Copper Islands Research Articles

Formation of copper islands on a native SiO 2 surface at elevated temperatures

A combination of in situ X-ray photoelectron spectroscopy analysis and ex situ scanning electron- and atomic force microscopy has been used to study the formation of copper islands upon Cu deposition at elevated temperatures as a basis for the guided growth of copper islands. Two different temperature regions have been found: (I) up to 250 °C only close packed islands are formed due to low diffusion length of copper atoms on the surface. The SiO 2 film acts as a barrier protecting the silicon substrate from diffusion of Cu atoms from oxide surface. (II) The deposition at temperatures above 300 °C leads to the formation of separate islands which are (primarily at higher temperatures) crystalline. At these temperatures, copper atoms diffuse through the SiO 2 layer. However, they are not entirely dissolved in the bulk but a fraction of them forms a Cu rich layer in the vicinity of SiO 2/Si interface. The high copper concentration in this layer lowers the concentration gradient between the surface and the substrate and, consequently, inhibits the diffusion of Cu atoms into the substrate. Hence, the Cu islands remain on the surface even at temperatures as high as 450 °C.

Influence of anion on the kinetics of copper island growth

We compare the kinetics of copper island growth in the kinetic regime in the presence and absence of anion adsorption and show that the growth exponents are inconsistent with a direct attachment model. In sulfate solution, anion adsorption leads to instantaneous nucleation, equal lateral and vertical growth exponents, and suppressed vertical growth. In perchlorate solution, island growth is characterized by progressive nucleation, enhanced vertical growth, and different lateral and vertical growth exponents. Avrami analysis and analysis of the surface power spectra suggest that island growth is controlled by surface diffusion to the island perimeter.

Methodical aspects of studying the electroreduction of nitrate on modified single crystal Pt(hkl) + Cu electrodes

Technique of modification of basal faces Pt(hkl) by adatoms and epitaxial copper deposits is developed. Analysis of potentiostatic current transients of copper deposition/dissolution and atomic force microscopy showed that the activity of Pt(hkl) faces regarding the processes of copper nucleation and epitaxial growth increases in the sequence of Pt(111) < Pt(110) < Pt(100). The reaction of nitrate anion reduction is sensitive towards the surface structure, not only in the case of platinum, but also in the case of copper deposits (including a monolayer of adatoms). The highest process rate is observed for the Pt(100) electrode modified by a monolayer of adatoms or islands of bulk copper; nitrate reduction at the lowest rate occurs at Pt(111) + Cu electrodes. Structure-sensitive competitive adsorption of background electrolyte and nitrate anions is the factor that largely determines the kinetics of nitrate reduction on different faces of platinum single crystal and copper deposits.

Growth kinetics of disk-shaped copper islands in electrochemical deposition

The ability to independently dictate the shape and crystal orientation of islands in electrocrystallization remains a significant challenge. The main reason for this is that the complex interplay between the substrate, nucleation, and surface chemistry is not fully understood. Here we report on the kinetics of island growth for copper on ruthenium oxide. The small nucleation overpotential leads to enhanced lateral growth and the formation of hexagonal disk-shaped islands. The amorphous substrate allows the nuclei to achieve the thermodynamically favorable orientation, i.e., a 111 surface normal. Island growth follows power law kinetics in both lateral and vertical directions. At shorter times, the two growth exponents are equal to 1/2 whereas at longer times lateral growth slows down while vertical growth speeds up. We propose a growth mechanism, wherein the lateral growth of disk-shaped islands is initiated by attachment of Cu adatoms on the ruthenium oxide surface onto the island periphery while vertical growth is initiated by two-dimensional nucleation on the top terrace and followed by lateral step propagation. These results indicate three criteria for enhanced lateral growth in electrodeposition: (i) a substrate that leads to a small nucleation overpotential, (ii) fast adatom surface diffusion on substrate to promote lateral growth, and (iii) preferential anion adsorption to stabilize the basal plane.

High-Rate Solar Photocatalytic Conversion of CO2 and Water Vapor to Hydrocarbon Fuels

Efficient solar conversion of carbon dioxide and water vapor to methane and other hydrocarbons is achieved using nitrogen-doped titania nanotube arrays, with a wall thickness low enough to facilitate effective carrier transfer to the adsorbing species, surface-loaded with nanodimensional islands of cocatalysts platinum and/or copper. All experiments are conducted in outdoor sunlight at University Park, PA. Intermediate reaction products, hydrogen and carbon monoxide, are also detected with their relative concentrations underlying hydrocarbon production rates and dependent upon the nature of the cocatalysts on the nanotube array surface. Using outdoor global AM 1.5 sunlight, 100 mW/cm(2), a hydrocarbon production rate of 111 ppm cm(-2) h(-1), or approximately 160 microL/(g h), is obtained when the nanotube array samples are loaded with both Cu and Pt nanoparticles. This rate of CO(2) to hydrocarbon production obtained under outdoor sunlight is at least 20 times higher than previous published reports, which were conducted under laboratory conditions using UV illumination.

Intensive Contact and the Copying of Paradigms: An Éven Dialect in Contact with Sakha (Yakut)

AbstractThe development of the unique structure of Copper Island Aleut, which displays a combination of Russian finite verb morphology and Aleut nominal and non-finite verbal morphology as well as lexicon has been the subject of heated debate. In the absence of other examples of similar inflectional paradigm copying, the processes leading to this development are hard to elucidate. This paper discusses examples of paradigms copied from the Siberian Turkic language Sakha (Yakut) into a dialect of the Northern Tungusic language Éven spoken in the village of Sebjan-Küöl in northeastern Siberia. These data demonstrate that paradigm copying can take place in a situation of widespread bilingualism, with code-switching playing a vital role. Furthermore, they provide evidence that such mixed forms have the potential of serving as conduits for further copying of grammatical forms, and that they play an important role in the linguistic identity of the speakers, as has been suggested previously for mixed languages such as Copper Island Aleut.

Formation of a Core/Shell Microstructure in Cu–Ni Thin Films

Electrodeposition of Cu–Ni thin films can result in phase separation characterized by the formation of nodular features that exhibit a uniform columnar core/shell structure with a copper-rich core and nickel-rich shell. Here, we show that the core/shell microstructure is the result of differences in the nucleation and growth rates of the two components. In the potential range where the core/shell structure is observed, copper deposition is fast, resulting in the formation of a relatively low density of large hemispherical islands. Nickel deposition is characterized by slower kinetics, resulting in the formation of a high density of small islands surrounding the copper islands. These results provide a basis for understanding the formation of this core/shell microstructure in binary alloy systems.

Growth of copper nanoislands on the Si(100)-c(4 × 12)-Al surface studied by scanning tunneling microscopy

The growth of copper island films on the Si(100)-c(4 × 12)-Al surface phase has been studied using scanning tunneling microscopy (STM). The dependence of the island concentration on their size and the substrate temperature is determined. The diffusion activation energy and the binding energy of copper dimers are determined from an analysis of the experimental STM data in terms of the kinetic theory of nucleation. The critical temperature corresponding to a change in the growth regime is evaluated from an Arrhenius plot. It is established that the c(4 × 12)-Al surface phase exhibits an orienting action on the crystalline structure of Cu nanoislands.

Electrochemical Nucleation and Growth of Copper from Acidic Sulfate Electrolytes on n-Si ( 001 ) : Effect of Chloride Ions

We present a study of the effect of chloride anion additions on the electrochemical nucleation and growth of copper on from acidic sulfate solutions. Chloride additions in the millimolar range depolarize copper reduction, while additions in the range and above have the opposite effect. Chlorides have no effect on the nucleation and growth mode. Deposition at constant potential in the presence of small amounts of chloride leads to an increased deposition overpotential, but contrary to the predictions of nucleation theory, this results in a lower density of nuclei with larger size. We hypothesize that adsorption of chlorides on the copper islands enhances surface diffusion processes which lead to island coarsening through Ostwald ripening. Morphological changes during deposition are possible at room temperature due to the high values of diffusivities at the electrolyte/solid interface.

Copper electrodeposition localized in picoliter droplets using microcantilever arrays

A patterning tool that allows electrochemical reactions to take place inside microdroplets is introduced. This tool relies on the use of a microfabricated array of silicon cantilevers that incorporate a channel into which a gold electrode is patterned. The deposition process consists in forming picoliter droplets onto a conducting surface by a contact method and applying a voltage between the surface and the cantilevers. To demonstrate the capability of the system, arrays of 20μm diameter copper islands are electrodeposited onto gold surfaces. As expected, the copper bumps exhibit increasing heights as a result of longer deposition times.

The morphology and nucleation kinetics of copper islands during electrodeposition

We describe a study, using in situ transmission electron microscopy, of the shape and nucleation kinetics of three-dimensional islands formed during the electrochemical deposition of copper. By operating an electrochemical cell within an electron microscope, we obtain real-time images of the formation of copper islands on a gold electrode while simultaneously recording electrochemical data such as voltage and current. We first present cyclic voltammetry, where the images show the deposition and stripping processes while the voltammogram demonstrates qualitatively the regimes in which diffusion and surface reaction are the rate limiting steps. We then examine island growth quantitatively under conditions of constant potential. Images recorded during growth at various potentials allow direct visualization of the differences between island shapes in the diffusion limited and kinetically limited growth regimes. Furthermore, a combined analysis of the current transients and the images allows parameters such as the diffusion coefficient, the rate constant and the critical nucleus size to be determined. We discuss these results in the context of electrochemical nucleation and growth models.

Growth of Copper on Reconstructed Pt(100)

The growth of copper on the hexagonally reconstructed Pt(100) surface has been studied in UHV by scanning tunneling microscopy (STM). The copper film grows anisotropically and initially almost in a bilayer fashion due to a high diffusion barrier at copper island edges and a reduced mobility on the Cu islands. The substrate reconstruction is lifted only locally by Cu island formation. This lifting leads to an ordered incorporation of platinum excess atoms into the first layer of the growing copper film: At 340 K, parallel, monatomically wide platinum chains are formed, whereas at 650 K, broader line structures appear in which the platinum atoms are again hexagonally arranged. Up to nine monolayers, the Cu films remain pseudomorphic with the Pt(100) substrate but undergo an fcc → bct transition beyond 4 monolayers.

Spectroscopic Studies on Electroless Deposition of Copper on Hydrogen-Terminated Si(111) Surface in NH4F Solution Containing Cu(II) Ions

The electroless deposition of copper on the hydrogen-terminated Si(111) surface was investigated by means of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy, scanning tunneling microscopy (STM), and energy-dispersive spectroscopy (EDS). The hydrogen-terminated Si(111) surface prepared was stable under air atmosphere for a day or more. It was found from ATR-FTIR that two bands centered at 2000 and 2260 <TEX>$cm^{-1}$</TEX> appeared after the H-Si(111) surface was immersed in 40% <TEX>$NH_4F$</TEX> solution containing 10 mM <TEX>$Cu^{2+}$</TEX>. On the other hand, STM image included the copper islands with a height of 5 nm and a diameter of 10-20 nm. The EDS data displayed the presence of copper, silicon and oxygen species. The results were rationalized in terms of the redox reaction of surface Si atoms and <TEX>$Cu^{2+}$</TEX> ions in solutions, which are changed into <TEX>$Si(OH)_x(F)_y$</TEX> containing <TEX>$SiF_6^{2-}$</TEX> ions and neutral copper islands.

Coexistance of Cu and Ag in surface nano-structure and the new behavior of glycine adsorption on silver surfaces

Novel properties of surfaces with heteronanostructures are of considerable interest in recent years. Although glycine can only be physisorbed on pure sil ver surfaces, our recent results show that monolayersilver islands deposited o n copper surfaces, with the help from the nearby nude copper surface areas, have the ability of chemisorption of glycine. In the present study we find that, on the other way round, copper islands deposited on the Ag(111) surface can also ma ke glycine being chemisorbed on the nude silver areas, although in the latter ca se the surface of a silver bulk is involved. This result demonstrates that it is the nanocoexistance of Cu and Ag on the surface, rather than any specific str ucture of the coexistance, that gives rise to the novel chemical property of the silver surfaces. Furthermore, we also find that all deposited copper islands will gradually be covered by a monolayer of Ag atoms, even at the room temperature , and thus lose the ability of helping the silver surface to gain the novel chem ical property. This indicates that in order to take advantage of the novel prope rties of heteronanostructures, we have to consider their longterm stabilit y and very often it is possible to choose the stable ones.

Metal island growth and dynamics on molybdenite surfaces

In order to understand the adsorption mechanism of metal atoms to semiconducting surfaces, we have studied, as a model system, the vapor phase adsorption of Ag, Au, and Cu on the (001) surface of molybdenite (MoS 2) and the subsequent surface diffusion of these adsorbates. Our scanning tunneling microscopy (STM) images show that, depending on the type of metal atom that is adsorbed, islands of a characteristic size (2 nm for Ag, 8 to 10 nm for Cu, two distinct sizes of 2 nm and 8 to 10 nm for Au), shape (well rounded in the lateral extension) and thickness (one monolayer for Ag, 1 to 1.5 nm for Cu) are formed during the initial stages of deposition. Whole islands are observed to surface diffuse without loss of size or shape. Despite the relatively large size of the copper islands on molybdenite, these islands surface diffuse extensively, suggesting that the Cu-S interaction is weak. Surface diffusion is only hindered once individual islands start to coalesce. As copper islands accumulate, the size and shape of the original islands can still be recognized, supporting the conclusion that these characteristics are constant and that monolayer growth occurs by the aggregation of islands across the surface. The strength and the nature of the Ag-S(MoS 2) bond were further investigated by using molecular orbital calculations, ultraviolet photoelectron spectroscopy (UPS) and scanning tunneling spectroscopy (STS). By applying quantum mechanical approaches using a two-dimensional periodic molybdenite slab and hexagonal MoS 2 clusters of different sizes with metal atoms adsorbed to them, it is possible to calculate the electron transfer between the mineral surface and the metal atom as well as the adsorption energy as a function of surface coverage. In addition, we used the results from the quantum mechanical runs to derive empirical potentials that model the characteristics of the forces within the crystal, within the adsorbed islands, and the metal and mineral surface. The combination of quantum mechanical calculations and empirical force field calculations explain the electronic structure and the highest stability of Ag islands that have seven atoms in diameter, which exactly agrees with the size of experimentally observed islands. UPS results also suggest that a specific new state is formed (approximately 4.5 eV into the valence band) which may describe the Ag-S bond because it does not occur in pure silver or molybdenite. This study shows how the combination of microscopic (STM), spectroscopic (STS, UPS), compositional (X-ray photoelectron spectroscopy, XPS) and molecular modeling (quantum mechanical and empirical) techniques is a useful approach to understand the nature of the metal to sulfide bond. Further insights may be gained concerning the natural association of certain metals with sulfides.

Analysis of Mitochondrial DNA Diversity in the Aleuts of the Commander Islands and Its Implications for the Genetic History of Beringia

The Aleuts are aboriginal inhabitants of the Aleutian archipelago, including Bering and Copper (Medny) Islands of the Commanders, and seem to be the survivors of the inhabitants of the southern belt of the Bering Land Bridge that connected Chukotka/Kamchatka and Alaska during the end of the Ice Age. Thirty mtDNA samples collected in the Commanders, as well as seven mtDNA samples from Sireniki Eskimos in Chukotka who belong to the Beringian-specific subhaplogroup D2, were studied through complete sequencing. This analysis has provided evidence that all 37 of these mtDNAs are closely related, since they share the founding haplotype for subhaplogroup D2. We also demonstrated that, unlike the Eskimos and Na-Dene, the Aleuts of the Commanders were founded by a single lineage of haplogroup D2, which had acquired the novel transversion mutation 8910A. The phylogeny of haplogroup D complete sequences showed that (1) the D2 root sequence type originated among the latest inhabitants of Beringia and (2) the Aleut 8910A sublineage of D2 is a part of larger radiation of rooted D2, which gave rise to D2a (Na-Dene), D2b (Aleut), and D2c (Eskimo) sublineages. The geographic specificity and remarkable intrinsic diversity of D2 lineages support the refugial hypothesis, which assumes that the founding population of Eskimo-Aleut originated in Beringan/southwestern Alaskan refugia during the early postglacial period, rather than having reached the shores of Alaska as the result of recent wave of migration from interior Siberia.

Infrared absorption study of CO chemisorption on copper island films

The infrared absorption spectra of chemisorbed CO at 90 K on copper island films deposited by electron-beam evaporation onto surface-oxidized silicon substrates have been measured at normal incidence of radiation. The integrated absorption intensity of the C–O stretch mode at saturation of CO adsorption depends on both the mass thickness of copper and the substrate temperature during copper deposition. The observed CO absorption features are quite different from those observed on bulk single crystals and thick polycrystalline films. The FE-SEM images reveal that at a given mass thickness the islands are larger when deposited at 296 K than when deposited at 473 K. This difference is reflected in the mass thickness dependence of the absorption intensity; the maximum absorption intensity is obtained at a thickness of 9 nm when deposited at 296 K, whereas the corresponding thickness is 16 nm when deposited at 473 K. A feature common to both films is partial aggregation of the islands. The integrated absorption intensity of the C–O stretch mode observed on the 16 nm film is 5 times larger than that observed on the bulk Cu(1 0 0) surface. This absorption enhancement is attributed to the transverse collective electron resonance of the copper islands.

STM studies of island nucleation during hyperthermal atom deposition

ABSTRACTWe report fundamental changes in island nucleation dynamics as the kinetic energy of the constituent particles used for film grown is increased. A hyperthermal energy ion beam-line with precise control over ion kinetic energy was used to grow copper islands on a Cu(100) substrate. Dramatic increases in island densities were observed with increasing kinetic energy from thermal energies to 150 eV. We find that sputter erosion and the formation of adatom-vacancy pairs contribute to this increase. In addition, variations in flux and temperature suggest that the mean-field scaling exponent is sensitive to atomistic mechanisms activated by the ion beam.

In situ variable temperature atomic force microscopy measurements of copper growth on MgO( [formula omitted

The nucleation properties and thermal stability of the copper on MgO(1 0 0) have been investigated with variable temperature atomic force microscopy. Using non-contact imaging techniques, the nucleation and diffusion of submonolayer coverages of copper on MgO(1 0 0) have been visualized over the temperature range of 300–750 K. For deposition at 300 K, islands one to two layers in thickness form across the terraces of the MgO surface. Upon annealing, the average diameter of the copper islands increases slowly between 300 and 550 K. At higher temperatures, a dramatic increase in island diameter and height occurs. Time dependent images of the Cu/MgO system at temperatures above 550 K reveal a complex evolution of the island structures involving growth, decay, coalescence, and diffusion of the islands.

Dynamical behaviour and sizedependence of 2D copper islands on the Cu(111) surface: amolecular dynamics study

Using molecular dynamics simulations based on an effectivepotential, in analogy to the tight binding theory in the second-moment approximation, we studied the vibrational behaviour of Cu islandsof various sizes on the Cu(111) surface. We found thatsmall clusters are contracted by as much 10% (for a dimer) from the relativepositions of the relaxed interlayers of the island's atoms;the effect is less important with increasing clustersize, N, attaining the same value as for the surface atoms for islandswith more than Nc = 33 atoms. In addition, from the calculatedmean-square displacements, we found that in the direction normal to thesurface the vibrational amplitudes of the island'satoms do not depend on the size of the cluster, while they areincreased by a factor of five in the in-plane directions. Thesefindings are compatible with the behaviour of the cluster phononmodes. Indeed, we found that small islands introduce newvibrational modes that diminish with increasing clustersize, regaining the surface vibrational behaviour. Theseresults suggest that small 2D islands (containing up to 10 atoms)formed on the Cu(111) face are strained; for largerislands this compressive stress is released and the islands exhibit thevibrational behaviour of the substrate for N>Nc. Thischaracteristic cluster size, Nc, coincides with the characteristicisland size above which the migration energy of the island'satoms saturates to the value of the step-energy barrier.