Julio Ramírez-Castellanos

Incorporation of Mn12 single molecule magnets into mesoporous silica

The incorporation of four Mn12 derivatives, namely [Mn12O12(O2CR)16(H2O)4] (R = CH3 (1), CH3CH2 (2), C6H5 (3), C6F5 (4)), into the hexagonal channels of the MCM-41 mesoporous silica has been studied. Only the smallest clusters 1 and 2, i.e. those with compatible size with the pores of MCM-41, could enter into the mesoporous silica. Powder X-ray diffraction analysis, HRTEM images and N2 adsorption–desorption isotherm experiments show that the well-ordered hexagonal structure of MCM-41 is preserved and that the Mn12 clusters are inside the pores. The magnetic properties of the MCM-41/2b nanocomposite material obtained in CH2Cl2 indicate that the structure of the cluster is maintained after incorporation on the MCM-41 walls, but some differences appear in the case of the MCM-41/1 and MCM-41/2a nanocomposite materials obtained in CH3CN. Calcination of the composite samples leads to materials with magnetic properties that are very different from those obtained after calcination of pristine 1 and 2. The incorporation of the Mn12 clusters inside the MCM-41 pores may induce the formation of new Mn–oxide nanoparticles after the loss of the carboxylate ligands. The incorporation of Mn12 derivatives into UVM-7, a mesoporous silica with a hierarchical pore system at two different length scales, gives rise to similar results.

Influence of Fe and Al doping on the stabilization of the anatase phase in TiO2 nanoparticles

Anatase TiO2 nanoparticles doped with Al or Fe have been synthesized via a modified Pechini method which allows us to reach high control in size and composition. Microstructural analysis confirms the good crystallinity of the doped anatase nanoparticles with average sizes around 5 nm and dopant cationic concentrations up to 30%. The anatase to rutile transition (ART) has been thermally driven and analyzed as a function of the doping. Thermo-diffraction measurements indicate that the phase transition can be either promoted or inhibited by Fe or Al doping, respectively. The influence of Al and Fe doping on the phase transition has been discussed by means of Raman spectroscopy, photoluminescence and X-ray photoelectron spectroscopy, with special attention paid to the role played by Ti3+ at the surface. The anatase phase has been stabilized up to temperatures above 900 °C by appropriate Al doping.

Cr doped titania microtubes and microrods synthesized by a vapor–solid method

Cr doped TiO2 rutile nanoparticles have been used as precursor of microrods and microtubes grown by a vapor–solid method. The grown microstructures have nearly square cross-sections of a few microns wide and lengths of up to about 100 microns. By longer thermal treatments or higher growth temperatures, the ratio of microrods to microtubes increases. The presence of partially filled openings with growth steps in the internal faces of the tubes indicates that the tubes transform into rods by extended or intense thermal treatments, which enables to control the nature of the microstructures, tubes or rods, by varying the parameters of the thermal treatment. Cr incorporation has been found to be homogeneous along the growth axis, with amounts in the range from 1.2 to 2.8% of cationic fraction, which depend on the Cr content in the precursor and on the growth parameters. Optical activation of the Cr ions has been demonstrated by cathodoluminescence in the scanning electron microscopy, and crystallographic assessment of the structures has been carried out using Raman spectroscopy and electron backscattered diffraction.

Microstructural characterization of Yba 2 Cu 3 O 7–δ thick films grown at very high rates and high temperatures by pulsed laser deposition

Microstructural and magnetic characterization were undertaken on high-rate, high-temperature grown YBa 2 Cu 3 O 7–δ (YBCO) films. The films were of approximately 1 μm thickness and were grown by pulsed laser deposition on (100) SrTiO 3 using a high-power industrial laser at growth temperatures between 750 °C and 870 °C and at growth rates of up to 4 μm/min. Two YBCO layers with different c -lattice parameters were observed in the films, the higher c value occurring near the substrate interface and arising from cation disorder and oxygen nonstoichiometry, and the lower one near the film surface arising from cation disorder alone. BaCuO 2 precipitates were present near the surface of the films, indicative of partial melting during growth. The amount of BaCuO 2 increased with growth temperature. Epitaxial Y 2 O 3 also formed in increasing amounts suggestive of a different partial melting reaction in the films compared to bulk YBCO, where Y 2 BaCuO 5 coexists with liquid. Around 1 MA/cm 2 values of high critical current density ( J c ) were observed in the films, and the in-field J c improved with growth temperature despite the fact that the superconducting transition width increased significantly.

The controlled transition-metal doping of SnO2 nanoparticles with tunable luminescence

SnO2 nanoparticles doped with transition metals (V, Cr, Mn) have been synthesized by both the hydrothermal method (HDT) in a basic media and the liquid mixed method (LQM) based on the Pechini method. Nanocrystalline particles obtained via a liquid mixed technique show a well-defined chemical composition and an average size of 6 nm, with a high degree of both crystallinity and chemical homogeneity. Nanoparticles prepared via a hydrothermal method exhibit a high dispersion in size as well as agglomeration effects. As the LQM demonstrates advantages with respect to the HDT, a more detailed investigation has been carried out on the SnO2 nanoparticles doped with V, Cr and Mn grown by this method. The microstructure of the materials was elucidated by means of X-ray Diffraction (XRD), Selected-Area Electron Diffraction (SAED), and High-Resolution Transmission Electron Microscopy (HRTEM). Luminescence from undoped and doped SnO2 nanoparticles was characterized by cathodoluminescence (CL). The luminescence studies demonstrate a strong dependence of CL signals with transition metal doping, thus inducing red, green or orange emissions when doping with Cr, V or Mn respectively.

Effect of lithium doping and precursors on the microstructural, surface electronic and luminescence properties of single crystalline microtubular tin oxide structures

Li doped SnO2 microtubes were obtained by thermal evaporation using two different starting materials as precursors: Li doped SnO2 nanoparticles synthesized by a soft chemistry method or a mixture of commercial SnO2 and Li2CO3 powders. In both cases the controlled lithium content was in the range from 10 to 30 cationic %. The microstructures are grown following a vapor–solid mechanism assisted by dislocations. The density of the obtained structures was slightly lower for the nanoparticle-based samples, although the treatments last for longer times in comparison to that of the commercial mixture-based samples. A study of the surface electronic properties and the luminescence emission of the different microtubes reveals a variable defect distribution in the samples depending on the precursor used. Oxygen vacancies and/or tin interstitials are generated to compensate for the charge imbalance due to the lithium incorporation in tin lattice sites together with the reduction of neighboring tin atoms. The microstructural characterization of the samples has been carried out by X-ray diffraction, Raman spectroscopy, and electron backscattered diffraction. X-ray photoemission spectroscopy and cathodoluminescence measurements allow comparison of pathways which favor the incorporation of the dopant into the structure and the interaction of lithium with native defects.

Silicon surface passivation by PEDOT: PSS functionalized by SnO2 and TiO2 nanoparticles

In this paper, we present a study of silicon surface passivation based on the use of spin-coated hybrid composite layers. We investigate both undoped poly(3,4-ethylenedioxythiophene)/poly-(styrenesulfonate) (PEDOT:PSS), as well as PEDOT:PSS functionalized with semiconducting oxide nanomaterials (TiO2 and SnO2). The hybrid compound was deposited at room temperature by spin coating-a potentially lower cost, lower processing time and higher throughput alternative compared with the commonly used vacuum-based techniques. Photoluminescence imaging was used to characterize the electronic properties of the Si/PEDOT:PSS interface. Good surface passivation was achieved by PEDOT:PSS functionalized by semiconducting oxides. We show that control of the concentration of semiconducting oxide nanoparticles in the polymer is crucial in determining the passivation performance. A charge carrier lifetime of about 275 μs has been achieved when using SnO2 nanoparticles at a concentration of 0.5 wt.% as a filler in the composite film. X-ray diffraction (XRD), scanning electron microscopy, high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray in an SEM, and μ-Raman spectroscopy have been used for the morphological, chemical and structural characterization. Finally, a simple model of a photovoltaic device based on PEDOT:PSS functionalized with semiconducting oxide nanoparticles has been fabricated and electrically characterized.

Controlled synthesis of lithium doped tin dioxide nanoparticles by a polymeric precursor method and analysis of the resulting defect structure

Recently, the high demand for the development of improved energy storage devices has brought the focus on tin oxide nanoparticles due to their ability to intercalate lithium. In order to strengthen the potential of the technological applications of Li doped SnO2, the synthesis of nanoparticles by a soft chemistry method with a highly homogeneous controlled size, high crystallinity and concentration of effectively incorporated Li over the range of 10 to 30 cat% is reported in this study. Li doped SnO2 nanoparticles were prepared with sizes between 4 nm (for undoped material) and 11 nm (for the 30 cat% Li doped nanoparticles). The samples were thoroughly characterized using different complementary techniques to determine the defect structure and electronic surface properties, whose knowledge is of the utmost relevance for achieving the desirable properties needed for implementation in different applications. The results indicate that lithium is incorporated mainly in substitutional sites in the rutile structure of tin oxide. On increasing the lithium content in the nanoparticles, the amount of oxygen vacancies reduced and a shift in the Fermi level, which was pinned at the surface defects, toward the maximum of the valence band was obtained. In the samples with the highest Li content, the local environment of lithium at the nanoparticle surface resembled the environment of Li in lithium oxide. Several intra-gap recombination defect levels were detected in the visible range, which are related to intrinsic tin oxide defects such as vacancies in different configurations. In addition, a defect level at 3.1 eV is also exhibited with increasing intensity in Li doped nanoparticles.

Understanding the effects of Cr doping in rutile TiO2 by DFT calculations and X-ray spectroscopy

The effects of Cr on local environment and electronic structure of rutile TiO2 are studied combining theoretical and experimental approaches. Neutral and negatively charged substitutional Cr impurities CrTi0 and CrTi1− as well as Cr-oxygen vacancy complex 2CrTi + VO are studied by the density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE) functional. Experimental results based on X-Ray absorption spectroscopy (XAS) and X-Ray photoelectron spectroscopy (XPS) performed on Cr doped TiO2 at the Synchrotron facility were compared to the theoretical results. It is shown that the electrons of the oxygen vacancy tend to be localized at the t2g states of the Cr ions in order to reach the stable oxidation state of Cr3+. Effects of Cr on crystal field (CF) and structural distortions in the rutile TiO2 cell were analyzed by the DFT calculations and XAS spectra revealing that the CF and tetragonal distortions in TiO2 are very sensitive to the concentration of Cr.

In situ local assessment of laser irradiation-induced phase transformations in hexagonal MoO3 microrods

Phase transformations in h-MoO3 microrods have been locally induced by laser irradiation and assessed in situ by micro-Raman and photoluminescence spectroscopy. The obtained phases and their time stability were found to depend on laser wavelength, power density and irradiation time. Red laser (633 nm) irradiation induces the stable formation of α-MoO3. The corresponding oxidation dynamics appear to be triggered by diffusion processes. Considering the diverse physical properties of the different Mo oxides, red laser irradiation appears as a suitable method to tailor the functionalities of individual h-MoO3 microstructures. UV (325 nm) irradiation also transforms h-MoO3 into α-MoO3, with the formation of Mo4O11 being observed in the initial stage of the irradiation process. However, this transformation seems to be unstable in time, finally giving rise to structural changes in the obtained α-MoO3 phase and to the appearance of other Mo oxides. Laser irradiation also modifies the luminescence properties of the h-MoO3 microrods, giving rise to the appearance of several emission bands related to complex defects involving oxygen vacancies.