Surface deposition and imaging of large Ag clusters formed in He droplets

The utility of a continuous beam of He droplets for the assembly and surface deposition of Ag(N) clusters, ~ 300-6000, is studied with transmission electron microscopy. Images of the clusters on amorphous carbon substrates obtained at short deposition times have provided for a measure of the size distribution of the metal clusters. The average sizes of the deposited clusters are in good agreement with an energy balance based estimate of Ag(N) cluster growth in He droplets. Measurements of the deposition rate indicate that upon impact with the surface the He-embedded cluster is attached with high probability. The stability of the deposited clusters on the substrate is discussed.     

Enhancement of ion intensity in time-of-flight secondary-ionization mass spectrometry

Enhancement of ion intensity in static secondary-ionization mass spectrometry (SIMS) has been achieved by using a matrix-assisted sample preparation technique. Previous investigations of polymers and biomolecules by SIMS indicated that secondary-ion (SI) yield is dependent on substrate coverage. Recently we discovered a sample preparation technique that enhanced the SI yield of cyclosporin A (CsA) in an allograft patient sample and neat samples of CsA (1202 u) and polystyrene (M _w=2650 u). The preparation technique involves deposition of a submonolayer of cocaine hydrochloride (5 µL of a 20-µg/mL MeOH solution) on an etched silver substrate, solvent evaporation, and subsequent deposition of the analyte. This preparation method resulted in ～300% increase in the SI yield of CsA and polystyrene when deposited from neat solutions. The original discovery was observed when a blood extract that contained CsA was deposited on an etched Ag substrate that had been soaking in a dilute cocaine solution for ～2 months. In these initial experiments, the SI yield of CsA was enhanced by over 1 order of magnitude.     

Evaluation of the structural,optical and electrical properties of AZO thin films prepared by chemical bath deposition for optoelectronics

Aluminum doped zinc oxide (AZO) thin films for electrode applications were deposited on glass substrates using chemical bath deposition (CBD) method. The influence of deposition time on the structural, morphological, and opto-electrical properties of AZO films were investigated. Structural studies confirmed that all the deposited films were hexagonal wurtzite structure with polycrystalline nature and exhibited (002) preferential orientation. There is no other impurity phases were detected for different deposition time. Surface morphological images shows the spherically shaped grains are uniformly arranged on to the entire film surface. The EDS spectrum confirms the presence of Zn, O and Al elements in deposited AZO film. The observed optical transmittance is high (87%) in the visible region, and the calculated band gap value is 3.27 eV. In this study, the transmittance value is decreased with increasing deposition time. The room temperature PL spectrum exposed that AZO thin film deposited at (60 min) has good optical quality with less defect density. The minimum electrical resistivity and maximum carrier concentration values were observed as 8.53 × 10⁻³(Ω cm) and 3.53 × 10¹⁸ cm⁻³ for 60 min deposited film, respectively. The obtained figure of merit (ϕ) value 3.05 × 10⁻³(Ω/sq)^{- 1} is suggested for an optoelectronic device.     

Doubly charged sodium clusters: plasmon response and photoproduction

The optical response of doubly charged sodium clusters Na _n+2 ⁺⁺ was measured for n = 20, 40, and 58 valence electrons, for which the jellium model predicts spherical clusters. A new experimental scheme was developed which allows to separate doubly charged clusters of even mass from the singly charged with half the mass. The optical spectra are dominated by a plasmon-like resonance which is blue shifted and narrower than that of the singly charged clusters. The smallest doubly charged cluster observed was Na ₉ ⁺⁺ . The photo ionization cross section for singly charged clusters was found to be typically 2.6·10^-19cm² per Na atom for photon energies of around 6 eV, which is a factor of 400 smaller than the maximum in the plasmon absorption in the region of hω=2.6 eV.     

Nucleation kinetics of heterogeneous catalyst: Pd/thin layer of MgO (100)

The kinetics describing the formation of the system Pd/thin layer of MgO (100) surface was investigated by applying programs developed on the basis of Fortran Software. The simulation is based on studies related to nucleation, crystallite growth, coalescence and diffusion of clusters on the surface of thin films. The density of Pd islands is obtained by simulating the deposition of 1 × 10¹³ atoms cm^–2 s^–1 in 150 s. The density of clusters first increases to reach a plateau. Then, it either remains some time at a nearly constant value or decreases slowly after the saturation regime gives way to the coalescence stage. This phenomenon is explained via island migration process at the surface. The coalescence time strongly depends on the deposition temperature, the coalescence occurs when the substrate temperature is high. Also, the surface coverage decreases when the substrate temperature increases. The coalescence is pronounced at a low surface coverage. It involves the migration on the surface, defined as the dynamic coalescence and precedes the process of the formation of immobile islands that predominates at high extents of surface coverage.     

The role of radicals and clusters in thermal plasma flash evaporation processing

The prominent features of plasma flash evaporation may be caused by cluster deposition under high net flux of radicals. In order to clarify such features quantitatively, the atomic oxygen flux (Γ _o ) and cluster size (d) in 200 Torr RF thermal Ar-O₂ plasma were measured on the substrate. It was found that Γ _o higher than 10¹⁹ atoms/cm²·s can be easily acheived in this processing and this value corresponds to frozen atomic oxygen fraction 7 %. The cluster size of YBCO was estimated to be about 0.3 to 10 nm. 0.8 nm cluster deposition under Γ _o higher than 6 × 10¹⁸ atoms/cm²·s made it possible to deposit fine epitaxial YBCO films with Tc = 90K on SrTiO₃ (100) with extremely high deposition rate around 2.2 μm/min.     

Palladium particles growth on various aluminas

Pd was deposited from a filament onto anα-Al₂O₃ single-crystal. The Pd 3dP ^5/2 binding energy change versus deposition time was used as a calibration of the Pd clusters size. The experiment was then reproduced onγ-Al₂O₃. The Pd 3d ^5/2 binding energy showed that the clusters growth mechanism was now different. Thermal treatments produced a dispersion of the metal or a clusters size increase depending on the temperature or the nature of the support. Oxygen exposure dispersed Pd in every case. A mechanism is proposed to interpret this phenomenon.     

An unexpected all-metal aromatic tetranuclear silver cluster in human copper chaperone Atox1

Metal clusters, such as iron–sulfur clusters, play key roles in sustaining life and are intimately involved in the functions of metalloproteins. Herein we report the formation and crystal structure of a planar square tetranuclear silver cluster when silver ions were mixed with human copper chaperone Atox1. Quantum chemical studies reveal that two Ag 5s¹ electrons in the tetranuclear silver cluster fully occupy the one bonding molecular orbital, with the assumption that this Ag₄ cluster is Ag₄²⁺, leading to extensive electron delocalization over the planar square and significant stabilization. This bonding pattern of the tetranuclear silver cluster represents an aromatic all-metal structure that follows a 4n + 2 electron counting rule (n = 0). This is the first time an all-metal aromatic silver cluster was observed in a protein.

Metal clusters, such as iron–sulfur clusters, play key roles in sustaining life and are intimately involved in the functions of metalloproteins.     

Fragmentation dynamics of ammonia cluster ions after single photon ionisation

A reflecting time of flight mass spectrometer (RETOF) is used to study unimolecular and collision induced fragmentation of ammonia cluster ions. Synchrotron radiation from the BESSY electron storage ring is used in a range of photon energies from 9.08 up to 17.7 eV for single photon ionisation of neutral clusters in a supersonic beam. The threshold photoelectron photoion coincidence technique (TPEPICO) is used to define the energy initially deposited into the cluster ions. Metastable unimolecular decay (µs range) is studied using the RETOF's capacity for energy analysis. Under collision free conditions the by far most prominent metastable process is the evaporation of one neutral NH₃ monomer from protonated clusters (NH₃) _{n ? 2}NH ₄ ⁺ . Abundance of homogeneous vs. protonated cluster ions and of metastable fragments are reported as a function of photon energy and cluster size up ton=10.     

Combined spectroscopic studies on post-functionalized Au25 cluster as an ATR-FTIR sensor for cations

Recently, significant research activity has been devoted to thiolate-protected gold clusters due to their attractive optical and electronic properties. These properties as well as solubility and stability can be controlled by post-synthetic modification strategies. Herein, the ligand exchange reaction between Au₂₅(2-PET)₁₈ cluster (where 2-PET is 2-phenylethanethiol) and di-thiolated crown ether (t-CE) ligands bearing two chromophores was studied. The post-functionalization aimed to endow the cluster with ion binding properties. The exchange reaction was followed in situ by UV-vis, ¹H NMR and HPLC. MALDI mass analysis revealed the incorporation of up to 5 t-CE ligands into the ligand shell. Once functionalized MALDI furthermore showed complexation of sodium ions to the cluster. ATR-FTIR spectroscopic studies using aqueous solutions of K⁺, Ba²⁺, Gd³⁺ and Eu³⁺ showed noticeable spectral shifts of the C–O stretching band around 1100 cm⁻¹ upon complexation. Further spectral changes point towards a conformational change of the two chromophores that are attached to the crown ether. Density functional theory calculations indicate that the di-thiol ligand bridges two staple units on the cluster. The calculations furthermore reproduce the spectral shift of the C–O stretching vibrations upon complex formation and reveal a conformational change that involves the two chromophores attached to the crown ether. The functionalized clusters have therefore attractive ion sensing properties due to the combination of binding properties, mainly due to the crown ether, and the possibility for signal transduction via an induced conformational change involving chromophore units.

Using ligand exchange reactions an atomically precise gold cluster was functionalized with a di-thiolated crown ether. Using in situ infrared spectroscopy films of the resulting composite were shown to incorporate metal cations.     

Optical properties of Cu nanoclusters supported on MgO(100)

The vertical transitions of Cu atoms, dimers, and tetramers deposited on the MgO surface have been investigated by means of ab initio calculations based either on complete active space second-order perturbation theory or on time-dependent density functional theory. Three adsorption sites have been considered as representative of the complexity of the MgO surface: regular sites at flat (100) terraces, extended defects such as monoatomic steps, and point defects such as neutral oxygen vacancies (F or color centers). The optical properties of the supported Cu clusters have been compared with those of the corresponding gas-phase units. Upon deposition a substantial modification of the energy levels of the supported cluster is induced by the Pauli repulsion with the substrate. This causes shifts in the optical transitions going from free to supported clusters. The changes in cluster geometry induced by the substrate have a much smaller effect on the optical absorption bands. On F centers the presence of filled impurity levels in the band gap of MgO results in a strong mixing with the empty levels of the Cu atoms and clusters with consequent deep changes in the optical properties of the color centers. The results allow to interpret electron energy loss spectra of Cu atoms deposited on MgO thin films.     

Optical extinction on free,supported and embedded metal clusters

To study changes of the optical properties of clusters due to interactions with matrices and substrates, clusters of several nm diameter were produced by the inert gas condensation and by the gas free expansion technique. Theoretical predictions for the cluster formation process could be verified. Optical extinction was measured on the free cluster beam, after deposition on quartz substrates and after embedding in quartz matrices. Several of the samples were analyzed by TEM. It is shown, that at high source temperatureT ₀ the free clusters remain liquid on their way through the vacuum chamber but solidify there ifT ₀ is decreased. At higherT ₀ oblate ellipsoids are formed during the deposition. The spectra of the free beam deviate from theoretical predictions.     

A Study About Nanocluster Deposition of Thin-film Formation by Molecular Dynamics Simulation

The thin-film growth has been confirmed to be assembled by an enormous number of clusters in ICBD method. In sequence of clusters’ depositions proceeds to form the thin-film to understand quantitatively the interaction mechanisms between the cluster atoms and the substrate atoms, we use molecular dynamics simulation with EAM potential. The quantitative of flatness of deposition and percent of disordered atoms were proposed to evaluate the property of thin-film. In this simulation, three different Co cluster sizes of 55, 70, and 100 atoms with different velocities (100 up to 800 m/s) were deposited on a Al(0 0 1) substrate whose temperatures were set between 300 and 500 K. The simulations begin at specific equilibrium temperature of clusters and the substrate. The simulations are performed at different temperatures of the clusters and substrate and for different sizes of clusters. We showed that the percent of disordered atoms of substrate are affected by the cluster size and velocity of the clusters. Temperature dependence of the number of disordered atoms for different cluster’s velocity was observed. We investigated the effect of cluster size and initial velocity of cluster on the value of flatness.     

Optical Activity of Metal Nanoclusters Deposited on Regular and Doped Oxide Supports from First-Principles Simulations

We report a computational study and analysis of the optical absorption processes of Ag₂₀ and Au₂₀ clusters deposited on the magnesium oxide (100) facet, both regular and including point defects. Ag₂₀ and Au₂₀ are taken as models of metal nanoparticles and their plasmonic response, MgO as a model of a simple oxide support. We consider oxide defects both on the oxygen anion framework (i.e., a neutral oxygen vacancy) and in the magnesium cation framework (i.e., replacing Mg⁺⁺ with a transition metal: Cu⁺⁺ or Co⁺⁺). We relax the clusters’ geometries via Density-Functional Theory (DFT) and calculate the photo-absorption spectra via Time-Dependent DFT (TDDFT) simulations on the relaxed geometries. We find that the substrate/cluster interaction induces a broadening and a red-shift of the excited states of the clusters, phenomena that are enhanced by the presence of an oxygen vacancy and its localized excitations. The presence of a transition-metal dopant does not qualitatively affect the spectral profile. However, when it lies next to an oxygen vacancy for Ag₂₀, it can strongly enhance the component of the cluster excitations perpendicular to the surface, thus favoring charge injection.     

Submonolayer-Deposition of Mass-Selected Au+ and Au n + (n = 3 and 7) on HOPG and Amorphous Carbon

Ions of gold monomer and clusters emitted from a liquid metal ion source were mass-selected, and deposited on cleaved HOPG (highly oriented pyrolytic graphite) surfaces and on amorphous carbon thin films at room temperature with the impinging energy E _i from 0 to 500 eV. The coverage of deposited ions were 1/100 and 1/1000 monolayers on HOPG surfaces and 1/3 monolayers on carbon films. Scanning tunneling microscopy of the HOPG surfaces deposited with low impinging energy (E _i<50 eV) revealed that large clusters with diameters ranging from 2 to 5 nm and height of 1–2 layers were present instead of isolated monomers and original clusters. When E _i was higher than 100 eV, HOPG surfaces were damaged and only bumpy surfaces were observed by STM. Transmission electron microscopy of Au⁺-deposited carbon films showed the formation of clusters with diameter 0.5–20 nm, depending on the E _i and the time elapsed after deposition.     

The band structure of submonolayer thorium coatings on a silicon oxide surface

Data are reported on the electronic structure of thorium-containing clusters formed on the surface of natural silicon oxide via electrochemical deposition. The mutual arrangement of bands in the clusters and silicon oxide substrate is restored with the use of X-ray photoelectron spectroscopy and electron energy loss spectroscopy. It is concluded that the studied cluster/substrate system may be promising for investigating the low-lying isomeric nuclear transition in ²²⁹Th isotope.     

Irreversible adsorption of latex particles on fibrinogen covered mica

Physicochemical properties of bovine plasma fibrinogen (Fb) in electrolyte solutions were characterized. These comprised the diffusion coefficient (hydrodynamic radius), determined by the DLS method, electrophoretic mobility and the isoelectric point. The hydrodynamic radius of Fb was 12 nm for pH<5. The number of uncompensated (electrokinetic) charges on the protein N _c was calculated from the electrophoretic mobility data. It was found that for pH<5.8 the electrokinetic charge was positive, independently of the ionic strength and negative for pH>5.8. For pH=3.5 the value of N _c , was 26 for 10^?3 M. Similar electrokinetic measurements were performed for the mica substrate using the streaming potential cell. It was shown that for pH=3.5 and 10^?3 M, the zeta potential of mica remained negative (?50 mV). This promoted an irreversible, electrostatically driven adsorption of Fb, which was confirmed in experiments carried out under diffusion-controlled transport. The surface concentration of Fb on mica was determined directly by AFM counting. By adjusting the time of adsorption, Fb monolayers of desired coverage were produced. Independently, the presence of Fb on mica was determined quantitatively by the colloid enhancement method, in which negatively charged latex particles were used, having the diameter of 800 nm. It was found that for Fb coverage below 0.05 the method was more sensitive than other indirect methods. The experimental data obtained in latex deposition experiments were adequately interpreted in terms of the random site model used previously for polyelectrolytes. It was shown that adsorption sites consisted of a cluster of two Fb molecules. It was concluded that the colloid enhancement method can be successfully used for detecting the presence of proteins at solid substrates and to determine the uniformity of monolayers in the nanoscale.     

Optical plasmon losses in stabilized Au55 clusters

Au₅₅ cluster compounds are investigated by optical spectroscopy and TEM. The optical spectra appear to be rather structureless, neither showing a collective excitation resonance nor exhibiting distinct absorption bands known from lower nuclearity clusters. We discuss changes of the electronic properties compared to larger Au clusters affecting both, 6sp electrons and5d-6sp interband transitions, the cluster-ligand-interaction being considered as a charge transfer process. We additionally report on a low temperature instability of the cluster compound, which results in changed optical extinction spectra. A characteristic absorption feature at λ=400 nm is attributed to small, ligand-free Au cluster fragments.     

Superconducting lead cluster films with particle size dependent reduction ofT c

To investigate films of metal clusters, a neutral lead cluster beam, generated by inert gas aggregation, is characterized via time-of-flight mass spectroscopy and subsequently used for growing a thin film (d=100 Å) on a cold sapphire substrate. In an annealing program the temperature dependence of the electrical resistance of the film is determined by the dc 4-probe technique. Directly after deposition a superconducting transition atT _c =5.6 K is observed which is shifted to higher temperatures with increasing annealing temperature (T _c =6.5 K after annealing at room temperature).     

钯团簇碰撞沉积钯/银基板的分子动力学模拟

运用分子动力学模拟方法研究了常温下较大的钯团簇以不同初始速度撞击不同硬度基板的微观过程,着重分析了沉积形貌的变化、团簇的嵌入深度和原子的扩散程度、基板碰撞接触区域的温度演变以及碰撞过程中团簇与基板间的能量转化,获得了沉积过程中变形形貌、结构特征及能量转化随团簇尺寸、初始速度及基板材质的变化规律.并进一步探究了第二颗团簇以不同时刻沉积时前一团簇的变形形貌及基板接触区域温度变化的特点,发现短时间间隔下第二颗团簇的沉积更有利于团簇与基板的结合.