Search

Binding of organic molecules on oxide mineral surfaces is a key process which impacts the fertility and stability of soils. Aluminium oxide and hydroxide minerals are known to strongly bind organic matter. To understand the nature and strength of sorption of organic carbon in soil, we investigated the binding of small organic molecules and larger polysaccharide biomolecules on α-Al 2 O 3 (corundum). We modelled the hydroxylated α-Al 2 O 3 (0001) surface, since these minerals' surfaces are hydroxylated in the natural soil environment. Adsorption was modelled using density functional theory (DFT) with empirical dispersion correction. Small organic molecules (alcohol, amine, amide, ester and carboxylic acid) were found to adsorb on the hydroxylated surface by forming multiple hydrogen bonds with the surface, with carboxylic acid as the most favourable adsorbate. A possible route from hydrogen-bonded to covalently bonded adsorbates was demonstrated, through co-adsorption of the acid adsorbate and a hydroxyl group to a surface aluminium atom. Then we modelled the adsorption of biopolymers, fragments of polysaccharides which naturally occur in soil: cellulose, chitin, chitosan and pectin. These biopolymers were able to adopt a large variety of hydrogen-bonded adsorption configurations. Cellulose, pectin and chitosan could adsorb particularly strongly, and therefore are likely to be stable in soil. This article is part of a discussion meeting issue ‘Supercomputing simulations of advanced materials’.

We evaluated the stability of organic molecules in soil by calculating these molecules' strength of adsorption on the α-Al2O3 mineral.

Understanding the interaction of the carboxylic group with TiO2 is crucial for photocatalytic degradation of pollutants, because aromatic molecules containing carboxylic acid groups are among the most common micropollutants. This study investigated the interactions of the anatase TiO2 (101) surface with several aromatic carboxylic acids: benzoic, nicotinic, salicylic and anthranilic acid, using dispersion-corrected density functional theory (DFT) calculations. For all the molecules studied, we found higher stabilities of monodentate adsorbed configurations over bidentate. Dispersion was found to have a significant effect on adsorption energies. In particular, dispersion gave rise to a tilted monodentate adsorption configuration, where adsorption through interfacial covalent and hydrogen bonds was additionally stabilised by dispersion interactions of the aromatic ring with the surface. Comparative calculations using DFT with empirical dispersion correction and van der Waals-corrected functionals found the relative stabilities of adsorbed structures to be independent of the method of describing dispersion. Thermodynamic probabilities of different adsorption configurations were evaluated using the Boltzmann distribution, and the tilted dispersion-stabilised structures were predicted to be by far the most abundant. Finally, the optical absorption of TiO2-acid systems was modelled, and TiO2-aromatic acid complexes were found to have their optical absorption extended into the visible range.

A series of structures based on graphitic carbon nitride (g-C3N4), a layered material composed of linked carbon–nitrogen heterocycles arranged in a plane, were investigated by density functional theory calculations. g-C3N4 is a semiconductor that absorbs UV light and visible light at the blue end of the visible spectrum, and is widely studied as a photocatalyst for water splitting; however, its photocatalytic efficiency is limited by its poor light-harvesting ability and low charge mobilities. Modifications to the g-C3N4 structure could greatly improve its optical and electronic properties and its photocatalytic efficiency. In this work, the g-C3N4 structure was modified by replacing the nitrogen linker with heteroatoms (phosphorus, boron) or aromatic groups (benzene, s-triazine and substituted benzenes). Two-dimensional (2D) sheets and three-dimensional (3D) multilayer structures with different stacking types were modelled. Several new structures were predicted to have electronic properties superior to g-C3N4 for use as water splitting photocatalysts. In particular, introduction of phosphorus, benzene and s-triazine groups led to band gaps smaller than in the standard g-C3N4 (down to 2.4 eV, corresponding to green light). Doping with boron in the linker positions dramatically reduced the band gap (to 1.6 eV, corresponding to red light); the doped material has the valence band position suitable for water oxidation. Our computational study shows that chemical modification of g-C3N4 is a powerful method to tune this material’s electronic properties and improve its photocatalytic activity.

Kinetic effects in monolayer self-assembly at liquid-solid interfaces are not well explored but can provide unique insights. We use variable-temperature scanning tunneling microscopy (STM) to quantify the desorption kinetics of 2,6-naphthalenedicarboxylic acid (NDA) monolayers at nonanoic acid-graphite interfaces. Quantitative tracking of the decline of molecular coverages by STM between 57.5 and 65.0 °C unveiled single-exponential decays over the course of days. An Arrhenius plot of rate constants derived from fits results in a surprisingly high energy barrier of 208 kJ mol-1 that strongly contrasts with the desorption energy of 16.4 kJ mol-1 with respect to solution as determined from a Born-Haber cycle. This vast discrepancy indicates a high-energy transition state. Expanding these studies to further systems is the key to pinpointing the molecular origin of the remarkably large NDA desorption barrier.

Encoding information in the chemical structure of tectons is the pivotal strategy in self-assembly for the realization of targeted supramolecular structures. However, frequently observed polymorphi...

Composites of titanium dioxide (TiO2) and reduced graphene oxide (RGO) have proven to be much more effective photocatalysts than TiO2 alone. However, little attention has been paid so far to the chemical structure of TiO2/RGO interfaces and to the role that the unavoidable residual oxygen functional groups of RGO play in the photocatalytic mechanism. In this work, we develop models of TiO2 rutile (110)/RGO interfaces by including a variety of oxygen functional groups known to be present in RGO. Using hybrid density functional theory calculations, we demonstrate that the presence of oxygen functional groups and the formation of interfacial cross-links (Ti–O–C covalent bonds and strong hydrogen bonds between TiO2 and RGO) have a major effect on the electronic properties of RGO and RGO-based composites. The electronic structure changes from semimetallic to semiconducting with an indirect band gap, with the lowest unoccupied band positioned below the TiO2 conduction band and largely localized on RGO oxygen and carbon orbitals, with some contributions of RGO-bonded Ti atoms. We suggest that this RGO-based lowest unoccupied band acts as a photoelectron trap and the indirect nature of the band gap hinders electron–hole recombination. These results can explain the experimentally observed extended lifetimes of photoexcited charge carriers in TiO2/RGO composites and the enhancement of photocatalytic efficiency of these composites.

The first page of this article is displayed as the abstract.

The interplay between self‐assembly and surface chemistry of 2,3,6,7,10,11‐hexaminotriphenylene (HATP) on Cu(111) was complementarily studied by high‐resolution Scanning‐Tunneling‐Microscopy (STM) and X‐ray Photoelectron Spectroscopy (XPS) under ultra‐high vacuum conditions. To shed light on competitive metal‐coordination, comparative experiments were carried out on pristine and nickel‐covered Cu(111). Directly after room temperature deposition of HATP onto pristine Cu(111) self‐assembled aggregates were observed by STM, while XPS indicated non‐deprotonated amino groups. Annealing up to 200 °C activated the progressive single deprotonation of all amino groups as indicated by chemical shifts of both N 1s and C 1s core levels in the XP spectra. This enabled the formation of topologically versatile π‐d conjugated coordination networks with intrinsic copper adatoms. The basic motif of these networks was a metal‐organic trimer, where three HATP molecules were coordinated by Cu3 clusters, as corroborated by accompanying Density Functional Theory (DFT) simulations. Additional deposition of more reactive nickel atoms resulted in both chemical and structural changes with deprotonation and formation of bis(diimino)‐Ni bonded networks already at room temperature. Even though fused hexagonal pores were observed, extended honeycomb networks remained elusive, as tentatively explained by a restricted reversibility of these metal‐organic bonds.

Mixed-halide perovskites prepared from methylammonium iodide and lead chloride (MAPbI 3-x Cl x ) precursors are becoming increasingly well understood, however the effect of non-stoichiometry in this system is still not clear. Here, we create MAPbI 3-x Cl x perovskites from starting mixtures containing an excess of MAI, and study them using a variety of structural and optical probes. Using grazing incidence X-ray scattering (GIWAXS) we demonstrate the existence of non-perovskite structures, and show that addition of hydroiodic acid (HI) also leads to similar low-dimensional phase formation. Photoluminescence spectroscopy performed at cryogenic temperatures indicates the existence of multiple emissive states between 510 nm and 605 nm resulting from a low dimensional phase (LDP) or multiple phases. By mapping the distribution of luminescence across the surface with submicron resolution, we found strong co-localisation of LDP emissive states. At certain blend ratios, emission is seen from both LDP states and methylammonium lead iodide perovskite (around 770 nm). Photoluminescence excitation spectroscopy of mixed-phase films reveals energy transfer, or a cascade, between different LDP states, but this process only occurs inefficiently to the surrounding perovskite. Time-resolved photoluminescence measurements demonstrate that LDP excited-state lifetimes decrease as a function of increasing temperature; a process consistent with a thermally-activated charge transfer process. Our work suggests that non-stoichiometric materials prepared via this processing route can lead to the formation of metastable LDPs with unique material properties that merit further investigation.

Even though the surface-assisted dehalogenative coupling constitutes the most abundant protocol in on-surface synthesis, its full potential will only become visible if selectivity issues with polybrominated precursors are comprehensively understood, opening new venues for both organometallic self-assembly and on-surface polymerization. Using the 3,3',5,5'-tetrabromo-2,2',4,4',6,6'-hexafluorobiphenyl (Br4F6BP) at Ag(111), we demonstrate a remote site-selective functionalization at room temperature and a marked temperature difference in double- vs. quadruple activation, both phenomena caused by conformational mechanical effects of the precursor-surface ensemble. The submolecularly resolved structural characterization was achieved by Scanning Tunneling Microscopy, the chemical state was quantitatively assessed by X-ray Photoelectron Spectroscopy, and the analysis of the experimental signatures was supported through first-principles Density-Functional Theory calculations. The non-planarity of the various structures at the surface was specifically probed by additional Near Edge X-ray Absorption Fine Structure experiments. Upon progressive heating, Br4F6BP on Ag(111) shows the following unprecedented phenomena: (1) formation of regular organometallic 1D chains via remote site-selective 3,5'-didebromination; (2) a marked temperature difference in double- vs. quadruple activation; (3) an organometallic self-assembly based on reversibility of C-Ag-C linkages with a thus far unknown polymorphism affording both hexagonal and rectangular 2D networks; (4) extraordinary thermal stability of the organometallic networks. Controlled covalent coupling at the previously Br-functionalized sites was not achieved for the Br4F6BP precursor, in contrast to the comparatively studied non-fluorinated analogue.

Small fluorescent π-conjugated indolyl-based molecules 4–6, 23 are prepared through direct Fischer synthesis or/and Stille cross-coupling method in appreciable yields. Our synthetic results have shown the benefits using Stille approach when Fischer double cyclisation method to access the bisindole dyads 4–6, 23 is not efficient. The synthetic routes to these materials have been designed to investigate the substrate requirements for the respective cyclisation and CC-coupling reactions and to evaluate their wider synthetic applicability. Comparative analysis of the different substituents and the different π-bridging units e. g. pyridine, thiophene and thiazole on the electronic and photophysical properties of the final compounds 4–6, 23 has been carried out. The structure−property relationship of the final bisindole dyads has been investigated via photophysical characterisation, and computational modelling. The obtained compounds absorb near-UV and visible (blue) light, with the spectral range dependent on the nature of the π-bridging units, and are capable of bright blue emission.

Composite systems of TiO2 with nanocarbon materials, such as graphene, graphene oxide, and carbon nanotubes, have proven to be efficient photocatalyst materials. However, a detailed understanding of their electronic structure and the mechanisms of the charge transfer processes is still lacking. Here, we use hybrid density functional theory calculations to analyze the electronic properties of the ideal rutile (110)–graphene interface, in order to understand experimentally observed trends in photoinduced charge transfer. We show that the potential energy surface of pristine graphene physisorbed above rutile (110) is relatively flat, enabling many possible positions of graphene above the rutile (110) surface. We verify that tensile and compressive strain has a negligible effect on the electronic properties of graphene at low levels of strain. By analyzing the band structure of this composite material and the composition of the valence and conduction band edges, we show that both the highest occupied states a...

Self-assembly of benzene-1,3,5-tricarboxylic acid (trimesic acid – TMA) monolayers at the alkanoic acid–graphite interface is revisited. Even though this archetypal model system for hydrogen bonded porous networks is particularly well studied, the analysis of routinely observed superperiodic contrast modulations known as moiré patterns lags significantly behind. Fundamental questions remain unanswered such as, are moiré periodicity and orientation always the same, i.e. is exclusively only one specific moiré pattern observed? What are the geometric relationships (superstructure matrices) between moiré, TMA, and graphite lattices? What affects the moiré pattern formation? Is there any influence from solvent, concentration, or thermal treatment? These basic questions are addressed via scanning tunneling microscopy experiments at the liquid–solid interface, revealing a variety of different moiré patterns. Interestingly, TMA and graphite lattices were always found to be ∼5° rotated with respect to each other. Consequently, the observed variation in the moiré patterns is attributed to minute deviations (

The effect of the concurrent action of intermolecular and molecule-substrate interactions on the two-dimensional (2-D) self-assembly of organic molecules on solid surfaces is investigated in a combined experimental and theoretical effort. Scanning tunnelling microscopy measurements of terephthalic acid on the Cu(111) surface, a model system where the interplay between the two interactions is particularly evident, are used to develop a general, simple and computationally inexpensive model that quantitatively accounts for the experimental observations. The model, related to the well-known Frenkel-Kontorova model, offers a comprehensive description of the ‘subtle interplay’ between intermolecular and molecule-substrate interactions and provides a qualitative and quantitative predictive capability in the design and fabrication of 2-D molecular nanostructures at surfaces.The effect of the concurrent action of intermolecular and molecule-substrate interactions on the two-dimensional (2-D) self-assembly of organic molecules on solid surfaces is investigated in a combined experimental and theoretical effort. Scanning tunnelling microscopy measurements of terephthalic acid on the Cu(111) surface, a model system where the interplay between the two interactions is particularly evident, are used to develop a general, simple and computationally inexpensive model that quantitatively accounts for the experimental observations. The model, related to the well-known Frenkel-Kontorova model, offers a comprehensive description of the ‘subtle interplay’ between intermolecular and molecule-substrate interactions and provides a qualitative and quantitative predictive capability in the design and fabrication of 2-D molecular nanostructures at surfaces.

Self-assembly of three related molecules – terephthalic acid and its hydroxylated analogues – at liquid/solid interfaces (graphite/heptanoic acid and graphite/1-phenyloctane) has been studied using a combination of scanning tunnelling microscopy and molecular mechanics and molecular dynamics calculations. Brickwork-like patterns typical for terephthalic acid self-assembly have been observed for all three molecules. However, several differences became apparent: (i) formation or lack of adsorbed monolayers (self-assembled monolayers formed in all systems, with one notable exception of terephthalic acid at the graphite/1-phenyloctane interface where no adsorption was observed), (ii) the size of adsorbate islands (large islands at the interface with heptanoic acid and smaller ones at the interface with 1-phenyloctane), and (iii) polymorphism of the hydroxylated terephthalic acids’ monolayers, dependent on the molecular structure and/or solvent. To rationalise this behaviour, molecular mechanics and molecular dynamics calculations have been performed, to analyse the three key aspects of the energetics of self-assembly: intermolecular, substrate–adsorbate and solvent–solute interactions. These energetic characteristics of self-assembly were brought together in a Born–Haber cycle, to obtain the overall energy effects of formation of self-assembled monolayers at these liquid/solid interfaces.

Monolayer self-assembly of a hexabrominated, three-fold symmetric aromatic molecule is studied at the heptanoic acid-graphite interface. Thermodynamical insights are obtained from an adapted Born-Haber cycle that is utilized to derive the overall enthalpy change including solvent effects. Comparison with theoretical entropy estimates suggests a minor influence of solvation.

Self-assembly of 1,3,5-tris(4-mercaptophenyl)benzene (TMB), a 3-fold symmetric, thiol-functionalized aromatic molecule, was studied on Au(111) with the aim of realizing extended Au-thiolate-linked molecular architectures. The focus lay on resolving thermally activated structural and chemical changes by a combination of microscopy and spectroscopy. Thus, scanning tunneling microscopy (STM) provided submolecularly resolved structural information, while the chemical state of sulfur was assessed by X-ray photoelectron spectroscopy (XPS). Directly after room-temperature deposition, only less well ordered structures were observed. Mild annealing promoted the first structural transition into ordered molecular chains, partly organized in homochiral molecular braids. Further annealing led to self-similar Sierpiński triangles, while annealing at even higher temperatures again resulted in mostly disordered structures. Both the irregular aggregates observed at room temperature and the chains were identified as metal-organic assemblies, whereby two out of the three intermolecular binding motifs are energetically equivalent according to density functional theory (DFT) simulations. The emergence of Sierpiński triangles is driven by a chemical transformation, i.e., the conversion of coordinative Au-thiolate to covalent thioether linkages, and can be further understood by Monte Carlo simulations. The great structural variance of TMB on Au(111) can on one hand be explained by the energetic equivalence of two binding motifs. On the other hand, the unexpected chemical transition even enhances the structural variance and results in thiol-derived covalent molecular architectures.

The injection time for transfer of an electron from photoexcited dodecathiophene or polythiophene to a silicon nanocrystal (2.2 nm diameter) is calculated by computing the retarded Green’s function for the system from the Hamiltonian and Kohn–Sham states produced by density functional calculations. We found that it can be of the order of 10–100 fs if the thiophene chain lies approximately parallel to the silicon surface. However, the electron injection time is 1–2 orders of magnitude longer if the oligothiophene chain lies perpendicular to the silicon surface. A chemisorption interaction between the thiophene chain and the nanocrystal provides a relatively small improvement (decrease) of injection times, much weaker than that achieved by enforcing the parallel arrangement of the chain with respect to the nanocrystal.

The process of electron recombination from semiconductor (TiO2) particles to oxidized dyes in dye-sensitized solar cells is investigated theoretically. The recombination rate is evaluated using nonadiabatic electron transfer theory with system parameters computed using ab initio density functional theory (DFT) calculations and derived from experimental sources. Our model for the recombination rate includes three contributions: the semiconductor-dye coupling term (calculated by partitioning the semiconductor-dye system into the semiconductor + anchoring group and the isolated dye), the Fermi-Dirac distribution of electrons in the semiconductor’s conduction band, and the Franck–Condon term (with the reorganization energy and the driving force evaluated for the isolated dye within an implicit solvent model, and the energy of the TiO2 conduction band edge taken from experimental reports). Recombination lifetimes for several organic dyes are evaluated for a realistic range of conduction band energies. The results are in good agreement with experiment for the NKX family of dyes with a systematic variation in the dyes’ structure; however, in a second considered family of dyes, complex adsorption and conformation flexibility of the molecules make quantitative prediction of recombination times more difficult. For all considered dyes, the range of the computed recombination lifetimes is in agreement with experimental data, and the relative ordering can be reproduced for dyes with predictable adsorption chemistry.

The attachment chemistry of the chromophore onto the semiconductor surface influences the efficiency of electron injection in dye-sensitized solar cells (DSSCs). In this work, we study injection times for dyes that bind to the semiconductor surface via the phosphonic acid anchoring group and the effect on the injection time of different binding modes (molecular or dissociative, monodentate or bidentate) of phosphonic acid for both TiO2 rutile (110) and anatase (101) surfaces. We calculate electron injection times for a large set of organic dyes on TiO2 rutile (110) and anatase (101) surfaces for the most stable adsorption geometries of the phosphonic acid anchoring group, using a model based on partitioning the semiconductor–chromophore system into fragments. We analyze the influence of the size and nature of the anchoring group on the injection times, performing calculations with larger models of the anchoring group (e.g., phenyl-phosphonic acid). Through the partitioning procedure, we are able to separa...

The objective of this work is to study both the dynamics and mechanisms of guest incorporation into the pores of 2D supramolecular host networks at the liquid-solid interface. This was accomplished by adding molecular guests to prefabricated self-assembled porous monolayers and the simultaneous acquisition of scanning tunneling microscopy (STM) topographs. The incorporation of the same guest molecule (coronene) into two different host networks was compared, where the pores of the networks either featured a perfect geometric match with the guest (for trimesic acid host networks) or were substantially larger than the guest species (for benzenetribenzoic acid host networks). Even the moderate temporal resolution of standard STM experiments in combination with a novel injection system was sufficient to reveal clear differences in the incorporation dynamics in the two different host networks. Further experiments were aimed at identifying a possible solvent influence. The interpretation of the results is aided by molecular mechanics (MM) and molecular dynamics (MD) simulations.

We present a theoretical modeling study of self-assembly of molecules into two-dimensional (2D) hydrogen-bonded networks. We compare two computational techniques, molecular dynamics (MD) and Monte Carlo (MC) calculations, in order to find out whether these computational techniques are efficient in modeling the process of self-assembly and in predicting the ordered supramolecular structures. Terephthalic acid, isophthalic acid, and phthalic acid, which have been widely studied experimentally, are used as test systems. According to both computational techniques, terephthalic acid molecules form ordered structures made of parallel molecular chains (same as observed in experiment) at high surface coverage, whereas less symmetric isophthalic and phthalic acid molecules mostly form disordered arrangements of zigzag chains or long isolated zigzag chains. Both computational techniques reproduce the supramolecular structure of terephthalic acid, the most symmetric of these molecules. However, MD simulations are mo...

We report a computational density-functional theory (DFT) study of planar hydrogen-bonded supramolecular structures (dimers, chains, and networks) of cyanuric acid (CA) and trithiocyanuric acid (TTCA). We confirm that the electronegative atom (oxygen or sulfur) influences both the geometry and the strength of hydrogen bonds and therefore affects the order of stabilities of the two-dimensional networks of CA and TTCA. The N−H···O bonds are stronger and have the energy of ∼ −0.27 eV in a CA dimer, compared to ∼ −0.18 eV for N−H···S bonds. Our analysis, based entirely on available hydrogen bonding connections, predicts that six 2D network structures are possible both for CA and TTCA. Our DFT calculations show that all these network structures are stable for CA, however, only four out of the six networks are stable for TTCA. We compare our results with the available experimental data on the structure of self-assembled networks on some flat crystal surfaces and with X-ray data on the bulk crystal structure of ...

We report on a joint experimental and theoretical study of the ordered structures of melamine molecules formed on the Au(111)-(22 × √3) surface. Scanning tunneling microscopy (STM) images taken under UHV conditions reveal two distinct monolayers one of which has never been reported before on gold. We also find that one of the structures may serve as a transition region ("domain wall") between islands formed by the other arrangement. Using state-of-the-art density functional calculations in conjunction with a systematic gas-phase analysis based on considering all planar structures melamine molecules can form with each other, we propose atomistic models for both structures and the transition region. © 2008 American Chemical Society.

The modelling of an excess electron in a semiconductor in a prototypical dye sensitised solar cell is carried out using two complementary approaches: atomistic simulation of the TiO2 nanoparticle surface is complemented by a dielectric continuum model of the solvent–semiconductor interface. The two methods are employed to characterise the bound (excitonic) states formed by the interaction of the electron in the semiconductor with a positive charge opposite the interface. Density-functional theory (DFT) calculations show that the excess electron in TiO2 in the presence of a counterion is not fully localised but extends laterally over a large region, larger than system sizes accessible to DFT calculations. The numerical description of the excess electron at the semiconductor–electrolyte interface based on the continuum model shows that the exciton is also delocalised over a large area: the exciton radius can have values from tens to hundreds of Angstroms, depending on the nature of the semiconductor (characterised by the dielectric constant and the electron effective mass in our model).

We have used a boroxine-based COF as a template for C60-fullerene self-assembly on graphite. Local removal of the COF by STM based nanomanipulation creates nanocorrals that may host other species crosscheck: This document is CrossCheck deposited related_data: Supplementary Information copyright_licence: The Royal Society of Chemistry has an exclusive publication licence for this journal copyright_licence: The accepted version of this article will be made freely available in the Chemical Sciences Article Repository after a 12 month embargo period history: Received 9 September 2015; Accepted 22 October 2015; Accepted Manuscript published 22 October 2015; Advance Article published 30 October 2015; Version of Record published 15 December 2015 ispartof: Chemical Communications vol:52 issue:1 pages:68-71 ispartof: location:England status: published

Fluorination of C 60 at 550°C leads to milligram quantities of two stable fullerene derivatives with 58-carbon cage structures: C 58 F 18 and C 58 F 17 CF 3 . The compounds were characterized by mass spectrometry and fluorine nuclear magnetic resonance spectroscopy, and the data support a heptagonal ring in the framework. The resulting strain, which has hindered past attempts to prepare these smaller quasi-fullerenes, is mitigated here by hybridization change of some of the carbons in the pentagons from sp 2 to sp 3 because of fluorine addition. The loss of carbon from C 60 is believed to occur via sequential fluorine addition to a C–C single bond and an adjacent C[dbond]C bond, followed by loss of a :CF 2 carbene.

We use DFT calculations to investigate the problem of hydrogen aggregation in silicon. We study atomic structures of finite hydrogen aggregates containing four or more hydrogen atoms. Beyond four hydrogen atoms, complexes consisting of Si-H bonds are likely to form, rather than aggregates of H2 molecules, which are the most stable diatomic hydrogen complex. Our calculations show that the basic structural unit of such complexes is a hydrogenated dislocation loop, which is formed spontaneously by a structural transformation of two H∗2 complexes. Hydrogen-induced formation of dislocation loops may account for the experimental observations of dislocation loops in proton-implanted or hydrogen plasma-treated silicon. We indicate the routes leading from H∗2 aggregates and hydrogenated dislocation loops to twodimensional hydrogen-induced platelets. We discuss the effect of hydrogen-catalysed formation of dislocation loops on the plasticity of silicon.

Pyrolytic trifluoromethylation of [70]fullerene with CF3CO2Ag at 300 degreesC results in the addition of up to 12 CF3 groups to the fullerene cage. Forty-six C-70(CF3)(n) derivatives (numbers in parentheses) were separated by two-stage high pressure liquid chromatography (HPLC) as follows: n = 2(2), 4(16), 6(9), 8(14) 10(5), some being characterised by F-19 NMR. The range of derivatives is much greater than for other [70]fullerene reactions, and as with [60] fullerene trifluoromethylation, no single derivative is dominant, indicating that kinetic stability mainly controls product formation. F-19 NMR spectra show most derivatives to be unsymmetrical, with combinations of quartets and septets (overlapping quartets) due to contiguous ('linear') addend arrays, having significantly different coupling constants of the 'terminal' quartets of between 9.1 and 17.7 Hz. These differences, together with those observed previously in trifluoromethylation of [60]fullerene are consistent with addition across both 6:6- and 5:6-ring junctions. Of the two C-70(CF3)(2) isomers, one has either C-s or C-2 symmetry, the other has C-1 symmetry, whilst the C-70(CF3)(4) derivatives fall into four categories: (i) symmetrical compounds (one gives only two singlets in the F-19 NMR); (ii) unsymmetrical compounds that show a 'linear' coupling sequence; (iii) unsymmetrical compounds having a remote pairs of adjacent groups; (iv) compounds having a coupled array of three CF3 groups, together with a remote group suggesting sterically-driven migration. The first evaluation of differential NMR couplings across 6:6- and 5:6-bonds in a fullerene has been made using C60F6 as a model. (C) 2004 Elsevier B.V. All rights reserved.

We report the results of first-principles calculations on the early stages of hydrogen aggregation in silicon and diamond. We demonstrate that the hydrogenated glide dislocation dipole is the preferred structure for small numbers of H atoms in silicon and that it expands by dislocation glide, with hydrogen condensing in the shuffle plane between the dislocations. This structure is a good candidate for the initial stage in the development of hydrogen-induced platelets. We investigate the effect of shear and dilation on the energies of hydrogenated structures and compare the relative stabilities of these structures in silicon and diamond. We describe the method of determination of the Burgers vectors of dilation and shear for the dislocation dipoles by varying the lattice vectors of their supercells.

Gezielt entworfene symmetrische Farbstoffe konnen dazu verwendet werden, die Ladungsrekombination in Farbstoffsolarzellen um zwei bis drei Zehnerpotenzen zu verlangsamen. Vermittelt eine konjugierte Brucke die elektronische Kopplung zwischen der Elektrode und dem Farbstoff, so konnen Systeme entworfen werden, in denen das HOMO nicht an den Halbleiter gekoppelt ist.

We review first-principles calculations of dislocation core structure in diamond, and draw out similarities with and differences from silicon. Primary differences are in hybridization changes in carbon and in the different behaviour of H interacting with dislocations. In both materials, condensation of a homogeneous distribution of H atoms should result, first, in formation of small H aggregates with the appearance of a glide dislocation dipole and, second, in formation of larger platelets based on the half-stacking-fault model.

The adsorption of p-aminobenzoic acid (pABA) on the anatase TiO2(101) surface has been investigated using synchrotron radiation photoelectron spectroscopy, near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and density functional theory (DFT). Photoelectron spectroscopy indicates that the molecule is adsorbed in a bidentate mode through the carboxyl group following deprotonation. NEXAFS spectroscopy and DFT calculations of the adsorption structures indicate the ordering of a monolayer of the amino acid on the surface with the plane of the ring in an almost upright orientation. The adsorption of pABA on nanoparticulate TiO2 leads to a red shift of the optical absorption relative to bare TiO2 nanoparticles. DFT and valence band photoelectron spectroscopy suggest that the shift is attributed to the presence of the highest occupied molecular orbitals in the TiO2 band gap region and the presence of new molecularly derived states near the foot of the TiO2 conduction band.

A direct calorimetric measurement of the overall enthalpy change associated with self-assembly of\ud organic monolayers at the liquid–solid interface is for most systems of interest practically impossible. In\ud previous work we proposed an adapted Born–Haber cycle for an indirect assessment of the overall\ud enthalpy change by using terephthalic acid monolayers at the nonanoic acid–graphite interface as a\ud model system. To this end, the sublimation enthalpy, dissolution enthalpy, the monolayer binding\ud enthalpy in vacuum, and a dewetting enthalpy are combined to yield the total enthalpy change. In the\ud present study the Born–Haber cycle is applied to 4,40\ud -stilbenedicarboxylic acid monolayers. A detailed\ud comparison of these two aromatic dicarboxylic acids is used to evaluate and quantify the contribution of\ud the organic backbone for stabilization of the monolayer at the nonanoic acid–graphite interface.

The problems with dislocations in semiconductors are becoming tractable with modern computing by hybrid techniques. These apply static first principles calculations of energetics for important processes (e.g. kink formation and migration energies) and kinetic Monte Carlo techniques to follow the dynamic interaction of these processes over length and time scales inaccessible to, for example, molecular dynamic simulation. The simplest model system for covalent and ceramic solids is silicon, but there is debate over the structure and properties of dislocations there. The movement of the dislocation by the simple bond switching mechanism was studied from first principles, finding activation energies close to experiment, but lately the alternative mechanism invoking free radicals or solitons was found to give similar energies. We report results from an n-fold way kinetic Monte Carlo approach, applied to a simple system to verify the standard model for kink pair nucleation limited dislocation glide (the Hirth–Lothe model). We then apply an improved technique to the kinetics of the soliton model and to hydrogen enhanced dislocation glide.

Pseudorotation (PR) of cyclic molecules is discussed. It is shown that PR can be considered as a special type of internal rotation with pseudorotational moment of inertia (PMI) as an important characteristic. The dependence of the PMI of a molecule upon the phase angle of PR and the nuclear trajectories is discussed. A method is proposed for the calculation of PMI from geometrical parameters of molecules. The moment of inertia of internal rotation for 1,2-dichloroethane in gauche conformation is calculated in order to test the proposed method. The obtained value is in good agreement with the values given by the Pitzer method. Pseudorotational moments of inertia of cyclopentane and its derivatives are calculated. Calculated values are compared with those adjusted to fit experimental entropy. The dependence of the PMI of cyclopentane derivatives upon their molecular mass is discussed.

Nanoscopic metal-molecule-metal junctions consisting of Fe-bis(terpyridine)-based ordered nanostructures are grown in layer-by-layer fashion on a solid support. Hopping is demonstrated as the main charge-transport mechanism both experimentally and theoretically.

The driving force for self-assembly is the associated gain in free energy with decisive contributions from both enthalpy and entropy differences between final and initial state. For monolayer self-assembly at the liquid-solid interface, solute molecules are initially dissolved in the liquid phase and then become incorporated into an adsorbed monolayer. In this work, we present an adapted Born-Haber cycle for obtaining precise enthalpy values for self-assembly at the liquid-solid interface, a key ingredient for a profound thermodynamic understanding of this process. By choosing terephthalic acid as a model system, it is demonstrated that all required enthalpy differences between well-defined reference states can be independently and consistently assessed by both experimental and theoretical methods, giving in the end a reliable value of the overall enthalpy gain for self-assembly of interfacial monolayers. A quantitative comparison of enthalpy gain and entropy cost reveals essential contributions from solvation and dewetting, which lower the entropic cost and render monolayer self-assembly a thermodynamically favored process.

The charge recombination reaction from the semiconductor (TiO(2)) conduction band to electron accepting electrolytes (I(2), I(2)(-), I(3)(-)) in dye-sensitised solar cells is investigated theoretically. The non-adiabatic theory of electron transfer has been adapted to compute the charge transfer rate measured in different experimental settings (namely with and without external illumination). In both cases we are able to provide an atomic level description of the charge recombination to the electrolyte (CRE), which is in good agreement with the experimental data available. The model employs a detailed density-functional theory (DFT) description of the semiconductor-electrolyte interface and the internal reorganization energy. A continuum dielectric model is used to evaluate the external component of the reorganization energy due to the solvent degrees of freedom. The intrinsic limitations of DFT are kept to a minimum by taking two key energetic parameters (the conduction band edge and the reaction energy) from the experiments. The proposed methodology correctly reproduces (i) the ratio between CRE rate to iodine and triiodide in dark, (ii) the absolute CRE rate to triiodide in dark, and (iii) the absolute CRE rate to I(2)(-) under illumination.

Metal-organic ruthenium-based dyes are often used as a source of photogenerated electrons in dye-sensitised solar cells and photocatalysis. Here, we study the relationship between adsorption geometry and electron injection properties of one of the most successful metal-organic dyes, N3 (cis-bis(isothiocyanato)-bis(4,4'-dicarboxy-2,2'-bipyridyl)-ruthenium(II)), on the TiO(2) rutile (110) surface. We systematically construct all possible adsorption configurations of the N3 molecule on this surface. By combining density-functional theory calculations and electron transfer calculations, we find that a large number of adsorption configurations are possible--more than ten structures, which differ in the number of carboxylic and thiocyanate groups adsorbed and in the adsorption mode of the carboxylic groups, have similar adsorption energies and similar electron injection times. Therefore, the observed fast electron injection from this dye may originate either from one adsorption configuration or from several co-existing configurations. Our results suggest that related substituted metal-organic dyes with fewer anchoring groups will also have good electron injection properties, even if only a small subset of adsorption configurations is available for them.