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Francesco ROSSELLA

Ricercatore t.d. art. 24 c. 3 lett. B
Dipartimento di Scienze Fisiche, Informatiche e Matematiche sede ex-Fisica

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2023 - Calibration-Free and High-Sensitivity Microwave Detectors Based on InAs/InP Nanowire Double Quantum Dots [Articolo su rivista]
Cornia, S; Demontis, V; Zannier, V; Sorba, L; Ghirri, A; Rossella, F; Affronte, M

At the cutting-edge of microwave detection technology, novel approaches which exploit the interaction between microwaves and quantum devices are rising. In this study, microwaves are efficiently detected exploiting the unique transport features of InAs/InP nanowire double quantum dot-based devices, suitably configured to allow the precise and calibration-free measurement of the local field. Prototypical nanoscale detectors are operated both at zero and finite source-drain bias, addressing and rationalizing the microwave impact on the charge stability diagram. The detector performance is addressed by measuring its responsivity, quantum efficiency and noise equivalent power that, upon impedance matching optimization, are estimated to reach values up to approximate to 2000 A W-1, 0.04 and root HZ, respectively. The interaction mechanism between the microwave field and the quantum confined energy levels of the double quantum dots is unveiled and it is shown that these semiconductor nanostructures allow the direct assessment of the local intensity of the microwave field without the need for any calibration tool. Thus, the reported nanoscale devices based on III-V nanowire heterostructures represent a novel class of calibration-free and highly sensitive probes of microwave radiation, with nanometer-scale spatial resolution, that may foster the development of novel high-performance microwave circuitries.

2023 - Measuring thermal conductivity of nanostructures with the 3ω method: the need for finite element modeling [Articolo su rivista]
Peri, L.; Prete, D.; Demontis, V.; Degoli, E.; Ruini, A.; Magri, R.; Rossella, F.

Conventional techniques of measuring thermal transport properties may be unreliable or unwieldy when applied to nanostructures. However, a simple, all-electrical technique is available for all samples featuring high-aspect-ratio: the 3? method. Nonetheless, its usual formulation relies on simple analytical results which may break down in real experimental conditions. In this work we clarify these limits and quantify them via adimensional numbers and present a more accurate, numerical solution to the 3? problem based on the Finite Element Method (FEM). Finally, we present a comparison of the two methods on experimental datasets from InAsSb nanostructures with different thermal transport properties, to stress the crucial need of a FEM counterpart to 3? measurements in nanostructures with low thermal conductivity.

2023 - Spin-Resolved Magneto-Tunneling and Giant Anisotropic g-Factor in Broken Gap InAs-GaSb Core-Shell Nanowires [Articolo su rivista]
Clericò, V.; Wójcik, P.; Vezzosi, A.; Rocci, M.; Demontis, V.; Zannier, V.; Díaz-Fernández, A.; Díaz, E.; Bellani, V.; Domínguez-Adame, F.; Diez, E.; Sorba, L.; Bertoni, A.; Goldoni, G.; Rossella, F.

: We experimentally and computationally investigate the magneto-conductance across the radial heterojunction of InAs-GaSb core-shell nanowires under a magnetic field, B, up to 30 T and at temperatures in the range 4.2-200 K. The observed double-peak negative differential conductance markedly blue-shifts with increasing B. The doublet accounts for spin-polarized currents through the Zeeman split channels of the InAs (GaSb) conduction (valence) band and exhibits strong anisotropy with respect to B orientation and marked temperature dependence. Envelope function approximation and a semiclassical (WKB) approach allow to compute the magnetic quantum states of InAs and GaSb sections of the nanowire and to estimate the B-dependent tunneling current across the broken-gap interface. Disentangling different magneto-transport channels and a thermally activated valence-to-valence band transport current, we extract the g-factor from the spin-up and spin-down dI/dV branch dispersion, revealing a giant, strongly anisotropic g-factor in excess of 60 (100) for the radial (tilted) field configurations.

2023 - Structural and Dynamic Characterization of Li-Ionic Liquid Electrolyte Solutions for Application in Li-Ion Batteries: A Molecular Dynamics Approach [Articolo su rivista]
Salvador, Ma; Maji, R; Rossella, F; Degoli, E; Ruini, A; Magri, R

Pyrrolidinium-based (Pyr) ionic liquids (ILs) have been proposed as electrolyte components in lithium-ion batteries (LiBs), mainly due to their higher electrochemical stability and wider electrochemical window. Since they are not naturally electroactive, in order to allow their use in LiBs, it is necessary to mix the ionic liquids with lithium salts (Li). Li-PF6, Li-BF4, and Li-TFSI are among the lithium salts more frequently used in LiBs, and each anion, namely PF6 (hexafluorophosphate), BF4 (tetrafluoroborate), and TFSI (bis(trifluoromethanesulfonyl)azanide), has its own solvation characteristics and interaction profile with the pyrrolidinium ions. The size of Pyr cations, the anion size and symmetry, and cation-anion combinations influence the Li-ion solvation properties. In this work, we used molecular dynamics calculations to achieve a comprehensive view of the role of each cation-anion combination and of different fractions of lithium in the solutions to assess their relative advantage for Li-ion battery applications, by comparing the solvation and structural properties of the systems. This is the most-comprehensive work so far to consider pyrrolidinium-based ILs with different anions and different amounts of Li: from it, we can systematically determine the role of each constituent and its concentration on the structural and dynamic properties of the electrolyte solutions.

2023 - Thermoelectric and Structural Properties of Sputtered AZO Thin Films with Varying Al Doping Ratios [Articolo su rivista]
Isram, Muhammad; MAGRIN MAFFEI, Riccardo; Demontis, Valeria; Martini, Leonardo; Forti, Stiven; Coletti, Camilla; Bellani, Vittorio; Mescola, Andrea; Paolicelli, Guido; Rota, Alberto; Benedetti, Stefania; DI BONA, Alessandro; Ribeiro, Joana M.; Tavares, C. J.; Rossella, Francesco

2023 - Unveiling the Thermoelectric Performances of Zn1−xFexSe Nanoparticles Prepared by the Hydrothermal Method [Articolo su rivista]
Isram, Muhammad; Demontis, Valeria; Magrin Maffei, Riccardo; Abbas Khan, Najaf; di Bona, Alessandro; Benedetti, Stefania; Amin, Nasir; Mahmood, Khalid; Rossella, Francesco

Fe2+-doped ZnSe nanoparticles, with varying concentrations of Fe2+ dopants, were prepared by the hydrothermal method and investigated using a multi-technique approach exploiting scanning electron microscopy (SEM), X-ray diffraction (XRD), and Raman spectroscopy, as well as measurement of the electrical transport properties and Seebeck coefficient (S). The doped nanoparticles appeared as variable-sized agglomerates on nanocrystallites upon SEM investigation for any doping level. Combined XRD and Raman analyses revealed the occurrence of a cubic structure in the investigated samples. Electric and thermoelectric (TE) transport investigations showed an increase in TE performance with an increase in Fe atom concentrations, which resulted in an enhancement of the power factors from 13 µWm−1K−2 to 120 µWm−1K−2 at room temperature. The results were also dependent on the operating temperature. The maximum power factor of 9 × 10−3 Wm−1K−2 was achieved at 150 °C for the highest explored doping value. The possible applications of these findings were discussed.

2022 - Giant reduction of thermal conductivity and enhancement of thermoelectric performance in twinning superlattice InAsSb nanowires [Articolo su rivista]
Peri, L.; Prete, D.; Demontis, V.; Zannier, V.; Rossi, F.; Sorba, L.; Beltram, F.; Rossella, F.

Semiconductor nanostructures hold great promise for high-efficiency waste-heat recovery exploiting thermoelectric energy conversion. They could significantly contribute to the implementation of environmentally friendly energy sources and to the realization of self-powered biomedical wearable devices. A crucial thermoelectric material requirement is a reduced thermal conductivity together with good electrical transport properties. In this work we demonstrate a drastic reduction of the thermal conductivity in III-V semiconductor nanowires as a result of the introduction of periodic crystal-lattice twin planes during growth. The electrical and thermal transport of these nanostructures, known as twinning superlattice nanowires, are probed and compared with their twin-free counterparts, showing a one order of magnitude decrease of thermal conductivity while maintaining unaltered electrical-transport properties and Seebeck coefficients. This leads to tenfold enhancement of the thermoelectric figure of merit, ZT. Our study reports for the first time the complete experimental measurement of electrical and thermal properties in twinning superlattice nanowires, demonstrating their emergence as a novel class of nanomaterials of great potential for high-efficiency thermoelectric-energy harvesting.

2022 - Particle swarm optimization of GaAs-AlGaAS nanowire photonic crystals as two-dimensional diffraction gratings for light trapping [Articolo su rivista]
Zagaglia, L.; Demontis, V.; Rossella, F.; Floris, F.

Semiconductor nanowire ordered arrays represent a class of bi-dimensional photonic crystals that can be engineered to obtain functional metamaterials. Here is proposed a novel approach, based on a particle swarm optimization algorithm, for using such a photonic crystal concept to design a semiconductor nanowire-based two-dimensional diffraction grating able to guarantee an in-plane coupling for light trapping. The method takes into account the experimental constraints associated to the bottom-up growth of nanowire arrays, by processing as input dataset all relevant geometrical and morphological features of the array, and returns as output the optimised set of parameters according to the desired electromagnetic functionality of the metamaterial. A case of study based on an array of tapered GaAs-AlGaAs core-shell nanowire heterostructures is discussed.

2022 - Polarization Control in Integrated Silicon Waveguides Using Semiconductor Nanowires [Articolo su rivista]
Kaplan, A. E.; Vitali, V.; Demontis, V.; Rossella, F.; Fontana, A.; Cornia, S.; Petropoulos, P.; Bellani, V.; Lacava, C.; Cristiani, I.

In this work, we show the design of a silicon photonic-based polarization converting device based on the integration of semiconduction InP nanowires on the silicon photonic platform. We present a comprehensive numerical analysis showing that full polarization conversion (from quasi-TE modes to quasi-TM modes, and vice versa) can be achieved in devices exhibiting small footprints (total device lengths below 20 µm) with minimal power loss (<2 dB). The approach described in this work can pave the way to the realization of complex and re-configurable photonic processors based on the manipulation of the state of polarization of guided light beams.

2022 - Ultrafast Photoacoustic Nanometrology of InAs Nanowires Mechanical Properties [Articolo su rivista]
Gandolfi, M.; Peli, S.; Diego, M.; Danesi, S.; Giannetti, C.; Alessandri, I.; Zannier, V.; Demontis, V.; Rocci, M.; Beltram, F.; Sorba, L.; Roddaro, S.; Rossella, F.; Banfi, F.

InAs nanowires are emerging as go-to materials in a variety of applications ranging from optoelectronics to nanoelectronics, yet a consensus on their mechanical properties is still lacking. The mechanical properties of wurtzite InAs nanowires are here investigated via a multitechnique approach, exploiting electron microscopies, ultrafast photoacoustics, and finite element simulations. A benchmarked elastic matrix is provided and a Young modulus of 97 GPa is obtained, thus clarifying the debated issue of InAs NW elastic properties. The validity of the analytical approaches and approximations commonly adopted to retrieve the elastic properties from ultrafast spectroscopies is discussed. The mechanism triggering the oscillations is unveiled. Nanowire oscillations in this system arise from a sudden expansion of the supporting substrate rather than the nanowire itself. This mechanism constitutes a new paradigm, being at variance with respect to the excitation mechanisms so far identified in ultrafast experiments on nanowires and on a plethora of nanosystems. The present findings are relevant in view of applications involving InAs nanowires, knowledge of their mechanical properties being crucial for any device engineering beyond a trial-and-error approach. The results bear generality beyond the specific case, the launching mechanism potentially encompassing a variety of systems serving as nano-optomechanical resonators.

2021 - Electron and Lattice Heating Contributions to the Transient Optical Response of a Single Plasmonic Nano-Object [Articolo su rivista]
Rouxel, R.; Diego, M.; Maioli, P.; Lascoux, N.; Vialla, F.; Rossella, F.; Banfi, F.; Vallee, F.; Del Fatti, N.; Crut, A.

The sudden absorption of light by a metal nanoparticle launches a series of relaxation processes (internal thermalization, acoustic vibrations, and cooling) which induce a transient modification of its optical response. In this work, the transient optical response associated with the internal thermalization of a single gold nanodisk (occurring on a few picoseconds time scale) was quantitatively investigated by time-resolved spectroscopy experiments, and the measured signals were compared with a model accounting for the effects of both electron and ionic lattice heating. We show that experimental time-resolved signals at delays posterior to nanodisk excitation and electron gas thermalization can be simply interpreted as a combination of electron and lattice temperature evolutions, with probe wavelength-dependent weights. This demonstrates the possibility to selectively probe the electronic or lattice dynamics, through the choice of specific probe wavelengths. Additionally, the time-dependent spectral shape of transient extinction cross-section changes is shown to be successively dominated by the effects of electron and lattice heating, which present distinct spectral signatures.

2021 - Electrostatic Control of the Thermoelectric Figure of Merit in Ion-Gated Nanotransistors [Articolo su rivista]
Prete, D.; Dimaggio, E.; Demontis, V.; Zannier, V.; Rodriguez-Douton, M. J.; Guazzelli, L.; Beltram, F.; Sorba, L.; Pennelli, G.; Rossella, F.

Semiconductor nanostructures have raised much hope for the implementation of high-performance thermoelectric generators. Indeed, they are expected to make available reduced thermal conductivity without a heavy trade-off on electrical conductivity, a key requirement to optimize the thermoelectric figure of merit. Here, a novel nanodevice architecture is presented in which ionic liquids are employed as thermally-insulating gate dielectrics. These devices allow the field-effect control of electrical transport in suspended semiconducting nanowires in which thermal conductivity can be simultaneously measured using an all-electrical setup. The resulting experimental data on electrical and thermal transport properties taken on individual nanodevices can be combined to extract ZT, guide device optimization and dynamical tuning of the thermoelectric properties.

2021 - Impact of electrostatic doping on carrier concentration and mobility in InAs nanowires [Articolo su rivista]
Prete, D.; Demontis, V.; Zannier, V.; Rodriguez-Douton, M. J.; Guazzelli, L.; Beltram, F.; Sorba, L.; Rossella, F.

We fabricate dual-gated electric double layer (EDL) field effect transistors based on InAs nanowires gated with an ionic liquid, and we perform electrical transport measurements in the temperature range from room temperature to 4.2 K. By adjusting the spatial distribution of ions inside the ionic liquid employed as gate dielectric, we electrostatically induce doping in the nanostructures under analysis. We extract low-temperature carrier concentration and mobility in very different doping regimes from the analysis of current-voltage characteristics and transconductances measured exploiting global back-gating. In the liquid gate voltage interval from -2 to 2 V, carrier concentration can be enhanced up to two orders of magnitude. Meanwhile, the effect of the ionic accumulation on the nanowire surface turns out to be detrimental to the electron mobility of the semiconductor nanostructure: The electron mobility is quenched irrespectively to the sign of the accumulated ionic species. The reported results shine light on the effective impact on crucial transport parameters of EDL gating in semiconductor nanodevices and they should be considered when designing experiments in which electrostatic doping of semiconductor nanostructures via electrolyte gating is involved.

2021 - Semiconductor nanowire arrays for optical sensing: A numerical insight on the impact of array periodicity and density [Articolo su rivista]
Zagaglia, L.; Demontis, V.; Rossella, F.; Floris, F.

Recent advances in the nanofabrication and modeling of metasurfaces have shown the potential of these systems in providing unprecedented control over light-matter interactions at the nanoscale, enabling immediate and tangible improvement of features and specifications of photonic devices that are becoming always more crucial in enhancing everyday life quality. In this work, we theoretically demonstrate that metasurfaces made of periodic and non-periodic deterministic assemblies of vertically aligned semiconductor nanowires can be engineered to display a tailored effective optical response and provide a suitable route to realize advanced systems with controlled photonic properties particularly interesting for sensing applications. The metasurfaces investigated in this paper correspond to nanowire arrays that can be experimentally realized exploiting nanolithography and bottom-up nanowire growth methods: the combination of these techniques allow to finely control the position and the physical properties of each individual nanowire in complex arrays. By resorting to numerical simulations, we address the near- and far-field behavior of a nanowire ensemble and we show that the controlled design and arrangement of the nanowires on the substrate may introduce unprecedented oscillations of light reflectance, yielding a metasurface which displays an electromagnetic behavior with great potential for sensing. Finite-difference time-domain numerical simulations are carried out to tailor the nanostructure parameters and systematically engineer the optical response in the VIS-NIR spectral range. By exploiting our computational-methods we set-up a complete procedure to design and test metasurfaces able to behave as functional sensors. These results are especially encouraging in the perspective of developing arrays of epitaxially grown semiconductor nanowires, where the suggested design can be easily implemented during the nanostructure growth, opening the way to fully engineered nanowire-based optical metamaterials.

2021 - Surface nano-patterning for the bottom-up growth of iii-v semiconductor nanowire ordered arrays [Articolo su rivista]
Demontis, V.; Zannier, V.; Sorba, L.; Rossella, F.

Ordered arrays of vertically aligned semiconductor nanowires are regarded as promising candidates for the realization of all-dielectric metamaterials and artificial electromagnetic materials, whose properties can be engineered to enable new functions and enhanced device performances with respect to naturally existing materials. In this review we account for the recent progresses in substrate nanopatterning methods, strategies and approaches that overall constitute the preliminary step towards the bottom-up growth of arrays of vertically aligned semiconductor nanowires with a controlled location, size and morphology of each nanowire. While we focus specifically on III-V semiconductor nanowires, several concepts, mechanisms and conclusions reported in the manuscript can be invoked and are valid also for different nanowire materials.

2020 - 30°-twisted bilayer graphene quasicrystals from chemical vapor deposition [Articolo su rivista]
Pezzini, S; Mišeikis, V; Piccinini, G; Forti, S; Pace, S; Engelke, R; Rossella, F; Watanabe, K; Taniguchi, T; Kim, P; Coletti, C.

2020 - Electrical probing of carrier separation in InAs/InP/GaAsSb core-dualshell nanowires [Articolo su rivista]
Salimian, S.; Arif, O.; Zannier, V.; Ercolani, D.; Rossi, F.; Momtaz, Z. S.; Beltram, F.; Roddaro, S.; Rossella, F.; Sorba, L.

We investigate the tunnel coupling between the outer p-type GaAsSb shell and the n-type InAs core in catalyst-free InAs/InP/GaAsSb core-dualshell nanowires. We present a device fabrication protocol based on wet-etching processes on selected areas of the nanostructures that enables multiple configurations of measurements in the same nanowire-based device (i.e. shell-shell, core-core and core-shell). Low-temperature (4.2 K) transport in the shell-shell configuration in nanowires with 5 nm-thick InP barrier reveals a weak negative differential resistance. Differently, when the InP barrier thickness is increased to 10 nm, this negative differential resistance is fully quenched. The electrical resistance between the InAs core and the GaAsSb shell, measured in core-shell configuration, is significantly higher with respect to the resistance of the InAs core and of the GaAsSb shell. The field effect, applied via a back-gate, has an opposite impact on the electrical transport in the core and in the shell portions. Our results show that electron and hole free carriers populate the InAs and GaAsSb regions respectively and indicate InAs/InP/GaAsSb core-dualshell nanowires as an ideal system for the investigation of the physics of interacting electrons and holes at the nanoscale. [Figure not available: see fulltext.].

2020 - Engineering the optical reflectance of randomly arranged self-assembled semiconductor nanowires [Relazione in Atti di Convegno]
Demontis, V; Marini, A; Floris, F; Sorba, L; Rossella, F

2020 - Fe-functionalized paramagnetic sporopollenin from pollen grains: one-pot synthesis using ionic liquids [Articolo su rivista]
Chiappe, C; Rodriguez Douton, Mj; Mozzati, Mc; Prete, D; Griesi, A; Guazzelli, L; Gemmi, M; Caporali, S; Calisi, N; Pomelli, Cs; Rossella, F

2020 - High-quality electrical transport using scalable CVD graphene [Articolo su rivista]
Pezzini, S; Mišeikis, V; Pace, S; Rossella, F; Watanabe, K; Taniguchi, T; Coletti, C

2020 - Morphology and Magneto-Transport in Exfoliated Graphene on Ultrathin Crystalline β-Si3N4(0001)/Si(111) [Articolo su rivista]
Salimian, S.; Xiang, S.; Colonna, S.; Ronci, F.; Fosca, M.; Rossella, F.; Beltram, F.; Flammini, R.; Heun, S.

This work reports the first experimental study of graphene transferred on β-Si3N4(0001)/Si(111). A comprehensive quantitative understanding of the physics of ultrathin Si3N4 as a gate dielectric for graphene-based devices is provided. The Si3N4 film is grown on Si(111) under ultra-high vacuum (UHV) conditions and investigated by scanning tunneling microscopy (STM). Subsequently, a graphene flake is deposited on top of it by a polymer-based transfer technique, and a Hall bar device is fabricated from the graphene flake. STM is employed again to study the graphene flake under UHV conditions after device fabrication and shows that the surface quality is preserved. Electrical transport measurements, carried out at low temperature in magnetic field, reveal back gate modulation of carrier density in the graphene channel and show the occurrence of weak localization. Under these experimental conditions, no leakage current between back gate and graphene channel is detected.

2020 - Optical and mechanical properties of streptavidin-conjugated gold nanospheres through data mining techniques [Articolo su rivista]
Peli, S; Ronchi, A; Bianchetti, G; Rossella, F; Giannetti, C; Chiari, M; Pingue, P; Banfi, F; Ferrini, G

2020 - Orbital Tuning of Tunnel Coupling in InAs/InP Nanowire Quantum Dots [Articolo su rivista]
Sadre Momtaz, Z.; Servino, S.; Demontis, V.; Zannier, V.; Ercolani, D.; Rossi, F.; Rossella, F.; Sorba, L.; Beltram, F.; Roddaro, S.

We report results on the control of barrier transparency in InAs/InP nanowire quantum dots via the electrostatic control of the device electron states. Recent works demonstrated that barrier transparency in this class of devices displays a general trend just depending on the total orbital energy of the trapped electrons. We show that a qualitatively different regime is observed at relatively low filling numbers, where tunneling rates are rather controlled by the axial configuration of the electron orbital. Transmission rates versus filling are further modified by acting on the radial configuration of the orbitals by means of electrostatic gating, and the barrier transparency for the various orbitals is found to evolve as expected from numerical simulations. The possibility to exploit this mechanism to achieve a controlled continuous tuning of the tunneling rate of an individual Coulomb blockade resonance is discussed.

2020 - Publisher Correction: Fe-functionalized paramagnetic sporopollenin from pollen grains: one-pot synthesis using ionic liquids (Scientific Reports, (2020), 10, 1, (12005), 10.1038/s41598-020-68875-6) [Articolo su rivista]
Chiappe, C.; Rodriguez-Douton, M. J.; Mozzati, M. C.; Prete, D.; Griesi, A.; Guazzelli, L.; Gemmi, M.; Caporali, S.; Calisi, N.; Pomelli, C. S.; Rossella, F.

In the original version of this Article, F. Rossella was incorrectly listed as a corresponding author. The correct corresponding author for this Article is C.S. Pomelli. Correspondence and request for materials should be addressed to This error has now been corrected in the HTML and PDF versions of the Article.

2020 - Strategy for accurate thermal biasing at the nanoscale [Articolo su rivista]
Denisov, Ao; Tikhonov, E; Piatrusha, Su; Khrapach, In; Rossella, F; Rocci, M; Sorba, L; Roddaro, S

2020 - Synthetic mycomelanin thin films as emergent bio-inspired interfaces controlling the fate of embryonic stem cells [Articolo su rivista]
Manini, P; Lucci, V; Lino, V; Sartini, S; Rossella, F; Falco, G; Chiappe, C; D’Ischia, M

2020 - Ultrafast Thermo-Optical Dynamics of a Single Metal Nano-Object [Articolo su rivista]
Rouxel, R; Diego, M; Medeghini, F; Maioli, P; Rossella, F; Vallée, F; Banfi, F; Crut, A; Del Fatti, N

2019 - 3D Multi-branched sno2 semiconductor nanostructures as opticalwaveguides [Articolo su rivista]
Rossella, F.; Bellani, V.; Tommasini, M.; Gianazza, U.; Comini, E.; Soldano, C.

Nanostructures with complex geometry have gathered interest recently due to some unusual and exotic properties associated with both their shape and material. 3D multi-branched sno2 one-dimensional nanostructrures, characterized by a "node"-i.e., the location where two or more branches originate, are the ideal platform to distribute signals of different natures. In this work, we study how this particular geometrical configuration affects light propagation when a light source (i.e., laser) is focused onto it. Combining scanning electron microscopy (SEM) and optical analysis along with Raman and Rayleigh scattering upon illumination, we were able to understand, in more detail, the mechanism behind the light-coupling occurring at the node. Our experimental findings show that multi-branched semiconductor 1D structures have great potential as optically active nanostructures with waveguiding properties, thus paving the way for their application as novel building blocks for optical communication networks.

2019 - Anisotropies of the g-factor tensor and diamagnetic coefficient in crystal-phase quantum dots in InP nanowires [Articolo su rivista]
Wu, S.; Peng, K.; Battiato, S.; Zannier, V.; Bertoni, A.; Goldoni, G.; Xie, X.; Yang, J.; Xiao, S.; Qian, C.; Song, F.; Sun, S.; Dang, J.; Yu, Y.; Beltram, F.; Sorba, L.; Li, A.; Li, B. -B.; Rossella, F.; Xu, X.

Crystal-phase low-dimensional structures offer great potential for the implementation of photonic devices of interest for quantum information processing. In this context, unveiling the fundamental parameters of the crystal phase structure is of much relevance for several applications. Here, we report on the anisotropy of the g-factor tensor and diamagnetic coefficient in wurtzite/zincblende (WZ/ZB) crystal-phase quantum dots (QDs) realized in single InP nanowires. The WZ and ZB alternating axial sections in the NWs are identified by high-angle annular dark-field scanning transmission electron microscopy. The electron (hole) g-factor tensor and the exciton diamagnetic coefficients in WZ/ZB crystal-phase QDs are determined through micro-photoluminescence measurements at low temperature (4.2 K) with different magnetic field configurations, and rationalized by invoking the spin-correlated orbital current model. Our work provides key parameters for band gap engineering and spin states control in crystal-phase low-dimensional structures in nanowires. [Figure not available: see fulltext.].

2019 - Conductometric sensing with individual InAs nanowires [Articolo su rivista]
Demontis, V.; Rocci, M.; Donarelli, M.; Maiti, R.; Zannier, V.; Beltram, F.; Sorba, L.; Roddaro, S.; Rossella, F.; Baratto, C.

In this work, we isolate individual wurtzite InAs nanowires and fabricate electrical contacts at both ends, exploiting the single nanostructures as building blocks to realize two different architectures of conductometric sensors: (a) the nanowire is drop-casted onto—supported by—a SiO2/Si substrate, and (b) the nanowire is suspended at approximately 250 nm from the substrate. We test the source-drain current upon changes in the concentration of humidity, ethanol, and NO2, using synthetic air as a gas carrier, moving a step forward towards mimicking operational environmental conditions. The supported architecture shows higher response in the mid humidity range (50% relative humidity), with shorter response and recovery times and lower detection limit with respect to the suspended nanowire. These experimental pieces of evidence indicate a minor role of the InAs/SiO2 contact area; hence, there is no need for suspended nanostructures to improve the sensing performance. Moreover, the sensing capability of single InAs nanowires for detection of NO2 and ethanol in the ambient atmosphere is reported and discussed.

2019 - III-V semicondutor nanostructures and iontronics: InAs nanowire-based electric double layer field effect transistors [Relazione in Atti di Convegno]
Prete, D.; Lieb, J.; Demontis, V.; Bellucci, L.; Tozzini, V.; Ercolani, D.; Zannier, V.; Sorba, L.; Ono, S.; Beltram, F.; Sacepe, B.; Rossella, F.

In the emerging interdisciplinary field of iontronics, ionic motion and arrangement in electrolyte media are exploited to control the properties and functionalities of electronic devices. This approach encompasses a wide range of applications across engineering and physical sciences including solid-state physics, electronics and energy storage. We briefly discuss the use of approaches and techniques characteristic of iontronics in nanoscale devices based on III-V semiconductor nanostructures, a versatile and promising platform for nanoscience and nanotechnology applications. Then, we report and discuss the operation of InAs nanowire-based electrolyte-gated transistors implemented using ionic liquids. We show that the ionic liquid gating outperforms the conventional solid-state back gate, and we compare the current modulation achieved in the same InAs NW using the ionic liquid gate or the back-gate. Finally, we highlight the capability of the liquid electrolyte to drastically change the resistance dependence on temperature in the nanowire. Our results suggest promising strategies toward the advanced field effect control of innovative III-V semiconductor nanowire-based devices for information and communication technologies at large.

2019 - Ionic-Liquid Gating of InAs Nanowire-Based Field-Effect Transistors [Articolo su rivista]
Lieb, Johanna; Demontis, Valeria; Prete, Domenic; Ercolani, Daniele; Zannier, Valentina; Sorba, Lucia; Ono, Shimpei; Beltram, Fabio; Sacépé, Benjamin; Rossella, Francesco

Here, the operation of a field-effect transistor based on a single InAs nanowire gated by an ionic liquid is reported. Liquid gating yields very efficient carrier modulation with a transconductance value 30 times larger than standard back gating with the SiO2/Si++ substrate. Thanks to this wide modulation, the controlled evolution from semiconductor to metallic-like behavior in the nanowire is shown. This work provides the first systematic study of ionic-liquid gating in electronic devices based on individual III–V semiconductor nanowires: this architecture opens the way to a wide range of fundamental and applied studies from the phase transitions to bioelectronics.

2019 - Management of the output electrical power in thermoelectric generators [Articolo su rivista]
Dimaggio, E.; Rossella, F.; Pennelli, G.

Thermoelectric Generators (TEGs) are devices for direct conversion of heat into electrical power and bear a great potential for applications in energy scavenging and green energy harvesting. Given a heat source, the conversion efficiency depends on the available temperature difference, and must be maximized for optimal operation of the TEG. In this frame, the choice of materials with high thermoelectric properties should be accompanied by the identification of criteria for an optimal exploitation of the electrical power output. In this work, we briefly review the main properties of TEGs, focusing on the electrical power output and the thermal-to-electrical conversion efficiency. Besides, we discuss principles of operation of TEGs enabling the optimization of the electrical power output, based on the suitable choice of the electrical load. In particular, we comparatively present and discuss the conditions for matching the electrical load—yielding to maximum power transfer—and those for maximizing the conversion efficiency. We compare the two conditions applying them to the exploitation of a heat reservoir for energy storage and to the recovery of heat from a heat exchanger. We conclude that the difference between the two conditions is not significant enough to justify the complexity required by the implementation of the maximum efficiency. In addition, we consider the effect of the thermal contact resistance on the electrical power output. Using a simple thermal-electrical model, we demonstrate that the equivalent electrical resistance measured between the terminals of the TEG depends on the thermal exchange. Hence, for maximum power transfer, the electrical load of the TEG should not match its parasitic resistance, but the equivalent electrical resistance in each specific operating conditions, which determine the thermal fluxes. The model can be applied for the development of efficient alternative algorithms for maximum power point tracking.

2019 - Microwave-Assisted Tunneling in Hard-Wall InAs/InP Nanowire Quantum Dots [Articolo su rivista]
Cornia, S.; Rossella, F.; Demontis, V.; Zannier, V.; Beltram, F.; Sorba, L.; Affronte, M.; Ghirri, A.

With downscaling of electronic circuits, components based on semiconductor quantum dots are assuming increasing relevance for future technologies. Their response under external stimuli intrinsically depend on their quantum properties. Here we investigate single-electron tunneling in hard-wall InAs/InP nanowires in the presence of an off-resonant microwave drive. Our heterostructured nanowires include InAs quantum dots (QDs) and exhibit different tunnel-current regimes. In particular, for source-drain bias up to few mV Coulomb diamonds spread with increasing contrast as a function of microwave power and present multiple current polarity reversals. This behavior can be modelled in terms of voltage fluctuations induced by the microwave field and presents features that depend on the interplay of the discrete energy levels that contribute to the tunneling process.

2019 - Polychromatic emission in a wide energy range from InP-InAs-InP multi-shell nanowires [Articolo su rivista]
Battiato, S.; Wu, S.; Zannier, V.; Bertoni, A.; Goldoni, G.; Li, A.; Xiao, S.; Han, X. D.; Beltram, F.; Sorba, L.; Xu, X.; Rossella, F.

InP-InAs-InP multi-shell nanowires (NWs) were grown in the wurtzite (WZ) or zincblende (ZB) crystal phase and their photoluminescence (PL) properties were investigated at low temperature (≈6 K) for different measurement geometries. PL emissions from the NWs were carefully studied in a wide energy range from 0.7 to 1.6 eV. The different features observed in the PL spectra for increasing energies are attributed to four distinct emitting domains of these nano-heterostructures: the InAs island (axially grown), the thin InAs capping shell (radially grown), the crystal-phase quantum disks arising from the coexistence of InP ZB and WZ segments in the same NW, and the InP portions of the NW. These results provide a useful frame for the rational implementation of InP-InAs-InP multi-shell NWs containing various quantum confined domains as polychromatic optically active components in nanodevices for quantum information and communication technologies.

2019 - Signatures of Small Morphological Anisotropies in the Plasmonic and Vibrational Responses of Individual Nano-objects [Articolo su rivista]
Medeghini, F.; Rouxel, R.; Crut, A.; Maioli, P.; Rossella, F.; Banfi, F.; Vallee, F.; Del Fatti, N.

The plasmonic and vibrational properties of single gold nanodisks patterned on a sapphire substrate are investigated via spatial modulation and pump-probe optical spectroscopies. The features of the measured extinction spectra and time-resolved signals are highly sensitive to minute deviations of the nanodisk morphology from a perfectly cylindrical one. An elliptical nanodisk section, as compared to a circular one, lifts the degeneracy of the two nanodisk in-plane dipolar surface plasmon resonances, which can be selectively excited by controlling the polarization of the incident light. This splitting effect, whose amplitude increases with nanodisk ellipticity, correlates with the detection of additional vibrational modes in the context of time-resolved spectroscopy. Analysis of the measurements is performed through the combination of optical and acoustic numerical models. This allows us first to estimate the dimensions of the investigated nanodisks from their plasmonic response and then to compare the measured and computed frequencies of their detectable vibrational modes, which are found to be in excellent agreement. This study demonstrates that single-particle optical spectroscopies are able to provide access to fine morphological characteristics, representing in this case a valuable alternative to traditional techniques aimed at postfabrication inspection of subwavelength nanodevice morphology.

2019 - Strong modulations of optical reflectance in tapered core-shell nanowires [Articolo su rivista]
Floris, F.; Fornasari, L.; Bellani, V.; Marini, A.; Banfi, F.; Marabelli, F.; Beltram, F.; Ercolani, D.; Battiato, S.; Sorba, L.; Rossella, F.

Random assemblies of vertically aligned core-shell GaAs-AlGaAs nanowires displayed an optical response dominated by strong oscillations of the reflected light as a function of the incident angle. In particular, angle-resolved specular reflectance measurements showed the occurrence of periodic modulations in the polarization-resolved spectra of reflected light for a surprisingly wide range of incident angles. Numerical simulations allowed for identifying the geometrical features of the core-shell nanowires leading to the observed oscillatory effects in terms of core and shell thickness as well as the tapering of the nanostructure. The present results indicate that randomly displaced ensembles of nanoscale heterostructures made of III-V semiconductors can operate as optical metamirrors, with potential for sensing applications.

2019 - Thermoelectric Conversion at 30 K in InAs/InP Nanowire Quantum Dots [Articolo su rivista]
Prete, D.; Erdman, P. A.; Demontis, V.; Zannier, V.; Ercolani, D.; Sorba, L.; Beltram, F.; Rossella, F.; Taddei, F.; Roddaro, S.

We demonstrate high-temperature thermoelectric conversion in InAs/InP nanowire quantum dots by taking advantage of their strong electronic confinement. The electrical conductance G and the thermopower S are obtained from charge transport measurements and accurately reproduced with a theoretical model accounting for the multilevel structure of the quantum dot. Notably, our analysis does not rely on the estimate of cotunnelling contributions, since electronic thermal transport is dominated by multilevel heat transport. By taking into account two spin-degenerate energy levels we are able to evaluate the electronic thermal conductance K and investigate the evolution of the electronic figure of merit ZT as a function of the quantum dot configuration and demonstrate ZT ≈ 35 at 30 K, corresponding to an electronic efficiency at maximum power close to the Curzon-Ahlborn limit.

2018 - Controlling the Quality Factor of a Single Acoustic Nanoresonator by Tuning its Morphology [Articolo su rivista]
Medeghini, Fabio; Crut, Aurélien; Gandolfi, Marco; Rossella, Francesco; Maioli, Paolo; Vallée, Fabrice; Banfi, Francesco; Del Fatti, Natalia

The mechanical vibrations of individual gold nanodisks nanopatterned on a sapphire substrate are investigated using ultrafast time-resolved optical spectroscopy. The number and characteristics of the detected acoustic modes are found to vary with nanodisk geometry. In particular, their quality factors strongly depend on nanodisk aspect ratio (i.e., diameter over height ratio), reaching a maximal value of ≈70, higher than those previously measured for substrate-supported nano-objects. The peculiarities of the detected acoustic vibrations are confirmed by finite-element simulations, and interpreted as the result of substrate-induced hybridization between the vibrational modes of a nanodisk. The present findings demonstrate novel possibilities for engineering the vibrational modes of nano-objects.

2018 - Measurement of the Thermoelectric Properties of Individual Nanostructures [Articolo su rivista]
Rossella, Francesco; Pennelli, Giovanni; Roddaro, Stefano

The advent of nanotechnology and nanomaterials is opening new perspectives for the achievement of efficient solid-state heat converters. After decades of slow progress, in recent years innovative ideas have been put forward to improve the thermodynamic conversion efficiency and, as a consequence, new thermoelectric nanomaterials have been developed. A key and challenging ingredient for the progress of this research ambit is today the refinement of precise methods for the measurement of the thermoelectric parameters of nanostructures.

2018 - Suspended InAs Nanowire-Based Devices for Thermal Conductivity Measurement Using the 3ω Method [Articolo su rivista]
Rocci, Mirko; Demontis, Valeria; Prete, Domenic; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Pennelli, Giovanni; Roddaro, Stefano; Rossella, Francesco

We demonstrated device architectures implementing suspended InAs nanowires for thermal conductivity measurements. To this aim, we exploited a fabrication protocol involving the use of a sacrificial layer. The relatively large aspect ratio of our nanostructures combined with their low electrical resistance allows to exploit the four-probe 3ω technique to measure the thermal conductivity, inducing electrical self-heating in the nanowire at frequency ω and measuring the voltage drop across the nanostructure at frequency 3ω. In our systems, field effect modulation of the transport properties can be achieved exploiting fabricated side-gate electrodes in combination with the SiO2/Si ++ substrate acting as a back gate. Our device architectures can open new routes to the all-electrical investigation of thermal parameters in III-V semiconductor nanowires, with a potential impact on thermoelectric applications.

2017 - Crystal Phases in Hybrid Metal-Semiconductor Nanowire Devices [Articolo su rivista]
David, J; Rossella, Francesco; Rocci, Mirko; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Gemmi, M; Roddaro, Stefano

We investigate the metallic phases observed in hybrid metal-GaAs nanowire devices obtained by controlled thermal annealing of Ni/Au electrodes. Devices are fabricated onto a SiN membrane compatible with transmission electron microscopy studies. Energy dispersive X-ray spectroscopy allows us to show that the nanowire body includes two Ni-rich phases that thanks to an innovative use of electron diffraction tomography can be unambiguously identified as Ni3GaAs and Ni5As2 crystals. The mechanisms of Ni incorporation leading to the observed phenomenology are discussed.

2017 - From pollen grains to functionalized microcapsules: a facile chemical route using ionic liquids [Articolo su rivista]
Chiappe, Cinzia; Demontis, Gian Carlo; Di Bussolo, Valeria; Rodriguez Douton, Maria Jesus; Rossella, Francesco; Pomelli, Christian Silvio; Sartini, Stefania; Caporali, Stefano; Chiappe, Cinzia; Demontis, Gian Carlo; Di Bussolo, Valeria; Rodriguez Douton, Maria Jesus; Rossella, Francesco; Pomelli, Christian Silvio; Sartini, Stefania; Caporali, Stefano

2017 - Physiological and biochemical impacts of graphene oxide in polychaetes: The case of Diopatra neapolitana [Articolo su rivista]
De Marchi, Lucia; Neto, Victor; Pretti, Carlo; Figueira, Etelvina; Brambilla, Luigi; Rodriguez Douton, Maria Jesus; Rossella, Francesco; Tommasini, Matteo; Furtado, Clascídia; Soares, Amadeu M. V. M; Freitas, Rosa

Graphene oxide (GO) is an important carbon nanomaterial (NM) that has been used, but limited literature is available regarding the impacts induced in aquatic organisms by this pollutant and, in particular in invertebrate species. The polychaete Diopatra neapolitana has frequently been used to evaluate the effects of environmental disturbances in estuarine systems due to its ecological and socio-economic importance but to our knowledge no information is available on D. neapolitana physiological and biochemical alterations due to GO exposure. Thus, the present study aimed to assess the toxic effects of different concentrations of GO (0.01; 0.10 and 1.00mg/L) in D. neapolitana physiological (regenerative capacity) and biochemical (energy reserves, metabolic activity and oxidative stress related biomarkers) performance, after 28days of exposure. The results obtained revealed that the exposure to GO induced negative effects on the regenerative capacity of D. neapolitana, with organisms exposed to higher concentrations regenerating less segments and taking longer periods to completely regenerate. GO also seemed to alter energy-related responses, especially glycogen content, with higher values in polychaetes exposed to GO which may result from a decreased metabolism (measured by electron transport system activity), when exposed to GO. Furthermore, under GO contamination D. neapolitana presented cellular damage, despite higher activities of antioxidant and biotransformation enzymes in individuals exposed to GO.

2017 - Self-Assembled InAs Nanowires as Optical Reflectors [Articolo su rivista]
Floris, Francesco; Fornasari, Lucia; Marini, Andrea; Bellani, Vittorio; Banfi, Francesco; Roddaro, Stefano; Ercolani, Daniele; Rocci, Mirko; Beltram, Fabio; Cecchini, Marco; Sorba, Lucia; Rossella, Francesco

Subwavelength nanostructured surfaces are realized with self-assembled vertically-aligned InAs nanowires, and their functionalities as optical reflectors are investigated. In our system, polarization-resolved specular reflectance displays strong modulations as a function of incident photon energy and angle. An effective-medium model allows one to rationalize the experimental findings in the long wavelength regime, whereas numerical simulations fully reproduce the experimental outcomes in the entire frequency range. The impact of the refractive index of the medium surrounding the nanostructure assembly on the reflectance was estimated. In view of the present results, sensing schemes compatible with microfluidic technologies and routes to innovative nanowire-based optical elements are discussed.

2016 - GHz Electroluminescence Modulation in Nanoscale Subwavelength Emitters [Articolo su rivista]
Rossella, Francesco; Piazza, Vincenzo; Rocci, Mirko; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Roddaro, Stefano

We investigate light emission from nanoscale point-sources obtained in hybrid metal-GaAs nanowires embedding two sharp axial Schottky barriers. Devices are obtained via the formation of Ni-rich metallic alloy regions in the nanostructure body thanks to a technique of controlled thermal annealing of Ni/Au electrodes. In agreement with recent findings, visible-light electroluminescence can be observed upon suitable voltage biasing of the junctions. We investigate the time-resolved emission properties of our devices and demonstrate an electrical modulation of light generation up to 1 GHz. We explore different drive configurations and discuss the intrinsic bottlenecks of the present device architecture. Our results demonstrate a novel technique for the realization of fast subwavelength light sources with possible applications in sensing and microscopy beyond the diffraction limit.

2016 - Gate-Tunable Spatial Modulation of Localized Plasmon Resonances [Articolo su rivista]
Arcangeli, Andrea; Rossella, Francesco; Tomadin, Andrea; Xu, Jihua; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Tredicucci, Alessandro; Polini, Marco; Roddaro, Stefano

We demonstrate localization and field-effect spatial control of the plasmon resonance in semiconductor nanostructures, using scattering-type scanning near-field optical microscopy in the mid-infrared region. We adopt InAs nanowires embedding a graded doping profile to modulate the free carrier density along the axial direction. Our near-field measurements have a spatial resolution of 20 nm and demonstrate the presence of a local resonant feature whose position can be controlled by a back-gate bias voltage. In the present implementation, field-effect induces a modulation of the free carrier density profile yielding a spatial shift of the plasmon resonance of the order of 100 nm. We discuss the relevance of our electrically tunable nanoplasmonic architectures in view of innovative optoelectronic devices concepts.

2016 - Local noise in a diffusive conductor [Articolo su rivista]
Tikhonov, E. S; Shovkun, D. V; Ercolani, Daniele; Rossella, Francesco; Rocci, Mirko; Sorba, Lucia; Roddaro, Stefano; Khrapai, V. S.

The control and measurement of local non-equilibrium configurations is of utmost importance in applications on energy harvesting, thermoelectrics and heat management in nano-electronics. This challenging task can be achieved with the help of various local probes, prominent examples including superconducting or quantum dot based tunnel junctions, classical and quantum resistors, and Raman thermography. Beyond time-averaged properties, valuable information can also be gained from spontaneous fluctuations of current (noise). From these perspective, however, a fundamental constraint is set by current conservation, which makes noise a characteristic of the whole conductor, rather than some part of it. Here we demonstrate how to remove this obstacle and pick up a local noise temperature of a current biased diffusive conductor with the help of a miniature noise probe. This approach is virtually noninvasive for the electronic energy distributions and extends primary local measurements towards strongly non-equilibrium regimes.

2016 - Nanostructured magnetic metamaterials based on metal-filled carbon nanotubes [Articolo su rivista]
Rossella, Francesco; Mozzati, M. C.; Bordonali, L.; Lascialfari, A.; Soldano, C.; Ortolani, L.; Bellani, V.

2016 - Ni-rich phases identification in GaAs nanowire devices by mean of electron diffraction tomography [Articolo su rivista]
David, Jérémy; Rossella, Francesco; Rocci, Mirko; Gemmi, Mauro; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Roddaro, Stefano

2016 - Noise thermometry applied to thermoelectric measurements in InAs nanowires [Articolo su rivista]
Tikhonov, E. S; Shovkun, D. V; Ercolani, Daniele; Rossella, Francesco; Rocci, Mirko; Sorba, Lucia; Roddaro, Stefano; Khrapai, V. S.

2016 - Tunable Esaki Effect in Catalyst-Free InAs/GaSb Core-Shell Nanowires [Articolo su rivista]
Rocci, Mirko; Rossella, Francesco; Gomes, U. P; Zannier, V; Rossi, F; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Roddaro, Stefano

We demonstrate tunable bistability and a strong negative differential resistance in InAs/GaSb core-shell nanowire devices embedding a radial broken-gap heterojunction. Nanostructures have been grown using a catalyst-free synthesis on a Si substrate. Current-voltage characteristics display a top peak-to-valley ratio of 4.8 at 4.2 K and 2.2 at room temperature. The Esaki effect can be modulated-or even completely quenched-by field effect, by controlling the band bending profile along the azimuthal angle of the radial heterostructure. Hysteretic behavior is also observed in the presence of a suitable resistive load. Our results indicate that high-quality broken-gap devices can be obtained using Au-free growth.

2015 - Complete thermoelectric benchmarking of individual InSb nanowires using combined micro-Raman and electric transport analysis [Articolo su rivista]
Yazji, Sara; Hoffman, Eric A.; Ercolani, Daniele; Rossella, Francesco; Pitanti, Alessandro; Cavalli, Alessandro; Roddaro, Stefano; Abstreiter, Gerhard; Sorba, Lucia; Zardo, Ilaria

Nanowires (NWs) are ideal nanostructures for exploring the effects of low dimensionality and thermal conductivity suppression on thermoelectric behavior. However, it is challenging to accurately measure temperature gradients and heat flow in such systems. Here, using a combination of spatially resolved Raman spectroscopy and transport measurements, we determine all the thermoelectric properties of single Se-doped InSb NWs and quantify the figure of merit ZT. The measured laser-induced heating in the NWs and associated electrical response are well described by a 1D heat equation model. Our method allows the determination of the thermal contact resistances at the source and drain electrodes of the NW, which are negligible in our system. The measured thermoelectric parameters of InSb NWs agree well with those obtained based on field-effect transistor Seebeck measurements.

2015 - Effective Wavelength Scaling of and Damping in Plasmonic Helical Antennae [Articolo su rivista]
Caridad, José M.; Mccloskey, David; Rossella, Francesco; Bellani, Vittorio; Donegan, John F.; Krstić, Vojislav

2014 - Electrostatic spin control in multi-barrier nanowires [Articolo su rivista]
Rossella, Francesco; Ercolani, Daniele; Sorba, Lucia; Beltram, Fabio; Roddaro, Stefano

2014 - Large thermal biasing of individual gated nanostructures [Articolo su rivista]
Roddaro, Stefano; Ercolani, Daniele; Mian Akif, Safeen; Rossella, Francesco; Piazza, Vincenzo; Francesco, Giazotto; Sorba, Lucia; Beltram, Fabio

2014 - Nanoscale spin rectifiers controlled by the Stark effect [Articolo su rivista]
Rossella, Francesco; Bertoni, Andrea; Ercolani, Daniele; Rontani, Massimo; Sorba, Lucia; Beltram, Fabio; Roddaro, Stefano

The control of orbitals and spin states of single electrons is a key ingredient for quantum information processing and novel detection schemes and is, more generally, of great relevance for spintronics. Coulomb and spin blockade in double quantum dots enable advanced single-spin operations that would be available even for room-temperature applications with sufficiently small devices. To date, however, spin operations in double quantum dots have typically been observed at sub-kelvin temperatures, a key reason being that it is very challenging to scale a double quantum dot system while retaining independent field-effect control of individual dots. Here, we show that the quantum-confined Stark effect allows two dots only 5 nm apart to be independently addressed without the requirement for aligned nanometre-sized local gating. We thus demonstrate a scalable method to fully control a double quantum dot device, regardless of its physical size. In the present implementation we present InAs/InP nanowire double quantum dots that display an experimentally detectable spin blockade up to 10 K. We also report and discuss an unexpected re-entrant spin blockade lifting as a function of the magnetic field intensity.

2014 - Tuning electronic transport in cobalt-filled carbon nanotubes using magnetic fields [Articolo su rivista]
Rossella, Francesco; Soldano, Caterina; Onorato, Pasquale; Bellani, Vittorio

Metal-filled and decorated carbon nanotubes represent a class of quasi one-dimensional hybrid systems with enormous potential for applications in nanoelectronics and spintronics. Here we show that is possible to control the electrical conduction in ferromagnetic metal-filled carbon nanotubes by means of external magnetic fields, suggesting specific dimensionality-dependent conduction regimes. By increasing the magnetic field, we drive the charge flow from a positive to a negative magneto-conductance, revealing channel-selective conduction. Furthermore, the zero-field current temperature dependence shows different regimes, suggesting that the inter-shell hopping, assisted by the cobalt clusters, plays a key role in the dimensional crossover. The possibility of engineering and controlling the nature and size of the conducting shells and the filling with magnetic materials can allow the implementation of these systems in tunable hybrid nano-sensors and multifunctional magnetic devices.

2013 - Giant Thermovoltage in Single InAs Nanowire Field-Effect Transistors [Articolo su rivista]
Roddaro, S; Ercolani, D; Safeen, Ma; Suomalainen, S; Rossella, F; Giazotto, F; Sorba, L; Beltram, F

2013 - Giant thermovoltage in single InAs-nanowire FETs [Poster]
Ercolani, Daniele; Roddaro, Stefano; Mian Akif, Safeen; Soile, Suomalainen; Rossella, Francesco; Francesco, Giazotto; Sorba, Lucia; Beltram, Fabio

The quest for solid-state thermoelectric (TE) conversion has been one of the prime driving forces behind semiconductor research before the discovery of the transistor effect. The achievement of efficient TE devices indeed requires a non-trivial trade-off between interdependent material characteristics and can be expressed in terms of the maximization of the figure of merit ZT = S2T/, where S is the Seebeck coefficient,  and  are the electrical and thermal conductivity, T the average operation temperature. Large ZT values, though, have proven elusive over the past decades despite the great design flexibility offered by semiconductor materials1. Here, we present results on thermovoltages in single InAs nanowire (NW) field effect transistors. Thanks to a buried heating scheme (see Fig. 1), we achieved both a large thermal bias, T > 10K and a strong field-effect modulation of electric conduction through nanostructures randomly positioned on oxidized silicon. This experimental arrangement allows a detailed mapping of S(; T) and the comparison with classic models for thermoelectric transport in degenerate semiconductors. The adopted approximations lead to a novel estimate of the electron mobility e~ 11000 cm2/Vs in the NW. Such value is significantly larger than the one obtain by field-effect, e, FE, on the same wire. The discrepancy can be understood in terms of history effects in the NW free charge filling by electric field, as a consequence of the slow surface and trap charge dynamics. On the other hand, the S vs  dependence is largely independent of spurious charge screening and hysteresis. Our results also indicate that special care should be taken in the interpretation of transport results based on field-effects in these nanostructures.

2013 - Quantum Hall effect in monolayer, bilayer and trilayer graphene [Articolo su rivista]
Cobaleda, C.; Diez, E; Amado, M.; Pezzini, S.; Rossella, F.; Bellani, V.; Maude, D. López-RomeroD. K.

We have performed magneto-transport experiments in monolayer, bilayer, trilayer and four layered graphene, at temperatures between 2 and 190 K and magnetic fields up to 28 T. In particular, in monolayer graphene we studied the quantum Hall effect and the metal-insulator transition. On the other hand, in bilayer graphene we observed quantum Hall plateaus at filling factor v = 4, 8, 12, 16, 20, ... and the v = 6 plateau in trilayer graphene, studying their temperature dependence. We have also studied the symmetry properties which are related with different contact configurations describing the method used to study inhomogeneus samples. Finally, four layered graphene we did not found quantum Hall plateaus, but we observed and investigated an ambipolar conduction effect.

2012 - Functionalization of reduced graphite oxide sheets with a zwitterionic surfactant [Articolo su rivista]
Martín-García, B.; Velázquez, M. M.; Rossella, F.; Bellani, V.; Diez, E.; Fierro, J. L. G.; Pérez-Hernández, J. A.; Hernández-Toro, J.; Claramunt, S.; Cirera, A.

Films of a few layers in thickness of reduced graphite oxide (RGO) sheets functionalized by the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDPS) are obtained by using the Langmuir–Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X-ray photoelectron spectroscopy, atomic force microscopy (AFM), field-emission scanning electron microscopy (SEM), micro-Raman spectroscopy, and electrical conductivity measurements. A modified Hummers method is used to obtain highly oxidized graphite oxide (GO) together with a centrifugation-based method to improve the quality of GO. The GO samples are reduced by hydrazine or vitamin C. Functionalization of RGO with the zwitterionic surfactant improves the degrees of reduction and defect repair of the two reducing agents and significantly increases the electrical conductivity of paperlike films compared with those prepared from unfunctionalized RGO.

2012 - High field quantum hall effect in disordered graphene near the dirac point [Capitolo/Saggio]
Escoffier, W.; Poumirol, J. M.; Amado, M.; Rossella, F.; Kumar, A.; Diez, E.; Goiran, M.; Bellani, V.; Raquet, B.

We investigate on the conductance properties of low mobility graphene in the quantum Hall regime at filling factor less than v = 2. For this purpose, we compare the high-field longitudinal and Hall resistances of two graphene samples with different mobility. We show that the presence of “charge density puddles”, most probably due to charged impurities, particularly affect the fundamental high field electronic properties of graphene. In particular, the Hall resistance plateau at R = h/2e2 is unstable and shows a non-monotonic behaviour when the system is driven close to the Dirac point. This phenomenon is ascribed to as Fermi level pinning in the Landau Level sub-bands of graphene, in the presence of disorder.

2012 - Magneto-transport of graphene and quantum phase transitions in the quantum Hall regime [Articolo su rivista]
Amado, M.; Diez, E.; Rossella, F.; Bellani, V.; Lopez-Romero, D.; Maude, D. K.

We studied the magneto-transport in SiO2 substrate-supported monolayer graphene and the quantum phase transitions that characterize the quantum Hall regime, using magnetic fields up to 28T and temperatures down to 4K. The analysis of the temperature dependence of the Hall and longitudinal resistivity reveals new non-universalities of the critical exponents of the plateau-insulator transition. These exponent depends on the type of disorder that governs the electrical transport, which knowledge is important for the design and fabrication of new graphene nano-devices.

2012 - Metal-filled carbon nanotubes as a novel class of photothermal nanomaterials [Articolo su rivista]
Rossella, F.; Soldano, C.; Bellani, V.; Tommasini, M.

Metal-filled carbon nanotubes represent a novel class of photothermal nanomaterials: when illuminated by visible light they exhibit a strong enhancement of the temperature at the metal sites, due to the enhanced plasmonic light absorption at the metal surface, which behaves as a heat radiator. Potential applications include nanomedicine, heat-assisted magnetic recording, and light-activated thermal gradient-driven devices.

2012 - Quantum Hall effect in bilayer and trilayer graphene [Articolo su rivista]
Cobaleda, C.; Rossella, F.; Pezzini, S.; Diez, E.; Bellani, V.; Maude, D. K.; Blake, P.

We have performed magneto-transport experiments in bilayer and trilayer graphene, at temperatures between 2 and 190 K and magnetic fields up to 22 T. Here we study the observation of the quantum Hall effect in bilayer and trilayer graphene. We have observed the quantum Hall plateaus at v = 4, 8, 12, 16, 20 in bilayer graphene and the quantum Hall plateaus v = +/- 6 and studied their temperature dependence. We have also studied the symmetry properties which are related with different contact configurations and describe the method used to study inhomogeneous samples

2012 - Trion confinement and exciton shrinkage in the 2DEG at high magnetic fields [Articolo su rivista]
Bellani, V.; Rossella, F.; Dionigi, F.; Goiran, M.; George, S.; Biasiol, G.; Sorba, L.

We study the photoluminescence from the negatively charged (X−) trions and neutral (X) excitons, in a diluted 2DEG in a magnetic field (B) up to 55 T. At zero B we analyze the evolution of the X− and X emission intensity, tuning it through the optical depletion effect. At non-zero field we find that the emission intensity of the singlet state of the trion and of X are in inverse proposition to B, revealing the effect and the mechanism of the magnetic confinement and of the excitonic shrinkage on the emission intensity.

2011 - Automated detection and characterization of graphene and few-layer graphite via Raman spectroscopy [Articolo su rivista]
Caridad, J. M.; Rossella, F.; Bellani, V.; Grandi, M. S.; Diez, E.

Several processes have to be automated in order to use graphene in future industrial applications. One of these is the detection and characterization of graphene and few-layer graphite (FLG) flakes on a substrate. Raman spectroscopy is an ideal tool for this purpose, as it allows not only the identification of these graphitic materials on arbitrary substrates but also monitoring the quality of flakes within the sample. In this paper, we report how graphene and FLG crystallites can be automatically detected and characterized by monitoring the evolution of Raman bands. We present an algorithm that achieves this purpose and thus has special potential in industrial applications of graphene.

2011 - Charge transport in graphene-polythiophene blends as studied by Kelvin Probe Force Microscopy and transistor characterization [Articolo su rivista]
Liscio, A.; Veronese, G. P.; Treossi, E.; Suriano, F.; Rossella, F.; Bellani, V.; Rizzoli, R.; Samorì, P.; Palermo, V.

Blends of reduced graphene oxide (RGO) and poly(3-hexylthiophene) (P3HT) are used as the active layer of field-effect transistors (FETs). By using sequential deposition of the two components, the density of RGO sheets can be tuned linearly, thereby modulating their contribution to the charge transport in the transistors, and the onset of charge percolation. The surface potential of RGO, P3HT and source–drain contacts is measured on the nanometric scale with Kelvin Probe Force Microscopy (KPFM), and correlated with the macroscopic performance of the FETs. KPFM is also used to monitor the potential decay along the channel in the working FETs.

2011 - Circularly Polarized Resonant Rayleigh Scattering and Skyrmions in the ν = 1 Quantum Hall Ferromagnet [Articolo su rivista]
Bellani, V.; Rossella, F.; Amado, M.; Diez, E.; Kowalik, K.; Biasiol, G.; Sorba, L.

We use the circularly polarized resonant Rayleigh scattering (RRS) to study the quantum Hall ferromagnet at ν = 1. At this filling factor, we observe a right-handed copolarized RRS which probes the Skyrmion spin texture of the electrons in the photoexcited ground states. The resonant scattering is not present in the left-handed copolarization, and this can be related to the correlation between Skymionic effects, screening, and spin wave excitations. These results evidence that RRS is a valid method for the study of the spin texture of the quantum Hall states.

2011 - Optical probing of quantum Hall effect of composite fermions and of the liquid-insulator transition [Articolo su rivista]
Rossella, Francesco; Bellani, Vittorio; Dionigi, Fabio; M., Amado; E., Diez; K., Kowalik; G., Biasiol; L., Sorba

In the photoluminescence spectra of a two-dimensional electron gas in the fractional quantum Hall regime we observe the states at filling factors ν = 4/5, 5/7, 4/11 and 3/8 as clear minima in the intensity or area emission peak. The first three states are described as interacting composite fermions in fractional quantum Hall regime. The minimum in the intensity at ν = 3/8, which is not explained within this picture, can be an evidence of a suppression of the screening of the Coulomb interaction among the effective quasi-particles involved in this intriguing state. The magnetic field energy dispersion at very low temperatures is also discussed. At low field the emission follows a Landau dispersion with a screened magneto-Coulomb contribution. At intermediate fields the hidden symmetry manifests. At high field above ν = 1/3 the electrons correlate into an insulating phase, and the optical emission behaviour at the liquid-insulator transition is coherent with a charge ordering driven by Coulomb correlations.

2011 - Optical probing of the metal-to-insulator transition in a two-dimensional high-mobility electron gas [Articolo su rivista]
Dionigi, Fabio; Rossella, Francesco; Bellani, Vittorio; M., Amado; E., Diez; K., Kowalik; G., Biasiol; L., Sorba

We study the quantum Hall liquid and the metal–insulator transition in a high-mobility two-dimensional electron gas, by means of photoluminescence and magnetotransport measurements. In the integer and fractional regime at nu > 1/3, by analyzing the emission energy dispersion we probe the magneto-Coulomb screening and the hidden symmetry of the electron liquid. In the fractional regime above nu = 1/3, the system undergoes metal-toinsulator transition, and in the insulating phase the dispersion becomes linear with evidence of an increased renormalized mass.

2011 - Quantum Hall effect in inhomogeneous trilayer graphene [Articolo su rivista]
Cobaleda, C.; Rossella, F.; Pezzini, S.; Diez, E.; Bellani, V.; Maude, D.; Blake, P.

We have performed magneto-transport experiments in trilayer graphene, at temperatures between 2 and 190 K and magnetic fields up to 22 T. Here we report the integer quantum Hall effect in trilayer graphene, showing the quantum Hall plateau at ν=6 and studying its temperature dependence. The symmetry properties of the resistance in the quantum Hall regime and at low magnetic fields are also investigated. The measurements obey the reciprocity relation, and reveal new symmetries that relate the resistances measured with different contact configurations.

2010 - Cobalt Nanocluster-Filled Carbon Nanotube Arrays: Engineered Photonic Bandgap and Optical Reflectivity [Articolo su rivista]
Soldano, C.; Rossella, F.; Bellani, V.; Giudicatti, S.; Kar, S.

Perfect vertically aligned and periodically arranged arrays of multidielectric heterostructures are ideal platforms both for photonic crystals and photonic bandgap materials. Carbon nanotubes grown inside anodic alumina templates form a novel class of heterostructured materials ideally suited for building such platforms. By engineering metallic (cobalt) nanoclusters inside the nanotubes, we present a novel method for tailoring the photonic bandgap as well as the magnitude of the reflectivity in these systems. We present spectroscopic ellipsometry (SE) and reflectivity measurements to investigate the effect of the presence of cobalt clusters on the optical response of multiwall carbon nanotubes (MWNT) grown in anodized alumina template. The real (ε1) and imaginary (ε2) part of the pseudodielectric function of the MWNT and Co-MWNT system have been studied in a wide energy range (1.4−5 eV). We found that the cobalt filling modifies the electronic structure of the nanotubes, suggesting that the insertion of the clusters leads to a semiconductor behavior. Angle-resolved reflectivity measurements further show that the metal filling drastically enhances the optical response up to 2 orders of magnitude.

2010 - Effects of particle contamination and substrate interaction on the Raman response of unintentionally doped graphene [Articolo su rivista]
Caridad, J. M.; Rossella, F.; Bellani, V.; Maicas, M.; Patrini, M.; Diez, E.

We investigated the inhomogeneities in the charge density of unintentionally doped graphene on SiO2 prepared by mechanical exfoliation. From the analysis of the G, D, and 2D phonon modes of the Raman spectra after displacing contaminants on graphene surface, and measuring the separation monolayer-substrate distance among zones with different doping levels, we deduce that the interaction with the substrate is the main cause of doping in graphene rather than particle contamination. In particular, we show how graphene doping levels vary within the same flake depending on the distance between graphene and the substrate.

2010 - Micro-Raman study of the role of sterilization on carbon nanotubes for biomedical applications [Articolo su rivista]
Belluucci, Stefano; Chiaretti, Massimo; Rossella, Francesco; Grandi Marco, Simone; Galinetto, Pietro; Sacco, Immacolata; Micciula, Federico

2010 - Optical detection of quantum Hall effect of composite fermions and evidence of the ν=3/8 state [Articolo su rivista]
Bellani, V.; Dionigi, F.; Rossella, F.; Amado, M.; Diez, E.; Biasiol, G.; Sorba, L.

In the photoluminescence spectra of a two-dimensional electron gas in the fractional quantum Hall regime we observe for the first time the states at filling factor ν=4/5, 5/7, 4/11, and 3/8 as clear minima in the emission peak intensity or area. The first three states are described as interacting composite fermions in fractional quantum Hall regime. The minimum in the intensity at ν=3/8, which is not explained within this picture, can be an evidence of a suppression of the screening of the coulomb interaction among the effective quasiparticles involved in this intriguing state.

2010 - Plateau–insulator transition in graphene [Articolo su rivista]
Amado, M.; Diez, E.; López-Romero, D.; Rossella, F.; Caridad, J. M.; Dionigi, F.; Bellani, V.; Maude, D. K.

We investigate the quantum Hall effect (QHE) in a graphene sample with Hall-bar geometry close to the Dirac point at high magnetic fields up to 28 T. We have discovered a plateau–insulator quantum phase transition passing from the last plateau for the integer QHE in graphene to an insulator regime ν=−2→ν=0. The analysis of the temperature dependence of the longitudinal resistance gives a value for the critical exponent associated with the transition equal to κ=0.58±0.03.

2010 - Role of oxygen content on the magnetic properties of epitaxial anatase and rutile TiO2 thin films [Articolo su rivista]
Galinetto, Pietro; Mozzati Maria, Cristina; Vercesi, Cristiano; Malavasi, Lorenzo; Diaz Fernandez Yuri, A; Rossella, Francesco; Drera, Giovanni; Sangaletti, Luigi

X-Ray diffraction, micro-Raman and static magnetization data at room temperature from epitaxial undoped anatase and rutile TiO(2) thin films are reported. Films are deposited by RF-magnetron sputtering on different substrates and at different deposition parameters. A weak ferromagnetic behaviour at room temperature is observed in each film series, often superimposed to a linear contribution. The different dependence of the magnetization on the oxygen content observed for the different TiO(2) phases is discussed in relation to the tensile strain induced by the different substrates. A complex interplay between oxygen sub-stoichiometry and the interface structural arrangement should be invoked to account for the observed magnetic behaviours.

2010 - TiO2 thin films for spintronics application: a Raman study [Articolo su rivista]
Rossella, Francesco; Galinetto, Pietro; Mozzati, MARIA CRISTINA; Malavasi, Lorenzo; DIAZ FERNANDEZ, YURI ANTONIO; Drera, Giovanni; Sangaletti, Luigi

We present the results of a systematic study performed by micro-Raman spectroscopy on pure anatase, pure rutile and mixed anatase-rutile TiO2 thin films, deposited by radio frequency magnetron sputtering on quartz substrates, with different thicknesses. The crystal structures of the as-deposited films were unambiguously determined and a good crystalline homogeneity was revealed by a systematic mapping of the samples. In the mixed-phase films, the relative amount of the two phases was monitored by a simple analysis of the components of the multi-Lorentzian fitting curves. For the single-phase films, the influence of the thickness and the effect of different thermal treatments, carried out to obtain series of thin films differing only for oxygen content, are discussed. The analysis of the scattered light has provided indication about the presence of an interface layer between the substrate and the film, which can play a role in driving the interesting magnetic properties exhibited by our samples, which are of potential usefulness for spintronics application. The results obtained from other techniques are briefly reported and discussed in relation to our systematic Raman characterization. This study points out how Raman investigation can provide suggestions toward the understanding of the complex physical phenomena leading to room-temperature ferromagnetism in TiO2 thin films.

2008 - Impurity Effects And Charge Transport In ABO3 Single Crystal Oxides: Photorefractivity-Oriented And Basic Studies [Articolo su rivista]
Rossella, F; Galinetto, P; Trepakov, V; Badalyan, Ag; Samoggia, G

2008 - Luminescence and Absorption Spectroscopy of Weakly Mn-Doped SrTiO3 Crystals [Articolo su rivista]
Potucek, Z; Trepakov, V; Deyneka, A; Bryknar, Z; Galinetto, P; Rossella, F

2007 - Influence of Cu ions on the photo-transport properties in KTaO3:(Cu,V) single crystals [Articolo su rivista]
Rossella, F; Perucchini, L; Galinetto, P; Samoggia, G; Mozzati, Mc; Azzoni, Cb; Badalyan, Ag; Trepakov, Va; Syrnikov, Pp

2007 - Photoconductive and Electro-optical properties of Hf doped lithium niobate crystals [Articolo su rivista]
Rossella, F; Grando, D; Galinetto, P; Degiorgio, V; Kokanyan, E

2007 - Photoconductivity and the structural phase transition in SrTiO3 [Articolo su rivista]
Rossella, F.; P, Galinetto; Samoggia, G.; Trepakov, V.; F, Rossella

2007 - Structural and optical properties of hafnium-doped lithium-niobate crystals [Articolo su rivista]
Galinetto, P; Rossella, F; Cristiani, I; Minzioni, P; Degiorgio, V; Kokanyan, Ep

2006 - EPR and optical absorption investigations of photochromic effect in nearly stoichiometric LiNbO3:Fe [Relazione in Atti di Convegno]
Mozzati Maria, Cristina; Badalyan, Andrei; Galinetto, Pietro; Rossella, Francesco; Azzoni Carlo, Bruno; Samoggia, Giorgio; Demaestri, G.; Bermudez, V.

EPR and optical absorption measurements have been performed on nearly stoichiometric lithium niobate single crystals with iron in traces and in 0.1% molar nominal amount. Thermal treatments in reducing atmosphere allowed to obtain samples with different Fe2+/Fe3+ ratios. The Fe2+/Fe3+ dynamic has been observed by irradiating the samples with UV and visible light in the temperature range 4-300 K: the photochromic effect resulted to be more evident in the strongly reduced crystals. A simple model to explain the photochromic behavior in nearly stoichiometric LiNbO3:Fe is proposed.

2006 - MicroRaman and photorefractivity study of hafnium-doped lithium niobate crystals [Articolo su rivista]
Galinetto, Pietro; Rossella, Francesco; Minzioni, Paolo; Razzari, Luca; Cristiani, Ilaria; Degiorgio, Vittorio; Kokanyan, Edvard

2006 - Structural phase transition and photo-charge carrier transport in SrTiO3 [Articolo su rivista]
Galinetto, P.; Rossella, F.; Samoggia, G.; Trepakov, V.; Kotomin, E.; Heifets, E.; Markovin, P.; Jastrabik, L.

The effect of the antiferrodistorsive phase transition observed at similar to 105 K on the photo-carriers transport and band structure of nominally pure SrTiO3 single crystals was investigated. Simultaneously the ab-initio analysis based on the DFT-HF hybrid approach was performed in order to elucidate the atomic and electronic structure changes. It was found that the structural transition play a minor role in photo-charge transport. However, in highly resistive specimens at low UV-light intensities or under visible-light, 514 nm excitation, weak photoconductivity features are observed in the transition region. Besides, in this region the temperature maximum was found for the photoconductivity spectra. Experimental results are discussed and agree well with theoretical predictions.

2006 - UV light-induced IR absorption and photoconductivity in KTa1-xNbxO3 [Articolo su rivista]
Trepakov, V. A.; Gubaev, A. I.; Kapphan, S. E.; Galinetto, P.; Rossella, F.; Boatner, L. A.; Syrnikov, P. P.; Jastrabik, L.

Observation and first studies of the UV light-induced optical absorption for KTa1-xNbxO3 single crystals (x = 0.004, 0.007, 0.012, 0.07) are reported. Under illumination (300-400 nm, 3.10-4.13 eV) at low temperatures a broad light-induced absorption band was found in NIR region, peaking at 5600 cm(-1) (1.79 mu, 0.69 eV T = 1.3 K)for x = 0.007. No light-induced absorption was found at room temperature in all Nb containing samples as well as in nominally pure KTaO3. Results of detailed studies, including photoconductivity and photoluminescence experiments, are associated with the localization of the charge carriers in the form of electronic polarons at low temperatures.

2005 - Characterization of structural and photoinduced defects in pure and doped lithium niobate [Articolo su rivista]
Mozzati, M. C.; Rossella, F.; Galinetto, P.; Bermudez, V.; Azzoni, C. B.; Samoggia, G.

In this work we report on the characterization of structural and defect-related properties of a series of lithium niobate single crystals with congruent and stoichiometric composition containing Fe and Mg dopant. In particular we investigate the role of UV and visible irradiation in the charge trapping mechanisms which control the photorefractive (PR) behaviour. Complementary techniques (Raman microscopy, Electron Paramagnetic Resonance, absorption measurements) were used to characterize the degree of crystal imperfections. This is the basic starting point for an exhaustive comprehension of PR behaviour, monitored observing photovoltaic current and photoconductivity.

2005 - Ferroelectricity in K0.992Rb0.008Ta0.96Nb0.04O3 [Articolo su rivista]
Giulotto, ENRICO VIRGILIO; Stasi, V.; Galinetto, Pietro; Rossella, Francesco; Samoggia, Giorgio; Trepakov, V.; Syrnikov, P.; Jastrabik, L.

The paper reports on the first crystal growth and Raman scattering studies in K0.992Rb0.008Ta0.96Nb0.04O3 (KRTN) and KTaO3:Rb (3% mol. in the melt), in which Rb+ ions (ionic radius 1.47 angstrom) substitute K+(1.33 angstrom). The experiments were performed in the range 10-300 K and focused on the behavior of TO1 and TO4 modes. The results confirm indications by previous dielectric permittivity data about the ferroelectric nature of the phase transition in KRTN. It appears that a small amount of Rb changes the character of the phase transition induced by Nb from mainly displacive to mainly order-disorder, whereas no evidence of ferroelectricity is revealed in the KTaO3:Rb sample.

2005 - Photoinduced charge transport in KTaO3:Be [Articolo su rivista]
Galinetto, P.; Rossella, F.; Giulotto, E.; Samoggia, G.; Trepakov, V.; Jastrabik, L.; Syrnkov, P.; Kapphan, S.

We present the first growth of KTaO3:Be (6% at. in the batch) single crystals, together with photoconductivity and thermally stimulated currents studies of this material. It is probably that very small Be2+ ions (ionic radius 0.35 Å ) mostly substitute K+(1.64 Å) and bounded and/or free electronic states formation including polaronic states. As a results Be-doping effect can strongly influence phase instability and electronic properties of KTaO3. It was found that Be2+ doping leads to appreciable photocurrents already at room temperature under irradiation of the samples with photons even at energies just below the fundamental optical gap. Photoconductivity increases strongly at low temperature. Intense thermally stimulated current peaks between 15 and 100 K are probably due to the presence of shallow hole traps and polaronic centres. The obtained results are compared to past observations in pure KTaO3 and KTaO3:Li

2004 - Relationship between photorefractive activity and Raman scattering in lithium niobate crystals [Articolo su rivista]
Giulotto, Enrico; De Contardi, Riccardo; Rossella, Francesco; Bermudez,

The paper reports on Raman scattering experiments in pure, Mg-doped and Fe-doped single crystals of lithium niobate. The measurements were carried out in 90degrees geometry at several laser beam intensities, and in several polarization arrangements. The possible occurrence of light-induced polarization-anisotropic scattering, and its effects on Raman spectra, were checked by simultaneously measuring the time dependence of the laser beam intensity transmitted by the sample with unchanged polarization, and the time dependence of the intensity of Raman lines from vibrational modes of known symmetry. The results give no indication of any effect of photorefractive activity on Raman observations in our pure and Mg-doped samples. On the contrary, in Fe-doped lithium niobate the build-up of light-induced o-e scattering strongly affected the relative intensities of Raman lines in specific experimental arrangements. We discuss the possible application of these effects to the characterization of lithium-niobate-based materials with strong photovoltaic and photorefractive activity.