Francesca Peiro - Profile on Academia.edu (original) (raw)
Papers by Francesca Peiro
![Research paper thumbnail of {"__content__"=>"Surface Chemistry and Nano-/Microstructure Engineering on Photocatalytic InS Nanocrystals.", "sub"=>[{"__content__"=>"2"}, {"__content__"=>"3"}]}](https://attachments.academia-assets.com/70299403/thumbnails/1.jpg)
](Langmuir : the ACS journal of surfaces and colloids, Jan 5, 2018
Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysi... more Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties, or a magnetic moment. Using high-throughput printing technologies, colloidal NCs can also be supported onto substrates to produce cost-effective electronic, optoelectronic, electrocatalytic, and sensing devices. For both catalytic and technological application, NC surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of InS, both as a colloid and as a supported layer. As a colloid, NCs stabilized by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization beco...
Langmuir
It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanopartic... more It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanoparticles (OA−IONPs) with the cetyltrimethylammonium (CTA +) surfactant induces the formation of spherical iron oxide nanoparticle clusters (IONPCs). However, the behavior and functional properties of IONPCs in chemical reactions have been largely neglected and are still not well-understood. Herein, we report an unconventional ligand-exchange function of IONPCs activated when dispersed in an ethyl acetate/ acetate buffer system. The ligand exchange can successfully transform hydrophobic OA−IONP building blocks of IONPCs into highly hydrophilic, acetate-capped iron oxide nanoparticles (Ac−IONPs). More importantly, we demonstrate that the addition of silica precursors (tetraethyl orthosilicate and 3-aminopropyltriethoxysilane) to the acetate/oleate ligandexchange reaction of the IONPs induces the disassembly of the IONPCs into monodispersed iron oxide−acetate−silica core− shell−shell (IONPs@acetate@SiO 2) nanoparticles. Our observations evidence that the formation of IONPs@acetate@SiO 2 nanoparticles is initiated by a unique micellar fusion mechanism between the Pickering-type emulsions of IONPCs and nanoemulsions of silica precursors formed under ethyl acetate buffered conditions. A dynamic rearrangement of the CTA + − oleate bilayer on the IONPC surfaces is proposed to be responsible for the templating process of the silica shells around the individual IONPs. In comparison to previously reported methods in the literature, our work provides a much more detailed experimental evidence of the silica-coating mechanism in a nanoemulsion system. Overall, ethyl acetate is proven to be a very efficient agent for an effortless preparation of monodispersed IONPs@acetate@SiO 2 and hydrophilic Ac−IONPs from IONPCs.
ACS Applied Materials & Interfaces
Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to rep... more Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to replace scarce and commercially expensive platinum electrodes. Thin film electrodes of metalcarbides are a promising alternative due to their reduced price and similar catalytic properties. However, most of the studied structures to date neglect long lasting chemical and structural stability, focusing only on electrochemical efficiency. Herein we report on a new approach to easily deposit and control the micro/nanostructure of thin film electrodes based on niobium carbide (NbC) and their electrocatalytic response. We will show that, by improving the mechanical properties of the NbC electrodes, microstructure and mechanical resilience can be obtained whilst maintaining high electro catalytic response. We also address the influence of
Nanoscale research letters, 2017
Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) perf... more Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core-disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO2 have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO2 and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO2 nanorods grown on F:SnO2 (FTO) substrate in order to ad...
Nano letters, Aug 8, 2016
The physicochemical properties used in numerous advanced nanostructured devices are directly cont... more The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.
Well surface roughness and fault density effects on the Hall mobility of In[sub x]Ga[sub 1−x]As/In[sub y]Al[sub 1−y]As/InP high electron mobility transistors
Journal of Vacuum Science Technology B Microelectronics and Nanometer Structures, 1997
ABSTRACT In this work, we present a correlation between the morphological characterization of Iny... more ABSTRACT In this work, we present a correlation between the morphological characterization of InyAl1-yAs/InxGa1-xAs heterostructures grown on InP substrates for high electron mobility transistors (HEMTs) applications as determined by transmission electron microscopy, and the electrical behavior of the two-dimensional electron gas (2DEG) confined in the InGaAs channel. Our main goal is to analyze the origin of the low and anisotropic values of 2DEG Hall mobilities, discussing the effect of the density and asymmetric distribution of stacking faults and the surface undulation induced by a three-dimensional (3D) growth mode, depending on the growth temperature (Tg) and thickness (tw) of the InxGa1-xAs well. Our results have shown that a high mobility for a matched channel is obtained if the In0.53Ga0.47As layer is grown at 530 °C. Lower temperatures reduce the mobility values and lead to higher mobilities for [11¯0] due to the surface corrugation along [110] induced by lateral decomposition of the InGaAs at low growth temperatures. For HEMT structures with strained In0.75Ga0.25As channels grown at 530 °C, within the range of the well thickness considered (5–10 nm), Hall mobilities are also more influenced by the surface roughness than by fault distribution. However, in this case, the observed roughness is not driven by alloy decomposition but by a strain-induced 3D growth mode. © 1997 American Vacuum Society.
Density Functional Theory Modeling of Low-Loss Electron Energy-Loss Spectroscopy in Wurtzite III-Nitride Ternary Alloys
Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada, Jan 12, 2016
In the present work, the dielectric response of III-nitride semiconductors is studied using densi... more In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calcula...
Studies on Surface Facets and Chemical Composition of Vapor Grown One-Dimensional Magnetite Nanostructures
Crystal Growth Design, 2009
Investigations on shape and chemical composition of one-dimensional magnetite nanostructures grow... more Investigations on shape and chemical composition of one-dimensional magnetite nanostructures grown by a catalyst-assisted vapor phase procedure are reported. Intrinsic crystal chemistry (preferred growth of most stable surfaces) could be modulated by seeding the magnetite ...
EELS and STEM Assessment of Composition Modulation in InAlAs Tensile Buffer Layers of InGaAs /InAlAs /(100)InP Structures
Springer Proceedings in Physics, 2008
ABSTRACT
Lead-Free α-La2WO6 Ferroelectric Thin Films
ACS Applied Materials & Interfaces, 2015
(001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented Sr... more (001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented SrTiO3 (STO) substrates. The α-phase (high-temperature phase in bulk) is successfully stabilized with an orthorhombic structure (a = 16.585(1) Å, b = 5.717(2) Å, c = 8.865(5) Å). X-ray-diffraction pole-figure measurements suggest that crystallographic relationships between the film and substrate are [100]LWO ∥ [110]STO, [010]LWO ∥ [11̅0]STO and [001]LWO ∥ [001]STO. From optical properties, investigated by spectroscopic ellipsometry, we extract a refractive-index value around 2 (at 500 nm) along with the presence of two absorption bands situated, respectively at 3.07 and 6.32 eV. Ferroelectricity is evidenced as well on macroscale (standard polarization measurements) as on nanoscale, calling for experiments based on piezo-response force-microscopy, and confirmed with in situ scanning-and-tunneling measurements performed with a transmission electron microscope. This work highlights the ferroelectric behavior, at room temperature, in high-temperature LWO phase when stabilized in thin film and opens the way to new functional oxide thin films dedicated to advanced electronic devices.
ECS Transactions, 2013
A detailed itinerary of our main results and current advances in Erdoped light emitting devices a... more A detailed itinerary of our main results and current advances in Erdoped light emitting devices are here reported. We start with the photoluminescence, then the electrical characterization and the electroluminescence of Er-doped capacitors and active slot waveguides. Finally a summary of the ongoing work focused on the combination of Er-doped semiconductor superlattices with more complex devices such as slot-based ring resonators or metal-oxidesemiconductor-field-effect transistors is presented.
Chemistry of Materials, 2015
The optimization of a material functionality requires both the rational design and precise engine... more The optimization of a material functionality requires both the rational design and precise engineering of its structural and chemical parameters. In this work, we show how colloidal chemistry is an excellent synthetic choice for the synthesis of novel ternary nanostructured chalcogenides, containing exclusively noble metals, with tailored morphology and composition and with potential application in the energy conversion field. Specifically, the Ag-Au-Se system has been explored from a synthetic point of view, leading to a set of Ag2Se-based hybrid and ternary nanoparticles, including the room temperature synthesis of the rare ternary Ag3AuSe2 fischesserite phase. An in-depth structural and chemical characterization of all nanomaterials has been performed, which proofed especially useful for unravelling the reaction mechanism behind the formation of the ternary phase in solution. The work is complemented with the thermal and electric characterization of a ternary Ag-Au-Se nanocomposite with promising results: we found that the use of the ternary nanocomposite represents a clear improvement in terms of thermoelectric energy conversion as compared to a binary Ag-Se nanocomposite analogue.
(S)TEM tomography of AlAs-GaAs coaxial Nanowires
2009 Spanish Conference on Electron Devices, 2009
(S) TEM tomography is used as a complementary tool to characterize AlAs-GaAs coaxial nanowires. T... more (S) TEM tomography is used as a complementary tool to characterize AlAs-GaAs coaxial nanowires. These nanostructures have been grown on two different GaAs substrate orientations using molecular-beam epitaxy (MBE). The results show that quantum wells synthesized on nanowires grown on different substrate orientations present different morphology, which induces changes on their photoluminescence properties.
Ultramicroscopy, 2012
Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape ... more Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape of nanostructured materials. Using a spectroscopic signal to achieve a reconstruction adds a fourth chemical dimension to the 3D structure. Up to date, energy filtering of the images in the transmission electron microscope (EFTEM) is the usual spectroscopic method even if most of the information in the spectrum is lost. Unlike EFTEM tomography, the use of electron energy-loss spectroscopy (EELS) spectrum images (SI) for tomographic reconstruction retains all chemical information, and the possibilities of this new approach still remain to be fully exploited. In this article we prove the feasibility of EEL spectroscopic tomography at low voltages (80 kV) and short acquisition times from data acquired using an aberration corrected instrument and data treatment by Multivariate Analysis (MVA), applied to Fe x Co (3 À x) O 4 @Co 3 O 4 mesoporous materials. This approach provides a new scope into materials; the recovery of full EELS signal in 3D.
Ultramicroscopy, 2012
EELS is nowadays a most relevant characterization tool as it provides chemical and electronic inf... more EELS is nowadays a most relevant characterization tool as it provides chemical and electronic information with an extraordinary spatial resolution. When a crystal is viewed in zone axis in the TEM, there is channelling of the electrons along the atom columns, which strongly reduce the EELS signal, so that it is generally advised to work slightly off the zone axis to collect EELS data, which may not always be possible or advantageous. In the present work, we demonstrate the use of precession to compensate for the reduction of EELS signal when in the zone axis.
Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes
Thin Solid Films, 2000
MgF2 is a current material for the optical applications in the UV and deep UV range. Nevertheless... more MgF2 is a current material for the optical applications in the UV and deep UV range. Nevertheless, modern applications still require improvement of the optical and structural quality of the deposited layers. In the present work, the composition and microstructure of MgF2 single layers grown on Si [100] substrate by physical vapour deposition (PVD) and ion beam sputtering (IBS) processes,
Surface Reactivity of Iron Oxide Nanoparticles by Microwave-Assisted Synthesis; Comparison with the Thermal Decomposition Route
The Journal of Physical Chemistry C, 2012
ABSTRACT Microwave-assisted chemistry is becoming very attractive in all areas of synthetic chemi... more ABSTRACT Microwave-assisted chemistry is becoming very attractive in all areas of synthetic chemistry; it is fast, easy to operate, efficient in terms of energy consumption and environmentally friendly. However, a quantitative assessment of this chemical procedure with respect to other widely used chemical routes is lacking. Focusing in the preparation of iron oxide nanoparticles of comparable sizes, we have analyzed the performance of microwave-assisted synthesized nanoparticles compared to those obtained by the widespread thermal decomposition process of metal complexes. On the basis of a multidisciplinary experimental approach, we have unveiled that microwave-synthesized nanoparticles exhibit a surface reactivity significantly smaller than their thermal decomposition counterparts. We ascribe such dissimilarities to the different configurations of crystallographic faceting planes resulting from the particularities of both synthesis routes. We also show that the microwave route allows a direct stabilization of the particles in organic or aqueous media by using either steric or electrostatic stabilizers. A simplified life cycle analysis, as a preliminary framework toward nanoparticles eco-design, shows also a cost-effective positive balance for the microwave synthesis. Our results are of relevance for a broad range of applications including health, information storage, environmental remediation, sensors, or catalysis.
Physical Review B, 2011
Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the... more Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3-37 nm) epitaxial films of NiFe 2 O 4 (NFO) on MgAl 2 O 4 (MAO) and SrTiO 3 (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices.
Lateral modulations in InAlAs/InP and InGaAs/InP systems
physica status solidi (a), 2003
ABSTRACT The influence of different technological parameters on the development of compositional ... more ABSTRACT The influence of different technological parameters on the development of compositional and morphological instabilities in InAlAs and InGaAs layers grown on InP substrates is studied in the present work by transmission electron microscopy (TEM). Results show that substrate misorientation plays a decisive role in the decomposition of strained layers. It is illustrated that the modulation of composition due to the layer morphological instability is a thermally activated process. The influence of lattice mismatch on composition modulation has been assessed by introducing changes of composition in the InGaAs layers. In this case, a change of the propagation direction in the modulation is found.
Nanoscale, 2013
High resolution scanning transmission electron microscopy with an aberration corrected and monoch... more High resolution scanning transmission electron microscopy with an aberration corrected and monochromated instrument has been used for the assessment of the silicon-based active layer stack for novel optoelectronic devices. This layer contains a multilayer structure consisting of alternate thin layers of pure silica (SiO 2) and silicon-rich silicon oxide (SRO, SiO x). Upon high temperature annealing the SRO sublayer segregates into a Si nanocluster (Si-nc) precipitated phase and a SiO 2 matrix. Additionally, erbium (Er) ions have been implanted and used as luminescent centres in order to obtain narrow emission at 1.54 mm. Our study exploits the combination of high angle annular dark field (HAADF) imaging with a sub-nanometer electron probe and electron energy loss spectroscopy (EELS) with an energy resolution below 0.2 eV. The structural and chemical information is obtained from the studied multilayer structure. In addition, the instrumental techniques for calibration, deconvolution, fitting and analysis of the EELS spectra are explained in detail. The spatial distribution of the Si-nanoclusters (Si-ncs) and the SiO 2 barriers is accurately delimited in the multilayer. Additionally, the quality of the studied multilayer in terms of composition, roughness and defects is analysed and discussed. Er clusterization has not been observed; even so, blue-shifted plasmon and interband transition energies for silica are measured, in the presence of Er ions and sizable nanometer-size effects.
Langmuir : the ACS journal of surfaces and colloids, Jan 5, 2018
Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysi... more Colloidal nanocrystals (NCs) compete with molecular catalysts in the field of homogenous catalysis, offering easier recyclability and a number of potentially advantageous functionalities, such as tunable band gaps, plasmonic properties, or a magnetic moment. Using high-throughput printing technologies, colloidal NCs can also be supported onto substrates to produce cost-effective electronic, optoelectronic, electrocatalytic, and sensing devices. For both catalytic and technological application, NC surface chemistry and supracrystal organization are key parameters determining final performance. Here, we study the influence of the surface ligands and the NC organization on the catalytic properties of InS, both as a colloid and as a supported layer. As a colloid, NCs stabilized by inorganic ligands show the highest photocatalytic activities, which we associate with their large and more accessible surfaces. On the other hand, when NCs are supported on a substrate, their organization beco...
Langmuir
It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanopartic... more It has been long known that the physical encapsulation of oleic acid-capped iron oxide nanoparticles (OA−IONPs) with the cetyltrimethylammonium (CTA +) surfactant induces the formation of spherical iron oxide nanoparticle clusters (IONPCs). However, the behavior and functional properties of IONPCs in chemical reactions have been largely neglected and are still not well-understood. Herein, we report an unconventional ligand-exchange function of IONPCs activated when dispersed in an ethyl acetate/ acetate buffer system. The ligand exchange can successfully transform hydrophobic OA−IONP building blocks of IONPCs into highly hydrophilic, acetate-capped iron oxide nanoparticles (Ac−IONPs). More importantly, we demonstrate that the addition of silica precursors (tetraethyl orthosilicate and 3-aminopropyltriethoxysilane) to the acetate/oleate ligandexchange reaction of the IONPs induces the disassembly of the IONPCs into monodispersed iron oxide−acetate−silica core− shell−shell (IONPs@acetate@SiO 2) nanoparticles. Our observations evidence that the formation of IONPs@acetate@SiO 2 nanoparticles is initiated by a unique micellar fusion mechanism between the Pickering-type emulsions of IONPCs and nanoemulsions of silica precursors formed under ethyl acetate buffered conditions. A dynamic rearrangement of the CTA + − oleate bilayer on the IONPC surfaces is proposed to be responsible for the templating process of the silica shells around the individual IONPs. In comparison to previously reported methods in the literature, our work provides a much more detailed experimental evidence of the silica-coating mechanism in a nanoemulsion system. Overall, ethyl acetate is proven to be a very efficient agent for an effortless preparation of monodispersed IONPs@acetate@SiO 2 and hydrophilic Ac−IONPs from IONPCs.
ACS Applied Materials & Interfaces
Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to rep... more Resistant and efficient electrocatalysts for hydrogen evolution reaction (HER) are desired to replace scarce and commercially expensive platinum electrodes. Thin film electrodes of metalcarbides are a promising alternative due to their reduced price and similar catalytic properties. However, most of the studied structures to date neglect long lasting chemical and structural stability, focusing only on electrochemical efficiency. Herein we report on a new approach to easily deposit and control the micro/nanostructure of thin film electrodes based on niobium carbide (NbC) and their electrocatalytic response. We will show that, by improving the mechanical properties of the NbC electrodes, microstructure and mechanical resilience can be obtained whilst maintaining high electro catalytic response. We also address the influence of
Nanoscale research letters, 2017
Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) perf... more Recently, colored H-doped TiO2 (H-TiO2) has demonstrated enhanced photoelectrochemical (PEC) performance due to its unique crystalline core-disordered shell nanostructures and consequent enhanced conduction behaviors between the core-shell homo-interfaces. Although various hydrogenation approaches to obtain H-TiO2 have been developed, such as high temperature hydrogen furnace tube annealing, high pressure hydrogen annealing, hydrogen-plasma assisted reaction, aluminum reduction and electrochemical reduction etc., there is still a lack of a hydrogenation approach in a controlled manner where all processing parameters (temperature, time and hydrogen flux) were precisely controlled in order to improve the PEC performance of H-TiO2 and understand the physical insight of enhanced PEC performance. Here, we report for the first time a controlled and local rapid thermal annealing (RTA) approach to prepare hydrogenated core-shell H-TiO2 nanorods grown on F:SnO2 (FTO) substrate in order to ad...
Nano letters, Aug 8, 2016
The physicochemical properties used in numerous advanced nanostructured devices are directly cont... more The physicochemical properties used in numerous advanced nanostructured devices are directly controlled by the oxidation states of their constituents. In this work we combine electron energy-loss spectroscopy, blind source separation, and computed tomography to reconstruct in three dimensions the distribution of Fe(2+) and Fe(3+) ions in a FeO/Fe3O4 core/shell cube-shaped nanoparticle with nanometric resolution. The results highlight the sharpness of the interface between both oxides and provide an average shell thickness, core volume, and average cube edge length measurements in agreement with the magnetic characterization of the sample.
Well surface roughness and fault density effects on the Hall mobility of In[sub x]Ga[sub 1−x]As/In[sub y]Al[sub 1−y]As/InP high electron mobility transistors
Journal of Vacuum Science Technology B Microelectronics and Nanometer Structures, 1997
ABSTRACT In this work, we present a correlation between the morphological characterization of Iny... more ABSTRACT In this work, we present a correlation between the morphological characterization of InyAl1-yAs/InxGa1-xAs heterostructures grown on InP substrates for high electron mobility transistors (HEMTs) applications as determined by transmission electron microscopy, and the electrical behavior of the two-dimensional electron gas (2DEG) confined in the InGaAs channel. Our main goal is to analyze the origin of the low and anisotropic values of 2DEG Hall mobilities, discussing the effect of the density and asymmetric distribution of stacking faults and the surface undulation induced by a three-dimensional (3D) growth mode, depending on the growth temperature (Tg) and thickness (tw) of the InxGa1-xAs well. Our results have shown that a high mobility for a matched channel is obtained if the In0.53Ga0.47As layer is grown at 530 °C. Lower temperatures reduce the mobility values and lead to higher mobilities for [11¯0] due to the surface corrugation along [110] induced by lateral decomposition of the InGaAs at low growth temperatures. For HEMT structures with strained In0.75Ga0.25As channels grown at 530 °C, within the range of the well thickness considered (5–10 nm), Hall mobilities are also more influenced by the surface roughness than by fault distribution. However, in this case, the observed roughness is not driven by alloy decomposition but by a strain-induced 3D growth mode. © 1997 American Vacuum Society.
Density Functional Theory Modeling of Low-Loss Electron Energy-Loss Spectroscopy in Wurtzite III-Nitride Ternary Alloys
Microscopy and microanalysis : the official journal of Microscopy Society of America, Microbeam Analysis Society, Microscopical Society of Canada, Jan 12, 2016
In the present work, the dielectric response of III-nitride semiconductors is studied using densi... more In the present work, the dielectric response of III-nitride semiconductors is studied using density functional theory (DFT) band structure calculations. The aim of this study is to improve our understanding of the features in the low-loss electron energy-loss spectra of ternary alloys, but the results are also relevant to optical and UV spectroscopy results. In addition, the dependence of the most remarkable features with composition is tested, i.e. applying Vegard's law to band gap and plasmon energy. For this purpose, three wurtzite ternary alloys, from the combination of binaries AlN, GaN, and InN, were simulated through a wide compositional range (i.e., Al x Ga1-x N, In x Al1-x N, and In x Ga1-x N, with x=[0,1]). For this DFT calculations, the standard tools found in Wien2k software were used. In order to improve the band structure description of these semiconductor compounds, the modified Becke-Johnson exchange-correlation potential was also used. Results from these calcula...
Studies on Surface Facets and Chemical Composition of Vapor Grown One-Dimensional Magnetite Nanostructures
Crystal Growth Design, 2009
Investigations on shape and chemical composition of one-dimensional magnetite nanostructures grow... more Investigations on shape and chemical composition of one-dimensional magnetite nanostructures grown by a catalyst-assisted vapor phase procedure are reported. Intrinsic crystal chemistry (preferred growth of most stable surfaces) could be modulated by seeding the magnetite ...
EELS and STEM Assessment of Composition Modulation in InAlAs Tensile Buffer Layers of InGaAs /InAlAs /(100)InP Structures
Springer Proceedings in Physics, 2008
ABSTRACT
Lead-Free α-La2WO6 Ferroelectric Thin Films
ACS Applied Materials & Interfaces, 2015
(001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented Sr... more (001)-Epitaxial La2WO6 (LWO) thin films are grown by pulsed laser deposition on (001)-oriented SrTiO3 (STO) substrates. The α-phase (high-temperature phase in bulk) is successfully stabilized with an orthorhombic structure (a = 16.585(1) Å, b = 5.717(2) Å, c = 8.865(5) Å). X-ray-diffraction pole-figure measurements suggest that crystallographic relationships between the film and substrate are [100]LWO ∥ [110]STO, [010]LWO ∥ [11̅0]STO and [001]LWO ∥ [001]STO. From optical properties, investigated by spectroscopic ellipsometry, we extract a refractive-index value around 2 (at 500 nm) along with the presence of two absorption bands situated, respectively at 3.07 and 6.32 eV. Ferroelectricity is evidenced as well on macroscale (standard polarization measurements) as on nanoscale, calling for experiments based on piezo-response force-microscopy, and confirmed with in situ scanning-and-tunneling measurements performed with a transmission electron microscope. This work highlights the ferroelectric behavior, at room temperature, in high-temperature LWO phase when stabilized in thin film and opens the way to new functional oxide thin films dedicated to advanced electronic devices.
ECS Transactions, 2013
A detailed itinerary of our main results and current advances in Erdoped light emitting devices a... more A detailed itinerary of our main results and current advances in Erdoped light emitting devices are here reported. We start with the photoluminescence, then the electrical characterization and the electroluminescence of Er-doped capacitors and active slot waveguides. Finally a summary of the ongoing work focused on the combination of Er-doped semiconductor superlattices with more complex devices such as slot-based ring resonators or metal-oxidesemiconductor-field-effect transistors is presented.
Chemistry of Materials, 2015
The optimization of a material functionality requires both the rational design and precise engine... more The optimization of a material functionality requires both the rational design and precise engineering of its structural and chemical parameters. In this work, we show how colloidal chemistry is an excellent synthetic choice for the synthesis of novel ternary nanostructured chalcogenides, containing exclusively noble metals, with tailored morphology and composition and with potential application in the energy conversion field. Specifically, the Ag-Au-Se system has been explored from a synthetic point of view, leading to a set of Ag2Se-based hybrid and ternary nanoparticles, including the room temperature synthesis of the rare ternary Ag3AuSe2 fischesserite phase. An in-depth structural and chemical characterization of all nanomaterials has been performed, which proofed especially useful for unravelling the reaction mechanism behind the formation of the ternary phase in solution. The work is complemented with the thermal and electric characterization of a ternary Ag-Au-Se nanocomposite with promising results: we found that the use of the ternary nanocomposite represents a clear improvement in terms of thermoelectric energy conversion as compared to a binary Ag-Se nanocomposite analogue.
(S)TEM tomography of AlAs-GaAs coaxial Nanowires
2009 Spanish Conference on Electron Devices, 2009
(S) TEM tomography is used as a complementary tool to characterize AlAs-GaAs coaxial nanowires. T... more (S) TEM tomography is used as a complementary tool to characterize AlAs-GaAs coaxial nanowires. These nanostructures have been grown on two different GaAs substrate orientations using molecular-beam epitaxy (MBE). The results show that quantum wells synthesized on nanowires grown on different substrate orientations present different morphology, which induces changes on their photoluminescence properties.
Ultramicroscopy, 2012
Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape ... more Electron tomography is a widely spread technique for recovering the three dimensional (3D) shape of nanostructured materials. Using a spectroscopic signal to achieve a reconstruction adds a fourth chemical dimension to the 3D structure. Up to date, energy filtering of the images in the transmission electron microscope (EFTEM) is the usual spectroscopic method even if most of the information in the spectrum is lost. Unlike EFTEM tomography, the use of electron energy-loss spectroscopy (EELS) spectrum images (SI) for tomographic reconstruction retains all chemical information, and the possibilities of this new approach still remain to be fully exploited. In this article we prove the feasibility of EEL spectroscopic tomography at low voltages (80 kV) and short acquisition times from data acquired using an aberration corrected instrument and data treatment by Multivariate Analysis (MVA), applied to Fe x Co (3 À x) O 4 @Co 3 O 4 mesoporous materials. This approach provides a new scope into materials; the recovery of full EELS signal in 3D.
Ultramicroscopy, 2012
EELS is nowadays a most relevant characterization tool as it provides chemical and electronic inf... more EELS is nowadays a most relevant characterization tool as it provides chemical and electronic information with an extraordinary spatial resolution. When a crystal is viewed in zone axis in the TEM, there is channelling of the electrons along the atom columns, which strongly reduce the EELS signal, so that it is generally advised to work slightly off the zone axis to collect EELS data, which may not always be possible or advantageous. In the present work, we demonstrate the use of precession to compensate for the reduction of EELS signal when in the zone axis.
Microstructure and composition of MgF2 optical coatings grown on Si substrate by PVD and IBS processes
Thin Solid Films, 2000
MgF2 is a current material for the optical applications in the UV and deep UV range. Nevertheless... more MgF2 is a current material for the optical applications in the UV and deep UV range. Nevertheless, modern applications still require improvement of the optical and structural quality of the deposited layers. In the present work, the composition and microstructure of MgF2 single layers grown on Si [100] substrate by physical vapour deposition (PVD) and ion beam sputtering (IBS) processes,
Surface Reactivity of Iron Oxide Nanoparticles by Microwave-Assisted Synthesis; Comparison with the Thermal Decomposition Route
The Journal of Physical Chemistry C, 2012
ABSTRACT Microwave-assisted chemistry is becoming very attractive in all areas of synthetic chemi... more ABSTRACT Microwave-assisted chemistry is becoming very attractive in all areas of synthetic chemistry; it is fast, easy to operate, efficient in terms of energy consumption and environmentally friendly. However, a quantitative assessment of this chemical procedure with respect to other widely used chemical routes is lacking. Focusing in the preparation of iron oxide nanoparticles of comparable sizes, we have analyzed the performance of microwave-assisted synthesized nanoparticles compared to those obtained by the widespread thermal decomposition process of metal complexes. On the basis of a multidisciplinary experimental approach, we have unveiled that microwave-synthesized nanoparticles exhibit a surface reactivity significantly smaller than their thermal decomposition counterparts. We ascribe such dissimilarities to the different configurations of crystallographic faceting planes resulting from the particularities of both synthesis routes. We also show that the microwave route allows a direct stabilization of the particles in organic or aqueous media by using either steric or electrostatic stabilizers. A simplified life cycle analysis, as a preliminary framework toward nanoparticles eco-design, shows also a cost-effective positive balance for the microwave synthesis. Our results are of relevance for a broad range of applications including health, information storage, environmental remediation, sensors, or catalysis.
Physical Review B, 2011
Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the... more Spinel ferrites are being considered for advanced spintronic applications. Here, we report on the magnetic properties of ultrathin (3-37 nm) epitaxial films of NiFe 2 O 4 (NFO) on MgAl 2 O 4 (MAO) and SrTiO 3 (STO) single crystalline substrates. It is found that NFO films on STO display superparamagnetic response down to 50 K, whereas films grown on MAO display ferrimagnetic response up to room temperature. Microstructural information indicates that this distinct response can be attributed to the different growth mechanisms of the spinel ferrite on the isostructural MAO substrate (two-dimensional growth) and the perovskite STO (Volmer-Weber three-dimensional growth). We discuss the reasons for this distinct behavior and its relevance for the integration of ferrites in epitaxial heterostructures for tunnel devices.
Lateral modulations in InAlAs/InP and InGaAs/InP systems
physica status solidi (a), 2003
ABSTRACT The influence of different technological parameters on the development of compositional ... more ABSTRACT The influence of different technological parameters on the development of compositional and morphological instabilities in InAlAs and InGaAs layers grown on InP substrates is studied in the present work by transmission electron microscopy (TEM). Results show that substrate misorientation plays a decisive role in the decomposition of strained layers. It is illustrated that the modulation of composition due to the layer morphological instability is a thermally activated process. The influence of lattice mismatch on composition modulation has been assessed by introducing changes of composition in the InGaAs layers. In this case, a change of the propagation direction in the modulation is found.
Nanoscale, 2013
High resolution scanning transmission electron microscopy with an aberration corrected and monoch... more High resolution scanning transmission electron microscopy with an aberration corrected and monochromated instrument has been used for the assessment of the silicon-based active layer stack for novel optoelectronic devices. This layer contains a multilayer structure consisting of alternate thin layers of pure silica (SiO 2) and silicon-rich silicon oxide (SRO, SiO x). Upon high temperature annealing the SRO sublayer segregates into a Si nanocluster (Si-nc) precipitated phase and a SiO 2 matrix. Additionally, erbium (Er) ions have been implanted and used as luminescent centres in order to obtain narrow emission at 1.54 mm. Our study exploits the combination of high angle annular dark field (HAADF) imaging with a sub-nanometer electron probe and electron energy loss spectroscopy (EELS) with an energy resolution below 0.2 eV. The structural and chemical information is obtained from the studied multilayer structure. In addition, the instrumental techniques for calibration, deconvolution, fitting and analysis of the EELS spectra are explained in detail. The spatial distribution of the Si-nanoclusters (Si-ncs) and the SiO 2 barriers is accurately delimited in the multilayer. Additionally, the quality of the studied multilayer in terms of composition, roughness and defects is analysed and discussed. Er clusterization has not been observed; even so, blue-shifted plasmon and interband transition energies for silica are measured, in the presence of Er ions and sizable nanometer-size effects.