**Abstract**

In this chapter Ag, Ni and Fe nanocolloids synthesized by "green" ultrashort pulse laser ablation of solid metal targets using different pulse energies and liquid media are characterized by different techniques. Optical extinction spectroscopy (OES), micro-Raman spectroscopy (MRS), transmission electron microscopy (TEM) and electron diffraction (ED) were independently used to analyze optical, morphological and compositional properties of the generated nanocolloids. In a deeper way, the stability characteristics of Ag nanocolloids in aqueous solutions with different stabilizers were studied owing to their potential use in biocompatible compounds. Besides, due to their interesting applications, few atoms Ag nanoclusters (NCs) were synthesized using the same ablation technique, analyzing their fluorescent and photocatalytic properties. On the other hand, to expand the characterization of the nanocolloids, their magnetic behavior was inspected for the Ni and Fe by vibrating sample magnetometry (VSM).

**Keywords:** nanocolloids, green synthesis, metal nanoparticles, nanoclusters, femtosecond laser ablation

#### **1. Introduction**

Interest in metal nanomaterials synthesis has grown rapidly in the last years due to their particular physical and chemical properties arising from atom interaction and quantum confinement at the nanoscale. Their applicability spans different fields of science and technology [1–3]. It is known that chemical synthesis methods tend to yield highly monodisperse colloidal suspensions, but mixed with unwanted chemical precursors, which often leads to purification steps to remove the chemical by-products and may derive in expensive and complicated procedures. For this reason, femtosecond laser ablation synthesis in solution (FLASiS) has emerged as a competitive and alternative method for synthesizing metallic nanomaterials without the intervention of unwanted chemical compounds. Besides, it has the ability of producing small spherical nanoparticles (NPs) [4, 5] as well as few atoms metal NCs [6].

In this chapter, different independent techniques are used to characterize the ablated nanomaterials. Altogether, they retrieve complementary and interrelated information about different NPs characteristics. TEM provides knowledge about morphology, internal structure, sphericity and size distribution in a small piece of sample. ED is an established technique that can identify different phase compositions and crystallinity type. Optical spectroscopy techniques (absorption, extinction, scattering and fluorescence) have the ability to interact with a very large number of NPs (on the order of cm 12 3<sup>−</sup> 10 ), enhancing statistics. OES together with Mie theory yields information related to size distribution, sphericity, configuration and composition of the NPs in the colloidal sample. MRS retrieves information about possible interactions between stabilizer solution molecules adsorbed to the NPs walls and the NP itself. In particular, for Ag nanocolloids generated in aqueous solutions with small concentrations of stabilizers, long term stability characteristics were studied, aiming to possible applications in biocompatible antibacterial compounds. For the case of magnetic metals NPs, magnetic nanocolloid properties were studied using VSM. Finally, fluorescent and photocatalytic properties of few atoms Ag NCs were analyzed.
