A. Paleari

SnO2 nanocrystals in SiO2: A wide-band-gap quantum-dot system

Nanoclusters of crystalline SnO2 have been grown in glass, starting from tin-doped silica prepared by a sol–gel method. Based upon a particular choice of molecular precursors, nanocrystallites with a mean radius of about 1 nm and a quite narrow size dispersion were obtained, resulting in wide-band-gap (>4 eV) quantum dots (QDs). In this system, differently from other semiconductor-doped glasses, both the glassy host and the nanophase are oxides of IV-group elements. Owing to their thermochemical compatibility, the two phases give stable optical-grade glass ceramics with potential applications in photonics, opening up the field to technological employments of wide-band-gap QDs.

Glycine-spacers influence functional motifs exposure and self-assembling propensity of functionalized substrates tailored for neural stem cell cultures.

The understanding of phenomena involved in the self-assembling of bio-inspired biomaterials acting as three-dimensional scaffolds for regenerative medicine applications is a necessary step to develop effective therapies in neural tissue engineering. We investigated the self-assembled nanostructures of functionalized peptides featuring four, two or no glycine-spacers between the self-assembly sequence RADA16-I and the functional biological motif PFSSTKT. The effectiveness of their biological functionalization was assessed via in vitro experiments with neural stem cells (NSCs) and their molecular assembly was elucidated via atomic force microscopy, Raman and Fourier Transform Infrared spectroscopy. We demonstrated that glycine-spacers play a crucial role in the scaffold stability and in the exposure of the functional motifs. In particular, a glycine-spacer of four residues leads to a more stable nanostructure and to an improved exposure of the functional motif. Accordingly, the longer spacer of glycines, the more effective is the functional motif in both eliciting NSCs adhesion, improving their viability and increasing their differentiation. Therefore, optimized designing strategies of functionalized biomaterials may open, in the near future, new therapies in tissue engineering and regenerative medicine.

Erbium doped nanostructured tin-silicate glass-ceramic composites

Er-doped tin–silicate glass–ceramic composites were synthesized from Si, Er and Sn molecular precursors by following a sol–gel method. Optical spectroscopy and electron microscopy showed that the resulting material is composed of an amorphous silica network that encloses submicrometric SnO2 crystalline clusters. Analysis of the luminescence properties shows that Er ions are, at least partially, trapped in the crystalline phase. Raman spectra show that nanostructured tin–silicate composites act as low phonon energy hosts for rare earth ions and are thus suitable for photonic applications.

Ultraviolet photoluminescence of porous silica

Excitation pattern and decay kinetics of ultraviolet photoluminescence of porous silica are investigated between 4.5 and 10 eV by means of synchrotron radiation. Spectra are dominated by a 3.7 eV emission similar to the recently observed ultraviolet emission of oxidized porous Si and Si nanostructures. Emission intensity is found to be controlled by the material specific surface. Other emissions are observed at 2.9, 3.8, and 4.2 eV. All emissions show lifetimes of a few nanoseconds. Spectral and kinetic features are sensibly different than in glassy SiO2, suggesting a revision of previous assignments of ultraviolet emissions in oxidized porous Si and Si nanostructures.

Thermal stability and structural transition of metastable Mn5O8: in situ micro-Raman study

The Raman spectrum of the metastable Mn5O8 phase, obtained from slow oxidation of Mn3O4 at low temperature, is presented and analysed for the first time and the thermal stability is monitored by the changes in Raman spectra due to laser-induced thermal treatments. A structural transformation toward the spinel phase Mn 3O4 is observed at temperature higher than 1000 K. Other Mn oxides, characterised by intermediate Mn oxidation states, are not detected below or during the transition. A compositional model of the sample grains is also proposed by comparing Raman data with X-ray diffraction and scanning electron microscopy measurements. q 1999 Elsevier Science Ltd. All rights reserved.

Fully inorganic oxide-in-oxide ultraviolet nanocrystal light emitting devices

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.

Photoluminescence of Sn-doped SiO2 excited by synchrotron radiation

Abstract Changes in the photoluminescence (PL) of amorphous SiO2 due to Sn-doping have been investigated by synchrotron radiation. Sn-doped SiO2 samples have been produced by a controlled sol–gel procedure as well as Ge-doped samples prepared for comparison. Detailed maps of the PL and PL excitation pattern have been obtained up to the band-to-band transition energy. The results confirm the analysis of Skuja as regards the emission at 3.1 and 4.2 eV excited at about 5 eV. At higher energies, our data show that the 3.1 and 4.2 eV PL bands have another excitation region with structures at 6.7, 7.2 and 8.0 eV. Lifetimes of about 10 ns for the 4.2 eV PL and 10 μs for the 3.1 eV PL are observed independently of the excitation energy. Data between 10 and 300 K are presented and compared with data from Ge-doped samples. The results show that high energy excitation of the 3.1 eV PL is not thermally activated, in contrast to the 4.9 eV excitation channel. Effects of the different spin–orbit coupling constants at Ge and Sn sites on PL intensity suggest that the high energy excitation channels arise from intra-center singlet-to-singlet transitions.

Ultraviolet free-exciton light emission in Er-passivated SnO2 nanocrystals in silica

SnO2 nanocrystals are grown in silica starting from a sol-gel method and using Er doping to passivate the cluster boundaries. As a result, emission at 3.8eV from the decay of SnO2 free excitons is observed in nanostructured SnO2:SiO2, besides the extrinsic 2eV luminescence of defects in SnO2 and ascribable to substoichiometric nanocluster boundaries. The analysis of the extrinsic emission competitive with the ultraviolet (UV) luminescence evidences the involvement of a phonon mode at 210cm−1 from a SnO-like phase. The feasibility of passivated wide-band-gap nanocrystals in silica gives interesting perspectives for UV-emitting optical devices.

Nickel-assisted growth and selective doping of spinel-like gallium oxide nanocrystals in germano-silicate glasses for infrared broadband light emission

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.

Ultra-broad optical amplification and two-colour amplified spontaneous emission in binary blends of insulated molecular wires

Conjugated polymers are receiving growing consideration thanks to their potential as optically active materials in light-emitting diodes (LEDs), [ 1 ] fi eld-effect transistors, photovoltaic cells, [ 2 ] microcavities [ 3 ] and all-organic lasers. [ 4–6 ] Virtually all those applications that exploit luminescence as their principal functional property (e.g. LEDs, microcavities, and lasers) can benefi t from control of intermolecular interactions between polymer chains, that often lead to formation of interchain aggregates with lower effi ciency and red-shifted emission. Non-covalent encapsulation of conjugated polymers by means of cyclodextrins (rotaxination) has enabled control of the secondary interactions between long conjugated chains, [ 7–11 ] and suppression of photoinduced charge generation (polarons) in the threaded backbone. [ 12 , 13 ] Such a property can be exploited to achieve broadband optical amplifi cation from polymer blends in which the gain