Gianluca Calestani

p-t-Butylcalix[4]arene tetra-acetamide: a new strong receptor for alkali cations [1]

From the reaction ofp-t-butylcalix[4]arene with α-chloro-N,N-diethyl acetamide a new lipophilic ether-amide ligand (2) has been obtained in high yield. Solution studies show (2) to be a very strong cation receptor for alkali cations, especially sodium and potassium. The X-ray crystal structure determination of the free ligand (2) and two potassium complexes (KI and KSCN) shows the calix[4]arene in a fixed ‘cone’ structure and the cation completely encapsulated in a polar cavity of eight oxygen atoms.

Zn vacancy induced green luminescence on non-polar surfaces in ZnO nanostructures

Although generally ascribed to the presence of defects, an ultimate assignment of the different contributions to the emission spectrum in terms of surface states and deep levels in ZnO nanostructures is still lacking. In this work we unambiguously give first evidence that zinc vacancies at the (1010) nonpolar surfaces are responsible for the green luminescence of ZnO nanostructures. The result is obtained by performing an exhaustive comparison between spatially resolved cathodoluminescence spectroscopy and imaging and ab initio simulations. Our findings are crucial to control undesired recombinations in nanostructured devices.

Magnetoelastic effects and magnetic anisotropy in Ni2MnGa polycrystals

Linear thermal expansion and magnetostriction measurements under a magnetic field up to 20 kOe, applied parallel and perpendicular to the measuring direction and in the temperature range of 10–300 K, have been performed on a Ni2MnGa polycrystal. The effects of zero-field cooling and field cooling on both the self-strain (H=0) and the magnetic-field-induced strain (MFIS) have been studied. We have found that the MFIS strongly depends on whether a magnetic field was applied during the cooling process. The applied magnetic field facilitates the growth of specific orientation variants along the field direction as the sample is cooled down through martensitic transformation. However, the application of a field in the martensitic phase induces a negligible motion of twin boundaries. On this basis, the singular point detection technique performed on polycrystalline specimens results in being the easiest and most direct way by which to determine the anisotropy field in this class of materials. The temperature beh...

Solid State Studies on p.t-Butyl-Calix[6]Arene Derivatives

Calix[n]arenes1 (1) are a class of phenol-formaldehyde macrocyclic oligomers which exhibit different sizes and shapes depending on the number of phenolic units in the cyclic array and on the nature of substituents on the aromatic nucleus (R1) and on the phenolic oxygen (R2).

Influence of Anions in Silver Supramolecular Frameworks: Structural Characteristics and Sorption Properties.

The complexation of a preorganized thioether-functionalized bis(pyrazolyl)methane ligand (L) with silver precursors produces supramolecular structures organized at two hierarchical levels: [AgL](6)(X)(6) metal-organic cyclic hexamers and their organization in 3D architectures. The cyclic toroidal hexamers of 22-26 Å external diameter are found to be stable already in solution before self-assembly into the crystalline state. In the 3D lattice, the hexameric building block are arranged in different highly symmetric space groups as a function of a variety of anions (cubic Fd3 with PF(6)(-) or BF(4)(-) and rhombohedral R3 with CF(3)SO(3)(-) or NO(3)(-)) and form cavities with the geometrical shapes of Platonic solids (tetrahedron and octahedron) that can be occupied by a variety of solvent molecules. Upon evacuation, cubic crystals can produce stable frameworks with permanent porosity, which can absorb reversibly several vapors, CO(2) and CH(4).

Reverse magnetostructural transformation in Co-doped NiMnGa multifunctional alloys

We studied the composition dependence of the structural and magnetic properties of Co-doped Ni–Mn–Ga alloys around the Mn-rich composition Ni50Mn30Ga20. By varying the Co and Mn content we have been able to tune the critical temperatures. In particular, in a suitable composition range, the Curie temperature of martensite is lower than Curie temperature of austenite and lower than martensitic transformation temperature, giving rise to a paramagnetic gap between magnetically ordered martensite and austenite and to the occurrence of a reverse magnetostructural transformation.

Composition effects on the formation and superconducting character of c≈31 Å and c≈37 Å phases in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems. X-ray and ESR analysis

Abstract In this paper we examine the effect of starting composition on the formation of the two crystallographic phases characterized by c≈31 A and c≈37 A in the Bi-Sr-Ca-Cu-O and Bi-Pb-Sr-Ca-Cu-O systems. The superconducting properties have been studied by field-modulated microwave non-resonant absorption. The c≈37 A phase was completely isolated in the Pb-containing system and shows a quite sharp superconductive transition at 105 K, well evidenced by the microwave absorption profile as a function of temperature. For the c≈31 A phase, obtained in a wide range of composition, the ESR analysis points out a more complex behaviour, with a superconductive transition taking place in the 75–110 K temperature range. Strong contribution to the 110 K transition has been observed in samples showing no evidence in the X-ray spectra of the typical c≈37 A reflections, nor of peak broadening due to stacking faults. A possible correlation of this phenomenon with the partial substitution of Bi in the Bi2O2 double layers by lighter Ca, Sr or Cu ions is discussed.

Ultrathin Bi2S3 nanowires: surface and core structure at the cluster-nanocrystal transition.

Herein, we present the structural characterization of the core and surface of colloidally stable ultrathin bismuth sulfide (Bi(2)S(3)) nanowires using X-ray Absorption Spectroscopy (EXAFS and XANES), X-ray Photoelectron Spectroscopy (XPS), and Nuclear Magnetic Resonance (NMR). These three techniques allowed the conclusive structural characterization of the inorganic core as well as the coordination chemistry of the surface ligands of these structures, despite the absence of significant translational periodicity dictated by their ultrathin diameter (1.6 nm) and their polycrystallinity. The atomic structure of the inorganic core is analogous to bulk bismuthinite, but Bi atoms display a remarkably higher coordination number than in the bulk. This can be only explained by a model in which each bismuth atom at the surface (or in close proximity to it) is bound to at least one ligand at any time.

The relationship between nanoscale architecture and charge transport in conjugated nanocrystals bridged by multichromophoric polymers

We report on the self-assembly and the electrical characterization of bicomponent films consisting of an organic semiconducting small molecule blended with a rigid polymeric scaffold functionalized in the side chains with monomeric units of the same molecule. The molecule and polymer are a perylene-bis(dicarboximide) monomer (M-PDI) and a perylene-bis(dicarboximide)-functionalized poly(isocyanopeptide) (P-PDI), which have been codeposited on SiO(x) and mica substrates from solution. These bicomponent films have been characterized by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM), revealing the relationship between architecture and function for various supramolecular nanocrystalline arrangements at a nanometer spatial resolution. Monomer-polymer interactions can be controlled by varying solvent and/or substrate polarity, so that either the monomer packing dictates the polymer morphology or vice versa, leading to a morphology exhibiting M-PDI nanocrystals connected with each other by P-PDI polymer wires. Compared to pure M-PDI or P-PDI films, those bicomponent films that possess polymer interconnections between crystallites of the monomer display a significant improvement in electrical connectivity and a 2 orders of magnitude increase in charge carrier mobility within the film, as measured in thin film transistor (TFT) devices. Of a more fundamental interest, our technique allows the bridging of semiconducting crystals, without the formation of injection barriers at the connection points.

Structural anomalies at the magnetic transition in centrosymmetric BiMnO3

The structural properties of