E. Axente

Comparative investigation of solar cell thin film processing using nanosecond and femtosecond lasers

The purpose of the present study was to examine the possibility of laser-machining of CuInSe2-based photovoltaic devices. Therefore, ablation thresholds and ablation rates of ZnO, CuInSe2 and Mo thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti : sapphire laser. The experimental results were compared with the theoretical evaluation of the samples heat regime obtained from numerical calculations. In addition, the photo-electrical properties of the solar cells were measured before and after laser-machining. Scanning electron microscopy and energy dispersive x-ray analyses were employed to characterize the laser-induced ablation channels. As a result, two phenomena were found to limit the laser-machining process: (i) residues of Mo that were projected onto the walls of the ablation channel and (ii) the metallization of the CuInSe2 semiconductor close to the channel. Both effects lead to a shunt in the device that decreases the photovoltaic efficiency. As a consequence of these limiting effects, micromachining of CuInSe2-based solar cells was not possible with nanosecond laser pulses. Only subpicosecond laser pulses provided selective or complete ablation of the thin layers without a relevant change in the photoelectrical properties.

Nanostructured ZnO coatings grown by pulsed laser deposition for optical gas sensing of butane

We report the detection of 100ppm of butane in superatmospheric N2 or air with an m-lines setup. The sensing elements are ZnO-nanostructured coatings prepared by pulsed laser deposition. The deposition technique was optimized to obtain highly transparent films of 1cm2 in area and several hundreds nanometer thick. ZnO structures preserve gas sensitivity even when deposited at room temperature. Refractive index variations down to 0.005 were detected and typical variations of about 20% were induced during ZnO film-butane contact.

Combinatorial matrix-assisted pulsed laser evaporation: Single-step synthesis of biopolymer compositional gradient thin film assemblies

We introduce a combinatorial approach for the fabrication of organic biopolymer thin films. Structures with compositional gradient are obtained by simultaneous laser vaporization of two distinct targets. Matrix-assisted pulsed laser evaporation deposition method was applied to obtain a compositional library of levan and oxidized levan in form of thin film. The gradient of film composition and structure was demonstrated by infrared spectroscopy while in vitro cell culture assays illustrated characteristic responses of cells to specific surface regions. The method can rapidly generate discrete areas of organic film compositions with improved properties than starting materials.

Strontium and zoledronate hydroxyapatites graded composite coatings for bone prostheses.

Both strontium and zoledronate (ZOL) are known to be useful for the treatment of bone diseases associated to the loss of bone substance. In this work, we applied an innovative technique, Combinatorial Matrix-Assisted Pulsed Laser Evaporation (C-MAPLE), to deposit gradient thin films with variable compositions of Sr-substituted hydroxyapatite (SrHA) and ZOL modified hydroxyapatite (ZOLHA) on Titanium substrates. Compositional gradients were obtained by simultaneous laser vaporization of the two distinct material targets. The coatings display good crystallinity and granular morphology, which do not vary with composition. Osteoblast-like MG63 cells and human osteoclasts were co-cultured on the thin films up to 21 days. The results show that Sr counteracts the negative effect of relatively high concentration of ZOL on osteoblast viability, whereas both Sr and ZOL enhance extracellular matrix deposition. In particular, ZOL promotes type I collagen production, whereas Sr increases the production of alkaline phosphatase. Moreover, ZOL exerts a greater effect than Sr on osteoprotegerin/RANKL ratio and, as a consequence, on the reduction of osteoclast proliferation and activity. The deposition method allows to modulate the composition of the thin films and hence the promotion of bone growth and the inhibition of bone resorption.

Enhanced gas sensing of Au nanocluster-doped or -coated zinc oxide thin films

We demonstrated that doping or covering with Au nanoclusters boosts gas sensing effectiveness of optical metal oxide sensors. The sensing response of pulsed laser deposited ZnO films as sensing element was tested by m-line technique for low concentration (1000ppm) of butane in environmental N2. The optical interrogation was performed for three types of coatings: undoped ZnO, undoped ZnO structures partially covered with Au nanoclusters, or obtained from Au (0.5wt%) doped ZnO targets. Nanocluster coating tripled the sensitivity, while doping resulted in an increase of up to 45% as compared with simple structures.

Accurate analysis of indium–zinc oxide thin films via laser-induced breakdown spectroscopy based on plasma modeling

We report on accurate analysis of indium–zinc oxide thin films via laser-induced breakdown spectroscopy (LIBS) based on the calculation of the spectral radiance of the nonuniform laser-produced plasma. A thin film sample with variable elemental composition was irradiated with ultraviolet nanosecond laser pulses and the plasma emission spectra were characterized using time-resolved optical emission spectroscopy. Thus, the spectrum recorded with an Echelle Spectrometer coupled to a gated detector was compared to the spectral radiance computed for a plasma in local thermodynamic equilibrium conditions. The time evolution of the plasma was studied to find optimized recording conditions for which the self-absorption of spectral lines is minimized. In addition, the time-resolved measurements allowed us to determine the Stark broadening parameters of spectral lines used for the LIBS analysis. The metal fractions measured via LIBS were found to be in good agreement with the values obtained by complementary measurements using energy dispersive X-ray spectroscopy. The relative decay between fractions measured with both methods was smaller than 5% over the entire measurement range that implied the variation by a factor of four in the case of zinc. The present results show that LIBS measurement procedures based on plasma modeling could be used for fast quality control in the industrial production of thin films.

Probing electron-phonon coupling in metals via observations of ablation plumes produced by two delayed short laser pulses

We investigate electron-phonon coupling in metals by analyzing the composition of plumes produced by laser ablation with two delayed short laser pulses. Samples of Ti, Zr, and Hf are chosen since they have similar thermo-physical properties but significant different atomic masses. It is shown that the atomic emission intensity increases with the interpulse delay, whereas the nanoparticle emission intensity decreases. The characteristic time of plume changes is found to depend linearly on the metals’ atomic mass. Theoretical considerations suggest that the measured times equal to the characteristic times of electron-lattice relaxation.

Active protein and calcium hydroxyapatite bilayers grown by laser techniques for therapeutic applications

Active protein and bioceramic calcium hydroxyapatite (HA) bilayers were grown by combining conventional pulsed laser deposition (PLD) and matrix-assisted pulsed laser evaporation (MAPLE) techniques. A pulsed UV KrF* excimer laser was used for the irradiations. The HA layers were grown by PLD. Proteins with antimicrobial action were attached to the bioceramic layers using MAPLE. The composite MAPLE targets were obtained by dissolving the proteins powder in distilled water. The crystalline status and chemical composition of the obtained structures were studied by X-ray diffractometry and Fourier transform infrared spectroscopy. The layers were grown for the design of advanced future metal implants coatings, ensuring both enhanced bone formation and localized antimicrobial therapy. Our results demonstrated that protein coatings improve bone cell proliferation in vitro. Immunofluorescence experiments show that actin filaments stretch throughout bone cells and sustain their optimal spreading.

Tailoring immobilization of immunoglobulin by excimer laser for biosensor applications

The sheltered transfer and immobilization of rabbit anti-human antiserum immunoglobulin G (IgG) by matrix-assisted pulsed laser evaporation (MAPLE) are reported. The iced targets submitted to laser irradiation consisted of 0.2-2 mg/mL IgG blended or not with lipid (L-α-phosphatidylcholine dipalmitoyl) dissolved in distilled water-based saline buffer. Thin IgG coatings were obtained at room temperature onto glass, fused silica, or silicon substrates. Ten thousand subsequent laser pulses of 0.33, 0.5, or 0.67 J/cm(2) fluence were applied for the synthesis of each sample. Morphology and composition of the thin films were studied by optical, scanning, and atomic force microscopy and Fourier transformed infrared spectrometry. Optical labeling methods such as spectrofluorimetry and fluorescence microscopy were selected to verify the biosensor transduction principle because of their high sensitivity for detecting low amounts of antigen (IgG). Protein immobilization to the substrate surface was demonstrated for all obtained structures after immersion in the donkey anti-rabbit secondary antibody solution. The IgG transfer and immobilization onto substrates were improved by addition of lipid to MAPLE solutions.

Ideal radiation source for plasma spectroscopy generated by laser ablation

Laboratory plasmas inherently exhibit temperature and density gradients leading to complex investigations. We show that plasmas generated by laser ablation can constitute a robust exception to this. Supported by emission features not observed with other sources, we achieve plasmas of various compositions which are both uniform and in local thermodynamic equilibrium. These properties characterize an ideal radiation source opening multiple perspectives in plasma spectroscopy. The finding also constitutes a breakthrough in the analytical field as fast analyses of complex materials become possible.