Giovanni Bruno - Academia.edu (original) (raw)

Papers by Giovanni Bruno

A key issue in the growth of wurtzite GaN films is the polar nature of the films arising from the... more A key issue in the growth of wurtzite GaN films is the polar nature of the films arising from the absence of a symmetry center making the (0001) and (000-1) surfaces inequivalent. The morphology, optical, and electrical properties of GaN films with Ga- and N-polarity have been demonstrated to be significantly different, and strongly impact device performance. Furthermore, which reaction occurs on each polarity surface, (0001) and (000-1) has been unclear yet. The present work aimed at providing further insight into the reactivity of GaN epitaxial films by investigating the interaction of GaN polar surfaces with atomic hydrogen produced by a remote r.f. H 2 plasma in order to distinguish not only film polarity, but also to discriminate different inversion domains (IDs) densities present in films with the same polarity. It is found that the surface reaction of atomic hydrogen with GaN epilayers is sensitive to GaN polarity and is useful for distinguishing films of mixed polarity, and ...

Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows th... more Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions mu...

A key issue for the entire field of III-Nitride materials growth is the unavailability of high qu... more A key issue for the entire field of III-Nitride materials growth is the unavailability of high quality lattice matched substrates. The most commonly used growth substrate, sapphire, still imposes constraints on the GaN film quality due to the lattice mismatch and control of polarity between sapphire and GaN . Therefore, the first stage of growth conditions (i.e. nitridation, types of buffer layers, growth ratio, annealing condition of buffer layer and so on) determines the characteristics (i.e. polarity, quality, and surface morphologies) of the subsequent GaN epitaxial layers. .Therefore, it is necessary to optimize the interface with the substrate and growth condition of buffer growth for the subsequent GaN epitaxial layer. In this work, we present the control of quality of GaN epitaxial layers with different nitridation temperatures. We present the dramatically improved structural quality of GaN epitaxial layers by optimized nitridation conditions.

Applied Sciences, 2021

Graphene is a material with exceptional optical, electrical and physicochemical properties that c... more Graphene is a material with exceptional optical, electrical and physicochemical properties that can be combined with dielectric waveguides. To date, several optical devices based on graphene have been modeled and fabricated operating in the near-infrared range and showing excellent performance and broad application prospects. This paper covers the main aspects of the optical behaviour of graphene and its exploitation as electrodes in several device configurations. The work compares the reported optical devices focusing on the wavelength tuning, showing how it can vary from a few hundred up to a few thousand picometers in the wavelength range of interest. This work could help and lead the design of tunable optical devices with integrated graphene layers that operate in the NIR.

Nanoscale, Jan 14, 2015

Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attr... more Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman s...

Scientific reports, Jan 25, 2015

In this paper, we report on the engineering and the realization of optically transparent graphene... more In this paper, we report on the engineering and the realization of optically transparent graphene-based microwave devices using Chemical Vapour Deposition (CVD) graphene whose sheet resistance may be tailored down to values below 30 Ω/sq. In particular, we show that the process was successfully used to realize and characterize a simple, optically transparent graphene-based wire-grid polarizer at microwave frequencies (X band). The availability of graphene operating in a quasi-metallic region may allow the integration of graphene layers in several microwave components, thus leading to the realization of fully transparent (and flexible) microwave devices.

Optics Express, 2014

A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to... more A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to enhance light absorption in a monolayer graphene exploiting guided mode resonances. Numerical findings reveal that the optimized configuration is able to absorb up to 60% of the impinging light at normal incidence for both TE and TM polarizations resulting in a theoretical enhancement factor of about 26 with respect to the monolayer graphene absorption (≈2.3%). Experimental results confirm this behaviour showing CVD graphene absorbance peaks up to about 40% over narrow bands of few nanometers. The simple and flexible design paves the way for the realization of innovative, scalable and easy-to-fabricate graphene-based optical absorbers.

Physical Review B, 2008

Indium kinetics and evidence for indium segregation on the GaN ͑0001͒ surface are investigated vi... more Indium kinetics and evidence for indium segregation on the GaN ͑0001͒ surface are investigated via in situ spectroscopic ellipsometry. Indium deposition exhibits two stable states at coverages of 1.0 and 1.7 ML within the temperature range of 630-688°C. Formation of each layer is governed by two kinetic processes: nuclei formation and nuclei-mediated layer adsorption. The measured desorption activation energies of nuclei of the first ͑2.04 eV͒ and second ͑2.33 eV͒ monolayers are lower than the desorption activation energies of the aggregated first ͑2.64 eV͒ and second ͑2.53 eV͒ monolayers, respectively. This suggests that adatoms preferentially interact with the nuclei and laterally aggregate.

The Journal of Physical Chemistry C, 2011

Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a gre... more Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a great challenge to control the thickness of graphene especially in real-time during the growth. So far, there are no reports on the real-time monitoring of graphene growth. Here, we show for the first time real-time in situ kinetic monitoring of graphene deposition by CVD on nickel. We demonstrate an optical nondestructive method of dynamic spectroscopic ellipsometry for controlling and optimizing the catalyst cleaning and annealing and, consequently, the graphene deposition and properties. The kinetic ellipsometry monitoring also highlights the mechanism of graphene formation. Discussion shows the applicability and industrial scalability of this ellipsometric method to the control of large-area graphene formation on any substrate. This approach opens a way to in-line real-time graphene metrology and is helpful in guiding the graphene growth process as we try to achieve reproducible and controllable research as well as industry processes for quality graphene formation.

Pure and Applied Chemistry, 1998

Surface modifications of InP and GaAs semiconductors induced by the interaction wil!i 112, N2 and... more Surface modifications of InP and GaAs semiconductors induced by the interaction wil!i 112, N2 and O2 plasmas are investigated in situ and in real time by ellipsometty. It will be shown that: (a) the kinetic ellipsometry allows to define the "border line" between the cleaning (oxide removal) and damage (phosphorus depletion) processes of InP which can occurr during exposure to H atoms; (b) electronic phenomena at the GaAsloxide interface play an important role in the oxidation kinetics; (c) the self-limiting kinetics of GaAs nitridation is due to the slow out-diffusion of As.

Pure and Applied Chemistry, 1985

Homogeneous and heterogeneous chemical mechanisms in discharges fed with halogenated gases are ex... more Homogeneous and heterogeneous chemical mechanisms in discharges fed with halogenated gases are examined in systems for etching or for deposition of polymeric and inorganic films. In particular, it is emphasized the importance of plasma parameters and of feed additives in changing the relative amounts of species active for etching and for deposition. '

Pure and Applied Chemistry, 1996

Plasma deposition of a-SiGe, a-Sic and a-SiN alloys starting from fluorinated precursors is overv... more Plasma deposition of a-SiGe, a-Sic and a-SiN alloys starting from fluorinated precursors is overviewed. The growth chemistries are examined on the basis of a unique chemisorption model. Some aspects on the role of F atoms in controlling the gas surface interactions and in determining the material properties are also evidenced.

Physical Review B, 2010

Gold nanoparticles ͑NPs͒ are directly deposited on a Si͑111͒ substrate by Ar sputtering to provid... more Gold nanoparticles ͑NPs͒ are directly deposited on a Si͑111͒ substrate by Ar sputtering to provide a semiconductor-based plasmonic platform. The Au NPs are characterized by interband transitions at energies above 2.5 eV, whose tails extend into the surface-plasmon resonance ͑SPR͒ region, hence affecting the SPR energy, amplitude, and broadening. Herein, we report the experimental evidence of the size dependence of the interband transitions for Au NPs supported on silicon and sapphire substrates by exploiting plasmonic ellipsometry. Providing the size dependence of the dielectric function of Au NPs in the extended spectral range of 190-1700 nm, and discussing the interconnection between the SPR and the interband transitions, this study as a whole represents a step ahead in the perspective of a better understanding of nanosize effect on optical properties and standardization of nanoparticles dielectric functions to rely on more accurate modeling of NPs behavior.

Physica E: Low-dimensional Systems and Nanostructures, 2003

Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited b... more Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited by co-sputtering of Er and Si. Films with different crystallinity, crystallite size and oxygen content have been obtained in order to investigate the effect of the microstructure on the photoluminescence properties. The correlation between the optical properties and microstructural parameters of the films is investigated by spectroscopic ellipsometry. PL response of the discussed structures covers both the visible wavelength range (a crystallite size-dependent photoluminescence detected for 5-6 nm sized nanocrystals embedded in a SiO matrix) and near IR range at 1:54 m (Er-related PL dominating in the films with 1-3 nm sized Si nanocrystals embedded in a-Si:H). It is demonstrated that the different PL properties can be also discriminated on the basis of ellipsometric spectra.

Materials, 2010

This review is an overview of our previous work on the synthesis and properties of poly(p-phenyle... more This review is an overview of our previous work on the synthesis and properties of poly(p-phenylenevinylene)s (PPVs) selectively fluorinated in different positions of the conjugated backbone. Both the synthetic challenges and the effects of functionalization with fluorine atoms on the optical behavior are discussed, highlighting the peculiarities and the interest of this class of conjugated polymers. A general polymerization protocol for PPVs, that is based on the Pd-catalyzed Stille cross-coupling reaction of bis-stannylated vinylene monomers with aromatic bis-halides, has been successfully extended to the synthesis of selectively fluorinated poly(p-phenylenevinylene)s. The properties of a series of these PPVs differing in the number and positions of the fluorine atoms on the conjugated backbone have been studied, even in comparison with the non-fluorinated counterparts. The intriguing optical features of the resulting materials are discussed considering not only the role of the electronic and steric effects induced by the fluorine substituents, but also the impact of the fluorination on the solid state organization and intermolecular interactions.

Le Journal de Physique IV, 1995

The effectiveness of hydrogen plasma for the reduction process of surface native oxide on InP sub... more The effectiveness of hydrogen plasma for the reduction process of surface native oxide on InP substrates is investigated by X-ray photoelectron spectroscopy (X'S) and by phase modulated spectroscopic ellipsometry PMSE). H2 plasmas, generated in a quartz tube by ap lying a r.f. field (13.56 MHz) to external electrodes, produce a very high H-atom flux (5.10~0 atoms/cm. g ec) in the d oregion. The ex-situ XPS and in-situ PMSE measurements indicate fhat the native oxide layer (25 A) is completely removed. The end point of the cleaning process is well detected by kinetic ellipsometry. The plasma treated surface shows a higher stability to reoxidation than that observed for wet etches samples.

Le Journal de Physique IV, 1995

Remote plasma metalorganic chemical vapor deposition (RP-MOCVD) technique, though relatively new,... more Remote plasma metalorganic chemical vapor deposition (RP-MOCVD) technique, though relatively new, is becoming more and more important in the processing of the 111-V semiconductor materials and, specifically, of indium phosphide. So far different processes have been designed for (a) the cleaning of InP substrates to remove surface native oxide by reduction with H2 plasnla and (b) the InP deposition under PH3 plasma preactivation. This paper deals with InP homoepitaxial growth by trimethylindium (TMI) and plasma preactivated PH3. Optical emission spcctroscopy (OES) measurements evidence the presence of 1' H and pH2 radicals, and of H-atoms in Lhe plasma phase. Mass spectrometry (MS) sampling close to the growth surface reveals the presence of alchylphosphinc ((CH3)2PH, CH3PH2). indium-phosphorus adduct and biphosphine (F2Hq). whose relative amounts depend on the growth conditions. Stoichiometric Id' epilayers having good structure and morphology. and with a very high photolurninescencc intensity are prepared under PH3 plasma preactivation, even at very low VIIII ratio (=20) and reduced temperalure (-550°C).

The European Physical Journal Applied Physics, 2004

Fully microcrystalline silicon, µc-Si, thin films (<100 nm) have been deposited at low temperatur... more Fully microcrystalline silicon, µc-Si, thin films (<100 nm) have been deposited at low temperature (60 • C) on Corning glass and plastic flexible polyimide substrates by plasma enhanced chemical vapor deposition (PECVD) using SiF4-H2-He. The effect of deposition temperature on the structure, i.e., crystallinity and density, of µc-Si films is investigated by spectroscopic ellipsometry in the 1.5−5.5 eV energy range. Modeling of spectroscopic ellipsometry data is used for highlighting crystallinity of the substrate/film interface, i.e., the absence of any amorphous incubation layer. It is found that film crystallinity does not depend on film thickness, and it increases with the decrease of deposition temperature. The temperature dependence is explained on the basis of a like-Arrhenius kinetic analysis of the etching process by atomic fluorine and hydrogen of both µc-Si and a-Si phases.

Applied Physics Letters, 2005

In this paper, we introduce an extensive study on nanostructured thin films of an organic small m... more In this paper, we introduce an extensive study on nanostructured thin films of an organic small molecule N,N′-diphenyl-N,N′-dip -tolylbenzene-1,4-diamine, (NTD). Further, the possibility of using NTD as a hole transport layer in optoelectronic devices is reported for the first time to the best of our knowledge. In this context, the thermal behaviour, crystal structure, optical absorption in ultraviolet-visible regions, and DC electrical conductivity of the as-deposited NTD thin films are investigated. The differential scanning calorimetry (DSC) investigations show that NTD has a phase transition at a high temperature of 190 °C which may not affect its morphological stability. Further, the XRD patterns reveal that NTD thin films have an as-amorphous nature with some crystals. Additionally, optical investigations indicate that the indirect electronic transition from valence to conduction band is the most probable transition which agrees well with the amorphous structure of NTD thin films. The value of the mobility gap decreases from 2.74 to 2.51 eV when the thickness of the films increases from 80 to 200 nm. Furthermore, the absorbance lies below 370 nm and the thinnest film of thickness 80 nm achieves the highest absorbance. Further, the DC electrical measurements show that the NTD film has an activation energy of 378 meV. The DC conductivity is interpreted in terms of the variable range hopping (VRH) model. Based on our studies, NTD thin films are proposed with thickness range 50-80 nm to serve as a hole transport layer in white organic light emitting diodes (OLEDs).

A key issue in the growth of wurtzite GaN films is the polar nature of the films arising from the... more A key issue in the growth of wurtzite GaN films is the polar nature of the films arising from the absence of a symmetry center making the (0001) and (000-1) surfaces inequivalent. The morphology, optical, and electrical properties of GaN films with Ga- and N-polarity have been demonstrated to be significantly different, and strongly impact device performance. Furthermore, which reaction occurs on each polarity surface, (0001) and (000-1) has been unclear yet. The present work aimed at providing further insight into the reactivity of GaN epitaxial films by investigating the interaction of GaN polar surfaces with atomic hydrogen produced by a remote r.f. H 2 plasma in order to distinguish not only film polarity, but also to discriminate different inversion domains (IDs) densities present in films with the same polarity. It is found that the surface reaction of atomic hydrogen with GaN epilayers is sensitive to GaN polarity and is useful for distinguishing films of mixed polarity, and ...

Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows th... more Among the several synthesis methodologies for graphene, chemical vapor deposition (CVD) allows the production of large area graphene as required for its application as transparent conductive layer substituting ITO. CVD-graphene presents a polycrystalline structure that strongly influences both mechanical and electrical properties. Nowadays, it is well consolidated, among different users of graphene, that analyzing graphene grains after growth, is important for quality-control. In fact, graphene is a well ordered material and contains internal boundaries, commonly known as "grain boundaries". When graphene is grown, the carbon atoms within each growing grain are lined up in a specific pattern, depending on the crystal structure of sample. With growth, each grain impact others and forms interfaces where the atomic orientations differ. It has been established that the transport properties of the graphene improve as the grain size increases. Therefore, the growth conditions mu...

A key issue for the entire field of III-Nitride materials growth is the unavailability of high qu... more A key issue for the entire field of III-Nitride materials growth is the unavailability of high quality lattice matched substrates. The most commonly used growth substrate, sapphire, still imposes constraints on the GaN film quality due to the lattice mismatch and control of polarity between sapphire and GaN . Therefore, the first stage of growth conditions (i.e. nitridation, types of buffer layers, growth ratio, annealing condition of buffer layer and so on) determines the characteristics (i.e. polarity, quality, and surface morphologies) of the subsequent GaN epitaxial layers. .Therefore, it is necessary to optimize the interface with the substrate and growth condition of buffer growth for the subsequent GaN epitaxial layer. In this work, we present the control of quality of GaN epitaxial layers with different nitridation temperatures. We present the dramatically improved structural quality of GaN epitaxial layers by optimized nitridation conditions.

Applied Sciences, 2021

Graphene is a material with exceptional optical, electrical and physicochemical properties that c... more Graphene is a material with exceptional optical, electrical and physicochemical properties that can be combined with dielectric waveguides. To date, several optical devices based on graphene have been modeled and fabricated operating in the near-infrared range and showing excellent performance and broad application prospects. This paper covers the main aspects of the optical behaviour of graphene and its exploitation as electrodes in several device configurations. The work compares the reported optical devices focusing on the wavelength tuning, showing how it can vary from a few hundred up to a few thousand picometers in the wavelength range of interest. This work could help and lead the design of tunable optical devices with integrated graphene layers that operate in the NIR.

Nanoscale, Jan 14, 2015

Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attr... more Graphene/metal heterojunctions are ubiquitous in graphene-based devices and, therefore, have attracted increasing interest of researchers. Indeed, the literature on the field reports apparently contradictory results about the effect of a metal on graphene doping. Here, we elucidate the effect of metal nanostructuring and oxidation on the metal work function (WF) and, consequently, on the charge transfer and doping of graphene/metal hybrids. We show that nanostructuring and oxidation of metals provide a valid support to frame WF and doping variation in metal/graphene hybrids. Chemical vapour-deposited monolayer graphene has been transferred onto a variety of metal surfaces, including d-metals, such as Ag, Au, and Cu, and sp-metals, such as Al and Ga, configured as thin films or nanoparticle (NP) ensembles of various average sizes. The metal-induced charge transfer and the doping of graphene have been investigated using Kelvin probe force microscopy (KPFM), and corroborated by Raman s...

Scientific reports, Jan 25, 2015

In this paper, we report on the engineering and the realization of optically transparent graphene... more In this paper, we report on the engineering and the realization of optically transparent graphene-based microwave devices using Chemical Vapour Deposition (CVD) graphene whose sheet resistance may be tailored down to values below 30 Ω/sq. In particular, we show that the process was successfully used to realize and characterize a simple, optically transparent graphene-based wire-grid polarizer at microwave frequencies (X band). The availability of graphene operating in a quasi-metallic region may allow the integration of graphene layers in several microwave components, thus leading to the realization of fully transparent (and flexible) microwave devices.

Optics Express, 2014

A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to... more A one-dimensional dielectric grating, based on a simple geometry, is proposed and investigated to enhance light absorption in a monolayer graphene exploiting guided mode resonances. Numerical findings reveal that the optimized configuration is able to absorb up to 60% of the impinging light at normal incidence for both TE and TM polarizations resulting in a theoretical enhancement factor of about 26 with respect to the monolayer graphene absorption (≈2.3%). Experimental results confirm this behaviour showing CVD graphene absorbance peaks up to about 40% over narrow bands of few nanometers. The simple and flexible design paves the way for the realization of innovative, scalable and easy-to-fabricate graphene-based optical absorbers.

Physical Review B, 2008

Indium kinetics and evidence for indium segregation on the GaN ͑0001͒ surface are investigated vi... more Indium kinetics and evidence for indium segregation on the GaN ͑0001͒ surface are investigated via in situ spectroscopic ellipsometry. Indium deposition exhibits two stable states at coverages of 1.0 and 1.7 ML within the temperature range of 630-688°C. Formation of each layer is governed by two kinetic processes: nuclei formation and nuclei-mediated layer adsorption. The measured desorption activation energies of nuclei of the first ͑2.04 eV͒ and second ͑2.33 eV͒ monolayers are lower than the desorption activation energies of the aggregated first ͑2.64 eV͒ and second ͑2.53 eV͒ monolayers, respectively. This suggests that adatoms preferentially interact with the nuclei and laterally aggregate.

The Journal of Physical Chemistry C, 2011

Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a gre... more Despite the recent advances in the chemical vapor deposition (CVD) of graphene, it is still a great challenge to control the thickness of graphene especially in real-time during the growth. So far, there are no reports on the real-time monitoring of graphene growth. Here, we show for the first time real-time in situ kinetic monitoring of graphene deposition by CVD on nickel. We demonstrate an optical nondestructive method of dynamic spectroscopic ellipsometry for controlling and optimizing the catalyst cleaning and annealing and, consequently, the graphene deposition and properties. The kinetic ellipsometry monitoring also highlights the mechanism of graphene formation. Discussion shows the applicability and industrial scalability of this ellipsometric method to the control of large-area graphene formation on any substrate. This approach opens a way to in-line real-time graphene metrology and is helpful in guiding the graphene growth process as we try to achieve reproducible and controllable research as well as industry processes for quality graphene formation.

Pure and Applied Chemistry, 1998

Surface modifications of InP and GaAs semiconductors induced by the interaction wil!i 112, N2 and... more Surface modifications of InP and GaAs semiconductors induced by the interaction wil!i 112, N2 and O2 plasmas are investigated in situ and in real time by ellipsometty. It will be shown that: (a) the kinetic ellipsometry allows to define the "border line" between the cleaning (oxide removal) and damage (phosphorus depletion) processes of InP which can occurr during exposure to H atoms; (b) electronic phenomena at the GaAsloxide interface play an important role in the oxidation kinetics; (c) the self-limiting kinetics of GaAs nitridation is due to the slow out-diffusion of As.

Pure and Applied Chemistry, 1985

Homogeneous and heterogeneous chemical mechanisms in discharges fed with halogenated gases are ex... more Homogeneous and heterogeneous chemical mechanisms in discharges fed with halogenated gases are examined in systems for etching or for deposition of polymeric and inorganic films. In particular, it is emphasized the importance of plasma parameters and of feed additives in changing the relative amounts of species active for etching and for deposition. '

Pure and Applied Chemistry, 1996

Plasma deposition of a-SiGe, a-Sic and a-SiN alloys starting from fluorinated precursors is overv... more Plasma deposition of a-SiGe, a-Sic and a-SiN alloys starting from fluorinated precursors is overviewed. The growth chemistries are examined on the basis of a unique chemisorption model. Some aspects on the role of F atoms in controlling the gas surface interactions and in determining the material properties are also evidenced.

Physical Review B, 2010

Gold nanoparticles ͑NPs͒ are directly deposited on a Si͑111͒ substrate by Ar sputtering to provid... more Gold nanoparticles ͑NPs͒ are directly deposited on a Si͑111͒ substrate by Ar sputtering to provide a semiconductor-based plasmonic platform. The Au NPs are characterized by interband transitions at energies above 2.5 eV, whose tails extend into the surface-plasmon resonance ͑SPR͒ region, hence affecting the SPR energy, amplitude, and broadening. Herein, we report the experimental evidence of the size dependence of the interband transitions for Au NPs supported on silicon and sapphire substrates by exploiting plasmonic ellipsometry. Providing the size dependence of the dielectric function of Au NPs in the extended spectral range of 190-1700 nm, and discussing the interconnection between the SPR and the interband transitions, this study as a whole represents a step ahead in the perspective of a better understanding of nanosize effect on optical properties and standardization of nanoparticles dielectric functions to rely on more accurate modeling of NPs behavior.

Physica E: Low-dimensional Systems and Nanostructures, 2003

Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited b... more Nanocrystalline silicon thin films codoped with erbium, oxygen and hydrogen have been deposited by co-sputtering of Er and Si. Films with different crystallinity, crystallite size and oxygen content have been obtained in order to investigate the effect of the microstructure on the photoluminescence properties. The correlation between the optical properties and microstructural parameters of the films is investigated by spectroscopic ellipsometry. PL response of the discussed structures covers both the visible wavelength range (a crystallite size-dependent photoluminescence detected for 5-6 nm sized nanocrystals embedded in a SiO matrix) and near IR range at 1:54 m (Er-related PL dominating in the films with 1-3 nm sized Si nanocrystals embedded in a-Si:H). It is demonstrated that the different PL properties can be also discriminated on the basis of ellipsometric spectra.

Materials, 2010

This review is an overview of our previous work on the synthesis and properties of poly(p-phenyle... more This review is an overview of our previous work on the synthesis and properties of poly(p-phenylenevinylene)s (PPVs) selectively fluorinated in different positions of the conjugated backbone. Both the synthetic challenges and the effects of functionalization with fluorine atoms on the optical behavior are discussed, highlighting the peculiarities and the interest of this class of conjugated polymers. A general polymerization protocol for PPVs, that is based on the Pd-catalyzed Stille cross-coupling reaction of bis-stannylated vinylene monomers with aromatic bis-halides, has been successfully extended to the synthesis of selectively fluorinated poly(p-phenylenevinylene)s. The properties of a series of these PPVs differing in the number and positions of the fluorine atoms on the conjugated backbone have been studied, even in comparison with the non-fluorinated counterparts. The intriguing optical features of the resulting materials are discussed considering not only the role of the electronic and steric effects induced by the fluorine substituents, but also the impact of the fluorination on the solid state organization and intermolecular interactions.

Le Journal de Physique IV, 1995

The effectiveness of hydrogen plasma for the reduction process of surface native oxide on InP sub... more The effectiveness of hydrogen plasma for the reduction process of surface native oxide on InP substrates is investigated by X-ray photoelectron spectroscopy (X'S) and by phase modulated spectroscopic ellipsometry PMSE). H2 plasmas, generated in a quartz tube by ap lying a r.f. field (13.56 MHz) to external electrodes, produce a very high H-atom flux (5.10~0 atoms/cm. g ec) in the d oregion. The ex-situ XPS and in-situ PMSE measurements indicate fhat the native oxide layer (25 A) is completely removed. The end point of the cleaning process is well detected by kinetic ellipsometry. The plasma treated surface shows a higher stability to reoxidation than that observed for wet etches samples.

Le Journal de Physique IV, 1995

Remote plasma metalorganic chemical vapor deposition (RP-MOCVD) technique, though relatively new,... more Remote plasma metalorganic chemical vapor deposition (RP-MOCVD) technique, though relatively new, is becoming more and more important in the processing of the 111-V semiconductor materials and, specifically, of indium phosphide. So far different processes have been designed for (a) the cleaning of InP substrates to remove surface native oxide by reduction with H2 plasnla and (b) the InP deposition under PH3 plasma preactivation. This paper deals with InP homoepitaxial growth by trimethylindium (TMI) and plasma preactivated PH3. Optical emission spcctroscopy (OES) measurements evidence the presence of 1' H and pH2 radicals, and of H-atoms in Lhe plasma phase. Mass spectrometry (MS) sampling close to the growth surface reveals the presence of alchylphosphinc ((CH3)2PH, CH3PH2). indium-phosphorus adduct and biphosphine (F2Hq). whose relative amounts depend on the growth conditions. Stoichiometric Id' epilayers having good structure and morphology. and with a very high photolurninescencc intensity are prepared under PH3 plasma preactivation, even at very low VIIII ratio (=20) and reduced temperalure (-550°C).

The European Physical Journal Applied Physics, 2004

Fully microcrystalline silicon, µc-Si, thin films (<100 nm) have been deposited at low temperatur... more Fully microcrystalline silicon, µc-Si, thin films (<100 nm) have been deposited at low temperature (60 • C) on Corning glass and plastic flexible polyimide substrates by plasma enhanced chemical vapor deposition (PECVD) using SiF4-H2-He. The effect of deposition temperature on the structure, i.e., crystallinity and density, of µc-Si films is investigated by spectroscopic ellipsometry in the 1.5−5.5 eV energy range. Modeling of spectroscopic ellipsometry data is used for highlighting crystallinity of the substrate/film interface, i.e., the absence of any amorphous incubation layer. It is found that film crystallinity does not depend on film thickness, and it increases with the decrease of deposition temperature. The temperature dependence is explained on the basis of a like-Arrhenius kinetic analysis of the etching process by atomic fluorine and hydrogen of both µc-Si and a-Si phases.

Applied Physics Letters, 2005

In this paper, we introduce an extensive study on nanostructured thin films of an organic small m... more In this paper, we introduce an extensive study on nanostructured thin films of an organic small molecule N,N′-diphenyl-N,N′-dip -tolylbenzene-1,4-diamine, (NTD). Further, the possibility of using NTD as a hole transport layer in optoelectronic devices is reported for the first time to the best of our knowledge. In this context, the thermal behaviour, crystal structure, optical absorption in ultraviolet-visible regions, and DC electrical conductivity of the as-deposited NTD thin films are investigated. The differential scanning calorimetry (DSC) investigations show that NTD has a phase transition at a high temperature of 190 °C which may not affect its morphological stability. Further, the XRD patterns reveal that NTD thin films have an as-amorphous nature with some crystals. Additionally, optical investigations indicate that the indirect electronic transition from valence to conduction band is the most probable transition which agrees well with the amorphous structure of NTD thin films. The value of the mobility gap decreases from 2.74 to 2.51 eV when the thickness of the films increases from 80 to 200 nm. Furthermore, the absorbance lies below 370 nm and the thinnest film of thickness 80 nm achieves the highest absorbance. Further, the DC electrical measurements show that the NTD film has an activation energy of 378 meV. The DC conductivity is interpreted in terms of the variable range hopping (VRH) model. Based on our studies, NTD thin films are proposed with thickness range 50-80 nm to serve as a hole transport layer in white organic light emitting diodes (OLEDs).