Antonino Licciardello

Effect of organic contaminants on the oxidation kinetics of silicon at room temperature

The oxidation kinetics of HF‐etched n‐ and p‐doped silicon in air at room temperature have been studied by electron spectroscopy for chemical analysis. No great differences have been found between the n‐ and p‐type oxidation kinetics at the low doping level of the studied samples. The rate of oxide growth on the HF‐etched surface is much lower than that on a silicon surface obtained by fracture in air of a silicon monocrystal. The behavior of a silicon sample fractured in de‐ionized water and then oxidized in air at room temperature is intermediate. The above findings have been interpreted on the basis of surface reactions involving the plasticizers of the HF and water containers. These reactions produce carbon‐rich hydrophobic surfaces which retard the silicon oxide growth. A mechanism for the involved surface reactions is proposed.

Pulsed radiofrequency glow discharge time of flight mass spectrometer for the direct analysis of bulk and thin coated glasses

Direct solid analysis of bulk and thin coated glasses by pulsed radiofrequency (rf) glow discharge time-of-flight mass spectrometry (GD-TOFMS) is investigated. Modulated low pressure plasma created by pulsed-rf-GD has been coupled to a fast TOFMS in order to obtain complete mass spectra information from the different GD pulse domains (pre-peak, plateau and afterglow). In particular, it was observed that the analytes show the highest atomic ion signals in the afterglow region some hundred microseconds after the maximum of the Ar ion signal. However, it should be highlighted that the analyte ions exhibited their peak maxima at different delay times, depending on the element, after the end of the GD pulse. Such ion signal delays have been measured for different selected isotopes, covering a mass spectrum from light to heavy isotopes at different conditions of pressure and applied power. The results showed that ion signal delays are influenced by both the isotope mass and the pressure of the GD. Furthermore, GD operating conditions (pressure and applied power) were optimised in terms of sensitivity, using a bulk glass certified reference material (NIST 1411). The best analytical performance was observed at low pressure (150–200 Pa) and high applied power (135 W). Moreover, different pulse GD duty cycles (relationship between pulse duration and pulse period) were investigated. An optimum value of 50–65% duty-cycle was selected considering the signal stability and the signal intensity. The previously optimized pulsed-rf-GD-TOFMS system was then evaluated for qualitative in-depth profile analysis of thin coatings deposited onto thick glass substrates. High depth resolution (nm range), comparable to that obtained using rf-GD-OES was achieved. However, the observed depth resolution using the ToF-SIMS system is still superior. In this sense, analytical figures of merit observed in our pulsed-rf-GD-TOFMS demonstrate its great analytical potential for high depth resolution analysis of coated glasses.

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.

Covalent surface modification of titanium oxide with different adhesive peptides: surface characterization and osteoblast-like cell adhesion.

A fundamental goal in the field of implantology is the design of innovative devices suitable for promoting implant-to-tissue integration. This result can be achieved by means of surface modifications aimed at optimizing tissue regeneration. In the framework of oral and orthopedic implantology, surface modifications concern both the optimization of titanium/titanium alloy surface roughness and the attachment of biochemical factors able to guide cellular adhesion and/or growth. This article focuses on the covalent attachment of two different adhesive peptides to rough titanium disks. The capability of biomimetic surfaces to increase osteoblast adhesion and the specificity of their biological activity due to the presence of cell adhesion signal-motif have also been investigated. In addition, surface analyses by profilometry, X-ray photoelectron spectroscopy, and time of flight-secondary ion mass spectrometry have been carried out to investigate the effects and modifications induced by grafting procedures.

Ion‐chain interaction in keV ion‐beam‐irradiated polystyrene

Molecular weight distribution has been measured in monodisperse polystyrene film (MW=9 000 amu) after ion bombardment, in the ion fluence range 1011–1013 ions/cm2. The chosen beams are 100 keV He, 200 keV Ne, and 400 keV Ar. The experimental data have been interpreted in terms of a simple statistical model for cross‐links. The chemical yield is found to be very high and equal to 0.30, about a factor of 10 higher than the values given in the literature for gamma irradiation [M. Dole, in The Radiation Chemistry of Macromolecules (Academic, New York, 1973), Vol. 2, Chap. 5, p. 57].

Pulsed radiofrequency glow discharge time-of-flight mass spectrometry for molecular depth profiling of polymer-based films

We demonstrate the potential of an innovative technique, pulsed radiofrequency glow discharge time-of-flight mass spectrometry, for the molecular depth profiling of polymer materials. The technique benefits from the presence, in the afterglow of the pulsed glow discharge, of fragment ions that can be related to the structures of the polymers under study. Thin films of different polymers (PMMA, PET, PAMS, PS) were successfully profiled with retention of molecular information along the profile. Multilayered structures of the above polymers were also profiled, and it was possible to discriminate among layers having similar elemental composition but different polymer structure.

How the Surface Nanostructure of Polyethylene Affects Protein Assembly and Orientation

Protein adsorption plays a key role in the biological response to implants. We report how nanoscale topography, chemistry, crystallinity, and molecular chain anisotropy of ultrahigh molecular weight polyethylene (UHMWPE) surfaces affect the protein assembly and induce lateral orientational order. We applied ultraflat, melt drawn UHMWPE films to show that highly oriented nanocrystalline lamellae influence the conformation and aggregation into network structures of human plasma fibrinogen by atomic force microscopy with unprecedented clarity and molecular resolution. We observed a transition from random protein orientation at low concentrations to an assembly guided by the UHMWPE surface nanotopography at a close to full surface coverage on hydrophobic melt drawn UHMWPE. This assembly differs from the arrangement at a hydrophobic, on the nanoscale smooth UHMWPE reference. On plasma-modified, hydrophilic melt drawn UHMWPE surfaces that retained their original nanotopography, the influence of the nanoscale surface pattern on the protein adsorption is lost. A model based on protein-surface and protein-protein interactions is proposed. We suggest these nanostructured polymer films to be versatile model surfaces to provide unique information on protein interactions with nanoscale building blocks of implants, such as nanocrystalline UHMWPE lamellae. The current study contributes to the understanding of molecular processes at polymer biointerfaces and may support their future design and molecular scale tailoring.

Primary chemical events in ion bombarded polystyrene films: an infrared study

Abstract The chemical phenomenology which occurs in polystyrene films bombarded with low doses 100 keV He+ ions has been investigated by infrared spectroscopy. The results obtained show that the polymer is rapidly transformed into a different chemical compound consistent with high cross-linking as indicated by low solubility in CHCI3. The I.R. finding is in agreement with XPS findings and indicates that the observed phenomenology occurs via a mechanism which eliminates the electronic “π” system of the phenyl ring and gives rise to alkene-type bonding.

Soft-landed protein voltammetry: a tool for redox protein characterization.

Microperoxidase-11 (MP-11) was first soft landed onto the gold surface of a screen-printed electrode. Intact protein deposition was verified by time-of-flight secondary ion mass spectrometry. The coupling of soft landing with electrochemical techniques allowed unique information to be obtained about the deposition features. A full characterization of the direct electron-transfer properties was performed by modeling data obtained from cyclic voltammetry experiments; calculated values of kinetic electron-transfer constant, formal redox potential, and reorganization energy allow us to hypothesize the mechanism involved in soft landing immobilization and demonstrate the different conformation of the enzyme deposited from two different charged species. The strong interaction between MP-11 and the gold surface and long-term stability of the functionalized electrode characterizes the peculiar features of this procedure, which enhance its power with respect to the existing immobilization procedure and ensure its suitability for those practical applications that could benefit from an unmediated bridgeless bioeletrochemical electron transfer (e.g., biosensor transducers or electrode elements in biofuel cells).

Crosslinking yield in particle beam irradiated polystyrene

Abstract The crosslinking yields in narrow molecular weight distribution polystyrene bombarded with H + and noble gas ions at energies of 100–400 keV have been compared with yields computed from literature data. The data have been plotted vs the linear energy deposition of the bombarding ions and a threshold located at ca. 10 eV/A has been found. This curve has been discussed and explained in terms of energy deposited along the ion track and in terms of type and concentration of intermediate chemical species produced by the particle beam passage.