Rogerio Paniago - Academia.edu (original) (raw)

Papers by Rogerio Paniago

ACS Applied Nano Materials, 2019

Nanoscale heterostructure engineering is the main target for the development of optoelectronic de... more Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is required to envisage technological applications. A number of local probe techniques that address single nanostructure signals were satisfactorily employed in semiconductor epitaxial systems. In this work we show that the use of chemically etched semiconductor nanomembranes allows carrying out scanning tunneling spectroscopy (STS) measurements in a post-processed system which was otherwise studied mainly under in-situ conditions that differ from operational regime. We were able to acquire STS spectra with energy level resolved response on InAs quantum dots grown within a 15 nm-thick GaAs singlecrystalline film transferred to a Au (111) surface. The presence of a native oxide layer does not affect the result, keeping the reliability of the usual UHV procedures. The use of nanomembranes also opens up the possibility of tailoring properties via additional variables such as nanomembrane thickness and surface charge depletion. Our method is applicable to a broad class of post-processed layers extracted in nanomembrane format from epitaxial systems that are potential candidates for optoelectronic applications.

The journal of physical chemistry letters, Jan 3, 2018

The goal of this work is to study transformations that occur upon heating Bi2Se3 to temperatures ... more The goal of this work is to study transformations that occur upon heating Bi2Se3 to temperatures up to 623 K. X-ray diffraction (XRD) and scanning tunneling microscopy (STM) and spectroscopy (STS) techniques were used in our investigation. XRD was measured following the 00L and 01L truncation rods. These measurements revealed that upon heating there is a coexistence of a major Bi2Se3 phase and other ones that present structures of quintuple-layers intercalated with Bismuth bilayers. STM measurements of the surface of this material showed the presence of large hexagonal BixSey domains embedded in a Bi2Se3 matrix. STS experiments were employed to map the local electronic density of states and characterize the modifications imposed by the presence of the additional phases. Finally, Density Functional Theory (DFT) calculations were performed to support these findings.

Nano letters, Jan 29, 2016

In this work we present unique signatures manifested by the local electronic properties of the to... more In this work we present unique signatures manifested by the local electronic properties of the topological surface state in Bi2Te3 nanostructures as the spatial limit is approached. We concentrate on the pure nanoscale limit (nanoplatelets) with spatial electronic resolution down to 1 nm. The highlights include strong dependencies on nanoplatelet size: (1) observation of a phase separation of Dirac electrons whose length scale decreases as the spatial limit is approached, and (2) the evolution from heavily n-type to lightly n-type surface doping as nanoplatelet thickness increases. Our results show a new approach to tune the Dirac point together with reduction of electronic disorder in topological insulator (TI) nanostructured systems. We expect our work will provide a new route for application of these nanostructured Dirac systems in electronic devices.

Physical Review B, 2002

The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic... more The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic and thermodynamic factors that determine their morphology and size. Some of the significant factors to their stability are strain and interdiffusion. Here we directly measure the gradient of composition and strain in Ge nanocrystals grown on Si͑001͒ using anomalous x-ray scattering. By combining our x-ray results, where we relate strain, interdiffusion, and shape with atomic force microscopy measurements, we have been able to determine the complete strain configuration of these islands. We show that the amount of elastic energy in pyramids and domes can be evaluated. The transition from pyramids to domes is accompanied by an increase of lattice parameter and enhancement of interdiffusion, both leading to a drastic decrease of the elastic energy stored per atom.

Physical Review B, 2010

Interdiffusion in self-assembled Ge:Si͑001͒ islands has been explained by models based on either ... more Interdiffusion in self-assembled Ge:Si͑001͒ islands has been explained by models based on either thermodynamic and/or surface kinetic considerations. In order to analyze the relevance of bulk and surface diffusion on the final composition state, we performed a set of controlled x-ray diffraction experiments to study both composition and atomic ordering in Ge/Si͑001͒ islands grown by different methods. Surface diffusion strongly enhances the overall interdiffusion during island growth by solid source molecular beam epitaxy while chemical-vapor-deposited islands are closer to thermodynamic model systems. The growth conditions play a crucial role on the appearance of atomic ordering. In particular, a remarkable correlation between atomic ordering and surface diffusion kinetics is found.

Physical Review B, 2010

The epitaxial growth by reactive deposition of-FeSi and ␤-FeSi 2 nanodots on Si͑111͒ is studied a... more The epitaxial growth by reactive deposition of-FeSi and ␤-FeSi 2 nanodots on Si͑111͒ is studied as a function of Fe coverage. The nanodots density, size, and strain were analyzed by atomic force microscopy and x-ray diffraction. Almost single phase-FeSi and ␤-FeSi 2 were formed at low and high iron coverage, respectively. A-FeSi to ␤-FeSi 2 change in phase formation is observed at Fe coverage of 5.5 nm, which is coincident with the coalescence of the nanodots, the relaxation of the strain in both phases and a discontinuous increase of the grain size of the ␤-FeSi 2 phase. A direct comparison of the diffraction and microscopy data shows that nanodots of different phases also exhibit different shapes, being the-FeSi ͑␤-FeSi 2 ͒ nanodots smaller ͑larger͒ and with a low ͑high͒ aspect ratio.

New Journal of Physics, 2010

Epitaxial growth of germanium on boron-terminated Si͑111͒ results in the formation of triangular ... more Epitaxial growth of germanium on boron-terminated Si͑111͒ results in the formation of triangular pyramidal Ge islands which are partially relaxed. We show that the termination of the Si͑111͒ surface with 1/3 ML of boron is essential for the formation of faceted islands. We have investigated the residual strain in the Ge islands using x-ray crystal truncation rod scattering, and developed an analytical expression for the scattered intensity from islands with a nonuniform lattice parameter. We compare the measured intensity to x-ray scattering profiles calculated on the basis of different strain models. It is found that the Ge lateral lattice parameter changes linearly from the bottom to the top of the islands.

Physical Review Letters, 2003

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 C were obta... more Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

Physical Review Letters, 1998

We have observed two length scales in a V 2 H crystal above the b 1-b 2 phase transition. A sharp... more We have observed two length scales in a V 2 H crystal above the b 1-b 2 phase transition. A sharp Lorentzian profile in the critical diffuse scattering was superimposed on a broader one for a low-energy x-ray experiment in reflection, while no sharp component appeared in transmission at high energy. For the sharp component, we extracted tricritical exponents n 1 and g 1 at small reduced temperatures t, where fluctuations are coherent over large distances. For larger t, where the correlation length is comparable to an effective defect size, there is a crossover to altered exponents. [S0031-9007(98)07111-7]

Physical Review E, 2013

Melting and premelting phenomena in self-organized organic systems have been extensively explored... more Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.

Journal of Nanoscience and Nanotechnology, 2007

The production and physical properties of nanowires and nanoribbons formed by methylphosphonic ac... more The production and physical properties of nanowires and nanoribbons formed by methylphosphonic acid (MPA)-CH 3 PO(OH) 2-were investigated. These structures are formed on an aluminum coated substrate when immersed in an ethanolic solution of MPA for several days. A careful investigation of the growth conditions resulted in a narrow window of solution concentrations and temperatures for the successful development of nanowires and nanoribbons. Several different techniques were employed to characterize these nanostructures: (1) Photoluminescence experiments showed a strong emission at 2.3 eV (green), which is visible to the naked eye; (2) X-ray diffraction experiments indicated a significant cristalinity, in agreement with atomic force microscopy (AFM) and transmission electron microscopy (TEM) morphology images, which show organized nano-scale wires and ribbons, (furthermore, AFM-Phase and TEM images also suggest that nanoribbons are formed by well-aligned nanowires); (3) Conductive-AFM experiments revealed an intermediary conductivity for these structures (10 −1 /Ohm • m), which is similar to some intrinsic semiconductors and; (4) finally, Infrared, Raman, and X-Ray Photoelectron Spectroscopies produced information about the contents, structure, and composition of both wires and ribbons.

Journal of Magnetism and Magnetic Materials, 2007

Journal of Applied Physics, 2006

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on... more We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft x-ray scattering and magnetic force microscopy. The results of the diffuse satellite x-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane monodomain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9...

Journal of Applied Physics, 2007

We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflecti... more We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction, and photoluminescence techniques. The InP dots present an efficient optical emission even when they are uncapped, which is attributed to the low surface recombination velocity in InP. We compare the difference in the optical properties between surface free dots, which are not covered by any material, with dots covered by a GaAs capping layer. We observed a bimodal dispersion of the dot size distribution, giving rise to two distinct emission bands. The results also revealed that the strain accumulated in the InP islands is slightly relieved for samples with large InP amounts. An unexpected result is the relatively large blue shift of the emission band from uncapped samples as compared to capped dots.

The European Physical Journal Special Topics, 2012

In this work we depict schematically the use of resonant (anomalous) X-ray diffraction as a tool ... more In this work we depict schematically the use of resonant (anomalous) X-ray diffraction as a tool to directly probe strain and composition of self-assembled semiconductor islands. By employing a direct analysis at the Eu L 3 edge its composition gradient is quantified for EuTe:SnTe capped islands. Projection maps are proposed to visualize the results, providing an alternative capability to infer quantum dot properties. A more complex methodology is applied to the study of InP:GaAs islands, in which complementary anomalous measurements are performed. For this system the number of samples analyzed allows us to extract the activation energy for Ga adatoms diffusion from the substrate to the islands. This work was supported by ABTLuS (Brazilian Synchrotron Association), FAPESP, and CNPq (Brazilian fuding agencies).

Chemical Physics Letters, 2011

The present study investigated the effect of the 6-monodeoxy-6-monoamine-b-cyclodextrin(Am-b-CD)/... more The present study investigated the effect of the 6-monodeoxy-6-monoamine-b-cyclodextrin(Am-b-CD)/ DNA (Am-b-CD/DNA) complex, as well as of culture medium components and proteins, at pH 7.4 and 5.0, on membranes of anionic and pH-sensitive liposomes comprised of DOPE-CHEMS, using energy dispersive X-ray diffraction (EDXD). At pH 7.4, the Am-b-CD/DNA complex induced the appearance of lamellar and hexagonal phases of DOPE. However, at pH 5.0, only non-lamellar phases could be observed. The presence of biological components led to a disruption of lipid order, but the pH-sensitivity of liposomes was maintained.

Brazilian Journal of Physics, 2004

In this work several structural and chemical properties of self-assembled InAs islands grown on G... more In this work several structural and chemical properties of self-assembled InAs islands grown on GaAs(001) are studied using surface x-ray scattering with synchrotron radiation. The technique of x-ray diffraction under grazing incidence condition was employed to differentiate coherent and incoherent islands. We used a model of a strained pyramidal island to interpret the x-ray results and correlate size and strain-state of these islands. The degree of GaAs interdiffusion in the islands was inferred from the variation of volume of the unit cell. The Poisson's ratio of the two materials involved establishes a limit of tetragonal distortion for this material. Any variation in this distortion is associated with the presence of Ga inside the islands.

Applied Physics Letters, 2014

We explore the donor/acceptor interaction between distinct polymers at sequential bilayer interfa... more We explore the donor/acceptor interaction between distinct polymers at sequential bilayer interfaces to understand if their emission is influenced by the solvent. Resonant soft X-ray reflectivity and near-edge X-ray absorption fine structure measurements were used to investigate optical and morphological properties of polymer stacks, which were made using polymer chain structures diluted with different or equal solvents. We identified coupled and uncoupled bilayer systems using as a probe the exciton energy transfer effect inferred from the donor/acceptor interdiffusion region at the heterojunction interfaces. We also show that the overall emission is dominated by the affinity of the chemical structures at the interface, regardless of the solvent used for the deposition of each layer.

Applied Physics Letters, 2007

This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitax... more This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitaxial and demonstrates the crystallographic relation between the two using grazing incidence synchrotron x-ray diffraction (XRD). The in-plane lattice match between the sapphire and the nanowires was directly probed by using XRD at grazing angles of incidence, where the lattice match between the (0001) plane of the sapphire and the (11−20) plane of the ZnO were observed simultaneously. It will also be shown that gold acts as a catalyst to initiate ZnO nanowire growth, but it does not interfere with the epitaxial mechanism between the nanowires and the sapphire substrate.

ACS Applied Nano Materials, 2019

Nanoscale heterostructure engineering is the main target for the development of optoelectronic de... more Nanoscale heterostructure engineering is the main target for the development of optoelectronic devices. In this sense, a precise knowledge of local electronic response after materials processing is required to envisage technological applications. A number of local probe techniques that address single nanostructure signals were satisfactorily employed in semiconductor epitaxial systems. In this work we show that the use of chemically etched semiconductor nanomembranes allows carrying out scanning tunneling spectroscopy (STS) measurements in a post-processed system which was otherwise studied mainly under in-situ conditions that differ from operational regime. We were able to acquire STS spectra with energy level resolved response on InAs quantum dots grown within a 15 nm-thick GaAs singlecrystalline film transferred to a Au (111) surface. The presence of a native oxide layer does not affect the result, keeping the reliability of the usual UHV procedures. The use of nanomembranes also opens up the possibility of tailoring properties via additional variables such as nanomembrane thickness and surface charge depletion. Our method is applicable to a broad class of post-processed layers extracted in nanomembrane format from epitaxial systems that are potential candidates for optoelectronic applications.

The journal of physical chemistry letters, Jan 3, 2018

The goal of this work is to study transformations that occur upon heating Bi2Se3 to temperatures ... more The goal of this work is to study transformations that occur upon heating Bi2Se3 to temperatures up to 623 K. X-ray diffraction (XRD) and scanning tunneling microscopy (STM) and spectroscopy (STS) techniques were used in our investigation. XRD was measured following the 00L and 01L truncation rods. These measurements revealed that upon heating there is a coexistence of a major Bi2Se3 phase and other ones that present structures of quintuple-layers intercalated with Bismuth bilayers. STM measurements of the surface of this material showed the presence of large hexagonal BixSey domains embedded in a Bi2Se3 matrix. STS experiments were employed to map the local electronic density of states and characterize the modifications imposed by the presence of the additional phases. Finally, Density Functional Theory (DFT) calculations were performed to support these findings.

Nano letters, Jan 29, 2016

In this work we present unique signatures manifested by the local electronic properties of the to... more In this work we present unique signatures manifested by the local electronic properties of the topological surface state in Bi2Te3 nanostructures as the spatial limit is approached. We concentrate on the pure nanoscale limit (nanoplatelets) with spatial electronic resolution down to 1 nm. The highlights include strong dependencies on nanoplatelet size: (1) observation of a phase separation of Dirac electrons whose length scale decreases as the spatial limit is approached, and (2) the evolution from heavily n-type to lightly n-type surface doping as nanoplatelet thickness increases. Our results show a new approach to tune the Dirac point together with reduction of electronic disorder in topological insulator (TI) nanostructured systems. We expect our work will provide a new route for application of these nanostructured Dirac systems in electronic devices.

Physical Review B, 2002

The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic... more The growth of strained epitaxial self assembled nanocrystals is comprised of a variety of kinetic and thermodynamic factors that determine their morphology and size. Some of the significant factors to their stability are strain and interdiffusion. Here we directly measure the gradient of composition and strain in Ge nanocrystals grown on Si͑001͒ using anomalous x-ray scattering. By combining our x-ray results, where we relate strain, interdiffusion, and shape with atomic force microscopy measurements, we have been able to determine the complete strain configuration of these islands. We show that the amount of elastic energy in pyramids and domes can be evaluated. The transition from pyramids to domes is accompanied by an increase of lattice parameter and enhancement of interdiffusion, both leading to a drastic decrease of the elastic energy stored per atom.

Physical Review B, 2010

Interdiffusion in self-assembled Ge:Si͑001͒ islands has been explained by models based on either ... more Interdiffusion in self-assembled Ge:Si͑001͒ islands has been explained by models based on either thermodynamic and/or surface kinetic considerations. In order to analyze the relevance of bulk and surface diffusion on the final composition state, we performed a set of controlled x-ray diffraction experiments to study both composition and atomic ordering in Ge/Si͑001͒ islands grown by different methods. Surface diffusion strongly enhances the overall interdiffusion during island growth by solid source molecular beam epitaxy while chemical-vapor-deposited islands are closer to thermodynamic model systems. The growth conditions play a crucial role on the appearance of atomic ordering. In particular, a remarkable correlation between atomic ordering and surface diffusion kinetics is found.

Physical Review B, 2010

The epitaxial growth by reactive deposition of-FeSi and ␤-FeSi 2 nanodots on Si͑111͒ is studied a... more The epitaxial growth by reactive deposition of-FeSi and ␤-FeSi 2 nanodots on Si͑111͒ is studied as a function of Fe coverage. The nanodots density, size, and strain were analyzed by atomic force microscopy and x-ray diffraction. Almost single phase-FeSi and ␤-FeSi 2 were formed at low and high iron coverage, respectively. A-FeSi to ␤-FeSi 2 change in phase formation is observed at Fe coverage of 5.5 nm, which is coincident with the coalescence of the nanodots, the relaxation of the strain in both phases and a discontinuous increase of the grain size of the ␤-FeSi 2 phase. A direct comparison of the diffraction and microscopy data shows that nanodots of different phases also exhibit different shapes, being the-FeSi ͑␤-FeSi 2 ͒ nanodots smaller ͑larger͒ and with a low ͑high͒ aspect ratio.

New Journal of Physics, 2010

Epitaxial growth of germanium on boron-terminated Si͑111͒ results in the formation of triangular ... more Epitaxial growth of germanium on boron-terminated Si͑111͒ results in the formation of triangular pyramidal Ge islands which are partially relaxed. We show that the termination of the Si͑111͒ surface with 1/3 ML of boron is essential for the formation of faceted islands. We have investigated the residual strain in the Ge islands using x-ray crystal truncation rod scattering, and developed an analytical expression for the scattered intensity from islands with a nonuniform lattice parameter. We compare the measured intensity to x-ray scattering profiles calculated on the basis of different strain models. It is found that the Ge lateral lattice parameter changes linearly from the bottom to the top of the islands.

Physical Review Letters, 2003

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 C were obta... more Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

Physical Review Letters, 1998

We have observed two length scales in a V 2 H crystal above the b 1-b 2 phase transition. A sharp... more We have observed two length scales in a V 2 H crystal above the b 1-b 2 phase transition. A sharp Lorentzian profile in the critical diffuse scattering was superimposed on a broader one for a low-energy x-ray experiment in reflection, while no sharp component appeared in transmission at high energy. For the sharp component, we extracted tricritical exponents n 1 and g 1 at small reduced temperatures t, where fluctuations are coherent over large distances. For larger t, where the correlation length is comparable to an effective defect size, there is a crossover to altered exponents. [S0031-9007(98)07111-7]

Physical Review E, 2013

Melting and premelting phenomena in self-organized organic systems have been extensively explored... more Melting and premelting phenomena in self-organized organic systems have been extensively explored in the literature, exploring distinct behaviors of different molecule lengths and morphologies. Nevertheless, the influence of the supramolecular assembly configuration on the occurrence of premelting remains poorly explored. Here we use phosphonic acids as model systems for self-organized molecular assemblies. These molecules exhibit long-range order on different types of substrates. The balance between chain-to-chain and head-to-head interactions leads to distinct types of stackings. Although their structural configurations are well understood, very little is known about their behavior near the melting transition. We show here that premelting occurs in lamellar structures and that its behavior depends directly on the ordered configuration assumed in the studied multilayers. Two molecules with different chain lengths were investigated: octadecyl phosphonic and octyl phosphonic acids. Although almost no dependence on the molecule length was observed, the occurrence of premelting is strongly influenced by their lamellar packing configuration. For tilted packings premelting is unfavored while in straight configurations, where alkyl chain interactions are weakened with respect to head-to-head interactions, strong premelting is observed. We find that the onset of premelting occurs at the domain boundaries with straight lamellar configurations and the domain sizes exhibit power law temperature dependences.

Journal of Nanoscience and Nanotechnology, 2007

The production and physical properties of nanowires and nanoribbons formed by methylphosphonic ac... more The production and physical properties of nanowires and nanoribbons formed by methylphosphonic acid (MPA)-CH 3 PO(OH) 2-were investigated. These structures are formed on an aluminum coated substrate when immersed in an ethanolic solution of MPA for several days. A careful investigation of the growth conditions resulted in a narrow window of solution concentrations and temperatures for the successful development of nanowires and nanoribbons. Several different techniques were employed to characterize these nanostructures: (1) Photoluminescence experiments showed a strong emission at 2.3 eV (green), which is visible to the naked eye; (2) X-ray diffraction experiments indicated a significant cristalinity, in agreement with atomic force microscopy (AFM) and transmission electron microscopy (TEM) morphology images, which show organized nano-scale wires and ribbons, (furthermore, AFM-Phase and TEM images also suggest that nanoribbons are formed by well-aligned nanowires); (3) Conductive-AFM experiments revealed an intermediary conductivity for these structures (10 −1 /Ohm • m), which is similar to some intrinsic semiconductors and; (4) finally, Infrared, Raman, and X-Ray Photoelectron Spectroscopies produced information about the contents, structure, and composition of both wires and ribbons.

Journal of Magnetism and Magnetic Materials, 2007

Journal of Applied Physics, 2006

We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on... more We investigated the thermal evolution of the magnetic properties of MnAs epitaxial films grown on GaAs(001) during the coexistence of hexagonal/orthorhombic phases using polarized resonant (magnetic) soft x-ray scattering and magnetic force microscopy. The results of the diffuse satellite x-ray peaks were compared to those obtained by magnetic force microscopy and suggest a reorientation of ferromagnetic terraces as temperature rises. By measuring hysteresis loops at these peaks we show that this reorientation is common to all ferromagnetic terraces. The reorientation is explained by a simple model based on the shape anisotropy energy. Demagnetizing factors were calculated for different configurations suggested by the magnetic images. We noted that the magnetic moments flip from an in-plane monodomain orientation at lower temperatures to a three-domain out-of-plane configuration at higher temperatures. The transition was observed when the ferromagnetic stripe width L is equal to 2.9...

Journal of Applied Physics, 2007

We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflecti... more We investigated structural and optical properties of type-II InP/GaAs quantum dots using reflection high energy electron diffraction, transmission electron microscopy, atomic force microscopy, grazing incidence x-ray diffraction, and photoluminescence techniques. The InP dots present an efficient optical emission even when they are uncapped, which is attributed to the low surface recombination velocity in InP. We compare the difference in the optical properties between surface free dots, which are not covered by any material, with dots covered by a GaAs capping layer. We observed a bimodal dispersion of the dot size distribution, giving rise to two distinct emission bands. The results also revealed that the strain accumulated in the InP islands is slightly relieved for samples with large InP amounts. An unexpected result is the relatively large blue shift of the emission band from uncapped samples as compared to capped dots.

The European Physical Journal Special Topics, 2012

In this work we depict schematically the use of resonant (anomalous) X-ray diffraction as a tool ... more In this work we depict schematically the use of resonant (anomalous) X-ray diffraction as a tool to directly probe strain and composition of self-assembled semiconductor islands. By employing a direct analysis at the Eu L 3 edge its composition gradient is quantified for EuTe:SnTe capped islands. Projection maps are proposed to visualize the results, providing an alternative capability to infer quantum dot properties. A more complex methodology is applied to the study of InP:GaAs islands, in which complementary anomalous measurements are performed. For this system the number of samples analyzed allows us to extract the activation energy for Ga adatoms diffusion from the substrate to the islands. This work was supported by ABTLuS (Brazilian Synchrotron Association), FAPESP, and CNPq (Brazilian fuding agencies).

Chemical Physics Letters, 2011

The present study investigated the effect of the 6-monodeoxy-6-monoamine-b-cyclodextrin(Am-b-CD)/... more The present study investigated the effect of the 6-monodeoxy-6-monoamine-b-cyclodextrin(Am-b-CD)/ DNA (Am-b-CD/DNA) complex, as well as of culture medium components and proteins, at pH 7.4 and 5.0, on membranes of anionic and pH-sensitive liposomes comprised of DOPE-CHEMS, using energy dispersive X-ray diffraction (EDXD). At pH 7.4, the Am-b-CD/DNA complex induced the appearance of lamellar and hexagonal phases of DOPE. However, at pH 5.0, only non-lamellar phases could be observed. The presence of biological components led to a disruption of lipid order, but the pH-sensitivity of liposomes was maintained.

Brazilian Journal of Physics, 2004

In this work several structural and chemical properties of self-assembled InAs islands grown on G... more In this work several structural and chemical properties of self-assembled InAs islands grown on GaAs(001) are studied using surface x-ray scattering with synchrotron radiation. The technique of x-ray diffraction under grazing incidence condition was employed to differentiate coherent and incoherent islands. We used a model of a strained pyramidal island to interpret the x-ray results and correlate size and strain-state of these islands. The degree of GaAs interdiffusion in the islands was inferred from the variation of volume of the unit cell. The Poisson's ratio of the two materials involved establishes a limit of tetragonal distortion for this material. Any variation in this distortion is associated with the presence of Ga inside the islands.

Applied Physics Letters, 2014

We explore the donor/acceptor interaction between distinct polymers at sequential bilayer interfa... more We explore the donor/acceptor interaction between distinct polymers at sequential bilayer interfaces to understand if their emission is influenced by the solvent. Resonant soft X-ray reflectivity and near-edge X-ray absorption fine structure measurements were used to investigate optical and morphological properties of polymer stacks, which were made using polymer chain structures diluted with different or equal solvents. We identified coupled and uncoupled bilayer systems using as a probe the exciton energy transfer effect inferred from the donor/acceptor interdiffusion region at the heterojunction interfaces. We also show that the overall emission is dominated by the affinity of the chemical structures at the interface, regardless of the solvent used for the deposition of each layer.

Applied Physics Letters, 2007

This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitax... more This letter shows that aligned zinc oxide (ZnO) nanowires growth on sapphire substrates is epitaxial and demonstrates the crystallographic relation between the two using grazing incidence synchrotron x-ray diffraction (XRD). The in-plane lattice match between the sapphire and the nanowires was directly probed by using XRD at grazing angles of incidence, where the lattice match between the (0001) plane of the sapphire and the (11−20) plane of the ZnO were observed simultaneously. It will also be shown that gold acts as a catalyst to initiate ZnO nanowire growth, but it does not interfere with the epitaxial mechanism between the nanowires and the sapphire substrate.