Spin coherence generation in negatively charged self-assembled (In,Ga)As quantum dots by pumping excited trion states

Spin coherence generation in an ensemble of negatively charged (In,Ga)As/GaAs quantum dots was investigated by picosecond time-resolved pump-probe spectroscopy measuring ellipticity. Robust coherence of the ground-state electron spins is generated by pumping excited charged exciton (trion) states. The phase of the coherent state, as evidenced by the spin ensemble precession about an external magnetic field, varies relative to spin coherence generation resonant with the ground state. The phase variation depends on the pump photon energy. It is determined by (a) pumping dominantly either singlet or triplet excited states, leading to a phase inversion, and (b) the subsequent carrier relaxation into the ground states. From the dependence of the precession phase and the measured g factors, information about the quantum dot shell splitting and the exchange energy splitting between triplet and singlet states can be extracted in the ensemble.

Influence of the deposition parameters on the morphology and electrical conductivity of PANI/PSS self-assembled films

The influence of deposition parameters, namely polymer concentration and pH of the deposition solution, cleaning, and drying steps on the morphology and electrical characteristics of polyaniline and sulfonated polystyrene (PANI/PSS) nanostructured films deposited by the self-assembly technique is evaluated by UV-Vis spectroscopy, optical and atomic force microscopy, and electrical resistance measurements. It is found that stirring the cleaning solution during the cleaning step is crucial for obtaining homogenous films. Stirring of the cleaning solution also influences the amount of PANI adsorbed in the films. In this regard, the drying process seems to be less critical since PANI amount and film thickness are similar in films dried with N-2 flow or with an absorbent tissue. It is observed, however, that drying with N-2 flow results in rougher films. As an additional point, an assessment of the influence of the deposition method (manual versus mechanical) on the film characteristics was carried out. A significant difference on the amount of PANI and film thickness between films prepared by different human operators and by a homemade mechanical device was observed. The variability in film thickness and PANI adsorbed amount is smaller in films mechanically assembled. (c) 2007 Elsevier B.V. All rights reserved.

Microelectrode array in mixed alkanethiol self-assembled monolayers: Electrochemical studies

We report an efficient alternative to obtain recessed microelectrodes device on gold electrode surface, in which mixed self-assembled monolayer of long and short carbon alkanethiol chains was used for this purpose. Development of the modified electrodes included the chemical adsorption of 11-mercaptoundecanoic acid and 2-mercaptoethanol solution, as well as their mixtures, on gold surface, resulting in the final mixed self-assembled monolayer configuration. For comparison, the electrochemical performance of self-assembled monolayer of 11-mercaptoundecanoic acid. 3-mercaptopropionic acid, 4-mercapto-1-butanol and 6-mercapto-1-hexanol modified electrodes was also investigated. It was verified that, in the mixed self-assembled monolayer, the 11-mercaptoundecanoic acid acts as a barrier for electron transfer while the short alkanethiol chair is deposited in an island-like shape through which electrons can be freely transferred to ions in solution, allowing electrochemical reactions to occur. The performance of the modified electrodes toward microelectrode behavior was investigated via cyclic voltammetry and electrochemical impedance spectroscopy measurements using [Fe(CN)(6)](3-/4-) redox couple as a probe. In this case, sigmoidal voltammetric responses were obtained, very similar to those observed for microelectrodes. Such behavior reinforces the proposition of electron transfer through the short alkanethiol chain layer and surface blockage by the long chain one. Electrochemical impedance results allowed calculated the mean radius value of each microelectrode disks of 3.8 mu m with about 22 mu m interval between them. The microelectrode environment provided by the mixed self-assembled monolayer can be conveniently used to provide an efficient catalytic conversion in biosensing applications. (C) 2012 Elsevier Ltd. All rights reserved.

Structural, Spectroscopic (NMR, IR, and Raman), and DFT Investigation of the Self-Assembled Nanostructure of Pravastatin-LDH (Layered Double Hydroxides) Systems

Layered double hydroxide (LDH) nanocontainers, suitable as carriers for anionic drugs, were intercalated with Pravastatin drug using magnesium-aluminum and zinc-aluminum in a M-II/Al molar ratio equal 2 and different Al3+/Pravastatin molar ratios. Postsynthesis treatments were used in order to increase the materials crystallinity. Hybrid materials were characterized by a set of physical chemical techniques: chemical elemental analysis, X-ray diffraction (XRD), mass coupled thermal analyses, vibrational infrared and Raman spectroscopies, and solid-state C-13 nuclear magnetic resonance (NMR). Results were interpreted in light of computational density functional theory (DFT) calculations performed for Sodium Pravastatin in order to assign the data obtained for the LDH intercalated materials. XRD peaks of LDH-Pravastatin material and the one-dimensional (1D) electron density map pointed out to a bilayer arrangement of Pravastatin in the interlayer region, where its associated carboxylate and vicinal hydroxyl groups are close to the positive LDH. The structural organization observed for the stacked assembly containing the unsymmetrical and bulky monoanion Pravastatin and LDH seems to be promoted by a self-assembling process, in which local interactions are maximized and chloride ion cointercalation is required. It is observed a high similarity among vibrational and C-13 NMR spectra of Na-Pravastatin and LDH-Pravastatin materials. Those features indicate that the intercalation preserves the drug structural integrity. Spectroscopic techniques corroborate the nature of the guest species and their arrangement between the inorganic layers. Changes related to carboxylate, alcohol, and olefinic moieties are observed in both vibrational Raman and C-13 NMR spectra after the drug intercalation. Thus, Pravastatin ions are forced to be arranged as head to tail through intermolecular hydrogen bonding between adjacent organic species. The thermal decomposition profile of the hybrid samples is distinct of that one observed for Na-Pravastatin salt, however, with no visible increase in the thermal behavior when the organic anion is sequestrated within LDH gap.

Atomically resolved study of the morphology change of InAs/GaAs quantum dot layers induced by rapid thermal annealing

The optoelectronic properties of InAs/GaAs quantum dots can be tuned by rapid thermal annealing. In this study, the morphology change of InAs/GaAs quantum dots layers induced by rapid thermal annealing was investigated at the atomic-scale by cross-sectional scanning tunneling microscopy. Finite elements calculations that model the outward relaxation of the cleaved surface were used to determine the indium composition profile of the wetting layer and the quantum dots prior and post rapid thermal annealing. The results show that the wetting layer is broadened upon annealing. This broadening could be modeled by assuming a random walk of indium atoms. Furthermore, we show that the stronger strain gradient at the location of the quantum dots enhances the intermixing. Photoluminescence measurements show a blueshift and narrowing of the photoluminescence peak. Temperature dependent photoluminescence measurements show a lower activation energy for the annealed sample. These results are in agreement with the observed change in morphology. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4770371]

Self-assembled films from chitosan and poly(vinyl sulfonic acid) on Nafion® for direct methanol fuel cell

Chitosan/poly(vinyl sulfonic acid) (PVS) films have been prepared on Nafion® membranes by the layer-by-layer (LbL) method for use in direct methanol fuel cell (DMFC). Computational methods and Fourier transform infrared (FTIR) spectra suggest that an ionic pair is formed between the sulfonic group of PVS and the protonated amine group of chitosan, thereby promoting the growth of LbL films on the Nafion® membrane as well as partial blocking of methanol. Chronopotentiometry and potential linear scanning experiments have been carried out for investigation of methanol crossover through the Nafion® and chitosan/PVS/Nafion® membranes in a diaphragm diffusion cell. On the basis of electrical impedance measurements, the values of proton resistance of the Nafion® and chitosan/PVS/Nafion® membranes are close due to the small thickness of the LbL film. Thus, it is expected an improved DMFC performance once the additional resistance of the self-assembled film is negligible compared to the result associated with the decrease in the crossover effect.

Synthesis of self-assembled tubes in direct conjugated porphyrin-gold nanohybrid solution

We report on the formation of self-assembled meso-tetrakis (p-sulfonatofenyl) porphyrin (H2 TPP'S POT. 4-''IND. 4') tubes stabilized by gold nanoparticles (NPs) in basic solution and on their spectroscopic chareterization. The role of the gold NPs in the aggregation dynamics of free-base sulfonated porphyrin (H2TPP'S POT. 4-''IND. 4') is also investigated. The direct conjugation of the gold NPs to the H2TPPS4 molecule quenches the fluorescence intensity, while absorption peaks are blue-shifted, indicating predominant H-type aggregation. It is observed that porphyrin molecules adsorbed on the surface of the gold NP interact and form tubes of maximum diameter ∼1.5 μm and length >100 μm. Steady-state and time-resolved spectroscopic techniques confirm nonradiative energy transfer from porphyrin to gold NP.

Structural, morphological, and magnetic characterization of In1-xMnxAs quantum dots grown by molecular beam epitaxy

In this paper, we present a method to order low temperature (LT) self-assembled ferromagnetic In1-xMnxAs quantum dots (QDs) grown by molecular beam epitaxy (MBE). The ordered In1-xMnxAs QDs were grown on top of a non-magnetic In0.4Ga0.6As/GaAs(100) QDs multi-layered structure. The modulation of the chemical potential, due to the stacking, provides a nucleation center for the LT In1-xMnxAs QDs. For particular conditions, such as surface morphology and growth conditions, the In1-xMnxAs QDs align along lines like chains. This work also reports the characterization of QDs grown on plain GaAs(100) substrates, as well as of the ordered structures, as function of Mn content and growth temperature. The substitutional Mn incorporation in the InAs lattice and the conditions for obtaining coherent and incoherent structures are discussed from comparison between Raman spectroscopy and x-ray analysis. Ferromagnetic behavior was observed for all structures at 2K. We found that the magnetic moment axis changes from [110] in In1-xMnxAs over GaAs to [1-10] for the ordered In1-xMnxAs grown over GaAs template. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4745904]