IR ns pulsed laser irradiation of Polydimethylsiloxane in vacuum

Highlights

• Virgin PDMS is very transparent to near IR lasers.

• PDMS with Au NPs is very absorbent IR lasers.

• Ns pulsed laser at 10¹⁰ W/cm2 produce a plasma at about 22 eV temperature.

• The irradiated PDMS shows formation of crystalline Si and SiC structures.

• The PDMS transparence reduces increasing the number of laser irradiation shots.

Abstract

Polydimethylsiloxane (PDMS) has been irradiated by a ns pulsed IR laser in vacuum and the radiation effects induced by the coherent radiation, above a threshold fluence, produce a non-equilibrium plasma and material ablation. The laser absorption can be enhanced using gold nanoparticles (AuNPs) embedded in the polymer. The laser-generated plasma has been characterized, whereas the ion emission from both the virgin PDMS and the PDMS + AuNPs sheets has been analysed both in mass and energy. Morphological investigations, optical spectroscopy and compositional analyses have been performed. The adopted laser treatment can be employed to modify the properties of PDMS with and without AuNPs inducing a local enrichment of microcrystalline silicon in both cases.

Introduction

Polydimethylsiloxane (PDMS) is a polymer belonging to the organosilicon compounds commonly referred to as silicones. PDMS is particularly known for its unusual rheological properties when it is partially cross-linked [1]. In its cross-linked state, the elastomer does not permanently deform under stress, and it is stable over wide ranges of temperature and humidity [2].

PDMS is optically transparent, inert, non-toxic, flexible, biocompatible and non-flammable [3]. It is composed of a siloxane (Si–O) backbone carrying two methyl groups, as organic hydrocarbon substituents, attached to Si atoms; its repeating unit has structural formula [-Si(CH₃)₂–O]. PDMS chains contain two vinyl end groups that react with a multifunctional cross-linker forming a three-dimensional network [4]. The polymerization reaction produces hydrochloric or acetic acid. PDMS density is 0.965 g/cm³, it withstands high temperatures up to about 300 °C, it is very flexible with a Young module below 1 MPa, has hydrophobic properties and high permeability to gases like helium, oxygen, nitrogen and carbon dioxide [5,6]. Some properties of PDMS are summarized in Table 1 [7,8].

Thanks to these properties PDMS is recognized as an optimum material for manufacturing micro-devices in microelectronics, engineering, bio-medicine and optics [9]. In fact, PDMS finds application as biomaterial for the realization of sensors and prosthesis, including porous PDMS membranes to control the release of drugs. It is also present in shampoos, food, lubricants and heat-resistant tiles. PDMS is also the most commonly used material in the soft lithography due to its soft and flexible nature, its transparency to ultraviolet and visible light and its permeability to oxygen, making it suitable as a substrate for cell culture [10]. Many of these applications can be improved using treatments with ion and laser beams [2] that modify the PDMS surface to improve its wettability and cell-adhesion.

In particular, the surface chemistry and topography simultaneous modifications induced by the laser beam can be employed to increase the PDMS hydrophilicity and to realize micrometric patterns with properties different from the bulk. In fact, as reported in the ref. [11], in many polymers, the irradiation of fast pulsed laser at high intensity in vacuum induces ionization, radicals, scissions and cross-links, gas desorption and plasma formation with emission of ions, electrons and X-rays. The fast and high energy density deposition produces ablation above a threshold fluence with material removing that can be deposited on suitable substrates [12]. Therefore, by performing the pulse laser deposition (PLD) of thin films and/or exploiting the laser radiation effects new macro- and micro-structures, material treatments, fast heating and quenching can be realized for the design of devices and processes useful for industry, research and engineering. Literature reports that also the gas permeability, the electrical and thermal properties and the mechanical properties change after the laser irradiations [13,14]. Furthermore, it is possible to induce a controlled laser writing in the polymer surface [15]. The controlled laser treatment on the PDMS sample, in fact, can be employed moving the target in front of the fixed laser beam to realize micrometric patterns, strips and other geometrical designs that may find useful applications in different scientific areas, as recent literature reports [16,17].

However, it is already known that the PDMS laser exposure produces surface degradation of the elastomer altering its optical properties: an increase of the linear absorption coefficient and a decrease of the threshold fluence in the laser ablation have been observed. Different photothermal and photochemical processes, depending on the laser wavelength, pulse length, fluence, number of pulses, and intrinsic properties of the polymer, influence the effects induced in the material [18]. Recently it has been shown that the use of gold nanoparticles (AuNPs) embedded in PDMS permits to modify drastically the optical properties making the material absorbent to specific wavelength bands and giving the material new properties dependent on the AuNPs concentration [19]. In fact, the surface plasma resonance (SPR) absorption effect confers absorption resonance due to the oscillating electrons on the nanoparticle surface, which occurs in the field of visible region (500–580 nm) for Au nanoparticles ranging in the diameter size within 5 nm and 100 nm, according to the literature [20].

In this paper a special attention is devoted to the characterization of plasma generated by an ns pulsed IR laser beam impinging on both a PDMS virgin sheet and a PDMS + AuNPs foil under high vacuum conditions. As far as we know such a characterization is not reported in the literature. Plasma induces polymer modifications which in air may increase the oxygen content while in vacuum generally produces oxygen and hydrogen depletions. This investigation has been developed to understand some aspects of the laser-matter interaction processes using PDMS with and without AuNPs. For this purpose the ion emission from the virgin PDMS and the PDMS + AuNPs foils has been analysed in both mass and energy in high vacuum conditions. Moreover, morphological investigations, optical spectroscopy and composition analyses have been carried out on the laser irradiated PDMS sheets with and without AuNPs.

A particular importance is also devoted to the laser processing of PDMS in air and gases, which generally have a different result with respect to vacuum conditions because the hot plasma cannot expands as in vacuum, it is confined between the solid target and the gas or air, shock waves are produced and the products of the laser ablation have low velocity. The emitted particles have a little free medium path, many collisions occur and a fast recombination effect neutralizes the ion charge states. The interaction with the gas molecules give rise to photoemission mainly in the visible and IR regions with a strong UV absorption Moreover, the presence of reactive gases such as oxygen produces effects of oxidations and other chemical reactions with the reactive radicals produced by the fast energy deposition in the polymer, according to the literature [21].