First-principles investigation of ag-doped gold nanoclusters

Gold nanoclusters have the tunable optical absorption property, and are promising for cancer cell imaging, photothermal therapy and radiotherapy. First-principle is a very powerful tool for design of novel materials. In the present work, structural properties, band gap engineering and tunable optical properties of Ag-doped gold clusters have been calculated using density functional theory. The electronic structure of a stable Au(20) cluster can be modulated by incorporating Ag, and the HOMO-LUMO gap of Au(20-) (n)Ag(n) clusters is modulated due to the incorporation of Ag electronic states in the HOMO and LUMO. Furthermore, the results of the imaginary part of the dielectric function indicate that the optical transition of gold clusters is concentration-dependent and the optical transition between HOMO and LUMO shifts to the low energy range as the Ag atom increases. These calculated results are helpful for the design of gold cluster-based biomaterials, and will be of interest in the fields of radiation medicine, biophysics and nanoscience.     

Evaluation of optical and electronic properties of silicon nano-agglomerates embedded in SRO: applying density functional theory

In systems in atomic scale and nanoscale such as clusters or agglomerates constituted by particles from a few to less than 100 atoms, quantum confinement effects are very important. Their optical and electronic properties are often dependent on the size of the systems and the way in which the atoms in these clusters are bonded. Generally, these nanostructures display optical and electronic properties significantly different to those found in corresponding bulk materials. Silicon agglomerates embedded in silicon rich oxide (SRO) films have optical properties, which have been reported to be directly dependent on silicon nanocrystal size. Furthermore, the room temperature photoluminescence (PL) of SRO has repeatedly generated a huge interest due to its possible applications in optoelectronic devices. However, a plausible emission mechanism has not been widely accepted in the scientific community. In this work, we present a short review about the experimental results on silicon nanoclusters in SRO considering different techniques of growth. We focus mainly on their size, Raman spectra, and photoluminescence spectra. With this as background, we employed the density functional theory with a functional B3LYP and a basis set 6-31G* to calculate the optical and electronic properties of clusters of silicon (constituted by 15 to 20 silicon atoms). With the theoretical calculation of the structural and optical properties of silicon clusters, it is possible to evaluate the contribution of silicon agglomerates in the luminescent emission mechanism, experimentally found in thin SRO films.     

Investigating the structural evolution of thiolate protected gold clusters from first-principles

Unlike bulk materials, the physicochemical properties of nano-sized metal clusters can be strongly dependent on their atomic structure and size. Over the past two decades, major progress has been made in both the synthesis and characterization of a special class of ligated metal nanoclusters, namely, the thiolate-protected gold clusters with size less than 2 nm. Nevertheless, the determination of the precise atomic structure of thiolate-protected gold clusters is still a grand challenge to both experimentalists and theorists. The lack of atomic structures for many thiolate-protected gold clusters has hampered our in-depth understanding of their physicochemical properties and size-dependent structural evolution. Recent breakthroughs in the determination of the atomic structure of two clusters, [Au(25)(SCH(2)CH(2)Ph)(18)](q) (q = -1, 0) and Au(102)(p-MBA)(44), from X-ray crystallography have uncovered many new characteristics regarding the gold-sulfur bonding as well as the atomic packing structure in gold thiolate nanoclusters. Knowledge obtained from the atomic structures of both thiolate-protected gold clusters allows researchers to examine a more general "inherent structure rule" underlying this special class of ligated gold nanoclusters. That is, a highly stable thiolate-protected gold cluster can be viewed as a combination of a highly symmetric Au core and several protecting gold-thiolate "staple motifs", as illustrated by a general structural formula [Au](a+a')[Au(SR)(2)](b)[Au(2)(SR)(3)](c)[Au(3)(SR)(4)](d)[Au(4)(SR)(5)](e) where a, a', b, c, d and e are integers that satisfy certain constraints. In this review article, we highlight recent progress in the theoretical exploration and prediction of the atomic structures of various thiolate-protected gold clusters based on the "divide-and-protect" concept in general and the "inherent structure rule" in particular. As two demonstration examples, we show that the theoretically predicted lowest-energy structures of Au(25)(SR)(8)(-) and Au(38)(SR)(24) (-R is the alkylthiolate group) have been fully confirmed by later experiments, lending credence to the "inherent structure rule".     

稀土元素在陶瓷材料中的应用

针对我国稀土资源得天独厚的现状,从稀土元素的原子结构和化学特性出发,较详细地阐述了稀土元素在Al2O3、Si3N4、ZrO2等结构陶瓷以及介电、压电、导电陶瓷等功能陶瓷和陶瓷色釉料中的应用以及作用机理。     

Defect Structures of Tin-Doped Indium Oxide

Defect structures associated with tin doping of indium oxide, an optically transparent conductor, have been characterized by atomistic simulations and first-principles density functional calculations. A comprehensive survey of defect clusters containing up to three tin dopants in the first and second cationic coordination shells of an oxygen interstitial has been conducted. The analysis of energetically favorable defects gives insights into the role and nature of defect clusters in the material. In particular, the origins of the experimentally postulated b-site preference of tin dopants have been examined. Our results show that b-site preference occurs only in defect clusters with oxygen interstitials and is not intrinsic to dopants. In contrast, in nearest coordination to an interstitial, a strong d-site preference is found. Density functional calculations in the discrete variational-embedded cluster approximation have been conducted on selected defect structures to illuminate the effect of clustering on partial atomic charges, bond-orders, and ¹¹⁹Sn Mössbauer parameters.     

氧化锆性质及其应用前景概述

氧化锆及制品是结构陶瓷、导电陶瓷、功能陶瓷、生物陶瓷的主要原料之一。氧化锆具有很多特性,如稳定的化学性质、熔点高、耐高温、热膨胀系数小、热稳定性好、可塑性好等。氧化锆性质的优越性,决定了氧化锆在军工、核反应、原子能领域、能源材料、冶金耐火材料以及电子、通讯、汽车、机械等高新技术领域的应用前景十分广泛。     

Structural and Kinetic DFT Characterization of Materials to Rationalize Catalytic Performance

This review shortly discusses recent results obtained by the application of density functional theory for the calculations of the adsorption and diffusion properties of small molecules and their reactivity on heterogenous catalytic systems, in the ambit of the Nanocat project. Particular focus has been devoted to palladium catalysts, either in atomic or small cluster form. Some protocols have been tested to obtain efficient ways able to treat the electronic and geometric influence of supports like zeolites and carbon nanotubes on the catalytic properties of palladium. The hydroisomerization of cis-but-2-ene is discussed as model reaction on supported and unsupported Pd clusters. Some preliminary results on the structural investigation of systems formed by a palladium clusters and block copolymers are also presented.     

In-Situ Configuration Studies on Segmented DNA Origami Nanotubes

One-dimensional nanotubes are of considerable interest in materials and biochemical sciences. A particular desire is to create DNA nanotubes with user-defined structural features and biological relevance, which will facilitate the application of these nanotubes in the controlled release of drugs, templating of other materials into linear arrays and the construction of artificial membrane channels. However, little is known about the structures of assembled DNA nanotubes in solution. Here we report an in situ exploration of segmented DNA nanotubes, composed of multiple units with set length distributions, by using synchrotron small-angle X-ray scattering (SAXS). Through joint experimental and theoretical studies, we show that the SAXS data are highly informative in the context of heterogeneous mixtures of DNA nanotubes. The structural parameters obtained by SAXS are in good agreement with those determined by atomic force microscopy (AFM), transmission electron microscopy (TEM), and dynamic light scattering (DLS). In particular, the SAXS data revealed important structural information on these DNA nanotubes, such as the in-solution diameters (≈25 nm), which could be obtained only with difficulty by use of other methods. Our results establish SAXS as a reliable structural analysis method for long DNA nanotubes and could assist in the rational design of these structures.     

Mixed monolayer protected gold atom-oxide cluster synthesis and characterization

Small atomic gold clusters in solution, Au(n), stabilized by cetyl trimethylammonium bromide (CTAB) and cysteine, have been synthesized potentiodynamically in quiescent aqueous solutions. The electrodissolution of gold to gold ions during an anodic scan and subsequent cluster formation during a cathodic scan in underpotential (UPDD) and overpotential dissolution-deposition (OPDD) regions were studied. The experimental potentiodynamic I-E profiles and chronoamperometric i-t transients are fit into reported theoretical models of adsorption and electrocrystallization. The plausible application of clusters/cluster film to cysteine sensing based on fluorescence quenching and square wave stripping voltammetry is demonstrated.     

Effects of chemical composition on the structural stability,elastic, vibrational,and electronic properties of Cs2NaLnX6 (Ln = La…Lu,X = F,Cl, Br,I) elpasolites

Elpasolite crystals are very important materials, both from the applied and fundamental points of view. Those elpasolites, which contain rare earth ions with a high atomic number Z, are very much suitable for the low-cost high-performance gamma-ray detection, applications in medicine, food industry, nuclear energy production, processing, and detection of nuclear proliferation. The thermal and structural stabilities are important parameters required for detecting applications, because the performance conditions for such devices are usually very harsh. Since it is widely believed that elpasolites may have even better detection properties, the lack of systematic studies on the elpasolites and thus the unavailability of reliable data on their physical properties and trends in their variation caused by chemical composition considerably hinders search for more efficient new materials. Therefore, to fill in this gap and provide with all essential information about a large number of elpasolites crystals, for the first time, the structural stability, elastic, vibrational, and electronic properties of 60 cubic elpasolite Cs₂NaLnX₆ (Ln = La, …, Lu, X = F, Cl, Br, I) crystals were consistently calculated in the framework of the same computational approach based on the density functional theory (DFT). Variation of all calculated parameters (such as the lattice constants, elastic constants, Debye temperature, normal vibrational modes frequencies, Mulliken effective charges, bond populations, and band gaps) across the considered groups of crystals was analyzed and several trends, which are important for the search and preparation of new stable materials with improved performance, were identified.     

C-H bond activation by oxygen-centered radicals over atomic clusters

Saturated hydrocarbons, or alkanes, are major constituents of natural gas and oil. Directly transforming alkanes into more complex organic compounds is a value-adding process, but the task is very difficult to achieve, especially at low temperature. Alkanes can react at high temperature, but these reactions (with oxygen, for example) are difficult to control and usually proceed to carbon dioxide and water, the thermodynamically stable byproducts. Consequently, a great deal of research effort has been focused on generating and studying chemical entities that are able to react with alkanes or efficiently activate C-H bonds at lower temperatures, preferably room temperature. To identify low-temperature methods of C-H bond activation, researchers have investigated free radicals, that is, species with open-shell electronic structures. Oxygen-centered radicals are typical of the open-shell species that naturally occur in atmospheric, chemical, and biological systems. In this Account, we survey atomic clusters that contain oxygen-centered radicals (O(-?)), with an emphasis on radical generation and reaction with alkanes near room temperature. Atomic clusters are an intermediate state of matter, situated between isolated atoms and condensed-phase materials. Atomic clusters containing the O(-?) moiety have generated promising results for low-temperature C-H bond activation. After a brief introduction to the experimental methods and the compositions of atomic clusters that contain O(-?) radicals, we focus on two important factors that can dramatically influence C-H bond activation. The first factor is spin. The O(-?)-containing clusters have unpaired spin density distributions over the oxygen atoms. We show that the nature of the unpaired spin density distribution, such as localization and delocalization within the clusters, heavily influences the reactivity of O(-?) radicals in C-H bond activation. The second factor is charge. The O(-?)-containing clusters can be negatively charged, positively charged, or neutral overall. We discuss how the charge state may influence C-H bond activation. Moreover, for a given charge state, such as the cationic state, it can be demonstrated that local charge distribution around the O(-?) centers can also significantly change the reactivity in C-H bond activation. Through judicious synthetic choices, spin and charge can be readily controllable physical quantities in atomic clusters. The adjustment of these two properties can impact C-H bond activation, thus constituting an important consideration in the rational design of catalysts for practical alkane transformations.     

Structural genomics of proteins involved in copper homeostasis

Genome sequencing projects have provided a wealth of data, most notably the primary sequences of all the proteins that a given organism can produce. The understanding of this information at the functional level is still in the beginning stages. Three-dimensional structural information is necessary to unravel at the atomic level the mechanisms by which a protein carries out its function, and such information can often be very useful to predict at least gross functional features, even in the absence of biochemical data. An exhaustive structural characterization of the proteins encoded in the genomes is thus highly desirable. To enhance the functional insights provided by genome-scale structural determination, we have prioritized our research to target specific processes of the cell, i.e., those responsible for controlling metal homeostasis. In this Account, we present the results obtained by the Magnetic Resonance Center of the University of Florence on proteins involved in the homeostasis of copper. The general research strategy is presented, followed by a discussion focused on different key experimental aspects. An overview of the initial results and of their relevance to the understanding of molecular function and cellular processes is also given.     

Synthesis and characterization of acrylic-based superabsorbents

This paper is devoted to the synthesis and characterization of superabsorbent polymers based on acrylic acid. These hydrogels were prepared by carrying the inverse suspension polymerization in an aromatic hydrocarbon. The dispersion is stabilized by the mixture of micromolecular and macromolecular emulsifiers. To obtain high swelling and appropriate absorption kinetics, parameters such as initial monomer and cross-linker concentration, range of neutralization, monomer addition rate, temperature, initiating system, stabilizing system, and nature of the organic phase were studied but not reviewed here. Thus, based on these topics a basic formula is obtained to display the effect of some structural parameters on behaviour of superabsorbents utilized. © 1993 John Wiley & Sons, Inc.     

Oxygen plasma modification of submicron vapor grown carbon fibers as studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed to monitor the changes in surface structure induced by oxygen plasma treatments of submicron vapor grown carbon fibers (VGCFs). It is shown that the fibers preserve their general smoothness upon plasma oxidation and that the structural changes brought about by this treatment essentially take place only at the atomic scale, where the relatively ordered domains typical of the untreated material are replaced by atomically rough and disordered structures. These atomic-scale changes imply the modification of some physico-chemical properties of the fiber surface, such as concentration of oxygen functionalities. The STM results, together with those obtained from nitrogen physical adsorption measurements, suggest that the potential improvement of plasma treatment in VGCF-matrix adhesion for application in composite materials should proceed mainly from chemical bonding due to the addition of functional groups rather than from increased mechanical interlocking.     

Platinum cross-linked chitosan hydrogels synthesized in water saturated with CO2 under high pressure

The creation of biocompatible composite hydrogels from renewable biopolymers with stabilized functional platinum nanoparticles seems to be an important scientific task. The formation of such hydrogels in a unique medium of carbonic acid under high pressure CO₂ is promising due to its sterilizing ability under pressure, biocompatibility after decompression and environmental friendliness. In the present work, stable chitosan hydrogels with platinum nanoparticles were obtained in such solutions. The hydrogel nature of the composites was confirmed by rotational rheology. The physicochemical characteristics were studied using Fourier transform infrared, X-ray photoelectron, ultraviolet–visible spectroscopies, transmission electron, scanning electron and atomic force microscopies. It was found that nanoparticles of oxidized Pt of approximately 4.5 nm in size are stabilized by chitosan in the composite. The resulting chitosan/Pt hydrogels were also tested for antimicrobial activity and shown strong activity against Gram-positive bacteria (B. subtilis and B. coagulans) and slight activity against E. coli.     

The controlled preparation and stability mechanism of partially stabilized zirconia by microwave intensification

Partially stabilized zirconia (PSZ) occupies an important application portion in ceramics materials and refractories materials. In this work, calcium oxide-partially stabilized zirconia (CaO-PSZ) ceramics were prepared from fused zirconia by microwave sintering, with its microstructure and stability properties characterized by XRD and SEM. Results indicated that the heating rate, cooling rate, quenching temperature and isothermal treatment time rendered different influence on the stability properties, which was mainly ascribed to the reversible martensitic transformation of zirconia ceramics. Additionally, a mixed-phase composed by cubic phase ZrO₂ (c-ZrO₂) and monoclinic phase ZrO₂ (m-ZrO₂) appeared after fused zirconia treated by microwave sintering at 1450 °C for 2 h, indicating the formation of CaO-PSZ ceramics, which the finding was consistent with the SEM and EDAX analysis. Meanwhile, CaO stabilizer precipitated behavior at the crystal boundary, with the formation of acicular grains and fine particles, further rendering a toughening effect to CaO-PSZ ceramics. This work can provide important theoretical and practical significance for applications of microwave sintering to prepare CaO-PSZ ceramics material, even extending further applications in functional materials and structural materials.     

Cluster-assembled metallic glasses

A bottom-up approach to nanofabricate metallic glasses from metal clusters as building blocks is presented. Considering metallic glasses as a subclass of cluster-assembled materials, the relation between the two lively fields of metal clusters and metallic glasses is pointed out. Deposition of selected clusters or collections of them, generated by state-of-the-art cluster beam sources, could lead to the production of a well-defined amorphous material. In contrast to rapidly quenched glasses where only the composition of the glass can be controlled, in cluster-assembled glasses, one can precisely control the structural building blocks. Comparing properties of glasses with similar compositions but differing in building blocks and therefore different in structure will facilitate the study of structure–property correlation in metallic glasses. This bottom-up method provides a novel alternative path to the synthesis of glassy alloys and will contribute to improving fundamental understanding in the field of metallic glasses. It may even permit the production of glassy materials for alloys that cannot be quenched rapidly enough to circumvent crystallization. Additionally, gaining deeper insight into the parameters governing the structure–property relation in metallic glasses can have a great impact on understanding and design of other cluster-assembled materials.     

Heterometallic cluster-based organic frameworks as highly active electrocatalysts for oxygen reduction and oxygen evolution reaction: a density functional theory study

Recently, metal–organic frameworks are one of the potential catalytic materials for electrocatalytic applications. The oxygen reduction reaction and oxygen evolution reaction catalytic activities of heterometallic cluster-based organic frameworks are investigated using density functional theory. Firstly, the catalytic activities of heterometallic clusters are investigated. Among all heterometallic clusters, Fe₂Mn–Mn has a minimum overpotential of 0.35 V for oxygen reduction reaction, and Fe₂Co–Co possesses the smallest overpotential of 0.32 V for oxygen evolution reaction, respectively 100 and 50 mV lower than those of Pt(111) and RuO₂(110) catalysts. The analysis of the potential gap of Fe₂M clusters indicates that Fe₂Mn, Fe₂Co, and Fe₂Ni clusters possess good bifunctional catalytic activity. Additionally, the catalytic activity of Fe₂Mn and Fe₂Co connected through 3,3′,5,5′-azobenzenetetracarboxylate linker to form Fe₂M–PCN–Fe₂M is explored. Compared with Fe₂Mn–PCN–Fe₂Mn, Fe₂Co–PCN–Fe₂Co, and isolated Fe₂M clusters, the mixed-metal Fe₂Co–PCN–Fe₂Mn possesses excellent bifunctional catalytic activity, and the values of potential gap on the Mn and Co sites of Fe₂Co–PCN–Fe₂Mn are 0.69 and 0.70 V, respectively. Furthermore, the analysis of the electron structure indicates that constructing a mixed-metal cluster can efficiently enhance the electronic properties of the catalyst. In conclusion, the mixed-metal cluster strategy provides a new approach to further design and synthesize high-efficiency bifunctional electrocatalysts.     

The interface chemistry between chalcogenide clusters and open framework chalcogenides

One of the most exciting recent developments concerning molecular architectures is the emerging field of crystalline chalcogenide superlattices that bridges two traditional but distinct areas of research: chalcogenide clusters and porous materials. By combining synthetic and structural concepts in these two areas, many crystalline solids containing spatially organized chalcogenide clusters have been created that exhibit varied properties ranging from microporosity, fast ion conductivity, and photoluminescence to narrow and tunable electronic band gaps. The potential applications of these materials extend beyond traditional areas such as acid catalysis or adsorption-based separation to include shape- or size-selective photocatalysis, solid-state ionics, and electrochemistry.