Structure Of Nanoislands Research Articles

Unraveling the electronic structure of cobalt oxide nanoislands on Au(111)

Unraveling the electronic structure of cobalt oxide nanoislands on Au(111)

Atomic Structures of Single-Layer Nanoislands of Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au Supported on Au(111) from Density Functional Theory Calculations

We have used density functional theory calculations to study the atomic structure of single-layer nanoislands of metal M (M=Ni, Cu, Rh, Pd, Ag, Ir, Pt, Au) supported on M(111) and Au(111) surfaces. Nanoislands of Cu, Pd, Ag, Pt, and Au have planar structures on Au(111), while nanoislands of Ni, Rh, and Ir are nonplanar. The calculations also show that nanoislands of Cu, Pd, Pt, and Au on Au(111) with a diameter below 3 nm can have one of several atomic structures. Two of these structures have atoms at the edges of the nanoislands located near bridge sites on Au(111), and the other structures have atoms at the edges and center of the nanoislands located near bridge sites. The relative stability of these atomic structures depends on the size and nature of the Au-supported nanoparticles. Our findings provided computational support for the work of Liao and Ya [J. Phys. Chem. C. 121 (2017) 19218–19225] reporting the formation of two phases of Pt nanoislands on Au(111). These findings also reveal the rich and complex atomic structures of small single-layer metal nanoislands supported on metal surfaces.

Strain and Low-Coordination Effects on Monolayer Nanoislands of Pd and Pt on Au(111): A Comparative Analysis Based on Density Functional Results.

Recent experiments demonstrated that the catalytic centers for the hydrogen evolution reaction (HER) are different on Pd and Pt nanoislands on Au(111). Inspired by these experiments, we examined the geometric, energetic, electronic and hydrogen adsorption properties of monolayer model nanoislands of Pd and Pt supported on Au(111) with density functional theory calculations. Accordingly, Au-tensile strain effects can be nearly 50% larger on the geometric structure of nanoislands of Pd on Au(111) than their Pt analogs, resulting on different electronic properties for these nanoislands. Despite these differences between Pd and Pt nanoisland on Au(111), our computational modelling of the hydrogen adsorption suggests that the unique catalytic centers for the HER on Pd and Pt nanoislands supported on Au(111) derive from the existence of low-coordinated adsorption sites and the intrinsic properties of Pd and Pt, but not from Au-tensile strain effects.

Structure, morphology and photocatalytic performance of BiVO4 nanoislands covered with ITO thin film

Epitaxial BiVO4 nanoislands and ITO–BiVO4 composite thin films were prepared on yttrium-stabilized zirconia (001) substrates by pulsed laser deposition. Structural characterization results indicated that the growth of ITO film followed the layer-by-layer mode and covered uniformly on the YSZ substrate and BiVO4 islands. The photocatalytic activities of ITO–BiVO4 composites were lower than bare BiVO4 islands, and decreased with the increase of thickness of deposited ITO films. Photocatalytic degradation experiments in anoxic solution showed that, photogenerated electrons in ITO–BiVO4 composites played a more important role in dye degradation than that in bare BiVO4. The complete coverage of ITO layer on BiVO4 nanoislands prevented the contact between photogenerated holes and RhB solution, leading to the accumulation of holes and the rapidly carrier recombination.

Anisotropic iron-doping patterns in two-dimensional cobalt oxide nanoislands on Au(111)

An integrated approach combining density functional theory (DFT) calculations and atomic resolution scanning tunneling microscopy (STM) is used to study well-defined iron-doped cobalt oxide nanoislands supported on Au(111). The focus is on the structure and distribution of Fe dopants within these nanoislands of CoO as a function of Fe to Co ratio. The DFT and STM results agree strongly and complement each other to allow for a more complete understanding of the dopant structure trends on the nanoscale. Using Fe as a marker, we first find that the stacking sequence of the moire structure of the host cobalt oxide nanoislands can be identified unambiguously through a combination of DFT and STM. Using the distinct contrast of the embedded Fe dopant atoms as observed with atom-resolved STM, we find correlations between Fe dopant position and the CoO/Au(111) moire pattern at varying Fe dopant densities. Formation of Fe-dopant clusters within the nanoislands is investigated in detail through DFT and found to agree with the dopant patterns observed in STM. We find that the structural effects of Fe dopants throughout the nanoislands with the basal planes and the two types of edges—the oxygen and metal edges—have different nature. Both DFT calculations and STM images show a strong preference for Fe dopants to be located directly on or near the oxygen edge of the nanoislands as opposed to being directly on or near the metal edge. Taken together, our results illustrate that Fe dopant incorporation and distribution within CoO nanoislands are highly anisotropic and governed by both the moire structure of the basal planes as well as nano-size effects present at the under-coordinated edges of different local geometry and chemistries.

Structure and Stability of Au-Supported Layered Cobalt Oxide Nanoislands in Ambient Conditions

Cobalt oxide is a promising earth-abundant electrocatalyst for water splitting; however, the structural complexity of oxides coupled with the difficulty of characterizing it in its operating environment means that fundamental understanding of its catalytic properties remains poor. In this study, we go beyond vacuum studies and investigate the morphological evolution of a CoOx/Au(111) model system from intermediate to high pressures of H2O vapor by means of scanning tunneling microscopy and near-ambient pressure and vacuum X-ray photoelectron spectroscopy. At elevated H2O pressure, we describe the formation of a well-defined Co(OH)2 nanoisland morphology with cobalt in the +2 oxidation state. In contrast, the presence of O2, in air and liquid water, results in only partially hydroxylated Co3+ phases comprising sheets of the CoO(OHx) trilayer, corresponding to a single sheet of cobalt(III)oxyhydroxide. We conclude that the oxyhydroxide structure, known to be the catalytically active phase for the oxygen evo...

Structural Changes to Supported Water Nanoislands Induced by Kosmotropic Ions

We report the influence of lithium ions on binding and structure of water nanoislands on Au(111) by temperature-programmed desorption and variable-temperature scanning tunneling microscopy. Water coverages between a fraction and full bilayer and two lithium coverages (<0.15% ML) are explored. Lithium enhances selectively the binding of some of the water molecules on precovered Au(111) as compared to water on pristine Au(111), which is revealed by an increase of the water desorption temperature by approx. 10 K. Surprisingly, the effect of lithium on the structure of water is much more extended than expected from these desorption experiments. A small amount of lithium changes the structure of water nanoislands drastically compared to those on pristine Au(111). On pristine Au(111), water ice grows in the form of crystalline islands that are two or three bilayers high. On Li precovered Au(111), the islands are more corrugated, at a 5 times broader apparent height distribution and much smaller, at a 4 times smaller area distribution. These changes reflect the influence of lithium as a structure maker, or kosmotrope, on water. Our study provides unprecedented real-space information of the influence of a kosmotrope on the water structure at the nanoscale. We utilize its kosmotropic behavior to provide real-space images of desorption.

The Electronic Structure of Graphene Nanoislands: A CAS-SCF and NEVPT2 Study

This paper presents a tight binding and ab initio study of finite graphene nanostructures. The attention is focused on three types of regular convex polygons: triangles, rhombuses, and hexagons, which are the most simple high-symmetry convex structures that can be ideally cut out of a graphene layer. Three different behaviors are evidenced for these three classes of compounds: closed-shells for hexagons; low-spin open-shells for rhombuses; high-spin open-shells for triangles.

Symmetry forbidden morphologies and domain boundaries in nanoscale graphene islands

The synthesis of graphene nanoislands with tailored quantum properties requires an atomic control of the morphology and crystal structure. As one reduces their size down to the nanometer scale, domain boundary and edge energetics, as well as nucleation and growth mechanisms impose different stability and kinetic landscape from that at the microscale. This offers the possibility to synthesize structures that are exclusive to the nanoscale, but also calls for fundamental growth studies in order to control them.By employing high-resolution scanning tunneling microscopy we elucidate the atomic stacking configurations, domain boundaries, and edge structure of graphene nanoislands grown on Ni(1 1 1) by CVD and post-annealed at different temperatures. We find a non-conventional multistep mechanism that separates the thermal regimes for growth, edge reconstruction, and final stacking configuration, leading to nanoisland morphologies that are incompatible with their stacking symmetry. Whole islands shift their stacking configuration during cooling down, and others present continuous transitions at the edges. A statistical analysis of the domain structures obtained at different annealing temperatures reveals how polycrystalline, ill-defined structures heal into shape-selected islands of a single predominant stacking. The high crystallinity and the control on morphology and edge structure makes these graphene nanoislands ideal for their application in optoelectronics and spintronics.

Modification of Magnetic Characteristics of Polycrystalline Nife Films at the Irradiation Laser Pulses and Formation of Regular Structure of Magnetic Nanoislands

This paper presents the results of experimental measurements of magnetic properties of thin NiFe films after irradiation with nanosecond laser pulses. The results showed that there is an optimal energy in the laser pulse, at which the magnetic film characteristics vary the most. For the film of 500 nm thick at magnetic susceptibility m for a Nd:YAG laser at l=1064 nm increased almost 3 times, and the coercive force Hc decreased about two. For the excimer laser at l=248 nm, we have received an increase in the magnetic susceptibility m almost ten times while a decrease in the coercive force Hc was 5-6 times. The results suggest the importance of non-thermal effects of laser radiation in the process of changing the structure and magnetic properties of magnetic films. All this shows that with the help of laser radiation it is possible to form a regular structure of nanoislands in thin magnetic nanocrystalline films.

First-principles theory of electronic structure and magnetism of Cr nano-islands on Pd(1 1 1)

We report on the electronic structure, magnetic moments and exchange interactions of one- and two-dimensional Cr clusters on a Pd(1 1 1) substrate, using a real-space method based on density functional theory in the local spin density approximation. We find in general that for the investigated clusters, the magnetic moments are sizeable and almost entirely of spin-character. We demonstrate that the interactions in general are dominated by nearest-neighbor antiferromagnetic Heisenberg form, which implies that Cr on Pd(1 1 1) forms an ideal model system, in which clusters of almost any shape and size can be investigated from a Heisenberg Hamiltonian, using a nearest-neighbor exchange model. We have also found that complex magnetic structures can be realized for linear chains of Cr, due to a competition between exchange interaction and a weaker Dzyaloshinskii–Moriya interaction.

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface.

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm2) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e., parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO2(111)-type O–Ce–O single and double trilayer structures for submonolayer coverages.

Study of the crystal structure of silicon nanoislands on sapphire

The results of studies of the crystal structure of silicon nanoislands on sapphire are reported. It is shown that the principal defects in silicon nanoislands on sapphire are twinning defects. As a result of the formation of such defects, different crystallographic orientations are formed in silicon nanoislands on sapphire. In the initial stages of the molecular-beam epitaxy of silicon on sapphire, there are two basic orientations: the (001) orientation parallel to the surface and the (001) orientation at an angle of 70° to the surface.

Monolayer Nanoislands of Pt on Au and Cu: A First-Principles Computational Study

Au- and Cu-supported Pt monolayer model islands with diameters of up to 2.7 nm were studied by density functional calculations to explore support effects on metal-supported metal nanoislands. We analyzed structure, energy, and reactivity aspects of Ptn monolayer species (n = 19–91) on Au(111) and Cu(111). Exploiting scaling relationships, we were able to quantify lateral and vertical interaction energies in terms of contributions of atoms at the edges and in the inner regions of nanoislands. According to our computational results, the supporting material hardly affects the structure of the Pt nanoislands. This also holds for the catalytic activity, as quantified by the d-band model, because for Au and Cu as supporting metals, the d-band centers of the Pt nanoislands differ on average by ∼0.2 eV only. In contrast, this difference is ∼1 eV for pseudomorphic Pt overlayers, indicating that pseudomorphic model systems do not adequately represent metal-supported metal nanoislands with diameters of a few nanometers. These results are expected to impact the interpretation of experimental and computational studies on metal-supported metal nanoparticles.

Cobalt epitaxial nanoparticles on CaF2/Si(111): Growth process, morphology, crystal structure, and magnetic properties

We study molecular beam epitaxy growth, morphology, crystal structure, and magnetic properties of Co nanoislands on CaF${}_{2}$/Si(111) surface. In order to have a full appreciation of complex growth kinetics at different stages, a comprehensive study of Co growth on CaF${}_{2}$ is carried out by atomic force, scanning electron, and transmission electron microscopies in the direct space, as well as by x-ray and electron diffraction in the reciprocal space. These experimental data are complemented by theoretical modeling. Magnetic properties are characterized by magneto-optical Kerr effect and superconducting quantum interference device magnetometries. Key effects influencing the Co growth on fluorite are addressed, including the sticking probability, the preferential nucleation sites, the size and shape time evolution, the dependence of Co morphology on temperature and Co exposure, and the coalescence mechanism. The two-stage deposition technique is developed, whereby the low-temperature seeding stage is used to facilitate Co nucleation, and the follow-up high-temperature deposition yields Co particles with high crystalline quality. Our results enable precise control over the resulting morphology, spatial ordering, and crystal structure affecting the magnetic properties. In particular, it is demonstrated that the transformation from dense to isolated Co nanoparticles leads to the change of the in-plane and out-of-plane magnetic anisotropy and also the sign of polar and longitudinal magneto-optical Kerr effects.

Growth and structure of CrN nanoislands on Cu(001) studied by scanning tunneling microscopy and X-ray photoemission spectroscopy

We report a method for the fabrication of high structural quality CrN nanoislands on Cu(001). The CrN nanoislands can be fabricated in ultrahigh vacuum conditions by means of Cr atoms deposition on saturated Cu(001)c(2×2)-N surface and subsequent annealing at 500°C. Existence of two types of nanoislands is shown by scanning tunneling microscopy investigations with atomic resolution for different CrN nanoislands formed on surface with 0.35 monolayer Cr coverage. The measured in-plane lattice constant (a=0.39±0.01nm) of the CrN nanoislands is 8% larger than the Cu substrate lattice. X-ray photoemission spectroscopy investigations reveal chromium nitride phase transition from CrN to Cr2N associated with changes of Cr concentration.

The effect of Fe-coverage on the structure, morphology and magnetic properties of α-FeSi2 nanoislands

Self-assembled α-FeSi2 nanoislands were formed using solid-phase epitaxy of low (∼1.2 ML) and high (∼21 ML) Fe coverages onto vicinal Si(111) surfaces followed by thermal annealing. At a resulting low Fe-covered Si(111) surface, we observed in situ, by real-time scanning tunneling microscopy and surface electron diffraction, the entire sequence of Fe–silicide formation and transformation from the initially two-dimensional (2 × 2)-reconstructed layer at 300 °C into (2 × 2)-reconstructed nanoislands decorating the vicinal step-bunch edges in a self-ordered fashion at higher temperatures. In contrast, the silicide nanoislands at a high Fe-covered surface were noticeably larger, more three-dimensional, and randomly distributed all over the surface. Ex situ x-ray photoelectron spectroscopy and high-resolution transmission electron microscopy indicated the formation of an α-FeSi2 island phase, in an α-FeSi2{112} ∥ Si{111} orientation. Superconducting quantum interference device magnetometry showed considerable superparamagnetism, with ∼1.9 μB/Fe atom at 4 K for the low Fe-coverage, indicating stronger ferromagnetic coupling of individual magnetic moments, as compared to high Fe-coverage, where the calculated moments were only ∼0.8 μB/Fe atom. Such anomalous magnetic behavior, particularly for the low Fe-coverage case, is radically different from the non-magnetic bulk α-FeSi2 phase, and may open new pathways to high-density magnetic memory storage devices.

Effect of Multiple Depositions and Annealing Treatments on the Erbium Silicide Nanoislands Self-Assembled on Si(OOl) Substrates

Erbium silicide nanoislands on Si(001) surface are fabricated by novel multiple depositions and annealing treatments method. The morphological investigations determine that the islands could grow with stable square shapes rather than the shape transformation exhibited in the traditional single time evaporation growth. Size distributions analyses further elucidate the effect of multiple depositions and annealing treatments on the nanoisland growth. It is suggested that strain relaxation and static coalescence play important roles in the cyclic growth. Specifically, after 15 times of the cycles, the larger islands are found to undergo the Ostwald ripening, which make the shape of nanoislands irregular. This gives us the direction to adjust the growth parameters to control the island morphology. Furthermore, the crystalline structure of the Er silicide nanoislands is efficiently characterized by grazing incidence synchrotron x-ray diffraction.

Diffraction Anomalous Fine Structure study and atomistic simulation of Ge/Si nanoislands

We applied Grazing Incidence Diffraction Anomalous Fine Structure to the study of the structure of Ge dome-shaped nanoislands, grown by Molecular Beam Epitaxy on Si (001) substrates at a temperature of 650°C. We determined the vertical composition of the islands showing the presence of a strong Ge/Si intermixing that is nearly constant from bottom to top. In particular, an abrupt change is found at the substrate interface where the composition switches from pure Si to Ge0.6Si0.4. The analysis of the Diffraction Anomalous Fine Structure oscillations of the spectra is crucial to obtain the true composition profile. We performed atomistic simulations to investigate the role of the strained substrate underneath the dome on the diffraction results and to quantify the resolution of our method. Anomalous Diffraction spectra and Diffraction Anomalous Fine Structure oscillations have been simulated for a real size and real shape cluster including faceting, giving a more detailed data interpretation and understanding of the Ge–Si intermixing mechanism.

Ge/Si nanostructures with quantum dots grown by ion-beam assisted heteroepitaxy

Effect of low-energy ion-beam irradiation during Ge on Si(100) growth on characteristics of Ge nanoislands array is studied. It is shown by STM that pulsed ion-beam action leads to an increase in Ge nanoislands density. The dependence of Ge nanoislands density upon ion energy is non-monotonous with maximum at 150 eV. RBS data indicate a perfect crystal structure of Ge nanoislands. By Raman spectroscopy the Ge content in SiGe quantum dots is estimated to be higher than 75%.