R. Lorenzi

Università degli Studi di Milano-Bicocca, Milano, Lombardy, Italy

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Publications (32)75.04 Total impact

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    ABSTRACT: We present a novel strategy for the synthesis of pure and Eu-doped γ-Ga2O3 nanoparticles (NP) with organic in situ capping resulting from a non-aqueous solution-based benzyl alcohol route. Photoluminescence spectroscopy points out concomitant benzoate-related and γ-Ga2O3 exciton-like Eu3+ excitation in the UV, and blue emission – superimposed to γ-Ga2O3 donor-acceptor recombination – ascribable to organic moieties different from benzoate.
    J. Mater. Chem. C. 10/2014;
  • 8 th Quantum Dots, Conference, Pisa, Italy, Pisa, Italy; 05/2014
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    ABSTRACT: Modification of the optical and dielectric response upon nanocrystallization represents a tool for functionalizing glass-based materials and an alternative approach for data storage in optical media. Here we report results for Ga-containing alkali-germanosilicate glass undergoing thermally activated secondary phase separation and crystallization of native nanoheterogeneities with formation of dense dispersions (105105 μμm−3) of γγ-Ga2O3 nanocrystals in transparent glass ceramics. The shift of the Urbach tail in the UV absorption and the refractive index increase upon nanocrystallization are analyzed following Duffy’s approach and the Maxwell–Garnett description of refractive index in composite systems. The results clarify that native nanoheterogeneities consist of Ge-containing Ga-oxide and, consequently, composition changes occur between matrix and nanophase during nanocrystallization. Complex impedance data are analyzed to disentangle the electronic and ionic contributions to the dielectric response, pointing to a lowering of ionic polarizability parallel to the electronic polarizability increase. The analysis, aided by 7171Ga NMR data, gives an insight into the relation between coordination changes and refractive and dielectric response, highlighting the role of non-bridging-oxygens and interphase.
    Acta Materialia 05/2014; 70:19–29. · 3.94 Impact Factor
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    ABSTRACT: Wide-bandgap nanocrystals are an inexhaustible source of tuneable functions potentially addressing most of the demand for new light emitting systems. However, the implementation of nanocrystal properties in real devices is not straightforward if a robust and stable optical component is required as a final result. The achievement of efficient light emission from dense dispersions of Ga-oxide nanocrystals in UV-grade glass can be a breakthrough in this regard. Such a result would permit the fabrication of low cost UV-to-visible converters for monitoring UV-emitting events on a large-scale - from invisible hydrogen flames to corona dispersions. From this perspective, γ-Ga2O3 nanocrystals are developed by phase separation in Ga-alkali-germanosilicate glasses, obtaining optical materials based on a UV transparent matrix. Band-to-band UV-excitation of light emission from donor-acceptor pair (DAP) recombination is investigated for the first time in embedded γ-Ga2O3. The analysis of the decay kinetics gives unprecedented evidence that nanosized confinement of DAP recombination can force a nanophase to the efficient response of exactly balanced DAPs. The results, including a proof of concept of UV-to-visible viewer, definitely demonstrate the feasibility of workable glass-based fully inorganic nanostructured materials with emission properties borrowed from Ga2O3 single-crystals and tailored by the nanocrystal size.
    Nanoscale 12/2013; · 6.73 Impact Factor
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    ABSTRACT: Herein we describe how UV excitation of localized electronic states in phosphate glasses can activate structural rearrangements that influence the kinetics of Au nanoparticle (NP) thermal growth in Au-doped glass. The results suggest a novel strategy to address the problem of controlling nano-assembly processes of metal NP patterns in fully inorganic and chemically stable hard materials, such as laser-quality glasses. We show that the mechanism is promoted by opening and subsequent cross-linkage of phosphate chains under UV excitation of non-bridging groups in the amorphous network of the glass, with a consequent modification of Au diffusion and metal NP growth. Importantly, the micro-Raman mapping of the UV-induced modifications demonstrates that the process is restricted within the beam waist region of the focused UV laser beam. This fact is consistent with the need for more than one excitation event, close in time and in space, in order to promote structural cross-linkage and Au diffusion confinement. The stability of the photo-induced modifications makes it possible to design new metal patterning approaches for the fabrication of three-dimensional metal structures in laser-quality materials for high-power nonlinear applications.
    Nanotechnology 04/2013; 24(22):225302. · 3.84 Impact Factor
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    ABSTRACT: In this Letter, we show functionalization of NiO-doped 7.5Li<sub>2</sub>O·2.5Na<sub>2</sub>O·20Ga<sub>2</sub>O<sub>3</sub>·35SiO<sub>2</sub>·35GeO<sub>2</sub> glass by space-selective nanocrystallization via exposure to the focused beam of a pulsed copper vapor laser (510.6 and 578.2 nm) at temperature close to the glass transition point (570°C). Irradiated areas drastically change their color, caused by electronic transitions of Ni<sup>2+</sup> dopant ions, without any alteration of the optical quality. Importantly, irradiated regions acquire broadband infrared luminescence (centered at about 1400 nm and possessing 400 nm effective bandwidth) typical of Ni<sup>2+</sup> ions in crystalline environment, and by positive change of refractive index (more than 10<sup>-3</sup>). Spectroscopic and diffractometric data of the irradiated regions indeed resemble those previously observed in thermally nanocrystallized glass, with Ni<sup>2+</sup> ions embedded in γ-Ga<sub>2</sub>O<sub>3</sub> nanocrystals. The results demonstrate the possibility of laser writing nanocrystallized multifunction patterns in germanosilicate glasses for the fabrication of active integrated devices.
    Optics Letters 02/2013; 38(4):492-4. · 3.39 Impact Factor
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    ABSTRACT: Nanoparticles in amorphous oxides are a powerful tool for embedding a wide range of functions in optical glasses, which are still the best solutions in several applications in the ever growing field of photonics. However, the control of the nanoparticle size inside the host material is often a challenging task, even more challenging when detrimental effects on light transmittance have to be avoided. Here we show how the process of phase separation and subsequent nanocrystallization of a Ga-oxide phase can be controlled in germanosilicates - prototypal systems in optical telecommunications - starting from a Ga-modified glass composition designed to favour uniform liquid-liquid phase separation in the melt. Small angle neutron scattering data demonstrate that nanosized structuring occurs in the amorphous as-quenched glass and gives rise to initially smaller nanoparticles, by heating, as in a secondary phase separation. By further heating, the nanophase evolves with an increase of nanoparticle gyration radius, from a few nm to a saturation value of about 10 nm, through an initial growing process followed by an Ostwald ripening mechanism. Nanoparticles finally crystallize, as indicated by transmission electron microscopy and X-ray diffraction, as γ-Ga(2)O(3) - a metastable gallium oxide polymorph. Infrared reflectance and photoluminescence, together with the optical absorption of Ni ions used as a probe, give an indication of the underlying interrelated processes of the structural change in the glass and in the segregated phase. As a result, our data give for the first time a rationale for designing Ga-modified germanosilicates at the nanoscale, with the perspective of a detailed nanostructuring control.
    Nanoscale 11/2012; · 6.73 Impact Factor
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    ABSTRACT: A spectroscopic protocol is proposed to implement confocal microfluorescence imaging to the analysis of microinhomogeneity in the nanocrystallization of the core of fibers belonging to a new kind of broadband fiber amplifier based on glass with embedded nanocrystals. Nanocrystallization, crucial for achieving an adequate light emission efficiency of transition metal ions in these materials, has to be as homogeneous as possible in the fiber to assure optical amplification. This requirement calls for a sensitive method for monitoring nanostructuring in oxide glasses. Here we show that mapping microfluorescence excited at 633 nm by a He-Ne laser may give a useful tool in this regard, thanks to quasi-resonant excitation of coordination defects typical of germanosilicate materials, such as nonbridging oxygens and charged Ge-O-Ge sites, whose fluorescence are shown to undergo spectral modifications when nanocrystals form into the glass. The method has been positively checked on prototypes of optical fibers--preventively characterized by means of scanning electron microscopy and energy dispersive spectroscopy--fabricated from preforms of Ni-doped Li₂O-Na₂O-Sb₂O₃-Ga₂O₃-GeO₂-SiO₂ glass in silica cladding and subjected to heat treatment to activate gallium oxide nanocrystal growth. The method indeed enables not only the mapping of the crystallization degree but also the identification of drawing-induced defects in the fiber cladding.
    Microscopy and Microanalysis 04/2012; 18(2):259-65. · 2.50 Impact Factor
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    ABSTRACT: The design of a process to create yttrium aluminosilicate microspheres with a core–shell structure is of interest in the field of cancer brachytherapy. Glass microspheres with yttrium-depleted shell may indeed reduce the risk of 90Y release into the organism. Here we show – by means of confocal micro-Raman scattering, microfluorescence, X-ray-fluorescence analysis, and IR spectroscopy – that yttrium depletion may be achieved by etching in HCl solution (pH 2) at a rate of 1 μm day−1 in bulk glass and 3 μm day−1 in glass microsphere (35 μm of diameter). Importantly, the spectroscopic results – collected in confocal configuration along the processed layer – indicate a high degree of structural reconstruction of the glass network, with the formation of an interconnected silicate-rich glass that surrounds a core of unmodified yttrium aluminosilicate. We also demonstrate that the process is driven by non-bridging oxygen sites, which regulate the hydroxylation and structural reconstruction of the glass within the Y-depleted layer. The analysis gives also some insight into open fundamental questions about the short-range structure and the chemical stability of this kind of glass, which is also important in photonics and nuclear waste disposal.
    Materials Chemistry and Physics. 03/2012; 133(1):24–28.
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    ABSTRACT: The target of taking advantage of the near-infrared light-emission properties of nickel ions in crystals for the design of novel broadband optical amplifiers requires the identification of suitable nanostructured glasses able to embed Ni-doped nanocrystals and to preserve the workability of a glass. Here we show that Ni doping of Li(2)O-Na(2)O-Ga(2)O(3)-GeO(2)-SiO(2) glass (with composition 7.5:2.5:20:35:35 and melting temperature 1480 °C, sensibly lower than in Ge-free silicates) enables the selective embedding of nickel ions in thermally grown nanocrystals of spinel-like gallium oxide. The analysis of transmission electron microscopy and x-ray diffraction data as a function of Ni-content (from 0.01 to 1 mol%) indicates that Ni ions promote the nanophase crystallization without affecting nanoparticle size (~6 nm) and concentration (~4 × 10(18) cm(-3)). Importantly, as shown by optical absorption spectra, all nickel ions enter into the nanophase, with a number of ions per nanocrystal that depends on the nanocrystal concentration and ranges from 1 to 10(2). Photoluminescence data indicate that fast non-radiative decay processes become relevant only at mean ion-ion distances shorter than 1.4 nm, which enables the incorporation of a few Ni ions per nanoparticle without too large a worsening of the light-emission efficiency. Indeed, at 0.1 mol% nickel, the room temperature quantum yield is 9%, with an effective bandwidth of 320 nm.
    Nanotechnology 01/2012; 23(1):015708. · 3.84 Impact Factor
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    ABSTRACT: Fluorinated silica glasses with improved vacuum ultraviolet (VUV) transmittance have been synthesized following a sol–gel route starting from fluorinated alkoxide precursors. Matrix dehydration and fluorination have been investigated by Raman, IR, and refractive index measurements. The results show that, during the densification step, the residual hydroxyl and fluorine content can be controlled by governing the kinetic equilibrium between matrix collapse and fluorine evolution. This highlights the pivotal role of in situ reactions of fluorine-containing molecular precursors in the networking mechanism of a sol–gel material after thermal activation. Superior VUV transmittance and radiation hardness with respect to pure commercial silica and undoped sol–gel silica have been confirmed by synchrotron light absorption measurements before and after X-ray exposure. These properties make the material very attractive for applications in VUV photolithography technologies and optical fibers, especially for use in environments with a high level of ionizing radiation, such as spacecrafts and nuclear power plants.
    Chemistry of Materials. 01/2012; 24(4):677-681.
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    ABSTRACT: The development of integrated photonics and lab-on-a-chip platforms for environmental and biomedical diagnostics demands ultraviolet electroluminescent materials with high mechanical, chemical and environmental stability and almost complete compatibility with existing silicon technology. Here we report the realization of fully inorganic ultraviolet light-emitting diodes emitting at 390 nm with a maximum external quantum efficiency of ~0.3%, based on SnO(2) nanoparticles embedded in SiO(2) thin films obtained from a solution-processed method. The fabrication involves a single deposition step onto a silicon wafer followed by a thermal treatment in a controlled atmosphere. The fully inorganic architecture ensures superior mechanical robustness and optimal chemical stability in organic solvents and aqueous solutions. The versatility of the fabrication process broadens the possibility of optimizing this strategy and extending it to other nanostructured systems for designed applications, such as active components of wearable health monitors or biomedical devices.
    Nature Communications 01/2012; 3:690. · 10.74 Impact Factor
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    Roberto Lorenzi, Yongmin Jung, Gilberto Brambilla
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    ABSTRACT: We present the absorption spectra collected with an evanescent-field absorption sensor. The device comprises a fluidic channel with an embedded fiber coil resonator. Deviations from Beer-Lambert law will be discussed in terms of adsorption mechanism.
  • Roberto Lorenzi, Yongmin Jung, Gilberto Brambilla
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    ABSTRACT: We fabricated and tested an evanescent-wave absorption sensor consisting of an optical microfiber coil resonator embedded in fluidic channel walls. Low concentrations of flowing analyte show optical losses in agreement with a modified Beer–Lambert law. Higher concentration causes a limit value of the measured optical losses arising from adsorption mechanisms.
    Applied Physics Letters 04/2011; 98(17):173504-173504-3. · 3.52 Impact Factor
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    ABSTRACT: We present the results of the analysis of the ultraviolet (UV) absorption edge of fluorine-modified sol-gel silica. UV transmission data, obtained by means of synchrotron radiation, have been analyzed in the spectral range 7.5–8.5eV, with a spectral resolution of about 10meV. Data on silica samples with different F content (from 0 to few 10−1mol%) have been analyzed and compared with literature data on quartz and pure synthetic commercial silica. The analysis allows us to discriminate between the effects of the fluorine addition and those ascribable to structural peculiarities of the sol-gel networking. The estimated Urbach energy EU(T=0) ranges between 45 and 55meV, higher that in crystalline quartz and lower than in commercial synthetic silica. The study of the temperature dependence of EU(T) shows that the fluorine modification of the silica network causes the lowering of the static disorder and the widening of the energy gap. However, there is also a relevant effect of the production process, since sol-gel silica samples show lower EU values with respect to other types of silica, quite independently of the fluorine content. The analysis of the Raman spectra however shows that the starting amount of fluorine-modified molecular precursor influences the network condensation process, independently of the final fluorine content into the matrix.
    Journal of Non-Crystalline Solids 01/2011; 357(8-9):1838-1841. · 1.72 Impact Factor
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    ABSTRACT: Silica thin films with embedded SnO2 nanoparticles have been grown on transparent substrates by the sol–gel method. Tin dioxide crystals with cassiterite structure are semiconductors with a wide band gap of ~ 3.6 eV. Optical absorption spectroscopy in the near ultraviolet–visible range has been exploited to probe nanostructuring features of such nanocrystals. The results show that the sintering conditions modify crystallite mean size and enable the occurrence of quantum confinement effects. The outcome is in accordance with transmission electron microscopy data conducted on analogous bulk samples.Research Highlights► Modified solgel synthesis of nanostructured SnO2:SiO2 film on transparent substrate. ► Quantum confinement effects on energy band gap inferred from UV-vis spectroscopy. ► Control of nanostructure morphology by thermal treatment atmosphere.
    Journal of Non-Crystalline Solids 01/2011; 357:1888-1891. · 1.72 Impact Factor
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    ABSTRACT: We have investigated the structural modifications that occur concomitantly with the generation of optical anisotropy in the binary glass with nominal starting composition 0.51(CaO)0.49(P2O5) as a result of hot-extrusion processes. Micro-Raman and X-ray-fluorescence micro-analysis have been used to map the structure and composition along the directions parallel and orthogonal to the extrusion axis, with respect to which we have detected birefringence and refractive index modifications up to 10−3. The results show an annular region, midway between the axis and the outer side, in which Ca is slightly depleted and the phosphate chains are shortened and packed through crosslinking sites in anisotropic arrangement.
    Materials Chemistry and Physics - MATER CHEM PHYS. 01/2011; 128(1):12-15.
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    ABSTRACT: Optical absorption and luminescence spectra of yellow corundum have been analyzed, both in magmatic and metamorphic materials, looking at the role of localized electronic transitions of transition metal ions at substituted Al sites. By the aid of energy dispersed x-ray fluorescence (EDXRF) elemental analysis and electron paramagnetic resonance (EPR) measurements, the results confirm that Fe3+ is the dominant impurity ion. However, the results also evidence that Cr3+ and Ti3+ contribute in determining the optical absorption and emission properties of this variety of gem-quality corundum, as we have recently found in Type 1 blue sapphires of metamorphic origin. Furthermore, preferentially but not exclusively in samples of metamorphic origin, Mn plays a role never evaluated up to now. Here we show how few ppm of Mn – below the detection limit of EDXRF, not revealed in the optical absorption spectra, but barely detected by EPR spectroscopy as Mn2+ ion – are active in photoluminescence, showing up with the characteristic emissions of Mn2+, Mn3+ and Mn4+.
    IOP Conference Series Materials Science and Engineering 11/2010; 15(1):012086.
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    ABSTRACT: In the blue colored sapphires of metamorphic origin and Verneuil synthetic studied here, the absorption-emission properties in the VIS-NIR range are largely determined by Cr3+ and Ti3+, as we have been able to demonstrate recently. In that work a sharp radio-luminescence band occurring at 870 nm in Verneuil blue sapphires was left unattributed: here we give evidence for the attribution of that band to the 2E emission transition of V2+, and for the existence of such an emission also in natural samples of metamorphic origin. After such a result, we accurately evaluated by EDXRF the V concentrations in various samples and found the ion more ubiquitous than foreseen. We then searched for and found, weak but diagnostic spin forbidden transitions and phonon sequencies in the absorption spectra of samples sufficiently rich in V. The experimental results just mentioned allowed us to discuss the effects of the overlap of V3+ and Cr3+ spin-allowed absorption bands on the spectrum of the varieties of corundum under study. To complete the updating of the interpretation, we spent a further effort to strengthen the attribution of the absorption band at 14500 cm−1 (currently interpreted as an IVCT (Fe2+ → Fe3+)) to the 5E→5T2 transitions of Cr2+.
    IOP Conference Series Materials Science and Engineering 11/2010; 15(1):012087.
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    ABSTRACT: The design of nanostructured materials with specific physical properties is generally pursued by tuning nanoparticle size, concentration, or surface passivation. An important step forward is to realize “active” systems where nanoparticles are vehicles for controlling, in situ, some specific, tuneable features of a responsive functional material. In this perspective, this work focuses on the rational design of a nanostructured glass with electrically tuneable dielectric function obtained by injection and accumulation of charge on embedded conductive nanocrystals. This enables electrically controlled switching of semiconducting nanophases to charged polarisable states to be achieved, which could lead to smart, field-enhancement applications in nanophotonics and plasmonics. Here, it is shown that such response switching can be obtained if a percolating charge-transport mechanism is activated through a disordered tree-like network, as is demonstrated to be possible in SiO2 films where suitable dispersions of SnO2 nanocrystals, with conductive interfaces, are obtained as a result of a new synthesis strategy.
    Advanced Functional Materials 10/2010; 20(20):3511-3518. · 10.44 Impact Factor

Publication Stats

45 Citations
75.04 Total Impact Points


  • 2010–2014
    • Università degli Studi di Milano-Bicocca
      • Department of Materials Science
      Milano, Lombardy, Italy
  • 2012–2013
    • Mendeleev Russian University of Chemical Technology
      • International Laboratory of Functional Glass-Based Materials
      Moscow, Moscow, Russia
  • 2007
    • Consorzio Nazionale Interuniversitario per le Scienze del Mare
      Roma, Latium, Italy