Island Shape Research Articles

Island shapes and aggregation steered by the geometry of the substrate lattice

We find that island shapes and aggregation in diindenoperylene deposited on Au(100), Au(110), and Au(111) single crystals are steered by the anisotropy due to the lattice geometry of the substrate. This phenomenon may be exploited as a tool for molecular patterning of surfaces.

Extraction of Gravity Anomalies Associated with Gold Mineralization: A Comparison of Singular Value Decomposition and Bi-Dimensional Empirical Mode Decomposition

Two methods of both the singular value decomposition (SVD) and the Bi-dimensional empirical mode decomposition (BEMD) were applied in extraction of gravity anomalies associated with gold mineralization in Tongshi gold field, respectively in this paper. Conclusions drawn by the comparison study are as follows: (a) The ore-controlling factor in the Tongshi gold field illustrated in the images obtained from the original gravity data by the two methods is the same that the Tongshi intrusions with a negative circular gravity anomaly and the ring contact metasomatic mineralization zone around the Tongshi intrusions with the positive gravity anomaly. (b) The two methods reveal the same spatial relationship between the ore-controlling factor and various gold mineralizations that the skarn and porphyry types of gold deposits are located within the complex pluton and the Carlin and Crypto-breccia types of gold deposits located within the contact metasomatic mineralization zone. (c) The image produced by BEMD not only reflects the structural features of the ore-controlling factor (Tongshi complex pluton), but also does the distributions of the other geological units in the Tongshi gold field such as the Mesozoic volcanic sedimentary basin in NW orientation with obvious negative gravity anomaly and the conceal metamorphic base swell in NW orientation with the positive gravity anomaly located between the Tongshi intrusions and the Mesozoic volcanic sedimentary basin. The image produced by SVD might depict in more detail the inner structure of the Tongshi intrusions and the ring contact metasomatic zone than that produced by BEMD. The higher gravity anomaly areas in island shape within the ring contact metasomatic zone may be caused by the skarn bodies with iron-copper-gold mineralization. (d) Under the constraints of the ore-forming geological setting, the results obtained from the original gravity data by combination of the two methods can depict the relationships between the ore-controlling factors and the gold mineralizations more exactly than the alternative methods.

Extraction of Gravity Anomalies Associated with Gold Mineralization: A Comparison of Singular Value Decomposition and Bi-Dimensional Empirical Mode Decomposition

Two methods of both the singular value decomposition (SVD) and the Bi-dimensional empirical mode decomposition (BEMD) were applied in extraction of gravity anomalies associated with gold mineralization in Tongshi gold field, respectively in this paper. Conclusions drawn by the comparison study are as follows: (a) The ore-controlling factor in the Tongshi gold field illustrated in the images obtained from the original gravity data by the two methods is the same that the Tongshi intrusions with a negative circular gravity anomaly and the ring contact metasomatic mineralization zone around the Tongshi intrusions with the positive gravity anomaly. (b) The two methods reveal the same spatial relationship between the ore-controlling factor and various gold mineralizations that the skarn and porphyry types of gold deposits are located within the complex pluton and the Carlin and Crypto-breccia types of gold deposits located within the contact metasomatic mineralization zone. (c) The image produced by BEMD not only reflects the structural features of the ore-controlling factor (Tongshi complex pluton), but also does the distributions of the other geological units in the Tongshi gold field such as the Mesozoic volcanic sedimentary basin in NW orientation with obvious negative gravity anomaly and the conceal metamorphic base swell in NW orientation with the positive gravity anomaly located between the Tongshi intrusions and the Mesozoic volcanic sedimentary basin. The image produced by SVD might depict in more detail the inner structure of the Tongshi intrusions and the ring contact metasomatic zone than that produced by BEMD. The higher gravity anomaly areas in island shape within the ring contact metasomatic zone may be caused by the skarn bodies with iron-copper-gold mineralization. (d) Under the constraints of the ore-forming geological setting, the results obtained from the original gravity data by combination of the two methods can depict the relationships between the ore-controlling factors and the gold mineralizations more exactly than the alternative methods.

Postplacement Voltage Island Generation

High power consumption will not only shorten the battery life of handheld devices, but also cause thermal and reliability problems. To lower power consumption, one way is to reduce the supply voltage as in multisupply voltage (MSV) designs. In region-based MSV, a circuit will be partitioned into “voltage islands” where each island occupies a contiguous physical space and operates at one supply voltage. In the work of Wu et al. [2005], this voltage supply problem is addressed, and the input placement is partitioned into a set of rectangular voltage islands by a slicing structure. However, the constraint of using a slicing structure prohibits better solutions in their approach. In the work of Ching et al. [2006], the constraint of obtaining rectangular shapes is relaxed; their method forms islands of very irregular shapes. In this article, we propose a method that focuses on forming rectangular voltage islands to minimize the power consumption, while at the same time favoring the power routing step. It is found that, even with this reduced flexibility on island shapes, we can still perform as well as, or in some cases, even better than the previous work of Ching et al. [2006] that does not control the shapes of the islands, in terms of power saving and island number.

Effects of Ad-atom Diffusivity Throughout Sb-Mediated Formation of Ge/Si Nanoislands

ABSTRACTThe effect of Sb on the formation of Ge nano islands in Si by means of molecular beam epitaxy is reported. We observe in the Ge/Si(100) system a non-monotonic dependence of the Stranski-Krastanov critical thickness of Ge islands formation on the adsorbed Sb amount. Dome- and hut-shaped Ge islands are replaced with the pyramids, when Ge is deposited on the Sb-covered Si(100) surface. The Sb-mediated conservation of the shape of Ge islands during embedding them in Si is shown. We assume that the decrease of the surface diffusion of Si and Ge ad-atoms causes these effects.

Atomistic modeling of alloy self-growth by vapor deposition: Ni and Al on NiAl(110)

ABSTRACTA multisite lattice-gas (msLG) model with realistic and precise surface diffusion kinetics is applied to provide a reliable description of the initial stages of non-equilibrium self-growth of a NiAl alloy by simultaneous stoichiometric codeposition of Ni and Al on the NiAl(110) surface. Deposition at 300 K produces intermixing but poor alloy ordering. Increasing temperature enhances alloy ordering to near perfection at 600 K, but island shapes remain un-equilibrated.

Tunable Localized Plasmon Transducers Prepared by Thermal Dewetting of Percolated Evaporated Gold Films

Gold island films displaying localized plasmon properties were prepared by evaporation of just-percolated Au films onto glass substrates followed by annealing at ≥550 °C. Annealing induces depercolation and formation of large, single-crystalline, well-separated islands, partially embedded in the glass. Two dewetting mechanisms were identified, depending on the initial film morphology. The variability of island sizes and shapes provides effective means of tuning the position of the localized surface plasmon resonance (LSPR) band in a wide wavelength range. With an increase in the Au nominal thickness a transition occurs from transducers dominated by absorbance to ones dominated by scattering. Numerical simulations taking into account the shape and size distribution in actual island samples are in agreement with the experimental spectra. Refractive index sensitivity (RIS) measurements at a constant wavelength or at a constant extinction, tailored to the specific transducer, provide superior sensitivity to refractive index change, up to ca. 600 nm RIU–1 in wavelength shift.

Effect of dislocations on the shape of islands during silicon growth on the oxidized Si(111) surface

The formation of dislocation-rich and dislocation-free silicon islands during growth in the absence of mechanical stresses has been studied by scanning tunneling microscopy. The rounded shape of islands obtained at growth temperatures of 400–500°C on the oxidized Si(111) surface is associated with the presence of dislocations within them. The transfer of atoms from the oxidized surface to the islands occurs due to the barrier of the potential energy at the SiO2/Si boundary. The {111} and {311} facets dominate in the shape of the islands grown at 500–550°C. Their appearance indicates the absence of the threading dislocations in the islands and that the growth is limited by the stage of the nucleation of a new atomic layer.

Electrochemical Synthesis of POT for Electrochromic and Solar Cell Materials

POT (poly (o-toluidine)) was synthesized on ITO/glass by using electrochemical method and the effect of different reaction conditions such as scan rate, the concentrations of monomer and sulfuric acid, and cycle times on the POT was studied. The resulting POT was characterized by spectroscopy such as FT-IR, Roman, and UV-vis methods. The surface morphology of protruding island shape was recorded by scanning electronic microscope (SEM). The electrochromic properties were found that the color of the polymer can turn from green to blue reversibly by increasing the applied voltage. The anode of the solar cell was made of FTO/glass covered with nano-TiO2/POT showed good photo-electron conversion performance which indicated the potential application in electrochromic and solar cell devices.

Morphology of epitaxial magnetic alloy nanostructures grown on WSe2(0001) studied by grazing-incidence small-angle X-ray scattering

The morphology of epitaxial alloy nanostructures grown on a van der Waals-type WSe2(0001) surface was studied using grazing-incidence small-angle X-ray scattering (GISAXS). Assemblies of 111-oriented islands of (Co,Cr)Pt3and (Co,Fe)Pt alloys were grown at different deposition temperatures, with nominal thicknesses from 0.1 to 3 nm, resulting in various island densities. Evaluation of the GISAXS patterns indicates that for similar growth conditions CrPt3islands are flatter than CoPt or FePt islands and exhibit larger island volumes. These features are correlated with the better wetting behaviour and more negative formation enthalpy of the CrPt3alloy. For dense arrays of self-assembled CoPt islands, much smaller island volumes are extracted from GISAXS experiments than are observed by scanning tunnelling microscope imaging, which indicates that only the upper parts of the islands contribute to the GISAXS signal. Another aspect that needs to be taken into account for interpreting GISAXS patterns is the sensitivity of GISAXS to facetting and thus its capacity to extract the island shape. The latter is strongly dependent on the island size. For islands with an average volume smaller than ∼20 nm3, the shape cannot be determined unequivocally. Furthermore, for dense island assemblies with some size dispersity, the identification of steep side-wall facets from the GISAXS patterns is not straightforward as observed for truncated tetrahedron-shaped CoPt3islands.

Analytical Study of Elastic Relaxation and Plastic Deformation in Nanostructures on Lattice Mismatched Substrates

In view of a continuously growing interest in monolithic integration of dissimilar semiconductor materials in a nanostructure form, we present an analytical study of elastic strain energy in nanostructures of different isotropic geometries grown on lattice mismatched substrates. An analytical solution for the elastic stress field is derived, which is not restricted by the small aspect ratio or particular geometry. It is shown that, at a large enough aspect ratio, the relaxation of displacement fields with the vertical coordinate is exponential. This allows us to find an analytical expression for the strain energy density. The cases of rigid and elastic substrates are considered simultaneously. By minimizing the total energy, we obtain the elastic energy density as a function of aspect ratio for given nanostructure geometry. Our data indicate that the elastic energy is highly dependent on the island shape, with the relaxation becoming faster as the contact angle increases. We also present a simple expressi...

Origin of the mosaicity in graphene grown on Cu(111)

We use low-energy electron microscopy to investigate how graphene grows on Cu(111). Graphene islands first nucleate at substrate defects such as step bunches and impurities. A considerable fraction of these islands can be rotationally misaligned with the substrate, generating grain boundaries upon interisland impingement. New rotational boundaries are also generated as graphene grows across substrate step bunches. Thus, rougher substrates lead to higher degrees of mosaicity than do flatter substrates. Increasing the growth temperature improves crystallographic alignment. We demonstrate that graphene growth on Cu(111) is surface diffusion limited by comparing simulations of the time evolution of island shapes with experiments. Islands are dendritic with distinct lobes, but unlike the polycrystalline, four-lobed islands observed on (100)-textured Cu foils, each island can be a single crystal. Thus, epitaxial graphene on smooth, clean Cu(111) has fewer structural defects than it does on Cu(100).

GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures: A way to get high luminescence efficiency in the near ultraviolet range

The epitaxial growth of GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures and their structural and optical properties are reported. The nanostructure formation results from a strain induced growth process (Stransky-Krastanov-like growth mode). Atomic force microscopy measurements show that depending on the amount of deposited GaN, the nanostructure shape evolves from an island shape to a string shape aligned along the [1-100] direction. Transmission electron microscopy experiments reveal that (11-20) and (11-23) lateral facets are formed, making with the (11-22) growth plane an angle of 32° and 12°, respectively, and giving a very asymmetric nanostructure shape. Photoluminescence (PL) experiments as a function of the excitation power and temperature show that the internal electric field is very low compared to the case of GaN/Al0.5Ga0.5N (0001) polar quantum dots (QDs). As a consequence, the PL emission is strongly shifted towards the UV range compared to polar QDs and the full width at half maximum of the PL peak is drastically reduced. Furthermore, a room temperature PL nanostructure emission 300 times more intense than that of the semi-polar GaN template is obtained despite a large density of defects in the surrounding matrix, as expected for efficient carrier localization inside the nanostructures.

Crystallization of pentacene thin films on polymeric dielectrics

Thin films of pentacene have been deposited on five different organic dielectric layers of three different classes of organic materials: a conventional polymer, a photoreactive polymer and a self assembled monolayer. The morphology and crystallographic properties of the pentacene films were investigated with atomic force microscopy and grazing incidence X-ray diffraction. It is found that the films form well known crystallographic structures but show unusual crystal orientations with respect to the surface. The (001) lattice plane of the Campbell phase is tilted approximately 13° from the substrate surface normal while tilts between 4° and 7° are observed for the thin film phase. The island shape and density are found to be inhomogeneous on all substrates. In films where mainly large dendritic grains are observed the thin-film phase is the prevalent crystallographic phase while films with a high island density and small irregular grains are dominated by the Campbell phase. It is observed that the mean surface roughnesses and mean surface energies of the substrate play a minor role for the crystallization while the substrate's morphology on nanoscopic scale is crucial for the thin film growth.

Determinants of plant species richness and patterns of nestedness in fragmented landscapes: evidence from land-bridge islands

Land-bridge islands formed by dam construction are considered to be “experimental” systems for studying the effects of habitat loss and fragmentation, offering many distinct advantages over terrestrial fragments. The Thousand Island Lake in Southeast China is one such land-bridge system with more than 1000 islands. Based on a field survey of vascular plant richness on 154 land-bridge islands during 2007–2008, we examined the effects of island and landscape attributes on plant species richness and patterns of species nestedness. We also examined the different responses of plant functional groups (classified according to growth form and shade tolerance) to fragmentation. We found that island area explained the greatest amount of variation in plant species richness. Island area and shape index positively affected species diversity and the degree of nestedness exhibited by plant communities while the perimeter to area ratio of the islands had a negative effect. Shade-tolerant plants were the most sensitive species group to habitat fragmentation. Isolation negatively affected the degree of nestedness in herb and shade-intolerant plants including species with various dispersal abilities in the fragmented landscape. Based on these results, we concluded that the effects of habitat loss and fragmentation on overall species richness depended mostly on the degree of habitat loss, but patterns of nestedness were generated from different ecological mechanisms due to species-specific responses to different characteristics of habitat patches.

Ge Growth on Vicinal Si(001) Surfaces: Island's Shape and Pair Interaction versus Miscut Angle

A complete description of Ge growth on vicinal Si(001) surfaces is provided. The distinctive mechanisms of the epitaxial growth process on vicinal surfaces are clarified from the very early stages of Ge deposition to the nucleation of 3D islands. By interpolating high-resolution scanning tunneling microscopy measurements with continuum elasticity modeling, we assess the dependence of island's shape and elastic interaction on the substrate misorientation. Our results confirm that vicinal surfaces offer an additional degree of control over the shape and symmetry of self-assembled nanostructures.

On Morphology and Strain Field of Ge/Si(001) Islands According to TEM Phase Imaging Method

The mechanism driving Germanium islands nucleation and self-assembly is an important effect for opto-electronic applications, still not fully understood. We demonstrate that the new transmission electron microscopy phase imaging method provides insights on the distribution of strain and composition fields in and around the islands on rather large areas. The method consists of retrieving the phase from a focus series of plane view images. The phase image is representative of morphology, composition and strain. The results show that whatever the islands size and shape is, a maximum compressive strain is obtained at the apex of the islands compensated by a maximum tensile strain in the substrate close to the islands perimeter. The maximum compressive strain is associated to a larger Ge concentration. The distribution of tensile strain varies with the shape of the islands: for square base pyramidal "hut" islands, a maximum tensile strain is obtained at the four corners of the pyramid base and for "dome" islands, the tensile strain is less pronounced and affects almost the whole island perimeter. These results are consistent with the higher strain relaxation level of "dome" islands in comparison to those of "hut" islands.

Anisotropy Driven Interrupted Coarsening of the Asaro-Tiller-Grinfeld Instability

We investigate the effects of surface energy anisotropy on the coarsening dynamics of the Asaro-Tiller-Grinfeld instability at stake in thin semiconductor films. We consider a continuum model which accounts for wetting interactions between the film and its substrate, elasticity driven mass currents and surface energy anisotropy. We derive an explicit non-linear, non-local evolution equation for the film height that we solve numerically. Anisotropy, which dictates the island shapes, impacts the growth kinetics by weakening the possible elastic relaxation, which can lead during annealing to an interruption of Ostwald ripening. The resulting stationary state is characterized by square-based pyramids separated by a wetting layer. It is found that the instability onset is delayed when the film thickness decreases above the critical thickness for the instability to occur. We characterize the influence of the growing flux used for the film deposition on the stationary state reached during subsequent annealing, and find that the density of the resulting self-organized islands increases with the flux.

Manipulation of magnetic anisotropy of Co ultrathin films by substrate engineering

The magnetic and structural properties of Co films prepared on various substrates were investigated in situ based on the surface-magneto-optical Kerr effect (SMOKE) and using reflection high-energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The magnetic signals of the Co films were found to change significantly depending on the underlying substrates, the film thickness, and the temperature. Both STM and RHEED observations revealed that the shape and atomic structure of the Co islands were very different, which explains the observed magnetic anisotropy in SMOKE. We also observed a steep increase in coercivity for Co films thicker than 1.5 bi-layers grown on an Ag(111) film. This increase may be interpreted as a fcc-to-hcp structural transformation.

CLASSICAL DYNAMICS OF ESCAPE IN A PERIODICALLY DRIVEN UNBOUND HAMILTONIAN SYSTEM: INITIAL PHASE EFFECTS

We consider the problem of a classical Morse oscillator driven by an external periodic field of controllable characteristics and study systematically the effects of the initial phase of the external field on the probability of escape upon molecular dissociation at the end of the interaction time. First, it is shown that such effects indeed exist and may become important depending on the field amplitude and frequency. Then, an explanation is given in terms of the energy exchanges between the basic periodic orbits and the external field and of the associated modifications of the shape of regular islands in phase space. Finally, we exploit this explanation in order to make predictions regarding the behavior of the escape probability as a function of the initial phase and frequency of the external field.