Mixtures of micron to submicron complex carbon phases, namely, defective graphene, graphene-like (multi-layered graphene, graphene oxide etc.), graphite flakes etc. as fine suspensions were obtained by pulsed laser ablation of highly purified microcrystalline graphite targets immersed in double distilled water. The fundamental wavelength (λ = 1064 nm) and the fourth harmonic (λFHG = 266 nm) of a Nd:YAG laser system (15 ns pulse duration, 10 Hz pulse repetition rate) were used in the fabrication process. The laser fluence value corresponding to the onset of the ablation process, the one initiating optical breakdown in water and an intermediate value were used for each of the wavelengths mentioned above. The morphology of the particles dispersed in water was studied by scanning electron microscopy (SEM). Their phase composition and structure were explored by Raman spectroscopy. It showed the presence of some traces of polymerized hydrocarbons (polystyrene, polybutadiene etc.) in addition to the main carbon phases: defected graphene, reduced graphene oxide and graphite.
The phase composition and morphology of nano-dispersed carbon phases obtained by nanosecond laser ablation of microcrystalline graphite target immersed in water were investigated. The second (λSHG= 532 nm) and the third (λTHG= 355 nm) harmonics of a Nd:YAG laser system were used to produce different water colloids of carbon. The values of the laser fluence applied for both wavelengths under the experimental conditions chosen were varied from several J/cm2 to tens of J/cm2 . Raman spectroscopy, X-ray photoelectron spectroscopy and SEM analyses were used to study the carbon target before and after laser treatment and the carbon colloids obtained. The study of the colloids is complemented through X-ray diffraction. A mix of different complex carbon phases and some hydrocarbon polymers (polystyrene predominantly) were found in the colloids.
Nanosecond pulsed laser ablation of a Ag target in water was applied to prepare noble metal nanoparticles. The fundamental (λ = 1064 nm) wavelength and the second harmonic (λSHG = 532 nm) of a Nd-YAG laser system were used for the fabrication procedure. The effect of the liquid level over the target surface on the characteristics of the nanoparticles was investigated using four different values (5, 8, 16, 29 mm). The liquid volume was constant and the duration of the ablation process was 5 min. A second set of experiments were aimed at clarifying the influence of the duration of ablation. The process was interrupted after 5, 10, 15, 20, 25 min and the mass concentration of Ag in the colloid was measured. This enabled us to establish the dependence of the mass concentration of the Ag on the duration of ablation. The profile of the optical extinction spectra of the colloids was helpful in assessing the state of the solid phase and the morphology of the material. Images obtained by transmission electron microscopy were used to visualize the morphology of the nanostructures produced.
In this work laser-assisted methods for metal nanostructures formation and their application as active substrates in Surface Enhanced Raman Spectroscopy are presented. The nanostructures are fabricated by laser processing of gold thin films deposited on low cost substrates as glass, ceramic, polymer and paper. The films are deposited by classical PLD technology. The produced films are then processed by nanosecond pulses delivered by nanosecond Nd:YAG laser system. At certain conditions the laser treatment leads to formation of discrete nanostructure on the substrate surface. Femtosecond Pulsed Laser Deposition in air is also applied for direct deposition of gold nanostructure. In another set of experiments gold nanoparticle colloids are fabricated by laser ablation of gold target in chloroform. The fabricated structures are then tested as active systems in SERS, as detection of pesticides (DDT), nitrates (NH4NO3), and drugs (Methylene blue) is demonstrated. The obtained results show that these nanostructures can be efficiently used in the detection and monitoring of materials with a high social impact.
This study presents results on pulsed laser ablation of metallic and bimetallic thin films immersed in liquids. The thin films are deposited by classical on-axis pulsed laser deposition technology by using targets consisted of one or two sections composed of different metals. Using this technique gold, silver and gold/silver thin films are deposited on quartz substrate. By changing the area of the different sections of the target, thin films with different concentrations of the two metals are obtained. The as prepared films are then placed on the bottom of glass vessel filled with double distilled water and irradiated by nanosecond laser pulses delivered by Nd:YAG laser system operated at λ = 1064 nm. This results in fabrication of colloids composed by metallic and bimetallic nanoparticles. The performed TEM analyses revealed spherical and spherical-like shape of the particles produced. The optical extinction spectra of the obtained colloidal nanoparticles show plasmon excitations as the resonance wavelength can be efficiently tuned in a wide range by changing the ratio of the basic metals in the films.
Pulsed laser ablation was used to create Ag nanostructures – nanoparticles and nanowires. Two different type of target motion were used – rotation and computer controlled by using an XY stage. The impact was investigated of the trajectory of the laser beam on the target surface on the characteristics of the ablation process itself and the nanostructures obtained. Two circular trajectories of different diameters generated at the target rotation were utilized. The fundamental (λ = 1064 nm) and the third harmonic (λTHG = 355 nm) wavelengths of a Nd-YAG laser system were used for the fabrication procedure. They were selected in order to study the influence of the two processes accompanying the ablation procedure, namely, scattering and absorption of the incident light by the nanostructures already created (the so-called self-absorption). The two possible nanostructures – nanoparticles and nanowires, were obtained by an appropriate choice of the laser fluence for each of the wavelengths selected. The optical extinction spectra of the fabricated colloids allowed an indirect assessment of the shape and size-distribution of the nanostructures obtained, whose size and morphology were visualized by transmission electron microscopy (TEM). It was established that the most appropriate processing conditions in terms of the efficiency of the ablation and reproducibility of the sample characteristics were achieved using a computer controlled XY stage.
Colloidal solutions of gold and silver nanoparticles (NPs) were prepared using a method pulsed laser ablation of target in
liquid media. A gold and silver targets immersed in double distilled water are irradiated for 20 min by laser pulses with
duration of 15 ns and repetition rate of 10 Hz. In order to investigate influences of laser wavelength and fluence on the
particle size, shape and optical properties the experiments were preformed by using two different wavelength - the
fundamental and the second harmonic (SH) (λ = 1064 and 532 nm, respectively) of a Nd:YAG laser system. Two
different values of the laser fluence for each wavelength at the experimental conditions chosen were used and thus it was
changed from several J/cm2 to tens of J/cm2. For characterization of the NPs shape and size distribution were used
transmission electron microscope (TEM) and optical transmission spectroscopy in the near UV and in the visible region.
Spherical shape of the nanoparticles at the low laser fluence and appearance of aggregation and building of nanowires at
the SH and high laser fluence is seen. Dependence of the mean particle size at the SH on the laser fluence was
established. The mean diameter of gold NPs became smaller with decrease in laser wavelength.
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