Distribution by Scientific Domains

Kinds of QD

  • mg qd

  • Terms modified by QD

  • qd layer

  • Selected Abstracts

    Influence of stability on the acute toxicity of CdSe/ZnS nanocrystals to Daphnia magna

    Heather E. Pace
    Abstract The acute toxicity of polymer-coated CdSe/ZnS quantum dots (QDs) to Daphnia magna was investigated using 48-h exposure studies. The principal objective was to relate the toxicity of QDs to specific physical and chemical aspects of the QD. As such, two different CdSe core diameters, 2,nm QDs (green-emitting) and 5,nm QDs (red-emitting), and two different surface coatings, polyethylene oxide (PEO) and 11-mercaptoundecanoic acid (MUA) were studied. The QDs were characterized before and after the 48-h exposure using fluorescence, ultrafiltrations (3 kDa), and inductively coupled plasma-atomic emission spectrometry (ICP-AES) metal analysis. In addition, flow field flow fractionation-inductively coupled plasma-mass spectrometry (Fl FFF-ICP-MS) was used as a more extensive characterization technique to determine particle size and composition as well as identify other potential constituents in the QD solutions. The more stable QDs (PEO) were found to be less acutely toxic than the QDs with accelerated dissolution (MUA), suggesting QD stability has significant impact on the nanoparticles' short-term toxicity. The emergence of dissolved Cd2+ in solution indicates that the toxicity of the MUA QDs is likely due to Cd poisoning, and a mass-based dose response occurred as a consequence of this mode of action. Alternatively, the PEO QDs caused acute toxicity without observed particle dissolution (i.e., no detectable metals were solubilized), suggesting an alternative mode of toxic action for these nanoparticles. Results of the present study suggest that using particle number, instead of mass, as a dose metric for the PEO QDs, produces markedly different conclusions, in that smaller core size does not equate to greater toxicity. Environ. Toxicol. Chem. 2010;29:1338,1344. © 2010 SETAC [source]

    Eslicarbazepine Acetate: A Double-blind, Add-on, Placebo-controlled Exploratory Trial in Adult Patients with Partial-onset Seizures

    EPILEPSIA, Issue 3 2007
    Christian Elger
    Summary:,Objective: To explore the efficacy and safety of eslicarbazepine acetate (BIA 2-093), a new antiepileptic drug, as adjunctive therapy in adult patients with partial epilepsy. Methods: A multicenter, double-blind, randomized, placebo-controlled study was conducted in 143 refractory patients aged 18,65 years with ,4 partial-onset seizures/month. The study consisted of a 12-week treatment period followed by a 1-week tapering off. Patients were randomly assigned to one of three groups: treatment with eslicarbazepine acetate once daily (QD, n = 50), twice daily (BID, n = 46), or placebo (PL, n = 47). The daily dose was titrated from 400 mg to 800 mg and to 1,200 mg at 4-week intervals. The proportion of responders (patients with a ,50% seizure reduction) was the primary end point. Results: The percentage of responders versus baseline showed a statistically significant difference between QD and PL groups (54% vs. 28%; 90% CI =,,, ,14; p = 0.008). The difference between the BID (41%) and PL did not reach statistical significance (90% CI =,,, ,1; p = 0.12). A significantly higher proportion of responders in weeks 5,8 was found in the QD group than in the BID group (58% vs. 33%, respectively, p = 0.022). At the end of the 12-week treatment, the number of seizure-free patients in the QD and BID groups was 24%, which was significantly different from the PL group. The incidence of adverse events was similar between the treatment groups and no drug-related serious adverse events occurred. Conclusion: Eslicarbazepine acetate was efficacious and well tolerated as an adjunctive therapy of refractory epileptic patients. [source]

    Nanoarrays: Cooperative Near-Field Surface Plasmon Enhanced Quantum Dot Nanoarrays (Adv. Funct.

    Abstract Fluorescence from quantum dots (QDs) sandwiched between colloidal gold nanoparticles and lithographically created metal nanoarrays is studied using engineered peptides as binding agents. For optimized structures, a 15-fold increase is observed in the brightness of the QDs due to plasmon-enhanced fluorescence. This enhanced brightness is achieved by systematically tuning the vertical distance of the QD from the gold nanoparticles using solid-specific peptide linkers and by optimizing the localized surface plasmon resonance by varying the geometric arrangement of the patterned gold nanoarray. The size and pitch of the patterned array affect the observed enhancement, and sandwiching the QDs between the patterned features and colloidal gold nanoparticles yields even larger enhancements due to the increase in local electromagnetic hot spots induced by the increased surface roughness. The use of bifunctional biomolecular linkers to control the formation of hot spots in sandwich structures provides new ways to fabricate hybrid nanomaterials of architecturally induced functionality for biotechnology and photonics. [source]

    Cooperative Near-Field Surface Plasmon Enhanced Quantum Dot Nanoarrays

    Kirsty Leong
    Abstract Fluorescence from quantum dots (QDs) sandwiched between colloidal gold nanoparticles and lithographically created metal nanoarrays is studied using engineered peptides as binding agents. For optimized structures, a 15-fold increase is observed in the brightness of the QDs due to plasmon-enhanced fluorescence. This enhanced brightness is achieved by systematically tuning the vertical distance of the QD from the gold nanoparticles using solid-specific peptide linkers and by optimizing the localized surface plasmon resonance by varying the geometric arrangement of the patterned gold nanoarray. The size and pitch of the patterned array affect the observed enhancement, and sandwiching the QDs between the patterned features and colloidal gold nanoparticles yields even larger enhancements due to the increase in local electromagnetic hot spots induced by the increased surface roughness. The use of bifunctional biomolecular linkers to control the formation of hot spots in sandwich structures provides new ways to fabricate hybrid nanomaterials of architecturally induced functionality for biotechnology and photonics. [source]

    An Incenter Nocturnal Hemodialysis Program,Three Years Experience

    M. Gene Radford
    We report our experience with a program of long, slow, overnight hemodialysis (HD) performed 3 times a week in an existing dialysis facility. Beginning in April 1999, 14 chairs in one bay of our facility were replaced with beds, subdued lighting was installed, and machine alarms were decreased to minimum volume. Fresenius F60 dialyzers were selected with a QB of 220,300 ml/min and a QD of 400,500 ml/min. Patients dialyze for 7,8 hrs overnight. Staffing is with 1 nurse and 1 PCT for 10 patients. Standard dialysate is used, and heparin is dosed 100 U/kg at treatment initiation and again at mid-treatment. All access types are utilized. The program is open to all patients in our area. A total of 65 patients have participated, with a current census of 20 patients. Participants have tried nocturnal dialysis for a variety of reasons including work/school schedules, excessive interdialytic weight gains, inadequate dialysis (due to poor access function or large body mass), and hemodynamic instability with standard daytime HD. Blood pressure control has improved among the participants in the program, perhaps due to more gentle ultrafiltration and improvement in maintenance of dry weight. Among 31 patients who remained on nocturnal dialysis for over 6 months, 21 started the program on an average of 2.5 antihypertensive agents (AHA). After 6 months, 9 patients no longer needed AHA while 12 patients remain on an average of 1.3 AHA. URR also improved by an average of 4.35 among 13 patients who transferred from standard incenter HD to the nocturnal program. In all, 45 patients have left the program, for reasons which include insomnia/social (15), death (9), transfer to home HD (8), renal transplantation (6), noncompliance (3), moved away (2), and other (2). In conclusion, long overnight HD can be performed in an existing dialysis facility, providing patients with another HD option. Patients who may benefit from this modality include those with daytime jobs, patients with inadequate clearance on standard HD, patients with excessive interdialytic weight gains, and those who poorly tolerate standard HD. [source]

    A new Groucho TLE4 protein may regulate the repressive activity of Pax5 in human B lymphocytes

    IMMUNOLOGY, Issue 4 2002
    Michčle Milili
    Summary During mouse B-cell development, Pax5 is an essential transcription factor that acts as an activator of B-cell-specific genes and as a repressor of alternative lineage fates. The repressive function is mediated by the recruitment of members of the Groucho co-repressor family. Using an RNA display approach, we have isolated a transcript, called QD, specifically expressed in human pro-B and pre-B cells, which is derived from the human Groucho TLE4 gene. The QD transcript contains the first four TLE4 exons and an intronic sequence 3, of exon 4, demonstrating that QD is a splice variant of TLE4. The putative resulting protein of 94 amino acids corresponds to approximately half of an N-terminal tetramerization domain. We also show specific expression of TLE4 transcripts in human B cells and of TLE4 proteins in B-cell nuclei. Moreover, we demonstrate that recombinant QD protein binds to the TLE4 Q domain and is able to abolish the TLE4/Pax5 interaction. Thus, QD could act as a negative regulator of TLE4 function, in early B-cell differentiation. [source]

    Two-Photon Excitation of Quantum-Dot-Based Fluorescence Resonance Energy Transfer and Its Applications,

    ADVANCED MATERIALS, Issue 15 2007
    R. Clapp
    Nonradiative fluorescence resonance energy transfer (FRET) between a luminescent quantum dot (QD) donor and a proximal dye brought in close proximity of the QD surface via conjugation with a dye-labeled peptide (or a protein) is shown (see figure). The system is excited with near IR irradiation (well below the absorption band of the QD), via a fast two-photon process, which produces a FRET signal with very low background contribution due to a substantially reduced nonlinear direct excitation of the dye. [source]

    Parallelization of the quantum dynamics code for cluster architecture and its applications to the Gross,Pitaevskii equation

    Adam Gorecki
    Abstract We present a parallel version of a quantum dynamics (QD) algorithm in a wave function representation. The algorithm has been optimized for a Linux cluster as well as for Cray T3E, using a parallel version of a three-dimensional FFT library. We have applied this algorithm for solving the nonlinear Gross,Pitaevskii equation (GPE), which describes the evolution of Bose,Einstein condensates (BEC) in the mean field approximation. Our method reduces the computational time and allows the study of nonlinear quantum systems. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006 [source]

    Non-Linear Effect of Modifier Composition on Enantioselectivity in Asymmetric Hydrogenation over Platinum Metals

    Wolf-Rüdiger Huck
    Abstract Prominent non-linear behavior was observed when mixtures of cinchona alkaloids were applied as chiral modifiers in enantioselective hydrogenations over Pt/Al2O3 and Pd/TiO2. The phenomenon is traced to differences in the strength and geometry of alkaloid adsorption on the metal surface. In ethyl pyruvate hydrogenation under close to ambient conditions the weaker adsorbing quinidine (QD) outperformed the generally preferred modifier cinchonidine (CD) and afforded the highest ee (96,98%) at 1,5,bar. In the partial hydrogenation of 4-methoxy-6-methyl-2-pyrone to the dihydro derivative 4 CD gave 73% ee to (S)- 4 and QD provided 72% ee to (R)- 4, and still the alkaloid mixture containing less than 5,mol,% CD afforded 15% ee to (S)- 4. This non-linear behavior may be advantageous in asymmetric synthesis as low purity chiral compounds can be applied as efficient modifiers for Pt or Pd. [source]

    Clinical trial: the effects of the proton pump inhibitor dexlansoprazole MR on daytime and nighttime heartburn in patients with non-erosive reflux disease

    R. FASS
    Summary Background, The proportion of patients who respond to proton pump inhibitor (PPI) therapy is about 20% lower in those with non-erosive reflux disease (NERD) than in those with erosive oesophagitis. Aim, To assess efficacy and safety of dexlansoprazole MR, a PPI using Dual Delayed Release technology, in NERD patients. Methods, In this 4-week, double-blind, placebo-controlled study, 947 NERD patients randomly received dexlansoprazole MR 30 mg, 60 mg or placebo once daily (QD). The percentages of 24-h heartburn-free days (primary) and nights without heartburn (secondary) were assessed from patients' daily diaries. Investigators also assessed symptoms. Patients completed validated quality of life and symptom severity questionnaires. Results, Dexlansoprazole MR provided significantly greater median percentages of 24-h heartburn-free days (54.9% and 50.0% for the 30- and 60-mg doses vs. 17.5% for placebo, P < 0.00001) and nights without heartburn (80.8% and 76.9% vs. 51.7%, P < 0.00001 vs. placebo). Dexlansoprazole MR also reduced symptom severity. Quality of life improvements in patients receiving dexlansoprazole MR were consistent with clinical efficacy endpoints. Percentages of patients experiencing treatment-emergent adverse events were similar among groups. Conclusions, Dexlansoprazole MR 30 and 60 mg were superior to placebo in providing 24-h heartburn-free days and nights in NERD patients. Treatment was well tolerated. [source]

    Cyclosporine exposure and calcineurin phosphatase activity in living-donor liver transplant patients: Twice daily vs. once daily dosing

    Masahide Fukudo
    We have compared the pharmacokinetics and pharmacodynamics of cyclosporine between once- and twice-daily dosing regimens in de novo patients of living-donor liver transplantation (LDLT). A total of 14 patients were enrolled in this study, who had received cyclosporine microemulsion (Neoral) twice a day (BID, n = 5) or once daily in the morning (QD, n = 9) after transplantation. On postoperative day (POD) 6, the QD regimen significantly increased cyclosporine exposure; the blood concentration at 2 hours postdose (C2) and area under the concentration-time curve (AUC) for 4 hours (AUC0,4), compared with the BID regimen. Moreover, the area under the calcineurin (CaN) activity in peripheral blood mononuclear cells time-curve (AUA) for 12 hours (AUA0,12) and 24 hours (AUA0,24) were decreased by approximately 42 and 25% with the QD regimen relative to the BID regimen, respectively. The C2 level was significantly correlated with the AUC0,4 (r2 = 0.95), which was negatively related to the AUA0,12 with a large interindividual variability (r2 = 0.59). However, a significant correlation was found between the AUA0,12 or AUA0,24 and CaN activity at trough time points. According to a maximum inhibitory effect attributable to the drug (Emax) model, the mean estimates of Emax and the Cb value that gives a half-maximal effect (EC50) for CaN inhibition were not significantly different between the 2 groups, respectively. These findings suggest that a once daily morning administration of cyclosporine may improve oral absorption and help to provide an effective CaN inhibition early after LDLT. Furthermore, CaN activity at trough time points would be a single surrogate predictor for the overall CaN activity throughout dosing intervals following cyclosporine administration. Liver Transpl 12:292,300, 2006. © 2006 AASLD. [source]

    Duloxetine for the Management of Diabetic Peripheral Neuropathic Pain: Response Profile

    PAIN MEDICINE, Issue 5 2007
    Yili L. Pritchett PhD
    ABSTRACT Objective., The current analysis examines the response profile in patients receiving duloxetine for the management of diabetic peripheral neuropathic pain (DPNP). Patients/Design., Data were pooled from three double-blind, randomized, placebo-controlled 12-week acute therapy trials of patients with DPNP of at least 6 months' duration. Study 1 (N = 457) had treatment groups of duloxetine 20 mg once daily (QD), 60 mg QD, 60 mg twice daily (BID), and placebo; Studies 2 (N = 334) and 3 (N = 348) compared duloxetine 60 mg QD and 60 mg BID with placebo. The primary efficacy measure in each study was the weekly mean score of the 24-hour average pain severity. Treatment response was defined as a 30% reduction in pain severity, although some analyses were repeated using alternative response criteria (50% reduction, or 2-point reduction, in pain severity). Results., Consistently across the three studies, response rates at endpoint were significantly higher among patients receiving duloxetine (60 mg QD or 60 mg BID) than among those receiving placebo, regardless of the chosen response criterion (30% reduction, 50% reduction, or 2-point reduction in weekly mean of 24-hour average pain severity). The proportion of patients achieving pain relief in the duloxetine treatment groups was significantly greater than that in the placebo group at Week 1 and at all subsequent study visits to the end of acute phase therapy. Using diary data (24-hour average pain severity) from the first 7 days of treatment, the first significant separation from placebo in pain severity reduction for duloxetine 60 mg QD occurred at Day 1 (Study 1), Day 2 (Study 2), and Day 4 (Study 3), while significant separation in response rates first occurred at Day 3 when using pooled data. Conclusions., Patients with DPNP receiving duloxetine 60 mg QD or 60 mg BID had significantly higher rates of treatment response, when compared with patients receiving placebo, regardless of the chosen response criterion. Response to duloxetine treatment tended to occur early in therapy. [source]

    HPA axis safety of fluticasone furoate nasal spray once daily in children with perennial allergic rhinitis

    Ita Tripathy
    The effects of intranasal corticosteroids (INSs) on the hypothalamic,pituitary,adrenal (HPA) axis should be assessed for any to be marketed INS. The objective of this study was to assess the effects of fluticasone furoate nasal spray (FFNS) on cortisol production (as a measure of HPA axis function) following 6 wk of treatment with FFNS 110 ,g once daily (QD) compared with placebo in pediatric patients with perennial allergic rhinitis (PAR). In this double-blind, parallel-group study, patients (n = 112) aged 2,11 yr with a 1-yr history of PAR (6 months for patients aged 2,3 yr) were randomized in a 1:1 ratio to either placebo or FFNS. Serum cortisol (SC) concentrations and urinary cortisol (UC) excretion were measured over a 24-h period at the randomization (baseline) and final treatment (week 6) visits for HPA axis evaluation in a domiciled environment (overnight in the clinic). Plasma samples were collected for FFNS at several time points over the 24 h after the final dose for pharmacokinetic analyses. FFNS was non-inferior to placebo with respect to change from baseline (expressed as a ratio) in 24-h SC weighted mean. The lower limit of the two-sided 95% confidence interval (CI) for the treatment ratio was greater than the pre-specified non-inferiority margin of 0.8 (treatment ratio = 0.97, 95% CI 0.88,1.07). UC excretion over 24 h at baseline and end of treatment was similar between treatment groups; no patients had 24-h excretion levels below normal range after 6 wk of treatment. Plasma concentrations of FFNS were generally non-quantifiable (<10 pg/ml). Results of the current study indicate that FFNS 110 ,g QD has no significant effect on HPA axis function in 2- to 11-yr-old pediatric patients with PAR. [source]

    Si-doped GaN/AlN quantum dot superlattices for optoelectronics at telecommunication wavelengths

    F. Guillot
    Abstract We report on the controlled growth of Si doped GaN/AlN quantum dot (QD) superlattices, in order to tailor their intersubband absorption within the 1.3,1.5 µm telecommunication wavelengths. The QD size is tuned by modifying the amount of GaN in the QDs and the growth temperature. Silicon can be incorporated in the QDs to populate the first electronic level, without significant perturbation of the QD morphology. As a proof of the capability of these structures for infrared detection, a quantum-dot intersubband photodetector at 1.38 µm with lateral carrier transport is demonstrated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Spacer layer thickness effects on the photoluminescence properties of InAs/GaAs quantum dot superlattices

    B. Ilahi
    Abstract InAs/GaAs vertically stacked self-assembled quantum dot (QD) structures with different GaAs spacer layer thicknesses are grown by solid source molecular beam epitaxy (SSMBE) and investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. An increase in the polarization anisotropy is observed when the spacer layer thickness decreases. For a 10 monolayer (ML) thick inter-dots GaAs spacer, the TEM image shows an increase in the QD size when moving to the upper layer accompanied by the generation of dislocations. Consequently, the corresponding temperature-dependant PL properties are found to exhibit an unusual behaviour. The main PL peak is quenched at a temperature around 190 K giving rise to a broad background correlated with the formation of a miniband in the growth direction due to the strong interlayer coupling. For a thicker GaAs spacer layer (30 ML), multilayer QDs align vertically in stacks with no apparent structural defects. Over the whole temperature range, the excitonic band energies are governed by the Varshni empirical relation using InAs bulk parameters and the PL line width shows a slight monotonic increase. For a thinner GaAs interlayer, the thermal activation energies of the carrier emission out of the quantum dots are found to be considerably small (about 25 meV) due to the existence of defects. By combining these structural and optical results, we can conclude that a thinner GaAs spacer has a poorer quality. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Effects of composition distribution on electronic structures of self-assembled InGaN/GaN quantum dots

    Wei-Yi Tsai
    Abstract In this study, we investigate the influences of different composition distributions on the electronic structures of truncated cone-shaped InGaN quantum dots (QDs). A varying parameter, , is defined as , in which w1 and w2 are the top and base diameter of QD, respectively. This factor is set to study the role of similar truncated cones with the same base diameter and height on electronic structures of QD. Three indium composition distributions are considered: ellipsoid, uniform, and linear. The single-band effective-mass equation and six-band k·p theory are used to calculate the transition energies of electrons and holes, respectively. The numerical results reveal that the parameter plays an important role in changing the piezoelectric potential. In addition, an InGaN quantum dot with a linear distribution of indium has the maximum transition energy, whereas one with an ellipsoidal distribution has the minimum value. It is noteworthy that the binding energy greatly decreases as increases for different indium distributions. [source]

    Multiphoton excitation of disc shaped quantum dot in presence of laser (THz) and magnetic field for bioimaging

    Siddhartha Lahon
    Abstract Recently, multiphoton processes in nanostructures have attracted much attention for their promising applications, especially in growing field of bioimaging. Here we investigate the optical response of quantum disc (QD) in the presence of laser and a static magnetic field. Floquet theory is employed to solve the equation of motion for laser driven intraband transitions between the states of the conduction band. Several interesting features namely dynamic stark shift, power broadening, and hole burning on excited levels degeneracy breaking are observed with variation of electric and magnetic field strengths. Enhancement and power broadening observed for excited states probabilities with increase of external fields are directly linked to the emission spectra of QD and will be useful for making future bioimaging devices. [source]

    CdSe quantum dot in a ZnSe nanowire as an efficient source of single photons

    A. Tribu
    Abstract We report on our development of fabrication of CdSe QD in ZnSe nanowire. We have been able to obtain high quality structures with very good optical properties. This has allowed us to measure photon emission from single quantum dots and to demonstrate photon antibunching. We show that this new type of II,VI quantum dot is very promising for high temperature operation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Control of single quantum dot emission characteristics and fine structure by lateral electric fields

    S. M. Ulrich
    Abstract The spontaneous emission characteristics of individual self-assembled (In,Ga)As/GaAs quantum dots have been investigated under the application of a tunable electric field in the lateral growth plane. For the neutral excitonic and bi-excitonic as well as a singly-charged (trionic) QD carrier configuration, similar quantum-confined Stark effects could be observed, thus enabling a direct and comparative determination of corresponding polarizability values ,. In addition we have applied a refined detection technique of high-resolution Fabry,Pérot interferometry on single QDs to investigate the anisotropy-induced excitonic fine structure and to monitor its tunability under a lateral electric field. Whereas most quantum dots reveal an oscillatory-type modulation and partial reduction in fine structure under the applied field, we also demonstrate the case of a selected QD where the initial fine structure could even be reversibly tuned down to zero. This fine structure tunability of single QDs represents an essential prerequisite for the realization of individually controlled entangled photon sources in the future. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Today's challenges in quantum dot materials research for tomorrow's quantum functional devices

    Richard Nötzel
    Abstract Size, shape, position control, and self-organized lateral ordering of epitaxial semiconductor quantum dot (QD) arrays are demonstrated. This constitutes the prerequisite for the ultimate control of the electronic and optical properties of man-made semiconductor heterostructures at the single and multiple charge, spin, and photon level, including their quantum mechanical and electromagnetic interactions in view of applications such as quantum information processing and computing. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Strong coupling in artificial semimagnetic Cd(Mn,Mg)Te quantum dot molecule

    S. V. Zaitsev
    Abstract Exciton photoluminescence in a pair of strongly coupled artificial asymmetric quantum dots (QDs) has been studied in a magnetic field up to 8 T. The QD molecules have been fabricated by a selective interdiffusion technique applied to asymmetric semimagnetic CdTe/Cd(Mg,Mn)Te double quantum wells. The lateral confinement potential within the plane, induced by the diffusion, gives rise to effective zero-dimensional exciton localization. In contrast to a typically positive exciton Lande g -factor, an exciton transition in the non-magnetic QD demonstrates a nearly zero g -factor, indicating a strong electron tunnel coupling between the QDs. The strong coupling results in the formation of an inter-QDs indirect exciton, which is a ground exciton state at high magnetic field, as found in the experiment and confirmed by our calculations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Magnetoabsorption coefficient of donor impurities in CdS quantum dot

    A. Didi Seddik
    Abstract A theoretical study of the donor-related absorption coefficient in spherical CdS quantum dots (QD) in the presence of a uniform magnetic field is performed. The wave functions and the eigenvalues of the initial and final states are obtained in the effective-mass approximation by using a variational method and a perturbation method, respectively. The results obtained show that the absorption coefficient presents essentially two peaks; one associated with on-center donors and another related to transitions involving donors at the QD edge. The dependences of the absorption coefficient on sample dimensions, the magnetic field strength and the light polarization are discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Investigation of spin state in a quantum dot by using strongly asymmetric tunnel barriers

    T. Hayashi
    Abstract We investigate the spin state of a GaAs lateral quantum dot (QD) by using strongly asymmetric tunnel barriers. The saturated current depends on the current polarity, which switches the direction of the dominant tunneling transition that increases or decreases the total spin. With this technique, the spin polarization and the Coulomb interaction are investigated. We find that high spin states appearing in some magnetic field regions can be understood by considering simple Coulomb interactions. [source]

    Single-photon and photon-pair emission from CdSe/Zn(S,Se) quantum dots

    S. Strauf
    Abstract We report on the generation of triggered single photons and photon pairs relying on pulsed optical excitation of epitaxially grown self-assembled CdSe quantum dots (QDs). Single-photon emission is studied on individual CdSe/Zn(S,Se) QDs for temperatures up to 200 K. At low temperatures nearly perfect single-photon emission is achieved whereas at higher temperatures an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring QDs is observed. The multi-photon emission probability of a bare QD (background subtracted) is strongly suppressed. Furthermore, the polarization cross correlation of the biexciton,exciton cascade has been investigated in a CdSe/ZnSe QD at low temperatures. A strong polarization correlation of the emitted photon pairs of ,74% is observed in a linear detection basis. [source]

    2nd International Conference on Semiconductor Quantum Dots (QD 2002) in Tokyo, Japan, 30 September,3 October 2002

    R. Heitz
    No abstract is available for this article. [source]

    Cascade upconversion of photoluminescence in ensembles of II-VI semiconductor nanocrystals

    E. Malainho
    Abstract We present the theory of the thermally activated photoluminescence upconversion, also known as anti-Stokes photoluminescence (ASPL), based on the quantum dot (QD) polaron effect and explaining all the principal features of the ASPL in colloidal solutions of chemically grown nanocrystals (NCs). The results obtained by Monte-Carlo simulations reproduce all the experimentally observed ASPL trends in colloidal NC solutions and are being extended to dense QD systems, such as films composed by layers of closely packed NCs (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Internal parameters and optical properties of green II-VI heterostructure lasers with active region composed of multi-sheet electronically-coupled CdSe quantum dots

    Aliaksei G. Vainilovich
    Abstract Optically pumped green-emitting ZnSe-based laser heterostructures of optimized design have been grown by molecular-beam epitaxy and studied. The structures containing five electronically-coupled stacked CdSe quantum dot (QD) sheets demonstrate the minimum threshold power density of 2.2 kW/cm2 and the maximum external quantum efficiency of , 50%. Internal laser characteristics were determined by measuring the differential laser efficiency and laser threshold of a series of samples with different cavity lengths. The characteristic gain and internal quantum efficiency of the structures have been shown to reach the values as high as ,G0 = 114 cm -1 and ,i = 65.4%, while the transparency threshold and internal losses are evaluated as low as IT = 1.22 kW/cm2 and ,i = 2.55 cm -1, respectively. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Effect of stack number on the threshold current density and emission wavelength in quantum dash/dot lasers

    D. Zhou
    Abstract InAs quantum dash and dot (QDH and QD) lasers grown by molecular beam epitaxy on InP substrate are studied. The grown lasers with active zone containing multiple stacked layers exhibit lasing wavelength at 1.55 ,m. On these devices, the experimental threshold current density reaches its minimum value for a double stacked QDH/QD structure. Other basic laser properties like gain and quantum efficiency are compared. QD lasers exhibit better threshold current densities but equivalent modal gain per layer than QDH. Finally, the analysis of the modal gain on QD laser structures shows a promising potential for improvement of the laser properties. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Deterministic self-organization: Ordered positioning of InAs quantum dots by self-organized anisotropic strain engineering on patterned GaAs (311)B

    E. Selçuk
    Abstract Laterally ordered InGaAs quantum dot (QD) arrays, InAs QD molecules, and single InAs QDs in a spot-like periodic arrangement are created by self-organized anisotropic strain engineering of InGaAs/GaAs superlattice (SL) templates on planar GaAs (311)B substrates in molecular beam epitaxy. On shallow- and deep-patterned substrates the respectively generated steps and facets guide the self-organization process during SL template formation to create more complex ordering such as periodic stripes, depending on pattern design. Here we demonstrate for patterns such as shallow- and deepetched round holes and deep-etched zigzag mesas that the self-organized periodic arrangement of QD molecules and single QDs is spatially locked to the pattern sidewalls and corners. This extends the concept of guided self-organization to deterministic self-organization. Absolute position control of the QDs is achieved without one-to-one pattern definition. This guarantees the excellent arrangement control of the ordered QD molecules and single QDs with strong photoluminescence emission up to room temperature, which is required for future quantum functional devices. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

    Dynamical model for coherent optical manipulation of a single spin state in a charged quantum dot

    Gabriela Slavcheva
    Abstract The optically-induced coherent spin dynamics of a single spin confined in a charged quantum dot (QD) is theoretically studied employing coupled vector Maxwell-pseudospin formalism. Generalized pseudospin master equation is derived for description of the time evolution of spin coherences and spin populations including spin population transfer and dissipation in the system through spin relaxation processes. The equation is solved in the time domain self-consistently with the vector Maxwell equations for the optical wave propagation coupled to it via macroscopic medium polarisation. Using the model the long-lived electron spin coherence left behind a single resonant ultrashort optical excitation of the electron-trion transition in a charged QD is simulated in the low- and high-intensity Rabi oscillations regime. Signatures of the polarised photoluminescence (PL), predicted by the model, such as the appearance of a second echo pulse after the excitation and characteristic PL trace shape, are discussed for realization of high-fidelity schemes for coherent readout of a single spin polarisation state. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]