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1.
The structures, spectra and electronic and magnetic properties of Ag4M and Ag4MCO (M?=?Sc–Zn) clusters have been studied using density functional theory and CALYPSO structure searching method. Structural searches show that M atoms except Zn tend to occupy the highest coordination position in the ground state Ag4M and Ag4MCO clusters. Carbon monoxide is most easily adsorbed on Ag atom of Ag4Zn and M atom of other Ag4M. Infrared and Raman spectra, photoabsorption spectra and photoelectron spectra of Ag4M and Ag4MCO clusters are forecasted and can be used to identify these clusters from experiment. Analysis of electronic properties indicates that the adsorption of CO on Ag4M clusters changes the zero vibrational energy (ZPVE) and increases stability of the host clusters. Dopant atoms except for Zn improve the stability of silver cluster. The Ag4Ni cluster shows high chemical activity and maximum adsorption energy for carbon monoxide. Magnetism calculations reveal that the magnetic moment of Ag4M (M?=?Mn–Ni) cluster adsorbed by carbon monoxide is decreased by 2 μB. The change of magnetic moment makes it possible to be used as a nanomaterial for carbon monoxide detection. Simultaneously, it is found that the adsorption of CO on Ag4Cu cluster is a physical adsorption.  相似文献   

2.
We present a joint experimental and theoretical gas phase study of photoabsorption and photofragmentation of silver cluster-biomolecule complexes. We demonstrate on the example of [ Trp.Ag3] + that binding of the metal cluster to a biomolecule leads to a significant enhancement of the photoabsorption in comparison with [Trp.H]+ and [Trp.Ag]+. This enhancement arises due to the coupling between the excitations in the metallic subunit with charge transfer excitations between silver cluster and tryptophan. Our experimental studies show that silver clusters up to eleven atoms can be bound to tryptophan and we present first results on the photofragmentation of the Trp.Ag11 + complex cation, in which properties of cluster subunit remain preserved.  相似文献   

3.
张安超  孙路石  向军  郭培红  刘志超  苏胜 《物理学报》2011,60(7):73103-073103
采用密度泛函理论中的广义梯度近似对Hg与小团簇Au qn (n=1—6, q=0, +1, -1)的相互作用进行了系统研究. 结果表明,除Au5+,-团簇外,前线分子轨道理论可以成功预测大部分Au n Hg q 复合物的最低能量结构. Aun团簇对Hg的吸附受团簇尺寸大小和团簇所携带电荷的影 关键词: 密度泛函理论 汞 金团簇 吸附能  相似文献   

4.
We investigated the single exciton and multiple exciton generation (MEG) behavior in Ag7 and single Cu atom-doped Ag7 quantum clusters using ab initio. MEG is observed for the first time in metal clusters. The results indicate that multiple excitons appear in the visible and near ultraviolet light ranges. Single excitations are main contribution for the optical spectra, while the multiple excitons merely contribute for some peaks at the higher energies. However, occurrence of MEG enhances the optical absorption in Ag7 cluster. The optical spectrum of pure Ag7 cluster obtained using the symmetry-adapted cluster theory with configuration interaction, and time-dependent density functional theory is in excellent agreement with experiment spectrum. As observed in both single Cu atom-doped Ag7 clusters, redshifts and suppressions of the MEG-related absorption peaks are observed compared with pure Ag7 cluster.  相似文献   

5.
To better understand experimentally observed surface‐enhanced Raman Scattering (SERS) of polychlorinated biphenyls (PCBs) adsorbed on nanoscaled silver substrates, a systematic theoretical study was performed by carrying out density functional theory and time‐dependent density functional theory calculations. 2,2′,5,5′‐tetrachlorobiphenyl (PCB52) was chosen as a model molecule of PCBs, and Agn (n = 2, 4, 6, and 10) clusters were used to mimic active sites of substrates. Calculated normal Raman spectra of PCB52–Agn (n = 2, 4, 6, and 10) complexes are analogical in profile to that of isolated PCB52 with only slightly enhanced intensity. In contrast, the corresponding SERS spectra calculated at adopted incident light are strongly enhanced, and the calculated enhancement factors are 104 ~ 105. Thus, the experimentally observed SERS phenomenon of PCBs supported on Ag substrates should correspond to the SERS spectra rather than the normal Raman spectra. The dominant enhancement in Raman intensities origins from the charge transfer resonance enhancement between the molecule and clusters. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

6.
We present structural and optical properties of silver clusters Agn (n=2, 4, 6, 8) at two model support sites of MgO, stoichiometric MgO(100) and FS-center defect, based on density functional theory and embedded cluster model. Our results provide the mechanism responsible for the absorption and emission patterns due to the specific interaction between the excitations within the cluster and the support site which is strongly cluster size and structure dependent. We propose Ag4 at stoichiometric site as well as Ag2, Ag4 and Ag6 at FS-center defects as good candidates for the emissive centers in the visible regime.  相似文献   

7.
An attempt has been made to explore the applicability of the complete neglect of differential overlap (CNDO) method for investigating clusters of silver atoms. A new parametrization for silver has been obtained by comparing charge distributions, as well as local and total density of states, from CNDO calculations with those from the Xα scattered wave (XαSW) method for an Ag7 cluster which represents a fragment of the silver lattice. These parameters have then been used for making CNDO calculations on four further clusters of the same type, namely Ag6, Ag10, Ag13 and Ag19, and the results are compared with previous XαSW calculations. These CNDO calculations give d-band widths in broad agreement with those from the XαSW method. The most significant difference is that the CNDO method gives less localization on central atoms with high coordination numbers than is found from the XαSW calculations. It is suggested that this apparent deficiency of the CNDO calculations may be less serious when the clusters are being used for modelling part of a solid metal rather than for specifically investigating the properties of small particles.  相似文献   

8.
Various dissociation channels of silver bromide cluster ion Ag2Br+ and silver cluster ion Ag3 + were observed in high-energy collisionally-activated dissociation (CAD) using a Cs target. The fragment patterns of the high-energy CAD were compared with those of the metastable dissociation and low-energy CAD. The difference in the fragment patterns between the high-energy CAD and the other dissociation methods was explained in terms of the internal energy distributions. The dissociation mechanisms of neutral silver bromide cluster Ag2Br and silver cluster Ag3 were also investigated by charge inversion mass spectrometry using the Cs target. While the fragment ions AgBr- and Ag2 - were dominantly observed in the charge inversion spectrum of Ag2Br+, the undissociated ion Ag3 - was observed as a predominant peak in the case of Ag3 +. The dissociation behavior of Ag2Br* can be explained on the basis of the calculated thermochemical data. Contrary to this, the predominant existence of the undissociated Ag3 - cannot be explained by the reported thermochemical data. The existence of undissociated Ag3 - suggests that the dissociation barrier is higher than the internal energy of Ag3 * (theoretical: 1.03 eV, experimental: 2.31 eV) estimated from the ionization potentials of Ag3 and Cs.  相似文献   

9.
We describe quantum‐size and binding‐site effects on the chemical and local field enhancement mechanisms of surface‐enhanced resonance Raman scattering (SERRS), in which the pyridine molecule is adsorbed on one of the vertices of the Ag20 tetrahedron. We first investigated the influence of the binding site on normal Raman scattering (NRS) and excited state properties of optical absorption spectroscopy. Second, we investigated the quantum‐size effect on the electromagnetic (EM) and chemical mechanism from 300 to 1000 nm with charge difference density. It is found that the strong absorption at around 350 nm is mainly the charge transfer (CT) excitation (CT between the molecule and the silver cluster) for large clusters, which is the direct evidence for the chemical enhancement mechanism for SERRS; for a small cluster the strong absorption around 350 nm is mainly intracluster excitation, which is the direct evidence for the EM enhancement mechanism. This conclusion is further confirmed with the general Mie theory. The plasmon peak in EM enhancement will be red‐shifted with the increase of cluster size. The influence of the binding site and quantum‐size effects on NRS, as well as chemical and EM enhancement mechanisms on SERRS, is significant. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

10.
Ab initio Molecular Dynamics (MD) method, based on density functional theory (DFT) with planewaves and pseudopotentials, was used to study the stability and internal motion in silver cluster Agn, with n =4-6. Calculations on the neutral, cationic and anionic silver dimer Ag2 show that the bond distance and vibrational frequency calculated by DFT are of good quality. Simulations of Ag4, Ag5, and Ag6 in canonical ensemble reveal distinct characteristics and isomerization paths for each cluster. At a temperature of 800 K, an Ag4 has no definite structure due to internal motion, while for Ag5 and Ag6the clusters maintain the planar structure, with atomic rearrangement observed for Ag5 but not for Ag6. At a temperature of 200 K, Ag4 can exist in two planar structures whilst Ag5 is found to be stable only in the planar form. In contrast Ag6 is stable in both planar trigonal and 3D pentagonal structures. Micro-canonical MD simulation was performed for all three clusters to obtain the vibrational density of states (DOS). Received 5 May 1999 and Received in final form 20 August 1999  相似文献   

11.
Adsorption energies and stable configurations of CO on the Pt clusters are investigated using the first-principles density-functional theory method. It is found that the adsorption of CO on the top site of the Pt4 cluster is more stable than that on the bridge site. The atomic charges are unevenly distributed within the charged Pt4 cluster, and the structural positions of the Pt atoms determine their charge states and therefore their activity. A systematic study on the effects of electrons and holes doping in the Pt4 clusters suggest an effective method to prevent the CO poisoning through regulating the total charge in Pt4 clusters. The graphene-based substrate is an ideal catalyst support, which makes the Pt catalyst lose electron and weakens the CO adsorption. The results would be of great importance for designing high active nanoscale Pt catalysts used for fuel cells.  相似文献   

12.
丁利苹  邝小渝  邵鹏  赵亚儒  李艳芳 《中国物理 B》2012,21(4):43601-043601
Using the meta-generalized gradient approximation (meta-GGA) exchange correlation TPSS functional, the geo- metric structures, the relative stabilities, and the electronic properties of bimetallic Ag n X (X=Au, Cu; n=1–8) clusters are systematically investigated and compared with those of pure silver clusters. The optimized structures show that the transition point from preferentially planar to three-dimensional structure occurs at n = 6 for the Ag n Au clusters, and at n = 5 for Ag n Cu clusters. For different-sized Ag n X clusters, one X (X=Au or Cu) atom substituted Ag n+1 structure is a dominant growth pattern. The calculated fragmentation energies, second-order differences in energies, and the highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO–LUMO) energy gaps show interesting odd–even oscillation behaviours, indicating that Ag 2,4,6,8 and Ag 1,3,5,7 X (X=Au, Cu) clusters keep high stabilities in comparison with their neighbouring clusters. The natural population analysis reveals that the charges transfer from the Ag n host to the impurity atom except for the Ag 2 Cu cluster. Moreover, vertical ionization potential (VIP), vertical electronic affinity (VEA), and chemical hardness (η) are discussed and compared in depth. The same odd–even oscillations are found for the VIP and η of the Ag n X (X=Au, Cu; n=1–8) clusters.  相似文献   

13.
The structural and electronic properties of silver-doped gold clusters Au n Ag v (2?≤?n?≤?10; v?=?0,?±1) have been systematically investigated using density functional theory. The results show that the ground state optimal structures of the cationic and neutral clusters are found to be planar up to n?=?3 and 9, respectively. However, for the anionic clusters, no three-dimensional lowest-energy structures are obtained according to DFT calculations. The calculated binding energy and dissociation energy as a function of cluster size exhibit odd–even alternations. The natural population analysis indicates that in Au n Ag v clusters charges transfer from the Ag atom to the Au frames. The trends for the vertical detachment energies, adiabatic electron affinities, adiabatic ionization potentials, and chemical hardness of Au n Ag v clusters, as the cluster size increases, are studied in detail and compared with the available experimental data.  相似文献   

14.
Classical molecular dynamics simulation (MD) with Sutton-Chen potential has been used to generate the minimum energy and to study the thermodynamic and dynamic properties of mixed transition metal cluster motifs of Ag n Ni(13?n) for n ?? 13. Literature results of thirteen particle clusters of neat silver and nickel atoms were first reproduced before the successive replacement of the silver atom by nickel. Calculation was repeated for both silver-centred and nickel-centred clusters. It was found that the nickel-centred clusters were more stable than the silver-centred clusters. Heat capacities and hence the melting points of silver and nickel-centred clusters were determined by using the Histogram method. Species-centric order parameters developed by Hewage and Amar were used to understand the dynamic behaviour in the transition of silver-centred clusters to more stable nickel-centred clusters. This species-centric order parameter calculation further confirmed the stability of nickel-centred clusters over those of silver-centred species.  相似文献   

15.
A model different from the Rayleigh model for Coulomb instability of charged metallic clusters is proposed. The two-component model of a metallic cluster in the quasi-classical approximation offers different critical charges depending on the type of charged particles. For small-sized parallelepiped clusters, the quantization of the electronic spectrum is taken into account. The critical sizes of Ag N 2? and Ag N 3? clusters are calculated in the framework of the proposed model. The results of calculations are in good agreement with experimental data. The Coulomb explosion of positively charged clusters Na N n+ at 3≤n≤5 is explained qualitatively.  相似文献   

16.
Bimetallic silver-gold clusters are well suited to study changes in metallic versus ionic properties involving charge transfer as a function of the size and the composition. We present structures, ionization potentials (IP) and vertical detachment energies (VDE) for neutral and charged bimetallic AunAgm ( 2(n + m)5) clusters obtained from density functional level of theory. In the stable structures of these clusters Au atoms assume positions which favor charge transfer from Ag atoms. In clusters with equal numbers of hetero atoms (n = m = 1- 4) heteronuclear bonding is preferred to homonuclear bonding, giving rise to large values of ionization potentials. For larger clusters (n=m=5, 10) stable structures do not favor neither hetero bonding nor segregation into the single components, although they exhibit more metallic than ionic features. This remains valid also for Au8Ag12 cluster characterized by strong charge transfer to gold subunit. The influence of doping of pure gold clusters with silver atoms on VDE and IP values is discussed in context of their reactivity towards O2 and CO molecules. As a starting point we consider reactivity towards CO and O2 molecules on the example of AgAu- dimer. The results show that the catalytic cycle can be fullfilled.  相似文献   

17.
We have found evidence of positive cluster formation during the laser ablation process of a silver target in aqueous solutions. In particular, by employing in situ shot-by-shot UV–vis spectroscopy in the early stages of the ablation, we observed a weak and unstable absorption band around 266 nm and a more stable one around 290 nm, which could be assigned to charged clusters like Ag 3 2+ and Ag 4 2+ , respectively. Surface-enhanced Raman scattering experiments performed with a test molecule adsorbed on a silver colloid obtained in pure water were compatible with the presence of Ag 4 2+ active sites on the surface of the Ag nanoparticles.  相似文献   

18.
《Current Applied Physics》2010,10(3):776-782
Herein a special nanoparticle cluster coated with a porous copolymer is designed and prepared. At first, Ag2O nanoparticles (secondary particles) were fabricated in gelatin solution by a facile chemical approach. Then these nanoparticles were entrapped in a copolymerization system containing gelatin, methyl methacrylate (MMA), an initiator, and using water as a solvent. The nanoparticle clusters coated with porous gelatin-g-PMMA copolymer (Ag2O/gelatin-g-PMMA) were prepared by grafting methyl methacrylate onto gelatin, followed by coating solidification. One significant feature for our approach is that every Ag2O aggregated cluster has been coated with porous gelatin-g-PMMA copolymer film in a unique way, and the Ag2O nanoparticle could penetrate and escape from the coating freely in water by ultrasonication. As a result, this study provides a new approach to prepare monodispersed nanoparticles by ordered porous copolymers with controlled releasing.  相似文献   

19.
The lifetime of positrons in nanostructured nonstoichiometric silver sulfide Ag2–δS samples with average nanoparticle sizes from 44 to 230 nm has been measured. It has been found that the lifetime spectra of Ag2?δS nanoparticles include two components corresponding to the capture and annihilation of positrons in two types of defects: structural vacancies of the metallic sublattice and vacancy clusters at the interfaces between nanoparticles. The long surface component depends on the size of silver sulfide particles and indicates a difference in their defect structure.  相似文献   

20.
Metastable fragmentation of silver bromide clusters   总被引:2,自引:0,他引:2  
The abundance spectra and the fragmentation channels of silver bromide clusters have been measured and analyzed. The most abundant species are AgnBrn - 1 + and AgnBrn + 1 - and Ag14Br13 + is a magic number, revealing their ionic nature. However, some features depart from what is generally observed for alkali-halide ionic clusters. From a certain size, AgnBrn - 1 + is no more the main series, and AgnBr n - 2, 3 + series become almost as important. The fast fragmentation induced by a UV laser makes the cations lose more bromine than silver ions and lead to more silver-rich clusters. Negative ions mass spectra contain also species with more silver atoms than required by stoichiometry. We have investigated the metastable fragmentation of the cations using a new experimental method. The large majority of the cations release mainly a neutral Ag3Br3 cluster. These decay channels are in full agreement with our recent ab initio DFT calculations, which show that Ag+-Ag+ repulsion is reduced due to a globally attractive interaction of their d orbitals. This effect leads to a particularly stable trimer (AgBr)3 and to quasi-planar cyclic structures of (AgBr)n clusters up to n = 6. We have shown that these two features may be extended to other silver halides, to silver hydroxides (AgOH)n, and to cuprous halide compounds. Received 9 November 2000 and Received in final form 25 January 2001  相似文献   

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