Quantum size effects in adatom island decay

The decay of hexagonal Ag adatom islands on top of larger Ag adatom islands on a Ag(111) surface is followed by a fast-scanning tunneling microscope. Islands do not always show the expected increase in decay rate with decreasing island size. Rather, distinct quantum size effects are observed where the decay rate decreases significantly for islands with diameters of 6, 9.3, 12.6, and 15.6 nm. We show that electron confinement of the surface state electrons is responsible for this enhancement of the detachment barrier for adatoms from the island edge.     

表面等离激元对氮化硅薄膜中Tb3+离子荧光寿命的影响

利用等离子增强化学气相沉积和离子注入方法,制备了铽掺杂的氮化硅薄膜,然后利用磁控溅射和热处理工艺在薄膜上沉积了不同颗粒尺寸的银薄膜来研究表面等离激元共振对铽离子荧光寿命的影响.实验结果表明氮化硅中Tb³⁺离子的光致荧光最强峰在547 nm,而银薄膜的存在会明显降低稀土离子Tb³⁺的荧光寿命,其寿命的改变是由于银薄膜的表面等离激元改变了电磁场的分布,从而影响了系统的局域光模密度(PMD),理论计算的结果也验证了这一点.     

Structural features of Ag-Cu alloy films produced by the codeposition of sputtered metals

Solid solutions in the form of alloy coatings are obtained in the entire concentration range of binary system using ion-plasma sputtering and the codeposition of Ag and Cu ultrafine particles. Sstudying the structure of solid solutions shows a change in the FCC-lattice parameters from silver to copper with a characteristic bend corresponding to the eutectic composition of the equilibrium phase diagram. Solid-solution decay begins at 100°C, and their complete decomposition occurs at 250°C. The surface structure of the initial solutions is presented by crystallites 20–30 nm in cross section; after decomposition, their size is less than 100 nm. For the latter, the distortions of the lattice parameters are 0.32% for a solution of Cu in Ag and 0.29% for a solution of Ag in Cu. The method of alloy production can be extended to the systems of metals immiscible in the solid phase.     

Excimer laser photofragmentation of metallic nanoparticles

Copper nanoparticles (Cu NPs) were prepared by different chemical methods possessing different sizes. While, silver nanoparticles (Ag NPs) were prepared by borohydride reduction method. The influences the changes in sizes of Ag NPs and Cu NPs were demonstrated by the absorption spectra. When Ag NPs and Cu NPs irradiated with 193 and 308 nm excimer laser, respectively; the maximum absorption decreased as the number of pulses increased up to 10 thousands pulse; due to the size reduction. The TEM photography gives good criteria about the size reduction process. Moreover, the mechanism of photofragmentation was described.     

Reduction of Cu2O islands grown on a Cu(100) surface through vacuum annealing

The reduction of Cu2O islands grown on Cu(100) surfaces through vacuum annealing was visualized by an in situ ultrahigh vacuum transmission electron microscope. The shrinkage of the island followed a linear decay behavior. The complete reduction of the oxide islands leads to the formation of nanoindentations on the Cu surfaces. A simple phenomenological kinetic model based on the dissociation along the island perimeter suitably describes the reduction behavior of the surface oxide islands.     

Formation of quasi-one-dimensional Cu2O structures by in situ oxidation of Cu(100)

Epitaxial Cu2O islands on a Cu(100) surface formed through oxidizing Cu(100) films at 600 degrees C in an ultrahigh vacuum transmission electron microscope were observed to undergo a shape transition from initially square shaped islands to elongated islands at a critical size of approximately 110 nm. Our experimental data on the elongation of Cu2O islands agree with the energetic calculations based on the balance between surface and interface energies and the elastic stress relaxation in the three-dimensional islands. We developed a kinetic model based on oxygen surface diffusion that fits well with the observed volume evolution of the Cu2O islands.     

理论预测晶体表面吸附二维原子岛的形状

提出了凝结势概念用以建立一种预测晶体表面吸附二维原子岛几何结构的理论方法.基于半经验相互作用势（SEAM势和OJ势）的计算表明,同相外延生长的二维原子岛在Cu和Ag的（111）面呈现正六边形结构, 而在Pt（111）面呈现截角三角形状;Cu和Ag的（100）面二维岛则形成正方形.这些理论预测均与实验观测结果一致.由于凝结势的计算不受原子数量的限制,该模型可普遍应用于预测各种表面二维原子岛形状. 关键词： 表面吸附 二维原子岛 量子点     

Ag-Cu双金属团簇中Ag原子偏析行为的 分子动力学研究

采用了恒温分子动力学方法系统模拟研究了不同尺寸不同组分的Ag-Cu双金属团簇的退火过程。分析低温退火结构可得团簇中Ag原子的偏析行为：在Ag原子所占比率较少时,Ag原子全部占据在团簇表面;随着Ag原子数的增多,直到Ag和Cu的比率接近时,绝大多数Ag原子仍占据在团簇表面;这与实验观测Ag-Cu混合团簇中Ag原子的偏析行为完全一致。通过细致研究Ag-Cu双金属团簇中Ag原子的偏析行为与团簇尺寸、组分和温度的关系发现：当Ag原子所占比重明显少于Cu原子时,在各不同尺寸下,Ag原子偏析温度点均高于熔点,即在熔点以上一定温度范围内仍会出现Ag原子的偏析现象;当Ag原子所占比重明显多于Cu原子时,在各不同尺寸下,体系偏析温度点均低于团簇熔点;对于较大尺寸团簇,在Ag原子所占比重接近于Cu原子时,体系偏析温度点与团簇熔点相同。     

Highly Luminescent Material Based on Alq3:Ag Nanoparticles

Tris (8-hydroxyquinoline) aluminum (Alq₃) is an organic semiconductor molecule, widely used as an electron transport layer, light emitting layer in organic light-emitting diodes and a host for fluorescent and phosphorescent dyes. In this work thin films of pure and silver (Ag), cupper (Cu), terbium (Tb) doped Alq₃ nanoparticles were synthesized using the physical vapor condensation method. They were fabricated on glass substrates and characterized by X-ray diffraction, scanning electron microscope (SEM), energy dispersive spectroscopy, atomic force microscope (AFM), UV-visible absorption spectra and studied for their photoluminescence (PL) properties. SEM and AFM results show spherical nanoparticles with size around 70–80 nm. These nanoparticles have almost equal sizes and a homogeneous size distribution. The maximum absorption of Alq₃ nanoparticles is observed at 300 nm, while the surface plasmon resonant band of Ag doped sample appears at 450 nm. The PL emission spectra of Tb, Cu and Ag doped Alq₃ nanoparticles show a single broad band at around 515 nm, which is similar to that of the pure one, but with enhanced PL intensity. The sample doped with Ag at a concentration ratio of Alq₃:Ag?=?1:0.8 is found to have the highest PL intensity, which is around 2 times stronger than that of the pure one. This enhancement could be attributed to the surface plasmon resonance of Ag ions that might have increased the absorption and then the quantum yield. These remarkable result suggest that Alq₃ nanoparticles incorporated with Ag ions might be quite useful for future nano-optoelectronic devices.     

Critical view on TL/OSL properties of Li2B4O7 nanoparticles doped with Cu,Ag and co-doping Cu,Ag: Dose response study

Small size (25 nm) Li₂B₄O₇ nanoparticles doped with different concentrations of Cu, Ag and co-doped with Cu, Ag were prepared by solid state sintering at 700 °C. The crystalline phase and particle sizes analysis were carried out by XRD and TEM. FTIR study reveals the formation of vibrational bonds at 1600–1200 cm⁻¹, 1500–700 cm⁻¹, 950–870 cm⁻¹ and 870–415 cm⁻¹. The kinetic parameters of the TL glow curves were evaluated using CGCD procedure in R-software. The CW-OSL decay curves were fitted with third order exponential decay curves and photoionization cross sections of each component were evaluated. The lifetime of the main TL dosimetric peak were also calculated to check the stability of the signal. Dose responses of the synthesized Li₂B₄O₇ nanoparticles for both the TL and CW-OSL were studied in the range of 0.02 mGy to50 Gy and found to be linear upto this range. Fading of the CW-OSL decay curves were also studied. The MDD of the synthesized samples were also calculated and observed to be 15 μGy.     

Real-time view of mesoscopic surface diffusion

Photoemission electron microscopy is used to study the thermal decay of Ag islands grown epitaxially on Si(001) surfaces. (2 x 3) Ag reconstructed zones, due to migrating Ag atoms supplied to the surface by the decaying islands, surround each of the islands. The shape of these reconstructed zones depends on the degree of diffusion isotropy in the system. We demonstrate that the imaging of these reconstructed "isocoverage zones" constitutes a unique experimental method for directly observing diffusion fields in epitaxial systems. We describe the dynamics of the thermal decay of the islands and the isozones in the context of a continuum diffusion model.     

Scaling of heteroepitaxial island sizes

Monte Carlo simulations of an atomistic solid-on-solid model are used to study the effect of lattice misfit on the distribution of two-dimensional islands sizes as a function of coverage θ in the submonolayer aggregation regime of epitaxial growth. Misfit promotes the detachment of atoms from the perimeter of large pseudomorphic islands and thus favors their dissolution into smaller islands that relieve strain more efficiently. The number density of islands composed of s atoms exhibits scaling in the form N_s(θ) ~ θ/〈s〉²g(s/〈s〉) where 〈s〉 is the average island size. Unlike the case of homoepitaxy, a rate equation theory based on this observation leads to qualitatively different behavior than observed in the simulations.     

3D KMC simulations of crater growth during the reduction of oxide nanoislands on metal surfaces

The homoepitaxial growth of Cu nanocraters induced by thermal reduction of Cu₂O nanoislands on Cu(100) surfaces is simulated using a three-dimensional (3D) kinetic Monte Carlo (KMC) model by incorporating surface diffusion, attachment and detachment Cu adatoms dislodged from reducing Cu₂O islands. The craters are observed to grow continuously in rim height and rim slopes while remaining relatively constant in rim width in the course of the oxide decomposition. Such a growth behavior is attributed to the climbing uphill of Cu adatoms released from the perimeter of the reducing Cu₂O island at the crater bottom. The observed decay of the rim height and slopes after completion of the reduction of oxide islands suggests that these surface craters are thermodynamically unstable at high temperatures.     

Cu-Au体系非对称异质外延行为的分子动力学研究

利用分子动力学模拟方法研究了Cu/Au(001)和Au/Cu(001)异质外延岛的演化行为. 研究结果显示：Cu-Au体系的相互外延行为呈现出明显的非对称性. Cu在Au(001)基体表面可以形成完整的外延结构，而Au在Cu(001)基体表面外延将导致失配位错的出现. 导致非对称外延生长行为的根本原因是外延岛的应变状态的差异和外延岛自身性质的不同. 随着外延岛的长大，Cu外延岛与Au(001)基体的微观失配度由最初的接近宏观失配度的9%左右迅速单调下降，并最终趋于晶格匹配；而Au在Cu(001)基体表面外延的微观失配度则呈现出振荡增加趋势. Cu/Au(001)体系的基体形变主要发生在外延岛的边缘，而Au/Cu(001)体系的基体形变主要发生在外延岛内部所对应的区域.     

Formation of nanoislands in the course of copper deposition on a Cu(111)-(9 × 9)-Ag surface

The process of copper deposition on a structured Cu(111)-(9 × 9)-Ag surface, which represents a (9 × 9) loop dislocation network, is studied by scanning tunneling microscopy. It is found that, when the substrate temperature is 100 K and the copper coverage is 0.1–0.4 of a monolayer, islands of a size no greater than 50 Å are formed at the Ag/Cu(111) interface. The islands remain stable as the sample is heated to room temperature. The shape and boundaries of the nanoislands follow the initial surface superstructure and are determined by the nonuniformity of the interaction of the upper silver layer with the copper substrate. The mechanism of island formation and the origin of their stability are explained in terms of the atom exchange between the adsorbate and substrate.     

Charge confinement in epitaxial metallic islands: a study at semiempirical level

Hartree-Fock calculations at semiempirical level, using the extended Debey-Hückel approximation, and tests calculations using LDA with pseudopotentials are used to describe the dispersion relationship, the density of states and the conduction charge of homoepitaxial islands formed by Cu and Ag. The islands have a three-dimensional shape with a dimension of the height and basis in the range of the bulk Fermi wavelength. The calculations indicate that at these small sizes the island steps, at variance with the ones on vicinal surfaces, act as weak and permeable barriers. In fact, the electronic charge retains bulk-like features, though faint traces of confinement can be identified.     

Metal film growth on regular and defective MgO(0 0 1) surface: A comparative ab initio simulation and thermodynamic study

In order to understand the difference in metallic film growth modes on perfect and defective oxide substrates, we have combined ab initio B3LYP periodic calculations on the slab models of the corresponding Me/MgO(0 0 1) interfaces (Me = Ag, Cu) with thermodynamic theory of solid solutions. For a defectless magnesia surface, we confirm the experimentally observed submonolayer growth of 3D metallic islands (Ag possesses a higher trend than Cu). Formation of F_s centers (neutral O vacancies) on the substrate markedly enhances metal atom adsorption as compared to physisorption over regular sites on a defect-free substrate. For the first time, we predict that the presence of these surface defects (beginning with concentrations of 5% for Cu and 22% for Ag) can stimulate the growth of uniform 2D metallic sublayers.     

New magic numbers in metallic clusters: an unexpected metal dependence

Using a many-body tight-binding potential within the second moment approximation in a quenched molecular dynamics simulation, we calculate the internal energy of free Cu, Ag and Au clusters of various sizes and morphologies. We find that the icosahedral structure, which is the equilibrium shape for small sizes at least for Cu and Ag, adopts a very inhomogeneous atomic relaxation. More surprisingly, introducing a vacancy at the center lowers the mean energy per atom for sufficiently large size icosahedra. This means that above a critical size, which decreases from Cu to Au, the icosahedron admits a constitutional vacancy. Taking into account the stability domain of the icosahedron relative to the fcc structure (namely the Wulff polyhedron), we find that there is a stability range of size for Cu and Ag icosahedra with a central vacancy, but not for Au icosahedra. This trend along the noble metal column is discussed in view of tight-binding potential parameters.     

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.     

On 308 nm photofragmentation of the silver nanoparticles

Silver nanoparticles (Ag NPs) were prepared by different chemical methods possessing different sizes 3 ± 2, 8 ± 2, and 20 ± 5 nm. The influence the size of Ag NPs was demonstrated by the absorption and fluorescence spectra, the maximum absorption of Ag NPs increases as the particle size increases. When Ag NPs irradiated with 308 nm excimer laser; the maximum absorption and the full width at half maximum decreased as the number of pulses increased up to 100,000 pulse; due to the size reduction. The fluorescence spectra of Ag NPs and irradiated Ag NPs with 308 nm excimer laser were recorded after excitation at 441.5 nm He-Cd laser, showing a red shift increasing as the particle size is increased.