Maria Elena Fragalà

Ion beam effects on the surface and on the bulk of thin films of polymethylmethacrylate

Abstract Poly(methyl methacrylate) (PMMA) is a prototype polymer for positive resist (scission rate higher than cross linking rate). Notwithstanding its importance the literature studies on the chemical effects induced by energetic beam irradiation on this polymer are relatively scarce. The interest for PMMA is considerable also because it has been reported in literature that beyond a given threshold fluence (that in turn depends on the ion) the resist turns negative: i.e. the cross linking rate becomes higher than the scission rate. No explanation has been reported till now in literature for this behaviour. In this paper we report a study on the molecular weight distribution (MWD) of thin PMMA films irradiated with energetic beams. In addition we have also performed a study of the effects induced by the bombardment on the surface of the polymer by using the XPS (X-ray photoelectron spectroscopy) technique. The main results are: i) There is a strong evidence of contemporary scission and cross linking events. The first ones predominate at low fluences, the second ones at high fluences. ii) While at low fluence there is a good agreement between the experimental results and a simple equation based on random scission model, with increasing fluence one observes larger and larger deviations from this equation. These results will be discussed and compared with the existing literature reports.

Fluorinated β-Diketonate Diglyme Lanthanide Complexes as New Second-Order Nonlinear Optical Chromophores: The Role of f Electrons in the Dipolar and Octupolar Contribution to Quadratic Hyperpolarizability

The second-order nonlinear optical (NLO) properties of [Ln(hfac)(3)(diglyme)] (hfac = hexafluoroacetylacetonate; diglyme = bis(2-methoxyethyl)ether; Ln = La, Ce, Pr, Sm, Eu, Gd, Er, Lu) complexes have been investigated by a combination of electric-field second harmonic generation (EFISH) and harmonic light scattering (HLS) techniques, providing evidence for the relevant role of f electrons in tuning the second-order NLO response dominated by the octupolar contribution. These lanthanide NLO chromophores allow a clean valuation of the influence of f electrons on the quadratic hyperpolarizability and on its dipolar and octupolar contributions. Molecular quadratic hyperpolarizability values measured by the EFISH method, beta(EFISH), initially increase rapidly with the number of f electrons, the value for the Gd complex being 11 times that of the La complex, whereas this increase is much lower for the last seven f electrons, the beta(EFISH) value of the Lu complex being only 1.2 times that of the Gd complex. The increase of beta(HLS), which is dominated by an octupolar contribution, is much lower along the Ln series. Remarkably, the good beta(HLS) values of these simple systems, well known for their luminescence properties, are reached at no cost of transparency.

AFM and XPS study of ion bombarded poly(methyl methacrylate)

Abstract An in situ XPS analysis was performed on nearly monodisperse poly(methyl methacrylate) (PMMA) samples (Mp = 31 kDa) deposited on silicon substrate. The samples have been bombarded with 5 keV Ar+ in a fluence range from 1012 to 5 × 1015cm−2. The experimental evidence inferred from the XPS findings is consistent with a loss of oxygen-rich moieties, connected to main chain scission of the polymer under bombardment. In addition to X-ray photoelectron spectroscopy we used the atomic force microscopy (AFM) technique in order to gain information on the changes in surface topography under ion irradiation. The main AFM result is that the bombardment induces a topographic reorganisation towards a less complex surface and a loss of the pristine high frequency features. We explain these data basing upon the fact that at high fluence (> 1014 cm−2) the PMMA (a positive resist) turns to a negative tone resist and cross-linking becomes the most probable effect giving rise to a 3D network. This heavy modification of the polymer skin under ion beam shrinks the polymer surface and is responsible for the topographic reorganisation detected by AFM.

Core–shell Zn-doped TiO2–ZnO nanofibers fabricated via a combination of electrospinning and metal–organic chemical vapour deposition

Zn-doped TiO2 nanofibers shelled with ZnO hierarchical nanoarchitectures have been fabricated combining electrospinning of TiO2 (anatase) nanofibers and metal–organic chemical vapor deposition (MOCVD) of ZnO. The proposed hybrid approach has proven suitable for tailoring both the morphology of the ZnO external shell as well as the crystal structure of the Zn-doped TiO2 core. It has been found that the Zn dopant is incorporated in calcined electrospun nanofibers without any evidence of ZnO aggregates. Effects of different Zn doping levels of Zn-doped TiO2 fibers have been scrutinized and morphological, structural, physico-chemical and optical properties evaluated before and after the hierarchical growth of the external ZnO shell over the electrospun nanofibers. Moreover, doping promotes the incipient transition from the anatase to rutile phase in the core–shell Zn-doped TiO2–ZnO nanostructures at lower temperature than that observed for pure TiO2. Finally, the present core–shell hierarchical nanofibers possess a very large surface to volume ratio and exhibit a marked cathodoluminescence with a strong UV and visible emission.

Interaction of Tricationic Corroles with Single/Double Helix of Homopolymeric Nucleic Acids and DNA

In this manuscript a multitechnique approach is proposed to characterize the interaction between new tri-N-methylpyridyl corrole (TMPC) and its germanium(IV) derivative (GeTMPC), with single- and double-stranded nucleic acid homopolymers and calf thymus DNA. The specificity of each spectroscopic technique has been exploited to analyze the different aspects of corrole binding. Noteworthy, this approach allows us to distinguish between H aggregation of TMPC in the presence of polyriboadenilic acid (poly(rA)) and J aggregates in the presence of polyribocytidinic acid (poly(rC)) as well as to identify the formation of GeTMPC dimers in the presence of single-stranded poly(rA) and pseudointercalation with single-stranded poly(rC).

ION BEAM ASSISTED UNZIPPING OF PMMA

Abstract Ion beam assisted thermal depolymerisation of poly (methyl methacrylate) (PMMA) thin films, 1 μm thick, has been studied in the temperature range RT – 400°C using a 300 keV He + beam at very low fluence ( 12 ions cm −2 ). The experiments have been performed using an in situ mass spectrometer in a 10 −7 torr vacuum chamber. A relevant methyl methacrylate monomer (MMA) evolution (mass signal m / z =100) at temperature (150°C) well below the polymer degradation temperature (360°C) has been detected during irradiation. These results have been compared with gas evolution during room temperature ion irradiation where volatile products different from monomer are known to be produced due to chain scission events. The low-temperature depolymerisation induced by ion irradiation performed on PMMA at temperature higher than T g (105°C) is promoted by the presence of beam-induced reactive species acting as initiators.

Sequence, Stoichiometry, and Dimensionality Control in Porphyrin/Bis‐calix[4]arene Self‐Assemblies in Aqueous Solution

The use of a water-soluble octacationic bis-calix[4]arene with divergent cavities (BC(4)) as a templating agent for the assembly of a tetraanionic porphyrin (CuTPPS) has allowed the noncovalent synthesis of 2D or 3D multiporphyrin assemblies. Self-assembly of CuTPPS and BC(4) molecules proceeded under hierarchical control in a stepwise fashion to yield discrete and isolable supramolecular nanostructures containing up to 33 molecular elements (i.e., the CuTPPS/BC(4) 17:16 assembly, obtained in less than three hours). The formation of these species could be conveniently monitored by means of UV/Vis spectroscopy by following the absorbance of the Soret band at 412 nm. In particular, the attainment of the pivotal CuTPPS/BC(4) 5:4 species with a cruciform structure, as the key fork-point intermediate for the subsequent formation of the higher 2D and 3D assemblies, has been demonstrated by light-scattering studies and by an unequivocal synthesis of mixed-porphyrin/calixarene 5:4 species involving the use of two different types of metallated porphyrins, namely CuTPPS and MnTPPS. The remarkable stability of these assemblies permits a stepwise synthesis that makes it possible to choose the desired porphyrin sequence.

Reversible photoswitching of stimuli-responsive Si(100) surfaces engineered with an assembled 1 -cyano-1-phenyl-2-[4/-(10-undecenyloxy)phenyl]-ethylene monolayer

Si(100) surfaces were molecularly engineered by covalent linkage of a monolayer of two stilbene-based chromophores, either 1-cyano-1-phenyl-2-[4′-(10-undecenyloxy)phenyl]-ethylene or its chlorine derivative, 1-cyano-1-(4-Cl-phenyl)-2-[4′-(10-undecenyloxy)phenyl]-ethylene. The hybrid systems have been probed by monochromatized angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and atomic force microscopy (AFM) measurements. Results indicated robust covalent linkage of stilbene molecules to the functionalized substrate surfaces. AFM lithography and contact angle (CA) analysis confirmed that the adopted molecular architectures proved to be well-suited for reversible cis–trans photoswitching promoted by UV irradiation in the solid state.

Ultrathin and nanostructured ZnO-based films for fluorescence biosensing applications

The fluorescence-based sensing capability of ultrathin ZnO-SiO(2) nanoplatforms, deposited by an integrated approach of colloidal lithography and metal organic chemical vapor deposition, has been investigated upon adsorption of fluorescein-labeled albumin, used as model analyte biomolecule. The protein immobilization process after spontaneous adsorption/desorption significantly enhances the green emission of the different ZnO-based films, as evidenced by scanning confocal microscopy, corresponding to a comparable protein coverage detected by X-ray photoelectron spectroscopy. Moreover, experiments of fluorescence recovery after photobleaching evidence that the protein lateral diffusion at the biointerface is affected by the chemical and/or topographical patterning of hybrid ZnO-SiO(2) surfaces. The used approach is very promising for biomolecular detection applications of these ZnO-SiO(2) nanoplatforms, by simple sizing of the 2D vs. 3D patterning design, which in turn is accomplished by the fine tuning of the integrated colloidal lithography-chemical vapor deposition processes.

Optical trapping of silver nanoplatelets

Optical trapping of silver nanoplatelets obtained with a simple room temperature chemical synthesis technique is reported. Trap spring constants are measured for platelets with different diameters to investigate the size-scaling behaviour. Experimental data are compared with models of optical forces based on the dipole approximation and on electromagnetic scattering within a T-matrix framework. Finally, we discuss applications of these nanoplatelets for surface-enhanced Raman spectroscopy.