Dot Clusters Research Articles

Interference between Visuospatial Dual Tasks and the Effects of Training

The mechanisms involved in reducing interference in visuospatial dual tasks were investigated. Three computerized visuospatial tasks, bargraphs, dot clusters, and occlusions (Boles, 1991, 1992, 1996) comprised the dual tasks. To compare the roles of automaticity and encapsulation, one group practiced dual tasks drawing upon separate processes while another practiced dual tasks drawing upon the same process. The results suggest automaticity plays a larger role in reducing interference than encapsulation. They also support the existence of resource training, in which practicing a mental resource in one task shows transfer to another task using the same resource.

InAs Quantum Dot Clusters Grown on GaAs Droplet Templates: Surface Morphologies and Optical Properties

GaAs nano-mounds formed by droplet epitaxy are used as templates for growth of self-assembled InAs quantum dot clusters (QDCs). These QDCs are found to contain an average of thirteen dots per cluster, of which there are two families of different sized quantum dots. Excitation intensity-dependent photoluminescence (PL) demonstrates that there is no lateral coupling between the two different size quantum dots. Lateral transfer of carriers is observed between different size quantum dots due to thermal activation as seen in their different temperature-dependent optical behaviors.

Spontaneous formation of a cluster of InAs dots along a ring-like zone on GaAs (1 0 0) by droplet epitaxy

We examine in detail the spontaneous formation process of a cluster of InAs dots on GaAs (1 0 0) by droplet epitaxy. A cluster of dots was fabricated by irradiating an As flux for 30 s to In droplets. The dots were found to be formed mainly along a ring-like zone stretching in the [0 1 1] direction, showing that In atoms migrated from the droplet and reacted with As atoms. This indicates the presence of not only the surface diffusion of In atoms but also the inward diffusion of As atoms. This is supported by the fact that such a ring was not visible when the As irradiation time was shortened (5 s). For photoluminescence measurements the dots were capped with a 50-nm-thick GaAs layer. Two peaks at 1.46 and 1.3 eV were observed in the sample annealed at 750 °C. These peaks were attributed to the wetting layer and the dots, respectively.

Magnetism of quantum dot clusters: a Hubbard model study

Magnetic properties of two and three-dimensional clusters of quantum dots are studied with exact diagonalization of a generalized Hubbard model. We study the weak coupling limit, where the electrons interact only within a quantum dot and consider cases where the second or third harmonic oscillator shell is partially filled. The results show that in the case of half-filled shell the magnetism is determined by the antiferromagnetic Heisenberg model with spin 1/2, 1 or 3/2, depending on the number of electrons in the open shell. For other fillings the system in most cases favors a large total spin, indicating a ferromagnetic coupling between the dots.

Perceptual segmentation and the perceived orientation of dot clusters: The role of robust statistics

We investigated perceptual segmentation in the context of a perceived-orientation task. Stimuli were dot clusters formed by the union of a large elliptical sub-cluster and a secondary circular sub-cluster. We manipulated the separation between the two sub-clusters, their common dot density, and the size of the secondary sub-cluster. As the separation between sub-clusters increased, the orientation perceived by observers shifted gradually from the global principal axis of the entire cluster to that of the main sub-cluster alone. Thus, with increasing separation, the dots within the secondary sub-cluster were assigned systematically lower weights in the principal-axis computation. In addition, this shift occurred at smaller separations for higher dot densities-consistent with the idea that reliable segmentation is possible with smaller separations when the dot density is high. We propose that the visual system employs a robust statistical estimator in this task and that data points are weighted differentially based on the likelihood that they arose from a separate generative process. However, unlike in standard robust estimation, weights based on residuals are insufficient to characterize human segmentation. Rather, these must be computed based on more comprehensive generative models of dot clusters.

Charge storage, photoluminescence, and cluster statistics in ensembles of Si quantum dots

In this paper, we provide a framework for the description of the basic transport processes in ensembles of disordered semiconductor quantum dots by studying solid systems of Si nanocrystallites embedded in an insulating matrix. Our combined structural, transport, charge storage, and optical studies enable us to evaluate these processes as a function of the density of the quantum dots. In particular, we found a remarkable matching between the onset of the formation of quantum dot clusters, the above properties, percolation-clusters theory, and intracluster carrier migration.

Incidence and clinical patterns of polypoidal choroidal vasculopathy in Korean patients

To determine the incidence, demographic features, and clinical characteristics of polypoidal choroidal vasculopathy (PCV) in Korean patients. A retrospective review was undertaken of 392 eyes of 321 symptomatic patients suspected of having exudative age-related macular degeneration (AMD) after their first visit to a tertiary hospital between February 2002 and May 2006. All patients underwent a complete ophthalmic examination, including fluorescein and indocyanine green angiography (ICGA). Of the 321 patients (392 eyes), 79 (98 eyes, 24.6%) were diagnosed with PCV. The mean PCV patient age was 64.6 +/- 7.6 years. PCV was more common in men (78.5%), and was usually unilateral (75.9%). In terms of PCV clinical manifestation, 52% of patients showed an exudative pattern, 34.7%, a hemorrhagic pattern, and 13.3%, an extensive hemorrhagic pattern. The mean visual acuity at presentation was 0.231 +/- 0.256. Classification was based on ICGA findings; 52% of patients showed relatively large aneurismal dilations, 25.5% showed atypical vessel deformations, and 22.5% showed dense clusters of numerous small hyperfluorescent dots. The incidence of PCV in Korean exudative AMD patients was relatively high compared with that in other ethnic groups. As in other Asian patient populations, PCV occurred more commonly in men and was predominantly unilateral.

Plasmonic Enhancement of Fluorescence and Raman Scattering by Metal Nanotips

We report modifications to the optical properties of fluorophores in the vicinity of noble metal nanotips. The fluorescence from small clusters of quantum dots has been imaged using an apertureless scanning near-field optical microscope. When a sharp gold tip is brought close to the sample surface, a strong distance-dependent enhancement of the quantum dot fluorescence is observed, leading to a simultaneous increase in optical resolution. These results are consistent with simulations of the electric field and fluorescence enhancement near plasmonic nanostructures. Highly ordered periodic arrays of silver nanotips have been fabricated by nanosphere lithography. Using fluorescence lifetime imaging microscopy, we have created high-resolution spatial maps of the lifetime components of vicinal fluorophores; these show an order of magnitude increase in decay rate from a localized volume around the nanotips, resulting in a commensurate enhancement in the fluorescence emission intensity. Spatial maps of the Raman scattering signal from molecules on the nanotips shows an enhancement of more than five orders of magnitude.

LATE PREHISTORIC CEREMONIAL TREATMENT OF A BISON SKULL IN SOUTHWESTERN MANITOBA

During the 2004 field season, a bison skull was excavated at the Crepeele site in southwestern Manitoba. The skull was in a supine position oriented along a proximal southwest to northeast axis. The bone was badly degraded and was transported back to the lab for careful exposure. It was found that only the horn cores, premaxilla, and the frontals were still intact. The most durable elements of the skull, the petrous bones, teeth and the occipital condyles had been removed, creating an open bowl defined by the remaining bone. As the soil was removed from the interior of the skull, a number of fired clay fragments were encountered and set aside. Upon examination it appeared that a shallow bowl of raw clay had been fired from within, at sufficient heat to bond with the surrounding sand matrix. The surrounding site matrix, and the material within the skull, were aeolian sands. It appears the clay had been introduced deliberately. The interior of the bowl was decorated with red ochre pigment using clusters of dots and other indeterminate shapes. While similar bison skull alignments have been noted elsewhere in the region, the fired and decorated clay bowl is unique.

What dot clusters and bar graphs reveal: Subitizing is fast counting and subtraction

In two studies, we found that dot enumeration tasks resulted in shallow-sloped response time (RT) functions for displays of 1-4 dots and steep-sloped functions for displays of 5-8 dots, replicating results implicating subitizing and counting processes for low and high ranges of dots, respectively. Extracting number from a specific type of bar graph within the same numerical range produced a shallow-sloped but scallop-shaped RT function. Factor analysis confirmed two independent subranges for dots, but all bar graph values defined a unitary factor. Significantly, factor scores and asymmetries both showed correlations of bar graph recognition to dot subitizing but not to dot counting, strongly suggesting that subitizing was used in both enumeration of low numbers of dots and bar graph recognition. According to these results, subitizing appears to be a nonverbal process operating flexibly in either additive or subtractive fashion on analog quantities having spatial extent, a conclusion consistent with a fast-counting model of subitizing but not with other models of the subitizing process.

Simulation of thermal-field directed self-assembly of epitaxial quantum dots

Thermal-field directed self-assembly (TDSA) of epitaxial self-assembled quantum dots (SAQDs) is the method of using spatially varying temperature patterns to influence SAQD growth kinetics and ultimately the order and positions of SAQDs. The effectiveness of TDSA in enhancing the size and spatial order or precision placement of single dot or dot cluster is investigated via a two dimensional (one dimensional surface) finite element based model of Ge dots grown on Si. Three different cases of heating are studied, namely, spatially constant heating, spatially periodic heating, and a Gaussian shaped hot spot. Statistical measures are used to study the evolution of quantum dot heights and spacings between the quantum dots for different temperatures and heating cases. Spatially periodic heating is investigated for a wavelength of about 185nm corresponding to the wavelength of an excimer laser. In order to investigate the effectiveness of spatially periodic heating in enhancing the spatial and size order of the SAQD array, the results are compared with those obtained from spatially uniform heating. Simulations using Gaussian shaped hot spot are performed to demonstrate its effectiveness in placing a distinct dot at a desired location. Results from the simulations indicate that spatially periodic heating proves to be an effective means for producing an array of uniformly sized and spaced quantum dots for a broad range of temperatures. Also, the results from the simulations using Gaussian shaped hot spots show that such a form of heating can effectively place a distinct quantum dot near a desired location for a broad range of temperature values.

Irradiation induced precipitates in vanadium alloys studied by atom probe microanalysis

Radiation-induced titanium precipitates are a major cause of irradiation hardening in vanadium alloys, but microstructural information is still far from comprehensive. In particular, the nature of black dot clusters observed to form during irradiation at low temperatures is still not clear while the large size Ti-precipitates with platelet shapes are relatively well characterized. V–4Cr–(0.1, 1 and 3 wt%)Ti alloys were irradiated up to 0.2 dpa at 350 °C. In TEM observations, platelet precipitates on {1 0 0} planes were observed in V–4Cr–3Ti and V–4Cr–1Ti, while a high density of small defect clusters was observed as black dot contrast in V–4Cr–1Ti. Enrichment of titanium as well as oxygen and titanium oxide in the matrix were observed by three-dimensional atom probe microanalysis in all alloys. Small clusters with sphere shape were observed in V–4Cr–0.1Ti, while the platelet morphology was seen in the other alloys. The mechanisms of nucleation and growth of precipitates in vanadium alloys are discussed.

A study of size dependent structure, morphology and luminescence behavior of CdS films on Si substrate

Size tunable cadmium sulfide (CdS) films deposited by a dip coating technique on silicon (100) and indium tin oxide/glass substrates have been characterized using X-ray diffraction, X-ray reflectivity, transmission electron microscopy, atomic force microscopy and photoluminescence spectroscopy. The structural characterization indicated growth of an oriented phase of cadmium sulfide. Transmission electron microscopy used to calculate the particle size indicated narrow size dispersion. The tendency of nanocrystalline CdS films to form ordered clusters of CdS quantum dots on silicon (100) substrate has been revealed by morphological studies using atomic force microscopy. The photoluminescence emission spectroscopy of the cadmium sulfide films has also been investigated. It is shown that the nanocrystalline CdS exhibit intense photoluminescence as compared to the large grained polycrystalline CdS films. The effect of quantum confinement also manifested as a blue shift of photoluminescence emission. It is shown that the observed photoluminescence behavior of CdS is substantially enhanced when the nanocrystallites are assembled on silicon (100) substrate.

Multiple vertically stacked quantum dot clusters with improved size homogeneity

Structures containing multiple layers of self-assembled InAs quantum dot clusters within a GaAs matrix are investigated on GaAs (1 0 0) by atomic force microscopy. Droplet homoepitaxy (GaAs nanostructures on planar GaAs) is used to create tiny GaAs nano-mound templates elongated along [0 1 −1]. Due to a high density of monolayer steps on the edge of nano-mounds, deposited InAs prefer to form in clusters around the nano-mound templates. By varying the subsequent InAs monolayer coverages and growth temperatures, two distinctive sizes of quantum dots (QDs) are formed around the nano-mounds for the layer stacking. With a fixed GaAs barrier thickness (10 nm) in between the layers, the resulting QDs from the stacked layers show significant improvement in their size uniformity.

Carrier‐induced coherent acoustic phonon excitation in strain engineered InAs/GaAs quantum dot clusters

AbstractStrain engineered InAs/GaAs quantum dot (QD) clusters grown on (311)B GaAs are studied by means of time‐resolved differential reflection spectroscopy revealing coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon modes. Photogenerated carriers which are captured within the ordered lateral QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain‐induced piezoelectric field within the QD clusters. The excited acoustic phonons modulate the optical properties of the QDs through the quantum‐confined Stark effect, causing oscillations of the reflection signal. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Annealing effect on GaAs droplet templates in formation of self-assembled InAs quantum dots

The authors report on the use of GaAs islands, formed by the droplet epitaxy growth technique, as a template for the growth of clusters of InAs quantum dots. Surface morphology measurements show that the shape and dimensions of the GaAs islands and consequently the formation of InAs quantum dots depend strongly on the annealing temperature and annealing time. This can be explained by the diffusion of gallium atom during the annealing process and the selective formation of InAs quantum dots on the misoriented GaAs island sidewalls.

Effect of Aluminum on the Formation of Silver Metal Quantum Dots in Sol–Gel Derived Alumino-Silicate Glass Film

The effect of aluminum incorporation on silver metal quantum dots formation in the alumino-silicate glass film processed by sol-gel process was investigated. The sol-gel derived glass was coated onto the silica glass plate by spin coating with the mixture solution of tetraethyl orthosilicate (TEOS), C2H5OH, H2O, AgNO3, Al(NO3)3. 39H2O, and HNO3 with the molar ratios of Ag/Si = 0.12 and Al/Si varying from 0 to 0.12. The formation of the silver metal quantum dots was confirmed by the measurements of the UV/VIS optical spectra, the X-ray diffraction patterns, and the transmission electron microscope images. While the radius of silver metal quantum dots increased with the increase of aluminum concentration, the concentration of the silver metal quantum dots decreased. The formation of the silver metal quantum dots was found strongly suppressed by incorporation of aluminum ions in the glass. The change in the glass structure due to the aluminum incorporation was investigated by the analysis of the Raman spectra. The silver ions in the glass contributed to form stable (Al:Ag)O4 tetrahedra by pairing with aluminum ions and thus clustering of silver metal quantum dots was hindered.

Coulomb Blockade and Transport in a Chain of One-Dimensional Quantum Dots

A long one-dimensional wire with a finite density of strong random impurities is modeled as a chain of weakly coupled quantum dots. At low temperature T and applied voltage V its resistance is limited by breaks: randomly occurring clusters of quantum dots with a special length distribution pattern that inhibit the transport. Because of the interplay of interaction and disorder effects the resistance can exhibit T and V dependences that can be approximated by power laws. The corresponding two exponents differ greatly from each other and depend not only on the intrinsic electronic parameters but also on the impurity distribution statistics.

The effect of misorientation of the GaAs substrate on the properties of InAs quantum dots grown by low-temperature molecular beam epitaxy

The structural and optical properties of arrays of InAs quantum dots grown on GaAs substrates at relatively low temperatures (250 and 350°C) and with various degrees of misorientation of the surface are studied. It is shown that low-temperature growth is accompanied by the formation of quantum dot clusters along the dislocation loops on the singular surface and along the steps caused by the surface vicinality on the misoriented surface. The formation of quantum dot clusters leads to the appearance of a new long-wavelength band in the exciton photoluminescence (PL) spectra. It is found that the degrees of polarization of the PL spectral band for clusters of various shapes are different.

Coherent acoustic phonons in strain engineered InAs∕GaAs quantum dot clusters

Coherent excitation of the quasilongitudinal and quasitransverse acoustic phonon mode in strain engineered InAs∕GaAs quantum dot (QD) clusters grown on (311)B GaAs is monitored by means of time-resolved differential reflection spectroscopy. Carrier capture within the ordered QD clusters initiate coherent acoustic phonon excitation, which induces a transient modulation of the local strain-induced piezoelectric field within the QD clusters. The excited acoustic phonons then modulate the optical properties of the QDs through the quantum-confined Stark effect, causing distinct oscillations of the differential reflection signal.