The optical response of Au–Ag bimetallic nanoalloys has been studied using pseudopotential time-dependent density-functional theory calculations. The structures included the magic-number icosahedral nanoparticles of 55 and 147 atoms, 37-atom pentagonal rods, and 20-atom tetrahedra. Our results show strong resonances for the pure Ag nanoparticles and strongly broadened spectra with many transitions for the pure gold structures, in qualitative agreement with available experiment and previous calculations. For bimetallic core–shell particles, the outer shell determines the overall character of the optical response; a single outer layer of Ag can produce an Ag-like resonance even in a gold-rich structure. The inclusion of a gold core within a silver shell leads to a distinct red-shift of the silver-like resonances as well as to some damping. The bimetallic nanoparticles are found to be very sensitive to the chemical configuration, the position of the atomic species in some cases outweighing the effect of chan...

Effect of Alloying on the Optical Properties of Ag–Au Nanoparticles

It is known that the surface-plasmon resonance (SPR) in small spherical Au nanoparticles of about 2 nm is strongly damped. We demonstrate that small Au nanorods with a high aspect ratio develop a strong longitudinal SPR, with intensity comparable to that in Ag rods, as soon as the resonance energy drops below the onset of the interband transitions due to the geometry. We present ab initio calculations of time-dependent density-functional theory of rods with lengths of up to 7 nm. By changing the length and width, not only the energy but also the character of the resonance in Au rods can be tuned. Moreover, the aspect ratio alone is not sufficient to predict the character of the spectrum; the absolute size matters.

Aspect-ratio- and size-dependent emergence of the surface-plasmon resonance in gold nanorods – an ab initio TDDFT study

Introduction: Bioanalytical sensing based on the principle of localized surface plasmon resonance experiences is currently an extremely rapid development. Novel sensors with new kinds of plasmonic ...

Localized surface plasmon resonance based biosensing.

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. © 2015 Wiley Periodicals, Inc. Mass Spec Rev. 00:1–41, 2015

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

The plasmonic behavior of size-selected supported silver clusters is studied by surface second harmonic generation spectroscopy for the first time. A blue shift of ∼0.2 eV in the plasmon resonance is observed with decreasing cluster size from Ag55 to Ag9. In addition to the general blue shift, a nonscalable size-dependence is also observed in plasmonic behavior of Ag nanoclusters, which is attributed to varying structural properties of the clusters. The results are in quantitative agreement with a hybrid theoretical model based on Mie theory and the existing DFT calculations.

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Plasmons in supported size-selected silver nanoclusters

A cavity ring-down spectrometer designed to investigate optical properties of size-selected clusters on surfaces under ultrahigh vacuum (UHV) conditions is presented. Clusters are produced using a laser vaporization cluster source with typical size-selected cluster currents of about 100 pA. The size of the deposition area can be controlled by means of a focusing octopole. Using the UHV compatible mirror exchanger, it is possible to have up to ten ring-down cavities and to adjust them while in vacuum. With ten cavities it is possible to cover a continuous spectral range as broad as 600 nm. The sensitivity of the method is shown to be about 5 ppm, which is two orders of magnitude better than commercial techniques. The optical spectra of small NiN clusters (N=7, 10, and 20) supported at amorphous SiO2 in the range between 417 and 669 nm are presented. Simple Mie theory calculations and oscillator-strength sum rule are in good agreement with the experimental data. The method proved to be sensitive enough for ...

Cavity ring-down spectrometer for measuring the optical response of supported size-selected clusters and surface defects in ultrahigh vacuum

The optical properties of supported gold nanoparticles with sizes of 1.3 nm, 
1.6 nm, 2.5 nm, and 2.9 nm have been studies by using cavity ring-down 
spectroscopy in the photon energy range between 1.8 eV and 3.0 eV. The 
obtained results show the possibility to obtain optical information of 
nanoassembled materials with high sensitivity. The experimental findings are 
compared to calculations using Mie-Drude theory. Whereas the broadening of 
the surface plasmon resonance with decreasing size is well described by this 
model, the observed blue-shift of the surface plasmon resonance contradicts 
the predictions of the Mie-Drude theory. The latter effect can be explained 
by the presence of a skin region with decreased polarizability typical for 
coinage metal particles. Furthermore, it is found that the supported gold 
nanoparticles are robust under ambient conditions, an important issue when 
using these materials for optical applications.

J. Peter

Papers

Cavity ring-down spectrometer for measuring the optical response of supported size-selected clusters and surface defects in ultrahigh vacuum

Cavity ring-down spectroscopy of metallic gold nanoparticles