Nanofilm versus Bulk Polymorphism in Wurtzite Materials

We generate a wide range of hexagonal sheet-based ZnO polymorphs inspired by enumeration of their characteristic underlying nets. Evaluating the bulk and nanofilm stabilities of these structures with ab initio calculations allows for an unprecedented overview of (nano)polymorphism in wurtzite materials. We find a rich low energy nanofilm polymorphism with a totally distinct stability ordering to that in the bulk. From this general basis we provide new insights into structural transitions observed during epitaxial growth and predictions for nanofilm stability with varying strain or thickness

Columnar-to-Disk Structural Transition in Nanoscale (SiO2)N Clusters

Extensive large-scale global optimizations refined by ab initio calculations are used to propose ( SiO 2 ) N N = 14 - 27 ground states. For N < 23 clusters are columnar and show N − odd - N − even stability, energetically and electronically. At N = 23 a columnar-to-disk structural transition occurs reminiscent of that observed for Si N . These transitions differ in nature but have the same basis, linking the nanostructural behavior of an element (Si) and its oxide ( SiO 2 ). Considering the impact of devices based on the nanoscale manipulation of Si / SiO 2 the result is of potential technological importance

Formation of Interstellar Silicate Dust via Nanocluster Aggregation: Insights From Quantum Chemistry Simulations

The issue of formation of dust grains in the interstellar medium is still a matter of debate. One of the most developed proposals suggests that atomic and heteromolecular seeds bind together to initiate a nucleation process leading to the formation of nanostructures resembling very small grain components. In the case of silicates, nucleated systems can result in molecular nanoclusters with diameters ≤ 2 nm. A reasonable path to further increase the size of these proto-silicate nanoclusters is by mutual aggregation. The present work deals with modeling this proto-silicate nanocluster aggregation process by means of quantum chemical density functional theory calculations. We simulate nanocluster aggregation by progressively reducing the size of a periodic array of initially well-separated nanoclusters. The resulting aggregation leads to a set of silicate bulk structures with gradually increasing density which we analyze with respect to structure, energetics and spectroscopic properties. Our results indicate that aggregation is a highly energetically favorable process, in which the infrared spectra of the finally formed amorphous silicates match well with astronomical observations

Efficiency of Interstellar Nanodust Heating: Accurate Bottom-up Calculations of Nanosilicate Specific Heat Capacities

Ultrasmall nanosized silicate grains are likely to be highly abundant in the interstellar medium. From sporadically absorbing energy from ultraviolet photons, these nanosilicates are subjected to significant instantaneous temperature fluctuations. These stochastically heated nanograins subsequently emit in the infrared. Previous estimates of the extent of the heating and emission have relied on empirical fits to bulk silicate heat capacities. The heat capacity of a system depends on the range of available vibrational modes, which for nanosized solids is dramatically affected by the constraints of finite size. Although experimental vibrational spectra of nanosilicates is not yet available, we directly take these finite size effects into account by using accurate vibrational spectra of low-energy nanosilicate structures from quantum chemical density functional theory calculations. Our results indicate that the heat capacities of ultrasmall nanosilicates are smaller than previously estimated, which would lead to a higher temperature and more intense infrared emission during stochastic heating. Specifically, we find that stochastically heated grains ultrasmall nanosilicates could be up to 35-80 K hotter than previously predicted. Our results could help to improve the understanding of infrared emission from ultrasmall nanosilicates in the ISM, which could be observed by the James Webb Space Telescope

Assessing the viability of silicate nanoclusters as carriers of the anomalous microwave emission: a quantum mechanical study

Nanosized silicate dust is likely to be abundant in many astronomical environments and it is a prime candidate for being the source of the anomalous microwave emission (AME). To assess the viability of silicate nanoclusters as AME carriers, their detailed properties need to be established. Using quantum chemical calculations, we compute the accurate chemical and electronic structures of three families of nanoclusters with astrophysically relevant compositions: Mg-rich olivine (Mg2SiO4)N, Mg-rich pyroxene (MgSiO3)N, and silicon monoxide (SiO)N, all in the ≤1 nm diameter size regime and for neutral and ± 1 charge states. From these fundamental data, we directly derive the shapes, ionization potentials, electron affinities, and dipole moments of all nanoclusters. The aspect ratio of the nanoclusters fluctuates significantly with N for small sizes, but especially for the olivine and pyroxene nanoclusters, it tends to stabilize towards ~1.3 for the largest sizes considered. These latter two nanocluster families tend to have mass distributions consistent with approximately prolate ellipsoidal shapes. Our calculations reveal that the dipole moment of all our nanoclusters can be substantially affected by changes in chemical structure (i.e. different isomers for a fixed N), ionisation, and substitution of Mg by Fe. Although all these factors are important, the dipole moments of our Mg-rich nanoclusters are always found to be large enough to account for the observed AME. However, (SiO)N nanoclusters are only likely to be potential AME contributors when they are both charged and their chemical structures are anisotropically segregated. We also model the emissivity per H of a representative (Mg2SiO4)3 nanocluster by directly calculating the quantum mechanical rotational energy levels and assuming a distribution of occupied levels in accordance with equilibrium Boltzmann statistics. We compare our bottom-up results with previously published classical models and show that a population of silicate nanoclusters containing only 1% of the total Si budget can reproduce the AME emissivity

An unconstrained approach to systematic structural and energetic screening of materials interfaces

From grain boundaries and heterojunctions to manipulating 2D materials, solid-solid interfaces play a key role in many technological applications. Understanding and predicting properties of these complex systems present an ongoing and increasingly important challenge. Over the last few decades computer simulation of interfaces has become vastly more powerful and sophisticated. However, theoretical interface screening remains based on largely heuristic methods and is strongly biased to systems that are amenable to modelling within constrained periodic cell approaches. Here we present an unconstrained and generally applicable non-periodic screening approach for systematic exploration of material's interfaces based on extracting and aligning disks from periodic reference slabs. Our disk interface method directly and accurately describes how interface structure and energetic stability depends on arbitrary relative displacements and twist angles of two interacting surfaces. The resultant detailed and comprehensive energetic stability maps provide a global perspective for understanding and designing interfaces. We confirm the power and utility of our method with respect to the catalytically important TiO2 anatase (101)/(001) and TiO2 anatase (101)/rutile (110) interfaces

Reduced ceria nanofilms from structure prediction

Experimentally, Ce2O3 films are used to study cerium oxide in its fully or partially reduced state, as present in many applications. We have explored the space of low energy Ce2O3 nanofilms using structure prediction and density functional calculations, yielding more than 30 distinct nanofilm structures. First, our results help to rationalize the roles of thermodynamics and kinetics in the preparation of reduced ceria nanofilms with different bulk crystalline structures (e.g. A-type or bixbyite) depending on the support used. Second, we predict a novel, as yet experimentally unresolved, nanofilm which has a structure that does not correspond to any previously reported bulk A2B3 phase and which has an energetic stability between that of A-type and bixbyite. To assist identification and fabrication of this new Ce2O3 nanofilm we calculate some observable properties and propose supports for its epitaxial growth

Global optimisation of hydroxylated silica clusters: a cascade Monte Carlo Basin Hopping approach

We report on a global optimisation study of hydroxylated silica nanoclusters (SiO2)/w(H2O)(N) with sizes M = 6, 8, 10 12, and for each size with a variable number of dissociatively chemisorbed water molecules (N = 1, 2, 3...). Due to the high structural complexity of these systems and the associated ruggedness of the underlying potential energy landscape, we employ a 'cascade' global optimisation approach. Specifically, we use Monte Carlo Basin Hopping (MCBH) where for each step we employ two energy minimisations with: (i) a lightly parameterised but computationally efficient interatomic potential (IP) which does not distinguish between H-bonded conformational isomers, and then (ii) a more sophisticated IP which accounts for polarisation and H-bonding. Final energies from the MCBH search are then refined with optimisations using density functional theory. The reliability of our approach is first established via comparison with previously reported results for the (SiO2)(8).(H2O)(N) case, and then applied to the M = 6, 10 and 12 systems. For all systems studied our results follow the trend in hydroxylation energy versus N, whereby the energy gain with hydroxylation is found to level off at a point where the average tetrahedral distortion of the SiO4 centres is minimised. This optimal hydroxylation point is further found to follow an inverse power law with increasing cluster size (M) with an exponent close to -2/3, further confirming work in previous studies for other cluster sizes. (C) 2016 Elsevier B.V. All rights reserved

Twistable Dipolar Aryl Rings as Electric Field Actuated Conformational Molecular Switches

The ability to control the chemical conformation of a system via external stimuli is a promising route for developing molecular switches. For eventual deployment as viable sub-nanoscale components that are compatible with current electronic device technology, conformational switching should be controllable by a local electric field (i.e. E-field gateable) and accompanied by a rapid and significant change in conductivity. In organic chemical systems the degree of π-conjugation is linked to the degree of electronic delocalisation, and thus largely determines the conductivity. Here, by means of accurate first principles calculations, we study the prototypical biphenyl based molecular system in which the dihedral angle between the two rings determines the degree of conjugation. In order to make this an E-field gateable system we create a net dipole by asymmetrically functionalising one ring with: (i) electron withdrawing (F, Br and CN), (ii) electron donating (NH2), and (iii) mixed (NH2/NO2) substituents. In this way, the application of an E-field interacts with the dipolar system to influence the dihedral angle, thus controlling the conjugation. For all considered substituents we consider a range of E-fields, and in each case extract conformational energy profiles. Using this data we obtain the minimum E-field required to induce a barrierless switching event for each system. We further extract the estimated switching speeds, the conformational probabilities at finite temperatures, and the effect of applied E-field on electronic structure. Consideration of these data allow us to assess which factors are most important in the design of efficient gateable electrical molecular switches

Understanding the nature and location of hydroxyl groups on hydrated titania nanoparticles

TiO2 nanoparticles (NPs) are intensively studied and widely used due to their huge potential in numerous applications involving their interaction with ultraviolet light (e.g. photocatalysis, sunscreens). Typically, these NPs are in water-containing environments and thus tend to be hydrated. As such, there is a growing need to better understand the physicochemical properties of hydrated TiO2 NPs in order to improve their performance in photochemical applications (e.g. photocatalytic water splitting) and to minimise their environmental impact (e.g. potential biotoxicity). To help address the need for reliable and detailed data on how nano-titania interacts with water, we present a systematic experimental and theoretical study of surface hydroxyl (OH) groups on photoactive anatase TiO2 NPs. Employing well-defined experimentally synthesised NPs and detailed realistic NP models, we obtain the measured and computed infrared spectra of the surface hydroxyls, respectively. By comparing the experimental and theoretical spectra we are able to identify the type and location of different OH groups in these NP systems. Specifically, our study allows us to provide unprecedented and detailed information about the coverage-dependent distribution of hydroxyl groups on the surface of experimental titania NPs, the degree of their H-bonding interactions and their associated assigned vibrational modes. Our work promises to lead to new routes for developing new and safe nanotechnologies based on hydrated TiO2 NPs