In-situ generation of Au nanoparticles in poly(methyl methacrylate) films via MeV proton irradiation

Abstract

It has been shown that a radiation-sensitive polymer such as PMMA, which is widely utilised in lithography as a positive resist, can be used to create a flexible nanocomposite with metal nanoparticles by proton irradiation without any additional treatment. Three 50-μm-thick free-standing polymer films with 4, 10 and 16 wt% of gold were prepared with the spin-coating technique from a mixture of PMMA and hydrogen tetrachloroaurate trihydrate and irradiated with 2-MeV protons. Fluences in the range of 2 × 1013–2 × 1014 protons/cm2 were used to reduce gold ions to neutral metal atoms with the subsequent formation of nanoparticles. UV–Vis and FTIR spectroscopy was employed to study the Au-nanoparticle coalescence after the radiation modification. AFM and SEM microscopy showed the morphology of the nanoparticles on the polymer surface, while TEM microscopy was used to visualise the nanoparticles formed inside the polymer. It was found that the fluence of the Au-nanoparticle formation can be lower than that used in proton-beam lithography on PMMA. Although the free-standing foil allowed generated gases to diffuse from both sides, it did not withstand the high fluence applied to it on the substrate. The concentration of the generated nanoparticles depended both on the primary Au-atom concentration in the PMMA matrix and on the proton irradiation fluence used. In Au–PMMA composites containing 10 and 16 wt% of gold, the fluence of 2 × 1013 protons/cm2 was enough to produce a sufficient amount of nanoparticles to obtain the resonance absorption of light, whereas the sample with 4 wt% of gold required a higher proton fluence to exhibit Au-atom coalescence. It has been ascertained that the amount of the nanoparticles synthesized by proton irradiation is directly proportional to the concentration of gold in PMMA and to the proton fluence used. As for the size of the nanoparticles, it is directly proportional to the concentration of gold in PMMA, but inversely proportional to the fluence used.