Emergent Spin Frustration in Neutral Mixed-Valence 2D Conjugated Polymers: A Potential Quantum Materials Platform.

Two-dimensional conjugated polymers (2DCPs)─organic 2D materials composed of arrays of carbon sp2 centers connected by π-conjugated linkers─are attracting increasing attention due to their potential applications in device technologies. This interest stems from the ability of 2DCPs to host a range of correlated electronic and magnetic states (e.g., Mott insulators). Substitution of all carbon sp2 centers in 2DCPs by nitrogen or boron results in diamagnetic insulating states. Partial substitution of C sp2 centers by B or N atoms has not yet been considered for extended 2DCPs but has been extensively studied in the analogous neutral mixed-valence molecular systems. Here, we employ accurate first-principles calculations to predict the electronic and magnetic properties of a new class of hexagonally connected neutral mixed-valence 2DCPs in which every other C sp2 nodal center is substituted by either a N or B atom. We show that these neutral mixed-valence 2DCPs significantly energetically favor a state with emergent superexchange-mediated antiferromagnetic (AFM) interactions between C-based spin-1/2 centers on a triangular sublattice. These AFM interactions are surprisingly strong and comparable to those in the parent compounds of cuprate superconductors. The rigid and covalently linked symmetric triangular AFM lattice in these materials thus provides a highly promising and robust basis for 2D spin frustration. As such, extended mixed-valence 2DCPs are a highly attractive platform for the future bottom-up realization of a new class of all-organic quantum materials, which could host exotic correlated electronic states (e.g., unusual magnetic ordering, quantum spin liquids).

Predicting observable infrared signatures of nanosilicates in the diffuse interstellar medium.

The destruction time scale of dust in the diffuse interstellar medium is estimated to be an order of magnitude shorter than its residence time. Nevertheless, dust is observed in the interstellar medium, leading to the conclusion that reformation and grain growth must take place. Direct observations of nanometre-sized silicate grains, the main constituent of interstellar dust, would provide a smoking gun for the occurrence of grain condensation in the diffuse interstellar medium. Here we employ quantum chemical calculations to obtain the mid-infrared (IR) optical properties of a library of Mg-end member silicate nanoparticles with olivine (Mg2SiO4) and pyroxene (MgSiO3) stoichiometries. We use this library as an input for a foreground-screen model to predict the spectral appearance of the absorption profile due to mixtures of bulk and nanoparticle silicates towards bright background sources. The mid-IR spectrum observed towards an O8V star or a carbon-rich Wolf-Rayet star starts to change when ∼3% of the silicate mass is in the form of nanosilicates. We predict that a 3-10% nanosilicate fraction can be detected with the James Webb Space Telescope (JWST) using the mid-IR instrument (MIRI). With our upcoming JWST observations using MIRI, we will be able to detect or place limits on the nanosilicate content in the diffuse interstellar medium, and thus potentially directly confirm interstellar dust formation.

Buckletronics: how compression-induced buckling affects the mechanical and electronic properties of sp2-based two-dimensional materials.

We explore the mechanical and electronic response of sp2-based two-dimensional materials under in-plane compression employing first principles density functional theory-based calculations. Taking two carbon-based graphynes (α-graphyne and γ-graphyne) as example systems, we show that the structures of both two-dimensional materials are susceptible to out-of-plane buckling, which emerges for modest in-plane biaxial compression (1.5-2%). Out-of-plane buckling is found to be more energetically stable than in-plane scaling/distortion and significantly lowers the in-plane stiffness of both graphenes. The buckling also gives rise to in-plane auxetic behaviour in both two-dimensional materials. Under compression, the induced in-plane distortions and out-of-plane buckling also lead to modulations of the electronic band gap. Our work highlights the possibility of using in-plane compression to induce out-of-plane buckling in, otherwise planar, sp2-based two-dimensional materials (e.g. graphynes, graphdiynes). We suggest that controllable compression-induced buckling in planar two-dimensional materials (as opposed to two-dimensional materials, which are buckled due to sp3 hybridization) could provide a route to a new 'buckletronics' approach for tuning the mechanical and electronic properties of sp2-based systems. This article is part of a discussion meeting issue 'Supercomputing simulations of advanced materials'.

A machine learning potential for simulating infrared spectra of nanosilicate clusters.

The use of machine learning (ML) in chemical physics has enabled the construction of interatomic potentials having the accuracy of ab initio methods and a computational cost comparable to that of classical force fields. Training an ML model requires an efficient method for the generation of training data. Here, we apply an accurate and efficient protocol to collect training data for constructing a neural network-based ML interatomic potential for nanosilicate clusters. Initial training data are taken from normal modes and farthest point sampling. Later on, the set of training data is extended via an active learning strategy in which new data are identified by the disagreement between an ensemble of ML models. The whole process is further accelerated by parallel sampling over structures. We use the ML model to run molecular dynamics simulations of nanosilicate clusters with various sizes, from which infrared spectra with anharmonicity included can be extracted. Such spectroscopic data are needed for understanding the properties of silicate dust grains in the interstellar medium and in circumstellar environments.

Can calculated harmonic vibrational spectra rationalize the structure of TiC-based nanoparticles?

Nanoscale titanium carbide (TiC) is widely used in composites and energy applications. In order to design and optimize these systems and to gain a fundamental understanding of these nanomaterials, it is important to understand the atomistic structure of nano-TiC. Cluster beam experiments have provided detailed infrared vibrational spectra of numerous TixCy nanoparticles with well defined masses. However, these spectra have yet to be convincingly assigned to TixCy nanoparticle structures. Herein, using accurate density functional theory based calculations, we perform a systematic survey of likely candidate nanoparticle structures with masses corresponding to those in experiment. We calculate harmonic infrared vibrational spectra for a range of nanoparticles up to 100 atoms in size, with a focus on systems based on removing either four carbon atoms or a single titanium atom from rocksalt-structured stoichiometric TiC nanoparticles. Our calculations clearly show that Ti-deficient nanoparticles are unlikely candidates to explain the experimental spectra as such structures are highly susceptible to C-C bonding, whose characteristic frequencies are not observed in experiment. However, our calculated infrared spectra for C-deficient nanoparticles have some matching features with the experimental spectra but tend to have more complex infrared spectra with more peaks than those obtained from experiment. We suggest that the discrepancy between experiment and theory may be largely due to thermally induced anharmonicities and broadening in the latter nanoparticles, which are not be accounted for in harmonic vibrational calculations.

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.

How graphenic are graphynes? Evidence for low-lying correlated gapped states in graphynes.

Graphynes can be structurally envisioned as 2D extensions to graphene, whereby linearly bonded carbon linkages increase the distance between trigonal carbon nodes. Many graphynes have been predicted to exhibit a Dirac-like semimetallic (SEM) graphenic electronic structure, which could potentially make them competitive with graphene for applications. Currently, most graphynes remain as attractive synthetic targets, and their properties are still unconfirmed. Here, we demonstrate that the electronic structure of hexagonal α-graphyne is analogous to that of biaxially strained graphene. By comparison with accurate quantum Monte Carlo results on strained graphene, we show that the relative energetic stability of electronic states in this correlated 2D system can be captured by density functional theory (DFT) calculations using carefully tailored hybrid functionals. Our tuned hybrid DFT approach confirms that α-graphyne has a low energy correlated Mott-like antiferromagnetic insulating (AFI) state, which competes with the SEM state. Our work shows that the AFI-SEM crossover in α-graphyne could be tunable by in-plane biaxial strain. Applying our approach to other graphynes shows that they should also exhibit correlated AFI states, which could be dominant even at zero strain. Calculations using an onsite Coulombic repulsive term (i.e., DFT + U) also confirm the predictions of our hybrid DFT calculations. Overall, our work strongly suggests that graphynes are not as graphenic (i.e., Dirac-like) as often previously predicted by DFT calculations using standard generalized gradient approximation functionals. However, due to the greater electronic versatility (e.g., tunable semiconducting bandgaps and accessible spin polarized states) implied by our study, graphynes could have novel device applications that are complementary to those of graphene.

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.

Room Temperature Methane Capture and Activation by Ni Clusters Supported on TiC(001): Effects of Metal-Carbide Interactions on the Cleavage of the C-H Bond.

Methane is an extremely stable molecule, a major component of natural gas, and also one of the most potent greenhouse gases contributing to global warming. Consequently, the capture and activation of methane is a challenging and intensively studied topic. A major research goal is to find systems that can activate methane, even at low temperatures. Here, combining ultrahigh vacuum catalytic experiments, X-ray photoemission spectra, and accurate density functional theory (DFT) based calculations, we show that small Ni clusters dispersed on the (001) surface of TiC are able to capture and dissociate methane at room temperature. Our DFT calculations reveal that two-dimensional Ni clusters are responsible for this chemical transformation, confirming that the lability of the supported clusters appears to be a critical aspect in the strong adsorption of methane. A small energy barrier of 0.18 eV is predicted for CH4 dissociation into adsorbed methyl and atomic hydrogen species. In addition, the calculated reaction free energy profile at 300 K and 1 atm of CH4 shows no effective energy barriers in the system. Comparison with other reported systems which activate methane at room temperature, including oxide and zeolite-based materials, indicates that a different chemistry takes place on our metal/carbide system. The discovery of a carbide-based surface able to activate methane at low temperatures paves the road for the design of new types of catalysts which can efficiently convert this hydrocarbon into other added-value chemicals, with implications in climate change mitigation.

Electronic, Structural and Functional Versatility in Tetrathiafulvalene-Lanthanide Metal-Organic Frameworks.

Tetrathiafulvalene-lanthanide (TTF-Ln) metal-organic frameworks (MOFs) are an interesting class of multifunctional materials in which porosity can be combined with electronic properties such as electrical conductivity, redox activity, luminescence and magnetism. Herein a new family of isostructural TTF-Ln MOFs is reported, denoted as MUV-5(Ln) (Ln=Gd, Tb, Dy, Ho, Er), exhibiting semiconducting properties as a consequence of the short intermolecular S⋅⋅⋅S contacts established along the chain direction between partially oxidised TTF moieties. In addition, this family shows photoluminescence properties and single-molecule magnetic behaviour, finding near-infrared (NIR) photoluminescence in the Yb/Er derivative and slow relaxation of the magnetisation in the Dy and Er derivatives. As such properties are dependent on the electronic structure of the lanthanide ion, the immense structural, electronic and functional versatility of this class of materials is emphasised.

Understanding H2 Formation on Hydroxylated Pyroxene Nanoclusters: Ab Initio Study of the Reaction Energetics and Kinetics.

The rate constants of H2 formation on five models of silicate nanoclusters with varying degrees of hydroxylation, (Mg4Si4O12)(H2O)N, were computed over a wide temperature range [180-2000 K]. We tested nine combinations of density functional methods and basis sets for their suitability for calculating reaction energies and barrier heights, and we computed the minimum energy H + H → H2 reaction paths on each nanocluster. Subsequently, we computed the rate constants employing three semiclassical approaches that take into account tunneling and nonclassical reflection effects by means of the zero curvature tunneling (ZCT), the small curvature tunneling (SCT), and the one-dimensional semiclassical transition state theory (SCTST) methods, which all provided comparable results. Our investigations show that the H2 formation process following the Langmuir-Hinshelwood (LH) mechanism is more efficient on the hydroxylated (N = 1-4) nanoclusters than on the bare (N = 0) one due to relatively higher reaction barrier height on the latter. H2 formation is found to have the smallest barrier and the most exothermic reaction for the moderately hydroxylated (Mg4Si4O12)(H2O)2 nanocluster for all nine considered methods. Overall, we conclude that all the considered nanoclusters are very efficient catalyzing grains for H2 formation in the physical conditions of the interstellar medium (ISM) with pyroxene nanosilicates having moderate to high hydroxylation being more efficient than bare nanograins.

Efficient preparation of TiO2 nanoparticle models using interatomic potentials.

Computational modeling has proven to be extremely useful for understanding how the morphology, size, and structure of TiO2 nanoparticles (NPs) affect their electronic properties and their usage in targeted applications (e.g., photocatalysis). Density functional theory (DFT) based calculations of NPs (on the order of hundreds to thousands of atoms) are, however, computationally highly demanding. Herein, we show that interatomic potentials (IPs) can provide a highly computationally efficient means to prepare NP structures which are sufficiently accurate to significantly reduce the computational cost of subsequent DFT calculations. We first compare the direct DFT optimization of faceted NPs directly cut from the anatase bulk crystal with the same calculation where the NP is preoptimized using four different IPs. We then establish the subsequent computational time saving for the respective complete DFT optimizations. We show that IP-based preoptimizing can greatly speed up DFT convergence, with speed-ups of 3×-10× for single point DFT energy evaluations. Moreover, as IP preoptimized NP structures can be closer to those of DFT energy minima, further speed-ups of 2× for DFT structure optimizations can be achieved. Finally, taking NPs derived from anatase spherical cuts, we show that IP-based molecular dynamics annealing gives rise to significant structural reconstruction with an associated high energetic stabilization, as confirmed by DFT calculations. Although similar results can be achieved using DFT tight binding methods, IP-based methods are 3-4 orders of magnitude faster and thus provide a particularly highly computationally efficient route to the preparation and design of large and diverse NP sets.

Operative Mechanism of Hole-Assisted Negative Charge Motion in Ground States of Radical-Anion Molecular Wires.

Charge transfer/transport in molecular wires over varying distances is a subject of great interest. The feasible transport mechanisms have been generally accounted for on the basis of tunneling or superexchange charge transfer operating over small distances which progressively gives way to hopping transport over larger distances. The underlying molecular sequential steps that likely take place during hopping and the operative mechanism occurring at intermediate distances have received much less attention given the difficulty in assessing detailed molecular-level information. We describe here the operating mechanisms for unimolecular electron transfer/transport in the ground state of radical-anion mixed-valence derivatives occurring between their terminal perchlorotriphenylmethyl/ide groups through thiophene-vinylene oligomers that act as conjugated wires of increasing length up to 53 Å. The unique finding here is that the net transport of the electron in the larger molecular wires is initiated by an electron-hole dissociation intermediated by hole delocalization (conformationally assisted and thermally dependent) forming transient mobile polaronic states in the bridge that terminate by an electron-hole recombination at the other wire extreme. On the contrary, for the shorter radical-anions our results suggest that a flickering resonance mechanism which is intermediate between hopping and superexchange is the operative one. We support these mechanistic interpretations by applying the pertinent biased kinetic models of the charge/spin exchange rates determined by electron paramagnetic resonance and by molecular structural level information obtained from UV-vis and Raman spectroscopies and by quantum chemical modeling.

Modeling hydroxylated nanosilica: Testing the performance of ReaxFF and FFSiOH force fields.

We analyze the performance of the FFSiOH force field and two parameterisations of the ReaxFF force field for modeling hydroxylated nanoscale silica (SiO2). Such nanosystems are fundamental in numerous aspects of geochemistry and astrochemistry and also play a key role during the hydrothermal synthesis of technologically important nanoporous silicas (e.g., catalysts, absorbents, and coatings). We consider four aspects: structure, relative energies, vibrational spectra, and hydroxylation energies, and compare the results with those from density functional calculations employing a newly defined dataset (HND: Hydroxylated Nanosilica Dataset). The HND consists of three sets of (SiO2)16(H2O)N nanoparticles (NPs), each with a different degree of hydroxylation and each containing between 23 and 26 distinct isomers and conformers. We also make all HND reference data openly available. We further consider hydroxylated silica NPs of composition (SiO2)M(H2O)N with M = 4, 8, 16, and 24 and infinite surface slabs of amorphous silica, both with variable hydroxylation. For energetics, both ReaxFF and FFSiOH perform well for NPs with an intermediate degree of hydroxylation. For increased hydroxylation, the performance of FFSiOH begins to significantly decline. Conversely, for the lower degree of hydroxylation both parameterisations of ReaxFF do not perform well. For vibrational frequencies, FFSiOH performs particularly well and significantly better than ReaxFF. This feature also opens the door to inexpensively calculating Gibbs free energies of the hydroxylated nanosilica systems in order to efficiently correct density functional theory calculated electronic energies. We also show how some small changes to FFSiOH could improve its performance for higher degrees of hydroxylation.

Under what conditions does (SiO)N nucleation occur? A bottom-up kinetic modelling evaluation.

Silicon monoxide (SiO) is a structurally complex compound exhibiting differentiated oxide-rich and silicon-rich nano-phases at length scales covering nanoclusters to the bulk. Although nano-sized and nano-segregated SiO has great technological potential (e.g. nano-silicon for optical applications) and is of enormous astronomical interest (e.g. formation of silicate cosmic dust) an accurate general description of SiO nucleation is lacking. Avoiding the deficiencies of a bulk-averaged approach typified by classical nucleation theory (CNT) we employ a bottom-up kinetic model which fully takes into account the atomistic details involved in segregation. Specifically, we derive a new low energy benchmark set of segregated (SiO)N cluster ground state candidates for N ≤ 20 and use the accurately calculated properties of these isomers to calculate SiO nucleation rates. We thus provide a state-of-the art evaluation of the range of pressure and temperature conditions for which formation of SiO will or will not proceed. Our results, which match with available experiment, reveal significant deficiencies with CNT approaches. We employ our model to shed light on controversial issue of circumstellar silicate dust formation showing that, at variance with the predictions from CNT-based calculations, pure SiO nucleation under such conditions is not viable.

Structural and electronic characterisation of π-extended tetrathiafulvalene derivatives as active components in field-effect transistors.

The electronic and structural properties of two tetrathiafulvalene derivatives bearing aromatic benzene rings are reported. Thin film transistors of these materials show p-type characteristics with comparable mobility values. It is found that the rigidification of the molecule is beneficial for reducing the reorganisation energy but also has an unfavorable impact on the electronic structure dimensionality.

Kondo effect in a neutral and stable all organic radical single molecule break junction.

Organic radicals are neutral, purely organic molecules exhibiting an intrinsic magnetic moment due to the presence of an unpaired electron in the molecule in its ground state. This property, added to the low spin-orbit coupling and weak hyperfine interactions, make neutral organic radicals good candidates for molecular spintronics insofar as the radical character is stable in solid state electronic devices. Here we show that the paramagnetism of the polychlorotriphenylmethyl radical molecule in the form of a Kondo anomaly is preserved in two- and three-terminal solid-state devices, regardless of mechanical and electrostatic changes. Indeed, our results demonstrate that the Kondo anomaly is robust under electrodes displacement and changes of the electrostatic environment, pointing to a localized orbital in the radical as the source of magnetism. Strong support to this picture is provided by density functional calculations and measurements of the corresponding nonradical species. These results pave the way toward the use of all-organic neutral radical molecules in spintronics devices and open the door to further investigations into Kondo physics.

Trends in the adsorption and reactivity of hydrogen on magnesium silicate nanoclusters.

We study nanoclusters of Mg-rich olivine and pyroxene (having (MgO)6(SiO2)3 and (MgO)4(SiO2)4 compositions) with respect to their reactivity towards hydrogen atoms, using density functional calculations. Ultrasmall silicate particles are fundamental intermediates in cosmic dust grain formation and processing, and are thought to make up a significant mass fraction of the grain population. Due to their nanoscale dimensions and high surface area to bulk ratios, they are likely to also have a disproportionately large influence on surface chemistry in the interstellar medium. This work investigates the potential role of silicate nanoclusters in vital interstellar hydrogen-based chemistry by studying atomic H adsorption and H2 formation. Our extensive set of calculations confirm the generality of a Brønsted-Evans-Polanyi (BEP) relation between the H2 reaction barrier and the 2Hchem binding energy, suggesting it to be independent of silicate dust grain shape, size, crystallinity and composition. Our results also suggest that amorphous/porous grains with forsteritic composition would tend to dissociate H2, but relatively Mg-poor silicate grains (e.g. enstatite composition) and/or more crystalline/compact silicate grains would tend to catalyse H2 formation. The high structural thermostability of silicate nanoclusters with respect to the heat released during exothermic H2 formation reactions is also verified.

HOMO stabilisation in π-extended dibenzotetrathiafulvalene derivatives for their application in organic field-effect transistors.

Three new organic semiconductors, in which either two methoxy units are directly linked to a dibenzotetrathiafulvalene (DB-TTF) central core and a 2,1,3-chalcogendiazole is fused on the one side, or four methoxy groups are linked to the DB-TTF, have been synthesised as active materials for organic field-effect transistors (OFETs). Their electrochemical behaviour, electronic absorption and fluorescence emission as well as photoinduced intramolecular charge transfer were studied. The electron-withdrawing 2,1,3-chalcogendiazole unit significantly affects the electronic properties of these semiconductors, lowering both the HOMO and LUMO energy levels and hence increasing the stability of the semiconducting material. The solution-processed single-crystal transistors exhibit high performance with a hole mobility up to 0.04 cm(2) V(-1) s(-1) as well as good ambient stability.