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International audience; A comparative experimental study of pulsed laser ablation in vacuum of two binary semiconductors, zinc oxide and indium phosphide, has been performed using IR-and visible laser pulses with particular attention to cluster generation. Neutral and cationic Zn_n O_m and In_n P_m particles of various stoichiometry have been produced and investigated by time-of-flight mass spectrometry. At ZnO ablation, large cationic (n>9) and all neutral clusters are mainly stoichiometric in the ablation plume. In contrast, indium phosphide clusters are strongly indium-rich with In_4 P being a magic cluster. Analysis of the plume composition upon laser exposure has revealed congruent vaporization of ZnO and a disproportionate loss of phosphorus by the irradiated InP surface. Plume expansion conditions under ZnO ablation are shown to be favourable for stoichiometric cluster formation. A delayed vaporization of phosphorus under InP ablation has been observed that results in generation of off-stoichiometric clusters.

Neutral and cationic Zn n O m clusters of various stoichiometry have been produced by nanosecond laser ablation of ZnO in vacuum and investigated by time-of-flight mass spectrometry. Particular attention was paid to the effect of laser wavelength (in the range from near-IR to UV) on cluster composition. Under 193-nm laser ablation, the charged clusters are essentially substoichiometric with $\mathrm{Zn}_{n}\mathrm{O}_{n-1}^{+}$ and $\mathrm{Zn}_{n}\mathrm{O}_{n-3}^{+}$ being the most abundant series. Both sub- and stoichiometric cationic clusters are generated in abundance at 532- and 1064-nm ablation whose composition depends on the cluster size. The reactivity of small stoichiometric $\mathrm{Zn}_{n}\mathrm{O}_{n}^{+}$ clusters (n

This work presents hybrid bulk heterojunction solar cells based on dye-sensitized zinc oxide (ZnO) nanorods blended with poly(3-hexylthiophene) (P3HT). Tetra(4-carboxyphenyl)porphyrin (TCPP) molecules were grafted onto the surface of ZnO nanorods to enlarge the absorption spectrum of the blend. We demonstrate that additional bands in the external quantum efficiency (EQE) spectra corresponding to Soret and Q-band absorption can already been observed at very low dye concentration at the ZnO surface. Therefore, direct grafting of TCPP onto ZnO nanorods leads to very efficient electron injection process into the ZnO nanorods after the light absorption of the dye. However, the overall photocurrent of the devices decreases gradually with TCPP concentration at the ZnO nanorod surface. The recombination dynamics of the photogenerated charges at the P3HT:ZnO interface are investigated by transient absorption spectroscopy on micro- to millisecond time scales. We observe that the lifetime of the P3HT polarons is red...

Pulsed laser deposited nanocrystalline V 2 O 5 thin films were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy (HRTEM) and optical spectroscopy. The films were deposited on amorphous glass substrates, keeping the O 2 partial pressure at 13.33 Pa and the substrate temperature at 220 °C. The characteristics of the films were changed by varying the laser fluence and repetition rate. XRD revealed that films are nanocrystalline with an orthorhombic structure. XPS shows the sub-stoichiometry of the films, that generally relies on the fact that during the formation process of V 2 O 5 films, lower valence oxides are also created. From the HRTEM images, we observed the size evolution and distribution characteristics of the clusters in the function of the laser fluence. From the spectral transmittance we determined the absorption edge using the Tauc plot. Calculation of the Bohr radius for V 2 O 5 is also reported.

Hybrid nanomaterials were synthesized by adopting femtosecond laser ablation of a ZnO target in pure ethanol. Dye molecules were grafted onto the ZnO nanoparticles by mixing colloidal ZnO–ethanol solution prepared by laser ablation to Tetramethylrhodamine B isothiocyante, or to Rhodamine B solutions. Strongly facetted nanohybrid particles were observed with an average size of 15 nm, by HRTEM measurements. From photoluminescence spectroscopy of the nanohybrids after single and multiphoton excitations, we observed energy transfer from the ZnO nanoparticles towards the attached dye molecules. Moreover, IR excitation spectra of the hybrid nanomaterials reveal the emission of the grafted dye via two photon absorption of the ZnO.

We present experimental and modeling studies of UV nanosecond pulsed laser desorption and ablation of (111) bulk silicon. The results involve a new approach to the analysis of plume formation dynamics under high-energy photon irradiation of the semiconductor surface. Non-thermal, photo-induced desorption has been observed at low laser fluence, well below the melting threshold. Under ablation conditions, the non-thermal ions have also a high concentration. The origin of these ions is discussed on the basis of electronic excitation of Si surface states associated with the Coulomb explosion mechanism. We present a model describing dynamics of silicon target excitation, heating and harge-carrier transport.

International audience; We study the steel surface modifications induced by irradiation of several lasers in air.The surface modifications are found to depend on the laser properties (Krf, 248 nm, 26 ns; XeCl, 308 nm, 25 ns; Nd:YAG, 1064 or 532 nm, 5 or 10 ns).The treated surface features (morphology, chemical composition and crystalline structure) are analysed by complementary techniques, such as X-ray photoelectron spectroscopy, nuclear reactions, scanning electron microscopy and grazing incidence X-ray diffraction.First of all, it is shown that the carbon coming from the machining oils is successfully removed under all the studied conditions.The incorporation of oxygen and nitrogen (beyond 1 mm in depth) to the surface is also shown.In addition to the original ferrite phase, new crystalline compounds are detected after treatment such as austenite, oxides (FeO or Fe2O3, Fe3O4) and nitride (εFe2-3 N).Depending on the laser characteristics, the main formed crystalline phases are different (oxide or nitride).A discussion of the expected surface modification mechanisms is presented. Under some experimental conditions, the formation of a backdeposition layer composed of oxides and hydroxides is observed

Rich populations of clusters have been observed after femtosecond laser ablation of bulk silicon in vacuum. Size and velocity distributions of the clusters as well as their charge states have been analyzed by reflectron time-of-flight mass spectrometry. An efficient emission of both neutral silicon clusters Si n (up to n =6) and their cations Si n + (up to n =10) has been observed. The clusters are formed even at very low laser fluences, below the ablation threshold, and their relative yield increases with fluence. We show the dependencies of the cluster yield as well as the expansion dynamics on both laser wavelength and laser fluence. The mechanisms of the cluster formation are discussed.

The condensation of zinc oxide nano clusters produced in gas phase has been evidenced in the ablation of a ZnO solid target by a pulsed ArF laser. We compare the spatio-temporal evolution of the shape of ZnO ablated plasma plume, both in vacuum and in controlled gas environments (Helium and/or Oxygen) from monitoring with CCD camera and from spectroscopic studies. The expansion of the plasma plume and the growth of the nanoclusters depend strongly upon the collisions between the ablated particles and surrounding gas molecules, as well as on the chemical reactions in the case of oxygen. The spectra of the observed plasma emittion are mainly due to atomic transitions in the neutral Zn. We have also observed the photoluminescence of the nanoclusters suspended in the gas and their decomposition in the beam of ArF laser., Comment: In French

International audience; This study focuses on the surface modification of a low alloy steel by XeCl laser under ambient conditions. These modifications were investigated employing two complementary methods. Auger electron spectroscopy was employed for chemical characterisation and depth profile and Scanning electron microscopy for the surface morphology after laser irradiation. It is shown that laser treatment induce the formation of Fe(III) oxide, mainly Fe2O3. This oxide can be described as a multilayer structure. One part, e.g. the upper layer, is formed with Fe2O3 aggregates due to the coalescence of small particles of Fe2O3 formed in the plasma by reaction of matter ejected from the surface with O2 gas in the atmosphere. Under this layer deposited on the surface, we find a Fe2O3 structured layer corresponding to the oxidation of the melt layer.

The response of cemented tungsten carbide and of titanium carbonitride was investigated with respect to damage and ablation properties, under interaction with ultrashort laser pulses. These carbide materials present high microhardness and are of significant interest for tribological applications. The experiments were carried out in air with a commercial Ti:sapphire laser at energy densities on the target up to 6.5 J/cm2. The irradiated target surfaces were analyzed with optical, SEM and AFM techniques and the damage and ablation threshold values were determined using the measured spot diameters and the calculated incident energy density distributions.

Several ultrahard materials and coatings of definite interest for tribological applications were tested with respect to their response when irradiated with fs laser pulses. Results on cemented tungsten carbide and on titanium carbonitride are reported for the first time and compared with outcomes of investigations on diamond and titanium nitride. The experiments were carried out in air, in a regime of 5–8 J/cm2 fluences, using the beam of a commercial Ti:sapphire laser. The changes induced in the surface morphology were analysed with a Nomarski optical microscope, and with SEM and AFM techniques. From the experimental data and from the calculated incident energy density distributions, the damage and ablation threshold values were determined. As expected, the diamond showed the highest threshold, while the cemented tungsten carbide exhibited typical values for metallic surfaces. The ablation rates determined (under the above-mentioned experimental conditions) were in the range 0.1–0.2 μm per pulse for all the materials investigated.

Experimental results on the gas-phase formation of neutral and cationic phosphorus clusters are presented. The clusters were synthesized by visible (532 nm) or UV (193 nm) laser ablation of crystalline red phosphorus under high vacuum conditions and were analyzed using TOF mass spectrometry. Neutral Pn clusters produced by 532-nm ablation are found to be even-numbered while P n + cations are mainly odd-numbered, with P 7 + and P 21 + being the most abundant ions. For UV laser ablation, stable compound clusters, neutral P7H3 and P23H5, and cations P23H 6 + were synthesized for the first time. The formation of Pn clusters by thermal vaporization of red phosphorus into a cold He gas was also investigated and only small clusters (n

We describe the basic principles of nanoparticle synthesis by conventional pulsed laser ablation. The generalization of the Zeldovich and Raizer theory of condensation has been performed for inhomogeneous laser-induced plume where the rates of nucleation as well as the condensation times are different for different parts of the plume. The theoretical development and analysis of the experimental results are given for condensation, expansion and properties of silicon nanoclusters.

The condensation of Si clusters, produced by ns-laser ablation, is completed typically after a few μs at sufficiently low pressure of background gas. The photoluminescence from free gas suspended Si-cluster was experimentally observed after a long delay time of the order of a few hundred μs. We show that this delay time is related to long cooling and crystallization processes necessary to reach the state with pronounced particle photoluminescence.

We report a photoluminescence study of silicon nanoclusters deposited by laser ablation. This technique allows to obtain clusters with a reduced size dispersion. Taking advantage of that as well as of the possibility to tune selectively the cluster size relatively to the preparation parameters, we are able to make a fine correlation between the emission band and the corresponding emitting size. The photoluminescence band can be tuned in a wide spectral region spreading from the near ultraviolet to the near infrared depending on preparation conditions. The correlation with atomic force microscopy measurements provides a size dependence of the luminescence which is fully consistent with the quantum confinement interpretation. To our knowledge, we present here the first observation of a wide spectral range tuning of the emission bands of nanocrystalline silicon by using a dry technique compatible with clean vacuum processing.

International audience; A three-dimensional Monte Carlo simulation of the laser desorbed flow of particles was performed. In addition to the gas-phase collisions, the chemical processes, i.e., recombination and dissociation are considered. The width of the angular distributions as a function of the radius of the evaporated area is analyzed. The comparison between the results obtained in the reactionless flow and in the flow with chemical reactions shows that in the presence of chemical processes the focusing effect of collisions is partly concealed and more wide angular distributions can be observed.

International audience; Laser ablation from one- and two-component targets into a diluted gas background is simulated by combined direct simulation-test particle Monte Carlo method. The spatial and velocity distributions of particles deposited at the plane substrate are calculated. The approach developed has allowed us to consider the influence of the collisions both among the ablated particles and between the ablated and ambient gas particles on the uniformity of film stoichiometry. It is found that the increase of the background gas pressure results in the more uniform distribution of the stoichiometrical ratio of the deposited particles with different masses.

Monte Carlo simulation is used to study the role of chemical reactions in the gas flow of particles laser desorbed from the target into a vacuum. The influence of recombination and dissociation processes on the properties of the gas flow is considered. It was found that chemical reactions have a significant effect on the composition of the desorption jet and on the angular and mean energy distributions of the desorbed particles. The study of these phenomena is of a particular interest for the understanding of the process of thin film deposition by pulsed laser ablation.

Spring Meeting of the European-Materials-Research-Society (E-MRS) / Symposium N / Symposium O / Symposium V on Laser Materials Processing for Micro and Nano Applications, Strasbourg, FRANCE, MAY 14-18, 2012; International audience; We investigate experimentally local non-linear absorption when 1.3 mu m wavelength femtosecond pulses are tightly focused inside n-type doped silicon. We show that 130 fs pulses with only few nanojoule energy is enough to initiate free-carrier generation. Our results also demonstrate that the laser energy deposition is independent on the doping concentration for substrates with free-carrier densities up to 10(18) cm(-3). For deep focusing experiments, the energy deposition can remain confined in micron-scale focal regions provided we perform the experiments with focused beam corrected for spherical aberration. The high degree of control observed in the experiments and the independence on doping are major assets for future 3D-micromachining technology developments based on this approach. (C) 2012 Elsevier B. V. All rights reserved.

17th International School on Quantum Electronics - Laser Physics and Applications (ISQE), Nessebar, BULGARIA, SEP 24-28, 2012; International audience; We investigate the non-linear absorption of 1.3 mu m femtosecond laser pulses strongly focused inside silicon and fused silica. Through transmission diagnostics, multiphoton initiated energy deposition is clearly observed inside these two materials with nanojoules laser pulse energy when using 0.3 numerical aperture objective. For silicon, the non-linear interaction is strongly dependent on the focusing depth due to the presence of spherical aberration contrarily to fused silica. Below the surface, we find a difference of three orders of magnitude between the intensity thresholds for nonlinear absorption at 1.3 mu m wavelength inside the two tested materials due to the difference of number of photons required for non-linear absorption. By measuring the transmission of the ionizing pulses during multiple pulse irradiation, irreversible modifications of the material are monitored inside fused silica in accordance with previous studies at 800 nm. For similar laser energy deposition, the response of bulk silicon remains unchanged over more than twenty thousands pulses suggesting no irreversible modifications are initiated.

An axisymetric two-dimensional modeling of laser-induced plume dynamics near the plasma ignition threshold in the ambient gas is carried out to provide a better understanding of the pulsed laser deposition process. In our modeling the ionization process is absent and the plume consists of neutral gas. The numerical investigations are performed with initial conditions close to an irradiance of about 100 MW/cm 2 . This model takes into account different initial conditions in the plume just after the ablation pulse and follows the hydrodynamic behaviour of the subsequent plume expansion. The hydrodynamics induced by the plume expansion are govemed by the two-dimensional unsteady Euler equation which are solved with the numerical method, based on a finite difference scheme associated with a flux corrected transport algorithm LCPFCT. The modeling was able to account for many important features of the gas plume dynamics. In particular, it is shown that the hydrodynamic behaviour and thermal relaxation of the gas plume are strongly dependent on the initial gradient of pressure and density between gas plume and background gas. Reversed flow of particles moving toward the target at certain distances along the R axis is clearly observed at a late expansion stage ( > 2 μs).

Pulsed laser ablation of a NbTe2 target was performed by an excimer laser in helium atmosphere at different fluences. Ablation plume characterization was realized by optical Time-of-Flight spectroscopy. Velocities and kinetic energies of the ablated particles were determined as a function of the fluence and of the background pressure. We observed an increase of the velocity of Te2 molecules when increasing the helium pressure. We show that this effect is due to three bodies associative reactions between monoatomic Te species and background gas particles.

Ablation of Si into He, Ar and O2 atmosphere by excimer laser has been investigated to understand the formation of molecules and clusters. Temperature is an obvious parameter controlling the stability of diatomic molecules. In this paper we report the measurements, by laser induced fluorescence (LIF), of the temperature of Si atoms and rotational temperature of SiO in laser induced plasma. Thermal relaxation processes of atomic Si in He and Ar atmospheres have been studied by measuring the Doppler profile of Si line using the 3d 1P0−3p2 1S transition centered at 263.128 nm. The thermalization of Si atoms is slower in Ar atmosphere than in He. The rotational temperature of SiO was measured by pumping the A1φ−X1Σ transition. In 5–400 μs delay after ablation pulse, the SiO temperature decreases from 2000 to 400 K. No evidence of nonequilibrium of the rotational and vibrational modes was observed. LIF measurements indicate that the formation of SiO molecules is better visualized as a gas phase equilibrium reaction rather than a ballistic collision in the temperature range 400–2000 K.

Experimental results are presented on the ablation of SiC ceramics in air under irradiation with XeCl and ArF excimer laser beam. X-Ray photoelectron spectroscopy analysis reveals the enrichment of the ablated areas in Si under laser beam energy densities of 1–3.5 J cm −2 . At higher energy densities the ablated areas contain Si oxide. The non-stoichiometric ablation of SiC allows one to activate selectively the SiC surface for metal deposition from electroless plating solution. Both Ni and Cu deposits show excellent adherence to both SiC ceramics and single-crystal SiC.

The synthesis and spectroscopic properties of a series of new organic electron donors derived from dibenzo[2,3:5,6]pyrrolizino-[1,7-bc]indolo[1,2,3-lm]carbazole are described. These compounds can be considered as planar hybrids of carbazole and p-phenylenediamine and possess high thermal stability. The electron donating properties and one- and two-photon spectroscopic features of methoxy derivatives are very sensitive to the number and positions of the methoxy groups. The X-ray structure of the 2,11-dimethoxy derivatives features slipped π-stacks and short interplanar distance between the molecules in the stacks similar to those found in the unsubstituted derivative.

Of a series of 4-substituted 1,8-naphthalimides, fluorescent 4-(6-piperidinyl-1,3-dioxo-1H-benzo[de]isoquinolin-2(3H)-yl)benzoic acid (4) was found to be a sensitive molecular probe for ZnO nanoparticles. We investigated in detail one- and two-photon absorption properties of this fluorophore. In nonpolar solvents, the acid 4 absorbs at about 400 nm and fluoresces at 500 nm with a fluorescence lifetime of about 7 ns, similar to the ester 6 and typical of the lifetimes of other derivatives of this type. Although the anionic form of this acid is not fluorescent, partial ionization of 4 in polar solvents, such as ethanol and acetonitrile, is not only accompanied by the expected decrease in the fluorescence quantum yield, but also gives rise to bathochromic shifts of both absorption and fluorescence and dual fluorescence with lifetimes of 0.2–0.3 ns and 6 ns ascribed to the formation of anionic complexes. The interaction with the ZnO surface brings about further considerable changes in the fluorescence patterns.

Conference on Frontiers in Ultrafast Optics - Biomedical, Scientific, and Industrial Applications XIII, San Francisco, CA, FEB 03-05, 2013; International audience; 3D laser microfabrication inside narrow band gap solids like semiconductors will require the use of long wavelength intense pulses. We perform an experimental study of the multiphoton-avalanche absorption yields and thresholds with tightly focused femtosecond laser beams at wavelengths: 1.3 mu m and 2.2 mu m. For comparisons, we perform the experiments in two very different materials: silicon (semiconductor, similar to 1.1 eV indirect bandgap) and fused silica (dielectric, similar to 9 eV direct bandgap). For both materials, we find only moderate differences while the number of photons required to cross the band gap changes from 2 to 3 in silicon and from 10 to 16 in fused silica.

Surface stability during laser pulsed melting of polymers and ceramics is studied theoretically. Irradiation conditions and material parameters are found giving rise to the suppression of surface wavy relief of a nonresonant type (with period Λ≫λ, where λ is the radiation wavelength) and thus to the smooth flat irradiation spots. For example, for the polymers considered this process takes place for wavelengths where the absorption coefficient is sufficiently high: α(λ)≳105 cm−1. Thus, it is shown that the formation of such spots, previously referred to as ‘‘clean ablation,’’ can be explained using only a thermal mechanism without reference to the concept of ‘‘photodecomposition.’’ Moreover, laser smoothing and polishing of a surface, if it had roughness before irradiation, can be achieved by appropriate matching of the characteristic size of this roughness along the surface with the values of α(λ) and laser fluence. Methods are proposed to decrease the parasitic influence of droplets on the deposition of ...

Pulsed laser deposition (PLD) of niobium telluride was performed by vaporisation of NbTe2 bulk targets with subsequent deposition of the ablation plume on heated silicon substrates. The laser energy density onto the target and the temperature of the substrate appear to play a major role in the quality of the deposited films. Correlations between these two parameters and the film composition on the one hand, and the structural properties on the other, were sought by systematic elemental and X-ray diffraction analysis. The deposition conditions for well-crystallized PLD films belonging to the Nb5Te4 (1D) structural type with strong (110) preferential orientation were demonstrated for the first time.

Experimental results are presented on the electroless metallization of SiO 2 using the Pd seeding layer obtained by either laser-assisted or thermal decomposition of a Pd acetylacetonate spun-on precursor layer. The processing of the seeding layer in acetic acid results in an enhanced adherence (up to 2 N/mm 2 ) of the Ni deposit to glass and fused quartz. XPS analysis of the seeding layer shows that processing in the acid removes the two acetylacetonate “wings” left after the decomposition of Pd(acac) 2 around Pd nuclei. Well-adherent Ni and Cu stripes are deposited onto glass and quartz substrates via laser decomposition of a spun-on Pd(acac) 2 layer with 30 μ m spatial resolution for a wide range of scanning velocities of the laser beam.

Silicon oxide, oxynitride, and nitride films are deposited, at low temperature (≤450 °C) by pulsed ArF excimer laser ablation from silicon, silicon monoxide, fused silica, and silicon nitride targets, performed under vacuum and in an oxygen atmosphere. The specific influence of laser fluence, target materials, substrate temperature, and oxygen pressure on the composition and final properties of SiOxNy grown layers is investigated using various complementary experiments such as infrared optical absorption, Rutherford backscattering, Auger electron spectroscopy, ellipsometry, and scanning electron microscopy. The process conditions are optimized in order to deposit good quality silicon oxide and silicon nitride thin films.

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm2 in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm2) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm2), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm2), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H (≈ 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.

A new method is proposed for the processing of experimental data from reflectivity measurements from a target-plume system. This method gives the possibility of finding the order of laser radiation absorption in the laser induced plume during the laser-target interaction time. We show that three-photon absorption is a main mechanism of plume shielding in the case of crystalline silicon ablation by ArF (λ=193 nm) laser.

Electroless copper metallization of polyphenylquinoxaline (PPQ) was carried out using a palladium acetyl acetonate (Pd(acac) 2 ) precursor. A continuous wave Ar ion laser was used to decompose the Pd(acac) 2 organometallic layer deposited onto the PPQ. The samples were then processed in a copper electroless plating solution of conventional composition. The sample surfaces were analysed by X-ray photoelectron spectroscopy before any treatment, after laser irradiation and at the very first stages of copper growth. An estimate of the temperature reached by the polymer under the laser beam was obtained by analysing the interference pattern reflected by the surface. Breakdown fields of the polymer dielectric were found in the region of 1 MV cm -1 and are weakly affected by the electrochemical processes

The optical emission from plasma produced by excimer laser (ArF, λ = 193 nm) ablation of Si, SiO and Si3N4 targets was studied in the visible range. We present results of the time integrated optical spectroscopy measurements and time-of-flight analysis of the particle expansion dynamics. Both methods show the formation of diatomic molecules within the laser induced plasma plume. The velocities of the ejected species are strongly affected by a chemical reaction inside the plasma plume.

Among the potential photovoltaic devices based on semiconductor oxides as active layer is cuprous oxide (Cu2O). Although the theoretical limit of Cu2O solar cell efficiency is 20%, the best efficiency obta ined up to now is only 2%. This is due to a very limited amount of work devoted to this semiconductor and only during last few years this material has been investigated for solar cells applications. In this work we report our results of optical, structural and surface morphology investigations of Cu2O films prepared by thermal oxidation of copper layer. The effects of oxidation temperature and oxygen partial pressure on surface morphology and crystalline structure of Cu2O films were studied. Scanning electron microscope results have shown that Cu2O films have microcrystalline structure with grain size of about 5-15 µm. Analysis of fine structure shows typical lattice spacing of cubic Cu2O structure. X-ray investigations have shown that the films consist of single Cu2O phase without any interstitial phase and have a nano-grain structure. The grains have an average dimensions about (33-41) nm. Optical investigations have shown that the absorption edge of prepared films is due to a direct allowed transition. The value of the optical band gap is 2.08 eV. Keywords: Cuprous oxide, nano-structure, thermal oxidation, solar cell

The time of flight measurements by optical and ion-probe methods have been performed during plasma expansion induced by picosecond laser pulse. The time and angle resolved measurements show two distinct ion and electron populations. Faster species show a broad angular distribution with shift of this distribution towards the laser beam. Slow species show a forward direction in space. The overall positive current was observed in the forward direction relatively to the target. At the higher angles, plasma expansion is characterized by time/space charge separation. The spatial edge and the time trailing edge of the plume exhibit overall neutrality.

Emission spectra from a high-current line plasma produced by means of a formed-ferrite plasma source have been measured in the 1200–2900 A spectral region. The formed-ferrite plasma source was driven by a 4.6 μF capacitor bank charged up to 25 kV. In order to obtain this intense flash light source, a conducting channel was first made on the ferrite surface which has a measurable high impedance, but orders of magnitudes less than the impedance of the unprocessed ferrite (>10 5 kΩ·cm). Two different methods were successfully used: laser ablation and explosion of a brass wire. A study of the formation and the characteristics of the channel and the surroundings on different ferrite materials has shown chemical and strong microcrystalline structure modifications. We report the electrical and optical properties of such a source by high voltage measurements, time integrated and time resolved VUV optical spectroscopy and frame photographs. An observation of the plasma expansion with the formation and the propagation of a blast wave (1.8 km/s) is presented for the first time. The possibility of a peak intensity shift of VUV radiation to shorter wavelengths is discussed.

Dans cet article, nous decrivons les principes de bases de la synthese des nanoagregats par ablation laser conventionnelle. La generalisation de la theorie de la condensation de Zeldovich et Raizer a ete appliquee pour decrire la condensation dans le panache d'ablation induit par laser. Nous avons considere le panache heterogene ou la vitesse de nucleation ainsi que le temps de condensation sont differents dans diverses parties de la plume. Le developpement theorique et l'analyse des resultats experimentaux sont presentes afin de decrire la condensation, l'expansion et les proprietes des nanoagregats de silicium.

The laser-induced plasma plume dynamics for picosecond laser ablation of silicon and YBaCuO targets have been studied using optical time-of-flight spectra. The picosecond pulse ablation allows one to reduce the temperature and velocity dispersion of the ablated species. The expansion velocities of the ionized species show an inverse square root dependence vs. the atomic mass and are proportional to their charge. The velocities of the neutral atoms exhibit a much weaker dependence vs. atomic mass. The formation of a charge separation region was observed during plasma plume expansion. These experimental results support the Coulomb expansion mechanism of the laser-induced plasma.

Diffusion of Au and Cu in amorphous germanium was produced by CW Ar laser excitation during furnace heating. Comparison with classical furnace annealing shows a lowering of the crystallization temperature and an increase of the diffusion rate. A transmission electron microscopy study (imaging and electronic diffraction) shows the formation of metastable compounds. The results are analyzed in relationship to the transient sequence of the produced metastable phases.

International audience; We report on picosecond UV-laser processing of hexagonal boron nitride (hBN) at moderately high pressures above 500 bar. The main effect is specific to the ambient gas and laser pulse duration in the ablation regime: when samples are irradiated by 5 ps or 0.45 ps laser pulses in nitrogen gas environment, multiple nucleation of a new crystalline product - BN nanorods - takes place. This process is triggered on structural defects, which number density strongly decreases upon recrystallization. Non-linear photon absorption by adsorbed nitrogen molecules is suggested to mediate the nucleation-growth. High pressure is responsible for the confinement and strong backscattering of ablation products. A strong surface structuring also appears at longer 150-ps laser irradiation in similar experimental conditions. However, the transformed product in this case is amorphous strongly contaminated by boron suboxides BxOy.

Based on experiments and a theoretical analysis, we raise questions on two fundamental mechanisms of femtosecond laser desorption/ablation of solids, namely Coulomb explosion (CE) and plasma etching. The effects of laser-induced ionization and surface charging are analyzed which can be responsible for ultrafast ions observed in time-of-flight mass-spectra under ultrashort laser irradiation of solids. The importance of surface charging in formation of velocity distributions of desorbed/ablated species has been revealed for conditions when the CE mechanism is inhibited. The influence of ambient plasma formation on the dynamics of heating of metallic targets by femtosecond laser pulses is studied based on 2D modeling of laser-induced target heating and dynamics of the ambient plasma. The calculations show an intriguing picture of the laser-induced ambient gas motion. We propose a model of laser-induced breakdown of an ambient gas in a region in front of the irradiated target and analyze plasma-chemical processes which can affect laser processing of surfaces in the presence of air or highly reactive media.

The conception of a totally new optoelectronic composite with a highly organized mesoporous silica core and a uniform ZnS shell (200 nm in thickness) with new optical properties is reported. The starting CMI-1 material was firstly functionalized with ethylenediamine groups and then the growth of the ZnS shell was targeted by the immersion of the functionalized CMI-1 material in a Zn(CH 3 COO) 2 solution followed by a reaction with sodium sulfide. The new optoelectronic nanocomposite has been characterized by a series of techniques such as SEM, TEM and nitrogen adsorption–desorption. The optical properties of the new nanocomposite have been evaluated by photoluminescent spectroscopy. No quantum size effect has been observed due to the large size aggregates in ZnS later.

Zinc oxide nanoparticles were introduced inside the porous structure of a Faujasite X zeolite via the impregnation method. N2 adsorption-desorption analysis, X-ray powder diffraction and photoluminescence spectroscopy were used to characterize the obtained nanocomposites ZnO/FAU. Textural and structural results showed that the presence of ZnO did affect the zeolite structure only when the ZnO precursor solution was quite concentrated. PL results reveal significant blue-shifted emission band which is attributed to quantum size effect (QSE).

We report investigations of a space-time structure of the electric field of laser plasma from a solid target. An ArF excimer laser with an intensity of I≅108 W/cm2 was used to produce the plasma on various targets placed in air at atmospheric pressure. A strong difference in both the amplitude (by more than 1–2 orders of magnitude) and the structure of the electric field for conductive and dielectric targets has been observed. The field distribution for a conductive target was found to correspond to a dipole configuration of charges in the laser plasma, while for the dielectric target a quadrupole configuration was revealed. Possible explanations and applications of the observed effect are discussed.

Zinc oxide is a promising material for creation of novel ultraviolet light sources. In this work we study random laser action in a thin ZnO nanocluster film under two-photon pumping. The results are compared with the case of single-photon pumping. A theoretical model is developed, which shows the effect of boundary conditions on lasing in the film. Measurement results of nonlinear transmission are presented and compared with classical two-photon absorption model.

International audience; Collisional processes leading to the formation of nanoparticles in a laser-ablated plume are numerically simulated with the aid of an atomistic-level model based on direct simulation Monte Carlo (DSMC) method. The formation of nanoparticles in nanosecond laser ablation of a mono-atomic target is investigated in the presence of an inert background gas. The DSMC procedure is modified in order to account for numerous plume species and to describe several reactions (i.e., recombination/dissociation, sticking, evaporation) taking place in the plume and affecting the size and spatial distribution of the produced nanoclusters. Calculation results allow us to visualize the nanoparticles and to correlate their space distributions with plume dynamics. In addition, cluster size distributions are investigated at different pressures. The effects of the background gas on cluster formation within the plume are furthermore shown.

The structural, optical and magnetic properties of nanostructured ZnO films co-doped with cobalt and aluminium have been studied. The nanocrystalline films, with cluster sizes in range 50–100 nm, were deposited by pulsed laser ablation in a mixed atmosphere of oxygen and helium. The nanocrystallites have the wurtzite structure and are highly oriented with the c-axis perpendicular to the substrate. Both optical and electron spin resonance (ESR) spectroscopy results show the substitutional incorporation of Co2+ ions on the Zn site inside the ZnO nanoclusters. The temperature dependence of the ESR spectra follows Curie law corresponding to a paramagnetic material.