Giuseppe Compagnini

Structural modification of polymer films by ion irradiation

Abstract The atomic and electronic structure of polymer films undergoes deep modifications during high energy (keV-MeV) ion irradiation, from molecular solid to amorphous material. At low energy density (1022–1024 eV cm 3 ) typical effects include chain scissions, crosslinks, molecular emission and double bonds formation. In hydrocarbon polymer (polystyrene, polyethylene) the main effect of irradiation is the formation of new bonds as detected by molecular weight distribution, solubility and optical measurements. Moreover the concentration of trigonal carbon (sp2) in the polymer changes with ion fluence (1011–1014 ions cm 2 ) and stabilizes to a value of 20% independently on the initial chemical structure of the irradiated sample. Photoemission spectroscopy shows an evolution of valence band states from localized to extended states. At high energy density (1024–1026 eV cm 3 ) the irradiated polymer continues to evolve showing spectroscopic characteristics close to those of hydrogenated amorphous carbon. Trigonal carbon concentration changes with ion fluence (1014–1016 ions cm 2 ) reaching the steady state value of 60% and the hydrogen concentration decreases to 20%. Moreover the values of the optical gap (2.5–0.5 eV) suggest the presence of medium range order in the obtained hydrogenated amorphous carbon. These values are consistent with the formation of graphitic clusters, whose size goes from 5 A to 20 A by changing the ion fluence (or energy density).

Production of gold nanoparticles by laser ablation in liquid alkanes

Pulsed laser ablation of Au target in n-alkanes generates Au sols. Experiments have been performed using the second harmonic of a Nd:yttritium–aluminum–garnet at 532 nm wavelength with a 5 ns pulse duration at a repetition rate of 10 Hz and fluences ranging from 1 to 200 J/cm2. Data coming from optical extinction show that the length of the hydrocarbon chain is able to tune the shape of the produced particles and the rate of formation in a controlled way. The data are supported by atomic force microscopy and transmission electron microscope observations.

Raman spectra of virgin and damaged graphite edge planes

Abstract First-order Raman spectra of virgin and ion irradiated highly oriented pyrolytic graphite were performed along the basal and edge planes. Ion irradiation was performed in the low fluence regime (≈ 1012–1013 ions cm−2) using a 400 keV Ar+ beam in order to a introduce controlled amount of defects in the structure. Virgin edge Raman spectra reveal the presence of a 1351 cm−1 structure (D∗) shifted 10 cm−1 away from the disorder induced D-line found upon irradiating or grinding the graphite surface. The 1351 cm−1 D∗ structure is then considered as the feature coming from the rupture of the D6h4 space group symmetry. A comparison between D∗ and the classical 1360 cm−1 D-line is discussed in terms of induced disorder.

Plasmon-enhanced optical trapping of gold nanoaggregates with selected optical properties.

We show how light forces can be used to trap gold nanoaggregates of selected structure and optical properties obtained by laser ablation in liquid. We measure the optical trapping forces on nanoaggregates with an average size range 20-750 nm, revealing how the plasmon-enhanced fields play a crucial role in the trapping of metal clusters featuring different extinction properties. Force constants of the order of 10 pN/nmW are detected, the highest measured on a metal nanostructure. Finally, by extending the transition matrix formalism of light scattering theory to the optical trapping of metal nanoaggregates, we show how the plasmon resonances and the fractal structure arising from aggregation are responsible for the increased forces and wider trapping size range with respect to individual metal nanoparticles.

High-rate in-plane micro-supercapacitors scribed onto photo paper using in situ femtolaser-reduced graphene oxide/Au nanoparticle microelectrodes

Direct laser-reduction of graphene oxide (GO), as a lithography-free approach, has been proven effective in manufacturing in-plane micro-supercapacitors (MSCs) with fast ion diffusion. However, the power density and the charge/discharge rate are still limited by the relatively low conductivity of electrodes. Here, we report a facile approach by exploiting femtolaser in situ reduction of the hydrated GO and chloroauric acid (HAuCl4) nanocomposite simultaneously, which incorporates both the patterning of rGO electrodes and the fabrication of Au current collectors in a single step. These flexible MSCs boast achievements of one-hundred fold increase in electrode conductivities of up to 1.1 × 106 S m−1, which provide superior rate capability (50% for the charging rate increase from 0.1 V s−1 to 100 V s−1), sufficiently high frequency responses (362 Hz, 2.76 ms time constant), and large specific capacitances of 0.77 mF cm−2 (17.2 F cm−3 for volumetric capacitance) at 1 V s−1, and 0.46 mF cm−2 (10.2 F cm−3) at 100 V s−1. The use of photo paper substrates enables the flexibility of this fabrication protocol. Moreover, proof-of-concept 3D MSCs are demonstrated with enhanced areal capacitance (up to 3.84 mF cm−2 at 1 V s−1) while keeping high rate capabilities. This prototype of all solid-state MSCs demonstrates the broad range of potentials of thin-film based energy storage device applications for flexible, portable, and wearable electronic devices that require a fast charge/discharge rate and high power density.

Synthesis, Properties and Potential Applications of Porous Graphene: A Review

Since the discovery of graphene, many efforts have been done to modify the graphene structure for integrating this novel material to nanoelectronics, fuel cells, energy storage devices and in many other applications. This leads to the production of different types of graphene-based materials, which possess properties different from those of pure graphene. Porous graphene is an example of this type of materials. It can be considered as a graphene sheet with some holes/pores within the atomic plane. Due to its spongy structure, porous graphene can have potential applications as membranes for molecular sieving, energy storage components and in nanoelectronics. In this review, we present the recent progress in the synthesis of porous graphene. The properties and the potential applications of this new material are also discussed.

Synthesis of gold colloids by laser ablation in thiol-alkane solutions

In this paper, we present a study on the formation of gold colloids by laser ablation of a gold metal target in alkanes and thiol-alkane solutions. The results show a decrease of the gold particles’ size up to 2 nm when thiol molecules are present in the liquid environment. The observation of a blue-shift of the surface plasmon resonance is discussed together with transmission electron microscopy analyses accounting the cluster size decrease and the stabilization of the obtained suspensions.

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.

The role of micro- and nanomorphology of rough silver surfaces of different nature in surface enhanced Raman scattering effect: A combined study of scanning force microscopy and low-frequency Raman modes

The micro- and nanomorphology of surface enhanced Raman scattering (SERS) active rough substrates obtained by plasma oxidation-reduction cycles onto original flat silver surfaces have been investigated by means of a dual technique approach. Scanning force microscopy and low-frequency Raman spectroscopy give complementary results when applied on very rough systems. Almost spherical silver colloids have been used as well-defined systems to model, by their stacking over flat silicon wafers, the plasma roughening process inducing SERS activity. The SERS activity results are strongly related to the micromorphology of the nanoparticles assembly, rather than to the silver cluster size. In particular an electromagnetic enhancement factor of 103 for the breathing mode of the polystyrene aromatic rings was found to be related to the vertical stacking of tens of clusters about 10 nm in diameter.

Pulsed laser ablation of a continuously-fed wire in liquid flow for high-yield production of silver nanoparticles

Using wires of defined diameters instead of a planar target for pulsed laser ablation in liquid results in significant increase of ablation efficiency and nanoparticle productivity up to a factor of 15. We identified several competitive phenomena based on thermal conductivity, reflectivity and cavitation bubble shape that affect the ablation efficiency when the geometry of the target is changed. On the basis of the obtained results, this work represents an intriguing starting point for further developments related to the up-scaling of pulsed laser ablation in liquid environments at the industrial level.