Size-selected Ag Research Articles

Photoelectron imaging of size-selected metal cluster anions in a quasi-continuous mode.

We present a novel high-repetition-rate photoelectron imaging (PEI) apparatus for exploring electronic structures of metal cluster anions. A continuous beam of mass-selected metal cluster anions, generated by a magnetron-sputtering cluster-ion source coupled with a quadrupole mass filter, is chopped into sub-megahertz ion bunches using a high-voltage pulser. The quasi-continuous anion beam is introduced into a PEI spectrometer, where the anions are photodetached using a 404nm (3.07eV) continuous-wave laser diode. As a demonstration, we acquire photoelectron images for size-selected Ag cluster anions, AgN - (N = 3, 7, 14), and show that each image can be obtained in a short accumulation time (50s) with a kinetic energy resolution (ΔE/E) of 4% at E = 1.77eV. The quasi-continuous PEI technique enables high-count-rate, space-charge-free acquisition of photoelectron spectra and angular distributions not only from size-selected metal cluster anions but also from anions prepared by other continuous ion sources, such as electrospray ionization.

Oxidation at the sub-nanoscale: oxygen adsorption on graphene-supported size-selected Ag clusters

Oxidizing size-selected Ag11 clusters on graphene reveals the formation of a bulk-like structure already for few-atom aggregates. The appearance of a bulk AgO crystal structure differs from that observed for metallic clusters of similar size.

Wettability Transition on the Fractal Surface Developed by Size-Selected Ag Nanoclusters

Wettability Transition on the Fractal Surface Developed by Size-Selected Ag Nanoclusters

Ecofriendly and environment-friendly synthesis of size-controlled silver nanoparticles/graphene composites for antimicrobial and SERS actions

Chemical and physical properties of silver nanoparticles (Ag NPs) are strongly influenced by NP size. The control of NP size is crucial for tuning or enhancing the properties of Ag NPs. In this paper, the size controlled Ag NPs have been achieved and dispersed on graphene (GE) sheets (Ag NPs/GE) using an eco-friendly and environment-friendly approach of thermal annealing method. The formation of size controlled Ag NPs has been restrained and it includes two stages. One is the formation of metallic Ag by reduction of Ag+ at low temperature (below 400 °C), wherein the ultrafine and highly dispersed Ag NPs can be obtained; another is growth and aggregation of Ag NPs at high temperature due to increasing surface diffusion coefficient of Ag atoms (above 400 °C). The Ag NPs/GE-400 presented an excellent antimicrobial activity due to the ultrafine size (∼9.6 nm), while the Ag NPs/GE-700 exhibited an excellent SERS activity and sensitivity because of the appreciate size (∼95.8 nm). The Ag NPs/GE-500 (∼25.3 nm) presented the modest antimicrobial and SERS properties. Thus, our work may provide a reference to synthesize size-selected Ag NPs-based materials with a certain application using the thermal annealing method.

Origin and Dynamics of Highly Efficient Broadband Photoluminescence of Aqueous Glutathione-Capped Size-Selected Ag–In–S Quantum Dots

The 2–3 nm size-selected glutathione-capped Ag–In–S (AIS) and core/shell AIS/ZnS quantum dots (QDs) were produced by precipitation/redissolution from an aqueous colloidal ensemble. The QDs reveal broadband photoluminescence (PL) with a quantum yield of up to 60% for the most populated fraction of the core/shell AIS/ZnS QDs. The PL band shape can be described by a self-trapped exciton model implying the PL band being a sequence of phonon replica of a zero-phonon line resulting from strong electron–phonon interaction and a partial conversion of the electron excitation energy into lattice vibrations. It can be concluded that the position and shape of the PL bands of AIS QDs originate not from energy factors (depth and distribution of trap states) but rather from the dynamics of the electron–phonon interaction and the vibrational relaxation in the QDs. The rate of vibrational relaxation of the electron excitation energy in AIS QDs is found to be size-dependent, increasing almost twice from the largest to the ...

Size-selected silver nanoparticles for MALDI-TOF mass spectrometry of amyloid-beta peptides.

Matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) is one of the most efficient mass spectrometric techniques for the analysis of high-molecular-weight compounds with superior selectivity and sensitivity. Common MALDI matrices are low molecular weight (LMW) organics and will therefore produce a large amount of matrix-related ion peaks, which limits the use of MALDI-MS for the detection of LMW molecules. A major breakthrough of this limitation was made by the introduction of surface assisted desorption/ionization techniques, with graphite particles firstly as the matrix, followed by expansion into other types of nanoparticles or nanostructures. However, previous studies failed to address well the optimum size and concentration of Ag NPs to be used as the MALDI matrix. In this study, to explore and compare the efficiency of different sized silver nanoparticles (Ag NPs) as the MALDI matrix for the detection of LMW molecules, three different sized Ag NPs (2.8 ± 1.0, 12.8 ± 3.2 and 44.2 ± 5.0 nm) have been successfully developed as the MALDI time-of-flight MS (MALDI-TOF MS) matrix and amyloid-beta (Aβ) peptides, crucially involved in Alzheimer's disease and a variety of cancers, were chosen as an example of LMW molecules in our MALDI-TOF MS analysis with Ag NPs as matrices. The results showed size-selected MS signals with the smallest (2.8 ± 1.0 nm) Ag NP matrix producing the highest spectral intensities, when compared with other larger sized Ag NP matrices and conventional matrices such as SA and DHB. Furthermore, the optimal concentrations for different sized Ag NPs as matrices were determined as follows: 0.125 nM (2.8 ± 1.0 nm Ag NPs), 0.0625 nM (12.8 ± 3.2 nm Ag NPs), and 0.03125 nM (44.2 ± 5.0 nm Ag NPs), respectively. These results not only corroborated that Ag NPs could act as a very suitable matrix to assist in the desorption/ionization of LMW molecules but also revealed size-selected mass spectrometry signals with smaller Ag NPs as the MALDI matrix bearing more advantages than their larger counterparts. These novel findings paved the way for wider applications of MALDI-MS using Ag NPs as matrices for the analysis of LMW molecules.

Quantitative Determination of the Raman Enhancement of Ag30(CO)25 and Ag50(CO)40 Matrix Isolated in Solid Carbon Monoxide

Size-selected Ag clusters in the range Ag3–Ag50 were prepared by sputtering a silver substrate, mass-selecting Ag cation clusters using a Wien filter, neutralizing and matrix-isolating them at cryogenic temperatures in solid CO. The Raman spectra of the resulting silver-cluster carbonyls were recorded using excitation wavelengths in the range 457.9 to 514.5 nm. For Ag30 and Ag50, the “adsorbed” carbon monoxide (which we estimated to number ∼25 and ∼40, respectively) gave rise to broad Raman bands centered at ∼2110 cm–1. Because both the metal cluster and the CO density were measured quantitatively, a good estimate was computed for the increase in the Raman scattering cross-section per CO molecule adsorbed on the silver particle. For Ag50 and 457.9 nm laser excitation, an enhancement of ∼1850 was measured, which dropped to ∼1350 at 514.5 nm excitation. For Ag30 (and 457.9 nm excitation) the enhancement was ∼530. The enhancements of Ag3, Ag5, and Ag9 were too low to measure accurately (i.e., <10). Extrapola...

Spectroscopic signatures of an ordered array of independent Ag heptamers

A periodic network of Ag heptamers forms on the carburized W(1 1 0)-R(15 × 12) surface, upon deposition of sub-monolayer amounts of Ag. We investigate the electronic structure and dimensionality of this system by angle-resolved photoemission spectroscopy. The observation of very well-defined Ag 4d-levels confirms the highly ordered growth of size-selected Ag nano-particles on the W(1 1 0)-R(15 × 12) template. The absence of energy dispersion of these states indicates negligible coupling among the Ag heptamers, and points out the local character of the heptamer–substrate interaction. The system can be described as an array of Ag heptamers with fully confined Ag 4d-levels.

Photoemission Enhancement Induced by Near-Fields via Local Surface Plasmon Resonance of Silver Nanoparticles on a Hydrogen-Terminated Si(111) Surface

The photoemission enhancement with local surface plasmon resonance (LSPR) was studied by two-photon photoemission (2PPE) spectroscopy for size-selected silver (Ag) and gold (Au) metal nanoparticles (NPs) deposited on a hydrogen-terminated Si(111)-(1 × 1) [H−Si(111)] surface. At 0.0015 monolayer equivalents (MLE) of Ag NPs, the photoemission enhancement was observed at a photon energy of 3.10 eV, which corresponds to the peak energy for the LSPR of isolated Ag NPs. A surface with around 0.005 MLE LSPR of size-selected Ag NPs exhibited three- and four-photon photoemission processes, implying monodispersed Ag NPs on H−Si(111). This enhancement could not be observed for Au NP deposition, even at 1.0 MLE in the photon energy range of 2.90−3.23 eV. Taking into account the polarization and photoemission-angle dependences, the photoemission enhancement could be accounted for by a mechanism involving the near-fields induced by the LSPR of Ag NPs. This mechanism is consistent with an analysis based on the effect of...

Size-Selected Ag Nanoparticles with Five-Fold Symmetry

Silver nanoparticles were synthesized using the inert gas aggregation technique. We found the optimal experimental conditions to synthesize nanoparticles at different sizes: 1.3 ± 0.2, 1.7 ± 0.3, 2.5 ± 0.4, 3.7 ± 0.4, 4.5 ± 0.9, and 5.5 ± 0.3 nm. We were able to investigate the dependence of the size of the nanoparticles on the synthesis parameters. Our data suggest that the aggregation of clusters (dimers, trimer, etc.) into the active zone of the nanocluster source is the predominant physical mechanism for the formation of the nanoparticles. Our experiments were carried out in conditions that kept the density of nanoparticles low, and the formation of larges nanoparticles by coalescence processes was avoided. In order to preserve the structural and morphological properties, the impact energy of the clusters landing into the substrate was controlled, such that the acceleration energy of the nanoparticles was around 0.1 eV/atom, assuring a soft landing deposition. High-resolution transmission electron microscopy images showed that the nanoparticles were icosahedral in shape, preferentially oriented with a five-fold axis perpendicular to the substrate surface. Our results show that the synthesis by inert gas aggregation technique is a very promising alternative to produce metal nanoparticles when the control of both size and shape are critical for the development of practical applications.

Dynamics of O 2 photodesorption from metal clusters: A significant difference from bulk behaviour

Photodesorption of O 2 from size-selected Ag n O 2 - cluster anions with n = 2, 3, 4 and 8 was studied using time-resolved photoelectron spectroscopy (TR-PES). The spectra indicate that relaxations of photo-excited Ag n O 2 - clusters with n = even numbers accompany ultrafast direct O 2 photodesorption. For the odd-numbered cluster Ag 3 O 2 - , in contrast, a long-lived excited state is observed, since O 2 might be dissociatively chemisorbed, suppressing direct photodesorption of oxygen. Both, direct desorption and long-lived excited states, have not been observed from adsorbate covered metal surfaces, suggesting unique photochemical properties of such small clusters.

The polymerization of acetylene on supported metal clusters

The polymerization of acetylene was studied by thermal programmed reaction on model catalysts consisting of size-selected Ag, Rh, and Pd atoms and Pdn (1⩽n⩽30) clusters on well-characterized MgO(111) thin films. In a single-pass heating cycle experiment, benzene, butadiene, and butane were catalyzed with different selectivities as function of cluster size: palladium and rhodium atoms selectively produce benzene, and the highest selectivity for butadiene is observed for Pd6, whereas Pd20 reveals the highest selectivity for butane. Ag atoms are inert. These results provide an atom-by-atom observation of the selectivity of small cluster catalysts.

Implantation of size-selected silver clusters into graphite

We report on a systematic study of the implantation of size-selected Ag N + clusters on a graphite sample, for different cluster sizes (N = 1,3,7,9,13) and different impact energies (E = 1-30 keV). Results show that the implantation depth scales linearly with the momentum of the cluster, with a stopping power which depends on cluster size. We have particularly investigated the effects of the size and the geometry of the cluster on the implantation into the graphite substrate. A sort of universal behavior, which unifies different elements and different cluster geometries, can be recognized by scaling the momentum with the cluster projected surface. The stopping power of the cluster while penetrating the HOPG surface has been investigated for each cluster size, and a “molecular effect” is recognized, meaning that the stopping power is not additive in the number of atoms of the cluster.

Metastable ordered arrays of size-selected Ag clusters on graphite

We employ high-resolution transmission electron microscopy to explore the deposition of large size-selected AgN clusters (N=1100 to 5000 atoms) on the model graphite surface. We find that Ag5000 clusters (only) form metastable, ordered arrays, nucleated at surface defects, which are stable for at least a day. The results suggest that the cluster coalescence is strongly dependent on cluster size, and that it should be possible to fabricate large-scale, two-dimensional, and metastable arrays of clusters in the size regime 5–10 nm.

Pinning of size-selected Ag clusters on graphite surfaces

We have explored the deposition of size-selected AgN+ clusters (N=50–200) onto the graphite surface (at room temperature) over the impact energy range of 250–2500 eV, via a combination of scanning tunneling microscopy experiments and molecular dynamics simulations. We show that the clusters are pinned to the surface when the impact energy exceeds a critical value, which is proportional to the cluster size, N, via the formation of a point defect at the impact site. This prevents lateral diffusion of the clusters even at room temperature.

Energy-dependent deposition processes of size-selected Ag nanoclusters on highly-oriented pyrolytic graphite

Singly charged cations of silver atoms and clusters (Agn+, n=1,3,5,7,9) were deposited on highly oriented pyrolytic graphite substrates at well-controlled, various collision energies. The total amount of Ag atoms remaining on the substrates after collision was quantified by measuring the Ag 3d5/2 photoelectron intensities. The collision energy dependence of the amount of deposited Ag atoms revealed that, for all the species, there are three distinct energy regions, for which soft landing, rebounding, or implantation is a dominant process, and that the energy ranges for the processes strongly depend on the cluster size. The deposition efficiency vs collision energy curve for each cluster is well fitted to that for Ag1, by considering the difference between the contact area of Ag1 with the surface and that of the cluster, reflecting the compactness of the clusters. Boundaries between the different deposition regimes for the clusters were less distinct than those for Ag1. Considering anisotropy in shape of a cluster, deposition efficiency around the implantation threshold was calculated, which well explains the indistinctness of the threshold observed in the experimental data. This supports the picture that the energy given to a unit surface area determines the collision process.

Absorption and fluorescence spectra of Ar-matrix-isolated Ag 3 clusters

The absorption and fluorescence spectra of silver trimers isolated in an Ar matrix show the existence of three distinct lattice sites whereby the population of each site depends on the conditions of matrix preparation. Irradiation of certain absorption bands of the trimer isolated in one of these sites results in an irreversible reorganization of the matrix surrounding and population of another site. The experiments were performed with matrices prepared either by the matrix isolation/aggregation method or by soft landing of size-selected Ag 3 species.

Etching fabrication of silicon nanostructures using deposited metal clusters as formation nuclei

Clusters of Au and Ag deposited on Si substrates were used as nuclei for self-formation of etching masks during electron cyclotron resonance plasma etching with SF 6 gas. As a result, Si pillars ~0.1 μm high were formed with a uniform diameter. The formation mechanism is concluded to be condensation of the reaction product of the etching gas S x F y , preferentially around the deposited clusters, forming etching masks when the substrate temperature was kept at − 130 °C, Au clusters yielded the average pillar diameter of 9 nm, while Ag clusters yielded a 16 nm diameter, indicating that the reactivity of the clusters with the reaction products plays a decisive role in the mask formation. In contrast, experiments using the deposition of size-selected Ag cluster ions indicate that the pillar diameter is almost independent of the cluster size. This feature is particularly advantageous in that equal size pillars can be fabricated without the need for an accurate control of the cluster size.

Trapping of size-selected Ag clusters at surface steps

We have investigated the deposition of size-selected metal clusters (Ag400) onto a stepped graphite surface. The clusters were produced with an inert gas condensation cluster source and were imaged on the surface with a high-resolution scanning electron microscope. For modest cluster deposition energies, cluster aggregation is much more limited at the steps than on the flat terrace regions of the surface, suggesting the possibility of the fabrication of structured arrays (e.g., chains) of size-selected particles. A theoretical analysis of the particle-particle gap size distribution along the step probes the 1D mobility of the particles trapped at the step.

Fabrication of 10-nm Si pillars with self-formed etching masks

It has been found that nanoscale etching masks self-form during plasma etching with SF6 when the sample is kept at around −130°C, at which condensation of the reaction products starts to occur. We have also found that if nucleation sites are present on the surface, the condensation occurs preferentially on the nucleation sites and the resulting condensates act as etching masks, leading to the fabrication of Si nano-pillars. The above mechanism has been confirmed by fabrication of Si nanopillars using size-selected Ag clusters as condensation nuclei.