120 research outputs found
Tailoring the Emission Color of Carbon Dots through Nitrogen-Induced Changes of Their Crystalline Structure
No full textNitrogen content in carbon dots (CDs) plays a crucial role both on the structure and on the optical properties. We synthesized two distinct families of CDs which differ both in structure and in optical emission, demonstrating how nitrogen determines the structure and the optical properties of N-CDs in two main cases: low content and high content of nitrogen. While the low-nitrogen-content family is characterized by blue-emitting nanoparticles with a N-doped hexagonal C-graphite crystalline core structure and a complex surface structure, the high-nitrogen-content family is composed of nanoparticles behaving as dual emitters (blue and green) with a hexagonal β-C3N4crystalline core structure and a high amount of amide groups grafted on the surface. Our data demonstrate clearly that the nitrogen not only is a doping agent but can also have a decisive role on the structure of CDs, which directly reflects into their optical properties. The emission mechanisms of the two types of N-doped CDs are discussed and are related to their different structures
β-C3N4 Nanocrystals: Carbon Dots with Extraordinary Morphological, Structural, and Optical Homogeneity
No full textCarbon nanodots are known for their appealing optical properties, especially their intense fluorescence tunable in the visible range. However, they are often affected by considerable issues of optical and structural heterogeneity, which limit their optical performance and limit the practical possibility of applying these nanoparticles in several fields. Here we developed a synthesis method capable of producing a unique variety of carbon nanodots displaying an extremely high visible absorption strength (ε > 3 × 106 M(dot)−1 cm−1) and a high fluorescence quantum yield (73%). The high homogeneity of these dots reflects in many domains: morphological (narrow size distribution), structural (quasiperfect nanocrystals with large electronic bandgaps), and optical (nontunable fluorescence from a single electronic transition). Moreover, we provide the proof of principle that an aqueous solution of these dots can be used as an active medium in a laser cavity, displaying a very efficient laser emission with dye-like characteristics, which reflects the benefits of such a highly homogeneous type of carbon-based nanodots
Disentangling size effects and spectral inhomogeneity in carbon nanodots by ultrafast dynamical hole-burning
No full textCarbon nanodots (CDs) are a novel family of nanomaterials exhibiting unique optical properties. In particular, their bright and tunable fluorescence redefines the paradigm of carbon as a "black" material and is considered very appealing for many applications. While the field keeps growing, understanding CDs fundamental properties and relating them to their variable structures becomes more and more critical. Two crucial problems concern the effect of size on the electronic structure of CDs, and to what extent their optical properties are influenced by structural disorder. Furthermore, it remains largely unclear whether traditional concepts borrowed from the photo-physics of semiconductor quantum dots can be applied to any type of CDs. We used femtosecond optical hole burning to address the excited-state properties of a family of CDs with the specific structure of β-C3N4. The experiments provide compelling evidence of the dramatic effects of structural heterogeneity on the optical spectra, and reveal the remarkably simple pattern of the electronic transitions of these CDs, normally obscured by disorder. Moreover, the data conclusively clarify the different effects of the nanometric size and of the disordered surface structure on the fluorescence tunability, ruling out for these CDs any quantum confinement effect comparable to semiconductor quantum dots
Fluorescent Boron Oxide Nanodisks as Biocompatible Multi-messenger Sensors for Ultrasensitive Ni2+ Detection
No full textBoron-based nanocomposites are very promising for
a wide range of technological applications, spanning from microelectronics to nanomedicine. A large variety of B-based nanomaterials has been already observed, such as borospherene, B
nanotubes and nanoparticles, and boron nitride nanoparticles.
However, their fabrication usually involves toxic precursors or
leads to very low yields or small boron atom concentration. In this
work, we report the synthesis of nanometric B2O3 nanodisks, a
family of nanomaterials with a quasi-2D morphology capable of
intense fluorescence in the visible range. Such as boron-based
nanomaterial, which we synthesized by pulsed laser ablation of a
boron target, is water-dispersible and nontoxic, and displays a highly
crystalline structure. Moreover, its bright blue photoluminescence is
highly sensitive and selective for the presence of Ni2+ ions in solution, down to extremely small concentrations in the picomolar
range. The results are very promising in view of the use of such novel B2O3 nanodisks as ultrasensitive multi-messenger Ni2+
nanosensors
One‐Step Fabrication of Carbon Dot‐Based Nanocomposites Powering Solid‐State Random Lasing
No full textCarbon dots (CDs) have attracted much attention for applications in photonics and optoelectronics because of their high emission efficiency and ease of synthesis. Although studies in solution are well established, solid-state applications are less common because of optical quenching phenomena that critically affect CDs. Herein, the synthesis of amorphous CDs from citric acid, operating as hosts of dye molecules, and their incorporation into organic–inorganic silica matrices through a fast photo-induced polymerization process are reported. The photocurable sol composition allows easy dispersion of nanometer-sized scattering centers, such as titania or gold nanoparticles (NPs), which have been incorporated, along with CDs, into nanocomposites. The combination of high-brightness CDs and nanoscatterers in the hybrid matrices allows for achieving and investigating the random lasing processes occurring in the orange-red range of the visible spectrum. In situ-grown gold NPs contribute to a significant improvement in solid-state lasing, enabling an emission as narrow as 5 nm and a laser threshold as low as 0.3 mJ pulse−1. The present approach reveals the technological and scientific potential of CDs when embedded in solid-state disordered active media
Quantitative High-Resolution Transmission Electron Microscopy of Single Atoms
No full textGamm B, Blank H, Popescu R, et al. Quantitative High-Resolution Transmission Electron Microscopy of Single Atoms. Microscopy and Microanalysis. 2012;18(01):212-217
The use of energy looping between Tm and Er ions to obtain an intense upconversion under the 1208 nm radiation and its use in temperature sensing
No full textThe upconversion phenomenon allows for the emission of nanoparticles (NPs) under excitation with near-infrared (NIR) light. Such property is demanded in biology and medicine to detect or treat diseases such as tumours. The transparency of biological systems for NIR light is limited to three spectral ranges, called biological windows. However, the most frequently used excitation laser to obtain upconversion is out of these ranges, with a wavelength of around 975 nm. In this article, we show an alternative – Tm/Er-doped NPs that can convert 1208 nm excitation radiation, which is in the range of the 2 biological window, to visible light within the 1 biological window. The spectroscopic properties of the core@shell NaYF:Tm@NaYF and NaYF:Er,Tm@NaYF NPs revealed a complex mechanism responsible for the observed upconversion. To explain emission in the studied NPs, we propose an energy looping mechanism: a sequence of ground state absorption, energy transfers and cross-relaxation (CR) processes between Tm ions. Next, the excited Tm ions transfer the absorbed energy to Er3+ ions, which results in green, red and NIR emission at 526, 546, 660, 698, 802 and 982 nm. The ratio between these bands is temperature-dependent and can be used in remote optical thermometers with high relative temperature sensitivity, up to 2.37%/°C at 57 °C. The excitation and emission properties of the studied NPs fall within 1 and 2 biological windows, making them promising candidates for studies in biological systems
Nanoscale copper sulfide hollow spheres with phase-engineered composition: covellite (CuS), digenite (Cu1.8S), chalcocite (Cu2S)
No full textTargeting assistance to the poor using household survey data
No full textIt is important that limited government resources be channeled to the poor, but it is not always easy to identify the poor. Which households should be given tranfers when reliable information on incomes is difficult to obtain? The authors of this paper present a simple method for targeting when income is not observable but other characteristics that are correlated with income can be observed. Using survey data taken from Cote d'Ivoire, they predict incomes based on observable characteristics and distribute transfers on the basis of those predictions. It appears that significant reductions in poverty can be achieved using this method.Environmental Economics&Policies,Rural Poverty Reduction,Services&Transfers to Poor,Safety Nets and Transfers,Poverty Assessment
INFRARED STUDY OF MOLECULAR MOTION IN LIQUID ISOPROPYLALCOHOL
No full textAuthor Institution: Department of Chemistry, Scientific Research Staff, Ford Motor CompanyRotational autocorrelation functions were compute from the 955- infrared band contour of pure liquid isopropylalcohol (at ) and of its solutions (0.1 molar) in CC1, and , respectively. The results indicate that the rotational motion of the alcohol molecule consists of orientation jumps of the order of 1/10 to 1/5 radian. This, strongly restricted, motion is in great contrast to what has been observed in near-ideal or weakly associated liquids, in which essentially unhindered rotational motion over angular distances of the order of one radian takes place, and is therefore believed to reflect the influence of the relatively strong hydrogen-bonding on the rotational motion of the alcohol molecules. The effects of various assumptions and simplifications, which have been made in order to extract this information from the measurements, are discussed
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