银团簇结构与特性的分子动力学研究

用分子动力学方法模拟了银团簇的结构与力能学。计算模拟中使用了一种基于第一性原理的原子间互作用多体势函数。通过分子动力学模拟确定了银徽团簇（原子个数3－13）的稳态结构;模拟了原子个数为13－141的银FCC晶体结构理想球形团簇的力能学,发现球形银簇形成三维紧密结构;计算了平均结合能,给出了结合能随团簇原子数N的变化图,发现随N增大团簇结合能逐渐接近块材的数值。     

火焰原子吸收光谱法测定含卤素金属原子簇化合物中银

本文介绍了火焰原子吸收光谱法测定含卤素金属原子簇化合物中银。研究了样品制备方法及常见共存元素干扰。平均回收率100%(99-101%),相对标准偏差<1%。用本法测定含卤素金属原子簇化合物中银,获得满意的结果。     

基于表面增强拉曼光谱及密度泛函理论的农药毒死蜱检测研究

毒死蜱作为一种广谱高效有机磷杀虫剂,在农业等领域被广泛使用。但是,环境毒理学研究发现,毒死蜱可直接施于土壤中,与土壤颗粒牢固结合,几乎不会迁移或挥发,而且水溶性低,容易造成药物残留,影响着农副产品食用的安全性,对生态环境具有潜在的危险性,许多国家对毒死蜱在农产品中的残留量有严格的规定。因此,检测毒死蜱残留的生态风险问题是当务之急。表面增强拉曼光谱(SERS)技术具有快捷、高效、灵敏度高等优势,已经成为光谱检测领域的热点研究技术;密度泛函理论被广泛用于分子结构与性质的理论模拟计算及光谱分析。基于表面增强拉曼光谱和密度泛函理论对杀虫剂毒死蜱的拉曼和表面增强拉曼光谱进行理论研究。首先,利用GaussView5.0对毒死蜱分子及加入银团簇基底的分子结构进行构型。其次,对毒死蜱分子采用6-31G基组,并基于密度泛函理论进行结构优化,利用Gaussian09模拟计算出其拉曼及表面增强拉曼光谱,并确定拉曼光谱和SERS光谱峰值归属。最后,从频移量角度分析银团簇Ag₂和Ag₃对毒死蜱拉曼光谱的增强效应,并进行频移量大小对比。研究发现,在两种尺寸银团簇作用下,拉曼光谱在326,463,741,781,1 068,1 294,1 435和1 602 cm-1波数处的特征峰强度均有明显的增强,且随着银团簇结构尺寸增大,拉曼信号增强效果更为明显;在不同银团簇增强作用下,一些特征峰发生偏移,其频移量与银团簇结构相关联,在Ag₂和Ag₃银团簇增强下,表面增强拉曼光谱在463,741～781 cm-1波数处均产生了较大的频移,其余特征峰波数处频移量较小,均在20 cm-1以下,毒死蜱分子分别与Ag₂和Ag₃入侵后的表面增强拉曼光谱进行对比,频移方向有很好的一致性。该研究结果为表面增强拉曼光谱技术在农药残留检测领域的应用提供了理论依据。     

A qualitative model for the growth mechanism of silver clusters in polymer solution

A simple and high-reproducible method for the synthesis of polymer-protected silver cluster of controlled size is described. UV-visible spectroscopy has been used for investigating the influence of the aging of the protective poly(vinylpyrrolidone) layer on the cluster growth rate at different reaction temperatures and poly(vinylpyrrolidone)/ethylene glycol weight ratios. The obtained results show that the aging time of the polymeric stabilizer solution plays a fundamental role in the reproducibility of the cluster growth process. A model for the metal cluster formation-grow process is also proposed. Received 18 July 2001 and Received in final form 3 October 2001     

Clustering of silver atoms in hydrogenated silver–sodium exchanged glasses

The diffusional growth regime of silver clusters in hydrogenated silver–sodium exchanged glasses has been numerically analysed. The diffusion coefficients of the three elements that contribute to silver cluster formation, namely H, Ag⁺, and Ag⁰, have been established. The activation energy of the reactant Ag⁰ diffusion coefficient is consistent in the three analysed different experimental results: Ag loss through evaporation, near-surface Ag accumulation, and optical absorption density. The calculated Ag⁰ diffusion coefficient accounts for the threshold time necessary to obtain a cluster size to give rise to a detectable absorption. Received: 27 September 1999 / Accepted: 11 October 1999 / Published online: 8 March 2000     

Valence band formation in small silver clusters

UV photoemission spectra of valence electrons in small silver clusters have been compared with spectra from bulk silver samples using synchrotron radiation, 16 <hν< 27 eV. Spectra for single silver atoms supported on carbon are indicative of a completely filled 4d¹⁰ initial state configuration. With increased cluster size, both density of states and valence band modulations with respect to photon energy resemble the bulk metal more closely. Spectral modulation, characteristic of conservation of crystal momentum, appears to require a cluster consisting of ～150 atoms.     

吸附在银溶胶颗粒上的品成像染料的拉曼散射增强

吸附在银溶胶颗粒上的品成像染料的拉曼散射增强刘春艳，王宇杰，李楠，臧泠，张振宗（中国科学院感光化学研究所北京１００１０１）ＳｔｕｄｉｅｓｏｎＴｈｅＩｎｔｅｒａｃｔｉｏｎｏｆＩｍａｇｉｎｇＤｙｅａｎｄＳｉｌｖｅｒＣｉｌｕｓｔｅｒｂｙＳＥＲＳ￥Ｃｈｕｎｙ...     

Softlanding and STM imaging of Ag<Subscript> 561</Subscript> clusters on a C<Subscript> 60</Subscript> monolayer

The low energy deposition of silver cluster cations with 561 (±5) atoms on a cold fullerene covered gold surface has been studied both by scanning tunneling microscopy and molecular dynamics simulation. The special properties of the C₆₀/Au(111) surface result in a noticeable fixation of the clusters without a significant change of the cluster shape. Upon heating to room temperature we observe a flattening or shrinking of the cluster samples due to thermal activation. Similar changes were observed also for mass selected Ag clusters with other sizes. For comparison we also studied Ag islands of similar size, grown by low temperature deposition of Ag atoms and subsequent annealing. A completely different behavior is observed with much broader size distributions and a qualitatively different response to annealing.     

Spin-dependent transport in cluster-assembled nanostructures: influence of cluster size and matrix material

Spin-dependent transport in granular metallic nanostructures has been investigated by means of a thermoelectric measurement. Cobalt clusters of well-defined size (〈n〉 = 15-600) embedded in copper and silver matrices show magnetic field responses of up to several hundred percent at low temperature. The experimental observations are attributed to spin mixing. The influence of cluster size and matrix are discussed.     

Direct observation of the non-supported metal nanoparticle electron density of states by X-ray photoelectron spectroscopy

Synchrotron-based X-ray photoelectron spectroscopy on copper and silver cluster beams created by a magnetron-based gas-aggregation source has allowed mapping the electron density of states (DOS) of free metallic nanoparticles. The cluster DOS profiles obtained in the experiments strongly resemble the infinite solid DOS shapes, but the extracted cluster work-functions are lower than those for the bulk metal. The latter observation is explained by the initial negative charge on most of the clusters, created by the source.     

A mechanism of the anti-Stokes luminescence of a dye-sensitized silver halide emulsion

A two-photon stepwise mechanism of the low-temperature anti-Stokes luminescence of a sensitized AgBrI emulsion, caused by photoexcitation of dye aggregates, is proved experimentally. According to this mechanism, the photoexcitation energy is transferred from the dye to the AgHal microcrystal through silver cluster centers. It is found that the luminescence with the maximum at λ ≈ 750 nm is caused by the recombination of an electron localized at the silver cluster with a free hole in the valence band of the AgHal microcrystal. The migration of an electron from a silver cluster to a iodine pair center with a captured hole creates a nonradiative recombination channel, which is responsible for the flare buildup of the green anti-Stokes luminescence upon excitation of dye aggregates.     

Stability competition between the layered and compact Cu16 clusters

In a recent study [P.H. Acioli, N. Ratanavade, M.R. Cline, S. Srinivas, Lect. Notes Comput. Sci. 5545, 203 (2009)] of the interaction of small silver clusters (Ag _n , n = 1?C4) with carbon monoxide we have found that the CO molecule can bond with the cluster either in a bent or in a linear configuration with respect to the silver carbon bond. These trends were explained by the interaction of the highest occupied molecular orbital (HOMO) of the cluster and the antibonding (?? ^?) orbital, the lowest unoccupied molecular orbital (LUMO) of CO. For a ??-type orbital of the cluster the CO molecule is bent with respect to the Ag-C bond, while for a ??-type HOMO the CO molecule is aligned with respect to the Ag-C bond. These trends tend to maximize the overlap of the CO molecule??s LUMO with the cluster??s HOMO. Furthermore, the CO molecules have a tendency to bond atop an atom rather than on bridge or face sites. In the present work we extend the investigation to clusters of up to seven atoms. The focus of this paper is on the 7-atom silver cluster which shows interesting complexities in that the cluster is characterized by a ??-like HOMO but has the CO bonded to a waist atom of the pentagonal bi-pyramid and bent with respect to the Ag-C bond, thus breaking the previously observed trend. In this work we provide an analysis of the potential energy surface of the CO bonded to Ag₇ and explain why the bonding differs from those of the smaller clusters. We find that the bonding is still explained by a ??-backdonation process. However, unlike the lowest size clusters there is an increase in overlap through bending and the complex prefers this conformation, rather than a linear Ag-C-O configuration.     

Processes involved in the formation of silver clusters on silicon surface

We analyze scanning electron microscopy measurements for structures formed in the deposition of solid silver clusters onto a silicon(100) substrate and consider theoretical models of cluster evolution onto a surface as a result of diffusion and formation of aggregates of merged clusters. Scanning electron microscopy (SEM) data are presented in addition to energy dispersive X-ray spectrometry (EDX) measurements of the these films. Solid silver clusters are produced by a DC magnetron sputtering source with a quadrupole filter for selection of cluster sizes (4.1 and 5.6 nm or 1900 and 5000 atoms per cluster in this experiment); the energy of cluster deposition is 0.7 eV/atom. Rapid thermal annealing of the grown films allows analysis of their behavior at high temperatures. The results exhibit formation of cluster aggregates via the diffusion of deposited solid clusters along the surface; an aggregate consists of up to hundreds of individual clusters. This process is essentially described by the diffusion-limited aggregation (DLA) model, and thus a grown porous film consists of cluster aggregates joined by bridges. Subsequent annealing of this film leads to its melting at temperatures lower than to the melting point of bulk silver. Analysis of evaporation of this film at higher temperatures gives a binding energy in bulk silver of ɛ₀= (2.74 ± 0.03) eV/atom. The text was submitted by the authors in English.     

Atom-by-atom extraction using the scanning tunneling microscope tip-cluster interaction

We investigate the atomistic details of a single atom-extraction process realized by using the scanning tunneling microscope tip-cluster interaction on a Ag(111) surface at 6 K. Single atoms are extracted from a silver cluster one atom at a time using small tunneling biases less than 35 mV. Combined total energy calculations and molecular dynamics simulations show a lowering of the atom-extraction barrier upon approaching the tip to the cluster. Thus, a mere tuning of the proximity between the tip and the cluster governs the extraction process. The atomic precision and reproducibility of this procedure are demonstrated by repeatedly extracting single atoms from a silver cluster on an atom-by-atom basis.     

Stability of a deposited liquid cluster

The fragmentation of silver liquid clusters deposited onto a silicon surface is observed after heating the clusters to 1073 K and subsequent annealing with the exposition time 3 min. This contradicts macroscopic models of a liquid drop deposited on a surface if we use critical parameters of bulk silver. Some versions are analyzed that are based on cluster properties and may explain the phenomenon of cluster fragmentation. An experiment is suggested for clarifying the nature of the cluster fragmentation phenomenon and cluster critical phenomena.     

Influence of Silver Nanoparticles on 2,3-Bis(Chloromethyl)Anthracene-1,4,9,10-Tetraone

Size effect of silver nano particles on the photophysical properties of 2,3-bis(chloromethyl)anthracene-1,4,9,10-tetraone (BCMAT) have been investigated using optical absorption and fluorescence emission techniques. Silver NPs of different sizes have been prepared by two different methods. Quenching of fluorescence of BCMAT has been found to increase with decrease in the size of the silver NPs. Stern–Volmer quenching constants have also been calculated.     

Electrical properties of silver-intercalated hafnium disulfide in DC and AC electric fields

The temperature dependences of the direct-current (dc) and alternating-current (ac) resistivities of silver intercalated compounds Ag _x HfS ₂ have been measured using impedance spectroscopy over a wide frequency range at different temperatures for the first time. The dc conductivity has an activation behavior and increases with increasing silver content in the samples. The results of the ac measurements have demonstrated that the ac conductivity has a frequency dispersion described by “the universal dynamic response.” It has been shown that the relaxation processes in an ac field are accelerated when the silver content in the sample and temperature increase.     

Agn-1Si（n=5-10）团簇结构与性质研究

采用密度泛函理论研究了Agn-1Si团簇（n = 5-10）的几何结构和物理性质。首先得到体系最低能量结构,其中Ag5Si 和 Ag7Si团簇的结构比之前研究中结构能量更低。通过分析相应结构的能隙,平均结合能,二阶能量差可以发现Si原子的加入可以加强团簇结构稳定性,使团簇更加紧凑。在团簇尺寸n = 5-10的范围里,拥有八个价电子的Ag4Si团簇在以上三个方面都显示出非常稳定的特点。通过分析Agn-1Si团簇 (n = 5-10)的差分电荷发现,电荷的转移主要发生在Si原子与其相邻的Ag原子之间, Si原子和附近的Ag原子之间产生了强烈的共价相互作用,是团簇稳定性增强的重要原因。     

Fluorescence Quenching of 1,4-Dihydroxy-2,3-Dimethyl-9,10-Anthraquinone by Silver Nanoparticles: Size Effect

Size effect of silver nano particles on the photophysical properties of 1,4-dihydroxy-2,3-dimethyl-9,10-anthraquinone (DHDMAQ) have been investigated using optical absorption and fluorescence emission techniques. Silver nanoparticles of different sizes have been prepared by Creighton method using magnetic stirrer and ultrasonic field. Quenching of fluorescence of DHDMAQ has been found to increase with decrease in the size of the silver nanoparticles. Stern–Volmer quenching constants have also been calculated.     

Electrical and emission properties of current-carrying silver cluster films detected by an emission electron microscope

Silver cluster films deposited on glass or sapphire have been investigated using an emission electron microscope when a voltage was applied across the film. The electric field distribution in the silver cluster film was measured exploiting the image distortion with increasing voltage. The electron and photon emission originating from the cluster film was visualized. The latter phenomena are correlated with a deviation from Ohmic behavior of the conductivity curve. PACS 68.37.Xy; 73.61.At; 79.90.+b