Roberto Lorenzi

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

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Publications (37)99.75 Total impact

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    ABSTRACT: The development of cheap and reliable time–temperature indicators (TTIs) can greatly improve customer satisfaction with respect to the correct handling and storage of perishable goods. A series of new irreversible thermochromic squaraine dyes enables the preparation of simple TTIs whose time–temperature regime can be controlled by selection of the appropriate molecule/substrate combinations.
    Advanced Optical Materials 06/2015; DOI:10.1002/adom.201500073 · 4.06 Impact Factor
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    ABSTRACT: Light emission of nanocrystals (NCs) can depend not only on NC size but also – and even more importantly in wide-band-gap NCs – on the occurrence of optically active sites, such as donor-acceptor pairs (DAPs). Here, we demonstrate that controlling the thermo-chemical conditions of NC nucleation when NCs are forming in a solid host – an approach often used for driving NC size dispersion – can be an innovative strategy for tailoring DAP population. Our data show that light emission from DAP recombination and decay in defect sites can be controlled in γ-Ga2O3 NCs in alkali-germanosilicate glass – a prototypal oxide-in-oxide nanostructured system – by oxygen and gallium vacancy formation during nucleation. Time-resolved UV-excited photoluminescence, combined with differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy, reveal how nucleation pretreatment determines, besides NC size and concentration, also DAP number via promotion of acceptor formation or their passivation during interaction with the host. The results envisage the possibility of nucleation-based design of light-emitting NCs in a wide range of oxide systems.
    03/2015; 3(17). DOI:10.1039/C4TC02837F
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    ABSTRACT: Phase transformations at the nanoscale represent a challenging field of research, mainly in the case of nanocrystals (NCs) in a solid host, with size-effects and interactions with the matrix. Here we report the study of the structural evolution of γ-Ga2O3 NCs in alkali-germanosilicate glass - a technologically relevant system for its light emission and UV-to-visible conversion - showing an evolution drastically different from the expected transformation of γ-Ga2O3 into β-Ga2O3. Differential scanning calorimetry registers an irreversible endothermic process at ∼1300 K, well above the exothermic peak of γ-Ga2O3 nano-crystallization (∼960 K) and below the melting temperature (∼1620 K). Transmission electron microscopy and X-ray diffraction data clarify that glass-embedded γ-Ga2O3 NCs transform into LiGa5O8via diffusion-driven kinetics of Li incorporation into NCs. At the endothermic peak, β-Ga2O3 forms from LiGa5O8 dissociation, following a nucleation-limited kinetics promoted by size-dependent order-disorder change between LiGa5O8 polymorphs. As a result of the changes, modifications of UV-excited NC light emission are registered, with potential interest for applications.
    Physical Chemistry Chemical Physics 01/2015; 17(7). DOI:10.1039/c4cp05485g · 4.20 Impact Factor
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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.
    10/2014; 3(1). DOI:10.1039/C4TC02118E
  • 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. DOI:10.1016/j.actamat.2014.02.022 · 4.47 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(3). DOI:10.1039/c3nr05210a · 6.74 Impact Factor
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    ABSTRACT: It is shown that a copper vapor laser can be used effectively for local crystallization of glasses with precipitation of active phases on a surface or in the interior volume of glass depending on its composition. Aprocedure is proposed for using a laser beam to form nanostructural waveguides with broadband near-IR luminescence peaking at 1300 – 1450 nm in glass with composition (mol.%) 7.5 Li2O, 2.5 Na2O, 20 Ga2O3, 35 GeO3 and 35 SiO2 doped with 0.1 – 0.5% NiO. A mechanism of self-limited laser heating, impeding subsequent growth of the nanocrystals formed, is demonstrated. The results of the investigations are of interest for developing active elements of integrated optics.
    Glass and Ceramics 07/2013; 70(3-4). DOI:10.1007/s10717-013-9525-6 · 0.38 Impact Factor
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    ABSTRACT: The mechanism of broadband near-IR luminescence in the process of nanostructuring of Ni2+-activated glasses in the system R2O–Ga2O3–SiO2–GeO2 (R = Li, Na) at the initial stage of phase separation is described.
    Glass and Ceramics 07/2013; 70(3-4). DOI:10.1007/s10717-013-9524-7 · 0.38 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. DOI:10.1088/0957-4484/24/22/225302 · 3.67 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. DOI:10.1364/OL.38.000492 · 3.18 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; 5(1). DOI:10.1039/c2nr32790b · 6.74 Impact Factor
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    ABSTRACT: The present work reports on the investigation of oxidation states, local symmetry, and optical properties of manganese and iron species in natural cordierite (Mg, Fe)2Al4Si5O18, by means of energy-dispersive X-ray fluorescence, electron paramagnetic resonance (EPR), photoluminescence (PL) and optical absorption techniques. In particular, specific EPR characteristics show the presence of distinct Mn2+ octahedral sites in the crystal lattice, and a small amount of Mn4+ ions in tetrahedral sites. The absence of EPR signal ascribable to Mn4+ in octahedral sites turns out to be consistent with the lack of narrow PL lines from d 3 ions in octahedral crystal field (CF). Evidence of Mn3+ in octahedral sites is instead found in steady-state and time-resolved PL measurements. EPR spectra give further details on iron site occupancy, showing the occurrence of Fe3+ ions in octahedral sites. Characteristics ascribable to all the identified metal species have been found in the optical absorption spectrum or in the PL excitation spectrum, as arising from the expected CF transitions of Fe2+, Fe3+, Mn2+, Mn3+, and Mn4+ ions. Interestingly, the results show a polarization-dependent correlation between the manganese luminescence and the pleochroic absorption band at about 17,000 cm−1. This result reveals a pleochroic contribution from the Mn3+ spin-allowed 5E→5T2 transition under the visible pleochroic band attributed to Fe2+(oct)→Fe3+(tet) transitions.
    European Journal of Mineralogy 06/2012; 24(3):447-456. DOI:10.1127/0935-1221/2012/0024-2177 · 1.51 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. DOI:10.1017/S1431927611012827 · 1.76 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. DOI:10.1016/j.matchemphys.2011.12.079 · 2.13 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 02/2012; 24(4-4):677-681. DOI:10.1021/cm202664a · 8.54 Impact Factor
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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 02/2012; 3:690. DOI:10.1038/ncomms1683 · 10.74 Impact Factor
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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. DOI:10.1088/0957-4484/23/1/015708 · 3.67 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 07/2011; 128(1):12-15. DOI:10.1016/j.matchemphys.2011.02.066 · 2.13 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.