Understanding MOF Flexibility: An Analysis Focused on Pillared Layer MOFs as a Model System.

Flexible porous frameworks are at the forefront of materials research. A unique feature is their ability to open and close their pores in an adaptive manner induced by chemical and physical stimuli. Such enzyme-like selective recognition offers a wide range of functions ranging from gas storage and separation to sensing, actuation, mechanical energy storage and catalysis. However, the factors affecting switchability are poorly understood. In particular, the role of building blocks, as well as secondary factors (crystal size, defects, cooperativity) and the role of host-guest interactions, profit from systematic investigations of an idealized model by advanced analytical techniques and simulations. The review describes an integrated approach targeting the deliberate design of pillared layer metal-organic frameworks as idealized model materials for the analysis of critical factors affecting framework dynamics and summarizes the resulting progress in their understanding and application.

Freeze-Thaw-Promoted Fabrication of Clean and Hierarchically Structured Noble-Metal Aerogels for Electrocatalysis and Photoelectrocatalysis.

Noble-metal aerogels (NMAs) have drawn increasing attention because of their self-supported conductive networks, high surface areas, and numerous optically/catalytically active sites, enabling their impressive performance in diverse fields. However, the fabrication methods suffer from tedious procedures, long preparation times, unavoidable impurities, and uncontrolled multiscale structures, discouraging their developments. By utilizing the self-healing properties of noble-metal aggregates, the freezing-promoted salting-out behavior, and the ice-templating effect, a freeze-thaw method is crafted that is capable of preparing various hierarchically structured noble-metal gels within one day without extra additives. In light of their cleanliness, the multi-scale structures, and combined catalytic/optical properties, the electrocatalytic and photoelectrocatalytic performance of NMAs are demonstrated, which surpasses that of commercial noble-metal catalysts.

Diffusion- and reaction-limited cluster aggregation revisited.

We simulated irreversible aggregation of non-interacting particles and of particles interacting via repulsive and attractive potentials explicitly implementing the rotational diffusion of aggregating clusters. Our study confirms that the attraction between particles influences neither the aggregation mechanism nor the structure of the aggregates, which are identical to those of non-interacting particles. In contrast, repulsive particles form more compact aggregates and their fractal dimension and aggregation times increase with the decrease of the temperature. A comparison of the fractal dimensions obtained for non-rotating clusters of non-interacting particles and for rotating clusters of repulsive particles provides an explanation for the conformity of the respective values obtained earlier in the well established model of diffusion-limited cluster aggregation neglecting rotational diffusion and in experiments on colloidal particles.

Optics, mechanics, and energetics of two-dimensional MoS2 nanostructures from a theoretical perspective.

CONSPECTUS: Nanostructures based on molybdenum disulfide (MoS2) are by far the most common and well-studied systems among two-dimensional (2D) semiconducting materials. Although still being characterized as a "promising material", catalytic activity of MoS2 nanostructures has been found, and applications in lubrication processes are pursued. Because exfoliation techniques have improved over the past years, monolayer MoS2 is easily at hand; thus, experimental studies on its electronic properties and applicability are in scientific focus, and some MoS2-based electronic devices have been reported already. Additionally, the improvement of atomic force microscopy led to nanoindentation experiments, in which the exceptional mechanical properties of MoS2 could be confirmed. In this Account, we review recent results from density-functional based calculations on several MoS2-based nanostructures; we have chosen to follow several experimental routes focusing on different nanostructures and their specific properties. MoS2-based triangular nanoflakes are systems that are experimentally well described and studied with a special focus on their optical absorption. The interpretation of our calculations fits well to the experimental picture: the absorption peaks in the visible light range show a quantum-confinement effect; they originate from excitations into the edge states. Additionally, delocalized metallic-like states are present close to the Fermi level, which do not contribute to photoabsorption in the visible range. Additionally, nanoindentation experiments have been simulated to obtain mechanical properties of the MoS2 material and to study the influence of deformation on the system's electronics. In these molecular-dynamics simulations, a tip penetrates a MoS2 monolayer, and the obtained Young's modulus and breaking stress agree very well with experimentally obtained values. Whereas the structural properties, such as bond lengths and layer contraction, vary locally differently upon indentation, the electronic structure in terms of the density of states, the gap between occupied and unoccupied states, or the quantum transport change only slightly. The robustness of the material with respect to electronic and mechanical properties makes monolayer MoS2 special. However, it is important to note that this robustness refers to a local disturbance through deformation and still seems to be dependent on the defect concentration. Finally, we present a comparison of the thermodynamic stabilities of different MoS2-based nanostructures with a focus on nanoflakes, fullerene-like nanooctahedra, and smaller Chevrel-type and non-Chevrel-type clusters (nanowires). All studied systems are stable in comparison to MoS2, Mo bulk, and the S8 crown, but only the studied nanoflakes and nanowires show specific stoichiometries, either sulfur-rich or sulfur-poor, whereas the nanooctahedra may adopt both. From the thermodynamic stabilities, it should be possible to deliberately choose specific nanostructures by thoughtful choices of the synthesis conditions. In conclusion, we present in this Account exceptional properties of MoS2-based nanostructures studied by means of density-functional theory. The focus lies on optical absorption in the visible range observed in triangular nanoflakes, which originate in the system's edge states, the robustness of monolayer MoS2 with respect to punctual loads regarding both mechanical and electronic properties, and the thermodynamic stability of most studied MoS2-based nanosystems revealing a correlation between composition and preferred morphology, particularly for 2D systems.

Is MoS2 a robust material for 2D electronics?

A nanoindentation computer experiment has been carried out by means of Born-Oppenheimer molecular-dynamics simulations employing the density-functional based tight-binding method. A free-standing MoS2 sheet, fixed at a circular support, was indented by a stiff, sharp tip. During this process, the strain on the nanolayer is locally different, with maximum values in the vicinity of the tip. All studied electronic properties-the band gap, the projected density of states, the atomic charges and the quantum conductance through the layer-vary only slightly before they change significantly when the MoS2 sheet finally is pierced. After strong local deformation due to the indentation process, the electronic conductance in our model still is 80% of its original value. Thus, the electronic structure of single-layer MoS2 is rather robust upon local deformation.

Isotope-selective pore opening in a flexible metal-organic framework.

Flexible metal-organic frameworks that show reversible guest-induced phase transitions between closed and open pore phases have enormous potential for highly selective, energy-efficient gas separations. Here, we present the gate-opening process of DUT-8(Ni) that selectively responds to D2, whereas no response is observed for H2 and HD. In situ neutron diffraction directly reveals this pressure-dependent phase transition. Low-temperature thermal desorption spectroscopy measurements indicate an outstanding D2-over-H2 selectivity of 11.6 at 23.3 K, with high D2 uptake. First-principles calculations coupled with statistical thermodynamics predict the isotope-selective gate opening, rationalized by pronounced nuclear quantum effects. Simulations suggest DUT-8(Ni) to remain closed in the presence of HT, while it also opens for DT and T2, demonstrating gate opening as a highly effective approach for isotopolog separation.

Disturbance-Promoted Unconventional and Rapid Fabrication of Self-Healable Noble Metal Gels for (Photo-)Electrocatalysis.

As an emerging class of porous materials, noble metal aerogels (NMAs) have drawn tremendous attention and displayed unprecedented potential in diverse fields. However, the development of NMAs is impeded by the fabrication methods because of their time- and cost-consuming procedures, limited generality, and elusive understanding of the formation mechanisms. Here, by revealing the self-healing behavior of noble metal gels and applying it in the gelation process at a disturbing environment, an unconventional and conceptually new strategy, i.e., a disturbance-promoted gelation method, is developed by introducing an external force field. It overcomes the diffusion limitation in the gelation process, thus producing monolithic gels within 1-10 min at room temperature, 2-4 orders of magnitude faster than for most reported methods. Moreover, versatile NMAs are acquired by using this method, and their superior (photo-)electrocatalytic properties are demonstrated for the first time in light of combined catalytic and optic properties.

Aspects of the proton transfer in liquid phosphonic acid.

Born-Oppenheimer molecular dynamics simulations have been performed to investigate the proton-transfer mechanism in liquid phosphonic acid at different temperatures. The transfers are observed if additional charge carriers are present. The liquid phosphonic acid has been characterized by pair-distribution functions, self-diffusion coefficients, and hopping rates. Moreover, we find that the proton transfer is prepared by the following necessary local geometrical criteria: (1) an O-H-O angle close to 180 degrees and (2) an O-O distance lower than 2.5 A. However, we observe in many cases a Zundel-like species, in which an excess proton is shared, complexed, and stabilized by two adjacent phosphonic acid molecules. These unsuccessful attempts to transfer a proton fulfill the same local criteria as successful transfers, and thus, we conclude that nonlocal requirements may play an important role for the success of the transfer too.

Specific ion effects directed noble metal aerogels: Versatile manipulation for electrocatalysis and beyond.

Noble metal foams (NMFs) are a new class of functional materials featuring properties of both noble metals and monolithic porous materials, providing impressive prospects in diverse fields. Among reported synthetic methods, the sol-gel approach manifests overwhelming advantages for versatile synthesis of nanostructured NMFs (i.e., noble metal aerogels) under mild conditions. However, limited gelation methods and elusive formation mechanisms retard structure/composition manipulation, hampering on-demand design for practical applications. Here, highly tunable NMFs are fabricated by activating specific ion effects, enabling various single/alloy aerogels with adjustable composition (Au, Ag, Pd, and Pt), ligament sizes (3.1 to 142.0 nm), and special morphologies. Their superior performance in programmable self-propulsion devices and electrocatalytic alcohol oxidation is also demonstrated. This study provides a conceptually new approach to fabricate and manipulate NMFs and an overall framework for understanding the gelation mechanism, paving the way for on-target design of NMFs and investigating structure-performance relationships for versatile applications.

The influence of C2 dimers on the stability of Ti(m)C(n) metcar clusters.

We present results of an investigation of small Ti(m)C(n) clusters with different stoichiometries in order to throw light on the occurrence of carbon dimers in these structures. Previous studies of transition metal (M) metallocarbohedrene (metcar) clusters M(m)C(n) have proposed that C(2) dimers play a special role. In the special case of Ti(m)C(n) metcars these dimers have been observed in several studies. We shall show that clusters containing C(2) dimers are energetically favored with respect to those containing only single carbon atoms or trimers, especially when the dimers occupy the corner positions of cubic clusters. Moreover, we find that cubic structures are more stable than corresponding double-cage metcars. Finally, a highly symmetric Ti(6)C(10) metcar cluster is presented and proposed to be the global-minimum structure of this stoichiometry.

Two-dimensional and tubular structures of misfit compounds: Structural and electronic properties.

Misfit layer compounds are structures that consist of two sublattices differing in at least one of their lattice constants. The two different layers are stacked either an alternating or in a more complex series resulting in mono- or multi-layer misfit compounds. To date, planar and bent misfit structures, such as tubes, scrolls or nanoparticles, have been synthesized and interesting magnetic and physical properties have been observed as a result of their special structures. Based on these observations, we present an overview of such misfit systems and summarize and discuss their electronic structure as well as the interlayer bonding behaviour, which is not completely understood yet. Furthermore, a more detailed insight into the SnS-SnS2 system is given, which was the first tubular misfit compound that has been synthesized and extensively investigated.

Optical Properties of Triangular Molybdenum Disulfide Nanoflakes.

The results from calculations of optical and electronic properties of triangular MoS2 nanoflakes with edge lengths ranging from 1.6 to 10.4 nm are presented. The optical spectra were calculated using the time-dependent extension of the density-functional tight-binding method (TD-DFTB). The size effect in the optical absorption spectra is clearly visible. With decreasing length of the nanoflakes edges, the long-wavelength absorption in the range of visible light is shifted toward short-wavelength absorption, confirming a quantum-confinement-like behavior of these flakes. In contrast, the edges of the nanoflakes exhibit a distinct metallic-like behavior. The relation of the absorption properties to the observed photoluminescence of MoS2 nanoflakes is discussed in a qualitative manner.

The virtues of magnetism.

Dislocations at grain boundaries in semiconducting two-dimensional transition-metal dichalcogenides have been found to be the origin of substantial intrinsic magnetic moments. Here, we introduce a first-principles study by Yakobson and co-workers, published in this issue of ACS Nano that shows the influence of grain-boundary tilt angle and doping level on the resulting magnetic moments. This specific property may help in designing new two-dimensional magnetic semiconductors.

Experimental and theoretical investigations of the ligand structure of water-soluble CdTe nanocrystals.

The unique optical properties, such as size-tunable absorption and emission, caused semiconductor nanocrystals to attract a great deal of interest for recent technological developments. For the evaluation of semiconductor nanocrystals as new materials for various applications like optoelectronic devices, knowledge of the structure-property relationships is indispensable, but still presents a challenge. Here, we address these challenges for thioglycolic acid-capped CdTe nanocrystals with a focus on the quantification of thiol ligands, identification of the ligand shell structure and their influence on the optical properties of these nanocrystals. We present the use of a simple analytical technique, the Ellman's test, and ICP-OES analysis for the study of the surface chemistry of these nanomaterials. Together with theoretical calculations, the results of these studies show the strong influence of the amount of Cd-thiolates present in the ligand shell on the concentration-dependent emission properties, thereby providing the basis for a better understanding of the chemical nature of the NC-ligand interface. In this context, the present work contributes to the establishment of a clearer picture and better control of the surface chemistry, which will provide the basis for the design of highly emitting nanocrystals and the prediction of their applicability.

Photoabsorption in sodium clusters on the basis of time-dependent density-functional theory.

The photoabsorption spectra of a continuous series of Na(n) clusters (n