Analysis of the applicability of Ni,Cu, Au,Pt, and Pd nanoclusters for data recording

The applicability of individual Ni, Cu, Au, Pt, and Pd nanoclusters as data bits in next generation memory devices constructed on the phase-change carrier principle is studied. To this end, based on the modified tight-binding potential (TB-SMA), structure formation from the melt of nanoparticles of these metals to 10 nm in diameter was simulated by the molecular dynamics method. The effect of various crystallization conditions on the formation of the internal structures of Ni, Cu, Au, Pt, and Pd nanoclusters is studied. The stability boundaries of various crystalline isomers are analyzed. The obtained systematic features are compared for nanoparticles of copper, nickel, gold, platinum, and palladium of identical sizes. It is concluded that platinum nanoclusters of diameter D > 8 nm are the best materials among studied metals for producing memory elements based on phase transitions.

     

The role of phase separation for self-organized surface pattern formation by ion beam erosion and metal atom co-deposition

We investigate the ripple pattern formation on Si surfaces at room temperature during normal incidence ion beam erosion under simultaneous deposition of different metallic co-deposited surfactant atoms. The co-deposition of small amounts of metallic atoms, in particular Fe and Mo, is known to have a tremendous impact on the evolution of nanoscale surface patterns on Si. In previous work on ion erosion of Si during co-deposition of Fe atoms, we proposed that chemical interactions between Fe and Si atoms of the steady-state mixed Fe _x Si surface layer formed during ion beam erosion is a dominant driving force for self-organized pattern formation. In particular, we provided experimental evidence for the formation of amorphous iron disilicide. To confirm and generalize such chemical effects on the pattern formation, in particular the tendency for phase separation, we have now irradiated Si surfaces with normal incidence 5 keV Xe ions under simultaneous gracing incidence co-deposition of Fe, Ni, Cu, Mo, W, Pt, and Au surfactant atoms. The selected metals in the two groups (Fe, Ni, Cu) and (W, Pt, Au) are very similar regarding their collision cascade behavior, but strongly differ regarding their tendency to silicide formation. We find pronounced ripple pattern formation only for those co deposited metals (Fe, Mo, Ni, W, and Pt), which are prone to the formation of mono and disilicides. In contrast, for Cu and Au co-deposition the surface remains very flat, even after irradiation at high ion fluence. Because of the very different behavior of Cu compared to Fe, Ni and Au compared to W, Pt, phase separation toward amorphous metal silicide phases is seen as the relevant process for the pattern formation on Si in the case of Fe, Mo, Ni, W, and Pt co-deposition.     

Admetal-induced substrate surface restructuring during metal-on-metal electrochemical deposition studied by in situ scanning tunneling microscopy

Based on time-dependent in situ scanning tunneling microscopy (STM) studies, we demonstrate that for Ni on Ag(111) and Ru on Au(111), electrochemical metal-on-metal deposition can result in pronounced substrate surface restructuring. For Ni/Ag(111), we observe that at low deposition flux and low coverage, Ni submonolayer islands at steps are partly embedded in the Ag terraces, whereas at higher deposition flux and higher coverage, substrate restructuring results in the formation of monolayer bays in the Ag terraces. We suggest that this restructuring process proceeds predominantly via step edge diffusion of Ag atoms. For Ru/Au(111), the formation of fjords and monolayer holes in the Au terraces is observed at low and high Ru coverage, respectively. The importance of the Au surface mobility for the restructuring process is demonstrated by comparing experiments in H₂SO₄ and HCl solutions, in which Au exhibits strongly different surface mobilities. For this system, restructuring involves Au diffusion along Au steps, Au atom detachment from the Au steps, and upward exchange diffusion. According to these observations and their comparison with similar findings for vacuum deposition, we conclude that this restructuring requires (i) a high substrate surface mobility and (ii) a stronger bonding of substrate atoms to deposit islands than to the substrate.     

On the size dependence of the heats of melting of metal nanoclusters

The size dependence of the heats of melting of transition metal nanoclusters (Ni, Au, Cu and Ag) is investigated using two alternative methods of computer simulation (isothermal molecular dynamics and Monte Carlo). Au and Cu nanoclusters are subjected to more detailed study over a wide range of their sizes. The heats of melting are shown to fall along with nanocluster size and to tend in some approximation to a linear dependence on inverse particle radii.     

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.     

Growth and sacrificial oxidation of transition metal nanolayers

Growth and oxidation of Au, Pt, Pd, Rh, Cu, Ru, Ni and Co layers of 0.3-4.3 nm thickness on Mo have been investigated with ARPES and AFM. Co and Ni layers oxidize while the Mo remains metallic. For nobler metals, the on top O and oxidation state of subsurface Mo increase, suggesting sacrificial e⁻ donation by Mo. Au and Cu, in spite of their significantly lower surface free energy, grow in islands on Mo and actually promote Mo oxidation. Applications of the sacrificial oxidation in nanometer thin layers exist in a range of nanoscopic devices, such as nano-electronics and protection of e.g. multilayer X-ray optics for astronomy, medicine and lithography.     

Effects of substrate shape, curvature and roughness on thin heteroepitaxial films of Pt on Au(1 1 1)

The growth of thin (1–10 nm) films of Pt on Au(1 1 1) was studied in order to understand and clarify differences in growth mode observed in ultra-high vacuum (UHV) studies and in electrochemical deposition studies. It was found that on flat Au(1 1 1), Pt grows in a layer-by-layer growth mode, but if the gold substrate is exposed to an acidic environment prior to Pt deposition, then the substrate becomes nanoscopically rough (islanded) and Pt growth follows a pseudo-Stranski–Krastanov (SK) growth mode in which an initially thin wetting layer becomes rougher with increasing film thickness. An analysis of curvature effects on epitaxial growth mode shows that thermodynamic curvature effects involving surface stress are negligible for the Pt/Au(1 1 1) system. Rather, the apparent SK growth is linked to kinetic effects associated with inhomogeneous in-plane elastic relaxation of Pt films on rough surfaces that drive Pt atoms from pits to the tops of islands in the early stages of growth. Implications for the control of epitaxial film roughness are discussed.     

Ordered growth of nanodots on a pre-structured metallic template: Au/N/Cu(0 0 1)

We have studied by Spot Profile Analysis Low Energy Electron Diffraction (SPA-LEED) the growth of gold particles on the N/Cu(0 0 1) self-organized surface. This template consists of nitrogen islands separated by bare Cu lines and forming a regular 2D array of period 5 nm. When Au is evaporated onto this surface, it mainly grows at the intersections between the Cu lines. The islands organization reproduces then the substrate 2D ordering.However, if the substrate temperature is too low, islands form everywhere. On the contrary, if the substrate temperature is too high, some nucleation sites are empty. By following the intensity of the diffraction satellites during the growth, we have observed that the ordering of the Au particles is optimum when the substrate temperature is between 210 and 290 K. Using both an analytical treatment based on the rate equations and kinetic Monte-Carlo simulations, we have determined the activation energy for the diffusion process and the energy of the traps.     

Estimating the Critical Glass Transition Rate of Pure Metals Using Molecular Dynamic Modeling

Doklady Physics - The critical cooling rates $${{{v}}_{{\text{c}}}}$$ at which pure metals Mg, Al, Ti, Fe, Co, Ni, Cu, Zr, Mo, Pd, Ag, Ta, W, Pt, Au, and Pb transit to an amorphous state (vitrify),...     

Au/Cu(001)异质外延岛演化的分子动力学研究

利用分子动力学弛豫方法模拟了Au/Cu(001)异质外延生长初期Au异质外延岛的形貌演化，分析了Au外延岛演化过程中的局域应力及与基体结合能随表面岛尺寸的变化. 研究结果表明：当异质外延岛小于7×7时，外延岛原子分布呈现赝Cu点阵形貌；当外延岛达到8×8后，外延岛内开始出现失配位错，失配位错数量随外延岛尺寸的增加而增加. 局域压力分析指出，外延岛上原子之间的近邻环境不同导致了所受应力的差异，而外延岛的形变则是由外延岛原子的应力分布所决定. 研究还发现，失配位错的产生导致错位原子与基体原子之间的结合强度减弱，但相对增加了非错位原子与基体原子之间的结合强度. 关键词： 异质外延 表面形貌 局域压力 分子动力学模拟     

Synthesis of bimetallic systems using replacement reactions

Series of bimetallic systems were prepared by replacement reactions and characterized by XRD and XPS. The results suggest that the ad-metals are monolayer dispersed on the surface of sub-metal in Pd(Pt, Cu)/Co(Ni) systems, while in Pd(Pt, Au)/Cu systems surface solid solution is formed. In Ag(Au)/Co(Ni) and Ag/Cu systems no interaction between the metals is observed just as in the simple mixture of the respective crystallites. The outermost electronic configurations, the atomic radius of the metals, and the low-preparation temperature seem to be important factors for the different states of these bimetallic catalysts.     

Structure and stability of clusters on metal surfaces

Small clusters of 3d metals Ni/Ni(001), Cu/Cu(001), 4d-Pd/Pd(001), Ag/Ag(001), 5d-Pt/Pt(001), and Au/Au(001) are investigated by semiempirical methods using multiparticle interatomic interaction potentials. It is shown that the same magic numbers (4, 6, and 9) are characteristic for all metals indicated; these numbers are determined by the symmetry characteristics of the clusters, related to the morphology of the fcc (001) substrate. It is shown for Pt/Pt(111) that small clusters of seven, ten, and more atoms are stable for the fcc (111) surface. This confirms that the magic numbers are associated with the symmetry of the clusters. Fiz. Tverd. Tela (St. Petersburg) 41, 1329–1334 (July 1999)     

Comparative investigation on decorating carbon nanotubes with different transition metals

The diffusion dynamics and structure evolvement of the transition metal (TM=Ni, Cu, Au, and Pt) atoms decorating carbon nanotubes (CNTs) with differences have been systematically studied by Monte Carlo (MC) simulation. The studies show that TM atoms can be encapsulated inside, aggregated and even wrapped on the surface of the CNT, which depend on the interactions among TM–TM and TM–C during the spontaneous diffusion process. The decorating effect is greatly influenced by the diameters of CNTs, TM atoms tend to be encapsulated inside the tube in the relatively large CNTs, while they are inclined to stack on the surface for the small ones. More interestingly, Au and Pt atoms would wrap around the smaller CNT, whereas Ni and Cu atoms are still clustering outside of the CNTs with the increase of the number of TM atoms. Simulation results indicate that Pt and Au possess a better wetting effect with CNT than Ni and Cu.     

Ethylene adsorption on thin films of Ni,Pd, Pt,Cu, Au and Al; Work function measurements

The changes in work function φ upon adsorption of C₂H₄ on clean film surfaces of six fcc metals (Ni, Pd, Pt, Cu, Au and Al) have been followed by means of photoelectron emission at 293 K. A marked difference was observed in the behaviour between Ni, Pd and Al on the one side and on Cu, Au and Pt on the other side: while with Ni, Pd and Al, φ as a function of coverage goes through a maximum, with Cu, Au and Pt, φ only decreases. In the discussion, the data obtained by work function measurements are related to other literature data. Several films covered with C₂H₄species were also submitted to a heat treatment while in other experiments H₂ was admitted to the surface covered by C₂H₄ species. In some experiments C₂h₄ was admitted to surfaces covered by H₂. In all cases φ was measured. The experiments reveal that C₂h₄is absorbed only reversibly on Cu and Au. On Ni, Pd and Pt, C₂H₄ is adsorbed initially with dissociation and this leads to an increase in φ on Ni and Pd and a decrease on Pt. Hydrogenated reactive species contribute to the lowering of φ observed with Ni, Pd and Pt. As with Cu and Au also on Ni, Pd and Pt a weakly bound C₂h₄is observed which leads to a decrease in φ as well. The behaviour of φ indicates that upon Al, C₂h₄ adsorbs first dissociatively to a small extent, while the weakly bound C₂H₄species act as intermediates for strongly adsorbed species which were observed after some time.     

Growth and morphology of thin films of aromatic molecules on metals: the case of perylene

The morphology and growth of perylene films on copper and gold surfaces have been characterized by XPS, AFM, SEM and polarization microscopy. Deposition at cryogenic temperatures leads to amorphous but homogeneous films whereas growth a room temperature results in a formation of disjointed crystalline islands. A similar morphology was observed after thawing the amorphous films which were grown before at low temperature and hence demonstrates a pronounced dewetting. Furthermore, it was found that the geometry of the resulting islands depends on the actual substrate surface which is attributed to the formation of seed layers and their influence on the subsequent film growth. The presently described dewetting and island formation appears to be a quite general phenomenon of organic film growth which needs to be considered in the interpretation of spectroscopic data and STM measurements for organic thin films. PACS 68.37.-d; 68.37.Ps; 68.55.Jk     

Morphology, optical and electrical properties of Cu-Ni nanoparticles in a-C:H prepared by co-deposition of RF-sputtering and RF-PECVD

We report optical and electrical properties of Cu-Ni nanoparticles in hydrogenated amorphous carbon (Cu-Ni NPs @ a-C:H) with different surface morphology. Ni NPs with layer thicknesses of 5, 10 and 15 nm over Cu NPs @ a-C:H were prepared by co-deposition of RF-sputtering and RF-Plasma Enhanced Chemical Vapor Deposition (RF-PECVD) from acetylene gas and Cu and Ni targets. A nonmetal-metal transition was observed as the thickness of Ni over layer increases. The surface morphology of the sample was described by a two dimensional (2D) Gaussian self-affine fractal, except the sample with 10 nm thickness of Ni over layer, which is in the nonmetal-metal transition region. X-ray diffraction profile indicates that Cu NPs and Ni NPs with fcc crystalline structure are formed in these films. Localized Surface Plasmon Resonance (LSPR) peak of Cu NPs is observed around 600 nm in visible spectra, which is widen and shifted to lower wavelengths as the thickness of Ni over layer increases. The variation of LSPR peak width correlates with conductivity variation of these bilayers. We assign both effects to surface electron delocalization of Cu NPs.     

面心立方金属声子色散曲线的数值模拟

把分析型嵌入原子法（AEAM）和晶格动力学理论相结合,在三维布喇菲晶格振动模型的基础上模拟了面心立方金属Au、Ag、Pt、Cu和Ni沿[00ζ]、[0ζζ]、[ζζζ]和[0ζ1]4个对称方向的声子色散曲线.结果表明：模拟曲线与实验点线符合的较好,特别在低频附近二者几乎一致,而在纵波模（L）q=（001）2π/a,q=...     

Backscattering correction for quantitative Auger analysis: II. Verifications of the backscattering factors through quantification by AES

The validity and utility of the backscattering correction factors obtained from Monte Carlo calculations for quantitative analysis by Auger electron spectroscopy (AES) were examined through practical quantification of surface concentrations of binary alloys. Quantifications were attempted, first, to access the surface composition of a sputter-deposited NiPt layer, which is probably the most appropriate test-sample with known surface composition for surface analysis. The quantification by AES has led to the result that the surface composition of the layer agrees well with the bulk composition of the sputtered NiPt alloy, as expected. The composition of a sputtered AuCu alloy surface was, then, examined according to the same correction procedure as for the NiPt layer, leading to the confirmation that no preferential sputtering is observed for AuCu alloys by AES as Färber et al. reported.     

Field emission properties and surface structure of nickel containing amorphous carbon

Nickel containing amorphous carbon (a-C:Ni) films have been deposited by filtered cathodic vacuum arc (FCVA) technique by introducing pure nickel into the graphite target. The field electron emission property of a-C:Ni was improved when compared to that of pure tetrahedral amorphous carbon (ta-C) by FCVA. The emission threshold field of a-C:Ni film is about 5 V μm⁻¹, whilst the threshold field of the ta-C film is about 13 V μm⁻¹. Raman spectroscopy suggests that the sp² clusters in the carbon film increase both in size and number when Ni is introduced. However, the emission was found to degrade to threshold fields beyond 20 V μm⁻¹ after the a-C:Ni film was left in ambient for a week. This observation is attributed to surface absorption of oxygen on the a-C:Ni film, as determined by X-ray Photoelectron Spectroscopy.     

Diffusion-limited electrodeposition of ultrathin Au films on Pt(1 1 1)

The electrodeposition of Au on Pt(1 1 1) from electrolytes containing μM concentrations of was studied by in situ scanning tunneling microscopy. Under these conditions the Au flux is limited by diffusion in the electrolyte over a wide potential range, which allows to assess the effect of the electrochemical environment on the growth kinetics. Similar to gas phase metal deposition Au film growth proceeds via nucleation and lateral growth of Au monolayer islands, with the saturation island density strongly depending on the deposition potential and on the anion species in the electrolyte. For deposition in H₂SO₄ solution the saturation island density continuously increases with increasing potential between −0.2 and 0.5 V (SCE), whereas in Cl-containing H₂SO₄ it first decreases and then increases again. Following nucleation and growth theories this behavior can be attributed to potential-induced changes of the Au surface mobility, caused by changes in the density and structure of coadsorbed sulfate/bisulfate and chloride adlayers. Under conditions of high Au surface mobility multilayer growth proceeds via a typical Stranski-Krastanov growth mode, with layer-by-layer growth of a pseudomorphic Au film up to 2 ML and 3D growth of structurally relaxed islands at higher coverage, indicating thermodynamic control under these conditions.