A tight-binding model for illustrating exciton confinement in semiconductor nanocrystals.

The Brus equation describes the relation between the lowest energy of an electron-hole pair and the size of a semiconductor crystallite. However, taking the strong confinement regime as a starting point, the equation does not cover the transition from weak to strong confinement, the accompanying phenomenon of charge-carrier delocalization, or the change in the transition dipole moment of the electron-hole pair state. Here, we use a one-dimensional, two-particle Hubbard model for interacting electron-hole pairs that extends the well-known tight-binding approach through a point-like electron-hole interaction. On infinite chains, the resulting exciton states exhibit the known relation between the Bohr radius, the exciton binding energy, and the effective mass of the charge carriers. Moreover, by introducing infinite-well boundary conditions, the model enables the transition of the exciton states from weak to strong confinement to be tracked, while straightforward adaptations provide insights into the relation between defects, exciton localization, and confinement. In addition, by introducing the dipole operator, the variation of the transition dipole moment can be mapped when shifting from electron-hole pairs in strong confinement to delocalized and localized excitons in weak confinement. The proposed model system can be readily implemented and extended to different multi-carrier states, thus providing researchers a tool for exploring, understanding, and teaching confinement effects in semiconductor nanocrystals under different conditions.

Triangular nanoperforation and band engineering of InGaAs quantum wells: a lithographic route toward Dirac cones in III-V semiconductors.

The design of two-dimensional periodic structures at the nanoscale has renewed attention for band structure engineering. Here, we investigate the nanoperforation of InGaAs quantum wells epitaxially grown on InP substrates using high-resolution e-beam lithography and highly plasma based dry etching. We report on the fabrication of a honeycomb structure with an effective lattice constant down to 23 nm by realising triangular antidot lattice with an ultimate periodicity of 40 nm in a 10 nm thick InGaAs quantum well on a p-type InP. The quality of the honeycomb structures is discussed in detail, and calculations show the possibility to measure Dirac physics in these type of samples. Based on the statistical analysis of the fluctuations in pore size and periodicity, calculations of the band structure are performed to assess the robustness of the Dirac cones with respect to distortions of the honeycomb lattice.

Transport Properties of a Two-Dimensional PbSe Square Superstructure in an Electrolyte-Gated Transistor.

Self-assembled nanocrystal solids show promise as a versatile platform for novel optoelectronic materials. Superlattices composed of a single layer of lead-chalcogenide and cadmium-chalcogenide nanocrystals with epitaxial connections between the nanocrystals, present outstanding questions to the community regarding their predicted band structure and electronic transport properties. However, the as-prepared materials are intrinsic semiconductors; to occupy the bands in a controlled way, chemical doping or external gating is required. Here, we show that square superlattices of PbSe nanocrystals can be incorporated as a nanocrystal monolayer in a transistor setup with an electrolyte gate. The electron (and hole) density can be controlled by the gate potential, up to 8 electrons per nanocrystal site. The electron mobility at room temperature is 18 cm2/(V s). Our work forms a first step in the investigation of the band structure and electronic transport properties of two-dimensional nanocrystal superlattices with controlled geometry, chemical composition, and carrier density.

Visual exploration of objects and scenes in patients with age-related macular degeneration.

OBJECTIVE: Studies on people with age-related macular degeneration (AMD) have shown that they are able to detect briefly displayed objects and scenes with high accuracy (above 80%). However, in everyday life we explore our environment to search and to recognize objects. We assessed visual exploration in people with AMD during the identification of objects and scenes. METHOD: Twenty patients with AMD, fifteen age-matched and twelve young controls participated. We used colored photographs of isolated objects, natural scenes and objects in scenes, displayed centrally on a monitor. Participants were asked to name the objects and scenes. Ocular movements were recorded during the identification task. Scan paths, saccades, fixations, and accuracy were also recorded. RESULTS: People with AMD exhibited lower accuracy (by about 30%). Eye movement parameters were impaired with a larger number of saccades, shorter fixation durations and a larger scan path than controls. CONCLUSIONS: Our results are consistent with studies on artificial scotoma in normally sighted people showing that a central scotoma impairs oculomotricity. In contrast to detection tasks, people with central vision loss exhibit impaired performance in identification of objects and scenes (62 to 66%). Eye movement studies suggest that the lower accuracy in patients is likely due to the use of peripheral vision and instability of fixation.

Topological states in multi-orbital HgTe honeycomb lattices.

Research on graphene has revealed remarkable phenomena arising in the honeycomb lattice. However, the quantum spin Hall effect predicted at the K point could not be observed in graphene and other honeycomb structures of light elements due to an insufficiently strong spin-orbit coupling. Here we show theoretically that 2D honeycomb lattices of HgTe can combine the effects of the honeycomb geometry and strong spin-orbit coupling. The conduction bands, experimentally accessible via doping, can be described by a tight-binding lattice model as in graphene, but including multi-orbital degrees of freedom and spin-orbit coupling. This results in very large topological gaps (up to 35 meV) and a flattened band detached from the others. Owing to this flat band and the sizable Coulomb interaction, honeycomb structures of HgTe constitute a promising platform for the observation of a fractional Chern insulator or a fractional quantum spin Hall phase.

Coulomb energy determination of a single Si dangling bond.

Determination of the Coulomb energy of single point defects is essential because changing their charge state critically affects the properties of materials. Based on a novel approach that allows us to simultaneously identify a point defect and to monitor the occupation probability of its electronic state, we unambiguously measure the charging energy of a single Si dangling bond with tunneling spectroscopy. Comparing the experimental result with tight-binding calculations highlights the importance of the particular surrounding of the localized state on the effective charging energy.

Electron-phonon coupling and intervalley splitting determine the linewidth of single-electron transport through PbSe nanocrystals.

The linewidth of the resonances in the single-electron tunneling spectra has been investigated for PbSe semiconductor nanocrystals (NCs) with scanning tunneling spectroscopy at low temperature. The linewidth of the resonances corresponding to tunneling through the first conduction and valence levels is found to increase with decreasing size of the NCs. Based on theoretical calculations, this broadening is mainly induced by the coupling between the tunneling electrons and the longitudinal optical phonon mode of the NC, and by the splitting of the degenerate electronic levels between the different L-valleys in the Brillouin zone. For the smallest sizes, it is shown that the intervalley splitting is the major source of broadening.

Probing the carrier capture rate of a single quantum level.

The performance of many semiconductor quantum-based structures is governed by the dynamics of charge carriers between a localized state and a band of electronic states. Using scanning tunneling spectroscopy, we studied the transport of inelastic tunneling electrons through a prototypical localized state: an isolated dangling-bond state on a Si(111) surface. From the saturation of the current at an energy resonant with this state, the hole capture rate by the dangling bond was determined. By further mapping the spatial extension of its wave function, the localized nature of the level was found to be consistent with the small magnitude of its cross section. This approach illustrates how the microscopic environment of a single defect critically affects its carrier dynamics.

Electron transport via local polarons at interface atoms.

Electronic transport is profoundly modified in the presence of strong electron-vibration coupling. We show that in certain situations, the electron flow takes place only when vibrations are excited. By controlling the segregation of boron in semiconducting Si(111)-square root 3 x square root 3 R 30 degrees surfaces, we create a type of adatom with a dangling-bond state that is electronically decoupled from any other electronic state. However, probing this state with scanning tunnelling microscopy at 5 K yields high currents. These findings are rationalized by ab-initio calculations that show the formation of a local polaron in the transport process.

[Familial case of Parkes Weber syndrome]. / Syndrome de Parkes-Weber familial.

INTRODUCTION: Parkes-Weber syndrome is usually described as a sporadic form of osteohypertrophic angiodysplasia. However, family forms of Klippel-Trenaunay syndrome have been described. We report the first familial case of Parkes-Weber syndrome. OBSERVATION: A boy born at 27 weeks and 6 days of amenorrhea with extensive plane angioma of the right lower limb, right lower part of the back and abdomen. We also noted hypertrophy of this member with venous dilatations. Arterial Doppler ultrasound of the right lower limb showed an aneurysmal varix between the vein and the common femoral artery, confirming a diagnosis of Parkes-Weber syndrome. His maternal first cousin, 10 years his senior, also presented Parkes-Weber syndrome of the right upper limb. DISCUSSION: This is the first observation of a familial case of Parkes-Weber syndrome in first cousins. Vascular malformations are transmitted in autosomal dominant fashion in the majority of infected families but with incomplete penetrance and variable expressivity. Symptoms appeared to worsen from generation to generation. In each generation of this family, we noted the presence of hemangiomas or capillary malformations with aggravation in the third generation and onset of Parkes-Weber syndrome. Genetic investigation with linkage analysis for the various members in order to identify a predisposing locus yielded little of interest.

Frequency-dependent spontaneous emission rate from CdSe and CdTe nanocrystals: influence of dark states.

We studied the rate of spontaneous emission from colloidal CdSe and CdTe nanocrystals at room temperature. The decay rate, obtained from luminescence decay curves, increases with the emission frequency in a supralinear way. This dependence is explained by the thermal occupation of dark exciton states at room temperature, giving rise to a strong attenuation of the rate of emission. The supralinear dependence is in agreement with the results of tight-binding calculations.

Semiconducting surface reconstructions of p-type Si(100) substrates at 5 K.

We report scanning tunneling microscopy (STM) studies of the technologically important Si(100) surface that reveal at 5 K the coexistence of stable surface domains consisting of the p(2 x 1) reconstruction along with the c(4 x 2) and p(2 x 2) reconstructions. Using highly resolved tunneling spectroscopic measurements and tight binding calculations, we prove that the p(2 x 1) reconstruction is asymmetric and determine the mechanism that enables the contrast variation observed in the formation of the bias-dependent STM images for this reconstruction.

Effect of quantum confinement on the dielectric function of PbSe.

Monolayers of lead selenide nanocrystals of a few nanometers in height have been made by electrodeposition on a Au(111) substrate. These layers show a thickness-dependent dielectric function, which was determined using spectroscopic ellipsometry. The experimental results are compared with electronic structure calculations of the imaginary part of the dielectric function of PbSe nanocrystals. We demonstrate that the size-dependent variation of the dielectric function is affected by quantum confinement at well-identifiable points in the Brillouin zone, different from the position of the band-gap transition.

Optical transitions in artificial few-electron atoms strongly confined inside ZnO nanocrystals.

We have studied the optical transitions in artificial atoms consisting of one to ten electrons occupying the conduction levels in ZnO nanocrystals. We analyzed near IR absorption spectra of assemblies of weakly coupled ZnO nanocrystals for a gradually increasing electron number and found four allowed dipole transitions with oscillator strengths in quantitative agreement with tight-binding theory. Furthermore, this spectroscopy provides the single-particle energy separation between the conduction levels of the ZnO quantum dots.

Dimensionality-dependent self-energy corrections and exchange-correlation potential in semiconductor nanostructures.

We study the quasiparticle gap in semiconductor nanostructures versus dimensionality and compare it to the value obtained in the local density approximation. We first develop general arguments based on the GW approach which we then substantiate numerically by a tight binding version of this theory. We show that the gap correction is dominated by a macroscopic surface self-polarization term and point out its nonmonotonic behavior versus dimensionality.

Excitonic and quasiparticle gaps in Si nanocrystals

We present calculations of the one- and two-particle excitations in silicon nanocrystals. The one-particle properties are handled in the GW approximation, and the excitonic gap is obtained from the Bethe-Salpeter equation. We develop a tight binding version of these methods to treat clusters up to 275 atoms. The self-energy and Coulomb corrections almost exactly cancel each other for crystallites with radius larger than 0.6 nm. The result of this cancellation is that one-particle calculations give quite accurate values for the excitonic gap of crystallites in the most studied range of sizes.

Urachal remnants: sonographic assessment.

OBJECTIVE: To evaluate the frequency of the visualization of urachal remnants (UR) with ultrasound and to determine their sonographic patterns. SUBJECTS AND METHODS: Two hundred and fifty consecutive patients were referred for abdominal and/or pelvic ultrasonography, 83 who had urinary tract symptoms. Patient age ranged from 1 month to 91 years (mean = 35 years). Patients were classified into four groups: (1) < 16 years (n = 47) (2) 16-35 years (n = 100), (3) 36-55 years (n = 49), (4) > or = 56 years (n = 54). Ultrasonography was performed using 3.75 MHz and 7.5 MHz transducers. Ultrasound criterion for diagnosis was a midline mass located between the rectus abdominus muscle and the upper part of the anterior bladder wall. RESULTS: UR were found in 90 cases (36%). UR demonstration was more frequent in groups 1 (61.7%) and 2 (49%) and 3 (20.4%) and 4 (3.7%). UR were nodular (87%) or tubular in structure (13%). Echogenicity was similar to or greater than adjacent muscle in 51% and less than in 49%. The length, width, and thickness mean and standard deviation values were 13.5 +/- 4.7 mm, 12.6 +/- 5 mm, and 5.2 +/- 1.5 mm, respectively. UR were observed in 50% of the asymptomatic patients of groups 1 and 2. CONCLUSION: Urachal remnants are commonly demonstrated with ultrasound, particularly in young patients. They should be considered to be a normal variant unless there is an increase in size or they are accompanied by clinical signs, without other possible causes for symptoms.