Thermoplasmonic maskless lithography assisted by gold nanostars

Introduction: The heat dissipation upon excitation of gold nanostars´ (AuNSs) plasmon resonance can produce more than 100 °C increase in the local temperature. On the other hand, polylactic acid (PLA) is a thermoplastic... [ view full abstract ]

Introduction:

The heat dissipation upon excitation of gold nanostars´ (AuNSs) plasmon resonance can produce more than 100 °C increase in the local temperature. On the other hand, polylactic acid (PLA) is a thermoplastic biodegradable polymer with a glass transition temperature around 60 °C. Here, we show that by covering the PLA films with AuNSs it is possible to locally modify them by harvesting the thermoplasmonic effect. Under a 976 nm focused laser beam, the local temperature in AuNSs surpasses the glass transition of the base polymer producing their attachment to its surface. The following dissolution of the unexposed material allows the precise control of the engraving process in the microscale. A computer numerical control system (CNC) was developed to transfer 2D patterns, opening up the thermoplasmonic lithography technique and the laser printing of nanoparticles on rigid and flexible substrates. Furthermore, by embedding Er³⁺ doped upconversion nanoparticles (UCNPs) which can act as primary nanothermometers into the polymer layer, it is possible to optically determine the local temperature, visualize the NIR laser spot and produce luminescent patterns. The methods developed were applied to produce patterned substrates for surface enhanced Raman spectroscopy (SERS), and optical encoding for anti-counterfeiting technologies.

Methods:

PLA films were formed by dissolving commercial 3D printing filaments in chloroform and depositing the solution by spin coating on different substrates (glass, silicon and polyimide tape). AuNSs of ca. 50 nm in diameter were synthesized by using spherical gold seed nanoparticles and by controlling the growth of the branches by stoichiometric addition of AgNO₃ and ascorbic acid at room temperature. Poly(styrenesulfonate) (PSS) was added to stabilize the AuNSs colloids.

Results:

The full characterization of the surface topography and the resolution of the technique is presented. Raman spectroscopy measurements of rhodamine 6G deposited inside and outside the engraved patterns of AuNSs reveal the SERS effect. Optical encoding was demonstrated by transferring a generated QR code on a PLA-UCNPs film.

Discussion:

The technique and the materials presented here are relevant for the development of plasmonic-based devices and as a fabrication tool that combines bottom-up and top-down approaches in a synergic way.