Thermoelectric effects in quantum dots

We report a numerical renormalization-group study of the thermoelectric effect in the single-electron transistor (SET) and side-coupled geometries. As expected, the computed thermal conductance and thermopower curves show signatures of the Kondo effect and of Fano interference. The thermopower curves are also affected by particle-hole asymmetry. © 2009 Elsevier B.V. All rights reserved.

Nanoscale manipulation of CdSe quantum dots in layer-by-layer films: influence of the host polyelectrolyte on the luminescent properties

Thioglycolic acid-capped Use quantum dots (QDs) were assembled on glass substrates with two distinct polyelectrolytes, viz poly(allylamine hydrochloride) (PAH) and poly(amidoamine) (PAMAM), generation 4 dendrimer, via the layer-by-layer (LbL) technique. Films containing up to 30 polyelectrolyte/QD bilayers were prepared. The growth of the multilayers was monitored with UV-vis spectroscopy, which showed an almost linear increase in the absorbance of the 2.8 nm QDs at 535 nm with the number of deposited bilayers. AFM measurements estimated a film thickness of 3 nm per bilayer for the PAH/Cdse films. The adsorption process and the optical properties of the PAMAM/CdSe LbL films were further analyzed layer-by-layer using surface plasmon resonance (SPR), from which a thickness of 3.2 nm was found for a PAMAM/CdSe bilayer. Photoluminescence measurements revealed higher photooxidation of the quantum dots in PAH/CdSe than in PAMAM/CdSe films. (c) 2004 Elsevier B.V. All rights reserved.

In Situ and Simultaneous UV-vis/SAXS and UV-vis/XAFS Time-Resolved Monitoring of ZnO Quantum Dots Formation and Growth

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Designing spatial correlation of quantum dots: towards self-assembled three-dimensional structures

Buried two-dimensional arrays of InP dots were used as a template for the lateral ordering of self-assembled quantum dots. The template strain field can laterally organize compressive (InAs) as well as tensile (GaP) self-assembled nanostructures in a highly ordered square lattice. High-resolution transmission electron microscopy measurements show that the InAs dots are vertically correlated to the InP template, while the GaP dots are vertically anti-correlated, nucleating in the position between two buried InP dots. Finite InP dot size effects are observed to originate InAs clustering but do not affect GaP dot nucleation. The possibility of bilayer formation with different vertical correlations suggests a new path for obtaining three-dimensional pseudocrystals.

Luminescence Enhancement of Carboxyl-Coated CdTe Quantum Dots by Silver Nanoparticles

Metallic nanoparticles (NPs) have been used to improve the sensibility of biosensors and bioassays either by enhancing radiative emission or inducing quenching process on fluorescent probes. The aim of this research was to study the interaction of silver and silver-pectin NPs with water-dispersed carboxyl-coated cadmium telluride (CdTe) quantum dots (QDs). Metallic NPs were observed to change the emission of these fluorophores through local field effects. In a solution-base platform, an increase of 82 % was observed for the CdTe emission due to the interaction of QDs and silver-pectin NPs. QDs interaction with silver NPs without pectin was also investigated and a smaller emission enhancement of 20 % was detected. We observed that the NPs' nature and QDs' surface charge and concentration are important parameters for NPs-QDs interaction. Moreover, the presence of the pectin polymer shows to be a key component to the observed fluorescence enhancement. © 2013 Springer Science+Business Media New York.

High fluorescence quantum efficiency of CdSe/ZnS quantum dots embedded in GPTS/TEOS-derived organic/silica hybrid colloids

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Técnica do Grupo de Renormalização Numérico (GRN) aplicada em quantum dots

Pós-graduação em Física - IGCE

On the nucleation of GaP/GaAs and the effect of buried stress fields

We have recently shown that spatial ordering for epitaxially grown InP dots can be obtained using the periodic stress field of compositional modulation on the InGaP buffer layer. The aim of this present work is to study the growth of films of GaP by Chemical Beam Epitaxy (CBE), with in-situ monitoring by Reflection High Energy Electron Diffraction (RHEED), on layers of unstressed and stressed GaAs. Complementary, we have studied the role of a buried InP dot array on GaP nucleation in order to obtain three-dimensional structures. In both cases, the topographical characteristics of the samples were investigated by Atomic Force Microscopy (AFM) in non-contact mode. Thus vertically-coupled quantum dots of different materials have been obtained keeping the in-place spatial ordering originated from the composition modulation. © 2006 Materials Research Society.

Universal zero-bias conductance through a quantum wire side-coupled to a quantum dot

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Nucleation and growth evolution of InP dots on InGaP/GaAs

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

SAXS and UV-Vis combined to Quick-XAFS monitoring of ZnO nanoparticles formation and growth

Time-resolved X-ray absorption-fine structure (Quick-XAFS) and UV-Vis absorption spectroscopies were combined for monitoring simultaneously the time evolution of Zn-based species and ZnO quantum dot (Qdot) formation and growth during the sol-gel synthesis from zinc oxy-acetate precursor solution. The time evolution of the nanostructural features of colloidal suspension was independently monitored in situ by small angle X-ray scattering (SAXS). In both cases, the monitoring was initialized just after the addition of NaOH solution (B = [OH]/[Zn] = 0.5) to the precursor solution at 40 degrees C. Combined time-resolved Quick-XAFS and UV-Vis data showed that the formation of ZnO colloids from the zinc oxy-acetate consumption achieves a quasi-steady-state chemical equilibrium in less than 200s. Afterwards, the comparison of the ZnO Qdots size and Guinier gyration radius evidences a limited aggregation process coupled to the Qdots growth. The analysis of the experimental results demonstrates that the nanocrystal coalescence and Ostwald ripening control the kinetics of the Qdot growth.

Photoluminescence in quantum-confined SnO2 nanocrystals: Evidence of free exciton decay

Nanocrystalline SnO2 quantum dots were synthesized at room temperature by hydrolysis reaction of SnCl2. The addition of tetrabutyl ammonium hydroxide and the use of hydrothermal treatment enabled one to obtain tin dioxide colloidal suspensions with mean particle radii ranging from 1.5 to 4.3 nm. The photoluminescent properties of the suspensions were studied. The particle size distribution was estimated by transmission electron microscopy. Assuming that the maximum intensity photon energy of the photoluminescence spectra is related to the band gap energy of the system, the size dependence of the band gap energies of the quantum-confined SnO2 particles was studied. This dependence was observed to agree very well with the weak confinement regime predicted by the effective mass model. This might be an indication that photoluminescence occurs as a result of a free exciton decay process. (C) 2004 American Institute of Physics.

Compositional modulation and surface stability in InGaP films: Understanding and controlling surface properties

We investigate the formation of compositional modulation and atomic ordering in InGaP films. Such bulk properties - as well as surface morphologies - present a strong dependence on growth parameters, mainly the V/III ratio. Our results indicate the importance of surface diffusion and, particularly, surface reconstruction for these processes. Most importantly from the application point of view, we show that the compositional modulation is not necessarily coupled to the surface instabilities, so that smooth InGaP films with periodic compositional variation could be obtained. This opens a new route for the generation of templates for quantum dot positioning and three-dimensional arrays of nanostructures. © 2007 American Institute of Physics.

Probing the antisymmetric Fano interference assisted by a Majorana fermion

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Geometry effects on the electronic properties of multi-open dots structures

In this paper we show that the electronic properties of multi-open dots structures are strongly modified by even smalt changes in their geometries. Our discussion of these effects is done in terms of the interaction among localized states (dot-like) and extended states (channel-like), from which a Fano resonance situation arises.