Femtosecond laser-induced periodic surface nanostructuring of sputtered platinum thin films

Highlights

• Femtosecond laser-induced surface nanostructures on sputtered platinum thin films.

• Three types of structures obtained: random nanostructures, LSFL and HSFL.

• Two different modification regimes have been established based on laser fluence.

Abstract

In this work, submicro and nanostructures self-formed on the surface of Platinum thin films under femtosecond laser-pulse irradiation are investigated. A Ti:Sapphire laser system was used to linearly scan 15 mm lines with 100 fs pulses at a central wavelength of 800 nm with a 1 kHz repetition rate. The resulting structures were characterized by scanning electron microscopy (SEM) and 2D-Fast Fourier Transform (2D-FFT) analysis. This analysis of images revealed different types of structures depending on the laser irradiation parameters: random nanostructures, low spatial frequency LIPSS (LSFL) with a periodicity from about 450 to 600 nm, and high spatial frequency LIPSS (HSFL) with a periodicity from about 80 to 200 nm. Two different modifications regimes have been established for the formation of nanostructures: (a) a high-fluence regime in which random nanostructures and LSFL are obtained and (b) a low-fluence regime in which HSFL and LSFL are obtained.

Introduction

In the last decade, Laser-Induced Periodic Surface Structures (LIPSS) have attracted increased research attention because of their applications in surface hydrophobic/hydrophilic properties [1], friction reduction [2], [3], control of the surface reflection [4], control of the cell growth direction [5], or Surface-Enhanced Raman Spectroscopy [6]. LIPSS have been induced in a wide variety of materials by pulsed lasers in the nanosecond (ns) to femtosecond (fs) regimes [3], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Two distinct types of LIPSS have been identified so far: Low Spatial Frequency LIPSS (LSFL) and High Spatial Frequency LIPSS (HSFL). Their orientations can be either parallel or perpendicular to the incident laser beam polarization, depending both on laser irradiation parameters and material properties [12], [19]. LSFL have a spatial period close to or slightly smaller than the irradiation laser wavelength. Their formation is considered a result of the interference between the incident laser beam and surface-scattered waves [21], [22] or Surface Plasmon Polaritons (SPP) [15], [23]. HSFL have a spatial period that is much smaller than the laser wavelength. Several theories have been proposed for HSFL formation mechanisms: second harmonic generation (SHG) [10], [12], [24], self-organization [25], or interference with modification of the optical properties during laser processing [26]. However, none of these theories gives a complete explanation of LIPSS formation; their formation is still debated. Consequently, further investigations on the resulting LIPSS on different materials and under different irradiation conditions can help to better understand their formation mechanism. LIPSS have generally been identified in the femtosecond laser regime on bulk materials, semiconductors [11], [15], dielectrics [17], [27] and metals [16], [23], [27], [28], [29], [30]. Among metallic materials, platinum is of particular interest due to its use as contact electrode in microsensing applications or as chemical catalyst, where high surface-to-volume ratio may lead to an increased sensitivity and/or enhanced performance [23]. The formation of LIPSS on bulk platinum has been already studied by other groups [23], [28], [29], [30]. These authors have reported LSFL perpendicularly oriented to the laser beam polarization with periods varying from 0.55 to 0.69 μm depending on the irradiation conditions. Table 1 provides a summary of the obtained LIPSS on this material, which includes the most relevant process parameters and the period and orientation of the resulting structures.

LIPSS formation on thin metal films instead has been less explored, although is getting increased interest in recent years [20], [31], [32]. The work here presented is centered on the experimental observation of LIPSS in Pt thin films under ultrafast laser pulses.