Development of the 'MN''SI IND. 1.7' phase in 'MN'-containing 'SI' films

Thin films of Si with Mn concentrations up to 20 at.% were prepared by conventional radio frequency sputtering. After deposition, the films were submitted to thermal annealing treatments and their properties were investigated by composition analysis, Raman scattering, microscopic techniques, optical transmission, and electrical transport. The experimental results show that all as-deposited films are amorphous, with the Mn atoms being effective and controllably incorporated into the Si matrix. Moreover, thermal annealing at increasing temperatures induces the crystallization of the films as well as the growth of the MnSi1.7 silicide phase in the Mn-containing samples. Along with sample crystallization, some films become covered by small structures that are randomly distributed all over their surfaces. These structures are essentially Mn-containing Si crystals with typical sizes in the (sub-) micrometer range and, as the thermal annealing advances, the density of structures increases at the expense of their individual average dimension. The development and characteristics of the observed superficial structures are discussed in view of the main structural and morphological properties of the samples.FAPESPCNP

Optical spectroscopy of Nd3+ ions in a nanostructured glass matrix

This paper presents the optical characterization of Nd3+ ions in nanostructured SiO2-Na2CO3-Al2O3-B2O3 (SNAB) CdS glass, synthesized by fusion. Radiative properties of the glass were determined by absorption, luminescence spectroscopy and lifetime measurements. Nd3+ emission enhancement and quenching were investigated in the presence of CdS nanocrystals. Nd3+-emission quenching was attributed to upconversion mechanisms and nonradiative processes such as multiphonon decay and energy transfer, while the Nd3+-emission enhancement was due to energy transfer from the CdS nanocrystals. Changes in the chemical environment around CdS nanocrystals were also confirmed by Judd-Ofelt calculations.CNPqFAPEMIGCAPE

Confirming the lattice contraction in CdSe nanocrystals grown in a glass matrix by Raman scattering

This work gives the evidence of the lattice contraction in CdSe nanocrystals (NCs) grown in a glass matrix. The CdSe NCs were investigated by atomic forcemicroscopy (AFM), optical absorption (OA), and Raman spectroscopy. The average size of CdSe NCs can be estimated by AFM images. Using the OA spectra and the effective-mass approximation, it was also possible to estimate the average sizes of CdSe NCs, which agree very well with the AFM data. These results showed that the CdSe NCs grow with increasing time of heat treatment. The blue shift of the longitudinal optical (LO)modes and surface optical (SO) phononmodes with an increase in the average radius of the NCs, shown in the Raman spectra, was explained by the lattice contraction in CdSe NCs caused by thermodynamic interactions at the interface with the host glass matrix.FAPEMIGMCT/CNP

The 2012–2013 Montes Claros earthquake series in the São Francisco\ud Craton, Brazil: new evidence for non-uniform intraplate stresses\ud in mid-plate South America

On 2012 May 19, an mb = 4 earthquake shook the town of Montes Claros, Brazil in the\ud middle of the S˜ao Francisco Craton. Because of the scarce seismicity in the area, an event\ud like this could provide valuable information to characterize the governing seismotectonics and\ud stress field for the region. Here, we present the results of more than 1 yr of local seismic\ud monitoring after the main shock. We found that the seismicity originated at approximately\ud 1-km depth in an NNW-oriented blind reverse fault, dipping to the E. The magnitude of\ud the main shock was 4mb, with aftershocks reaching up to 3.6mb. Focal mechanisms from\ud first motion polarities and waveform moment tensor inversions indicate a reverse faulting in\ud agreement with the orientation of the aftershock locations. In addition, we derived a new 1-D\ud local velocity model using a simultaneous inversion of hypocentres and velocity layers. The\ud results indicate P-wave velocities of 4.5 km s−1 for the upper layer of carbonate rocks and 5.23\ud and 5.69 km s−1 for the lower fractured and compact crystalline basement layers, respectively.\ud Higher Vp/Vs ratios were obtained for the upper two layers compared to the lowermost layer,\ud possibly indicating presence of rock fracturing and percolated water. The calculated stress drop\ud for the main event is 0.33 MPa, which is a relatively low value for an intraplate earthquake\ud but still within the observed range. The inversion of the main shock focal mechanism and\ud previously published focal mechanisms suggests a compressional stress regime in the central\ud part of the S˜ao Francisco Craton, which is different from the strike-slip regime in the southern\ud part, although both have an EW-oriented σ1. On the other hand, focal mechanisms of events\ud located to the west of the craton indicate an NW–SE oriented σ1 for central Brazil. This\ud variability highlights the importance of local sources of stresses (e.g. flexural stresses) in\ud mid-plate South America, unlike other mid-plate areas of the world, such as central and east\ud North America, where a more uniform stress field is observedPetrobras - BRASIS projec

Thermal diffusivity of a SNAB glass system doped with CdS nanocrystals and Nd3+

This work reports on the thermal diffusivity of the SiO2-Na2CO3-B2O3-Al2O3 (SNAB) glass system doped with semiconductor nanocrystals of CdS and Nd3+ ions. Thermal diffusivity (D) was obtained by the Thermal Lens technique. It is shown that D decreases up to 30% when SNAB is doped with CdS nanocrystals. The effect is discussed in terms of heat transport by phonons as well as interface and surface scattering.CAPESFAPEMIGCNP

ZnTe nanocrystal formation and growth control on UV-transparent substrate

ZnTe nanocrystals (NCs) were successfully grown in a UV transparent, vitreous substrate synthesized by fusion and then annealed. The formation of dot structures, emitting in the UV-range, was investigated by optical absorption (OA), atomic force microscopy (AFM) and Raman spectroscopy. Dot growth was evidenced by an OA band red-shift with increasing annealing time. Average sizes of the ZnTe dots were determined using the effective mass fit model with OA spectra and comparing the results with estimates from AFM images. A UV-transparent PZABP vitreous matrix was used because it allowed ZnTe NC growth and displayed typical Raman active phonon modes.MCT/CNPqFAPEMIGFAPESPCAPE

Reduced graphene oxide multilayers for gas and liquid phases chemical sensing

Graphene oxide (GO) multilayers were produced by the layer-by-layer technique after successive immersions of quartz slides into aqueous suspensions of cationic poly(diallyldimethyl ammonium chloride) (PDAC) and anionic GO. The adsorbed amount of GO within the multilayers measured ex situ by UV-vis spectroscopy was found to increase linearly with the number of PDAC-GO bilayers. UV-vis and Raman spectra confirmed the conversion of GO to its reduced form, namely reduced graphene oxide (RGO), when the multilayers were subjected to hot hydrazine. According to AFM images, multilayers are flat with GO sheets forming edge structures. Additionally, impedance spectroscopy provided information regarding the multilayer growth mechanism, which starts with isolated GO sheets that bridge each other after deposition of five PDAC-GO bilayers. As a proof of principle, it was demonstrated that a sensor array composed by reduced multilayers deposited onto interdigitated microelectrodes and interrogated by impedance spectroscopy is capable of discriminating vapours of volatile solvents, including toluene, gasoline, ethanol, chloroform, and acetone, as well as chemicals in aqueous solutions, such as hydrochloric acid, sodium chloride, ammonium hydroxide, and sucrose. This capability was made possible only because the LbL assembly permitted one to tune the sensors' sensitivity with the number of PDAC-GO bilayers. The results presented herein suggest that the reduced PDAC-GO multilayers are promising elements for non-specific chemical sensors.CNPq (308038/2012-6)CAPE

Adsorption of cobalt ferrite nanoparticles within layer-by-layer films: a kinetic study carried out using quartz crystal microbalance

The paper reports on the successful use of the quartz crystal microbalance technique to assess accurate kinetics and equilibrium parameters regarding the investigation of in situ adsorption of nanosized cobalt ferrite particles (CoFe2O4-10.5 nm-diameter) onto two different surfaces. Firstly, a single layer of nanoparticles was deposited onto the surface provided by the gold-coated quartz resonator functionalized with sodium 3-mercapto propanesulfonate (3-MPS). Secondly, the layer-by-layer (LbL) technique was used to build multilayers in which the CoFe2O4 nanoparticle-based layer alternates with the sodium sulfonated polystyrene (PSS) layer. The adsorption experiments were conducted by modulating the number of adsorbed CoFe2O4/PSS bilayers (n) and/or by changing the CoFe2O4 nanoparticle concentration while suspended as a stable colloidal dispersion. Adsorption of CoFe2O4 nanoparticles onto the 3-MPS-functionalized surface follows perfectly a first order kinetic process in a wide range (two orders of magnitude) of nanoparticle concentrations. These data were used to assess the equilibrium constant and the adsorption free energy. Alternatively, the Langmuir adsorption constant was obtained while analyzing the isotherm data at the equilibrium. Adsorption of CoFe2O4 nanoparticles while growing multilayers of CoFe2O4/PSS was conducted using colloidal suspensions with CoFe2O4 concentration in the range of 10-8 to 10-6 (moles of cobalt ferrite per litre) and for different numbers of cycles n = 1, 3, 5, and 10. We found the adsorption of CoFe2O4 nanoparticles within the CoFe2O4/PSS bilayers perfectly following a first order kinetic process, with the characteristic rate constant growing with the increase of CoFe2O4 nanoparticle concentration and decreasing with the rise of the number of LbL cycles (n). Additionally, atomic force microscopy was employed for assessing the LbL film roughness and thickness. We found the film thickness increasing from about 20 to 120 nm while shifting from 3 to 10 CoFe2O4/PSS bilayers, using the 8.9 × 10-6 (moles of cobalt ferrite per litre) suspension.MCT/CNPqFINEPCAPESFUNAPEFINATE

Influence of crystal field potential on the spectroscopic parameters of\ud SiO2.B2O3.PbO glass doped with Nd2O3

This paper presents the optical characteristics of Nd3+ silicate glass (SiO2-B2O3-PbO), synthesized by the fusion method. Two sets of samples were prepared: glass and corresponding glass ceramics. Optical absorption, luminescence, Raman spectroscopy and atomic force microscopy (AFM) measurements were performed in order to determine the structural properties of the systems and the radiative characteristics of Nd3+ ions. Near infrared luminescence exhibited typical Nd3+ bands. Raman and AFM measurements indicated nanocrystal growth with thermal treatment of the glass ceramics. Judd-Ofelt calculations also confirmed that heat treatment induced structural rearrangement of the samples that was dependent on Nd2O3 concentration. This resulted in changes in the optical and physical properties of the samples, including stimulated emission cross section and rigidity.CNPqFAPEMIGCAPE