Formation, structure and nonlinear optical properties of carbon nanoparticles synthesized by pulsed laser ablation

Carbon nanoparticles were prepared by pulsed laser ablation in argon gas for studying their formation, properties and applications. The nanoparticles were produced at different background pressures and different substrate temperatures, respectively. Micro-Raman spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to analyze the samples. Nonlinear optical limiting properties of the carbon nanoparticles were characterized using nanosecond laser pulses. Uniform carbon nanoparticles can be synthesized in a background gas for nonlinear optical applications. PACS 61.46.+w; 81.07.-b; 78.20.-e     

Structural and optical properties of silicon nanoparticles prepared by pulsed laser ablation in hydrogen background gas

We studied the structural and optical properties of silicon (Si) nanoparticles (np-Si) prepared by pulsed laser ablation (PLA) in hydrogen (H₂) background gas. The mean diameter of the np-Si was estimated to be approximately 5 nm. The infrared absorption corresponding to Si-H_n (n=1,2,3) bonds was observed at around 2100 cm^-1, and a Raman scattering peak corresponding to crystalline Si was observed at around 520 cm^-1. These results indicate that nanoparticles are not an alloy of Si and hydrogen but Si nanocrystal covered by hydrogen or hydrogenated silicon. This means that surface passivated Si nanoparticles can be prepared by PLA in H₂ gas. The band-gap energy of np-Si prepared in H₂ gas (1.9 eV) was larger than that of np-Si prepared in He gas (1.6 eV) even though they are almost the same diameter. After decreasing the hydrogen content in np-Si by thermal annealing, the band-gap energy decreased, and reached the same energy level as np-Si prepared in He gas. Thus, the optical properties of np-Si were affected by the hydrogenation of the surface of np-Si. PACS 81.15.Fg; 61.46.+W; 78.67.Bf     

Synthesis of CaWO<Subscript>4</Subscript> nanocolloidal suspension via pulsed laser ablation and its optical properties

Pulsed laser ablation (PLA) in the liquid phase was successfully employed to synthesize calcium tungstate (CaWO₄) nanocolloidal suspension. The crystalline phase, particle morphology and laser ablation mechanism for the colloidal nanoparticles were investigated using XRD, TEM and SEM. The obtained colloidal suspension consisted of well-dispersed CaWO₄ nanoparticles which showed a spherical shape with sizes ranging from 5 to 30 nm. The laser ablation and the nanoparticle forming process were discussed under consideration of the photo-ablation mechanism, where the nanoparticles were generated by rapid condensation of the plume in high pressured ethanol vapor. The optical properties of the prepared CaWO₄ colloidal nanoparticles were analyzed in detail using XPS, Raman spectroscopy, UV-Vis spectroscopy and PL spectrophotometry. The optical band gap was estimated by Tauc and Menths law. PACS 42.62.-b; 82.70.Dd; 78.55.Hx; 81.07.Wx     

Ag nanoparticles obtained by pulsed laser ablation in water: surface properties and SERS activity

We studied the surface properties and reactivity of silver nanoparticles obtained by picosecond or nanosecond pulsed laser ablation in water and with 1064‐nm wavelength. Ultraviolet–visible spectroscopy results and subsequent modelling by Mie theory indicated the presence of an oxide layer on the nanoparticle surface, which favours the colloidal stability, but reduces the interaction with the environment. The oxide layer is also responsible for the reduced surface enhanced Raman spectroscopy (SERS) activity of these colloids with respect to those obtained by chemical reduction. However, SERS activation can be efficiently obtained by addition of chloride ions to the colloids, leading to SERS enhancement factors that are comparable with those of the chemically prepared counterparts. Copyright © 2015 John Wiley & Sons, Ltd.     

Surface modification of the Cr-based coatings by the pulsed TEA CO2 laser

The interaction of a transversely excited atmospheric (TEA) CO₂ laser with chromium oxynitride (CrON) coating deposited on a AISI 304 steel substrate was considered. The results have shown that CrON was surface-modified by the laser beam of 45 J/cm² energy density. The energy absorbed from the TEA CO₂ laser beam was partially converted into thermal energy, which has generated a series of effects such as melting, vaporization of the molten material, and shock waves in the vapor and in the solid. Morphological manifestations on the CrON coating surface can be summarized as follows: non-uniform features with ablation and appearance of crater-like form (central zone of interaction); appearance of three damaged areas and presence of hydrodynamic effects with resolidified droplets (periphery zone of interaction). In case of applied energy density the interaction of laser radiation with CrON has been always followed by plasma creation in front of the coating. PACS 79.20.Ds; 61.80.Ba     

Structural,optical, and nonlinear optical properties of indium nanoparticles prepared by laser ablation

A study of indium nanoparticles prepared by two laser ablation techniques is reported. The suspensions of indium nanoparticles were prepared using the laser ablation of bulk indium in liquids. The prepared suspensions of indium nanoparticles were analyzed by the X-ray fluorescence spectroscopy and absorption spectroscopy. The position of the surface plasmon resonance of In-containing suspensions (350 nm) was consistent with the estimations taking into account the average size of In nanoparticles (43 nm) measured using the X-ray fluorescence spectroscopy. The nonlinear optical parameters of indium nanoparticles-containing liquids were studied by the z-scan technique using a picosecond Nd:YAG laser. We compare the laser ablation in liquids with the laser ablation of indium in vacuum at the tight and weak focusing conditions of a Ti:sapphire laser and analyze the 60 nm indium nanoparticles synthesized in the latter case. PACS 42.65.An; 42.65.Hw; 42.65.Jx; 61.46.Df; 78.67.Bf     

Surface charging and impulsive ion ejection during ultrashort pulsed laser ablation

We report time-resolved studies using femtosecond laser pulses, accompanied by model calculations, that illuminate the difference in the dynamics of ultrashort pulsed laser ablation of different materials. Dielectrics are strongly charged at the surface on the femtosecond time scale and undergo an impulsive Coulomb explosion. This is not seen from metals and semiconductors where the surface charge is effectively quenched.     

Fabrication and transformation of dense SnO2 via laser ablation condensation

Dense SnO₂ nanocondensates with fluorite-type-related structures have been synthesized via severe heating-cooling under energetic Nd-YAG laser pulse irradiation of the Sn target in oxygen ambient. Transmission electron microscopic observations indicated that the fluorite type transformed in a martensitic manner into a baddeleyite-type structure with accompanied dislocations, twinning, commensurate shearing and shape change. The Pa3¯-modified fluorite-type structure was hardly observed possibly due to its transformation into α-PbO₂ type and then rutile-type structures in the dynamic process.     

The production and oxidation of uranium nanoparticles produced via pulsed laser ablation

Depleted uranium samples were ablated using five nanosecond pulses from a Nd:YAG laser and produced films of ∼1600 Å thickness that were deposited with an angular distribution typical of a completely thermal ablation (cos¹ θ). The films remained contiguous for many months in vacuum but blistered due to tensile stress induced in the films several days after being brought into air. While under vacuum (2 × 10⁻¹⁰ Torr base pressure) the films were allowed to oxidize from the residual gases, of which water vapor was found to be the primary oxidizer. During the oxidation, the samples were monitored with both X-ray and ultraviolet photoemission spectroscopy (XPS and UPS) and were found to oxidize following Langmuir kinetics. That a 2D-surface growth model described the oxidation indicates that, even at these low pressures, oxygen accumulation on the surface is a much faster process than diffusion into the bulk. While bulk diffusion did occur, the oxygen present at the surface saturated the measurements taken using photoemission and diffusion was difficult to observe. A method for determining oxide concentration via photoemission from the valence level, as opposed to the more conventional core levels, is also presented.     

Optical properties of GaAs films deposited via pulsed ion ablation

Optical reflectance and absorbance of gallium arsenide films formed on polycrystalline corundum, quartz glass, and copper foil are investigated in the energy interval of 1.1–6.2 eV. The films have been deposited from ablation plasma induced by a high-power ion beam. The exponential and interband absorbance spectra of the material of films are determined by defects in the GaAs crystalline lattice and the intricate composition of the material with predominance of nanocrystalline inclusions in the amorphous phase. Films deposited on polycor at the plasma flame center with the use of a low-resistance target have optimal properties for application in devices of optoelectronics and solar power engineering. Thermal vacuum treatment at 300–850 K modifies the optical properties of films owing to annealing of defects and changing of the structural-phase composition of a material.     

Investigation of Ag nanoparticles produced by nanosecond pulsed laser ablation in water

A study is presented of the properties of Ag nanoparticles produced by nanosecond pulsed laser ablation in twice-distilled water. An Ag target was immersed in the liquid and irradiated by the fundamental, second, third and fourth harmonics of a Nd:YAG laser system to create different colloids. Two specific boundary values of the laser fluence were applied for each wavelength. The properties of the nanoparticles at different wavelengths of the laser radiation were examined. The characterization of the colloids was performed immediately after their fabrication. Spherical and spherical-like shapes of the nanoparticles created were established. The formation of nanowires was observed when the second and the third harmonics of the laser were used. It is connected with self-absorption of the incident laser light from the already-created nanoparticles and depends also on the laser fluence. The size distribution of the nanoparticles is estimated by transmission electron microscopy. Generally, their mean size and standard deviation decreased as the wavelength of the incident laser light was increased and increased with the increase of the laser fluence. The substantial discrepancy between the results already commented on for both characteristics considered and others, obtained by dynamic light scattering, is discussed. The structure of the nanoparticles was established to be single and polycrystalline, and the phase composition in both cases is identified as consisting of cubic silver. The nanoparticles are slightly oxidized.     

Phase transformation during surface ablation of cobalt-cemented tungsten carbide with pulsed UV laser

Surface ablation of cobalt-cemented tungsten carbide hard metal has been carried out in this work using a 308 nm, 20 ns XeCl excimer laser. Surface microphotography and XRD, as well as an electron probe have been used to investigate the transformation of phase and microstructure as a function of the pulse-number of laser shots at a laser fluence of 2.5 J/cm². The experimental results show that the microstructure of cemented tungsten carbide is transformed from the original polygonal grains of size 3 μm to interlaced large, long grains with an increase in the number of laser shots up to 300, and finally to gross grains of size 10 μm with clear grain boundaries after 700 shots of laser irradiation. The crystalline structure of the irradiated area is partly transformed from the original WC to βWC_1-x, then to αW₂C and CW₃, and finally to W crystal. It is suggested that the undulating ‘hill–valley’ morphology may be the result of selective removal of cobalt binder from the surface layer of the hard metal. The formation of non-stoichiometric tungsten carbide may result from the escape of elemental carbon due to accumulated heating of the surface by pulsed laser irradiation. Received: 13 July 2000 / Accepted: 27 October 2000 / Published online: 10 January 2001     

Optical and structural properties of pulsed laser ablation deposited ZnO thin film

A limited number of reports exists in the literature concerning the systematic study of the structural and optical properties of ZnO thin films, produced by pulsed laser ablation, in correlation with the deposition parameters adopted. In this paper we present a characterization of a sample prepared by this technique and studied by photoelectron spectroscopy and X-ray diffraction. The dielectric function of both target and films has been deduced by reflection electron energy loss spectroscopy.     

Structural and optical properties of pulsed laser deposited ZnO thin films

Nanocrystalline ZnO thin films were grown by means of pulsed laser deposition. The ablation process was carried out at relatively low background oxygen gas pressure (10 Pa) and by varying the substrate temperature up to 600 °C. Information on the structural and morphological properties of the deposited thin films have been obtained by means of X-ray photoelectron, Raman spectroscopies, X-ray diffraction (XRD) and atomic force microscopy (AFM). The results showed that all the deposited films are sub-stoichiometric in oxygen and with a hexagonal wurtzite crystalline structure, characterized by features of some tens of nanometers in size. An improvement of the films' crystalline quality was observed for the deposition temperature of 300 °C while the further increase of the deposition temperature up to 600 °C induces a worsening of the material's structural properties with the development of a large amount of nanoparticle's clusters. The analysis of the XRD patterns shows a growth crystallographic preferential direction as a function of the deposition temperature, in agreement with the appearance of the only E₂ optical phonon mode in the Raman spectra. Such findings are compatible with the changes observed in the photoluminescent (PL) optical response and was related to the modification of the ZnO thin film structural quality.     

Growth and optical characterization of diamond-shaped zinc oxide nanostructures deposited by pulsed laser ablation

We report on the growth of zinc oxide diamond-shaped nanostructured thin films by pulsed laser deposition technique on silicon substrate at different substrate temperatures (room temperature to 600  $^{\circ }$ C) and at fixed background pressure of oxygen using Nd:YAG laser at a wavelength of 532 nm. The influence of substrate temperature on the grain size, surface morphology and optical properties is characterized by x-ray diffraction, field-emission scanning electron microscope, photoluminescence and Raman spectroscopy. x-ray diffraction results show that the grain size increases with increasing substrate temperature during the growth of ZnO thin films due to improved crystallinity but at 450  $^{\circ }$ C the crystallinity degrades. It is attributed to the formation of diamond-shaped ZnO nanostructures as supported by the field emission scanning electron microscope images. Consequently, increase in photoluminescence and Raman intensities is also attributed to the formation of diamond-shaped structures. The growth of diamond-shaped structure is discussed in the light of growth of various planes in the hexagonal structure of ZnO.     

Initiation of martensitic transformation and shape memory by pulsed electron beams

The change in phase composition of alloys based on titanium nickelide as a result of pulsed electron bombardment is investigated. The chemical composition of the alloy (and hence the initial phase composition) and the electron-beam power are varied. In special experiments, restoration of the shape of irradiated samples is observed.V. D. Kuznetsov Siberian Physicotechnical Institute, Tomsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 69–73, May, 1993.     

Synthesis and properties of Si and Ge nanoclusters produced by pulsed laser ablation

We show that conventional pulsed laser ablation (PLA) of Si and Ge targets in inert buffer gases is an efficient method of nanocluster synthesis. From a photoluminescence study of Si and Ge nanoclusters produced by PLA we have demonstrated the possibility of tuning the luminescence band from the near infrared to the near ultraviolet regions. The stabilization of the properties of Si nanoclusters by reactive (H₂ gas) PLA synthesis was proved by photoluminescence measurements. Finally, we report a photoluminescence study of gas-suspended Ge nanoclusters during their preparation. They exhibit a broad luminescence spectrum extended from UV to the blue-green region and modulated by a molecule-like structure. We propose an interpretation of the vibronic structure involving Ge-O-Ge vibrations at the surface of photo-excited clusters. To the best of our knowledge, we report here the first observation of vibrational effects from gas-suspended Ge nanoclusters.     

Nickel aluminate oxides/hydroxides by pulsed laser ablation of NiAl2O4 powder in water

Submicron-sized NiAl_2+X O₄ fragments and nanocondensates of Ni-doped γ-Al₂O₃, Al-doped NiO and β-Ni(OH)₂ were synthesized simultaneously by pulsed laser ablation of NiAl₂O₄ powder in water and characterized using X-ray/electron diffraction and optical spectroscopy. The NiAl_2+X O₄ is Al-enriched spinel with dislocations and subgrains. The Ni-doped γ-Al₂O₃ spinel has paracrystalline distribution (i.e., with fair constant longitudinal spacing, but variable relative lateral translations) of defect clusters and intimate intergrowth of θ-Al₂O₃ and 2x( \( \overline{1} \) 3 \( \overline{1} \) ) commensurate superstructure. The Al-doped NiO has perfect cubo-octahedron shape and as small as 5 nm in size. The β-Ni(OH)₂ and 1-D turbostratic hydroxide lamellae occurred as a matrix of these oxide nanoparticles. The colloidal suspension containing the composite phases has a minimum band gap of 5.3 eV for potential photocatalytic applications.