentally. [ABSTRACT FROM AUTHOR]

Based on systematic first-principles density functional calculations, we predict that Li-decorated monolayer black phosphorus (MBP) is a hydrogen storage medium with great potential. It is found that pure MBP has only weak H2 adsorption energies. Surprisingly, its hydrogen storage properties can be significantly improved after lithium decoration. The calculated Li-binding energy on MBP is higher than the cohesive energy of bulk Li, ruling out the possibility of cluster formations of Li on the MBP surface. Our study shows that Li-decorated MBP can achieve a hydrogen storage capacity as high as 8.11 wt% . The calculated H2 adsorption energies fall within the range of 0.13–0.18 eV. This is a remarkable result indicating another important application of MBP. [ABSTRACT FROM AUTHOR]