A Prato Tour on Carbon Nanotubes: Raman Insights

The functionalisation of carbon nanotubes has been instrumental in broadening its application field, allowing especially its use in biological studies. Although numerous covalent and non-covalent functionalisation methods have been described, the characterisation of the final materials has always been an added challenge. Among the various techniques available, Raman spectroscopy is one of the most widely used to determine the covalent functionalisation of these species. However, Raman spectroscopy is not a quantitative technique, and no studies are reported comparing its performance when the same number of functional groups are added but using completely different reactions. In this work, we have experimentally and theoretically studied the functionalisation of carbon nanotubes using two of the most commonly used reactions: 1,3-dipolar cycloaddition of azomethylene ylides and diazonium-based radical addition. The number of groups introduced onto the tubes by these reactions has been determined by different characterisation techniques. The results of this study support the idea that data obtained by Raman spectra are only helpful for comparing functionalisations produced using the same type of reaction. However, they should be carefully analysed when comparing functionalisations produced using different reaction types.The functionalisation of carbon nanotubes using Prato reaction and diazonium-based radical addition has been studied. The results support the idea that data obtained by Raman spectroscopy are only helpful for comparing functionalisations produced using the same type of reaction. However, they should be carefully analysed when comparing different reaction types.imag

High-Resolution Rotational Spectroscopy of a Cyclic Ether

The conformational landscape of crown ethers has constituted a central topic in the development of host–guest supramolecular chemistry. We report a high-resolution rotational study of a crown ether, 1,4,7,10,13-pentaoxacyclopentadecane (15-<i>crown</i>-5), by means of molecular beam Fourier transform microwave spectroscopy. The considerable size and the broad range of conformations allowed by the flexibility of the cyclic backbone of this ether pose important challenges to spectroscopy approaches. In this investigation, three stable rotamers of the 15-<i>crown</i>-5 ether have been identified and characterized through their rotational constants and centrifugal distortion coefficients. Ab initio quantum calculations at the MP2 level predict these conformers as the most stable ones for the title system and reproduce accurately their distinct structural features. The results pave the ground for an extensive survey of the conformational landscape of the 15-<i>crown</i>-5 and related cyclic ethers in the near term

High-Resolution Rotational Spectroscopy of a Cyclic Ether

The conformational landscape of crown ethers has constituted a central topic in the development of host–guest supramolecular chemistry. We report a high-resolution rotational study of a crown ether, 1,4,7,10,13-pentaoxacyclopentadecane (15-<i>crown</i>-5), by means of molecular beam Fourier transform microwave spectroscopy. The considerable size and the broad range of conformations allowed by the flexibility of the cyclic backbone of this ether pose important challenges to spectroscopy approaches. In this investigation, three stable rotamers of the 15-<i>crown</i>-5 ether have been identified and characterized through their rotational constants and centrifugal distortion coefficients. Ab initio quantum calculations at the MP2 level predict these conformers as the most stable ones for the title system and reproduce accurately their distinct structural features. The results pave the ground for an extensive survey of the conformational landscape of the 15-<i>crown</i>-5 and related cyclic ethers in the near term

Unknown Aspects of Self-Assembly of PbS Microscale Superstructures

A lot of interesting and sophisticated examples of nanoparticle (NP) self-assembly (SA) are known. From both fundamental and technological standpoints, this field requires advancements in three principle directions: (a) understanding the mechanism and driving forces of three-dimensional (3D) SA with both nano- and microlevels of organization; (b) understanding disassembly/deconstruction processes; and (c) finding synthetic methods of assembly into continuous superstructures without insulating barriers. From this perspective, we investigated the formation of well-known star-like PbS superstructures and found a number of previously unknown or overlooked aspects that can advance the knowledge of NP self-assembly in these three directions. The primary one is that the formation of large seemingly monocrystalline PbS superstructures with multiple levels of octahedral symmetry can be explained only by SA of small octahedral NPs. We found five distinct periods in the formation PbS hyperbranched stars: (1) nucleation of early PbS NPs with an average diameter of 31 nm; (2) assembly into 100–500 nm octahedral mesocrystals; (3) assembly into 1000–2500 nm hyperbranched stars; (4) assembly and ionic recrystallization into six-arm rods accompanied by disappearance of fine nanoscale structure; (5) deconstruction into rods and cuboctahedral NPs. The switches in assembly patterns between the periods occur due to variable dominance of pattern-determining forces that include van der Waals and electrostatic (charge–charge, dipole–dipole, and polarization) interactions. The superstructure deconstruction is triggered by chemical changes in the deep eutectic solvent (DES) used as the media. PbS superstructures can be excellent models for fundamental studies of nanoscale organization and SA manufacturing of (opto)electronics and energy-harvesting devices which require organization of PbS components at multiple scales

Temporal Structures in Positron Spectra and Charge-Sign Effects in Galactic Cosmic Rays

International audienceWe present the precision measurements of 11 years of daily cosmic positron fluxes in the rigidity range from 1.00 to 41.9 GV based on 3.4×106 positrons collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The positron fluxes show distinctly different time variations from the electron fluxes at short and long timescales. A hysteresis between the electron fluxes and the positron fluxes is observed with a significance greater than 5σ at rigidities below 8.5 GV. On the contrary, the positron fluxes and the proton fluxes show similar time variation. Remarkably, we found that positron fluxes are modulated more than proton fluxes with a significance greater than 5σ for rigidities below 7 GV. These continuous daily positron fluxes, together with AMS daily electron, proton, and helium fluxes over an 11-year solar cycle, provide unique input to the understanding of both the charge-sign and mass dependencies of cosmic rays in the heliosphere

Properties of Daily Helium Fluxes

International audienceWe present the precision measurement of 2824 daily helium fluxes in cosmic rays from May 20, 2011 to October 29, 2019 in the rigidity interval from 1.71 to 100 GV based on 7.6×108 helium nuclei collected with the Alpha Magnetic Spectrometer (AMS) aboard the International Space Station. The helium flux and the helium to proton flux ratio exhibit variations on multiple timescales. In nearly all the time intervals from 2014 to 2018, we observed recurrent helium flux variations with a period of 27 days. Shorter periods of 9 days and 13.5 days are observed in 2016. The strength of all three periodicities changes with time and rigidity. In the entire time period, we found that below ∼7  GV the helium flux exhibits larger time variations than the proton flux, and above ∼7  GV the helium to proton flux ratio is time independent. Remarkably, below 2.4 GV a hysteresis between the helium to proton flux ratio and the helium flux was observed at greater than the 7σ level. This shows that at low rigidity the modulation of the helium to proton flux ratio is different before and after the solar maximum in 2014