Evidence for electronic shell structure of small silver clusters in the optical absorption spectra

Temperature and Composition Dependent Structural Evolution: Thermodynamics of CunAg135-n (n = 0-135) Nanoalloys during Cooling

Molecular dynamics simulations are performed to investigate the changes of packing structures, and thermodynamic quantities including internal energy, entropy, and free energy are used to determine temperature regime and transition time of atomic packing structures. The simulation results show different packing structures as the component composition changes, and there are different packing patterns during cooling. For these Cu-Ag alloy clusters containing only a small number of atoms of Cu, they present FCC packing structures in different parts at high temperatures, and then there are transformations to icosahedral structures. With the increase in content of Cu atoms, there is a transition mechanism from molten state to icosahedron. When the content of Cu atoms is appropriate, core-shell structures can be formed at room temperature.

Chitosan-stabilized silver nanoclusters with luminescent, photothermal and antibacterial properties

The aim of this paper is to achieve in situ photochemical synthesis of silver nanoclusters (AgNCs) stabilized by the multiple-amine groups of chitosan (Ch@AgNCs) with luminescent and photothermal properties. Ch@AgNCs were obtained by applying a fast and simple methodology previously described by our group. Direct functionalization of AgNCs with chitosan template provided new nanohybrids directly in water solution, both in the presence or absence of oxygen. The formation of hybrid AgNCs could be monitored by the rapid increase of the absorption and emission maximum band with light irradiation time. New Ch@AgNCs not only present photoluminescent properties but also photothermal properties when irradiated with near infrared light (NIR), transducing efficiently NIR into heat and increasing the temperature of the medium up to 23 °C. The chitosan polymeric shell associated to AgNCs works as a protective support stabilizing the metal cores, facilitating the storage of nanohybrids and preserving luminescent, photothermal and bactericide properties.

Optical properties of size selected neutral Ag clusters: electronic shell structures and the surface plasmon resonance

We present optical absorption spectra from the ultraviolet to the visible for size selected neutral Agn clusters (n = 5-120) embedded in solid Ne. We compare the spectra to time-dependent density functional calculations (TDDFT) that address the influence of the Ne matrix. With increasing size, several highly correlated electron excitations gradually develop into a single surface plasmon. Its energy is situated between 3.9 and 4.1 eV and varies with size according to the spherical electronic shell model. The plasmon energy is highest for clusters with atom numbers fully filling states with the lowest radial quantum number (e.g. 1s, 1p, 1d,...). TDDFT calculations for clusters with several candidate geometrical structures embedded in Ne show excellent agreement with the experimental data, demonstrating that the absorption bands depend only weakly on the exact structure of the cluster.

Low-frequency polarization in molecular-scale noble-metal/metal-oxide nanocomposites

Materials with high dielectric permittivity are highly desirable in the electronics industry. One avenue for enhancing the permittivity of standard metal oxide and ceramic dielectrics is to incorporate nanoscale Ag and Au inclusions in the material. Given the small scale of modern day devices, these inclusions will necessarily be up to a few nanometers in size. We develop methodology by which polarization in nanocomposites with molecular-scale inclusions can be obtained from first-principles calculations, and partitioned into inclusion and matrix contributions. The methodology is applied to a model Ag8/MgO nanocomposite. A 4% volume loading of Ag8 nanoparticles leads to a 30% enhancement of the dielectric permittivity. The enhancement arises from both the electronic polarization of the nanoparticle and the additional polarization of matrix ions in the interfacial region.

Soft- and reactive landing of ions onto surfaces: Concepts and applications

Soft- and reactive landing of mass-selected ions is gaining attention as a promising approach for the precisely-controlled preparation of materials on surfaces that are not amenable to deposition using conventional methods. A broad range of ionization sources and mass filters are available that make ion soft-landing a versatile tool for surface modification using beams of hyperthermal (<100 eV) ions. The ability to select the mass-to-charge ratio of the ion, its kinetic energy and charge state, along with precise control of the size, shape, and position of the ion beam on the deposition target distinguishes ion soft landing from other surface modification techniques. Soft- and reactive landing have been used to prepare interfaces for practical applications as well as precisely-defined model surfaces for fundamental investigations in chemistry, physics, and materials science. For instance, soft- and reactive landing have been applied to study the surface chemistry of ions isolated in the gas-phase, prepare arrays of proteins for high-throughput biological screening, produce novel carbon-based and polymer materials, enrich the secondary structure of peptides and the chirality of organic molecules, immobilize electrochemically-active proteins and organometallics on electrodes, create thin films of complex molecules, and immobilize catalytically active organometallics as well as ligated metal clusters. In addition, soft landing has enabled investigation of the size-dependent behavior of bare metal clusters in the critical subnanometer size regime where chemical and physical properties do not scale predictably with size. The morphology, aggregation, and immobilization of larger bare metal nanoparticles, which are directly relevant to the design of catalysts as well as improved memory and electronic devices, have also been studied using ion soft landing. This review article begins in section 1 with a brief introduction to the existing applications of ion soft- and reactive landing. Section 2 provides an overview of the ionization sources and mass filters that have been used to date for soft landing of mass-selected ions. A discussion of the competing processes that occur during ion deposition as well as the types of ions and surfaces that have been investigated follows in section 3. Section 4 discusses the physical phenomena that occur during and after ion soft landing, including retention and reduction of ionic charge along with factors that impact the efficiency of ion deposition. The influence of soft landing on the secondary structure and biological activity of complex ions is addressed in section 5. Lastly, an overview of the structure and mobility as well as the catalytic, optical, magnetic, and redox properties of bare ionic clusters and nanoparticles deposited onto surfaces is presented in section 6.

Correlations of the stability, static dipole polarizabilities, and electronic properties of yttrium clusters

Static dipole polarizabilities for the ground-state geometries of yttrium clusters (Yn, n < or = 15) are investigated by using the numerically finite field method in the framework of density functional theory. The structural size dependence of electronic properties, such as the highest occupied molecular orbital-lowest occupied molecular orbital (HOMO-LUMO) gap, ionization energy, electron affinity, chemical hardness and softness, etc., has been determined for yttrium clusters. The energetic analysis, minimum polarizability principle, and principle of maximum hardness are used to characterize the stability of yttrium clusters. The correlations of stability, static dipole polarizabilities, and electronic properties are analyzed especially. The results show that static polarizability and electronic structure can reflect obviously the stability of yttrium clusters. The static polarizability per atom decreases slowly with an increase in the cluster size and exhibits a local minimum at the magic number cluster. The ratio of the mean static polarizability to the HOMO-LUMO gap has a much lower value for the most stable clusters. The static dipole polarizabilities of yttrium clusters are highly dependent on their electronic properties and are also partly related to their geometrical characteristics. A large HOMO-LUMO gap of an yttrium cluster usually corresponds to a large dipole moment. Strong correlative relationships of the ionization potential, softness, and static dipole polarizability are observed for yttrium clusters.