Flat Dots Research Articles

Hyperfine interaction for holes in quantum dots: k·p model

We formulate the multi-band kp theory of hyperfine interactions for semiconductor nanostructures in the envelope function approximation. We apply this theoretical description to the fluctuations of the longitudinal and transverse Overhauser field experienced by a hole for a range of InGaAs quantum dots of various compositions and geometries. We find that for a wide range of values of $d$-shell admixture to atomic states forming the top of the valence band, the transverse Overhauser field caused by this admixture is of the same order of magnitude as the longitudinal one, and band mixing adds only a minor correction to this result. In consequence, the kp results are well reproduced by a simple box model with the effective number of ions determined by the wave function participation number, as long as the hole is confined in the compositionally uniform volume of the dot, which holds in a wide range of parameters, excluding very flat dots.

The Impact of Printing Substrate on Dot Deformation in Flexography

This study characterizes and compares the dot properties in flexography on coated and uncoated paper and on OPP film, with the use of conventional AM screening technology and photopolymer plate with flat top dots. A monochrome test form was created and printed with cyan ink on three different printing substrates. For the study of the dot deformation, microscopic images were taken from the prints and analyzed using ImageJ software. The primary area of the study are highlight tones since the reproduction of small dots is synonymous with high quality reproduction. All significant parameters of the dot deformation were evaluated, including the dot geometry, dot sharpness and uniformity of density. The image analysis showed that dot deformation is the result of spreading and penetration of the ink. The microscopic snapshots of the test areas on the prints show that ink penetrates significantly into the pores and holes of the uncoated paper, resulting in a non-homogeneous dot, thus losing its original shape. On coated paper and OPP film, the ink spreads more on the surface of the printing substrate and in such way gives a more homogeneous dot shape.

Magnetostatic Green's functions for the description of spin waves in finite rectangular magnetic dots and stripes

We present derivation of the magnetostatic Green's functions used in calculations of spin-wave spectra of finite-size non-ellipsoidal (rectangular) magnetic elements. The elements (dots) are assumed to be single domain particles having uniform static magnetization. We consider the case of flat dots, when the in-plane dot size is much larger than the dot height (film thickness), and assume the uniform distribution of the variable magnetization along the dot height. The limiting cases of magnetic waveguides with rectangular cross-section and thin magnetic stripes are also considered. The developed method of tensorial Green's functions is used to solve the Maxwell equations in the magnetostatic limit, and to represent the Landau–Lifshitz equation of motion for the magnetization of a magnetic element in a closed integro-differential form.

Controlling Profiles of Polymer Dots by Switching between Evaporation and Condensation

We found that the profiles of the dots formed from the drying droplets of polymer solution can be modified by switching between the evaporation and condensation processes. When a polymer dot is exposed to solvent vapor during a certain time and is dried again, the dot profile changes from ringlike to flat. To obtain a flat dot, there exists an optimal exposure time. We conjecture that the change of the dot profile is due to the refluidization of the polymer film. Our results imply a new possibility for controlling the dot profile in inkjet printing technologies.

Magnetic hysteresis of interface-biased flat iron dots

We report a theoretical study of the coercivity and bias of iron dots exchange coupled with an antiferromagnetic substrate. We show that flat dots, with height close to the iron exchange length, and lateral dimensions of a few exchange lengths, exhibit large enhancement of coercivity and exchange bias. For small interface field strength the magnetization reversal is nearly a coherent rotation with symmetrical loops. Interface pinning leads to large reduction in coercivity and asymmetrical loops, if the interface field strength is comparable to the value of the iron exchange field. We discuss the impact of geometrical confinement and interface pinning on the magnetization reversal mechanisms. We show that small area dots with height larger than the exchange length display stronger interface effects.

Pinning of graphene to Ir(111) by flat Ir dots

Compact flat Ir islands form and are stable at 400 K when $l0.1$ monolayer of Ir is evaporated onto a graphene flake preadsorbed on Ir(111). Local density approximation calculations account for the Ir islands' two dimensionality and their preferred sites on the substrate. They show that local $s{p}^{3}$ bonding at once chemisorbs the dots above the graphene and firmly pins the graphene layer to the underlying metal.

Optical properties of (In,Ga)As capped InAs quantum dots grown on [11k] substrates

Using three-dimensional k∙p calculation including strain and piezoelectricity, the authors showed that the size of the quantum dot (QD) in the growth direction determines the influence of the (In,Ga)As capping layer on the optical properties of [11k] grown InAs QDs, where k=1,2,3. For flat dots, increase of In concentration in the capping layer leads to a decrease of the transition energy, whereas for large dots an increase of the In concentration in the capping layer is followed by an increase of the transition energy up to a critical concentration of In, after which the optical transition energy starts to decrease.

Photoluminescence and magnetophotoluminescence of vertically stacked InAs/GaAs quantum dot structures

We report on photoluminescence of a series of vertically stacked multilayer quantum dot structures. Our analysis of the data taken at different excitation powers and in the magnetic field up to 24T yields a useful insight into the electronic structure of the InAs/GaAs quantum dot structures. The data are compared with results of model electronic–structure calculations for flat dots, including the effects of the strain field. The calculations suggest that the difference between the energies of the ground and first excited transitions is mainly determined by the lateral dimensions of the dots. A comparison of the experimental results with the theoretical ones reveals an increase of the dot dimensions (both height and diameter) and a decrease of the aspect ratio (height/diameter) with increasing number of the quantum dot layers. For seven-layer samples, the wavelength of 1.3μm has been achieved. The increase of the thickness of the spacing layers between adjacent dot layers leads to a decrease of the lateral dimensions of the dots, accompanied by an increase of the aspect ratio and of the energy separation between the ground and first excited transitions.

Optical Spectroscopy of Quantum Dots in High Magnetic Fields

Optical spectroscopy measurements of single InAs/GaAs self-assembled quantum dots have been performed. The multiexcitonic emission from the s-, p-, and d-shells of a single dot is observed and investigated in magnetic field up to 28 T. The Zeeman splitting of the s-shell excitons has been found to depend on the dot morphology. While the energy-splitting in flat (2 nm height) dots linearly increases with magnetic field, its significant non-linearity is observed for larger in size, tall (4 nm height) dots. The effect of magnetic field on the orbital motion of carriers in dots has also been investigated. It has been found that the modified Fock–Darwin pattern explains the observed evolution of the emission from highly-excited single tall quantum dot.

Piling-to-buckling transition in the drying process of polymer solution drop on substrate having a large contact angle

We studied the drying process of polymer solution drops placed on a substrate having a large contact angle with the drop. The drying process takes place in three stages. First, the droplet evaporates keeping the contact line fixed. Second, the droplet shrinks uniformly with receding contact line. Finally the contact line is pinned again, and the droplet starts to be deformed. The shape of the final polymer deposit changes from concave dot, to flat dot, and then to concave dot again with the increase of the initial polymer concentration. This shape change is caused by the gradual transition from the solute piling mechanism proposed by Deegan to the crust buckling mechanism proposed by de Gennes and Pauchard.

Dependence of the electronic structure of self-assembled (In,Ga)As∕GaAs quantum dots on height and composition

While electronic and spectroscopic properties of self-assembled In1−xGaxAs∕GaAs dots depend on their shape, height, and alloy compositions, these characteristics are often not known accurately from experiment. This creates a difficulty in comparing measured electronic and spectroscopic properties with calculated ones. Since simplified theoretical models (effective mass, k∙p, parabolic models) do not fully convey the effects of shape, size, and composition on the electronic and spectroscopic properties, we offer to bridge the gap by providing accurately calculated results as a function of the dot height and composition. Prominent features of our results are the following: (i) Regardless of height and composition, the confined electron energy levels form shells of nearly degenerate states with a predominant s,p,… orbital character. On the contrary, the confined hole energy levels form shells only in flat dots and near the highest hole level (HOMO). (ii) In alloy dots, the electrons s-p splitting depends weakly on height, while the p-p splitting depends nonmonotonically due to alloy fluctuations. In pure, nonalloyed InAs∕GaAs dots, both these splittings depend weakly on height. Furthermore, the s-p splitting is larger, while the p-p has nearly the same magnitude. For hole levels in alloy dots, the s-p splitting decreases with increasing height (the splitting in tall dots being about four times smaller than in flat dots), whereas the p-p splitting remains nearly unchanged. Shallow, pure, nonalloyed dots have a s-p splitting of nearly the same magnitude, whereas the p-p splitting is about three times larger. (iii) As height increases, the s and p characters of the wave function of the HOMO becomes mixed, and so does its heavy-hole and light-hole characters. (iv) In alloy dots, regardless of height, the wave function of low-lying hole states are localized inside the dot. Remarkably, in nonalloyed InAs∕GaAs dots these states become localized at the interface as height increases. The localized states are nearly degenerate and polarized along [11¯0] and [110]. This localization is driven by the peculiarities of the biaxial strain present in the nanostructure.

Amplatzer Atrial Septal Defect Occluder for Pediatric Patients: Radiographic Appearance

To describe the chest radiographic appearance of the Amplatzer septal occluder (ASO) (AGA Medical Corporation, Golden Valley, Minn) for atrial septal defects (ASDs) in pediatric patients. Two radiologists independently reviewed frontal and lateral chest radiographs obtained in young patients 24 hours after transcatheter ASD closure with the ASO. The appearance (flat disks or dots) and location of the ASO were recorded. The location was related to that of a thoracic vertebral body on frontal and lateral chest radiographs and to a line drawn between the anterior margin of the right hilum and the posterior margin of the inferior vena cava (hilar-caval line) on lateral radiographs; this line corresponded to the expected position of the interatrial septum. The relationship between ASO appearance and patient age was assessed with logistic regression and cumulative probability plots. Sixty-eight pediatric patients (age range, 1 month to 18 years; mean age, 4.2 years; 24 boys and 44 girls) were included. On frontal radiographs, the ASO center projected between T7 and T9, either to the right of or over the spinous processes of the vertebral body. On lateral radiographs, the ASO projected over (n = 66) or anterior to (n = 2) the hilar-caval line. On frontal radiographs, it appeared as one or two flat disks (n = 61) or as two metallic dots (n = 7). On lateral radiographs, it appeared as two flat disks (n = 54) or as two metallic dots (n = 14). The relationship between increasing patient age and the metallic dot appearance on frontal and lateral radiographs and on the combination of frontal and lateral radiographs was highly significant in each case (P < .001, likelihood ratio chi(2) test), with r(2) values of 0.35, 0.20, and 0.28, respectively. ASDs were successfully occluded with the ASO in all patients except one, in whom trivial shunting was seen at 12-month follow-up. The ASO in pediatric patients has a characteristic radiographic appearance when properly positioned.

Micromagnetic model of magnetization reversal of magnetically hard ultrathin dots and stripes

We propose improvements and extensions to an analytical model of magnetization reversal in ultrathin flat dots and stripes with in-plane uniaxial anisotropy. Owing to the localized character of nucleation volumes in hard magnetic materials, we use the concept of edge demagnetizing torque, where all demagnetizing effects are applied at the dot's edge. The magnetization state and the reversal field h̃r are predicted as a function of magnetization, dot thickness and in-plane edge orientation. An excellent agreement is found with numerical simulations. An approximate but accurate scaling law is proposed for an easy computation of h̃r. The model is shown to be valid for dots thinner than both exchange length and wall width, and lateral size well above each of these lengths.

Management of bleeding peptic ulcer: Current status of intravenous proton pump inhibitors

Over 80% of ulcer bleeding stops spontaneously, but associated mortality and morbidity remain high. Occurrence of re-bleeding increases mortality 10-fold. Endoscopic findings in those that have bled predict the risk of recurrent bleeding. Patients whose ulcers show a 'flat dot' or clean base (Forrest Class 3) rarely rebleed or need hospitalization. However, actively bleeding ulcers or those with evidence of recent hemorrhage (Forrest Classes 1 and 2) are likely to re-bleed and may need intensive care. Meta-analyses indicate that endoscopic hemostasis has reduced re-bleeding and surgical intervention by over 60% and mortality by 45%. Beyond these reductions, data indicate that (unlike H2-receptor antagonists use, largely devoid of benefits in this area) continuous intravenous infusions of high doses of proton pump inhibitors reduce rebleeding, re-endoscopy, blood transfusion and surgical intervention, but have little effect (beyond endoscopic therapy) on associated mortality, much of it due to conditions other than rebleeding.

Compositional and size-dependent spectroscopic shifts in charged self-assembledInxGa1−xAs/GaAsquantum dots

Atomistic pseudopotential many-body calculations of excitonic (X) recombination in charged, self-assembled ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ dots predict and explain remarkable trends. (i) The redshift of the exciton energy upon negative charging is rapidly reduced with increasing the In content and increasing the dot height. The opposite behavior is observed upon positive charging. (ii) The recombination peak energies of different charge states show intriguing symmetries and alignments, e.g., ${X}^{\ensuremath{-}}$ aligns with ${X}^{2\ensuremath{-}}$ and ${X}^{3\ensuremath{-}}$ aligns with ${X}^{4\ensuremath{-}}.$ (iii) The ${X}^{3\ensuremath{-}}$ spectrum shows that a triplet initial state is lower in energy for flat dots (yielding two spectral lines), whereas the singlet state is lower in energy for taller dots (yielding a single line). These trends are explained theoretically in terms of a crossover occurring at a critical In concentration and dot height at which the electron wave functions becomes more localized than the hole wave functions.

Self-organized growth of nanosized flat dots and vertical magnetic Co pillars on Au(1 1 1)

We report various experiments that cast some light on the growth process of self-organized dots and vertical Co pillars on Au(1 1 1). We first report grazing incidence small angle X-ray scattering (GISAXS) on Co dots and films, that allowed us to perform a crystallographic study of the order in the self-organized array, and also yielded a statistical view of the coalescence process. We then report scanning tunneling microscopy (STM) and spectroscopy (STS) images performed during the course of vertical growth. Details about the atomic growth process and the possible influence of the growth technique used are reported.

Growth of self-organized nanosized Co pillars in Au(111) using an alternating deposition process

Nanosized Co pillars were grown in Au(111) using a novel deposition process. Starting from a self-organized array of Co dots on the Au(111) surface, we alternatively deposit Au (0.9 ML) and Co (0.1 ML). Under appropriate conditions, the newly deposited Co atoms assemble into dots, vertically self-aligned and in direct contact with the Co dots from the previous layers. This way, pillars 8 nm high and with a 2:1 vertical aspect ratio were fabricated. It is argued that the growth process is driven by parameter misfit and surface energy effects, and should thus occur for other pairs of elements than (Co,Au). From the superparamagnetic regime, we deduce that each pillar behaves like a single magnetic entity. The onset of perpendicular remanence occurs around room temperature, instead of 20 K for the initial flat dots. In terms of applications, the main drawbacks of self-assembled and organized nanosized magnetic systems are the small amounts of material contributing to magnetic effects, and the superparamagnetism blocking temperature, generally well below RT. The pillar growth process is a good candidate to solve them simultaneously.

Self-Organized Growth of Nanosized Vertical Magnetic Co Pillars on Au(111)

Starting from the conventional self-organized arrays of flat Co dots on Au(111), we extended the growth vertically to fabricate Co pillars. The principle is to deposit sequentially a fraction $x$ of an atomic layer (AL) of Co and $1\ensuremath{-}x$ AL of Au. At each step, the deposited Co atoms aggregate on top of the previous dots, increasing their height by 1 AL. Pillars with a 2:1 vertical aspect ratio were achieved. Because of the large number of atoms per pillar, the superparamagnetic blocking temperature is 300 K in the pillars, whereas it is only 20 K in conventional flat dots with the same lateral density.

The evolution of stigmata of hemorrhage in bleeding peptic ulcers: a sequential endoscopic study.

Stigmata of hemorrhage in bleeding peptic ulcers have prognostic characteristics. In the present study, the evolution of these stigmata was studied prospectively using daily endoscopic examinations. From January 1989 to October 1989, 778 consecutive patients with bleeding peptic ulcers underwent endoscopy within 24 hours of admission. The bleeding peptic ulcers were assigned by three endoscopists to five categories, those with: a) active bleeding, b) a nonbleeding visible vessel, c) adherent clot, d) dot, or e) a clean base. Actively bleeding ulcers were treated by epinephrine injection. Ulcers with nonbleeding visible vessels, adherent clots, or dots were left untreated. Daily endoscopic examinations were carried out for three subsequent days, or until the ulcer base became clean. On day 0, there were 56 actively bleeding ulcers (7%), 62 ulcers with visible vessels (8%), 104 with adherent clots (13%), 182 with flat dots (23%), and 374 with a white base (48%). On the subsequent three days, 24 of 62 ulcers with visible vessels (39%), 30 of 104 with adherent clots (29%), 24 of 182 with dots (13%), and 19 of 374 with a clean base (5%) on day 0 re-bled endoscopically or clinically, or both. The overall rebleeding risk was 9.9%, 4.9%, and 2.7% on days 1, 2, and 3, respectively. Stigmata of hemorrhage in bleeding peptic ulcers are predictive of rebleeding. They represent intermediate phases in the evolution of bleeding vessels into clean-based ulcers. The associated rebleeding risk diminishes as the vessel disappears from the ulcer base.