Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1999
ABSTRACT In this work we studied the possibility of improving InGaAs/InAlAs-on-InP high electron ... more ABSTRACT In this work we studied the possibility of improving InGaAs/InAlAs-on-InP high electron mobility transistor (HEMT) devices by using temperature-graded InAlAs buffer layers with growing temperatures in the range of T-g = 250-560 degrees C. Our specimens were grown by molecular beam epitaxy and we analyzed them using plane view and cross-sectional transmission electron microscopy, atomic force microscopy, scanning electron microscopy and Hall effect measurements. We found that growth at the optimum temperature (T-g = 530 degrees C) of a thin InAlAs buffer sublayer between the InP substrate and the thick InAlAs buffer layer (grown at low or high T-g) can dramatically improve the crystalline quality of our HEMT devices. Therefore the growth temperature of the buffer could be used as engineering parameter to tailor the electronic properties of InAlAs layers without interfering with the layers' crystalline quality, already assured by the first optimized nanometers. (C) 1999 American Vacuum Society. [S0734-211X(99)00506-5].
We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted ... more We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.
... Nano Lett. 4 (2004), pp. 1919–1924. Full Text via CrossRef | View Record in Scopus | Cited By... more ... Nano Lett. 4 (2004), pp. 1919–1924. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (247). [11] G. Faglia, C. Baratto, G. Sberveglieri, M. Zha and A. Zappettini, Appl. Phys. Lett. 86 (2005), p. 011923. Full Text ...
ABSTRACT TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The start... more ABSTRACT TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The starting amorphous TiO2 nanoparticles were prepared by hydrolytic processing of TiCl4-derived solutions. A V-containing solution, prepared from methanolysis of VCl4, was added to the TiO2 suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state 51V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti–O–V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V–O–V network. After heating at 500 °C, V2O5 phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of about 5 nm, proving the successful synthesis of the colloidal counterpart of the well-known TiO2–V2O5 catalytic system. Hence, and also due to the complete elimination of organic residuals, this sample was used for processing chemoresistive devices. Ethanol was used as a test gas, and the results showed the beneficial effect of the V surface modification of anatase, with a response improvement up to almost 2 orders of magnitude with respect to pure TiO2. Moreover, simple comparison of the temperature dependence of the response clearly evidenced the catalytic effect of V addition.
In this work, we have used mesoporous silica SBA-15 and KIT-6 as templates for the synthesis of d... more In this work, we have used mesoporous silica SBA-15 and KIT-6 as templates for the synthesis of different WO3 mesostructures. These materials show a small particle size, about 5–15nm, and a large surface area. We report the synthesis pathways of these mesoporous oxides and their structural characterization. In the case of the KIT-6 replica sample, a single crystal composes the
Langmuir : the ACS journal of surfaces and colloids, Jan 29, 2015
A cation exchange-based route was used to produce Cu2ZnSnS4 (CZTS)-Ag2S nanoparticles with contro... more A cation exchange-based route was used to produce Cu2ZnSnS4 (CZTS)-Ag2S nanoparticles with controlled composition. We report a detailed study of the formation of such CZTS-Ag2S nanoheterostructures and of their photocatalytic properties. When compared to pure CZTS, the use of nanoscale p-n heterostructures as light absorbers for photocatalytic water splitting provides superior photocurrents. We associate this experimental fact to a higher separation efficiency of the photogenerated electron-hole pairs. We believe this and other type-II nanoheterostructures will open the door to the use of CZTS, with excellent light absorption properties and made of abundant and environmental friendly elements, to the field of photocatalysis.
ABSTRACT TiO2-WO3 nanocomposites were prepared by colloidal processing in solvothermal conditions... more ABSTRACT TiO2-WO3 nanocomposites were prepared by colloidal processing in solvothermal conditions, with W:Ti nominal atomic ratio of 0.16 and 0.64. By exploiting the different chemical properties of the precursors, it was possible to separately nucleate TiO2 anatase nanocrystals and tiny species of W oxides. After heat-treatment at 500 °C, TiO2 nanocrystals with a size of about 8 nm were obtained, among which WO3 monoclinic nanocrystals were dispersed, as ensured by X-ray diffraction, High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The resulting chemoresistive gas-sensors were tested against acetone vapors as a sample target analyte. While pure TiO2 displayed very low response to acetone in all the operating conditions, the addition of the WO3 component boosted the sensor response, which became comparable or even larger than pure WO3. The analysis of the electrical properties of the nanocomposites showed that the response enhancement was not due to doping effects, indicating a cooperative effect between the two materials oxide components.
EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008
ABSTRACT Molecular Beam Epitaxy has been for the last two decades the most established technique ... more ABSTRACT Molecular Beam Epitaxy has been for the last two decades the most established technique for the realization of atomically precise nanostructures such as quantum wells, wires and dots. To date the epitaxial precision has been limited to the plane of growth.
EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008
ABSTRACT Mixed valence manganites, such as La2/3Ca1/3MnO3 (LCMO), have been the focus of a great ... more ABSTRACT Mixed valence manganites, such as La2/3Ca1/3MnO3 (LCMO), have been the focus of a great deal of research efforts due their ferromagnetic and metallic character that can be useful as electrodes in magnetic tunnel junctions. These devices ar based in naometric layers of ferromagnetic electrodes separated by an insulating barrier, and it is of major relevance the control of the thin film properties for the final device functionality. When grown as thin films, it has been reported that film magnetic properties are much poorer than those of their bulk counterparts [1,2]. The origin of this decrease in the magnetic properties has been ascribed to the loss of the ferromagnetic metallic mixed valence state of the Mn ions (Mn3+/4+) through the development of electronic phase separated states (ferromagnetic insulating Mn3+ and Mn4+ or non-magnetic Mn ions) along the layer [8]. Electron energy loss spectroscopy is a tool to determine the local chemistry and Mn oxidation state at the nanoscale. In this sense, several studies have been carried out in mixed valence manganites grown with (001) orientation [3—7]. Yet, very little attention has been paid to mixed valence manganites grown with other orientation, although recently (110) oriented manganite films have been reported to display enhanced magnetic properties [8].
La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray ... more La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1-10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic
CuO nanowires (NWs) were grown by thermal oxidation of metallic Cu thin layer deposited by sputte... more CuO nanowires (NWs) were grown by thermal oxidation of metallic Cu thin layer deposited by sputtering on different substrates. Optimal growth parameters were found, studying the influence of the oxidizing temperature and the atmosphere during the oxidation process. A strong relation between oxidation parameters and morphology has been detected. The preliminary response of this sensing material, using nanowire's mat-based device, to various oxidizing and reducing target gases has been evaluated, in order to corroborate the functional properties of the CuO NWs as potential sensing material under certain conditions and to confirm the p-type conductometric response of the material.
Journal of Vacuum Science & Technology B: Microelectronics and Nanometer Structures, 1999
ABSTRACT In this work we studied the possibility of improving InGaAs/InAlAs-on-InP high electron ... more ABSTRACT In this work we studied the possibility of improving InGaAs/InAlAs-on-InP high electron mobility transistor (HEMT) devices by using temperature-graded InAlAs buffer layers with growing temperatures in the range of T-g = 250-560 degrees C. Our specimens were grown by molecular beam epitaxy and we analyzed them using plane view and cross-sectional transmission electron microscopy, atomic force microscopy, scanning electron microscopy and Hall effect measurements. We found that growth at the optimum temperature (T-g = 530 degrees C) of a thin InAlAs buffer sublayer between the InP substrate and the thick InAlAs buffer layer (grown at low or high T-g) can dramatically improve the crystalline quality of our HEMT devices. Therefore the growth temperature of the buffer could be used as engineering parameter to tailor the electronic properties of InAlAs layers without interfering with the layers' crystalline quality, already assured by the first optimized nanometers. (C) 1999 American Vacuum Society. [S0734-211X(99)00506-5].
We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted ... more We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.
... Nano Lett. 4 (2004), pp. 1919–1924. Full Text via CrossRef | View Record in Scopus | Cited By... more ... Nano Lett. 4 (2004), pp. 1919–1924. Full Text via CrossRef | View Record in Scopus | Cited By in Scopus (247). [11] G. Faglia, C. Baratto, G. Sberveglieri, M. Zha and A. Zappettini, Appl. Phys. Lett. 86 (2005), p. 011923. Full Text ...
ABSTRACT TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The start... more ABSTRACT TiO2 anatase nanocrystals were surface modified by deposition of V(V) species. The starting amorphous TiO2 nanoparticles were prepared by hydrolytic processing of TiCl4-derived solutions. A V-containing solution, prepared from methanolysis of VCl4, was added to the TiO2 suspension before a solvothermal crystallization step in oleic acid. The resulting materials were characterized by X-ray diffraction, transmission electron microscopy (TEM), Fourier transform infrared, Raman, and magic angle spinning solid-state 51V nuclear magnetic resonance spectroscopy (MAS NMR). It was shown that in the as-prepared nanocrystals V was deposited onto the surface, forming Ti–O–V bonds. After heat treatment at 400 °C, TEM/electron energy loss spectroscopy and MAS NMR showed that V was partially inserted in the anatase lattice, while the surface was covered with a denser V–O–V network. After heating at 500 °C, V2O5 phase separation occurred, further evidenced by thermal analyses. The 400 °C nanocrystals had a mean size of about 5 nm, proving the successful synthesis of the colloidal counterpart of the well-known TiO2–V2O5 catalytic system. Hence, and also due to the complete elimination of organic residuals, this sample was used for processing chemoresistive devices. Ethanol was used as a test gas, and the results showed the beneficial effect of the V surface modification of anatase, with a response improvement up to almost 2 orders of magnitude with respect to pure TiO2. Moreover, simple comparison of the temperature dependence of the response clearly evidenced the catalytic effect of V addition.
In this work, we have used mesoporous silica SBA-15 and KIT-6 as templates for the synthesis of d... more In this work, we have used mesoporous silica SBA-15 and KIT-6 as templates for the synthesis of different WO3 mesostructures. These materials show a small particle size, about 5–15nm, and a large surface area. We report the synthesis pathways of these mesoporous oxides and their structural characterization. In the case of the KIT-6 replica sample, a single crystal composes the
Langmuir : the ACS journal of surfaces and colloids, Jan 29, 2015
A cation exchange-based route was used to produce Cu2ZnSnS4 (CZTS)-Ag2S nanoparticles with contro... more A cation exchange-based route was used to produce Cu2ZnSnS4 (CZTS)-Ag2S nanoparticles with controlled composition. We report a detailed study of the formation of such CZTS-Ag2S nanoheterostructures and of their photocatalytic properties. When compared to pure CZTS, the use of nanoscale p-n heterostructures as light absorbers for photocatalytic water splitting provides superior photocurrents. We associate this experimental fact to a higher separation efficiency of the photogenerated electron-hole pairs. We believe this and other type-II nanoheterostructures will open the door to the use of CZTS, with excellent light absorption properties and made of abundant and environmental friendly elements, to the field of photocatalysis.
ABSTRACT TiO2-WO3 nanocomposites were prepared by colloidal processing in solvothermal conditions... more ABSTRACT TiO2-WO3 nanocomposites were prepared by colloidal processing in solvothermal conditions, with W:Ti nominal atomic ratio of 0.16 and 0.64. By exploiting the different chemical properties of the precursors, it was possible to separately nucleate TiO2 anatase nanocrystals and tiny species of W oxides. After heat-treatment at 500 °C, TiO2 nanocrystals with a size of about 8 nm were obtained, among which WO3 monoclinic nanocrystals were dispersed, as ensured by X-ray diffraction, High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The resulting chemoresistive gas-sensors were tested against acetone vapors as a sample target analyte. While pure TiO2 displayed very low response to acetone in all the operating conditions, the addition of the WO3 component boosted the sensor response, which became comparable or even larger than pure WO3. The analysis of the electrical properties of the nanocomposites showed that the response enhancement was not due to doping effects, indicating a cooperative effect between the two materials oxide components.
EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008
ABSTRACT Molecular Beam Epitaxy has been for the last two decades the most established technique ... more ABSTRACT Molecular Beam Epitaxy has been for the last two decades the most established technique for the realization of atomically precise nanostructures such as quantum wells, wires and dots. To date the epitaxial precision has been limited to the plane of growth.
EMC 2008 14th European Microscopy Congress 1–5 September 2008, Aachen, Germany, 2008
ABSTRACT Mixed valence manganites, such as La2/3Ca1/3MnO3 (LCMO), have been the focus of a great ... more ABSTRACT Mixed valence manganites, such as La2/3Ca1/3MnO3 (LCMO), have been the focus of a great deal of research efforts due their ferromagnetic and metallic character that can be useful as electrodes in magnetic tunnel junctions. These devices ar based in naometric layers of ferromagnetic electrodes separated by an insulating barrier, and it is of major relevance the control of the thin film properties for the final device functionality. When grown as thin films, it has been reported that film magnetic properties are much poorer than those of their bulk counterparts [1,2]. The origin of this decrease in the magnetic properties has been ascribed to the loss of the ferromagnetic metallic mixed valence state of the Mn ions (Mn3+/4+) through the development of electronic phase separated states (ferromagnetic insulating Mn3+ and Mn4+ or non-magnetic Mn ions) along the layer [8]. Electron energy loss spectroscopy is a tool to determine the local chemistry and Mn oxidation state at the nanoscale. In this sense, several studies have been carried out in mixed valence manganites grown with (001) orientation [3—7]. Yet, very little attention has been paid to mixed valence manganites grown with other orientation, although recently (110) oriented manganite films have been reported to display enhanced magnetic properties [8].
La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray ... more La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1-10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic
CuO nanowires (NWs) were grown by thermal oxidation of metallic Cu thin layer deposited by sputte... more CuO nanowires (NWs) were grown by thermal oxidation of metallic Cu thin layer deposited by sputtering on different substrates. Optimal growth parameters were found, studying the influence of the oxidizing temperature and the atmosphere during the oxidation process. A strong relation between oxidation parameters and morphology has been detected. The preliminary response of this sensing material, using nanowire's mat-based device, to various oxidizing and reducing target gases has been evaluated, in order to corroborate the functional properties of the CuO NWs as potential sensing material under certain conditions and to confirm the p-type conductometric response of the material.
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Papers by Jordi Arbiol