J. M. J. Santillán

Analysis of the structure, configuration, and sizing of Cu and Cu oxide nanoparticles generated by fs laser ablation of solid target in liquids

We report on the analysis of structure, configuration, and sizing of Cu and Cu oxide nanoparticles (Nps) produced by femtosecond (fs) laser ablation of solid copper target in liquids. Laser pulse energy ranged between 500 μJ and 50 μJ. Water and acetone were used to produce the colloidal suspensions. The study was performed through optical extinction spectroscopy using Mie theory to fit the full experimental spectra, considering free and bound electrons size dependent contributions to the metal dielectric function. Raman spectroscopy and AFM technique were also used to characterize the sample. Considering the possible oxidation of copper during the fabrication process, two species (Cu and Cu2O) arranged in two structures (bare core or core-shell) and in two configuration types (Cu-Cu2O or Cu2O-Cu) were considered for the fitting depending on the laser pulse energy and the surrounding media. For water at high energy, it can be observed that a Cu-Cu2O configuration fits the experimental spectra of the collo...

Quantitative optical extinction-based parametric method for sizing a single core–shell Ag–Ag2O nanoparticle

This paper develops a parametric method for determining the core radius and shell thickness in small silver?silver-oxide core?shell nanoparticles (Nps) based on single particle optical extinction spectroscopy.The method is based on the study of the relationship between plasmon peak wavelength, full width at half maximum (FWHM) and contrast of the extinction spectra as a function of core radius and shell thickness. This study reveals that plasmon peak wavelength is strongly dependent on shell thickness, whereas FWHM and contrast depend on both variables. These characteristics may be used for establishing an easy and fast stepwise procedure to size core?shell NPs from single particle absorption spectrum. The importance of the method lies in the possibility of monitoring the growth of the silver-oxide layer around small spherical silver Nps in real time.Using the electrostatic approximation of Mie theory, core?shell single particle extinction spectra were calculated for a silver particles core size smaller than about 20?nm and different thicknesses of silver oxide around it. Analysis of the obtained curves shows a very particular characteristic of the plasmon peak of small silver?silver-oxide Nps, expressed in the fact that its position is strongly dependent on oxide thickness and weakly dependent on the core radius. Even a very thin oxide layer shifts the plasmon peak noticeably, enabling plasmon tuning with appropriate shell thickness.This characteristic, together with the behaviour of FWHM and contrast of the extinction spectra can be combined into a parametric method for sizing both core and shell of single silver Nps in a medium using only optical information. In turn, shell thickness can be related to oxygen content in the Nps surrounding media.The method proposed is applied to size silver Nps from single particle extinction spectrum. The results are compared with full optical spectrum fitting using the electrostatic approximation in Mie theory. The method may be the basis for developing a plasmonic sensor for O2 concentration based on Ag single NP spectroscopy.

Influence of size-corrected bound-electron contribution on nanometric silver dielectric function. Sizing through optical extinction spectroscopy

The study of metal nanoparticles (NPs) is of great interest due to their ability to enhance optical fields on the nanometric scale, which makes them interesting for various applications in several fields of science and technology. In particular, their optical properties depend on the dielectric function of the metal, its size, shape and surrounding environment.This work analyses the contributions of free and bound electrons to the complex dielectric function of spherical silver NPs and their influence on the optical extinction spectra. The contribution of free electrons is usually corrected for particle size under 10?nm, introducing a modification of the damping constant to account for the extra collisions with the particles boundary.For the contribution of bound electrons, we considered the interband transitions from the d-band to the conduction band including the size dependence of the electronic density states for radii below 2?nm. Bearing in mind these specific modifications, it was possible to determine optical and band energy parameters by fitting the bulk complex dielectric function. The results obtained from the optimum fit are: Kbulk?=?2???1024 (coefficient for bound-electron contribution), Eg?=?1.91?eV (gap energy), EF?=?4.12?eV (Fermi energy), and ?b?=?1.5???1014?Hz (damping constant for bound electrons).Based on this size-dependent dielectric function, extinction spectra of silver particles in the nanometric?subnanometric radius range can be calculated using Mies theory, and its size behaviour analysed. These studies are applied to fit experimental extinction spectrum of very small spherical particles fabricated by fs laser ablation of a solid target in water. From the fitting, the structure and size distribution of core radius and shell thickness of the colloidal suspension could be determined. The spectroscopic results suggest that the colloidal suspension is composed by two types of structures: bare core and core?shell. The former is composed by Ag, while the latter is composed by two species: silver?silver oxide (Ag?Ag2O) and hollow silver (air?Ag) particles. High-resolution transmission microscopy and atomic force microscopy analysis performed on the dried suspension agree with the sizing obtained by optical extinction spectroscopy, showing that the latter is a very good complementary technique to standard microscopy methods.

Determination of nanometric Ag2O film thickness by surface plasmon resonance and optical waveguide mode coupling techniques

There is a continuing need for measuring nanometric film thicknesses for a wide variety of industrial and scientific purposes. Kretschmann-type sensors are well-known multilayer nanometric sensing devices. This work is focused on two objectives: firstly, the design of an Ag2O Kretschmann sensor and, secondly, the development of a measurement protocol for determining silver oxide thickness when a silver film of given initial thickness is gradually converted into silver oxide by exposure to a controlled oxygen-rich atmosphere. The particular characteristics of the reflectivity curves of this multilayer structure are studied for both p- and s-wave polarization as a function of the incidence angle and layer thickness. In the former, the surface plasmon resonance (SPR) dip position as well as its FWHM depends strongly on silver oxide thickness. For s-wave polarization, a broad dip due to optical waveguide mode coupling is observed for angles larger than the total internal reflection (TIR) angle when sufficiently large silver oxide thicknesses are studied. Besides, reflectivity at fixed angles for both polarizations was studied as a function of the silver oxide layer. Each of these relations may be represented by different continuous functions defined for successive ranges that can be used as calibration curves. Taking into account all these features, a measurement protocol is proposed for determining silver oxide thickness when a 45 nm initial silver film is gradually converted into silver oxide by exposure to an oxygen-rich atmosphere. This new approach is an alternative to the traditional methods of full angular interrogation in the Kretschmann configuration. Based on this procedure, it is possible to measure Ag2O thickness in the range 0–70 nm.

Size dependent Cu dielectric function for plasmon spectroscopy: Characterization of colloidal suspension generated by fs laser ablation

Copper metal nanoparticles (Nps) have received increasing interest during the last years due to their potential applications in several fields of science and technology. Their optical properties depend on the characteristics of the dielectric function of the metal, their size, and the type of environment. The contribution of free and bound electrons on the dielectric function of copper Nps is analyzed as well as their influence on its plasmonic properties. The contribution of free electrons is corrected for particle size under 10 nm, introducing a term inversely proportional to the particles radius in the damping constant. For bound electron contribution, interband transitions from the d-band to the conduction band are considered. For particles with sizes below 2 nm, the larger spacing between electronic energy levels must be taken into account by making the electronic density of states in the conduction band size-dependent. Considering these specific modifications, optical parameters and band energy val...

Optical and Magnetic Properties of Fe Nanoparticles Fabricated by Femtosecond Laser Ablation in Organic and Inorganic Solvents

Magnetic nanoparticles have attracted much interest due to their broad applications in biomedicine and pollutant remediation. In this work, the optical, magnetic, and structural characteristics of colloids produced by ultrashort pulsed laser ablation of a solid Fe target were studied in four different media: HPLC water, an aqueous solution of trisodium citrate, acetone, and ethanol. Optical extinction spectroscopy revealed an absorption band in the UV region for all, in contrast to the results obtained with nanosecond lasers. Micro-Raman spectroscopy showed that the samples are heterogeneous in their composition, with hematite, maghemite, and magnetite nanoparticles in all four solvents. Similar results were obtained by electron diffraction, which also found α-Fe. Magnetic properties were studied by vibrating-sample magnetometry, and showed nanoparticles in the superparamagnetic state. Under certain experimental conditions, submicrometer-sized iron oxide nanoparticles agglomerate into fractal patterns that show self-similar properties. Self-assembled annular structures on the nanometer scale were also observed and are reported for the first time.

Plasmonic properties and sizing of core-shell Cu-Cu2O nanoparticles fabricated by femtosecond laser ablation in liquids

The synthesis and study of optical properties of copper nanoparticles are of great interest since they are applicable to different areas such as catalysis, lubrication, conductive thin films and nanofluids. Their optical properties are governed by the characteristics of the dielectric function of the metal, its size and environment. The study of the dielectric function with radius is carried out through the contribution of free and bound electrons. The first one is corrected for size using the modification of the damping constant. The second one takes into consideration the contribution of the interband transitions from the d-band to the conduction band, considering the larger spacing between electronic energy levels as the particle decreases in size below 2 nm. Taking into account these specific modifications, it was possible to fit the bulk complex dielectric function, and consequently, determine optical parameters and band energy values such as the coefficient for bound electron contribution Qbulk = 2 x 1024, gap energy Eg = 1.95 eV, Fermi energy EF = 2.15 eV and damping constant for bound electrons γb = 1.15 x 1014 Hz. The fit of the experimental extinction spectra of the colloidal suspensions obtained by 500 μJ ultrashort pulse laser ablation of solid target in water and acetone, reveals that the nanometric and subnanometric particles have a Cu- Cu2O structure due to an oxidation reaction during the fabrication. The results were compared with those obtained by AFM, observing a very good agreement between the two techniques, showing that Optical Extinction Spectroscopy (OES) is a good complementary technique to standard electron microscopy.

Structure, configuration, and sizing of Ni nanoparticles generated by ultrafast laser ablation in different media

In recent years, nickel nanoparticles (NPs) have increased scientific interest because of their extensive prospects in catalysts, information storage, large-scale batteries and biomedicine. Several works on Ni NPs generation by laser ablation have appeared in the literature in the last years, using different pulsed laser regimes and different media have been published recently. In this work we analyze the characteristics of species, structure (bare core or core-shell), configuration and size distribution of NPs generated by fs pulse laser ablation over a Ni solid target in n-heptane and water. We explore the presence of NiO-Ni core-shell and hollow Ni (or air-Ni) NPs in the colloids obtained. These were experimentally characterized using AFM and TEM microscopy, as well as Optical Extinction Spectroscopy (OES). Extinction spectra were modeled using Mie theory through an appropriate modification of the complex experimental dielectric function, taking into account a size-dependent corrective term for each free and bound electron contribution. Experimental UVvisible- NIR spectra were reproduced considering a size distribution of bare core, hollow and core-shell structures NPs. In both media, Ni NPs shape and size distribution agrees with that derived from TEM and AFM analysis.