Due to their peculiar qualities, metal-based nanostructures have been extensively used in applications such as catalysis, electronics, photography, and information storage, among others. New applications for metals in areas such as photonics, sensing, imaging, and medicine are also being developed. Significantly, most of these applications require the use of metals in the form of nanostructures with specific controlled properties. The properties of nanoscale metals are determined by a set of physical parameters that include size, shape, composition, and structure. In recent years, many research fields have focused on the synthesis of nanoscale-sized metallic materials with complex shape and composition in order to optimize the optical and electrical response of devices containing metallic nanostructures. The present paper aims to overview the most recent results—in terms of fabrication methodologies, characterization of the physico-chemical properties and applications—of complex-morphology metal-based nanostructures. The paper strongly focuses on the correlation between the complex morphology and the structures’ properties, showing how the morphological complexity (and its nanoscale control) can often give access to a wide range of innovative properties exploitable for innovative functional device production. We begin with an overview of the basic concepts on the correlation between structural and optical parameters of nanoscale metallic materials with complex shape and composition, and the possible solutions offered by nanotechnology in a large range of applications (catalysis, electronics, photonics, sensing). The aim is to assess the state of the art, and then show the innovative contributions that can be proposed in this research field. We subsequently report on innovative, versatile and low-cost synthesis techniques, suitable for providing a good control on the size, surface density, composition and geometry of the metallic nanostructures. The main purpose of this study is the fabrication of functional nanoscale-sized materials, whose properties can be tailored (in a wide range) simply by controlling the structural characteristics. The modulation of the structural parameters is required to tune the plasmonic properties of the nanostructures for applications such as biosensors, opto-electronic or photovoltaic devices and surface-enhanced Raman scattering (SERS) substrates. The structural characterization of the obtained nanoscale materials is employed in order to define how the synthesis parameters affect the structural characteristics of the resulting metallic nanostructures. Then, macroscopic measurements are used to probe their electrical and optical properties. Phenomenological growth models are drafted to explain the processes involved in the growth and evolution of such composite systems. After the synthesis and characterization of the metallic nanostructures, we study the effects of the incorporation of the complex morphologies on the optical and electrical responses of each specific device.

Metal nanostructures are, nowadays, extensively used in applications such as catalysis, electronics, sensing, optoelectronics and others. These applications require the possibility to design and fabricate metal nanostructures directly on functional substrates, with specifically controlled shapes, sizes, structures and reduced costs. A promising route towards the controlled fabrication of surface-supported metal nanostructures is the processing of substrate-deposited thin metal films by fast and ultrafast pulsed lasers. In fact, the processes occurring for laser-irradiated metal films (melting, ablation, deformation) can be exploited and controlled on the nanoscale to produce metal nanostructures with the desired shape, size, and surface order. The present paper aims to overview the results concerning the use of fast and ultrafast laser-based fabrication methodologies to obtain metal nanostructures on surfaces from the processing of deposited metal films. The paper aims to focus on the correlation between the process parameter, physical parameters and the morphological/structural properties of the obtained nanostructures. We begin with a review of the basic concepts on the laser-metal films interaction to clarify the main laser, metal film, and substrate parameters governing the metal film evolution under the laser irradiation. The review then aims to provide a comprehensive schematization of some notable classes of metal nanostructures which can be fabricated and establishes general frameworks connecting the processes parameters to the characteristics of the nanostructures. To simplify the discussion, the laser types under considerations are classified into three classes on the basis of the range of the pulse duration: nanosecond-, picosecond-, femtosecond-pulsed lasers. These lasers induce different structuring mechanisms for an irradiated metal film. By discussing these mechanisms, the basic formation processes of micro- and nano-structures is illustrated and justified. A short discussion on the notable applications for the produced metal nanostructures is carried out so as to outline the strengths of the laser-based fabrication processes. Finally, the review shows the innovative contributions that can be proposed in this research field by illustrating the challenges and perspectives.

Nanostructured Au films were deposited on Si(111) by room-temperature sputtering. By the atomic force microscopy technique we studied the evolution of the Au film morphology as a function of the film thickness $h$ and annealing time $t$ at 873 K. By the study of the evolution of the mean vertical and horizontal sizes of the islands forming the film and of their fraction of covered area as a function of $h$ from $1.7\times {10}^{17}$ to $1.0\times {10}^{18}\text{Au}/{\text{cm}}^2$ we identified four different growth stages such as: (1) $1.7\times {10}^{17}\le h\le 3.0\times {10}^{17}\text{Au}/{\text{cm}}^2$ , nucleation of nanometric three-dimensional (3D) hemispherical Au clusters; (2) $3.0\times {10}^{17}<h\le 5.2\times {10}^{17}\text{Au}/{\text{cm}}^2$ , lateral growth of the Au clusters; (3) $5.2\times {10}^{17}<h\le 7.7\times {10}^{17}\text{Au}/{\text{cm}}^2$ , coalescence of the Au clusters; (4) $7.7\times {10}^{17}<h\le 1.0\times {10}^{18}\text{Au}/{\text{cm}}^2$ , vertical growth of the coalesced Au clusters. The application of the dynamic scaling theory of growing interfaces allowed us to calculate the dynamic scaling exponent $z=3.8\pm 0.3$ , the dynamic growth exponent $\beta =0.38\pm 0.03$ , the roughness exponent $\alpha =1.4\pm 0.1$ and the Avrami exponent $m=0.79\pm 0.02$ . Finally, the study of the evolution of the mean Au clusters size as a function of annealing time at 873 K allowed us to identify the thermal-induced self-organization mechanism in a surface diffusion limited ripening of 3D structures and also the surface diffusion coefficient of Au on Si(111) at 873 K was estimated in $\left(8.2\times {10}^{-16}\right)\pm \left(3\times {10}^{-17}\right){\text{m}}^2/\text{s}$ .

Very thin Au layers were deposited on SiC hexagonal and $\mathrm{Si}{\mathrm{O}}_2$ substrates by sputtering. The Au surface diffusion, clustering, and self-organization of Au nanoclusters on these substrates, induced by thermal processes, were investigated by Rutherford backscattering spectrometry, atomic force microscopy, scanning electron microscopy, and transmission electron microscopy. On both types of substrates, clustering is shown to be a ripening process of three-dimensional structures controlled by surface diffusion and the application of the ripening theory allowed us to derive the surface diffusion coefficient and all other parameters necessary to describe the entire process. The system Au nanoclusters/SiC and Au nanoclusters/$\mathrm{Si}{\mathrm{O}}_2$ are proposed as nanostructured materials for nanoelectronic and nanotechnology applications.