Laser induced synthesis of nanoparticles in liquids

The review of results on nanoparticles formation is presented under laser ablation of Ag, Au, and Cu-containing solid targets in liquid environments (H₂O, C₂H₅OH, C₂H₄Cl₂, etc.). X-ray diffractometry (XRD), UV-vis optical transmission spectrometry, and high resolution transmission electron microscopy (HRTEM) characterize the nanoparticles. The morphology of nanoparticles is studied as the function of both laser fluence and nature of the liquid. The possibility to control the shape of nanoparticles by ablation of an Au target by an interference pattern of two laser beams is demonstrated. Formation of alloyed Au-Ag and Ag-Cu nanoparticles is reported under laser exposure of a mixture of individual nanoparticles. The effect of internal segregation of brass nanoparticles is discussed due to their small lateral dimensions. The factors are discussed that determine the distribution function of particles size under laser ablation. The influence of laser parameters as well as the nature on the liquid on the properties of nanoparticles is elucidated.     

Synthesis of silver nanoparticles and antimony oxide nanocrystals by pulsed laser ablation in liquid media

Pulsed laser ablation in liquid media (PLALM) is a prominent technique for the controlled fabrication of nanomaterials via rapid reactive quenching of ablated species at the interface between the plasma and liquid. Results on nanoparticles and nanocrystals formed by PLALM of silver (Ag) and antimony (Sb) solid targets in different liquid environments (Sodium Dodecyl Sulfate, distilled water) are presented. These experiments were done by irradiating solid targets of Ag and Sb with a nanosecond pulsed Nd:YAG laser output of wavelength 532 nm. Nanoparticles of silver and nanocrystals of antimony oxide (Sb₂O₃) obtained were characterized using UV-Vis spectrometry, Scanning Electron Microscopy (SEM), transmission electron microscopy (TEM), X-ray Energy Dispersion Analysis (EDAX) and X-ray diffractometry (XRD). The morphology of nanomaterials formed is studied as a function of surfactant environment. The silver nanoparticles obtained were spherical of size in the order of 10–35 nm in solution of SDS having different concentrations. In case of the Sb target, ablation was performed in two different molarities of SDS solution and distilled water. Nanocrystals of Sb₂O₃ in powder form having cubic and orthorhombic phases were formed in SDS solution and as fibers of nanocrystals of cubic Sb₂O₃ in distilled water.     

Nanoparticle formation during laser ablation of solids in liquids

The experimental data on the generation of metal and semiconductor nanoparticles during their laser ablation in liquids is reviewed. The dependence of the morphology of noble metal nanoparticles on the liquid type and laser parameters is discussed. The data on the kinetics of the formation of alloyed Au-Ag nanoparticles under laser irradiation of a mixture of colloid solutions of individual nanoparticles are presented. The effect of femtosecond laser beam self-action during metal ablation in liquids via the second harmonic generation at Ag nanoclusters is discussed. The data on the generation of core-shell nanoparticles during laser ablation of alloys and in the presence of the chemical interaction of formed nanoparticles with surrounding liquid are presented. It was shown that laser ablation of CdS and ZnSe crystals leads to the formation of quantum dots of these semiconductors in solution. The parameters controlling the properties of nanoparticles during ablation in liquids and possible applications of the method are discussed.     

Pulsed laser induced synthesis of scheelite-type colloidal nanoparticles in liquid and the size distribution by nanoparticle tracking analysis

Pulsed laser ablation (PLA) of ceramic target in liquid phase was successfully employed to prepare calcium tungstate (CaWO₄) and calcium molybdate (CaMoO₄) colloidal nanoparticles. The crystalline phase, particle morphology and optical property of the colloidal nanoparticles were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and Raman spectroscopy. The produced stable colloidal suspensions consisted of the well-dispersed nanoparticles showing a spherical shape. The mechanism for the laser ablation and nanoparticle forming was discussed under consideration of photo-ablation process. Nanoparticle tracking analysis using optical microscope combined with image analysis was proposed to determine the size distribution function of the prepared colloidal nanoparticles. The mean size of the CaWO₄ and CaMoO₄ colloidal nanoparticles were 16 and 29 nm, with a standard deviations of 2.1 and 5.2 nm, respectively.     

Generation of higher harmonics of femtosecond radiation in clusters

The frequency conversion of laser radiation in cluster media is studied by the method of laser ablation of surfaces containing different nanoparticles (Cr₂O₃, In₂O₃, Ag, Sn, Au, and Cu). Using plasma that contains In₂O₃ nanoparticles as an example, it is shown that the resonant amplification of certain harmonics in the plateau-like distribution of harmonics, which is characteristic of the ablation of certain solid targets, is considerably modified in the case of targets that contain cluster formations. An increase in the conversion efficiency to harmonics in nanoparticle-containing media is discussed. In order to determine optimal characteristics of laser plasma with nanoparticles, their morphology is studied before and after laser ablation.     

Elaboration and characterization of bimetallic nanoparticles obtained by laser ablation of Ni75Pd25 and Au75Ag25 targets in water

A YAG laser operating at the second harmonic wavelength (532 nm, 10 Hz, 8 ns and 40 mJ) was used to elaborate bimetallic nanoparticles by laser ablation of Ni₇₅Pd₂₅ and Au₇₅Ag₂₅ targets in water. TEM–EDX, UV–Vis spectroscopy and PIXE measurements were performed to obtain information on their mean sizes, size distributions and chemical composition as a function of the time of laser ablation. The surface of the laser impacted regions of the targets were characterized by RBS in order to check their composition after the laser ablation. The so-obtained bimetallic nanoparticles always show a homogeneous composition. However, while the composition of Au–Ag nanoparticles was found to be very similar to the one of the alloy target, the composition of the Ni–Pd nanoparticles can be different from the nominal composition of the alloy target. Segregation phenomena can be invoked to explain the difference between the Ni–Pd nanoparticles and the Au–Ag nanoparticles compositions obtained in the same conditions. However, an influence of chemical reactions occurring in the high pressure plasma created locally at liquid–solid interface (called ‘reactive quenching’) cannot be completely ruled out.     

Laser ablation of silver and gold in liquid ammonia

Laser ablation of a silver (Ag) and/or gold (Au) target was performed in liquid ammonia (l-NH₃) at 233 K using nanosecond laser pulses of 1064, 532 and 355 nm wavelengths. An “in situ” monitoring of the ablation process by UV/vis/NIR spectroscopy has shown the evolution of the surface plasmon extinction band of silver or gold nanoparticles and thus confirmed their formation. While sols of Au nanoparticles in l-NH₃ are quite stable in air, those of Ag nanoparticles undergo oxidation to Ag(I) complexes with NH₃ ligands. On the other hand, formation of solvated electrons, namely of the (e⁻)NH₃ solvates, has not been unequivocally confirmed under the conditions of our laser ablation/nanoparticle fragmentation experiment, since only very weak vis/NIR spectral features of these solvates were observed with a low reproducibility. Reference experiments have shown that the well-known chemical production of these solvates is hindered by the presence of Ag and Au plates. Ag and Au targets can thus possibly act as electron scavengers in our ablation experiments.     

Enhanced luminescence of silver nanoclusters in mesoporous film

Luminescence of silver nanoparticles photodeposited on titan dioxide nanoparticles of mesoporous film is studied. Luminescence was registered under the two-photon excitation by femtosecond laser pulses of Ti:sapphire laser. It occurs that Ag/TiO₂ mesoporous films have high concentration of bright luminescence spots which reveal stability to degradation under long illumination. Various configurations of silver nanoparticles are analyzed to explain the physics of bright luminescence spots (“hot spots”). Luminescence intensity reveals “hot spots” dependence on excitation laser pulse polarization. Properties of Ag/TiO₂ system show its promising usage for single molecule spectroscopy and biological objects visualization.     

Comparison of silver nanoparticles confined in nanoporous silica prepared by chemical synthesis and by ultra-short pulsed laser ablation in liquid

Hexagonally ordered mesoporous silica materials, MCM-41 and SBA-15, have been synthesized and loaded with Ag nanoparticles, utilizing both chemical synthesis and ultra-short pulsed laser ablation in liquid. In laser ablation, a silver target, immersed in aqueous suspension of ordered mesoporous silica SBA-15, was irradiated by ultra-short laser pulses to generate silver nanoparticles. For comparison, samples of similar silver contents were prepared either by incorporating silver into the SBA-15 during a hydrothermal synthesis or by introducing silver in MCM-41 by template ion-exchange. Samples were characterized by XRD, N₂ physisorption, TEM and UV–vis spectroscopy. All preparations contained significant amount of 5–50 nm size silver agglomerates on the outer surface of the silica particles. The laser ablation process did not cause significant destruction of the SBA-15 structure and metallic silver (Ag⁰) nanoparticles were mainly generated. It is demonstrated that by laser ablation in aqueous silica suspension smaller and more uniform metallic silver particles can be produced and loaded on the surface of the silica support than by synthesis procedures. Catalytic properties of the samples have been tested in the total oxidation of toluene. Because of its favorable Ag dispersity, the Ag/SBA-15 catalyst, generated by the laser ablation method, had better catalytic stability and, relative to its Ag load, higher activity than the conventional Ag/SBA-15 preparations.     

Self-influence of a femtosecond laser beam upon ablation of Ag in liquids

Formation of nanoparticles of either Au or Ag is reported under ablation of metallic targets exposed to radiation of a femtosecond Ti:sapphire laser (wavelength of 810 nm, pulse width of 120 fs) in either water or ethanol. Nanoparticles are characterized by UV-visible spectroscopy and transmission electron microscopy. Nanoparticles of Ag are several times smaller than those of Au at otherwise equal conditions. The effect is attributed to the self-influence of a laser beam via generation of the second harmonics of the laser radiation on Ag clusters and its good matching to the plasmon resonance of Ag. PACS 42.62.-b; 61.46.+w; 78.66.-w     

Ultra fine carbon nitride nanocrystals synthesized by laser ablation in liquid solution

Crystalline carbon nitride nanopowders and nanorods have been successfully synthesized at room temperature and pressure using the novel technique of pulsed laser ablation of a graphite target in liquid ammonia solution. High-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and Fourier transform infrared spectroscopy (FTIR) were used to systematically study the morphology, nanostructure and chemical bonding. The experimental composition and structure of the nanoparticles are consistent with the theoretical calculations for α-C₃N₄. After 2 h ablation the particles had a size distribution ∼8–12 nm, whereas after 5 h ablation the particles had grown into nanorod-like structures with a crystalline C₃N₄ tip. A formation mechanism for these nanorods is proposed whereby nanoparticles are first synthesized via rapid formation of an embryonic particle, followed by a slow growth, eventually leading to a one-dimensional nanorod structure.     

Valency control in MoO<Subscript>3−δ</Subscript> nanoparticles generated by pulsed laser liquid solid interaction

The possibility of synthesizing binary oxides nanoparticles in a nano-scaled form by laser liquid solid interaction using a NdYAG “1.064 μm” as an irradiating laser source is reported. The case of MoO_3−δ is emphasized. Furthermore, it is demonstrated that the Mo–O electronic valence can be controlled through the coupling effects of oxygen enriched nature of the used coating liquid layer, namely pure H₂O or H₂O₂ and the laser beam fluence. Dark blue hydrated molybdic pentoxide Mo₂O₅·xH₂O and yellow molybdenum trioxide MoO₃ nano-suspensions were reproducibly synthesized with hydrogen peroxide and water, respectively, at a relatively high ablation rate. The average size of the molybdenum trioxide nanoparticles was about <ϕ>~8 nm, slightly larger than the molybdic pentoxide ones “<ϕ>~6.2 nm”.     

Accelerated alpha decay under laser exposure of metallic nanoparticles in aqueous solutions of uranium salt

The recent experimental results on the acceleration of alpha decay under laser exposure of metallic nanoparticles in aqueous solutions (D₂O or H₂O) of uranium salt UO₂Cl₂ are reviewed. It is demonstrated that the rate of the alpha decay of uranium branching depends on the laser wavelength and laser peak power both under exposure of the solution of uranium salt in colloidal solutions of metallic nanoparticles and under laser ablation of a bulk target in an aqueous solution. Tuning the laser wavelength to the plasmon resonance allows observation of large deviations of the contents of radio-nuclides of ²³⁸U branching from their equilibrium values. The effect of laser exposure is characterized by high isotopic selectivity towards the decay of different nuclides. Possible mechanisms of the laser-induced alpha decay of uranium isotopes and their potential applications are discussed.     

Dispersion of zirconium dioxide by pulsed laser radiation

A technique for producing ZrO₂ dioxide nanoparticles under the action of pulsed laser radiation is developed. By the methods of transmission electron microscopy and X-ray phase analysis, it is shown that the high-temperature cubic phase of ZrO₂ is formed during laser ablation. The dependence of the size of ZrO₂ dispersed particles on the laser radiation intensity is determined. A thermodynamic one-dimensional model of laser ablation of zirconium dioxide is analyzed. The results of analytical computations of ablation of ZrO₂ particles are confirmed by experimental data.     

Hydrogenation of carbon fragments after femtosecond laser ablation of solid C60

We have observed several kinds of hydrocarbon cations after the nanosecond and the femtosecond laser ablation (nsLA and fsLA) of solid C₆₀. The observation indicates that the carbon fragments produced just after laser ablation of the C₆₀ molecule react with the hydrogen atoms and ions coexisting in the ablation plume. In the case of fsLA, clear dependence of the product hydrocarbon species on the ablation laser power has been observed although the dependence is not clearly observed in nsLA. The production of C_nH₅⁺ (n = 8, 10, and 12) is only observed in fsLA suggesting the unique nature of the transient carbon fragments produced by fsLA.     

Enhancement of ethanol sensing of TeO2 nanorods by Ag functionalization

Gas sensors based on Ag–TeO₂ composite nanorods were fabricated using thermal evaporation and sputtering techniques. The morphology, structure and phase composition of the as-prepared nanofibers were characterized by scanning electron microscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD), respectively. TEM and XRD showed that the nanorods and nanoparticles on them were tetragonal-structured single crystal TeO₂ and a mainly amorphous phase, respectively. The multiple-networked bare TeO₂ nanorod sensors exhibited a response of ～219% at 25 ppm C₂H₅OH at 300 °C, whereas the Ag-functionalized TeO₂ nanorod sensors showed a response of ～808% under the same conditions. The mechanism by which the sensing properties of the TeO₂ nanorods were enhanced by functionalization with Ag is also discussed.     

Nonlinear optical characteristics of nanoparticles in suspensions and solid matrices

The study of the nonlinear optical parameters of suspensions and glass matrices doped with various nanoparticles (Ag, Cu, Co, Au, Pt, GaAs, CdS, As₂S₃) are presented. Various techniques for the preparation of nanoparticles in suspensions and solid matrices, in particular laser ablation, chemical methods, and ion implantation are discussed. We present our measurements of the nonlinear refractive indices, nonlinear absorption coefficients, saturation intensities, and nonlinear susceptibilities of nanoparticles-containing media using laser pulses generating in various spectral and temporal ranges. PACS 42.65.An; 42.65.Hw; 42.65.Jx; 61.46.Df; 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     

Small size TiO2 nanoparticles prepared by laser ablation in water

Titanium dioxide nanoparticles in distilled H₂O solvent were prepared by laser ablation. The experiments were performed irradiating a Ti target with a second harmonic (532 nm) output of a Nd:YAG laser varying the operative fluence between 1 and 10 J cm⁻² and for an ablation time ranging from 10 to 30 min. Electron microscopy measurements have evidenced the predominant presence of nanoparticles with diameter smaller than 10 nm together with agglomerations of 100-200 nm whose content increases with the laser fluence. At low laser fluence the particles’ size distribution shows that more than 85% of the nanoparticles have a size smaller than 5 nm while at mid and high fluences the presence of 5-7 nm nanoparticles is predominant. XPS analysis has revealed the presence of different titanium suboxide phases with the prevalence of Ti-O bonds from TiO₂ species. The optical bandgap values, determined by UV-vis absorption measurements, are compatible with the anatase phase.     

Formation of silicon nanoparticles and web-like aggregates by femtosecond laser ablation in a background gas

We show that the mechanism of nanoparticle formation during femtosecond laser ablation of silicon is affected by the presence of a background gas. Femtosecond laser ablation of silicon in a H₂ or H₂S background gas yields a mixture of crystalline and amorphous nanoparticles. The crystalline nanoparticles form via a thermal mechanism of nucleation and growth. The amorphous material has smaller features and forms at a higher cooling rate than the crystalline nanoparticles. The background gas also results in the suspension of plume material in the gas for extended periods, resulting in the formation (on a thin film carbon substrate) of unusual aggregated structures including nanoscale webs that span tears in the film. The presence of a background gas provides additional control of the structure and composition of the nanoparticles during short pulse laser ablation. PACS 81.16.-c