Nano-sized Islands Research Articles

Short-Time Magnetron Sputtering for the Development of Carbon-Palladium Nanocomposites.

In recent nanomaterials research, combining nanoporous carbons with metallic nanoparticles, like palladium (Pd), has emerged as a focus due to their potential in energy, environmental and biomedical fields. This study presents a novel approach for synthesizing Pd-decorated carbons using magnetron sputter deposition. This method allows for the functionalization of nanoporous carbon surfaces with Pd nano-sized islands, creating metal-carbon nanocomposites through brief deposition times of up to 15 s. The present research utilized direct current magnetron sputtering to deposit Pd islands on a flexible activated carbon cloth substrate. The surface chemistry, microstructure, morphology and pore structure were analyzed using a variety of material characterization techniques, including X-ray photoelectron spectroscopy, X-ray diffraction, Raman spectroscopy, gas sorption analysis and scanning electron microscopy. The results showed Pd islands of varying sizes distributed across the cloth's carbon fibers, achieving high-purity surface modifications without the use of chemicals. The synthesis method preserves the nanoporous structure of the carbon cloth substrate while adding functional Pd islands, which could be potentially useful in emerging fields like hydrogen storage, fuel cells and biosensors. This approach demonstrates the possibility of creating high-quality metal-carbon composites using a simple, clean and economical method, expanding the possibilities for future nanomaterial-based applications.

A Metastable p-Type Semiconductor as a Defect-Tolerant Photoelectrode.

A p-type Cu3Ta7O19 semiconductor was synthesized using a CuCl flux-based approach and investigated for its crystalline structure and photoelectrochemical properties. The semiconductor was found to be metastable, i.e., thermodynamically unstable, and to slowly oxidize at its surfaces upon heating in air, yielding CuO as nano-sized islands. However, the bulk crystalline structure was maintained, with up to 50% Cu(I)-vacancies and a concomitant oxidation of the Cu(I) to Cu(II) cations within the structure. Thermogravimetric and magnetic susceptibility measurements showed the formation of increasing amounts of Cu(II) cations, according to the following reaction: Cu3Ta7O19 + x/2 O2 → Cu(3−x)Ta7O19 + x CuO (surface) (x = 0 to ~0.8). With minor amounts of surface oxidation, the cathodic photocurrents of the polycrystalline films increase significantly, from <0.1 mA cm−2 up to >0.5 mA cm−2, under visible-light irradiation (pH = 6.3; irradiant powder density of ~500 mW cm−2) at an applied bias of −0.6 V vs. SCE. Electronic structure calculations revealed that its defect tolerance arises from the antibonding nature of its valence band edge, with the formation of defect states in resonance with the valence band, rather than as mid-gap states that function as recombination centers. Thus, the metastable Cu(I)-containing semiconductor was demonstrated to possess a high defect tolerance, which facilitates its high cathodic photocurrents.

Wafer-scale fabrication of isolated luminescent silicon quantum dots using standard CMOS technology

A wafer-scale fabrication method for isolated silicon quantum dots (Si QDs) using standard CMOS technology is presented. Reactive ion etching was performed on the device layer of a silicon-on-insulator wafer, creating nano-sized silicon islands. Subsequently, the wafer was annealed at 1100 °C for 1 h in an atmosphere of 5% H2 in Ar, forming a thin oxide passivating layer due to trace amounts of oxygen. Isolated Si QDs covering large areas (∼mm2) were revealed by photoluminescence (PL) measurements. The emission energies of such Si QDs can span over a broad range, from 1.3 to 2.0 eV and each dot is typically characterized by a single emission line at low temperatures. Most of the Si QDs exhibited a high degree of linear polarization along Si crystallographic directions [] and []. In addition, system resolution-limited (250 μeV) PL linewidths (full width at half maximum) were measured for several Si QDs at 10 K, with no clear correlation between emission energy and polarization. The initial part of PL decays was measured at room temperature for such oxide-embedded Si QDs, approximately several microseconds long. By providing direct access to a broad size range of isolated Si QDs on a wafer, this technique paves the way for the future fabrication of photonic structures with Si QDs, which can potentially be used as single-photon sources with a long coherence length.

Growth of single crystalline films on lattice-mismatched substrates through 3D to 2D mode transition

Regarding crystalline film growth on large lattice-mismatched substrates, there are two primary modes by which thin films grow on a crystal surface or interface. They are Volmer-Weber (VW: island formation) mode and Stranski-Krastanov (SK: layer-plus-island) mode. Since both growth modes end up in the formation of three-dimensional (3D) islands, fabrication of single crystalline films on lattice-mismatched substrates has been challenging. Here, we demonstrate another growth mode, where a buffer layer consisting of 3D islands initially forms and a relaxed two-dimensional (2D) layer subsequently grows on the buffer layer. This 3D-2D mode transition has been realized using impurities. We observed the 3D-2D mode transition for the case of ZnO film growth on 18%-lattice-mismatched sapphire substrates. First, nano-sized 3D islands grow with the help of nitrogen impurities. Then, the islands coalesce to form a 2D layer after cessation of the nitrogen supply, whereupon an increase in the surface energy may provide a driving force for the coalescence. Finally, the films grow in 2D mode, forming atomically flat terraces. We believe that our findings will offer new opportunities for high-quality film growth of a wide variety of materials that have no lattice-matched substrates.

Observation of Coulomb Gap and Enhanced Superconducting Gap in Nano-Sized Pb Islands Grown on SrTiO3**Supported by the National Natural Science Foundation under Grant Nos. 11674191 and 11825404, the National Key Program for S&T Research and Development under Grant No. 2016YFA0301002, and the Beijing Advanced Innovation Center for Future Chip (ICFC). W.L. was also supported by Beijing Young Talents Plan.

We report high-resolution scanning tunneling microscopy (STM) study of nano-sized Pb islands grown on SrTiO3, where three distinct types of gaps with different energy scales are revealed. At low temperature, we find that the superconducting gap (Δs) in nano-sized Pb islands is significantly enhanced from the one in bulk Pb, while there is no essential change in superconducting transition temperature Tc, giving rise to a larger BCS ratio 2Δs/kBTc ∼ 8.31 and implying stronger electron-phonon coupling. The stronger coupling can originate from the interface electron-phonon interactions between Pb islands and SrTiO3. As the superconducting gap is totally suppressed under applied magnetic field, the Coulomb gap with apparent V-shape emerges. Moreover, the size of Coulomb gap (ΔC) depends on the lateral size of Pb islands (R) with ΔC ∼ 1/R0.35, indicating that quantum size effect can significantly influence electronic correlations. Our experimental results shall shed important light on the interplay among superconductivity, quantum size effect and correlations in nano-sized strong-coupling superconductors.

Cobalt-Based Fischer–Tropsch Synthesis: A Kinetic Evaluation of Metal–Support Interactions Using an Inverse Model System

Metal–support interactions in the cobalt–alumina system are evaluated using an inverse model system generated by impregnating Co3O4 with a solution of aluminum sec-butoxide in n-hexane. This results in the formation of nano-sized alumina islands on the surface of cobalt oxide. The activated model systems were kinetically evaluated for their activity and selectivity in the Fischer–Tropsch synthesis under industrially relevant conditions (220 °C, 20 bar). The kinetic measurements were complemented by H2-chemisorption, CO-TPR, and pyridine TPD. It is shown that the introduction of aluminum in the model system results in the formation of strong acid sites and enhanced CO dissociation, as evidenced in the CO-TPR. The incorporation of aluminum in the model systems led to a strong increase in the activity factor per surface atom of cobalt in the rate expression proposed by Botes et al. (2009). However, the addition of aluminum also resulted in a strong increase in the kinetic inhibition factor. This is accompanied by a strong decrease in the methane selectivity, and an increase in the desired C5+ selectivity. The observed activity and selectivity changes are attributed to the increase in the coverage of the surface with carbon with increasing aluminum content, due to the facilitation of CO dissociation in the presence of Lewis acid sites associated with the alumina islands on the catalytically active material.

Nano-sized islands growth control in plasma-condensate devices

In this article, we provide a theoretical study of nanostructured thin films growth in a prototype model of plasma-condensate systems. By taking into account anisotropy in the transfer of adatoms between neighbor layers induced by electric field near substrate, we derive the generalized model, describing adsorbate evolution on an intermediate layer of the multi-layer system. In the framework of stability analysis and numerical simulations, we define ranges of the control parameters, responsible for pattern formation. We will discuss the influence of the adsorption coefficient, interaction strength and strength of the anisotropy in transference of adatoms between layers on the dynamics of the pattern formation and statistical properties of the surface structures. We will show that the derived 1-layer model allows one to reproduce the multi-layer pyramidal-like surface structures. This study provides insight into details of self-organization of adatoms into nano-sized adsorbate islands in plasma-condensate systems.

Cu, Mo-doped and pristine-BiVO4 thin films prepared by rf sputtering process for photocatalytic applications

Cu, Mo-doped and pristine BiVO4 thin films were realized by coupling rf sputtering and mechanochemical synthesis technique. These monoclinic thin films were deposited over silicon substrate under the pure argon atmosphere (Ar: 72 sccm). The sputtering targets were prepared by the respective ball-milled powders hydraulically pressed at optimal conditions. XRD analysis shows that the as-deposited thin films are amorphous and the monoclinic structure is obtained with a post-annealing process in the air at 450 °C for 2 h. Compositional, morphological and optical studies were thoroughly investigated by using FE-SEM, AFM and UV–Vis absorption spectroscopies respectively and discussed as a function of photocatalytic degradation of methyl orange organic dyes in solution. The pseudo first-order degradation rate constant values were estimated as 0.028, 0.031 and 0.022 min−1 for Cu, Mo-doped and pristine BiVO4 thin films respectively. Degradation performances are correlated as a function of their structural, surface morphological and film thickness aspects as well as the role of doping agent. Mo-doped high crystalline BiVO4 thin films with thickness 130 nm and morphology of small nano-sized islands on the surface (RMS = 5.08 nm) contributed well to the amplification of photocatalytic performance and followed by Cu doped and pristine BiVO4 thin films.

On the growth mechanisms of polar (100) surfaces of ceria on copper (100)

We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction.

Dimensionality and Valency Dependent Quantum Growth of Metallic Nanostructures: A Unified Perspective

Quantum growth refers to the phenomena in which the quantum mechanically confined motion of electrons in metallic wires, islands, and films determines their overall structural stability as well as their physical and chemical properties. Yet to date, there has been a lack of a unified understanding of quantum growth with respect to the dimensionality of the nanostructures as well as the valency of the constituent atoms. Based on a first-principles approach, we investigate the stability of nanowires, nanoislands, and ultrathin films of prototypical metal elements. We reveal that the Friedel oscillations generated at the edges (or surfaces) of the nanostructures cause corresponding oscillatory behaviors in their stability, leading to the existence of highly preferred lengths (or thicknesses). Such magic lengths of the nanowires are further found to depend on both the number of valence electrons and the radial size, with the oscillation period monotonously increasing for alkali and group IB metals, and monotonously decreasing for transition and group IIIA-VA metals. When the radial size of the nanowires increases to reach ∼10 Å, the systems equivalently become nanosize islands, and the oscillation period saturates to that of the corresponding ultrathin films. These findings offer a generic perspective of quantum growth of different classes of metallic nanostructures.

Oxygen content modulation by nanoscale chemical and electrical patterning in epitaxial SrCoO3−δ (0 < δ ≤ 0.5) thin films

Fast controllable redox reactions in solid materials at room temperature are a promising strategy for enhancing the overall performance and lifetime of many energy technology materials and devices. Easy control of oxygen content is a key concept for the realisation of fast catalysis and bulk diffusion at room temperature. Here, high quality epitaxial brownmillerite SrCoO2.5 thin films have been oxidised to perovskite (P) SrCoO3 with NaClO. X-ray diffraction, scanning probe microscopy and x-ray photoelectron spectroscopy measurements were performed to investigate the structural and electronic changes of the material. The oxidised thin films were found to exhibit distinct morphological changes from an atomically flat terrace structure to forming small nanosized islands with boundaries preferentially in [100] or [010] directions all over the surface, relaxing the in-plane strain imposed by the substrate. The conductivity, or oxygen content, of each single island is confined by these textures, which can be locally patterned even further with electric poling. The high charging level at the island boundaries indicates a magnified electric capacity of SCO thin films, which could be exploited in future device geometries. This finding represents a new way of oxygen modulation with associated self-assembled charge confinement to nanoscale boundaries, offering interesting prospects in nanotechnology applications.

Thermal effects in nano-sized adsorbate islands growth processes at vapor deposition

We study a model of pattern formation in adsorptive systems with a local change in the surface temperature due to adsorption/desorption processes. It is found that thermal effects shrink the domain of main system parameters, when pattern formation is possible. It is shown that an increase in a surface reheat efficiency delays ordering processes. We have found that a distribution of adsorbate islands over sizes depends on relaxation and reheat processes. We have shown that the mean linear size of stationary adsorbate islands is of nano-meter range.

Statistical properties of abnormal nano-sized adsorbate islands growth in a model of adsorptive multilayer system

Abstract A generalized model for nano-sized islands growth in substratum-mediated adsorption multilayer system is proposed. It is shown that the system manifests cascade of first order transitions related to the number of growing atomic layers. We have found that pattern formation on low layers is governed by a rate of adatom transference between neighboring layers. Critical coverage and critical island size governing adsorbate islands formation are defined. Anomalous dynamics of adsorbate island size growth and a crossover of dynamical regimes are discussed. Distributions of adsorbate/vacancy islands over their sizes are studied in details. It is shown that the linear size of vacancy/adsorbate islands varies from 20 to 100 nm.

Interface controlled oxidation states in layered cobalt oxide nanoislands on gold.

Layered cobalt oxides have been shown to be highly active catalysts for the oxygen evolution reaction (OER; half of the catalytic "water splitting" reaction), particularly when promoted with gold. However, the surface chemistry of cobalt oxides and in particular the nature of the synergistic effect of gold contact are only understood on a rudimentary level, which at present prevents further exploration. We have synthesized a model system of flat, layered cobalt oxide nanoislands supported on a single crystal gold (111) substrate. By using a combination of atom-resolved scanning tunneling microscopy, X-ray photoelectron and absorption spectroscopies and density functional theory calculations, we provide a detailed analysis of the relationship between the atomic-scale structure of the nanoislands, Co oxidation states and substrate induced charge transfer effects in response to the synthesis oxygen pressure. We reveal that conversion from Co(2+) to Co(3+) can occur by a facile incorporation of oxygen at the interface between the nanoisland and gold, changing the islands from a Co-O bilayer to an O-Co-O trilayer. The O-Co-O trilayer islands have the structure of a single layer of β-CoOOH, proposed to be the active phase for the OER, making this system a valuable model in understanding of the active sites for OER. The Co oxides adopt related island morphologies without significant structural reorganization, and our results directly demonstrate that nanosized Co oxide islands have a much higher structural flexibility than could be predicted from bulk properties. Furthermore, it is clear that the gold/nanoparticle interface has a profound effect on the structure of the nanoislands, suggesting a possible promotion mechanism.

Thermal evolution of the morphology of Ni/Ge(111)-c(2 × 8) surface

Abstract The thermal evolution of the interface formed by room temperature (RT) deposition of Ni atoms (coverage 0.1, 0.5, 1.2 ML) onto a Ge(111)-c(2 × 8) surface has been studied with the use of scanning tunneling microscopy (STM). Atomically resolved STM images revealed that, at RT, the boundaries between the different c(2 × 8) domains acted as nucleation sites for Ni atoms. After annealing the surface with deposited material at 473 to 673 K the formation of nano-sized islands of NixGey compounds was observed. In addition, the occurrence of ring-like structures was recorded. Based on the dual-polarity images the latter were assigned to Ni atoms adsorbed on Ge adatoms.

Selective Growth of CdTe on Nano-patterned CdS via Close-Space Sublimation

Selective-area deposition of CdTe on CdS via close-space sublimation is used to study the effect of window size (2 μm and 300 nm) on grain growth. The basic fabrication procedures for each of the layers (CdS, SiO2, and CdTe) and for achieving selective-area growth are presented. Selective-area growth of both micro- and nano-scale CdTe islands on CdS substrates using close-spaced sublimation is demonstrated. Scanning electron microscopy and electron backscatter diffraction microstructure analysis show that the micro-scale CdTe islands remain polycrystalline. However, when the island size is reduced to 300 nm, single crystal CdTe can be achieved within the windows. The CdTe grains were most often in the (101) orientation for both the micro- and nano-sized CdTe islands.

Influence of SiH&lt;sub&gt;4&lt;/sub&gt; Flow Rate on GaN Films with &lt;i&gt;In Situ&lt;/i&gt; SiNx Mask on Sapphire Surface&lt;sup&gt;&lt;/sup&gt;

Sapphire substrate was treated by SiH4 under NH3 atmosphere before GaN growth and nanosize islands SiNx mask was formed on the substrate. Properties of GaN films were investigated by high resolution X-ray diffraction, photoluminescence and transmission electron microscopy. The results indicated that the SiH4 flow rate is an important factor affecting the size and density of SiNx mask besides SiH4 treatment time.The GaN films grown on the SiNx-patterned sapphire substrate revealed an epitaxial lateral overgrowth mode, which affected 3D to 2D growth time and the microstructures of GaN films. The lowest FWHM value of (102) rocking curve was 286 arcsec. as the SiH4 flow rate of 0.5sccm, with the calcultated edge-type dislocations density of 4.28×109cm-2.

Formation of Ni-driven nano-sized structures on Ag/Ge(111)-&amp;radic;3 × &amp;radic;3 surface: STM study

The formation of nano-sized structures upon room temperature (RT) deposition of small amounts of Ni (~0.1 ml) onto an Ag/Ge(111)-√3 × √3 surface was investigated by scanning tunnelling microscopy. The RT behaviour was dominated by the formation of Ni clusters, which did not interact chemically with the surface. The images as obtained after annealing within the temperature range between 100oC and 500oC corresponded to the formation of clusters as well as triangular, elongated and hexagonal-shaped nano-sized islands. Annealing above 550oC resulted in the transformation of the √3 × √3 phase into a c(2 × 8) phase. This process was accompanied by the thermal decomposition of the majority of the islands, except the hexagonal-shaped ones. The latter remained on the surface in an unaltered form upon annealing even at 800oC. We propose that the hexagonal-islands are made of Ni and Ge compounds rather than pure Ni atoms.

Nanostructure effects on the kinetics and deactivation at reactions over multifunctional catalysts

This study brings into focus some nanostructure-size effects on the deactivation kinetics at reactions over supported multifunctional catalysts. The active phase dispersion as nano-sized islands on the support predetermines diversities in the action of active sites depending on their location and structure. In view of this, the applied approach assumes participation of active site types differing by coordination, configuration and contribution to various reaction routes. The suggested model concerns regularities associated with the availability of active surface atoms in proper arrangements which facilitate structure-sensitive reactions. Problems linking the vulnerability of active sites with their geometry and structure are put to discussion. Furthermore, the model relates the probabilities for action of different site types to the size of active-phase islands. The effect of site blockage on the probabilities for existence of multi-site active centers (catalytic clusters) facilitating structure-sensitive reactions is explored. The performed analysis points out that, when matter concerns reactions facilitated by multi-centered active sites, two distinct reasons can be specified, by virtue of which coke species may affect the activity of catalytic clusters: (i) canceling the action of partially or totally coke-covered cluster configurations and (ii) reducing the capability of the adjacent active atoms to construct multi-centered active configurations.

Nickel-containing nano-sized islands grown on Ge(111)-c(2 × 8) and Ag/Ge(111)-(√3 × √3) surfaces.

The formation of nano-islands on both a Ge(111)-c(2 × 8) surface and an Ag/Ge(111)-(√3 × √3) surface evaporated with 0.1 ML Ni was investigated by scanning tunneling microscopy (STM). We have noticed that at temperatures lower than 670 K, the reaction between Ni and the individual substrate surfaces proceeds to form different structures: flat-topped islands with a 2√7 × 2√7 or a 3 × 3 reconstruction on the Ni/Ge(111)-c(2 × 8) surface vs. islands with a 7 × 7 reconstruction on the Ni/Ag/Ge(111)-(√3 × √3) surface. From this we have inferred that within a temperature range between room temperature and 670 K, the intermediate Ag layer retards mixing between Ni and Ge atoms. As a result, the grown islands are composed of pure Ni atoms. Within a temperature range from 670 to 770 K, most islands produced on the Ag/Ge(111)-(√3 × √3) surface are identical with those formed on the Ni/Ge(111)-c(2 × 8) surface, suggesting that above 670 K, Ni atoms are likely to bind with Ge atoms. However, an essential difference between STM images of the surfaces under study exists in the appearance of large elongated islands on the Ni/Ag/Ge(111)-(√3 × √3) surface. The formation of the latter is explained in terms of a difference in energy for Ni diffusion on the Ge(111)-c(2 × 8) and Ag/Ge(111)-(√3 × √3) surfaces.