Three in One: Three Different Molybdates Trapped in a Thiacalix[4]arene Protected Ag72 Nanocluster for Structural Transformation and Photothermal Conversion.

Polyoxometalates (POMs) represent crucial intermediates in the formation of insoluble metal oxides from soluble metal ions, however, the rapid hydrolysis-condensation kinetics of MoVI or WVI makes the direct characterization of coexisted molecular species in a given medium extremely difficult. Silver nanoclusters have shown versatile capacity to encapsulate diverse POMs, which provides an alternative scene to appreciate landscape of POMs in atomic precision. Here, we report a thiacalix[4]arene protected silver nanocluster (Ag72b) that simultaneously encapsulates three kinds of molybdates (MoO4 2- , Mo6 O22 8- and Mo7 O25 8- ) in situ transformed from classic Lindqvist Mo6 O19 2- , providing more deep understanding on the structural diversity and condensation growth route of POMs in solution. Ag72b is the first silver nanocluster trapping so many kinds of molybdates, which in turn exert collective template effect to aggregate silver atoms into a nanocluster. The post-reaction of Ag72b with AgOAc or PhCOOAg produces a discrete Ag24 nanocluster (Ag24a) or an Ag28 nanocluster based 1D chain structure (Ag28a), respectively. Moreover, the post-synthesized Ag28a can be utilized as potential ignition material for further application. This work not only provides an important model for unlocking dynamic features of POMs at atom-precise level but also pioneers a promising approach to synthesize silver nanoclusters from known to unknown.

Sulfur-deficient edges as active sites for hydrogen evolution on MoS2.

A grand-canonical approach is employed to calculate the voltage-dependent activation energy and estimate the kinetics of the hydrogen evolution reaction (HER) on intrinsic sites of MoS2, including edges of varying S-coverage as well as S-vacancies on the basal plane. Certain edge configurations are found to be vastly more active than others, namely S-deficient edges on the Mo-termination where, in the fully S-depleted case, HER can proceed with activation energy below 0.5 eV at an electrode potential of 0 V vs. SHE. There is a clear distinction between the performance of Mo-rich and S-rich adsorption sites, as HER at the latter sites is characterized by large (generally above 1.5 eV) Heyrovsky and Tafel energy barriers despite near-thermoneutral hydrogen adsorption energy. Thus, exposing Mo-atoms on the edges to which hydrogen can directly bind is crucial for efficient hydrogen evolution. While S-vacancies on the basal plane do expose Mo-rich sites, the energy barriers are still significant due to high coordination of the Mo atoms. Kinetic modelling based on the voltage-dependent reaction energetics gives a theoretical overpotential of 0.25 V and 1.09 V for the Mo-edge with no S atoms and the weakly sulfur-deficient (2% S-vacancies) basal plane, respectively, with Volmer-Heyrovsky being the dominant pathway. These values coincide well with reported experimentally measured values of the overpotential for the edges and basal plane. For the partly Mo-exposed edges, the calculated overpotential is 0.6-0.7 V while edges with only S-sites give overpotential exceeding that of the basal plane. These results show that the overpotential systematically decreases with increased sulfur-deficiency and reduced Mo-coordination. The fundamental difference between Mo- and S-rich sites suggests that catalyst design of transition metal dichalcogenides should be focused on facilitating and modifying the metal sites, rather than activating the chalcogen sites.

Is the doped MoS2 basal plane an efficient hydrogen evolution catalyst? Calculations of voltage-dependent activation energy.

Transition metal dichalcogenides are cheap and earth-abundant candidates for the replacement of precious metals as catalyst materials. Experimental measurements of the hydrogen evolution reaction (HER), for example, have demonstrated significant electrocatalytic activity of MoS2 but there is large variation depending on the preparation method. In order to gain information about the mechanism and active sites for the HER, we have carried out calculations of the reaction and activation energy for HER at the transition metal doped basal plane of MoS2 under electrochemical conditions, i.e. including applied electrode potential and solvent effects. The calculations are based on identifying the relevant saddle points on the energy surface obtained from density functional theory within the generalized gradient approximation, and the information on energetics is used to construct voltage-dependent volcano plots. Doping with 3d-metal atoms as well as Pt is found to enhance hydrogen adsorption onto the basal plane by introducing electronic states within the band gap, and in some cases (Co, Ni, Cu, Pt) significant local symmetry breaking. The Volmer-Heyrovsky mechanism is found to be most likely and the associated energetics show considerable dopant and voltage-dependence. While the binding free energy of hydrogen can be tuned to be seemingly favorable for HER, the calculated activation energy turns out to be significant, at least 0.7 eV at a voltage of -0.5 V vs. SHE, indicating low catalytic activity of the doped basal plane. This suggests that other sites are responsible for the experimental activity, possibly edges or basal plane defects.

Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

Utilizing quantum effects in complex oxides, such as magnetism, multiferroicity and superconductivity, requires atomic-level control of the material's structure and composition. In contrast, the continuous conductivity changes that enable artificial oxide-based synapses and multiconfigurational devices are driven by redox reactions and domain reconfigurations, which entail long-range ionic migration and changes in stoichiometry or structure. Although both concepts hold great technological potential, combined applications seem difficult due to the mutually exclusive requirements. Here we demonstrate a route to overcome this limitation by controlling the conductivity in the functional oxide hexagonal Er(Mn,Ti)O3 by using conductive atomic force microscopy to generate electric-field induced anti-Frenkel defects, that is, charge-neutral interstitial-vacancy pairs. These defects are generated with nanoscale spatial precision to locally enhance the electronic hopping conductivity by orders of magnitude without disturbing the ferroelectric order. We explain the non-volatile effects using density functional theory and discuss its universality, suggesting an alternative dimension to functional oxides and the development of multifunctional devices for next-generation nanotechnology.

Publisher Correction: Conductivity control via minimally invasive anti-Frenkel defects in a functional oxide.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The Role of Temperature and Lipid Charge on Intake/Uptake of Cationic Gold Nanoparticles into Lipid Bilayers.

Understanding the molecular mechanisms governing nanoparticle-membrane interactions is of prime importance for drug delivery and biomedical applications. Neutron reflectometry (NR) experiments are combined with atomistic and coarse-grained molecular dynamics (MD) simulations to study the interaction between cationic gold nanoparticles (AuNPs) and model lipid membranes composed of a mixture of zwitterionic di-stearoyl-phosphatidylcholine (DSPC) and anionic di-stearoyl-phosphatidylglycerol (DSPG). MD simulations show that the interaction between AuNPs and a pure DSPC lipid bilayer is modulated by a free energy barrier. This can be overcome by increasing temperature, which promotes an irreversible AuNP incorporation into the lipid bilayer. NR experiments confirm the encapsulation of the AuNPs within the lipid bilayer at temperatures around 55 °C. In contrast, the AuNP adsorption is weak and impaired by heating for a DSPC-DSPG (3:1) lipid bilayer. These results demonstrate that both the lipid charge and the temperature play pivotal roles in AuNP-membrane interactions. Furthermore, NR experiments indicate that the (negative) DSPG lipids are associated with lipid extraction upon AuNP adsorption, which is confirmed by coarse-grained MD simulations as a lipid-crawling effect driving further AuNP aggregation. Overall, the obtained detailed molecular view of the interaction mechanisms sheds light on AuNP incorporation and membrane destabilization.

Varying oxygen coverage on Cu55 and its effect on CO oxidation.

Adsorption of molecular oxygen on a Cu55 cluster and the resulting oxidation effects have been investigated by spin-polarized density functional theory (DFT). The optimal structure for each Cu55O2N (N = 1-20) complex has been obtained via a sequential addition of O2 and systematic screening of the preferable adsorption sites. Upon structural optimization, several O2 molecules dissociate readily on Cu55 at different oxygen coverages, and further DFT molecular dynamics simulations at 300 K confirm the instability (small dissociation barrier) of the remaining O2 and a spontaneous movement of some oxygen atoms from the surface sites towards the cluster interior. The Cu55 cluster and its oxidized derivatives have been placed on a γ-Al2O3(100) surface to study the cluster-support interaction, and furthermore, CO oxidation reactions on both Cu55(O)2N and Cu55(O)2N/γ-Al2O3(100) have been studied as a function of oxygen coverage. The CO oxidation reaction barrier is rather insensitive to the oxygen coverage regardless of the support, indicating a small increase in activity with the number of surface oxygen atoms.

Molecular-Scale Ligand Effects in Small Gold-Thiolate Nanoclusters.

Because of the small size and large surface area of thiolate-protected Au nanoclusters (NCs), the protecting ligands are expected to play a substantial role in modulating the structure and properties, particularly in the solution phase. However, little is known on how thiolate ligands explicitly modulate the structural properties of the NCs at atomic level, even though this information is critical for predicting the performance of Au NCs in application settings including as a catalyst interacting with small molecules and as a sensor interacting with biomolecular systems. Here, we report a combined experimental and theoretical study, using synchrotron X-ray spectroscopy and quantum mechanics/molecular mechanics simulations, that investigates how the protecting ligands impact the structure and properties of small Au18(SR)14 NCs. Two representative ligand types, smaller aliphatic cyclohexanethiolate and larger hydrophilic glutathione, are selected, and their structures are followed experimentally in both solid and solution phases. It was found that cyclohexanethiolate ligands are significantly perturbed by toluene solvent molecules, resulting in structural changes that cause disorder on the surface of Au18(SR)14 NCs. In particular, large surface cavities in the ligand shell are created by interactions between toluene and cyclohexanethiolate. The appearance of these small molecule-accessible sites on the  NC surface demonstrates the ability of Au NCs to act as a catalyst for organic phase reactions. In contrast, glutathione ligands encapsulate the Au NC core via intermolecular interactions, minimizing structural changes caused by interactions with water molecules. The much better protection from glutathione ligands imparts a rigidified surface and ligand structure, making the NCs desirable for biomedical applications due to the high stability and also offering a structural-based explanation for the enhanced photoluminescence often reported for glutathione-protected Au NCs.

Coexisting Honeycomb and Kagome Characteristics in the Electronic Band Structure of Molecular Graphene.

We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures.

The electrooxidation-induced structural changes of gold di-superatomic molecules: Au23vs. Au25.

The gold cluster compounds Au38(SC2H4Ph)24 and [Au25(PPh3)10(SC2H4Ph)5Cl2](2+) are known to possess bi-icosahedral Au23 and Au25 cores, respectively, inside their ligand shells. These Au cores can be viewed as quasi-molecules composed of two Au13 superatoms sharing three and one Au(+) atoms, respectively. In the present work, we studied the structural changes of these gold di-superatomic molecules upon electrooxidation via spectroelectrochemical techniques, X-ray absorption fine structure analysis, and density functional theory calculations. The Au23 core was electrochemically stable, but the Au25 core underwent irreversible structural change. This marked difference in the stability of the oxidized states is ascribed to differences in the bonding scheme of Au13 units and/or the bonding nature of the protecting ligands.

Collective excitations and viscosity in liquid Bi.

The analysis of extensive density functional/molecular dynamics simulations (over 500 atoms, up to 100 ps) of liquid bismuth at four temperatures between 573 K and 1023 K has provided details of the dynamical structure factors, the dispersion of longitudinal and transverse collective modes, and related properties (power spectrum, viscosity, and sound velocity). Agreement with available inelastic x-ray and neutron scattering data and with previous simulations is generally very good. The results show that density functional/molecular dynamics simulations can give dynamical information of good quality without the use of fitting functions, even at long wavelengths.

Network topology for the formation of solvated electrons in binary CaO-Al2O3 composition glasses.

Glass formation in the CaO-Al2O3 system represents an important phenomenon because it does not contain typical network-forming cations. We have produced structural models of CaO-Al2O3 glasses using combined density functional theory-reverse Monte Carlo simulations and obtained structures that reproduce experiments (X-ray and neutron diffraction, extended X-ray absorption fine structure) and result in cohesive energies close to the crystalline ground states. The O-Ca and O-Al coordination numbers are similar in the eutectic 64 mol % CaO (64CaO) glass [comparable to 12CaO·7Al2O3 (C12A7)], and the glass structure comprises a topologically disordered cage network with large-sized rings. This topologically disordered network is the signature of the high glass-forming ability of 64CaO glass and high viscosity in the melt. Analysis of the electronic structure reveals that the atomic charges for Al are comparable to those for Ca, and the bond strength of Al-O is stronger than that of Ca-O, indicating that oxygen is more weakly bound by cations in CaO-rich glass. The analysis shows that the lowest unoccupied molecular orbitals occurs in cavity sites, suggesting that the C12A7 electride glass [Kim SW, Shimoyama T, Hosono H (2011) Science 333(6038):71-74] synthesized from a strongly reduced high-temperature melt can host solvated electrons and bipolarons. Calculations of 64CaO glass structures with few subtracted oxygen atoms (additional electrons) confirm this observation. The comparable atomic charges and coordination of the cations promote more efficient elemental mixing, and this is the origin of the extended cage structure and hosted solvated (trapped) electrons in the C12A7 glass.

Atomistic simulations of anionic Au144(SR)60 nanoparticles interacting with asymmetric model lipid membranes.

Experimental observations indicate that the interaction between nanoparticles and lipid membranes varies according to the nanoparticle charge and the chemical nature of their protecting side groups. We report atomistic simulations of an anionic Au nanoparticle (AuNP(-)) interacting with membranes whose lipid composition and transmembrane distribution are to a large extent consistent with real plasma membranes of eukaryotic cells. To this end, we use a model system which comprises two cellular compartments, extracellular and cytosolic, divided by two asymmetric lipid bilayers. The simulations clearly show that AuNP(-) attaches to the extracellular membrane surface within a few tens of nanoseconds, while it avoids contact with the membrane on the cytosolic side. This behavior stems from several factors. In essence, when the nanoparticle interacts with lipids in the extracellular compartment, it forms relatively weak contacts with the zwitterionic head groups (in particular choline) of the phosphatidylcholine lipids. Consequently, AuNP(-) does not immerse deeply in the leaflet, enabling, e.g., lateral diffusion of the nanoparticle along the surface. On the cytosolic side, AuNP(-) remains in the water phase due to Coulomb repulsion that arises from negatively charged phosphatidylserine lipids interacting with AuNP(-). A number of structural and dynamical features resulting from these basic phenomena are discussed. We close the article with a brief discussion of potential implications.

CO oxidation catalyzed by neutral and anionic Cu20 clusters: relationship between charge and activity.

Reactions of CO and O2 on neutral and anionic Cu20 clusters have been investigated by spin-polarized density functional theory. Three reaction mechanisms of CO oxidation are explored: reactions with atomic oxygen (dissociated O2) as well as reactions with molecular oxygen, including Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The adsorption energies, reaction pathways, and reaction barriers for CO oxidation are calculated systematically. The anionic Cu20(-) cluster can adsorb CO and O2 more strongly than the neutral counterpart due to the superatomic shell closing of 20 valence electrons which leaves one electron above the band gap. The activation of O2 molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu20 and Cu20(-) clusters, when O2 is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu20. In general, CO oxidation occurs more readily on the anionic Cu20(-) (effective reaction barriers 0.1-0.3 eV) than on the neutral Cu20 cluster (0.3-0.5 eV). Moreover, Cu20(-) exhibits enhanced binding for CO2. From the analysis of the reverse direction of CO oxidation, it is observed that the transition of CO2 to CO + O can occur on the Cu20(-) cluster, which demonstrates that Cu clusters may serve as good catalyst for CO2 chemistry.

Hydrogen evolution descriptors of 55-atom PtNi nanoclusters and interaction with graphite.

Density functional simulations have been performed for PtnNi55-nclusters (n=0,12,20,28,42,55) to investigate their catalytic properties for the hydrogen evolution reaction (HER). Starting from the icosahedralPt12Ni43, hydrogen adsorption energetics and electronicd-band descriptors indicate HER activity comparable to that of purePt55(distorted reduced core structure). The PtNi clusters accommodate a large number of adsorbed hydrogen before reaching a saturated coverage, corresponding to 3-4 H atoms per icosahedron facet (in total ∼70-80). The differential adsorption free energies are well within the window of|ΔGH|<0.1 eV which is considered optimal for HER. The electronic descriptors show similarities with the platinumd-band, although the uncovered PtNi clusters are magnetic. Increasing hydrogen coverage suppresses magnetism and depletes electron density, resulting in expansion of the PtNi clusters. For a single H atom, the adsorption free energy varies between -0.32 (Pt12Ni43) and -0.59 eV (Pt55). The most stable adsorption site is Pt-Pt bridge for Pt-rich compositions and a hollow site surrounded by three Ni for Pt-poor compositions. A hydrogen molecule dissociates spontaneously on the Pt-rich clusters. The above HER activity predictions can be extended to PtNi on carbon support as the interaction with a graphite model structure (w/o vacancy defect) results in minor changes in the cluster properties only. The cluster-surface interaction is the strongest forPt55due to its large facing facet and associated van der Waals forces.

Exceptional Microscale Plasticity in Amorphous Aluminum Oxide at Room Temperature.

Oxide glasses are an elementary group of materials in modern society, but brittleness limits their wider usability at room temperature. As an exception to the rule, amorphous aluminum oxide (a-Al2 O3 ) is a rare diatomic glassy material exhibiting significant nanoscale plasticity at room temperature. Here, it is shown experimentally that the room temperature plasticity of a-Al2 O3 extends to the microscale and high strain rates using in situ micropillar compression. All tested a-Al2 O3 micropillars deform without fracture at up to 50% strain via a combined mechanism of viscous creep and shear band slip propagation. Large-scale molecular dynamics simulations align with the main experimental observations and verify the plasticity mechanism at the atomic scale. The experimental strain rates reach magnitudes typical for impact loading scenarios, such as hammer forging, with strain rates up to the order of 1 000 s-1 , and the total a-Al2 O3 sample volume exhibiting significant low-temperature plasticity without fracture is expanded by 5 orders of magnitude from previous observations. The discovery is consistent with the theoretical prediction that the plasticity observed in a-Al2 O3 can extend to macroscopic bulk scale and suggests that amorphous oxides show significant potential to be used as light, high-strength, and damage-tolerant engineering materials.

From local structure to nanosecond recrystallization dynamics in AgInSbTe phase-change materials.

Phase-change optical memories are based on the astonishingly rapid nanosecond-scale crystallization of nanosized amorphous 'marks' in a polycrystalline layer. Models of crystallization exist for the commercially used phase-change alloy Ge(2)Sb(2)Te(5) (GST), but not for the equally important class of Sb-Te-based alloys. We have combined X-ray diffraction, extended X-ray absorption fine structure and hard X-ray photoelectron spectroscopy experiments with density functional simulations to determine the crystalline and amorphous structures of Ag(3.5)In(3.8)Sb(75.0)Te(17.7) (AIST) and how they differ from GST. The structure of amorphous (a-) AIST shows a range of atomic ring sizes, whereas a-GST shows mainly small rings and cavities. The local environment of Sb in both forms of AIST is a distorted 3+3 octahedron. These structures suggest a bond-interchange model, where a sequence of small displacements of Sb atoms accompanied by interchanges of short and long bonds is the origin of the rapid crystallization of a-AIST. It differs profoundly from crystallization in a-GST.

Machine-learned model Hamiltonian and strength of spin-orbit interaction in strained Mg2X (X = Si, Ge, Sn, Pb).

Machine-learned multi-orbital tight-binding (MMTB) Hamiltonian models have been developed to describe the electronic characteristics of intermetallic compounds Mg2Si, Mg2Ge, Mg2Sn, and Mg2Pb subject to strain. The MMTB models incorporate spin-orbital mediated interactions and they are calibrated to the electronic band structures calculated via density functional theory by a massively parallelized multi-dimensional Monte-Carlo search algorithm. The results show that a machine-learned five-band tight-binding (TB) model reproduces the key aspects of the valence band structures in the entire Brillouin zone. The five-band model reveals that compressive strain localizes the contribution of the 3sorbital of Mg to the conduction bands and the outer shellporbitals of X (X = Si, Ge, Sn, Pb) to the valence bands. In contrast, tensile strain has a reversed effect as it weakens the contribution of the 3sorbital of Mg and the outer shellporbitals of X to the conduction bands and valence bands, respectively. Theπbonding in the Mg2X compounds is negligible compared to theσbonding components, which follow the hierarchy|σsp|>|σpp|>|σss|, and the largest variation against strain belongs toσpp. The five-band model allows for estimating the strength of spin-orbit coupling (SOC) in Mg2X and obtaining its dependence on the atomic number of X and strain. Further, the band structure calculations demonstrate a significant band gap tuning and band splitting due to strain. A compressive strain of-10%can open a band gap at the Γ point in metallic Mg2Pb, whereas a tensile strain of+10%closes the semiconducting band gap of Mg2Si. A tensile strain of+5%removes the three-fold degeneracy of valence bands at the Γ point in semiconducting Mg2Ge. The presented MMTB models can be extended for various materials and simulations (band structure, transport, classical molecular dynamics), and the obtained results can help in designing devices made of Mg2X.

Steered molecular dynamics simulations of ligand-receptor interaction in lipocalins.

Retinol binding protein (RBP) and an engineered lipocalin, DigA16, have been studied using molecular dynamics simulations. Special emphasis has been placed on explaining the ligand-receptor interaction in RBP-retinol and DigA16-digoxigenin complexes, and steered molecular dynamics simulations of 10-20 ns have been carried out for the ligand expulsion process. Digoxigenin is bound deep inside the cavity of DigA16 and forms several stable hydrogen bonds in addition to the hydrophobic van der Waals interaction with the aromatic side-chains. Four crystalline water molecules inside the ligand-binding cavity remain trapped during the simulations. The strongly hydrophobic receptor site of RBP differs considerably from DigA16, and the main source of ligand attraction comes from the phenyl side-chains. The hydrogen bonds between digoxigenin and DigA16 cause the rupture forces on ligand removal in DigA16 and RBP to differ. The mutated DigA16 residues contribute approximately one-half of the digoxigenin interaction energy with DigA16 and, of these, the energetically most important are residues His35, Arg58, Ser87, Tyr88, and Phe114. Potential "sensor loops" were found for both receptors. These are the outlier loops between residues 114-121 and 63-67 for DigA16 and RBP, respectively, and they are located near the entrance of the ligand-binding cavity. Especially, the residues Glu119 (DigA16) and Leu64 (RBP) are critical for sensing. The ligand binding energies have been estimated based on the linear response approximation of binding affinity by using a previous parametrization for retinoids and RBP.