S.W. da Silva

Optical and magnetic properties of Co-doped ZnO nanoparticles and the onset of ferromagnetic order

In this study, we report on the optical and magnetic properties of Co-doped ZnO nanoparticles with increasing Co-content (CoxZn1−xO; x = 0.000, 0.005, 0.010, 0.030, 0.050, 0.070, and 0.100) synthesized by the combustion reaction method. The X-ray diffraction patterns and the Raman spectra of all samples indicated the formation of the ZnO hexagonal wurtzite phase (space group C46V). The Raman data also show the formation of a secondary Co3O4 phase, which is barely seen in the X-ray spectra. Photoacoustic spectroscopy and electron paramagnetic resonance confirm the presence of the two phases (CoxZn1−xO and Co3O4). Vibrating sample magnetometer measurements performed at room temperature exhibited hysteresis loops, indicating the presence of long-range magnetic ordering in the samples. Analysis of the magnetization as a function of magnetic field and temperature shows that the ferromagnetism in the as-synthesized samples comes from small Co-metallic inclusions, with an estimated radius of about 4.8 nm and blo...

Experimental evidence of asymmetric carrier transport in InGaAs quantum wells and wires grown on tilted InP substrates

The influence of the interface morphology upon the electron–hole transport in intrinsic In0.53Ga0.47As/InP quantum structures was investigated by scanning the photoluminescence intensity profile on the sample surface. The results suggest that the carrier diffusion is very sensitive both to the roughness of the interfaces and the presence of finite-width terraces. It was found that the carrier density profile shows asymmetric diffusion normal to the terraces whereas it shows symmetric expansion along the terraces. Simulations of the asymmetry in the carrier density profile using a non-Fickian diffusion equation described by the Levy statistics show a excellent agreement with the experimental data.

Defect-limited carrier diffusion in In0.53Ga0.47As-InP single quantum well

Abstract The diffusion of electron–hole plasma in an intrinsic In0.53Ga0.47As single quantum well (SQW) was investigated by measurements of the PL intensity profile around the illuminated area. We found that the carrier diffusion length increases with the temperature, from 85 to 300 K, according to a defect-limited carrier diffusion. A change in the carrier expansion is observed at about 200 K, which appears to be correlated with the thermal activation of a defect center with activation energy of 120 meV. An Arrhenius function of the PL emission intensity confirms that a nonradiative recombination channel becomes visible with an energy of about 120 meV.

Raman spectroscopy in oleoylsarcosine-coated magnetic fluids: A surface grafting investigation

Raman spectroscopy is used to investigate magnetite nanoparticles pre-coated with oleoylsarcosine and dispersed as stable water-based (WE) and hydrocarbon-based (HE) magnetic fluids (MFs). The Raman spectra obtained from the MF samples and from liquid water were compared and the data discussed in terms of the suppression of the symmetrical hydroxyl (OH) vibrational modes. The effective grafting coefficient of the oleoylsarcosine in the WB-MF, with respect to the HB-MF, was estimated through the OH-stretching Raman modes of the OH-group chemisorbed at the nanoparticle surface. It was found that the OH-grafting coefficient in the HB-MF sample is about 3 times lower than the grafting coefficient in the WB-MF sample.

Time-resolved measurements and spatial photoluminescence distribution in InAs/AlGaAs quantum dots

Abstract We report the results of time-integrated and time-resolved photoluminescence spectroscopy on red-emitting self-organized InAs/Al0.6Ga0.4As quantum dots with indirect barriers. Spatially resolved PL measurements confirm that carriers excited in the Al0.6Ga0.4As barriers are consistent with a carrier hopping process between dots, a result also supported by time-resolved PL experiments.

Excitation Transfer through Quantum Dots Measured by Microluminescence: Dependence on the Quantum Dot Density

In this study, spatially resolved cw photoluminescence of self-assembled InAs quantum dots (QDs) on a GaAs matrix has been observed using the microluminescence surface scan technique. A sample containing QD densities from 0 up to 1600 dots/μm 2 was investigated. It was found that the QD density has a strong influence on the diffusion process and there is evidence of two different mechanisms operating at low and high QD concentration ranges that we interpret as photon recycling and interdot carrier tunneling, respectively.

Study of the interactions between the surface chemisorbed layer and the surrounding media in magnetite-coated nanoparticles using Raman spectroscopy

Raman spectroscopy is used to investigate surface-coated magnetite nanoparticles (9 nm diameter) dispersed in aqueous medium. In a first sample, the magnetite nanoparticle is coated with dextran. In a second sample the magnetite nanoparticle is double-coated with a dodecanoic layer following an ethoxylated polyalcohol layer. The Raman measurements were focused on the typical intra-molecular OH-stretching mode chemiosorbed at the nanoparticle surface. The data are discussed in terms of the suppression of the symmetric and asymmetric hydrogen bonded mode in both samples.

Optical phonon spectra of GaSb/AlSb superlattices: Influence of strain and interface roughnesses

Raman spectroscopy indicates the formation of extended intermixture alloy regions at the interfaces in the GaSb/AlSb superlatttices, which suppress the optical confined modes. A misfit strain was found to be responsible for the high‐frequency shift of the optical phonons confined in the AlSb layers. It turned out that, in the ultrathin layer GaSb/AlSb superlattices with layer thicknesses less than 7 monolayers, the strain was completely relaxed in the alloy interface regions.

Maghemite–gold core–shell nanostructures (γ-Fe2O3@Au) surface-functionalized with aluminium phthalocyanine for multi-task imaging and therapy

In this study we report on the elaboration and characterization of core–shell maghemite–gold nanoparticles with the shell modulated for different thicknesses below 2 nm. Gold-shelled maghemite nanoparticles with an average core size of about 9 nm were developed using a single-step protocol involving reduction of Au3+ in the presence of citrate-coated maghemite nanoparticles. Additionally, post-functionalization of the core–shell structures with aluminium phthalocyanine was successfully accomplished, aimed at the production of a material platform for photodynamic therapy. The as-produced samples were structurally, morphologically, magnetically and optically characterized and presented long-term colloidal stability at physiological pH. Impressively, we found the as-produced samples showed good X-ray attenuation properties, making them able to be used as nanoprobes for targeted computed tomography. Moreover, in vitro nanocytotoxicity tests confirmed a superior biocompatibility of the as-produced samples, making them a very promising multi-task platform for in vivo applications.

Evolution of the doping regimes in the Al-doped SnO2 nanoparticles prepared by a polymer precursor method.

In this study, we report on the structural and hyperfine properties of Al-doped SnO2 nanoparticles synthesized by a polymer precursor method. The x-ray diffraction data analysis carried out using the Rietveld refinement method shows the formation of only rutile-type structures in all samples, with decreasing of the mean crystallite size as the Al content. A systematic study of the unit cell, as well as the vicinity of the interstitial position show strong evidence of two doping regimes in the rutile-type structure of SnO2. Below 7.5 mol% doping a dominant substitutional solution of Al(+3) and Sn(4+)-ions is determined. However, the occupation of both substitutional and interstitial sites is determined above 7.5 mol% doping. These findings are in good agreement with theoretical ab initio calculations.