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Optical glassceramics are an important class of functional materials mainly employed in photonic and optoelectronic applications, as well as for advanced fiber optics. Here we present recent results on phase separation processes and optical properties of alkali germanosilicate glasses containing gallium oxide nanoparticles. Samples, with typical composition 7.5Li2O–2.5Na2O–20Ga2O3–35GeO2–35SiO2 (mol%), are obtained from the melt-quenching method. The resulting glasses are completely amorphous, but show phase separation, on the nanometer scale, deriving from liquid–liquid phase separation in the melt. Nanocrystallization of gallium oxide, in its metastable gamma phase, is thermally activated inside the matrix at about 615 °C. Treatment at higher temperature causes the formation of LiGa5O8 polymorph by Li ion diffusion from the surrounding matrix to the nanocrystals. In a second step, a further increment of temperature induces an order-to-disorder LiGa5O8 change that finally dissociates causing the formation of beta-Ga2O3. Neutron scattering, X-ray diffraction analysis, differential scanning calorimetry, and transmission electron microscopy studies have been performed so as to assess nucleation and crystal growth mechanisms. Parallel to morphological studies, we also present optical response of these systems, characterized by a strong UV-excited blue luminescence due to intrinsic defects and its dependence on thermal treatments. - See more at: http://www.european-mrs.com/2015-fall-symposium-w-european-materials-research-society#sthash.pRj5G4Gf.dpuf

The ability to incorporate crystalline nanostructures in amorphous glass is a successfully strategy for the design of novel functional and structural materials. On the one hand, the oxide matrix ensures ideal workability, chemical inertness, and mechanical robustness. On the other hand, crystalline nanoparticles may give rise to unprecedented optical, electrical, or mechanical behaviour. In this view, alkali germano-silicate glasses containing γ-Ga2O3 nanoparticles, prepared by the melt-quenched method, represent an interesting solution as multipurpose photonic materials. In fact, the low refractive mismatch between crystals and matrix as well as the ability of obtaining nanostructures with dimensions of the order of few nanometers enables the possibility of having a transparent material with peculiar optical properties. γ-Ga2O3 is indeed a wide-band gap semiconductor (EG = 4.9 eV) showing strong blue luminescence upon excitation at wavelength shorter than 280 nm and related to recombination at donor and acceptor pairs. For these reasons Ga2O3-based materials would be particularly suitable, in principle, for the fabrication of simple and robust UV-to-visible converters. The photoluminescence excitation threshold at 280 nm coincides with the UV-C region above which the solar spectrum does not give any background to the detection of UV-emitting events such as flames, electric sparks, and corona dispersions, which can significantly affect safety conditions in working places, energy-saving, and electric power distribution reliability. Beyond this intrinsic luminescence, γ-Ga2O3 may also host other optically-active ions so as to enrich the photophysics of the system. Here we present our recent results on the optimization of the intrinsic emission features of these glassceramics and their application as UV-light converter in solar-blind passive devices. Moreover, we also discuss the optical behaviour of samples doped with rare-earth and transition metal ions. In particular, we will focus on samples doped with Nickel ions that show strong broadband infrared emission, and Gadolinium doped samples where energy-transfer processes take place thus enabling rare-earth emission with large excitation cross section.

We elucidate here the nature of the emissive states in Rb7Sb3Cl16 nanocrystals (NCs) for which we report a synthesis. These NCs have a crystal structure comprising both isolated [SbCl6]3- octahedra and isolated [Sb2Cl10]4- dimers of octahedra. The optical properties of Rb7Sb3Cl16 NCs are compared with those of Sb-doped Rb3InCl6 NCs, the latter containing only isolated [SbCl6]3- octahedra. The remarkably similar behaviors of the two systems indicate that the optical emission in both materials originates from the isolated octahedra. Density functional theory calculations suggest that the [SbCl6]3- octahedra are optically active in emission because the local arrangement of the Rb+ ions around the octahedra limits the elongation of the Sb-Cl bonds upon excitation, helping to confine the self-trapped exciton in them. Conversely, in the [Sb2Cl10]4- dimers the constraining effect of the surrounding Rb+ ions is less marked and the Sb-Cl bonds fully break upon photoexcitation, opening up an efficient nonradiative channel for the self-trapped excitons.

In this work, we report the hot-injection synthesis of Cs3ZnCl5 colloidal nanocrystals (NCs) with tunable amounts of Cu+ and Mn2+ substituent cations. All the samples had a rodlike morphology, with a diameter of 14 nm and a length of 30-100 nm. Alloying did not alter the crystal structure of the host Cs3ZnCl5 NCs, and Cu ions were mainly introduced in the oxidation state +1 according to X-ray photoelectron and electron paramagnetic resonance spectroscopies. The spectroscopic analysis of unalloyed, Cu-alloyed, Mn-alloyed, and Cu, Mn coalloyed NCs indicated that (i) the Cs3ZnCl5 NCs have a large band gap of 5.35 eV; (ii) Cu(I) aliovalent alloying leads to an absorption shoulder/peak at 4.8 eV and cyan photoluminescence (PL) peaked at 2.50 eV; (iii) Mn(II) isovalent alloying leads to weak Mn PL, which intensifies remarkably in the coalloyed samples, prompted by an energy transfer (ET) process between the Cu and Mn centers, favored by the overlap between the lowest (6A1 → 4T1) transition for tetrahedrally coordinated Mn2+ and the PL profile from Cu(I) species in the Cs3ZnCl5 NCs. The efficiency of this ET process reaches a value of 61% for the sample with the highest extent of Mn alloying. The PL quantum yield (QY) values in these Cu, Mn coalloyed NCs are lower at higher Mn contents. The analysis of the Mn PL dynamics in these samples indicates that this PL drop stems from inter-Mn exciton migration, which increases the likelihood of trapping in defect sites, in agreement with previous studies.

Nanocrystals (NCs) of transparent conducting oxides with a localized surface plasmon resonance (LSPR) in the near-infrared (NIR) spectral region show promising electrochromic properties for the development of a new generation of dynamic "smart windows". In this regard, we exploit thin films of F, In-codoped CdO (FICO) NCs as active coatings for electrochromic devices. The control over the dopants concentration in FICO NCs results in fine tuning of their LSPR across the NIR region of the electromagnetic spectrum. Highly transparent mesoporous electrodes were prepared from colloidal FICO NCs by in situ ligand exchange of the pristine organic capping ligands. This approach preserves the optical and electrical properties of native NCs and delivers highly homogeneous, nonscattering films with a good electronic coupling between the NCs. We achieved a dynamic control over the LSPR frequency by reversible electrochemical doping, hence a spectrally selective modulation of the optical transmittance in the NIR region of the solar spectrum, which carries nearly 50% of the whole solar heat. Spectroelectrochemical characterization, coloration efficiency, and switching kinetics results indicate that thin film based on FICO NCs are potential candidates for plasmonic electrochromic applications. Moreover, the high electron mobility and wide optical bandgap of FICO makes NCs of this material suitable for large-area devices capable of dynamically controlling the heat load coming from the solar infrared radiation, without affecting the visible light transmittance.

We report the composition-dependent optical properties of Bi-doped Cs2Ag1-xNaxInCl6 nanocrystals (NCs) having a double perovskite crystal structure. Their photoluminescence (PL) was characterized by a large Stokes shift, and the PL quantum yield increased with the amount of Na up to ∼22% for the Cs2Ag0.4Na0.6InCl6 stoichiometry. The presence of Bi3+ dopants was crucial to achieve high PL quantum yields (PLQYs) as nondoped NC systems were not emissive. Density functional theory calculations revealed that the substitution of Ag+ with Na+ leads to localization of AgCl6 energy levels above the valence band maximum, whereas doping with Bi3+ creates BiCl6 states below the conduction band minimum. As such, the PL emission stems from trapped emission between states localized in the BiCl6 and AgCl6 octahedra, respectively. Our findings indicated that both the partial replacement of Ag+ with Na+ ions and doping with Bi3+ cations are essential in order to optimize the PL emission of these systems.

We synthesize colloidal nanocrystals (NCs) of Rb3InCl6, composed of isolated metal halide octahedra (“0D”), and of Cs2NaInCl6 and Cs2KInCl6 double perovskites, where all octahedra share corners and are interconnected (“3D”), with the aim to elucidate and compare their optical features once doped with Sb3+ ions. Our optical and computational analyses evidence that the photoluminescence quantum yield (PLQY) of all these systems is consistently lower than that of the corresponding bulk materials due to the presence of deep surface traps from under-coordinated halide ions. Also, Sb-doped “0D” Rb3InCl6 NCs exhibit a higher PLQY than Sb-doped “3D” Cs2NaInCl6 and Cs2KInCl6 NCs, most likely because excitons responsible for the PL emission migrate to the surface faster in 3D NCs than in 0D NCs. We also observe that all these systems feature a large Stokes shift (varying from system to system), a feature that should be of interest for applications in photon management and scintillation technologies. Scintillation properties are evaluated via radioluminescence experiments, and re-absorption-free waveguiding performance in large-area plastic scintillators is assessed using Monte Carlo ray-tracing simulations.

We report here the synthesis of undoped and Cu-doped Cs2ZnCl4 nanocrystals (NCs) in which we could tune the concentration of Cu from 0.7 to 7.5%. Cs2ZnCl4 has a wide band gap (4.8 eV), and its crystal structure is composed of isolated ZnCl4 tetrahedra surrounded by Cs+ cations. According to our electron paramagnetic resonance analysis, in 0.7 and 2.1% Cu-doped NCs the Cu ions were present in the +1 oxidation state only, while in NCs at higher Cu concentrations we could detect Cu(II) ions (isovalently substituting the Zn(II) ions). The undoped Cs2ZnCl4 NCs were non emissive, while the Cu-doped samples had a bright intragap photoluminescence (PL) at similar to 2.6 eV mediated by band-edge absorption. Interestingly, the PL quantum yield was maximum (similar to 55%) for the samples with a low Cu concentration ([Cu]

We devised a hot-injection synthesis to prepare colloidal double-perovskite Cs2NaBiCl6 nanocrystals (NCs). We also examined the effects of replacing Na+ with Ag+ cations by preparing and characterizing Cs2Na1-x Ag x BiCl6 alloy NCs with x ranging from 0 to 1. Whereas Cs2NaBiCl6 NCs were not emissive, Cs2Na1-x Ag x BiCl6 NCs featured a broad photoluminescence band at ∼690 nm, Stokes-shifted from the respective absorption by ≥1.5 eV. The emission efficiency was maximized for low Ag+ amounts, reaching ∼3% for the Cs2Na0.95Ag0.05BiCl6 composition. Density functional theory calculations coupled with spectroscopic investigations revealed that Cs2Na1-x Ag x BiCl6 NCs are characterized by a complex photophysics stemming from the interplay of (i) radiative recombination via trapped excitons localized in spatially connected AgCl6-BiCl6 octahedra; (ii) surface traps, located on undercoordinated surface Bi centers, behaving as phonon-assisted nonradiative decay channels; and (iii) a thermal equilibrium between trapping and detrapping processes. These results offer insights into developing double-perovskite NCs with enhanced optoelectronic efficiency.

We employed oleylphosphonic acid (OLPA) for the synthesis of CsPbBr3 nanocrystals (NCs). Compared to phosphonic acids with linear alkyl chains, OLPA features a higher solubility in apolar solvents, allowing us to work at lower synthesis temperatures (100 °C), which in turn offer a good control over the NCs size. This can be reduced down to 5.0 nm, giving access to the strong quantum confinement regime. OLPA-based NCs form stable colloidal solutions at very low concentrations (∼1 nM), even when exposed to air. Such stability stems from the high solubility of OLPA in apolar solvents, which enables these molecules to reversibly bind/unbind to/from the NCs, preventing the NCs aggregation/precipitation. Small NCs feature efficient, blue-shifted emission and an ultraslow emission kinetics at cryogenic temperature, in striking difference to the fast decay of larger particles, suggesting that size-related exciton structure and/or trapping-detrapping dynamics determine the thermal equilibrium between coexisting radiative processes.

Nanocrystals (NCs) surface characterization is a fundamental step for understanding the physical and chemical phenomena involved at the nanoscale. Surface energy and chemistry depend on particle size and composition, and, in turn, determine the interaction of NCs with the surrounding environment, their properties and stability, and the feasibility of nanocomposites. This work aims at extracting more information on the surface of different titanium dioxide polymorphs using1H-TD-NMR of water. Taking advantage of the interaction between water molecules and titanium dioxide NCs, it is possible to correlate the proton transverse relaxation times (T2) as the function of the concentration and the specific surface area (δp·Cm) and use it as an indicator of the crystal phase. Examples of three different crystals phase, rutile, anatase, and brookite, have been finely characterized and their behavior in water solution have been studied with TD-NMR. The results show a linear correlation between relaxivity (R2) and their concentration Cm. The resulting slopes, after normalization for the specific surface, represent the surface/water interaction and range from 1.28 g m-2s-1of 50 nm rutile nanocrystals to 0.52 for similar sized brookite. Even higher slopes (1.85) characterize smaller rutile NCs, in qualitative accordance with the trends of surface energy. Thanks to proton relaxation phenomena that occur at the NCs surface, it is possible to differentiate the crystal phase and the specific surface area of titanium dioxide polymorphs in water solution.

We show here the first colloidal synthesis of double perovskite Cs2AgInCl6 nanocrystals (NCs) with a control over their size distribution. In our approach, metal carboxylate precursors and ligands (oleylamine and oleic acid) are dissolved in diphenyl ether and reacted at 105 °C with benzoyl chloride. The resulting Cs2AgInCl6 NCs exhibit the expected double perovskite crystal structure, are stable under air, and show a broad spectrum white photoluminescence (PL) with quantum yield of ∼1.6 ± 1%. The optical properties of these NCs were improved by synthesizing Mn-doped Cs2AgInCl6 NCs through the simple addition of Mn-acetate to the reaction mixture. The NC products were characterized by the same double perovskite crystal structure, and Mn doping levels up to 1.5%, as confirmed by elemental analyses. The effective incorporation of Mn ions inside Cs2AgInCl6 NCs was also proved by means of electron spin resonance spectroscopy. A bright orange emission characterized our Mn-doped Cs2AgInCl6 NCs with a PL quantum yield as high as ∼16 ± 4%.

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 the 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.

The aim of this PhD project, also called “warm white light project”, is to propose an alternative solid state lighting device that works using the same principles of color-conversion WLEDs. The idea is to create a polymer based nanocomposite that could work as a transparent glass in off-mode and as a “lamp” in on-mode. The new concept is to spatially decouple the light source and the converters in order to avoid all the thermal problems that affect the current LEDs (in which the color is “generated” within the chip and the epoxy case). To do so, a blue LED, made of Indium gallium nitride (InGaN), is coupled to a bulk polymer nanocomposite sheet where the actual conversion of light takes place. For emitting white light it is necessary that the nanocomposite would not only convert partially the blue light into warmer colors (like yellow and red) but also diffuse the blue light outside in order to have a color mixing.

We report a new colloidal synthesis of niobium-doped TiO2 anatase nanocrystals (NCs) that allows for the preparation of ∼10 nm NCs with control over the amount of Nb doping up to ∼14%. The incorporation of niobium ions leads to the appearance of a tunable, broad absorption peak that ranges from the visible range to the mid-infrared. This optical behavior is attributed to the substitution of Nb5+ on Ti4+ sites generating free carriers inside the conduction band of the TiO2 NCs as supported by optical and electron paramagnetic resonance spectroscopic investigations. At the same time, the incorporation of progressively more niobium ions drives an evolution of the shape of the NCs from tetragonal platelets to "peanutlike" rods. © 2013 American Chemical Society.

We describe two synthesis approaches to colloidal Cu(3)P nanocrystals using trioctylphosphine (TOP) as phosphorus precursor. One approach is based on the homogeneous nucleation of small Cu(3)P nanocrystals with hexagonal plate-like morphology and with sizes that can be tuned from 5 to 50 nm depending on the reaction time. In the other approach, metallic Cu nanocrystals are nucleated first and then they are progressively phosphorized to Cu(3)P. In this case, intermediate Janus-like dimeric nanoparticles can be isolated, which are made of two domains of different materials, Cu and Cu(3)P, sharing a flat epitaxial interface. The Janus-like nanoparticles can be transformed back to single-crystalline copper particles if they are annealed at high temperature under high vacuum conditions, which makes them an interesting source of phosphorus. The features of the Cu-Cu(3)P Janus-like nanoparticles are compared with those of the striped microstructure discovered more than two decades ago in the rapidly quenched Cu-Cu(3)P eutectic of the Cu-P alloy, suggesting that other alloy/eutectic systems that display similar behavior might give origin to nanostructures with flat, epitaxial interface between domains of two diverse materials. Finally, the electrochemical properties of the copper phosphide plates are studied, and they are found to be capable of undergoing lithiation/delithiation through a displacement reaction, while the Janus-like Cu-Cu(3)P particles do not display an electrochemical behavior that would make them suitable for applications in batteries.

We report a high-yield, low cost synthesis route to colloidal Cu1-xInS2 nanocrystals with a tunable amount of Cu vacancies in the crystal lattice. These are then converted into quaternary Cu-In-Zn-S (CIZS)nanocrystals by partial exchange of Cu+ and In3+ cations with Zn2+ cations. The photoluminescence quantum yield of these CIZS nanocrystals could be tuned up to a record 80\%, depending on the amount of copper vacancies.