Search

Increased demand for light alkenes has motivated research on the catalytic dehydrogenation of the light alkanes and on understanding the role of catalyst composition on the activity, selectivity, and stability of Pt-based catalysts used for this purpose. The present study examines the structure and performance of Pt-Ir catalysts for ethane and propane dehydrogenation, and compares them with the performance of Pt and Pt-Sn catalysts. Nanoparticles of Pt, PtSn, and PtIr were prepared in a colloidal suspension and then dispersed onto calcined hydrotalcite (Mg(Al)O). After characterization to confirm formation of a bimetallic alloy, it was observed that at high conversions, Pt3Ir/Mg(Al)O exhibited lower initial activity than Pt3Sn/Mg(Al)O but greater stability to coke deposition. Intrinsic rate measurements at low feed residence time revealed the following trend in activity: Pt3Sn > Pt3Ir > Pt. DFT calculations carried out on tetrahedral clusters (Pt4, Pt3Ir, Pt3Sn) reveals that this trend in activity can be replicated and Ir is capable of alkane activation, a trait unique to this bimetallic system.

A summary of the technical advances that are incorporated in the fourth major release of the Q-Chem quantum chemistry program is provided, covering approximately the last seven years. These include developments in density functional theory methods and algorithms, nuclear magnetic resonance (NMR) property evaluation, coupled cluster and perturbation theories, methods for electronically excited and open-shell species, tools for treating extended environments, algorithms for walking on potential surfaces, analysis tools, energy and electron transfer modelling, parallel computing capabilities, and graphical user interfaces. In addition, a selection of example case studies that illustrate these capabilities is given. These include extensive benchmarks of the comparative accuracy of modern density functionals for bonded and non-bonded interactions, tests of attenuated second order Møller-Plesset (MP2) methods for intermolecular interactions, a variety of parallel performance benchmarks, and tests of the accuracy of implicit solvation models. Some specific chemical examples include calculations on the strongly correlated Cr2 dimer, exploring zeolite-catalysed ethane dehydrogenation, energy decomposition analysis of a charged ter-molecular complex arising from glycerol photoionisation, and natural transition orbitals for a Frenkel exciton state in a nine-unit model of a self-assembling nanotube.

Microscale objects responding to chemical gradients by migrating toward or away from a preferred species is a simple yet constitutive mechanism by which transport occurs in biological organisms. Synthetic chemotaxis provides key physical descriptions of simplified systems that can be used in biological models, or in the creation of advanced responsive material systems. In this article, we provide a quantitative framework for understanding synthetic chemotaxis of microparticles which involves a competition between phoresis and osmosis. We present separate quantitative measurements of phoresis and osmosis acting on individual taxing particles, finding that phoresis follows the long-predicted [Formula: see text] scaling while the osmotic contribution depends on the geometry and details of the system, and must be solved on a case-by-case basis. Through this, we are able to develop a more accurate picture of particle transport at the single particle level. Equipped with this approach, we go on to describe how high concentrations of particles in a symmetric chemical gradient grow close-packed hives that reach a steady-state size tunable through light intensity or particle size. Last, we demonstrate that mixed particles experiencing the same chemical gradient will selectively migrate toward or away depending on the nature of the particle surface, thereby locally sorting out a particular species. We anticipate these results will be important in describing both biological and synthetic chemotaxis in phoretic systems and should bring a wealth of studies that take advantage of competing osmotic flows to illicit unexpected dynamic active behavior., Competing Interests: Competing interests statement:The authors declare no competing interest.

Structures of molecular crystals are identified using scattering techniques because we cannot see inside them. Micrometre-sized colloidal particles enable the real-time observation of crystallization with optical microscopy, but in practice this is still hampered by a lack of 'X-ray vision'. Here we introduce a system of index-matched fluorescently labelled colloidal particles and demonstrate the robust formation of ionic crystals in aqueous solution, with structures that can be controlled by size ratio and salt concentration. Full three-dimensional coordinates of particles are distinguished through in situ confocal microscopy, and the crystal structures are identified via comparison of their simulated scattering pattern with known atomic arrangements. Finally, we leverage our ability to look inside colloidal crystals to observe the motion of defects and crystal melting in time and space and to reveal the origin of crystal twinning. Using this platform, the path to real-time analysis of ionic colloidal crystallization is now 'crystal clear'., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Metal phthalocyanines, a highly versatile class of aromatic, planar, macrocyclic molecules with a chelated central metal ion, are topical objects of ongoing research and particularly interesting due to their magnetic properties. However, while the current focus lies almost exclusively on spin-Zeeman-related effects, the high symmetry of the molecule and its circular shape suggests the exploitation of light-induced excitation of 2-fold degenerate vibrational states in order to generate, switch, and manipulate magnetic fields at the nanoscale. The underlying mechanism is a molecular pseudorotation that can be triggered by infrared pulses and gives rise to a quantized, small, but controllable magnetic dipole moment. We investigate the optical stimulation of vibrationally induced molecular magnetism and estimate changes in the magnetic shielding constants for confirmation by future experiments.

Driven systems composed largely of droplets and fuel make up a significant portion of microbiological function. At the micrometer scale, fully synthetic systems that perform an array of tasks within a uniform bulk are much more rare. In this work, we introduce an innovative design for solid-in-oil composite microdroplets. These microdroplets are engineered to nucleate an internal phase, undergo inflation, and eventually burst, all powered by a steady and uniform energy input. We show that by altering the background input, volumetric change and burst time can be tuned. When the inflated droplets release the inner contents, colloidal particles are shown to transiently attract to the release point. Lastly, we show that the system has the ability to perform multiple inflation-burst cycles. We anticipate that our conceptual design of internally powered microdroplets will catalyze further research into autonomous systems capable of intricate communication as well as inspire the development of advanced, responsive materials., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

Sub-nanometer metal clusters have special physical and chemical properties, significantly different from those of nanoparticles. However, there is a major concern about their thermal stability and susceptibility to oxidation. In situ X-ray Absorption spectroscopy and Near Ambient Pressure X-ray Photoelectron spectroscopy results reveal that supported Cu 5 clusters are resistant to irreversible oxidation at least up to 773 K, even in the presence of 0.15 mbar of oxygen. These experimental findings can be formally described by a theoretical model which combines dispersion-corrected DFT and first principles thermochemistry revealing that most of the adsorbed O 2 molecules are transformed into superoxo and peroxo species by an interplay of collective charge transfer within the network of Cu atoms and large amplitude "breathing" motions. A chemical phase diagram for Cu oxidation states of the Cu 5 -oxygen system is presented, clearly different from the already known bulk and nano-structured chemistry of Cu., (© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The theoretical investigation of gas adsorption, storage, separation, diffusion, and related transport processes in porous materials relies on a detailed knowledge of the potential energy surface of molecules in a stationary environment. In this article, a new algorithm is presented, specifically developed for gas transport phenomena, which allows for a highly cost-effective determination of molecular potential energy surfaces. It is based on a symmetry-enhanced version of Gaussian process regression with embedded gradient information and employs an active learning strategy to keep the number of single point evaluations as low as possible. The performance of the algorithm is tested for a selection of gas sieving scenarios on porous, N-functionalized graphene and for the intermolecular interaction of CH4 and N2., (© 2023 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).)

Nanoparticles with diameters in the range of a few nanometers, consisting of gold and vanadium oxide, are synthesized by sequential doping of cold helium droplets in a molecular beam apparatus and deposited on solid carbon substrates. After surface deposition, the samples are removed and various measurement techniques are applied to characterize the created particles: scanning transmission electron microscopy (STEM) at atomic resolution, temperature dependent STEM and TEM up to 650 °C, energy-dispersive X-ray spectroscopy (EDXS) and electron energy loss spectroscopy (EELS). In previous experiments we have shown that pure V 2 O 5 nanoparticles can be generated by sublimation from the bulk and deposited without affecting their original stoichiometry. Interestingly, our follow-up attempts to create Au@V 2 O 5 core@shell particles do not yield the expected encapsulated structure. Instead, Janus particles of Au and V 2 O 5 with diameters between 10 and 20 nm are identified after deposition. At the interface of the Au and the V 2 O 5 parts we observe an epitaxial-like growth of the vanadium oxide next to the Au structure. To test the temperature stability of these Janus-type particles, the samples are heated in situ during the STEM measurements from room temperature up to 650 °C, where a reduction from V 2 O 5 to V 2 O 3 is followed by a restructuring of the gold atoms to form a Wulff-shaped cluster layer. The temperature dependent dynamic interplay between gold and vanadium oxide in structures of only a few nanometer size is the central topic of this contribution to the Faraday Discussion.

In the year 1933, Herzberg and Teller realized that the potential energy surface of a triatomic, linear molecule splits into two as soon as the molecule is bent. The phenomenon, later dubbed the Renner-Teller effect due to the detailed follow-up work of Renner on the subject, describes the coupling of a symmetry-reducing molecular vibration with degenerate electronic states. In this article, we show that a very similar type of nonadiabatic coupling can occur for certain translational degrees of freedom of diatomic, electronically degenerate molecules when trapped in a nearly spherical or cylindrical quantum confinement, e.g., realized through electromagnetic fields or molecular encapsulation. We illustrate this on the example of fullerene-encapsulated nitric oxide, and provide a prediction of its interesting, perturbed vibronic spectrum.

The electronic structure of subnanometric clusters, far off the bulk regime, is still dominated by molecular characteristics. The spatial arrangement of the notoriously undercoordinated metal atoms is strongly coupled to the electronic properties of the system, which makes this class of materials particularly interesting for applications including luminescence, sensing, bioimaging, theranostics, energy conversion, catalysis, and photocatalysis. Opposing a common rule of thumb that assumes an increasing chemical reactivity with smaller cluster size, Cu 5 clusters have proven to be exceptionally resistant to irreversible oxidation, i.e., the dissociative chemisorption of molecular oxygen. Besides providing reasons for this behavior in the case of heavy loading with molecular oxygen, we investigate the competition between physisorption and molecular chemisorption from the perspective of nonadiabatic effects. Landau-Zener theory is applied to the Cu 5 (O 2 ) 3 complex to estimate the probability for a switching between the electronic states correlating the neutral O 2 + Cu 5 (O 2 ) 2 and the ionic O 2 - + (Cu 5 (O 2 ) 2 ) + fragments in a diabatic representation. Our work demonstrates the involvement of strong nonadiabatic effects in the associated charge transfer process, which might be a common motive in reactions involving subnanometric metal structures.

We present path integral molecular dynamics (PIMD) calculations of an electron transfer from a heliophobic Cs2 dimer in its (3Σu) state, located on the surface of a He droplet, to a heliophilic, fully immersed C60 molecule. Supported by electron ionization mass spectroscopy measurements (Renzler et al., J. Chem. Phys. 2016 , 145 , 181101), this spatially quenched reaction was characterized as a harpoon-type or long-range electron transfer in a previous high-level ab initio study (de Lara-Castells et al., J. Phys. Chem. Lett. 2017 , 8 , 4284). To go beyond the static approach, classical and quantum PIMD simulations are performed at 2 K, slightly below the critical temperature for helium superfluidity (2.172 K). Calculations are executed in the NVT ensemble as well as the NVE ensemble to provide insights into real-time dynamics. A droplet size of 2090 atoms is assumed to study the impact of spatial hindrance on reactivity. By changing the number of beads in the PIMD simulations, the impact of quantization can be studied in greater detail and without an implicit assumption of superfluidity. We find that the reaction probability increases with higher levels of quantization. Our findings confirm earlier, static predictions of a rotational motion of the Cs2 dimer upon reacting with the fullerene, involving a substantial displacement of helium. However, it also raises the new question of whether the interacting species are driven out-of-equilibrium after impurity uptake, since reactivity is strongly quenched if a full thermal equilibration is assumed. More generally, our work points towards a novel mechanism for long-range electron transfer through an interplay between nuclear quantum delocalization within the confining medium and delocalized electronic dispersion forces acting on the two reactants.

Metal clusters have drawn continuous interest because of their high potential for the assembly of matter with special properties that may significantly differ from the corresponding bulk. Controlled combination of particular elements in one nanoparticle can increase the options for the creation of new materials for photonic, catalytic, or electronic applications. Superfluid helium droplets provide confinement and ultralow temperature, i.e. an ideal environment for the atom-by-atom aggregation of a new nanoparticle. This perspective presents a review of the current research progress on the synthesis of tailored metal and metal oxide clusters including core-shell designs, their characterization within the helium droplet beam, deposition on various solid substrates, and analysis via surface diagnostics. Special attention is given to the thermal properties of mixed metal clusters and questions about alloy formation on the nanoscale. Experimental results are accompanied by theoretical approaches employing computational chemistry, molecular dynamics simulations and He density functional theory.

Orbital-free approaches might offer a way to boost the applicability of density functional theory by orders of magnitude in system size. An important ingredient for this endeavor is the kinetic energy density functional. Snyder et al. [ Phys. Rev. Lett. 2012, 108, 253002] presented a machine learning approximation for this functional achieving chemical accuracy on a one-dimensional model system. However, a poor performance with respect to the functional derivative, a crucial element in iterative energy minimization procedures, enforced the application of a computationally expensive projection method. In this work we circumvent this issue by including the functional derivative into the training of various machine learning models. Besides kernel ridge regression, the original method of choice, we also test the performance of convolutional neural network techniques borrowed from the field of image recognition.

The temperature-induced structural changes of Fe-, Co-, and Ni-Au core-shell nanoparticles with diameters around 5 nm are studied via atomically resolved transmission electron microscopy. We observe structural transitions from local toward global energy minima induced by elevated temperatures. The experimental observations are accompanied by a computational modeling of all core-shell particles with either centralized or decentralized core positions. The embedded atom model is employed and further supported by density functional theory calculations. We provide a detailed comparison of vacancy formation energies obtained for all materials involved in order to explain the variations in the restructuring processes which we observe in temperature-programmed TEM studies of the particles., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Locating the minimum energy structure of molecules, typically referred to as geometry optimization, is one of the first steps of any computational chemistry calculation. Earlier research was mostly dedicated to finding convenient sets of molecule-specific coordinates for a suitable representation of the potential energy surface, where a faster convergence toward the minimum structure can be achieved. More recent approaches, on the other hand, are based on various machine learning techniques and seem to revert to Cartesian coordinates instead for practical reasons. We show that the combination of Gaussian process regression with those coordinate systems employed by state-of-the-art geometry optimizers can significantly improve the performance of this powerful machine learning technique. This is demonstrated on a benchmark set of 30 small covalently bonded molecules.

The localization of transition states and the calculation of reaction pathways are routine tasks of computational chemists but often very CPU-intense problems, in particular for large systems. The standard algorithm for this purpose is the nudged elastic band method, but it has become obvious that an "intelligent" selection of points to be evaluated on the potential energy surface can improve its convergence significantly. This article summarizes, compares, and extends known strategies that have been heavily inspired by the machine learning developments of recent years. It presents advantages and disadvantages and provides an unbiased comparison of neural network based approaches, Gaussian process regression in Cartesian coordinates, and Gaussian approximation potentials. We test their performance on two example reactions, the ethane rotation and the activation of carbon dioxide on a metal catalyst, and provide a clear ranking in terms of usability for future implementations.

An ab initio study of the interaction of O 2 , the most abundant radical and oxidant species in the atmosphere, with a Cu 5 cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for by using the multireference perturbation theory, allowing the experimentally confirmed resistivity of Cu 5 clusters toward oxidation to be investigated. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O 2 with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multireference approach, the transition barrier for O 2 activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu 5 arranged in a bipyramidal structure, the O-O bond cleavage is confirmed as the rate-determining step. However, for planar Cu 5 , the high energy barrier for O 2 activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case., Competing Interests: The authors declare no competing financial interest.

Vanadium oxide clusters with a mean diameter below 10 nm are investigated by high resolution Scanning Transmission Electron Microscopy (STEM), Electron Energy Loss Spectroscopy (EELS) and UV-vis absorption spectroscopy. The clusters are synthesised by sublimation from bulk vanadium(v) oxide, in combination with a pick-up by superfluid helium droplets. The latter act as reaction chambers which enable cluster growth under fully inert and solvent-free conditions. High-resolution STEM images of deposited vanadium oxide particles allowing for the determination of lattice constants, clearly indicate a dominating presence of V 2 O 5 . This finding is further supported by UV-vis absorption spectra of nanoparticles after deposition on fused silica substrates, which indicates that the oxidation state of the material is preserved over the entire process. From the results of the UV-vis measurement, the band gap of the nanosized V 2 O 5 could be determined to be 3.3 eV. The synthesis approach provides a route to clean V 2 O 5 clusters as it does not involve any surfactant or solvents, which is crucial for an unbiased measurement of intrinsic catalyst properties.

In this work, we explore the decomposition of CO 2 on unsupported and TiO 2 -supported Cu 5 clusters via computational modeling, using both finite cluster and periodic slab structures of the rutile TiO 2 (110) surface. While the energy needed for C=O bond breaking is already significantly reduced upon adsorption onto the unsupported metal catalyst (it drops from 7.8 to 1.3 eV), gas desorption before bond activation is still the inevitable outcome due to the remaining barrier height even at 0 K. However, when the Cu 5 cluster itself is supported on TiO 2 , reactant and product adsorption is strongly enhanced, the barrier for bond breaking is further reduced, and a spontaneous decomposition of the molecule is predicted. This finding is linked to our previous work on charge-transfer processes in the Cu 5 -TiO 2 system triggered by solar photons, since a combination of both phenomena at suitable temperatures would allow for a photoinduced activation of CO 2 by sunlight., Competing Interests: The authors declare no competing financial interest., (Copyright © 2019 American Chemical Society.)

Structural changes of Ni-Au core-shell nanoparticles with increasing temperature are studied at atomic resolution. The bimetallic clusters, synthesized in superfluid helium droplets, show a centralized Ni core, which is an intrinsic feature of the growth process inside helium. After deposition on SiN x , the nanoparticles undergo a programmed temperature treatment in vacuum combined with an in situ transmission electron microscopy study of structural changes. We observe not only full alloying far below the actual melting temperature, but also a significantly higher stability of core-shell structures with decentralized Ni cores. Explanations are provided by large-scale molecular dynamics simulations on model structures consisting of up to 3000 metal atoms. Two entirely different diffusion processes can be identified for both types of core-shell structures, strikingly illustrating how localized, atomic features can still dictate the overall behavior of a nanometer-sized particle., Competing Interests: The authors declare no competing financial interest.

The 0.4 K internal temperature of superfluid helium nanodroplets is believed to guarantee a corresponding ground-state population of dopant atoms and molecules inside this cryogenic matrix. We have recorded 6s ← 5p excitation spectra of indium atoms in helium droplets and found two absorption bands separated by about 2000 cm - 1 , a value close to the spin-orbit (SO) splitting of the In 2 P ground state. The intensities of the bands agree with a thermal population of the 2 P 1/2 and 2 P 3/2 states at 870 K, the temperature of the In pick-up cell. Applying femtosecond pump-probe spectroscopy, we found the same dynamical response of the helium solvation shell after the photoexcitation of the two bands. He-density functional theory simulations of the excitation spectra are in agreement with the bimodal structure. Our findings show that the population of SO levels of hot dopants is conserved after pick-up inside the superfluid droplet. Implications for the interpretation of experiments on molecular aggregates are discussed.

The oxidation of Fe@Au core@shell clusters with sizes below 5 nm is studied via high resolution scanning transmission electron microscopy. The bimetallic nanoparticles are grown in superfluid helium droplets under fully inert conditions, avoiding any effect of solvents or template structures, and deposited on amorphous carbon. Oxidation resistivity is tested by exposure to oxygen at ambient conditions. The passivating effect of Au-shells is studied in detail and a critical Au shell thickness is determined which keeps the Fe core completely unharmed. Additionally, we present the first synthesis of Fe@Au@Fe-oxide onion-type structures., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The decoration of semiconductors with subnanometer-sized clusters of metal atoms can have a strong impact on the optical properties of the support. The changes induced differ greatly from effects known for their well-studied, metallic counterparts in the nanometer range. In this work, we study the deposition of Cu 5 clusters on a TiO 2 surface and investigate their influence on the photon-absorption properties of TiO 2 nanoparticles via the computational modeling of a decorated rutile TiO 2 (110) surface. Our findings are further supported by selected experiments using diffuse reflectance and X-ray absorption spectroscopy. The Cu 5 cluster donates an electron to TiO 2 , leading to the formation of a small polaron Ti 3+ 3d 1 state and depopulation of Cu(3d) orbitals, successfully explaining the absorption spectroscopy measurements at the K-edge of copper. A monolayer of highly stable and well fixated Cu 5 clusters is formed, which not only enhances the overall absorption, but also extends the absorption profile into the visible region of the solar spectrum via direct photo-induced electron transfer and formation of a charge-separated state.

Oxide nanoparticles in the size range of a few nanometers are typically synthesized in solution or via laser ablation techniques, which open numerous channels for structural change via chemical reactions or fragmentation processes. In this work, neutral vanadium oxide nanoparticles are instead synthesized by sublimation from bulk in combination with a pickup by superfluid helium droplets. Mass spectroscopy measurements clearly demonstrate the preservation of the bulk stoichiometric ratio of vanadium to oxygen in He-grown nanoparticles, indicating a tendency towards tetrahedral coordination of the vanadium centers in finite geometries. This unexpected finding opens up new possibilities for a combined on-the-fly synthesis of nanoparticles consisting of metal and metal-oxide layers. In comparison to mass spectra obtained via direct ionization of vanadium oxide in an effusive beam, where strong fragmentation occurred, we observe a clear preference for (V 2 O 5 ) n oligomers with even n inside the He nanodroplets, which is further investigated and explained using the electronic structure theory.

We present a new class of tunable light-driven oscillators based on mm-scale objects adsorbed at fluid interfaces. A fixed light source induces photothermal surface tension gradients (Marangoni stresses) that drive nanocomposite hydrogel discs away from a stable apex position atop a drop of water. The capillary forces on the disc increase with surface curvature; thus, they act to restore the disc to its original position. As the disc reenters the light source it again experiences Marangoni propulsion, leading to sustained oscillation for appropriate conditions. Propulsive forces can be modulated with incident light intensity, while the restoring force can be tuned via surface curvature-i.e., drop volume-providing highly tunable oscillatory behaviors. To our knowledge, this is the first example where Marangoni and capillary forces combine to incite sustained motion. As such, a model was developed that describes this behavior and provides key insights into the underlying control parameters. We expect that this simple approach will enable the study of more complex and coupled oscillatory systems.

The observation of chemical reactions on the time scale of the motion of electrons and nuclei has been made possible by lasers with ever shortened pulse lengths. Superfluid helium represents a special solvent that permits the synthesis of novel classes of molecules that have eluded dynamical studies so far. However, photoexcitation inside this quantum solvent triggers a pronounced response of the solvation shell, which is not well understood. Here, we present a mechanistic description of the solvent response to photoexcitation of indium (In) dopant atoms inside helium nanodroplets (He N ), obtained from femtosecond pump-probe spectroscopy and time-dependent density functional theory simulations. For the In-He N system, part of the excited state electronic energy leads to expansion of the solvation shell within 600 fs, initiating a collective shell oscillation with a period of about 30 ps. These coupled electronic and nuclear dynamics will be superimposed on intrinsic photoinduced processes of molecular systems inside helium droplets.

In a recent study [Angew. Chem. Int. Ed., 2014, 53, 9957-9960] a new concept of chiral separation has been suggested, which is based on functionalized, nanoporous sheets of graphene. In this follow-up article we discuss the underlying principle in greater detail and make suggestions for suitable pore functionalizations with respect to a selection of chiral prototype molecules. Considering drug molecules as future targets for a chiral separation via membranes, the necessary pore sizes represent a big challenge for standard methods of computational chemistry. Therefore, we test two common force fields (GAFF, CGenFF) as well as a semiempirical tight-binding approach recently developed by the Grimme group (GFN-xTB) against the computationally much more expensive density functional theory. We identify the GFN-xTB method as the most suitable approach for future simulations of functionalized pores for the given purpose, as it is able to produce reaction pathways in very good agreement with density functional theory, even in cases where force fields tend to an extreme overestimation of barrier heights., (© 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Alloying processes in nanometre-sized Ag@Au and Au@Ag core@shell particles with average radii of 2 nm are studied via high resolution Transmission Electron Microscopy (TEM) imaging on in situ heatable carbon substrates. The bimetallic clusters are synthesized in small droplets of superfluid helium under fully inert conditions. After deposition, they are monitored during a heating cycle to 600 K and subsequent cooling. The core-shell structure, a sharply defined feature of the TEM High-Angle Annular Dark-Field images taken at room temperature, begins to blur with increasing temperature and transforms into a fully mixed alloy around 573 K. This transition is studied at atomic resolution, giving insights into the alloying process with unprecedented precision. A new image-processing method is presented, which allows a measurement of the temperature-dependent diffusion constant at the nanoscale. The first quantification of this property for a bimetallic structure <5 nm sheds light on the thermodynamics of finite systems and provides new input for current theoretical models derived from bulk data.

An ab initio study of quantum confinement of deuterium clusters in carbon nanotubes is presented. First, density functional theory (DFT)-based symmetry-adapted perturbation theory is used to derive parameters for a pairwise potential model describing the adsorbate-nanotube interaction. Next, we analyze the quantum nuclear motion of N D 2 molecules (N < 4) confined in carbon nanotubes using a highly accurate adsorbate-wave-function-based approach, and compare it with the motion of molecular hydrogen. We further apply an embedding approach and study zero-point energy effects on larger hexagonal and heptagonal structures of 7-8 D 2 molecules. Our results show a preference for crystalline hexagonal close packing hcp of D 2 molecules inside carbon nanotubes even at the cost of a reduced volumetric density within the cylindrical confinement.

An ab initio study of a long-range electron transfer or "harpoon"-type process from Cs and Cs 2 to C 60 in a superfluid helium droplet is presented. The heliophobic Cs or Cs 2 species are initially located at the droplet surface, while the heliophilic C 60 molecule is fully immersed in the droplet. First, probabilities for the electron transfer in the gas phase are calculated for reactants with velocities below the critical Landau velocity of 57 m/s to account for the superfluid helium environment. Next, reaction pathways are derived that also include the repulsive contribution from the extrusion of helium upon the approach of the two reactants. Our results are in perfect agreement with recent experimental measurements of electron ionization mass spectroscopy [ Renzler , M. ; et al., J. Chem. Phys. 2016 , 145 , 181101 ], showing a high possibility for the formation of a Cs 2 -C 60 complex inside of the droplet through a direct harpoon-type electron transfer involving the rotation of the molecule but a negligibly low reactivity for atomic Cs.

Electronic excitations of an electron bound to an alkali metal ion inside a droplet of superfluid 4 He are computed via a combination of helium density functional theory and the numerical integration of the Schrödinger equation for a single electron in a modified, He density dependent atomic pseudopotential. The application of a spectral method to the radial part of the valence electron wavefunction allows the computation of highly excited Rydberg states. For low principal quantum numbers, the energy required to push the electron outward is larger than the solvation energy of the ion. However, for higher principal quantum numbers the situation is reversed, which suggests the stability of a system where the ion sits inside the droplet while the valence electron orbits the nanodroplet.

We present time-resolved transmission electron microscopy studies of the degradation of Au, Ag, Cu and Ni nanowires deposited on a heated support. The wires are grown under fully inert conditions in superfluid helium droplets and deposited onto amorphous carbon. The inherent stability of these pristine metal nanowires with diameters below 10 nm is investigated in the absence of any stabilizers, templates or solvents. The phenomenon of Rayleigh-breakup, a consequence of diffusion processes along the wire surfaces, is analysed in situ via scans over time and support temperature. Our experimental efforts are combined with simulations based on a novel model featuring a cellular automaton to emulate surface diffusion. Based on this model, correlations between the material parameters and actual breakup behaviour are studied.

The formation of well-defined and functional three-dimensional (3D) structures by buckling of thin sheets subjected to spatially nonuniform stresses is common in biological morphogenesis and has become a subject of great interest in synthetic systems, as such programmable shape-morphing materials hold promise in areas including drug delivery, biomedical devices, soft robotics, and biomimetic systems. Given their ability to undergo large changes in swelling in response to a wide variety of stimuli, hydrogels have naturally emerged as a key type of material in this field. Of particular interest are hybrid systems containing rigid inclusions that can define both the anisotropy and spatial nonuniformity of swelling as well as nanoparticulate additives that can enhance the responsiveness and functionality of the material. In this Account, we discuss recent progress in approaches to achieve well-defined shape morphing in hydrogel hybrids. First, we provide an overview of materials and methods that facilitate fabrication of such systems and outline the geometry and mechanics behind shape morphing of thin sheets. We then discuss how patterning of stiff inclusions within soft responsive hydrogels can be used to program both bending and swelling, thereby providing access to a wide array of complex 3D forms. The use of discretely patterned stiff regions to provide an effective composite response offers distinct advantages in terms of scalability and ease of fabrication compared with approaches based on smooth gradients within a single layer of responsive material. We discuss a number of recent advances wherein control of the mechanical properties and geometric characteristics of patterned stiff elements enables the formation of 3D shapes, including origami-inspired structures, concatenated helical frameworks, and surfaces with nonzero Gaussian curvature. Next, we outline how the inclusion of functional elements such as nanoparticles can enable unique pathways to programmable and even reprogrammable shape-morphing materials. We focus to a large extent on photothermally reprogrammable systems that include one of a variety of additives that serve to efficiently absorb light and convert it into heat, thereby driving the response of a temperature-sensitive hydrogel. Such systems are advantageous in that patterns of light can be defined with very high spatial and temporal resolution in addition to offering the potential for wavelength-selective addressability of multiple different inclusions. We highlight recent advances in the preparation of light-responsive hybrid systems capable of undergoing reprogrammable bending and buckling into well-defined 3D shapes. In addition, we describe several examples where shape tuning of hybrid systems enables control over the motion of responsive hydrogel-based materials. Finally, we offer our perspective on open challenges and future areas of interest for the field.

A recent experimental study [Renzler et al., J. Chem. Phys., 2016, 145, 181101] on superfluid helium nanodroplets reported different reactivities for Cs atoms and Cs 2 dimers with C 60 fullerenes inside helium droplets. Alkali metal atoms and clusters are heliophobic, therefore typically residing on the droplet surface, while fullerenes are fully immersed into the droplet. In this theoretical study, which combines standard methods of computational chemistry with orbital-free helium density functional theory, we show that the experimental findings can be interpreted in the light of a quenched electron-transfer reaction between the fullerene and the alkali dopant, which is additionally hindered by a reaction barrier stemming from the necessary extrusion of helium upon approach of the two reactants.

A recent experimental study [ Ohba, Sci. Rep. 2016, 6, 28992 ] of gas adsorption on single-walled carbon nanotubes at temperatures between 2 and 5 K reported a quenched propagation of helium through carbon nanotubes with diameters below 7 Å despite the small kinetic diameter of helium atoms. After assessing the performance of a potential model for the He-nanotube interaction via ab initio calculations with density functional theory-based symmetry adapted perturbation theory, we apply orbital-free helium density functional theory to show that the counterintuitive experimental result is a consequence of the exceptionally high zero-point energy of helium and its tendency to form spatially separated layers of helium upon adsorption at the lowest temperatures. Helium filling factors are derived for a series of carbon nanotubes and compared to the available experimental data.

Alkali metal atoms and small alkali clusters are classic heliophobes and when in contact with liquid helium they reside in a dimple on the surface. Here we show that alkalis can be induced to submerge into liquid helium when a highly polarizable co-solute, C 60 , is added to a helium nanodroplet. Evidence is presented that shows that all sodium clusters, and probably single Na atoms, enter the helium droplet in the presence of C 60 . Even clusters of cesium, an extreme heliophobe, dissolve in liquid helium when C 60 is added. The sole exception is atomic Cs, which remains at the surface.

Platinum-based bimetallic nanoparticles are analyzed by the application of density functional theory to a series of tetrahedral Pt3X cluster models, with element X taken from the P-block, preferably group 14, or from the D-block around group 10. Almost identical cluster geometries allow a systematic investigation of electronic effects induced by different elements X. Choosing the propane-to-propene conversion as the desired dehydrogenation reaction, we provide estimates for the activity and selectivity of the various catalysts based on transition state theory. No significant Brønsted-Evans-Polanyi-relation could be found for the given reaction. A new descriptor, derived from an energy decomposition analysis, captures the effect of element X on the rate-determining step of the first hydrogen abstraction. Higher activities than obtained for pure Pt4 clusters are predicted for Pt alloys containing Ir, Sn, Ge and Si, with Pt3Ir showing particularly high selectivity.

We investigate the properties of alkali-alkaline earth diatomic molecules in the lowest Σ(+) states of the doublet and quartet multiplicity by ab initio calculations. In all sixteen cases studied, the permanent electric dipole moment points in opposite directions for the two spin states. This peculiarity can be explained by molecular orbital theory. We further discuss dissociation energies and bond distances. We analyze trends and provide an empirically motivated model for the prediction of the permanent electric dipole moment for combinations of alkali and alkaline earth atoms not studied in this work.

Silver and gold nanowires are grown within superfluid helium nanodroplets and investigated by high resolution electron microscopy after surface deposition. The wire morphologies depend on the rate of metal atom doping in the pickup sequence. While high doping rates result in a polycrystalline face-centered cubic nanowire structure, at lower doping rates the initial fivefold-symmetry seems to be preserved. An explanation for this observation is given by computer simulations, which allow the derivation of timescales for the nanowire growth process inside helium nanodroplets.

Ag nanowires with diameters below 6 nm are grown within vortex containing superfluid helium nanodroplets and deposited onto a heatable substrate at cryogenic temperatures. The experimental setup allows an unbiased investigation of the inherent stability of pristine silver nanowires, which is virtually impossible with other methods due to chemical processes or templates involved in standard production routes. We demonstrate by experiment and by adaption of a theoretical model that initially continuous wires disintegrate into chains of spheres. This phenomenon is well described by a Rayleigh-like breakup mechanism when the substrate is heated to room temperature. Our findings clarify the recent discussions on the cause of the observed segmented patterns, where a breakup during deposition [Gomez et al., Phys. Rev. Lett., 2012, 108, 155302] or mechanisms intrinsic to the helium droplet mediated growth process [Spence et al., Phys. Chem. Chem. Phys., 2014, 16, 6903] have been proposed. The experimental setup confirms the validity of previous suggestions derived from bulk superfluid helium experiments [Gordon et al., Phys. Chem. Chem. Phys., 2014, 16, 25229] for the helium droplet system, and further allows a much more accurate determination of the breakup temperature.

Collision times for the coinage metal atoms Cu, Ag and Au in He-droplets are derived from helium density functional theory and molecular dynamics simulations. The strength of the attractive interaction between the metal atoms turns out to be less important than the mass of the propagating metal atoms. Even for small droplets consisting of a few thousand helium atoms, the collision times are shortest for Cu, followed by Ag and Au, despite the higher binding energy of Au2 compared to Cu2.

Patterning deformation within the plane of thin elastic sheets represents a powerful tool for the definition of complex and stimuli-responsive 3D buckled shapes. Previous experimental methods, however, have focused on sheets that access a limited number of shapes pre-programmed into the sheet, restricting the degree of dynamic control. Here, we demonstrate on-demand reconfigurable buckling of poly(N-isopropylacrylamide-co-acrylic acid) (PNIPAM) hydrogel network films containing gold nanoparticles (AuNPs) by patterned photothermal deswelling. Predictable, easily controllable, and reversible transformations from a single flat gel sheet to numerous different three-dimensional forms are shown. Importantly, the response time is limited by poroelastic mass transport, rather than photochemical switching kinetics, enabling reconfiguration of shape on timescales of several seconds, with further increases in speed possible by reducing film thickness., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We propose a new method of chiral separation using functionalized nanoporous graphene as an example. Computational simulations based on density functional theory show that the attachment of a suitable chiral "bouncer" molecule to the pore rim prevents the passage of the undesired enantiomer while letting its mirror image through., (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)