- Full Professor, State University of Campinas, UNICAMP. Master (1989) and PhD in Physics (1984), Postdoctoral at Harva... moreFull Professor, State University of Campinas, UNICAMP. Master (1989) and PhD in Physics (1984), Postdoctoral at Harvard University, USA (1989 -1991), visiting professor at the University of Utah, USA (1998-1999) and visiting researcher at SCIRO Melbourne, Australia (2010-2011). Works in Condensed Matter Physics in the study of optoelectronic, structural and thermomechanical properties of amorphous and nanostructured materials for electronic and photovoltaic devices.edit
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In recent years, there has been a dramatic progress in the photonics field of disordered media, ranging from applications in solar collectors, photocatalyzers, random lasing, and other novel photonic devices, to investigations into... more
In recent years, there has been a dramatic progress in the photonics field of disordered media, ranging from applications in solar collectors, photocatalyzers, random lasing, and other novel photonic devices, to investigations into fundamental topics, such as localization of light and other phenomena involving photon interactions. Anderson localization of light is an open researcher frontier, which has greatly attracted the attention of researchers in the past few decades. In this work, we study the transport of light in a strongly disordered optical medium composed by core-shell nanoparticles (TiO2@Silica) suspended in ethanol solution. We demonstrate the crossover from a diffusive transport to a localization transition regime as TiO2@Silica nanoparticle concentration is increased. A striking phenomenon of enhanced absorption, mainly near the input border, arises at the localization transition, from which an increase of refractive index was inferred. An increase of the density of l...
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We study the light-matter coupling by Raman scattering in colloidal suspensions composed by core-shell TiO2@Silica (Rutile@Silica) nanoparticles suspended in ethanol and water solutions. Strong enhancement of the Raman signal per particle... more
We study the light-matter coupling by Raman scattering in colloidal suspensions composed by core-shell TiO2@Silica (Rutile@Silica) nanoparticles suspended in ethanol and water solutions. Strong enhancement of the Raman signal per particle is observed as [TiO2@Silica] is increased above a threshold, being stronger in ethanol suspensions. Moreover, above this [TiO2@Silica] threshold, the optical transmittance of the ethanol suspension starts to be considerably lower than in water, despite scattering strength being higher in water. These results are attributed to localization of light induced by strong correlation in the scatterers' position as a consequence of the long-range Coulomb interaction between the TiO2@Silica nanoparticles. Light diffraction in TiO2@Silica suspensions (water and ethanol) shows strong correlation in the scatterers' position (structure seemingly cubic), being stronger in ethanol than in water (longer-range Coulomb interaction). As a result, we demonstra...
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Perovskite solar cells (PSCs) technology is now reaching its full potential in terms of power conversion efficiency, but still presenting problems related to long-term stability under operating conditions. One of the most promising... more
Perovskite solar cells (PSCs) technology is now reaching its full potential in terms of power conversion efficiency, but still presenting problems related to long-term stability under operating conditions. One of the most promising alternatives to PSCs is the layered PSCs (2D-PSCs). Layered perovskites present a huge compositional variety, which can be used to directly tune photophysical characteristics that influence the operational mechanisms of the devices. This review addresses the structural organization of both the organic and inorganic sublattices, focusing on how the structure influences the quantum and dielectric confinement, phonons and charge carriers' dynamics, charge mobility, and structural defects. We discuss the relation between the structure-properties of layered perovskites with the performance of solar cells. We, then, offer insights into how these characteristics have been controlled in the assembly of 2D-PSCs to improve their efficiency and stability. We conclude by giving a perspective of future developments and open areas of exploration that might impact the progress of this rapidly growing technology.
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The high contamination by the SARS-Cov-2 virus has led to the search for ways to minimize contagion. Masks are used as part of a strategy of measures to suppress transmission and save lives. However, they are not sufficient to provide an... more
The high contamination by the SARS-Cov-2 virus has led to the search for ways to minimize contagion. Masks are used as part of a strategy of measures to suppress transmission and save lives. However, they are not sufficient to provide an adequate level of protection against COVID-19. Activated charcoal has an efficient antibacterial action, adsorption and low cost. Here, the interaction between two molecules of activated carbon was analyzed, interacting with two structures of the SARS-Cov-2, through docking and molecular dynamics using the platforms Autodock Vina 4.2.6, Gaussian 09 and Amber 16. As a result, the complexes from ozone-functionalized coal to viral structures happen mainly through hydrophobic interactions at the binding site of each receptor. The values of the mean square deviations of the two systems formed by ligands/receptors and showed better stability. The results of Gibbs free energy showed a better interaction between proteins and functionalized charcoal, with △G...
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Density Functional Theory and canonical ensemble were used to investigate thermodynamic properties of Syngas and its mixture with natural gas. The following thermodynamic potentials were obtained: internal energy, enthalpy, entropy and... more
Density Functional Theory and canonical ensemble were used to investigate thermodynamic properties of Syngas and its mixture with natural gas. The following thermodynamic potentials were obtained: internal energy, enthalpy, entropy and Gibbs free energy for temperatures ranging from 0.5 K to 1500 K. It was observed that CO and H2 were the most stable Syngas components, possessing the ability to render Syngas less favorable to the temperature increase. Also, we verified that Syngas presents properties similar to an antiknock agent for natural gas, raising its resistance to temperature increases. Were determined the Poisson coefficients and Bulk modulus for Natural gas/Syngas mixtures and Shomate equation coefficients for some Syngas types, providing a more complete thermodynamic description for these gases. Additionally, thermodynamic potentials of combustion for Natural gas/Syngas mixtures were predicted, showing that this biofuel can reduce the calorific power of natural gas and makes its combustion less favorable due its antiknock behavior. However, a mixture with 30% of Syngas may be useful for natural gas combustion, since it present a calorific power between 73.41% and 79.49% of that of natural gas, which is a substantial fraction of energy released during combustion, showing good future prospects to the Natural gas/Syngas mixture to the renewable energy generation.
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Upgraded metallurgical grade silicon (UMG-Si) was obtained through metallurgical methods using two steps. First, metallurgical grade silicon was purified by the vacuum degassing technique using an electron-beam system. An ingot was then... more
Upgraded metallurgical grade silicon (UMG-Si) was obtained through metallurgical methods using two steps. First, metallurgical grade silicon was purified by the vacuum degassing technique using an electron-beam system. An ingot was then produced through Czochralski (CZ) growth. This later process was also used to reduce impurities through the segregation phenomenon in the CZ technique, producing a material of 99.9993% purity, one order of magnitude less pure than the minimum required for solar grade silicon. Solar cells fabricated with polycrystalline silicon with that amount of impurities are of low efficiency. Thus, the CZ technique was also adopted to supply monocrystalline silicon in order to avoid additional defects due to the grain boundary of polycrystalline wafers. Adopting this procedure, we produced solar cells with an efficiency of 13%, using a very simple fabrication process.
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Here we demonstrate a low-temperature process for the synthesis of pinhole-free lead iodide and perovskite (CH3NH3PbI3) films. The approach consists in converting amorphous sputtered lead sulfide films onto lead iodide (PbI2) films... more
Here we demonstrate a low-temperature process for the synthesis of pinhole-free lead iodide and perovskite (CH3NH3PbI3) films. The approach consists in converting amorphous sputtered lead sulfide films onto lead iodide (PbI2) films through an iodination process. The following step consists of the conversion of PbI2 into CH3NH3PbI3 through dipping into methylammonium iodide (CH3NH3I) solution. The effectiveness of this new route was investigated through X-ray diffraction, optical transmittance, scanning electron microscopy and X-ray photoelectron spectroscopy. This method is comprehensibly compatible with large-scale and large-area deposition and provides a convenient and highly reproducible route for the preparation of perovskite thin films.
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ABSTRACTOrganolead iodide perovskites, CH3NH3PbI3, have attracted the attention of researchers around the world due to their optical and electrical properties. Their main characteristics include, direct band-gap (1.4 to 3.0 eV), large... more
ABSTRACTOrganolead iodide perovskites, CH3NH3PbI3, have attracted the attention of researchers around the world due to their optical and electrical properties. Their main characteristics include, direct band-gap (1.4 to 3.0 eV), large absorption coefficient in the visible spectrum, long carrier diffusion length and ambipolar charge transport. Aside that, perovskite thin films can be produced with low cost and are compatible with large-scale manufacture. Perovskite thin films have been synthesized mainly by spin-coating technique and thermal evaporation, which can be executed in one or two steps. Aiming to increase the light absorption, nanostructured perovskite thin films are also under intense study, since the nanostructures can absorb more light than a flat film. Thus, in this work, we reported the synthesis of perovskite (CH3NH3PbI3) nanorods by means of conversion of lead sulphide quantum dots (PbSQD). The perovskite nanorods were grown by exposing the PbSQD to a highly concentr...
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ABSTRACTThe effect of surface-enhanced Raman spectroscopy (SERS) was investigated in N719 dye thin films deposited on silicon wafer with a thin film of silver nanoparticles (Ag-NPs) fabricated by laser ablation in an aqueous solution,... more
ABSTRACTThe effect of surface-enhanced Raman spectroscopy (SERS) was investigated in N719 dye thin films deposited on silicon wafer with a thin film of silver nanoparticles (Ag-NPs) fabricated by laser ablation in an aqueous solution, using a NdYAG laser (λ = 1064nm). Optical absorption spectroscopy of the Ag-NPs colloidal solution shows an absorption peak at λ = 400nm, associated with a localized surface plasmon resonance in the Ag-NPs. Scanning electron microscopy (SEM) reveals that these NPs have an approximately spherical shape, with their diameter being tunable by laser power intensity. Raman spectroscopy measurements were performed using low laser power to avoid damage to the N719 dye films. Thus, a small Raman signal is obtained. The Raman intensity was greatly increased when the N719 film was deposited on a substrate with a thin film of Ag-NPs due to the SERS effect. The process was also used in Rhodamine-B to clearly demonstrate the SERS effect obtained by the use of these ...
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Abstract Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission... more
Abstract Photoelectron emission spectroscopy in air (PESA) has been used to investigate titanium dioxide (TiO2) deposited by atomic layer deposition (ALD). A procedure has been developed to unambiguously determine the photoemission threshold energy (also referred to as the “ionization potential”) of TiO2 thin films, avoiding inherent artifacts due to photoelectron emission from the substrate, which supplies misleading results. This has been achieved using PESA measurements performed as a function of TiO2 film thickness on two substrates with different work functions. We find that proper measurements of the photoemission threshold energy (including work function and ionization potential) of thin films by PESA require the use of films much thicker than their electron attenuation length (EAL). A photoemission threshold energy of 5.0 ± 0.2 eV is obtained for TiO2 and has been attributed to a trap level due to oxygen vacancies, which lie within the band gap of the TiO2. The analysis of the photoemission decay with film thickness also provides a method for determining a “practical” (or effective) EAL at excitation energy slightly above the photoemission threshold energy of the material. We extract an EAL for the deposited TiO2 of 0.65 ± 0.02 nm (at 0.5 eV). The procedure can also be adopted for determining the thickness of extremely thin films, provided their thickness is smaller than their EAL.
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We show that superlattice (SL) of PbS quantum dots (QD) can be easily prepared by drop casting of colloidal QD solution onto glass substrate and the ordering level can be controlled by the substrate temperature. A QD solution was dropped... more
We show that superlattice (SL) of PbS quantum dots (QD) can be easily prepared by drop casting of colloidal QD solution onto glass substrate and the ordering level can be controlled by the substrate temperature. A QD solution was dropped on glass and dried at 25, 40, 70 and 100°C resulting in formation of different SL structures. X-ray diffractograms (XRD) of deposited films show a set of sharp and intense peaks that are higher order satellites of a unique peak at 1.8 degrees (two theta), which corresponds, using the Bragg’s Law, to an interplanar spacing of 5.3 nm. The mean particles diameter, calculated through the broadening of the (111) peak of PbS using the Scherrer’s formula, were in agreement with the interplanar spacing. Transmission electron microscopy (TEM) measurements were also used to study the SL structure, which showed mainly a face centered cubic (FCC) arrangement of the QD. The photoluminescence (PL) spectrum of QD in the SL showed a shift toward lower energy compar...
Research Interests: Materials Science and MRS
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ABSTRACT Hybrid photovoltaic (PV) devices based on a poly(3-hexylthiophene) (P3HT) layer and chemically modified silicon wafers with thiophene groups are reported for the first time. The chemical modification was performed by linking... more
ABSTRACT Hybrid photovoltaic (PV) devices based on a poly(3-hexylthiophene) (P3HT) layer and chemically modified silicon wafers with thiophene groups are reported for the first time. The chemical modification was performed by linking thiophene units directly onto silicon atoms at the n-type silicon wafer surface. To achieve the silicon modification, two-step, chlorination/alkylation reactions were used to convert Si–H into Si–thiophene bonds. Such interfacial modification increased the compatibility and the physical contact between the organic and inorganic phases as well as it promoted a more favorable alignment of band-edge energies. Thus, the hybrid photovoltaic devices based on n-Si/thiophene/P3HT achieved a power conversion efficiency of 8.0% (under simulated air mass 1.5 solar irradiation at 100 mW cm−2), which was higher than the device containing the silicon wafer with terminal Si–H bonds.
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ABSTRACT In this study, we aimed at systematically determining the potential of the zone melting (ZM) technique to remove impurities from Metallurgical Grade Silicon (MG- Si) in an Electron Beam Furnace (EBF), using a water-cooled copper... more
ABSTRACT In this study, we aimed at systematically determining the potential of the zone melting (ZM) technique to remove impurities from Metallurgical Grade Silicon (MG- Si) in an Electron Beam Furnace (EBF), using a water-cooled copper crucible. Our focus was on obtaining solar grade silicon, with the purity between Electronic Grade Silicon (EG-Si) and MG-Si, at lower cost than the silicon obtained by the Siemens process. The MG- Si (99.855% purity in mass, or 1,450 ppm of impurities) was processed by 1 and 2 passes of ZM at speed of 1 mm/min and 10 mm/min. The ZM process reduced in 98% the total amount of impurities present in the MG-Si, increasing the purity from 99% to 99.999%, an intermediate stage to achieve the electronic grade (> 99.9999%). Boron remained near the same after the ZM due to its vapor pressure be lower than the pressure of the furnace chamber and due its distribution coefficient in silicon be near the unit. Carbon and oxygen in the MG-Si were reduced from 106 to 35 ppm and from 30 ppm to 5 ppm, respectively, after ZM, and these values are very close to the levels in the electronic grade silicon. The electrical resistivity showed to be dependent on the boron concentration, but not on the phosphorus or the total amount of impurities. All ingots processed by ZM exhibited p-type characteristics, and it means that boron was really the dominant dopant.
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Foram preparados filmes compactos de dióxido de titânio por Deposição por Camadas Atômicas (ALD) para aplicação como blocking layer e camada antirrefletora para células solares. Os filmes foram depositados a 200°C e recozidos a 300°C,... more
Foram preparados filmes compactos de dióxido de titânio por Deposição por Camadas Atômicas (ALD) para aplicação como blocking layer e camada antirrefletora para células solares. Os filmes foram depositados a 200°C e recozidos a 300°C, 400°C e 500°C. Foram feitas caracterizações das propriedades termomecânicas a partir de medidas da tensão residual em função da temperatura para uma variedade de substratos, a partir da qual se obtém o coeficiente de dilatação térmica dos filmes; e nanoindentação, a partir do qual se determina o módulo de Young e a razão de Poisson dos filmes.
Filmes finos de ZnO tem várias aplicações em dispositivos eletrônicos como em diodos, transistores, dispositivos piezoelétricos e células solares. Neste trabalho desenvolvemos este material pela técnica ALD (Atomic Layer Deposition)... more
Filmes finos de ZnO tem várias aplicações em dispositivos eletrônicos como em diodos, transistores, dispositivos piezoelétricos e células solares. Neste trabalho desenvolvemos este material pela técnica ALD (Atomic Layer Deposition) utilizando um equipamento recém adquirido pelo laboratório de pesquisas fotovoltaicas do IFGW. Esta técnica permite deposições camada-por-camada (layer-by-layer) dos filmes de maneira conformal (o filme se deposita assumindo a forma da superfície do substrato). Investigaremos o efeito da temperatura entre 100 ºC e 300 ºC.
Lead iodide (PbI 2) is a precursor for the preparation of the organolead iodide perovskite (CH 3 NH 3 PbI 3), which has been used in the fabrication of highly efficient solar cells. In this work, a novel route for the deposition of PbI 2... more
Lead iodide (PbI 2) is a precursor for the preparation of the organolead iodide perovskite (CH 3 NH 3 PbI 3), which has been used in the fabrication of highly efficient solar cells. In this work, a novel route for the deposition of PbI 2 thin films is performed by rf sputtering a target made from compressed PbI 2 powder. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) revealed that the PbI 2 films produced were uniform, pinhole-free, polycrystalline, and had low roughness. A small concentration of Pb nanocrystals observed within the films is attributed to differences in the sputtering yield of lead and iodide from the PbI 2 target. A dependence of band gap on rf sputtering power was observed, which was associated with a reduction in the concentration of Pb nanocrystals. The PbI 2 films were efficiently converted into CH 3 NH 3 PbI 3 perovskite films through the immersion into a methylammonium iodide (MAI) solution, which also converted the remaining Pb nanocrystals into perovskite. This methodology has the potential to forge the way toward a new method for the fabrication of large-area perovskite solar cells.
Perovskite solar cells (PSCs) technology is now reaching its full potential in terms of power conversion efficiency, but still presenting problems related to long-term stability under operating conditions. One of the most promising... more
Perovskite solar cells (PSCs) technology is now reaching its full potential in terms of power conversion efficiency, but still presenting problems related to long-term stability under operating conditions. One of the most promising alternatives to PSCs is the layered PSCs (2D-PSCs). Layered perovskites present a huge compositional variety, which can be used to directly tune photophysical characteristics that influence the operational mechanisms of the devices. This review addresses the structural organization of both the organic and inorganic sublattices, focusing on how the structure influences the quantum and dielectric confinement, phonons and charge carriers' dynamics, charge mobility, and structural defects. We discuss the relation between the structure-properties of layered perovskites with the performance of solar cells.
We, then, offer insights into how these characteristics have been controlled in the assembly of 2D-PSCs to improve their efficiency and stability. We conclude by giving a perspective of future developments and open areas of exploration that might impact the progress of this rapidly growing technology.
We, then, offer insights into how these characteristics have been controlled in the assembly of 2D-PSCs to improve their efficiency and stability. We conclude by giving a perspective of future developments and open areas of exploration that might impact the progress of this rapidly growing technology.
The high contamination by the SARS-Cov-2 virus has led to the search for ways to minimize contagion. Masks are used as part of a strategy of measures to suppress transmission and save lives. However, they are not sufficient to provide an... more
The high contamination by the SARS-Cov-2 virus has led to the search for ways to minimize contagion.
Masks are used as part of a strategy of measures to suppress transmission and save lives.
However, they are not sufficient to provide an adequate level of protection against COVID-19. Activated
charcoal has an efficient antibacterial action, adsorption and low cost. Here, the interaction
between two molecules of activated carbon was analyzed, interacting with two structures of the
SARS-Cov-2, through docking and molecular dynamics using the platforms Autodock Vina 4.2.6,
Gaussian 09 and Amber 16. As a result, the complexes from ozone-functionalized coal to viral
structures happen mainly through hydrophobic interactions at the binding site of each receptor. The
values of the mean square deviations of the two systems formed by ligands/receptors and showed
better stability. The results of Gibbs free energy showed a better interaction between proteins and
functionalized charcoal, with Gtotal values of −48.530 and −38.882 kcal/mol. Thus, the set formed
by combinations of proteins with functionalized activated carbon tends to more efficiently adsorb
the protein components of the coronavirus to the pores of the activated carbon with ozone during
filtration.
Masks are used as part of a strategy of measures to suppress transmission and save lives.
However, they are not sufficient to provide an adequate level of protection against COVID-19. Activated
charcoal has an efficient antibacterial action, adsorption and low cost. Here, the interaction
between two molecules of activated carbon was analyzed, interacting with two structures of the
SARS-Cov-2, through docking and molecular dynamics using the platforms Autodock Vina 4.2.6,
Gaussian 09 and Amber 16. As a result, the complexes from ozone-functionalized coal to viral
structures happen mainly through hydrophobic interactions at the binding site of each receptor. The
values of the mean square deviations of the two systems formed by ligands/receptors and showed
better stability. The results of Gibbs free energy showed a better interaction between proteins and
functionalized charcoal, with Gtotal values of −48.530 and −38.882 kcal/mol. Thus, the set formed
by combinations of proteins with functionalized activated carbon tends to more efficiently adsorb
the protein components of the coronavirus to the pores of the activated carbon with ozone during
filtration.
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ABSTRACTBismuth triiodide (BiI3) is a potential candidate for application in solar cell due to its good optoelectronic properties and because it is free of toxic elements. It can be used as the absorber material in solar cells or... more
ABSTRACTBismuth triiodide (BiI3) is a potential candidate for application in solar cell due to its good optoelectronic properties and because it is free of toxic elements. It can be used as the absorber material in solar cells or converted into the perovskite-like material MA3Bi2I9, suitable also for photovoltaic applications. Bismuth triiodide has been prepared by physical vapour transport (PVT) and by solution process through spin coating. In this work we present optical and structural/topological properties of BiI3 deposited by thermal evaporation under high vacuum. The films are slightly tensile, polycrystalline, homogenously distributed and with good adherence on several substrates, with an indirect bandgap of 1.81 eV, index of refraction of 3.3 (630 nm), photoluminescence centered at 1.74 eV and a Raman peak at 118cm-1 associated with the Ag mode.
Research Interests: Perovskites and MRS
ABSTRACTBismuth triiodide (BiI3) has been studied aiming the development of lead-free photovoltaic materials. It can also be used as X-ray detectors due to the high density of its elements (bismuth and iodine). We investigate the... more
ABSTRACTBismuth triiodide (BiI3) has been studied aiming the development of lead-free photovoltaic materials. It can also be used as X-ray detectors due to the high density of its elements (bismuth and iodine). We investigate the mechanical stress, hardness, and elastic properties of BiI3 thin films deposited by thermal evaporation. The stress was determined by the bending beam technique using the Stoney equation. The films are tensile with stress of approximately 27 MPa. The hardness and the elastic modulus were determined by nanoindentation technique using a Berkovich diamond tip. The hardness of the films is approximately 0.8 GPa and the reduced Young´s modulus is ∼28 GPa for maximum penetration depth of 10% of the film thickness.
Research Interests: Perovskites and MRS
The electronic structures and optical properties of triphenylamine-functionalized graphene (G-TPA) doped with transition metals, using water as a solvent, were theoretically investigated to verify the efficiency of photocatalytic hydrogen... more
The electronic structures and optical properties of triphenylamine-functionalized graphene (G-TPA) doped with transition metals, using water as a solvent, were theoretically investigated to verify the efficiency of photocatalytic hydrogen production with the use of transition metals. This study was performed by Density Functional Theory and Time-dependent Density Functional Theory through Gaussian 09W software, adopting the B3LYP functional for all structures. The 6-31g(d) basis set was used for H, C and N atoms, and the LANL2DZ basis set for transition metals using the Effective Core Potentials method. Two approaches were adopted: (1) using single metallic dopants (Ni, Pd, Fe, Os and Pt) and (2) using combinations of Ni with the other dopants (NiPd, NiPt, NiFe and NiOs). The DOS spectra reveal an increase of accessible states in the valence shell, in addition to a gap decrease for all dopants. This doping also increases the absorption in the visible region of solar radiation where ...
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In the last few years, research on dye-sensitised devices has been focused on the development of solar cells, based on CHNHPbX (X = I, Br, Cl) composites with perovskite structure. The deposition of perovskite thin films is usually... more
In the last few years, research on dye-sensitised devices has been focused on the development of solar cells, based on CHNHPbX (X = I, Br, Cl) composites with perovskite structure. The deposition of perovskite thin films is usually carried out by solution-based processes using spin-coating techniques that result in the production of high quality films. Solar cells made by this method exceed 20% efficiency, with the potential for use in large scale production through ink print or screen printing techniques. As an alternative route, perovskite thin films can be deposited through thermal evaporation. A new method is proposed to produce CHNHPbI, based on a radio-frequency (rf) -sputtering technique that results in a high reproducibility of the films and is compatible with roll-to-roll processes. We deposited thin films of lead-sulphide (PbS) and converted them into perovskite by placing the films in an iodine atmosphere, followed by dipping in a solution of methylammonium iodide (CHNHI)...
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Electron spin resonance of graphite-like a-C thin films is investigated in the 20 K up to 340 K temperature range. The films with sp2 concentration of about 90 % (determined by electron energy loss spectroscopy), with no measurable... more
Electron spin resonance of graphite-like a-C thin films is investigated in the 20 K up to 340 K temperature range. The films with sp2 concentration of about 90 % (determined by electron energy loss spectroscopy), with no measurable optical band gap, were prepared by ion beam assisted sputtering. The results revealed an unexpected low density of paramagnetic centers, ascribed to itinerant states (conduction electrons) and not to localized states usually reported for a-C with band gap higher than 1.0 eV.
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Density functional theory was performed for thermodynamic predictions on natural gas, whose B3LYP/6-311++G(d,p), B3LYP/6-31+G(d), CBS-QB3, G3, and G4 methods were applied. Additionally, we carried out thermodynamic predictions using G3/G4... more
Density functional theory was performed for thermodynamic predictions on natural gas, whose B3LYP/6-311++G(d,p), B3LYP/6-31+G(d), CBS-QB3, G3, and G4 methods were applied. Additionally, we carried out thermodynamic predictions using G3/G4 averaged. The calculations were performed for each major component of seven kinds of natural gas and to their respective air + natural gas mixtures at a thermal equilibrium between room temperature and the initial temperature of a combustion chamber during the injection stage. The following thermodynamic properties were obtained: internal energy, enthalpy, Gibbs free energy and entropy, which enabled us to investigate the thermal resistance of fuels. Also, we estimated an important parameter, namely, the specific heat ratio of each natural gas; this allowed us to compare the results with the empirical functions of these parameters, where the B3LYP/6-311++G(d,p) and G3/G4 methods showed better agreements. In addition, relevant information on the the...
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Aqueous colloidal silver nanoparticles have substantial potential in biological application as markers and antibacterial agents and in surface-enhanced Raman spectroscopy applications. A simple method of fabrication and encapsulation into... more
Aqueous colloidal silver nanoparticles have substantial potential in biological application as markers and antibacterial agents and in surface-enhanced Raman spectroscopy applications. A simple method of fabrication and encapsulation into an inert shell is of great importance today to make their use ubiquitous. Here we show that colloids of silver-core/silica-shell nanoparticles can be easily fabricated by a laser-ablation-assisted chemical reduction method and their sizes can be tuned in the range of 2.5 to 6.3 nm by simply choosing a proper water-ethanol proportion. The produced silver nanoparticles possess a porous amorphous silica shell that increases the inertness and stability of colloids, which decreases their toxicity compared with those without silica. The presence of a thin 2 to 3 nm silica shell was proved by EDX mapping. The small sizes of nanoparticles achieved by this method were analyzed using optical techniques, and they show typical photoluminescence in the UV-vis r...
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The influence of the deposition condition on the band-gap of amorphous germanium nitrogen alloys is presented. The films were prepared by rf-reactive sputtering a crystalline germanium target in an argon plus ammonia atmosphere. The... more
The influence of the deposition condition on the band-gap of amorphous germanium nitrogen alloys is presented. The films were prepared by rf-reactive sputtering a crystalline germanium target in an argon plus ammonia atmosphere. The optical gap of the material could be tailored from approximately 1.0 eV to about 3.0 eV by varying the rf power, substrate temperature, nitrogen and hydrogen partial pressure. The rf power, and not the gas composition, was found to be the most important parameter in determining the nitrogen concentration. The hydrogen concentration, on the other hand, has a small influence on the band gap, while the temperature plays an important role on the determination of the alloy structure.
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High-quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively... more
High-quality amorphous hydrogenated germanium has been deposited using the diode rf glow discharge method out of a gas plasma of GeH4 and H2. The optical, electrical, and structural properties of this material have been extensively characterized. The optical and electrical properties are all consistent with material containing a low density of defect related states in the energy gap. In particular,
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ABSTRACTStructural and mechanical properties of hydrogenated amorphous germanium carbon (a-Ge1-xCx:H) alloys are presented. The films were prepared by the rf-co-sputtering technique using a graphite/germanium composed target. The carbon... more
ABSTRACTStructural and mechanical properties of hydrogenated amorphous germanium carbon (a-Ge1-xCx:H) alloys are presented. The films were prepared by the rf-co-sputtering technique using a graphite/germanium composed target. The carbon and germanium relative concentrations were determined by RBS, and the total hydrogen concentration by ERDA measurements. An increase in the optical gap was measured for low carbon content (0 < × < 0.15). For higher values of x the optical gap is almost constant. Infrared transmission absorption spectra show several absorption bands related to Ge-C stretching, C-Hn (n = 1,2,3) and Ge-H stretching and bending modes. The mechanical internal stress was strongly affected by the incorporation of carbon. The trends of the optical gap, refractive index, infrared absorption and mechanical stress as a function of the carbon content suggest that the high carbon concentration alloys have polymeric and/or graphite-like contribution in their structure.